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Langmuir 2007, 23, 12760-12764
High Activity of Pt4Mo Alloy for the Electrochemical Oxidation of Formic Acid Snezˇana Lj. Gojkovic´,*,† Amalija V. Tripkovic´,‡ Rade M. Stevanovic´,‡ and Nedeljko V. Krstajic´† Faculty of Technology and Metallurgy, UniVersity of Belgrade, KarnegijeVa 4, P.O. Box 3503, 11000 Belgrade, Serbia, and ICTM-Institute of Electrochemistry, UniVersity of Belgrade, NjegosˇeVa 12, P.O. Box 473, 11000 Belgrade, Serbia ReceiVed August 1, 2007. In Final Form: September 25, 2007 Surface processes on Pt4Mo alloy well-defined by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were studied in acid solution by cyclic voltammetry. It was established that Mo in the alloy is much more resistant toward electrochemical dissolution than pure Mo. During the potential cycling of Pt4Mo surfaces in completely quiescent electrolyte, hydrous Mo-oxide could be generated on Mo sites. Investigation of the formic acid oxidation revealed that this type of Mo-oxide enhances the reaction rate by more than 1 order of magnitude with respect to pure Pt. Surface poisoning by COads is significantly lower on Pt4Mo alloy than on pure Pt. The effect of hydrous Mo-oxide on the HCOOH oxidation rate was explained through the facilitated removal of the poisoning species and through its possible influence on the intrinsic rate of the direct reaction path.
Introduction Oxidation of formic acid on Pt has been extensively studied not only as a model reaction in electrocatalysis but also as a candidate for the anodic reaction in a proton exchange membrane fuel cell.1 Although it is a relatively simple reaction involving only two electrons, its mechanism is still not fully understood. According to Capon and Parsons,2 the reaction proceeds through two pathways. After adsorption, HCOOH molecules undergo dehydration, forming COads which is further oxidized to CO2 (an indirect pathway), and dehydrogenation in which CO2 is produced, circumventing COads (a direct pathway). Adsorbed formate, HCOOads, was proposed as an active intermediate in the direct path,3 but it was recently suggested4 that bridge-bonded HCOOads, although present on the electrode surface, is not the intermediate in the direct reaction pathway. Thus, the triple path mechanism including the indirect pathway, the formate pathway, and the direct pathway was envisaged.4 Adsorbed CO was identified as the poisoning species and detected by infrared spectroscopy.5 Platinum is the most active material for the adsorption of HCOOH, but as it is readily poisoned by COads, its practical application requires modification of the surface by some foreign atoms. A significant increase of the HCOOH oxidation rate was found on Pt-Pb,6-8 Pt-Bi,9-12 Pt-Sn,13 and Pt-Pd13-15 surfaces, * To whom correspondence should be addressed. E-mail: sgojkovic@ tmf.bg.ac.yu. Fax: +381 11 3370 387. † Faculty of Technology and Metallurgy. ‡ ICTM-Institute of Electrochemistry. (1) Jarvi, T. D.; Stuve, E. M. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; pp 75-153. (2) Capon, A.; Parsons, R. J. Electroanal. Chem. 1973, 44, 1-7. (3) Miki, A.; Ye, S.; Osawa, M. Chem. Commun. 2002, 1500-1501. (4) Chen, Y. X.; Heinen, M.; Jusys, Z.; Behm, R. J. Angew. Chem., Int. Ed. 2006, 45, 981-985. (5) Park, S.; Xie, Y.; Weaver, M. J. Langmuir 2002, 18, 5792-5798. (6) Xia, X.; Iwasita, T. J. Electrochem. Soc. 1993, 140, 2559-2565. (7) Solis, V.; Pletcher, D. J. Electroanal. Chem. 1982, 131, 309-323. (8) Casado-Rivera, E.; Volpe, D. J.; Alden, L.; Lind, C.; Downie, C.; Vaı`zquezAlvarez, T.; Angelo, A. C. D.; DiSalvo, F. J.; Abrun˜a, H. D. J. Am. Chem. Soc. 2004, 126, 4043-4049. (9) Adzˇic´, R.; Tripkovic´, A.; Vesˇovic´, V. J. Electroanal. Chem. 1986, 204, 329-341. (10) Clavilier, J.; Fernandez-Vega, A.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1989, 258, 89-100.
