Hydrogen Electrosorption and Oxidation of Formic Acid on Platinum

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Chapter 34

Hydrogen Electrosorption and Oxidation of Formic Acid on Platinum Single-Crystal Stepped Surfaces N. M . Marković, Α. V. Tripković, N. S. Marinković, and R. R. Adžić Institute of Electrochemistry, Institute of Chemistry, Technology, and Metallurgy, and Center for Multidisciplinary Studies, University of Belgrade, P.O. Box 815, Belgrade, Yugoslavia Hydrogen adsorption and oxidation of formic a c i d show a pronounced dependence on the structure of single c r y s t a l surfaces. The influence o f the terrace and step orientation and step density is r e f l e c t e d i n both reactions on step surfaces. The multiple states of hydrogen adsorption can be correlated with the nature of adsorption s i t e s . There i s a n e g l i g i b l e e f f e c t o f adsorbate-adsorbate i n t e r a c t i o n on step surfaces. Some l a t e r a l repulsion of hydrogen adsorbed on Pt(111) could be i n f e r r e d . A strong adsorption o f bisulphate and sulphate anions on the (111) oriented terraces and step s i t e s considerably affects both reactions. These data show that each crystallographic o r i e n t a t i o n of the electrode surfaces gives a different electrochemical entity. A remarkable progress has been made i n the l a s t several years i n e l e c t r o c a t a l y s i s on single c r y s t a l surfaces. This p a r a l l e l s the progress i n surface science and i t has been p a r t l y stimulated by developments i n that f i e l d , mostly regarding the preparation and characterization o f surfaces. New advances i n preparation of surfaces outside o f high vacuum, achieved i n e l e c t r o c a t a l y t i c studies, also helped t h i s trend. E l e c t r o c a t a l y s i s i s a complex area which has lacked so far a molecular-level understanding of elementary steps in e l e c t r o c a t a l y t i c reactions. This lack has been due to unsuitable techniques f o r i n s i t u i d e n t i f i c a t i o n of reaction intermediates and products and to poor characterization of surfaces, despite the high s e n s i t i v i t y of electrochemical techniques. A l l e x i s t i n g

c

0097-6156/88A)378-0497$06.25/0 1988 American Chemical Society

Soriaga; Electrochemical Surface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

498

E L E C T R O C H E M I C A L SURFACE SCIENCE

concepts, however, are based on the data obtained with p o l y c r y s t a l l i n e electrodes. I t already appears that the concepts r e s u l t i n g from studies with single c r y s t a l surfaces w i l l cast doubts on e a r l i e r established ideas. Many i n t e r e s t i n g features, not recognizable with p o l y c r y s t a l l i n e surface, have been observed. These studies, with in situ spectroscopic investigations, could provide a rapid development of this e x c i t i n g area. Since the f i r s t observation of the s t r u c t u r a l dependence of hydrogen adsorption on Pt single c r y s t a l electrodes (1), many experiments have been reported on t h i s topic including the transfer of the samples from the UHV chambers into the electrochemical c e l l (2-5). Another important development i n surface preparation i s the flame annealing technique (6) with i t s modifications (7). Only recently have data from various laboratories started to agree. Oxidation of formic acid shows a pronounced s t r u c t u r a l dependence which i s well i l l u s t r a t e d by data on the low index planes (8-10) and preliminary data on stepped single c r y s t a l surfaces (11). In t h i s work further investigations of hydrogen adsorption and oxidation of formic acid on single c r y s t a l Pt surfaces with 15 orientations are reported.

Experimental Single c r y s t a l electrodes were obtained from Metal C r y s t a l Ltd. (Cambridge, England) oriented and cut to better than l o . The surfaces have been prepared by / ° 2 ^ annealing at 1200K, followed by cooling i n down to 400K, and were protected by a drop of water while being transferred into the c e l l . The surface structures of several electrodes have been checked by LEED. However, no direct transfer from the UHV chamber into the c e l l has been achieved. The solutions have been prepared from the organic-free water obtained from 18 MftMillipore water i r r a d i a t e d by a UV-radiation f o r 24 h or distilled from a l k a l i n e permanganate to remove traces of organics. The e l e c t r o l y t e s were Merck reagent grade acids. Platinum f o i l served as a counter electrode while Hg/HgCl^ and Hg/HgS0 were used as a reference. A l l potentials are given against the saturated calomel electrode. H

