Fluorine-Induced Surface Metallization for Ammonia Synthesis under Photoexcitation up to 1550 nm

Recent Advances in Ammonia Synthesis: From Haber-Bosch Process to External Field Driven Strategies

Ammonia, a pivotal chemical feedstock and a potential hydrogen energy carrier, demands efficient synthesis as a key step in its utilization. The traditional Haber-Bosch process, known for its high energy consumption, has spurred researchers to seek ammonia synthesis under milder conditions. Advances in surface science and characterization technologies have deepened our understanding of the microscopic reaction mechanisms of ammonia synthesis. This article concentrates on gas-solid phase ammonia synthesis, initially exploring the latest breakthroughs and improvements in thermal catalytic synthesis. Building on this, it especially focuses on emerging external field-driven alternatives, such as photocatalysis, photothermal catalysis, and low-temperature plasma catalysis strategies. The paper concludes by discussing the future prospects and objectives of nitrogen fixation technologies. This comprehensive review is intended to provide profound insights for overcoming the inherent thermodynamic and kinetic constraints in traditional ammonia synthesis, thereby fostering a shift towards "green ammonia" production and significantly reducing the energy footprint.

Electron repulsion tuned electronic structure of TiO2 by fluorination for efficient and selective photocatalytic ammonia generation

The photocatalytic conversion of nitrogen into high-value ammonia products holds tremendous potential in the global nitrogen cycle. However, the activation of N2 and competition of hydrogen evolution limit the improvement of nitrogen fixation performance. In this study, we developed a fluorinated TiO2 (F-TiO2) using a hydrothermal-annealing method. The incorporation of F dopants not only enhances the adsorption and activation of N2 through electronic structure regulation, but also facilitates an in situ increase in active sites via the electron repulsion effect between F and Ti atoms. In addition, the presence of F on the surface effectively improved the nitrogen supply problem and optimized the nitrogen fixation selectivity for its hydrophobic modulation. The NH3 yield of the F-TiO2 photocatalyst reached 63.8 μmol h-1 g-1, which was 8.5 times higher than that of pure TiO2. And the selectivity experiment showed that the electronic ratio of NH3 to H2 production reached 0.890. This research offers valuable insights for the design of highly efficient and selective nitrogen-fixing photocatalysts.

Interfacial Engineering of Fluorinated TiO2 Nanosheets with Abundant Oxygen Vacancies for Boosting the Hydrogen Storage Performance of MgH2

The interaction between fluorinated surface in the partially reduced nano-crystallite titanium dioxide (TiO2-x(F)) and MgH2 is studied for the first time. Compared with pristine MgH2 (416 °C), the onset desorption temperature of MgH2+5 wt.% TiO2-x(F) composite can be dramatically lowered to 189 °C. In addition, the composite exhibits remarkable dehydrogenation kinetics, which can release 6.0 wt.% hydrogen thoroughly within 6 min at 250 °C. The apparent activation energy for dehydriding is decreased from 268.42 to 119.96 kJ mol-1. Structural characterization and theoretical calculations indicate that the synergistic effect between multivalent Ti species, and the in situ formed MgF2 and MgF2-xHx is beneficial for improving the hydrogen storage performance of MgH2. Moreover, oxygen vacancies can accelerate the electron transportation and facilitate hydrogen diffusion. The study provides a novel perspective on the modification of MgH2 by fluorinated transition metal oxide catalyst.

