UHV and electrochemical studies of CO and methanol adsorbed at platinum/ruthenium surfaces, and reference to fuel cell catalysis

Abstract

This paper reviews surface chemistry of carbon monoxide and methanol in ultra high vacuum (UHV) and in the electrochemical environment on clean and Ru modified Pt single crystal surfaces, and on Pt and Pt/Ru nanoparticles. The results show that CO behaves very similarly in UHV and in the electrochemical environment. Cyclic voltammetry (CV), temperature programmed desorption (TPD) and radioactive labeling all show similar behavior in terms of numbers of peaks, peak splitting etc. Both UHV and CV measurements show that there is about a 200-meV change in the potential for CO removal in the presence of ruthenium. Earlier 13C EC-NMR data indicated a 30% reduction in the E f-LDOS of CO bound to Ru islands deposited on platinum, and 15% of CO bound to Pt sites, and TPD and CV also show that the binding of CO is modified. The present data confirm that Pt atoms away from Ru are only weakly affected, and the overall CO binding energy modification is quite moderate. We conclude that the changes in the CO binding energy only play a small role in enhancing methanol electrooxidation rates. Instead, the main effect of the ruthenium is to activate water to form OH. Quantitative estimates of the reduction in CO desorption barrier indicate that the effect of bifunctional mechanism is about four times larger than that of ligand effect. In contrast to the results for CO, methanol behaves quite differently in UHV and in an electrochemical environment. Pt(111) is unreactive at room temperature in UHV, while Pt(110) is quite reactive. Initially, clean Pt(111) is less reactive than clean Pt(110) even in the electrochemical environment. However, Pt(110) is quickly poisoned in the electrochemical environment, so at steady state, Pt(111) is more reactive than Pt(110). Another issue is that the mechanism of methanol decomposition is quite different in UHV and in the electrochemical environment. There are three pathways in UHV, a simple decomposition via a methoxonium (CH 3O (ad)) intermediate, an S N1 pathway via a methoxonium cation ([CH 3OH 2] +), and an S N2 pathway via a methoxonium intermediate. So far, none of these pathways have been observed in an electrochemical environment. Instead, the decomposition goes mainly through a hydroxymethyl (CH 2OH (ad)) intermediate. These results show that there are both similarities and differences in the behavior of simple molecules in UHV and in the electrochemical environment.