CO adsorption and oxidation on Pt(111) electrodes modified by irreversibly adsorbed arsenic in sulphuric acid medium. Comparison with bismuth-modified electrodes

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ELSEVIER

Journal of Electroanalytical Chemistry 393 (1995) 87-96

CO adsorption and oxidation on Pt(111) electrodes modified by irreversibly adsorbed arsenic in sulphuric acid medium. Comparison with bismuth-modified electrodes E. Herrero, A. Rodes, J.M. P@ez, J.M. Feliu, A. Aldaz Departament de Qulmica-Fisica, Universitat d'Alacant, Ap. Correus 99, E-03003 Alacant, Spain

Received 14 November 1994; in revised form 13 February 1995

Abstract CO adsorption and stripping on Pt(111) electrodes modified by irreversibly adsorbed bismuth and arsenic were studied in sulphuric acid medium. Coadsorbed bismuth and CO form a mixed adlayer, whereas arsenic tends to desorb in the presence of CO. Both adatoms modify the CO stripping process, resulting in the catalysis of CO oxidation, in which arsenic is more effective than bismuth. Fourier transform IR (FTIR) studies demonstrate that bismuth stabilizes the adsorbed CO, while arsenic shows the opposite effect. In both cases, linear CO is the only species present on the electrode surface at high adatom coverage. The catalysis mechanism is an adatom-mediated oxygen transfer for both cases, with an additional electronic effect when arsenic is on the surface. Using the CO stripping charge after adequate correction, the values of the CO coverage for the different adatom coverages were calculated. The results of the CO coverage agree well with the behaviour shown in the FTIR experiments. Keywords: CO adsorption/oxidation; P t ( l l l ) electrode; Bismuth- and arsenic-modified electrodes

I. Introduction CO is a small molecule that adsorbs on most metal surfaces. The CO bond with the electrode surface depends on the surface energy and properties, and therefore CO adsorption shows a structure-sensitive behaviour [1,2]. Such characteristics make CO adsorption on single crystal surfaces a test reaction in both ultrahigh vacuum (UHV) and electrochemical environments. In addition, when CO is adsorbed on the surface, it can be oxidized to CO 2. This process is important not only in itself, but also because CO is formed as a by-product in the electrochemical oxidation of many organic molecules [3]. In electrochemical environments, CO adsorption was carried out on platinum single crystal electrodes after suitable preparation methods for electrodes had been developed [4]. Since then, the adsorptive properties of the CO molecule have been studied using pure electrochemical methods [5,6], in situ techniques, such as IR spectroscopy [7-12] or STM [13,14], and ex situ UHV techniques [15,16]. The maximum CO coverage attainable on P t ( l l l ) electrodes is 0.75, which corresponds to a (2 x 2)-3CO ordered structure [14]. In this structure, CO is bonded to the surface in two different configurations: as linear CO 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved

SSDI 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 3 9 6 6 - X

(bonded to only one platinum atom) or multibonded CO (bonded to three platinum atoms) [11]. This structure is stable at potentials lower than 0.5 V. At this potential, a small amount of CO oxidation takes place and a ( 1 ~ x lV"~-) R23.4 ° CO structure is obtained (~9co = 0.68) [13,14]. The transformation from the initial structure to the latter implies the disappearance of multibonded CO and the appearance of bridged bonded CO (bonded to two platinum atoms) [11,14]. The onset of the main CO oxidation process under voltammetric conditions in 0.5 M H 2SO4

is 0.75 v [6]. P t ( l l l ) electrodes can be easily modified by adatoms such as bismuth and arsenic [17,18]. It has been shown that the presence of bismuth on the electrode surface alters its behaviour towards CO adsorption and oxidation [19,20]. The changes can be summarized as follows. (1) At high bismuth coverages, the only species is linear CO [19]. (2) Bismuth and CO oxidize simultaneously, indicating the presence of a C O - B i mixed adlayer [20]. (3) Bismuth catalyses CO oxidation through an adatommediated oxygen transfer [20]. Similar results have been reported for adsorbed antimony on P t ( l l l ) electrodes [21].

