some earlier works from this laboratory were concerned with electrosorption and electroreactivity of small organic molecules at well - ordered pt single - crystal surfaces , including : aliphatic oximes [ 1 , 2 ] and guanidinium [ g / nh2  =  c(nh2)2]-type ions [ 35 ] . with respect to g ions , their presence in solution
was revealed in substantial effects on the voltammetric profiles for upd of h , both in acidic and in alkaline media , owing to two - dimensional interaction effects between the adsorbed organic cations and electrosorbed bi(sulfate ) ( or oh ) species .
these effects were also characterized in detail by in situ fourier transform infrared spectroscopy ( ftir ) experiments at pt(111 ) and ( 100 ) planes , as well as by alternate current ( ac ) impedance spectroscopy kinetic investigations , carried out in 0.5  m h2so4 solution at the ( 100 ) surface . urea [ o  =  c(nh2)2 ] is a structure - related molecule to that of guanidine .
hence , climent et al . in refs .   and   examined in situ ftir and cyclic voltammetry behavior of urea in 0.1  m hclo4 at pt(100 ) and ( 111 ) planes , respectively .
urea was shown there to undergo dissociative electrosorption on the surface of pt in an anodic ( 1- or 2-electron transfer ) oxidation process [ 6 , 7 ] .
similar cyclic voltammetry experiments on adsorption of urea in hclo4 at various low - index pt surfaces were reported by rubel et al . .
in addition , important radiochemistry measurements [ 9 , 10 ] were conducted in order to facilitate derivation of the potential and concentration dependence of urea coverage on pt , including the surface structure for the urea adlayer on pt(100 ) plane .
the key aim of this work was to present the kinetic aspects of the process of electrosorption of urea on the pt(100 ) plane in 0.5  m h2so4 , especially in reference to those recently reported for guanidinium cations under analogous experimental conditions .
the above was accomplished through derivation of the faradaic resistance and electrosorption pseudocapacitance components associated with upd of h and electrosorption of u , carried - out at several working electrode potentials .
pt single - crystal of the ( 100 ) orientation was prepared from 1  mm diameter 99.9985  % pt wire ( aesar / puratronic ) by employing the techniques and procedures for preparation of pt single crystals developed by clavilier et al . .
high - purity , aqueous 0.5  m h2so4 solution was prepared from sulfuric acid of highest purity available ( seastar chemicals ) with water derived from an 18.2 m direct - q3 uv ultrapure water system from millipore .
poland ) was used to prepare acidic solutions , at a concentration of 1    10  m u. all prepared solutions were de - aerated with high - purity argon ( ar 6.0 grade , linde ) , which was also purged above the solutions during impedance measurements .
ac impedance measurements were conducted by means of the solartron 12,608  w full electrochemical system , consisting of 1,260 frequency response analyzer ( fra ) and 1,287 electrochemical interface .
all potential measurements were referred to the potential of a reversible hydrogen electrode ( rhe ) , in the same test solution .
the 1,260 fra generator provided an output signal of known amplitude ( 5  mv ) and the frequency range was usually swept between 1.0    10 and 0.1  hz . the instruments were controlled by zplot 2.9 software for windows ( scribner associates , inc . ) .
presented impedance results were obtained through selection and analysis of representative series of experimental data .
reproducibility of such - obtained results was typically below 10  % from one measurement to another .
the impedance data analysis was performed with zview 2.9 software package , where the spectra were fitted by means of a complex , nonlinear , least - squares immitance fitting program , levm 6 , written by macdonald .
figure  1 below presents the adsorption behavior of urea   ( at a concentration of 6    10  m u ) at the pt(100 ) plane in 0.5  m h2so4 .
the adsorption effects of u are characterized by the potential range for upd of h being  squeezed  and considerably ( by ca .
the voltammetric behavior of urea is analogous to that previously reported for guanidinium ions at the pt(100 ) surface [ 3 , 4 ] .
1 ) could conveniently be explained in terms of attractive , ion pairing interactions between co - adsorbed urea molecules and hso4 ions ( see structure  1 below ) thus referring to those previously examined for the g ion by means of cyclic voltammetry and ftir measurements in refs .   and .fig .
