Absorption and oxidation of hydrogen at Pd and Pd–Au (1 1 1) surfaces

The kinetics of the H2–O2 reaction over Pd, Pd70Au30 and Au (1 1 1)-oriented surfaces has been studied in a static reactor near room temperature, using pressures in the torr range. The Pd–Au surface is highly enriched in gold (75 at.%) and unreconstructed, as shown by previous AES, LEIS and LEED investigations. While Au(1 1 1) is inert, Pd(1 1 1) and Pd–Au(1 1 1) are very active to catalyze the oxidation of hydrogen. The clean Pd surface is firstly more efficient to form water than the Pd–Au surface, but deactivates upon exposure to the reactive mixture. A careful analysis of the reactant partial pressures allows deriving the amount of hydrogen absorbed in the crystal during each reaction cycle. This shows that Pd–Au absorbs much more hydrogen than Pd, and that bulk dissolution directly competes with surface oxidation of hydrogen. Gold-induced lowering of the oxygen sticking probability and absorption-induced structural modification of Pd(1 1 1), through formation of a surface hydride, are invoked to rationalize the results.