SPECULAR REFLECTANCE STUDY OP CATALYTIC EFFECTS OF MONOLAYER OF LEAD ON OXIDATION OF FORMIC ACID ON PLATINUM ELECTRODE

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SPECULAR REFLECTANCE STUDY OP

CATALYTIC EFFECTS OF MONOLAYER OF LEAD

ON OXIDATION OF FORMIC ACID ON PLATINUM

ELECTRODE

R. Adžij, M. Podlavicky

To cite this version:

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SPECULAR REFLECTANCE STUDY OF CATALYTIC EFFECTS

OF MONOLAYER OF LEAD ON OXIDATION

OF FORMIC ACID ON PLATINUM ELECTRODE

R. R.

ADZIC

and M. M. PODLAVICKY

Institute of Electrochemistry, ICTM, and Center for Multidisciplinary Studies, University of Belgrade, Belgrade, Yugoslavia

Résumé.

-

La déposition de monocouche de plomb sur platine et son effet catalytique 31 I'oxidation de l'acide formique était étudiée par la technique de reflectance spéculaire. La monocouche de plomb produit un grand changement de réflectance de l'électrode de platine. Nous avons déterminé la dépendance du changement relative de réflectance en fonction de la longueur d'onde. Les inter- médiaires de l'oxydation de l'acide formique produisent un petit changement de réflectance de l'électrode de platine. Ces espèces n'avaient pas d'effet sur l'adsorption des adatomes de plomb. Il

était confirmé que l'effet catalytique des adatomes de plomb prévenait la formation des intermé- diaires qui sont attachés fortement h la surface du platine. Les adatomes de plomb diminuent l'adsorption d'hydroghne sur le platine qui participe à la formation des intermédiaires. L'adsorption de cadmium est influencée par la présence de l'acide formique. Cela explique l'effet plus faible des adatomes de cadmium.

Abstract.

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The underpotential deposition of monolayer of lead on platinum and its catalytic effects on oxidation of formic acid were studied by specular reflectance technique. A monolayer of lead causes a large change of reflectivity of platinum electrode. The wavelength dependence of relative change of reflectivity has been determined. The intermediates in oxidation of formic acid, however, cause a small change of reflectivity of platinum electrode. These species do not affect the adsorption of lead adatoms. It was confbned that the catalytic effect of lead adatoms originates in prevention of formation of strongly bound intermediates. Lead adatoms decrease the hydrogen adsorption on platinum which participates in formation of such poisoning species. The adsorption of cadmium is affected by the presence of formic acid. This explains a smaller catalytic of its adatorns.

1. Introduction.

-

The application of optical techniques, such as a specular reflectance spectroscopy and ellipsometry, has been quite successful in a number of electrochernical systems. These two techniques, in combination with some electrochemical ones, provide a new insight in processes at electrochemical interfaces. The properties of metal surfaces with adsorbed foreign atoms are of general interest in surface che- mistry and physics. In several recent papers the optical properties of electrodes covered by foreign metal monolayers have been investigated [l-51. Such layers are formed at potentials more positive than the reversible ones in the case of electrodeposition of metals on foreign substrates (underpotential deposition). This is essentially an adsorption process which gives neutral, or slightly charged adatoms [6-91. The process offers a convenient method of formation of metal monolayers. By variation of the electrode potential and concentration of cations in solution, the coverage of foreign adatoms can be controlled down to 2

%.

We have shown recently that foreign metal monolayers have striking catalytic effects on the rates of several electrochemical reactions which include FeZ+/Fe3+ redox reaction on gold [IO] and Pt [Il], oxygen reduction on gold [121, oxidation of formic acid on Pt [13-151 and R h 1161.

In this work the use of specular reflectance will be made in studing these effects with a hope of obtaining new information about them. The attention will be focused on electrocatalysis, and we will try to illustrate the usefulness of optical studies of such processes. Less attention wil be paid to the optical properties of electrode with adsorbed foreign atoms per seconds.

