Interaction between Residues of Different Organic Compounds on Platinum: A Mass Spectrometric Study

ácido fórmico e álcool propargílico a do ácido fórmico e o etanol platina meio ácido foram estudados por espectrometria de massa “on line” (DEMS). oxidação espécies co-adsorvidas que permaneceram na superfície eletrodo após experimento célula de fluxo resultou somente CO 2 . Usando H 13 COOH isotopicamente modificado, a contribuição ácido fórmico du rante a eletro-oxidação dos co-adsorvatos pode ser distinguida daquelas do etanol ou de resíduos do álcool propargílico. Foi verificado que o etanol substitui os resíduos do ácido fórmico, enquanto que a absorção do álcool propargílico é modificada pela presença do ácido fórmico na superfície. O ácido fórmico não pode quimisorvido numa superfície de platina coberta por resíduos do álcool propargílico, reage sem haver substituição platina modificada por resíduos de etanol. the elec trode sur face af ter a flow-cell ex per i ment yields only CO 2 . Using iso to pi cally la beled H 13 COOH, the con tri bu tion of for mic acid dur ing the elec tro-oxidation of the coadsorbates can be dis tin guished from that of eth a nol or propargyl al co hol res i dues. It is found that eth a nol re places for mic acid res i dues, whereas the ad sorp tion of propargyl al co hol is mod i fied by the pres ence of for mic acid on the sur face. For mic acid can not chemisorb on a plat i num sur face cov ered by propargyl al co hol res i dues, but re acts with out re place ment with plat i num mod i fied by eth a nol res i dues.

The con sec u tive ad sorp tion of for mic acid and propargyl al co hol, as well as that of for mic acid and eth a nol on plat i num in acid me dia were stud ied by on-line mass spec trom e try (DEMS). Ox i da tion of the coadsorbed spe cies re main ing on the elec trode sur face af ter a flow-cell ex per i ment yields only CO 2. Using iso to pi cally la beled H 13 COOH, the con tri bu tion of for mic acid dur ing the elec tro-oxidation of the coadsorbates can be dis tin guished from that of eth a nol or propargyl al co hol res i dues. It is found that eth a nol re places for mic acid r es i dues, whereas the ad sorp tion of propargyl al co hol is mod i fied by the pres ence of for mic acid on t he sur face. For mic acid can not chemisorb on a plat i num sur face cov ered by propargyl al co hol r esi dues, but re acts with out re place ment with plat i num mod i fied by eth a nol res i dues.
Key words: or ganic adlayers, mass spec trom e try, iso to pic la bel ing, ad sor bate replace ment

In tro duc tion
The na ture of the cat a lytic poi sons formed dur ing the suc ces sive ad sorp tion of dif fer ent or ganic com pounds obvi ously de pends strongly on the struc ture of the ini tial compound 1-6 , and con se quently, the elec tro-oxidation of these res i dues oc curs in dif fer ent po ten tial re gions, de pend ing on the com po si tion of the adlayer. The aim of the pres ent work is to elu ci date the pro cesses tak ing place when a sec ond com pound re acts on a sur face al ready mod i fied by the presence of a first layer of or ganic res i dues. With this pur pose, three sim ple mol e cules were cho sen in or der to study the inter ac tion be tween their res i dues on plat i num: for mic acid (HCOOH) 1,2 , eth a nol (CH 3 CH 2 OH) 3 , and propargyl al cohol (HC ≡CCH 2OH) 4-6 .
The above-mentioned com pounds form dif fer ent adsor bates on the Pt sur face. For mic acid pro duces mainly CO ad 1 , al though COH ad spe cies may also be pres ent 2 . CO ad ox i da tion oc curs in the dou ble layer re gion. In the case of eth a nol, the res i dues are also mainly ox i dized in the dou ble layer re gion. How ever, a con tri bu tion in the plat i num ox ide re gion may be ob served. Al though CO ad is formed, most of the ad sor bates con tain the C-C chain (O-CH 2 -CH 3 , COCH 3, and =COHCH 3 ) 3 . A max i mum cov er age of 0.85 is at tained at an ad mis sion po ten tial of E ad = 0.35 V for eth anol, and at E ad = 0.15-0.35 V for for mic acid 7 . Ad sor bates from propargyl al co hol re tain the C 3 struc ture, with only a very small amount of CO ad be ing de tected 6 . Com plete cover age was ob served for E ad > 0.15 V 6,8 , ox i da tion of these spe cies occuring at po ten tials in the plat i num ox ide re gion.
Ac cord ing to pre vi ous stud ies, the res i dues from for mic acid, eth a nol, and propargyl al co hol on plat i num in acid me dia are dif fer ent, and there fore, changes should be expected when one of these com pounds re acts on plat i num modifed by the res i dues of one of the other com pounds. Dif fer en tial elec tro chem i cal mass spec trom e try (DEMS) was se lected as the ap pro pri ate tech nique for these stud ies.

