Catalytic Consequences of Particle Size and Chloride Promotion in the Ring-Opening of Cyclopentane on Pt/Al2O3

Ring-opening of cyclopentane on alumina-supported Pt particles was studied as a function of Pt particle size in the presence of different Cl contents. With catalysts prepared from a Cl-free precursor, measured turnover rates increased monotonically with increasing Pt particle size (1-15 nm). On catalysts derived from a Cl-containing precursor, the turnover rates fell into two separate trends with the change of Pt particle size, depending on the extent of Cl removal by increasing thermal treatment temperature. In both cases, catalytic activity increased with increasing particle size in the examined ranges of dispersions (D = 0.7-1.0 and 0-0.6) and for both series of catalysts, the apparent activation energies were higher on large Pt particles than on small ones, with only small differences in the reaction orders for H-2 and cyclopentane on particles of widely varying average sizes. Therefore, the effect of particle size on the turnover rates stems mainly from intrinsic rate constants, rather than from coverage effects. The relative adsorption coefficients of toluene and benzene indicated lower electron densities at the surface Pt atoms in the catalysts prepared from the Cl-containing precursor than in those from the Cl-free precursor. This subtle electron deficiency, which seems not to stem from the local Cl enrichment near Pt, affects both the concentration of chemisorbed hydrogen under reaction conditions and the barrier for C-C bond cleavage. The Cl postintroduced to the catalyst, in contrast, does not induce a similarly positive effect.