2. Experimental. - The optical and electrochemical systems were the same as reported earlier [2]. It can be breifly mentioned that it consisted of a cylindrical optical electrochemical cell, tungsten-halogen light source, second order linear interference fillter, Hama- matsu R-374 photomultiplier and PAR 129 lock-in amplifier. The standard electronic circuitry included a Tacussel potentiostat, PAR Universal programmer and IIewlett-Packard XY recorder. Al1 reflectance measurements were done with parallel polarization at an angle of incidence of 450. In modulation measurements the electrode potential was modulated with an a. c. signal of 100 mV p-p.

The electrolyte was 1 M HC104 prepared from triply destilled water. Nitrogen gas wasused todeoxyde- nate the solutions. It was purified in usual way [2]. The reference electrode was a saturated sulphate electrode, while platinum served as a counter electrode. The

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C5-194 R. R. ADZIC AND M. M. PODLAVICKY

working electrode was a platinum plate polished to obtaine a mirror finish.

3. Results and discussion.

-

3 . 1 OPTICAL PRO-

PERTIES OF Pt ELECTRODE WITH ADSORBED LEAD ATOMS.

-

The specular reflectivity of Pt electrode has been followed during a linear potential sweep. The change in relative reflectivity caused by lead adatoms has been calculated for a given potential E, from the expression,

where 0 is a coverage of Pb,,. Ro falls in the oxide potential range for Pt with no Pb on the surface. There is no better potential for a reference since lead adsorbes at oxide free Pt surface.

Figure l a shows a voltammetry curve of Pt electrode in 1 M HClO, in the absence, and in the presence of 1 x M P b Z f . The adsorption of Pb commences at the first oxide-free Pt surface as seen from the increase of the current at potentials of oxide reduction. At potentials close to 0.0 V the coverage of lead reaches its saturation. It is seen that on such a surface there is no hydrogen adsorption-desorption processes. It will be shown later that this is of upmost importance for the catalytic effect of Pb on oxidation of HCOOH. A desorption of Pb in anodic sweep coincides with oxide formation on Pt.

I

f ! I I 6

o

04 a8 12 E ( V ) *

FIG. 1. - (a) Voltammetry curves of Pt electrode in 1 M HC104 in the presence (full line) and in the absence (dashed line) of Pbad. (b) Corresponding reflectivity-potential curves.

R = 470 nm ; sweep rate 50 mV/s.

Figure lb displays corresponding reflectivity-poten- tial curves. A small decrease of reflectivity is caused by hydrogen adsorption, while the oxide formation causes a pronounced decrease. This is in agreement with the data in the literature [17]. The adsorption of Pb causes a large change of reflectivity. A linearity of potential dependence of reflectivity indicates that the adsorption of Pb obeys the Temkin isotherm if the change of reflectivity is proportional to the lead coverage. This was ascertained by potential step measurements of lead adsorption under diffusion control. A linear plot of AR/Ro us. t 1 / 2 , indicates a diffusion control of lead adsorption and a linear relationship between AR/Ro and O,, [2]. The plot AR/Ro us. charge passed in deposition of Pb is not possible to obtaine, because of the problems in separation of the currents of adsorption of hydrogen and lead. McIntyre [18] has shown that the electroreflectance of Pt, arising from the increase in electronic surface-charge density, causes the increase of reflectivity. Since lead has the opposite effect, it seems that the whole change of reflectivity is caused by the surface concentration of Pb. This must be a conse- quence of the strong interaction of adsorbed lead atoms with the band structure of platinum.

IN PHASE

FIG. 2. -Differential reflectivity-potential curves of Pt elec- trode in the presence (full line) and in the absence (dashed line) of Pbad. Only in-phase component is shown. a. c. modulation potential, 100 mV p-p ; 33 Hz, sweep potential rate 20 mV/s ;

1 = 470 nm.

