Inf luence of Surface Structure of Pla tinum Electrodes on Electroox idation of CO 3

Transcription

1 CH EM. R ES. CH IN ESE U. 2003, 19 (4), Inf luence of Surface Structure of Pla tinum Electrodes on Electroox idation of CO 3 X IA X ing2hua 3 3, ZHAN G D ai and SON G Yan2yan S ta te K ey L abora tory of Coord ina tion Chem istry, Institu te of A na ly tica l S cience, N anj ing U n iveristy, N anj ing , P. R. C h ina R eceived O cṫ 24, 2002 T he oxidation of CO on p latinum electrodes in an acid so lution w as studied w ith the conventional electro2 chem ical m ethods and the on2line electrochem ical m ass spectro scopy. It w as found that th is reaction is strongly determ ined by the surface m o rpho logy of p latinum. T he p retreatm ent of p latinum electrodes can change the su rface p roperties dram atically, in con sequence it can im p rove the electrocatalytic activity tow ards the electrooxidation of CO. T he existence of surface active sites on the roughened p latinum electrodes can be used to exp lain its h igh electrocatalysis tow ards the oxidation of CO. Ke yw o rds E lectrocatalysis, P latinum, Surface p roperty, M echanism A rtic le ID (2003) In troduction T he studies of the adso rp tion and the ox ida2 tion of CO on nob le m etals have attracted m uch at2 ten tion of som e scien tists in the fields of electro2 chem istry and su rface science [1 4 ]. T h is is becau se it is, on one hand, an unexpected impurity in H 2 gas in now adays MW fuel cell un its [5 ], CO has been p roved to be the general po ison ing in term edi2 ate in the ox idation of sm all o rgan ic compounds (i. e., HCOOH, HCHO, and CH 3OH ) w h ich are su itab le fuels fo r direct ox idation fuel cells [6, 7 ]. O n the o ther hand, it is u sed to be a model mo lecu le to p robe the su rface p roperties of variou s su rface o ri2 en tation ṡ T here have been variou s statem en ts concern2 ing the po ten tial at w h ich the ox idation of CO starts and the range of the po ten tial over w h ich it occu rṡ It is genera lly know n tha t a lthough p la t2 inum is rega rded a s the best elect roca ta lyst, the ox idation of CO on smoo th p latinum in an acid so2 lu tion occu rs at a h igh over2po ten tial (ca V vs. RH E) due to the fo rm ation of strongly bound CO adso rbates [8, 9 ]. B inary o r m u lti2compo sition electrocatalysts w ere once p ropo sed to be u sed to overcom e the h igh over2po ten tial fo r CO ox idation. M o st in terestingly, such a h igh over2po ten tial cou ld be overcom e simp ly by changing the su rface p roperties v ia electrochem ical p re2treatm en ṫ K ita [8 ] et a l. repo rted that the po ten tial requ ired fo r the electroox idation of the disso lved CO on smoo th P t cou ld be low ered to ca V simp ly by ho lding the electrode po ten tial in the hydrogen region be2 fo re the adm ission of CO in to the electro lyte. Such phenom ena on po lycrystalline [9 ] and single crystal p latinum electrodes [10 ] w ere confirm ed lately by Gu tierrez and h is co2w o rkers. T hese au tho rs re2 po rted that fo r the electroox idation of a simp le mo lecu le CO, its electrochem ical behaviou r is qu ite comp licated. U nder certain experim en tal condi2 tion s, up to fou r peak s in CV du ring the anodic scan app ea red sim u ltaneou sly. T h is find ing led them to m ake a conclu sion tha t the th ree su rface species w ere p resen t on P t su rface con stitu ting a [11 14 ] comp lete mono layer. R ecen tly, Sun et a l. have ob served the abno rm al op tic p roperties of a dispersed p latinum layer in FT IR reflection spec2 tro scopy fo r CO adso rp tion. O n a dispersed p lat2 inum electrode, the natu re of the adso rbed CO is a2 gain b ridged and linearly bonded. How ever, the in2 ten sity of the IR adso rp tion bands is enhanced as compared w ith that on the smoo th electrodes. T hese facts imp ly unam b iguou sly that the su rface structu re of electrodes p lays an impo rtan t ro le in elect roca ta lysis. U p to now, system atical data on the ox idation of CO on p latinum electrodes w ith differen t struc2 3 Suppo rted by the N ationaln atural Science Foundation of Ch ina (N o , ) and the Starting Fund from the M inistry of Education of Ch ina. 3 3 To w hom co rrespondence should be addressed.