E. Herrero et aL /Journal of Electroanalytical Chemistry 393 (1995) 87-96

88

The aim of this work was to study the behaviour of arsenic- modified P t ( l l l ) electrodes for CO adsorption and oxidation. The comparison with bismuth-modified electrodes serves to clarify the effect of adatom modification on the surface properties. Electrochemical and Fourier transform IR (FTIR) measurements were carried out to obtain a more detailed knowledge of the CO-adatom system.

It was equipped with a prismatic CaF 2 window bevelled at 60 °. As a test electrolyte, 0.1 M HESO4 was used to avoid extensive damage to the CaF 2 window. The experimental set-up allows the electrochemical characterization of the electrode surface prior to the FTIR experiment. P-polarized radiation was used in all experiments.

3. Results and discussion 2. Experimental details

3.1. Electrochemical behaviour of adsorbed CO on arsenic- modified Pt (111) electrodes

Pt(111) electrodes used in the electrochemical experiments were obtained from single crystal beads, which were oriented within + 3', cut, polished down to 0.25 /zm with diamond paste and annealed according to the technique of Clavilier et al. [22]. The same procedure was also employed to obtain the samples used in the FTIR experiments with a surface area of 12 mm 2. All coverages were defined as the number of adsorbed species per platinum atom. All potentials were measured vs. the reversible hydrogen electrode (RHE). The irreversible adsorption of bismuth or arsenic was carried out spontaneously by immersion of the clean P t ( l l l ) electrode in a Bi(III)- or As(III)-containing solution for several seconds, as described elsewhere [17,18], followed by rinsing with water. This modified electrode was characterized in the test electrolyte (0.5 M H2SO 4) in a potential range which ensures the stability of the electrode substrate structure as well as the stability of the adsorbed adatom on the surface. The adatom coverage was calculated from the charge of the surface redox process of the adsorbed adatom, assuming that bismuth exchanges two electrons and arsenic exchanges three electrons, as well as from the remaining platinum adsorption charge [17,18] using the following equations ~Bi =

1Qai/241 /zC cm

2

~gA~= ~QAs/241 /zC cm -2 Bad = 3(1 -- Q~°t/241 /xC cm - 2 )

(1) (2) (3)

where QBi and QAs are the bismuth and arsenic oxidation charges respectively, Qadpt is the adsorption charge of the electrode covered by the adatom and 241 /zC cm -2 is the platinum adsorption charge of the unmodified Pt(111) electrode for a monoelectronic surface adsorption process involving all surface platinum atoms (1.5 × 1015 atoms cm-2). Eqs. (1) and (2) or (2) and (3) give the same coverage within experimental error, enabling the presence of surface contamination to be checked. The experimental protocol for the electrochemical measurements of CO adsorption on modified electrodes has been described elsewhere [20]. The FTIR measurements were carried out with a Nicolet 5PC equipped with a liquid nitrogen-cooled MCT detector. The spectroelectrochemical external reflection cell has been described elsewhere [23].

Fig. 1 shows the results of the CO stripping experiments for unmodified and arsenic- and bismuth-modified P t ( l l l ) electrodes. CO can block all the platinum adsorption sites, resulting in a flat voltammetric profile in the region 0.06-0.5 V. From the voltammograms depicted in Fig. 1, it is clear that both adatoms on the electrode surface modify the CO oxidation process. On clean P t ( l l l ) surfaces, the onset of CO oxidation is at 0.75 V (excluding small waves at 0.5-0.6 V (Fig. I(A))) with a sharp peak around 0.83 V. The presence of the adatom modifies the CO oxidation process, lowering the onset of CO oxidation to a value close to the onset of adatom oxidation in the absence of adsorbed CO. However, the redox peak of the adsorbed adatom is not resolved in the presence of adsorbed CO. After CO oxidation, in the subsequent negative going scan, the reduction process of adsorbed arsenic can be detected, indicating that the adatom has been oxidized together with CO. In the case of arsenic, the charge measured under the reduction peak is the same as that in the subsequent voltammetric scans. Therefore, during the process of CO oxidation, all surface arsenic adatoms have been oxidized. Unlike arsenic, bismuth oxidation is not complete after the CO stripping process, and the first bismuth reduction peak is lower than that recorded in subsequent scans. It has been shown that oxidized bismuth transfers the - O H group necessary for the oxidation of adsorbed CO, yielding reduced bismuth adatoms [20]. These reduced adatoms do not contribute to the reduction peak obtained immediately after CO oxidation. Assuming that arsenic also transfers - O H groups to CO, the total oxidation of the arsenic adatoms at the upper potential limit indicates that the transfer is faster in the case of arsenic (see next point), allowing the immediate reoxidation of the arsenic adatom. CO is a strong adsorbate and can be used to displace adsorbed species on the electrode surface [24-26]. In this way, CO may displace bismuth or arsenic adatoms from the electrode. In order to verify this possibility, voltammograms of the adatom- modified electrodes before CO adsorption and after CO oxidation were compared. Arsenic adlayers display a high instability on CO adsorption. As shown in Fig. 2, there is a large increase in the platinum adsorption states parallel to the decrease in the arsenic

