Industrial Haber-Bosch Process Research Articles

(Invited) Boron and Nitrogen Co-Doped Porous Carbon Nanofibers As Metal-Free Electrocatalysts for Highly Efficient Ammonia Electrosynthesis

NH3 synthesis via the electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions has been regarded as a sustainable strategy for fertilizer production, hydrogen fuel storage, and mitigating greenhouse gas emissions. Electrochemical reduction reaction of nitrogen to ammonia has been widely investigated as a potential technology to overcome thermodynamic barrier of high-temperature and high-pressure needed for industrial Haber-Bosch process. The design of construction of heteroatoms (e.g., B, N, P, S) doped carbon materials have become a research focus as metal-free electrocatalysts owing to their good electrochemical stability and high electronic conductivity. N-doping carbon materials could induce the protonated pyridinic N and adjacent atomic vacancy site, which are favor of N2 adsorption. Besides, on account of charge density difference, B-doping carbon materials could induce inhomogeneity for carbon to adsorb N2 molecules. Compared with single heteroatom dopant, heteroatom co-doping could bring in charge redistribution of carbon, which can facilitate the chemisorption of N2 molecules and following electron transfer.Herein, we present a metal-free B and N co-doped porous carbon nanofibers (B/N-CNF) fabricated by an electrospinning and calcination strategy. The FESEM image of B/N-CNF showed the typical morphology of the cross-linked carbon networks composed of nanofibers with a diameter of ~100 nm. The contents of B and N dopant for B/N-CNF were 29.9% and 27.6%, respectively. The N2 adsorption-desorption isotherms curve of B/N-CNF exhibited a high BET specific surface area of 748 m2 g-1 and a mesopores structure with pore diameter of around 4 nm and volume of ~0.39 cm3 g-1. Benefitting from a high surface area and synergistic effect between B and N dopants, the B/N-CNF exhibited a high electrocatalytic activity for NRR in a basic electrolyte, achieving the highest FE of 13.2% at -0.5 V and the NH3 yield rate of 32.5 μg h-1 mg-1 cat. at -0.7 V. Such a high NRR activity was superior to most of the previously reported B- or N- doped carbon materials. It was found that B atoms facilitated the cleavage of N≡N bonds, and the N atoms enhanced the electronic conductivity of CNF nanofibers.The B/N-CNF delivered superior catalytic activity for electrochemical NRR, featured by the highest FE of 13.2% at -0.5 V and NH3 yield of 32.5 μg h-1 mg-1 cat. at -0.7 V in basic electrolytes. The high NRR performance could be attributed to the synergistic effect between B and N co-dopants. The FE value of B/N-CNF was almost the highest among all previously reported carbon-based materials with single N- or B-doping. Moreover, an electrolyzer with B/N-CNF as the cathode and commercial Ir/C as the anode was successfully fabricated to confirm the NRR activity of B/N-CNF driven by alkaline batteries and solar panels. The developed B/N-CNF presented in this work paves a promising pathway for rational design of highly efficient dual heteroatoms doped carbon materials for not only NRR, but other electrochemical energy conversion reactions.

Self-supporting CoMoC nanoporous catalysts for N2 reduction reaction under ambient conditions

Electrochemical nitrogen reduction is expected to replace the industrial Haber-Bosch process due to its environment-friendly and sustainable development route, while the main challenge is to develop active and selective N2 reduction reaction (NRR) electrocatalysts. Here, we prepare the self-supporting CoMoC nanoporous catalyst by electrochemical dealloying method. Due to the nanoporous structure and MoC compositions, the CoMoC catalyst exhibits high catalytic activity and outstanding selectivity to NRR. The intimate between Co and MoC favors in electron transport, resulting in promoting the activity of the whole nitrogen reduction reaction process. In 0.1 M PBS (PH = 7), the NH3 yield rate and faradaic efficiency (FE) at 0.1 V (vs. RHE) are 18.9 ug cm−2 h−1 and 8.9%, respectively. This work opens up new opportunities to develop self-supporting CoMoC nanoporous catalysts with good conductivity and activity for NRR.

Low-Coordinate Step Atoms via Plasma-Assisted Calcinations to Enhance Electrochemical Reduction of Nitrogen to Ammonia.

