Development of a transient kinetic model for SO2 storage/release on γ-Al2O3 and Pt/γ-Al2O3 catalysts

Tayebeh  Hamzehlouyan, University of Houston

SO2 storage and release on/from γ-Al2O3 and Pt/γ-Al2O3 were studied in order to understand the transient phenomena that occur upon exposure of a Pt/Al2O3 diesel oxidation catalyst (DOC) to SO2. Temperature-programmed desorption (TPD) experiments were performed and a transient kinetic model was developed based on the experimental data. Diffuse reflectance infrared spectroscopy (DRIFTS) experiments were also used to identify the surface species. To decouple the support effect, TPD experiments were performed in a flow reactor system using γ-Al2O3 and Pt/γ-Al2O3 samples. Adsorption data demonstrate that γ-Al2O3, as a catalyst support, significantly affects the SO2 storage/release on the Pt/γ-Al2O3 catalyst, as previously reported in the literature [1]. Based on the DRIFTS and TPD results, different sulfur species were identified during SO2 adsorption on γ-Al2O3 and Pt/γ-Al2O3. In the case of SO2 TPD on γ-Al2O3, the most stable species was assigned to bulk aluminum sulfate with its desorption peak centered at ~850°C, whereas the other species with less stability were identified as weakly adsorbed SO2 (with its desorption peak centered at 250°C) and surface sulfites or sulfates (with a peak maximum in the range of 400-500°C). Similar surface species were identified for SO2 adsorption on Pt/γ-Al2O3, however, with a higher contribution of the high temperature peaks. These results indicate that Pt has a promoting effect on surface sulfate formation as well as on spillover of surface sulfates into the bulk. Based on the DRIFTS and TPD study, a multi-step reaction mechanism was proposed for SO2 adsorption/desorption on γ-Al2O3 and a transient kinetic model was developed. The kinetic parameters were optimized to describe the TPD experimental data. A similar methodology was also applied in the Pt/γ-Al2O3 model development. The transient models for γ-Al2O3 and Pt/γ-Al2O3 were able to accurately predict the experimental behavior of the catalyst. The model is also able to describe different stages of adsorption, for example the front portion of the catalyst being impacted by sulfur with initial exposure and as the exposure time increases, the poisoning front moves progressively along the channel. This leads to non-uniform coverage becoming nearly uniform with time.

[1] F. J. Gracia, S. Guerrero, E. E. Wolf, J. T. Miller and A. J. Kropf, Journal of Catalysis, vol. 233, pp. 372-387, 2005.