**Plasma-Catalysis for Volatile Organic Compounds Decomposition: Complexity of the Reaction Pathways during Acetaldehyde Removal**

### **Arlette Vega-González \* , Xavier Duten and Sonia Sauce**

CNRS, UPR 3407, Laboratoire des Sciences des Procédés et des Matériaux, LSPM, Université Sorbonne Paris Nord, F-93430 Villetaneuse, France<EMAIL_ADDRESS>(X.D<EMAIL_ADDRESS>(S.S.)

**\*** Correspondence<EMAIL_ADDRESS>Tel.: +33-1-49-40-34-31

Received: 4 September 2020; Accepted: 30 September 2020; Published: 3 October 2020

**Abstract:** Acetaldehyde removal was carried out using non-thermal plasma (NTP) at 150 J·L −1 , and plasma-driven catalysis (PDC) using Ag/TiO2/SiO2, at three different input energies—70, 350 and 1150 J·L −1 . For the experimental configuration used, the PDC process showed better results in acetaldehyde (CH3CHO) degradation. At the exit of the reactor, for both processes and for all the used energies, the same intermediates in CH3CHO decomposition were identified, except for acetone which was only produced in the PDC process. In order to contribute to a better understanding of the synergistic effect between the plasma and the catalyst, acetaldehyde/catalyst surface interactions were studied by diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS). These measurements showed that different species such as acetate, formate, methoxy, ethoxy and formaldehyde are present on the surface, once it has been in contact with the plasma. A reaction pathway for CH3CHO degradation is proposed taking into account all the identified compounds in both the gas phase and the catalyst surface. It is very likely that in CH3CHO degradation the presence of methanol, one of the intermediates, combined with oxygen activation by silver atoms on the surface, are key elements in the performance of the PDC process.

**Keywords:** non-thermal plasma; catalysis; VOC; DRIFTS; acetaldehyde

#### **1. Introduction**

Diphasic processes combining a non-thermal plasma at atmospheric pressure and a catalytic bed have become of major interest for air-pollutant removal, and particularly volatile organic compounds (VOC) [1–3]. Indeed, compared to classical air cleaning techniques (thermal catalysis or photocatalysis), plasma-driven catalysis can lead to similar or better VOC degradation rates with less energy injected into the reactor and without catalyst deactivation issues [4,5]. Moreover, in the case of indoor air cleaning, where low VOC concentrations (in the ppm order) have to be removed, this diphasic process seems to be the most suitable [6]. Reviews dealing with the synergistic effects and mechanism of a non-thermal plasma catalysis system in volatile organic compound removal have recently been published [7–9]. Among the studies dealing with the different aspects of such a complex process, we can mention studies on plasma/pollutant interactions [10], surface charging [11,12], adsorption [13,14] or catalyst activation by non-thermal plasma (NTP) [15].

Given the fact that the induced heterogeneous reactions and the plasma are interdependent due to the plasma's continuous discharge during processing, it is essential to make further progress on identifying the surface species forming on the catalyst under the plasma effect. The in-situ Fourier transformed infrared spectroscopy (FTIR)/diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) approach for studying catalysts under working conditions is gaining importance in the

framework of the plasma-driven catalysis (PDC) process. To our knowledge, the first of these studies was carried out on isopropanol conversion on γ-Al2O3 [16]. The authors used a dielectric barrier discharge (DBD) plasma reactor, under static conditions, coupled to in-situ FTIR spectroscopy. They were able to show that unlike the thermal catalysis pathway, with the DBD process the first reaction intermediate is acetone. Plasma-assisted hydrocarbon selective catalytic reduction [17], catalytic steam-reforming of methane [18], reverse water–gas [19]/water–gas shift [20] and plasma-assisted CO2 hydrogenation [21] processes have also been studied using in-situ DRIFTS. These works pointed out the fact that NTP is able to activate some of the species present either in the gas phase or on the surface, thus improving the performance of the PDC process. Indeed, the role of oxygen in the PDC process has been investigated via isotopically-labeled molecular oxygen (18O2); the results showed that oxygen was fixed onto the surface of the catalyst by the action of the NTP discharge, and was able to survive in such a state for about 30 min [22]. What is more, this same study demonstrated that when using Ag nanoparticles supported on catalysts, they served as oxygen reservoirs. As for VOC removal with non-thermal plasma-assisted catalysis, isopropanol [23,24], toluene [24,25] and acetone removal [26] have been studied using an FTIR in-situ technique. This approach allowed the identification of reaction intermediates, and the following of the evolution of secondary compounds arising from the VOC oxidation.

In a previous work [27] we have showed that, by combining a DBD generated at atmospheric pressure and a silver-supported nanostructured catalyst, acetaldehyde can be removed up to 98% with the production of mainly CO, CO2 and O3. In addition, we have also studied acetaldehyde adsorption and ozonation over the same silver-based catalyst, and the obtained results support the fact that PDC is a rather complex process in which homogeneous and heterogeneous chemistry are closely interconnected [28]. In the present study we analyse the changes that occurred on an Ag/TiO2/SiO2 catalyst, used for acetaldehyde removal, after plasma exposure. We investigate which intermediate species were formed on the surface after acetaldehyde adsorption and discharge-assisted conversion by using DRIFT spectroscopy. Gaseous by-products resulting from this process were monitored and quantified, with the aim to contribute to a better understanding of the mechanism of acetaldehyde oxidation. Essentially based on the reactor outlet analysis, the surface composition and literature data, an acetaldehyde removal pathway is proposed considering both the homogeneous and heterogeneous chemistry of the process.

#### **2. Results**

The results obtained when studying acetaldehyde removal using non-thermal plasma, NTP-plasma driven catalysis or just a catalyst will be briefly presented. In addition, we have previously shown that VOC adsorption and heterogeneous NTP-induced chemistry have to be taken into account when associating NTP in air with a catalyst. These results, discussed in more detail in previous works [27–29], were obtained at ambient temperature and atmospheric pressure, but in different configurations of the plasma discharge (i.e., corona or DBD discharge, diameter of the catalyst support, mass of catalyst, energy injected).

#### *2.1. Acetaldehyde Decomposition Using the Plasma-Driven Catalysis Process*

The results obtained for acetaldehyde decomposition with the single stage plasma-driven catalysis, and the plasma-alone processes are presented in Table 1. It is worth noting that the degradation of acetaldehyde by the catalyst alone in the DBD reactor was also studied at room temperature and atmospheric pressure. In this case, neither any degradation of acetaldehyde nor by-product formation is observed.

As can be seen from the results presented in the table above, at atmospheric pressure and ambient temperature the catalyst is not active for acetaldehyde removal. With the plasma-alone process acetaldehyde can be removed up to 55% with an energy consumption of 150 J·L −1 (~250 mW power consumption). As soon as the silver nanostructured catalyst is introduced into the discharge zone, acetaldehyde removal increases, already allowing, at 70 J·L −1 (~120 mW), 33% to be obtained and up to 98% at 1150 J·L −1 (~2 mW). Acetaldehyde removal (%) 0 55 33 87 98

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**Table 1.** Acetaldehyde removal obtained using the plasma-driven catalysis and the non-thermal plasma (NTP) processes. As can be seen from the results presented in the table above, at atmospheric pressure and ambient temperature the catalyst is not active for acetaldehyde removal. With the plasma-alone

The main gaseous by-products at the exit of the DBD reactor are CO and CO2, but other organic by-products have also been identified: methanol (Me), acetic acid (AcA), acetone (Ace), methyl formate (MeF), methyl acetate (MeA), 1,2-ethanediol monoformate (EmF), 1,2-ethanediol diformate (EdF), nitromethane (Nm), methyl nitrate (MN) and 2-ethoxyethanol (Eet). Their distribution at the DBD reactor exit is presented in Figure 1, except for the ones present in trace amounts (Eet), or those that could not be quantified (MN). Among these organic products we recognize compounds belonging to different functional groups such as hydroxyl, carboxyl, carbonyl, carboalkoxy and nitro compounds, reflecting the complexity of the chemistry involved in such processes. by-products have also been identified: methanol (Me), acetic acid (AcA), acetone (Ace), methyl formate (MeF), methyl acetate (MeA), 1,2-ethanediol monoformate (EmF), 1,2-ethanediol diformate (EdF), nitromethane (Nm), methyl nitrate (MN) and 2-ethoxyethanol (Eet). Their distribution at the DBD reactor exit is presented in Figure 1, except for the ones present in trace amounts (Eet), or those that could not be quantified (MN). Among these organic products we recognize compounds belonging to different functional groups such as hydroxyl, carboxyl, carbonyl, carboalkoxy and nitro compounds, reflecting the complexity of the chemistry involved in such processes.

**Figure 1.** Comparison of the composition of the dielectric barrier discharge (DBD) reactor effluent obtained with the plasma-alone and the plasma-driven catalysis process, as a function of SIE. **Figure 1.** Comparison of the composition of the dielectric barrier discharge (DBD) reactor effluent obtained with the plasma-alone and the plasma-driven catalysis process, as a function of SIE.

