Dramatic improvement of NO reduction activity via reversible re-dispersion of CeO2 nanoparticles into Ce+3 atoms on alumina under high temperature reactive treatment

Ceria nanoparticles supported on gamma-alumina prepared via wet impregnation and sourced commercially have low activity for industrially relevant NO reduction by CO in the presence of steam. These supports contain ceria nanoparticles as well as small (~1%) amount of Ce atomically dispersed and anchored by penta-Al sites. We discovered that treatment of these catalysts at temperatures ~750-950 oC under the flow of CO and NO in the presence of steam, which typically leads to catalyst deterioration and sintering, in fact, leads to dispersion of ceria nanoparticles into isolated Ce+3 atoms. We extensively characterize them with XPS, FTIR and HAADF-STEM imaging. Their presence changes the alumina surface, as evidenced by XPS and FTIR with probe molecules. Ce+3 ions show dramatically enhanced NO reduction ability in the presence of CO and steam. Infra-red studies reveal close interaction of NO molecules on Ce+3/Alumina surfaces with the formation of N2O species. Heating these samples in oxygen (in wet or dry streams) at 800 oC and above leads to coalescence of Ce+3 into CeO2 nanoparticles, resulting in reversible loss of activity. Further, reactive treatment of CeO2/Al2O3 under high temperature reaction conditions restores Ce+3 cations as well as catalytic activity. Our study shows reversible redispersion of ceria into isolated Ce+3 cations under conditions where typical catalyst sintering is generally assumed to occur and suggests a pathway to utilize these materials as supports for more effective catalysis. Indeed, supporting only 0.1-0.5 wt% Rh on these CeAl supports, shows synergies between Rh and atomically dispersed Ce ions with excellent activity and stability for NO reduction with CO.