1cyclic voltammograms for pt(100 ) in 0.5  m h2so4 recorded at a sweep rate of 50  mv  s and in the presence of u at the concentration indicated ; voltammograms were recorded on the third cycle ( from ref . )
2a complex plane impedance plot for pt(100 ) in contact with 0.5  m h2so4 in the presence of 1    10  m u recorded at 293  k for the stated potential value .
the solid line corresponds to representation of the data according to the equivalent circuit shown in fig . 
b bode phase - angle plot for impedance behavior at pt(100 ) in contact with 0.5  m h2so4 in the presence of 1    10  m u , for the stated potential value ( other details as in fig . 
2a ) cyclic voltammograms for pt(100 ) in 0.5  m h2so4 recorded at a sweep rate of 50  mv  s and in the presence of u at the concentration indicated ; voltammograms were recorded on the third cycle ( from ref . )
a complex plane impedance plot for pt(100 ) in contact with 0.5  m h2so4 in the presence of 1    10  m u recorded at 293  k for the stated potential value .
the solid line corresponds to representation of the data according to the equivalent circuit shown in fig . 
b bode phase - angle plot for impedance behavior at pt(100 ) in contact with 0.5  m h2so4 in the presence of 1    10  m u , for the stated potential value ( other details as in fig . 
2a ) although at the pt(100 ) plane , the recorded   voltammetric behavior of u was comparable with that observed for the g ions , on the ( 111 ) surface , urea was suggested   to exhibit significantly reduced ion pairing interactions with bi(sulfate ) species than the corresponding guanidinium ions . however , it has to be stressed that it is predominantly sulfate ( not bisulfate ) species that becomes adsorbed on the pt(111 ) plane in sulfuric acid solution ( see recent works by su et al .
, by garcia - araez et al . , and by yeh et al . ) .
thus , in this case , no attractive interactions between the adsorbed , unprotonated urea molecules and sulfate species could be envisaged [ contrast to the behavior at the ( 100 ) plane , see structure  1 again ] .
consequently , for the ( 111 ) plane , the recorded displacement of the voltammetric profile in the presence of urea ( see fig .
7a in ref . ) is most likely the result of partial protonation of urea in the supporting electrolyte ( please note that about 10  % of u molecules at ph    1 exist in a cationic form [ 3 , 4 , 6 ] ) .
the above - made conclusions get support from the previously performed   ftir experiments in the presence of urea , which indicated considerably less extensive chemisorption of u on the pt(111 ) surface , as compared to that exhibited at the ( 100 ) plane .
the impedance behavior of urea , at a concentration of 1    10  m u in 0.5  m h2so4 at the pt(100 ) plane is shown in fig . 
2a and b and in table  1 . thus , the impedance behavior of u at potentials close to that of the capacitive peak in fig . 
1 ( 200 , 250 , 300 , and 350  mv vs. rhe ) is characterized by the appearance of two partial semicircles and a capacitive line at an inclination to the z axis different from 90. the smaller semicircle ( see the impedance spectrum recorded at 300  mv in inset to fig . 
2a ) , observed at high frequencies , corresponds to the process of upd of h ( in relation to the charge transfer resistance , rh ) , and the part of a large - diameter semicircle ( see fig . 
2a again ) , observed throughout the intermediate frequency range , is associated with the charge transfer process ( with respect to the ru parameter in table  1 ) accompanying electrosorption of urea on the pt surface ( see eq .  1 below ) .
moreover , existence of two maxima is clearly discernible in the corresponding bode phase - angle plot in fig . 
2b ( please evaluate the fitting quality by the derived chi - squared :  parameter values given in table  1).fig . 3equivalent circuit for an adsorption process such as upd of h , exhibiting faradaic pseudocapacitance , cph , charged via a faradaic resistance ,
rh in the presence of co - adsorbed urea ( ru and cu components ) , in a parallel combination with the double - layer capacitance , cdl , jointly in series with an uncompensated solution resistance , rs ; w diffusional warburg element , employed as an empirical termtable 1resistance and capacitance parameters for upd of h and the process of electrosorption of u molecule ( at a concentration of 1    10  m ) on pt(100 ) plane in 0.5  m h2so4 ( recorded at 293  k ) , obtained by finding the equivalent circuit which best fitted the impedance data , as shown in fig .  3 (  refers to the recorded chi - squared parameter values by the zview fitting software)e / mvrh/ cm10cph / f  cm10cdl / f  cmru/ cm10cu / f  cm2000.45    0.01395.7    3.328.5    1.7267.0   
6.5304.5    6.34.7    102500.55    0.02378.8    3.836.5    1.9133.1    5.62,089    959.4    103001.85    0.03159.4    1.743.2   
1.8148.8    3.2291.4    5.55.6    103504.70    0.2261.8    2.055.4    2.2513.8    12.6205.2    3.97.2    10 equivalent circuit for an adsorption process such as upd of h , exhibiting faradaic pseudocapacitance , cph , charged via a faradaic resistance , rh in the presence of co - adsorbed urea ( ru and cu components ) , in a parallel combination with the double - layer capacitance , cdl , jointly in series with an uncompensated solution resistance , rs ; w diffusional warburg element , employed as an empirical term resistance and capacitance parameters for upd of h and the process of electrosorption of u molecule ( at a concentration of 1    10  m ) on pt(100 ) plane in 0.5  m h2so4 ( recorded at 293  k ) , obtained by finding the equivalent circuit which best fitted the impedance data , as shown in fig .  3 (  refers to the recorded chi - squared parameter values by the zview fitting software ) as compared to the case of pure 0.5  m h2so4 supporting electrolyte ( e.g. , see table 1 in ref . ) ,
the kinetics of the process of upd of h have become significantly slowed down in the presence of u , beyond the potential of 250  mv vs. rhe ( table  1 ) .
