NOx Storage and Reduction with H2 on Pt/Rh/BaO/CeO2: Effects of Rh and CeO2 in the Absence and Presence of CO2 and H2O

Abstract

The storage and reduction features of a family of Pt/Rh/BaO/CeO2/Al2O3 washcoated monolith catalysts are compared in terms of NOx conversion and product selectivity using H2 as the reductant. The catalyst composition, monolith temperature, regeneration time, and presence of H2O and CO2 in the feed were systematically varied to identify trends and to elucidate effects. In addition to cycling, experiments involving the reduction of a fixed amount of prestored NOx help to isolate differences in the regeneration features of the catalysts. The addition of both CeO2 and Rh to Pt/BaO increases the cycle-averaged NOx conversion and selectivity to N2, but oxygen storage on the ceria expectedly leads to the consumption of additional reductant during the regeneration. CeO2 is shown to be an inferior NOx storage component, but as a supplement to BaO, CeO2 provides the role of promoting the oxidation of NH3 to N2. The same stored oxygen is the likely cause for the enhanced NH3 oxidation. On the other hand, Pt/BaO is shown to be the most effective catalyst for converting NOx to NH3. Fixed NOx storage experiments show the existence of at least three rate-controlling regimes: one that is reductant-feed-rate-limited, another that is limited by NOx storage phase diffusion, and a third that has a chemical or textural origin. On Pt/BaO, the first two regimes are clearly distinguishable, whereas a more complex picture emerges for Pt/CeO2, with which a fraction of the stored NOx is kinetically inaccessible to reduction. Water inhibits the oxidation of NO on each catalyst, but the cycle-averaged NOx conversion is largely unaffected. In contrast, CO2 has only a minor effect on the conversion during steady state NO oxidation but significantly inhibits the NOx conversion during cyclic storage and reduction. This effect is attributed to the known higher stability of BaCO3 compared with BaO/Ba(OH)2. The conversion of H2 to the less effective reductant CO via reverse water gas shift chemistry is a contributing factor based on steady-state activity tests. This pathway is shown to be most important for the catalysts, containing Rh/CeO2, a known effective water gas shift catalyst. Building on the existing literature, most of the observed trends are interpreted in terms of the likely reaction pathways and transport processes. The findings are assessed in terms of identifying the catalyst best suited to the specific NOx trap application, be it a stand-alone reactor or one coupled with downstream NH3-based selective catalytic reduction.