Short communicationNitrogen electroreduction on two-dimensional π-conjugated metal bis(dithiolene) complex nanosheets: A density functional theory study
Graphical abstract
Introduction
As an important chemical raw material, ammonia (NH3) is widely used in various industries and is closely related to our lives [[1], [2], [3]]. The production of ammonia is predominantly made via the conversion of nitrogen (N2), which is referred to as nitrogen reduction reaction (NRR) or nitrogen fixation. Although N2 is the most abundant molecular species in the atmosphere, it is relatively inert and inactive due to the strong triple N=N chemical bond. In nature, nitrogen is reduced by nitrogenizes, which is a type of enzyme widely existing in certain bacteria [4]. Currently, the industrial process for ammonia synthesis relies on the Haber–Bosch process, which produces ammonia from nitrogen and hydrogen using Fe- or Ru- based catalysts under high operating pressure and temperature. Although this process requires a significant energy input, which annually is around 1.4% of global energy resources, the yield is very low with a single-pass conversion ratio of nearly 10–15% [4,5]. Therefore, numerous efforts have been devoted in the past years to find an environmentally friendly and efficient alternative technology to produce ammonia, such as electrochemical synthesis from nitrogen, protons and electrons under ambient conditions [6,7]. Although electrochemical reduction of nitrogen has not been realized on the industrial scale, significant progression has been made in the past decade. Many excellent NRR catalysts have been reported, including metal-free nanomaterials [8], metal-based materials [9], complex compounds [10] and molecular catalysts [11].
Metal organic frameworks (MOFs), which consist of well-organized metal centers and organic linkers, have high specific surface area and well-defined porous structures [[12], [13], [14], [15]]. The evenly distribution of coordination metals in MOFs provides a unique opportunity to construct catalyst with sufficient dispersion of active sites. The high specific surface area and controllable porous structure of MOFs will be favorable to the adequate exposure of active sites and good transport properties of NRR-relevant species. Lee et al. [16] reported that ZIF-coated Ag–Au platform (Ag-Au@ZIF) shows excellent NRR catalytic activity with selectivity of 90%. Shah et al. [17] found that Ni-MOF-74 shows higher ammonia yield than pure Ni metal with ammonia yields being as high as 0.23 g-NH3 (kWh-g-catalyst)−1. Zhang et al. [18] used metal–organic framework derived shuttle-like V2O3/C as a NRR catalyst, and found that V2O3/C shows remarkable NH3 yield,~12.3 μgh−1mg−1cat, and a high faradic efficiency of 7.28%.
π-conjugated metal bis(dithiolene) complexes (MC4S4) nanosheets, which are two-dimensional (2D) MOFs materials, exhibit unique physical and chemical properties. Since Kambe et al. [19] first synthesized the planar nickel bis(dithiolene) complex nanosheet and proved that it has high conductivity, and the stacked nanosheets have a metallic nature, numerous MC4S4nanosheets have been studied. Tie et al. [20] investigated the electronic structures and carriers mobilities of MC4S4 (M = Ni, Pd, Pt), where they discovered that MC4S4 nanosheets are semiconductors with narrow band gap and have potential applications in electronic devices with high electron mobilities [20]. Courtney et al. [21] reported that the hydrogen evolution reaction activity of MC4S4 follows the order: FeC4S4 < NiC4S4 < CoC4S4, and is closely related to the thickness of MC4S4. Tang et al. [22] reported that NiC4S4 can bind and release ethylene molecules in neutral and reduction conditions and can be developed as an electrocatalyst in olefin separation and purification. Our previous work also found that IrC4S4 shows excellent ORR catalytic activity [23].
In this work, the catalytic activity of NRR on MC4S4 with different transition metal center (Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cr, Mn, Mo and W) has been studied systematically based on density functional theory (DFT) computations. It was found that the central transition metal atom determines the NRR catalytic activity. A volcano-type curve was found between the reaction free energy (ΔG) value of the rate-determining step in NRR and the adsorption energy (Ead) of N2H intermediate species. Among the thirteen MC4S4 nanosheets investigated, OsC4S4 and RuC4S4 nanosheets exhibit better catalytic activity than the other MC4S4 nanosheets due to the smaller ΔG value of the rate-determining step, which results from the favorable adsorption properties.
Section snippets
Computational methods
All calculations are performed within the DFT framework by DMol3 code [24]. The generalized gradient approximation (GGA) with PW91 was used to characterize the exchange and correlation effects [25]. The all-electron relativistic core treat method was selected to treat the relativistic effect [26]. The double numerical plus polarization (DNP) was implemented for the atomic orbital basis set [24]. A smearing value of 0.005 Ha was used to accelerate the electronic convergence. To ensure high
Results and discussion
The adsorption of N2 on the surface of the catalyst is the first step in the NRR process [9]. The favorable adsorption of N2 on MC4S4 is a prerequisite for NRR proceeding on the electrocatalyst surface. Before studying the NRR mechanism(s), the adsorption properties of N2 on thirteen MC4S4 nanosheets were studied and these are summarized in Fig. S1 (Electronic Supporting Information, ESI) and Table S2 (ESI). N2 can adsorb on the MC4S4 via two configurations: side-on and end-on structures (Fig.
Conclusions
In this work, the NRR catalytic properties on the MC4S4 (M = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cr, Mn, Mo and W) nanosheets have been investigated based on DFT calculations. It was found that the catalytic activity of MC4S4 is dependent on the central atoms. The ΔG of rate-determining step of NRR on the MC4S4 increases in the order: OsC4S4 < RuC4S4 < MnC4S4 < CrC4S4 < FeC4S4 < IrC4S4 < RhC4S4 < CoC4S4 < NiC4S4 < PtC4S4 < MoC4S4 < PdC4S4 < WC4S4. OsC4S4 and RuC4S4 nanosheet materials display
Declaration of Competing Interest
None
Acknowledgements
This work was supported by the Natural Science Fund for Colleges and Universities in the Jiangsu Province (17KJB430008) and the Senior Intellectuals Fund of Jiangsu University (No. 12JDG094).
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