N2 Selectivity Research Articles

Co‐Catalytic Metal‐Support Interactions Design on Single‐Atom Alloy for Boosted Electro‐Reduction of Nitrate to Nitrogen

AbstractThe past decades have seen considerable imbalances in the nitrogen cycle due to the excessive use of nitrate in agriculture and industry. Electrocatalytic reduction of nitrate (NO3RR) to nitrogen (N2) holds significant potential for addressing nitrate pollution in wastewater but suffers from nitrite formation and the sluggish hydrogeneration process. Here a single atom alloy (SAA) catalyst featuring atomically dispersed Ru on 2D Ni metal (Ru1Ni), proving remarkable nitrate reduction performance (NO3−–N conversion (≈93%) and N2 selectivity (≈99%)) through the co‐catalytic metal‐support interactions (CMSI) effect is reported. Significantly, the Ru1Ni SAA achieves NO3RR removal capacity as high as 11.1 mg L−1 h−1 cm−2 with 20 cycles stability (9 h per cycle), surpassing most previously reported works. The core of the boosting lies in the synergistically promoted nitrate activation and accelerated hydrogenation of nitrogen oxide intermediates on the Ru site and Ni substrate, respectively, revealed by various in situ experiments and theoretical simulations. DFT calculations indicate electron transfer from substrate Ni to Ru and more robust interaction between NO3− and Ru–Ni in comparison to that of Ni–Ni. This work offers a resilient methodology for the rational design of highly efficient electrocatalysts with CMSI modulation for NO3RR, illuminating the arena of wastewater treatment and the nitrogen cycle.

Utilizing the immanent chloride ions in wastewater for reactive chlorine species photogeneration towards effective ammonia nitrogen removal

Utilizing the immanent salt (Cl–) in wastewater to generate reactive chlorine species (RCS) satisfies the future needs for sustainable, on-demand, and decentralized ammonia nitrogen (NH4+-N) removal. The microstructural evolution of bismuth oxychloride (BiOCl) photocatalyst under irradiation benefits for overcoming the competition between Cl– and H2O oxidation and achieving high N2 selectivity. Herein, internal electric field (IEF) engineering by carbon doping is applied to promote photocatalytic RCS generation rate over BiOCl. The carbon dopant not only enhances the IEF intensity of BiOCl by a factor of 2.1 times for carrier dynamics promotion, but also improves Cl– oxidation against H2O oxidation. An RCS production rate of 0.23 mg/L min−1 is realized over carbon-doped BiOCl (C-BiOCl) through the accelerated interaction between lattice and environmental Cl–, which is twice that of the undoped BiOCl (0.11 mg/L min−1). The relation between IEF intensity and RCS generation rate over C-BiOCl manifests a correlation coefficient of 0.96. Effective NH4+-N degradation with N2 selectivity of >99% is validated. The system is competent in dealing with saline NH4+-N wastewater with a wide range of NH4+-N (20∼200 mg N/L) and NaCl (6∼10 g/L) concentrations. In the treatment of realistic landfill leachate (884 mg N/L NH4+-N), C-BiOCl displays a photocatalytic NH4+-N removal efficiency of up to 95.6% while maintaining the NO2−-N (∼2 mg N/L) and NO3−-N (∼40 mg N/L) levels. This system provided an interesting and innovative idea for wastewater self-purification.

Pt/Al2O3@Ce/ZrO2-S bifunctional catalysts prepared by mechanically milling for selective catalytic oxidation of high-concentration ammonia.