Ex per i men tal
The so lu tions were pre pared with Millipore-MilliQ * wa ter and an a lyt i cal grade chem i cals. 0.1 M HCOOH, CH 3 CH 2 OH and HC ≡CCH 2 OH were added to the sup porting elec tro lyte (0.5 M H 2 SO 4 ). Iso to pically-labeled 13 C for mic acid (Cam bridge Iso tope Lab o ra tories, 13 C 99%, chem i cal pu rity 94.5%) was em ployed with out fur ther puri fi ca tion. All ex per i ments were per formed at room temper a ture un der ar gon at mo sphere.
The elec tro chem i cal cell was a flow cell con tain ing approx. 2 cm 3 so lu tion. The work ing elec trode was a plat inum layer sput tered on a microporous PTFE mem brane (Scimat 200/40/60). The real area, mea sured by H ad sorption, var ied be tween 4 and 20 cm 2 . The elec trode was ac tivated by po ten tial cy cling at 0.10 V s -1 in the sup port ing elec tro lyte so lu tion be tween the on set po ten tials for hy drogen and ox y gen evo lu tion. A plat i num wire was the coun ter elec trode, and a re vers ible hy dro gen elec trode (RHE) in the sup port ing elec tro lyte was used as the ref er ence. The DEMS cell was di rectly at tached to the vac uum cham ber con tain ing the mass spec trom e ter (Balzers QMG 112) with a Far a day cup de tec tor. More de tails have been de scribed else where 7,8 .

Ex per i men tal pro ce dure
Af ter ac ti va tion of the elec trode, the po ten tial was set at the ad mis sion po ten tial E ad = 0.30 or 0.35 V, the so lu tion con tain ing the first or ganic com pound was in tro duced into the cell and the cur rent tran sient was re corded for 3 min. The or ganic so lu tion was then com pletely re placed by pure sup port ing elec tro lyte at E ad . This pro ce dure was re peated for the coadsorption of a sec ond com pound in creas ing the ad sorp tion time to 10 min. The charge den si ties ob served upon ad mis sion of each com pound, Q t1 and Q t2 , were obtained by in te gra tion of the cur rent tran sients. Finally, a for ward po ten tial scan start ing at E ad and go ing up to 1.50 V was per formed at a scan rate of 0.01 V s -1 . Suc ces sive cyclic voltammograms (CVs) and mass spec tro met ric cy clic voltammograms (MSCVs) for CO 2 were si mul ta neously re corded be tween 0.05 and 1.50 V. The to tal charge den sity in volved in the elec tro-oxidation pro cess Q ox was cal culated by in te grat ing the an odic cur rents in the CVs and substracting the plat i num ox ide cur rent. The in te grated ion charge, Q I , was ob tained from the MSCVs.

