(123d) 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.

Methodology

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 [4].

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.   

REFERENCES

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