The selective catalytic oxidation (SCO) is an effective method for removing slipped high-concentration ammonia from NH3-fueled engine exhaust gas. Herein a novel bifunctional catalyst was synthesized by mechanically mixing sulfated Ce/ZrO2 (Ce/ZrO2-S) with a small fraction of Pt/Al2O3 (Pt 0.1 wt.%) for SCO of NH3. As expected, the introduction of a small amount of Pt/Al2O3 significantly improved NH3 conversion ability of Ce/ZrO2-S catalysts toward low-temperature direction. When the mass ratio of Pt/Al2O3 to Ce/ZrO2-S was 7.5% (the corresponding mixed catalyst was denoted as P@CZS-7.5), T90 temperature was 312°C. More importantly, P@CZS-7.5 catalyst exhibited a much better N2 selectivity (> 96%) in a wide temperature range (320 ~ 450°C). H2-TPR results revealed that the addition of a trace amount of Pt/Al2O3 significantly led to a distinct shift of reduction temperature peak toward low-temperature direction, thereby greatly improved the low-temperature redox performance of mixed catalysts. Furthermore, NH3-TPD and BET results showed that P@CZS-7.5 catalyst exhibited a similar NH3 adsorption capacity to Ce/ZrO2-S catalyst, while the former had a relatively higher specific surface area than the latter. It was considered as a crucial factor for P@CZS-7.5 catalyst maintaining superior N2 selectivity in high-concentration NH3 (5000ppm) removal processes. In situ DRIFTS results indicated that P@CZS-7.5 catalyst followed the internal selective catalytic reduction (i-SCR) mechanism in NH3-SCO reactions.

Lewis acid-rich SrFexTi1−xO3/TiO2 to enhance the photocatalytic reduction of nitrate to N2

The photocatalytic reduction of nitrate has received considerable attention due to its high efficiency and environmentally friendly nature. The excessive addition of hole scavengers is the most commonly used method to increase the nitrate reduction efficiency. However, achieving high selectivity in the photocatalytic reduction of nitrate to N2 with low concentration of hole scavengers remains challenging. In this study, the SrFexTi1−xO3/TiO2 S-scheme heterojunction photocatalysts with many Lewis acidic adsorption sites have been developed. The experimental results demonstrated that the incorporation of 6% Fe into SrFe0.06Ti0.94O3/TiO2 (SFTO6) resulted in the nitrate conversion rate of 97.68% and the N2 selectivity reached 96.35% with 25 mmol/L formic acid. Moreover, it also exhibited excellent stability and recycle ability. After 5 cycles, SFTO6 still exhibited a stable photocatalytic denitration efficiency of 92.94%, highlighting its potential for practical application. Through comprehensive mechanistic investigations, enhancing direct reduction process is considered the key to its high reduction efficiency with low formic acid. And the Lewis acidic adsorption sites enhance N2 selectivity by enriching NOx− on the surface of the material. Overall, this study provides a novel approach for achieving efficient photocatalytic reduction of nitrate to N2 under conditions with low concentration of hole scavengers.

Active learning driven discovery of novel alloyed catalysts for selective ammonia oxidation

Alloy catalysts design faces the contradiction of small sample and huge screen space, especially in rational design with multi-objective and multi-constraints. Here, we conduct the active learning enabled innovative catalyst screening in alloy space composed of transition metal elements. First, the small dataset and large search space are constructed based on knowledge informed feature group. Then, the surrogate model based on Gaussian process regression are trained for predicting catalytic activity and N2 selectivity descriptors for selective catalytic oxidation of ammonia (NH3-SCO) process, merely based on the small dataset. Furthermore, coupling pareto solution and Bayesian optimization, the active learning driven material searching are performed in the huge alloy spaces with more than 1000 configurations. After 3 iterations with high throughput calculation on 30 alloyed configurations and further theoretical validation based on microkinetic and stability analysis, 9 ensemble configurations are considered as innovative alloy catalysts of NH3-SCO with reactivity and selectivity trade-off. Our study provides the reliable framework for catalyst design with multi-objective and multi-constraints when facing with small sample data and huge searching space and gives the specific guidance for rational design of NH3−SCO alloy catalysts.

