Photo-illuminated diamond as a solid-state source of solvated electrons in water for nitrogen reduction
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.
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TL;DR: It is demonstrated that efficient fixation of N2 to NH3 can proceed under room temperature and atmospheric pressure in water using visible light illuminated BiOBr nanosheets of oxygen vacancies in the absence of any organic scavengers and precious-metal cocatalysts.
Abstract: Even though the well-established Haber–Bosch process has been the major artificial way to “fertilize” the earth, its energy-intensive nature has been motivating people to learn from nitrogenase, which can fix atmospheric N2 to NH3 in vivo under mild conditions with its precisely arranged proteins Here we demonstrate that efficient fixation of N2 to NH3 can proceed under room temperature and atmospheric pressure in water using visible light illuminated BiOBr nanosheets of oxygen vacancies in the absence of any organic scavengers and precious-metal cocatalysts The designed catalytic oxygen vacancies of BiOBr nanosheets on the exposed {001} facets, with the availability of localized electrons for π-back-donation, have the ability to activate the adsorbed N2, which can thus be efficiently reduced to NH3 by the interfacial electrons transferred from the excited BiOBr nanosheets This study might open up a new vista to fix atmospheric N2 to NH3 through the less energy-demanding photochemical process
1,345 citations
TL;DR: This tutorial review gives an overview of the different catalytic systems, highlight the recent breakthroughs, pinpoint common grounds and discuss the bottlenecks and challenges in catalytic reduction of dinitrogen.
Abstract: Ammonia is an important nutrient for the growth of plants. In industry, ammonia is produced by the energy expensive Haber–Bosch process where dihydrogen and dinitrogen form ammonia at a very high pressure and temperature. In principle one could also reduce dinitrogen upon addition of protons and electrons similar to the mechanism of ammonia production by nitrogenases. Recently, major breakthroughs have taken place in our understanding of biological fixation of dinitrogen, of molecular model systems that can reduce dinitrogen, and in the electrochemical reduction of dinitrogen at heterogeneous surfaces. Yet for efficient reduction of dinitrogen with protons and electrons major hurdles still have to be overcome. In this tutorial review we give an overview of the different catalytic systems, highlight the recent breakthroughs, pinpoint common grounds and discuss the bottlenecks and challenges in catalytic reduction of dinitrogen.
1,098 citations
TL;DR: In this article, the rational design of electrocatalysts and photo(electro) catalysts for N2 reduction to NH3 under ambient conditions is highlighted, with a special emphasis on the relationship between their physicochemical properties and NH3 production performance.
Abstract: As one of the most important chemicals and carbon-free energy carriers, ammonia (NH3) has a worldwide annual production of ∼150 million tons, and is mainly produced by the traditional high-temperature and high-pressure Haber–Bosch process which consumes massive amounts of energy. Very recently, electrocatalytic and photo(electro)catalytic reduction of N2 to NH3, which can be performed at ambient conditions using renewable energy, have received tremendous attention. The overall performance of these electrocatalytic and photo(electro)catalytic systems is largely dictated by their core components, catalysts. This perspective for the first time highlights the rational design of electrocatalysts and photo(electro)catalysts for N2 reduction to NH3 under ambient conditions. Fundamental theory of catalytic reaction pathways for the N2 reduction reaction and the corresponding material design principles are introduced first. Then, recently developed electrocatalysts and photo(electro)catalysts are summarized, with a special emphasis on the relationship between their physicochemical properties and NH3 production performance. Finally, the opportunities in this emerging research field, in particular, the strategy of combining experimental and theoretical techniques to design efficient and stable catalysts for NH3 production, are outlined.
1,098 citations
TL;DR: In this paper, the authors summarized the recent progress on the electrochemical nitrogen reduction reaction (NRR) at ambient temperature and pressure from both theoretical and experimental aspects, aiming at extracting instructive perceptions for future NRR research activities.