89

E. Herrero et al. /Journal of Electroanalytical Chemistry 393 (1995) 87-96

5O

l I f'.j/

50

<

. . . .

0

<~ 0 --1 .,-.n

\~ l

./-\

~-~.

E/V (RHE)

EIV (RHE)

II

II [ 1

(#.i

Fig. 2. Comparison of the voltammetric profiles prior to CO adsorption (full line) and after CO stripping (broken line) for an arsenic-modified electrode with @n~= 0.10. ,< =1.

50

L00 t

b_

it. . . .

'

-\

'

E/V (RHE)

~ ~1 I

!I V

(B)

<

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2

1,,-1

(c)

f

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Fig. 1. CO stripping experiment for an unmodified Pt(lll) electrode (A), a Pt(lll) electrode with @A~= 0.10 (B) and a Pt(lll) electrode with ~Bi = 0.10 (C): (a) stripping of adsorbed CO; (b) recovery of the adsorptive properties of the electrode after CO stripping. Scan rate of 50 mV s-1 in all voltammograms.

redox peak. The same desorptive behaviour was observed for potential-controlled CO adsorption. Nevertheless, no additional modification of the electrode properties was observed. The relationship between the platinum adsorption charge and arsenic oxidation charge given by the following equation [18] is always fulfilled OpatS+ QAs = 241 /~C cm -2

(4)

Bismuth is more stable on the surface on CO adsorption. Only a small decrease in the bismuth redox peak is observed after CO adsorption and oxidation. The charge decrease under the bismuth oxidation peak is always lower than 10%, much less than that measured for arsenic. The adatom desorption induced by CO adsorption on the electrode surface indicates that the CO adsorption energy is higher than that of both adatoms on the P t ( l l l ) electrode surface. Comparing the desorption behaviour observed for bismuth and arsenic on CO adsorption, the higher arsenic desorption suggests a lower adsorption energy of arsenic than bismuth on the Pt(111) electrode in the absence of potentiostatic control. It is important to note that the reported electrode changes increase with the CO adsorption time. This suggests that the changes are mainly caused by CO adsorption and not by the stripping process. Owing to the arsenic desorption induced by CO adsorption, two different coverages can be calculated, one for the initial state of the modified electrode and the other for the final state after CO oxidation. All the coverage values given in this work are referred to the final arsenic coverage, as they reflect the exact adatom coverage when CO and the adatom coexist on the electrode surface. The presence of a C O - B i mixed adlayer has been postulated by IR [19] and electrochemical [20] studies. In the electrochemical studies, the presence of the mixed

