Co3O4 is a promising, noble metal-free oxidation catalyst but a better understanding of the active catalyst is required. We have performed operando studies of functioning catalysts by synchrotron-based near atmospheric pressure X-ray photoelectron spectroscopy (NAP-XPS) and X-ray absorption spectroscopy (XAS), complemented by lab-based infrared (FTIR, PM-IRAS). We employed a two-fold approach, using industrial-grade catalysts (commercial Co3O4 and CoO) as well as surface science based planar model catalysts (ultrathin films of Co3O4 and CoO grown in ultrahigh vacuum (UHV) on Ir(100) single crystals). Commercial catalysts were monitored under steady-state and dynamic conditions, unfolding a complex reaction network: Mars-van-Krevelen (MvK; oxygen vacancies), CO dissociation, and carbonate formation. Whereas the amount of reduced Co2+ at/near the Co3O4 surface was low &lt;200°C, only in pure CO about 15% of surface reduction were detected, suggesting that the active sites are a minority species. Carbonates are rather spectators and also CO dissociation followed by carbon reoxidation seemed a minor route. Commercial mesoscopic CoO (particle size ~1 mm) and nanosized CoO (20-50 nm particle size) were also examined. After CoO pre-oxidation, electron diffraction (SAED) detected thin surface layers of Co3O4 on CoO, but the activity of nanosized Co3O4 was still not reached. For nanosized CoO (surface layers on vacuum-reduced Co3O4), operando NAP-XPS/XAS revealed CoO oxidation to Co3O4 above 150°C, yielding the activity of nanosized Co3O4. The nanoscale CoO shell on a Co3O4 core more easily transformed to Co3O4 than mesoscopic CoO with large grains. For thin Co3O4 and CoO model films the interaction with CO was examined using isotopes, 18O2 for oxide growth and 13C16O for adsorption. CO formed carbonates and desorbed as 13C16O18O, corroborating MvK. Catalytic activity and composition changes in CO+O2 at mbar pressure were simultaneously monitored by MS and NAP-XPS, allowing to identify the more active state. Similar to commercial catalysts, CoO transformed to Co3O4 under reactive conditions.

Operando studies of cobalt oxide by NAP-XPS, XAS and IR: Commercial and model catalysts

2019 Fall National Meeting

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