Effect of cycle time on NH3 generation on low Pt dispersion Pt/BaO/Al2O3 catalysts: Experiments and crystallite-scale modeling

Abstract

A crystallite-scale model that explicitly accounts for the diffusion of stored NOx in the storage phase is used to explain the effects of cycle time, Pt dispersion and loading on NOx conversion and NH3 selectivity on low Pt dispersion NOx storage/reduction (NSR) catalysts. The low Pt dispersion or aged NSR catalysts are known to give high selectivity to NH3 which is essential for the combined NSR/SCR (selective catalytic reduction) system. It is shown through both experiments and modeling that for a fixed ratio of the moles of NO:H2 fed per cycle, a longer diluted rich feed for fixed lean time and a shorter overall cycle is beneficial in terms of NOx conversion for both 3% and 8% dispersion catalysts. Model predicts that more stored NOx is allowed to diffuse back to the Pt sites during a longer diluted regeneration while excessive NOx slip is prevented in a shorter overall cycle due to frequent regeneration, both of which leads to higher NOx conversion. The NH3 selectivity which depends on local NO:H2 ratio decreased with increase (decrease) in rich time (total cycle time) for 8% catalyst. Similar NH3 selectivity trend as a function of overall cycle time was also observed for 3% catalyst. The findings are helpful in assessing the viability of these catalysts (3% and 8%) to be used in conjunction with the SCR catalyst. Finally, the model is also used to gain additional insights on the impact of Pt loading and dispersion to provide important guidelines for NSR catalyst design for combined NSR/SCR application.