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The wavelength dependence of the relative reflectance changes are given in figure 3 for three potentials of Pt electrode. At 0.0, 0.2 and 0.4 V the lead coverage is 0.65, 0.57 and 0.50 respectively. It was calculated from the charge obtained from voltammetry curve and roughness factor of Pt elcctrode of 1.7. This factor was determined from the charge of hydrogen adsorption, using the charge of 230 pC/cm2 for a monolayer coverage at smooth surfaces. These are the saturation coverages at given potentials. Although the calculation shows that 8 < 1, it might be that at E = 0.0 V the coverage is compatible with monolayer i. e., no appreciable adsorption will occur at more cathodic potentials. In calculating a coverage of lead it is assumed that two electrons are exchanged in adsorption process. This should be justified by sepa- rate determination of electrosorption valence of adsorbed lead. I t is, however, beyond the scope of this paper.

+

O r A CALCULATED

I I I I

LOO 500 600 700 800

'

X lnm)

FIG. 3. -Wavelength dependence of normalized change of reflectivity for Pt electrode with Pbaa at thrce potentials (see text). Calculated curve obtained from data for bulk metals

(see text).

A monolayer, or submonolayer of lead adatoms on platinum should have optical properties which differ substantially from those of bulk lead because of two- dimensional nature of the layer and the strong interaction with the band structure of platinum substrate. Nonetheless, it is interesting to compare the reflectance changes calculated for a metallic lead layer using the refractive index of bulk lead with experimental data. The calculation has been carried out with the equation derived by McIntyre and Aspnes [19] as a linear approximation of Fresnel formula for a three layer system.

For a parallel polarization we have,

A A 8 dn, cos cpl (ARIR), = Im

[k

-

8 3 A] (2) A where h n n

si,

E~ and e3 are the complex dielectric constants of the solution, lead layer and platinum substratc, respectively ; nl is the refractive index of the solution ; d is the thickness of the lead layer ; and 40, angle of incidence. For the 45O angle of incidence used in the present work, the term A contributes relatively little to the wave- length dependence compared to the other term within the bracket in eq. (2) and will be approximated as 1 (2). Eq. (2) can be rearranged to the form,

U = (8 dnn, cos cpl)/l.

The wavelength dependence of (ARIR), have been calculated using the optical constants for Pt from the literature [20] and from the ellipsometric measurements for bulk lead [21]. The result is inserted in figure 3. The calculation shows an increase of reflectivity while experimentally a decrease was obtained. This discrepancy is not surprising since the optical properties of the monolayer should be quite different from those of lead. Further, the electronic properties and optical constants of the surface layer of platinum should be strongly perturbed by the lead.

The experimental curves show a monotonic increase of ARIR, in the wavelength region 500-750 nm. An indication of shoulder appears at 450-480 nm. We have refrained from speculating as to the origin of this shoulder at present. The three-layer model used in obtaining eq. (2) is too oversimplified, as evident from figure 3. Watanabe et al. [22] for platinum electrode with adsorbed copper atoms have also obtained the increase of reflectivity, calculated using the same equa- tions, while the experiment has shown the opposite. However, the same calculation of AdiiC et al. [2] for lead adsorbed on gold has reproduced the basic features of experimental spectrum. These results are considered as preliminary. More experimental work, and possibly, a refinement of a three-layer model seems to be neces- sary before reaching more definitive conclusions.

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C5-196 R. R. ADZIC AND M. M. PODLAVICKY

bound intermediate xxxCOH, which require three platinum sites for its adsorption [23]. It is formed in reaction of hydrogen adsorbed on platinum with the formic acid radical. The existance of another poisoning species, namely ..C(OH),, which adsorbes on two platinum sites, has been postulated. These species block the platinum surface for oxidation of HCOOH, which results in a rather small rate of this reaction on such surfaces. This is seen in figure 4, where a voltammetry curves for oxidation of HCOOH on Pt (dashed line), and Pt with Pb,, (full line), are given. Lead causes a large catalytic effect increasing the current of the peak in anodic direction by two orders of magnitude.

