Ensemble size effects on toluene hydrogenation and hydrogen chemisorption by supported bimetallic particle catalysts

Silica-supported bimetallic catalysts were prepared by decomposing the mole cular clusters Pt2Au4(C=(CBu)-Bu-t)(8) and Pt2Cu4(C dropC(t)Bu)(8). The res ulting catalysts were characterized with hydrogen, oxygen, and carbon monox ide chemisorptions. These indicated that CO chemisorption is the preferred method for evaluating metal dispersion. Chemisorption results also suggeste d that different types of substrate binding sites may be present on the bim etallic particles and that a significant fraction of the available platinum atoms may not be capable of activating hydrogen, Catalytic activity was ev aluated with the toluene hydrogenation reaction. The new catalysts were fou nd to be relatively inactive for this reaction at 60 degreesC, despite havi ng high platinum dispersions. Apparent activation energies for toluene hydr ogenation were evaluated with Arrhenius plots. Although activities were low at 60 degreesC, the cluster-derived catalysts had smaller apparent activat ion energies than a traditionally prepared Pt/silica catalyst. Trends in hy drogen chemisorption and toluene hydrogenation are discussed in terms of po ssible electronic, particle size, and ensemble size effects.