Abstract: DOI: 10.1002/aenm.201800369 reactions involved.[1] In recent years, tremendous progress has been achieved in the field of heterogeneous electrocatalysis, with rapid development of multifarious electocatalysts toward oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and carbon dioxide reduction reaction (CO2RR). However, electrocatalysts for the reduction of dinitrogen (N2) to ammonia (NH3) at room temperature and atmospheric pressure remain largely underexplored, despite the fact that investigations on catalysts and reaction systems for artificial nitrogen fixation have been continued for more than 100 years.[2–4] Ammonia is primarily used for producing fertilizers to sustain the world’s population.[5] It also serves as a green energy carrier and a potential transportation fuel.[6] Currently, ammonia synthesis is dominated by the industrial Haber–Bosch process using heterogeneous iron-based catalysts at high temperature (300–500 °C) and high pressure (150–300 atm),[7] accounting for more than 1% of the world’s energy supply and generating more than 300 million metric tons of fossil fuel–derived CO2 annually.[8,9] Hence, it is desirable to develop alternative processes that have the potential to overcome the limitations of the Haber–Bosch process including harsh conditions, complex plant infrastructure, centralized distribution, high energy consumption, and negative environmental impacts. In nature, biological N2 fixation occurs under mild conditions via nitrogenase enzymes that contain FeMo, FeV, or FeFe cofactor as catalytic active sites.[10,11] Developed man-made catalysts are therefore stimulated to reduce N2 upon the addition of protons and electrons, which is similar to the nitrogenase catalytic process. Transition metal–dinitrogen complexes such as the molybdenum–, iron–, and cobalt–dinitrogen complexes have been proposed as homogeneous catalysts for the reduction of N2 into NH3 under ambient conditions;[12] however, the stability and recycling issues are challenging.[13] On the other hand, electrochemical and photochemical reduction processes using heterogeneous catalysts benefit from clean and renewable energy sources and are promising for achieving NH3 production directly from N2 and water.[14] The electrochemical reduction of N2 to NH3 can be more efficient than the photochemical counterpart. This is because not all of the photons in the photochemical reduction process can The production of ammonia (NH3) from molecular dinitrogen (N2) under mild conditions is one of the most attractive topics in the field of chemistry. Electrochemical reduction of N2 is promising for achieving clean and sustainable NH3 production with lower energy consumption using renewable energy sources. To date, emerging electrocatalysts for the electrochemical reduction of N2 to NH3 at room temperature and atmospheric pressure remain largely underexplored. The major challenge is to achieve both high catalytic activity and high selectivity. Here, the recent progress on the electrochemical nitrogen reduction reaction (NRR) at ambient temperature and pressure from both theoretical and experimental aspects is summarized, aiming at extracting instructive perceptions for future NRR research activities. The prevailing theories and mechanisms for NRR as well as computational screening of promising materials are presented. State-of-the-art heterogeneous electrocatalysts as well as rational design of the whole electrochemical systems for NRR are involved. Importantly, promising strategies to enhance the activity, selectivity, efficiency, and stability of electrocatalysts toward NRR are proposed. Moreover, ammonia determination methods are compared and problems relating to possible ammonia contamination of the system are mentioned so as to shed fresh light on possible standard protocols for NRR measurements.
848 citations
TL;DR: A protocol for the electrochemical reduction of nitrogen to ammonia enables isotope-sensitive quantification of the ammonia produced and the identification and removal of contaminants, and should help to prevent false positives from appearing in the literature.
Abstract: The electrochemical synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative1–4 to the energy-intensive Haber–Bosch process, which dominates industrial ammonia production. However, there are considerable scientific and technical challenges5,6 facing the electrochemical alternative, and most experimental studies reported so far have achieved only low selectivities and conversions. The amount of ammonia produced is usually so small that it cannot be firmly attributed to electrochemical nitrogen fixation7–9 rather than contamination from ammonia that is either present in air, human breath or ion-conducting membranes9, or generated from labile nitrogen-containing compounds (for example, nitrates, amines, nitrites and nitrogen oxides) that are typically present in the nitrogen gas stream10, in the atmosphere or even in the catalyst itself. Although these sources of experimental artefacts are beginning to be recognized and managed11,12, concerted efforts to develop effective electrochemical nitrogen reduction processes would benefit from benchmarking protocols for the reaction and from a standardized set of control experiments designed to identify and then eliminate or quantify the sources of contamination. Here we propose a rigorous procedure using 15N2 that enables us to reliably detect and quantify the electrochemical reduction of nitrogen to ammonia. We demonstrate experimentally the importance of various sources of contamination, and show how to remove labile nitrogen-containing compounds from the nitrogen gas as well as how to perform quantitative isotope measurements with cycling of 15N2 gas to reduce both contamination and the cost of isotope measurements. Following this protocol, we find that no ammonia is produced when using the most promising pure-metal catalysts for this reaction in aqueous media, and we successfully confirm and quantify ammonia synthesis using lithium electrodeposition in tetrahydrofuran13. The use of this rigorous protocol should help to prevent false positives from appearing in the literature, thus enabling the field to focus on viable pathways towards the practical electrochemical reduction of nitrogen to ammonia. A protocol for the electrochemical reduction of nitrogen to ammonia enables isotope-sensitive quantification of the ammonia produced and the identification and removal of contaminants.
