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Abstract
Atom trapping allows to prepare catalysts with atomically dispersed Ru ions Ru1O5 on (100) facets of ceria, as identified by EXAFS, infra-red spectroscopy and DFT calculations. This is a new class of ceria-based materials with Ru properties drastically different from the known M/ceria materials. They show excellent activity in industrially important catalytic NO oxidation reaction, a critical step that requires use of 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 unprecedentedly high NOx storage properties during cold start due to formation of stable Ru-NO complexes as well as high spill-over rate of NOx to the support. Only ~0.05 wt% of Ru is required to have excellent NOx storage. Single Ru ions forming Ru1O5 sites exhibit remarkably higher stability during calcination in air/steam up to 750 ºC in contrast to RuO2 nanoparticles that become volatile at temperature slightly above ambient. We clarify the location of Ru(II) ions on the ceria surface and experimentally identify mechanism of NO oxidation (as well as reactive storage) using DFT calculations and in-situ DRIFTS/Mass-spectroscopy measurements. Furthermore, we show the high reactivity of Ru1/CeO2 for NO reduction by CO: only 0.1-0.5 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 and XPS measurement, we uncover the individual elementary steps of NO reduction by CO on an atomically dispersed ceria-supported catalyst, highlighting the unique properties of Ru1/CeO2 catalyst and its propensity to form oxygen vacancies/Ce+3 sites that are critical for NO reduction even at low Ru loadings. Our study highlights the applicability of novel ceria-based single-atom catalysts to industrially relevant NO and CO abatement.