The electrochemical N2 reduction reaction (NRR) is emerging as a promising alternative to the industrial Haber-Bosch process for distributed and modular production of NH3 . Nevertheless, developing high-efficiency catalysts to simultaneously realize both high activity and selectivity for the development of a sustainable NRR is very critical but extremely challenging. Here, a unique plasma-assisted strategy is developed to synthesize iridium diphosphide nanocrystals with abundant surface step atoms anchored in P,N-codoped porous carbon nanofilms (IrP2 @PNPC-NF), where the edges of the IrP2 nanocrystals are extremely irregular, and the ultrathin PNPC-NF possesses a honeycomb-like macroporous structure. These characteristics ensure that IrP2 @PNPC-NF delivers superior NRR performance with an NH3 yield rate of 94.0 µg h-1 mg-1 cat. and a faradaic efficiency (FE) of 17.8%. Density functional theory calculations reveal that the unique NRR performance originates from the low-coordinate step atoms on the edges of IrP2 nanocrystals, which can lower the reaction barrier to improve the NRR activity and simultaneously inhibit hydrogen evolution to achieve a high FE for NH3 formation. More importantly, such a plasma-assisted strategy is general and can be extended to the synthesis of other high-melting-point noble-metal phosphides (OsP2 @PNPC-NF, Re3 P4 @PNPC-NF, etc.) with abundant step atoms at lower temperatures.

CoS2 Nanoparticles-Embedded N-Doped Carbon Nanobox Derived from ZIF-67 for Electrocatalytic N2-to-NH3 Fixation under Ambient Conditions

NH3 is an indispensable chemical with numerous applications; however, its large-scale synthesis highly relies on the industrial Haber–Bosch process, which is performed under harsh conditions (high ...

Understanding of nitrogen fixation electro catalyzed by molybdenum–iron carbide through the experiment and theory

The electrochemical method is considered being a sustainable alternative to the industrial Haber-Bosch process (150–350 atm, 350–550 °C) because it can produce ammonia (NH3) from nitrogen (N2) and water (H2O) at room temperature and pressure. However, since the N≡N triple bond in N2 is one of the strongest bonds in nature, it requires a more negative potential for N2 reduction, which often leads to violent hydrogen evolution reaction (HER) in aqueous electrolysis systems. Therefore, it is a great challenge for the electrocatalytic N2 reduction reaction (ENRR) to find catalysts that can reduce the energy barrier of N2 fixation and inhibit the HER. Herein, inspired by the Mo–Fe site in the biological nitrogenase, we found that the catalyst containing Mo3Fe3C active material has excellent N2-fixing catalytic performance and can effectively inhibit the HER. At −0.05 V vs RHE, the Faraday efficiency (FE) of ENRR was as high as 27.0%. In addition, we innovatively used the Fourier-transformed alternating current voltammetry (FTACV) to explore the electron transfer process in ENRR, indicating that Mo3Fe3C is more conducive to reducing N2 at low potential. According to density functional theory (DFT) calculations, compared with Mo2C and Fe3C, Mo3Fe3C is more helpful in promoting N2 activation and hydrogenation. Due to the synergistic effect of the Mo–Fe site, N2 hydrogenation needs to overcome a lower energy barrier in potential-determining step (PDS). Our research extends the knowledge into bimetallic active sites in ENRR and provides a new insight for the subsequent synthesis of high selectivity catalysts.

Identification of a uranium–rhodium triple bond in a heterometallic cluster

The chemistry of d-block metal-metal multiple bonds has been extensively investigated in the past 5 decades. However, the synthesis and characterization of species with f-block metal-metal multiple bonds are significantly more challenging and such species remain extremely rare. Here, we report the identification of a uranium-rhodium triple bond in a heterometallic cluster, which was synthesized under routine conditions. The uranium-rhodium triple-bond length of 2.31 Å in this cluster is only 3% longer than the sum of the covalent triple-bond radii of uranium and rhodium (2.24 Å). Computational studies reveal that the nature of this uranium-rhodium triple bond is 1 covalent bond with 2 rhodium-to-uranium dative bonds. This heterometallic cluster represents a species with f-block metal-metal triple bond structurally authenticated by X-ray diffraction. These studies not only demonstrate the authenticity of the uranium-metal triple bond, but also provide a possibility for the synthesis of other f-block metal-metal multiple bonds. We expect that this work may further our understanding of the bonding between uranium and transition metals, which may help to design new d-f heterometallic catalysts with uranium-metal bonds for small-molecule activation and to promote the utilization of abundant depleted uranium resources.