2 N o. 4 X IA X ing2hua et al. 475 tu res have no t yet been availab le. T herefo re, the aim of th is paper is to shed som e ligh t on the effect of su rface characteristics con tro lled by the p retreat2 m en t on the electroox idation of CO in acid so lu2 tion ṡ Bo th stationary and dynam ic electrochem ical m ethods w ere u sed to test the influence of the p re2 treatm en t on the electroox idation of the disso lved and ad so rbed CO in an acid ic m ed ium. T he en2 hancem en t in electrocatalytic activity tow ards the electroox idation of CO is discu ssed in term s of the su rface active sites created by the p retreatm en ṫ Exper im en ta l 1 Chem ica ls T he so lu tion s w ere p repared w ith M illipo re w ater (R > 18 M 8 ). T he suppo rting electro lytes w ere either 011 o r 015 mo lgl HC lo 4. A rgon 418 (991998%, M esser Griesheim ) w as u sed to deaer2 ate the so lu tion o r to elim inate the disso lved CO. 2 In strum en ta tion s A th ree2electrode flow cell con tro lled by a tra2 ditional th ree2electrode po ten tio stat w as u sed in the electrochem ical experim en ts, in w h ich p latinum sheets w ith geom etrical areas of ca. 2 cm 2 ( its real su rface area w ill be stated at respective p laces) and ca. 410 cm 2 w ere u sed as the w o rk ing electrode and the coun ter elect rode, resp ect ively. A reversib le hydrogen electrode w as u sed as the reference elec2 trode in all the experim en ts, and the po ten tials re2 po rted in th is paper refered to th is electrode in the sam e electro lyte. T he po ten tial of the electrochem 2 ical system w as con tro lled by a CH I electrochem ical w o rk station (CH In strum en ts, U SA ). T he real su rface area of the electrode w as determ ined in a so lu tion of 015 mo lgl H 2SO 4, by graph ical in tegra2 t ion of the cha rge under the hyd rogen ad so rp t ion peak from the doub le layer region to 0108 V at the cu rren t2po ten tial cu rve. A fractional coverage of 0177 w as adop ted, and the real su rface area w as calcu lated on the basis of assum ing that a mono lay2 er of the ad so rbed hyd rogen requ ires 210 ΛCg cm 2[15 ]. T he roughness facto r f of an electrode w as calcu lated by m ean s of equation (1) : f = A realga geo (1) w here A real and A geo are the real and the geom etric su rface areas of an electrode, respectively. O n2line m ass spectrom etry (D EM S) w as u sed to co rrela te the cu rren t s, ob served du ring the vo ltammogram m easu rem en t of the electroox i2 dation of CO in an acidic so lu tion on po rou s po ly2 crystalline p latinum electrodes w ith a copo lym er m em b rane pain ted o r sputtered (SC IM A T 200g40g 60, m ean th ickness: 60 Λm, po ro sity: 50%, and m ean po re size: Λm ), w ith the vo latile reac2 tion p roducts. T he experim en tal details of th is m ethod are described elsew here [15, 16 ]. Results 1 Genera l Fea tures of Pla tinum Electrodes with D ifferen t Rough Surfaces T he electrocatalytic ox idation of CO satu rated in so lu tion at p latinum electrodes w ith differen t roughness facto rs in 011 mo lgl HC lo 4 is show n in F ig11. F ig11 Cyclic voltammogram s for the electroox idation of the d issolved CO ( saturated and with CO bubbl ing) on Pt electrodes with d ifferen t surface roughness factors(r f) in a solution of 011 m olgl HClO 4. Scan rate of 10 mv gs. (A ) Smoo th P t (R f = 211 ) ; (B) sputtered P t on copo lym er m em brane (R f= 200). A s can be seen from F ig11, the electrocatalytic activity fo r CO ox idation depends strongly on the su rface structu re of electrodes. Fo r smoo th p lat2 inum elect rodes w ith a roughness facto r of 211 [ F ig11 (A ) ], the electroox idation of CO show s a sharp peak at abou t 0190 V in the anodic sw eep. Below th is po ten tial, the anodic cu rren t app roaches zero due to the po ison ing effect of a mono layer of the adso rbed CO. A t the po ten tials above 0190 V, the strongly bound CO is comp letely ox idised due to the fo rm ation of the su rface ox ides. In con se2 quence, the ox idation of the disso lved CO reaches its diffu sion2lim ited level. O n ly at the po ten tial

