/g and pore volumes from 0.5 to

and it reversibility, with a simple adsorption-desorption kinetics.

due to its low storage density and the high costs involved [1].

cates, CMK have specific surface areas from 1000 to 2000 m2

volumes, which make effective H2 physisorption feasible.

and three-dimensional conducting network [10,11].

metals or cations are added, improving hydrogen sorption [6–9].

was fully reversible and higher than that in carbon hosts [12,13].

#### **2.1. Templates preparation: MCM-48 and SBA-15**

The mesoporous material MCM-48 was prepared as the template for the CMK-1. The meso‐ porous silicate was synthesized by hydrolysis of tetraethylorthosilicate (TEOS, 98%, Sigma– Aldrich) at room temperature, in an aqueous solution, using cetyltrimethylammonium bromide (CTAB, Sigma–Aldrich) as a surfactant. The procedure designed is described as follows: Sodium hydroxide (NaOH, Sigma–Aldrich) and CTAB were dissolved in deionized water, and then tetraethyl orthosilicate (TEOS) was added. The molar composition of the gel was 1 M TEOS: 0.48 M NaOH: 0.48 M CTAB: 55 M H2O. After stirring for about 1 h, the solution was transferred into a Teflon bottle and there it was heated up to a temperature of 373 K. After 72 h, the mixture was kept at RT and a solution of HCl was used to turn the pH to 7. Afterwards, the solution was kept at 373 K for 72 h. In order to wash and dry the product, the solution was filtered [14]. The template was extracted by thermal treatment performed up to 373 K, using N2 (20 mL/min) and afterwards a calcination at 723 K was implemented, raising the tempera‐ ture 3 K/min [15].

Ordered mesoporous silica SBA-15 was prepared using the triblock copolymer, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly (ethylene glycol), (EO20PO70EO20, P123- Sigma–Aldrich), as surfactant, and tetraethyl orthosilicate (TEOS, Sigma–Aldrich, reagent grade 98%) as the source of silica. The technique planned was: A solution was prepared with P123 (20 g) in 400 ml of HCl 1 M at 323 K. Then, TEOS (40 g) was drop under stirring during 24 h at the same temperature. The subsequent white combination was moved to a propylene flask and matured by 72 h at 373 K. The material was filtered out and then washed with deionized water until pH ~ 6. Its molar composition was 1 Si:0.018 EO20PO70EO20:2.08 HCl:112 H2O. The template was removed by immersing the material in ethanol reflux for 6 h and then calcining it at 823 K in air for 6 h. The product was finally filtered, washed, and dried in air at 363 K [16].

#### **2.2. Synthesis of the replicas CMK-1 and CMK-3**

Mesoporous carbons CMK-1 and CMK-3 were prepared via a two step impregnation of the mesopores of MCM-48 and SBA-15 with a solution of sucrose using an incipient wetness method. Briefly, 1.0 g of the as-prepared silica material MCM-48 for CMK-1 and SBA-15 for CMK-3 was soaked with 1.1 g of sucrose and 0.14 g of H2SO4 in 5.0 g of deionized water solution. The mixture was then dried at 373 K for 6 h and then at 433 K for 6 h. The silica sample, which now contained partially polymerized and carbonized sucrose, had 0.65 g of sucrose, 90 mg of H2SO4, and 5.0 g of deionized water added to it, and was treated again at 373 K and 433 K. To complete the carbonization, the sucrose-silica composite was heated at 1173 K for 4 h under nitrogen flow (20 mL/min).

The silica template was dissolved with 5 wt% hydrofluoric acid at room temperature. This step was done two times in order to dissolve entirely the template. The template-free carbon product thus obtained was filtered, washed with deionized water and ethanol, and dried [12, 13].

#### **2.3. Synthesis of Zn-CMK-1**

Zn-CMK-1 was prepared using as a source of zinc Zn nitrate (Sigma–Aldrich 98%, hexahy‐ drate), dissolved in ethanol, and then mixed with a portion of CMK-1 at room temperature. The suspension was left static for 24 h, before being dried at 373 K for a period of 18 h, and subsequently desorbed from 298 K to 473 K in an inert atmosphere (N2), at a rate of 10 K/min. It was held at this latter temperature for 5 h. During the whole process, the nitrogen flow was 20 ml/min. To obtain the reduced sample of Zn-CMK-1, it was placed under H2 flow at 773 K for more than 6 h with a ramp rate 10 K/min. The sample was denoted as Zn-CMK-1. ICP analysis showed that the pristine sample of Zn-CMK-1 contained 5.5 wt% of Zn before reduction and 5 wt% afterwards [13].

#### **2.4. Synthesis of Ni-CMK-1**

Ni-CMK-1 was obtained by impregnating CMK-1 using NiCl2.6H2O (Sigma–Aldrich 98%) dissolved in ethanol, which was stirred vigorously for 20 min. Then, the sample was dried under vacuum at 308 K, to achieve complete dryness by evaporation of the solvent. The powder obtained was heated to 353 K to dry and then placed in a tubular furnace at 773 K under controlled flow of H2 for reduction during 6 h. It was determined by ICP that the reduced sample contained 5.5 wt% Ni [13].

#### **2.5. Synthesis of Pt-CMK-3**

The sample was prepared by wetness impregnation using chloroplatinic acid (H2PtCl6⋅H2O-Sigma–Aldrich) as Pt source. Chloroplatinic acid in ethanol was added to the carbon solution at room temperature. Excess of ethanol was removed by placing the solution in a rotary evaporator at around 333 K and 60 revolutions per minute. The sample was then dehydrated at 373 K during 18 h. The resultant solid was heated under nitrogen flow from 298 K to 473 K (ramp 10 K/min) and maintained at 473 K for 5 h. Afterwards, it was heated up to 743 K (slope 10 K/min), where it was kept for another 5 h. The nitrogen flow was kept constant at 20 ml/ min. The sample was also subjected to H2 flow at 773 K for 6 h with a ramp rate of 10 K/min. The sample was denoted as Pt-CMK-3. 1% wt of Pt in the final Pt-CMK-3 material was obtained [12].

#### **2.6. Synthesis of TiO2-CMK-3**

TiO2 nanoparticles were incorporated into ordered mesoporous carbon CMK-3 using wetness impregnation and tetrabutyl orthotitanate (TBOT) as a titanium source. The metal precursor (TBOT) was dissolved in 10 mL of ethanol with vigorous stirring, to reach a nominal content of 5 wt% of Ti in the final solid. CMK-3 was incorporated in the solution, and both were placed in a rotary evaporator to remove excess of ethanol at about 313 K and 60 rpm. The powder was then dried at 373 K overnight. Finally, the resulting material was heated in a dynamic inert atmosphere (nitrogen flow of 20 mL/min) from 298 to 473 K with a slope of 4 K/min; temper‐ ature was then increased to 743 K with a slope of 10 K/min and kept at this temperature during 5 h.

#### **2.7. Synthesis of V-CMK-3**

The silica template was dissolved with 5 wt% hydrofluoric acid at room temperature. This step was done two times in order to dissolve entirely the template. The template-free carbon product thus obtained was filtered, washed with deionized water and ethanol, and dried [12,

Zn-CMK-1 was prepared using as a source of zinc Zn nitrate (Sigma–Aldrich 98%, hexahy‐ drate), dissolved in ethanol, and then mixed with a portion of CMK-1 at room temperature. The suspension was left static for 24 h, before being dried at 373 K for a period of 18 h, and subsequently desorbed from 298 K to 473 K in an inert atmosphere (N2), at a rate of 10 K/min. It was held at this latter temperature for 5 h. During the whole process, the nitrogen flow was 20 ml/min. To obtain the reduced sample of Zn-CMK-1, it was placed under H2 flow at 773 K for more than 6 h with a ramp rate 10 K/min. The sample was denoted as Zn-CMK-1. ICP analysis showed that the pristine sample of Zn-CMK-1 contained 5.5 wt% of Zn before

Ni-CMK-1 was obtained by impregnating CMK-1 using NiCl2.6H2O (Sigma–Aldrich 98%) dissolved in ethanol, which was stirred vigorously for 20 min. Then, the sample was dried under vacuum at 308 K, to achieve complete dryness by evaporation of the solvent. The powder obtained was heated to 353 K to dry and then placed in a tubular furnace at 773 K under controlled flow of H2 for reduction during 6 h. It was determined by ICP that the reduced

The sample was prepared by wetness impregnation using chloroplatinic acid (H2PtCl6⋅H2O-Sigma–Aldrich) as Pt source. Chloroplatinic acid in ethanol was added to the carbon solution at room temperature. Excess of ethanol was removed by placing the solution in a rotary evaporator at around 333 K and 60 revolutions per minute. The sample was then dehydrated at 373 K during 18 h. The resultant solid was heated under nitrogen flow from 298 K to 473 K (ramp 10 K/min) and maintained at 473 K for 5 h. Afterwards, it was heated up to 743 K (slope 10 K/min), where it was kept for another 5 h. The nitrogen flow was kept constant at 20 ml/ min. The sample was also subjected to H2 flow at 773 K for 6 h with a ramp rate of 10 K/min. The sample was denoted as Pt-CMK-3. 1% wt of Pt in the final Pt-CMK-3 material was obtained

TiO2 nanoparticles were incorporated into ordered mesoporous carbon CMK-3 using wetness impregnation and tetrabutyl orthotitanate (TBOT) as a titanium source. The metal precursor

13].

**2.3. Synthesis of Zn-CMK-1**

54 Microporous and Mesoporous Materials

reduction and 5 wt% afterwards [13].

**2.4. Synthesis of Ni-CMK-1**

sample contained 5.5 wt% Ni [13].

**2.5. Synthesis of Pt-CMK-3**

**2.6. Synthesis of TiO2-CMK-3**

[12].

Vanadium nanoparticles were incorporated into ordered mesoporous carbon CMK-3 using wetness impregnation and VCl3 as source of Vanadium. The metal precursor (VCl3) was dissolved in 20 mL of ethanol with vigorous stirring, to have a nominal content of 1 wt% of V in the final solid. The solution was placed in a rotary evaporator to remove excess of ethanol at about 323 K and 50 rpm. The obtained powder was then dried at 373 K overnight. The resulting material was heated in a dynamic inert atmosphere (nitrogen flow of 20 mL/min) from 298 to 473 K with a slope of 4 K/min, and then, the temperature was increased to 743 K with a slope of 10 K/min and kept at this temperature during 5 h. A reduction procedure in H2 flow (20 mL/min) was performed to the sample at 1023 K using the same procedure described earlier.

#### **2.8. Characterization of the samples**

ICP (ICP-AES; VISTA-MPX) was used to determine the elemental composition, working at a frequency of 1.5 kW under airflow 12 L/min. In order to determine textural properties, such as surface area and pore volume, N2 adsorption/desorption isotherms at 77 K were measured on ASAP 2020. To realize studies of the adsorption isotherms, the samples were purged at 673 K, and size pore distribution assessed by means of Barrett, Joyner, and Halenda (BJH) algorithm. The hydrogen chemisorption characterization was realized in the Micromeritics Chemisorb 2720 apparatus, equipped with a detector TCD, at RT. The cleaned samples were titrated with H2 pulses at 295 K under flow of N2 up to a persistent output signal of TCD, which indicates saturation.

X-ray diffraction profiles of samples were recorded with a X'PertPro PANalytical diffractom‐ eter equipped with a Cu Kα radiation (λ = 0.154 nm, X-ray generator current and voltage set at 40 mA and 45 kV). The samples were crushed previously and placed on an aluminum sample holder. The diffraction patterns were recorded in steps over a range of Bragg angles (2θ) between 0.5° and 5° (and for wide angle between 20 and 70°), at a scanning rate of 0.02 per step and an accumulation time of 20 s. Diffractograms were analyzed with the X'Pert High Score Plus software. X-ray photoelectron spectra (XPS) were obtained on a Microtech Multilb 3000 spectrometer, furnished with a hemispherical electron analyzer and MgKα (hν = 1253.6 eV) photon source. An estimated error of ±0.1 eV can be obtained for all ranges. Peak intensity was determined from each peak areas after background removal and spectrum suitable by a combination of Gaussian/Lorentzian functions. Surface relation ratios (50–100 A of Deep) were extracted as a function of the intensities of the signals corrected by the sensitivity factors with an estimated precision of ±7%.

Raman spectrum was acquired from an InVia Reflex Raman microscope and spectrometer using a 532 nm diode laser excitation. The transmission electron microscopy (TEM) micro‐ graphs were recorded in a Philips EM 301, with accelerating voltage of 200 kV (point resolution of 0.17 nm).

To determine hydrogen storage isotherms, we employed an ASAP 2050 apparatus properly calibrated, at 77 K and at low and high pressures (up to 10 bar). Previous to all the adsorption experiments, the samples were degassed at 523 K during 8 h under vacuum conditions, to prevent the probable decomposition of the metal nanoparticles at higher temperature. Afterwards, the samples were cooled at RT. The adsorption experiments were carried out with high-purity hydrogen gas (99.9999%), and in a pressure range from 0 to 10 bar.

#### **3. X-ray diffraction studies**

**Figure 1** shows typical XRD patterns for the MCM-48 silica template and the carbon CMK-1 produced with the aforementioned procedures. The low-angle XRD pattern of MCM-48 points to a high degree of structural order for the cubic crystallographic space group Ia3d (**Table 1**). However, a different pattern relative to MCM-48 can be observed for the carbon CMK-1, obtained by removing the silica wall after carbonization. As opposed to CMK-2, 3, and 4, the structure of the CMK-1 obtained through the use of silicate MCM-48 templates was not found to be an exact negative replica of the template. This was due to a transformation of the mesostructure after the template wall had dissolved. This structural transformation was mainly evidenced by the appearance of a strong low-angle diffraction peak (h k l [1 1 0] at 2θ = 1.67°) on the X-ray diffraction (XRD) patterns of CMK-1 (see **Figure 1**), which was not reliable with the symmetry of MCM-48 [12,17].

**Figure 1.** Small-angle XRD patterns of MCM-48 and CMK-1.

As shown by the low-angle powder XRD pattern, a structural transformation of the inter‐ laced carbon framework occurs after the silica wall has dissolved. A displacement of the sub-frameworks with respect to each other may lead to the formation of contacts. A distor‐ tion of the frameworks may also take place as a consequence of further transformations. The principal indicator of structural transformation of the material is the strong low-angle dif‐ fraction peak appearing in the XRD powder pattern, which is inconsistent with the space group Ia3d of the MCM-48 template. This reflection is indexed as [1 1 0], and the explana‐ tion of its emergence may lie in a decrease in the material symmetry. The complexity of the framework structure of CMK-1 is such that it fails to be described by simple terms such as "pore," "wall," or "diameter," which are commonly used in descriptions of the texture of mesoporous materials and molecular sieves. However, an estimation of its geometric charac‐ teristics might be in place. It is possible to determine the diameter of the roughly cylindrical framework segments, which coincide with the Wyckoff symmetry positions 12c and 12d of the I4132 space group [18].

