To obtain insight into the structure and surface stoichiometry of copper-based catalysts in commercially important chemical reactions such as the oxygen-assisted water-gas shift reaction, we perform density-functional theory calculations to investigate the relative stability of low-index copper oxide surfaces. By employing the technique of ``ab initio atomistic thermodynamics,'' we identify low-energy surface structures that are most stable under realistic catalytic conditions are found to exhibit a metallic character. Three surfaces are shown to have notably lower surface free energies compared to the others considered and could be catalytically relevant; in particular, under oxygen-rich conditions, they are the ${\mathrm{Cu}}_{2}\mathrm{O}(110):\mathrm{Cu}\mathrm{O}$ surface, which is terminated with both Cu and O surface atoms, and the ${\mathrm{Cu}}_{2}\mathrm{O}(111)\text{\ensuremath{-}}{\mathrm{Cu}}_{\mathrm{CUS}}$ surface, which contains a surface (coordinatively unsaturated) Cu vacancy, while for the oxygen-lean conditions, the ${\mathrm{Cu}}_{2}\mathrm{O}(111)$ surface with a surface interstitial Cu atom is found to be energetically most favorable, highlighting the importance of defects at the surface.

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Thermodynamic stability and structure of copper oxide surfaces: A first-principles investigation

As a first step towards gaining microscopic understanding of copper-based catalysts, e.g., for the low-temperature water-gas shift reaction and methanol oxidation reactions, we present density-functional theory calculations investigating the chemisorption of oxygen, and the stability of surface oxides on $\mathrm{Cu}(111)$. We report atomic geometries, binding energies, and electronic properties for a wide range of oxygen coverages, in addition to the properties of bulk copper oxide. Through calculation of the Gibbs free energy, taking into account the temperature and pressure via the oxygen chemical potential, we obtain the $(p,T)$ phase diagram of $\mathrm{O}∕\mathrm{Cu}(111)$. Our results show that for the conditions typical of technical catalysis the bulk oxide is thermodynamically most stable. If, however, formation of this fully oxidized surface is prevented due to a kinetic hindering, a thin surface-oxide structure is found to be energetically preferred compared to chemisorbed oxygen on the surface, even at very low coverage. Similarly to the late $4d$ transition metals (Ru, Rh, Pd, Ag), sub-surface oxygen is found to be energetically unfavorable.

Oxygen adsorption and stability of surface oxides on Cu ( 111 ) : A first-principles investigation

Atomic layer deposition-Sequential self-limiting surface reactions for advanced catalyst "bottom-up" synthesis

A Cu(111) surface displays a low activity for the oxidation of carbon monoxide (2CO + O(2) → 2CO(2)). Depending on the temperature, background pressure of O(2), and the exposure time, one can get chemisorbed O on Cu(111) or a layer of Cu(2)O that may be deficient in oxygen. The addition of ceria nanoparticles (NPs) to Cu(111) substantially enhances interactions with the O(2) molecule and facilitates the oxidation of the copper substrate. In images of scanning tunneling microscopy, ceria NPs exhibit two overlapping honeycomb-type moire structures, with the larger ones (H(1)) having a periodicity of 4.2 nm and the smaller ones (H(2)) having a periodicity of 1.20 nm. After annealing CeO(2)/Cu(111) in O(2) at elevated temperatures (600-700 K), a new phase of a Cu(2)O(1+x) surface oxide appears and propagates from the ceria NPs. The ceria is not only active for O(2) dissociation, but provides a much faster channel for oxidation than the step edges of Cu(111). Exposure to CO at 550-750 K led to a partial reduction of the ceria NPs and the removal of the copper oxide layer. The CeO(x)/Cu(111) systems have activities for the 2CO + O(2) → 2CO(2) reaction that are comparable or larger than those reported for surfaces of expensive noble metals such as Rh(111), Pd(110), and Pt(100). Density-functional calculations show that the supported ceria NPs are able to catalyze the oxidation of CO due to their special electronic and chemical properties. The configuration of the inverse oxide/metal catalyst opens new interesting routes for applications in catalysis.

T. Yamada

Papers

Scanning tunneling microscopy studies of oxygen adsorption on Cu(111)

STM observation of oxygen adsorption on Cu(111).