(Invited) Plasmon-Driven Chemical Reactions

Plasmonic metal nanocrystals can interact strongly with light, efficiently converting light into heat and generating hot charge carriers. Both plasmonic photothermal conversion and hot carrier generation can accelerate chemical reactions. The use of plasmons to drive chemical reactions has recently become a new and active research field (Adv. Mater. 2014, 26, 5274). Plasmonic hot charge carriers can not only enhance the reaction yield and selectivity, but also introduce new reaction pathways. The lifetime of plasmonic hot charge carriers is on the femtosecond scale. Without immediate usage, they will rapidly relax, converting their energy into heat. We have employed two approaches to enable the use of plasmonic hot charge carriers. The first is the integration of Au or Ag nanoparticles with Pd or Pt nanoparticles. We have applied this approach to enhance Suzuki coupling reactions by localized plasmons (J. Am. Chem. Soc. 2013, 135, 5588; Adv. Funct. Mater. 2017, 27, 1700016). The second is the integration of plasmonic metals with semiconductors. A barrier is formed at the interface. The generated hot electrons and holes can quickly inject into the conduction or valence band and therefore get separated. The injected charge carriers can then drive chemical reactions. We have synthesized Au/TiO2 (Energy Environ. Sci. 2014, 7, 3431; J. Am. Chem. Soc. 2018, 140, 8497), Au/CeO2 (ACS Nano 2014, 8, 8152) and Au/BiOCl hybrid structures (J. Am. Chem. Soc. 2017, 139, 3513) as plasmonic photocatalysts to drive the photo-generation of reactive oxygen species, the selective oxidation of alcohols, and N2 photofixation. Specifically, oxygen vacancies have been introduced in the metal oxides to “work in-tandem” together with plasmonic hot charge carriers. For example, during the use of Au/BiOCl hybrid structures for the photocatalytic selective oxidation of benzyl alcohol, oxygen vacancies on BiOCl facilitate the trapping and transfer of plasmonic hot electrons to adsorbed O2, producing superoxide anion radicals, while plasmonic hot holes remaining on the Au surface mildly oxidize benzyl alcohol to corresponding carbon-centered radicals. The ring addition between these two radical species leads to the production of benzaldehyde along with an unexpected oxygen atom transfer from O2 to the product. In contrast, the oxygen atom in the product is usually from the benzyl alcohol reactant. In another example, Au nanoparticles are anchored on ultrathin TiO2 nanosheets with oxygen vacancies. The oxygen vacancies on the nanosheets chemisorb and activate N2 molecules, which are subsequently reduced to ammonia by hot electrons generated from plasmon excitation of the Au nanoparticles. The hybrid photocatalyst can accomplish photodriven N2 fixation in the “working-in-tandem” pathway at room temperature and atmospheric pressure. The apparent quantum efficiency of 0.82% at 550 nm for the conversion of incident photons to ammonia is higher than those reported so far. The oxygen vacancies play three roles. They act as the active sites for N2 molecules; they cause defect states within the bandgap for trapping plasmonic hot electrons and therefore lengthening their lifetime; and they function as a bridge between plasmonic hot electrons and the activated N2 molecules. This work offers a new approach for the rational design of efficient catalysts towards sustainable N2 fixation through a less energy-demanding photochemical process compared to the industrial Haber-Bosch process.

Promoting nitrogen electroreduction to ammonia with bismuth nanocrystals and potassium cations in water

The electrochemical nitrogen reduction reaction (ENRR) can allow the production of ammonia from nitrogen and water under ambient conditions and is regarded as a sustainable alternative to the industrial Haber–Bosch process. However, electrocatalytic systems that selectively and efficiently catalyse nitrogen reduction remain elusive due to the strong competition with the hydrogen evolution reaction. Here, we report a strategy to simultaneously promote ENRR selectivity and activity using bismuth nanocrystals and potassium cations. Bismuth exhibits higher intrinsic ENRR activity than transition metals due to the strong interaction between the Bi 6p band and the N 2p orbitals. Potassium cations stabilize key nitrogen-reduction intermediates and regulate proton transfer to increase the selectivity. A high Faradaic efficiency of 66% and ammonia yield of 200 mmol g–1 h–1 (0.052 mmol cm–2 h–1) are obtained in aqueous electrolyte under ambient conditions. This strategy represents a general method to expand the library of catalysts and promoters for the selective electrochemical reduction of stable molecules. The electrochemical reduction of nitrogen to ammonia represents a challenge of major interest that would substantially decrease the burden of the energy-consuming Haber–Bosch process. Now, Yin, Yan, Zhang, Si and colleagues achieve high ammonia yield and Faradaic efficiency over 66% using bismuth nanocatalysts promoted by alkali cations.

Carbon-Based Metal-Free Catalysts for Electrocatalytic Reduction of Nitrogen for Synthesis of Ammonia at Ambient Conditions.