Performance of ammonia catalytic combustion over Cu-CeOx catalyst and the impact of oxygen concentration

Ammonia has long been regarded as a high-quality zero-carbon fuel; however, limitations in its combustion characteristics have restricted its practical applications. Catalytic combustion is considered an effective solution to overcome the challenges associated with ammonia combustion. In this study, we successfully synthesized a copper-cerium binary catalyst (Cu-CeOx) using the sol-gel method, demonstrating outstanding performance for NH3 catalytic combustion. Through in-situ DRIFTS results, we observed that the 15Cu-CeOx catalyst exhibits both i-SCR and -NH mechanisms. The oxygen concentration affects the N2 selectivity by modulating the overoxidation of NH3. Under high oxygen concentrations, high temperatures promote the excessive oxidation of NH3, and ammonia combustion through the i-SCR mechanism. Conversely, the oxygen concentration was equal to NH3 reaction stoichiometry, the overoxidation of NH3 is limited, and ammonia combustion proceeds through the NH mechanism, resulting in higher N2 selectivity.

Nonradical-dominated pathway toward selective catalytic ozonation of NH4+-N to N2 in drinking water over birnessite

Ammonia nitrogen (NH4+-N), which exists in the dominant form of ammonium ion (NH4+) under the pH of 6.5–8.5, is one of the regular indexes and the most important pollutants in drinking water. The selective conversion of NH4+-N into harmless substances is urgently needed and also a great challenge. Herein we propose a new NH4+-N conversion pathway based on a catalytic ozonation process with birnessite-type MnO2 (δ-MnO2) catalysts. The δ-MnO2 exhibits a superior performance for NH4+-N (1.0 mg/L) removal in adsorption (8.26 %) and oxidation (44.6 %) as well as up to 95.2 % of N2 selectivity, surpassing almost all the reported O3-based catalysts. Unlike the traditional radical oxidation, a singlet oxygen (1O2)-dominated nonradical oxidation pathway is confirmed for the efficiently selective conversion of NH4+-N to N2 through the successive dehydrogenation, leading to a 15.0-fold enhancement in NH4+-N oxidation rate compared with ozone oxidation. The Mn(III)/Mn(IV) redox sites and surface lattice oxygen in δ-MnO2 act as the active sites promoting the decomposition of O3 into •O2− and then 1O2 via electrons transfer. In application, δ-MnO2 catalytic ozonation effectively removes NH4+-N in real drinking water and presents excellent stability. This work finds a nonradical reaction mechanism in catalytic ozonation for NH4+-N selective conversion to N2, and solidly solves the problem of low-concentration NH4+-N removal from drinking water under the limited pH of 6.5–8.5 condition.

Tailoring Ni 3d orbitals in Ni-Fe/Ni foam heterostructures for enhanced H* adsorption and boosted electrocatalytic nitrate reduction performance

The electrochemical nitrate (NO3−) reduction (ENR) in wastewater, a promising remediation strategy, is often impeded by the essential use of costly noble metals. This study introduces an innovative non-noble Ni-Fe/Ni foam cathode fabricated via one-step electrodeposition technique. Efficient nitrate removal (99.4 %) was achieved with the selectivity of 83.6 % to NH3. The exceptional performance can be ascribed to the augmented active sites and the synergistic interaction between Ni and Fe. The leftward shift of the 3d orbitals density of Ni and the elevation of density at the Fermi level on Ni foam heterostructures, consequent to Fe doping, enhance the adsorption of H*. The addition of reactive chlorine species, generated through anodizing in Cl− mediated system, further improves N2 selectivity by oxidizing intermediate NH4+ product. The coupled anodic oxidation method employed herein achieves nearly 100 % removal efficiency of total nitrogen (TN) for initial concentration of 100 mg/L within 180 min, under the conditions of 1.5 g/L Cl− concentration and a current density of 25 mA cm−2. The findings underscore the potential of the Ni-Fe/Ni foam cathode within a Cl− mediated electrocatalytic framework for nitrate wastewater treatment, providing novel insights into the design of cathode catalysts and anodic coupling strategies for efficient TN removal.

