Photocatalytic reduction of dinitrogen to ammonia over noble-metal-loaded TiO2
TL;DR: The photocatalytic reduction of dinitrogen to ammonia is influenced by the nature and amount of metal loading on TiO2 and the optimum metal content varies depending on the nature of the metal as mentioned in this paper.
Abstract: The photocatalytic reduction of dinitrogen to ammonia is influenced by the nature and amount of metal loading on TiO2. The optimum metal content varies depending on the nature of the metal. A correlation between the ammonia yield and the intermediary MH bond strength is established (low bond strength gives rise to low ammonia yield).
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TL;DR: The field of surface science provides a unique approach to understand bulk, surface and interfacial phenomena occurring during TiO2 photocatalysis as mentioned in this paper, including photon absorption, charge transport and trapping, electron transfer dynamics, adsorbed state, mechanisms, poisons and promoters, and phase and form.
1,768 citations
TL;DR: It is demonstrated that illuminated hydrogen-terminated diamond yields facile electron emission into water, thus inducing reduction of N₂ to NH₃ at ambient temperature and pressure.
Abstract: The photocatalytic reduction of N₂ to NH₃ is typically hampered by poor binding of N₂ to catalytic materials and by the very high energy of the intermediates involved in this reaction. Solvated electrons directly introduced into the reactant solution can provide an alternative pathway to overcome such limitations. Here we demonstrate that illuminated hydrogen-terminated diamond yields facile electron emission into water, thus inducing reduction of N₂ to NH₃ at ambient temperature and pressure. Transient absorption measurements at 632 nm reveal the presence of solvated electrons adjacent to the diamond after photoexcitation. Experiments using inexpensive synthetic diamond samples and diamond powder show that photocatalytic activity is strongly dependent on the surface termination and correlates with the production of solvated electrons. The use of diamond to eject electrons into a reactant liquid represents a new paradigm for photocatalytic reduction, bringing electrons directly to reactants without requiring molecular adsorption to the surface.
777 citations
TL;DR: It is reported that a commercially available TiO2 with a large number of surface oxygen vacancies, when photoirradiated by UV light in pure water with N2, successfully produces NH3.
Abstract: Ammonia (NH3) is an essential chemical in modern society. It is currently manufactured by the Haber–Bosch process using H2 and N2 under extremely high-pressure (>200 bar) and high-temperature (>673 K) conditions. Photocatalytic NH3 production from water and N2 at atmospheric pressure and room temperature is ideal. Several semiconductor photocatalysts have been proposed, but all suffer from low efficiency. Here we report that a commercially available TiO2 with a large number of surface oxygen vacancies, when photoirradiated by UV light in pure water with N2, successfully produces NH3. The active sites for N2 reduction are the Ti3+ species on the oxygen vacancies. These species act as adsorption sites for N2 and trapping sites for the photoformed conduction band electrons. These properties therefore promote efficient reduction of N2 to NH3. The solar-to-chemical energy conversion efficiency is 0.02%, which is the highest efficiency among the early reported photocatalytic systems. This noble-metal-free TiO2 ...
628 citations
TL;DR: In this paper, the state-of-the-art engineering of efficient photocatalysts for dinitrogen (N2) fixation toward NH3 synthesis is reviewed and the challenges, outlooks and future prospects at the forefront of this research platform are presented.
Abstract: 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.
527 citations
TL;DR: In this article, the authors review the history of the photocatalytic nitrogen fixation and examine current progress toward understanding and improving photofixation of nitrogen, supplemented by a quantitative review of the thermodynamic considerations and limitations for various reaction mechanism.
Abstract: 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...
389 citations
References
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TL;DR: In this paper, the dependence of the exchange current for the electrolytic evolution of hydrogen on metals (i 0,H ) on the work function is analyzed on the basic of a new list of polycrystalline surfaces.
1,474 citations
TL;DR: The photolysis of chemisorbed water on incompletely outgassed TiO/sub 2/ powder yields H 2 and O 2 in the molar ratio of 2 : 1 if conducted under argon in the presence of molecular nitrogen as mentioned in this paper.
Abstract: The photolysis of chemisorbed water on incompletely outgassed TiO/sub 2/ powder yields H/sub 2/ and O/sub 2/ in the molar ratio of 2 : 1 if conducted under argon In the presence of molecular nitrogen, O/sub 2/ is still formed but the evolution of H/sub 2/ is inhibited as chemisorbed nitrogen is reduced to NH/sub 3/ and traces of N/sub 2/H/sub 4/ according to N/sub 2/ + 3H/sub 2/O + nhv yields 2NH/sub 3/ + 150/sub 2/ and N/sub 2/ + 2H/sub 2/O + mhv yields N/sub 2/H/sub 4/ + O/sub 2/ Iron doping enhances the photocatalytic reactivity of rutile and provides prototypes of solar cells for photochemical ammonia synthesis from N/sub 2/ and H/sub 2/O
817 citations
TL;DR: In this article, the improvement of H/sub 2/ evolution from two different types of catalytic p-type photocathode surfaces has been examined, and a comparison of the naked p-Si, the simply platinized, and the (PQ/sup 2 +//sup ///sup +/.)sub n/.nPt(0))/sub surf/ system compared to the same surface directly platinised confirm an important difference in the mechanism of H /sub 2 / evolution catalysis for the two surface catalyst systems.