Enhanced Photocatalysis of Electrically Polarized Titania Nanosheets

Titania (TiO2) nanosheets are crystals with controlled, highly ordered structures that improve the functionality of conventional TiO2 nanoparticles. Various surface modification methods have been studied to enhance the effectiveness of these materials as photocatalysts. Surface modifications using electrical polarization have attracted considerable attention in recent years because they can improve the function of titania without changing its composition. However, the combination of facet engineering and electrical polarization has not been shown to improve the functionality of TiO2 nanosheets. In the present study, the dye-degradation performance of polarized TiO2 nanosheets was evaluated. TiO2 nanosheets with a F/Ti ratio of 0.3 were synthesized via a hydrothermal method. The crystal morphology and structure were evaluated using transmission electron microscopy and X-ray diffraction. Then, electrical polarization was performed under a DC electric field of 300 V at 300 °C. The polarized material was evaluated using thermally stimulated current measurements. A dye-degradation assay was performed using a methylene blue solution under ultraviolet irradiation. The polarized TiO2 nanosheets exhibited a dense surface charge and accelerated decolorization. These results indicate that electrical polarization can be used to enhance the photocatalytic activity of TiO2.

Li-intercalated CeO2 as an ideal substrate for boosting ammonia synthesis

We present a Li-intercalated-CeO2 catalyst that exhibits outstanding activity for ammonia synthesis. The incorporation of Li significantly reduces the activation energy and suppresses hydrogen poisoning of the Ru co-catalysts. As a result, the lithium intercalation enables the catalyst to achieve ammonia production from N2 and H2 at substantially lower operating temperatures.

Engineering photocatalytic ammonia synthesis

Photocatalytic ammonia synthesis (PAS) is an emerging zero carbon emission technology, which is critical for mitigating energy crises and achieving carbon neutrality. Herein, we summarize the recent advances and challenges in PAS from an engineering perspective based on its whole chain process, i.e., materials engineering, structure engineering and reaction engineering. For materials engineering, we discuss the commonly used photocatalytic materials including metal oxides, bismuth oxyhalides and graphitic carbon nitride and emerging materials, such as organic frameworks, along with the analysis of their characteristics and regulation methods to enhance the PAS performance. For structure engineering, the design of photocatalysts is described in terms of morphology, vacancy and band, corresponding to the crystal, atom and electron scales, respectively. Moreover, the structure-performance relationship of photocatalysts has been deeply explored in this section. For reaction engineering, we identify three key processes from the chemical reaction and mass transfer, i.e., nitrogen activation, molecule transfer and electron transfer, to intensify and optimize the PAS reaction. Hopefully, this review will provide a novel paradigm for the design and preparation of high-efficiency ammonia synthesis photocatalysts and inspire the practical application of PAS.

Dual Charge-Transfer Channels Harmonize Carrier Separation for Efficient U(VI) Photoreduction

The low efficient transfer of photogenerated electrons to an active catalytic site is a pivotal problem for the photoreduction of highly soluble hexavalent uranium [U(VI)] to low soluble tetravalent uranium [U(IV)]. Herein, we successfully synthesized a TiO_2-x/1T-MoS₂/reduced graphene oxide heterojunction (T_2-xTMR) with dual charge-transfer channels by exploiting the difference in Fermi levels between the heterojunction interfaces, which induced multilevel separation of photogenerated carriers. Theoretical and experimental results demonstrate that the presence of the electron buffer layer promoted the efficient migration of photogenerated electrons between the dual charge-transfer channels, which achieved effective separation of photogenerated carriers in physical/spatial dimensions and significantly extended the lifetime of photogenerated electrons. The migration of photogenerated electrons to the active catalytic site after multilevel spatial separation enabled the T_2-xTMR dual co-photocatalyst to remove 97.4% of the high concentration of U(VI) from the liquid-phase system within 80 min. This work provides a practical reference for utilizing multiple co-catalysts to accomplish directed spatial separation of photogenerated carriers.

Efficient full-spectrum driven ammonia synthesis over heterostructured TiO2 nanosheet arrays

Here, we report that efficient photocatalytic ammonia synthesis was realized across the entire solar spectrum by using Ru modified anatase/TiO₂(B) heterostructured nanosheet arrays. The superior NH₃ production rates of 2004 μg h^-1 g^-1 and 521 μg h^-1 g^-1 were achieved under visible light (400 nm) and near-infrared-light (1550 nm) irradiation, respectively.