Photocatalytic Ammonia Synthesis Research Articles

Computational Study of Transition-Metal Substitutions in Rutile TiO2 (110) for Photoelectrocatalytic Ammonia Synthesis

Synthesis of ammonia through photo- and electrocatalysis is a rapidly growing field. Titania-based catalysts are widely reported for photocatalytic ammonia synthesis and have also been suggested as electrocatalysts. The addition of transition-metal dopants is one strategy for improving the performance of titania- based catalysts. In this work, we screen d-block transition- metal dopants for surface site stability and evaluate trends in their performance as the active site for the reduction of nitrogen to ammonia on TiO2. We find a linear relationship between the d-band center and metal substitution energy of the dopant site, while the binding energies of N2, N2H, and NH2 all are strongly correlated with the cohesive energies of the dopant metals. The activity of the metal-doped systems shows a volcano type relationship with the NH2 and N2H energies as descriptors. Some metals such as Co, Mo, and V are predicted to slightly improve photo- and electrocatalytic performance, but most metals inhibit the ammonia synthesis reaction. The results provide insight into the role of transition-metal dopants for promoting ammonia synthesis, and the trends are based on unexpected electronic structure factors that may have broader implications for single-atom catalysis and doped oxides.

Photocatalytic synthesis of N2/H2O to ammonia on coal based GO/SiO2 supported Ru composite catalyst

Graphene oxide/silica (GO/SiO2) containing various inorganic particles is obtained by using the Xinjiang Wucaiwan coal as carbon source and silicon source. The ruthenium/graphene oxide/silica (Ru/GO/SiO2) composite was prepared by loading Ru nanoparticles on the surface of GO/SiO2. Then, the photocatalytic ammonia synthesis performance was investigated while characterizing the structure of Ru/GO/SiO2. On the one hand, the Ru can be sufficiently dispersed to make the surface plasmon resonance (SPR) effect enhance significantly. On the other hand, the quantum confinement effect of the defect structure on the Ru/GO/SiO2 can effectively separate electron and hole, and it can participate in the reducing/oxidizing reaction of photocatalytic ammonia synthesis.

Formation of BNC Coordination to Stabilize the Exposed Active Nitrogen Atoms in g-C3 N4 for Dramatically Enhanced Photocatalytic Ammonia Synthesis Performance.

It is an important issue that exposed active nitrogen atoms (e.g., edge or amino N atoms) in graphitic carbon nitride (g-C3 N4 ) could participate in ammonia (NH3 ) synthesis during the photocatalytic nitrogen reduction reaction (NRR). Herein, the experimental results in this work demonstrate that the exposed active N atoms in g-C3 N4 nanosheets can indeed be hydrogenated and contribute to NH3 synthesis during the visible-light photocatalytic NRR. However, these exposed N atoms can be firmly stabilized through forming BNC coordination by means of B-doping in g-C3 N4 nanosheets (BCN) with a B-doping content of 13.8 wt%. Moreover, the formed BNC coordination in g-C3 N4 not only effectively enhances the visible-light harvesting and suppresses the recombination of photogenerated carriers in g-C3 N4 , but also acts as the catalytic active site for N2 adsorption, activation, and hydrogenation. Consequently, the as-synthesized BCN exhibits high visible-light-driven photocatalytic NRR activity, affording an NH3 yield rate of 313.9 µmol g-1 h-1 , nearly 10 times of that for pristine g-C3 N4 . This work would be helpful for designing and developing high-efficiency metal-free NRR catalysts for visible-light-driven photocatalytic NH3 synthesis.

RuO2-loaded TiO2–MXene as a high performance photocatalyst for nitrogen fixation

In this work, we report RuO2-loaded TiO2–MXene hybrid nanostructure as a promising photocatalyst possessing remarkable activity for photocatalytic nitrogen fixation in water at room temperature. The MXene was synthesized by HF etching of the precursor Ti3AlC2 and used as the substrate for the growth of RuO2 nanoparticles and TiO2 through a hydrothermal reaction. The benign activity is mainly attributed to the synergetic effect among MXene, TiO2, and RuO2 in photocatalytic ammonia synthesis by combining light harvesting and nitrogen activation.