3 4 76 CH EM. R ES. CH IN ESE U. V o l. 19 over V does the cu rren t decrea se aga in be2 cau se of the fo rm ation of the strongly bound su r2 face ox ide (i. e., P to 2) w h ich po ison s the electrode su rface as w ell. D u ring the cathodic scan, the dif2 fu sion cu rren t m ain tain s a po ten tial at w h ich nearly half of the su rface ox ides is reduced. A t th is po ten2 tial the cu rren t exh ib its a m ax im um, th is fact is in good agreem en t w ith the reactan t pairs m echan ism suggested by Gilm an [17 ]. W hen the electrode po ten2 tial reached to a m uch low er value, the electrode su rface w as qu ick ly covered by CO adso rbate and lo st its activity again as indicated by the decreased cu rren ṫ In con trast, the electroox idation of CO on the rough su rfaces took p lace already at the po ten2 tials in the doub le layer region [ F ig11 (B ), w ith roughness facto r = 200 ) a s ind ica ted by the in2 creased cu rren ṫ It w as also ob served, how ever, that at mo re po sitive po ten tials the ox idation cu r2 ren t per real su rface is m uch low er than that at the smoo th p latinum electrodes due to the slow diffu2 sion rate of CO in to the po res of the roughed p lat2 inum electrodes. In addition, the sp ike at 0190 V fo r CO ox idation on the smoo th electrodes decreas2 es rap idly w ith the increase of the roughness facto r of the electrodes. T h is imp lies that there ex ists a certa in co rrela t ion betw een the app ea rance of the h igh electrocatalytic activity at a low po ten tial and the roughness facto r of an electrode. B y com p a ring the resu lt s a t the rough elec2 trodes w ith tho se at the smoo th ones, som e o ther featu res in CV are w o rth emphasizing. F irstly, in the p resence of CO in the so lu tion ( satu rated con2 cen tration ), the ex ten t of the supp ression of the adso rp tion and the deso rp tion peak s of hydrogen depends strongly on the su rface structu re of elec2 t rodes. Fo r the elect rodes w ith roughness facto rs low er than 2010, the adso rp tion and the deso rp tion of hydrogen in CV at 10 mv gs is comp letely sup2 p ressed [F ig11 (A ) ]. Fo r the electrodes w ith a larg2 er roughness facto r ( i. e., f = 200), on ly a sm all part of the hydrogen adso rp tiongdeso rp tion is sup2 p ressed. A fu ll mono layer of CO ad can on ly be ob2 tained at a longer adso rp tion tim e, i. e., 10 m in is m uch longer than that needed fo r a po ten tial scan at 10 mv gṡ O bviou sly, the m ass tran sfer in the po res of po rou s electrodes lim its the rate of CO ad2 so rp tion [F ig11 (B ) ]. 2 Effect of Adsorption Poten tia l on the Electroox ida tion of CO W e stud ied the dep endence of the elect roca t2 alytic activity of p latinum tow ards CO ox idation on the po ten tial at w h ich CO w as adm itted in to the electro lyte. T he po ten tial of the smoo th po lycrys2 talline P t in a 011 mo lgl HC lo 4 so lu tion w as held at a certain value (E ad) befo re the anodic po ten tial sw eep ing, then argon w as bubb led fo r th ree m in2 u tes. Sub sequen tly, CO w as bubb led fo r 400 s (th is adso rp tion tim e is long enough fo r the fo rm ation of a fu ll mono layer of CO on a smoo th p latinum elec2 trode). T hen the po ten tial sw eep ing in the CO sat2 u rated so lu tion w as resum ed at 20 mv gs in a qu ies2 cen t electro lyte. T he resu lts are show n in F ig12. T he cu rren