The motivation
Ammonia is an essential chemical feedstock and a promising carbon-free energy carrier. However, the Haber-Bosch process remains highly energy-intensive and contributes significantly to global CO 2 emissions. Electrochemical nitrate reduction reaction (NO 3 RR) offers a sustainable alternative by converting nitrate pollutants in wastewater into ammonia under ambient conditions using renewable electricity.
Despite rapid progress, achieving high NH 3 selectivity at industrial current densities remains extremely challenging. One of the major bottlenecks originates from the imbalance in active hydrogen (*H) utilization. During NO 3 RR, surface *H species are required for stepwise hydrogenation of nitrogen intermediates, yet excessive *H tends to participate in HER instead of nitrate reduction.
Moreover, pristine Cu 2 O catalysts exhibit a hydrogenation mechanism: the early-stage nitrate reduction follows a Langmuir–Hinshelwood (L-H) pathway involving surface *H, while the later NO-to-NH 3 conversion mainly proceeds through an Eley–Rideal (E-R) pathway independent of adsorbed hydrogen. This may lead to residual *H accumulation and accelerated HER under high current densities.
We therefore asked: can we redesign the catalytic active sites to continuously channel active hydrogen toward nitrogen intermediate hydrogenation rather than hydrogen evolution?
The origin of the idea
Copper-based catalysts are widely regarded as one of the most promising systems for NO 3 RR because of their favorable nitrate adsorption and moderate binding strength toward nitrogen intermediates. However, their hydrogenation pathways are difficult to regulate.
We hypothesized that introducing rare-earth elements with strong electronic modulation capability could alter the adsorption configuration and hydrogenation behavior of key intermediates. Cerium, with its dynamic Ce redox properties and strong oxygen affinity, emerged as an ideal candidate.
This concept led us to design a Ce-doped Cu 2 O catalyst, where atomically dispersed Ce induces the formation of Cu-O-Ce active centers alongside the intrinsic Cu 0 -Cu + sites. We envisioned that these new interfacial sites could activate an additional *H-mediated hydrogenation route and improve active hydrogen utilization efficiency.
Our approach: dual active-site for hydrogen pathway regulation
We developed a Ce-Cu 2 O catalyst containing synergistic Cu 0 -Cu + and Cu-O-Ce dual active centers. Structural characterization confirmed that Ce atoms were uniformly incorporated into the Cu 2 O lattice without forming CeO 2 aggregates.
The newly formed Cu-O-Ce sites introduced an additional hydrogenation pathway involving the *NHOH intermediate, fundamentally changing the reaction mechanism of NO 3 RR. Instead of relying solely on the traditional E-R hydrogenation route, Ce-Cu 2 O enabled cooperative L-H and E-R hydrogenation processes throughout nitrate reduction.
This design effectively redirected excessive surface *H toward nitrogen intermediate hydrogenation, thereby suppressing HER and enhancing ammonia selectivity under high current densities.
What we did and key experiments
To validate the mechanism and catalytic origin, we combined advanced operando spectroscopy with theoretical calculations:
Why it matters
This work introduces a new catalyst design principle based on active hydrogen pathway regulation. Rather than simply increasing catalytic activity, we demonstrate that controlling how surface hydrogen participates in elementary reaction steps is crucial for achieving efficient nitrate-to-ammonia conversion.
The discovery of cooperative L-H and E-R hydrogenation pathways provides new mechanistic insight into multi-step electrocatalytic hydrogenation reactions. More importantly, the Cu-O-Ce dual-site strategy offers a general framework for regulating hydrogen utilization in other electrochemical systems, including CO2 reduction, nitrite reduction, and biomass upgrading.
From a sustainability perspective, this work advances the development of decentralized ammonia production technologies powered by renewable electricity while simultaneously enabling nitrate wastewater remediation.
Outlook
We envision several promising future directions:
We hope this work encourages the community to view active hydrogen not merely as a reaction intermediate, but as a controllable reaction resource whose transfer pathway can fundamentally determine catalytic efficiency and selectivity.
Science Bulletin
Experimental study