Unsaturated p-Metal-Based Metal–Organic Frameworks for Selective Nitrogen Reduction under Ambient Conditions

Abstract

Electrochemical ammonia synthesis through the nitrogen reduction reaction (NRR) using renewable electricity has recently attracted significant attention. Of particular importance is the development of efficient electrocatalysts at low costs. Herein, highly selective nitrogen capture using porous aluminum-based metal-organic frameworks (MOFs) materials, MIL-100 (Al), is first designed for the electrochemical NRR in alkaline media under ambient conditions. Owing to the unique structure, MIL-100 (Al) exhibits remarkable electrocatalytic performances (NH3 yield: 10.6 µg h -1 cm -2 mgcat. -1 , Faradaic efficiency: 22.6%) at a low overpotential (177 mV). Investigation indicates that the catalyst shows excellent N2-selective captures due to the unsaturated metal sites binding with N2. More specifically, Al as a main-group metal shows a highly selective affinity to N2 due to the strong interaction between the Al 3p band and N 2p orbitals. The manipulation of multifunctional MOFs delivers both high N2 selectivity and abundant catalytic sites, leading to remarkable efficiency for NH3 production. TOC GRAPHICSAmmonia (NH3) is an important chemical for many applications including pharmaceutical, synthetic fibres, and fertilizer production, while also showing great promise for energy storage systems (i.e. hydrogen (H2) storage). [1][2][3][4] Importantly, it is the only currently known carbon-free energy carrier that does not release carbon dioxide (CO2). Therefore, it is expected to play a significant role in the future hydrogen economy. However, at present, the traditional energy-intensive Haber-Bosch process is used for ammonia production at high temperature and pressure (300-550 °C and 200-350 atm) with substantial greenhouse gas emission. 5 There is a growing interest in developing alternative and sustainable approaches for ammonia synthesis. Up to now, environmentally benign processes such as photocatalysis and electrocatalysis without using hydrogen as a reactant through nitrogen reduction reaction (NRR) process under ambient conditions is a rapidly expanding field of research. [6][7][8][9][10][11] The electrochemical synthesis of ammonia was first discovered in 1807 by Davy et al. who used only water and dissolved air between two gold electrodes. 12 The main challenge for electrochemical method is the slow kinetics of N2 adsorption and subsequent N≡N triple bond cleavage. In addition, due to the standard reduction potential for the hydrogen evolution reaction (HER) (0 V vs. Standard Hydrogen Electrode (SHE)) is close to that of NRR (0.057 V vs. Reversible Hydrogen Electrode (RHE)), the competing HER significantly reduces the Faradaic efficiency (FE) of the NRR. 5 The cell reactors are categorized as either alkaline or acidic systems, but the HER is more dominant in acids as evidenced by the two orders of magnitude higher current density in acidic medium than that in alkaline electrolytes. 13 In this regard, developing efficient electrochemical NRR catalyst in alkaline media is of great significance.A key considerati...

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Metal‐Organic Frameworks Nanocomposites with Different Dimensionalities for Energy Conversion and Storage

“…[ 318–322 ] Attributed to the advantages of highly ordered microporous structure in gas‐phase catalysis, MOFs are favorable for promising electrocatalytic NRR performance. [ 323–325 ] Ling et al. fabricated a ZIF‐coated Ag‐Au platform (Ag‐Au@ZIF) for electrocatalytic NRR.…”

Elemental 2D Materials: Solution‐Processed Synthesis and Applications in Electrochemical Ammonia Production

“…This energy-intensive process accounts for 1-2% of the world's annual energy consumption and is also responsible for 1.2% of global CO 2 emissions. [234][235][236] Therefore, the search for alternative strategies that are efficient and sustainable to replace the traditional Haber-Bosch process is an active area of research.…”