Promoted catalytic property of Cu/SSZ-13 by introducing a minority of Mn for NO removal from diesel engine exhaust

The Cu-exchanged SSZ-13 with the small-pore chabazite framework is considered as a highly efficient catalyst for selective catalytic reduction of NO with NH3 (NH3-SCR). In order to further improve the catalytic property, a series of Mn ion-assisted Cu/SSZ-13 powder catalysts were prepared by co-exchange method and stepwise exchange method. It is found that the NH3-SCR activity, N2 selectivity, hydrothermal stability and sulfur resistance of Cu/SSZ-13 are promoted by introducing a minority of Mn (0.15%–0.23%(mass)) through co-exchange method. Characterization results revel that the Cu,Mn co-exchange enables the higher amounts of Cu2+ active sites, the abundant medium strong and strong acid, the optimized ratio of lewis acid to Brønsted acid, etc., which are required for a good NH3-SCR catalytic property over broad temperature range and under harsh working environment. Moreover, a monolithic catalyst was prepared by impregnating a cordierite ceramic support into the coating slurry containing the optimized CuMn/SSZ-13 powder. The diesel engine bench tests show that Cu,Mn co-exchange gives the monolith catalyst a better catalytic property than commercial catalysts. This work provides an important guidance for the rational design of secondary-ion-assisted zeolites applied in NH3-SCR.

TiO2 modified limonite for selective catalytic reduction of NO from cement kiln flue gas with NH3

Limonite was used as a precursor and TiO2 as a modifier to synthesize TiO2/Limonite composites through the wet impregnation method. The influences of the TiO2 doping amount, the calcination temperature on the denitration efficiency and the structure and properties of composites were systematically studied. The results showed that the modified catalyst over the Ti/Fe mass ratio was 0.07 and the calcination temperature was 300°C exhibited excellent catalytic activity, good stability, and remarkable resistance to water and sulfur dioxide within reaction temperature of 100 ∼ 240°C. When NO initial concentration was 0.05 % and gas hourly space velocity (GHSV) was 36,000 h−1 and the reaction temperature was 130°C, the NO conversion efficiency reached 100 %, and the N2 selectivity was 99.6 %. When the catalyst was exposed to 10 %H2O at 150°C, it maintained a 100 % NO conversion efficiency. In the presence of 10 %H2O and 50 ppm SO2 at 200°C, the NO conversion efficiency remained at 100 %. The TiO2/Limonite catalyst possessed improved performance due to the introduction of the anatase TiO2, which led to more lattice defects, weak Lewis acid sites, and higher adsorbed oxygen mobility.

Preparation of Mn/TiO2 catalysts using recovered manganese from spent alkaline batteries for low-temperature NH3-SCR

Black mass (BM) from spent alkaline Zn-MnO2 batteries was used for the first time as a Mn source in the preparation of Mn/TiO2 catalysts for low-temperature NH3-selective catalytic reduction (SCR) of NOx. To recover Mn species and eliminate alkali and Zn species, BM powder underwent DI-water washing, followed by carbothermal reduction. The resulting slags were further dissolved in HNO3, loaded onto TiO2 particles with ball milling, and then subjected to calcination. Nearly 100% of Zn species were removed from the BM via carbothermal reduction at 950 °C for 4 h with 5.0 wt% activated carbon. The resulting catalyst, derived from the treated BM, achieved similar NOx conversion (97%) as the catalyst prepared using a reagent-grade Mn chemical at 160 °C but a higher NOx-to-N2 conversion rate at 78%. The promoted N2 selectivity was attributed to a high Mn4+/Ti ratio and the presence of impurities from BM, such as Fe3+ ions, which enhanced oxidation ability of the catalyst. Conversely, insufficient removal of Zn or carbon additives in the slags led to a decreased Mn concentration, an increased proportion of Mn2+/Mn3+ species, increased surface OH groups, and reduced oxidation ability on the surface, thus reducing NOx conversion and N2 selectivity.

