Insight into the effect of surface structure on H2 adsorption and activation over different CuO(111) surfaces: A first-principle study

Using density functional theory (DFT) calculations, the interactions of H and H-2 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 H-2 adsorption and activation. Our results show that the outermost surface oxygen site (O-SUF) 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 (O-pre) on the pre-covered oxygen CuO(111) surface. In the case of H-2 adsorption and activation, H-2 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 (Cu-SUB), which ultimately contributes to the formation of oxygen-vacancy site on CuO(111) surface. For the oxygen-vacancy surface, H-2 adsorbed at the singly (Cu-1) and doubly (Cu-2) 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, H-2 dissociation is difficult to occur on the oxygen-vacancy surface. For the pre-covered oxygen surface, H-2 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, H-2 initially adsorbed at Opre site with the side-on mode is also the dissociative adsorption. Thus, H-2 dominantly exists in the form of the dissociative adsorption with the adsorbed H atoms on the perfect and pre-covered oxygen surfaces, whereas, H-2 mainly exists in the form of molecular adsorption H-2 on the oxygen-vacancy surface, suggesting that the adsorption and activation behaviors of H-2 over CuO(111) surface are particularly sensitive to the surface structure. (C) 2016 Elsevier B.V. All rights reserved.