Electrochemical nitrate (NO₃⁻) reduction reaction (NO₃RR) offers a promising route for NO₃⁻ remediation and sustainable ammonia (NH₃) synthesis, yet its efficiency is often constrained by the hydrogenation of nitrogen-containing intermediates. Herein, we report Fe single atoms anchored on oxygen-vacancy-rich TiO₂ nanosheet assemblies (Fe_SA-TiO₂-O_v) for efficient NO₃⁻ to NH₃ conversion. The Fe_SA-TiO₂-O_v catalyst achieves a high NH₃ yield rate of 16.6 mg·h⁻¹·cm⁻² at −0.5 V versus reversible hydrogen electrode (vs. RHE), accompanied by a maximum Faradaic efficiency of 92% and excellent durability over 40 h. Operando X-ray absorption fine structure (XAFS) spectroscopy reveals a gradual decrease in Fe valence and contraction of the Fe–O coordination shell, confirming the formation of reconstructed low-valent Fe single-atom active sites during NO₃RR. Theoretical calculations and spectroscopic analysis further indicate that Fe sites are effective for *NO hydrogenation to the *NOH intermediate, thereby promoting the efficient formation of NH₃. These findings identify the reconstructed low-valent Fe single atoms as the active sites for selective electrosynthesis of NH₃, providing a mechanistic framework for designing single-atom catalysts applicable to multistep electrocatalytic reduction reactions.

Nitrogen monoxide (NO), a critical precursor to particulate matter with an aerodynamic diameter ≤ 2.5 μm (PM_2.5) and ozone formation, requires efficient abatement at atmospheric low concentrations (< 1 ppm), yet conventional photocatalysts struggle with deep NO purification due to inefficient trace O₂/NO co-activation. Herein, we engineer Fe single atoms on defective TiO₂ catalyst (Fe_SA/OV-TiO₂, OV refers to oxygen vacancy) with orbital-modulated dual-active sites. Density functional theory calculations and in situ characterizations reveal that surface oxygen vacancies drive robust O₂ activation to generate reactive oxygen species (ROS). Adjacent Fe single atoms enable targeted NO chemisorption via their d-orbital hybridization with antibonding π* orbital of NO. This synergy shifts the NO oxidation pathway from ·O₂⁻-dominated Eley–Rideal (E–R) to dual-activated Langmuir–Hinshelwood (L–H) pathways, where pre-adsorbed NO directly reacts with ROS, forming thermodynamically stable bidentate nitrate. Crucially, Fe_SA/OV-TiO₂ achieves 75% NO conversion efficiency with 98% nitrate selectivity under visible light irradiation, outperforming defective TiO₂-OV by 1.4-fold while maintaining the good activity over five cycles without deactivation. The work establishes orbital-level dual-site engineering as a strategy for developing high-efficiency photocatalysts for air pollution remediation.

Crystallinity and crystal structure greatly influence the photocatalytic behavior of photocatalysts. Pristine g-C₃N₄ produced by traditional thermal-induced polycondensation reaction bears low crystallinity and thus poor photoactivity, which originates from the incomplete polymerization of the precursor containing amine groups, abundant hydrogen bonds, and unreacted amino, as well as cyanide functional groups in the skeleton. During photocatalytic process, these residual functional groups often work as electron trap sites, which may hinder the transfer of electrons on the plane, resulting in low photoactivity. Fortunately, crystalline carbon nitride (CCN) was reported as a promising photocatalyst because its increased crystallinity not only reduces the number of carriers recombination centers, but also increases charge conductivity and improves light utilization due to extended π-conjugated systems and delocalized π-electrons. As such, we summarize the recent studies on CCN-based photocatalysts for the photoactivity enhancement. Firstly, the unique structure and properties of CCN materials are presented. Next, the preparation methods and modification strategies are well outlined. We also sum up the applications of CCN-based materials in the environmental purification and energy fields. Finally, this review concerning CNN materials ends with prospects and challenges in the obtainment of high crystallinity by effective techniques, and the deep understanding of photocatalytic mechanism.

The widespread nitrogen oxides (NO_x, mainly in NO) in the atmosphere have threatened human health and ecological environment. The dilute NO (ppb) is difficult to efficiently remove via the traditional process due to its characteristics of low concentration, wide range, large total amount, etc. Photocatalysis can utilize solar energy to purify NO pollutants under mild conditions, but its application is limited due to the low selectivity of nitrate and poor activity of NO removal. The underlying reason is that the interface mechanism of NO oxidation is not clearly understood, which leads to the inability to accurately regulate the NO oxidation process. Herein, the recent advances in the photocatalytic oxidation of NO are summarized. Firstly, the common strategies to effectively regulate carrier dynamics such as morphology control, facet engineering, defect engineering, plasma coupling, heterojunction and single-atom catalysts are discussed. Secondly, the progress of enhancing the adsorption and activation of reactants such as NO and O₂ during NO oxidation is described in detail, and the corresponding NO oxidation mechanisms are enumerated. Finally, the challenges and prospects of photocatalytic NO oxidation are presented in term of nanotechnology for air pollution control. This review can shed light on the interface mechanism of NO oxidation and provide illuminating information on designing novel catalysts for efficient NO_x control.