**Table 1.** XRD parameters of samples.

extracted as a function of the intensities of the signals corrected by the sensitivity factors with

Raman spectrum was acquired from an InVia Reflex Raman microscope and spectrometer using a 532 nm diode laser excitation. The transmission electron microscopy (TEM) micro‐ graphs were recorded in a Philips EM 301, with accelerating voltage of 200 kV (point resolution

To determine hydrogen storage isotherms, we employed an ASAP 2050 apparatus properly calibrated, at 77 K and at low and high pressures (up to 10 bar). Previous to all the adsorption experiments, the samples were degassed at 523 K during 8 h under vacuum conditions, to prevent the probable decomposition of the metal nanoparticles at higher temperature. Afterwards, the samples were cooled at RT. The adsorption experiments were carried out with

**Figure 1** shows typical XRD patterns for the MCM-48 silica template and the carbon CMK-1 produced with the aforementioned procedures. The low-angle XRD pattern of MCM-48 points to a high degree of structural order for the cubic crystallographic space group Ia3d (**Table 1**). However, a different pattern relative to MCM-48 can be observed for the carbon CMK-1, obtained by removing the silica wall after carbonization. As opposed to CMK-2, 3, and 4, the structure of the CMK-1 obtained through the use of silicate MCM-48 templates was not found to be an exact negative replica of the template. This was due to a transformation of the mesostructure after the template wall had dissolved. This structural transformation was mainly evidenced by the appearance of a strong low-angle diffraction peak (h k l [1 1 0] at 2θ = 1.67°) on the X-ray diffraction (XRD) patterns of CMK-1 (see **Figure 1**), which was not reliable

As shown by the low-angle powder XRD pattern, a structural transformation of the inter‐ laced carbon framework occurs after the silica wall has dissolved. A displacement of the

high-purity hydrogen gas (99.9999%), and in a pressure range from 0 to 10 bar.

an estimated precision of ±7%.

56 Microporous and Mesoporous Materials

**3. X-ray diffraction studies**

with the symmetry of MCM-48 [12,17].

**Figure 1.** Small-angle XRD patterns of MCM-48 and CMK-1.

of 0.17 nm).

**Figure 2** shows the low-angle X-ray diffraction patterns of CMK-1, Ni-CMK-1, and Zn-CMK-1 samples. Retention of the overall pore structure after the addition of the different metals is indicated by the low-angle diffraction peaks (**Table 1**). However, the peak intensity of the samples is noticeably lower than that of the parent CMK-1. This intensity loss may be due to the introduction of scattering material into the pores, leading to an increased phase cancellation

**Figure 2.** Small-angle XRD patterns of CMK-1, Ni-CMK-1, and Zn-CMK-1.

between scattering from the walls and the pore regions [19]. In the case of the Zn-CMK-1 sample, the absence of XRD reflections can be ascribed to a very high degree of pore filling.

**Figure 3** shows a typical XRD pattern for the silica template SBA-15 and the carbon CMK-3 yielded by the procedures described. The low-angle XRD pattern of SBA-15 points to a high degree of structural order for the hexagonal P6mm crystallographic space group (**Table 2**). A similar pattern relative to SBA-15 can be observed for the carbon CMK-3 obtained by removing the silica wall after carbonization. The structure of the CMK-3 obtained by the use of a silicate SBA-15 template was found to be an exact negative replica of the template. The structural transformation was mainly evidenced by the appearance of CMK-3 on the X-ray diffraction (XRD) patterns, which was consistent with the symmetry of SBA-15 [19].

**Figure 3.** Small-angle XRD patterns of SBA-15 and CMK-3.

**Table 2.** XRD parameters of samples.

**Figure 4** shows the low-angle X-ray diffraction patterns of CMK-3 and of the modified samples. The overall pore structure is retained after the addition of Pt, TiO2, and V, which is indicated by the low-angle diffraction peaks (**Table 2**). However, the signal intensity corresponding to the [1 1 0] diffraction plane of the samples shows a slow decrease in relation to the parent CMK-3. The introduction of scattering material into the pores generally leads to an increased phase cancellation between scattering from the wall and the pore regions. The intensity loss of CMK-3 typical Bragg reflections is caused by the introduction of scattering material (Pt, TiO2, and V) into the pores [19].

between scattering from the walls and the pore regions [19]. In the case of the Zn-CMK-1 sample, the absence of XRD reflections can be ascribed to a very high degree of pore filling.

**Figure 3** shows a typical XRD pattern for the silica template SBA-15 and the carbon CMK-3 yielded by the procedures described. The low-angle XRD pattern of SBA-15 points to a high degree of structural order for the hexagonal P6mm crystallographic space group (**Table 2**). A similar pattern relative to SBA-15 can be observed for the carbon CMK-3 obtained by removing the silica wall after carbonization. The structure of the CMK-3 obtained by the use of a silicate SBA-15 template was found to be an exact negative replica of the template. The structural transformation was mainly evidenced by the appearance of CMK-3 on the X-ray diffraction

(XRD) patterns, which was consistent with the symmetry of SBA-15 [19].

**Sample SBA-15 [h k l] CMK-3 [h k l] a0 (nm)**

SBA-15 8.6 5.0 4.3 – – – 10.0 CMK-3 – – – 10.5 6.1 5.2 12.1 Pt-CMK-3 – – – 8.4 4.9 4.2 9.70 TiO2–CMK-3 – – – 7.1 4.4 3.8 8.60 V-CMK-3 – – – 7.2 4.2 3.9 8.50

**d (nm) d (nm)**

**[1 0 0] [1 1 0] [2 0 0] [1 0 0] [1 1 0] [2 0 0]**

**Figure 4** shows the low-angle X-ray diffraction patterns of CMK-3 and of the modified samples. The overall pore structure is retained after the addition of Pt, TiO2, and V, which is indicated by the low-angle diffraction peaks (**Table 2**). However, the signal intensity corresponding to the [1 1 0] diffraction plane of the samples shows a slow decrease in relation to the parent

**Figure 3.** Small-angle XRD patterns of SBA-15 and CMK-3.

**Table 2.** XRD parameters of samples.

58 Microporous and Mesoporous Materials

**Figure 4.** Small-angle XRD patterns of CMK-3 (a), V-CMK-3 (b), Pt-CMK-3 (c), and TiO2–CMK-3 (d).

**Figure 5** shows the wide-angle diffraction region of the Zn-modified sample (Zn-CMK-1). XRD pattern at the wide-angle range (10–60°) of the CMK-1 host (see the inset of **Figure 5**) exhibits two broad diffraction peaks which can be indexed as [0 0 2] and [1 0 0] diffraction for typical graphite carbons [20].

The pattern of Zn-CMK-1 shows ZnO and metallic Zn signals [21,22], probably due to the reduction process not being complete. The fact that this pattern exhibits no prominent reflections is an indicator that no crystalline bulk material was formed outside the pore system. The Zn/ZnO clusters are confined for the most part in the nanostructured carbon [23], showing nanometric size and high dispersion which can be seen in **Table 1** and TEM studies.

**Figure 5.** Wide-angle XRD patterns of Zn-CMK-1 and CMK-1 (inset).

The XRD pattern of Ni-CMK-1 (**Figure 5**) shows intensive peaks at 2θ = 44.5, 51.8, and 76.3°, corresponding to metallic Ni diffraction planes [1 1 1], [2 0 0], and [2 2 0], [45]. These are characteristic patterns of an fcc lattice consisting of nickel nanoparticles. NiO species may be present, since the NiO signals were very close to metallic Ni signals and would overlap [24] in the pattern, as TEM-EDS and XPS studies will show later. In agreement with XRD studies, the characteristic structure of CMK-1 is maintained after the metal is within the host, whereas the nanomaterial area becomes significantly smaller (**Table 1**).

**Figure 6** displays the diffraction region of 20–70° 2Ө of samples. Typical XRD pattern of mesoporous materials was exhibited by SBA-15 sample. Two broad diffraction peaks were found in CMK-3 pattern which can be ascribed to [0 0 2] and [1 0 0] diffraction indexes for a characteristic graphite carbons [20].

The final sample of Pt-CMK-3 pattern displays metallic platinum peaks [25], pointing to a welldeveloped reduction process. In the case of Pt clusters, the absence of prominent reflections indicates that no crystalline bulk material formation had occurred outside the pore system [23], with nanometric size and high dispersion (very broad XRD signal of Pt, **Figure 6**) as shown in **Table 2**. The reason for this may be found in a relatively low scattering contrast between the pores and the walls of the mesoporous materials caused by the formation of reduced platinum which indicates fine dispersion on CMK-3 with a narrow size distribution [24].

**Figure 6.** Wide-angle XRD patterns of SBA-15, CMK-3, and Pt-CMK-3.

**Figure 7** shows the wide-angle X-ray diffraction patterns for CMK-3 and TiO2-CMK-3. In both cases, two broad diffraction peaks are distinguished, which can be indexed as [0 0 2] and [1 0 0] diffraction for typical graphite carbons [20].

The pattern of TiO2-CMK-3 shows characteristic signals of TiO2 in anatase phase [26], indicat‐ ing a particularly good synthesis process. The absence of prominent reflections in TiO2 clusters

Synthesis and Characterization of CMK Porous Carbons Modified with Metals Applied to Hydrogen Uptake and Storage http://dx.doi.org/10.5772/63710 61

indicates that no crystalline bulk material has been formed [23], with nanometric size and high dispersion (very broad XRD TiO2 signals, **Figure 7**). This can be ascribed to a relatively low scattering contrast between the pores and walls of mesoporous materials, due to the formation of anatase nanoclusters that depict fine dispersion on CMK-3 with a narrow size distribution [24].

**Figure 7.** Wide-angle XRD patterns of SBA-15, CMK-3, and TiO2–CMK-3.

The XRD pattern of Ni-CMK-1 (**Figure 5**) shows intensive peaks at 2θ = 44.5, 51.8, and 76.3°, corresponding to metallic Ni diffraction planes [1 1 1], [2 0 0], and [2 2 0], [45]. These are characteristic patterns of an fcc lattice consisting of nickel nanoparticles. NiO species may be present, since the NiO signals were very close to metallic Ni signals and would overlap [24] in the pattern, as TEM-EDS and XPS studies will show later. In agreement with XRD studies, the characteristic structure of CMK-1 is maintained after the metal is within the host, whereas

**Figure 6** displays the diffraction region of 20–70° 2Ө of samples. Typical XRD pattern of mesoporous materials was exhibited by SBA-15 sample. Two broad diffraction peaks were found in CMK-3 pattern which can be ascribed to [0 0 2] and [1 0 0] diffraction indexes for a

The final sample of Pt-CMK-3 pattern displays metallic platinum peaks [25], pointing to a welldeveloped reduction process. In the case of Pt clusters, the absence of prominent reflections indicates that no crystalline bulk material formation had occurred outside the pore system [23], with nanometric size and high dispersion (very broad XRD signal of Pt, **Figure 6**) as shown in **Table 2**. The reason for this may be found in a relatively low scattering contrast between the pores and the walls of the mesoporous materials caused by the formation of reduced platinum

**Figure 7** shows the wide-angle X-ray diffraction patterns for CMK-3 and TiO2-CMK-3. In both cases, two broad diffraction peaks are distinguished, which can be indexed as [0 0 2] and [1 0

The pattern of TiO2-CMK-3 shows characteristic signals of TiO2 in anatase phase [26], indicat‐ ing a particularly good synthesis process. The absence of prominent reflections in TiO2 clusters

which indicates fine dispersion on CMK-3 with a narrow size distribution [24].

the nanomaterial area becomes significantly smaller (**Table 1**).

**Figure 6.** Wide-angle XRD patterns of SBA-15, CMK-3, and Pt-CMK-3.

0] diffraction for typical graphite carbons [20].

characteristic graphite carbons [20].

60 Microporous and Mesoporous Materials

**Figure 8** shows the wide-angle X-ray diffraction patterns for CMK-3 and V-CMK-3. In both cases, two broad diffraction peaks are distinguished which can be indexed as [0 0 2] and [1 0 0] diffraction for typical graphite carbons [20].

**Figure 8.** Wide-angle XRD patterns of CMK-3 and V-CMK-3.

In the wide-angle region, practically no reflections typical of vanadium have been found. The absence of these prominent reflections indicates that no crystalline bulk materials have been formed outside the pore system and it is an evidence that the clusters have nanometric size and high dispersion [23].

#### **3.1. Cluster size estimation using Scherrer's formula**

Analysis of the XRD patterns makes it possible to determine the size of crystals of metallic particles in the samples and to evaluate the effect on metal dispersion. The average cluster size was thus estimated based on the width of the diffraction peaks corresponding to different [h k l] Miller indices using Scherrer's formula (Eq. (1)) [27].

$$d = \frac{K \,\lambda}{\beta \cos \theta} \tag{1}$$

where d is the average size of the crystals or domains (the crystal mean size defined as the cube root of the material volume). K is a shape factor of the nanocluster, dimensionless, generally has a typical value of 0.89 or 0.83 (if the particles are approximately spherical or not), λ is the wavelength of the X-rays (1.54 Å) used for performing the analysis, β is the full width at half-maximum (FWHM, in radians) of the XRD signal, and θ is the Bragg angle (in radians) [28].

In order to apply Scherrer's formula, [1 1 1], [2 0 0], and [2 2 0] refractions plains for Pt-CMK-3 were employed. The average diameter of the metal particles, as obtained by Scherrer's formula, was ~1.76 nm. In the case of TiO2-CMK-3, the average diameters of anatase particles obtained by Scherrer's formula were ~5 nm, employing [1 0 1], [1 1 2], and [2 0 0] refractions plains.

In the case of the sample CMK-1 modified with Ni, the planes [101], [200], and [220] were used. For the Zn-CMK-1, [100] and [101] refraction planes for Zn0 and for ZnO the planes [002] and [110] were used. Using Scherrer's formula, the average size of metal nanoclusters for the sample modified with Ni were 13 nm, and 2.3 nm and 2.6 nm for the Zn0 and the ZnO particles.

