Photocatalytic Nitrogen Reduction Reaction Research Articles

Schottky Junctions with Bi@Bi2MoO6 Core-Shell Photocatalysts toward High-Efficiency Solar N2-to-Ammonnia Conversion in Aqueous Phase.

The photocatalytic nitrogen reduction reaction (NRR) in aqueous solution is a green and sustainable strategy for ammonia production. Nonetheless, the efficiency of the process still has a wide gap compared to that of the Haber-Bosch one due to the difficulty of N2 activation and the quick recombination of photo-generated carriers. Herein, a core-shell Bi@Bi2MoO6 microsphere through constructing Schottky junctions has been explored as a robust photocatalyst toward N2 reduction to NH3. Metal Bi self-reduced onto Bi2MoO6 not only spurs the photo-generated electron and hole separation owing to the Schottky junction at the interface of Bi and Bi2MoO6 but also promotes N2 adsorption and activation at Bi active sites synchronously. As a result, the yield of the photocatalytic N2-to-ammonia conversion reaches up to 173.40 μmol g-1 on core-shell Bi@Bi2MoO6 photocatalysts, as much as two times of that of bare Bi2MoO6. This work provides a new design for the decarbonization of the nitrogen reduction reaction by the utilization of renewable energy sources.

Enhanced Photocatalytic Nitrogen Reduction via Bismuth Nanoparticle-Decorating ZnCdS Solid Solution.

The construction of photocatalysts with a surface plasmon resonance effect (SPR) has been demonstrated as a highly effective strategy for enhancing photocatalytic efficiency. In this paper, we synthesized a catalyst with bismuth metal loaded on ZnCdS nanospheres for an efficient photocatalytic nitrogen reduction reaction (PNRR). The SPR effect induced by Bi nanoparticles under light excitation significantly promoted the ammonia production efficiency of the photocatalyst. Under air ambient conditions with lactic acid as the sacrificial agent, the photocatalytic NH4+ yield of 3% Bi@ZnCdS was 58.93 μmol·g-1·h-1, which exhibited an approximately 7.7 times that of the pure phase ZnCdS. The experimental characterization results demonstrate that the incorporation of metallic bismuth enhances the light absorption capacity of the catalyst and improves the separation efficiency of the photogenerated carriers. Theoretical calculations proved that Bi NPs provide more photogenerated electrons to convert N2 to NH3 for solid-solution ZnCdS. This work presents a novel concept for the development of advanced plasma nanomaterials to enhance the photocatalytic nitrogen fixation reaction.

A novel two-dimensional Janus TiSiGeN4 monolayer with N vacancies for efficient photocatalytic nitrogen reduction.

The photocatalytic nitrogen reduction reaction (pNRR) is a clean technology that converts H2O and N2 into NH3 under environmental conditions using inexhaustible sunlight. Herein, we designed a novel two-dimensional (2D) Janus TiSiGeN4 structure and evaluated the pNRR performance of the structure with the presence of nitrogen vacancies at different positions using density functional theory (DFT) calculations. The intrinsic dipoles in the Janus TiSiGeN4 structure generate a built-in electric field, which promotes the migration of photogenerated electrons and holes towards the (001) and (00-1) surfaces, respectively, to achieve efficient charge separation. For the pNRR, the Si atoms exposed after the formation of top N vacancies can realize the efficient activation of N2 through the "acceptance-donation" mechanism, while the presence of middle N vacancies not only suppresses the hydrogen evolution reaction, a competition reaction, but also lowers the reaction barrier for the protonation of N atoms. The limiting potential of TiSiGeN4 with the coexistence of both top and middle N vacancies (TiSiGeN4-VN-mt) is as low as -0.44 V. In addition, the introduction of N vacancies generates defect levels, narrowing the band gap and improving the light response. This work provides theoretical guidance for the design of efficient pNRR photocatalysts under mild conditions.

