New insights into the promoting effect of H2O on a model Pt/Ba/Al2O3 NSR catalyst

The effects of H2O on the performance of a model Pt/Ba/Al2O3 catalyst and regeneration of the catalyst surface were investigated, both in the absence and presence of CO2. In the absence of CO2, an unexpected promotional effect of H2O was observed at low temperature. For example, in the presence of H2O, 90% NOX conversion was obtained at 150 °C, which is the same as that at 350 °C under otherwise identical conditions. The results demonstrate that regeneration, the rate-limiting step at low temperature, occurs through hydrogen spillover to the nitrates, and not through reverse NOX species migration to the Pt sites, and the presence of H2O greatly promotes the hydrogen spillover rate by providing and stabilizing surface hydroxyl groups. The promotional effect of H2O gradually decreases with increasing temperature, and shows no positive effect above 350 °C. In the presence of CO2, however, the presence of H2O always results in improved performance in the entire temperature range. Besides enhanced hydrogen spillover, another contribution of H2O when CO2 is present, is to weaken the detrimental effect of CO. CO is formed via the reverse water–gas shift reaction and poisons Pt sites as well as forms barium isocyanates. The H2O decreases the amount of CO formed as well as hydrolyzes the –NCO species.