#### **4. Nitrogen adsorption/desorption isotherm analysis**

**Figure 9** shows the N2 adsorption/desorption isotherms for MCM-48, CMK-1, Zn-CMK-1, and Ni-CMK-1 samples, while **Table 3** shows the textural properties obtained from nitrogen physisorption analysis. The nitrogen adsorption–desorption is other for MCM-48 has a typical type IV curve (according to the IUPAC classification), which is a sure indicator of the meso‐ porous nature of the material. The isotherm for CMK-1 and Zn-CMK-1 and Ni-CMK-1 shows hysteresis loops at a relative pressure range of 0.4–0.6, which can be caused by capillary condensation-evaporation from the mesopores. Zn-CMK-1and Ni-CMK-1 clearly reveal a reduced specific surface area (as well as a narrowed pore size), as opposed to those of pure MCM-48 and CMK-1 (**Table 2**), with the corresponding incorporated metallic species.

Synthesis and Characterization of CMK Porous Carbons Modified with Metals Applied to Hydrogen Uptake and Storage http://dx.doi.org/10.5772/63710 63

**Figure 9.** N2 adsorption–desorption isotherms of MCM-48, mesoporous carbon CMK-1, Zn-CMK-1, and Ni-CMK-1.

**Table 3.** Textural and structural properties of the samples.

In the wide-angle region, practically no reflections typical of vanadium have been found. The absence of these prominent reflections indicates that no crystalline bulk materials have been formed outside the pore system and it is an evidence that the clusters have nanometric size

Analysis of the XRD patterns makes it possible to determine the size of crystals of metallic particles in the samples and to evaluate the effect on metal dispersion. The average cluster size was thus estimated based on the width of the diffraction peaks corresponding to different [h

> cos *<sup>K</sup> <sup>d</sup>*

b q

l

where d is the average size of the crystals or domains (the crystal mean size defined as the cube root of the material volume). K is a shape factor of the nanocluster, dimensionless, generally has a typical value of 0.89 or 0.83 (if the particles are approximately spherical or not), λ is the wavelength of the X-rays (1.54 Å) used for performing the analysis, β is the full width at half-maximum (FWHM, in radians) of the XRD signal, and θ is the Bragg angle (in radians)

In order to apply Scherrer's formula, [1 1 1], [2 0 0], and [2 2 0] refractions plains for Pt-CMK-3 were employed. The average diameter of the metal particles, as obtained by Scherrer's formula, was ~1.76 nm. In the case of TiO2-CMK-3, the average diameters of anatase particles obtained by Scherrer's formula were ~5 nm, employing [1 0 1], [1 1 2], and [2 0 0] refractions plains.

In the case of the sample CMK-1 modified with Ni, the planes [101], [200], and [220] were used. For the Zn-CMK-1, [100] and [101] refraction planes for Zn0 and for ZnO the planes [002] and [110] were used. Using Scherrer's formula, the average size of metal nanoclusters for the

**Figure 9** shows the N2 adsorption/desorption isotherms for MCM-48, CMK-1, Zn-CMK-1, and Ni-CMK-1 samples, while **Table 3** shows the textural properties obtained from nitrogen physisorption analysis. The nitrogen adsorption–desorption is other for MCM-48 has a typical type IV curve (according to the IUPAC classification), which is a sure indicator of the meso‐ porous nature of the material. The isotherm for CMK-1 and Zn-CMK-1 and Ni-CMK-1 shows hysteresis loops at a relative pressure range of 0.4–0.6, which can be caused by capillary condensation-evaporation from the mesopores. Zn-CMK-1and Ni-CMK-1 clearly reveal a reduced specific surface area (as well as a narrowed pore size), as opposed to those of pure MCM-48 and CMK-1 (**Table 2**), with the corresponding incorporated metallic species.

sample modified with Ni were 13 nm, and 2.3 nm and 2.6 nm for the Zn0

**4. Nitrogen adsorption/desorption isotherm analysis**

<sup>=</sup> (1)

and the ZnO particles.

and high dispersion [23].

62 Microporous and Mesoporous Materials

[28].

**3.1. Cluster size estimation using Scherrer's formula**

k l] Miller indices using Scherrer's formula (Eq. (1)) [27].

The sharp peak at pore size distribution of CMK-1 and Zn- and Ni-containing CMK-1 (**Figure 10**) indicates a fairly regular range of nanopores in carbonaceous materials, in correspondence with TEM observations.

The isotherm for CMK-3 and Pt-CMK-3 samples shows hysteresis loops at a relative pressure range of 0.4–0.8, which may be caused by capillary condensation–evaporation from the mesopores (**Figure 11**). The material appears in the form of fairly uniform carbon rods (see TEM images) with a higher pore volume as a result of the presence of mesopores between carbon rods. Irregularities (i.e., the presence of a fraction of nonlinear channels) can be attributed to SBA-15 meso-tunnels being incompletely filling with carbon precursor. The pore size distribution of CMK-3 shows a pronounced peak at 4 nm that points to a quite regular array of nanopores.

**Figure 10.** Pore size distribution of MCM-48, CMK-1, Zn-CMK-1, and Ni-CMK-1.

**Figure 11.** Nitrogen adsorption (solid symbols)–desorption isotherm (open symbols) at 77 K and pore size distribution of (a): SBA-15, (b) CMK-3, (c) Pt-CMK-3, (d) TiO2-CMK-3, and (e) V-CMK-3.

On the contrary, the Pt-CMK-3 pore size distribution shows a broad peak with a maximum at approx. 3.8 nm, which is consistent with primary mesopores. The appearance of a wide peak can be ascribed to the filling of pores with Pt nanoclusters and the increase in the irregularity of nanopores. Around 6 nm an overlapping peak appears, corresponding to CMK-3 pores that are not filled with Pt nanoclusters. These analyses of pore size (see **Figure 11**) were consistent with observations from TEM.

**Figure 11** also shows, the N2 adsorption/desorption isotherms for SBA-15, CMK-3 and TiO2- CMK-3 samples at 77 K; **Table 4** displays the textural properties determined from nitrogen physisorption analysis. The isotherm for SBA-15 is a typical type IV curve with hysteresis loop, according to IUPAC classification, revealing the mesoporous nature of the material.

The nitrogen adsorption/desorption isotherms for CMK-3 and TiO2-CMK-3 are typical type IV curves exhibiting hysteresis loops type H2, according to IUPAC classification, typical of mesoporous solids.

The materials obtained exhibit capillary condensation to relative pressures about 0.40 and 0.45, which can be related to the pore blocking effect of influencing pressure where evaporation/ desorption pore occurs. The increase in adsorption at low relative pressures is associated with the presence of micropores or a strong adsorbate-adsorbent interaction.

TiO2-CMK-3 reveals a reduced specific surface area, in comparison with those of pure SBA-15 and CMK-3, with incorporated anatase nanoparticles.

The CMK-3 pore size distribution shows a sharp peak at 4.2 nm that points to a quite regular array of nanopores. On the other hand, the pore size distribution of TiO2-CMK-3 exhibits a broad peak with a maximum at approximately 6 nm, which is consistent with primary mesopores.

VTP, total pore volume; SBET, BET surface area; VμP, micropore volume; Wp, wide mesopore.

**Table 4.** Textural properties of the materials and composite.

carbon rods. Irregularities (i.e., the presence of a fraction of nonlinear channels) can be attributed to SBA-15 meso-tunnels being incompletely filling with carbon precursor. The pore size distribution of CMK-3 shows a pronounced peak at 4 nm that points to a quite regular

**Figure 11.** Nitrogen adsorption (solid symbols)–desorption isotherm (open symbols) at 77 K and pore size distribution

On the contrary, the Pt-CMK-3 pore size distribution shows a broad peak with a maximum at approx. 3.8 nm, which is consistent with primary mesopores. The appearance of a wide peak can be ascribed to the filling of pores with Pt nanoclusters and the increase in the irregularity

**Figure 10.** Pore size distribution of MCM-48, CMK-1, Zn-CMK-1, and Ni-CMK-1.

of (a): SBA-15, (b) CMK-3, (c) Pt-CMK-3, (d) TiO2-CMK-3, and (e) V-CMK-3.

array of nanopores.

64 Microporous and Mesoporous Materials

N2 adsorption/desorption isotherms and pore size distribution for CMK-3 and V-CMK-3 samples at 77 K shown **Figure 11** and **Table 4**, displays the textural properties determined from nitrogen physisorption analysis. The nitrogen adsorption–desorption isotherms for CMK-3 and V-CMK-3 are typical type IV curves exhibiting hysteresis loops type H2, according to IUPAC classification, typical of mesoporous solids.

Materials obtained exhibit capillary condensation to relative pressures about 0.40 and 0.45, which can be related to the pore blocking effect of affecting the pressure where evaporation/ desorption pore occurs. The increase in adsorption at low relative pressures of the samples CMK-3 instead V-CMK-3 is associated with the presence of micropores or a strong adsorbateadsorbent interaction.

V-CMK-3 reveals a reduced specific surface area, in comparison with those of pure CMK-3 (**Table 4**), with the corresponding incorporated metallic species.

#### **5. XPS studies**

#### **5.1. Ni-CMK-1, Zn-CMK-1, Pt-CMK-3, and V-CMK-3 XPS characterization**

Zn-CMK-1 XPS data exhibited that zinc appears as Zn0 or Zn2+ ions (**Figure 12**), according to the binding energies (BE) of Zn (2p3/2) electron at 1022.5 eV. The Zn 2p3/2 spectrum of Zn oxide, though not presenting multiple splitting or other complex effects, overlaps with Zn0 species peak BE [29]. According to Woll [30], the Zn 2p3/2 lines for Zn and ZnO are quoted at 1021.4 eV and 1021.7 eV.

**Figure 12.** Zn-CMK-1 XPS spectrum.

The split 2p electron energy levels for Ni, Ni (2p1/2), and Ni (2p3/2) and seemed at 865–885 and 850–865 eV, respectively (**Figure 13**). These bond energies are lower by 1.5–2.0 eV for pure NiO [31]. This abnormality is possibly because the variance in organization of Ni2+ in NiO and on CMK-1. It can confidently be said that Ni2+ ions are coordinated in higher symmetry in CMK-1, having a well-ordered crystal structure compared to that of NiO. The satellite band which appears at higher bond energy values also indicates the typical oxide structure.

CMK-3 instead V-CMK-3 is associated with the presence of micropores or a strong adsorbate-

V-CMK-3 reveals a reduced specific surface area, in comparison with those of pure CMK-3

the binding energies (BE) of Zn (2p3/2) electron at 1022.5 eV. The Zn 2p3/2 spectrum of Zn oxide, though not presenting multiple splitting or other complex effects, overlaps with Zn0 species peak BE [29]. According to Woll [30], the Zn 2p3/2 lines for Zn and ZnO are quoted at 1021.4

The split 2p electron energy levels for Ni, Ni (2p1/2), and Ni (2p3/2) and seemed at 865–885 and 850–865 eV, respectively (**Figure 13**). These bond energies are lower by 1.5–2.0 eV for pure NiO [31]. This abnormality is possibly because the variance in organization of Ni2+ in NiO and on

or Zn2+ ions (**Figure 12**), according to

(**Table 4**), with the corresponding incorporated metallic species.

Zn-CMK-1 XPS data exhibited that zinc appears as Zn0

**5.1. Ni-CMK-1, Zn-CMK-1, Pt-CMK-3, and V-CMK-3 XPS characterization**

adsorbent interaction.

66 Microporous and Mesoporous Materials

**5. XPS studies**

eV and 1021.7 eV.

**Figure 12.** Zn-CMK-1 XPS spectrum.

**Figure 13.** XPS binding energies spectra of the Ni 2p3/2 and Ni 2p1/2 peaks region separate by a vertical line. Gaussian component: Ni0 solid line; NiO dot line and satellite peak of NiO dash dot line.

Activation under H2 causes a partial reduction of the surface Ni2+ species, which yields Ni0 (reduced catalyst). In addition to Ni0 , the reactive surface contains mixed Ni0 + NiO. The mixed nickel compounds (such as nickel oxides and hydroxides) can appear in relatively large particles, where the NiO compound can be present on the outer surface, and metallic Ni is located in a the sub-surface close to the support, respectively. This distribution can be due to the drastic changes of the deposits' electronic structure that take place as the cluster size increases. Cluster size has a direct impact on the final state in the photoemission process because, once the process is over, the positive charge that remains on the aggregate can be less screened delocalized, compared with an extended metallic system. This entails a shift of the XPS spectrum corresponding to the Coulomb energy of localized charge. The shift proved to be proportional to the reciprocal particle diameter [32]. As a consequence, the binding energies of small metal aggregates should be found at higher values, as was in fact observed in a large number of cases. Elemental XPS analysis (Zn 2p3/2. O1s and C1s) showed that the chemical composition for Zn, O, and C, 0.2 %, 0.16%, and 99.64% (at.%), respectively, at 50–100 Å of depth for Zn-CMK-1 sample. Nevertheless, EDS study (**Figure 18c** and **d**) showed that the Zn content was 4.85 wt%, which suggests the Zn0 (74%) lies inside the mesoporous of CMK-1 sample. Furthermore, by XPS, the oxygen concentration was around 80%, giving support to the knowledge that the ZnO lie on the external Zn-CMK-1 surface.

XPS study of Ni-CMK-1 point to a composition for Ni, O, and C as follows: 0.625, 0.375, and 99 (at.%), whereas by EDS analysis (5.3 and 0.6 wt% and for Ni and O, respectively). Therefore, a large amount of NiO is located on the outside of CMK-1 (>40%), consistent to the inferior quantities of Ni0 create in Ni-CMK-1 mesoporous material. The EDS and XPS studies for Zn and Ni support the difference in cluster size obtained by XRD (Scherrer's formula) and TEM, shown in **Figure 18a** and **b** (2.5 nm and 12 nm, respectively). This is an indicator that the nanostructure of CMK-1 prevents the growth of large crystals in its nanopores, especially during the addition of zinc.

The XPS technique is normally used to determine the nature and oxidation state of Pt species (Pt0 , Pt2+, and Pt4+), and in particular the Pt (4f) peak study is employed (**Figure 14**). Metallic Pt0 is known to have binding energies of 70.7–70.9 and 74.0–74.1 eV for 4f7/2 and 4f5/2 electrons, respectively [33]. In oxidized states, the binding energies exhibited by Pt2+ and Pt4+ are much higher: 72.8–73.1 eV (4f7/2) and 76.3–76.4 eV (4f5/2) for Pt2+ and 74.6–74.9 eV (4f7/2) and 78.1–78.2 eV (4f5/2) for Pt4+ [34].

