Deep oxidation of propane on Pt-supported catalysts: Drastic turnover rate enhancement using zeolite supports

Abstract

The combustion of propane was studied on Pt supported on MgO, alumina, and zeolites KL, HY, ZSM5 and Beta. Samples contained a similar amount of Pt, between 0.32 and 0.44%, and were characterized by employing a variety of physical and spectroscopic techniques. The catalyst activities were evaluated through both conversion versus temperature (light-off curves) and conversion versus time catalytic tests. Kinetic studies showed that the reaction is first order in propane, and zero (Pt/Al2O3, Pt/MgO) or negative (Pt/zeolites) orders in oxygen. Apparent activation energies (Ea) and pre-exponential factors (A) were determined and it was verified that the experimental data obey a Constable relation (ln A = mEa + c). Pt/Al2O3 catalysts of different metallic dispersions were prepared for investigating the effect of Pt crystallite size on combustion activity. It was found that propane oxidation is a structure insensitive reaction on Pt/Al2O3. Propane oxidation turnover rates (TOF) followed the order: Pt/MgO < Pt/Al2O3 Pt/KL < Pt/HY ≤ Pt/ZSM5 < Pt/Beta. The TOF values on Pt/acid zeolites were more than two orders of magnitude higher than on Pt/Al2O3. Propane oxidation activity was also significantly higher on Pt/KL as compared to Pt/Al2O3, despite that Al2O3 and zeolite KL supports exhibited similar acid sites density and strength. This result showed that the support acid strength did not have a major influence on propane combustion activity. Areal propane uptake was more than one order of magnitude higher on Pt/zeolites than on Pt/Al2O3 and this drastic increase in the density of propane adsorbed species may promote the alkane oxidation rate. It is proposed that the enhanced combustion activity obtained on Pt/zeolites is associated with an additional oxidation pathway from propane adsorbing on the metal-oxide interface region and reacting with oxygen spilled-over from the metal surface.