The electrocatalytic nitrogen reduction reaction (NRR) is a promising catalytic system for N2 fixation in ambient conditions. Currently, metal-based catalysts are the most widely studied catalysts for electrocatalytic NRR. Unfortunately, the low selectivity and poor resistance to acids and bases, and the low Faradaic efficiency, production rate, and stability of metal-based catalysts for NRR make them uncompetitive for the synthesis of ammonia in comparison to the industrial Haber-Bosch process. Inspired by applications of carbon-based metal-free catalysts (CMFCs) for the oxygen reduction reaction (ORR) and CO2 reduction reaction (CO2 RR), the studies of these CMFCs in electrocatalytic NRR have attracted great attention in the past year. However, due to the differences in electrocatalytic NRR, there are several critical issues that need to be addressed in order to achieve rational design of advanced carbon-based metal-free electrocatalysts to improve activity, selectivity, and stability for NRR. Herein, the recent developments in the field of carbon-based metal-free NRR catalysts are presented, along with critical issues, challenges, and perspectives concerning metal-free catalysts for electrocatalytic reduction of nitrogen for synthesis of ammonia at ambient conditions.

Electrochemical Fabrication of Porous Au Film on Ni Foam for Nitrogen Reduction to Ammonia.

Electrochemical reduction of N2 to NH3 provides an alternative strategy to replace the industrial Haber-Bosch process for facile and sustainable production of NH3 . The development of efficient electrocatalysts for the nitrogen reduction reaction (NRR) is highly desired. Herein, a micelle-assisted electrodeposition method is presented for the direct fabrication of porous Au film on Ni foam (pAu/NF). Benefiting from its interconnected porous architectonics, the pAu/NF exhibits superior NRR performance with a high NH3 yield rate of 9.42 µg h-1 cm-2 and a superior Faradaic efficiency of 13.36% at -0.2 V versus reversible hydrogen electrode under the neutral electrolyte (0.1 m Na2 SO4 ). The proposed micelle-assisted electrodeposition strategy is highly valuable for future design of active NRR catalysts with desired compositions toward various electrocatalysis fields.

The role of bridging ligands in dinitrogen reduction and functionalization by uranium multimetallic complexes.

Cooperativity between metal centres is identified as a crucial step in dinitrogen reduction both for the industrial Haber-Bosch process and for the natural fixation of nitrogen by nitrogenase enzymes, but the mechanism of N2 reduction remains poorly understood. This is in large part because multimetallic complexes that reduce and functionalize dinitrogen in the absence of strong alkali reducing agents are crucial to establish a structure-activity relationship, but remain extremely rare. Recently, we reported a multimetallic nitride-bridged diuranium(III) complex capable of reducing and functionalizing dinitrogen. Here we show that an analogous complex assembled with an oxo instead of a nitride linker also effects the four-electron reduction of dinitrogen, but the reactivity of the resulting oxo-(N2) complex differs significantly from that of the nitride-(N2). Computational studies show a different bonding scheme for the dinitrogen where the bridging nitride does participate in the binding and consequent activation of N2, while the oxide does not.

High-Efficiency "Working-in-Tandem" Nitrogen Photofixation Achieved by Assembling Plasmonic Gold Nanocrystals on Ultrathin Titania Nanosheets.

The fixation of atmospheric N2 to NH3 is an essential processfor sustaining life. One grand challenge is to develop efficient catalysts to photofix N2 under ambient conditions. Herein we report an all-inorganic catalyst, Au nanocrystals anchored on ultrathin TiO2 nanosheets with oxygen vacancies. It can accomplish photodriven N2 fixation in the "working-in-tandem" pathway at room temperature and atmospheric pressure. The oxygen vacancies on the TiO2 nanosheets chemisorb and activate N2 molecules, which are subsequently reduced to NH3 by hot electrons generated from plasmon excitation of the Au nanocrystals. The apparent quantum efficiency of 0.82% at 550 nm for the conversion of incident photons to NH3 is higher than those reported so far. Optimizing the absorption across the overall visible range with the mixture of Au nanospheres and nanorods further enhances the N2 photofixation rate by 66.2% in comparison with Au nanospheres used alone. This work offers a new approach for the rational design of efficient catalysts toward sustainable N2 fixation through a less energy-demanding photochemical process compared to the industrial Haber-Bosch process.