l a m e

2

4

Results and Discussion Hydrogen Adsorption on the Low-Index Surfaces. The data on the hydrogen electrosorption on the low-index planes of Pt d i f f e r i n some d e t a i l , as w i l l be discussed below with the help of r e s u l t s for stepped surfaces. Voltammetry curves f o r a l l three low-index surfaces are given i n F i g . 1. Hydrogen adsorption at P t ( l l l ) , the process at -0.25 < Ε < -0.05 V i n F i g . 1, i s not affected by the nature of the anion (such as SO 2-, C10 ~ or F-) (12) . The lack of a well defined peak, i n the drawn-out curve of F i g . 1 c l e a r l y indicates a strong l a t e r a l repulsion between adsorbed hydrogen adatoms. This i s probably a consequence of a p a r t i a l l y charge on the adsorbed hydrogen adatoms which, i n turn, does not allow the 4


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Hydrogen Electrosorption and Formic Acid Oxidation 499

MARKOVIC E T A L .

Figure 1. C y c l i c voltammetry f o r Pt single c r y s t a l s i n 0.05 M 2 4* P mV/s. H

S 0

S w e e

r

a

t

e

5

0


500


coverage to reach a f u l l monolayer (12). Hydrogen adsorption on the P t ( l l l ) plane could be described by the following equation: ί

H+ +\e

ΐ

_

λ

)

+

> Η . (D ad w h e r e ^ i s a p a r t i a l charge transfer c o e f f i c i e n t . In contrast to curve i n F i g . 1, two small peaks on a f l a t portion of the curve at E=-0.025 V and E=-0.15 V have been observed by several authors (4-6) . Our data to be shown i n the next section strongly suggest that such peaks are due to a stepped surface with a small step density, while a well-ordered P t ( l l l ) surface should give a curve as i n F i g . 1. The i n t e r p r e t a t i o n of voltammetry curve f o r the Pt(100) surface poses some problems, e.g. the o r i g i n of the peak at E=-0.15 V (Fig. 1) . Markovic et a l . (12) ascribed t h i s peak to hydrogen adsorption on p a r t i c u l a r surface imperfections, the (111)-oriented step s i t e s . The height of t h i s peak varies from one set of data to another, indicating a lack of control of the surface structure. Further support of t h i s view w i l l be shown below with the data for stepped surfaces. The sharp and narrow peak at -0.15 V on the Pt(110) plane indicates on a coupling of hydrogen adsorption/desorption and HSO^and S0 2- desorption/adsorption. This w i l l be further discussed i n connection with the data for stepped surfaces. Positive to the hydrogen adsorption region, the curve for the P t ( l l l ) surface shows "anomalous peaks" which, according to the predominant view, are due to sulphate adsorption (li, 13) . C l a v i l i e r ascribed these peaks to adsorption of 1/31 of a monolayer of strongly-bound hydrogen. The charge associated with these peaks i s 75 jaC cm-2, which corresponds to 1/31 of a monolayer of a completely discharged univalent species. Such a discharge i s possible for oxy-anions such as HS0 ~ and S0 2-, since t h e i r adsorption i s very strong at (111) s i t e s . This I i s due to the compatibility of t h e i r tetrahedral structure with (111) surface symmetry (14). I t was assumed that one oxy-anipn can cover at least 3 Pt atoms. Such adsorption can r e s u l t i n ι almost complete discharge of the overlayer of anions (14). A sizable discharge of S0 2-, HSO - on Au(111) can be also deduced from the shape of C-E curves (15), which suggests that the (111) symmetry i s more important i n t h i s p a r t i c u l a r case than the nature of the electrode material. A strong adsorption of S0 2- and HS0 ~ on P t ( l l l ) i s also r e f l e c t e d i n the beginning or oxide formation which, on t h i s plane, i s s h i f t e d to high p o s i t i v e p o t e n t i a l s (Fig. 1) . A sharp r e v e r s i b l e p a i r of peaks at E=0.2 V (which merge with the process at E=0.15 V) require a two-dimensional order, since they do not occur at stepped surfaces with (111)-oriented terraces (12). This unusual p a i r of peaks require further study which i s outside of scope of t h i s paper. A d i r e c t evidence of the way of tetrahedral anion adsorption at three-fold s i t e s and the degree of hydratation i s not available at present. However, a strong i n d i c a t i o n of such adsorption of sulphates i s found i n voltammetry on gold (14) and i n our data for platinum surfaces (12). A pronounced difference between the sulphate and perchlorate adsorption e f f e c t s i s 4

1

4

4

4

4


4

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MARKOVIC ET AL.