From the obtained results we can say that with the plasma-alone process neither acetone nor nitromethane are produced at 150 J∙L-1; and compared with the PDC process, more acetic acid and methanol are formed. With the PDC process at 70 J∙L, aside from methyl formate and the emergence of acetone, all the other by-products are produced in lower amounts compared with NTP alone, which is certainly due to the lower acetaldehyde conversion. The by-products' formation is promoted as soon as the specific input energy (SIE) increases. Between 70 and 350 J∙L-1, there is a significant rise in each by-product concentration, 150% being the lowest increase observed. Between 350 and 1150 J∙L-1, an almost constant production of CO2, methanol, acetone, methyl formate and nitromethane is observed; whereas an increase between 70/80% in CO, MeA, AcA and EdF, and of 108% in EmF is still observed. Even if the amount of MN could not be quantified, the analyses of the DBD reactor effluent have shown that it is formed in higher quantities in the PDC process, and its production increases with SIE. Furthermore, introducing a catalyst in the discharge zone induces an increase in COx selectivity. In fact, NTP at 150 J∙L-1 has a COx selectivity of 35% while in the PDC process this From the obtained results we can say that with the plasma-alone process neither acetone nor nitromethane are produced at 150 J·L −1 ; and compared with the PDC process, more acetic acid and methanol are formed. With the PDC process at 70 J·L, aside from methyl formate and the emergence of acetone, all the other by-products are produced in lower amounts compared with NTP alone, which is certainly due to the lower acetaldehyde conversion. The by-products' formation is promoted as soon as the specific input energy (SIE) increases. Between 70 and 350 J·L −1 , there is a significant rise in each by-product concentration, 150% being the lowest increase observed. Between 350 and 1150 J·L −1 , an almost constant production of CO2, methanol, acetone, methyl formate and nitromethane is observed; whereas an increase between 70/80% in CO, MeA, AcA and EdF, and of 108% in EmF is still observed. Even if the amount of MN could not be quantified, the analyses of the DBD reactor effluent have shown that it is formed in higher quantities in the PDC process, and its production increases with SIE. Furthermore, introducing a catalyst in the discharge zone induces an increase in COx selectivity. In fact, NTP at 150 J·L −1 has a COx selectivity of 35% while in the PDC process this selectivity is equal to 36, 50 and 60% for 70, 350 and 1150 J·L −1 , respectively.

selectivity is equal to 36, 50 and 60% for 70, 350 and 1150 J∙L-1, respectively.

#### *2.2. Acetaldehyde Adsorption on Ag*/*TiO2*/*SiO2 2.2. Acetaldehyde Adsorption on Ag/TiO2/SiO2*

In order to characterize the catalyst surface before turning the plasma on, in-situ DRIFTS was used to monitor the Ag/TiO2/SiO2 surface during acetaldehyde adsorption. The adsorption capacity of the catalyst, at the experimental conditions used, was evaluated to be 698 ± 3 µmol/gcatalyst. In order to characterize the catalyst surface before turning the plasma on, in-situ DRIFTS was used to monitor the Ag/TiO2/SiO2 surface during acetaldehyde adsorption. The adsorption capacity of the catalyst, at the experimental conditions used, was evaluated to be 698 ± 3 mol/gcatalyst.

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Figure 2 shows the infrared spectra of the silver-based catalyst surface after 1 and 30 min of exposition to 1000 ppmC of CH3CHO at 298 K and atmospheric pressure. Acetaldehyde shows peaks in the 3000–2600 cm−1 region and 1800–1500 cm−1 region, as shown in the figure. After 1 min acetaldehyde adsorption, bands at 2936, 2922, 2866, 2770, 2748, 1759, 1713, 1555 and 1242 cm−1 are observed. Once the saturation is attained, after 30 min, the previously mentioned absorption bands increase, and a new band appears at 2974 cm−1 . In addition, broadband absorption features, associated with the peaks at 1713, 1555 and 1242 cm−1 are observed. The first broadband presents peaks at 1720 and 1713 cm−1 ; a small shoulder at 1697 cm−1 , and a large shoulder in the 1690–1650 cm−1 range. The second broadband still showing a maximum at 1555 cm−1 , and many smaller peaks in the 1590–1500 cm−1 range are visible after saturation of the surface with acetaldehyde. The last broadband also presents smaller peaks in the 1280–1210 cm−1 range. Figure 2 shows the infrared spectra of the silver-based catalyst surface after 1 and 30 min of exposition to 1000 ppmC of CH3CHO at 298 K and atmospheric pressure. Acetaldehyde shows peaks in the 3000–2600 cm−1 region and 1800–1500 cm−1 region, as shown in the figure. After 1 min acetaldehyde adsorption, bands at 2936, 2922, 2866, 2770, 2748, 1759, 1713, 1555 and 1242 cm−1 are observed. Once the saturation is attained, after 30 min, the previously mentioned absorption bands increase, and a new band appears at 2974 cm−1. In addition, broadband absorption features, associated with the peaks at 1713, 1555 and 1242 cm−1 are observed. The first broadband presents peaks at 1720 and 1713 cm−1; a small shoulder at 1697 cm−1, and a large shoulder in the 1690–1650 cm−1 range. The second broadband still showing a maximum at 1555 cm−1, and many smaller peaks in the 1590–1500 cm−1 range are visible after saturation of the surface with acetaldehyde. The last broadband also presents smaller peaks in the 1280–1210 cm−1 range.

**Figure 2. D**iffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra recorded after **Figure 2.** Diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra recorded after 1 and 30 min of acetaldehyde adsorption on Ag/TiO2 /SiO2 at 298 K and atmospheric pressure.

1 and 30 min of acetaldehyde adsorption on Ag/TiO2/SiO2 at 298 K and atmospheric pressure. **Table 1.** Adsorbed acetaldehyde: observed bands and their assignments. Peak assignments are presented in Table 2. In both spectra, bands in the 3000–2800 cm−1 region, the CH stretching region, can be assigned to symmetric and asymmetric ν(CH3) and ν(CH2) modes [30] and bands in the 1700–1680 cm−1 region can be assigned to ν(C=O) mode [31].

**Vibrational mode gas phase Reference CH3CHO adsorbed on Table 2.** Adsorbed acetaldehyde: observed bands and their assignments.

**Vibrational assignments (cm−1)**

the CH stretching region, can be assigned to symmetric and asymmetric (CH3) and (CH2) modes [30] and bands in the 1700–1680 cm−1 region can be assigned to (C=O) mode [31]. From the literature, most of the bands in the CH stretching region could be assigned to adsorbed From the literature, most of the bands in the CH stretching region could be assigned to adsorbed acetaldehyde [32–34]. However significant overlap of much of this region exists between acetaldehyde

region.

and the products of its heterogeneous reactions on oxide surfaces. Indeed, the bands at 2922, 2830 and 2748 cm−1 could be assigned to crotonaldehyde [33] whereas the band at 2936 cm−1 may also be assigned to other condensation products derived from surface reactions [31], or to acetate species [32]. The formation of crotonaldehyde is supported by the presence of the peak at 1720 cm−1 and the broad shoulder at 1690–1650 cm−1 [31,33,37–39] and the formation of acetate species is in agreement with the emergence of the broadband centered at 1555 cm−1 [36]. The dominant spectral feature in the ν(C=O) region, at 1713 cm−1 , which is assigned to adsorbed acetaldehyde, shifts to lower wavenumbers with respect to the same band of CH3CHO in the gas phase. This indicates that acetaldehyde is bound to the surface TiO2 through its carbonyl group by H-bridge bonding [37,38,40]. In addition, the growth of other infrared spectral features in this region points out the probable formation of other products such as acetone (1697 cm−1 [41]) or formaldehyde. Indeed, the band at 1242 cm−1 can be assigned to molecularly adsorbed formaldehyde [42,43], which has also been reported to present bands at 2913, 2863, 2759, 1648 and 1413 cm−1 [42,44], supporting the presence of formaldehyde on the silver-based catalyst surface after acetaldehyde adsorption. acetaldehyde and the products of its heterogeneous reactions on oxide surfaces. Indeed, the bands at 2922, 2830 and 2748 cm−1 could be assigned to crotonaldehyde [33] whereas the band at 2936 cm−1 may also be assigned to other condensation products derived from surface reactions [31], or to acetate species [32]. The formation of crotonaldehyde is supported by the presence of the peak at 1720 cm−1 and the broad shoulder at 1690–1650 cm−1 [31,33,37–39] and the formation of acetate species is in agreement with the emergence of the broadband centered at 1555 cm−1 [36]. The dominant spectral feature in the (C=O) region, at 1713 cm−1, which is assigned to adsorbed acetaldehyde, shifts to lower wavenumbers with respect to the same band of CH3CHO in the gas phase. This indicates that acetaldehyde is bound to the surface TiO2 through its carbonyl group by H-bridge bonding [37,38,40]. In addition, the growth of other infrared spectral features in this region points out the probable formation of other products such as acetone (1697 cm−1 [41]) or formaldehyde. Indeed, the band at 1242 cm−1 can be assigned to molecularly adsorbed formaldehyde [42,43], which has also been reported to present bands at 2913, 2863, 2759, 1648 and 1413 cm−1 [42,44], supporting the presence of formaldehyde on the silver-based catalyst surface after acetaldehyde adsorption.

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Although at the end of the adsorption step the Ag/TiO2/SiO2 catalyst surface is mainly covered with adsorbed acetaldehyde, the other species which are also present have to be considered in order to better understand the acetaldehyde degradation mechanism in the following step of the process. Although at the end of the adsorption step the Ag/TiO2/SiO2 catalyst surface is mainly covered with adsorbed acetaldehyde, the other species which are also present have to be considered in order to better understand the acetaldehyde degradation mechanism in the following step of the process.

#### *2.3. Surface Species Formed during the PDC Process 2.3. Surface Species Formed During the PDC Process*

Once saturated with acetaldehyde, the Ag/TiO2/SiO2 catalyst is used in order to decompose the VOC with the PDC process. Three different energies were used—70, 350 and 1150 J·L −1 . Figure 3 shows the surface spectra of the catalyst before and after being exposed to the DBD discharge for 30 min. It is clear that the surface species present on the surface after the adsorption process take part in different surface reactions while the plasma is on, leading to the modification of the surface composition as can be seen from the FTIR band features. Once saturated with acetaldehyde, the Ag/TiO2/SiO2 catalyst is used in order to decompose the VOC with the PDC process. Three different energies were used—70, 350 and 1150 J∙L-1. Figure 3 shows the surface spectra of the catalyst before and after being exposed to the DBD discharge for 30 min. It is clear that the surface species present on the surface after the adsorption process take part in different surface reactions while the plasma is on, leading to the modification of the surface composition as can be seen from the FTIR band features.

