894
J. Phys. Chem. 1990, 94, 894-900
right-hand side. This difference corresponds well to the higher reducibility of PMo,, observed in the temperature-programmed reduction8J0and the redox potential." Another difference is that R, was greater for PMo,,. This indicates that the reactivity between protons and oxygen atoms of the polyanion was higher for PMoI2,although R , and R2 were much smaller. The reason why the reduction of PMo,, was nearly independent of the surface area may now be obvious. In the initial stage of the reduction, there was an approximate relationship, R , - R , = 1 SR,, and in the very late stage R , - R2 = R3 (Table I). Hence, in any case, the rate of H2 uptake (=r(H,) = R, - R 2 ) was controlled mainly by R3, which is the rate of water formation in the bulk. Since this reaction mostly takes place in the bulk, as discussed above, r(H2) depended to a much smaller extent on the specific surface area than the case when the reduction took place only near the surface. This is what we observed i n the present study. as shown in Figures 1 and 2 . Mechanism of Catalytic Oxidation of H , over PMoI2. If the catalytic oxidation proceeds by the repetition of reduction and reoxidation of the catalyst (a redox or Mars-van Krevelen mechanism), r(H2) and r ( 0 2 ) , which were measured for the catalyst at the stationary state, must coincide. This kind of measurement is possible and meaningful for PW,, and PMo,,, because the reduction and reoxidation can proceed uniformly throughout the bulk, and in fact the results obtained previously proved the redox mechanism.) Similar measurements were performed for PMo,,'s having different specific surface areas. The data are collected in Figure 7. While the degree of reduction at the stationary state differed among the catalysts having different and surface areas, r(H2) (shown by one solid line), r(02) (O), r(cat.) ( 0 )for each catalyst agreed well as shown in Figure 7. (10) Katamura, K.; Nakamura, T.; Sakata. K.; Misono, M.; Yoneda, Y Chem. Lett. 1981, 89. ( I 1) Pope. M. T. Heteropoly and Isopoly Oxometallates; Springer: Berlin, 1983.
r(H,) depended little on the surface area, while r ( 0 2 ) was proportional to the surface area, if they are compared for a given oxidation state.3 Therefore, the degree of reduction at the stationary state, at which the two rates agree, became smaller for catalysts having greater surface area. Furthermore, the change of r(H2) was rather small in the range of the degree of reduction examined in the present study (0.094.17 electrons/anion), so that r(cat.) did not much depend on the surface area, as observed. Conclusion (1) H,-D2 reactions (equilibration, exchange, and reduction of the catalyst) over PMo,, were very different in features from those for PW,,. Isotopic equilibration of H2 and D2 and exchange between H2 (D,)in the gas phase and protons in the catalyst were much slower than PW,,, while the reduction of catalyst (H2 uptake) was faster. It was quantitatively demonstrated based on numerical simulation that the equilibration and exchange were apparently very slow because the reduction of catalyst (the H2 uptake and subsequent formation of water) proceeded at a rate comparable with the dissociation of H2 (D2). In the case of PW,,, the dissociation of H2 and its recombination were much more rapid than the H, uptake and subsequent water formation. (2) In spite of the marked difference between PMo12and PW,,, the catalytic oxidation of H2 over PMo,, as well as the reduction of PMo,, by H 2 depended little on the specific surface area of PMo,,, if the surface area was greater than 1 m2.g-I. It was also shown quantitatively that this is because the second step of the reduction, that is, the formation of water from proton and polyanion (eq 5 ) , proceeded nearly uniformly throughout the catalyst bulk. Acknowledgment. This study was supported in part by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture (No. 01550610 and 62470070). Registry No. H,PMo,,Oa, 12026-57-2; Na2HMo120,0,55624-58-3; HZ. 1333-74-0; CO, 630-08-0.
Coadsorption of Copper with Anions on Platinum(111): The Role of Surface Redox Chemistry in Determining the Stability of a Metal Monolayer J. H. White* and H. D. Abruiia* Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853- 1301 (Received: July 25, 1989)
The voltammetry for the underpotential deposition (UPD) of copper from aqueous sulfuric acid solution on flame-annealed Pt( 1 1 1 ) electrodes pretreated with or in the presence of C1-, Br-, I-, or S2- is reported. We find that the potential for the UPD of copper on Pt( 1 11) decreases in the order C1- > Br- > I- > S2-and is very linearly related to the standard reduction potential for the half-cell reaction CuX + e- Cu + X-. This is consistent with the near-edge features of the observed X-ray absorption spectrum and with a model involving partial charge transfer from the adsorbed copper to the coadsorbate, which is most likely initially present on the platinum surface in a nonanionic (i.e., neutral) state. In addition, the kinetics of cooper UPD for the different cases were also found to vary linearly with the aforementioned standard potential.
-
Introduction The underpotential deposition (UPD) of copper on Pt has been the subject of a number of recent publications dealing with the atomic level structure of this and related systems. It has been demonstrated that copper deposition of Pt(l1 l ) , upon which a well-defined iodine adlattice exists, occurs in such a manner so as to produce superlattices of copper and iodine.' Using surface EXAFS,, we have shown that copper UPD on iodine-coated (1) Stickney, J. L.; Rosasco, S . D.; Hubbard, A. T. J . Electrochem. Soc. 1984, 131, 260.
0022-3654/90/2094-0894$02.50/0
Pt( 11 1) produces structures that give rise to very-pronounced fine structure, suggesting the presence of copper clusters on the electrode surface at low (