Ammonia nitrogen removal from coal gasification wastewater using a falling-film dielectric barrier discharge reactor: Process optimization and toxicity analysis

The continuous use of coal gasification technology for green energy generation to supplement the dwindling energy reserves generates significant amounts of wastewater containing ammonia nitrogen, posing a significant risk to the finite drinking water sources and aquatic lives. Herein, two falling-film dielectric barrier discharge (DBD) reactors with copper and titanium high-voltage electrodes, respectively, were designed to remove ammonia nitrogen (NH3−N) and chemical oxygen demand (COD) simultaneously from coal gasification wastewater. The operational parameters were screened using the Plackett-Burman design, and the significant parameters were further optimized via the response surface methodology. The results showed that the power parameter significantly influenced NH3-N removal, and COD removal was influenced considerably by power and treatment time. Under the optimum conditions (treatment time = 20.32 min, pH = 3, airflow rate = 1 L/min, power = 116 W, and recirculating flow rate = 50.02 mL/min), a removal ratio of 62.33 % and 37.35 % was achieved for NH3-N and COD for the reactor with the copper electrode, and 67.66 % and 44.90 % for NH3-N, and COD for the reactor with the titanium electrode. Additionally, the concentrations of O3, ·OH, and H2O2 produced by the reactors were quantified, and their crucial roles in the removal process of NH3-N were evaluated. Although smaller concentrations of NO2− and NO3− were detected in the treated wastewater, over 97 % of N2 selectivity encompassed NH3-N removal. Finally, acute phytotoxicity tests were performed, and the results indicated that at 100 mL/L dosage, the treated wastewater from both reactors did not restrict Barbarea verna growth.

Effect of sulfation on the catalytic performance of Mn-Co composite oxide catalyst for the selective catalytic reduction of NOx by NH3

The effect of sulfation on the catalytic performance of Mn-Co composite oxide catalyst for the selective catalytic reduction of NOx by NH3 (NH3-SCR) has been investigated. Different concentrations of SO2 have been used to sulfate Mn-Co composite oxide catalyst. The catalyst sulfated with 30 ppm SO2 showed the widest activity temperature window, over which 80% NOx conversion was obtained in the temperature range of 80–350 °C. The sulfation leads to the appropriate redox property of Mn-Co composite oxide, thus improving the N2 selectivity. Meanwhile, the sulfation contributes to the generation of more Brønsted acid sites, which is crucial for the enhanced activity. In situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS) demonstrated that the adsorbed NH3 species on both Lewis acid and Brønsted acid sites, monodentate nitrite, bidentate nitrate and cis-nitrite species are the reactive intermediates in the NH3-SCR reaction. This study provides an idea for the regulating the activity temperature window of NH3-SCR catalyst to adapt to its practical application.

Designing ammonia oxidation catalysts via physical mixing of Pt/SiO2 and Cu/ZSM-5

Ammonia (NH3) is emerging as a carbon-neutral fuel alternative; however, its use poses environmental and health risks due to NH3 slip. The NH3-selective catalytic oxidation (NH3-SCO) process, which converts NH3 into N2 and H2O, offers a solution. However, it faces challenges with respect to the formation of byproducts, particularly N2O and NOx. This study focuses on designing NH3-SCO catalysts to enhance low temperature NH3 conversion efficiency while minimizing undesired byproducts. Pt/SiO2 catalysts were synthesized with adjusted Pt particle sizes to improve the NH3-SCO activity and were physically mixed with Cu/ZSM-5 to overcome the low N2 selectivity of Pt-based catalysts by utilizing the selective catalytic reduction (SCR) ability of Cu/ZSM-5. Pt1Cu3 catalyst, a physical mixture of Pt/SiO2 and Cu/ZSM-5 in a 1:3 mass ratio, was found to be the most effective, achieving complete NH3 oxidation at 250 ℃ while maintaining a high N2 selectivity of approximately 80%. Further investigation revealed that the performance of the Pt1Cu3 catalyst was significantly influenced by the arrangement and physical proximity of the Pt/SiO2 and Cu/ZSM-5 catalysts, with a closely contacted mixture showing synergistic effects that enhanced activity and selectivity. These results suggest that the Pt1Cu3 catalyst offers a promising solution for NH3 emission control by utilizing a balanced approach that combines SCO and SCR reactions.