t2po ten tial cu rves clearly show that the electrocatalytic activity of the P t electrode tow ards the CO elect roox ida t ion dep end s st rong ly on the F ig12 The f irst cyclic voltammogram s for the ox idation of the dissolved CO at smooth Pt electrodes in a solution of 015 molgl HClO 4 at differen t adsorption poten tials(e ad) of CO. Scan rate: 20 mv gs. E adgmv : a. 80; b. 200; c. 285; d adso rp tion po ten tial (E ad ). T he ox idation cu rren t fo r CO electroox idation at a low er po ten tial in2 creased w ith the decrease of the adso rp tion po ten2 tial. In addition, the adso rp tion po ten tial E ad af2 fects no t on ly the shape of vo ltammogram bu t also the on set and the peak po ten tials. W hen the adm is2 sion po ten tial w as h igher than 0135 V, on ly a sin2 gle sharp cu rren t peak (dashed2do tted cu rve) ap2 peared at ca. 019 V in the first anodic vo ltammo2 gram [ F ig11 (A ) ]. Below th is po ten tial almo st no cu rren t w as ob served due to the po ison ing effect of the strongly adso rbed CO. In terestingly, w hen the adm ission po ten tial of CO w as set at a value below 0130 V, tw o anodic cu rren t peak s in the first anod2 ic vo ltammogram appeared (F ig12). T heir peak po2 ten t ia ls sh ifted to a low er va lue a s the adm ission po ten tial of CO decreased. T he peak cu rren t of the first peak increases w ith the decrease of the amoun t of CO adm ission, w h ile the cu rren t of the second peak decreases. T h is fact demon strates that the in2 crease of the CO ox idation cu rren t of the first peak

4 N o. 4 X IA X ing2hua et al. 477 occu rs at the expen se of that of the second peak. W hen the adso rp tion po ten tial of CO w as set at 0108 V (F ig12a), the ox idation cu rren t started al2 ready at 0122 V, and the second peak sp lit in to tw o peak s ( the natu re of the sp ike w ill be discu ssed in the D EM S section). Such comp licated peak charac2 teristics disappeared in the sub sequen t vo ltammo2 gram s, and on ly one cu rren t peak sim ilar to the dashed2do tted cu rve of F ig12 appeared irrespective of differen t adso rp tion po ten tials of CO, nam ely, the ox idation of the disso lved CO occu rred on ly af2 ter a mono layer of the adso rbed CO w as ox idised. T hese phenom ena demon strate that the in teresting su rface st ructu ra l cha racterist ics of the sm oo th p la t inum elect rode a re crea ted by the d ifferen t ho lding po ten tials at w h ich the CO adm itted is re2 versib le and is, how ever, operative on ly in the first scan, i. e., such a special su rface structu re disap2 pears by the po ten tial sw eep in the oxygen region o r the repeated CO adso rp tion. 3 Effect of Hold ing Tim e on the Ox ida tion of Strongly Adsorbed CO T he influence of the ho lding tim e on the elec2 troox idation of CO w as also ob served. A n electrode w ith a longer ho lding tim e show ed a m uch h igher electrocatalytic activity. It w as found that the peak cu rren t at a low er overpo ten tial increased linearly w ith the increase of square roo t of the scan rate. In addition, a linear relation sh ip betw een the peak cu rren t and the square roo t of the ro tating frequen2 cy w as also ob served. Bo th the resu lts clearly demon strate that the peak at a low er overpo ten tial is due to the diffu sion2lim ited CO ox idation. T he resu lts fo r the influence of the cu rren t on the ox i2 dation of CO at the po ten tials in the first peak show that the ob served ox idation cu rren t w as sta2 b le du ring ou r experim en t tim e period of 8 h. T he influence of the ho lding tim e on the elec2 troox idation of the adso rbed CO w as investigated. T he electrode w as held at 0108 V du ring the anodic po ten tial scan and the so lu tion w as