390165 Transient Kinetic Study of Sulfur Oxide Adsorption/Desorption over Al2O3 and Pt/Al2O3 Catalysts
Sulfur oxides in the engine exhaust interact with diesel aftertreatment catalysts and deactivate them. SO2 is the main sulfur species exiting the engine, which in turn can be further oxidized to SO3 over the diesel oxidation catalyst (DOC). SO2 and SO3 may have different impacts on the DOC activity, and downstream catalysts, with SO3 leading to more severe deactivation. In the present work, in order to study the activity recovery of a Pt/Al2O3 DOC after sulfur poisoning, sulfur storage/release was studied using temperature-programmed desorption (TPD) experiments and a transient kinetic model was developed based on the experimental data. Diffuse reflectance infrared spectroscopy (DRIFTS) experiments were also utilized to identify the surface species.
To separate out the support effect, TPD experiments were performed in a flow reactor system using Al2O3 and Pt/Al2O3 samples. An Al2O3-coated monolith with a loading of 1.60 g/in3 and a Pt/Al2O3-coated monolith with a Pt loading of 50 g/ft3 were used in the experiments. During the TPD experiments, the catalyst was saturated with a specific concentration of either SO2 or SO3, and a temperature ramp from the saturation temperature up to ~900°C was used to desorb the surface species. For the SO3 TPD experiments, a separate reactor upstream of the main reactor was used where SO2 was completely oxidized to generate SO3 required in the feed. SO2 and SO3 TPD experiments were carried out under four different sets of operating conditions where saturation temperatures of 120°C and 160°C and approximate concentrations of 50 and 100 ppm for the adsorbing gas species were employed. Temperature ramps of 20 and 10°C/min were used in the SO2 and SO3 TPD experiments, respectively. For all flow reactor experiments, the outlet gas concentrations were measured using a MKS MultiGas MG-2030 FT-IR analyzer. In the DRIFTS experiments, pellets of Al2O3 and Pt/Al2O3 were prepared from the monolith washcoat and IR studies were performed with SO2 and SO3 concentrations similar to those employed in the flow reactor experiments.
Results and Discussion
According to the DRIFTS and TPD results, three different sulfur species were identified during SO2 adsorption on Al2O3. A small high temperature peak in the TPD spectra, i.e. above 850°C, was assigned to bulk aluminum sulfate, 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 in the case of SO2 TPD on Pt/Al2O3, however, with a higher contribution of the high temperature peaks indicating the promoting effect of Pt on the sulfate formation as well as on the spillover of the surface sulfates to the bulk. 1.45×10-4 mol SO2 adsorbed was calculated based on the SO2 TPD for Pt/Al2O3. The adsorbed amount of sulfur oxides significantly increased, to 1.50×10-3 mol, when exposed to SO3 instead, suggesting a higher level of sulfur storage upon exposure to SO3. Also according to these results, SO3 exposure resulted in the formation of stable sulfate species on the catalyst. The SO3 TPD results obtained with the Al2O3 and Pt/Al2O3 samples indicated that SO2 and SO3 release patterns were strongly affected by thermodynamic equilibrium. The important role of the Al2O3 support in the catalyst sulfation, as previously reported in the literature [1-3], was also confirmed in the experimental study.
A three site adsorption mechanism, with S1, S2 and S3 representing the adsorption sites with an increasing order of strength, was assumed to develop a transient kinetic model for the SO2 storage/release over Al2O3 and Pt/Al2O3. First order kinetics were assumed for the SO2 adsorption on the S1 (weak) and S3 (strong) sites, whereas higher order kinetics, i.e. second or third order depending on the catalyst, was assumed for SO2 adsorption on the S2 site. The S2 kinetics are explained by the structure of the surface sulfates being formed as a result of SO2 adsorption on the under-saturated oxygen anions on the surface of alumina. These oxygen species were recently proposed as possible sites for the strong adsorption of SO2 in the form of surface sulfates .
A uniform temperature profile was assumed for the reactor and mass transfer limitations were neglected in the radial direction of the monolith channel. A set of governing equations were solved to obtain the concentration and surface coverages as a function of time and axial position. Some of the kinetic parameters were optimized to describe the experimental data with thermodynamic constraints imposed upon the optimized parameters. A similar methodology was also applied in the development of the SO3 TPD model over Al2O3 and Pt/Al2O3. The kinetic model was validated using other sets of experimental data which were not included in the fitting process. The transient models were able to accurately predict the experimental behavior of the catalyst. According to the modeling results, the binding energy of the adsorbed species on the S1 and S2 sites are coverage dependent, indicating surface interactions between the adsorbed species as well as the heterogeneous nature of the alumina surface sites. Coverages of different sulfur species as well as their variation as a function of temperature were also predicted by the model indicating differences in the SO2 and SO3 storage mechanism.
- O. Saur M. Bensitle, S. Mohammed, J.C. Lavalley, C.P. Tripp and B.A. Morrow, Journal of Catalysis 110(1986)104.
- F. Gracia, S. Guerrero, E. Wolf, J. Miller and A. Kropf, Journal of Catalysis 233(2005)372.
- J.H. Pazmiño, C. Bai, J.T. Miller, F.H. Ribeiro and W.N. Delgass, Catalysis Letters 143(2013)1098.
- M.Y. Smirnov, A.V. Kalinkin, A.V. Pashis, A.M. Sorokin, A.S. Noskov, K.C. Kharas and V.I. Bukhtiyarov The Journal of Physical Chemistry B 109(2005)11712.
See more of this Group/Topical: Environmental Division