ACTIVATION OF THE PT DEPOSITED RH(100) BIMETALLIC SURFACE BY CHEMICAL ORDERING

When the Pt deposited Rh(100) surface, Pt/Rh(100), was exposed to 10(-7) Torr of O2 at 780 K, Rh atoms are segregated and the LEED pattern changes from a c(2 X 2) to a p(3 X 1) + p4g p(2 X 2) pattern. This p(3 X 1) + p4g LEED pattern changed to a p(1 X 1) surface at room temperature by exposing it to H2 and was recovered at room temperature by exposure to O2. The p(3 X 1) + p4g surface is stable at 780 K in vacuum but changes to a p(1 X 1) surface at 1000 K. If the p(1 X 1) surface obtained by annealing at 1000 K is heated in 10(-7) Torr of O2 at 780 K, the p(3 X 1) + p4g surface is again recovered. The p(1 X 1) surface annealed at 1000 K is composed of the Pt-enriched topmost layer on a Rh rich 2nd layer but the p(3 X 1) + p4g surface prepared by treating with 02 at 780 K is composed of the Rh-0 surface compound on the Pt enriched 2nd layer. The catalytic reaction of NO + H-2 was compared on the two surfaces, the p(3 X 1) + p4g Rh-rich surface and the p(1 X 1) Pt-rich surface. The surface composed of Rh-0 on the Pt layer was very active for the reaction but the Pt enriched p(1 X 1) surface was inactive for the reaction. From these results, we concluded that the Rh-0 surface compound ordered on the Pt layer is an active structure for the catalysis.