[Paper Review] Elucidating the active phases of CoOx films on Au(111) in the CO Oxidation Reaction

Hao Chen, Lorenz J. Falling+11 more|arXiv (Cornell University)|Apr 26, 2023

Catalytic Processes in Materials ScienceMaterials Science3 citations

TL;DR

This study identifies three distinct active phases of CoOx films on Au(111) during CO oxidation, depending on gas-phase CO/O2 stoichiometry: partially oxidized CoO under O2-lean conditions, poisoned CoO2 by carbonate formation under moderate CO exposure, and catalytically active Co3+ species under O2-rich conditions. Resonant photoemission spectroscopy and DFT calculations reveal Co3+ as the key active site, with low free energy barriers for CO oxidation, demonstrating dynamic catalyst restructuring under reaction conditions.

ABSTRACT

Using CoOx thin films supported on Au(111) single crystal surfaces as model catalysts for the CO oxidation reaction we show that three reaction regimes exist in response to chemical and topographic restructuring of the CoOx catalyst as a function of reactant gas phase CO/O2 stoichiometry a finding that highlights the versatility of catalysts and their evolution in response to reaction conditions. Under oxygen-lean conditions and moderate temperatures (below 150C degrees) partially oxidized films containing CoO were found to be efficient catalysts. In contrast, stoichiometric CoO films containing only Co2+ form carbonates in the presence of CO that poison the reaction below 300 C degrees. Under oxygen-rich conditions a more oxidized catalyst phase forms containing Co3+ species that is effective in a wide temperature range. Resonant photoemission spectroscopy (ResPES) revealed the unique role of Co3+ sites in catalyzing the CO oxidation. DFT calculations provided deeper insights into the pathway and free energy barriers for the reactions on these oxide phases.

Motivation & Objective

• To identify the active catalytic phases of CoOx films on Au(111) under varying CO/O2 reaction conditions.
• To understand how chemical and topographic restructuring of CoOx films influences catalytic activity in CO oxidation.
• To determine the role of different cobalt oxidation states (Co2+, Co3+) in reaction pathways and selectivity.
• To correlate spectroscopic observations with DFT-calculated free energy barriers for mechanistic insight.

Proposed method

• Preparation of epitaxial CoOx thin films on single-crystal Au(111) substrates as model catalysts.
• In situ resonant photoemission spectroscopy (ResPES) to probe oxidation states and electronic structure of Co species under reaction conditions.
• Temperature-programmed reaction and XPS analysis to track surface composition and carbonate formation.
• Density functional theory (DFT) calculations to determine reaction pathways and free energy barriers on different CoOx phases.
• Systematic variation of CO/O2 stoichiometry to probe phase transitions and activity regimes.
• Correlation of spectroscopic data with catalytic activity to identify active sites.

Experimental results

Research questions

• RQ1 What are the distinct catalytic phases formed in CoOx films on Au(111) under different CO/O2 ratios during CO oxidation?
• RQ2 How does the oxidation state of cobalt (Co2+ vs. Co3+) influence catalytic activity and stability?
• RQ3 Why does stoichiometric CoO deactivate under CO-rich conditions, and what is the nature of the poisoning species?
• RQ4 What is the role of Co3+ in enabling efficient CO oxidation across a wide temperature window?
• RQ5 How do the reaction pathways and free energy barriers differ across the various CoOx phases?

Key findings

• Under O2-lean conditions and temperatures below 150 °C, partially oxidized CoO films exhibit high catalytic activity for CO oxidation.
• Stoichiometric CoO films form surface carbonates under CO exposure, which poison the catalyst and suppress activity below 300 °C.
• Under O2-rich conditions, a more oxidized phase containing Co3+ species forms, which remains active over a broad temperature range.
• Resonant photoemission spectroscopy (ResPES) confirms that Co3+ sites are the primary active centers for CO oxidation.
• DFT calculations show that the CO oxidation pathway on Co3+ sites has a low free energy barrier, explaining their high activity.
• The study demonstrates that CoOx catalysts undergo dynamic chemical and topographic restructuring in response to reaction conditions, leading to distinct active phases.