Unraveling Working-State Catalysis in Mixed-Metal Oxides: Active-Site Structure, Reaction Mechanisms, and Quantitative Descriptors for Environmental Remediation

Mixed-metal oxides (MMOs) allow local coordination, cation distribution, redox chemistry, oxygen mobility, defects, and interfaces to be tuned within one material family. These properties acquire mechanistic meaning only when they can be tied to the catalyst state that exists under turnover and to a kinetically relevant elementary step. This Review builds a working-state account of MMO catalysis that connects crystal chemistry, defect and interface physics, active-site reconstruction, mechanistic evidence, and quantitative kinetics. Environmental oxidation and reduction reactions serve as demanding testbeds, not as the organizing scope, because solution composition, oxidant identity, photon input, leaching, and mass transfer readily expose weak structure–activity assignments. This review gives particular attention to oxygen-vacancy assignments, lattice-oxygen participation, mixed-valence redox cooperation, interfacial charge transfer, reconstruction, and the gap between apparent and intrinsic activity. Four evidence levels (observation, correlation, mechanistic support, and working-state demonstration) are defined and applied to active-site identification, operando spectroscopy, isotope and transient methods, transport-free kinetics, and catalytic benchmarking. SMART (Structure, Mechanism, Activity, Realism, Translation)-MMO condenses these principles into a sequential decision tool with explicit failure conditions; it organizes established catalysis concepts and does not claim to be a new theory. By linking structure to working state, mechanism, rate, stability, and reaction environment, this Review shows which descriptors can be treated as causal, which remain correlative, and which measurements are needed before a mechanistic interpretation can transfer from one mixed-oxide system to another.