Advancing Ammonia Synthesis through Plasma-Assisted Catalysis

Plasmas can enhance the selectivity, activity, and product yields for many chemical reactions. In the presence of a catalyst, synergy between the catalyst and the plasma exists to assist in the activation of reactants to promote the chemical transformation. We have been particularly interested in the use of non-thermal plasmas for the catalytic production of ammonia from nitrogen/hydrogen gas mixtures. The industrial Haber-Bosch process for ammonia synthesis from nitrogen and hydrogen is carried out at high pressures (100-200 bar) and temperatures (400-500 °C). N2 dissociation is the fundamental barrier for this reaction, and plasma-catalysis has been suggested as viable way to activate N2 under ambient conditions. We have demonstrated that is possible to produce ammonia at atmospheric pressure and temperatures between 100 - 200 °C by coupling the catalyst with a non-thermal plasma. It is speculated that the plasma assists in activating the source gas by generating reactive species, such as vibrationally or electronically excited N2*, ions, and radicals, which interact on the catalyst surface to produce ammonia. In this presentation, we will discuss our experimental results from varying the gas composition, bulk gas temperature, plasma input power, and reactor space velocity and how these conditions influence the catalyst-plasma interactions using a variety of catalysts (Fe/Al2O3, Ru/Al2O3, Pt/Al2O3, Ni/Al2O3, and Co/Al2O3).

Nitrogen reduction and functionalization by a multimetallic uranium nitride complex.

Molecular nitrogen (N2) is cheap and widely available, but its unreactive nature is a challenge when attempting to functionalize it under mild conditions with other widely available substrates (such as carbon monoxide, CO) to produce value-added compounds. Biological N2 fixation can do this, but the industrial Haber-Bosch process for ammonia production operates under harsh conditions (450 degrees Celsius and 300 bar), even though both processes are thought to involve multimetallic catalytic sites. And although molecular complexes capable of binding and even reducing N2 under mild conditions are known, with co-operativity between metal centres considered crucial for the N2 reduction step, the multimetallic species involved are usually not well defined, and further transformation of N2-binding complexes to achieve N-H or N-C bond formation is rare. Haber noted, before an iron-based catalyst was adopted for the industrial Haber-Bosch process, that uranium and uranium nitride materials are very effective heterogeneous catalysts for ammonia production from N2. However, few examples of uranium complexes binding N2 are known, and soluble uranium complexes capable of transforming N2 into ammonia or organonitrogen compounds have not yet been identified. Here we report the four-electron reduction of N2 under ambient conditions by a fully characterized complex with two Uiii ions and three K+ centres held together by a nitride group and a flexible metalloligand framework. The addition of H2 and/or protons, or CO to the resulting complex results in the complete cleavage of N2 with concomitant N2 functionalization through N-H or N-C bond-forming reactions. These observations establish that a molecular uranium complex can promote the stoichiometric transformation of N2 into NH3 or cyanate, and that a flexible, electron-rich, multimetallic, nitride-bridged core unit is a promising starting point for the design of molecular complexes capable of cleaving and functionalizing N2 under mild conditions.

Buckminsterfullerenes: a non-metal system for nitrogen fixation.

In all nitrogen-fixation processes known so far--including the industrial Haber-Bosch process, biological fixation by nitrogenase enzymes and previously described homogeneous synthetic systems--the direct transformation of the stable, inert dinitrogen molecule (N2) into ammonia (NH3) relies on the powerful redox properties of metals. Here we show that nitrogen fixation can also be achieved by using a non-metallic buckminsterfullerene (C60) molecule, in the form of a water-soluble C60:gamma-cyclodextrin (1:2) complex, and light under nitrogen at atmospheric pressure. This metal-free system efficiently fixes nitrogen under mild conditions by making use of the redox properties of the fullerene derivative.

METAL-DINITROGEN COMPLEXES

Two unique metal-dinitrogen complexes are offering new perspectives on the important phenomena of dinitrogen bonding to metal centers. Chemists at the University of Missouri, Columbia, have created an unusual dinitrogen-bridged gold cluster, while researchers at the University of British Columbia, Vancouver, have made a dinitrogen-zirconium complex with singular reactivity toward hydrogen. Metal-dinitrogen complexes are the key to nitrogen fixation, the catalytic reduction of molecular nitrogen (N 2 ) to ammonia. Nature easily fixes N 2 with enzymes called nitrogenases that are found in certain bacteria. Humans have a harder time accomplishing the same feat: The industrial Haber-Bosch process requires high pressure and temperature to convert N 2 and hydrogen to ammonia over an iron catalyst. Neither the mechanism of the enzymatic reaction nor the mechanism of the industrial process is clearly understood. Now, Missouri chemistry professor Paul R. Sharp and graduate students Hui Shan, Yi Yang, and Alan J. J...