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.