then bubb led w ith argon fo r differen t length s of tim e. A fter the adso rp tion tim e of CO lasted fo r 15 m in, the so lu2 tion w as again bubb led w ith argon fo r ano ther 15 m in in o rder to elim inate the disso lved CO. T hen an anodic po ten tial scan w as started from 0108 V at 20 mv gṡ T he resu lts are show n in F ig13. Gener2 ally, there ex ist tw o cu rren t peak s co rresponding to the elect roox ida t ion of the st rong ly ad so rbed CO. A sm all shou lder at the h igher po ten tial side F ig13 The f irst voltammogram s of the electroox idation of the adsorbed CO formed at 0108 V with differen t holding time on a smooth platinum electrode in a solution of 011 m olgl HClO4 at a scan rate of 20 mvgṡ T he electrode w as held at 0108 V fo r 0 (a), 400 (b) and 1600 s (c) argon bubbling, and then CO w as adm itted into the so lution fo r 10 m in. A fter the disso lved CO w as removed w ith argon bubbling fo r 15 m in, a CV w as re2 sum ed at a scan rate of 20 mv gs. of the m ain cu rren t peak is ob served. T he peak po2 ten tial fo r the first peak sh ifts to a low er value w ith increasing the ho lding tim e (see the in set in F ig13), w h ile the peak cu rren t show s no obviou s depen2 dence on the ho lding tim e. Such a negative sh ift of the peak po ten tial reach s a m ax im um w hen the ho lding tim e is 1600 s. In con trast, the ho lding tim e does no t affect the peak po ten tial of the sec2 ond m ain peak, bu t show s a strong effect on the peak cu rren ṫ It has to be emphasised that the on2 set po ten tial of bo th the peak s are sh ifted tow ards low er values. A t the ho lding tim e ta r= 1600 s, the electroox idation of the adso rbed CO takes p lace al2 ready at 0122 V. T he resu lts clearly demon strate that the ho lding tim e has a strong effect on the su r2 face compo sition of the mono layer of the adso rbed CO and in tu rn affects the electrocatalytic activity tow ards the electroox idation of the disso lved CO. D iscussion T he adso rp tion of CO on a smoo th p latinum electrode at a low er adso rp tion po ten tial occu rs very fast, and a fu ll mono layer of the adso rbed CO is comp leted w ith in a tim e sho rter than 5 m in. O n such a CO 2covered elect rode su rface, there is no free p latinum site availab le fo r the direct electroox2 idation of CO in the so lu tion. A t first sigh t, w e shou ld no t ob serve the diffu sion2lim ited cu rren t fo r the electroox idation of the disso lved CO. How ever, in the study of the effect of the ho lding tim e on the electroox idation of the adso rbed CO, w e found that

5 4 78 CH EM. R ES. CH IN ESE U. V o l. 19 w hen the adso rp tion po ten tial of CO is set in the po ten tial region fo r hydrogen adso rp tiongdeso rp2 tion, u sually a p repeak befo re the m ain cu rren t peak is ob served (F ig13). W hen the adso rp tion po2 ten tial of CO w as set at a po ten tial beyond the hy2 drogen region, on ly the m ain electroox idation peak appeared (no t show n here). O u r FT IR resu lts of the electroox idation of m ethano l on single crystal p latinum electrodes show that ou t of the fu ll mono2 layer of CO fo rm ed in the hyd rogen reg ion, tw o differen t types of CO adso rbates ex ist, nam ely, the b ridged bound CO and the linearly bound CO [6, 18 ]. W hen the mono layer of the adso rbates fo rm ed at a po ten tial beyond the hydrogen region, on ly the lin2 early bound CO w as ob served. T hat does no t m ean that the m ain cu rren t peak co rresponds to the elec2 troox idation of the linearly bound CO and the p re2 peak to the electroox idation of the b ridged bound CO since the FT IR resu lts show bo th the b ridged bound CO and the linearly bound CO co rresponding to the p repeak. How ever, on the basis