Modified activities and selectivities of silated-oxidized-reduced Pd and Pt catalysts

Supported nanoscale Pd and Pt particles lose their ability to catalyze hydrogenation and to hydrogenolyze triethylsilane when silated with either trimethyl silane or silane. Subsequently exposing these silated catalysts to oxygen restores their hydrogenation activities to various extents depending on their metal particle sizes. Large particles (<50%D) experience partial restoration, whereas small particles (>50%D) experience virtually complete restoration (85-111%) of their original alkene hydrogenation activity. Benzene hydrogenation activity on Pt and triethyl silane hydrogenolysis activities on Pd are restored less than alkene hydrogenation activities on Pt and Pd. Additional data from XPS, FTIR, DSC, TGA, and hydrogen chemisorption measurements point to a mechanism in which silation produces surface silicon, some of which migrates into the metal particle and some of which remains on the surface and poisons hydrogenation and hydrogenolysis activities. Oxidation produces PdO2 (or PtO2) and SiO2 at the surface, and because the surface Si concentration has been reduced, interior Si migrates back to the surface where it is also oxidized. Hydrogen does not reduce SiO2 but does reduce PdO2 to Pd metal, which now presents a new reconstructed surface for hydrogenation. Thereby, Si alloyed to nanoscale Pd particles imparts not only new chemical characteristics, but also new physical characteristics. The reconstructed larger Pd-Si particles are not able to form as many favorable hydrogenation sites as before silation-oxidation-reduction, whereas the reconstructed smaller Pd-Si particles are able to form more hydrogenation sites than before. Likely the smallest Pd particles experience instabilities, rendering them incapable of maintaining many active hydrogenation sites; however, alloying with Si enlarges and stabilizes them such that they are capable of maintaining more active sites than before. (C) 1996 Academic Press, Inc.