(628b) Ammonia Production during Three-Way Catalysis Is Due to Atomically Dispersed Rhodium

Rhodium plays a key role in Three Way Catalysts (TWCs) to control NO_x emissions in gasoline engines by reducing NO by CO. Despite extensive investigation, there remains uncertainty about NO reduction mechanisms. Particularly, which sites are responsible for the unselective reduction of NO to NH₃ under light-off conditions or in CO-rich feed. Here, we elucidate structure-function relationships for oxide supported Rh catalysts for the reduction of NO by CO under dry conditions and in the presence of H₂O.

To develop structure-function relationships for NH₃ production on Rh catalysts, we synthesized Rh catalysts on γ-Al₂O₃ and CeO₂ at weight loadings from 0.05-5 wt% where the predominant Rh structure varies from atomically dispersed species (single atoms) to a mixture of single atoms, clusters and nanoparticles. CO probe molecule Fourier-transform infrared spectroscopy (FTIR) provided evidence of the expected dependence of Rh structure on weight loading. Catalytic activity was measured by flowing simulated exhaust conditions (5000 PPM CO/1000 PMM NO + 0 or 2% H₂O) over catalysts while the temperature was increased at a controlled rate to observe temperature dependent reactivity during start up conditions. Under dry conditions, where no NH₃ is produced because there is no H-source, atomically dispersed Rh species are less reactive than Rh clusters, with ~25 °C difference in light off temperatures. The addition of H₂O significantly promotes the reactivity of atomically dispersed Rh and results in 100% selectivity to NH₃ at temperatures < 200 °C on Al₂O₃ and CeO₂ supports. Rh clusters are less reactive than atomically dispersed species when H₂Ois co-fed and the reaction primarily leads to N₂ formation. We will also discuss the mechanism by which NO is reduced into NH₃ over atomically dispersed Rh. Thus, less atomically dispersed Rh in the catalytic converter may be beneficial to prevent NH₃ production in TWCs.