90

E. Herrero et aL /Journal of Electroanalytical Chemistry 393 (1995) 87-96

adlayer was suggested by the joint oxidation of CO and bismuth and the influence of CO on the adatom redox peak. The similar electrochemical behaviour of adsorbed arsenic suggests that a C O - A s mixed adlayer is also possible. However, non-saturated CO coverages on adatom-modified surfaces display an important difference. When the CO molecules do not cover the electrode surface completely, the redox peak of the adsorbed arsenic shifts towards negative potentials, whereas the bismuth redox peak shifts in the opposite direction under the same conditions (Fig. 3). This reveals a different interaction between Bi and CO and As and CO. In the first case, the presence of adsorbed CO stabilizes the Bi ° species on the electrode surface as a consequence of attractive C O - B i interactions [27], whereas in the latter case, As(Ill) species are stabilized rather than As ° species. Thus a repulsive interaction between CO molecules and arsenic adatoms can be proposed. Normally, repulsive interactions tend to create segregated domains, and therefore the possible formation of a CO-AS mixed adlayer must be re-examined. When CO is adsorbed on an arsenic-modified P t ( l l l ) electrode, the first adsorption step is to occupy all free platinum sites, since the replacement of an arsenic adatom is energetically hindered with respect to the adsorption on a free platinum site. After this, three different adlayers can be formed: a pure CO adlayer, a microscopically mixed C O - A s adlayer and CO and As in segregated domains. Owing to the high arsenic desorption rates in the presence of CO, the most stable structure must be the pure CO adlayer, which has a lower energy than both C O - A s adlayers. After all platinum sites have been blocked, CO molecules tend to replace arsenic adatoms and the arsenic coverage decreases. Thus the formation of a segregated adlayer does not take place, since the driving force in adlayer formation is arsenic replacement and not C O - A s repulsion. Probably, non-replaced arsenic adatoms will maintain their initial position, uniformly distributed over the surface, giving a C O - A s mixed adlayer. For higher arsenic coverages (Fig. 3(B)), the redox peak of arsenic in the presence of CO is split, suggesting different environments for the arsenic adatoms. Those adatoms whose oxidation potentials are lower probably have more CO molecules in their vicinity than the other adatoms or these molecules are arranged in a more closepacked island. It is noteworthy that, in both cases, the loss of arsenic from the surface is significantly less than that observed when CO is adsorbed at saturation. Therefore arsenic desorption is more effectively forced in the later stages of CO adsorption. Non-saturated coverages can also be obtained by partial CO stripping (Fig. 4). These experiments allow the intimate relationship between adatom oxidation and CO stripping to be demonstrated. CO partial stripping experiments on arsenic- modified P t ( l l l ) electrodes have been performed using an upper potential limit of 0.58 V (Fig. 4). At this upper potential limit, simultaneous CO and arsenic

A

ii 50

I

b

"--'. j -

.

,...- ....

~'

~

,:o

~l I,'

E/V (RHE)

(A)/ '1

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i, l

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(B)

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(c) | Fig. 3. CO stripping experiments for modified P t ( l l l ) electrodes at partial CO coverages ((A) OAs = 0.05; (B) OAs = 0.15; (C) OBi = 0.05): (a) initial electrode state after CO adsorption; (b) CO stripping; (c) recovery of the adsorptive properties of the electrode after CO stripping.

oxidation starts. Arsenic oxidation can be detected in the voltammetric profile by the characteristic presence of the redox peak, which appears at a lower potential than for CO-free surfaces as well as for non- saturated CO cover-

E. Herrero et al. /Journal of Electroanalytical Chemistry 393 (1995) 87-96

ages. Nevertheless, oxidation charges under the peak are always larger than reduction charges, indicating that an additional species is oxidized on the electrode surface. This species must be adsorbed CO. In the first six voltammetric scans, the development of the redox peak of adsorbed arsenic at 0.51 V can be observed (Fig. 4(A)). This redox peak has a smaller charge than that measured after CO stripping. In contrast, no increase in the adsorption charge is observed in the platinum adsorption region (potentials lower than 0.5 V). Therefore partial CO oxidation gives a less compact C O As adlayer but no free platinum sites. In the less compact areas, water adsorption takes place more easily and arsenic oxidation can occur. Lateral interaction between CO and arsenic leads to lower potential values for arsenic oxidation. After the sixth voltammetric cycle (Fig. 4(B)), the arsenic redox peak continues its development and shifts towards positive potentials. The reduction peak is increas-