**Figure 14.** Core level Pt4f XPS spectra of Pt-CMK-3.

The spectra showed two pairs of peaks. As we can see all spectra displays signals at low and high energy (doublets), Pt 4f7/2: 71.8 eV and 4f5/2: 75.1 eV, demonstrating that Pt nanoclus‐ ters are metallic (**Figure 14**). The Pt0 peak moves slightly to higher binding energies; this phenomena is in agreement to the characteristic effect of small particle sizes, as published by Roth et al. [34] and Takasu et al. [35] or for charge transfer between carbon and platinum described by Aricò et al. [36]. The bands that appeared in the second pair of platinum signs, on the other hand, appears around 74.9 and 77.1 eV, which has been reported in the literature to be due to the presence of PtO or PtO2 [37].

According to the XPS analysis, the Pt concentration at 50 Å of depth was 0.38 wt%, whereas ICP and EDS analyses pointed to a nominal Pt = 0.9 wt% on Pt-CMK-3. This led to the suggestion that the majority of Pt0 lies inside the nanostructure of the CMK-3 sample. More‐ over, about 80 wt% of oxygen was on the outer surface of CMK, with the correspondingly lower amount of not reduced Pt species (Pt2,4+).

**Figure 15** shows the binding energy for V 2p3/2core level of XPS spectra for V-CMK-3. The only one contribution located about 515.0–515.2 eV is assigned to V3+ in the form of V2O3. Vanadium oxidation states is consistent with XPS spectra previously taken of V2O3, which we attributed to 2p3/2 core electrons [38]. The peak at 525 eV is consistent with the 2p1/2 peak of vanadium [38].

Vanadium 3+ has a d2 electronic configuration and, from inspection of the Tanabe-Sugano diagram for a d2 ion [39], is expected to have three spin-allowed ground-state absorption transitions in both tetrahedral and octahedral coordination. Therefore, 3+ is a possible oxidation state of vanadium in CMK-3 according to XPS data.

**Figure 15.** Core level V2p XPS spectra of V-CMK-3.

XPS study of Ni-CMK-1 point to a composition for Ni, O, and C as follows: 0.625, 0.375, and 99 (at.%), whereas by EDS analysis (5.3 and 0.6 wt% and for Ni and O, respectively). Therefore, a large amount of NiO is located on the outside of CMK-1 (>40%), consistent to the inferior

and Ni support the difference in cluster size obtained by XRD (Scherrer's formula) and TEM, shown in **Figure 18a** and **b** (2.5 nm and 12 nm, respectively). This is an indicator that the nanostructure of CMK-1 prevents the growth of large crystals in its nanopores, especially

The XPS technique is normally used to determine the nature and oxidation state of Pt species

The spectra showed two pairs of peaks. As we can see all spectra displays signals at low and high energy (doublets), Pt 4f7/2: 71.8 eV and 4f5/2: 75.1 eV, demonstrating that Pt nanoclus‐ ters are metallic (**Figure 14**). The Pt0 peak moves slightly to higher binding energies; this

, Pt2+, and Pt4+), and in particular the Pt (4f) peak study is employed (**Figure 14**). Metallic Pt0 is known to have binding energies of 70.7–70.9 and 74.0–74.1 eV for 4f7/2 and 4f5/2 electrons, respectively [33]. In oxidized states, the binding energies exhibited by Pt2+ and Pt4+ are much higher: 72.8–73.1 eV (4f7/2) and 76.3–76.4 eV (4f5/2) for Pt2+ and 74.6–74.9 eV (4f7/2) and 78.1–78.2

create in Ni-CMK-1 mesoporous material. The EDS and XPS studies for Zn

quantities of Ni0

(Pt0

during the addition of zinc.

68 Microporous and Mesoporous Materials

eV (4f5/2) for Pt4+ [34].

**Figure 14.** Core level Pt4f XPS spectra of Pt-CMK-3.

#### **6. TiO2-CMK-3 Raman studies**

**Figure 16** shows Raman spectra of SBA-15, pristine host CMK-3, TiO2-CMK-3, and pure anatase and rutile in the range of 200–1200 cm-1.

For SBA-15, Raman bands at 430, 800 and 1080 cm-1 are observed. The bands at 430 and 1080 cm-1 can be attributed to symmetric and asymmetric vibrations of the Si-O-Si unit, respectively. The band at 800 cm-1 is the symmetric stretching mode of the tetrahedral [SiO4] unit [40–42]. **Figure 16** also shows the spectra of TiO2 onto CMK-3; the characteristic signals of anatase are detected in the sample if compared with those of pure anatase [41,43,44] and pure rutile.

In inset **Figure 16**, we illustrated the Raman spectra of bare CMK-3 and TiO2-CMK-3 (from 1100 to 1700 cm-1). The Raman spectrum of the obtained CMK-3 as well as of TiO2-CMK-3 shows two peaks, at 1578 and 1353 cm-1. The peak at 1578 cm-1 corresponds to an E2g mode of graphite and is called G band, related to C═C-double-bonded carbon vibration in hexagonal lattice, while the peak at 1353 cm-1 is associated with vibrations of C-C bond, referred to as D band, attributed to the configuration of disordered graphite [45–47]. In the Raman spectrum of graphitic materials such as activated charcoal, carbon black, and CMK, this second line

**Figure 16.** Raman spectra of SBA-15, TiO2–CMK-3, rutile, anatase, and CMK-3 (inset).

appears. The relative intensity of the two lines depends on the type of graphitic material. The intensity of 1353 cm-1 line increases with an increase in carbon in the samples and with a decrease in the graphite crystal size. Hence, a signal at 1355 cm-l might possibly be ascribed to a diamond-like atomic arrangement in the graphite samples, probably as partially tetrahedral bonded [48]. This agrees with the proposal of the graphite-like structure for CMK-3 [49]. It could be clearly seen that G and D band intensity of TiO2/CMK-3 was not perturbed signifi‐ cantly (inset **Figure 16**).

#### **7. TEM studies**

**6. TiO2-CMK-3 Raman studies**

70 Microporous and Mesoporous Materials

anatase and rutile in the range of 200–1200 cm-1.

**Figure 16** shows Raman spectra of SBA-15, pristine host CMK-3, TiO2-CMK-3, and pure

For SBA-15, Raman bands at 430, 800 and 1080 cm-1 are observed. The bands at 430 and 1080 cm-1 can be attributed to symmetric and asymmetric vibrations of the Si-O-Si unit, respectively. The band at 800 cm-1 is the symmetric stretching mode of the tetrahedral [SiO4] unit [40–42]. **Figure 16** also shows the spectra of TiO2 onto CMK-3; the characteristic signals of anatase are detected in the sample if compared with those of pure anatase [41,43,44] and pure rutile.

In inset **Figure 16**, we illustrated the Raman spectra of bare CMK-3 and TiO2-CMK-3 (from 1100 to 1700 cm-1). The Raman spectrum of the obtained CMK-3 as well as of TiO2-CMK-3 shows two peaks, at 1578 and 1353 cm-1. The peak at 1578 cm-1 corresponds to an E2g mode of graphite and is called G band, related to C═C-double-bonded carbon vibration in hexagonal lattice, while the peak at 1353 cm-1 is associated with vibrations of C-C bond, referred to as D band, attributed to the configuration of disordered graphite [45–47]. In the Raman spectrum of graphitic materials such as activated charcoal, carbon black, and CMK, this second line

**Figure 16.** Raman spectra of SBA-15, TiO2–CMK-3, rutile, anatase, and CMK-3 (inset).

**Figure 17** shows TEM images of CMK-1. Ordered structure was shown to be slightly damaged by thermal treatments.

**Figure 17.** TEM images of CMK-1.

The TEM micrographs of Ni-CMK-1 and Zn-CMK-1 were depicted in **Figure 18**, showing. The ordered cubic Ia3d mesostructure of Metal-CMK-1 (**Figure 18a** and **b**), indicating that the ordered structure of CMK-1 is retained after the incorporation of Ni and Zn nanoparticles.

**Figure 18.** TEM images of (a) Ni-CMK-1 and (b) Zn-CMK-1. (c and d) EDS spectrum of: (c) Ni-CMK-1 and (d) Zn-CMK-1.

**Figure 19.** TEM images of SBA-15. (a) [1 0 0] and (b) [1 1 0] reflection planes.

The larger dark areas over the channels most likely correspond to NiO nanoparticle agglom‐ erates on the external surface with average diameter of 11–13 nm (**Figure 18a**). The small dark spots in the image (**Figure 18b**) could be ascribed to Zn and ZnO nanoparticles with average diameter of ~2.5–3 nm probably located into the host pores. EDS analysis was performed to estimate the presence of Zn- and Ni-containing CMK-1. A careful evaluation of the elemental compositions for C, O, and Zn and Ni nanoparticles was performed (**Figure 18c** and **d**).

According to the maps collecting five spots, the average atomic weight % of C, O and Zn nanoclusters in Zn-CMK-1 is about 94.94, 0.26, and 4.80%, respectively. This shows that only 20% of the Zn species appear as ZnO. On the other hand, the average atomic weight % for Ni-CMK-1 was 94.1, 0.6, and 5.3 wt% for C, O, and Ni, clearly indicating that the higher Ni content appears as oxide (about 40%).

**Figure 19a** shows TEM images of synthesized SBA-15 used as an inorganic template. In **Figure 19b**, an ordered mesoporous array of longitudinal nanochannels can be seen, with pore

**Figure 20.** TEM images of CMK-3. (a) Nanoparticles (b) [1 0 0] and (c) [1 1 0] reflection planes.

diameter of 7.5 nm. TEM images of CMK-3 are shown in **Figure 20**. Ordered structure proved to be slightly damaged by thermal treatments and exhibits well-organized pores parallel to one another. The white lines correspond to the mesopores generated in the space where the walls of the SBA-15 template had been.

**Figure 21.** TEM images (a, b) and EDS (c) spectrum of Pt-CMK-3.

The larger dark areas over the channels most likely correspond to NiO nanoparticle agglom‐ erates on the external surface with average diameter of 11–13 nm (**Figure 18a**). The small dark spots in the image (**Figure 18b**) could be ascribed to Zn and ZnO nanoparticles with average diameter of ~2.5–3 nm probably located into the host pores. EDS analysis was performed to estimate the presence of Zn- and Ni-containing CMK-1. A careful evaluation of the elemental compositions for C, O, and Zn and Ni nanoparticles was performed (**Figure 18c** and **d**).

**Figure 19.** TEM images of SBA-15. (a) [1 0 0] and (b) [1 1 0] reflection planes.

According to the maps collecting five spots, the average atomic weight % of C, O and Zn nanoclusters in Zn-CMK-1 is about 94.94, 0.26, and 4.80%, respectively. This shows that only 20% of the Zn species appear as ZnO. On the other hand, the average atomic weight % for Ni-CMK-1 was 94.1, 0.6, and 5.3 wt% for C, O, and Ni, clearly indicating that the higher Ni content

**Figure 19a** shows TEM images of synthesized SBA-15 used as an inorganic template. In **Figure 19b**, an ordered mesoporous array of longitudinal nanochannels can be seen, with pore

**Figure 20.** TEM images of CMK-3. (a) Nanoparticles (b) [1 0 0] and (c) [1 1 0] reflection planes.

appears as oxide (about 40%).

72 Microporous and Mesoporous Materials

**Figure 22.** Histograms of PSDs measured from full images of Pt-CMK-3 shown in **Figure 21**.

**Figure 21** offers an illustration of TEM micrographs of Pt-CMK-3. The ordered mesostructure of the sample can be seen in **Figure 21a** and **b**, indicating that the ordered structure of CMK-3 persists after Pt nanoparticles are incorporated. The darker contrast (small dark spots) which can be seen in some places may be attributed to the presence of regularly dispersed Pt particles. These images evidence the fine dispersion of Pt0 on CMK-3, a tinny size dissemination (**Figure 22**), and mean diameter of ~1 to 2 nm inside the mesopores of CMK-3 nanostructure. The existence of Pt0 inside the pore system of CMK-3 indicates an enhancement in the dispersion phenomena compared with the metal on the external carbon surface. The Pt nanoclusters are active sites for hydrogen adsorption and narrow pore size distribution is vital because the level of dispersion is better and the metal active area is superior.

The metal particles dispersed in CMK-3 carbon have from 1 to 2.5 nm size, and the average particle diameter are approximately to 1.74 nm, showing that utmost of Pt nanoclusters are mainly in the nanostructured intra-channel system of CMK-3.

Dispersion and size distribution of metal particles of Pt toughly depends on the technique of deposition and the characteristics of carbon support as published by Kuppan and Selvam [25], suggesting that reduction with paraformaldehyde of platinum nanoparticles deposited on mesoporous carbon CMK-3 is superior to other methods, including H2 reduction. In this work, we show that the nature of Pt incorporation to CMK-3 and the preparation of Pt-CMK-3 before Pt reduction (under N2 atmosphere and controlled temperature removal of the Pt precursor) impact the subsequent size of Pt-reduced nanoparticles, resulting in a narrow distribution of around 1.74 nm (**Figure 22**). If the first elimination of Pt precursors-CMK-3 occurs in an oxidative atmosphere, the likelihood of generating larger Pt clusters increases. Similar consequence on dispersion of iridium over SBA-16 was reported recently [37]. The factor of the Pt source removal (chloroplatinic acid in ethanol-CMK-3) is endothermic and not oxidative under N2 flow and controlled temperature desorption, avoiding the relocation and accumu‐ lation of platinum particles after it reduction.

In order to determine the presence of Pt in the prepared Pt-CMK-3, EDS analysis was employed to carefully evaluate the elemental compositions for C, O, and Pt nanoparticles (**Figure 21c**). Maps of elements were collected from four spots and used to evaluate the presence of all elements. This showed that the average atomic percentage of C, O, and Pt nanoclusters in Pt-CMK-3 is about 98.82, 0.28, and 0.9%, respectively.

TEM images for TiO2-CMK-3 (**Figure 23**) indicated an ordered structure slightly damaged by thermal treatments; they also exhibit well-organized pores parallel to each other. The white lines correspond to the mesopores generated in the space previously occupied by the walls of SBA-15 template. In **Figure 23b**, a dark spot indicates anatase nanoclusters. **Figure 23c** shows a TEM image with negative contrast, displaying nanoclusters with bright spots, allowing the observation of nanoclusters. Particle size distribution ranged from 1 to 11 nm and mean particle diameter was found to be close to 5 nm, indicating that most of the parti‐ cles reside within of the pore system.

