Speakers
Description
Cobalt-based oxides are excellent catalysts for oxidation reactions, both in thermal catalysis1 and in electrocatalysis.2 Here, we present an overview of combined operando X-ray spectroscopic and operando transmission electron microscopic studies of cobalt spinel oxides (Co3O4) and perovksites (LaCoxFe1-xO3) in low-temperature oxidation reactions, focusing on the evolution of the electronic and geometric surface structures under reaction conditions and the occurrence of common motifs between all systems.
ILIAS (identical location imaging and spectroscopy) in combination with a quasi in situ approach allows the effects of temperature and gas phase on the catalyst surface to be disentangled from those of the electron beam. As a proof of concept, the structural evolution of a Co3O4 spinel catalyst was investigated, tracking the oxidation state changes across the surface during different heat treatments, which can be correlated with the catalytic activity by CO titration.
For Co3O4, a complex network of solid-state diffusional processes controlling the activity of the selective oxidation of 2-propanol could be unraveled. These include exsolution of reduced CoOx, vacancy formation and collapse, as well as recrystallization and give rise to a unique splitting of the activity into a low- and a high-temperature activity regime,3 possibly with a frustrated state at the transition between them.4 On LaCoxFe1-xO3, distinct differences between dry and wet feeds were observed: the presence of water not only inhibits the reduction of cobalt to Co2+ and carbonaceous deposits, but also increases the number of exsolved nanoparticles at the surface and leads to higher surface hydroxide coverages.
Furthermore, the evolution of Co3O4 during the calcination process between 300 and 600 °C was analyzed, giving rise to short-range atomic migration, micropore formation, and local structural instability at lower temperatures, as well as larger, more defined pores, smoother surfaces and more stable structures at higher temperatures.
In the oxygen evolution reaction on sputter-deposited cobalt thin films, the formation of electrophilic oxygen species was investigated, which were found to correlate with the oxidation waves in the voltammogram – their identity was confirmed using DFT.
Many of these systems are characterized by similar structural features, such as exsolution of nanoparticles, surface roughening or void formation, suggesting the existence of similar processes despite the different chemical potentials.
1. Najafishirtari, S. et al. A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols. Chem. Eur. J. 27, 16809 (2021).
2. Song, F. et al. Transition Metal Oxides as Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Solutions: An Application-Inspired Renaissance. J. Am. Chem. Soc. 140, 7748 (2018).
3. Götsch, T. et al. Local Solid-State Processes Adjust the Selectivity in Catalytic Oxidation Reactions. Nat Catal. in Revision.
4. Schlögl, R. Heterogeneous Catalysis. Angew. Chem. Int. Ed. 54, 3465 (2015).