but only a modest effect was found on Pt-Ru surfaces.16-18 The action of Sn and Ru was discussed in terms of the bifunctional mechanism.13,16 The effect of Bi was ascribed to a combination of the third-body effect and the electronic effects,10,11,19 though bifunctional action of hydroxylated Bi species was also considered.12 For submonolayers of Pd on nanosized Pt, it was demonstrated that the catalyst remained reasonably active even with substantial COads coverage; that is, the direct path was accelerated.14,15 Similarly, it was concluded that Pb adatoms acted as a true catalyst in the direct oxidation path.6 To date, electrocatalysts based on Pt and Mo or its oxide have been investigated with respect to electrochemical oxidation of CO and alcohols, but surveying the literature we have found only two publications dealing with the influence of Mo on HCOOH oxidation. Wu et al.20 showed that polyaniline-PtHxMoO3 was slightly more active than a Pt electrode, while Song et al.21 found improvement in the performance of HCOOH fuel cells when Pt or PtRu blacks were modified with Mo-oxide. Thus, we examined HCOOH oxidation on well-defined Pt4Mo alloy and correlated the reaction rate to the electrochemical pretreatment of the Pt4Mo surface. The results are discussed in terms of the third-body effect, the bifunctional mechanism, and the electronic effects. (11) Casado-Rivera, E.; Gaı`l, Z.; Angelo, A. C. D.; Lind, C.; DiSalvo, F. J.; Abrun˜a, H. D. ChemPhysChem 2003, 4, 193-199. (12) Tripkovic´, A. V.; Popovic´, K. Dj.; Stevanovic´, R. M.; Socha, R.; Kowal, A. Electrochem. Commun. 2006, 8, 1492-1498. (13) Chetty, R.; Scott, K. J. New Mater. Electrochem. Syst. 2007, 10, 135142. (14) Zhao, M.; Rice, C.; Masel, R. I.; Waszczuk, P.; Wieckowski A. J. Electrochem. Soc. 2004, 151, A131. (15) Rice, C.; Masel, R. I.; Wieckowski, A. J. Power Sources 2003, 115, 229. (16) Markovic´, N. M.; Gasteiger, H. A.; Ross, P. N., Jr.; Jiang, X.; Villegas, I.; Weaver, M. J. Electrochim. Acta 1995, 40, 91-98. (17) Ross, P. N., Jr. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998. (18) Tripkovic´, A. V.; Gojkovic´, S. Lj.; Popovic´, K. Dj.; Lovic´, J. D.; Kowal, A. Electrochim. Acta. 2007, 53, 887-893. (19) Oana, M.; Hoffmann, R.; Abruna, H. D.; DiSalvo, F. J. Surf. Sci. 2005, 574, 1-16. (20) Wu, Y. M.; Li, W. S.; Lu, J.; Du, J. H.; Lu, D. S.; Fu, J. M. J. Power Sources 2005, 145, 286-291. (21) Song, C.; Khanfar, M.; Pickup, P. J. Appl. Electrochem. 2006, 36, 339.
