An investigation of the reaction mechanism for the promotion of propane oxidation over Pt/Al2O3 by SO2

Total oxidation of propane with oxygen in the absence and the respective presence of SO2 in the feed gas was studied over 1 wt% Pt on gamma -Al2O3 using transient experiments with TAP (temporal analysis of products) and in situ DRIFT spectroscopy. The TAP experiments confirm the promoting role of SO2 on the conversion of propane and, moreover, show an inhibiting effect from SO2 on the conversion of the intermediate products propene and CO. In situ DRIFT spectroscopy reveals the formation of sulphate species on the catalyst surface and indicates the formation of an allylic intermediate in the absence of SO2 only. Interaction between hydrocarbon intermediates and the sulphate species is confirmed by the appearance of thiol fragments in the mass spectra. The transient experiments show that propane in the first step is irreversibly adsorbed at the surface. Analysis of the surface residence times of the products formed at the propane pulse in pump-probe experiments provides information about the reaction pathways. Without S02 in the feed gas, propane reacts consecutively in the sequence propane --> propene -->. ethane --> CO --> CO2. In the presence of SO2 the first step of the main route is the breaking of a C-C bond in propane producing ethane and a C-1 fragment, which then form CO and eventually CO2. It is suggested that the promoting effect by SO2 on the conversion of propane is due to the formation of an acidic site at the Pt/Al2O3/SO42- interface. (C) 2001 Academic Press.