A density functional study on properties of a Cu3Zn material and CO adsorption onto its surfaces

Abstract

Prior experimental and theoretical efforts have provided strong evidence that the formation of α-brass such as Cu3Zn alloys in Cu/ZnO/Al2O3 CO2/CO hydrogenation catalysts enhances dramatically the catalytic activity toward methanol synthesis. In this work, a density functional theory (DFT) slab model has been adopted to get information concerning the bulk and surface properties of DO23-like Cu3Zn and to explore CO molecular adsorption, which will help pave the way to future rationalization of the impact of surface alloying on Cu/ZnO-based catalysis for CO2 and CO hydrogenations. Our calculations imply that the bulk modulus and cohesive energy of the binary solid solution lie between the corresponding ones for the individual components, but only the former quantity equals its composition weighted average. From the DFT-computed surface energies, the stability of Cu3Zn surfaces was predicted to be reinforced in the sequence (110)<(101)<(111)<(100)=(001)<(214)<(114), which can be interpreted as sensitive to the density change of surface dangling bonds. The downward shifts in the C–O stretch frequency measured experimentally over methanol synthesis catalysts at successively elevated reduction temperatures were correctly reproduced by the present simulation for the adsorption of CO to take place at Cu3Zn(114), Cu3Zn(214) and, as a reference, Cu(111). This agreement confirms the total energy results that indicate that the flat (114) and stepped (214) facets are the most stable and abundant ones in the Cu3Zn particles formed. It was found that a subtle compromise between the cost of fragment distortions and the large stabilization due to molecule–surface interaction is the way to control and optimize the reactivity of the Cu-based alloy to CO chemisorption. Intriguingly, electronic structure evaluation reveals that as far as all the alloy surfaces under scrutiny are concerned, a layer of CO brought a decrease, not an increase, in work function for (101)Zn and (110)CuZn, though the electrons always flowed from the substrate to the adsorbate. The finding is not trivial at all since it counters the classical rule that an electronegative species raises the work function of the underlying surface. The bonding of CO to the Cu3Zn systems via C–Cu contacts was identified as being primarily covalent rather than ionic. A simple d-band energy model is able to capture the bonding tendency observed.