**Figure 3.** Ag/TiO2/SiO2 catalyst surface DRIFTS spectra after saturation with CH3CHO; and NTP effect on surface species at different plasma discharge energies: (**a**) C-H stretching region; (**b**) bending **Figure 3.** Ag/TiO2 /SiO2 catalyst surface DRIFTS spectra after saturation with CH3CHO; and NTP effect on surface species at different plasma discharge energies: (**a**) C-H stretching region; (**b**) bending region.

From Figure 3a it is clear that, after the PDC process, some of the bands disappear (2974 and 2770 cm−1) or almost disappear (2748 cm−1). The relative intensity of the bands in the 2970–2900 cm−1 region has clearly changed, with visible peaks at 2946, 2955 and 2964 cm−1; the band in the 2900–2800 cm−1 region decreases significantly; and a new broad band with peaks at 2600 and 2581 cm−1 appears. From Figure 3a it is clear that, after the PDC process, some of the bands disappear (2974 and 2770 cm−1 ) or almost disappear (2748 cm−1 ). The relative intensity of the bands in the 2970–2900 cm−1 region has clearly changed, with visible peaks at 2946, 2955 and 2964 cm−1 ; the band in the 2900–2800 cm−1 region decreases significantly; and a new broad band with peaks at 2600 and 2581 cm−1 appears. Figure 3b shows an evident broadening of the bands centered at 1713, 1555 and

Figure 3b shows an evident broadening of the bands centered at 1713, 1555 and 1242 cm−1 in the

1242 cm−1 in the acetaldehyde-saturated surface. As the plasma input energy increases, the maximum of the ν(C=O) band is redshifted and its intensity decreases for an SIE higher than 70 J·L −1 . At the same time, a band with increasing intensity develops in the 1300–1000 cm−1 region.

The loss of the bands at 2974 and 2770 cm−1 (assigned to adsorbed acetaldehyde), along with an intensity decrease of the peaks at 2866 and 1713 cm−1 indicates a gradual depletion of acetaldehyde surface species once the catalyst is in contact with the plasma. Likewise, the loss in intensity of the bands at 2922, 2830, 2748 and 1720 cm−1 indicates that less crotonaldehyde is present on the surface. Under the plasma effect, the 2950–2900 cm−1 band broadens, and presents different small peaks and shoulders with maximum intensities at 2946 and 2936 cm−1 . The former peak may be assigned to surface methoxy [45]; indeed, along with the peak at 2946 cm−1 , the presence of a small shoulder at 2924 cm−1 and a peak at 2855 cm−1 supports the possibility of the formation of surface methoxy [45–47]. Concerning the peak at 2936 cm−1 , its attribution is less straightforward as it can be related to different surface species such as formate, ethoxy or acetate species [32,44,45]. On the one hand, the infrared features at 2964, 2955, 2936, 2924, 2866 and 2735 cm−1 bring out the formation of surface formate [45,48]. On the other hand, the peaks at 2964, 2936, 2866 and 2855 cm−1 have also been assigned to ethoxy species formed on a TiO2 surface [37,44,49]. Furthermore, the peaks at 2964 and 2936 cm−1 have also been assigned to methyl acetate [35], whereas the one at 2936 cm−1 is also assigned to acetic acid [50]. The broad band between 2650 and 2550 cm−1 that appears after the plasma catalysis process supports the formation of formate species as it has been attributed to formic acid [51]. As the energy increases up to 350 J/L, the 2900–2800 cm−1 band loses intensity, the loss being more pronounced for the peak at 2936 cm−1 ; whereas the peaks at 2964 and 2955 cm−1 , as well as a new peak at 2914 cm−1 become more visible. This latter peak may be attributed either to acetaldehyde [36], or to an intermediate formed from formaldehyde oxidation or disproportionation to give formate or methoxide, respectively [52,53].

These attributions can be backed by the infrared features in the 2000–950 cm−1 region. Significant overlapping of the peaks in this wavelength range complicates precise identification of surface species but, based on litterature data, the bands generated on the plasma-exposed surface can be related to distinct vibrational modes of acetate, formate, methoxy, ethoxy, carbonate and formaldehyde species adsorbed on the surface of the Ag/TiO2/SiO2 catalyst. Thus, we may assign some of the bands as presented in Table 3.

**Table 3.** Infrared (IR) bands assignment of the major surface species formed during the PDC process, in the 2000–950 cm−1 region.

The change in the vibrational spectrum of the catalyst surface following its exposure to NTP suggests a modification of its composition, which is directly related to the acetaldehyde decomposition mechanism.

#### **3. Discussion**

In this section we will propose a reaction pathway that takes into account acetaldehyde adsorption and its degradation on the surface with the PDC process. On the basis of the DBD reactor effluent analysis, and the infrared spectroscopic measurements carried out on the catalyst surface, it is evident that acetaldehyde undergoes a rather complex chemistry during the process. For this reason, a detailed description of the acetaldehyde decomposition mechanism is out-of-reach at this point. Alternatively, we propose a series of simplified adsorption/degradation routes in order to account for the key aspects of surface activity that lead to the formation of the different identified surface species, and a subsequent increase in acetaldehyde degradation, which results in an increase in the amount of gaseous by-products.

#### *3.1. Acetaldehyde Adsorption Mechanism*

In different studies concerning the photocatalytic degradation pathway of acetaldehyde on a TiO2 surface, it has been noticed that, upon adsorption of acetaldehyde, different species are already formed without illuminating the catalyst. Two compounds, 3-hydroxybutanal and crotonaldehyde, are formed through an aldol condensation, while bidentate acetate species have been detected to be formed by an oxidation process occurring on the TiO2 surface [40].

As suggested from the spectra analysis of the saturated surface, when acetaldehyde is brought into contact with the nanostructured TiO2, it will certainly bind to the surface through the oxygen atom of its carbonyl group. Then, two acetaldehyde molecules adsorbed on neighboring sites can react through a β-aldolization reaction to give crotonaldehyde, CH3(CH)2CHO [28,31,33,64–66]. It has been shown that acetaldehyde undergoes aldol condensation forming 3-hydroxybutanal (CH3CH(OH)CH2CHO) which, upon dehydration forms crotonaldehyde [31,34,40]:

$$\text{CH}\_3\text{CHO}\_{\text{ads}} + \text{CH}\_3\text{CHO}\_{\text{ads}} \rightarrow \text{CH}\_3-\text{CH}=\text{CH}-\text{CHO}\_{\text{ads}} + \text{H}\_2\text{O} \tag{1}$$

Formation of acetate species can result from the direct oxidation on the surface of acetaldehyde [30, 32,46,66], and to a lesser extent of crotonaldehyde [40], through a surface oxygen (Os):

$$\text{CH}\_3\text{CHO}\_{\text{ads}} + \text{O}\_{\text{s}} \rightarrow \text{CH}\_3\text{COO}\_{\text{ads}}^- + \text{H}\_{\text{ads}}^+\tag{2}$$

Besides, a Cannizzaro disproportionation of two acetaldehyde molecules involving a vacancy (VO) and surface oxygen would result in the formation of acetate and ethoxy groups. This pathway has been proposed for acetaldehyde adsorption on rutile TiO2 [33]:

$$2\text{CH}\_3\text{CHO}\_{\text{ads}} + \text{V}\_\text{O}/\text{O}\_\text{s} \to \text{CH}\_3\text{COO}\_{\text{ads}}^- + \text{CH}\_3\text{CH}\_2\text{O}\_{\text{ads}}^-\tag{3}$$

Despite the fact that it is difficult to ascertain the presence of ethoxy species from the spectra in Figure 2, their presence at much smaller levels than acetates cannot be completely ruled out.

Concerning acetone formation on oxide surfaces from aldehydes, two pathways are mainly known. The first is via a two-step reaction: oxidation to carboxylates followed by the coupling of two carboxylates to form one molecule of ketone. The second pathway is through the reaction of an adsorbed acyl group, RC=O, with an alkyl group [32]. Examples of acetone formation from acetaldehyde via these two pathways have been reported, but they concern studies carried out at temperatures well above the ambient temperature [32,67–69]. Another possibility is acetone formation through 3-hydroxybutanal. Indeed, at ambient temperature this compound is very reactive and, either it undergoes fast dehydration on the surface to form crotonaldehyde; or it can be converted via intramolecular H transfer to 4-hydroxybutan-2-al (CH3C(O)CH2CH2OH). The latter can be further transferred into acetone (CH3C(O)CH3) and formaldehyde (HC(O)H) via reverse reaction to aldol condensation [34]:

$$\text{C}2\text{CH}\_3\text{CHO}\_{\text{ads}} \rightarrow \text{CH}\_3\text{CH}(\text{OH})\text{CH}\_2\text{CHO}\_{\text{ads}} \leftrightarrow \text{CH}\_3\text{C}(\text{O})\text{CH}\_2\text{CH}\_2\text{OH} \rightarrow \text{HC}(\text{O})\text{H} + \text{CH}\_3\text{C}(\text{O})\text{CH}\_3 \tag{4}$$

This mechanism is a very plausible one, as it explains acetone and formaldehyde formation after acetaldehyde adsorption at ambient temperature.

