Non-noble metal single-atom catalysts with phosphotungstic acid (PTA) support: A theoretical study of ethylene epoxidation

Abstract

Geometric and electronic structures of phosphotungstic acid (PTA) supported single transition metal atom (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt) catalysts have been systematically investigated by using the first-principles theoretical methods. Possible reaction mechanism for ethylene epoxidation was explored. The most possible anchoring site for the single transition metal atom is the fourfold hollow site on PTA. As the non-noble metal Fe1-PTA system possesses considerable adsorption energies towards both O2 and C2H4, the strong bonding interaction between Fe1 and PTA cluster was analyzed. It is found that the electron transfers from Fe atom to PTA cluster and strong covalent metal-support interactions (CMSI) between the Fe 3d orbitals and O 2p orbitals of PTA lay the foundation of high stability. The proposed catalytic reaction mechanism for ethylene epoxidation on Fe1- PTA single-atom catalyst (SAC) includes three steps: the O2 adsorbs on Fe1-PTA via electron transfer; the first ethylene attacks the adsorbed O2 molecule on Fe1-PTA followed by the formation of C2H4O; finally, the O atom remained on Fe1-PTA reacts with a second ethylene to form the product and accomplish the catalytic cycle. The Fe1-PTA has high selectivity and catalytic activity for ethylene epoxidation via an Eley–Rideal mechanism with low energy barriers. A potentially competitive pathway for the formation of acetaldehyde is not kinetically favorable. These results provide insights for the development of highly efficient heterogeneous SACs for ethylene epoxidation with non-noble metals.