Microkinetic analysis of acetone hydrogenation over Pt/SiO₂

The kinetics of vapor-phase acetone hydrogenation were investigated over a Pt/SiO₂ catalyst through a range of temperatures (303–435 K) and reactant partial pressures (p_ACE = 0.3–140 Torr, p_H2 = 100–930 Torr). We compile macroscopically observable reaction orders, apparent barriers, kinetic isotope effects, and reaction rates; and we present a microkinetic analysis that considers multiple rate control scenarios in two Horuiti-Polanyi-type mechanisms. Overall, our data are best described by a two-site model wherein atomic hydrogen adsorbs at a distinct set of active sites from organic species. We further propose that two surface reactions each exert partial rate control, and we illustrate that degree of rate control is sensitive to reaction conditions. Shifts in rate control are attributed primarily to changing surface coverage regimes, which highlights the significance of both free energies of activation and chemical-potential driving forces in dictating the kinetic significance of elementary steps. Despite our conclusion that surface reactions involving H-X bond formation are likely rate controlling, we observe that H₂/D₂ switching minimally perturbs acetone hydrogenation rates. We interpret this by considering that observed kinetic isotope effects reflect a convolution of kinetic and thermodynamic impacts, which can obscure the elementary effects of changes in isotopic mass.