Operando XAS and NAP-XPS investigation of CO oxidation on meso- and nanoscale CoO catalysts

• Published in: Catalysis today Vol. 336; pp. 139 - 147
• Main Authors: Lukashuk, Liliana, Yigit, Nevzat, Li, Hao, Bernardi, Johannes, Föttinger, Karin, Rupprechter, Günther
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.2019
• Subjects: CO oxidation; Co3O4; CoO; NAP-XPS; Operando; TEM; XAS; CoO; Operando; CO oxidation; NAP-XPS; XAS; TEM; Co3O4
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2018.12.052

[Display omitted] •Mesoscopic (˜1 μm) and nanosized (20–50 nm) CoO studied by operando XAS and NAP-XPS.•Pre-oxidation of mesoscopic CoO produced Co3O4 surface layers.•Vacuum-reduction of nanosized Co3O4 produced CoO surface layers.•During CO oxidation no bulk phase changes on mesoscopic CoO.•Nanosized CoO reoxidized to Co3O4 under active conditions. In contrast to Co3O4, CoO has been much less studied for CO oxidation. Herein, the phase changes of commercial mesoscopic CoO (particle size ˜1 μm) and nanosized CoO (20–50 nm particle size), the latter prepared by vacuum reduction of commercial Co3O4, were examined by operando X-ray absorption (XAS) and near-ambient pressure X-ray photoemission (NAP-XPS) spectroscopy during CO oxidation, as well as ex situ by transmission electron microscopy and diffraction (TEM/SAED). Commercial mesoscopic CoO exhibited CO oxidation activity at ˜200 °C, but even up to 530 °C in pure O2 no substantial (bulk) oxidation was observed by operando XAS, likely due to the large grains and bulk nature of CoO. After pre-oxidation at 400 C, electron diffraction detected thin surface layers of Co3O4. This increased activity but the activity of nanosized Co3O4 of equal surface area was still not reached. For nanosized CoO (surface layers on vacuum-reduced Co3O4), operando NAP-XPS/XAS, acquired during CO oxidation, revealed oxidation of CoO to Co3O4 above 150 °C, yielding the activity of nanosized Co3O4. Evidently, the nanoscale CoO shell on a Co3O4 core with small grains more easily and more completely transformed to Co3O4 than mesoscopic (bulk) CoO with large grains. Our study demonstrates how flexible and dynamic surfaces of cobalt oxide materials adjust to various reaction environments, which also depends on grain size and morphology (bulk vs. thin layers), illustrating the importance of operando techniques to determine active catalyst phases under reaction conditions.