Temperature-programmed oxidation of coke deposited on cracking catalysts: Combustion mechanism dependence

Coke deposited on catalytic cracking catalysts has been investigated by continuously monitoring evolved CO and CO2 during temperature-programmed oxidation (TPO) in a 1% O-2/N-2 mixture. Analyses were carried out on spent samples recovered from an industrial catalytic cracker and on coke prepared in the laboratory by exposure of fresh catalyst to l-hexene and cyclohexene at 500 degrees C. A maximum of three peaks in the rate of carbon dioxide evolution and one carbon monoxide peak are apparent in the TPO spectra. The small lowest temperature CO2 peak, which is only observed for the TPO of the spent catalyst, is assigned to either highly reactive coke or coke in the vicinity of trace metals. Two larger overlapping CO2 peaks occur at higher temperatures and are attributed to competing coke oxidation mechanisms. The first of these larger peaks coincides with the CO evolution, while for the final peak only CO2 evolution prevails. This reduction in CO and increase in CO2 are also observed for the TPO of powdered charcoal. The temperature dependence of carbon oxide evolution is attributed to changes in the combustion rate-determining steps due to different pre-exponential factors and activation energies. This indicates that the combustion reaction mechanism can affect the shape of the TPO spectra and so must be included in the interpretation of catalytic coke oxidation data.