RARE-EARTH METAL-DOPED CATALYSTS FOR VOLATILE ORGANIC COMPOUND OXIDATION: MECHANISMS, STRUCTURE–ACTIVITY RELATIONSHIPS, AND FUTURE DIRECTIONS

Volatile organic compounds (VOCs) are important atmospheric and indoor pollutants because they participate in photochemical ozone and secondary aerosol formation and include compounds with substantial toxicity. Catalytic oxidation is attractive because VOCs can be converted, ideally, to CO2 and H2O at temperatures substantially below those required for thermal incineration. Rare-earth (RE) elements are particularly useful catalyst modifiers because their ionic size, coordination chemistry, redox behavior, oxygen-storage characteristics, and ability to generate lattice defects can be tuned across the lanthanide series. This review critically examines original experimental studies on RE-doped and RE-modified catalysts for thermal, photothermal, photocatalytic, and plasma-assisted VOC degradation, with emphasis on La, Ce, Pr, Nd, Sm, Eu, Gd, Ho, and Y. The evidence shows that RE incorporation can increase oxygen-vacancy concentration, alter the Mn³⁺/Mn²⁺ or Ce³⁺/Ce⁴⁺ redox balance, improve lattice-oxygen mobility, modify adsorption strength, stabilize active phases, and enhance resistance to water or chlorine. Representative data include T90 values of 162 °C for Nd-modified Co₃O₄ during toluene oxidation, 204 °C for CuHoCeOx at high gas hourly space velocity, 202 °C for complete toluene conversion over 1% Sm/CeO₂, 255 °C for 4 mol% La-doped CuMnOx, and 120 °C for complete formaldehyde oxidation over 4% Eu/CeO₂ nanosheets. However, direct ranking of catalysts is inappropriate unless VOC concentration, space velocity, catalyst loading, oxygen concentration, humidity, reactor configuration, and conversion definition are matched. The review therefore emphasizes structure–property–performance relationships rather than nominal light-off temperatures alone. Remaining challenges include standardization of performance reporting, validation with realistic VOC mixtures, long-term stability under humidity and sulfur/chlorine contaminants, scalable monolith design, and quantitative descriptors linking RE ionic characteristics with oxygen-vacancy formation and reaction kinetics.