a significant increase of the rh parameter ( from 0.45    cm at 200  mv to 4.70    cm at 350  mv ) can be explained in terms of the ion pairing mechanism ( originally proposed for the g ions in ref . ) , where the presence of surface - adsorbed bisulfate species , arising at significantly lower electrode potentials together with co - adsorbed u molecules ( structure  1 ) , appreciably influences the kinetics of upd of h on this pt plane .
in addition , changes of the hydrogen adsorption capacitance ( cph ) strictly follow those of the rh parameter ; thus , the cph dramatically declines ( table 
1 ) from 395.7  f  cm ( at 200  mv ) to 61.8  f  cm ( at 350  mv).1 on the other hand , the adsorption charge transfer resistance for urea ( ru ) reaches its minimum value of 133.1    cm at the potential close to that of the peak current  density in fig .  1 ( 250  mv ) , which implies that the kinetics of adsorption of u on the pt(100 ) plane are dramatically slower than those of upd of h on this surface . again
, minimum of the ru value coincides with a very large value ( 2,089  f  cm ) of the adsorption pseudo - capacitance parameter ( cu ) , recorded at the potential of 250  mv .
furthermore , the recorded double - layer capacitance values ( cdl ) in table  1 oscillate between ca .
these cdl values are considerably higher than 20  f  cm , i.e. , a commonly quoted double - layer capacitance value in literature for smooth and homogeneous surfaces [ 16 , 17 ] , which implies some contribution to the recorded cdl from the adsorption capacitance components ( table  1 ) .
moreover , an observed deviation from the purely capacitive , 90 phase angle behavior ( also expressed by  depressed 
the phenomenon of capacitance dispersion is typically visualized as corresponding either to slow adsorption / desorption processes or as the effect of increasing pt surface inhomogeneity , especially important when extensive potentiostatic impedance measurements take place [ 1820 ] .
all fittings of the recorded impedance data were performed by means of the double - adsorbate equivalent circuit ( see fig . 
3 ) with employment of a warburg diffusional element ( w ) , regarded here as an empirical term   involved in the kinetics of urea electrosorption on the pt(100 ) plane.fig . 4total capacitance ( ctotal ) as a function of the applied potential ( vs. rhe ) for the desorption of h and adsorption of u ( at a concentration of 1    10  m ) on pt(100 ) , in contact with 0.5  m h2so4 solution .
the solid line corresponds to the ctotal calculated from the cv profile ( ctotal  =  current  density / sweep rate ) and the experimental points are the impedance - derived values of total capacitance total capacitance ( ctotal ) as a function of the applied potential ( vs. rhe ) for the desorption of h and adsorption of u ( at a concentration of 1    10  m ) on pt(100 ) , in contact with 0.5  m h2so4 solution .
the solid line corresponds to the ctotal calculated from the cv profile ( ctotal  =  current  density / sweep rate ) and the experimental points are the impedance - derived values of total capacitance as an internal check on the impedance results , fig . 