FIG. 4. - Voltammetry curves of Pt electrode in 1 M HC104 containing 0.265 M HCOOH in the absence (dashed line) and

in the presence (full line) of Pbad.

It has been demonstrated that there are two origins of this catalytic effect [13]. First, the lead adatoms prevent a formation of the main poison xxxCOH by suppression of hydrogen adsorption on Pt (Fig. 1). Second, the adsorbed atoms protect a part of Pt surface from adsorption of the species xxC(OH),, since it adsorbes on two platinum sites.

There are several questions concerning this explana- tion. The adsorption of lead may be affected by adsorption of formic acid and its intermediates. In electrochemical experiments this cannot be detected. Besides that, it has to be rationalize why cadmium, which also decreases the hydrogen adsorption on platinum, exhibits a much smaller catalytic effect than lead.

These questions have been answered with the help of specular reflectance measurements.

Figure 5 shows the reflectivity-potential curves of platinum electrode during oxidation of HCOOH. For the purpose of comparison the surve in the absence of formic acid is also given (Fig. 5a). It is seen that the formic acid, and its intermediates, produce a small decrease of reflectivity of platinum electrode. Barrett and Parsons [24] have found a small increase of reflectivity caused by the adsorption of methanol. It is believed that these two molecules form similar intermediates in oxidation and their different effect on reflectivity of Pt electrode is surprising.

FIG. 5.

-

Reflectivity-potential curves of Pt electrode in the absence of HCOOH and Pbad (dashed line). Full lines shows the same curve in the presence of HCOOH (a), and both HCOOH

and Pbad (b). Sweep rate 50 mV/s, 1 = 470 nm.

In the oxide formation region, from 0.8 to 1.2 V, the sweep in anodic direction shows an increase of reflectivity. It cannot be caused by a decrease of oxide formation on Pt since the loop caused by oxide is reproduced at other potentials. The voltammetry curve shows a peak at these potentials, which has been ascribed 1241 to the oxidation of the species xxxCOH

.

This reaction probably causes a reduction of platinum oxide, which is reformed at higher anodic potentials.

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One should be reminded of the fact that during recording of above reflectivity curve HCOOH is oxidized with the current density of the order of 100 mAcm-', as seen in figure 4. Simultaneously we follow the potential dependence of

e,,,

whose adsorption involves three orders of magnitude smaller current densities. This experiment illustrates remarkable possibilities of the specular reflectance technique in application to the studies of electrocatalysis. The separation of the currents associated with oxidation of HCOOH and adsorption of Pb seems otherwise impossible.

Figure 6 shows the potential dependence of differen- tial reflectance coefficient of platinum electrode for oxidation of HCOOH in the absence and in the presence of Pb,,. The curves differ considerably in the lead adsorption potential region. The comparison with figure 2 shows that the curves for Pt electrode with Pb,, are practically the same. This is even better evidence that the lead atoms are stronger adsorbed on Pt surface than the intermediates ...COH and ..C(OH),, since the a. c. electromodulation technique is more sensitive than the recording of reflectivity during potential sweep.

-+

I N PHASE

6

I I I I I I I

0 0 4 0 8 1 2 16 b

E I V )

FIG. 6. -Differential reflectivity-potential curves of Pt elec- trode in the presence of HCOOH (dashed line) and HCOOH and

Pbad (full line). Other data same as for figure 2.

3 . 3 CATALYTIC EFFECTS OF Cd ADATOMS ON OXIDA-

TION OF HCOOH ON Pt. - Cadmium adatoms exhi- bited much smaller catalytic effects than lead. This is evident from figure 7 which displays the voltammetry curves for oxidation of HCOOH on Pt electrode in the absence and in the presence of Cd,,. The peak in anodic direction is considerably increased, while in cathodic direction only the shoulder is changed, which caused its transformation into a peak.