819 citations
References
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TL;DR: Water photolysis is investigated by exploiting the fact that water is transparent to visible light and cannot be decomposed directly, but only by radiation with wavelengths shorter than 190 nm.
Abstract: ALTHOUGH the possibility of water photolysis has been investigated by many workers, a useful method has only now been developed. Because water is transparent to visible light it cannot be decomposed directly, but only by radiation with wavelengths shorter than 190 nm (ref. 1).
27,819 citations
TL;DR: In this article, it is shown that in principle three reference levels can be chosen to measure an absolute value of the electrode potential, and a thermodynamic analysis of the components of the emf of an elec- trochemical cell is shown.
Abstract: The document begins with the illustration of the most widespread misunderstandings in the literature about the physical meaning of absolute electrode potential. The correct expression for this quantity is then de— rived by a thermodynamic analysis of the components of the emf of an elec— trochemical cell. It is shown that in principle three reference levels can be chosen to measure an absolute value of the electrode potential. Only one of these possesses all the requisites for a meaningful comparison on a con— mon energy scale between electrochemical and physical parameters. Such a comparison is the main problem for which the adoption of a correct scale for absolute electrode potentials is a prerequisites. The document ends with the recommendation of a critically evaluated value for the absolute potential of the standard hydrogen electrode in water and in a few other protic solvents. The \"electrode potential\" is often misinterpreted as the electric potential difference between a point in the bulk of the solid conductor and a point in the bulk of the electrolyte solution (L4) (Note a). In reality, the transfer of charged particles across the electrode/electrolyte solution interface is controlled by the difference in the energy levels of the species in the two phases (at constant T and p), which includes not only electrical (electric potential difference) but also chemical (Gibbs energy difference) contributions since the two phases are compositionally dissimilar (refs. 1,2). The value of the tjq of a \"single\" electrode, e.g. one consisting of an electronic conductor in contact with an ionic conductor, is not amenable of direct experimental determination. This is because the two metallic probes from the measuring instruments, both made of the same material, e.g. a metal M1, have to be put in contact with the bulk of these two phases to pick up the signal there. This creates two additional interfaces: a M1/solution interface, and a M1/electrode metal interface. The experimental set-up can be sketched as follows: M1 SIMIMI (1) where M is the metal of the electrode under measure, S is the electrolyte solution, M1 is the metal of the \"connections\" to the measuring instrument and the prime on M indicates that this terminal differs from the other one (M1) by the electrical state only. It is expedient to replace the M1/S interface with a more specific, reproducible and stable system known as the reference electrode. It ensues that an electrode potential can only be measured against a reference system. The measured quantity is thus a relative electrode potential. For the specific example of cell (1), the measured quantity E, the electrode potential of M relative to M1 (Note b), is conventionally split into two contributions, each pertaining to one of the electrodes: EEM_EM1 (2) EM and EM1 can be expressed in their own on a potential scale referred to another reference electrode. In this respect, the hydrogen electrode is conventionally taken as the universal Note a: This quantity, known as the Galvani potential difference between M and 5, has been defined in ref. 3. Note b: In accord with the IUPAC convention on the sign of electrode potentials, all electrode potentials in this document are to be intended as \"reduction potentials\", i.e. the electrode reaction is written in the direction of the reduction (refs. 3,4). 956 Absolute electrode potential (Recommendations 1986) 957 (for solutions in protic solvents) reference electrode for which, under standard conditions, E°(H/H2) = 0 at every temperature (Note c). Since EM as measured is a relative value, it appeals to many to know what the absolute value may be: viz. , the value of EM measured with respect to a universal reference system not including any additional metal/solution interface. Actually, for the vast majority of practical electrochenilcal problems, there is no need to bring in absolute potentials . The one outstanding example where this concept is useful is the matching of semiconductor energy levels and solution energy levels . However, from a fundamental point of view, this problem comes necessarily about in every case one wants to connect the \"relative\" electrode potential to the \"absolute\" physical quantities of the given system. On a customary basis, since the electrode potential is envisaged as the electric potential drop between M and S, the cell potential difference for system (1) is usually written as the electric potential difference between the two metallic terminals: EMi M1 (3) Since three interfaces are involved in cell (1), eqn.(3) can be rewritten as: E (M{ M) + (M S) + (S Mi) (4) Comparison of eqn. (4) with eqn. (2) shows that the identification of the absolute electrode potential with (M S) is not to be reconmended because it is conceptually misleading. Since M' and M are in electronic equilibrium, then (ref. 3): (4M ) = ('/F pr/F) (5) where the right hand side of eqn. (5) expresses the difference in chemical potential of electrons in the two electrode metals. Substitution of eqn.(5) into eqn.(4) gives: E = (p ii'/F) (E'q p'/F) (6) The two exoressions in brackets do not contain quantities pertaining to the other interfaces. They can thus be defined as single electrode potentials (Note d). Since eqn. (6) has been obtained with the two electrodes assembled into a cell, it is possible that terms common to both electrodes do not appear explicitly in eqn. (6) because they cancel out ultimately. The relationship between the truly absolute electrode potential and the single electrode potential in eqn.(6) can thus be written in the form (Note e) (ref. 5): EM(abs) = EM(r) + K (7) where K is a constant depending on the \"absolute\" reference system, and
1,205 citations
TL;DR: In this paper, the authors explored the catalytic reduction of dinitrogen by molybdenum complexes that contain the [HIPTN3N]3- ligand.