Re sults and Dis cus sion 1 st HCOOH 2 nd HC ≡CCH 2 OH con sec u tive ad sorp tion
A small an odic cur rent tran sient of 6 µC cm -2 was obtained dur ing the ad sorp tion of propargyl al co hol at E ad = 0.30 V on a plat i num sur face cov ered by for mic acid res i dues (Ta ble 1). The CV (solid line in Fig. 1) dis plays two con tri bu tions, at 0.78 V (in the dou ble layer re gion) and at around 1.13 V (in the po ten tial re gion of Pt ox ide forma tion). In or der to es tab lish the dif fer ences be tween this 108 Pas tor et al. J. Braz. Chem. Soc.

Ta ble 1.
Charge den si ties (Q t) of the tran sients pro duced by ad mis sion at E ad = 0.30 V of for mic acid, propargyl al co hol, and the two com pounds added suc ces sively (see text), and the an odic charge den sity (Q ox ) and peak po ten tials (E p1 and E p2 ) ob tained in a sub se quent CV.  adlayer and those ob tained for each com pound sep a rately, the in di vid ual ad sorp tion of for mic acid and propargyl alco hol was per formed at the same E ad . Ad sorp tion charges and peak po ten tials for the first for ward po ten tial scan are sum ma rized in Ta ble 1.
The value of Q t1 for the cur rent tran sient at 0.30 V of pure for mic acid on a "clean" plat i num sur face, i.e . not covered with or ganic res i dues, is 1500 µC cm -2 , much higher than a 2emonolayer (~400 µC cm -2 ), which means that at this E ad , bulk ox i da tion of for mic acid oc curs si mul ta neous with the ad sorp tion. Dur ing the first an odic strip ping voltammogram, a peak at 0.76 V with a charge den sity of 230 µC cm -2 ap pears. In the case of pure propargyl al co hol, the an odic tran sient, also at 0.30 V, is much smaller (Q t1 = 40 µC cm -2 ), but Q ox is sig nif i cantly higher (730 µC cm -2 ) 6,7 . The ox i da tion takes place in the plat i num ox ide region, pro duc ing a broad peak at 1.18 V 6,7 .
Ac cord ing to the above re sults for the in di vid ual adsorp tion of for mic acid and propargyl al co hol, the peak at around 0.80 V in the CV for the strip ping of the coadsorbed layer ( Fig. 1 -solid line) seems to cor re spond to the ox i dation of for mic acid res i dues, whereas the an odic peak at 1.13 V could be as signed to propargyl al co hol res i dues. Com paring the value of Q ox = 520 µC cm -2 ob tained from the 1 st HCOOH 2 nd HC ≡CCHOH coadsorption pro cess with Q ox for for mic acid (see Ta ble 1), the charge den sity in creases by 290 µC cm -2 . In pre vi ous re search, a max i mum cov er age of 0.85 was es tab lished for pure for mic acid res idues at E ad = 0.30 V 7 , 15% of the sur face re main ing free. Then an ex cess of only 730 x 0.15  110 µC cm -2 should be ex pected if there is no re place ment of for mic acid adspecies by propargyl al co hol res i dues. Since the ex per i men tal value is 290 µC cm -2 , it seems that there is a re place ment of for mic acid res i dues by propargyl al co hol. How ever, cy clic voltammetry can not pro vide clear proof of this re placement.
The DEMS tech nique us ing iso to pi cally la beled H 13 COOH makes it pos si ble to dis tin guish each con tri bution in the coadsorbed layer, pro vid ing un ques tion able ev idence of whether the re place ment re ac tions oc cur or not. Fig. 2 dis plays the MSCVs for m/z = 44 ([ 12 CO 2 ] •+ ) re lated to the pro duc tion of CO 2 from propargyl al co hol (solid line), and m/z = 45 ([ 13 CO 2 ] •+ ) cor re spond ing to for mic acid res i due ox i da tion to CO 2 (dot ted line) in a 1 st H 13 COOH 2 nd HC ≡CCH 2 OH ex per i ment . The re sults from this ex per i ment, from a sim i lar one but with re verse adsorp tion or der, and from the two pure com pounds are shown in Ta ble 2. The ion charge of the m/z = 45 sig nal for the ox i da tion of for mic acid spe cies in the coadsorbate (14.0 a.u.) in the ex per i ment of Fig. 2 is sim i lar to that for the ad sorp tion of pure for mic acid (15.1 a.u.), the dif ference be ing within ex per i men tal er ror. Thus, it is clear that no re place ment of for mic acid res i dues by propargyl al cohol oc curs. On the con trary, Q I for the m/z = 44 sig nal from propargyl al co hol ox i da tion in the coadsorbate (9.8 a.u.) is about 50% of the ion charge for the ox i da tion of pure propargyl al co hol res i dues. How ever, the charge den sity cor re spond ing to the an odic ad mis sion tran sient for propargyl al co hol in the ex per i ment of Fig. 2, Q t2 = 6 µC cm -2 , is only 15% of the an odic tran sient of 40 µC cm -2 for the ad mis sion of pure propargyl al co hol (Ta ble 1). As no re place ment of for mic acid res i dues is ob served, a mod i fica tion in the com po si tion of the ad sorbed layer of propargyl al co hol pos si bly oc curs, es pe cially since it has been shown that propargyl al co hol res i dues are a mix ture Vol. 8, No. 2, 1997 Or ganic Com pounds on Plat i num 109  It should be men tioned that propargyl al co hol res i dues ox i dize not only at po ten tials in the Pt ox ide re gion, but also in the dou ble layer re gion in a first peak at around 0.80 V (see MSCV in Fig. 2 -solid line). A sim i lar MSCV was observed for pure propargyl al co hol ad sorbed at E ad = 0.05 V. Thus, propargyl al co hol seems to ad sorb in the same way on a Pt sur face cov ered by H or for mic acid res i dues 7 .
Com paring the MSCVs in Fig. 2 with those from the ox i da tion of the res i dues formed from pure propargyl al cohol and for mic acid, it can be con cluded that both compounds form do mains that main tain their in di vid ual char ac ter is tics.

