Kinetic study of adsorption and desorption of SO2 over γ-Al2O3 and Pt/γ-Al2O3

SO2 storage and release on/from γ-Al2O3 and Pt/γ-Al2O3 were studied in order to understand the transient phenomena that occur upon a diesel oxidation catalyst’s (DOC) exposure to SO2. Temperature programmed desorption (TPD) and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were performed to understand the adsorption, surface reaction and desorption dynamics. Adsorption data verified that γ-Al2O3 as a catalyst support significantly affects the SO2 storage and release of a Pt/γ-Al2O3 catalyst. Multiple sulfur species formed during SO2 adsorption on γ-Al2O3 and Pt/γ-Al2O3. SO2 adsorbed as molecular SO2 on Al sites, as surface sulfites/bi-sulfites and surface sulfates on electron deficient oxygen sites, as well as bulk aluminum sulfate were identified. The results show that Pt has a promoting effect on surface sulfate formation as well as on spillover of surface sulfates into the bulk alumina support. Based on the DRIFTS and TPD studies, multi-step reaction mechanisms were proposed for SO2 adsorption and desorption on/from both γ-Al2O3 and Pt/γ-Al2O3. The kinetic parameters were optimized to describe the TPD experimental data and the kinetic models were able to accurately predict the experimental behavior of the catalyst. Similar steady state coverage profiles were predicted by the model irrespective of inlet concentrations for long exposure times, suggesting that low SO2 concentrations typically seen in diesel exhaust can be simulated with higher concentrations to accelerate the sulfur effects.