4 below shows total capacitance , ctotal , where ctotal  =  cdl  +  cp ( adsorption pseudocapacitance components ) , as a function of potential for the adsorption behavior of u on the pt(100 ) single - crystal surface .
this figure compares ctotal obtained from the impedance results with that directly calculated from the voltammetric profile [ 5 , 22 ] under comparable experimental conditions ( 1    10  m  u ) .
it can be seen in fig .  4 that the ctotal values are in fairly good agreement with each other for all four  probing  potential values.structure 1ion pairing interactions between co - adsorbed urea molecules and hso4 ions ion pairing interactions between co - adsorbed urea molecules and hso4 ions with respect to the impedance behavior previously recorded in the presence of the guanidinium cation , the process of electrosorption of urea at the pt(100 ) plane is characterized by significantly increased values of the charge transfer resistance parameter ( ru vs. rg+ in ref . ) .
in fact , the ru / rg+ ratio comes to ca . 5.6 for the minimum electrosorption resistance values , recorded for u and g species , respectively .
the above strongly supports the formerly derived conclusions   on the ion pairing ( dipole  dipole type ) between the electrosorbed u molecules and bisulfate species being significantly weaker than the corresponding g / hso4 ( ion dipole ) interactions ( not particularly evident from the comparative : u vs. g cyclic voltammetry behavior for this pt plane ) . in this respect ,
the electrochemical behavior of urea on the pt(100 ) plane is somewhat analogous to that recorded for n , n - dimethyl - guanidinium ( dmg ) cation in ref . .
figure  1 below presents the adsorption behavior of urea   ( at a concentration of 6    10  m u ) at the pt(100 ) plane in 0.5  m h2so4 .
the adsorption effects of u are characterized by the potential range for upd of h being  squeezed  and considerably ( by ca .
the voltammetric behavior of urea is analogous to that previously reported for guanidinium ions at the pt(100 ) surface [ 3 , 4 ] .
1 ) could conveniently be explained in terms of attractive , ion pairing interactions between co - adsorbed urea molecules and hso4 ions ( see structure  1 below ) thus referring to those previously examined for the g ion by means of cyclic voltammetry and ftir measurements in refs .   and .fig .
1cyclic voltammograms for pt(100 ) in 0.5  m h2so4 recorded at a sweep rate of 50  mv  s and in the presence of u at the concentration indicated ; voltammograms were recorded on the third cycle ( from ref . )
2a complex plane impedance plot for pt(100 ) in contact with 0.5  m h2so4 in the presence of 1    10  m u recorded at 293  k for the stated potential value .
the solid line corresponds to representation of the data according to the equivalent circuit shown in fig . 
b bode phase - angle plot for impedance behavior at pt(100 ) in contact with 0.5  m h2so4 in the presence of 1    10  m u , for the stated potential value ( other details as in fig . 
2a ) cyclic voltammograms for pt(100 ) in 0.5  m h2so4 recorded at a sweep rate of 50  mv  s and in the presence of u at the concentration indicated ; voltammograms were recorded on the third cycle ( from ref . )
a complex plane impedance plot for pt(100 ) in contact with 0.5  m h2so4 in the presence of 1    10  m u recorded at 293  k for the stated potential value .
the solid line corresponds to representation of the data according to the equivalent circuit shown in fig . 
b bode phase - angle plot for impedance behavior at pt(100 ) in contact with 0.5  m h2so4 in the presence of 1    10  m u , for the stated potential value ( other details as in fig . 
2a ) although at the pt(100 ) plane , the recorded   voltammetric behavior of u was comparable with that observed for the g ions , on the ( 111 ) surface , urea was suggested   to exhibit significantly reduced ion pairing interactions with bi(sulfate ) species than the corresponding guanidinium ions . however , it has to be stressed that it is predominantly sulfate ( not bisulfate ) species that becomes adsorbed on the pt(111 ) plane in sulfuric acid solution ( see recent works by su et al .
, by garcia - araez et al . , and by yeh et al . ) .
thus , in this case , no attractive interactions between the adsorbed , unprotonated urea molecules and sulfate species could be envisaged [ contrast to the behavior at the ( 100 ) plane , see structure  1 again ] .
consequently , for the ( 111 ) plane , the recorded displacement of the voltammetric profile in the presence of urea ( see fig .
7a in ref . ) is most likely the result of partial protonation of urea in the supporting electrolyte ( please note that about 10  % of u molecules at ph    1 exist in a cationic form [ 3 , 4 , 6 ] ) .
the above - made conclusions get support from the previously performed   ftir experiments in the presence of urea , which indicated considerably less extensive chemisorption of u on the pt(111 ) surface , as compared to that exhibited at the ( 100 ) plane .
the impedance behavior of urea , at a concentration of 1    10  m u in 0.5  m h2so4 at the pt(100 ) plane is shown in fig . 
thus , the impedance behavior of u at potentials close to that of the capacitive peak in fig . 
