On the Nature of Support Effects of Metal Dioxides MO₂ (M = Ti, Zr, Hf, Ce, Th) in Single-Atom Gold Catalysts: Importance of Quantum Primogenic Effect

Abstract

Fundamental understanding of support effects and metal–support interaction is critical in heterogeneous catalysis. In this work, theoretical investigations are carried out to study the nature of support effects of different tetravalent-metal dioxides of MO₂ (M = Ti, Zr, Ce, Hf, Th) in single-atom gold catalysts using density functional theory with on-site Coulomb interactions (DFT+U). The properties of gold adatom on the stoichiometric (MO₂) and reduced (MO_2–<i>x</i>) surfaces as well as CO adsorption on Au₁/MO₂ and Au₁/MO_2–<i>x</i> have been investigated systematically. Our calculations indicate that the fundamental <i>quantum primogenic effect</i> that causes the radial contraction and low orbital energies of 3d and 4f orbitals in these MO₂ oxides plays a vital role in determining the valence states and charge distribution of single-atom gold as well as the adsorption modes of CO on various MO₂ supports. We find that gold atoms supported on different surfaces exhibit oxidation states from Au­(−I) to Au(0) to Au­(I), depending on the nature of the metal oxide supports. The support-dependent oxidation states and charge distribution of Au can further influence the adsorption mode of CO. While CO adsorbs strongly on Au­(I) in Au₁/MO₂ (M = Ti, Ce) via Au–C σ-bonding, weaker adsorption occurs on Au(0) in Au₁/MO₂ (M = Zr, Hf, Th) with charge back-donation to CO 2π* antibonding orbitals. In contrast, at Au₁/MO_2–<i>x</i> of reduced support, CO adsorption is stronger for M = Zr, Hf, and Th than for M = Ce. These results provide essential understanding on the nature of support effects of metal oxides in heterogeneous catalysis