Edith Cowan University Research Online ECU Publications Post 2013 2020 Rational catalyst design for N2 reduction under ambient conditions: Strategies towards enhanced conversion efficiency Lei Shi Edith Cowan University Yu Yin Edith Cowan University Shaobin Wang Hongqi Sun Edith Cowan University Follow this and additional works at: https://ro.ecu.edu.au/ecuworkspost2013 Part of the Engineering Commons 10.1021/acscatal.0c01081 Shi, L., Yin, Y., Wang, S., & Sun, H. (2020). Rational catalyst design for N2 reduction under ambient conditions: Strategies towards enhanced conversion efficiency. ACS Catalysis. 10, 6870 - 6899. https://doi.org/10.1021/ acscatal.0c01081 This Journal Article is posted at Research Online. https://ro.ecu.edu.au/ecuworkspost2013/8184 This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes. pubs.acs.org/acscatalysis Review Rational Catalyst Design for N2 Reduction under Ambient Conditions: Strategies toward Enhanced Conversion Efficiency Lei Shi,* Yu Yin, Shaobin Wang, and Hongqi Sun* Cite This: ACS Catal. 2020, 10, 6870−6899 Read Online ACCESS Metrics & More Article Recommendations ABSTRACT: Ammonia (NH3), one of the basic chemicals in most fertilizers and a promising carbon-free energy storage carrier, is typically synthesized via the Haber−Bosch process with high energy consumption and massive emission of greenhouse gases. The photo/ electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions has attracted increasing interests recently, providing alternative routes to realize green NH3 synthesis. Despite rapid advances achieved in this most attractive research field, the unsatisfactory ffi ffi conversion e ciency including a low NH3 yield rate, and limited Faradaic e ciency or apparent quantum efficiency still remains as a great challenge. The NRR performance is intrinsically related to the electronic and surface structure of catalysts. Rational design and preparation of advanced catalysts are indispensable to improve the performance (e.g., activity and selectivity) of NRR. In this Review, various strategies for the development of desirable catalysts are comprehensively summarized, mainly containing the defect engineering, structural manipulation, crystallographic tailoring, and interface regulation. State-of-the-art heteroge- neous NRR catalysts, prevailing theories and underlying catalytic mechanisms, together with current issues, critical challenges, and perspectives are discussed. It is highly expected that this Review will promote the understanding of recent advances in this area and stimulate greater interests for designing promising NRR catalysts in future. KEYWORDS: nitrogen reduction, ammonia synthesis, catalyst design, electrocatalysis, photocatalysis 1. INTRODUCTION Currently, the industrial production of NH3 is mainly − Ammonia (NH ) is one of the basic chemicals widely used in realized with the Haber Bosch process, at high temperatures 3 (300−600 °C) and pressures (150−300 atm).6 This process the agriculture, and in the chemical industry as an upstream − reagent for producing value-added chemicals.1 As a replace- accounts for 1 2% of the total global energy consumption. In addition to the heavy energy inputs, the used H2 is primarily ment of hydrogen (H2), NH3 containing 17.6 wt % H is also Downloaded via EDITH COWAN UNIV on June 18, 2020 at 01:18:32 (UTC). considered as an important hydrogen carrier that can be easily stemmed from the natural gas reforming, discharging millions stored and transported compared with gaseous H . Particularly, of tons of CO2 each year. Considering these energy and 2 environment costs, it is desirable to develop alternative serving as a carbon-free energy storage intermediate, the ffi See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. complete burning of NH generates nitrogen (N ) and water processes to achieve highly e cient N2 reduction for NH3 3 2 synthesis.7 (H2O), leading to no emission of harmful gases, like carbon monoxide (CO) and greenhouse gases, for instance, carbon In the past few years, emerging photo/electrocatalytic reduction processes have received enormous interests to dioxide (CO2), into the atmosphere. fi directly synthesize NH3 from N2 and H2O at ambient In nature, biological N2 xation occurs under ambient 8,9 conditions (temperature and pressure) with the assistance of conditions. These fascinating catalytic procedures possess nitrogenase enzymes in certain bacteria. As key catalytic many advantages, for example, mild reaction conditions, simple centers, the cofactors like FeMo, FeFe, and FeV in the infrastructures, less environmental pollutions, and renewable enzymes are responsible