Tuning metal catalysts via nitrogen-doped nanocarbons for energy chemistry: From metal nanoparticles to single metal sites

Most processes in energy chemistry require suitable catalysts to decrease activation energy, control reaction rate and increase selectivity. As a kind of very important supports, nanocarbons are widely used for constructing various metal-based heterogenous catalysts owing to their abundant microstructures and morphologies, tunable surface area, high stability, low cost and excellent electrical conductivity. Nitrogen-doped nanocarbons are the even more attractive for the modified electronic structure and enhanced interaction with the supported species. With the assistance of N participation, metal catalysts have been constructed on N-doped nanocarbons from highly dispersed nanoparticles to sub-nanometer clusters and single sites. The metal catalysts supported on N-doped nanocarbons have exhibited unique advantages of modified electronic structure, facilitated charge transfer and high metal utilization, hence show wide applications in various energy-related reactions. This review firstly elucidates the roles of different types of nitrogen dopants for anchoring metal species from theoretical viewpoint, then summarizes the synthetic strategies of various N-doped nanocarbons and the related metal catalysts from high dispersion to single sites. Then their typical performances in energy chemistry are reviewed which ranges from electrocatalytic applications including oxygen reduction, alcohol oxidation, hydrogen oxidation, water splitting, CO₂ reduction and nitrogen reduction to thermal catalytic reactions including Fischer-Tropsch synthesis, H₂ production, hydrogenation and oxidation, as well as to photocatalytic applications and beyond. The structure-performance correlations are discussed in depth to highlight the contribution of N-doped nanocarbons. The facing challenges and research trends are also discussed for better understanding the development of advanced heterogeneous catalysts based on the heteroatom-doped nanocarbons for energy applications.