Synthesis and Characterization of CMK Porous Carbons Modified with Metals Applied to Hydrogen Uptake and Storage http://dx.doi.org/10.5772/63710 75

**Figure 21** offers an illustration of TEM micrographs of Pt-CMK-3. The ordered mesostructure of the sample can be seen in **Figure 21a** and **b**, indicating that the ordered structure of CMK-3 persists after Pt nanoparticles are incorporated. The darker contrast (small dark spots) which can be seen in some places may be attributed to the presence of regularly dispersed Pt particles.

**22**), and mean diameter of ~1 to 2 nm inside the mesopores of CMK-3 nanostructure. The existence of Pt0 inside the pore system of CMK-3 indicates an enhancement in the dispersion phenomena compared with the metal on the external carbon surface. The Pt nanoclusters are active sites for hydrogen adsorption and narrow pore size distribution is vital because the level

The metal particles dispersed in CMK-3 carbon have from 1 to 2.5 nm size, and the average particle diameter are approximately to 1.74 nm, showing that utmost of Pt nanoclusters are

Dispersion and size distribution of metal particles of Pt toughly depends on the technique of deposition and the characteristics of carbon support as published by Kuppan and Selvam [25], suggesting that reduction with paraformaldehyde of platinum nanoparticles deposited on mesoporous carbon CMK-3 is superior to other methods, including H2 reduction. In this work, we show that the nature of Pt incorporation to CMK-3 and the preparation of Pt-CMK-3 before Pt reduction (under N2 atmosphere and controlled temperature removal of the Pt precursor) impact the subsequent size of Pt-reduced nanoparticles, resulting in a narrow distribution of around 1.74 nm (**Figure 22**). If the first elimination of Pt precursors-CMK-3 occurs in an oxidative atmosphere, the likelihood of generating larger Pt clusters increases. Similar consequence on dispersion of iridium over SBA-16 was reported recently [37]. The factor of the Pt source removal (chloroplatinic acid in ethanol-CMK-3) is endothermic and not oxidative under N2 flow and controlled temperature desorption, avoiding the relocation and accumu‐

In order to determine the presence of Pt in the prepared Pt-CMK-3, EDS analysis was employed to carefully evaluate the elemental compositions for C, O, and Pt nanoparticles (**Figure 21c**). Maps of elements were collected from four spots and used to evaluate the presence of all elements. This showed that the average atomic percentage of C, O, and Pt nanoclusters in Pt-

TEM images for TiO2-CMK-3 (**Figure 23**) indicated an ordered structure slightly damaged by thermal treatments; they also exhibit well-organized pores parallel to each other. The white lines correspond to the mesopores generated in the space previously occupied by the walls of SBA-15 template. In **Figure 23b**, a dark spot indicates anatase nanoclusters. **Figure 23c** shows a TEM image with negative contrast, displaying nanoclusters with bright spots, allowing the observation of nanoclusters. Particle size distribution ranged from 1 to 11 nm and mean particle diameter was found to be close to 5 nm, indicating that most of the parti‐

on CMK-3, a tinny size dissemination (**Figure**

These images evidence the fine dispersion of Pt0

74 Microporous and Mesoporous Materials

lation of platinum particles after it reduction.

CMK-3 is about 98.82, 0.28, and 0.9%, respectively.

cles reside within of the pore system.

of dispersion is better and the metal active area is superior.

mainly in the nanostructured intra-channel system of CMK-3.

**Figure 23.** (a) and (b) TEM images TiO2–CMK-3 and (c) Negative contrast image. (d) and (e) particle size distribution histogram corresponding to (a) and (c) TEM images, respectively.

In this work, we show that the nature of TiO2 incorporation into CMK-3 and the preparation of TiO2-CMK-3 (under N2 atmosphere and controlled temperature elimination of Ti precursor), influence posterior TiO2 nanoparticle size, leading to a narrow distribution as shown in the histograms (**Figure 23a** and **c**).

Vanadium nanoclusters were not seen in TEM images of V-CMK-3 due to a low contrast in the TEM image (**Figure 24**).

**Figure 24.** TEM images of V-CMK-3.

#### **8. Hydrogen uptake measurements**

The capacity of hydrogen storage was evaluated at low and high pressures and cryogenic temperatures (77 K).

The experimental data were fitted by Freundlich isotherm [50], that is, a purely empirical formula for gaseous adsorbates. The Freundlich model (Eq. (2)) is described by the following equation:

$$
\underline{Q} = K\_F \stackrel{\underline{\mathbb{L}}}{P^n} \tag{2}
$$

Here, Q is the quantity per unit mass of adsorbent, P is the adsorbate pressure, and KF and n are experiential constants (Freundlich constants) for, respectively, adsorbent-adsorbate pair at a constant temperature. A measure of the adsorption intensity is characterized by n, and KF is the adsorption measurement [50]. This equation was applied to the experimental data using nonlinear least-square regression for arbitrary fit functions and minimizing the objective function by means of the Levenberg-Marquardt method. The fitting accuracy was R2 = 0.98.

The hydrogen storage is defined as "uptake," the carbon material behaves as a sponge, due to the fact that the process is completely reversible and the curve remains in the same place as each point returned to baseline when measurements were carried out without the bath of liquid N2. We performed the treatment at high temperatures (except TiO2-CMK-3 sample where treatment is done only under N2 flow) is to reduce the oxide cluster to a metallic cluster because the electronic configuration of the metal cluster interacts better with hydrogen molecules and for high H2 storage. The amount of hydrogen uptake of all samples is higher than carbon samples. The Zn-CMK-1 sample has the highest hydrogen uptake (0–10 bar). Yet, at high pressures, the sample with Zn shows greater H2 adsorption than all the other samples. In samples that contain metal clusters, there are at least two ways in the process of hydrogen physisorption on metal-CMK-1/CMK-3 surface. Hydrogen molecules spill over onto the CMK-1 nanopores/micropores and are adsorbed onto nanometric metal clusters. Hydrogen adsorption proved to be fully reversible, which indicates that there was no chemical reaction or strong bond between hydrogen and metal nanocluster or the CMK-1 framework.

The proposed mechanism for hydrogen storage in metal-CMK-1/CMK-3 agreed with Kim and Park [51]. Hence, a dipole-induced model is proposed. The first layer of hydrogen molecules could react with the metal cluster because of the high oxidation capacity of metals interacting as dihydrogen complex [35], but the level of this initial interaction is not significant. The second layer of hydrogen molecules is physically adsorbed by dipole-induced interaction. Hydrogen molecules are basically non-polar, but the strong interaction of the metal particles leads to the dipole-induced effects of the hydrogen molecules. The third layer and any other layer above the hydrogen molecules could interact with metal cluster by the same mechanism, but the force of the dipole-induce bond is weak and decreases in inverse proportion with the distance from the surface. Consequently, this mechanism can be applied at higher pressures, since metal-CMK-1/CMK-3 adsorbs a larger amount of hydrogen than carbon at higher pressures.

**Figure 24.** TEM images of V-CMK-3.

76 Microporous and Mesoporous Materials

temperatures (77 K).

equation:

**8. Hydrogen uptake measurements**

The capacity of hydrogen storage was evaluated at low and high pressures and cryogenic

The experimental data were fitted by Freundlich isotherm [50], that is, a purely empirical formula for gaseous adsorbates. The Freundlich model (Eq. (2)) is described by the following

1

Here, Q is the quantity per unit mass of adsorbent, P is the adsorbate pressure, and KF and n are experiential constants (Freundlich constants) for, respectively, adsorbent-adsorbate pair at a constant temperature. A measure of the adsorption intensity is characterized by n, and KF is the adsorption measurement [50]. This equation was applied to the experimental data using nonlinear least-square regression for arbitrary fit functions and minimizing the objective

The hydrogen storage is defined as "uptake," the carbon material behaves as a sponge, due to the fact that the process is completely reversible and the curve remains in the same place as each point returned to baseline when measurements were carried out without the bath of liquid N2. We performed the treatment at high temperatures (except TiO2-CMK-3 sample where treatment is done only under N2 flow) is to reduce the oxide cluster to a metallic cluster because

function by means of the Levenberg-Marquardt method. The fitting accuracy was R2

*<sup>n</sup> Q KP* <sup>=</sup> *<sup>F</sup>* (2)

= 0.98.

Theoretically, the 3d states of metals mainly contributed to ground-state electronic structure near the Fermi level of metal-CMK-1 and, to some degree, the p states of carbon. Those contributions of 3d states decrease as we move across the periodic table from Mn to Zn following the increase of d orbital occupancies [52].

Hydrogen bonds to the metal result in an opening of HOMO-LUMO energy gap in the case of Ni-CMK-1, while it shows a reduction in HOMO-LUMO energy gap for Zn-CMK-1, resulting in the appearance of molecular electronic states which allow a better H2 physisorption.

The understanding at an atomic scale of the hydrogen spillover mechanism for storage of hydrogen in metal-doped carbon materials and metal-organic frameworks is discussed by means of a critical assessment of recent computational and experimental studies. It is claimed that the spillover mechanism includes (a) generation and desorption of mobile H atoms on metal nanoparticles; (b) diffusion of H atoms in weakly bound states on the support; (c) sticking and immobilization of H atoms at preferential locations of the receptor where barriers to sticking are lower, and (d) Eley-Rideal recombination of the adsorbed H atoms with diffusing mobile H atoms to form H2 [53].

In the TiO2-CMK-3, surface can adsorb hydrogen molecules by spill over onto CMK-3 nano‐ pores/micropores and adsorbed onto nanometric anatase clusters. First, we could attribute this adsorption to the different particle surface areas, since the adsorbed quantity might be primarily related to the available free surface area. Second, we could attribute it to the fact that nanometric carbon promoting the high-surface areas of TiO2 nanoclusters. CMK-3 surface promotes the high-surface areas of TiO2 nanoclusters may possibly modify oxidation state and control porosity feature, which is important to cause hydrogen adsorption (H2 molecules) by weak chemical pathways (such as dihydrogen complex interaction), although anatase or rutile is generally weak in hydrogen storage [54]. Through XRD and TEM observation, we also noticed that with the assistance of CMK-3, the fabricated TiO2 nanoparticles show small size and high dispersion, contributing to the substantial increase in H2 adsorption. In other words, the presence of TiO2 nanoclusters in CMK-3 results in formation of better-localized states of electrons at TiO2 nanoclusters and CMK-3 interface. These localized states may distribute electron concentration at the surface of TiO2-CMK-3 composite and causes a few reactions between hydrogen molecules and surface TiO2 nanoclusters [54]. According to theoretical investigations [55], TiO2 and CMK-3 interaction is not affected by the physisorption of the first H2 molecule because it does not impact the distance between Ti and the nearest C. The H-H distance is influenced (elongated) by bonds Ti-C sp2d because the strong repulsive phenomena on the adsorbed H2. The hydrogen atoms of the adsorbed H2 molecule should have negative charges, and the H2 molecule can be regarded as trapped by Ti cation by means of the charge polarization mechanism. This is an indicator that Ti donates electrons to the nearest C atoms on CMK-3, where the d-orbitals of the Ti atom overlap the sp2 orbitals of the Ti-C bonds to form the mixed sp2 d hybridization. This charge transfer is possible because the Ti atom is in cationic form and renders extensive hetero-polar bonding between the Ti atom and its neighboring C atoms, resulting in an increase in H2 molecule uptake. Moreover, CMK-3 approaching the positively charged Ti cation leads to loss of d-orbital degeneracy since the electrons of CMK-3 will be closer to some d-orbitals while being farther away from others. Thus, those d-orbitals that are closer to CMK-3 have a higher degree of energy than those farther away. This results in d-orbitals splitting in energy to reduce the total energy and stabilize the system, as explained by the ligand field theory [56].

Therefore, the first layer of hydrogen molecules could be reacted with the metal cluster due to high oxidation/reduction metals capacity interacting like as dihydrogen complex [35], similar metal-carbon samples; yet, the amount of this initial interaction is negligible. The second layer of hydrogen molecules is physically adsorbed by dipole-induced interaction as metal-carbon samples. Hydrogen molecules are non-polar, but the strong interaction of anatase particles leads to dipole-induced effects in H2. The third level of the layers and any higher layer of H2 molecules could bond with metal cluster by the same way; however, the dipole-induced interaction is weak, decreasing as separation to the surface increases. At higher pressure, this mechanism can be applied; consequently, the adsorption of hydrogen to TiO2-CMK-3 is higher than CMK-3 at greater pressures. The understanding of the distribution of frontier molecular orbitals around a nanostructure would be a significant guideline to design new functionalized materials for hydrogen storage. While strong localization of the highest occupied molecular orbital (HOMO) occurs on Ti cation located at the surface on anatase nanocluster, strong delocalization of the lowest unoccupied molecular orbital (LUMO) occurs at the base area. The strong localization of HOMO on Ti cation at the surface explains the enhanced adsorption interactions of H2 molecules [55]. Moreover, hydrogen bonds to anatase result in an opening of HOMO-LUMO energy gap. A reduction in HOMO-LUMO energy gap for TiO2-CMK-3 results in the appearance of molecular electronic states that allow a better H2 physisorption [52].