st HC≡CC H2OH 2 nd HCOOH con sec u tive ad sorp tion
The ab sence of for mic acid ad sorp tion at E ad = 0.30 V on a plat i num elec trode cov ered by propargyl al co hol res idues can be seen in the CV in Fig. 1 (dashed line), which coin cides with that ob tained for propargyl al co hol res i dues 6,7 . No cur rent tran sient is ob served upon the ad mis sion of formic acid, and the charge Q ox for the ox i da tion of propargyl al co hol res i dues is the same as for pure propargyl al co hol (Ta ble 1). No po ten tial-dependent mass sig nal for m/z = 45 was ob served 7 , which shows the ab sence of for mic acid res i dues. These ex per i ments con firm that the re ac tiv ity of for mic acid on plat i num is com pletely in hib ited by propargyl al co hol res i dues.

st HCOOH 2 nd CH 3 CH 2 OH con sec u tive ad sorp tion
For this study, E ad was set at 0.35 V, be cause at this poten tial the max i mum cov er age for the in di vid ual ad sorption of eth a nol was ob served. The an odic cur rent tran sient dur ing the ad sorp tion of eth a nol on a plat i num sur face mod i fied by for mic acid res i dues has a charge of Q t2 = 95 µC cm -2 (Ta ble 3) . Fig ure 3a shows the first and sec ond CVs for the ox i da tion of the res i dues. Dur ing the first for ward scan, an ox i da tion peak ap pears at about 0.70 V, in volv ing a charge den sity of 265 µC cm -2 (Ta ble 3).
Both eth a nol and for mic acid mainly ox i dize in the same po ten tial re gion, i.e. be fore the on set of plat i num oxide for ma tion, and there fore, the peak po ten tial can not give any in for ma tion on the na ture of the coadsorbed layer. It should be men tioned that eth a nol also shows a small con tribu tion in the PtO re gion dur ing the ox i da tion of its res idues 3 which is not pres ent in the case of for mic acid ad sorbates 7 . The Q t1 for pure for mic acid and eth a nol pro cesses are 7180 and 460 µC cm -2 , whereas Q ox is 205 µC cm -2 for for mic acid ad sor bates and 300 µC cm -2 for eth a nol. The value of Q ox for the 1 st HCOOH 2 nd CH 3 CH 2 OH

