NOx reduction on Ceria: Impact of lean-rich cycling

Abstract

NOx storage and reduction (NSR) is a cyclic catalytic process that eliminates NOx from lean burn vehicles. Lean-rich switching can achieve a higher conversion than steady state operation. There is an active debate about the NOx reduction mechanism for NSR catalysts containing CeO2 (ceria), particularly at high temperatures and fast cycle frequency. In order to isolate the role of ceria, we examine the performance of a ceria-washcoated monolith for a wide range of operating conditions, including temperature, cycling frequency, reductant type and feed concentration of oxidants (O2, CO2 and H2O). The results reveal enhancement of NO conversion with faster cycling particularly at elevated temperatures (> 550 °C). The data are consistent with a cyclic mechanism in which oxygen vacancies are created during the rich feed through reduction by H2, CO, or C3H6 and filled during the lean feed through NO and/or O2 oxidation. An excess of O2 is detrimental to NO conversion, due to the competing oxidation of reduced ceria by O2. The data reveal that at least two types of vacancy sites participate in the cyclic redox process. Surface vacancy sites provide rapid NO reduction, while utilization of bulk vacancy sites is slowed by solid-state diffusion limitations. For a fixed duty cycle, lean/rich switching operation is superior to steady-state operation and an optimal lean/rich switching frequency exists for different reaction conditions. Fast cycling is especially favorable for stoichiometric feeds or cycle-averaged rich feeds.