10.1021/la702344s CCC: $37.00 © 2007 American Chemical Society Published on Web 11/08/2007
Formic Acid Oxidation on Platinum-Molybdenum Alloy
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Experimental Section Pt4Mo alloy was produced by arc-melting of high purity components under Ar atmosphere. The previous characterization of the alloy by X-ray diffraction (XRD) showed a single phase structure of face-centered cubic (fcc) symmetry with the peaks corresponding to Pt.22,23 This is characteristic of the solid solution of Mo in Pt, which is to be expected based on the similarity of the Pt and Mo atomic radii (0.139 and 0.140 nm, respectively). However, X-ray photoelectron spectroscopy (XPS) detected electron transfer from the hyper-d-electronic antibonding Pt band to the deficient hypod-electronic Mo bonding band, indicating the formation of the intermetallic phase.22,23 Ab initio calculations also predicted the compound Pt4Mo.24 By integrating the XPS spectra, the surface Pt/Mo atomic ratio was calculated to be 3.6:1. The small deviation from the bulk composition (4:1) was explained by an experimental error or a possible slight segregation of Mo on the sample surface.22 A cylindrical specimen of Pt4Mo alloy or pure polycrystalline Pt was fitted into the Teflon holder and attached to the rotating disk assembly (Pine Instrument, Inc.). Before each experiment, the electrode surface was polished with an Al2O3 water suspension (particle size ) 0.05 µm) and cleaned ultrasonically in high purity water. A standard glass cell was used with a Pt wire as the counter electrode, and a saturated calomel electrode was used as the reference electrode. All the potentials are expressed on the scale of the reversible hydrogen electrode (RHE). The electrolyte contained 0.1 M H2SO4 and 0.1 M HCOOH. High purity water (Millipore, 18 MΩ cm) and p.a. grade chemicals (Merck) were used. In some experiments, the concentration of the supporting electrolyte was varied between 0.001 and 0.1 M H2SO4, and those results were presented on the scale of the standard hydrogen electrode (SHE). The electrolyte was deaerated by high purity N2. The cell was thermostated at 30.0 °C. A Pine RDE4 potentiostat and a Philips PM 8143 X-Y recorder were employed. After having immersed a Pt4Mo electrode in the supporting electrolyte, the potential was cycled between 0.05 and 1.15 V at 100 mV s-1 until a steady-state voltammogram was obtained. Further, in the positive going sweep, the potential was held at 0.1 V, HCOOH was added into the electrolyte, and after 2 min the sweep was continued at the rate of 1 mV s-1. Potentiodynamic polarization curves were recorded at 50 mV s-1 after the slow sweep experiments were accomplished. The procedure for the experiments with the Pt electrode was the same. All the polarization curves for HCOOH oxidation were recorded at the stationary electrode with the stream of N2 being passed over the electrolyte. It is reasonable to assume that oxidation of HCOOH on Pt4Mo alloy takes place on Pt sites, because Pt is much more active for the electrooxidation of organic molecules than other metals. Because of that, the current densities for HCOOH oxidation were calculated with respect to the real surface area of Pt on the Pt4Mo surface. For each particular experiment, the steady-state cyclic voltammogram of Pt4Mo in the supporting electrolyte was integrated in the potential region of the hydrogen desorption and, assuming 0.210 mC cm-2 for the monolayer of adsorbed hydrogen, the real surface area of Pt was calculated. In this way, comparison between the current densities for HCOOH oxidation on pure Pt and on Pt4Mo alloy provides insight into the influence of Mo on the reaction kinetics. Although one can suspect that such a determination of the Pt surface could give underestimated values because of a possible Mo influence on the capability of Pt to adsorb atomic hydrogen, it appears that this effect is not significant. When leaching of Mo from the alloy surface was facilitated by prolonged cycling up to 1.3 V (see Figure 3), the charge for hydrogen desorption increased by 35%, which is close to 25% expected from the alloy composition. The difference can be also ascribed to the roughening of the surface upon leaching of Mo. It should be stressed that the roughness factor of the Pt4Mo sample (22) Vracˇar, Lj.; Krstajic´, N.; Neophytides, S. G.; Jaksˇic´, J. Int. J. Hydrogen Energy 2004, 29, 835-842. (23) Jaksic, J. M.; Vracˇar, Lj.; Neophytides, S. G.; Zafeiratos, S.; Papakonstantinou, G.; Krstajic, N. V.; Jaksic, M. M. Surf. Sci. 2005, 598, 156-173. (24) Curtarolo, S.; Morgan, D.; Ceder, G. Calphad 2005, 29, 163-211.
Figure 1. Cyclic voltammograms of a Pt4Mo surface upon extending the anodic potential limit, recorded in a quiescent solution of 0.1 M H2SO4 at the sweep rate of 100 mV s-1. Inset: Dependence of the potential of the anodic redox peak on the concentration of H+ ions. Geometric surface area ) 0.33 cm2. after the leaching of Mo was found to be about 1.9, which is almost the same as that for the pure Pt electrode polished in the same manner.
