CHEMICAL RECONSTRUCTION OF PT-RH(100) ALLOY AND PT/RH(100), RH/PT(100) BIMETALLIC SURFACES

When Cu(110), Ni(110), Ag(110) surfaces are exposed to O-2 at room temperature, one dimensional metal-oxygen strings grow in the < 001 > direction of the (110) surfaces. A similar phenomenon occurs in the adsorption of H-2 on Ni(110) surface at room temperature, where the one dimensional strings grow along the < 1T0 > direction. These phenomena are undoubtedly different from the adsorption induced reconstruction but are explained by the chemical reconstruction involving the formation of quasi-compounds and their self-ordering on the metal surfaces. The chemical reconstruction is indispensablly important to understand the structure and catalysis of alloy and bimetallic surfaces. Pt0.25Rh0.75(100) alloy surface being active for the reaction of NO with H-2 is an interesting example. When the Pt-Rh(100) alloy surface is exposed to NO or O-2 at arround 500 K, a p(3 x 1) ordered Rh-O overlayer is obtained on a Pt-enriched 2nd layer by the chemical reconstruction. Ordering of Rh-O in the p(3 x 1) structure on the Pt(100) surface was reproduced by heating a Rh/Pt(100) bimetallic surface in O-2, and the chemical reconstruction making the p(3 x 1) Rh-O overlayer on a Pt enriched 2nd layer was also proved by heating a Pt/Rh(100) bimetallic surface in O-2 or NO. The activation mechanism of the Pt-Rh alloy and the Pt/Ph bimetallic surfaces by the chemical reconstruction was evidently shown by using a Pt deposited Rh(100), Pt/Rh(100), surface. That is, the Pt/Rh(100) is not so active for the reaction of NO with H-2, but the reconstructed p(3 x 1)Rh-O/Pt-layer/Rh(100) surface is very active for the reaction. Therefore, it was concluded that the chemical reconstruction of the Pt-Ph catalyst makes the active surface which is composed of Rh-O and a Pt layer.