Catalysis with monometallic and bimetallic films: Trimerization of acetylene to benzene on Pd/Ru(001) and (Pd+Au)/Ru(001)

Au and Pd are essentially insoluble in Ru(001), forming pseudomorphic epita xial overlayers which grow layer by layer at room temperature. XP spectra i ndicate only weak electronic interaction between Au and Ru, whereas the Pd (3d(5/2)) binding energy suggests charge transfer from Pd to Ru which decre ases with increasing Pd coverage. With bimetallic (Pd + Au) monolayer films the dependence of the Pd (3d(5/2)) binding energy on Pd loading is similar , and these electronic effects are reflected in the catalytic behaviour of Pd and (Pd + Au) overlayers. Acetylene trimerisation to benzene was studied on monometallic (Pd) films in the coverage range 0 < theta(Pd) < 3 monolay ers. The desorption spectra of reactively formed benzene are characterised by three desorption states assigned to benzene molecules present in flat, t ilted and island-edge adsorption geometries on the Pd film. These results a lso confirm that at lower temperatures (<700 K) the Pd film exists as islan ds, even at very low Pd coverages. In contrast, temperature programmed XPS data indicate that at higher temperatures (>1300 K) the Pd film is disperse d. The catalytic behaviour of bimetallic (Pd-x + Au1-x) monolayer films was similarly investigated, with 0 < x < 1. For x > 0.60 the benzene yield is reduced compared to that from pure Pd films. However, for x = 0.4 a 40% hig her benzene yield is obtained, illustrating the importance of ensemble effe cts in the chemistry of benzene production. (C) 1999 Elsevier Science B.V. All rights reserved.