Sulfur Poisoning of Small Pore Cu-exchanged Chabazite (CHA) Catalysts for Catalytic Reduction of NOx with NH3

The main advantages of using compression ignition, diesel engines instead of gasoline spark-ignited engines are better fuel economy and the coincident lower greenhouse gas emissions. Unlike gasoline engines, diesel engines operate at air-to-fuel ratios higher than stoichiometric, known as “lean” conditions. Reducing NOx in the presence of excess oxygen is challenging and cannot be achieved using the traditional three-way catalyst technology. Selective catalytic reduction (SCR) of NOx using NH3 has been widely used/studied as a promising technology for NOx abatement in mobile applications. The state-of-the-art catalysts for NH3-SCR are Cu-exchanged in small pore molecular sieves with a chabazite (CHA) structure; namely Cu-SSZ-13 and Cu-SAPO-34. Compared to other candidates, such materials have shown both better activity and thermal stability under simulated exhaust conditions. However, these materials are prone to sulfur poisoning. Even with current ultra-low sulfur diesel fuel (containing less than 15 ppm S), the cumulative lifetime exposure of catalysts to such fuel may lead to significant effects on performance/stability.
In this study, the chemistry of Cu-CHA sulfur poisoning was studied with the focus on understanding the poisoning mechanism and development of a low-temperature SCR deactivation/regeneration model.
To this end, firstly, temperature programmed desorption (TPD) and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were performed to evaluate SO2 poisoning effects on SCR activity and oxidation functionality of Cu-SAPO-34. The results show that SO2 significantly inhibits low-temperature (< 350 °C) SCR and oxidation functionality of Cu-SAPO-34.
Secondly, studying sulfation and desulfation of Cu-SSZ-13 with different Si:Al ratios revealed that depending on the Cu active site, i.e. Z2Cu or ZCuOH, SO2 interaction with Cu-CHA results in either formation of highly stable sulfite/sulfate species or ammonium sulfate depending on whether NH3 is present or not. In-situ DRIFTS and transient kinetic studies revealed Z2Cu and ZCuOH have different responses to SO2 exposure corresponding to different sulfur intermediates that form. Based on these findings, a low-temperature sulfur poisoning mechanism was proposed.
Finally, based on the proposed mechanism, a multi-site mechanism-based kinetic model was developed capable of describing S adsorption/desorption behavior and SCR activity on fresh, sulfated and regenerated catalyst.