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Pushing the Limits of Precious Metal Atom Economy for Three-Way-Catalysts (TWC): Thermally Stable and Highly Active Single Rh Atom Catalysts (Rh1/ceria) Under Both Dry and Industrially Relevant Conditions for NO Abatement

preprint
submitted on 06.04.2020 and posted on 07.04.2020 by Konstantin Khivantsev, Carlos Garcia Vargas, Jinshu Tian, Libor Kovarik, Nicholas R. Jaegers, János Szanyi, Yong Wang

We show for the first time that single positively-charged Rh atoms on ceria, prepared via high-temperature atom trapping synthesis, are the highly active species for (CO+NO) reaction both under dry and wet, industrial conditions. This provides a direct link between organometallic homogeneous Rh(I) complexes capable of catalyzing dry (CO+NO) reaction and supported Rh single atoms, the latter being much more active than their homogeneous analogues. Decreasing the Rh loading from to 0.1 wt% leads to a catalyst with uniform Rh ions on the surface of ceria, which is very active (full NO conversion >120 ⁰C, TOF per Rh atom at 120 ⁰C ~ 330 hr-1) and thermodynamically stable. Under dry conditions, the main product above 100 ⁰C is N2 with N2O being the minor product. Water promotes low-temperature activity of 0.1 Rh/CeO2 starting 50 ⁰C with full NO conversion above 125 ⁰C in the wet stream. In this case, however, ammonia and nitrogen are the main products with only minor N2O amounts. NH3 formation at such relatively low temperatures is attractive because of the potential to use this as a passive SCR system. Because of the uniformity of Rh ions on the support, we are able to clarify the mechanistic aspects of this reaction. More specifically, we show that ammonia formation correlates with the WGS activity of the material and thus, rhodium hydride Rh-H species are believed to be involved in this reaction. These findings provide new mechanistic understanding for the catalytically active species in TWC catalysis and open up a new avenue for the synthesis of industrially relevant emissions control catalysts with 100% atom economy of ultra-expensive precious metals such as Rh.

Funding

The research at PNNL was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences, and Geosciences Catalysis Program (DEAC05-RL01830, FW-47319). 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 Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for the 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 emissions regulations.

History

Email Address of Submitting Author

Konstantin.Khivantsev@pnnl.gov

Institution

Pacific Northwest National Laboratory

Country

United States

ORCID For Submitting Author

0000-0002-4810-586X

Declaration of Conflict of Interest

The authors are filing for a patent.

Version Notes

Version 1.0

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