The hydrogenolysis of ethane over RePt(111) and PtRe(0001) bimetallic crystal surfaces

Abstract

Platinum-rhenium bimetallic surfaces were prepared by condensing both rhenium on the (111) face of platinum, and platinum on the (0001) face of rhenium from the vapor under ultrahigh vacuum. Using a high-pressure microreactor, the hydrogenolysis of ethane was investigated over the small-area catalysts in the temperature range 300–350 °C, with pressures of 5–10 Torr ethane and 100–1000 Torr hydrogen. The reaction is structure sensitive on both platinum and rhenium as indicated by the large increase in the initial rates of reaction observed after the Pt(111) and Re(0001) surfaces were roughened by argon ion bombardment. The Re(0001) surface was two orders of magnitude more active than the Pt(111) surface, yet a bimetallic surface of the stoichiometry Re 2Pt was found to be the most active surface for ethane hydrogenolysis—about one order of magnitude more active than the Re(0001) surface. Thus the activity of the bimetallic catalyst is not a linear combination of the activities due to the two metallic components. Hydrogen pressure dependence studies show that a bimetallic surface composed of 0.3 monolayer of rhenium on Pt(111) had an activity close to that of a monometallic Re(0001) surface, yet displayed a hydrogen partial pressure dependence closer to that of a Pt(111) surface. These results suggest that an electronic interaction exists between platinum and rhenium metals that strongly influence the catalytic hydrogenolysis of ethane. The accumulation of carbonaceous deposits was fairly insensitive to temperature and hydrogen pressure on rhenium and bimetallic PtRe surfaces. However, the accumulation of carbonaceous deposits on monometallic Pt(111) surfaces was highly sensitive to the reaction conditions, and adsorbed on this surface more tenaciously than on bimetallic PtRe surfaces.