Dewetting Transition of CoO/Pt(111) in CO Oxidation Conditions Observed In Situ by Ambient Pressure STM and XPS

Abstract

Ultrathin cobalt oxide films supported on noble-metal surfaces have received much attention as interesting examples of low-temperature CO oxidation catalysts. It is expected that the activity of the cobalt oxides is closely linked with the structure and morphology of the film, but a direct operando correlation between CO oxidation activity, nanoscale structure, oxidation state, and surface composition has been missing. Here, we use a combination of operando ambient pressure scanning tunneling microscopy and ambient pressure X-ray photoelectron spectroscopy to investigate varying submonolayer coverages of CoO supported on Pt(111) under CO oxidation conditions. The goal is to compare the structural and spectroscopic features as the samples are exposed to O-rich CO/O 2 gas mixtures at millibar pressure and brought to temperatures where CO oxidation occurs. Upon first exposure to millibar gas mixtures, the initial bilayer CoO film is oxidized to trilayer CoO 2 , characterized by a preserved film morphology and Co in a predominant 3+ oxidation state. However, upon temperature increase during the CO oxidation reaction, the cobalt oxide ultrathin film undergoes dewetting into nanoparticles. On the basis of the XPS signature, we conclude that these nanoparticles have a Co 3 O 4 -like structure. The results underline the importance of operando observations of surface structures. This new insight into the Co oxide/metal interface may aid in our understanding of reactivity of metal oxide coated noble-metal particles in general.

Elucidating the active phases of CoOx films on Au(111) in the CO oxidation reaction

“…Free energies of CO and CO 2 gas were approximated using their translational and rotational partition functions 37 . The structure of ultrathin CoO x films on the hexagonal (111) surfaces of Au and Pt by STM has been studied extensively in the past 6 , 10 , 22 , 38 . In all cases the films form hexagonal lattices with slightly different unit cells from that of the metal substrate leading to formation of Moire patterns.…”

Hydroxide Structure-Dependent OH Promotion Mechanism over a Hydroxylated CoOx/Pt(111) Catalyst toward CO Oxidation

“…Designing multicomponent catalysts, such as constructing metal–oxide interface, is an effective strategy to achieve excellent catalytic performance. − In many cases, however, the active structure of the catalysts often evolves under reaction conditions, which can result in activation or deactivation of catalysts or changes in product selectivity. − H 2 or H 2 O molecules are often present as reactants or products in many catalytic reactions, such as the water–gas shift (WGS) reaction, − CO 2 hydrogenation, ,,− and Fischer–Tropsch synthesis, − or exist in the reaction as impurities. , The introduction of H 2 O or H 2 can lead to surface hydroxylation of many metal oxides, such as cobalt oxides, − zinc oxides, − titanium oxides, , and nickel oxides, through dissociative adsorption of H 2 O or hydrogen spillover. − The surface hydroxylation has been shown to be correlated with the high reactivity of metal oxides in WGS, , acetylene hydrogenation, and CO oxidation reactions …”