91

ingly more symmetric and the adsorption states of platinum begin to appear. Once a less compact CO adlayer is achieved, partial CO oxidation leads to the liberation of platinum adsorption sites. The appearance of these sites implies a lower interaction between the CO molecules and arsenic adatoms and the displacement of the redox peak towards positive potentials. The arsenic redox peak is symmetric (without splitting), which suggests that the structure of the mixed adlayer is rather homogeneous (Fig. 4(C)). The existence of different local structures for the arsenic adatoms with different numbers of surrounding CO molecules would lead to a split peak, as observed in Fig. 3. After the 20th cycle, there is an important amount of non- blocked platinum sites, and the peak potential of the redox process of adsorbed arsenic is almost the same as that obtained in CO-free As + P t ( l l l ) electrodes (Fig. 4(D)). The oxidation charge under the redox peak is the same as that measured after complete CO oxidation if the upper potential limit is set at 0.85 V. This is in agreement

50

<

0

r"

"

"

~ 0 "

.6

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-

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i

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i

.,In

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50 C

">,"2 ,<

.6

:::I.

1 .~0

,ii

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l

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Fig. 4. Partial CO stripping experiment on an arsenic-modified electrode with OAs = 0.11: (A) lst-6th cycles; (B) 7th-15th cycles; (C) 16th cycle; (D) 20th cycle and total CO stripping. (a) and (b) as in Fig. i.

E. Herrero et al. / Journal of Electroanalytical Chemistry 393 (1995) 87-96

92

potential value. Arsenic oxidation on the electrode surface starts at a potential 50 mV lower than bismuth oxidation. It is possible that the difference in the oxidation rates is a consequence of this difference, i.e. for the same potential arsenic adatoms are more easily oxidized. In order to verify this possibility, two oxidation transients were recorded, one for bismuth-modified electrodes at 0.65 V and one for arsenic-modified electrodes at 0.60 V, both potentials 0.025 V higher than the adatom redox peak. As shown in Fig. 6, the difference in the oxidation rates depends not only on the oxidation potential of the adatom but also on its chemical nature, since arsenic is a much more effective catalyst for CO oxidation than bismuth. In polycrystalline electrodes, the same behaviour was found [28]. It is important to note that an increase in the CO oxidation rate is only found at potentials at which the adatoms are oxidized, which supports the concept of catalysis through an adatom-mediated oxygen transfer.

30

20 ...t

10

/

(A)

0

\

25

50

75

t/s

100 o -n

50 3.3. In situ FTIR spectroscopy experiments on bismuthand arsenic-modified electrodes (B)

0

5

10

1'5

20

25

t/s Fig. 5. Transient CO oxidation at 0.7 V for P t ( l l l ) electrodes with (A) ~ B i = 0.10 and Oco = 0.56 (full line) and @Bi = 0.00 and @co = 0.70 (broken line), and (B) @As = 0.04 and Oco = 0.68.

with the previous statement that arsenic desorption occurs during CO adsorption and not during the stripping process. This complete CO stripping process is the same as that obtained on As + Pt(111) electrodes partially covered by CO.

The different interactions of CO with bismuth- and arsenic- modified electrodes can be characterized by in situ FTIR spectroscopy. The information provided by this technique is very useful in understanding the greater catalytic effect of arsenic with respect to bismuth. The set-up of the spectroelectrochemical cell allows the characterization of the modified electrodes in the meniscus configuration prior to the FTIR experiment. After the electrode characterization, CO was bubbled for 1 min through the test solution. Then, excess CO was removed by bubbling with argon for 5 min and the complete blocking of the surface was checked electrochemically. Subsequently, the electrode was pushed down against the

3.2. Enhancement of the CO oxidation rate by arsenic and bismuth adatoms In the case of bismuth-modified P t ( l l l ) electrodes, a catalytic enhancement of CO oxidation was found [20]. Bismuth acts as a bifunctional catalyst, transferring the oxygen group needed for CO oxidation. In the previous section, catalysis by arsenic-modified electrodes has also been found. Therefore it is interesting to compare this behaviour with that obtained for bismuth. To study this enhancement, several current-time plots were recorded. In these experiments, after CO adsorption, the electrode was immersed at 0.1 V. A potential step was applied and the transient current was recorded. Fig. 5 shows the CO oxidation transients at 0.70 V for different samples: saturated CO coverage on an unmodified P t ( l l l ) electrode and on bismuth- and arsenic-modified P t ( l l l ) electrodes. As can be seen, arsenic adatoms are much more effective in the catalysis of CO oxidation. Oxidation rates for arsenic-modified electrodes are five times faster than for bismuth-modified electrodes for this