Results and Discussion Cyclic Voltammetry of the Pt4Mo Surface. Cyclic voltammograms of a Pt4Mo electrode in a quiescent solution of 0.1 M H2SO4 are presented in Figure 1. A profile with weak features of hydrogen adsorption/desorption was obtained when the potential was cycled up to 0.4 V. In the first cycle toward more positive potentials (0.6 V), a broad anodic peak with a maximum at about 0.47 V appeared. In the second cycle, this peak decreased and vanished after about 20 cycles (Figure 1a). With further extension of the positive potential limit (1.15 V), an increase in the anodic current was observed at 0.7 V (Figure 1b). In the second cycle, the broad anodic peak at 0.47 V reappeared and increased over time. The voltammogram depicted as a thick solid line in Figure 1b with the developed anodic peak and its smaller cathodic counterpart was obtained after about 15 min of cycling. When the redox peaks once developed, resetting the positive potential limit to 0.7 V, that is, before the beginning of the anodic process which induced their growth, showed no influence on them. Essentially the same voltammogram was obtained even when, after holding the potential at 0.7 V for 10 min, the electrode was transferred into the fresh electrolyte. This indicates that Mo was not completely leached from the surface. Extension of the positive potential limit (Figure 1b) was also followed by a gradual development of the peaks for hydrogen adsorption/desorption on Pt, which could be induced by partial dissolution of Mo from
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Figure 2. Steady-state cyclic voltammogram of a Pt4Mo surface recorded in 0.1 M H2SO4 at the rotating electrode (solid line) and tenth cycle after the rotation was switched off (dotted line). Sweep rate ) 100 mV s-1. Geometric surface area ) 0.33 cm2.
GojkoVic´ et al.
Mo and of Mo in Pt4Mo alloy is in accord with the XPS measurements which indicated a redistribution of charges around the Pt and Mo atoms, compared with their pure metallic state.22 The anodic peak at 0.47 V appearing in the first cycle with the positive potential limit of 0.6 V (Figure 1a) can be ascribed to the oxidation of a Mo-oxide from the bimetallic surface to a higher oxidation state. Since the peak disappeared over time, but Mo was not significantly leached from the surface, probably some Mo species formed at the Mo surface sites, which can be only partially reduced during the cathodic scan. Interestingly, the same anodic peak starts to develop again upon extending the positive potential limit. dos Anjos et al.,26 who also observed the same redox couple on Pt-Mo alloys, ascribed it to MoO3/HxMoO3 (x > 0) and postulated that Mo oxo species generated in the solution readsorbed at the Pt surface at E < 0.4 V. As the increase of the redox peaks was not followed by the decrease of the hydrogen adsorption/desorption peaks of Pt, it seems that Mo oxo species are readsorbed on Mo-oxide sites. A gradual increase of the redox peaks and their sensitivity to the rotation of the electrode suggest that a loosely bound hydrated Mo-oxide was growing on Mo sites on the alloy surface. In the potential region of the redox peaks, which are postulated to be the oxidation and reduction of hydrated Mo-oxide, several electrochemical reactions are possible:26
MoO2 + 4H+ + e- ) Mo3+ + 2H2O
Eθ ) 0.311 V
(1)
H2MoO4(aq) + 2H+ + 2e- ) MoO2 + 2H2O Eθ ) 0.39 V (2) HMoO4- + 3H+ + 2e- ) MoO2 + 2H2O Eθ ) 0.429 V (3)
Figure 3. First cyclic voltammograms of Pt4Mo recorded with the anodic limit of 1.3 V at the stationary electrode (dotted line) and steady-state cyclic voltammogram at the rotating electrode (solid line). Thick gray line represents the voltammogram of a pure Mo wire. Solution ) 0.1 M H2SO4 and sweep rate ) 100 mV s-1. Geometric surface area ) 0.33 cm2.
the surface followed by the recrystallization of Pt, or more likely by the effect of the Pt surface cleaning at positive potentials. It is well-known that a pure Pt surface has to be cycled in the potential range from H2 to O2 evolution to get fully developed and resolved hydrogen adsorption/desorption features. After having initiated the rotation of the electrode on which the redox peaks were previously developed, the peaks started to decrease and they almost disappeared after about 10 cycles up to 1.15 V. When the rotation was switched off, the peaks started to grow again (Figure 2). This shows that the substantial leaching of Mo from the alloy did not occur below 1.15 V. However, when the anodic limit was set to 1.3 V and the electrode was rotated during the cycling, the redox peaks irreversibly disappeared and the voltammogram gained the characteristics of pure Pt (Figure 3). The cyclic voltammogram of pure Mo wire in 0.1 M H2SO4, also given in Figure 3, shows that Mo starts to dissolve already at 0.4 V. During potential cycling with the positive limit of 1 V, no passivation peak was observed but a dark-brown powder (probably MoO3)25 was falling off from the electrode surface. The difference in the electrochemical behavior of pure (25) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solution, 2nd ed.; National Association of Corrosion Engineers: Houston, TX, 1974; p 272.
