Cyclic Lean Reduction of NO by CO in Excess H2O on Pt–Rh/Ba/Al2O3: Elucidating Mechanistic Features and Catalyst Performance

Abstract

This study provides insight into the mechanistic and performance features of the cyclic reduction of NOx by CO in the presence and absence of excess water on a Pt–Rh/Ba/Al2O3 NOx storage and reduction catalyst. At low temperatures (150–200 °C), CO is ineffective in reducing NOx due to self-inhibition while at temperatures exceeding 200 °C, CO effectively reduces NOx to main product N2 (selectivity >70 %) and byproduct N2O. The addition of H2O at these temperatures has a significant promoting effect on NOx conversion while leading to a slight drop in the CO conversion, indicating a more efficient and selective lean reduction process. The appearance of NH3 as a product is attributed either to isocyanate (NCO) hydrolysis and/or reduction of NOx by H2 formed by the water gas shift chemistry. After the switch from the rich to lean phase, second maxima are observed in the N2O and CO2 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 Al2O3 and BaO and their reactivity appears to be an important pathway during cyclic operation on Pt–Rh/Ba/Al2O3 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 O2 upon switching to the subsequent lean phase, as evidenced by the evolution of N2, N2O and CO2. 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 NH3, which results in a deeper regeneration of the catalyst due in part to the reaction of the NH3 with stored NOx. The data reveal more efficient utilization of CO for reducing NOx 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 NOx by CO under dry and wet conditions.