1 ( 200 , 250 , 300 , and 350  mv vs. rhe ) is characterized by the appearance of two partial semicircles and a capacitive line at an inclination to the z axis different from 90. the smaller semicircle ( see the impedance spectrum recorded at 300  mv in inset to fig . 
2a ) , observed at high frequencies , corresponds to the process of upd of h ( in relation to the charge transfer resistance , rh ) , and the part of a large - diameter semicircle ( see fig . 
2a again ) , observed throughout the intermediate frequency range , is associated with the charge transfer process ( with respect to the ru parameter in table  1 ) accompanying electrosorption of urea on the pt surface ( see eq .  1 below ) .
moreover , existence of two maxima is clearly discernible in the corresponding bode phase - angle plot in fig . 
2b ( please evaluate the fitting quality by the derived chi - squared :  parameter values given in table  1).fig . 3equivalent circuit for an adsorption process such as upd of h , exhibiting faradaic pseudocapacitance , cph , charged via a faradaic resistance , rh in the presence of co - adsorbed urea ( ru and cu components ) , in a parallel combination with the double - layer capacitance , cdl , jointly in series with an uncompensated solution resistance , rs ; w diffusional warburg element , employed as an empirical termtable 1resistance and capacitance parameters for upd of h and the process of electrosorption of u molecule ( at a concentration of 1    10  m ) on pt(100 ) plane in 0.5  m h2so4 ( recorded at 293  k ) , obtained by finding the equivalent circuit which best fitted the impedance data , as shown in fig .  3 (  refers to the recorded chi - squared parameter values by the zview fitting software)e / mvrh/ cm10cph / f  cm10cdl / f  cmru/ cm10cu / f  cm2000.45    0.01395.7    3.328.5    1.7267.0   
6.5304.5    6.34.7    102500.55    0.02378.8    3.836.5    1.9133.1    5.62,089 
  959.4    103001.85    0.03159.4    1.743.2    1.8148.8    3.2291.4    5.55.6    103504.70    0.2261.8    2.055.4   
2.2513.8    12.6205.2    3.97.2    10 equivalent circuit for an adsorption process such as upd of h , exhibiting faradaic pseudocapacitance , cph , charged via a faradaic resistance , rh in the presence of co - adsorbed urea ( ru and cu components ) , in a parallel combination with the double - layer capacitance , cdl , jointly in series with an uncompensated solution resistance , rs ; w diffusional warburg element , employed as an empirical term resistance and capacitance parameters for upd of h and the process of electrosorption of u molecule ( at a concentration of 1    10  m ) on pt(100 ) plane in 0.5  m h2so4 ( recorded at 293  k ) , obtained by finding the equivalent circuit which best fitted the impedance data , as shown in fig .  3
(  refers to the recorded chi - squared parameter values by the zview fitting software ) as compared to the case of pure 0.5  m h2so4 supporting electrolyte ( e.g. , see table 1 in ref . ) , the kinetics of the process of upd of h have become significantly slowed down in the presence of u , beyond the potential of 250  mv vs. rhe ( table  1 ) .
a significant increase of the rh parameter ( from 0.45    cm at 200  mv to 4.70    cm at 350  mv ) can be explained in terms of the ion pairing mechanism ( originally proposed for the g ions in ref . ) , where the presence of surface - adsorbed bisulfate species , arising at significantly lower electrode potentials together with co - adsorbed u molecules ( structure  1 ) , appreciably influences the kinetics of upd of h on this pt plane .
in addition , changes of the hydrogen adsorption capacitance ( cph ) strictly follow those of the rh parameter ; thus , the cph dramatically declines ( table  1 ) from 395.7  f  cm ( at 200  mv ) to 61.8  f  cm ( at 350  mv).1 on the other hand , the adsorption charge transfer resistance for urea ( ru ) reaches its minimum value of 133.1    cm at the potential close to that of the peak current  density in fig .  1 ( 250  mv ) , which implies that the kinetics of adsorption of u on the pt(100 ) plane are dramatically slower than those of upd of h on this surface .
again , minimum of the ru value coincides with a very large value ( 2,089  f  cm ) of the adsorption pseudo - capacitance parameter ( cu ) , recorded at the potential of 250  mv .
furthermore , the recorded double - layer capacitance values ( cdl ) in table  1 oscillate between ca .
these cdl values are considerably higher than 20  f  cm , i.e. , a commonly quoted double - layer capacitance value in literature for smooth and homogeneous surfaces [ 16 , 17 ] , which implies some contribution to the recorded cdl from the adsorption capacitance components ( table  1 ) .
moreover , an observed deviation from the purely capacitive , 90 phase angle behavior ( also expressed by  depressed 
the phenomenon of capacitance dispersion is typically visualized as corresponding either to slow adsorption / desorption processes or as the effect of increasing pt surface inhomogeneity , especially important when extensive potentiostatic impedance measurements take place [ 1820 ] .
all fittings of the recorded impedance data were performed by means of the double - adsorbate equivalent circuit ( see fig . 
