Methanol oxidation over model cobalt catalysts: Influence of the cobalt oxidation state on the reactivity

Abstract

X-ray photoelectron and absorption spectroscopies (XPS and XAS) combined with on-line mass spectrometry were applied under working catalytic conditions to investigate the methanol oxidation on cobalt. Two cobalt oxidation states (Co3O4 and CoO) were prepared and investigated as regards their influence on the catalytic activity and selectivity. In addition adsorbed species were monitored in the transition of the catalyst from the non-active to the active state. It was unequivocally shown that the surface oxidation state of cobalt is readily adapted to the oxygen chemical potential in the CH3OH/O2 reaction mixture. In particular, even in rich to oxygen mixtures the Co3O4 surface is partially reduced, while the degree of surface reduction is higher as the methanol concentration in the mixture increases. The reaction selectivity depends on the cobalt oxidation state with the more reduced samples favouring the partial oxidation of methanol to formaldehyde. In the absence of oxygen, methanol is effectively reducing cobalt to the metallic state, promoting also hydrogen and CO production. Direct evidence of methoxy and formate species adsorbed on the surface upon reaction was found by analysing the O 1s and C 1s photoelectron spectra. However, the surface coverage of those species was not proportional to the catalytic activity, indicating that in the absence of surface oxygen, these species might act also as reaction inhibitors.

A comparative in situ XPS study of PtRuCo catalyst in methanol steam reforming and water gas shift reactions

“…Depth dependent measurements indicted that the 529.4 eV component is located in deeper layers compared to that at 531.2 eV. In accordance, the BE at 529.4 eV is indicative of lattice oxygen due to CoO and/or Co 3 O 4 (529.5 ± 0.2 eV see also above) [31], in full agreement with the cobalt oxide evidence in the Co 2p 3/2 region. As described above the broad surface component at 531.2 eV is a convolution of various oxygen species including defective cobalt sub-oxides (CoO x ), adsorbed OH groups, methoxy and formate species [14] and [24].…”

“…Depth dependent measurements using higher photon energies (see also below) indicated that the peak at 530.3 eV is to a large extent due to sub-surface species. However the position of this peak is shifted about 0.8 eV as compared to that of the lattice oxygen peak of CoO and Co 3 O 4 oxides (at 529.5 ± 0.2 eV), reported in the literature [30] and measured on reference compounds in the same set-up [31]. The interpretation of 530.3 eV peak will be given below in combination with other results.…”

Characterization and Catalytic Activity of Mn-Co/TiO2 Catalysts for NO Oxidation to NO2 at Low Temperature

“…It was reported that cobalt oxides possess better oxidation ability [12][13][14]. Co 2+ /Co 3+ oxidation states were found to interconvert readily in oxidizing and reducing conditions [15].…”

Electronic states and magnetic structure at the Co3O4(110) surface: A first-principles study

“…1,2 Recently, this material has attracted further interest as a promising catalyst for energy and environment-related applications such as low-temperature CO oxidation, 3 water splitting, 4 and the oxygen reduction reaction. 5 Surfaces have a key role in these applications, and a detailed understanding of the physical and chemical properties of 3 and is believed to be mainly responsible for the oxidation reactivity 6 of this material.…”