Synergism of heterovalent valence state and oxygen vacancy defect engineering in Co/S co-doped TiO2 for nitrogen photoreduction to ammonia

Herein, we demonstrate a Co/S co-doped TiO2 bimetal sulfur oxide catalyst with heterovalent Co states and abundant oxygen vacancy defects for the photocatalytic nitrogen reduction reaction in an ambient environment.

Recent advances in tunable metal–support interactions for enhancing the photocatalytic nitrogen reduction reaction

Photocatalytic nitrogen reduction using inexhaustible solar energy has been considered as a promising strategy for NH3 synthesis. The design of heterogeneous photocatalysts with tunable MSIs is of great significance.

Efficient photofixation of nitrogen to ammonia over an S-scheme-based NiSnO3-g-C3N4 heterojunction

A limited injection of photogenerated electrons in N2 molecules is the core problem limiting the performance of heterojunction materials in photofixing N2 to ammonia. In this study, an inexpensive Sn-based perovskite (NiSnO3, NSO) was anchored over the g-C3N4 (gCN) sheets under mild reaction conditions to develop an S-scheme heterojunction for the photocatalytic nitrogen reduction reaction. The NSO-gCN heterojunction, utilizing methanol as a sacrificial agent, showed remarkable N2 reduction capability with an impressive ammonia evolution of 566 µmolg−1h−1, surpassing that of gCN and NSO by the factor of 3.6 and 40, respectively, in visible light. The significantly improved performance of the material was attributed to the S-scheme charge transfer mechanism. The NSO-gCN photocatalyst showed good stability with only a marginal decrease measured in the activity over 3 cycles of the photocatalytic reaction. The study reveals a new strategy to design the highly efficient S-scheme-based heterojunctions by using Sn-based perovskite for sustainable ammonia synthesis.

Influence of B-atom in g-C3N4 matrix to enhance the photocatalytic dinitrogen to ammonia conversion

Boron-doped metal-free catalysts (B-g-C3N4) were successfully synthesized via the facile thermal polymerization method. In B-doped g-C3N4 synthesis, boric acid was varied viz; 1%, 3%, 6%, and 9% as compared to 2 g of melamine. 6% B-doped g-C3N4 was found to be the optimized catalyst showcasing enhanced performance for photocatalytic nitrogen reduction reaction (NRR). XRD and FT-IR results revealed that B doping did not change the basic chemical structure of g-C3N4. The presence of boron in the synthesized materials was confirmed via EDS, Elemental mapping, and XPS analysis. Boron doping also increased the carrier lifetime of g-C3N4 confirmed by time-resolved photoluminescence analysis. 6% B-g C3N4 showed the best photo-catalytic ammonia production among all the synthesized catalysts. It could produce ammonia up to 345.35 μmol.L−1.h−1 as compared to pure g-C3N4 (112.06 μmol. L −1.h −1). There was no loss of N2 reduction activities after 5 cycles of catalyst reuse. Mott-Schottky, photocurrent, and EIS were used to determine the flat band potential, electron recombination rate, and charge transfer resistance. Post N2 reduction, XRD, and XPS analysis of pure g-C3N4 and 6% B-g-C3N4 were almost identical revealing high stability of the doped g-C3N4. The doped boron enhances the optical and electrical properties and enhances photocatalytic activity for the production of NH3.

In-situ construction of WS2/ZIF-8 composites with an electron-rich interface for enhancing nitrogen photofixation

Photocatalytic nitrogen reduction reaction (PNRR) is an environmentally friendly synthesis method. It has been regarded as a promising approach for future NH3 preparation, which can reduce the natural fuel consumption and pollution of the Haber Bosch process. Nevertheless, this method exists poor activity for mass production, so it is urgent but challenging to explore highly efficient catalysts. Here, the novel WS2/ZIF-8 composites are reported, DFT and XPS indicate the transfer direction of electrons is from ZIF-8 to WS2, forming an electron-rich interface between WS2 and ZIF-8, thus it endows the more powerful photocatalytic nitrogen reduction ability for 2-WS2/ZIF-8 than monomer material. Meanwhile, 2-WS2/ZIF-8 exhibits admirable photocatalytic nitrogen reduction performance under real and simulated sunlight or in tap water, further attesting its excellent stability and practicability.