#### Synthesis and Characterization of CMK Porous Carbons Modified with Metals Applied to Hydrogen Uptake and Storage http://dx.doi.org/10.5772/63710 79

The hydrogen storage on V-CMK-3 sample could be attributed to a high V2O3 particle surface areas and could be attributed to the fact that nanometric carbon helping the high-surface areas of V2O3 nanoclusters. However, the high hydrogen adsorption may be explain because hydrogen storage might increase via the spill-over phenomenon. Specifically, hydrogen molecules are dissociated on the surface of the introduced transition metal cation (V+3). The other potential explanation is that the hydrogen molecules are attracted into the graphite pores due to formed weak dipoles with V2O3 nanoclusters. Metal cation increases and quickens hydrogen storage through a weak chemical reaction (from either the spill-over phenomenon or dipole attraction) (**Figure 25**).

control porosity feature, which is important to cause hydrogen adsorption (H2 molecules) by weak chemical pathways (such as dihydrogen complex interaction), although anatase or rutile is generally weak in hydrogen storage [54]. Through XRD and TEM observation, we also noticed that with the assistance of CMK-3, the fabricated TiO2 nanoparticles show small size and high dispersion, contributing to the substantial increase in H2 adsorption. In other words, the presence of TiO2 nanoclusters in CMK-3 results in formation of better-localized states of electrons at TiO2 nanoclusters and CMK-3 interface. These localized states may distribute electron concentration at the surface of TiO2-CMK-3 composite and causes a few reactions between hydrogen molecules and surface TiO2 nanoclusters [54]. According to theoretical investigations [55], TiO2 and CMK-3 interaction is not affected by the physisorption of the first H2 molecule because it does not impact the distance between Ti and the nearest C. The H-H distance is influenced (elongated) by bonds Ti-C sp2d because the strong repulsive phenomena on the adsorbed H2. The hydrogen atoms of the adsorbed H2 molecule should have negative charges, and the H2 molecule can be regarded as trapped by Ti cation by means of the charge polarization mechanism. This is an indicator that Ti donates electrons to the nearest C atoms

d hybridization. This charge transfer is possible because the Ti atom is in

cationic form and renders extensive hetero-polar bonding between the Ti atom and its neighboring C atoms, resulting in an increase in H2 molecule uptake. Moreover, CMK-3 approaching the positively charged Ti cation leads to loss of d-orbital degeneracy since the electrons of CMK-3 will be closer to some d-orbitals while being farther away from others. Thus, those d-orbitals that are closer to CMK-3 have a higher degree of energy than those farther away. This results in d-orbitals splitting in energy to reduce the total energy and

Therefore, the first layer of hydrogen molecules could be reacted with the metal cluster due to high oxidation/reduction metals capacity interacting like as dihydrogen complex [35], similar metal-carbon samples; yet, the amount of this initial interaction is negligible. The second layer of hydrogen molecules is physically adsorbed by dipole-induced interaction as metal-carbon samples. Hydrogen molecules are non-polar, but the strong interaction of anatase particles leads to dipole-induced effects in H2. The third level of the layers and any higher layer of H2 molecules could bond with metal cluster by the same way; however, the dipole-induced interaction is weak, decreasing as separation to the surface increases. At higher pressure, this mechanism can be applied; consequently, the adsorption of hydrogen to TiO2-CMK-3 is higher than CMK-3 at greater pressures. The understanding of the distribution of frontier molecular orbitals around a nanostructure would be a significant guideline to design new functionalized materials for hydrogen storage. While strong localization of the highest occupied molecular orbital (HOMO) occurs on Ti cation located at the surface on anatase nanocluster, strong delocalization of the lowest unoccupied molecular orbital (LUMO) occurs at the base area. The strong localization of HOMO on Ti cation at the surface explains the enhanced adsorption interactions of H2 molecules [55]. Moreover, hydrogen bonds to anatase result in an opening of HOMO-LUMO energy gap. A reduction in HOMO-LUMO energy gap for TiO2-CMK-3 results in the appearance of molecular electronic states that allow a better H2 physisorption [52].

orbitals of the Ti-C bonds to

on CMK-3, where the d-orbitals of the Ti atom overlap the sp2

stabilize the system, as explained by the ligand field theory [56].

form the mixed sp2

78 Microporous and Mesoporous Materials

**Figure 25.** Isotherms of hydrogen at 77 K on the different materials studied at a range of pressures (0–10 bar).

**Figure 26.** Scheme for the proposed mechanism of metal-CMK and hydrogen interaction.

The scheme showed in **Figure 26** suggests a mechanism for hydrogen storage in carbon mesoporous CMK, modified with metals in agreement with Kim et al.

#### **9. Conclusions**

We have shown that a hopeful hydrogen storage material can be obtained by ordered porous carbons CMK-1/CMK-3 modified with Ni, Zn, Pt, Ti, and V species that were synthesized by replication using MCM-48 and SBA-15 as hard template, respectively. The metal-metal/oxide nanoparticles-containing carbon was obtained by wetness impregnation considering the thermal treatments employing H2. Zn species mostly found as metallic Zn while Ni as NiO, with the highest average cluster size (12 nm), indicating greater dispersion of zinc and its incorporation in the nanoporous of mesoporous carbons but without occlusion by their small size compared with NiO clusters. The higher Zn dispersion probably causing a high metal surface area and a better use of the support allowed to a superior performance to H2 uptake of Zn-CMK-1. Pt-CMK-3 sample present an improvement in the capacity of hydrogen uptake than CMK-3 carbon due to the better dispersion of uniform platinum nanoparticles as well as better utilization of the support, which may probably was originate of a high-surface area and pore volume, that allow a large dispersion of Pt. Pt nanoparticles in Pt-CMK-3 have smaller size (~1.7 nm average) and have a quite well dispersion inside the nanochannels of CMK-3. This shows the better hydrogen uptake of the Pt-CMK-3 than CMK-3 carbon.

TiO2-CMK-3 can significantly enhance hydrogen adsorption capacity and hydrogen storage performance of CMK-3 material, proving to prospective candidates for application in hydro‐ gen storage. CMK-3 modified with TiO2 in anatase phase shows an enhanced capacity for H2 storage than that of the nanoporous CMK-3 sample. TiO2-CMK-3 improved performance and larger activity for H2 uptake is attributed to improved dispersion of uniform anatase nano‐ particles in addition to efficient use of the CMK-3, probably originate in the high pore volume and surface area, allowing a great dispersion of anatase. TiO2 (anatase phase) nanoparticles in TiO2-CMK-3 have a smaller size (~5 nm average, obtained by XRD and TEM) and a quite good dispersion.

Vanadium-modified carbon sample has higher hydrogen storage than CMK-3 pristine material and is the best material for hydrogen adsorption using CMK-3 nanometric carbon as support but lower than Zn-CMK-1.

A hydrogen uptake mechanism on metal/carbon surfaces was proposed. We can conclude by indicating that CMK-1/CMK-3 hydrogen storage capacity was increased by addition of metal clusters. Moreover, hydrogen storage behaviors onto metal/metal oxides-carbons can be optimized by controlling the metal cluster size and dispersion and by increasing the carbonspecific surface area.

#### **Acknowledgements**

The scheme showed in **Figure 26** suggests a mechanism for hydrogen storage in carbon

We have shown that a hopeful hydrogen storage material can be obtained by ordered porous carbons CMK-1/CMK-3 modified with Ni, Zn, Pt, Ti, and V species that were synthesized by replication using MCM-48 and SBA-15 as hard template, respectively. The metal-metal/oxide nanoparticles-containing carbon was obtained by wetness impregnation considering the thermal treatments employing H2. Zn species mostly found as metallic Zn while Ni as NiO, with the highest average cluster size (12 nm), indicating greater dispersion of zinc and its incorporation in the nanoporous of mesoporous carbons but without occlusion by their small size compared with NiO clusters. The higher Zn dispersion probably causing a high metal surface area and a better use of the support allowed to a superior performance to H2 uptake of Zn-CMK-1. Pt-CMK-3 sample present an improvement in the capacity of hydrogen uptake than CMK-3 carbon due to the better dispersion of uniform platinum nanoparticles as well as better utilization of the support, which may probably was originate of a high-surface area and pore volume, that allow a large dispersion of Pt. Pt nanoparticles in Pt-CMK-3 have smaller size (~1.7 nm average) and have a quite well dispersion inside the nanochannels of CMK-3.

mesoporous CMK, modified with metals in agreement with Kim et al.

This shows the better hydrogen uptake of the Pt-CMK-3 than CMK-3 carbon.

TiO2-CMK-3 can significantly enhance hydrogen adsorption capacity and hydrogen storage performance of CMK-3 material, proving to prospective candidates for application in hydro‐ gen storage. CMK-3 modified with TiO2 in anatase phase shows an enhanced capacity for H2 storage than that of the nanoporous CMK-3 sample. TiO2-CMK-3 improved performance and larger activity for H2 uptake is attributed to improved dispersion of uniform anatase nano‐ particles in addition to efficient use of the CMK-3, probably originate in the high pore volume and surface area, allowing a great dispersion of anatase. TiO2 (anatase phase) nanoparticles in TiO2-CMK-3 have a smaller size (~5 nm average, obtained by XRD and TEM) and a quite good

Vanadium-modified carbon sample has higher hydrogen storage than CMK-3 pristine material and is the best material for hydrogen adsorption using CMK-3 nanometric carbon as

A hydrogen uptake mechanism on metal/carbon surfaces was proposed. We can conclude by indicating that CMK-1/CMK-3 hydrogen storage capacity was increased by addition of metal clusters. Moreover, hydrogen storage behaviors onto metal/metal oxides-carbons can be optimized by controlling the metal cluster size and dispersion and by increasing the carbon-

**9. Conclusions**

80 Microporous and Mesoporous Materials

dispersion.

specific surface area.

support but lower than Zn-CMK-1.

Juliana M. Juarez, Marcos Gomez Costa, Oscar A. Anunziata, NANOTEC, CONICET, UTN-FRC, Maestro Lopez y Cruz Roja Argentine, Cordoba Argentine. The authors thank to CONICET Argentina, PIP CONICET<PHONE_NUMBER>0218CO. 2014–2016.

#### **Author details**

Marcos B. Gómez Costa, Juliana M. Juárez and Oscar A. Anunziata\*

\*Address all correspondence to<EMAIL_ADDRESS>
Centre for Research in Nanoscience and Nanotechnology (NANOTEC), Cordoba Regional Faculty, National Technological University, Maestro López y Cruz Roja Argentine, Córdoba, Argentina

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## **Mesoporous Carbons for Energy-Efficient Water Splitting to Produce Pure Hydrogen at Room Temperature**

Mohindar S. Seehra and Vishal Narang

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62503

#### **Abstract**

Theoretical and experimental aspects of the use of mesoporous carbons in carbonassisted water electrolysis (CAWE) to produce pure hydrogen at room temperature are presented. It is shown that the electrical energy requirements for CAWE can be as low as 20% ofthe energy needed for conventional water electrolysis, the extra energy coming from the electrochemical oxidation of carbon occurring at room temperature. Al‐ though CO2 is produced at the anode in this process, it is well separated from pure H2 produced at the cathode. Experimentalresults are reviewed for a variety of carbons with the major focus on the results obtained with carbon BP2000, which has both meso‐ pores and micropores and a nanocarbon produced by the hydrothermal treatment of microcrystalline cellulose.

**Keywords:** Hydrogen, Carbons, Electrolysis, Energy efficiency, Carbon dioxide

#### **1. Introduction**

Mesoporous materials with pore size from 2 to 50 nm fall in the middle range between microporous (pore size <2 nm) and macroporous (pore size >50 nm) materials. In a recent book edited by Titirici [1] and a review by Liang et al. [2], various processes for producing mesoporous carbon materials have been described. Sevilla and Fuertes [3] and Seehra et al. [4, 5] used hydrothermal treatment of cellulose and lignin to produce nanoparticles of carbon. Liu and Guo [6] compared the characteristics of carbons produced by the hydrothermal treatment of holocellulose and crude biomass. Other methods of preparing mesoporous

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

carbons have been reported by Zhou et al. [7] for supercapacitor applications. Such carbons produced by different methods have applications in separation processes, catalysis, energy storage (such as super capacitors), and energy conversion. A very popular commercial source of mesoporous and microporous carbons is non-graphitized Black Pearls (BP) carbon blacks manufactured by Cabot Corporation. The pore structures of the BP carbons have been reported by Kruk et al. [8].

The focus of this review is on the use and the science of mesoporous/microporous carbons for the electrochemical production of pure hydrogen at room temperature employing electrical energy as low as 20% of the energy used in ordinary water electrolysis (WE). This process was first proposed and demonstrated by Coughlin and Farooque [9, 10] and developed more fully in the recent investigations of Seehra et al. using a variety of carbons and termed as carbonassisted water electrolysis (CAWE) [11, 12]. Some follow-up studies of this process have been reported by Dubey et al. [13] using a carbon nanotube anode and by Giddy et al. [14] using a solid-state electrolytic cell. A different variation of this process involving hydrogen generation by laser irradiation of a carbon powder suspension in water has been reported by Akimoto et al. [15]. Results of the hydrogen evolution rate RH using a variety of carbons show some proportionality to the surface area of a carbon which in turn is related to the pore structure of the carbons. The best results so far have been obtained with carbon BP2000, which as shown later has both micropores and mesopores and a surface area of about 1500 m2 /g.

This review covers the science behind CAWE followed by select experimental results and a comparative energy analysis of the process vis-à-vis conventional WE. Such comparative energy assessment of different processes to produce H2 is essential for considering practical applications of the different processes. The remaining sections are organized as follows. In Section 2, prominent analytical techniques used in the structural characterization of porous carbons are reviewed with some examples imported from the published literature. Section 3 deals with the science behind CAWE and the experimental results on the electrochemical production of hydrogen reported in the literature using different carbons. The comparative energy analysis of the CAWE and WE processes is presented in Section 4. Concluding remarks of this review are given in Section 5.

#### **2. Structural characterization of porous carbons**

The porous structure of materials is often determined by means of adsorption of gases such as nitrogen and argon over a wide range of pressures. For activated carbons, the resulting structure depends on the process of activation, the activation agent, and the nature of the raw material used for producing a carbon. Kruk et al. [8] reported the surface and structural parameters (standard BET surface area SBET, external surface area Sex, micropore volume Vmi, and total pore volume Vt ) of six carbon blacks produced by Cabot Corporation. These are named as Cabot BP120, BP280, BP460, BP800, BP1300, and BP2000 with SBET = 30, 41, 78, 242, 520, and 1450 m2 /g, respectively. The pore volume distribution of these six carbons determined from nitrogen adsorption and desorption isotherms at 77.35 K and reproduced from Ref. [8] is shown in **Figure 1**. From these graphs, it is evident that BP 120 and BP 280 have only small amount of pores, mostly in macroregion. BP460 has some mesoporosity, whereas BP800 and BP1300 are largely mesoporous with some microporosity. In contrast, BP2000 contains large fraction of micropores in addition to mesopores and some macropores. The ratio of SBET/Vt for BP120, BP280, BP460, BP800, BP1300, and BP2000, based on the numbers given in Ref. [8] are 375, 410, 269, 356, 571, and 829 m2 /mL, respectively, showing that shapes and sizes of the particles and pores for the BP1300 and BP 2000 are quite different from the other four carbons.

carbons have been reported by Zhou et al. [7] for supercapacitor applications. Such carbons produced by different methods have applications in separation processes, catalysis, energy storage (such as super capacitors), and energy conversion. A very popular commercial source of mesoporous and microporous carbons is non-graphitized Black Pearls (BP) carbon blacks manufactured by Cabot Corporation. The pore structures of the BP carbons have been

The focus of this review is on the use and the science of mesoporous/microporous carbons for the electrochemical production of pure hydrogen at room temperature employing electrical energy as low as 20% of the energy used in ordinary water electrolysis (WE). This process was first proposed and demonstrated by Coughlin and Farooque [9, 10] and developed more fully in the recent investigations of Seehra et al. using a variety of carbons and termed as carbonassisted water electrolysis (CAWE) [11, 12]. Some follow-up studies of this process have been reported by Dubey et al. [13] using a carbon nanotube anode and by Giddy et al. [14] using a solid-state electrolytic cell. A different variation of this process involving hydrogen generation by laser irradiation of a carbon powder suspension in water has been reported by Akimoto et al. [15]. Results of the hydrogen evolution rate RH using a variety of carbons show some proportionality to the surface area of a carbon which in turn is related to the pore structure of the carbons. The best results so far have been obtained with carbon BP2000, which as shown

This review covers the science behind CAWE followed by select experimental results and a comparative energy analysis of the process vis-à-vis conventional WE. Such comparative energy assessment of different processes to produce H2 is essential for considering practical applications of the different processes. The remaining sections are organized as follows. In Section 2, prominent analytical techniques used in the structural characterization of porous carbons are reviewed with some examples imported from the published literature. Section 3 deals with the science behind CAWE and the experimental results on the electrochemical production of hydrogen reported in the literature using different carbons. The comparative energy analysis of the CAWE and WE processes is presented in Section 4. Concluding remarks

The porous structure of materials is often determined by means of adsorption of gases such as nitrogen and argon over a wide range of pressures. For activated carbons, the resulting structure depends on the process of activation, the activation agent, and the nature of the raw material used for producing a carbon. Kruk et al. [8] reported the surface and structural parameters (standard BET surface area SBET, external surface area Sex, micropore volume Vmi,

named as Cabot BP120, BP280, BP460, BP800, BP1300, and BP2000 with SBET = 30, 41, 78, 242,

from nitrogen adsorption and desorption isotherms at 77.35 K and reproduced from Ref. [8]

) of six carbon blacks produced by Cabot Corporation. These are

/g, respectively. The pore volume distribution of these six carbons determined

/g.

later has both micropores and mesopores and a surface area of about 1500 m2

reported by Kruk et al. [8].

