Review on Palladium-Catalyzed CO2 Hydrogenation to Formic Acid: From Nanoparticles, Clusters, to Single Atoms

CO2 hydrogenation to formic acid, a key catalytic topic for carbon neutrality, converts inert CO2 into a valuable chemical and hydrogen carrier. Palladium-based catalysts stand out for their exceptional ability to activate hydrogen. However, their catalytic performance is not determined by composition alone. The structural dimension of Pd species─from nanoparticles, clusters, to single atoms─plays a decisive role. This review systematically examines how the atomic arrangement, particle size, and interfacial electronic structure of Pd-based catalysts influence catalytic activity, formic acid selectivity, and operational stability. A clear structure–activity relationship across dimensions was revealed. Pd nanoparticles offered high hydrogen activity but were prone to overhydrogenation and sintering. Pd clusters bridged the gap between single atoms and nanoparticles, achieving a balanced performance. Single-atom Pd catalysts delivered excellent selectivity under mild conditions yet faced challenges in long-term stability. This review will provide an understanding of these dimensional effects, offering a rational basis for designing efficient and selective Pd-based catalysts for CO2 hydrogenation to formic acid.