Sulfur Poisoning and Regeneration of Aftertreatment Oxidation Catalysts

Catalytic converters are the current technology for emission control from internal combustion engines. Depending on the operating mode of the engine and the composition of the pollutants present in the exhaust gas, the aftertreatment system components may vary. Nevertheless, an oxidation catalyst is always present, or a component of the catalyst has oxidation function, regardless of the engine type. It is responsible for mitigating emissions of unburned hydrocarbons, CO, and other pollutants. However, small amounts of sulfur originating from both fuel and lubricating oils are the cause of chemical degradation of oxidation catalysts due to the strong chemisorption of sulfur on the catalyst surface. Exposure to sulfur forms numerous sulfur species that inhibit the active material in these catalysts from carrying out oxidation reactions. In this work, we assessed the effects of sulfur poisoning of new formulations of oxidation catalysts, enhanced regeneration strategies to recover catalytic activity, and further investigated the particle size effects on sulfur speciation after aftertreatment catalyst aging protocols.
We assessed the effects of sulfur poisoning and regeneration on a new methane oxidation catalyst formulation that integrates a spinel oxide layer as the oxygen storage material for natural gas exhaust applications. We studied the methane oxidation performance under periodic conditions and the oxygen storage capacity of a bilayer Pt- Pd/Al2O3 + Mn0.5Fe2.5O4 catalyst before and after SO2 exposure, and after simulated regeneration conditions. Prior to SO2 exposure, under both simple feed and more complex simulated exhaust conditions, CH4 conversion at low temperatures was improved under periodic conditions compared to steady-state conditions. Methane oxidation activity and oxygen storage capacity of the spinel-based oxygen storage material were affected after SO2 exposure. Common literature regeneration protocols were applied, and while all regeneration protocols did improve CH4 oxidation activity, the utilization of regeneration methods under periodic conditions induced greater sulfur species desorption from the catalyst surface, ultimately resulting in a higher recovery of oxidation activity. Key parameters of this enhanced regeneration protocol under periodic conditions – temperature, feed composition, modulation amplitude and frequency – could be optimized to improve regenerability after sulfur poisoning.
The development of sulfur-resistant materials presents another avenue to alleviate the effects of sulfur poisoning of oxidation catalysts. The performance of a bimetallic Pd- Cu diesel oxidation catalyst in the presence of competitive adsorbates and its response to sulfur poisoning was studied The addition of Cu to Pd-based diesel oxidation catalysts (DOCs) does offer some resistance to SO2 poisoning based on the lower amount of SO2 adsorbed during the SO2 exposure, compared to the SO2 adsorbed on the monometallic Pd sample. Plus, after sulfur exposure at 100 C, the regeneration protocol under reducing conditions can reverse the effects of SO2 poisoning for both CO oxidation and CO+NO co-oxidation conditions.
As an extension of understanding sulfur poisoning, we investigated the influence of precious metal particle size effects on sulfur speciation after aftertreatment catalyst aging protocols. To avoid particle size heterogeneity, we synthesized uniform Pd nanoparticles with two particle sizes (3.4 nm and 13.1 nm) and deposited them onto supports relevant to aftertreatment applications, Al2O3 and CeO2. The gas composition of aging protocols (hydrothermal vs. thermal) and particle size influence sulfur speciation for both Al2O3-supported catalysts. In the case of CeO2-supported catalysts, particle size does not lead to changes in sulfur speciation. CO pulse injection measurements were used to evaluate the changes in particle size that occurred during aging and show that catalysts with a smaller particle size are more susceptible to sintering compared to catalysts with a larger particle size. Even though aging does affect CO oxidation activity for catalysts with a smaller particle size, they exhibit higher resistance to sulfur poisoning. Conversely, the catalysts with a larger particle size experience the opposite effect.