#### *3.2. Surface Species Formed during the PDC Process: Formation Mechanism*

Before turning the plasma on, the catalyst surface is mainly composed of adsorbed acetaldehyde. Part of this acetaldehyde yields crotonaldehyde (Reaction (1)); acetate (Reactions (2) and (3)), acetone and formaldehyde (Reaction (4)). DRIFTS measurements carried out on the catalyst exposed to the plasma reveal that acetaldehyde degradation proceeds with the formation of different surface compounds such as acetate, methoxy, ethoxy, formate and carbonate species, as well as formaldehyde and formic and acetic acid. It should be noted that, despite being referred to as formic acid and acetic acid, it is most likely formate and acetate species on the surface [40]. Indeed, previous studies have shown that photocatalytic oxidation of acetaldehyde could yield acetic and formic acid that remain adsorbed on the surface in a dissociated form, i.e., as acetate and formate, respectively (degradation routes Reactions (5) and (6)) [55,69].

$$\text{CH}\_3\text{CHO}\_\text{a} \rightarrow \text{CH}\_3\text{COO}\_\text{a}^- + \text{H}\_\text{a}^+ \tag{5}$$

$$\text{CH}\_3\text{CHO}\_3 \rightarrow \text{HCHO}\_3 \rightarrow \text{HCOO}\_3^- + \text{H}\_4^+ \tag{6}$$

The formation of acetate species on the catalyst surface has already been presented in the previous section. Nevertheless, once the plasma is turned on, two more routes may be considered. One route involves the acetyl radical resulting from adsorbed acetaldehyde decomposition by the plasma; which in turn can react with gas-phase oxygen to form acetate species (Reaction (7)) [56]. The other one is the reaction of gaseous acetaldehyde with an activated surface oxygen (Reaction (8)) [70]—indeed, it has been shown that the presence of silver atoms enhances the adsorption of oxygen on the catalyst and promotes its activation [22,71]. *above above above*

−−−−−−−−−−−−−→ *below below below*

$$\mathrm{CH\_3CHO\_a} \rightarrow \mathrm{CH\_3CO\_a} + \mathrm{H\_a} \xrightarrow{+\mathrm{O\_2}} \mathrm{CH\_3COO\_a^-} \tag{7}$$

$$\rm{CH\_3OH\_8^- + O\_a \to CH\_3COO\_a^- + H\_a^+} \tag{8}$$

Regarding formate species, its formation through oxidation of different compounds such as acetone (Reaction (9)) [55], acetate species (Reactions (10) and (11)) [46,55], formaldehyde (Reaction (12)) [55,72–74] and crotonaldehyde (Reaction (13)) [64] should be considered.

$$2\text{ CH}\_3\text{C(O)CH}\_3 \xrightarrow{\text{O}\_2} 2\text{HCOO}\_3^- + \text{H}\_a^+ + \text{CO}\_2 \tag{9}$$

$$\rm{CH\_3COO\_4^- + H^+ \to HCOO\_4^- + H\_a^+} \tag{10}$$

$$\rm CH\_3COO\_a^- \rightarrow CH\_3O\_a \rightarrow CH\_2O\_a \rightarrow HCOO\_a^- \tag{11}$$

$$\rm{HCHO} + \rm{O\_{a}} \rightarrow \rm{HCOO\_{a}^{-}} \tag{12}$$

$$\rm CH\_3(CH)\_2CHO\_a \to \rm HCOO\_a^- + H\_a^+ \tag{13}$$

Methoxy species may be formed following different routes, one of which involves methanol, a by-product of acetaldehyde decomposition by NTP. The proposed routes are direct oxidation of surface acetate species (Reaction (14)) [46], and dissociative adsorption of methanol (Reaction (15)) [75,76]. Indeed, on the basis of reaction mechanisms derived from model studies carried out under ultrahigh vacuum conditions on noble metal surfaces, methanol is activated by surface oxygen so that adsorbed methoxy species can form [77].

$$\rm{CH\_3COO\_4^-} \rightarrow \rm{CH\_3O\_4 + CO\_2} \tag{14}$$

$$\rm{CH\_3OH\_g} + \rm{S} \rightarrow \rm{CH\_3O\_a} + \rm{H\_a^+} \tag{15}$$

where S is a site on the oxide surface.

Concerning ethoxy species, one possible pathway has already been proposed through the Reaction (3) route. Moreover, it has been shown that TiO2-supported catalysts may significantly promote the reaction of acetaldehyde with adsorbed hydrogen formed in previous steps to form ethoxy species [54,66] (Reaction (16)).

$$\text{CH}\_3\text{CHO}\_4 + \text{H}\_4^+ \rightarrow \text{CH}\_3\text{CH}\_2\text{O}\_4^- \tag{16}$$

As for carbonate species, they may be produced from acetates (Reactions (17) and (18)) [56,67,70]. Furthermore, an investigation carried out using supported TiO2 catalysts in the presence of H2O2 [58] has shown that formate species interaction with the surface can lead to carbonate and acetone formation (Reaction (19)). This latter possibility should be considered in our case, as NTP is able to generate H2O2.

$$\rm{CH\_3COO\_3^-} + \rm{OH\_s} \rightarrow \rm{CO\_3^{2-}} + \rm{CH\_4} \tag{17}$$

$$\rm{2CH\_3COO\_3^-} \rightarrow \rm{CH\_3C(O)CH\_3\, a + CO\_3^{2-}} \tag{18}$$

$$\rm{HCOO\_{3}^{-}} \xrightarrow{H\_{2}O\_{2}} \rm{CO\_{3}^{2-}} \tag{19}$$

Regarding formaldehyde formation, in addition to Reaction (4) route, other routes may also be allowed for. These include formation of formaldehyde from acetaldehyde on TiO2 surfaces, either directly or through an acetate intermediary (Reaction (20)) [55], from acetaldehyde oxidation by surface oxygen (Reaction (21)) [54], from surface ethoxy and methoxy species oxidation (Reactions (22) and (23)) [44,46,57] and as an intermediate in methanol oxidation (Reaction (24)) [74].

$$\text{CH}\_3\text{CHO}\_\text{a} \xrightarrow{\text{(CH}\_3\text{COO}\_3^-)} \text{HCHO}\_\text{a} + \text{HCOOH}\_\text{a} \tag{20}$$

$$\text{CH}\_3\text{CHO}\_4 + 2\text{O}\_8 \rightarrow \text{HCHO}\_4 + \text{HCOOH}\_4 \tag{21}$$

$$\rm CH\_3CH\_2O\_a^- \rightarrow \rm HCHO\_a \tag{22}$$

$$\rm{CH\_3O\_a} \rightarrow \rm{HCHO\_a} + \frac{1}{2} \rm{H\_2}\_{\rm g} \tag{23}$$

$$\text{CH}\_3\text{OH} \rightarrow \text{HCHO}\_4 + \text{H}\_2\text{ads} \tag{24}$$

It can be seen that routes through Reactions (20) and (21) are not only a possible source of formaldehyde, but also of formic acid.

Some authors working with photocatalytic degradation of acetaldehyde have also observed that the initially formed species, 3-hydroxybutanal and crotonaldehyde, are converted to other intermediates upon illumination of the catalyst. Among the main intermediates, they have identified formic acid, acetic acid and formaldehyde, and this is in accordance with our results [40,64].

It is important to bear in mind that some of the species identified on the catalyst surface after the PDC process can also be formed directly in the gas phase. Indeed, gas-phase acetaldehyde decomposition by NTP can lead to the formation of methoxy radical, as well as acetone, acetic acid and formaldehyde [78], which can subsequently adsorb on the surface.

#### *3.3. Proposed Simplified Mechanism for Acetaldehyde Decomposition Using the Plasma*/*Catalysis Process*

From the identified gaseous by-products, surface IRbands' assignment and proposed adsorption/ degradation routes—a schematic diagram of the possible pathways for acetaldehyde degradation over Ag/TiO2/SiO2 in the PDC process is presented in Figure 4. It is worth noting that we have not detected formaldehyde at the exit of the DBD reactor whereas it has been detected on the catalyst surface. There are two reasons for this: formaldehyde is a short-lived species [77] and it is strongly adsorbed onto silver catalyst [79].

*Catalysts* **2020**, *10*, x FOR PEER REVIEW 10 of 16

selectivity is dependent on the O3:HCHO ratio and the relative humidity (RH). In this work, for a O3:HCHO ratio of 5 (best ratio) and 20% RH, 90% of HCHO removal and a CO/CO2 ratio of 1.5 were

**Figure 4.** Schematic diagram of the proposed reaction pathway of the PDC degradation of acetaldehyde on Ag/TiO2/SiO2. The line patterns (solid line, dotted line, dashed line) represent, respectively, either direct routes or routes involving reactions with simple radicals/atoms/sites on the surface (H/OH/O/VO/S), routes involving two surface species and routes leading to the formation of by-products observed only at high energy. Blue color compounds: not detected/identified. **Figure 4.** Schematic diagram of the proposed reaction pathway of the PDC degradation of acetaldehyde on Ag/TiO2 /SiO2 . The line patterns (solid line, dotted line, dashed line) represent, respectively, either direct routes or routes involving reactions with simple radicals/atoms/sites on the surface (H/OH/O/VO/S), routes involving two surface species and routes leading to the formation of by-products observed only at high energy. Blue color compounds: not detected/identified.