of the po2 ten tial dependen t band sh ift of the b ridged bound CO and the linearly bound CO, w e found that there ex ist tw o differen t CO dom ain s, nam ely, the is2 land2like CO and the loo sely d ist ribu ted CO. T he electroox idation of the CO in loo se state accoun ts fo r the p repeak, and the electroox idation of the CO in the m anner of island fo r the m ain cu rren t peak. T he electroox idation of the adso rbed CO co rre2 sponding to the p repeak w ill release the free p lat2 inum sites. T hese free p latinum sites can, in tu rn, be u sed fo r the direct electroox idation of the dis2 so lved CO in the so lu tion. R ecen tly, po ten tial dependen t bands fo r the OH stretch ing and HOH bending modes of w ater have been ana lyzed and in terp reted in term s of the in teraction betw een w ater mo lecu les and the elec2 tric field of the doub le layer and w ith the m etal su r2 face [19 ]. T he sh ift of the band fo r the bending mode at the po ten tials in the range of V (vs. RH E ) is indicative of O 2adso rbed w ater mo lecu les suffering from a p rog ressive o rien ta t ion of the mo lecu lar p lane from a tilted to a perpendicu lar po2 sition. W ith increasing anodic po ten tials w ater dipo les acqu ire a configu ration of the m in im um in2 teraction energy w ith the electric field on the in ter2 face. T h is geom etry cau ses an increasing participa2 tion of the 3a1 o rb it of w ater in the su rface bond. T he in teraction leads to a partial dissociation of w a2 ter, p roducing the adso rbed OH. T hu s it is gener2 a lly accep ted tha t the elect roox ida t ion of the ad2 so rbed CO occu rs w ith the dissociation of w ater due to the reaction of CO w ith OH. How ever, it is dif2 ficu lt to exp lain the ob served electroox idation cu r2 ren t of CO at a po ten tial of 0120 V. T h is on set po2 ten tial fo r CO electroox idation is m uch low er than 014 V at w h ich the su rface OH species is fo rm ed. T herefo re, w e m u st con sider the participation of w ater mo lecu les near the adso rbed CO in the elec2 t roox ida t ion of the ad so rbed CO on the elect rode su rface. In th is m echan ism, the CO in the loo sely adso rbed state reacts w ith its nearest w ater mo lecu les to give the final p roduct CO 2 and in the m ean tim e to resu lt in one free p latinum site. T h is free p latinum site is in tu rn ready fo r the adso rp2 tion of CO from the so lu tion. In th is w ay, the elec2 troox idation of the disso lved CO in the so lu tion at a po ten tial as low as 0120 V can be expected. T he elect roox ida t ion of the ad so rbed CO occu rs very fasṫ In the w ho le reaction p rocess, the diffu sion of CO from the so lu tion to the electrode su rface is the ra te2determ in ing step. T herefo re, a d iffu sion2con2 tro lled ox idation cu rren t is ob served. Based on the above reaction m echan ism, the enhanced electrocatalytic activity of a smoo th p lat2 inum electrode tow ards the electroox idation of the disso lved CO simp ly by ho lding the electrode in the po ten tial region fo r the adso rp tiongdeso rp tion of hydrogen can be exp lained as fo llow s. (1) Ho lding the elect rode in hyd rogen reg ion p roduces su rface active sites due to the fast reaction of adso rp tion and deso rp tion of hydrogen atom ṡ (2) T hese new fo rm ed active sites are in equ ilib rium w ith the no r2 m al sites on the electrode su rface. T hey w ill be stab ilized by the adso rp tion of CO, bu t deactivated by the fo rm ation of su rface ox ides at h igher po ten2 tials. (3) T he num ber of these sites increases w ith the ho lding tim e. (4) T he CO mo lecu les adso rbed on these active sites in loo sely adso rbed CO state, w h ich can be ox idised at low er overpo ten tials, are the p recu rso r fo r the direct ox idation of the dis2 so lved CO. T he ox idation of the p recu rso r is v ia an in teraction of the p recu rso r w ith the w ater mo lecu les nex t to them, bu t no t the adso rbed OH species w h ich is fo rm ed at a m uch h igher po ten tial. In term s of th is model, it is suggested that such ac2 t ive sites do ex ist a lready on the rough su rface of the electrodes. T he num ber of such active sites is m uch b igger on a rougher electrode than that on a smoo th electrode. T herefo re, the po rou s electrodes

