Surface Redox Behavior of a High Entropy Spinel Catalyst during Propene Oxidation

Abstract

High entropy (HE) oxides have the potential to deliver a step-change in heterogeneous catalysts. HE oxides allow for unprecedented compositional flexibility with a diverse range of possible active sites, promoting higher catalyst activity and long-term stability. However, due to the increased complexity of HE surfaces, little is understood about the operando behavior or redox chemistry of a surface containing over five cations. Herein, a seven-metal HE spinel oxide (M3O4, where M is an equimolar contribution of Cr, Mn, Fe, Co, Ni, Cu, and Zn) demonstrates impressive catalytic performance with respect to propene (C3H6) oxidation. Catalytic activity was significantly impacted by reductive (H2) or oxidative (O2) surface pre-treatment. A 31% lower temperature to reach 10% conversion (T10) was required under H2 pre-treatment when compared to O2 pre-treatment. Above 60% propene conversion, this performance enhancement was lost, and catalytic activity converged with the O2 pre-treatment. In-situ and operando near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) studies revealed both transient Cu(I)/Cu(0) surface species and the reduction of cobalt to Co(II) which has not been observed previously ex-situ. Catalyst deactivation was shown to be a slow, multifaceted process where active sites re-oxidized slowly, and catalytically active cations (e.g., Fe and Mn) migrate from the surface into the bulk. This work combines several characterization methods, including operando and in-situ techniques, to expand the study of structure-property relationships to non-model chemical reactions and complex material systems.