Computational Study on M/POM Single-Atom Catalysts (M = Cu, Zn, Ag, and Au; POM = [PWO]): Metal-Support Interactions and Catalytic Cycle for Alkene Epoxidation.

Geometrical structures, metal-support interactions, and infrared (IR) spectroscopy of a series of M/POM (M = Cu, Zn, Ag, and Au; POM = [PWO]) single-atom catalysts (SACs), and catalytic cycle for alkene epoxidation catalyzed by M/POM SACs were studied using density functional theory (DFT) calculations. The calculations demonstrate that the most probable anchoring sties for the isolated single atoms studied here in the M/POM SACs are the fourfold hollow sites on the surface of POM support. The bonding interaction between single metal atom and surface of POM support comes from the molecular orbitals with a mixture of d atomic orbital of metal and 2p group orbital of surface oxygen atoms of POM cage. The calculated adsorption energy of isolated metal atoms in these M/POM SACs indicates that the early transition metals (Cu and Zn) have high thermal stability. The DFT-derived IR spectra show that the four characteristic peaks of free Keggin-type POM structure split into six because of introduction of isolated metal atom. Compared with other metal atoms, the Zn/POM SAC has the high reactivity for activity of dioxygen molecule, because the dioxygen moiety in Zn/POM SAC displays O· radical feature with [POM·ZnO·] configuration. Finally, a catalytic cycle for ethylene epoxidation by O catalyzed by Zn/POM SAC was proposed based on our DFT calculations. Supported noble-metal SACs are among the most important catalysts currently. However, noble metals are expensive and of limited supply. Development of non-noble-metal SACs is of essential importance. Therefore, the reported Zn/POM SAC would be very useful to guide the search for SACs into non-noble metals.