30

,¢ 20

10

100

200

300

400

t/s Fig. 6. Transient CO oxidation for an arsenic-modified Pt(111) electrode with OA~ = 0.04 and ¢9co = 0.68 at E = 0.60 V (full line) and for a bismuth-modified P t ( l l l ) electrode with OBi = 0 . 0 8 and @co = 0.59 (broken line).

93

E. Herrero et al. /Journal of Electroanalytical Chemistry 393 (1995) 87-96

optical window and the spectra were collected at different potentials. Each spectrum was referred to that obtained at 0.8 V after CO stripping. Finally, the voltammogram of the electrode after CO stripping was recorded to measure the final adatom coverage. No differences in the electrochemical behaviour were found when 0.1 M H2SO 4 solution was used instead of 0.5 M H2SO 4 solution. Figs. 7 and 8 show the CO spectra at 0.25 V for the bismuth- and arsenic-modified electrodes and the unmodified P t ( l l l ) electrode. For the unmodified electrode, the spectrum obtained displays two bands associated with two different species [11]: the band at 2072 cm -1 (v t) corresponds to linear CO and the band at 1787 cm 1 (urn) corresponds to multibonded CO. The assignment of the IR bands to the different adsorbed CO species may be misleading in some cases [29]. However, the agreement between the FTIR and STM [14] data seems to confirm the present assignment. When bismuth is added to the electrode surface (Fig. 7), the v t band shifts towards lower wavenumbers, as reported in Refs. [19] and [27], and at high bismuth coverages the multibonded CO band disappears. The shift of v t has been interpreted as the result of electron donation from the bismuth to the platinum surface [27] which increases the back-donation to the CO molecule. The higher back-donation from the platinum surface resuits in a weakening of the C - O bond and a stabilization of adsorbed CO. In the electrochemical studies, a shift of the redox peak of adsorbed bismuth towards positive potentials was found, indicating attractive interactions between CO and bismuth and a stabilization of both species

1787 cm -;

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//

17++

v2064 cm"

2057 cm"

| 5 10" LkR./R 3_

2200

I

I

I

I

2100

2000

1900

1800

1700

Were" Fig. 7. FTIR spectra for modified electrodes: (a) 0.12; (c) ~gBi= 0.22.

Bad = 0.00;

(b) OBi =

/

1836 cm 1

~J 2074 c m ;

2200

I

I

I

I

2100

2000

1900

1800

1700

++lena" Fig. 8. FTIR spectra for modified electrodes: (a) tgad= 0.00; (b) 0.07; (c) 6)A~= 0.16.

(if)As =

on the surface. The peak potential shift agrees well with the u t band shift, since both indicate the reciprocal stabilization of both adsorbed species, CO and bismuth. Previous results on bismuth-modified electrodes in 0.1 M HC104 [19] did not show the multibonded band. Instead, a band at 1830-1840 cm 1 appeared, which is associated with bridged bonded CO. The difference between these results and those given here is probably due to the lower CO coverage in the previous case. For unmodified P t ( l l l ) electrodes, the multibonded CO band only appears at the maximum CO coverage (~9co = 0.75) [14]. A decrease in CO coverage leads to a transformation of the multibonded CO band into the bridged bonded CO band [11,14,30]. For arsenic-modified electrodes, the linear CO band moves in the opposite direction, i.e. towards higher wavenumbers (Fig. 8). This means that the back-donation to CO has decreased and CO is less stable on the electrode surface, in agreement with the repulsive interaction between CO and arsenic reported in the previous sections. Another important characteristic of the spectrum of arsenic-modified electrodes was the absence of the multibonded CO band and the appearance of the bridged bonded CO band. An increase in CO coverage leads to the disappearance of this latter band. It is worthwhile to mention that the linear CO band is broader for the adatom-modified electrodes than for the unmodified P t ( l l l ) electrodes. This may be the result of different a d a t o m - C O environments. We have seen that the band shift is dependent on the adatom coverage. Different

