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

Magnetic photocatalysts were synthesized by coating titanium dioxide particles onto colloidal magnetite and nano-magnetite particles. The photoactivity of the prepared coated particles was lower than that of single-phase TiO2 and was found to decrease with an increase in the heat treatment. These observations were explained in terms of an unfavorable heterojunction between the titanium dioxide and the iron oxide core, leading to an increase in electron−hole recombination. Interactions between the iron oxide core and the titanium dioxide matrix upon heat treatment were also seen as a possible cause of the observed low activities of these samples. Other issues considered include the physical and chemical characteristics of the samples, such as surface area and the presence of surface hydroxyl groups. Depending on the calcination conditions, these photocatalysts were found to suffer from varying degrees of photodissolution. Photodissolution tests revealed the greater the extent of the heat treatment, the low...

Novel Photocatalyst: Titania-Coated Magnetite. Activity and Photodissolution

An exhaustive review on the photochemical properties of iron-doped TiO2 semiconductors is presented. Photocatalytic reactions (reductions and oxidations) using Fe-containing TiO2 on different organic and inorganic substrates are reported. Different aspects relating to structural, surface and photophysical properties of these photocatalysts are extensively discussed. The origin of the photoactivity of this kind of mixed oxides is considered with regards to previously proposed physical and chemical processes and on the role of the iron content.

Photocatalytic properties of iron-doped titania semiconductors

Pure and Ag doped TiO2 powders were prepared by the sol-gel process. The effect of Ag doping on TiO2 anatase to rutile phase transformation was investigated by means of XRD, TEM, SEM, DSC-TG, and surface morphological characterization. It is found that the Ag doping promotes the phase transformation but has a depression effect on the anatase grain growth. The mechanism is proposed. With a suitable amount (ca. 2–6 mol%), the Ag dopant reduces anatasegrain size and increases the specific surface area of TiO2 powder, which exhibits a great potential in improving the TiO2 photocatalytic activity.

Effect of silver doping on the phase transformation and grain growth of sol-gel titania powder

Iron-doped titania photocatalysts with different iron contents were prepared by using a sol−gel method in acidic media. The crystalline structures of the various phases calcined at temperatures ranging from 70 to 800 °C were studied by using the Rietveld technique in combination with XRD experiments. The average crystallite size of the phases, lattice cell parameters, phase concentrations, and titanium cationic defects in the crystalline structures of different samples were quantitatively determined. Both iron content and calcination temperature strongly affected phase transformation and solid-state reaction mechanism. Below 400 °C of calcination, all the samples had some brookite and a majority of anatase phase. Iron ions were uniformly distributed in the interstices of titania crystals to form a titanium−iron solid solution when the samples were calcined at 80, 200, and 400 °C. However, when the temperature was 800 °C, Fe2TiO5 was produced in the sample containing 5 wt % Fe by a reaction between interst...

Quantitative Determination of Titanium Lattice Defects and Solid-State Reaction Mechanism in Iron-Doped TiO2 Photocatalysts

Some additional contributions to structural and surface features of powder iron-doped titania photocatalysts have been investigated by applying a variety of complementary techniques: temperature programmed desorption-mass spectrometry analysis, non-isothermal thermogravimetric analysis, differential thermoanalysis, infrared spectroscopy, scanning electron microscopy and X-ray diffraction analysis. The effects of iron concentration and pretreatment temperature, over the polymorphic transformation anatase-rutile, were analysed. The results obtained were used in an attempt to correlate structural and surface aspects of catalysts, with their reported activity versus inactivity to photoreduce dinitrogen to ammonia.

M. Gonzalez-Catalan

Papers

Bulk and surface characterization of powder iron-doped titania photocatalysts