3 ) with employment of a warburg diffusional element ( w ) , regarded here as an empirical term   involved in the kinetics of urea electrosorption on the pt(100 ) plane.fig . 4total capacitance ( ctotal ) as a function of the applied potential ( vs. rhe ) for the desorption of h and adsorption of u ( at a concentration of 1    10  m ) on pt(100 ) , in contact with 0.5  m h2so4 solution .
the solid line corresponds to the ctotal calculated from the cv profile ( ctotal  =  current  density / sweep rate ) and the experimental points are the impedance - derived values of total capacitance total capacitance ( ctotal ) as a function of the applied potential ( vs. rhe ) for the desorption of h and adsorption of u ( at a concentration of 1    10  m ) on pt(100 ) , in contact with 0.5  m h2so4 solution .
the solid line corresponds to the ctotal calculated from the cv profile ( ctotal  =  current  density / sweep rate ) and the experimental points are the impedance - derived values of total capacitance as an internal check on the impedance results , fig . 
4 below shows total capacitance , ctotal , where ctotal  =  cdl  +  cp ( adsorption pseudocapacitance components ) , as a function of potential for the adsorption behavior of u on the pt(100 ) single - crystal surface .
this figure compares ctotal obtained from the impedance results with that directly calculated from the voltammetric profile [ 5 , 22 ] under comparable experimental conditions ( 1    10  m  u ) .
it can be seen in fig .  4 that the ctotal values are in fairly good agreement with each other for all four  probing  potential values.structure 1ion pairing interactions between co - adsorbed urea molecules and hso4 ions ion pairing interactions between co - adsorbed urea molecules and hso4 ions with respect to the impedance behavior previously recorded in the presence of the guanidinium cation , the process of electrosorption of urea at the pt(100 ) plane is characterized by significantly increased values of the charge transfer resistance parameter ( ru vs. rg+ in ref . ) .
in fact , the ru / rg+ ratio comes to ca . 5.6 for the minimum electrosorption resistance values , recorded for u and g species , respectively .
the above strongly supports the formerly derived conclusions   on the ion pairing ( dipole  dipole type ) between the electrosorbed u molecules and bisulfate species being significantly weaker than the corresponding g / hso4 ( ion dipole ) interactions ( not particularly evident from the comparative : u vs. g cyclic voltammetry behavior for this pt plane ) .
in this respect , the electrochemical behavior of urea on the pt(100 ) plane is somewhat analogous to that recorded for n , n - dimethyl - guanidinium ( dmg ) cation in ref . .
application of ac impedance spectroscopy to study adsorption behavior of urea at pt(100 ) single - crystal surface provided support for the cooperative : u / hso4 electrosorption mechanism for u ( also with respect to significance of upd of h ) , based on the impedance - derived charge transfer resistance and capacitance components .
moreover , significantly increased values of the adsorption charge - transfer resistance for urea ( with respect to those previously recorded for guanidinium ion at pt(100 ) plane in ref . )
suggest considerably slower electrosorption of urea ( and respectively weakened u / hso4ion pairing ) than that previously reported for g on this pt plane .
 the present paper reports an alternate current impedance spectroscopic study on adsorption of urea ( u ) at pt(100 ) single - crystal surface , examined in 0.5  m h2so4 supporting electrolyte . the resulted information provided confirmation of the role of electrosorption of urea on the pt(100 ) plane through evaluation of the associated charge transfer resistance and capacitance parameters . 
 obtained impedance results were compared to those previously recorded for guanidinium cation ( g+ ) under analogous experimental conditions , especially with respect to the so - called ion pairing mechanism , as originally proposed for the g+ ion and bi(sulfate)/oh species , based on the voltammetric and in situ fourier transform infrared spectroscopy results . 
Introduction
Experimental
Results and discussion
Cyclic voltammetry behavior of urea at Pt(100) single-crystal plane in 0.5M H
AC impedance behavior of urea at Pt(100) plane in 0.5M H
Conclusions