Unravelling the effects of layered supports on Ru nanoparticles for enhancing N2 reduction in photocatalytic ammonia synthesis

Harnessing the vast supply of solar energy as the driving force to produce ammonia from abundant nitrogen gas and water is beneficial for both relieving energy demands and developing sustainable chemical industry. Bulk carbon nitride (B-g-C3N4), exfoliated carbon nitride (E-g-C3N4) and graphite (g-C) supported Ru-K catalysts, denoted as Ru-K/B-g-C3N4, Ru-K/E-g-C3N4 and Ru-K/g-C, respectively, with the layered materials serving both as supports and light harvesters, were designed for photocatalytic ammonia synthesis. It was discovered that, besides the light harvesting properties of the catalysts which played roles in photocatalytic reactions, the structure of the supports influenced greatly the preferential locations of Ru species, which further exerted effects on the N2 activation process and ultimately impacted the ammonia production rate. The fine Ru nanoparticles uniformly and randomly dispersed on the monolayered E-g-C3N4 did not provide outstanding activity in ammonia photosynthesis; in contrast, Ru nanoparticles at the step edges of bulk g-C3N4 exhibited lower overall barriers for N2 activation and a much enhanced photocatalytic ammonia synthesis rate due to the synergy effects between metal and support as confirmed by density functional theory (DFT) calculations. The discovery of the relationship between reactivity and support geometry in this study will be important in guiding the rational predesign of efficient photocatalysts.

Synthesis and visible-light photocatalytic N2/H2O to ammonia of ZnS nanoparticles supported by petroleum pitch-based graphene oxide

It is of significant academic value to find a new type of eco-friendly photocatalyst with high-efficiency, low-cost, which can be applied to the photocatalytic synthesis of N2/H2O to ammonia. In this paper, pitch-based graphene oxide (GO) is obtained by using petroleum pitch as a carbon source (Hummers Method). Then, the ZnS nanoparticles were loaded onto the pitch-based GO sheet to get the composite photocatalyst ZnS/GO of different GO contents (Hydrothermal Method). The performance of ZnS/GO photocatalytic synthesis of N2/H2O to ammonia was studied while characterizing the material structure and photoelectric properties. The results showed a similar structure and performance with general graphite-based GO. The presence of GO structure not only enables the composite catalyst to enhance electron transfer by inhibiting the recombination of the photogenerated electrons and holes pairs, but also improves activation and adsorption N2 properties and photocatalytic ammonia synthesis with N2/H2O performance. The NH4+ yield was 910.6 μmol/L·g cat after 6 h photocatalytic reaction, which was about five times of NH4+ yields on pure ZnS as a photocatalyst under the same conditions.

Novel Photocatalytic NH3 Synthesis by NO3− Reduction over CuAg/TiO2

The highly effective reaction system was investigated for the photocatalytic ammonia synthesis from the reduction of nitrate ions by using the semiconductor photocatalyst, Cu and Ag doped on TiO2 (CuAg/TiO2) at room temperature under UV light irradiation (max. 352 nm). In this study, CuAg/TiO2 gave the high efficiency and the selectivity for the ammonia synthesis by the photoreduction of nitrate in the presence of methanol as a hole scavenger. For the evaluation of the photocatalytic activity over CuAg/TiO2, various TiO2 samples, such as standard TiO2, Cu/TiO2, and Ag/TiO2, were evaluated in the same procedure. The chemical properties were investigated by XRD, TEM, XPS, PL, and DRS. We examined the optimum conditions for the experimental factors and the important issues, including the effect of the molar ratio of Cu and Ag onto TiO2, the optimization of the CuAg amount loaded on TiO2, the influence of the loading amount of the catalyst on the reduction of nitrate ions, the exploration of the optimum hole scavenger, and the reusability of the optimum photocatalyst. The very efficient conversion of nitrate ions (95%) and the highest selectivity (86%) were achieved in the reaction with the optimum conditions. Here, we reported the process that nitrate ions can efficiently be reduced, and ammonia can be selectively synthesized over CuAg/TiO2.

Progress of Metal Oxide (Sulfide)-Based Photocatalytic Materials for Reducing Nitrogen to Ammonia

The Haber–Bosch process has been an important approach to produce ammonia for meeting the food need of increasing population and the worldwide need of nitrogenous fertilizers since 1913. However, the traditional ammonia production process is a high energy-consumption process, which usually produces 1 metric ton ammonia with releasing around 1.9 metric tons CO2. Photocatalytic ammonia synthesis under solar light as energy source, an attractive and promising alternative approach, is a very challenging target of reducing fossil energy consumption and environmental pollution. Therefore, photocatalytic ammonia production process would emerge huge opportunities by directly providing nitrogenous fertilizers in a distributed manner as needed in the agricultural fields. In this article, different metal oxide (sulfide)-based photocatalytic materials for reducing nitrogen to ammonia under ambient conditions are reviewed. This review provides insights into the most recent advancements in understanding the photocatalyst materials which are of fundamental significance to photocatalytic nitrogen reduction, including the state-of-the-art, challenges, and prospects in this research field.

