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In situ/operando studies of a heterogeneous catalyst are particularly valuable for achieving a fundamental understanding of catalytic mechanisms at a molecular level by establishing a correlation between the observed catalytic performance and the corresponding surface chemistry during catalysis. Herein, CO oxidation on cobalt oxides was studied via ambient pressure X-ray photoelectron spectroscopy (AP-XPS). During CO oxidation on CoO in the temperature range of 140-180 °C, the active surface phase of CoO progressively transforms to CoO. Kinetic studies of CO oxidation on the surface phase CoO at 80-120 °C and on the formed CoO at 160-220 °C show that CoO and CoO exhibit different activation barriers: 49.3 kJ mol for CoO and 36.9 kJ mol for CoO. This study demonstrates the transition of the active surface phase of a transition metal oxide-based catalyst under catalytic conditions with no change in the bulk phase of the catalyst.

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