The effect of ruthenium on the binding of CO, H-2, and H2O on Pt(110)

Ruthenium is known to improve the CO tolerance of platinum based fuel cell catalysts, but the mechanism is unclear. Some investigators believe that th e main role of ruthenium is to dissociate water, and thereby provide a fast er route for the oxidation of CO. Other investigators believe that rutheniu m changes the electronic structure of platinum, in a way that lowers the bi nding energy of the CO and thereby promotes easy reaction. In this study, w e have used TPD to measure the magnitude of the two effects in UHV. When we deposit 0.25 monolayers of ruthenium on Pt(110), we find that the adsorpti on properties of the surface change substantially. The surface only adsorbs about half as much CO and H-2. The sticking probability of water is also r educed. The beta (2) hydrogen peak disappears, whereas the alpha (1), CO pe ak is attenuated by a factor of 2. The binding energy of the alpha (1) CO d ecreases from 31 to 29 kcal/mol when ruthenium is added to the surface, whe reas the binding energy of the alpha (2) CO decreases from 25 to 23 kcal/mo l. The, exchange of O-18 into (H2O)-O-16 is substantially enhanced showing that the activation barrier for OH recombination is reduced from 12 kcal/mo l to between 7 and 9 kcal/mol. Together these effects would be expected to produce between a 170 and 260 meV reduction in the activation barrier for C O removal from the surface. This is in close agreement with the 200 meV red uction in the potential for CO removal measured electrochemically by previo us workers. Quantification of our results shows that only about 40meV of th e total reduction is associated with the ligand effect, whereas the remaini ng portion is associated with the bifunctional mechanism. These results sho w that ruthenium on platinum has very different properties than ruthenium m etal. It slightly weakens the CO binding, and has a more substantial effect on activating water. There also is a site blocking effect that needs to be explained.