As presented in Section 2.1, except from Ace and the fraction corresponding to the unknown compounds, the same by-products are obtained with the NTP and PDC processes. This fact allows the drawing of an initial conclusion concerning Ace and MeF formation. Indeed, at the experimental conditions used, Ace seems to be generated only on the surface, and most certainly through the ketonization of acetic acid/acetate species. In addition, with NTP at 150 J∙L-1 there is no formation of MeF, whereas with the PDC process it is already formed at 70 J∙L-1, and in equivalent amounts to the other acetate/formate compounds. Thus, it is clear that the catalyst surface promotes MeF formation. As mentioned earlier, Me can be activated by surface oxygen. In a study on methanol oxidation over silver catalysts, the authors reported that the catalytic oxidative reaction of methanol to methyl formate is related to a synergic process concerning oxygen species on the silver surface. The It is very likely that EdF and EmF are formed through the esterification of ethylene glycol (EG) and formic acid, even if there is no evidence of the presence of EG either on the effluent or on the surface. However, it is well-known that silver catalysts are commonly used in the manufacturing process for the direct oxidation of ethylene to ethylene oxide (EO) [80], and EO is a precursor of EG [81,82]. Besides, ethylene and EO can be formed from acetaldehyde decomposition on the surface [83–86]. Thus, EdF and EmF formation through an EG pathway is very plausible in the PDC process. In addition, low-energy, electron-induced processes on Me have been proven to induce the formation of methoxy (CH3O) and hydroxymethyl (CH2OH) radicals via electron impact excitation, which, followed by radical–radical coupling, can lead to EG and formic acid formation [87], therefore explaining EmF and EdF production with NTP alone.

mechanism proposed in this Os-Me system starts with methanol dehydrogenation to form adsorbed formaldehyde and methoxy; thereafter, formaldehyde reacts with adsorbed OH or O to form adsorbed formate, that reacts with methoxy and forms methyl formate [79,90]. Thus, Me formed as a by-product of acetaldehyde decomposition can be the precursor of MeF, with formaldehyde and CH3O as intermediates. In addition, as presented in Figure 1, NTP at 150 J∙L-1 produces more Me and AcA than the PDC process. Thus, we can suggest that the lower amount of Me is related to its decomposition into formaldehyde and CH3O, that will in turn produce MeF; and the lower amount of AcA is probably related to the preferential decomposition of acetate through other routes than the one at its origin. From spectra in Figure 3 we have seen that bands corresponding to methoxy, acetate and formaldehyde are present at 70 J∙L-1, which stresses the feasibility of the proposed mechanisms for Ace and MeF formation. Increasing SIE from 70 to 350 J∙L-1 increases acetaldehyde degradation from 33% to 87%, which leads to an important rise in the concentration of all the identified by-products. CO concentration increases 8-fold, whereas the other compounds experience a 2.5- to 4-fold increase. This result confirms the fact that acetaldehyde degradation follows a pathway favoring CO formation. In this diagram we did not include the pathways leading to CO2 and CO formation as there are many possibilities. The different routes to COx can involve many of the different adsorbed species or gaseous organic compounds. Nevertheless, it is worth noting that increasing the energy in the PDC process also increases the selectivity towards CO. In order to explain this result, we have proposed in a previous work a decarbonylation process of acetaldehyde on the silver-based catalyst [27]. However, this higher CO selectivity can also be the result of the by-products' decomposition pathways. According to a study on the mechanism of the heterogeneously-catalyzed oxidation of organic molecules on metal oxides, it has been stated that formate ions rather easily decompose to CO over metal oxides [88]. Furthermore, an experimental study of ozone catalytic oxidation of gaseous formaldehyde using a TiO2-supported catalyst at room temperature has shown that COx selectivity is dependent on the O3:HCHO ratio and the relative humidity (RH). In this work, for a O3:HCHO ratio of 5 (best ratio) and 20% RH, 90% of HCHO removal and a CO/CO2 ratio of 1.5 were achieved, making CO the major product [89]. Similar results regarding higher conversion towards CO than CO2 where obtained when using TiO2-supported catalysts for ethanol oxidation; and it is well-known that acetaldehyde is one of the major intermediate species in ethanol decomposition [70].

Concerning the catalyst surface composition, in Figure 3b we can see the emergence of a peak at 2914 As presented in Section 2.1, except from Ace and the fraction corresponding to the unknown compounds, the same by-products are obtained with the NTP and PDC processes. This fact allows the drawing of an initial conclusion concerning Ace and MeF formation. Indeed, at the experimental conditions used, Ace seems to be generated only on the surface, and most certainly through the

ketonization of acetic acid/acetate species. In addition, with NTP at 150 J·L −1 there is no formation of MeF, whereas with the PDC process it is already formed at 70 J·L −1 , and in equivalent amounts to the other acetate/formate compounds. Thus, it is clear that the catalyst surface promotes MeF formation. As mentioned earlier, Me can be activated by surface oxygen. In a study on methanol oxidation over silver catalysts, the authors reported that the catalytic oxidative reaction of methanol to methyl formate is related to a synergic process concerning oxygen species on the silver surface. The mechanism proposed in this Os-Me system starts with methanol dehydrogenation to form adsorbed formaldehyde and methoxy; thereafter, formaldehyde reacts with adsorbed OH or O to form adsorbed formate, that reacts with methoxy and forms methyl formate [79,90]. Thus, Me formed as a by-product of acetaldehyde decomposition can be the precursor of MeF, with formaldehyde and CH3O as intermediates. In addition, as presented in Figure 1, NTP at 150 J·L −1 produces more Me and AcA than the PDC process. Thus, we can suggest that the lower amount of Me is related to its decomposition into formaldehyde and CH3O, that will in turn produce MeF; and the lower amount of AcA is probably related to the preferential decomposition of acetate through other routes than the one at its origin. From spectra in Figure 3 we have seen that bands corresponding to methoxy, acetate and formaldehyde are present at 70 J·L −1 , which stresses the feasibility of the proposed mechanisms for Ace and MeF formation.

Increasing SIE from 70 to 350 J·L −1 increases acetaldehyde degradation from 33% to 87%, which leads to an important rise in the concentration of all the identified by-products. CO concentration increases 8-fold, whereas the other compounds experience a 2.5- to 4-fold increase. This result confirms the fact that acetaldehyde degradation follows a pathway favoring CO formation. Concerning the catalyst surface composition, in Figure 3b we can see the emergence of a peak at 2914 cm−1 , which has been assigned in Section 2.3 either to adsorbed acetaldehyde or to an intermediate formed from formaldehyde oxidation or disproportionation. Considering all the above, this peak could rather be assigned to the intermediate formed from formaldehyde oxidation to formate species, that will subsequently lead to the formation of the different formate compounds. This is supported by the loss of the band centered at 1870 cm−1 , attributed to formate species, for which desorption from the surface may be favored at this energy. Similarly, the growth of a rather broad band in the 1300–950 cm−1 region observed in Figure 3a should rather be assigned to methoxy species that can also promote formate species formation.

When increasing SIE to 1150 J·L −1 , two different behaviors among the by-products were observed: (i) those whose concentration remains rather constant (CO2, MeO, MeF and Ace), (ii) those whose concentration almost doubled (CO, MeA, EdF, EmF and AcA). Therefore, it seems that once formate/ formaldehyde are formed, the path that would be favored is the one towards formic acid and then EmF and EdF. Additionally, acetates will be transformed into AcA and MeA rather than into Ace.

#### **4. Materials and Methods**

#### *4.1. Catalyst Preparation*

Silver nanoparticles deposition was performed following the method presented in a previous study [91]. In this same study, atomic forcemicroscopy (AFM, Veeco, Munich, Germany), scanning electron microscopy (SEM, Supra 40 VP, Zeiss, France) and transmission electron microscopy (TEM, 200 kV JEM 2011, JEOL, France) measurements showing a homogeneous dispersion of silver nanoclusters on the TiO2 monolayer can be found. The silver nanoparticles were deposited on 150-µm-diameter SiO2 pellets (Sigma-Aldrich silica gel, St. Louis, MO, USA). The pellets were first covered by a size-selected titanium oxo-alkoxy (TOA, Sigma-Aldrich, St. Louis, MO, USA, 99.999% purity)) nanoparticles monolayer synthesized in a rapid micromixing sol-gel reactor. Then, the silver nanoclusters (10 nm mean diameter) were grown on the TOA monolayer by photocatalytic reduction of an Ag+ aqueous solution (AgNO3, Prolabo, Prolabo, Paris, France, 99.8% purity), under ultraviolet (UV) irradiation (Philips, Haarlem, The Netherlands, operating at 362 ± 10 nm, 8W). Finally, the pellets were washed in distilled water and

dried at 80 ◦C for 4 h. A 303 m2g −1 BET (Brunauer–Emmet–Teller) (Beckman Coulter, Brea, CA, USA, SATM 3100) specific surface area was measured.

#### *4.2. Plasma*/*Catalysis Process*

A DBD reactor previously described [27], was used for acetaldehyde removal at room temperature and atmospheric pressure. The DBD reactor is composed of a quartz cylinder externally covered by a grounded stainless steel. A sinusoidal high voltage is applied to a stainless steel cylindrical center electrode through a 5-mm gap. This high voltage is obtained using a voltage amplifier (TREK, 20/20 C, France), coupled to a frequency generator. Different values of the specific input energy (SIE in J·L −1 ), defined as the ratio of the discharge power (W) over the flow rate (L·s −1 ), were obtained by changing the frequency of the signal. When using the PDC process, the Ag/TiO2/SiO2 nanostructured catalyst was placed in the plasma discharge zone.

The pollutant mixture was composed of acetaldehyde (1000 ppmC), oxygen (20%) and nitrogen (balance) (Air Liquide AlphagazTM 1), and the flow rate was set to 100 mL·min−1 (Bronkhorst El-Flow® Select). The gas was admitted at the bottom of the DBD reactor and flowed upward through the fluidized bed. At the exit, it was sent to a set of analyzers: gas chromatography (Shimadzu GC-2110, Shimadzu, Noisiel, France), for quantitative analysis of residual acetaldehyde; gas chromatography coupled to mass spectrometry, for identification of the gaseous by-products (Shimadzu 2110-GC coupled to a QP2010S-MS, Shimadzu, Noisiel, France); and an infrared multigas analyzer (MIR 9000, Environment SA, France), for CO2 and CO quantification. The plasma-catalysis experiments were carried out in two stages: (i) the catalyst surface was saturated by acetaldehyde at room temperature and atmospheric pressure, and (ii) the plasma discharge was switched on for 30 min to induce acetaldehyde removal on the surface in the same temperature and pressure conditions. The surface was then characterized by DRIFTS analysis.

#### *4.3. DRIFTS Measurements*

DRIFTS analyses were performed with a Shimadzu IRPrestige-21 spectrophotometer (DLATGS detector, 100 scans accumulation, and 4 cm−1 resolution, Shimadzu, France). A Pike DiffusIRTM accessory (Pike, Eurolabo, France) containing a chamber equipped with a KBr window was adapted on the FTIR spectrophotometer. This chamber can contain a cup with approximately 10 mg of catalyst. All DRIFTS spectra presented were acquired in transmittance mode.