Figure 8a shows the voltammetry curve for under-

potential deposition of Cd. It is seen that its adsorption also decreases the hydrogen adsorption on Pt. Its desorption, however, occurs at lower potentials than that of Pb. This causes somewhat larger currents in the hydrogen adsorption region. They are mostly due to desorption of Cd and partly to desorption of residual Had. Figures 8b and 8c display the corresponding reflec-

FIG. 7.

-

Voltammetry curves of Pt electrode in 1 M HC104 containing 0.265 M HCOOH in the absence (dashed line) and

in the presence (full line) of Pb,d.

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C5-198 R. R. ADZIC AND M. M. PODLAVICKY

tivity-potential curves. It is seen that Cd causes a small decrease of reflectivity (Fig. 8b). In the presence of HCOOH the reflectivity is slightly incrcased figure 8c. The curve actually looks like one in figure 5 , which was

obtained in the absence of adsorbed Cd atoms. This simply means that HCOOH and its interrncdiates cause a desorption of Cd atoms. This seems to be the reason for a smaller catalytic effect of Cd adatoms. A relatively weak adsorption of Cd atoms results in low electrosorption valence. Schultze and Vetter [25] have reported yCd2+

--

0.3 at 8 = 0.1. This means that adsorption of Cd is mainly electrostatic, with a smal charge transfer. It appears that intermediates of formic acid oxidation are stronger bound to the surface than cadmium adatoms.

3 . 4 M O D ~ L OF THE CATALYTIC EFFECT OF kOREIGN METAL ADATOMS.

-

I t has been mentioned above that the catalytic effect of Pb adatoms has two origins. They prevent the formation of ...COH intermediate and impede the adsorption of ,,C(OH),. The Iatter requires two sites for adsorption, and lead atoms will protect one neighbouring Pt atom. Therefore, for t),, = 0.5, Or, = 0.5 i. e. a half of the Pt surface will be available

for a reaction. This is the basis of the model which was successfully used in interpretation of voltammetry curves [15]. Some indications wcre found that the bare atoms of thc substrate were not affected by the neighbouring adsorbed foreign atoms. In heterogenous catalysis a similar behaviour of alloy catalyst is treated by the minimum polnrity model [26]. In its simplest form the model assumes that each component in alloy has the electronic structure as in its pure phase. The underpotential deposition in general does not result in alloy formation and the comparison with the catalysis by an alloy surface may be an oversimplification. However, it seems that a minimum polarity model is applicable to thcse systems.

The results of specular reflectance measurements indicate that indirect interactions (vertical) between Pb and Pt atoms are strong, which probably determines the wavelength dependence. This is substantiated by a linear dependence of the change of reflectivity with lead coverage. Direct interaction (horizontal) between the lead adatoms would be expected to be partly coulombic

due to small residual charge, and relatively weak under these circumstances.

A linear sweep voltammetry offers the evidence which supports these views. Figure 9 shows the voltammetry curves of Pt electrode in the presence of several concen- trations of Pb2+ in the electrolyte. At the same sweep rate different coverages of Pb,, are obtained in this way. The hydrogen adsorption decreases as O,, increases. However, the positions of the peaks for weakly and strongly bound hydrogen remain the same although their heights decrease. This is a dear evidence that Pb atoms interact only with Pt atoms on which they are adsorbed. This does not affect the neighbouring Pt atoms on which the hydrogen adsorption occurs as the lead atoms were not present.

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t

I

-

.

0 0 0 5

l o E I V ) 1.5

FIG. 9.

-

Voltammetry curves of Pt electrode in 1 M HCIO4,

1, in the absence and 2, 3, 4, 5 in the presence of 0.005, 0.01, 0.1 and 1 mM Pbz+ respectively.

Acknowledgments.

-

The authors are indebted to

Funds for Research of the Republic of Serbia for financial support.