88 Microporous and Mesoporous Materials

of this review are given in Section 5.

and total pore volume Vt

520, and 1450 m2

**2. Structural characterization of porous carbons**

**Figure 1.** Pore size distribution of the six carbon blacks. (Reproduced with permission from Kruk et al. [8], copyright Elsevier 1996).

Characterization of seven activated carbons from PICA-USA for supercapacitor applications is reported by Gamby et al. [16]. Their BET surface areas ranged from a low of 1200 m2 /g to a high of SBET = 2315 m2 /g. These carbons contained roughly equal fractions of microporous and mesoporous volume fractions with total porous volume ≈1 cm3 /g for pore sizes <50 nm. The microporous volume increases nearly linearly with increase in the surface area. The specific capacitance increased by about 40% with increase in SBET from about 1000 m2 /g to about 2000 m2 /g. This improvement in specific capacitance was tentatively related to increase in the mesoporous volume of the carbons.

**Figure 2.** Wide-angle X-ray diffraction patterns of the three PICA carbons. (Reproduced with permission from Mani‐ vannan et al. [17], copyright Elsevier 1999).

Manivannan et al. [17] reported characterization of three PICA carbons viz. GX203, P1400, and Med50 with SBET = 1000, 1150 and 2000 m2 /g, respectively, using the experimental techniques of wide-angle X-ray diffraction (WAXD), Raman spectroscopy, magnetometry, and electron spin resonance (ESR) spectroscopy. In **Figure 2**, we show the WAXD patterns of the three carbons using Cu-Kα source, with the broad lines near 2θ = 24° and 44° due to graphitic crystallites corresponding to the (002) and (101)/(101) Bragg lines, respectively. The widths of these lines are then used to determine Lc (La) as the crystallite sizes along the c(a) directions, yielding Lc ≈ 1 nm and La ≈ 3 nm for these carbons. The Raman spectra of the three carbons, shown in **Figure 3**, can also be used to determine La from the empirical relation: La (nm) = 4.4/ R, where R = I(1350)/I(1600) is the ratio of the intensities of the D band near 1350 cm−1 and the G band near 1600 cm−1. This analysis also showed La ≈ 3 nm in agreement with the results from WAXD. A similar comparison of the use of WAXD and Raman spectroscopy for characterizing carbons is given by Cuesta et al. [18]. ESR and magnetometry was used by Manivannan et al. [17] to determine the density of ESR active surface dangling bonds and nature of magnetic impurities which otherwise could not be detected by WAXD.

From the WAXD data shown in **Figure 2**, it is evident that for the lower 2θ values, the scattered intensity of X-ray photons increases sharply with decrease in 2θ. This is a characteristic feature of many amorphous materials. Measurements of the intensity of scattered X-rays covering smaller angles from 2θ = 0.1° to 10° are termed small angle X-ray scattering (SAXS), and it can provide very useful information about pore sizes and structural aspects of macromolecules between 5 and 25 nm. In this case, the quantity plotted is the intensity I(q) of the scattered Xrays of wavelength λ as a function of momentum transfer q = 4π·sinθ/λ and such a plot can separate out contributions from micro and macropores [19]. In general, the intensity I(q) ~ q−α where α = 6−D, with D being the dimensionality of pore-boundary surface [20]. Experiments of SAXS in a lignite coal showed α = 3.5 and so D = 2.5 signifying fractal nature of the surface. Peaks in the I(q) versus q provides information on the size of macromolecules as for example observed in carbon nanotube suspensions [21].

**Figure 3.** Raman spectra of the three PICA carbons. (Reproduced with permission from Manivannan et al. [17], copy‐ right Elsevier 1999).

Techniques other than those described above that have also been used for the characterization of carbons include Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). FTIR spectroscopy is particularly useful for determining the nature of surface functional groups. The use of all these techniques except XPS was employed in the characterization of the carbons produced from the hydrothermal carbonization of microcrystalline cellulose [4]. SEM/TEM is indispensable tools for visualiza‐ tion of the size and morphology of the particles [1–5], and TGA provides good information on the decomposition and oxidation characteristics of carbons [4]. The technique of XPS is often used for determining the elemental composition of a material. In summary, techniques that are used for structural characterization of other materials can also be used for carbons.

#### **3. Carbon-assisted WE**

**Figure 2.** Wide-angle X-ray diffraction patterns of the three PICA carbons. (Reproduced with permission from Mani‐

Manivannan et al. [17] reported characterization of three PICA carbons viz. GX203, P1400, and

of wide-angle X-ray diffraction (WAXD), Raman spectroscopy, magnetometry, and electron spin resonance (ESR) spectroscopy. In **Figure 2**, we show the WAXD patterns of the three carbons using Cu-Kα source, with the broad lines near 2θ = 24° and 44° due to graphitic crystallites corresponding to the (002) and (101)/(101) Bragg lines, respectively. The widths of these lines are then used to determine Lc (La) as the crystallite sizes along the c(a) directions, yielding Lc ≈ 1 nm and La ≈ 3 nm for these carbons. The Raman spectra of the three carbons, shown in **Figure 3**, can also be used to determine La from the empirical relation: La (nm) = 4.4/ R, where R = I(1350)/I(1600) is the ratio of the intensities of the D band near 1350 cm−1 and the G band near 1600 cm−1. This analysis also showed La ≈ 3 nm in agreement with the results from WAXD. A similar comparison of the use of WAXD and Raman spectroscopy for characterizing carbons is given by Cuesta et al. [18]. ESR and magnetometry was used by Manivannan et al. [17] to determine the density of ESR active surface dangling bonds and nature of magnetic

From the WAXD data shown in **Figure 2**, it is evident that for the lower 2θ values, the scattered intensity of X-ray photons increases sharply with decrease in 2θ. This is a characteristic feature of many amorphous materials. Measurements of the intensity of scattered X-rays covering smaller angles from 2θ = 0.1° to 10° are termed small angle X-ray scattering (SAXS), and it can provide very useful information about pore sizes and structural aspects of macromolecules between 5 and 25 nm. In this case, the quantity plotted is the intensity I(q) of the scattered Xrays of wavelength λ as a function of momentum transfer q = 4π·sinθ/λ and such a plot can separate out contributions from micro and macropores [19]. In general, the intensity I(q) ~ q−α where α = 6−D, with D being the dimensionality of pore-boundary surface [20]. Experiments of SAXS in a lignite coal showed α = 3.5 and so D = 2.5 signifying fractal nature of the surface.

/g, respectively, using the experimental techniques

vannan et al. [17], copyright Elsevier 1999).

90 Microporous and Mesoporous Materials

Med50 with SBET = 1000, 1150 and 2000 m2

impurities which otherwise could not be detected by WAXD.

The basic reactions for the well-known process of WE and Carbon-assisted water electrolysis (CAWE), the latter proposed by Coughlin and Farooque [9, 10], are given below:

#### **3.1. Water electrolysis (WE)**

$$\text{Anode:}\ 2\text{H}\_2\text{O} \rightarrow \text{O}\_2 + 4\text{H}^+ + 4\text{e}^-\tag{1}$$

$$\text{Cathode:}\ 4\text{H}^+\text{+}4\text{e}^-\rightarrow 2\text{H}\_2\tag{2}$$

$$\text{Net reaction:}\,2\text{H}\_2\text{O} \to 2\text{H}\_2 + \text{O}\_2\tag{3}$$

#### **3.2. Carbon-assisted water electrolysis (CAWE)**

$$\text{Anode:}\,\text{C}\left(\text{s}\right) + 2\text{H}\_2\text{O} \rightarrow \text{CO}\_2 + 4\text{H}^+ + 4\text{e}^-\tag{4}$$

$$\text{Cathode:}\ 4\text{H}^+ + 4\text{e}^- \to 2\text{H}\_2(\text{g})\tag{5}$$

$$\text{Net reaction: }\mathrm{C}(\text{s}) + 2\mathrm{H}\_{2}\mathrm{O}(l) \rightarrow \mathrm{CO}\_{2}(\text{g}) + 2\mathrm{H}\_{2}(\text{g})\tag{6}$$

To understand why CAWE is more energy efficient than WE, the change in the Gibbs-free energy ΔG = ΔH—TΔS needs to be determined where ΔH (ΔS) is the change in enthalpy (entropy) of the system at temperature T. This is valid for the reactions under constant temperature and constant pressure conditions. Enthalpy is best understood as standard heat of formation of a compound from its basic elements at 25°C, the enthalpy being zero for the elements. For a compound to be stable, ΔH has to be negative. A reaction is favored if ΔG = ΔG (products) − ΔG (reactants) < 0 which implies that a reaction is favored if, ΔH < 0 and ΔS > 0. We now apply these concepts to reactions for WE and CAWE listed in Eqs. (1)–(6).

At 25°C, ΔH (*l*) = −68.32 kcal/mol for water and ΔH (*g*) = −94.05 kcal/mol for CO2 [22]. The standard molar entropies (in units of cal/mol K) are 31.2, 49.0, 16.7, and 51.0 for H2(*g*), O2(*g*), H2O(*l*), and CO2(*g*), respectively. Using these numbers for reaction (3) gives ΔH = 136.64 kcal and TΔS = 23.23 kcal for 298 K, yielding ΔG = ΔH−TΔS = 113.41 kcal > 0. ΔG being >0 means that the reaction (3) is not favored and energy must be supplied to split water into H2 and O2. Finally, ΔG is related to the electrical potential Eo generated in the reaction by the relation [22]:

$$
\Delta \mathbf{G} = -\mathbf{n} \mathbf{F} \mathbf{E}^o \tag{7}
$$

Here, n is the number of electrons involved in the reaction and F = 96484.56 C/mol is the Faraday constant. For WE, n = 4 according to Eq. (1) leading to Eo = −1.23 V. The negative sign implies that a minimum of Eo = 1.23 V must be applied to split water. Note that we have used 1 cal = 4.184 J and volt = Joules /Coulomb.

For CAWE, defined by reaction (6), a similar analysis yields ΔH = 42.59 kcal, ΔS = 80.05 cal leading to ΔG = 18.74 kcal and Eo = −0.20 V. This suggests that for reaction (6) to proceed, a minimum Eo = 0.20 V needs to be applied which is a factor of 1.23/0.20 ≃ 6 smaller than that needed for reaction (3) of WE. This extra energy comes from carbon which gets oxidized to CO2 in the process, making CAWE six times more energy efficient than WE, at least theoreti‐ cally. This process of CAWE may thus be classified as electrochemical gasification of carbon at room temperature producing pure H2 at the cathode which is well separated from pure CO2 produced at the anode.

**3.1. Water electrolysis (WE)**

92 Microporous and Mesoporous Materials

**3.2. Carbon-assisted water electrolysis (CAWE)**

Finally, ΔG is related to the electrical potential Eo

4.184 J and volt = Joules /Coulomb.

constant. For WE, n = 4 according to Eq. (1) leading to Eo

<sup>+</sup> Anode: 2H O O +4H +4e 2 2 ® - (1)

Net reaction: 2H O 2H O 2 22 ® + (3)

Anode: C 2H O CO 4H 4e ( ) 2 2 *s* + - + ®++ (4)

Net reaction: C 2H O CO g 2H (g) (*s l* ) + ®+ 2 22 ( ) ( ) (6)

generated in the reaction by the relation [22]:

= −1.23 V. The negative sign implies

<sup>o</sup> ΔG nFE = - (7)

<sup>+</sup> Cathode: 4H +4e 2H2

<sup>+</sup> Cathode: 4H +4e 2H (g) <sup>2</sup>

To understand why CAWE is more energy efficient than WE, the change in the Gibbs-free energy ΔG = ΔH—TΔS needs to be determined where ΔH (ΔS) is the change in enthalpy (entropy) of the system at temperature T. This is valid for the reactions under constant temperature and constant pressure conditions. Enthalpy is best understood as standard heat of formation of a compound from its basic elements at 25°C, the enthalpy being zero for the elements. For a compound to be stable, ΔH has to be negative. A reaction is favored if ΔG = ΔG (products) − ΔG (reactants) < 0 which implies that a reaction is favored if, ΔH < 0 and ΔS > 0. We now apply these concepts to reactions for WE and CAWE listed in Eqs. (1)–(6).

At 25°C, ΔH (*l*) = −68.32 kcal/mol for water and ΔH (*g*) = −94.05 kcal/mol for CO2 [22]. The standard molar entropies (in units of cal/mol K) are 31.2, 49.0, 16.7, and 51.0 for H2(*g*), O2(*g*), H2O(*l*), and CO2(*g*), respectively. Using these numbers for reaction (3) gives ΔH = 136.64 kcal and TΔS = 23.23 kcal for 298 K, yielding ΔG = ΔH−TΔS = 113.41 kcal > 0. ΔG being >0 means that the reaction (3) is not favored and energy must be supplied to split water into H2 and O2.