94

E. Herrero et al. /Journal of Electroanalytical Chemistry 393 (1995) 87-96

2100

k . . . ~ . .....A..........- A ' " ' " ' ' A

e~

•

.[~,.. o-- TJ

flu'

.f s • s S

,,IS,'•

2050

0.0

I

I

I

I

I

O.l

0.2

0.3

0.4

0.5

0.6

EIV (RHE) Fig. 9. Evolution of linear CO band position with the electrode potential: O, Oad =0.00; D, ¢9ai=0.12; II, @ai =0.22; ,x, Ogs=0.07; A, @As = 0.16.

Once the participation of the oxidized adatom in the mechanism of CO oxidation has been established, the partial CO oxidation experiment of Fig. 4 can be explained. CO oxidation starts with the CO molecules neighbouring an oxidized adatom. The oxidation of the first CO molecules does not liberate any platinum adsorption sites, but a less compact adlayer is created. In order to keep all the platinum sites blocked, with a lower CO coverage, a transformation from linear CO to bridged bonded CO must occur. This less compact adlayer allows easier oxidation of the surface adatoms and the adatom redox peak appears, although displaced from its normal value due to the lateral effects between the adatom and CO molecules. In the next step, CO oxidation continues in the vicinity of the adatom, and liberates platinum adsorption sites close to the adatom. The CO-adatom distance increases and the lateral effects decrease, displacing the adatom redox peak towards its normal potential value. Once the surface is partially deblocked, the CO oxidation can continue either by adatommediated oxygen transfer or by water-mediated oxygen transfer. The high mobility of the CO molecules on the electrode surface at the oxidation potential [13] suggests that the oxidation is mainly an adatom-mediated oxygen transfer. 3.4. CO charge measurements

local adatom coverages produce different shifts in the linear CO band and the final result is a broadening of the band. These inhomogeneities in the CO-adatom adlayer were also detected in the voltammetric behaviour of the adatoms (Fig. 3(B)). It is well known that the CO band position shifts as a function of the applied potential to the electrode. The shifts for the linear CO band are given in Fig. 9. For arsenicmodified and bare P t ( l l l ) electrodes, a variation in the band peak with the potential of approximately 30 cm -1 V -1 is obtained. For high bismuth coverages, the band shift is 70 cm -1 V -1, suggesting the presence of an additional electronic effect that changes not only the band position but also the shift with potential. These FTIR results serve to explain the different catalytic activities of the two adatoms. In both cases, the main catalytic mechanism is an adatom-mediated oxygen transfer, since oxidation enhancement is found only when the adatom is oxidized. However, the 1.,t band shift indicates that arsenic and bismuth display a completely different electronic effect on the surface. Arsenic tends to destabilize CO on the surface, whereas bismuth has the opposite effect. If CO is less stable on the surface, its oxidation is easier and faster. Thus the fast oxidation of CO on arsenic-modified electrodes is a result of two combined effects: first, oxygen transfer from the oxidized arsenic adatom; second, an electronic effect that destabilizes CO on the surface. For bismuth-modified electrodes, catalysis is only a consequence of bismuth-mediated oxygen transfer, since electronic effects should inhibit CO oxidation.

The most important problem in making a quantitative analysis of the CO coverage is to determine the correction which has to be applied to the charge measurements of the CO stripping process. In this work, the correction criterion and parameters described in Refs. [6] and [20] were used. For the correction of the CO oxidation charge, anion adsorption on the free platinum sites must be taken into account in all cases [24,26,31], as well as the adatom contribution to the overall process measured in the subsequent cycle. In the case of bismuth, the adatom contribution has been measured from the first reduction peak after CO oxidation, since some of the bismuth adatoms have already been reduced by the CO molecules. At this point, we will correlate the electrochemical behaviour of the modified electrodes, the FFIR experiments and the CO charge measurements. The oxidation charge measured for an unmodified P t ( l l l ) electrode is 352 _+ 14/xC cm -2, which gives a value of (9co = 0.73 _+ 0.03. This value is within the experimental error of the value obtained from STM measurements [14]. This high value of CO coverage has been associated with the presence of multibonded CO bands [11,14]. It is known that the multibonded band disappears at potentials higher than 0.4 V, and it is substituted by a bridged bonded CO band [11,14]. From STM measurements, a (V'~- × 1 ~ ) R23.4 ° structure with a CO coverage of 0.68 is found at potentials higher than 0.4 V [13,14]. The change in the CO binding mode and adlayer structure is also reflected in the voltammogram. At 0.50 V, a small oxidation wave appears in the