Hydrogen Electrosorption and Formic Acid Oxidation

p a r t i a l l y due to t h e i r d i f f e r e n t hydratation on the surface. There are indications that sulphates are hydrated by only two, while perchlorates by 7-8 water molecules (14). A high hydratation of perchlorates may be cause of a small difference between adsorption of these anions and f l u o r i d e s . Two other small peaks appear at E=0.45 and 0.7 V on the P t ( l l l ) surface. They are shifted to more negative p o t e n t i a l s with increasing 2 4 concentration, suggesting t h e i r l i n k with anion adsorption. A peak at 0.35 V for the Pt(100) surface, where one would expect the so-called double-layer region for Pt, i s also i n t r i g u i n g . I t remains to be seen whether or not i t i s due to sulphate adsorption on the (111)-oriented imperfections. The curve for Pt(110) surface shows a wide double layer region, indicating that the anion adsorption i s completed concurrently with hydrogen desorption. The anions are strongly held i n the "troughs" of the Pt(110) surface, i . e . i n the 2(111)-(111) terrace-step s i t e s . This point w i l l be touched upon connection with the stepped surfaces. The C-E curves f o r Au(110) i n 2 4 c l e a r l y show a completion of anion adsorption on that surface at very negative p o t e n t i a l s (15). A pronounced s t r u c t u r a l s e n s i t i v i t y of the oxidation of Pt surfaces i s also seen i n F i g . 1. The reaction takes place at the most p o s i t i v e p o t e n t i a l on P t ( l l l ) . This i s probably due to e f f e c t i v e blocking of the surface by oxy-anions with the t r i g o n a l symmetry, compatible with the (111) o r i e n t a t i o n . A detailed analysis of t h i s reaction on Au(111) has been recently performed by Angerstein-Kozlowska et a l . (14). No such blocking i s possible for the Pt(100) and Pt(110) surfaces with four-fold and two-fold symmetries. Consequently, the oxidation commences at more negative p o t e n t i a l s , probably predominantly determined by the surface energy as found with Au (16). H

H

S 0

S 0

Hydrogen Adsorption on Platinum Single C r y s t a l Stepped Surfaces. Few data are available f o r hydrogen adsorption on stepped surface (12, 17, 20-22). The reaction on several surfaces from the [lio] zone has recently been analyzed i n some d e t a i l (12). F i g . 2 shows voltammetry curves for hydrogen adsorption i n a l l three zones of the stereographic t r i a n g l e . For the [ l i o ] zone (Fig. 2a) the reaction i n the region -0.25 < Ε < -0.075 V has been ascribed to hydrogen adsorption on the (111)-oriented terraces. The peak at -0.025 V has been found due to adsorption on the step s i t e s of the (111)-(100) or (100)-(111) intersections of terraces and steps f o r surfaces v i c i n a l to the (111) and (100) planes, respectively. The t r i g o n a l s i t e s formed by intersections on the (111) and (100)-oriented steps and terraces are responsible f o r a strong adsorption of oxy-anions at steps. I t appears that upon anion desorption, hydrogen adsorbs f i r s t at the (100)-oriented s i t e s (12). This size of t h i s peak at -0.025 V i s proportional to the step density. Hydrogen adsorption i s concurrent with the bisulphate/sulphate desorption. The s i m i l a r i t y of these s i t e s , which can bee seen from models i n F i g . 3, causes the appearance of the peak at almost the same p o t e n t i a l i n the whole zone. For surfaces v i c i n a l to the (111) plane (Fig. 2a) i n the p o t e n t i a l region 0.15 < Ε < 0.5 V a broad peak has been ascribed


501

502


Figure 2A. C y c l i c voltammetry for the single c r y s t a l surfaces laying on the 111 zone l i n e . Sweep rate 50 mV/s.


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MARKOVIC ET AL.


ι U50uAcm-2

1

1

Γ

0.05M HjSO*

#

I -0.4

Figure 2B. laying on

ι 0

ι

ι

(M

0.8

Β ι

E

/

Y

I

1.2

Cyclic voltammetry f o r the single c r y s t a l surfaces the 100 zone l i n e . Sweep rate 50 mV/s.


504



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Figure 3. Models f o r the stepped surfaces, with tetrahedral anions adsorbed on the (111) trigonal sites for a) n(lll)-(100), b) n(100)-(lll) , c) n(100)-(100) and d) η(111)-(111) terrace-step o r i e n t a t i o n s .


