Functional differentiation of active hydrogen on Pd/Cu catalysts for efficient electrocatalytic nitrate reduction to ammonia.

Electrocatalytic nitrate reduction (NO3RR) offers a promising strategy to remediate NO3- pollution and synthesize value-added ammonia (NH3), but its efficiency is limited by the sluggish nitrite reduction and insufficient NH3 selectivity over Cu catalysts. Herein, Pd nanoparticle-decorated Cu submicron particles (PdNPs/Cu SPs) were developed to enhance the conversion of NO3- to NH3 through promoting active hydrogen (*H) generation. PdNPs/Cu SPs achieved a high conversion of 95.05% for NO3- to NH3 with a Faradaic efficiency of 95.65%. Mechanistic investigations revealed the generation of *H species with distinct binding states on PdNPs/Cu SPs. Lattice-associated hydrogen (*Hlat) was found to modify the local electronic structure of Pd and strengthen NO3- adsorption, whereas adsorbed hydrogen on the surface (*Hads) served as the primary reactive *H species participating in reduction and hydrogenation of nitrogen intermediates. The cooperative functions of *Hlat and *Hads promoted both NO3- adsorption and subsequent hydrogenations. DFT calculations further showed that *H accumulation on PdNPs/Cu SPs rendered the critical *NO → *HNO thermodynamically favorable (ΔG = -0.13 eV), thereby shifting the potential-determining step from *NO hydrogenation to NH3 desorption. In-situ mass spectrometry and spectroscopy elucidate the dominant reaction pathway and confirm the proposed synergy: *NO3→*NO2→*NO→*HNO→*NH2OH→*NH2→*NH3→NH3. This work provides an efficient catalyst for sustainable nitrogen cycling and offers a novel reference for leveraging distinct *H species with different binding states in multi-step electrocatalytic reactions.