Catalysis on Singly Dispersed Rh Atoms Anchored on an Inert Support

A metal catalyst supported on an inert substrate could consist of both metal nanoparticles and singly dispersed metal atoms. Whether these singly dispersed metal atoms are active and how different their catalytic mechanism could be in contrast to a supported metal catalyst are fundamentally important for understanding catalysis on a supported metal or oxide. By taking reduction of NO with CO on singly dispersed Rh atoms anchored on an inert support SiO₂ as a probe system (Rh₁/SiO₂), here we demonstrated how singly dispersed metal atoms on an inert support could perform a complex multi-step catalytic cycle through a mechanism distinctly different from that for a supported metal nanoparticle with continuously packed metal sites. These singly dispersed Rh₁ atoms anchored on SiO₂ are active in reducing nitric oxide with carbon monoxide through two reaction pathways that are different from those of Rh nanoparticles. In situ IR studies show that a CO molecule and a NO molecule coadsorb on a singly dispersed Rh atom, Rh₁ anchored on SiO₂, and couple to form an N atom to adsorb on the surface and a CO₂ molecule to desorb. The adsorbed N atom further couples with another CO molecule in the gas phase to form an intermediate -NCO on Rh₁; this intermediate can directly couple with an NO molecule adsorbed on the same Rh₁ to form N₂ and CO₂. In another pathway, the adsorbed N atom can couple with a coadsorbed NO on the same Rh₁ to form N₂O; N₂O further reacts with adsorbed CO on the same Rh₁ to form N₂ and CO₂ through a high activation barrier that can be overcome at a high temperature. Our studies show that the singly dispersed metal atoms on an inert support have great potential to perform selective transformation of chemicals. The confirmed catalysis with a singly dispersed Rh₁ on SiO₂ through a mechanism different from a metal nanoparticle supported on the same substrate suggests the significance of taking the single-atom catalysis (SAC) into fundamental studies of catalysis of a supported metal catalyst, since metal nanoparticles and singly dispersed metal atoms likely coexist on the inert support of many supported catalysts.