Photocatalytic Nitrogen Reduction: Challenging Materials with Reaction Engineering

Unveiling the potential of step-scheme and Type II photocatalysts in dinitrogen reduction to ammonia

Innovative photocatalytic systems designed to enhance efficiency of nitrogen fixation processes, specifically focusing on sustainable ammonia (NH3) production strategies via dinitrogen (N2) reduction into ammonia (NH3). This process is critical for sustainable agriculture and energy production. To improve photocatalyst activity, catalyst stability and reusability, reduction efficiency due to electron/hole recombination, and light-absorption efficiency has drawn extensive attention. Herein, a broad range of research progress and comprehensive overview of step-scheme/type-II heterojunctions focusing on dinitrogen (N2) reduction are reviewed with focus on general synthesis, characterization by their unique charge separation mechanisms that improve light absorption and electron-hole pair utilization. The review highlights recent advancements in material design, which have shown promising results in enhancing photocatalytic activity under visible light irradiation. A significant portion of the review delves into the underlying mechanisms which these heterojunctions operate. Despite the promising literature results, several challenges facing this field, such as scalability, stability of photocatalysts, and environmental impact under operational conditions were also discussed. In summary, this review provides valuable insights into the potential of step-scheme/type-II photocatalysts for dinitrogen reduction to ammonia. The need for interdisciplinary approaches to overcome existing challenges such as incorporation of piezoelectric biomaterials and unlocking the full potential of these materials in addressing global nitrogen demands sustainably are highlighted, outlining future directions for further research and innovations.

Light-driven nitrogen fixation routes for green ammonia production

The global goal for decarbonization of the energy sector and the chemical industry could become a reality by a massive increase in renewable-based technologies. For this clean energy transition, the versatile green ammonia may play a key role in the future as a fossil-free fertilizer, long-term energy storage medium, chemical feedstock, and clean burning fuel for transportation and decentralized power generation. The high energy-intensive industrial ammonia production has triggered researchers to look for a step change in new synthetic approaches powered by renewable energies. This review provides a comprehensive comparison of light-mediated N2 fixation technologies for green ammonia production, including photocatalytic, photoelectrocatalytic, PV-electrocatalytic and photothermocatalytic routes. Since these approaches are still at laboratory scale, we examine the most recent developments and discuss the open challenges for future improvements. Last, we offer a technoeconomic comparison of current and emerging ammonia production technologies, highlighting costs, barriers, recommendations, and potential opportunities for the real development of the next generation of green ammonia solutions.

Photocatalytic Dinitrogen Reduction to Ammonia over Biomimetic FeMoSx Nanosheets

Reduction of atmospheric dinitrogen (N2) to ammonia (NH3) using water and sunlight in the absence of sacrificial reducing reagents at room temperature is very challenging and is considered an eco-friendly approach to meet the rapidly increasing demand for nitrogen storage, fertilizers, and a sustainable society. Currently, ammonia production via the energy-intensive Haber-Bosch process causes ∼350 million tons of carbon dioxide (CO2) emission per year. Interestingly, natural N2 fixation by the nitrogenase enzyme occurs under ambient conditions. Unfortunately, N2 fixation on biomimetic catalysts has rarely been studied. To mimic biological nitrogen fixation, herein, we synthesized the novel iron molybdenum sulfide (FeMoSx) micro-/nanosheets via a simple hydrothermal approach for the first time. Further, we successfully demonstrated the photochemical conversion of N2 to NH3 over a biomimetic FeMoSx photocatalyst. The estimated yield is around 99.79 ± 6.0 μmol/h/g photocatalyst with a quantum efficiency of ∼0.028% at 532 nm visible-light wavelength. Besides, we also systematically studied the influence of key factors to further improve NH3 yields. Overall, this study paves a new pathway to fabricate carbon-free, photochemical N2 fixation materials for future applications.

Bridging Together Theoretical and Experimental Perspectives in Single-Atom Alloys for Electrochemical Ammonia Production

Ammonia is an essential commodity in the food and chemical industry. Despite the energy-intensive nature, the Haber-Bosch process is the only player in ammonia production at large scales. Developing other strategies is highly desirable, as sustainable and decentralized ammonia production is crucial. Electrochemical ammonia production by directly reducing nitrogen and nitrogen-based moieties powered by renewable energy sources holds great potential. However, low ammonia production and selectivity rates hamper its utilization as a large-scale ammonia production process. Creating effective and selective catalysts for the electrochemical generation of ammonia is critical for long-term nitrogen fixation. Single-atom alloys (SAAs) have become a new class of materials with distinctive features that may be able to solve some of the problems with conventional heterogeneous catalysts. The design and optimization of SAAs for electrochemical ammonia generation have recently been significantly advanced. This comprehensive review discusses these advancements from theoretical and experimental research perspectives, offering a fundamental understanding of the development of SAAs for ammonia production.

