Filling Mesopores of Conductive Metal–Organic Frameworks with Cu Clusters for Selective Nitrate Reduction to Ammonia

The electrocatalytic nitrate reduction reaction (NO3-RR) to ammonia (NH3) under ambient conditions not only has the benefit of lowering energy consumption, but also helps remove nitrate contamination. Inspired by the unique structure of nitrate/nitrite reductase with the active spheroproteins encapsulated by larger enzymes, herein, we develop an in situ synthetic strategy for the construction of metal cluster-conductive metal-organic framework (MOF) composite electrocatalysts. The metallic Cu clusters are filled into the mesopores of a conductive copper-based MOF (i.e., CuHHTP); meanwhile, CuHHTP with a porous structure provides an internal environment to limit the growth of metallic Cu clusters with an ultrasmall size (i.e., 1.5 ± 0.2 nm) and restrains their aggregation. The obtained Cu@CuHHTP exhibits superb performance for NO3-RR. In a neutral electrolyte with 500 ppm NO3-, Cu@CuHHTP shows a high NO3- conversion of 85.81% and a selectivity for NH3 of 96.84%. 15N isotope labeling experiments confirm that the formation of NH3 originates from the process of NO3-RR. Theoretical calculations confirm that Cu clusters are the active sites in the composite electrocatalysts, in which the proper d-band center and the "accept-donate" mechanism in charge transfer are the key factors for the improvement of the electrocatalytic performance.