6 N o. 4 X IA X ing2hua et al. 479 show a m uch h igher elect roca ta lyt ic act ivity to2 w ards the ox idation of CO at low er overpo ten tials than smoo th ones (F ig11). In reference[ 9 ], Caram and Gu tierrez repo rted that under certain experim en tal condition s up to fou r peak s in CV of a smoo th p latinum electrode in a CO satu rated so lu tion of 015 mo lgl HC lo 4 du ring anod ic scan app ea red sim u ltaneou sly. T hese au2 tho rs conclude tha t the first p eak loca ted a t ca V co rresponds to the electroox idation of the disso lved CO in the so lu tion, its electroox idation cu rren t is diffu sion2con tro lled by the disso lved CO. To exp lain th is finding of the ox idation of the dis2 so lved CO at a p latinum electrode covered w ith a mono layer of the adso rbed CO, Caram and Gu tier2 rez po stu late that under their experim en tal condi2 tion s, th ree su rface species are p resen t on the P t su rface con stitu ted in a comp lete mono layer. Species 1, although m ino r, can m ediate the dis2 so lved CO ox idation on the ox ide2free P t; species 2 is ox idized at a po ten tial of 0120 V w h ich is mo re po sitive than it is needed fo r the ox idation of the disso lved CO and therefo re can no t m ediate th is re2 action. T he electron tran sfer relating to the ox ida2 tion of the disso lved CO takes p lace th rough a layer of the inactive species 2. Species 3 co rresponding to the sp ike (peak 3, located at ca V ) is a true po ison ing in term ediate fo r the CO ox idation, its appearance is at the expen se of o ther peak ṡ How 2 ever, FT IR resu lts clearly show that there are on ly tw o su rface species ex isting in the mono layer of the adso rbed CO. In o rder to understand the natu re of the sp ike located at 0190 V (F ig11), the sam e experim en ts as show n in F ig12 bu t on a sputtered p latinum elec2 trode (roughness facto r of 190) fo r diffren tial elec2 trochem ical m ass spectro scopy (D EM S ) in 011 mo lgl HC lo 4 w ere perfo rm ed. D u ring the anodic po ten tial scan the po ten tial w as held at 0108 V at w h ich argon w as bubb led fo r 200 ṡ A fter the ad2 so rp tion tim e of CO lasting fo r 10 m in, a CV w as resum ed in the CO satu rated so lu tion at 10 mv gṡ the resu lt s a re show n in F ig14. O n the CV ob2 tained by m ean s of th is po rou s p latinum electrode, th ree peak s appeared. T he first cu rren t peak is lo2 ca ted a t 0148 V a s in F ig12, the second cu rren t peak is located at 0169 V, and the sp ike is located a t 0190 V. T he cu rren t of the first p eak ha s a som ew hat sm aller value as compared w ith the re2 su lts in F ig11, w h ich can be due to the fact that the concen tration of the disso lved CO at the electro lyte side is relatively sm aller in the D EM S experim en t becau se of the p ressu re difference on the in terface betw een the electro lyte and the m ass spectrom eter. T he sim u ltaneou sly reco rded m ass signal (M SCV ) fo r CO 2 [m ge= 44, F ig14 (B ) ] delivers m uch mo re info rm ation. In the M SCV there appear on ly tw o CO 2 peak ṡ F rom the location s of the peak po ten2 tials it can be concluded that they co rrespond to the first tw o peak s in CV. Fo