Unifying the Nitrogen Reduction Activity of Anatase and Rutile TiO2 Surfaces

TiO₂ is a model transition metal oxide that has been applied frequently in both photocatalytic and electrocatalytic nitrogen reduction reactions (NRR). However, the phase which is more NRR active still remains a puzzle. This work presents a theoretical study on the NRR activity of the (001), (100), (101), and (110) surfaces of both anatase and rutile TiO₂ . We found that perfect surfaces are not active for NRR, while the oxygen vacancy can promote the reaction by providing excess electrons and low-coordinated Ti atoms that enhance the binding of the key intermediate (HNN*). The NRR activity of the eight facets can be unified into a single scaling line. The anatase TiO₂ (101) and rutile TiO₂ (101) surfaces were found to be the most and the second most active surfaces with a limiting potential of -0.91 V and -0.95 V respectively, suggesting that the TiO₂ NRR activity is not very phase-sensitive. For photocatalytic NRR, the results suggest that the anatase TiO₂ (101) surface is still the most active facet. We further found that the binding strength of key intermediates scale well with the formation energy of oxygen vacancy, which is determined by the oxygen coordination number and the degree of relaxation of the surface after the creation of oxygen vacancy. This work provides a comprehensive understanding of the activity of TiO₂ surfaces. The results should be helpful for the design of more efficient TiO₂ -based NRR catalysts.

Recent progress of photocatalysts based on tungsten and related metals for nitrogen reduction to ammonia

Photocatalytic nitrogen reduction reaction (NRR) to ammonia holds a great promise for substituting the traditional energy-intensive Haber–Bosch process, which entails sunlight as an inexhaustible resource and water as a hydrogen source under mild conditions. Remarkable progress has been achieved regarding the activation and solar conversion of N₂ to NH₃ with the rapid development of emerging photocatalysts, but it still suffers from low efficiency. A comprehensive review on photocatalysts covering tungsten and related metals as well as their broad ranges of alloys and compounds is lacking. This article aims to summarize recent advances in this regard, focusing on the strategies to enhance the photocatalytic performance of tungsten and related metal semiconductors for the NRR. The fundamentals of solar-to-NH₃ photocatalysis, reaction pathways, and NH₃ quantification methods are presented, and the concomitant challenges are also revealed. Finally, we cast insights into the future development of sustainable NH₃ production, and highlight some potential directions for further research in this vibrant field.

Comparative Evaluation of Light-Driven Catalysis: A Framework for Standardized Reporting of Data

Light-driven homogeneous and heterogeneous catalysis require a complex interplay between light absorption, charge separation, charge transfer, and catalytic turnover. Optical and irradiation parameters as well as reaction engineering aspects play major roles in controlling catalytic performance. This multitude of factors makes it difficult to objectively compare light-driven catalysts and provide an unbiased performance assessment. This Scientific Perspective highlights the importance of collecting and reporting experimental data in homogeneous and heterogeneous light-driven catalysis. A critical analysis of the benefits and limitations of the commonly used experimental indicators is provided. Data collection and reporting according to FAIR principles is discussed in the context of future automated data analysis. The authors propose a minimum dataset as a basis for unified collecting and reporting of experimental data in homogeneous and heterogeneous light-driven catalysis. The community is encouraged to support the future development of this parameter list through an open online repository.

A minireview on catalysts for photocatalytic N2 fixation to synthesize ammonia

Ammonia (NH3) is an important feedstock in chemical industry. Nowadays NH3 is mainly produced via the industrialized Haber-Bosch process, which requires substantial energy input, since it operates at high temperatures (400-650 °C) and high pressures (20-40 Mpa). From the energy conservation point of view, it is of great significance to explore an alternative avenue to synthesize NH3, which is in line with the concept of sustainable development. Very recently, photocatalytic N2 fixation (PNF) has been discovered as a safe and green approach to synthesize NH3, as it utilizes the inexhaustible solar energy and the abundant N2 in nature to synthesize NH3 under mild conditions. A highly efficient catalyst is the core of PNF. Up to now, extensive studies have been conducted to design efficient catalysts for PNF. Summarizing the catalysts reported for PNF and unraveling their reaction mechanisms could provide guidance for the design of better catalysts. In this review, we will illustrate the development of catalysts for PNF, including semiconductors, plasmonic metal-based catalysts, iron-based catalysts, ruthenium-based catalysts and several other catalysts, point out the remaining challenges and outline the future opportunities, with the aim to contribute to the development of PNF.

Radiometric measurement techniques for in-depth characterization of photoreactors – part 1: 2 dimensional radiometry

A spatially resolving radiometric scanning method for the evaluation of the radiation field of photoreactors is presented, allowing for a knowledge driven development of highly efficient photoreactors and scale-up.