Formation of a 2D Meta-stable Oxide by Differential Oxidation of AgCu Alloys

Abstract

Metal alloy catalysts can develop complex surface structures when exposed to reactive atmospheres. The structures of the resulting surfaces have intricate relationships with a myriad of factors, such as the affinity of the individual alloying elements to the components of the gas atmosphere and the bond strengths of the multitude of low-energy surface compounds that can be formed. Identifying the atomic structure of such surfaces is a prerequisite for establishing structure–property relationships, as well as for modeling such catalysts in ab initio calculations. Here, we show that an alloy, consisting of an oxophilic metal (Cu) diluted into a noble metal (Ag), forms a meta-stable two-dimensional oxide monolayer, when the alloy is subjected to oxidative reaction conditions. The presence of this oxide is correlated with selectivity in the corresponding test reaction of ethylene epoxidation. In the present study, using a combination of in situ, ex situ, and theoretical methods (NAP-XPS, XPEEM, LEED, and DFT), we determine the structure to be a two-dimensional analogue of Cu2O, resembling a single lattice plane of Cu2O. The overlayer holds a pseudo-epitaxial relationship with the underlying noble metal. Spectroscopic evidence shows that the oxide’s electronic structure is qualitatively distinct from its three-dimensional counterpart, and because of weak electronic coupling with the underlying noble metal, it exhibits metallic properties. These findings provide precise details of this peculiar structure and valuable insights into how alloying can enhance catalytic properties.

Single-Atom Alloy Catalysis

“…Recent work by Schweinar and coworkers examined the structure of dilute CuAg alloys after oxygen exposure. 269 They found that oxygen treatment leads to formation of a thin 2D copper oxide film, structurally similar to Cu 2 O, with distinct electronic properties due to the influence of the underlying noble metal.…”

Plasma-assisted oxidation of Cu(100) and Cu(111)

“…8, the Cu L-edge NEXAFS spectra measured aer each O 2 plasma treatment are shown, as well as the intensity of various components, Cu, Cu 2 O and CuO, as determined by a linear combination (LC) analysis based on NEXAFS ngerprints of the different Cu species. 58,59 The details regarding the LC analysis can be found in the ESI (Fig. S5(a) and S6 † for Cu Ledge and with Fig.…”

In Situ Characterization of Catalysis and Electrocatalysis Using APXPS

“…These inhomogeneities in chemical composition and electronic The same group further combined APXPS, PEEM, LEED, and DFT to study the atomic structure of Cu x O y surface oxide and tried to establish a structure−property relationship. 75 They found a two-dimensional metastable Cu surface oxide on AgCu alloy surfaces. Spectroscopic results showed that the Cu x O y had a similar oxidation state to Cu 2 O with a p2 structure.…”

“…The same group further combined APXPS, PEEM, LEED, and DFT to study the atomic structure of Cu x O y surface oxide and tried to establish a structure–property relationship . They found a two-dimensional metastable Cu surface oxide on AgCu alloy surfaces.…”