Structurally Regenerable High Entropy Aluminate Spinel Catalysts for Dry Reforming of Methane

Abstract

The dry reforming of methane reaction is a promising means to convert two potent greenhouse gases, methane and carbon dioxide, into industrially valuable synthesis gas. However, the presence of reducing gases and high operating temperatures degrade conventional nickel catalysts via excessive coke formation and particle sintering. These catalysts are not readily regenerated because the oxidative heat treatments employed to remove coke further promote active particle sintering. Herein, we designed high entropy aluminate spinel oxides (MAl₂O₄ where M = Co, Mg, Ni, and divalent site vacancies in nominal equimolar concentration) as selective and regenerable reforming catalysts. Under reaction conditions, reducible nickel and cobalt cations exsolved from the spinel lattice to form highly selective bimetallic particles on the oxide surface. Instead of sintering, these particles uniquely redissolved back into the aluminate lattice upon reoxidation and regained the original spinel structure. This phenomenon is ascribed to entropic stabilization, wherein an increase in configurational entropy creates a thermodynamic driving force for redispersing supported metal particles back into the multi-cationic oxide structure. During the dry reforming reaction, nickel atoms similarly exsolved from a NiAl₂O₄ sample and reduced to form metallic nickel particles. However, subsequent oxidation of this sample promoted sintering and oxidation of the nickel particles to an inactive state. High entropy materials thus provide a unique mechanism of regeneration, which is inaccessible in conventional catalysts.