Co doping regulating electronic structure of Bi2MoO6 to construct dual active sites for photocatalytic nitrogen fixation

Although photocatalytic nitrogen reduction reaction (PNRR) is a green ammonia synthesis technology, it still encounters low adsorption/activation efficiency of N2 and lack of reaction active sites. Element doping is an efficient strategy to regulate electronic structure of catalyst. Nevertheless, the mechanism of the effect of doping elements on the N2 adsorption/activation, reaction active site and energy barriers is not well unraveled. Taking Co doped Bi2MoO6 (Co-Bi2MoO6) as a model photocatalyst, density functional theory (DFT) and experiment study were used to investigate the mechanism of Co doping on the PNRR performance over Bi2MoO6. DFT results reveal that Co doping regulates the electronic structure, activates Bi sites of Co-Bi2MoO6 and provides new Co active sites, thus constructing dual active sites for PNRR. Benefited from dual active sites for effectively adsorption/activation N2, the as-fabricated 3% Co-Bi2MoO6 exhibit the maximum NH3 generation rate of 95.5 μmol·g−1·h−1 without sacrificial agents, which is 7.2 times that of Bi2MoO6. Furthermore, the detail mechanism of NN bond adsorption/activation and hydrogenation reaction on Co-Bi2MoO6 was also proposed according to in-situ FTIR and DFT results. This study provides a promising strategy to design catalysts with dual active sites for PNRR, which is of great significance to the popularization of other material systems.

Boosting photocatalytic nitrogen reduction reaction by Jahn-Teller effect

Compared with traditional the Haber-Bosch process, photocatalytic ammonia production has attracted a considerable attention due to its advantages of low energy consumption and sustainability. In this work, we mainly study the photocatalytic nitrogen reduction reaction (NRR) on MoO3·0.55H2O and α-MoO3. Structure analysis shows that compared to α-MoO6, the [MoO6] octahedrons in MoO3·0.55H2O obviously distort (Jahn-Teller distortion), leading to the formation of Lewis acid active sites that favors the adsorption and activation of N2. X-ray photoelectron spectroscopy (XPS) further confirms the formation of more Mo5+ as Lewis acid active sites in MoO3·0.55H2O. Transient photocurrent, photoluminescence and electrochemical impedance spectra (EIS) confirmed that MoO3·0.55H2O has a higher charge separation and transfer efficiency than α-MoO3. Density functional theory (DFT) calculation further confirmed that the N2 adsorption on MoO3·0.55H2O is more favorable thermodynamically than that on α-MoO3. As a result, under visible light irradiation (λ ≥ 400 nm) for 60 min, an ammonia production rate of 88.6 μmol·gcat-1 was achieved on MoO3·0.55H2O, which is about 4.6 times higher than that on α-MoO3. In comparison to other photocatalysts, MoO3·0.55H2O exhibits an excellent photocatalytic NRR activity under visible light irradiation without using sacrificial agent. This work offers a new fundamental understanding to photocatalytic NRR from the viewpoint of crystal fine structure, which benefits designing efficient photocatalysts.

Photocatalytic Synthesis of Ammonia from Pinecone Graphite-Phase Carbon Nitride Loaded with MoS2 Nanosheets as Co-catalysts.