506


to bisulphate/sulphate adsorption on the (111)-oriented terraces (12). Its size i s inversely proportional to the step density. No such peak i s seen f o r the Pt(311) surface whose structure, being 2(111)- (100), cannot accommodate the tetrahedral anions i n the terrace. Such anions, adsorbed i n the step, block the whole 2-atom long terrace (12). Voltammetry curves f o r surfaces from the [lOOJ zone are displayed i n F i g . 2b. These are the (210), (310) and (610) faces, which i n Lang et a l . (23) notation are 2(100)-(100), 3(100)-(100) and 6(100)-(100), respectively. The curve for the (320) surface, on the other side of the (210) orientation which i s the "turning point" of the zone, i s also shown. A common feature of these curves i s a peak at -0.025 V, the same p o t e n t i a l as for the [ l i o ] zone. This peak has been explained above by p a r t i c u l a r atomic arrangements i n the step. The question a r i s e s whether the same arrangements are formed i n t h i s zone? A close inspection of the stepped surfaces from t h i s zone (Fig. 3) shows that geometrically similar atomic arrangement can be obtained by a combination of the (100) and (110) oriented terraces and steps. Again, a t r i g o n a l symmetry of Pt atoms can be formed, providing s i t e s f o r a strong adsorption of bisulphate or sulphate. Although three-fold binding s i t e s are found i n the [lOO] zone, the surface coordination, surface energy and o r b i t a l orientation are d i f f e r e n t shape. The peak occurs, however, at almost the same p o t e n t i a l . The size of the peak i s proportional to the step density, as has been checked by combining our data with the data of Motoo and Furuya (22) who reported a d d i t i o n a l three surfaces v i c i n a l to the (100) plane. The peak associated with a strongly bound hydrogen at E*0.15 V i n F i g . 2b decreases as the terrace width decreases, giving no i n d i c a t i o n of i t s presence for the Pt(210)=2(100)-(100) or 2(110)-(100) surface. The highly active s i t e i s provided by the (100) o r i e n t a t i o n , which decreases with decreasing terrace width. For the Pt(210) surface adsorption on the terrace i s apparently strongly affected by the anion adsorption i n the step, which blocks the 2-atom wide terrace. Consequently, no s i t e s f o r a strong adsorption of hydrogen are a v a i l a b l e . On the basis of the models i n F i g . 3 and comparison with the peak f o r the [ l i o ] zone, the reaction i n the p o t e n t i a l range -0.25 < Ε < -0.15 V could be ascribed to the hydrogen adsorption on the (111)-oriented step s i t e s . The behaviour of the Pt(110) surface, as discussed above, i s largely determined by a strong adsorption of anions i n the steps of i t s 2(111)-(111) structure. The same i s v a l i d f o r v i c i n a l surfaces such as Pt(320), which gives a sizable peak at the same p o t e n t i a l as Pt(110). For most of the surfaces from the [lTo] zone, F i g . 2c, hydrogen adsorption i s predominantly associated with the single peak occurring at almost the same p o t e n t i a l for a l l the surfaces investigated. That peak, in the potential region -0.25 < Ε < -0.1 V, i s determined by hydrogen adsorption, coupled with anion desorption, from the (111)-(111) or (110)-(111) terrace-step s i t e s (Fig. 2c). They coincide with the peak f o r the


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MARKOVIC ET AL.


Pt(llO) surface. As explained above, t h i s surface behaves as the " f u l l y stepped" surface with the o r i e n t a t i o n 2(111)-(111). The coupling of H adsorption with HS0 - and S0 2- desorption can be written i n the following way: 4

y"

+

+

A

( z

ad ~^

) _

+

(

y W

e

H

> y ad

4

( 1 _ > H ) +

+

z

* ~

( 2 )