E. Herrero et al. /Journal of Electroanalytical Chemistry 393 (1995) 87-96

1.0

%%

400

%~

~ '?

0.8

%%%

t,~'~

%%%

300

06

=

o

® 200

%%~

100 0 0.0

0.4

% 0.2 i

I

I

0.1

0.2

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0.0

Fig. 10. Evolution of the CO stripping charge (Qco) and Oco with O~d:

voltammogram, which comprises a charge of 38 ~ C cm -2 [6]. If this charge is subtracted from the total oxidation charge, a coverage of 0.67 is obtained, which is in agreement with that obtained by STM [14]. Therefore the presence of oxidation waves in the region 0.50-0.60 V in the voltammetric profile can be associated with the transformation of the multibonded CO band to the bridged bonded CO band and a change in the CO adlayer structure. For the bismuth- and arsenic-modified electrodes, the values of Qco (Oco) are plotted vs. Oad in Fig. 10. If the adatoms act as a simple third body and the CO populations remain unaffected, a linear decrease in Oco is expected, with Oco = 0 for the adatom coverage at which all platinum atoms have been blocked (Oad = 0.33). This kind of behaviour is depicted by the full line. The broken line represents the behaviour expected if the only species blocking platinum atoms is linear CO and the adatom behaves as a third body. For bismuth-modified electrodes (Fig. 10, circles), the points deviate from the full line towards the broken line. This indicates that increasing bismuth coverage tends to give linear CO as the only species on the surface. This is well correlated with the FTIR spectra. At bismuth coverages of 0.22, the multibonded band has disappeared and only the linear CO band is visible. Moreover, the oxidation wave between 0.50 and 0.60 V, indicative of the presence of the multibonded CO band, appears for low and intermediate bismuth coverages (Fig. I(C)), but not for coverages higher than 0.15. The case of arsenic-modified electrodes is more complicated. The study was limited to arsenic coverages lower

95

than 0.10, since arsenic desorption does not allow higher arsenic coverages to be obtained with a suitable CO adsorption time. In this case, the points (Fig. 10, triangles) correlate well with the full line. This is the result of two different effects caused by arsenic adatoms on the surface. In the electrochemical measurements, no evidence of a CO oxidation wave at 0.50 V was found, indicating that no transformation of the multibonded CO band takes place. The FTIR measurements show why this transformation does not occur; no multibonded CO band is present, but instead a bridged bonded CO band appears. As noted earlier, multibonded CO bands are associated with compressed CO structures. If the presence of arsenic on the surface destabilizes the adsorbed CO, it is logical that the formation of compressed CO structures will not take place and more open structures will be formed, as revealed by the presence of the bridged bonded CO band. Considering only this effect, the points in Fig. 10 should deviate to lower CO coverages of the full line since the CO structure is less compact. However, another effect caused by the presence of arsenic must be considered. The FTIR experiments for the arsenic-modified electrodes show that, for high arsenic coverages, the bridged bonded CO band almost disappears and, consequently, linear CO is the only species on the surface. According to this effect, points should tend to approximate to the behaviour depicted by the broken line. The two effects in the arsenic coverage range studied counteract one another, yielding no deviation from the full line.

Acknowledgements

This work was partially supported by the DGICYT project PB 94-0944. One of us (E.H.) acknowledges the Conselleria de Educaci6 i Ci~ncia of the Generalitat Valenciana for an FPI grant. The authors are indebted to Dr. T. Iwasita for providing the spectroelectrochemical cell and for her advice on the improvement of the F'FIR set-up.

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