for binding, activating, and reducing energy consumptions from solar energy or electrical energy provided by solar cells and wind turbines. N2, by consuming the energy from the adenosine triphosphate molecules.2,3 Enlightened by this attractive process, several Received: March 5, 2020 molecular catalysts have been reported to conduct N2 reduction via multiple proton/electron-involved procedures.4,5 Revised: May 19, 2020 However, the ultralow yield rate in the natural process could Published: May 26, 2020 not satisfy the actual demands. Moreover, the poor stability and recycling issues of nitrogenase enzymes and molecular catalysts also restrict their large-scale applications. © XXXX American Chemical Society https://dx.doi.org/10.1021/acscatal.0c01081 6870 ACS Catal. 2020, 10, 6870−6899 ACS Catalysis pubs.acs.org/acscatalysis Review fi Figure 1. (a) Molecular structure of N2. (b) Schematics of N atomic orbitals and N2 molecular orbitals. (c) Simpli ed schematic of N2 bonding to fi 3 TMs. (d) Electronic con guration of pure B atom and B atom with sp hybridization. (e) N2 binding motifs to the B atom on the substrate. Reproduced with permission from ref 12. Copyright 2018 American Chemical Society. a Table 1. Applied Potentials of Hydrogenation Reactions Related to the NRR reaction equilibrium Eo/V +− R1 N6H6e2NH2 ++↔3 +0.55 vs NHE (pH 0) +− R2 2H2eH+↔2 0 vs SHE (pH 0) − − − R3 N22++↔+ 6H O 6e 2NH 3 6OH 0.736 vs SHE (pH 14) − − − R4 2HO22+↔+ 2e H 2OH 0.828 vs SHE (pH 14) − − − − R5 Ne22+↔ N 4.20 vs NHE or 3.37 vs RHE (pH 14) +− − R6 NH22++↔ e NH 3.20 vs RHE +− − R7 N2H2eNH222++↔ 1.10 vs RHE +− − R8 N4H4eNH22++↔4 0.36 vs RHE aNHE: normal hydrogen electrode; SHE: standard hydrogen electrode; RHE: reversible hydrogen electrode. 2. PRINCIPLES AND CHALLENGES IN NITROGEN the negative electron affinity (−1.9 eV), as well as a high REDUCTION REACTION (NRR) ionization potential (15.85 eV) of N2 molecule reduces its 2.1. Thermodynamics of NRR. reactivity; and (iv) even in view of kinetics, the large energy The N2 molecule consists ∼ of two linearly combined N atoms (Figure 1a). Each N atom gap ( 10.82 eV) between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital has a pair of electrons located in the 2s orbital with the 10,11 opposite spin direction and three lone-pair electrons dispersed (LUMO) is not favorable for electron transfer. in 2p orbitals with same spin direction (Figure 1b). After the The primary challenges in the catalytic NRR can be further hybridization of atomic orbitals, new bonding orbitals (σ and π disclosed from the thermodynamic constraints arising from the orbitals) and antibonding orbitals (σ* and π* orbitals) are intermediates. The hydrogenation processes involved in the generated, with the shared electrons in π and 2σ orbitals to NRR and the related equilibrium potentials are provided in 13,14 form a highly intensive triple bond (NN). Attributed to the Table 1. Obviously, these unfavorable potentials for the ffi intrinsically inert properties of N2, its cleavage and reduction at intermediate reactions clarify the thermodynamic di culty for ambient conditions are confronted with several challenges. For N2 hydrogenation. example, (i) the N2 molecule has a high bond energy of 941 kJ 2.2. N2 Molecule Chemisorption and Activation. It is −1 mol , and the thermodynamically strong cleavage energy (410 well-known that the chemisorption of N2 molecules onto the kJ mol−1) of the first bond demonstrates the critical challenge active centers and the subsequent activation with receiving in the NN dissociation; (ii) the first-H addition process with electron from the catalysts are considered as prerequisite steps Δ 0 −1 N2 is endothermic ( H = 37.6 kJ mol ), revealing that direct for NRR. Traditional transition metals (TMs, e.g., Fe, Mo, and protonation process is forbidden in the thermodynamics; (iii) Ti) have been theoretically predicted to strongly interact with 6871 https://dx.doi.org/10.1021/acscatal.0c01081 ACS Catal. 2020, 10, 6870−6899 ACS Catalysis pubs.acs.org/acscatalysis Review Figure 2. Figure 2. (a) Schematic illustrations of the dissociative and associative pathways (including distal, alternating, and enzymatic pathways) for NRR. Reproduced with permission from ref 11. Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Schematic diagram of the photocatalytic H2O splitting and NH3 synthesis. Reproduced with permission from ref 21. Copyright 2017 American Chemical Society. (c) 22 23 24 Electronic energy-level diagram of typical photocatalysts, e.g., Bi5O7Br, CuCr-LDH, and Cu-TiO2, employed for the photocatalytic NH3 synthesis. − fi N2 via the formation of