Structural Changes in Monolayer Cobalt Oxides under Ambient Pressure CO and O2 Studied by In Situ Grazing-Incidence X-ray Absorption Fine Structure Spectroscopy.

Dorotea Gajdek,Lindsay R. Merte,Sara Blomberg,Per-Anders Carlsson,Pär A. T. Olsson,Dörthe Haase,Edvin Lundgren,Johan Gustafson

doi:10.1021/acs.jpcc.1c10284

Abstract

We have used grazing incidence X-ray absorption fine structure spectroscopy at the cobalt K-edge to characterize monolayer CoO films on Pt(111) under ambient pressure exposure to CO and O2, with the aim of identifying the Co phases present and their transformations under oxidizing and reducing conditions. X-ray absorption near edge structure (XANES) spectra show clear changes in the chemical state of Co, with the 2+ state predominant under CO exposure and the 3+ state predominant under O2-rich conditions. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that the CoO bilayer characterized in ultrahigh vacuum is not formed under the conditions used in this study. Instead, the spectra acquired at low temperatures suggest formation of cobalt hydroxide and oxyhydroxide. At higher temperatures, the spectra indicate dewetting of the film and suggest formation of bulklike Co3O4 under oxidizing conditions. The experiments demonstrate the power of hard X-ray spectroscopy to probe the structures of well-defined oxide monolayers on metal single crystals under realistic catalytic conditions.

Highlights

Ultrathin or two-dimensional transition metal oxides are active catalysts for several reactions and useful models for understanding the behavior of noble metal/reducible oxide catalysts, where wetting of the metal occurs and where interfacial reactions are of interest.[1−4] Supported oxides have been shown in particular to provide enhanced activity for lowtemperature CO oxidation[5,6] and the water gas shift reaction,[7] for example.Fundamental studies have shown that these oxides display rather complex behavior upon exposure to reactant gases by changing their structure
The film grown immediately before X-ray absorption fine structure spectroscopy (XAFS) experiments was characterized by lowenergy electron diffraction (LEED) (Figure 3b)
The changes observed in X-ray absorption near edge structure (XANES) and Extended X-ray absorption fine structure spectroscopy (EXAFS) spectra with increasing temperature changing white line profiles and the appearance of configurations were 3d84s1 (Co)−Co scattering are consistent with a dewetting transition from an initial 2D layer to 3D islands

Summary

Introduction

Ultrathin or two-dimensional transition metal oxides are active catalysts for several reactions and useful models for understanding the behavior of noble metal/reducible oxide catalysts, where wetting of the metal occurs and where interfacial reactions are of interest.[1−4] Supported oxides have been shown in particular to provide enhanced activity for lowtemperature CO oxidation[5,6] and the water gas shift reaction,[7] for example.Fundamental studies have shown that these oxides display rather complex behavior upon exposure to reactant gases by changing their structure. Cobalt oxides are active catalysts for oxidation reactions[11,12] and form hexagonal bilayer films on Pt(111),[13−16] Pd(100),[17] and Au(111),[14−16] which are similar to FeO These CoO films exhibit similar structural changes as FeO when reduced and oxidized in high vacuum: in particular, a bilayer to CoO2 (or CoOOH) trilayer transformation has been identified upon oxygen exposure.[16,18] as with FeO, the edges of CoO islands were shown to be active sites for reactions such as water dissociation, and after exposure to electrochemical conditions, formation of the 2D hydroxides/oxyhydroxides Co(OH)[2] and CoOOH was reported.[18] Kersell et al.[19] recently reported the formation of a stable carbonate at CoO island edges while exposing the film to CO and O2 at room temperature

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Conclusion