The burgeoning development of ammonia (NH3) synthesis technology addresses the urgency of food intake required to sustain the population growth of the last 100 years. To date, NH3 has mostly been synthesized by the Haber–Bosch process in industry. Under the ever-increasing pressure of the fossil fuel depletion crisis and anthropogenic global climate change with continuous CO2 emission in the 21st century, research targeting the synthesis of NH3 under mild conditions in a sustainable and environment friendly manner is vigorous and thriving. Therefore, the focus of this review is the state-of-the-art engineering of efficient photocatalysts for dinitrogen (N2) fixation toward NH3 synthesis. Strenuous efforts have been devoted to modifying the intrinsic properties of semiconductors (i.e. poor electron transport, rapid electron–hole recombination and sluggish reaction kinetics), including nanoarchitecture design, crystal facet engineering, doping and heterostructuring. Herein, this review provides insights into the most recent advancements in understanding the charge carrier kinetics of photocatalysts with respect to charge transfer, migration and separation, which are of fundamental significance to photocatalytic N2 fixation. Subsequently, the challenges, outlooks and future prospects at the forefront of this research platform are presented. As such, it is anticipated that this review will shed new light on photocatalytic N2 fixation and NH3 synthesis and will also provide a blueprint for further investigations and momentous breakthroughs in next-generation catalyst design.

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Photocatalytic fixation of nitrogen to ammonia: state-of-the-art advancements and future prospects

Over the last century, the industrialization of agriculture and the consumption of fossil fuels have resulted in a significant imbalance and redistribution in nitrogen-containing resources. This has sparked an interest in developing more sustainable and resilient approaches for producing nitrogen-containing commodities such as fertilizers and fuels. One largely neglected but emerging approach is photocatalytic nitrogen fixation. There is significant evidence that this process occurs spontaneously in terrestrial settings, and it has been demonstrated in numerous engineered systems. Yet many questions still remain unanswered regarding the rates, mechanisms, and impacts of photocatalytically producing fixed nitrogen “out of thin air”. This work reviews the fascinating history of the reaction and examines current progress toward understanding and improving photofixation of nitrogen. This is supplemented by a quantitative review of the thermodynamic considerations and limitations for various reaction mechanism...

Photon-Driven Nitrogen Fixation: Current Progress, Thermodynamic Considerations, and Future Outlook

Achieving robust catalysis of the nitrogen reduction reaction (NRR) is an ongoing scientific challenge, which needs activation and cleavage of the extremely strong N–N triple bond (bond energy: 940.95 kJ mol−1). The famous Haber–Bosch process requires enormous energy and extreme conditions in the presence of an iron catalyst to achieve the above-mentioned process. Environmentally friendly solar nitrogen fixation at room temperature and atmospheric pressure is an appealing way, but its efficiency is far from satisfactory for industrialization. Herein, we report for the first time on the design and preparation of MXene-derived TiO2@C/g-C3N4 heterojunctions where two-dimensional (2D) planar carbon nanosheet-supported TiO2 is thermally derived from MXene Ti3C2Tx and effectively coupled with in situ formed g-C3N4 nanosheets. Such a structure features abundant surface defects, high electron-donating ability, suitable light harvesting ability, excellent charge transport properties, and strong nitrogen activation ability and is demonstrated to be a robust photocatalyst for the NRR and achieves an NH3 production rate of 250.6 μmol g−1 h−1 under visible light irradiation. This performance compares favorably with those of previously reported photocatalysts for the NRR. The present finding would enrich our knowledge on designing and developing photocatalytically active catalysts for the NRR.