Furthermore, the electrochemical reduction of Mo(VI) species produces electrodeposited material in which hydrogen is intercalated in the oxide film:27
MoO3 + xH+ + xe- ) HxMoO3
(4)
The dependence of the cathodic peak of the redox pair on the H+ concentration is difficult to determine precisely because of its overlapping with the adsorption of hydrogen on Pt atoms, but for the anodic peak potential (inset in Figure 1) a linear dependence with the slope of 60 mV dec-1 was found. This suggests that an equal number of electrons and H+ ions participate in the process, that is, reactions 2 or 4 are the most probable. Oxidation of Formic Acid. The oxidation of HCOOH was investigated on Pt4Mo surfaces preconditioned by potential cycling up to 1.15 V in the supporting electrolyte on the stationary electrode and on the rotating electrode. The polarization curve was also recorded for the pure Pt electrode. Quasi steady-state polarization curves presented in Figure 4 show that the Pt4Mo surface preconditioned in the quiescent electrolyte is much more active than pure Pt. At 0.4 V, the enhancement factor is 40. The Pt4Mo surface at which the growth of hydrous Mo-oxide was disturbed by the convection of the electrolyte performed similarly to pure Pt at low potentials. However, from the potential at which the redox process on Mo sites commences (about 0.4 V), the current densities increased faster and at 0.7 V reached the activity of Pt4Mo preconditioned in the quiescent electrolyte. The (26) dos Anjos, D. M.; Kokoh, K. B.; Leger, J. M.; de Andrade, A. R.; Olivi, P.; Tremiliosi-Filho, G. J. Appl. Electrochem. 2006, 36, 1391-1397. (27) Pereira, A. C.; Ferreira, T. L.; Kosminsky, L.; Matos, R. C.; Berotti, M.; Tabacniks, M. H.; Kiyohara, P. J.; Fantini, M. C. A. Chem. Mater. 2004, 16, 2662-2668.
Formic Acid Oxidation on Platinum-Molybdenum Alloy
Figure 4. Tafel plots for the oxidation of formic acid in 0.1 M HCOOH + 0.1 M H2SO4 on pure Pt and on Pt4Mo alloy preconditioned in quiescent and stirred electrolyte. Sweep rate ) 1 mV s-1. Inset: Chronopotentiograms for HCOOH oxidation on pure Pt and on Pt4Mo alloy previously cycled in quiescent electrolyte, recorded by stepping the potential from 0.1 to 0.4 V.
Figure 5. Potentiodynamic polarization curves for the oxidation of formic acid in 0.1 M HCOOH + 0.1 M H2SO4 on pure Pt and on Pt4Mo alloy preconditioned in stirred and in quiescent electrolyte, recorded in the positive going scan at 50 mV s-1.
chronoamperograms recorded at 0.4 V (inset of Figure 4) corroborated the difference in the activities of Pt and Pt4Mo alloy that was found under the slow sweep conditions. Since all the current densities in Figure 4 are expressed as per Pt surface area, the increase in the reaction rate is exclusively the consequence of the hydrous Mo-oxide presence near the Pt sites at which the reaction takes place. If the activities of Pt4Mo alloy and pure Pt were compared with respect to geometric surface area, the enhancement factor at 0.4 V would be 26 instead of the previously mentioned 40. Insight into the amount of the poisoning species on the electrode surface can be obtained when the reaction is examined under transient conditions. Potentiodynamic polarization curves recorded in the positive going scans at 50 mV s-1 when the slow sweep experiments were accomplished are displayed in Figure 5. The oxidation rate of HCOOH on pure Pt reached a plateau at about 0.55 V, which was followed by the ascending current from 0.8 V and a high maximum at 0.88 V. Such a profile suggests that HCOOH oxidation at low potentials proceeds through the direct path with the simultaneous formation of COads in the indirect