Photocatalytic nitrogen fixation is a promising alternative to the Haber-Bosch process to alleviate the energy and environmental crises. Here, we designed a pinecone-shaped graphite-phase carbon nitride (PCN) catalyst supported with MoS2 nanosheets by a supramolecular self-assembly method. The catalyst shows an excellent photocatalytic nitrogen reduction reaction (PNRR) due to the larger specific surface area and the enhancement of visible light owing to the reduced band gap. Under simulated sunlight, the sample of PCN loaded with 5 wt % MoS2 nanosheets (MS5%/PCN) shows a PNRR efficiency of 279.41 μmol g-1 h-1, which is 14.9 times that of bulk graphite-phase carbon nitride (g-C3N4), 4.6 times that of PCN, and 5.4 times that of MoS2, respectively. The unique pinecone-like structure of MS5%/PCN not only improves the ability of light absorption but also assists in the uniform loading of MoS2 nanosheets. Likewise, the existence of MoS2 nanosheets improves the light absorption ability of the catalyst and reduces the impedance of the catalyst. Furthermore, as a co-catalyst, MoS2 nanosheets can efficiently adsorb nitrogen (N2) and serve as active N2 reduction sites. From the perspective of structural design, this work can offer novel solutions for the creation of effective N2-fixing photocatalysts.

Electro- and Photocatalytic Conversion of N2 to NH3 by Chemically Modified Transition Metal Dichalcogenides, MoS2, and WS2

Electro- and photocatalytic reduction of N2 to NH3—the nitrogen reduction reaction (NRR)—is an environmentally- and energy-friendly alternative to the Haber-Bosch process for ammonia production. There is a great demand for the development of novel semiconductor-based electrocatalysts with high efficiency and stability for the direct conversion of inert substrates—including N2 to ammonia—using visible light irradiation under ambient conditions. Herein we report electro-, and photocatalytic NRR with transition metal dichalcogenides (TMDCs), viz MoS2 and WS2. Improved acid treatment of bulk TMDCs yields exfoliated TMDCs (exTMDCs) only a few layers thick with ∼10% S vacancies. Linear scan voltammograms on exMoS2 and exWS2 electrodes reveal significant NRR activity for exTMDC-modified electrodes, which is greatly enhanced by visible light illumination. Spectral measurements confirm ammonia as the main reaction product of electrocatalytic and photocatalytic NRR, and the absence of hydrazine byproduct. Femtosecond-resolved transient absorption studies provide direct evidence of interaction between photo-generated excitons/trions with N2 adsorbed at S vacancies. DFT calculations corroborate N2 binding to exMoS2 at S-vacancies, with substantial π-backbonding to activate dinitrogen. Our findings suggest that chemically functionalized exTMDC materials could fulfill the need for highly-desired, inexpensive catalysts for the sustainable production of NH3 using Sunlight under neutral pH conditions without appreciable competing production of H2.

Plasmonic silver nanoparticle-deposited n-Bi2S3/p-MnOS diode-type catalyst for enhanced photocatalytic nitrogen fixation: Introducing the defective p-MnOS

Photocatalytic nitrogen reduction reaction (PNRR) is clean and sustainable, considered an ideal synthesis technology for ammonia (NH3). In this study, we in situ developed the n-Bi2S3 nanorod/p-MnOS cubic composite catalyst for the first time using a one-step hydrothermal method, as MnOS, listed in the X-ray diffraction standard data file as an uncharacterized compound, has not been reported since 1965. Then, using a photodeposition technique, Ag nanoparticles (NPs) were added to the surface of the Bi2S3/MnOS to enhance PNRR via the surface plasmon resonance. After different characterization techniques for evaluation, Ag-Bi2S3/MnOS is found to be highly effective in the spatial separation of photogenerated electron-hole pairs due to the charge transfer at the composite interface. Photodeposited Ag is applied as an electron trapper to enhance the spatial separation of charge carriers. Based on their ability to reduce nitrogen under simulated solar light irradiation, the PNRR activities of Bi2S3, MnOS, Bi2S3/MnOS, and 0.5 to 2 % Ag-Bi2S3/MnOS were evaluated. After two hours, the PNRR activity of 1 % Ag-Bi2S3/MnOS produced ammonia 5.9 mmol/g, double the amount of Bi2S3/MnOS composite and approximately 5 times the amounts of Bi2S3 and MnOS materials. PNRR enhancement involves the internal static electron field created at the n-type Bi2S3/p-type MnOS interface and the improved electron conductivity with the photodeposition of Ag NPs, to speed up the reaction of reactive species. The importance of solid solution engineering in material design is demonstrated in this work.

