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; Pär A T Olsson; Sara Blomberg; Johan Gustafson; Per-Anders Carlsson; Dörthe Haase; Edvin Lundgren; Lindsay R Merte

doi:10.1021/acs.jpcc.1c10284

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

J Phys Chem C Nanomater Interfaces. 2022 Feb 24;126(7):3411-3418. doi: 10.1021/acs.jpcc.1c10284. Epub 2022 Feb 16.

Authors

Dorotea Gajdek^{1

2}, Pär A T Olsson^{1

3}, Sara Blomberg^{4

2}, Johan Gustafson⁵, Per-Anders Carlsson^{6

7}, Dörthe Haase⁸, Edvin Lundgren^{5

2}, Lindsay R Merte^{1

2}

Affiliations

¹ Department of Materials Science and Applied Mathematics, Malmö University, SE-211 19 Malmö, Sweden.
² NanoLund, Lund University, Box 118, SE-221 00 Lund, Sweden.
³ Division of Mechanics, Lund University, Box 118, SE-221 00 Lund, Sweden.
⁴ Department of Chemical Engineering, Lund University, Box 118, SE-221 00 Lund, Sweden.
⁵ Division of Synchrotron Radiation Research, Lund University, Box 118, SE-221 00 Lund, Sweden.
⁶ Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden.
⁷ Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden.
⁸ MAX IV Laboratory, Lund University, Box 118, SE-221 00 Lund, Sweden.

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.