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path.28 Increasing coverage of COads reduces the amount of Pt surface available for the direct path, and the current density reaches a plateau. Subsequent formation of the oxygen containing species on Pt enables the oxidative removal of COads, manifested as the current peak. The potentiodynamic curves for HCOOH oxidation on Pt4Mo electrodes preconditioned in stirred electrolyte and in quiescent electrolyte both point to quite different behavior. At low potentials, the current densities are much higher than those on pure Pt, especially on the electrode preconditioned in quiescent electrolyte, which is in accord with quasi-steady-state polarization curves. The oxidation of COads is displayed as the low peak or the shoulder at the descending part of the curve. This indicates significantly less poisoned Pt4Mo surfaces by COads compared to pure Pt, resulting in the enhanced rate of the direct path. Role of Mo in the Promotion of the Oxidation of Formic Acid. Foreign atoms on a Pt surface can improve its electrocatalytic behavior either by diminishing the poisoning reaction or by accelerating the main reaction. Several mechanisms have been proposed in the literature to explain their action, and the electrocatalytic activity of the Pt4Mo surface toward HCOOH oxidation is discussed here with respect to them. Surfaces of Pt4Mo alloy preconditioned in quiescent electrolyte and in stirred electrolyte should not differ significantly regarding the size of the ensembles of Pt atoms. When Pt4Mo alloy was preconditioned in the stirred electrolyte, the amount of loosely bound hydrous Mo-oxide was low, if any, but Mo was still present at the surface, probably in some form of a more compact surface Mo-oxide. Even if some spilling of the hydrous oxide to the nearby Pt sites is possible, cyclic voltammetry did not show a significant difference in the Pt surface area with two types of Mo-oxide, and the difference in the HCOOH oxidation rate on two Pt4Mo surfaces is indeed too large to be explained by geometric restriction, that is, the third-body mechanism. Since the presence of hydrated Mo-oxide was found to be essential for the enhancement of the HCOOH reaction rate, the bifunctional mechanism seems to be more plausible. Applying the rationale of Watanabe and Motoo for the oxidation of CH3OH on Pt-Ru alloy,29 the oxidation of HCOOH on Pt4Mo alloy could be envisaged as proceeding through HCOOH adsorption on the Pt sites, producing COads, possibly formate, and some other active intermediate, while neighboring hydrous Mo-oxide facilitates the oxidation of COads. This is in accord with the activity of Pt-Mo bimetallic catalysts for CO oxidation reported in the literature.26,30-32 Indeed, our potentiodynamic measurements indicated a lower amount of COads in the presence of hydrated Mo-oxide but not its complete absence. Thus, it can be disputed that the enhancement by more than 1 order of magnitude is caused only by the reduction of COads coverage, keeping in mind that on pure Pt it amounts to up to 0.6.5 It can also be possible that hydrous Mo-oxide, besides the suppressing COads coverage, enhances the intrinsic rate of the direct oxidation path, as it was proposed for the influence of Pd,14,15 Pb,6 and Bi19 on the HCOOH oxidation rate on the Pt catalyst.
Conclusion Investigation of Pt4Mo surfaces by cyclic voltammetry showed that hydrous Mo-oxide is generated on the surface during the (28) Lovic´, J. D.; Tripkovic´, A. V.; Gojkovic´, S. Lj.; Popovic´, K. Dj.; Tripkovic´, D. V.; Olszewski, P.; Kowal, A. J. Electroanal. Chem. 2005, 581, 294-302. (29) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267-273. (30) Grgur, B. N.; Zhuang, G.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. B 1997, 101, 3910-3913. (31) Grgur, B. N.; Markovic, N. M.; Ross, P. N. J. Electrochem. Soc. 1999, 146, 1613-1619. (32) Samjeske´, G.; Wang, H.; Lo¨ffler, T.; Baltruschat, H. Electrochim. Acta 2002, 47, 3681-3692.
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cycling of the electrode potential in the quiescent electrolyte. The activity of such Pt4Mo surfaces for HCOOH oxidation was found to be much higher than the activity of the surface previously cycled in the stirred electrolyte as well as the activity of pure Pt. It is suggested that hydrous Mo-oxide enhances the HCOOH oxidation rate by diminishing COads coverage, but the intrinsic enhancement of the direct reaction path should also be considered.
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Acknowledgment. The authors are grateful to Professor P. L. H. Notten from Philips Research Laboratory, Eindhoven, The Netherlands, for supplying the alloy. This work was financially supported by the Ministry of Science, Republic of Serbia, under Contract No. 142056. LA702344S
