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 H2 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.