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
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.