Kinetic Modeling of Spillover and Temperature-Programmed Oxidation of Oxy-Carbon Surface Species on Pt/Al2O3

During propane oxidation over Pt/Al2O3 in the 200–300 °C temperature range, many different oxygenated carbonaceous (oxy-carbon) surface species spillover from the platinum nanoparticles and grow on the Al2O3 support. The rate of oxy-carbon species growth on the Pt/Al2O3 surface is consistent with a diffusion-limited process where the rate of oxy-carbon species diffusion on the Al2O3 support is the rate-determining step. A model based on Fick's second law for two-dimensional radial diffusion is used to analyze the kinetics of oxy-carbon species spillover on the Al2O3 support. An Arrhenius expression describing the rate of oxy-carbon species diffusion on the Al2O3 support, which has a pre-exponential factor of 7.9 × 10–14 cm2/s and an activation barrier of 24 kJ/mol, was extracted from the kinetic analysis. Following propane oxidation, the oxy-carbon surface species were completely oxidized to CO2 during temperature-programmed oxidation (TPO). During TPO of the oxy-carbon species, diffusion of the oxy-carbon species on the Al2O3 support is relatively fast, and a surface reaction on the platinum nanoparticles is the rate-determining step. TPO of oxy-carbon surface species was simulated using surface reaction rate expressions with three different reaction orders with respect to the surface carbon concentration (first-order, second-order, and power-law). The kinetics of TPO of the oxy-carbon surface species is most accurately represented by second-order kinetic rate expressions with activation barriers of 147 kJ/mol for oxidation of acetate species and 112 kJ/mol for oxidation of higher reactivity enolate, aliphatic ester, and acetone species. Formation of platinum oxides during propane oxidation increase the activity of the catalyst for TPO of the oxy-carbon species. This work reveals quantitative mechanistic insights into both the carbon growth and burnoff processes, which is important for designing efficient hydrocarbon conversion processes.