Here, n is the number of electrons involved in the reaction and F = 96484.56 C/mol is the Faraday

that a minimum of Eo = 1.23 V must be applied to split water. Note that we have used 1 cal =

**Figure 4.** A schematic diagram of the three-electrode cell with Pt plate as anode (surface area = 6.8 cm2 ) and Pt coil as cathode (surface area = 2.5 cm2 ). A magnetic stirrer was used to stir the contents of the anode. (Reproduced with per‐ mission from Seehra et al. [11], copyright AIP Publishing LLC 2007).

Experimentally, an electrochemical cell of the type shown in **Figure 4** is often used. Because of the over-potentials of the electrodes which are related to their intrinsic properties and surface structure, Eo considerably greater than the theoretical 1.23 V in WE and the theoretical 0.20 V in CAWE needs to be applied to initiate H2 evolution. Electrodes made of Pt and Pd are often used because they have the lowest over-potentials [23]. For each applied voltage Eo between the cathode and the anode, the quantities measured are as follows: (i) current Io; (ii) the time tH needed to produce the same amounts of H2 for each Eo as measured by gas chromatography; and the computed quantities such as the hydrogen evolution rate RH = 1/tH and the efficiency factor AH = RH/Eo Io representing evolved H2 per kWh of energy used. The voltage Eo is with respect to the standard hydrogen electrode (SHE), and it is determined from the Eq.: Eo = E(P) + 0.22 V where E(P) is the voltage measured by the potentiostat with respect to the reference electrode Ag/AgCl with Eo = 0.22 V (SHE). It is noted that in many publications, voltages listed are simply E(P), and this needs to be kept in mind while comparing with the theoretical estimates.

**Figure 5.** Forward and reverse current–voltage characteristics of the reaction, with and without adding carbon BP2000. (Reproduced with permission from Seehra and Bollineni [12], copyright Elsevier 2009).

In **Figure 5**, the effect of adding carbon BP2000 to the anode in the amount of 0.08 gm/cm3 of the electrolyte (3.7 M H2SO4) is shown in terms of current–voltage plot. Without added carbon, an increase in current indicative of the start of WE only begins for Eo > 2 V. With the addition of carbon BP2000 to the anode, a rapid increase in current accompanied by evolution of H2 gas at the cathode was observed for Eo > 0.4 V. These results are consistent with the theoretical predictions mentioned earlier in that the presence of carbon in the anode significantly lowers the threshold voltage to initiate WE and produce H2 gas at the cathode.

It is noted that 3.7 M H2SO4 is chosen for the electrolyte since this particular concentration was found to have the highest electrical conductivity [10]. Similarly, the carbon concentration of 0.08 g/cm3 was determined to be most efficient and practical by varying the concentration of the added carbon [11, 12]. A number of carbons were tried for their effects on the hydrogen generation rate RH (**Figure 6**). As evident in **Figure 6**, carbon BP2000 yielded the best RH values and so most of the follow up experiments were made with carbon BP2000 [12]. Another carbon that produced similar RH values was the carbon produced by the hydrothermal treatment of microcrystalline cellulose using water as solvent and treated at 250°C for 60 min under the maximum pressure of 800 psi. This hydrothermal carbonization yielded spherical carbon particles of about 210 nm [4]. In the following section, the results obtained with carbon BP2000 [12] and hydrothermally treated cellulose are compared. The liquid-phase products obtained in the hydrothermal treatment of microcrystalline cellulose have also been analyzed [24].

the Eq.: Eo = E(P) + 0.22 V where E(P) is the voltage measured by the potentiostat with respect

voltages listed are simply E(P), and this needs to be kept in mind while comparing with the

**Figure 5.** Forward and reverse current–voltage characteristics of the reaction, with and without adding carbon BP2000.

In **Figure 5**, the effect of adding carbon BP2000 to the anode in the amount of 0.08 gm/cm3

the electrolyte (3.7 M H2SO4) is shown in terms of current–voltage plot. Without added carbon,

of carbon BP2000 to the anode, a rapid increase in current accompanied by evolution of H2 gas

predictions mentioned earlier in that the presence of carbon in the anode significantly lowers

It is noted that 3.7 M H2SO4 is chosen for the electrolyte since this particular concentration was found to have the highest electrical conductivity [10]. Similarly, the carbon concentration of 0.08 g/cm3 was determined to be most efficient and practical by varying the concentration of the added carbon [11, 12]. A number of carbons were tried for their effects on the hydrogen generation rate RH (**Figure 6**). As evident in **Figure 6**, carbon BP2000 yielded the best RH values and so most of the follow up experiments were made with carbon BP2000 [12]. Another carbon that produced similar RH values was the carbon produced by the hydrothermal treatment of microcrystalline cellulose using water as solvent and treated at 250°C for 60 min under the maximum pressure of 800 psi. This hydrothermal carbonization yielded spherical carbon particles of about 210 nm [4]. In the following section, the results obtained with carbon BP2000

(Reproduced with permission from Seehra and Bollineni [12], copyright Elsevier 2009).

an increase in current indicative of the start of WE only begins for Eo

the threshold voltage to initiate WE and produce H2 gas at the cathode.

at the cathode was observed for Eo

= 0.22 V (SHE). It is noted that in many publications,

of

> 2 V. With the addition

> 0.4 V. These results are consistent with the theoretical

to the reference electrode Ag/AgCl with Eo

theoretical estimates.

94 Microporous and Mesoporous Materials

**Figure 6.** Comparison of hydrogen evolution rate RH measured at Eo = 1.12 V for several carbons plotted vs. oxidation temperatures of carbons determined by thermo-gravimetric analysis. MWCNT and SCNT stand for multi-walled and single-walled carbon nanotubes, respectively (Reproduced with permission from Seehra and Bollineni [12], copyright Elsevier 2009).

In the experiments reported in Refs. [4] and [12], H2 was produced electrochemically first by conventional WE using 3.7 M H2SO4 as the electrolyte and then by adding carbons and cellulose-derived nanocarbons to the anode in separate experiments, repeating the experi‐ ments each time by measuring current Io and hydrogen solution rate RH as a function of the voltage Eo applied between the cathode and anode. Evolved hydrogen at the cathode was measured by a calibrated gas chromatograph. The efficiency factor AH = RH/Eo Io representing evolved H2 per kWh of energy used is then computed for each Eo . The plots of current in the circuit, RH and AH as a function of applied voltage Eo (relative to the SHE) are shown in **Figure 7** for three cases: (i) electrolyte only, (ii) carbon BP2000 added to the anode in the amount of 0.08 gm/mL of the electrolyte, and (iii) the carbon produced by the hydrothermal treatment of cellulose added to the anode in the amount of 0.08 gm/mL of the electrolyte. In the WE process, a noticeable current and RH is observed only if Eo > 2 V. However, for both BP2000 and hydrothermally treated cellulose, significant Io and RH are observed at Eo = 0.7 V. In **Figure 7**, the horizontal dotted line is drawn to show that using commercial carbon BP2000 with surface area of 1500 m2 /g, same RH is obtained at applied voltage Eo = 0.7 V using carbon BP2000 as obtained by applying Eo = 2.6 V in conventional WE. For carbon produced by hydrothermal treatment of cellulose, Eo = 1.2 V is needed to produce the same RH. A detailed analysis of the comparison of the efficiencies of the three processes is given in Section 4.

**Figure 7.** Comparison of current, hydrogen evolution rate RH, and efficiency factor AH against applied voltage Eo (vs. standard hydrogen electrode) for HTP-cellulose treated at 250°C for 60 min, carbon BP2000 and electrolyte only. (Re‐ produced with permission from Seehra et al. [4], copyright Elsevier 2012).

According to the Eqs. (4) and (5) valid for CAWE, H2 should be produced at the cathode and CO2 at the anode in the molar ratio of H2/CO2 =2. With careful quantitative measurements, this indeed was found to be valid using carbon BP2000 [12]. However, using carbon produced from the hydrothermal treatment of cellulose, CO2 at the anode could not be detected with certainty suggesting that the reaction with this carbon is more complicated perhaps because of the presence of surface group present in this carbon [4]. Another difficulty in the quantification of CO2 is that its peak in gas chromatography is quite weak relative to peaks from H2 and O2. Regardless, CO2 produced in CAWE at the anode is well separated from H2 produced at the cathode. This is important since CO2 is a green-house gas and so it can be collected in relatively pure form and so appropriately used or sequestered.

Of all the carbons results for which are shown in **Figure 6**, BP2000 was found to be the most efficient for producing H2. It is very likely that this is due to surface area effect since BP2000 has high surface area of 1500 m2 /g. However, a direct correlation between the surface area and RH has not yet been established experimentally. It may be possible to do so by comparing RH yields for the various BP carbons with different surface areas shown in **Figure 1**.

#### **4. Comparative energy analysis**

treatment of cellulose, Eo

96 Microporous and Mesoporous Materials

= 1.2 V is needed to produce the same RH. A detailed analysis of the

comparison of the efficiencies of the three processes is given in Section 4.

**Figure 7.** Comparison of current, hydrogen evolution rate RH, and efficiency factor AH against applied voltage Eo (vs. standard hydrogen electrode) for HTP-cellulose treated at 250°C for 60 min, carbon BP2000 and electrolyte only. (Re‐

According to the Eqs. (4) and (5) valid for CAWE, H2 should be produced at the cathode and CO2 at the anode in the molar ratio of H2/CO2 =2. With careful quantitative measurements, this indeed was found to be valid using carbon BP2000 [12]. However, using carbon produced from the hydrothermal treatment of cellulose, CO2 at the anode could not be detected with certainty suggesting that the reaction with this carbon is more complicated perhaps because of the presence of surface group present in this carbon [4]. Another difficulty in the quantification of CO2 is that its peak in gas chromatography is quite weak relative to peaks from H2 and O2. Regardless, CO2 produced in CAWE at the anode is well separated from H2 produced at the cathode. This is important since CO2 is a green-house gas and so it can be collected in relatively

Of all the carbons results for which are shown in **Figure 6**, BP2000 was found to be the most efficient for producing H2. It is very likely that this is due to surface area effect since BP2000

RH has not yet been established experimentally. It may be possible to do so by comparing RH

yields for the various BP carbons with different surface areas shown in **Figure 1**.

/g. However, a direct correlation between the surface area and

produced with permission from Seehra et al. [4], copyright Elsevier 2012).

pure form and so appropriately used or sequestered.

has high surface area of 1500 m2

Understanding the efficiencies of various processes to produce hydrogen is very important for practical considerations. In this regard, Rosen and Scott [25] have published a comparative energy analysis of the various processes currently in use for producing H2. The summary of this analysis, reproduced from their paper, is given in **Table 1**. According to this analysis, the energy efficiency of the conventional WE for producing H2 gas is only about 30% compared to about 85% for the steam methane reforming (SMR) and about 60% for coal gasification (CG) processes. If heat losses to the environment are also included (exergy) in the analysis, then these efficiencies are reduced by an additional few percent. The energy efficiency here is defined as the ratio of the energy contained in the produced hydrogen to the input energy. Because of the higher efficiencies of the SMR and CG processes, these are currently the favored techniques for producing H2 although both of these processes also produce CO2.

**Table 1.** Hydrogen production processes and their efficiencies considering fuel and/or a hypothetical heat source as the external energy inputs.

In the recent paper [4], energy efficiencies of the WE and CAWE processes were considered in terms of the energy equivalence of electrical energy in kWh (kilo-watt-hour) and liters (L) of produced H2. Using the data available in the literature, it can be shown that one gasoline gallon equivalent (GGE) equals 33.4 kWh of electricity which in turn equals 10.1 m3 of H2 gas at STP (standard temperature and pressure). According to this equivalence, one kWh of electricity equals 300 L (liters) of hydrogen implying that one kWh of electricity used in electrolysis must produce 300 L of H2 @ STP for 100% energy efficiency. This equivalence is used here to determine the energy efficiencies of WE and CAWE processes, the latter using carbon BP2000 and the carbon produced from the hydrothermal treatment of microcrystalline cellulose [4].

Examining the results shown in **Figure 7**, it is evident that for electrolyte only representing conventional WE, Eo > 2 V is needed to start the reaction, reaching close to saturation at Eo = 2.6 V. At this voltage, RH is about 15 mL/h of hydrogen and AH equals 100 L of H2 produced per kWh of electrical energy used. As noted earlier, if process was 100% energy efficient, AH = 300 L/kWh should have been produced. Thus, conventional WE is only about 33% (100/300) efficient, in close agreements with the results from the analysis of Rosen and Scott shown in **Table 1**.

Following the horizontal dotted line drawn in **Figure 7**, it is evident that using commercial carbon BP2000 with surface area of 1500 m2 /g, same RH is obtained at applied voltage Eo = 0.7 V using carbon BP2000 as obtained by applying Eo = 2.6 V in conventional WE. However, at Eo = 0.7 V with carbon BP2000, AH = 450 L/kWh of H2 is produced resulting in energy efficiency of 150% (450/300) compared to about 33% for conventional WE. This extra energy representing nearly fivefold improvement in energy efficiency over conventional WE is coming from the extra electrons provided by use of carbon BP2000. This electrochemical gasification occurring at room temperature does produce CO2 at the anode, but it is well separated from the H2 produced at the cathode.

In **Figure 7**, the data of H2 production using carbon produced from hydrothermally treated cellulose instead of carbon BP2000 is also shown. With the use of carbon produced from hydrothermally treated cellulose, again following the horizontal line, somewhat lower AH ≃ 350 L/kWh is obtained at Eo = 0.7 V with essentially similar evolution rate RH of H2. This yields slightly lower energy efficiency of about 120% (350/300) which is still a factor of four improve‐ ment over conventional WE. This electrochemical process with cellulose-derived nanocarbon has the added advantage that no CO2 could be detected at the anode presumably because cellulose-derived nanocarbons have surface functional groups [4] unlike carbon BP2000. It is noted that in doing these comparisons of energy efficiencies, the cost of producing the carbons is not included. However, BP2000 is available commercially in large quantities and hydro‐ thermal carbonization is environmentally friendly process employing water as solvent under subcritical temperature–pressure conditions [24].

In summary, the comparative energy analysis described here shows factors of four to five improvements in the energy efficiencies for producing H2 electrochemically using commercial carbon BP2000 and cellulose-derived nanocarbons. Pure CO2 is produced with the use of carbon BP2000, but it is well separated from H2. The use of post-HTP cellulose has the advant‐ age that no CO2 could be detected in the process. For more practical issues related to the electrochemical production of hydrogen from WE, the reader is referred to the recent review by Wang et al. [23].

#### **5. Concluding remarks**