Enhanced transport in washcoated monoliths: Application to selective lean NOx reduction and ammonia oxidation

Abstract

Washcoated monolithic reactors are extensively used in vehicle emission control systems to reduce pollutants from engine exhaust. Selective catalytic reduction (SCR) with the downstream ammonia slip catalyst (ASC) are deployed in diesel emission control to remove NOx and prevent ammonia release. Earlier works have shown that washcoat diffusion can be rate controlling for both SCR and ASC. Here we use sacrificial agents (polymer, yeast) to enhance the conversion without compromising the selectivity for SCR and ASC. The agents are removed during calcination, creating inter-crystallite pores that increase the washcoat macroporosity. Steady state reaction studies of SCR on a Cu-SSZ-13 coated monolith show a noted increase in NOx conversion during standard SCR (NH3 + NO + O2). The improved performance is sustained for hydrothermally aged samples. Similar conversion enhancement is observed for NH3 oxidation over the single-layer Cu-SSZ-13 washcoat. Conversion increases are also encountered for the dual-layer ASC (top Cu-SSZ-13 and base Pt/Al2O3 layer) during the selective oxidation of NH3 to N2. For both SCR and ASC, the N2 product selectivity is negligibly impacted on the modified catalysts. Sharper NH3 desorption profiles are obtained during NH3 temperature programmed desorption (TPD) experiments with Cu-SSZ-13. The results show that modified washcoat may reduce the amount of catalyst to maintain a desired conversion. For the ASC up to a 70% reduction in the Pt loading is demonstrated by using the sacrificial agents to achieve the same conversion. The implication for both SCR and ASC catalyst design and performance is discussed.