#### **5. Conclusions**

In summary, we used DRIFT spectroscopy to identify the adsorbed species formed on an Ag/TiO2/SiO2 surface during acetaldehyde adsorption and degradation with a PDC process. The measurements showed that different species such as acetate, formate, methoxy, ethoxy and formaldehyde are present on the surface, once it has been in contact with the plasma.

A pathway that considers the formation of these intermediates via the interaction of adsorbed acetaldehyde, and the plasma-generated species and/or by-products of acetaldehyde decomposition in the gas phase is proposed. The main carbonaceous intermediates identified allowed more detailed mechanism for acetaldehyde degradation to be given, as well as the formation of most of the identified by-products detected in the gas phase leaving the reactor to be explained.

Further investigations are needed to include nitrogenous species detected in the gas phase in the mechanism of acetaldehyde degradation with the PDC process. It is very likely that in CH3CHO degradation the presence of methanol, one of the intermediate by-products in CH3CHO oxidation, combined with oxygen activation by silver atoms on the surface, are key elements in the performance of the PDC process.

**Author Contributions:** Conceptualization, A.V.-G., X.D. and S.S.; Formal analysis, A.V.-G., X.D. and S.S.; Funding acquisition, X.D.; Investigation, A.V.-G., X.D. and S.S.; Methodology, A.V.-G., X.D. and S.S.; Supervision, A.V.-G. and X.D.; Validation, A.V.-G. and X.D.; Writing—original draft, A.V.-G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the ANR Programme Blanc grant ANR-08-BLAN-015–01 à 04)) and Region Ile de France grant IF-110005975 and 10005961).

**Acknowledgments:** The authors wish to thank Nicolas Fagnon for the Labview program used for the automated data acquisition during the DRIFTS experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*


**Author details**

Nivetha Martin Department of Mathematics, Arul Anandar College (Autonomous), Madurai, Tamil Nadu, India

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© 2021 The Author(s). Licensee IntechOpen. 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.

*Plithogenic SWARA-TOPSIS Decision Making on Food Processing Methods with Different… DOI: http://dx.doi.org/10.5772/intechopen.100548*


**3. Blood-brain barrier damage**

Morphology and function of BBB are linked as shown by microscopic observation. Early ultrastructural studies where performed by administration of silver nitrate in the drinking water of rodents [33]. They showed the presence of very scarce quantity of silver around the capillaries in the cerebral cortex, medulla, and cerebellum. In such brain regions, capillaries are continuous. In other areas, such as neurohypofisis, area postrema, pineal body and intercolumnar tubercle, heavy accumulation of silver was present around fenestrated capillaries [34]. Using lanthanum and horseradish peroxidase as tracers, via intravenous injection, it has been demonstrated that these substances are unable to penetrate between the endothelial cells because of the tight junctions presence (zonulae occludentes). Regarding the contribution of astrocytes to the BBB, it is well known that its end-feet form a relatively complete layer, but the junctions between are gap junctions and not of the occluding kind [35]. Perivascular endfeet of astrocytes do not provide an effective barrier even if substances should pass through the endothelial cells into the brain. Indeed, after intravenous infusion of peroxidase, endothelial cells show micropinocytotic vesicles containing the tracer [36]. Such a transendothelial cell barrier is very selective and based on carrier-mediated transports, but is not furtherly

Caveolae are small, bulb-shaped, plasma membrane invaginations. They have been described to have a function in endocytosis and transcytosis and in in maintaining the lipid composition

While endothelial cells in peripheral organs, such as the lung and heart, are enriched in caveolae, in BBB only a small number of caveolae are detectable [24]. Mfsd2a (transporter of the major facilitator superfamily domain-containing (Mfsd) family) contributes to the regulation of vesicular traffic in BBB endothelial cells [37] through the transport of the essential omega-3 fatty acid docosahexaenoic acid (DHA). The expression of Mfsd2a becomes upregulated in ECs with the maturation of BBB. Gene ablation of Mfsd2a in mice results in BBB leakiness and

Caveolins (Cavs) are thought to play a role in the regulation of BBB function. Cavs, are a family of integral membrane proteins which represent both positive and negative regulators of intracellular signaling as scaffolding proteins that regulate the intracellular distribution of the signaling molecules. Cav-1 overexpression protects the integrity of the BBB mainly by

Cav-1 is a marker of caveolae in endothelial cells and is important in the regulation of various functions like endocytosis, transcytosis, signal transduction, and molecular transport. Recent studies on mice indicate that the suppression of the caveolae pathway requires the transport of lipids, notably DHA-containing phospholipids, by Mfsd2a to regulate CNS endothelial cell

Moreover, Cav-1 regulate the angiogenic response by influencing VEGF receptor 2 (VEGFR2)

In BBB the basement membrane (BM) represents the noncellular component. Astrocytes, PCs and ECs synthetize and secrete molecules which constitute the BM surrounding the external surface of the endothelial cell, composed by type IV collagen, fibronectin, heparan sulfate,

plasma membrane composition and to inhibit caveolae vesicle formation [39].

of the membrane, as well as acting as signaling background.

preventing the degradation of TJ proteins in rats [38].

phosphorylation and internalization [40, 41].

mechanically regulated.

132 Liquid Biopsy

increased vesicular traffic in ECs.

The endothelial cells (ECs) compose the wall of vessels and capillaries and represent the primary blood-tissue barrier. The ECs acting as a protective filter are able to regulate the passage of molecules and immune cells and the level of specialization of each blood-barrier is determined by the functions of endothelial-wall. In the brain, there is the higher level of specialization of the endothelial wall. The filter function of BBB is carried out by ECs strictly interconnect with numerous tight junctions.

Pathological conditions within the central nervous system like ischemia, inflammation or tumor growth lead to blood-brain barrier (BBB) dysfunction, emphasizing that the permeability barrier regulation is principally provided by the local microenvironment and its maintenance is a necessary condition in any circumstance. In many brain tumors morphological irregularities of the perivascular space correlate with a breakdown of the BBB [45–47].

Wolburg et al. [48] found that claudin-3, a key component of BBB tight junctions, is lost in glioblastoma. Further evidence on claudin-1 loss in tumor microvessels, as well as downregulation of claudin-5 and occludin in hyperplastic vasculature, result in a phenotypic change in BBB function due to leaky tight junctions and hyperpermeable endothelial cells.

A recent study by Watkins et al. [49] using a mouse model demonstrates that astrocytic endfeet displace from their position alongside endothelial cells with disruption of the communication between the astrocytes and vasculature and that single glioma cells were sufficient to produce local BBB opening.

The structural perturbations of the vascular barrier during tumor progression, in other organs was demonstrated, are successive to the release in the tumor microenvironment of specific cytokines that downregulate the transcription of these structural proteins [50].

The crucial point is the vascular damage that the tumor direct and indirect operas through the release on the next microenvironment of chemical factors able to increase the permeability of the tissue vessels reducing their protective barrier function. This could explain the pathogenesis of the BBB damage occurring during the development of glioblastoma. The BBB damage induced by the glioblastoma represents a strategy employed to control the BBB opening to allow the passage of drugs [51].

The detection of endogenous circulating molecules normally restricted by the BBB, namely albumin, immunoglobulin G, or fibrinogen, in the brain parenchyma, using immunohistochemistry or immunofluorescence is one of the most straightforward in situ techniques used to assess BBB impairment [52].

Measurement of exogenous tracer extravasation remains a technique of choice in preclinical studies testing for BBB permeability. A variety of detectable exogenous tracers has become available, and methods have been developed to evaluate the kinetics of their extravasation. Due to variations in tightness of the BBB, the extravasation of different BBB permeability tracers across an impaired BBB critically depends on their physicochemical properties and molecular weight.

In recent years, different in vivo imaging techniques have contributed to the understanding of BBB dysfunction in disease [52]. Among these, noteworthy:

• In vivo confocal microscopy, an important tool for the high-resolution dynamic fluorescence imaging in BBB research. This technique has the advantage of time-lapse imaging of the same subject. The apparent disadvantages of this technique are the technical complexity and invasive nature, because it requires opening of a cranial window in experimental animals.

**4. Glioblastoma's angiogenesis**

et al. IJC Heart & Vasculature [106]).

active, and predisposed to cellular necrosis [56].

Gliomas are a heterogeneous group of neoplasms derived from glial cells that account for 40–45% of all intracranial tumors. The most malignant type of glioma, Glioblastoma multiforme (GBM), account for approximately 12–15% of all intracranial neoplasms and 50–60% of all astrocytic tumors. In most European and North American countries, incidence is approximately 2–3 new cases per 100,000 people per year.  The World Health Organization defines GBM as a cancer of grade IV characterized as malignant, mitotically

**Figure 2.** Vessel's arrangement of endothelial cells acting as protective filter (A) mimics an array of warriors (B) placed to protect the tissues. In (A), primary human endothelial culture. Scale bar 100 μm (figure partially modified from Malara

Blood-Brain Barrier Breakdown by Combined Detection of Circulating Tumor and Endothelial…

http://dx.doi.org/10.5772/intechopen.80594

135

Untreated patients with glioblastoma multiform uniformly die within 3 months from the diagnosis. Treated patients with a protocol including surgical resection, radiation therapy, and chemotherapy, have a median survival of 14.6 months, with fewer than 25% of patients surviving up to 2 years and fewer than 10% up to 5 years [52, 57, 58]. Moreover, males had a slight preponderance over females, with a male-to-female ratio of 1.6:1. GBM may manifest in persons of any age, but it affects adults preferentially, with a peak incidence at 45–70 years [55]. Glioblastomas are tumors that display extensive morphological and molecular heterogeneity, and thus may reflect their origin from different population of astrocytes, and possibly from oligodendrocytes and ependymal cell lineages. GBM rarely metastasize outside the brain [59]. GBM can be classified into primary type and secondary type, according to whether they are generated de novo or by progression of lower-grade tumors. Histologically, primary and secondary glioblastomas are largely indistinguishable, but they differ in their genetic and epigenetic profiles [60]. GBM is a highly aggressive tumor with distinct histopathological features, including high proliferation, necrosis and considerable neovascularization (i.e., angiogenesis), leading to vessels that exhibit morphological abnormalities and "leakiness" [60]. It is generally accepted that the degree of angiogenesis is correlated to the malignancy of the tumor [58, 61].