Evidence for t h i s coupling was obtained by a comparison of the data for sulphuric and p e r c h l o r i c acids (12). The reaction at -0.1 < Ε < 0.15 V seems to be associated with hydrogen adsorption on the (111) oriented terrace s i t e s for the surfaces v i c i n a l to the (111) plane. This can be deduced from a comparison with the P t ( l l l ) surface. A small peak at E«*0.35 V, seen for the Pt(332) and Pt(221) surfaces, i s due to bisulphate/sulphate adsorption on the (111)-oriented terraces. This point has been discussed i n d e t a i l f o r the Pt(332) and Pt(755) surfaces (12). I d e n t i f i c a t i o n of Peaks f o r Hydrogen Adsorption on the Disordered Low Index Planes. Besides the major objective f o r studying e l e c t r o c a t a l y s i s on single c r y s t a l stepped surfaces mentioned above, these studies o f f e r a wealth of information on the behaviour of p o l y c r y s t a l l i n e surfaces, of p r e f e r e n t i a l l y oriented surfaces and, as we suggested recently (12), of disordered low-index surface. A single sweep into the oxide formation with the P t ( l l l ) surface causes a considerable change of voltammetry curve for a well-ordered surface, as can be seen i n F i g . 4a. Two new small peaks are seen on the curve of the o r i g i n a l P t ( l l l ) . These peaks can be seen on the curves f o r surfaces by believed to be "well-ordered" by several authors (10, 17, 22). To unravel the o r i g i n of the two peaks at E=-0.15 and -0.075 V, i t i s necessary to be able to introduce, i n a c o n t r o l l e d manner, deviations from the perfect well-ordered l a t t i c e structure. This i s offered by the use of the stepped surfaces. F i g . 4b shows that the sharp peaks of the Pt(755)=6(111)-(100) and Pt(332)=6(111)-(111) coincide with the above peaks. This strongly suggests that these peaks are due to the introduction of the (100)and (111)-oriented steps upon c y c l i n g into the oxide formation region. Introduction of steps, i . e . , more active s i t e s on the well-ordered (111) surface, i s also seen i n the oxide formation region which now commences at much less p o s i t i v e p o t e n t i a l . I t appears that the surface energy plays important r o l e i n surface oxidation (15), although the lack of sulphate adsorption at the top step s i t e s c e r t a i n l y has an e f f e c t . Also, the sharp peak at E=0.2 V i s l o s t and the peak of sulphate adsorption is diminished. A l l these processes are now t y p i c a l f o r a stepped surface. The curve f o r Pt(100) shows some features which could be due to the presence of the surface imperfections. Defect formation on the Pt(100) surface by an Η ^ Λ ^ l Y electrochemical oxidation may not be s u r p r i s i n g , considering r e s u l t s from the gas-phase (24) which showed that the reaction of 0 with surfaces f

a

m

e

o r

b


507

508


Figure 4. C y c l i c voltammetry f o r well ordered ( - - - - ) and disordered ( ) P t ( l l l ) , f o r well ordered Pt(755) and Pt(332) i n 0.05 M H SO . Sweep rate 50 mV/s.


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MARKOVIC ET AL.


from the [lOo] zone produced reconstructions. I t remains t o be seen how much cooling i n H prevents reconstructing. The e f f e c t of a single p o t e n t i a l excursion into the oxide region causes a considerable decrease of the peak of strongly bound hydrogen and an increase o f the peak a t E=~0.15 V (Fig. 5a). At the same p o t e n t i a l a small peak i s seen for the Pt(100), prepared as described above. A s i m i l a r , or even larger, peak i s seen with a l l the curves published so f a r (21, 22, 23). The curves for vicinal ( Pt(11,1,1)=6(100)-(111) and Pt(610)=6(100)-(100) ) surfaces (Fig. 5b) show much larger peaks at the same p o t e n t i a l s . These data strongly suggest that a small Peak at E=-0.16 V f o r Pt(100) i s due t o the contribution of the (111) step s i t e s . Oxidation of Formic Acid. The oxidation of formic a c i d on Pt has been the subject of numerous studies on p o l y c r y s t a l l i n e (26) and single c r y s t a l (8-11) electrodes. However, no consensus on the mechanism has been reached so f a r . It i s accepted that i n the f i r s t step the C-H bond i s broken: HCOOH

