Structure and reactivity of intermediates. N-overlayer on Pd(100), Rh(100) and Pt-Rh(110) and its reaction with H₂

Abstract

Rh(100), Pt(100), and Pt-Rh(100) surfaces are inert for the dissociative adsorption of N₂, but they are active for the catalytic reaction of NO with H₂ During the reaction on Rh(100) and Pt-Rh(100) surfaces, N atoms are accumulated by making a c(2x2)-N overlayer, but no accumulation of N atoms occurs on Pt(100) surface. The fact that N atoms on the Pt-Rh(100) surface gives the c(2x2) structure indicates that the N atoms have equal affinity to Pt and Rh on the alloy surface. When the c(2x2)-N surface was exposed to H₂ of 10^-7 to 10^-8 Torr, a prominent loss peak being assignable to NH_x appeared at 3200 – 3240 cm^-1 at around 400 K. The in-situ HREELS study proved that NH are prominent species which are formed during the hydrogenation of the c(2x2)-N, that is, a quasi-equilibrium of N + 1/2 H₂ - NH is established. When a clean Pt-Rh(100) (Pt/Rh = 1/3) alloy surface is exposed to NO at about 440 K, the LEED pattern changes sequentially as (1x1) → c(2x2) → c(2x2) + p(3x1) → p(3x1), where the c(2x2) pattern appears instantaneously on the alloy surface of any Pt/Rh ratio but the p(3x1) pattern accompanies a certain characteristic interval times being responsible to the segregation of Rh. The p(3x1) surface reflects the formation of an intermediate of Rh-O complex overlayer and it reacts rapidly with H₂.