References

[I] TAKAMURA, T., TAKAMURA, K., NTPPE, W. and YEAGER, E., [7] KOLB, D. M., PRZASNYSKY, M. and GEKISCHER, H., J. EIec-

J. Electrochem. Soc. 117 (1 970) 626. troanat. Chem. 54 (1974) 25.

[2] ~ ~ i r t , R., YEAGER, E. and CAHAN, B. D., Ibid. 121 (1976) _[g]_CONWAY,_B._{E., ANGERSTEIN-KOZLOWSKA,}_{H. and SHARP,}_W.

474. _{B. A.,}_2._{Phys. Chem.}_N._F.₉₈_{(1975) 61.}

[3] TAKAMURA, T. and SATO, Y., J. Efectroanat. Chem. 47 (1973)

245. [9] TRASATTI, S., ibid. 98 (1975) 75.

[4] KOLB, D. M., LEUTLOFP, D. and PRZASNYSKI, M., Surf. Sci. [lo] AD'C*

-

.,.-

R. and DESPIC* A. J. Phys.

47 (1975) 622. 348L.

[5] BEWICK, A. and THOMAS, B., J. Efectroanal. Chcm. 65 (1975) [I1] ADilt, R. R., NIKOLIC, B. Z- and DEsplC* A. R.* 27th ISE

321. Meeting, Zurich, 1976.

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[13] A ~ i r d , R. R., S I M I ~ , D. N., DESPIC, A. R. and DRAZIC, D.

M., J. Electroanal. Chem. 61 (1975) 117 and 65 (1976) 587.

[14] An2.16, R. R., DESPIC, A. R., SIMIC, D. N. and DRAilt, D. M., Nat. Bur. Stand. Spec. Publ. 455. Electrocatalysis on Non-Metallic Surfaces, Proc. of NBS Workshop, Dec. (1975).

[15] ADirt, R. R., Srwd, D. N., DESPIC, A. R. and D R A ~ I ~ , D.

M., J. Electroanat. Chern., in press.

[I61 Des~rC, A. R., A ~ i r k , R. R. and T R ~ P K O V I ~ , A. V., Elektro-

khirniya, in press.

[17] MCINTYRE, J. D. E. and K o ~ n , D. M., Symp. Faraduy Soc. 4

(1970) 56.

[18] MCINTYRE, J. D. E. and PECK, W. F., Symp. Furaday Soc.

(1973).

[19] MCINTYRE, J. D. E. and ASPNES, D. E., Surf: Sci. 24 (1971) 417.

[20] LANDOLT-BORNSTTIN, 6. Aufl., 11 Band, 8 Teil (Springer- Verlag, Berlin) 1962, p. 1-1.

[21] HORKANS, J., Ph. D. Thesis, Case Western Reserve Univ. Cleveland, Ohio (1973).

[22] WATANABE, F., TAKAMURA, K. and TAKAMURA, T., Denki

Kagaku 43 (1975) 469.

[23] CAPON, A. and PARSONS, R., J. Electroanal. Chem. 45 (1973) 205.

[24] BARRFITT, M. A. and PARSONS, R., Symp. Faraday Soc. 4 (1 970) 72.

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[26] LAND, N. D. and EHRENREICH, H., Phys. Rev. 168 (1968) 605.

DISCUSSION

S. GOTTESFELD. - Can you please give an idea on the extent of improvement in the formic acid oxidation process thanks to the adsorbed lead atoms, specifically in long term oxidations.

R.

R. A D ~ I ~ . - The steady-state measurements showed that the effect of lead is large and stable. (Cf.

R.

AdiiC, et al., J. Electroanal. Chem. 65 (1975) 585.) The oxidation of formic acid is known as a reaction which causes that the establishment of equili- brium on platinum electrodes requires long times. In the presence of lead this happens much faster.

W. HANSEN. -- Would YOU further explain what you

meant when you said that your electroreflectance measurement : do not agree with McIntyre theory ?

In particular, have you taken into account the four valence electrons which might be contributed by each Pb atom ?