> -COOH + H+ + e-

I t i s also agreed that further reaction goes through two p a r a l l e l pathways, ^»CO + H+ + e-COOH poisoning species one leading to a formation of C 0 the other to a formation of strongly bound species, which may be eventually be oxidized to CO a t high p o s i t i v e p o t e n t i a l s . The H-C bond i s broken f i r s t , unlike i n the gas phase where 0-H bond reacts f i r s t (27). This electrochemical behaviour has been confirmed by comparison of the kinetics of DCOOD, DCOOH and HCOOH on p o l y c r y s t a l l i n e Pt modified by Pb (28) and on Pt(100) and Pt(110) electrodes (29). A long disputed issue o f the nature o f strongly bound species i n t h i s reaction has been recently revived with the v i b r a t i o n a l spectroscopy studies of Bewick e t a l . (30) using EMIRS technique and of Kunimatsu and Kita (31) using p o l a r i z a t i o n modulation IR-reflection-absorption technique. These data indicated the only CO i s a strongly bound intermediate. Heitbaum et a l . (32) on the other hand advocate COH, and most recently HCO (33), as the poisoning species on the basis o f d i f f e r e n t i a l electrochemical mass spectroscopy (DEMS). To allow for CO formation i n the H adsorption region one can modify the reaction proposed e a r l i e r for formation of COH (31): 2#

COOH + H > CO + H 0 a a a 2 Outside the hydrogen adsorption reaction (3): o

(3) region CO can be formed by the


510


Figure 5. C y c l i c voltammetry f o r Pt(100) before ( - - - - ) and after ( ) disordering of the surface and f o r ordered P t ( l l , l , l ) and Pt(610) surfaces i n 0.05 M H SO.. Sweep rate 50 mV/s. 2


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MARKOVIC ET AL.


2COOH > -CO + H.O + CO (4) a a /. The oxidation of formic a c i d was the f i r s t e l e c t r o c a t a l y t i c reaction which c l e a r l y showed a pronounced influence of the structure on i t s kinetics and mechanism (8-10). Despite these studies, a thorough understanding of the reaction mechanism on various planes has not been obtained. This applies e s p e c i a l l y to the negative-going sweeps f o r the Pt(100) and Pt(110) planes a f t e r reversal at 1.1 - 1.2 V. Upon "activation" of the electrode at these p o t e n t i a l s , the Pt(110) surface on the negative going sweep immediately (once the oxide layer i s reduced) gives high current density (Fig. 6). For Pt(100), on the other hand, one obtains a gradual increase of the current as the p o t e n t i a l becomes more negative, i . e . the current increases with decreasing the overpotential for the reaction. The Pt(100) surface (Fig.6a) i s blocked, most probably by CO, up to the p o t e n t i a l of the oxide formation. Upon sweep r e v e r s a l , the reduction of a p a r t i a l l y oxidized surface takes place gradually. Instead of the expected fast r i s e of the current for the oxidation of HCOOH on clean surface, one sees a gradual increase of the current as the p o t e n t i a l becomes more negative. The Pt(110) surface exhibits a quite d i f f e r e n t behaviour. F i r s t , the positive-going sweep gives a large peak commencing at ~0.65 V. This indicates that upon oxidation of the blocking species the reaction takes place at a high rate at that high overpotential. Upon sweep reversal s i m i l a r high currents are observed. This behaviour, e s p e c i a l l y the cathodic sweep, contrasts with the behaviour of the Pt(100) surface. The P t ( l l l ) plane shows no a c t i v a t i o n e f f e c t , i . e . no poisoning species seem to be formed on that plane. The sweeps i n anodic and cathodic directions almost retrace, with the current peak at 0.5 V. This peak was found a consequence of anion adsorption (29). I t i s noteworthy that the oxidation of CH^OH i s less structure sensitive (9). Anion adsorption i s apparently determining the negative going sweeps for P t ( l l l ) and Pt(100). This w i l l be i l l u s t r a t e d by following discussion. For Pt(110), the i n d i r e c t arguments given in the 4 voltammetry section indicate that sulphate desorption occurs at very negative p o t e n t i a l s , while the oxidation of HCOOH i n the cathodic sweep takes place at much more negative p o t e n t i a l s on t h i s surface than on the others. The adsorption of sulphates at those potentials (0.7 V) i s c l e a r l y very strong. There i s no d i r e c t data available on the adsorption of sulphate on Pt single c r y s t a l surfaces. For Au, on the other hand, there e x i s t s a considerable body of experimental work (15, 34) . These data show a pronounced role of crystallographic orientation on adsorption of sulphate. The data of Strbac et a l . (15) corroborate the conclusions reached f o r Pt based on hydrogen adsorption and i t s r e l a t i o n to adsorption of anions. These data show that at Pt(110) sulphate adsorption i s largely completed at Ε < 0.05 V. The oxidation of HCOOH at Pt(110) at E-vO.65 V i n sweeps i n both d i r e c t i o n s now appears as a contradiction to the above analysis. Why doesn't sulphate block the reaction, since they are H

S 0

2


511

512


Figure 6. Oxidation of HCOOH on the Pt(100) and Pt(llO) single c r y s t a l surfaces i n 0.5 M H SO . Sweep rate 50 mV/s.