Oxygen Vacancy Engineering Promoted Photocatalytic Ammonia Synthesis on Ultrathin Two-Dimensional Bismuth Oxybromide Nanosheets

The catalytic conversion of nitrogen to ammonia is one of the most important processes in nature and chemical industry. However, the traditional Haber-Bosch process of ammonia synthesis consumes substantial energy and emits a large amount of carbon dioxide. Solar-driven nitrogen fixation holds great promise for the reduction of energy consumption and environmental pollution. On the basis of both experimental results and density functional theory calculations, here we report that the oxygen vacancy engineering on ultrathin BiOBr nanosheets can greatly enhance the performance for photocatalytic nitrogen fixation. Through the addition of polymetric surfactant (polyvinylpyrrolidone, PVP) in the synthesis process, VO-BiOBr nanosheets with desirable oxygen vacancies and dominant exposed {001} facets were successfully prepared, which effectively promote the adsorption of inert nitrogen molecules at ambient condition and facilitate the separation of photoexcited electrons and holes. The oxygen defects narrow the bandgap of VO-BiOBr photocatalyst and lower the energy requirement of exciton generation. In the case of the specific surface areas are almost equal, the VO-BiOBr nanosheets display a highly improved photocatalytic ammonia production rate (54.70 μmol·g-1·h-1), which is nearly 10 times higher than that of the BiOBr nanoplates without oxygen vacancies (5.75 μmol·g-1·h-1). The oxygen vacancy engineering on semiconductive nanomaterials provides a promising way for rational design of catalysts to boost the rate of ammonia synthesis under mild conditions.

PdSn/NiO/NaTaO3:La for photocatalytic ammonia synthesis by reduction of NO3− with formic acid in aqueous solution

Photocatalytic ammonia synthesis is of great importance but remains very challenging in photocatalysis. Here, we fabricated a photocatalytic reaction system for the efficiently photocatalytic nitrate reduction to ammonia under UV irradiation using PdSn/NiO/NaTaO3:La as a photocatalyst in the presence of formic acid. Several important factors of the photocatalytic performance were investigated, including initial concentration of the nitrate solution, initial pH value and cocatalyst loadings. Nitrate conversion reaches 100% and ammonia selectivity is 72% over the optimum photocatalyst with 5% bimetallic PdSn alloy and 0.2% NiO loadings. The outstanding performance of photocatalyst can be ascribed to both the strong adsorption of nitrite on NiO and the efficient separation of electron-hole pairs by PdSn bimetallic alloy. Besides as the hole scavenger, formic acid also provides not only carboxyl anion radicals for nitrate reduction but also the in-situ buffer effect on ammonia formation. These results give inspiration for designing the reactive system with efficient photocatalytic ammonia synthesis.

Interference Effect of Alcohol on Nessler’s Reagent in Photocatalytic Nitrogen Fixation

Photocatalytic ammonia synthesis is another important reaction to mimic natural nitrogen fixation, which has attracted more and more attention. In recent reports, sacrificial agents are often used to promote charge separation, and high-activity photocatalysts are discovered by using Nessler’s reagent method as a detection technique of ammonia production. However, there is an open question on the rationality and accuracy of the ammonia production amount in the presence of the sacrificial agent and Nessler’s reagent detection method. In this report, P25 TiO2 is employed as a model photocatalyst and alcohol as sacrificial agent, and both Nessler’s reagent and cation exchange chromatography are employed as ammonia detection methods. The different ammonia production amount was found by the different detection method. HPLC and 1H NMR results indicate that carbonyl compounds (formaldehyde, acetaldehyde, and acetone) are produced in the reaction. When the carbonyl compound was added to the ammonia standard soluti...

Unoccupied surface state induced by ozone and ammonia on H-terminated diamond electrodes for photocatalytic ammonia synthesis

This work follows upon the recent demonstration of the photocatalytic conversion of N2 to NH3 in aqueous solutions via H-terminated, polycrystalline diamond electrodes. X-ray absorption spectroscopy serves as an atom- and bond-specific probe of the local bonding of oxygen and nitrogen species at the surface of the electrodes. Ozone treatment of the electrodes creates a sharp C1s transition to an unoccupied surface state at 2.5 eV below the onset of the transitions to the bulk conduction band. A similar transition occurs at 2.6 eV below the onset after ammonia treatment. It is surprising to obtain such a well-defined surface state on complex, real-life electrode materials, which suggests a characteristic local bonding configuration. An extensive comparison with spectra from reference molecules reveals keto groups (C=O) at the ozone-treated surface, with the possibility of additional tertiary alcohols (C-OH). The electron–hole interaction is taken into account for estimating the absolute energy of the surface state in the bandgap of diamond.

Photocatalytic ammonia synthesis in a fluidised bed reactor

Photocatalytic ammonia synthesis was successfully performed in a fluidised reactor of parallel-wall configuration by irradiating iron-doped titanium dioxide with near ultraviolet light. The catalyst was prepared in such a way that good quality of fluidisation could be obtained. Mixing the catalyst with γ-alumina was found to affect the fluidisation behaviour. Ammonia production was increased when the catalyst was suitably fluidised because of enhanced utilisation of light energy. Quantum efficiencies of more than 10% have been reached. The parallel-wall reactor configuration was found to give higher energy conversion than a cylindrical one [1a].