r peak g, there is no co rresponding CO 2 signal ob served, there is no o th2 er detectab le m a ss signa l either. A cco rd ing ly, peak s g and g co rrespond to the ox idation of CO, and Peak g resu lt s from a nonfa rada ic p rocesṡ Based upon the fact that peak g appears at a rela2 tively h igher po ten tial, the po ssib le sp ike is due to a delayed fo rm ation of the su rface ox ides becau se of the inh ib ition by the adso rbed CO, as p reviou sly [20 repo rted by W o lter and H eitbaum et a l. ]. F ig14 The f irst cyclic voltammogram s for the electrooxidation of the dissolved CO at a porous platinum electrode ( sputtered) in a CO saturated solution of 011 molgl HClO 4. The experim ental p rocedures are the sam e as tho se in F ig12 but the CO adso rp tion at V fo r 3 m in after 100 s argon bubbling. a. The current2po tential p rofile ( CV ) ; b. the sim ultaneously reco rded m ass signal (M SCV ) fo r CO 2, m ge= 44. Conclus ion s W e have demon strated that the electrocatalytic activity of a p latinum electrode tow ards the elec2 troox idation of CO cou ld be ach ieved simp ly by set2

7 4 80 CH EM. R ES. CH IN ESE U. V o l. 19 ting the adm ittance po ten tial of CO at a po ten tial in the hydrogen region. In th is w ay, the overpo ten tial fo r the elect roox ida t ion of the d isso lved CO is sigan ifican tly low ered. T he electrocatalytic activity increases w ith low ering the adm ittance po ten tial of CO. T he enhanced electrocatalytic activity can be ob served under bo th po ten tio static and po ten tiody2 nam ic condition ṡ Such a p romo ting effect can be due to the crea t ion of su rface act ive sites fo r the electroox idation of the adso rbed CO at low er po ten2 tials. T he ex istence of su rface active sites can also be u sed to exp la in the ob served h igh elect roca t2 alytic activity of a po rou s p latinum electrode to2 w ards the electroox idation of CO. Acknowledgem en t W e thank P rof. T. Iw asita and P rof. W. V ielstich f or their v a luable d iscussions and the ava ilability of the D EM S equ ipm en t a t U n iversity of B onn. Re fe re nce s [ 1 ] M arkovic N. M., Grgur B. N., L ucas C. A., et a l., L ang2 m u ir, 2000, 16, 1998 [ 2 ] PerezM. C., R incon A., Gutierrez C., J. E lectroanal. Chem., 2001, 511, 39 [ 3 ] Jusys Z., Kaiser J., Behm R., J. P hy ṡ Chem. Chem. P hy ṡ, 2001, 3, 4650 [ 4 ] Schm idt T. J., Ro ss P. N., M arkovic N. M., J. P hy ṡ Chem. B, 2001, 105, [ 5 ] Stonehart P., B er. B unseng eṡ P hy ṡ Chem., 1990, 94, 913 [ 6 ] X ia X. H., Iw asita T., Ge F. Y., et al., E lectroch im. A c2 ta, 1996, 41, 711 [ 7 ] V ielstich W., Iw asita T., D albeck R., H and book f or H etro2 g eneous Cataly sis, V o l. 17, Eds., E rtl G., Kno β zinger, V CH, N ew Yo rk, 1995, [ 8 ] Kita H., Sh im azu K., Kunim atsu K., J. E lectroanal. Chem., 1988, 241, 163 [ 9 ] Caram J. A., Gutierrez C., J. E lectroanal. Chem., 1991, 305, 259 [ 10 ] W ieckow sk i A., Rubel M., Gutierrez C., J. E lectroanal. Chem., 1995, 382, 97 [ 11 ] L u G. Q., Sun S. G., Chen S. P., et al., E lectrochem. S oc. P roc., 1996, 96, 8 [ 12 ] Gong H., Chen S. P., Zhou Z. Y., et al., Ch in. S ci. B ull., 2001, 46, 1612 [ 13 ] Zheng M. S., Sun S. G., J. E lectroanal. Chem., 2001, 500, 223 [ 14 ] L in W. G., Sun S. G., Zhou Z. Y., et al., J. P hy ṡ Chem. B, 2002, 106, [ 15 ] X ia X. H., Iw asita T., J. E lectrochem. S oc., 1994, 140, 2559 [ 16 ] X ia X. H., V ielstich W., E lectroch im. A cta, 1994, 39, 13 [ 17 ] Gilm an S., J. P hy ṡ Chem., 1964, 68, 70 [ 18 ] Iw asita T., N art F. C., L opez B., et al., E lectroch im. A c2 ta, 1992, 37, 2361 [ 19 ] Iw asita T., X ia X. H., J. E lectroanal. Chem., 1996, 411, 95 [ 20 ] W o lter O., H eitbaum J., B er. B unseng eṡ P hy ṡ Chem., 1984, 88, 6