Despite this advance the problem to track BBB opening remains for insufficient selectivity, specificity of the tools provided and for the no sufficiently abundancy [52] of the biologicsbased molecular imaging probes today available. In this prospective the liquid biopsy could represent an opportunity to track in real time the BBB opening. The concept is based on the causal release of tumor cells in the systemic circulation through the BBB damage. The structural perturbation of the BBB and the consequent modification of its permeability could induce cell spread independently by the molecular equipment expressed by cancer cell. In this direction, the detection of the circulating cancer cells in glioblastoma cases could acquire a specific significance related to the permeability of the BBB more than a prognostic value related to the dissemination phase of the disease (**Figure 2**)**.**

Blood-Brain Barrier Breakdown by Combined Detection of Circulating Tumor and Endothelial… http://dx.doi.org/10.5772/intechopen.80594 135

**Figure 2.** Vessel's arrangement of endothelial cells acting as protective filter (A) mimics an array of warriors (B) placed to protect the tissues. In (A), primary human endothelial culture. Scale bar 100 μm (figure partially modified from Malara et al. IJC Heart & Vasculature [106]).


**3. Nurse prescribing in Iran**

Similar to the UK, in Iran especially in critical care situations, nurses assess the patients and after diagnosing the problem, they wait for the physician to confirm and stamp the diagnosis and prescription taken by the nurse. Therefore, in future possible scenarios for nursing prescription are the works in the UK that have been done. In other words, countries such as Iran (which is in the infancy period) could use the UK experiences to develop their nursing prescription ability. In primary care, which can enable the level of access and cheaper services, or in the clinical areas such as chronic diseases (diabetes, cardiovascular disease, cerebrovascular accidents …) nurse prescriptions can be started from the list of drugs. And after some time, the independent prescription of drugs will become official from the national formulary, and registered nurses who have at least 3 years of work experience will be allowed to participate in prescription training courses.


**4. Conclusions**

The present investigation made possible an analysis on how the ozone-based oxidative processes influence the reduction of the ecotoxicity risk caused by emergent pollutants. The obtained results have showed algae's behavioral changes among the four examined laboratories–hormones, solids, dermo-cosmetics and psychotropics–, comparing raw effluent samples, treated effluent samples and control.

Significant alterations on *Euglena gracillis* behavior were observed for the effluents from the laboratories of hormones and dermo-cosmetics, with modifications in general speed and velocity of surface ascent. The psychotropics and solids laboratories have not presented significant statistically difference over the algae's behavior. However, complementary studies are appropriate, in order to confirm the long-term toxicity, since the algae find compensatory mechanisms to fit the adversities.

The variations on algae's behavior before the exposure to different pollutants have suggested it is important to distinguish the effluent treatment, according with the characteristics of the substances manipulated in each laboratory, to reduce the environmental toxicity risks.

Therefore, the found biomonitoring data were relevant for better knowing and to be more aware of the issue, indicating the environmental toxicity caused by the effluents from magistral pharmacies may provoke great impact to the environment if revisions of actions and legislation are not performed in a way that in long term the environmental points related to this type of emergent pollutants are reduced.

For future studies, the evaluation of fish's behavior, before similar conditions, may point out better comprehension over the influence of pharmaceutical ecosystem risks.

### **Acknowledgements**

The researchers thank Univille and the Research Support Funds, that make possible the Environmental Impacts Integrated Project (ECOSAM) development.


**2.2 Biological role of EVs**

The key biological function of EVs is cell to cell communication and the transfer of biological materials that act closely, but also, and more importantly, remotely. Cargo within the EVs is protected from degradation in the bloodstream and can be successfully transferred to specific cells of interest, affecting several biological functions of these cells. EVs can transfer a wide variety of molecules: heat shock proteins (HSP-90, HSP-70), interleukins (IL), such as tumor necrosis factor-alpha (TNFα), acute phase proteins, such as serum amyloid A [20], enzymes, peptides, growth factors [14]. Therefore, EVs have a wide range of biological functions including immune response, antigen presentation, and the transfer of RNA, including micro RNA (miR) and DNA. Given the fact that EVs migrate through the bloodstream they can have pleiotropic effects that are likely to affect every tissue in the body [14]. In immunity, they modulate immune cells, cell–cell interactions, and transfer of cytokines and chemokines. In the heart and vessels, they stimulate

#### **Figure 1.**

*Classification of EVs. EVs can be classified according to their size (Small <100 nm or < 200 nm, Medium/ large >200 nm), density (Low, Medium, High) with a defined density range, biogenesis pathway (Exosomes; endosomal derived, Microvesicles; membrane derived and Apoptotic bodies; released upon cell apoptosis) or biochemical composition defining EVs origin, surface proteins or cargo. Created with BioRender.com.*

coagulation and thrombosis, modulate angiogenesis, calcification and vascular repair. In the adipose tissue, they modulate angiogenesis, inflammation, cell differentiation and secretion of cytokines. In the bone marrow, they are involved in cell–cell cooperation, cell proliferation, differentiation and maturation. In the central nervous system, they are involved in the integration of neurons and various glial cells, modulate angiogenesis, neuronal plasticity and myelination. In the blood, they influence activation and aggregation of platelets, are directly involved in coagulation, as well as cargo transfer of procoagulant or anticoagulant molecules, cytokines and growth factors [14].


**1. Introduction**

In the realm of hydrogenation reactions, the selective hydrogenation of unsaturated carboncarbon bonds is very attractive [1]. Asymmetric and partial hydrogenation reactions are pivotal for the access of important compounds employed as pharmaceuticals, food additives, pesticides, flavors, and fragrances [2]. In this respect, α,β-unsaturated carbonyl compounds present an interesting group of chemicals whose enantio- [3], chemo [4], and regio-selectivities [5] are highly challenging. For instance, Lindlar's [6] and Lindlar types' catalysts [7] are widely used to chemoselectively hydrogenate alkynes to the corresponding alkene. In parallel, a variety of different selective hydrogenation methods have been developed [8]. In particular, bimetallic catalysts [9], noble metal-based catalysts supported onto carbon-based materials [10] or inorganic matrixes [11], in combination with supercritical solvents [12] and "*flow*" reactors [13], have been studied. Also for the partial alkyne hydrogenation, a great deal of catalytic systems were proposed based on the use of noble metals and different approaches like reactions conducted in gas [14] and liquid phase [15], combined with ultrasound techniques [16] and biphasic catalytic systems [17].

The interest on polymer-based catalysts applied for hydroformylation reactions [18], hydrotreatment [19], and strong acid/base [20, 21] catalyzed reactions is steadily increasing. Since functional polymer-based catalytic systems generally exhibit notable advantages over traditional catalysts such as the tunable solubility and improved catalytic selectivity [22], the biodegradable polyester, poly(lactide) (PLA) was used as a polymer matrix [23], which is rather straightforwardly end-capped as reported by Helle et al. [24]. The possibility to produce carboxylic end-capped PLA chains functionalized with nitrogen containing moieties, called macroligands by Giachi et al. [25], allowed the synthesis of well-defined macrocomplexes containing metal cations, such as Pd(II). The latter Pd-based macrocomplexes were efficiently used to catalyze hydrogenation reactions conducted in the homogeneous phase followed by an easy separation step, as reported by Giachi et al. [26] and Bartoli et al. [27]. Isotactic endfunctionalized PLA with opposite tacticity can be combined to form a supramolecular structure named stereocomplex [28]. Functionalized PLA stereocomplexes show high resistance against hydrolysis and thermal degradation and in addition, a poor solubility in all organic solvents. This latter property was exploited by Petrucci et al. [29] and Oberhauser et al. [30] for catalytic purposes to easily separate PLA-stereocomplex-based catalysts from catalytic solution.

In the following sections, synthesis, characterization, and catalytic application of homogeneous and heterogeneous Pd-catalysts supported on end-functionalized PLA were reported.


**3. Application of enzyme in pseudocereals**

Enzymes are necessary for the production of compounds from grains that are used in contemporary foods and beverages. Enzymes have been identified as having the ability to improve the processing behavior or qualities of cereal and pseudocereals meals such as flavor, texture, and shelf-life with minimal impact on nutritional content [38].

Grain contains endogenous and exogenous enzymes. Endogenous enzymes are found naturally in grain kernels and are primarily found in the outer layer, fiber, and bacteria. Meanwhile, exogenous enzymes are created by bacteria on the surface. These enzymes influence grain raw materials quality and processing properties, mainly when humidity and time are present. Some enzymes are found in cereals, are frequently of microbial origin, and are given as pretreatment and manufacturing agents. Enzymes can contribute significantly by increasing the usage of raw materials and improving the impact of food and beverages [39]. **Table 3** shows the benefits of exogenous and endogenous enzymes in the diet. In general, both exogenous and endogenous enzymes influence the quality of the pseudocereal grains as listed in **Table 3** below. Apparently, the existence of endogenous enzyme gives a huge impact on the physical-chemical properties of pseudocereal grain compared with exogenous enzyme.

#### **Table 3.**

*The advantages of exogenous and endogenous enzymes in food [38, 40, 41].*

The areas of enzyme in grain processing are pervasive in terms of raw material and enzyme-catalyzed processes. The most important contemporary sectors involving enzyme treatment of grains are energy supply, bioethanol, biomaterials, digestible films, and sustainable biomass utilization. The enzymes work with polysaccharides and proteins to control and use starch structure, which is significant in the food and beverage industries [42].