Design of dual-ligand coordination in metal organic frameworks for breaking the seesaw effect between de-NOx activity and N2 selectivity

In the selective catalytic reduction (SCR) of NOx with NH3, a catalyst usually exhibits a seesaw effect on de-NOx activity and N2 selectivity at low temperatures. Herein, we designed metal-organic frameworks (MOFs) with dual-ligand coordination to regulate the electronic and morphological structure for a breakthrough of the seesaw effect. The developed catalyst attained a noteworthy NOx conversion exceeding 90% within 60–330 ℃ with increased N2 selectivity. The dual-ligand coordination leads to more defects and a more uniform electron distribution for delocalization of electrons, thereby improving de-NOx activity. Furthermore, the dual-ligand coordination manifests a Si-O-Mn bond for amplifying both de-NOx activity and thermal stability. Additionally, the dual ligands and Mn produce a hollow sea-urchin-like morphology with more exposed active sites for concentrating reactants and intermediates to improve electron transfer efficiency in N2 selectivity. This work clarifies the mechanism of dual-ligand regulation for electron-morphology to simultaneously enhance de-NOx activity and N2 selectivity.

In-situ interzeolite transformation synthesis Cu-SSZ-13 from coal fly ash-derived zeolite for NH3-SCR reaction

Cu-SSZ-13 catalyst has excellent performance for removing NOx. In this study, the in-situ inter-zeolite transformation (IZT) method was proposed to prepare Cu-SSZ-13 catalyst. The coal fly ash-derived zeolite acted as a raw material rather than the chemical silicon and aluminum source. A series of catalysts were prepared using the in-situ IZT method with different ratios of the two template agents. The NO conversion of Cu-SSZ-13 catalysts was above 90 % within the temperature range of 200–700 °C, whereas the N2 selectivity was maintained at about 100 %. The as-synthesized catalysts had higher conversion rates for Si and Al, higher copper loading and more isolated Cu2+. The selective catalytic reduction (SCR) reaction mechanisms of Cu-SSZ-13 catalyst prepared using the in-situ IZT method followed both the Eley-Rideal (E-R) mechanism and the Langmuir-Hinshelwood (L-H) mechanism, though it was mainly based on the E-R reaction. In this study, the Cu-SSZ-13 catalysts were synthesized using coal fly ash based zeolite in one step. This study further improved the synthesis process route from coal based solid waste to SCR catalyst, shortened the preparation cycle, and improved catalyst performance.

Revealing the roles of Zr on enhanced H2-SCR performances on Pt/TiO2 catalyst

The development of Hydrogen Internal Combustion Engine (H2-ICEs) to replace conventional fuel engines is expected to be an effective way to reduce CO2 emissions, but some NOx emitted from the H2-ICEs still should be eliminated by the selective catalytic reduction of NOx with H2 (H2-SCR). Large amount of platinum oxide existed on Pt/TiO2 leads to its low H2-SCR activity. In contrast, the Zr-modified Pt/TiO2 catalysts exhibit a higher NOx conversion and N2 selectivity in a wider temperature window (100–180 °C) than those of Pt/TiO2. The appropriately weakened strong interaction between Pt and TiO2 by Zr induces more Pt0 species, which are active sites for the activation of NOx and H2 in H2-SCR reaction. More oxygen vacancies caused by the formation of Zr-Ov-Ti3+ bands involve in the activation of NOx and the adsorption of nitrates (NOx−) species. Furthermore, in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTs) demonstrates that Pt/ZrxTi1-x catalysts possess more adsorbed NOx− species and a faster reduction rate for them. Monodentate and bridged nitrates are considered to be more reactive with H2 than bidentate nitrates at relatively low temperatures. This work provides directions for the development of high-performance H2-SCR catalysts and the modulation of the conversion temperature.

