Stabilizing Frustrated Phase Transitions in Selective Oxidation Reactions.

Abstract

Frustrated phase transitions represent the ideal working state of a heterogeneous catalyst. These states exist within a narrow parameter window, making them difficult to stabilize. Here, it is shown for the selective oxidation of 2-propanol to acetone over Co₃O₄ spinels that the addition of water extends the stability regime of the relevant frustrated phase transition. This conclusion is based on results obtained from multi-modal experiments, including operando scanning electron microscopy (OSEM), near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), transmission electron microscopy (TEM), and computer vision analysis. It is found that the most selective state for acetone formation coincides with a dynamic spinel structure that fluctuates through reversible redox processes. At elevated temperatures, this metastable state undergoes a complete phase transition into the rock-salt CoO phase characterized by low acetone selectivity. This process is found to be mediated by the generation of mobile vacancies. The addition of water vapor mitigates vacancy mobility and stabilizes the selective, but thermodynamically frustrated, state. As such, the study conceptualizes a strategy to extend the lifetime of a catalyst during reaction by the adequate addition of a co-reactant.