Insight into the effect of surface structure on H2 adsorption and activation over different CuO(1 1 1) surfaces: A first-principle study

Abstract

Using density functional theory (DFT) calculations, the interactions of H and H2 with different types of CuO(111) surfaces, including the perfect, oxygen-vacancy and pre-covered oxygen surfaces, have been systematically investigated to probe into the effect of surface structures on H2 adsorption and activation. Our results show that the outermost surface oxygen site (OSUF) is the active center for H adsorption both on the perfect and oxygen-vacancy surfaces, while H favorably adsorbs at the pre-covered oxygen site (Opre) on the pre-covered oxygen CuO(111) surface. In the case of H2 adsorption and activation, H2 is the dissociative adsorption on the perfect surface, two dissociative H atoms adsorbed at the same OSUF site pull out the OSUF atom from the surface to form a H2O molecule adsorbed at the subsurface copper site (CuSUB), which ultimately contributes to the formation of oxygen-vacancy site on CuO(111) surface. For the oxygen-vacancy surface, H2 adsorbed at the singly (Cu1) and doubly (Cu2) coordinated CuSUB sites via the side-on mode are two favorable configurations, moreover, the dissociation barriers for these two favorable configurations is higher than their corresponding adsorption energies, namely, H2 dissociation is difficult to occur on the oxygen-vacancy surface. For the pre-covered oxygen surface, H2 initially adsorbed at OSUF site with the side-on or end-on modes is the dissociative adsorption, in which one H is still adsorbed at OSUF site, the other H is adsorbed at the adjacent pre-covered oxygen site; meanwhile, H2 initially adsorbed at Opre site with the side-on mode is also the dissociative adsorption. Thus, H2 dominantly exists in the form of the dissociative adsorption with the adsorbed H atoms on the perfect and pre-covered oxygen surfaces, whereas, H2 mainly exists in the form of molecular adsorption H2 on the oxygen-vacancy surface, suggesting that the adsorption and activation behaviors of H2 over CuO(111) surface are particularly sensitive to the surface structure.