#### **3.1 The involvement of enzymes in seed germination**

Seed germination is a phenomenon governed by various mechanisms required to transform a seed into a new plant. The mature seed includes the necessary components to participate in the processes that control germination, including the enzymes that will aid in the process. Germination demonstrates the nutritional value and availability of proteins and amino acids while decreasing the amount of anti-nutritive substances [43]. Enzymes help restore broken DNA during the drying and germination of the grain, resulting in proper seedling growth.

Unrefined cereal crops need a variety of pretreatment processes, including the use of enzymes in addition to standard techniques. By altering the molecular structure and the amount and quality of nutrients, phytochemicals, and harmful compounds, enzyme pretreatment improves processability, safety, stability, or technical and nutritional utility [43]. For example, enzymes can reduce mycotoxin levels by biotransforming mycotoxin into harmless metabolites [38].

Research demonstrated that enzymatic mycotoxin destruction depends on the enzyme-producing source, its concentration, and the circumstances used [44]. Susanna and Prabhasankar [45] developed an outstanding quality hypoimmunogenic pasta using a blend of xylanase, protease, and transglutaminase, which might be a gluten-free alternative [45, 46] and showed the use of enzymes in cereal grain polishing. Depolymerization of bran carbohydrates happens due to cell wall degrading enzymes in this process, altering phenolic mobilization and dietary fiber solubilization. Carbohydrate-cleaving enzymes, such as cellulases (e.g., endoglucanase, exoglycanases, and beta-glucosidase), xylanases, glucanases, and esterases, undertake enzyme biopolishing [46]. **Table 4** shows the enzymes utilized and the progress

*Rheological Stability, Enzyme Activity, and Incorporation of Pseudocereal Powder… DOI: http://dx.doi.org/10.5772/intechopen.101890*

#### **Table 4.**

*The types of enzymes used and the improvement toward pseudocereals [38].*

toward pseudocereals. In general, these enzymes have been used in improving nutrition level, quality, taste, color, surface structure, strength, and size. Apparently, amylases and hemicellulase are the two enzymes that are important for pseudocereal improvement. The taste and quality of pseudocereal as shown in **Table 4** were affected by the same group of enzymes, namely amylases, proteases, glucose oxidases, hemicellulases, and lipoxygenases.

#### **3.2 Fermentation in pseudocereal processing**

Fermentation is an ancient and cost-effective way of generating and storing foods that may be applied to grain processing. Fermentation is the process that releases energy by oxidizing carbohydrates without the need of an external electron acceptor. Most of the time, enzymes are required in the fermentation process to speed up the reaction. For example, alcohol cannot be produced without the enzyme amylase, which breaks down starch into simple sugar. Moreover, fermentation is impossible

without enzymes. Fermentation is one of the oldest and most cost-effective methods of food preservation and processing. Due to the enzymatic degradation of antinutritional compounds such as phytate, fermentation enhances the availability of particular amino acids, B vitamins, and minerals, including iron, zinc, and calcium [42]. Certain antinutrient substances in pseudocereals such as phytic acid, polyphenols, and protein inhibitors could negatively impact on malnutritions [47]. Fermentation with Lactic Acid Bacteria (LAB) can increase pseudocereals' nutritional and functional qualities [48].

Cereal and pseudocereal fermentation is critical in the creation of chemicals that have a significant impact on organoleptic features such as scent, taste, and texture as well as the enhancement of nutritional values, all of which have a good impact on human health [47]. Microorganisms may be found in practically every biological niche; cereals and pseudocereals generally provide an excellent substrate for microbial fermentations. Polysaccharides are abundant, which microorganisms may use as a carbon and energy source during fermentation. Fermented items made from common cereals and pseudocereals are ubiquitous worldwide [48]. LAB, enterobacteria, aerobic spore formers, and other microbiota fighting for resources are typical in cereal and pseudocereal grains. The pH value, water activity, salt concentration, temperature, and food matrix composition all influence the kind of bacteria present in each fermented meal [48].


**5. Inactivation of non-enveloped viruses by oxidizers**

An oxidizer or oxidizing agent is a chemical that has the ability to oxidize other molecules, i.e., to accept their electrons. Common oxidizing agents used for disinfection, sterilization, or antisepsis include hydrogen peroxide, peracetic acid, ozone, and halogen-containing compounds such as sodium hypochlorite (bleach), hypochlorous acid, povidone-iodine, chlorohexidine, and chlorine dioxide, etc. These compounds can react with and alter the proteins and nucleic acids of nonenveloped viruses and render them noninfectious. Oxidizers comprise a large group of chemicals, and the relative order of susceptibility of non-enveloped viruses to oxidizers seems to vary by specific type of active ingredient (**Table 4**).

Parvoviruses are generally among the least susceptible viruses to various types of oxidizers, including sodium hypochlorite, hydrogen peroxide, and peracetic acid. However, for sodium hypochlorite, minute virus of mice appears to be more susceptible than porcine parvovirus and canine parvovirus. All picornaviruses appear to exhibit a similar degree of susceptibility to sodium hypochlorite; but within the family of *Caliciviridae*, feline calicivirus appears to be more susceptible than murine norovirus. Both adenovirus and rotavirus are susceptible to sodium hypochlorite.

The trending for hydrogen peroxide seems more complex than that for sodium hypochlorite. For example, there seems a higher level of variability within the *Picornaviridae* family. Rhinovirus is quite susceptible to hydrogen peroxide, whereas hepatitis A virus is much less susceptible. Poliovirus seems to be more susceptible than hepatitis A virus but less susceptible than rhinovirus. Similar to the case for sodium hypochlorite, feline calicivirus seems more susceptible than murine norovirus to hydrogen peroxide. Interestingly, adenovirus and rotavirus, two larger non-enveloped viruses, seem to be less susceptible than rhinovirus, a smaller virus, to inactivation by hydrogen peroxide. This is another case where the size of viral particle alone does not appear to dictate the level of susceptibility to a microbicide.

For peracetic acid, hepatitis A virus also seems less susceptible than poliovirus. Both feline calicivirus and murine norovirus are susceptible to peracetic acid and so is adenovirus.

*Variability and Relative Order of Susceptibility of Non-Enveloped Viruses to Chemical… DOI: http://dx.doi.org/10.5772/intechopen.102727*

*Disinfection of Viruses*

*a See Table 1 for abbreviations used for viruses.*

*bBSA, bovine serum albumin; PBS, phosphate buffered saline; medium, culture medium; RT, room temperature. c Viral-inoculated lettuce was washed with PAA solution for a defined period of time.*

*Entries in purple font indicate results from undiluted original or diluted formulations with microbicidal active ingredients.*

#### **Table 4.**

*Efficacy of oxidizers against non-enveloped viruses.*


**1. Introduction**

Currently, light-gas cannons are one of the most effective devices for accelerating shells of relatively large mass to speeds comparable to the orbital. In this area of research, intensive and extensive experimental work is being carried out to study high-speed impact, resistance of materials to high-speed action of solid particles, hypersonic flows around axisymmetric bodies and are closely related to the development of aviation and space technology. In this case, some types of guns with the replacement of the accelerating channel by the Laval nozzle can be effectively used to study hypersonic flows around bodies, since the parameters of the working gas required for high-speed throwing and generation of a hypersonic air flow are identical. High-speed throwing system parameters and technical requirements are described in [1–5]. Since the 40s of the 20th century the generation of hypersonic flows, development of measurement methods and study of such flows structure over various test bodies were carried out by researchers from all over the world [6–16]. Numerical modeling methods mentioned, for example in [10, 11, 17–19], were added to the experimental tools of the research with the development of computer technology. However, it should be noted that experimental studies of hypersonic flows around the objects are still preferred. Due to the importance of problems to be solved flight tests are often used for higher authenticity of the experimental data. Usually total and static pressure is measured by means of Pitot-Prandtl probe to determine

the Mach number, but in large number of research works measurements are not carried out due to complicated experimental techniques. Researchers [6, 7, 11–16] used the initial Mach number calculated from Laval nozzle geometrical parameters. Pressure measurement using a Pitot-Prandtl probe and temperature measurement using a thermocouple significantly distort the flow structure, complicate the design due to the need to place measuring elements inside and on the surface of the models under study, which, in the absence of flow visualization, can introduce uncontrollable errors in the experimental results. In hypersonic aerodynamic facilities preferable methods of research are optical methods with high-speed cameras as the reg-istering devices. Methods of optical visualization of supersonic flows include a method of a laser knife, smoke visualization, visualization by means of the spark discharge, etc. [13, 15, 16, 18]. Optical methods based on the relationship between the optical and thermodynamic properties of the medium (shadow methods, interference methods, spectral diagnostics) allow measurements in hypersonic flows with high accuracy. In some cases, to visualize and measure the temperature field on the surface of the model, a thermosensitive coating is used as an alternative to measurements using thermocouples [8, 9]. Research in the field of high-speed impact and diagnostics of hypersonic flow are often interrelated tasks, which makes it necessary to create experimental installations equipped with systems for optical visualization and diagnostics of the flow and measurement of the Mach number, which makes it possible to study hypersonic flow around objects without suppressing the investigated gas flow.


*Edited by Suna Sabuncuoglu*

*Botulinum Toxin - Recent Topics and Applications* provides doctors and researchers with the most up-to-date information on the structure and action of botulinum toxins as well as their usage to treat a wide range of disorders. The book covers the fundamentals of botulinum toxins, including improvements in our understanding of their molecular structure and mechanism of action. Furthermore, it covers the production and formulation of botulinum toxins for clinical applications. The information contained herein will hopefully shift the paradigm of botulinum toxin research, moving it away from a focus on toxin attributes toward a focus on toxin engineering based on patient requests and physician experiences in everyday practice.

ISBN 978-1-80355-228-6

Botulinum Toxin - Recent Topics and Applications

Published in London, UK © 2022 IntechOpen © Anna Kim / iStock

## Botulinum Toxin Recent Topics and Applications

*Edited by Suna Sabuncuoglu*