PNNL

Abstract

Atom trapping leads to catalysts with atomically dispersed Ru1O5 sites on (100) facets of ceria, as identified by 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 catalytic NO oxidation, a critical step that requires use of large loadings of expensive noble metals in diesel aftertreatment systems. Ru1/CeO2 is stable during continuous cycling, ramping and cooling as well as presence of moisture. Furthermore, Ru1/CeO2 shows very high NOx storage properties due to formation of stable Ru-NO complexes as well as high spill-over rate of NOx onto CeO2. Only ~0.05 wt% of Ru is required for excellent NOx storage. Ru1O5 sites exhibit much higher stability during calcination in air/steam up to 750 ºC in contrast to RuO2 nanoparticles. We clarify the location of Ru(II) ions on the ceria surface and experimentally identify mechanism of NO storage and oxidation using DFT calculations and in-situ DRIFTS/Mass-spectroscopy. Moreover, we show excellent reactivity of Ru1/CeO2 for NO reduction by CO at low temperatures: only 0.1-0.5 wt% of Ru is sufficient to achieve high activity. Modulation-excitation in-situ infrared and XPS measurements reveal the individual elementary steps of NO reduction by CO on an atomically dispersed Ru ceria catalyst, highlighting unique properties of Ru1/CeO2 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 NO and CO abatement. The research at Pacific Northwest National Laboratory (PNNL) was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Vehicle Technology Office. Experiments were conducted in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research at PNNL. PNNL is a multiprogram national laboratory operated for the Department of Energy (DOE) by Battelle Memorial Institute under Contract DE-AC06-76RL01830. We acknowledge the support of CLEERS (Crosscut Lean Exhaust Emissions Reduction Simulations). CLEERS is an initiative funded by the U.S. Department of Energy (DOE) Vehicle Technologies Office to support the development of accurate tools for use in the design, calibration, and control of next generation engine/emissions control systems that maximize efficiency, while complying with emission regulations. HAA gratefully acknowledges the support by Bulgarian National Science Fund (project DN-19/2). GNV acknowledges the support of the project EXTREME, funded by the Bulgarian Ministry of Education and Science, D01-76/30.03.2021 through the program “European Scientific Networks”. Computational resources are provided by Avitohol supercomputer, supported by Bulgarian Ministry of Education and Science via National Roadmap for Research Infrastructures. The previous version of this paper was uploaded on the pre-print server ChemRxiv on July 23, 2020: DOI: 10.26434/chemrxiv.12692264.v1.