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.

Identification of single Ru(II) ions on ceria as a highly active catalyst for abatement of NOx pollutants

Atom trapping allows to prepare catalysts with atomically dispersed Ru ions anchored to the ceria support. The resulting catalysts free of expensive noble metals such as Pt, Pd, Rh (whose prices are ~8-60 times higher than Ru on the per-molar basis) with Ru loadings of only 0.25-0.5 wt% show excellent activity in industrially important catalytic NO oxidation reaction, a critical step that requires use of relatively large loadings of expensive noble metals in diesel aftertreatment systems. Ru1/CeO2 catalysts are stable during continuous cycling, ramping and cooling as well as presence of moisture. Furthermore, Ru1/CeO2 shows excellent NOx storage properties during cold start, with improved NO adsorption compared with the best described Pd/Zeolite NO adsorbers with ~2-3 times higher Pd loadings. We clarify the location of Ru(II) ions on the ceria surface and identify mechanism of NO oxidation (as well as reactive storage) using DFT calculations and in-situ DRIFTS/Mass-spectroscopy measurements. Furthermore, we show the possible applications of Ru1/CeO2 in gasoline engines for NO reduction by CO: only 0.1 wt% of atomically dispersed Ru is sufficient to achieve high activity at low temperatures. With the aid of excitation-modulation in-situ infra-red measurements, we uncover the elementary steps of NO reduction by CO on an atomically dispersed ceria-supported catalyst. Our study highlights the potential applicability of single-atom catalysts to industrially relevant NO and CO abatement.