MXene-derived TiO2@C/g-C3N4 heterojunctions for highly efficient nitrogen photofixation

Bismuth-rich bismuth oxyhalides for environmental and energy photocatalysis

As an "up-and-coming" two-dimensional (2D) material, black phosphorus (BP) has attracted much attention due to its abundant metal-free properties and broad application prospects in photocatalysis. This study introduces a promising sunlight-driven metal-free photocatalyst for oxytetracycline hydrochloride degradation and hexavalent chromium reduction in water and wastewater. The roles of BP quantum dots (BPQDs) in the distribution of electrons and photocatalytic performances were well identified by experimental and density functional theory (DFT) calculations. As expected, the specially designed 0D/1D structure shows unusual photocatalytic efficiency toward the degradation of oxytetracycline hydrochloride (0.0276 min−1) and reduction of hexavalent chromium (0.0404 min−1). Reactive species, namely, O2− and h+ comprised the primary photocatalytic mechanisms for oxytetracycline hydrochloride degradation. This work highlights that the combination of tubular g-C3N4 (TCN) with BPQDs facilitates the charge spatial separation in the photocatalytic process, and provides alternative strategy for design of highly active and metal-free nanomaterials toward environmental remediation and sustainable solar-to-chemical energy conversion.

1D porous tubular g-C3N4 capture black phosphorus quantum dots as 1D/0D metal-free photocatalysts for oxytetracycline hydrochloride degradation and hexavalent chromium reduction

Ni2P loading on Cd0.5Zn0.5S solid solution for exceptional photocatalytic nitrogen fixation under visible light

Photocatalytic conversion of CO2 to solar fuels is considered an alternative approach for simultaneously mitigating the greenhouse effect and solving energy shortage. The efficient light harvesting and the thermochemical conversion have been demanding quests in photocatalysis due to the relatively low solar energy utilization efficiency. In this work, oxygen vacancies are induced in MoO3 for improving photo-thermal CO2 reduction efficiency by capturing near-infrared (NIR) photons. The localized surface plasmon resonance (LSPR) of MoO3−x triggered by oxygen vacancies enables the efficient capture of NIR photons. Additionally, oxygen vacancies can promote the carrier separation, improve CO2 adsorption on the defective surface and lower the barrier of CO2 hydrogenation during the conversion process. As a result, MoO3−x displayed dramatically enhanced photo-thermal synergistic CO2 reduction under simulated sunlight (UV-Vis-IR) irradiation than MoO3. The amount of CO produced by MoO3−x can reach 10.3 μmol g−1 h−1, which is 20 times higher than that of MoO3 (0.52 μmol g−1 h−1). And the CH4 production of MoO3−x can reach 2.08 μmol g−1 h−1, which is 52 times higher than that of MoO3 (0.04 μmol g−1 h−1). In situ FT-IR and theoretical calculations also proved the enhanced activity of MoO3−x. This work highlights the significance of defect engineering for improving the photo-thermal catalytic conversion of CO2.

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Sunlight induced photo-thermal synergistic catalytic CO2 conversion via localized surface plasmon resonance of MoO3−x

The development of low cost, metal free semiconductor photocatalysts for CO2 reduction to fuels and valuable chemical feedstocks is a practically imperative for reducing anthropogenic CO2 emissions. In this work, black phosphorus quantum dots (BPQDs) were successfully dispersed on a graphitic carbon nitride (g-C3N4) support via a simple electrostatic attraction approach, and the activities of BP@g-C3N4 composites were evaluated for photocatalytic CO2 reduction. The BP@g-C3N4 composites displayed improved carrier separation efficiency and higher activities for photocatalytic CO2 reduction to CO (6.54 µmol g−1 h−1 at the optimum BPQDs loading of 1 wt%) compared with pure g-C3N4 (2.65 µmol g−1 h−1) . This work thus identifies a novel approach towards metal free photocatalysts for CO2 photoreduction.

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Jue Li

Papers

Ni2P loading on Cd0.5Zn0.5S solid solution for exceptional photocatalytic nitrogen fixation under visible light

Sunlight induced photo-thermal synergistic catalytic CO2 conversion via localized surface plasmon resonance of MoO3−x

Black phosphorus quantum dot/g-C 3 N 4 composites for enhanced CO 2 photoreduction to CO

Solar-energy-driven photothermal catalytic C–C coupling from CO2 reduction over WO3–

Industrial TiO2 based nacreous pigments as functional building materials: Photocatalytic removal of NO