Chemistry of copper overlayers on zinc oxide single-crystal surfaces: model active sites for copper/zinc oxide methanol synthesis catalysts

Abstract

Copper has been evaporated onto chemically different single-crystal surfaces of zinc oxide in ultrahigh vacuum to model Cu/ZnO methanol synthesis catalysts. The formation of the copper overlayers from less than 0.1 monolayer (ML) to several ML on the (0001), (0001), and (10 0) surface planes is followed with core-level X-ray photoelectron spectroscopy, valence band photoelectron spectroscopy with both resonance discharge sources and synchrotron radiation, and low-energy electron diffraction. At room temperature, the first monolayer grows in a two-dimensional fashion, with low coverage copper existing as isolated atoms/small islands on all surfaces. Surface perturbations show submonolayer copper supported on the Zn2+-terminated (0001) surface to be most prone to high-temperature clustering and reaction with molecular oxygen. Copper on the oxide-terminated (0001) surface is much less reactive, while copper on the (1010) dimer surface shows intermediate reactivity. Low-temperature carbon monoxide (CO) chemisorption experiments indicate that highly dispersed copper on the (0001) and (lOTO) surfaces chemisorb CO with approximately the same affinity as copper metal (A/fads = 15-16 kcal/mol), while chemisorption on Cu/(0001) was much weaker (A/7ads < 12 kcal/mol). High-affinity (A7/ads = 21 kcal/mol) CO chemisorption, often associated with the catalytic active site, is shown to occur at a coordinatively unsaturated tetrahedral Cu+ site created on the (0001) surface upon annealing in oxygen. Chemisorption to the Cu+ site perturbs the CO electronic structure to a much greater extent than chemisorption to either Cu°or Zn2+, with valence band PES indicating both strong and 7 interactions. The implications of these results with respect to CO activation and the catalytic activity of the Cu/ZnO system are discussed.with Cu/ZnO as compared to ~30 kcal/mol on ZnO.8 Metallic copper is reported to show no measurable activity.8,9 Additionally, a significant amount of CO chemisorbs to the Cu-promoted catalyst more strongly than to either Cu or ZnO, indicating that the Cu/ZnO combination contains a detectable number of surface

Interaction of platinum films with the (0001̄) and (0001) surfaces of ZnO

“…Upward band-bending accompanied by charge transfer from the oxide to the metal overlayer has also been observed in the case of Cu deposition on ZnO(0001). 38 The formation of a Schottky barrier is also consistent with the observed increase in the normalized phonon intensity upon deposition of 0.1 ML of Pt on ZnO(OOOI). As was shown in the results section, the (0001) surface contains an accumulation layer of charge carriers.…”

Oxidation of Cu on ZnO(0001)-Zn: Angle-Resolved Photoelectron Spectroscopy and Low-Energy Electron Diffraction Study

“…A similar band-bending behavior is also found in the work by Yoshihara et al [15], while an initial dip is missing in the works by Lazcano et al [17] and by Didziulis et al [18]. Although the cause of such a difference is not known at present, we speculate that the…”

Morphology evolution and local electric properties of Au nanoparticles on ZnO thin films

“…7͒ and Pt. 8 Conversely, Cu on ZnO displays a two-dimensional ͑2D͒ island growth, 1 or Stranski-Krastanov nucleation, 9 whereas Pd shows a layer-by-layer, 7 or Volmer-Weber three-dimensional ͑3D͒ island clusters formation. 10 The purpose of our investigations is the study of the evolution of the shape and size of Au particles on the surface of ZnO thin films in view of the determination of the growth mode, as a function of increasing Au coverage.…”