Cyclic Lean Reduction of NO by CO in Excess HO on Pt-Rh/Ba/AlO: Elucidating Mechanistic Features and Catalyst Performance.

This study provides insight into the mechanistic and performance features of the cyclic reduction of NO by CO in the presence and absence of excess water on a Pt-Rh/Ba/AlO NO storage and reduction catalyst. At low temperatures (150-200 °C), CO is ineffective in reducing NO due to self-inhibition while at temperatures exceeding 200 °C, CO effectively reduces NO to main product N (selectivity >70 %) and byproduct NO. The addition of HO at these temperatures has a significant promoting effect on NO conversion while leading to a slight drop in the CO conversion, indicating a more efficient and selective lean reduction process. The appearance of NH as a product is attributed either to isocyanate (NCO) hydrolysis and/or reduction of NO by H formed by the water gas shift chemistry. After the switch from the rich to lean phase, second maxima are observed in the NO and CO concentrations versus time, in addition to the maxima observed during the rich phase. These and other product evolution trends provide evidence for the involvement of NCOs as important intermediates, formed during the CO reduction of NO on the precious metal components, followed by their spillover to the storage component. The reversible storage of the NCOs on the AlO and BaO and their reactivity appears to be an important pathway during cyclic operation on Pt-Rh/Ba/AlO catalyst. In the absence of water the NCOs are not completely reacted away during the rich phase, which leads to their reaction with NO and O upon switching to the subsequent lean phase, as evidenced by the evolution of N, NO and CO. In contrast, negligible product evolution is observed during the lean phase in the presence of water. This is consistent with a rapid hydrolysis of NCOs to NH, which results in a deeper regeneration of the catalyst due in part to the reaction of the NH with stored NO. The data reveal more efficient utilization of CO for reducing NO in the presence of water which further underscores the NCO mechanism. Phenomenological pathways based on the data are proposed that describes the cyclic reduction of NO by CO under dry and wet conditions. [ABSTRACT FROM AUTHOR]