Abstract text (incl. figure legends and references)

Selective oxidation reactions, in many cases catalyzed by noble metals, are some of the most important industrial processes, used to synthesize base chemicals such as acetone from 2-propanol. Consequently, finding more efficient catalysts that are based on cheaper and more abundant materials is desirable. One example is Co₃O₄, which is highly active in the oxidation of 2-propanol to acetone.

On this catalyst, the reaction is split into two distinctive activity regimes: one centered around 150 °C ("low-temperature") and one up to 300 °C ("high-temperature"). While the catalyst is very selective at low temperature, it deactivates rapidly. At higher temperature, there is no deactivation, although total oxidation to CO₂ and H₂O becomes a prominent side reaction. The mechanism behind the low temperature deactivation is still unclear, with different reasons being postulated such as changes in oxidation state or simple carbon deposition.^[1]

Here, we present a combined operando investigation of the selective 2-propanol oxidation using operando TEM (OTEM) together with synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) under near-identical conditions in order to unravel the origins behind the low-temperature activity region and its deactivation. OTEM was conducted using a DENSsolutions Climate holder and a self-built gas feeding and analysis setup^[2] at an image-corrected ThermoFisher Scientific Titan 80–300.

After pre-sintering (600 °C, 20% O₂/He) of the nanocrystalline catalyst, hexagonal Co₃O₄ platelets derived from thermal decomposition of Co(OH)₂ precursors, the introduction of the reaction feed (4.5% 1:1 O₂/2-propanol in He, total pressure of 500 mbar) leads to the formation of nanoparticles beneath the {111} facets already at room temperature (see Fig. 1), which coincides with a slight reduction of the cobalt species as evidenced by NAP-XPS. Subsequent increases in temperature elicit an exsolution of these particles before they coalesce again to an overlayer at around 200 °C, coinciding with the transition to the high-temperature regime. At 300 °C, they roughen up significantly, accompanied by a continuous increase of Co(II) in the XPS spectra and increased conversion. According to HRTEM recorded under reaction conditions, these exsolved particles are CoO, although there is no bulk phase transition occurring. Cooling down from 300 °C back to room temperature reveals a lattice expansion, most likely due to the formation of O vacancies.

When re-oxidizing in O₂/He at 600 °C, the nanoparticles are dissolved, and the CoO signals in the SAED patterns disappear. Additionally, the original lattice parameter is recovered too.

As no carbon deposition is observed in OTEM and NAP-XPS reveals no significant build-up during the reaction either, this investigation indicates that the rapidly deactivating nature of the low-temperature regime is not simply due to carbon accumulation. Instead, because the partial reduction of Co(III) to Co(II) already occurs at room temperature, there possibly is a more complex reason for the deactivation. More experiments are currently being conducted to probe the chemical nature of the catalyst surface.

(1) Anke et al. ACS Catal. 2019, 9, 5974–5985

(2) Plodinec et al. Microsc. Microanal. 2020, 26, 220–228

Fig. 1: Operando bright field images during the oxidation of 2-propanol on Co₃O₄ show the restructuring of the {111} facets.