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.

Catalysis plays a critical role in our daily life, from energy to healthcare and from clothing to the environment. Heterogeneous catalysis usually occurs on solid surfaces that provide the appropriate electronic and/or geometric environment. The ultimate goal of research on heterogeneous catalysis is to provide a fundamental understanding of the nature of active sites toward the design of efficient heterogeneous catalysts that provide highest activity, 100 % selectivity, and long-term stability. A catalytic reaction, by its nature, involves atomic-scale and nanoscale entities that interact dynamically with reactant molecules. To better understand the complex processes of heterogeneous catalysis requires better control of the structure and chemistry of the active sites. Advancement in synthesis of nanostructured catalysts, such as shape and size control of particles or pores, modification of surface adsorption energies by compositional and structural modulation, fabrication of unique active sites, and manipulation of particle-support interactions, etc., not only provides better-controlled active sites for fundamental studies of heterogeneous catalysis, but also has practical implications for developing improved or novel industrial catalysts for applications in energy, environment, and sustainability.