Abstract
When a clean Pt-Rh(100) alloy surface was exposed to NO at T > 440K, the LEED pattern changed sequentially as p(1 × 1) → c(2 × 2) → c(2 × 2) + p(3 × 1) → p(3 × 1), where the c(2 × 2) pattern appeared immediately after the exposure to NO. Contrary to this, the formation of the p(3 × 1) needs some interval time which depends strongly on the initial Rh concentration of the alloy surface as adjusted by annealing in vacuum. When the p(3 × 1) surface was exposed to H 2 by adding H 2 to the NO gas, the AES intensity of O(a) decreased and while that of N(a) increased markedly. At the same time, the LEED pattern changed from p(3 × 1) to c(2 × 2). These results suggest that N(a) has equal affinity to the Pt and Rh sites on the alloy surface so that it is difficult for N(a) to distinguish the Pt and Rh atoms. On the other hand, O(a) prefers to make a stronger bond with Rh sites and prolonged exposure induces Rh surface segregation. The reaction of Rh atoms with O(a) on the Pt-Rh(100) surface yields a surface compound of p(3 × 1) structure.
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