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

Silica-supported bimetallic catalysts were prepared by decomposing the molecular clusters Pt₂Au₄(C triple bond C^tBu)₈ and Pt₂Cu₄(C triple bond C^tBu)₈. The resulting catalysts were characterized with hydrogen, oxygen, and carbon monoxide chemisorptions. These indicated that CO chemisorption is the preferred method for evaluating metal dispersion. Chemisorption results also suggested that different types of substrate binding sites may be present on the bimetallic particles and that a significant fraction of the available platinum atoms may not be capable of activating hydrogen. Catalytic activity was evaluated with the toluene hydrogenation reaction. The new catalysts were found to be relatively inactive for this reaction at 60°C, despite having high platinum dispersions. Apparent activation energies for toluene hydrogenation were evaluated with Arrhenius plots. Although activities were low at 60°C, the cluster-derived catalysts had smaller apparent activation energies than a traditionally prepared Pt/silica catalyst. Trends in hydrogen chemisorption and toluene hydrogenation are discussed in terms of possible electronic, particle size, and ensemble size effects.