Structurally Accurate Model for the “29”-Structure of Cu_xO/Cu(111): A DFT and STM Study

Abstract

Copper is a common catalyst for many important chemical reactions including low-temperature water gas shift, selective catalytic reduction of NO x , methanol synthesis, methanol steam reforming, and partial oxidation of methanol. The degree of surface oxidation, or the oxidation state of the active site, during these reactions has been debated and is known to have a large influence on the reaction rates. Therefore, elucidating the atomic-scale structure of copper surface oxides is an important step toward a fuller understanding of reaction mechanisms in heterogeneous catalysis. The so-called “29” monolayer oxide film is a common intermediate in the oxidation of Cu­(111). The large size of its unit cell has thus far prevented the development of a definitive model for its structure. Using high-resolution scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we arrive at a model for the “29” Cu x O film on Cu(111). There is very good agreement between experimental and computational STM images over a range of biases. Through the construction of a phase diagram from first-principles, we further find that the “29” structure derived from the DFT calculations is indeed the most stable structure under the experimental conditions considered. This work yields an accurate picture of the atomic scale structure of the “29” oxide film and therefore a basis for beginning to understand adsorption sites and reaction mechanisms on this catalytically relevant surface.

High-temperature oxidation and reduction of the inverse ceria/Cu(111) catalyst characterized by LEED, STM, nc-AFM and KPFM

“…This creates locally a different structure as shown by the orange parallelogram, which has a much smaller dimension with a = (8.8 ± 0.5) Å, b = (17.2 ± 0.5) Å and α = (86 ± 1)°. The size of this local structure is comparable with the one of the "29" oxide structure previously observed by STM [56,57,60,26,61,59].…”

“…Another regular observation is that the atomic contrast of the oxidized Cu(111) surface considerably changes when changing the bias voltage, e.g., from 1.0 to 0.08 V, as shown by the three images in figure 3(e). A similar voltage dependence was also observed before when imaging the "29" and "44" oxide structure [61,59]. Because of this and because we also regularly observe the above mentioned contrast changes upon tip changes, we do not compare the relative atomic contrast of the images.…”

“…The oxidation of the Cu(111) surface was extensively studied by LEED [62,57], µLEED/LEEM [63], XPS [64] and STM [56,57,60,58,61,65,64,59], in particular in dependence on the temperature of the O 2 annealing (from RT to 450°C) and post-annealing [60,57]. The overall conclusion is that the surface reconstructs at temperatures starting at ∼300 °C, forming the "29" and "44" oxide structures [56].…”

Molecular scale insights into interaction mechanisms between organic inhibitor film and copper

“…As for the decomposition of the O 1s spectra, two components O 1 and O 2 are observed at binding energies of 531.1 eV and 529.6 eV, with a full width at half-maximum (FWHM) of 1.0 eV and 2.5 eV, respectively. According to previous studies on the "29" structure formed in these pre-oxidation conditions 29,30 , atomically accurate models were proposed with oxygen adsorbed in two different environments: O anions in Cu-O hexagonal ring and O adatom in the center of the ring. The component O 2 at lower binding energy is usually attributed to the bulk oxide 31,32 , which could in this case be assigned to O anion bonded to copper in the hexagonal ring.…”

“…conditions, a 2D surface oxide with "29" superstructure was formed 29,30,37 . A 3D oxide was also prepared by exposing the metallic Cu(111) surface to air.…”