One-step hydrothermal synthesis of novel flower-like Bi2Mn4O10 anchored on BiOI1−xBrx nanosheets for efficient photocatalytic nitrogen fixation

The reduction of nitrogen molecules (N2) to ammonia (NH3) is a promising method for energy sources and chemical manufacturing, owing to its sustainability and environmentally friendly process. The photocatalytic nitrogen reduction reaction (PNRR) is particularly recognized as a potential synthesis method. In this study, we developed n-type flower-like of Bi2Mn4O10 (BiMnO) nanoparticles in different percentages (5%, 10%, 15%, and 20%) anchored on the surface of n-type BiOI0.6Br0.4 (BiOIBr) nanosheets to form a heterojunction 5–20-BiMnO/BiOIBr using one-step hydrothermal method. According to Mott-Schottky plot analysis, BiMnO has a low Fermi level than BiOIBr, leading to spontaneous electron transfer from BiOIBr to BiMnO. In contrast, holes move in the opposite direction, forming a type-II heterojunction. Electrochemical impedance spectroscopy, linear sweep voltammetry, cyclic voltammetry, and transient photocurrent tests demonstrated that the 10-BiMnO/BiOIBr heterojunction is highly effective in the spatial separation photogenerated electron-hole pair due to the synergetic impact of the energy-level discontinuity at the interface. Consequently, outstanding N2 reduction activity was achieved under simulated solar light, with an NH3 production yield of 6.2 mmol g−1 over a 4-h run, significantly higher than that of pristine BiMnO and the solid solution of BiOIBr in 10% methanol aqueous solution. This work provides a systematic method for the fabrication of novel and efficient heterojunctions for nitrogen fixation.

Regulating bimetallic active centers for exploring the structure–activity relationship toward high-performance photocatalytic nitrogen reduction

Photocatalytic ammonia (NH3) synthesis from N2 is a strategy conducive to carbon neutrality because it avoids high energy consumption and high carbon emission process in the industrial synthesis of NH3. However, the structure–activity relationship of bimetallic active centers in heterogeneous catalysts is still unclear for high-performance photocatalytic nitrogen reduction reaction (pNRR). Here, we explore in-depth the structure–activity relationship of high-performance pNRR by regulating the semi- or full-encapsulated structure model formed by the noble metal nanoparticles (Pd, Au, and Pt) and mesoporous TiO2 nanorods (NRs). Further mechanism studies revealed that the Ti–Pd bimetallic active centers at the interface of semi-encapsulated palladium-decorated mesoporous TiO2 NRs possesses ultra-high adsorption energy, which can effectively weaken the triple bond of N2 and reduce the activation energy barrier of pNRR process. Accordingly, the optimal se-Pd/m-TiO2 NRs (2.71%) photocatalyst demonstrates a NH3 production rate of up to 635.73 μg/gcat./h, reaching a high apparent quantum efficiency of 0.37% at 375 nm under mild conditions. This work not only opens an avenue to expand high-performance catalysts for N2 photofixation under mild conditions via regulating bimetallic centers but also offers atomic-level insights for understanding the internal pNRR procedure.

Back Cover Image, Volume 5, Number 3, March 2023

Back cover image: Back cover image: Ammonia (NH3) plays an important role in modern industry, the photocatalytic nitrogen reduction reaction (NRR) is expected to achieve clean and sustainable NH3 production. In this review, the catalysts, reaction mechanisms, and NH3 generation detection method are summarized. Furthermore, the research strategies for improving photocatalytic activity are discussed. Finally, the challenges and prospects of photocatalytic NRR are presented.

