Bridging Nickel-MOF and Copper Single Atoms/Clusters with H-Substituted Graphdiyne for the Tandem Catalysis of Nitrate to Ammonia

Interfacial engineering of synergistic catalysts is one of the keys to achieving multiple proton-coupled electron transfer processes in nitrate-to-ammonia conversion. Herein, by joining ultrathin nickel-based metal–organic framework (denoted Ni-MOF) nanosheets with few-layered hydrogen-substituted graphdiyne-supported copper single atoms and clusters (denoted HsGDY@Cu), a tandem catalyst of Ni-MOFs@HsGDY@Cu with dual-active interfaces was developed for the concerted catalysis of nitrate-to-ammonia. In such a system, the sandwiched HsGDY layer could serve as a bridge to connect the coordinated unsaturated Ni²⁺ sites with Cu single atoms/clusters in a limited range of 0 to 3.6 nm. From Ni²⁺ to Cu, via the hydrogen spillover process, the hydrogen radicals (H⋅) generated at the unsaturated Ni²⁺ sites could migrate across HsGDY to the Cu sites to participate in the transformation of *HNO₃ to NH₃. From Cu to Ni²⁺, bypassing the higher reaction energy for *HNO₃ formation on the Ni²⁺ sites, the NO₂⁻ detached from the Cu sites could diffuse onto the unsaturated Ni²⁺ sites to form NH₃ as well. The combined results make this hybrid a tandem catalyst with dual active sites for the catalysis of nitrate-to-ammonia conversion with improved Faradaic efficiency at lower overpotentials.