Abstract: The improvement of H/sub 2/ evolution from two different types of catalytic p-type photocathode surfaces has been examined. p-Type Si has been platinized by phtotelectrochemically plating Pt(0) onto the Si surface. Such a photocathode shows significant improvement (compared to naked p-type Si) for photochemical H/sub 2/ evolution with respect to output photovoltage, fill factor, and overall efficiency. Such photocathodes having an optimun amount of Pt(0) give a pH-dependent output voltage with respect to the H/sub 2/O/H/sub 2/ couple, but the dependence is not a simple 59-mV/pH dependence. No pH dependence would be expected if Pt(0) formed a Schottky barrier when plated onto p-type Si. A second kind of H/sub 2/ evolution catalyst has been confined to the surface of p-type Si. Polymeric quantities of an electroactive N,N'-dialkyl-4,4'-bipridinium reagent, (PQ/sup 2 +/.)/sub n/, have been confined to the surface. The Br/sup -/ counterions of the polymer are then exchanged by PtCl/sub 6//sup 2 -/. Photoreduction then yields Pt(0) dispersed in the polymer. Such a surface is again significantly improved compared to naked p-type Si with respect to H/sub 2/ evolution. A comparison of the naked p-Si, the simply platinized, and the (PQ/sup 2 +//sup ///sup +//sub n/.nPt(0))/sub surf./ system is mademore » and contrasted to the expected behavior of an external Schottky barrier photocell driving an electrolysis cell with a Pt cathode. Experiments with n-type MoS/sub 2/, n-type Si, Pt, Au, and W cathodes functionalized with the (PQ/sup 2 +//sup ///sup +/.)sub n/.nPt(0))/sub surf./ system compared to the same surface directly platinized confirm an important difference in the mechanism of H/sub 2/ evolution catalysis for the two surface catalyst systems. p-Type Si modified with optimum amounts of Pt(0) by direct platinization appears to give improved H/sub 2/ evolution efficiency by a mechanism where the Pt(0) serves as a catalyst that does not alter the interface energetics of the semiconductor.« less
318 citations
TL;DR: The quantum yield of hydrogen production was increased greatly by supporting metals or metal complexes on the TiO2 surface, amounting to 38% for a Pt-TiO2 photocatalyst.
218 citations
TL;DR: In this paper, the Fermi level difference of p-InP and H/sup +//H/sub 2/ (0.9 +/- 0.2 eV) was shown to be a function of metal work functions.
Abstract: Noble metal incorporation in the surface of p-type semiconductor photocathodes to catalyze hydrogen evolution leads to efficient solar to chemical conversion if a set of energetic and kinetic criteria are satisfied: (1) the semiconductor-catalyst junction barrier height must be equal to or greater than that of the semiconductor H/sup +//H/sub 2/ junction; (2) the recombination velocity of photogenerated electrons at the semiconductor-catalyst interface must be low; (3) the overpotential for hydrogen evolution at solar cell current densities (approx.30 mA/cm/sup 2/) must be minor. Because of substantial differences in the vacuum work functions of Pt, Rh, Ru, and the (redox potential of the) H/sup +//H/sub 2/ couple, the barrier heights for junctions of each of the four systems with p-InP ought to vary widely. Yet experiments show that all p-InP(M)/H/sup +//H/sub 2/ junctions, where M = Pt, Rh, Ru, or no metal, have essentially the same approx.0.7-V gain in onset potential for hydrogen evolution relative to Pt/H/sup +//H/sub 2/. We attribute the similarity to the known lowering of metal work functions upon hydrogen alloying. Such alloying increases the barrier height and thereby the gain in onset potential over that anticipated from the vacuum work functions. The barrier height, measured as themore » limiting value of onset potential gain at high irradiance, approaches in all cases the Fermi level difference of p-InP and H/sup +//H/sub 2/ (0.9 +/- 0.2 eV). That Fermi level pinning by interfacial states is not the cause of the similar barriers is evident from the reversible decrease in onset potential with hydrogen depletion and by a unity diode perfection factor of the p-InP(Rh)/H/sup +//H/sub 2/ photocathode, which indicates no measurable interfacial recombination of photogenerated carriers. In agreement, the quantum efficiency of carrier collection (hydrogen evolution) nears unity.« less
184 citations