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strongly bound a t these potentials? Oxide formation requires more p o s i t i v e potentials and can be ruled out as a p o s i t i v e "trigger" of the reaction. From the above data i t follows that the Pt(llO) surface behaves as " f u l l y " stepped surface with the orientation 2(111)-(111), the anions are probably strongly adsorbed i n "troughs" of that surface. Their repulsion prevents the additional adsorption of tetrahedral anions on "ridges" the top s i t e s . HCOOH can, however, react with these s i t e s , r e s u l t i n g i n current peaks a t Ε 0.65 V. The question arises as to why the reaction "waits" f o r such p o s i t i v e potentials? This i s due to immediate poisoning with CO a t lower p o t e n t i a l . CO i s oxidized a t the same p o t e n t i a l as HCOOH on that plane (35). From gas phase measurements CO i s known to prefer top s i t e s on a l l three low index faces, with the CO molecule perpendicular to the surface and bonded through the carbon end of the molecule except a t high coverages (27). I t i s l i k e l y that HCOOH and COOH are adsorbed i n a s i m i l a r way. I t i s not l i k e l y that they could "enter" the "troughs", which seems to be possible f o r anions. For Pt(100) on the other hand, upon sweep reversal and gradual oxide reduction, anions are immediately adsorbed on that "flat" surface. They block adsorption o f HCOOH. Adsorption o f anions decreases as p o t e n t i a l becomes more negative. The oxidation of HCOOH commences and the rate increases as a t more negative p o t e n t i a l s , i . e . at lower overpotential. A competition between anions and HCOOH adsorption explains t h i s apparently anomalous behaviour. The explanation of the "anomalous" behaviour o f the Pt(110) surface can be also found i n the data f o r stepped surface v i c i n a l to the (100) and (110) orientations. F i g . 7 gives the curve f o r the Pt(11,1,1)=6(100)-(111). Introduction o f steps i n the (100) f l a t surface causes the appearance of a new peak i n cathodic sweep a t Ε 0.75 V. The peak grows as the step density increases (29). Sulphate i s strongly adsorbed i n the steps o f these surfaces just as i n the "troughs" of the (110) plane. The top step s i t e s should be again available f o r the oxidation of HCOOH, as strongly indicated by F i g . 7. The data f o r the Pt (332)=6(111) - (111) surface provide further support of t h i s analysis. As with P t ( l l l ) t h i s surface shows no poisoning e f f e c t (as do other low index planes) (Fig. 7), giving a peak i n anodic sweep. However, the (111)-(111) terrace-step combination gives the s i t e s o f the (110) geometry. This causes the appearance of the peak at 0.7 V, as on Pt(110). F i g . 8 shows a strong s t r u c t u r a l dependence on surfaces from the 3 zones. General discussion and conclusions. Both the data on hydrogen adsorption and formic acid oxidation show pronounced s t r u c t u r a l s e n s i t i v i t y , thus confirming a paramount r o l e of surface structure i n e l e c t r o c a t a l y t i c reactions. I t can be concluded that each crystallographic orientation represents a d i s t i n c t electrochemical (chemical) e n t i t y . The investigation of stepped surfaces seems t o be necessary to reach an understanding o f these systems on a molecular l e v e l . Hydrogen adsorption shows dependences on the terrace orientation, step orientation, and step density. A l l the


514


1

τ

1

1

Γ

.

ι

ι

ι

I

0.0 OA 0.8 1.2 E/V Figure 7. Oxidation of HCOOH on the P t ( l l , l , l ) and Pt(332) single c r y s t a l stepped surfaces i n 0.5 M H SO . Sweep rate 50 mV/s.


