Interactions between Sulfur Oxides and a Pt-Based Diesel Oxidation Catalyst



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Sulfur is a common poison for automotive catalysts. Sulfur oxides in the engine exhaust adsorb onto and interact with catalytic converters leading to catalyst deactivation. SO2 is the main sulfur species exiting the engine, which can be further oxidized to SO3 over an oxidation catalyst with subsequent H2SO4 formation in the presence of water. For diesel engines in particular, SO2, SO3 and H2SO4 may have different impacts on the diesel oxidation catalyst (DOC) activity, as well as the activity of catalysts further downstream. In the present PhD research project, the interactions between sulfur oxides and a model Pt/γ-Al2O3 DOC were investigated. SO2 oxidation on a Pt/γ-Al2O3 catalyst was experimentally studied and a steady-state kinetic model was developed to describe SO2 oxidation on the catalyst. The modeling results reveal that the relative importance of the individual rates in the reaction mechanism, as well as the surface coverages, were strongly temperature dependent. SO2 storage and release on/from γ-Al2O3 and Pt/γ-Al2O3 were studied to understand the transient phenomena that occur upon a DOC’s exposure to SO2. Temperature-programmed desorption (TPD) and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were performed to identify sulfur species formed on both catalysts. Adsorption data verified that γ-Al2O3 as a catalyst support significantly affects SO2 storage and release over Pt/γ-Al2O3. 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 and kinetic models were developed. SO3 adsorption and desorption on γ-Al2O3 and Pt/γ-Al2O3 were studied using TPD and DRIFTS experiments. It was found that similar species form on γ-Al2O3 and Pt/γ-Al2O3 during SO3 adsorption, however upon the adsorbed species decomposition, different sulfur species are released from the two samples. Different impacts of SO2, SO3 and H2SO4 on Pt/γ-Al2O3 were studied using TPD experiments. The results show that the sulfur uptake as well as the contribution of stable sulfates on the alumina support decreases in the following order: H2SO4 > SO3 > SO2. The results suggested that the degree of catalyst sulfation upon its saturation with different forms of sulfur could represent the extent of deactivation.



Sulfur deactivation, SO2 oxidation, SO2 and SO3 adsorption and desorption, Kinetic modeling, Diesel exhaust, Pt/Al2O3 diesel oxidation catalyst


Portions of this document appear in: Hamzehlouyan, Tayebeh, Chaitanya Sampara, Junhui Li, Ashok Kumar, and William Epling. "Experimental and kinetic study of SO 2 oxidation on a Pt/γ-Al 2 O 3 catalyst." Applied Catalysis B: Environmental 152 (2014): 108-116. And in: Hamzehlouyan, Tayebeh, Chaitanya S. Sampara, Junhui Li, Ashok Kumar, and William S. Epling. "Kinetic study of adsorption and desorption of SO 2 over γ-Al 2 O 3 and Pt/γ-Al 2 O 3." Applied Catalysis B: Environmental 181 (2016): 587-598.