In situ DRIFTS study and mechanism analysis of selective catalytic reduction of NOx by NH3 over ions doping MnCeOx/GR catalyst

The present study demonstrates an innovative approach of doping MnCeOx/GR catalyst with three transition metal ions, namely Fe3+, Co3+, and Ni2+. Subsequently, the catalyst exhibiting superior performance was carefully selected. Meanwhile, the micro-reaction process of MnCeOx/GR and ion-doped MnCeOx/GR catalysts were established by in situ DRIFTS. The phenomena occurred in the experiment process were thoroughly examined through the application of various characteristic analysis, for instance, XRD, Raman, BET, XPS, H2-TPR, and NH3-TPD to analyze that Co3+ doping had a good improvement of activity and N2 selectivity on MnCeOx/GR catalyst at low temperature. This was evidenced by the activity test experiments, which revealed that CoMnCeOx/GR catalyst has the highest NO conversion within 150–225 °C temperature range, surpassing 90 %. Our findings showed that Co3+ doping not only enhanced redox capability and surface acidity, but also raised the relative Mn4+, Ce3+, and Oα content compared to MnCeOx/GR catalyst, which eventually contributed that the reactant molecules can be more effectively adsorbed onto the catalyst, thereby enhancing activation of NH3 and O2, thus improving efficiency of CoMnCeOx/GR catalyst. Additionally, it was proved that the Eley-Rideal and Langmuir-Hinshelwood mechanisms co-existed on CoMnCeOx/GR catalyst in a competitive way by in situ DRIFTS test.

Tandem photocatalysis of nitrate to nitrogen on Cu@CN/TiOx S-scheme heterojunction

The persistent challenge of nitrate (NO3–) pollution in water systems, profoundly impacting ecosystems and human health, demands effective remediation strategies. Here, we present an innovative S-scheme Cu@CN/TiOx heterojunction for high-efficiency photocatalytic denitrification (PCDN). Evaluation of its performance demonstrates exceptional NO3– removal efficiency (99.5%) and remarkable N2 selectivity (95.5%), with 13.7 and 4.1 times higher rate constants of NO3– removal than the individual Cu@CN and TiOx components. Notably, this catalyst exhibits impressive stability across multiple cycles, maintaining consistent high conversion rates and selectivity. Mechanistic insights unveiled a tandem reaction pathway where Cu@CN selectively reduced NO3– to NO2–, followed by the TiOx-mediated CO2•– generation, enabling highly selective NO2– to N2 conversion. This unique catalytic system, featuring spatially separated redox-active sites, presents a promising avenue for efficient, selective, and stable denitrification processes, offering significant potential in addressing water pollution challenges and advancing tandem catalytic systems.

Optimized synthesis of Fe-doped Ohmic Schottky heterojunctions for efficient NO3− photocatalytic reduction to generate N2

Capacitive Fe-doped Ni2P(Fe2xNi2(1-x)P) is synthesized via the modified solvothermal method to form Ohmic Schottky heterojunctions with monolayer Ti3C2 (MLTC). The composite (10 % 0.1FNP/CdS-2 % MLTC) achieved a high hydrogen evolution rate (HER) of 19.18 mmol h−1 with a 50 mg sample. In a mere span of 3 h, an impressive NO3− reduction of 0.42 mg/L was achieved, accompanied by an exceptional 92 % N2 selectivity. Quantum efficiency (QE) can reach 56.22 % (λ = 475 nm) by adding 0.1FNP and MLTC. In contrast to previous catalysts, the QE of visible light after 500 nm has been improved to 45.2 % (λ = 500 nm) or 36.4 % (λ = 550 nm). The significant enhancement of HER and NO3− reduction is attributed to the enhanced adsorption of oxygen-containing reactants, the lowering of the reduction overpotential, further enhanced separation efficiency of photogenerated carriers through the ohmic Schottky heterojunction after iron doping. Additionally, the observed capacitor-like nature of 0.1FNP is also an essential factor for the enhanced reduction ability of the composites.