Abstract: This Account explores the catalytic reduction of dinitrogen by molybdenum complexes that contain the [HIPTN3N]3- ligand ([HIPTN3N]3- = [(HIPTNCH2CH2)3N]3-, where HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3) at room temperature and pressure with protons and electrons. A total of 7−8 equiv of ammonia is formed out of ∼12 possible (depending upon the Mo derivative employed). No hydrazine is formed. Numerous X-ray studies of proposed intermediates in the catalytic cycle suggest that N2 is being reduced at a sterically protected, single Mo center operating in oxidation states between MoIII and MoVI. Subtle variations of the [HIPTN3N]3- ligand are not as successful as a consequence of an unknown shunt in the catalytic cycle that consumes reduction equivalents to yield (it is proposed) dihydrogen.
1,149 citations
IBM1
TL;DR: In this paper, the secondary-electron energy distributions were analyzed for an unreconstructed diamond (111) surface (type-$\mathrm{II}b), gem-quality blue-white semiconductor).
Abstract: Quantum photoyield and secondary-electron distributions are presented for an unreconstructed diamond (111) surface (type-$\mathrm{II}b$, gem-quality blue-white semiconductor). This chemically inert surface exhibits a negative electron affinity, resulting in a stable quantum yield that increases linearly from photothreshold (5.5 eV) to \ensuremath{\sim}20% at 9 eV, with a very large yield of \ensuremath{\sim}40%-70% for $13\ensuremath{\lesssim}h\ensuremath{
u}\ensuremath{\lesssim}35$ eV. For all photon energies, secondary-electron energy distributions show a dominant \ensuremath{\sim}0.5-eV-wide emission peak at the conduction-band minimum (${\ensuremath{\Delta}}_{1}^{min}=5.50\ifmmode\pm\else\textpm\fi{}0.05$ eV above the valence-band maximum ${{\ensuremath{\Gamma}}_{25}}^{\ensuremath{'}}$). In contrast with recent self-consistent calculations [J. Ihm, S. G. Louie, and M. L. Cohen, Phys. Rev. B 17, 769 (1978)] no occupied intrinsic surface states with ionization energies in the fundamental gap (the Fermi level was 1 eV above ${{\ensuremath{\Gamma}}_{25}}^{\ensuremath{'}}$) were observed. Likewise, the measured photothreshold (${E}_{\mathrm{vac}}\ensuremath{-}{{\ensuremath{\Gamma}}_{25}}^{\ensuremath{'}}$) is significantly smaller than calculated (7.0\ifmmode\pm\else\textpm\fi{}0.7 eV).
959 citations
TL;DR: The data and features that have been added or replaced since the previous edition of HITRAN are described and instances of critical data that are forthcoming are cited.
Abstract: Nineteen ninety-eight marks the 25th anniversary of the release of the first HITRAN database. HITRAN is recognized as the international standard of the fundamental spectroscopic parameters for diverse atmospheric and laboratory transmission and radiance calculations. There have been periodic editions of HITRAN over the past decades as the database has been expanded and improved with respect to the molecular species and spectral range covered, the number of parameters included, and the accuracy of this information. The 1996 edition not only includes the customary line-by-line transition parameters familiar to HITRAN users, but also cross-section data, aerosol indices of refraction, software to filter and manipulate the data, and documentation. This paper describes the data and features that have been added or replaced since the previous edition of HITRAN. We also cite instances of critical data that is forthcoming. A new release is planned for 1998.
828 citations