Anchored Cu Single Atoms on TiO₂ for Highly Selective Nitrite-to-Ammonia Electrocatalysis

Nitrite pollution remains a persistent challenge for the environment in agricultural regions, industrial wastewater, and aging water infrastructure. There is also the well-known health risk, since it’s considered a probable carcinogen. Because of that, regulators have set strict limits, however, actually keeping nitrite below those limits in real water systems is difficult. The conventional treatments do work, but each has its own limitation. For instance, biological denitrification is great until the microbial balance shifts, while chemical treatments can feel like trading one problem for another because they often generate side products. On the other hand, physical methods, while straightforward on paper, usually demand more energy or infrastructure than makes sense for most communities. Therefore, the field has slowly gravitated toward approaches that do something more constructive—ideally turning nitrite into a useful product rather than just removing it. Electrochemical reduction fits that mindset. Since the process relies on electrons instead of chemical additives, it allows nitrite to be pushed toward more reduced nitrogen species with relatively few complications. Among the possible end products, ammonia is the one that has caught the most attention. Besides its established role in fertilizers, there’s growing interest in using ammonia as an energy carrier, so the idea of converting a pollutant directly into something valuable feels intuitively appealing. The challenge, though, is that the reaction from NO₂⁻ to NH₃ involves multiple electron transfers, and at each step the system can wander off into unwanted territory—hydrogen evolution, hydrazine formation, or simply low selectivity. Many catalysts show hints of promise but lose efficiency when operated in neutral solutions, which is typically where real wastewater sits.

This is partly why single-atom catalysts have become so intriguing. When a metal exists as individual atoms on a support, its local coordination can be manipulated in ways bulk materials can’t achieve. Copper single-atom catalysts, in particular, have shown signs that they can steer nitrite along the right reaction path, but their performance still suffers from stability issues and gaps in our understanding of how Cu interacts with key intermediates. TiO₂, on the other hand, has earned a reputation for anchoring isolated metal atoms securely. However, its ability to stabilize higher densities of Cu single atoms for nitrite electroreduction hasn’t been fully examined, which leaves an opening that this study tries to address. To this account, new research paper published in Advanced Functional Materials and conducted by Dr. Mingming Sun, Dr. Wenrui Wan, Dr. Xiao Zhao, Dr. Chunfeng Shao, Dr. Ning Liu and Professor Jianji Wang from the Henan Normal University alongside Professor Maohong Fan from the University of Wyoming, researchers developed a TiO₂-supported copper single-atom catalyst in which Cu atoms are stabilized in a well-defined Cu–O₄ coordination environment. This atomic dispersion reshapes the electronic structure of TiO₂, narrows its bandgap, and enables rapid, selective nitrite conversion to ammonia. Their mechanistic analysis shows that the catalyst stabilizes *NO and *NH₂OH intermediates while lowering the barrier for ammonia release.

The research team designed a family of Cu-decorated TiO₂ catalysts using a hydrothermal method that allowed them to finely tune the copper loading while maintaining atomic dispersion. The researchers performed structural characterization which showed why the material behaved differently from conventional Cu/TiO₂ systems. XRD patterns retained the signatures of anatase TiO₂ without any discernible copper reflections, implying an absence of metallic nanoparticles. TEM and HAADF-STEM images reinforced this conclusion: Cu atoms appeared as isolated bright spots, uniformly scattered across the TiO₂ lattice. The absence of any aggregated species, even at higher loadings, indicated that the support created a sufficiently strong coordination environment to stabilize single atoms across the surface. Complementary XPS, EPR, and XANES spectra confirmed that copper resided primarily in a Cu²⁺–O coordination, with EXAFS fitting revealing an average Cu–O bond length of ~1.94 Å, consistent with an atomically anchored species.

The authors carried out electrochemical measurements in a neutral K₂SO₄ electrolyte containing 0.1 M NaNO₂ and they found as soon as nitrite was introduced, the linear sweep voltammetry curves shifted sharply, highlighting the catalyst’s strong affinity for the reduction reaction. Among the series, the sample with 1.45 wt.% Cu consistently delivered the highest current density and the fastest charge-transfer kinetics, supported by its markedly reduced Nyquist arc radius. When held at −1.0 V versus RHE, this catalyst achieved a Faradaic efficiency of 96% and produced ammonia at a rate of 21.2 mg h⁻¹ mg–cat⁻¹ (nearly triple the output of pristine TiO₂). It is important to mention, the product distribution contained neither hydrazine nor significant H₂, which suggests that the isolated Cu centers effectively suppressed competing pathways. The team evaluated as well the stability and reliability of the system and over repeated cycles, they found that both Faradaic efficiency and ammonia yield remained steady, and structural analyses after prolonged electrolysis showed no formation of Cu nanoparticles. Isotopic labeling with ¹⁵NO₂⁻ produced the expected doublet in the ¹H NMR spectrum, confirming that ammonia originated solely from nitrite reduction. The catalyst also fully removed 0.1 M nitrite within eight hours, reducing the concentration below drinking-water limits. Moreover, the authors performed in-situ ATR-FTIR measurements and noticed distinct vibrational features associated with *NO₂, *NO, *NH₂OH, and *NH₃ appeared at progressively more negative potentials, with intensities much stronger on Cu₁/TiO₂ than on TiO₂ alone. DFT calculations provided a consistent interpretation: N-end adsorption of nitrite at the Cu site required significantly lower energy than at TiO₂, and the critical *NO₂ → *NO transition carried a smaller barrier. The final desorption of *NH₃—identified as the rate-determining step—was also more favorable on the single-atom catalyst. The combined evidence supports a pathway in which the Cu–O₄ site facilitates both electron transfer and intermediate stabilization, enabling rapid, selective conversion under mild conditions.

In conclusion, Professor Jianji Wang and colleagues first established that single-atom copper, when anchored within a carefully engineered TiO₂ coordination environment, can fundamentally reshape the energetics of nitrogen oxyanion conversion. Many catalysts have shown isolated successes—good activity at the expense of selectivity, or good selectivity at the expense of productivity—but few have approached the combination achieved in the study: strong suppression of side products, rapid electron transport, and nearly quantitative nitrite conversion under neutral aqueous conditions. That mixture of environmental relevance and catalytic rigor positions this system as far more than an incremental improvement. Second, the work provides mechanistic clarity by tracking the emergence of intermediates during reaction and correlated spectroscopic observations with theoretical modeling and the identification of *NO and *NH₂OH as decisive intermediates, alongside the determination that NH₃ desorption is the rate-determining step, is an important advancement in existing mechanistic assumptions. Third, the study points toward broader applications in nitrogen-cycle remediation. Because the bottleneck for nitrate reduction often lies in its conversion to nitrite and subsequently to ammonia, a catalyst that excels at the second step could be incorporated into tandem or cascade systems that treat more complex wastewater chemistries. The neutral-pH operation is a notable advantage for real-world deployment, where buffer selection, corrosion concerns, and electrolyte cost often constrain otherwise promising electrocatalysts. Finally, the work of Henan Normal University scientists has important environmental relevance and the authors’ demonstration of complete nitrite removal after eight hours makes a compelling case for scaling, testing in flow systems, and integrating into modular treatment units. If adapted appropriately, such systems could help close the anthropogenic nitrogen loop in a more sustainable and economically rational manner.