Ta ble 3.
Charge den si ties (Q t) of the tran sients pro duced by ad mis sion at E ad = 0.30 V of for mic acid, eth a nol, and the two com pounds added succes sively (see text), and the an odic charge den sity (Q ox ) and peak po ten tials (E p1 and E p2 ) ob tained in a sub se quent CV.  The MSCVs for the mass sig nals m/z = 45 of H 13 COOH ox i da tion (dot ted line) and m/z = 44 for eth a nol res i dues (solid line) re corded si mul ta neously with the CV for the strip ping of the coadsorbate are given in Fig. 3b. The in tegrated ion cur rents are 6.2 a.u. for CO 2 from eth a nol and 12.4 a.u. for CO 2 from for mic acid (Ta ble 4). The sig nal related to eth a nol is much higher than 15% of the value of 21.2 ob tained with pure eth a nol. Since for the ad sorp tion of for mic acid a value of Q I = 16.0 a.u. is mea sured, a re place -ment of about 25% of for mic acid res i dues by eth a nol is es tab lished.

st CH 3CH 2OH 2 nd HCOOH con sec u tive ad sorp tion
The an odic cur rent tran sient of 2950 µC cm -2 ob tained at 0.35 V for for mic acid on a plat i num sur face poi soned by eth a nol res i dues sug gests that bulk ox i da tion of for mic acid takes place at this mod i fied elec trode. The CVs for the first and sec ond po ten tial cy cles af ter the coadsorption of 1 st CH 3 CH 2 OH 2 nd HCOOH are shown in Fig. 4a. Two po tential re gions for the ox i da tion of the ad sor bates are dis tinguished: the first be tween 0.50 and 0.90 V with an an odic peak at 0.70 V, and the sec ond for E > 0.90 V as a broad peak. These CVs are sim i lar to those for the ad sorp tion exper i ment with eth a nol 3 . Thus, it can be es tab lished that the ox i da tion of eth a nol pre dom i nates, but the pres ence of formic acid res i dues can not be dis re garded. The MSCVs in Fig. 4b dem on strate that a small amount of for mic acid is coadsorbed (dot ted line), in volv ing an ion charge for m/z = 45 of 2.1 a.u., in rea son able agree ment with the adsorp tion of for mic acid on 15% of free Pt sites af ter eth a nol ad sorp tion (16.0 x 0.15 = 2.4). In the same way, the ion charge for m/z = 44, Q I = 19.8 a.u., ob tained for the con tribu tion of eth a nol in the coadsorbate (solid line in Fig. 4b) co in cides with the value of Q I = 21.2 a.u. for pure eth a nol within ex per i men tal er ror. Thus, it is con cluded that no replace ment takes place, for mic acid only ad sorb ing on the free sites of the Pt sur face.

Con cluding Re marks
The ap pli ca tion of DEMS us ing iso to pi cally la beled com pounds to study multicomponent sys tems makes it pos si ble to dis tin guish the con tri bu tions of the dif fer ent com pounds in the coadlayer. Thus, the na ture of the in terac tion be tween the chemisorbates of each com pound can be es tab lished. No re place ment oc curs dur ing the ad sorption of propargyl al co hol on a plat i num sur face cov ered by for mic acid res i dues, in op po si tion to the case of eth a nol. The re ac tiv ity of for mic acid with a plat i num sur face is inhib ited by propargyl al co hol res i dues, whereas it can re act with out re place ment on the free plat i num sites of the surface mod i fied by the eth a nol ad sorbed layer. Vol. 8, No. 2, 1997 Or ganic Com pounds on Plat i num 111