N2 reduction in uranium-doped C2N/C3N4 monolayers: a DFT computational study

Ammonia (NH3) synthesis under ambient conditions is a challenge in chemistry and the electrocatalytic method is expected to replace the currently and widely used Haber-Bosch process. Uranium (U) doped on substrates could act as efficient single-atom catalysts in electrocatalytic NH3 synthesis with high stability, selectivity, and activity as U has a relatively large ionic radius and a 5f orbital that can participate in covalent bonding with N-2. Although U has low radioactivity, U-containing materials have been reported in many catalytic reactions (NRR, HER, ORR) because their half-life is long enough. Moreover, C2N and C3N4 can be excellent substrates due to their high adsorption properties and chemical stability. Through the investigation of the properties of the U-doped C2N/C3N4 monolayers and NRR on their surfaces based on DFT calculations, the results indicate that the U atoms can be strongly embedded in the monolayers, and N-2 can be adsorbed strongly on the surfaces with the adsorption energies in the range of -0.69 to -0.98 eV. As for the NH3 formation process on these catalysts, the limiting potentials are relatively low, especially for U-doped C2N (-0.44 V). In addition, we have also considered the competing HER and found that the NRR is predominant. Overall, the study demonstrates for the first time that U-doped substrates can be used as efficient SACs for NRR.