Molybdenum-Based Catalysts for Electrochemical Nitrogen Reduction to Ammonia under Ambient Conditions

鉬基催化劑常溫常壓下電催化還原氮氣合成氨性能研究

Student thesis: Doctoral Thesis

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Award date30 Sep 2020

Abstract

Ammonia (NH3) is considered as a green energy carrier and a potential transportation fuel. Nitrogen fixation, the reduction of atmospheric nitrogen (78% of the atmosphere) to NH3, is an attractive route to producing ammonia. For more than a century, NH3 production dominantly relies on the Haber–Bosch process in industrial-scale. This process requires operation at high temperatures and pressures with the assistance of catalysts, giving rise to the mass consumption of energy and pure hydrogen as well as greenhouse-gas emissions. In the aspect of sustainable development, electrochemical nitrogen-reduction reaction (e-NRR) is an attractive strategy because it can be powered by renewable electricity and derived from N2 and H2O at room temperature and atmospheric pressure. However, e-NRR is hindered by the sluggish cleavage of N≡N bonds and poor selectivity of e-NRR over the competitive hydrogen evolution reaction (HER). Accordingly, for efficient electrocatalytic NH3 generation, active catalysts need to be developed under low overpotential. Non-noble metal molybdenum (Mo) is a key element in the highly abundant nitrogenase for biological N2 fixation. Due to their optimized N adsorption energy and favorable electronic configuration to activate N≡N bonds, Mo-based catalysts with favorable performance deserve further exploration according to rational design. Therefore, the design, fabrication, characterization and performance of functional Mo-based electrocatalysts were systematically studied for e-NRR, including molybdenum carbide, bimetallic molybdenum nanoparticle and bimetallic molybdenum oxide.

Molybdenum carbide (Mo2C) with unoccupied d orbitals has great potential to activate the N2 molecule. Herein, Mo2C on nitrogen-doped carbon (denoted as Mo2C/NC) was prepared directly derived from Mo/Zn-doped zeolitic imidazolate frameworks (ZIFs) precursors, as an efficient e-NRR electrocatalyst under ambient conditions. In 0.10 M Na2SO4 electrolyte, this catalyst achieved a maximum NH3 yield rate of 70.60 μmol h–1 gcat.–1 and a Faradaic efficiency (FE) of 12.3% at –0.20 V vs. RHE. The NH3 yield of Mo2C/NC was superior to that of nitrogen-doped carbon (NC), confirming that Mo2C formation is responsible for enhanced e-NRR. Additionally, Mo2C/NC displayed high electrochemical selectivity and stability, attributed to the structural and electrochemical stability of ZIFs-derived carbon framework and Mo2C. This work provides new perspectives upon metal carbides and their compounds as catalysts for efficient e-NRR.

Up to now, no study has been conducted on the use of non-noble bimetals on nitrogen-doped carbon as efficient e-NRR electrocatalysts, considered from either side of the theoretical volcano plot for e-NRR. Herein, Mo-Co bimetallic nanoparticles anchored onto ZIFs-derived NC (Mo-Co/NC) were developed for the first time and utilized as a cost-effective catalyst for e-NRR. Compared with the single-metallic Co/NC, the bimetallic Mo-Co/NC catalyst exhibited enhanced activity and selectivity of e-NRR with an NH3 yield of 89.80 μmol h–1 gcat.–1 and an FE of 13.50% at a low operating potential of –0.10 V (vs. RHE) in 0.10 M Na2SO4. The relationship between nanoparticle size of Mo-Co and e-NRR activity was also explored, by tuning the zinc dopant content in predesigned bimetallic Mo-Co-Zn/ZIFs. Moreover, Mo-Co/NC showed high electrochemical stability and selectivity during e-NRR for 50000 s. Such favorable electrocatalytic activity is due to the synergistic effect of the strongly adsorbed Mo and the weakly adsorbed Co composite, large electrochemical active surface area, and conductive carbon frameworks. This work provides an easy-operating strategy towards the design of nonprecious metallic electrocatalysts for efficient e-NRR under ambient conditions.

Lastly, inspired by the advantages of the Mo-Co synergistic effect, the incorporation of Co into MoO3 to produce bimetallic oxide catalyst can be a feasible way towards e-NRR with inhibited HER, which has not yet been previously reported. Accordingly, an easy operating, hydrothermal and low-temperature oxidization strategy was used to fabricate CoMoO4 nanorod (CMO-NR) as a noble-metal-free e-NRR electrocatalyst. The bimetallic CMO-NR electrocatalyst achieved higher FE (22.76%) and NH3 yield rate (79.87 μmol h-1 gcat.-1) in 0.10 M Na2SO4 electrolyte than those of monometallic counterparts (CoO and MoO3 nanorods). Moreover, long-term electrochemical durability was also achieved over CMO-NR during e-NRR. This work can serve as a reference for establishing methods of designing bimetallic oxides to apply in catalyzing e-NRR.