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peaks observed with voltammetry are directly due to surface structure, rather than to induced heterogeneity. The same conclusions hold for the oxidation on formic acid. On the basis of the anodic sweep, the most active surface appears to be Pt(lll), due to the lack of its poisoning. Although the structure of some of the surfaces used has been checked by LEED, i t appears necessary to prove by that technique that the flame-annealing-hydrogen cooling method gives ordered surfaces. The systematic changes of hydrogen adsorption and of HCOOH oxidation with step density indicate on a high probability that the surface structures were well-ordered and well-oriented. Further work involving a fast transfer from the UHV into the cell seems desirable. Acknowledgments The authors are indebted to the Research Fund of S.R. Serbia, Yugoslavia and Department of Energy, Washington, USA contract 553 for financial support. Literature Cited 1. F.G.Will, J.Electrochem.Soc., 112 (1965) 481. 2. A.T.Hubbard, R.M.Ishikawa and J.Katekaru, J.Electroanal.Chem., 861 (1978) 271. 3. E.Yeager, W.E.O'Grady, M.Y.C.Woo and P.Hagans, J.Electrochem.Soc., 125 (1978) 348. 4. P.N.Ross, J.Electroanal.Chem., 150 (1983) 141. 5. D.Aberdam, R.Durand, R.Faure and F.El-Omar, Surf.Sci., 171 (1986) 303. 6. J.Clavilier, J.Electroanal.Chem., 107 (1980) 211. 7. N.Marković, M.Hanson, G.McDougal and E.Yeager, J.Electroanal.Chem., 214 (1986) 555. 8. R.R.Adžić, W.O'Grady and S.Srinivasan, Surf.Sci., 94 (1980) L191. 9. R.R.Adžić, A.V.Tripković and W.O'Grady, Nature (London) 196 (1982) 137. 10. J.Clavilier and G.Sun, J.Electroanal.Chem., 199 (1986) 479. 11. R.R.Adžić, A.V.Tripković and V.B.Vešović, J.Electroanal.Chem., 204 (1986) 329. 12. N.M.Markovic, N.S.Marinkovic and R.R.Adžić, J.Electroanal.Chem., 241 (1988) 309. 13. D.M.Kolb, Zeitschrift fur Phys.Chem. Neue Folge, 154 (1987) 179. 14. H.Angerstein-Kozlowska, B.Conway, A.Hamelin and L.Stoicoviciu, Electrochem.Acta, 31 (1986) 1051; J.Electroanal.Chem., 228 (1987) 429. 15. S.Štrbac, R.R.Adžić and A.Hamelin, J.Electroanal.Chem., in press. 16. R.R.Adžić and S.Štrbac, J.Serb.Chem.Soc., 52 (1987) 587. 17. P.N.Ross, J.Electrochem.Soc., 126 (1979) 67. 18. C.L.Scortichini and C.N.Reilley, J.Electroanal.Chem., 139 (1982) 247. 19. A.N.Tripković and R.R.Adžić, J.Electroanal.Chem., 205 (1986) 335.


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20. B.Love, K.Seto and J.Lipkowski, J.Electroanal.Chem., 199 (1986) 259. 21. J.Clavilier, D.Armand, S.Sun and M.Petit, J.Electroanal.Chem., 205 (1980) 267. 22. S.Motoo and N.Furuya, Ber.Buns.Ges, 101 (1987) 624. 23. B.Lang, R.W.Joyner and G.A.Somorjai, Surf.Sci., 30 (1972) 440. 24. P.Blakely and G.A.Somorjai, Surf.Sci., 65 (1977) 419. 25. J.Clavilier and D.Armand, J.Electroanal.Chem., 199 (1986) 187. 26. See e.g. R.R.Adžić, Advances in Electrochemistry and Electrochemical Engineering, vol. 13, H.Gerisher ed., 159-260, J.Willey, New York, (1985). 27. R.Madix, Advances in Catalysis, vol. 29, (1980). 28. A.Razaq and D.Pletcher, J.Electrochem.Soc., 129 (1984) 322. 29. R.R.Adžić and A.V.Tripković, unpublished. 30. A.Bewick and S.Pons In Advances in Infrared and Raman Spectroscopy, Ed. R.E.Nexter and R.Clarke, Heyden and Sun, London 1984. 31. Kunimatsu and H.Kita, J.Electroanal.Chem., 218 (1987) 155. 32. J.Wilsau and J.Heitbaum, Electrochim.Acta, 31 (1986) 8. 33. O.Wolter, J.Wilsau and J.Heitbaum, J.Electrochem.Soc., 132 (1985) 1635. 34. A.Hamelin, Modern Aspects of Electrochemistry, Ed. B.Conway, J.Bockris and K.White, vol. 16, Chapter 1, Plenum Press, New York, (1985). 35. C.Lamy, J.M.Leger, J.Clavilier and R.Parsons, J.Electroanal.Chem., 150 (1983) 71. RECEIVED May 17, 1988


Hydrogen Electrosorption and Oxidation of Formic Acid on Platinum

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