Solvent-in-Gas System for Promoted Photocatalytic Ammonia Synthesis on Porous Framework Materials.

Photocatalytic nitrogen reduction reaction (PNRR) is emerging as a sustainable ammonia synthesis approach to meet global carbon neutrality. Porous framework materials with well-designed structures have great opportunities in PNRR; however, they suffer from unsatisfactory activity in the conventional gas-in-solvent system (GIS), owing to the hindrance of nitrogen utilization and strong competing hydrogen evolution caused by overwhelming solvent. In this study, porous framework materials are combined with a novel "solvent-in-gas" system, which can bring their superiority into full play. This system enables photocatalysts to directly operate in a gas-dominated environment with a limited proton source uniformly suspended in it, achieving the accumulation of high-concentrated nitrogen within porous framework while efficiently restricting the solvent-photocatalyst contact. An over eightfold increase in ammonia production rate (1820.7 µmol g-1 h-1 ) compared with the conventional GIS and an apparent quantum efficiency as high as ≈0.5% at 400 nm are achieved. This system-level strategy further finds applicability in photocatalytic CO2 reduction, featuring it as a staple for photosynthetic methodology.

Recent progress in research and design concepts for the characterization, testing, and photocatalysts for nitrogen reduction reaction

AbstractThe reduction of molecular nitrogen (N2) to ammonia (NH3) under mild conditions is one of the most promising studies in the energy field due to the important role of NH3 in modern industry, production, and life. The photocatalytic reduction of N2 is expected to achieve clean and sustainable NH3 production by using clean solar energy. To date, the new photocatalysts for photocatalytic reduction of N2 to NH3 at room temperature and atmospheric pressure have not been fully developed. The major challenge is to achieve high light‐absorption efficiency, conversion efficiency, and stability of photocatalysts. Herein, the methods for measuring produced NH3 are compared, and the problems related to possible NH3 pollution in photocatalytic systems are mentioned to provide accurate ideas for measuring photocatalytic efficiency. The recent progress of nitrogen reduction reaction (NRR) photocatalysts at ambient temperature and pressure is summarized by introducing charge transfer, migration, and separation in photocatalytic NRR, which provides guidance for the selection of future photocatalyst selection. More importantly, we introduce the latest research strategies of photocatalysts in detail, which can guide the preparation and design of photocatalysts with high NRR activity.

First-principles-driven catalyst design protocol of 2D/2D heterostructures for electro- and photocatalytic nitrogen reduction reaction.

Ammonia synthesis from nitrogen is a vital process and a necessity in a variety of applications including energy, pharmaceutical, agricultural, and chemical applications. The electro- and photocatalytic nitrogen reduction reactions (NRRs) are promising sustainable processes operated under milder conditions than the conventional Haber-Bosch process. However, the main pain points of these catalytic processes are their low selectivity and low efficiency. This perspective presents the recent status and the design protocols for developing promising 2D/2D heterojunction catalysts for the NRR, using the first-principles approach. The current theoretical studies are briefly discussed, and available methods are suggested for the development and design of new potential 2D/2D heterojunctions as efficient electro- and photo-NRR catalysts.

The role of 13X molecular sieves in photocatalytic nitrogen fixation.

To overcome the weak adsorption and difficult activation of N2 on catalysts in the photocatalytic nitrogen reduction reaction (NRR), we put forward that the introduction of molecular sieve 13X may realize the enrichment and activation of N2. 13X and the photoactive substrate BiOBr were assembled electrostatically to construct composite catalysts. In the presence of 13X, they are rich in nitrogen adsorption and activation sites, and the highest ammonia yield can reach 360.5 μmol h-1 gcat-1. It is surprising to find that 13X is able to optimize the photoelectric properties. This work extends the function of molecular sieves in the NRR and offers guidance to design catalysts with high photocatalytic activity.