Hydrothermal synthesis and characterization under dynamic conditions of cobalt oxide nanoparticles supported over magnesium oxide nano-plates

Abstract

A nano-catalyst comprised of oxidized Co NPs supported on MgO nano-plates was synthesized via a hydrothermal co-precipitation strategy and calcination in O₂ and subsequently in H₂ at 250 °C. Spectro-microscopy characterization was performed by scanning transmission electron microscopy, electron energy loss spectroscopy and scanning X-ray transmission microscopy. Surface measurements under H₂ and H₂ + CO atmospheres were obtained by ambient pressure X-ray photoelectron spectroscopy and in situ X-ray absorption spectroscopy in the 225–480 °C range. These measurements at the atomic and microscopic levels demonstrated that the oxidized Co nanoparticles uniformly decorated the MgO nano-plates. The surfaces are enriched with Co, and with a mixture of Co(OH)₂ and CoO under H₂ and H₂ + CO atmospheres. Under a H₂ atmosphere, the outermost surfaces were composed of (lattice) O²⁻, CO₃²⁻ and OH⁻. No inorganic carbonates were observed in the bulk. Chemisorbed CO, likely on the oxidized Co surfaces, was observed at the expense of O²⁻ under 300 mTorr H₂ + CO (2 : 1) at 225 °C. Gas phase CO₂ was detected under 32 Torr H₂ + CO (2 : 1) at 225 °C upon prolonged reaction time, and was attributed to a surface chemical reaction between O²⁻ and chemisorbed CO. Furthermore, sp³ like carbon species were detected on the otherwise carbon free surface in H₂ + CO, which remained on the surface under the subsequent reaction conditions. The formation of sp³ like hydrocarbons was ascribed to a surface catalytic reaction between the chemisorbed CO and OH⁻ as the apparent hydrogen source.