AVS 45th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS2-TuM

Paper SS2-TuM8
Suppression of Water Formation by Adsorbed Gold on Pt(335)

Tuesday, November 3, 1998, 10:40 am, Room 309

Session: Noble Metal Catalysis
Presenter: D.C. Skelton, Michigan State University/General Motors R & D Center
Authors: D.C. Skelton, Michigan State University/General Motors R & D Center
R.G. Tobin, Tufts University
D.K. Lambert, General Motors R & D Center
C.L. DiMaggio, General Motors R & D Center
G.B. Fisher, General Motors R & D Center
Correspondent: Click to Email
Platinum, an excellent catalyst for water formation, has its catalytic activity significantly reduced by a submonolayer coverage of gold. As part of a larger study on surface reactions on modified electrodes for chemical gas sensors, temperature programmed reaction on a stepped platinum surface, Pt(335), at three gold coverages was studied. Reactive surfaces were prepared in UHV with systematic doses of hydrogen and oxygen to obtain submonolayer coverages of adsorbed atomic hydrogen and oxygen. Gold coverage was measured by comparing the relative intensities of the 64 and 69 eV Auger peaks of platinum and gold, respectively. At low coverages gold adsorbs into the step sites on Pt(335) [4(111) × (100) in terrace step notation] and fills in to form islands which merge into a semi-continuous surface with openings to the platinum approximately 50 to 100 @Ao@ in diameter at 0.7 monolayers. This presumably leaves 30% of the surface platinum atoms available for chemisorption. However, hydrogen and oxygen in adsorption are reduced to 15% and 9% of their values on bare Pt(335), respectively, and bonding in step sites is reduced to 3% and 6%, respectively. Neither molecule bonds to the gold covered areas. The temperature-programmed reaction data for both Pt(335) and 0.7Au/Pt(335) show that this reduction in reactant coverage accounts for only part of the reduction in water production. Blockage of step sites by gold eliminates a major low-temperature reaction pathway seen on Pt(335) and a slight reduction in the desorption temperature of terrace hydrogen shifts hydrogen desorption into the same temperature range as water formation. At intermediate coverages (40% of the maximum hydrogen coverage and 50% of the maximum oxygen coverage) on each surface, the resulting competition between desorption and reaction lowers the reaction probability per hydrogen atom from 0.6 on Pt(335) to 0.2 on 0.7Au/Pt(335).