Boosting Nitrogen Reduction to Ammonia on FeN4 Sites by Atomic Spin Regulation

Abstract Understanding the relationship between the electronic state of active sites and N2 reduction reaction (NRR) performance is essential to explore efficient electrocatalysts. Herein, atomically dispersed Fe and Mo sites are designed and achieved in the form of well‐defined FeN4 and MoN4 coordination in polyphthalocyanine (PPc) organic framework to investigate the influence of the spin state of FeN4 on NRR behavior. The neighboring MoN4 can regulate the spin state of Fe center in FeN4 from high‐spin (d xy 2 dyz 1 dxz 1 dz2 1 dx2−y2 1) to medium‐spin (dxy 2 dyz 2 dxz 1 dz2 1), where the empty d orbitals and separate d electron favor the overlap of Fe 3d with the N 2p orbitals, more effectively activating N≡N triple bond. Theoretical modeling suggests that the NRR preferably takes place on FeN4 instead of MoN4, and the transition of Fe spin state significantly lowers the energy barrier of the potential determining step, which is conducive to the first hydrogenation of N2. As a result, FeMoPPc with medium‐spin FeN4 exhibits 2.0 and 9.0 times higher Faradaic efficiency and 2.0 and 17.2 times higher NH3 yields for NRR than FePPc with high‐spin FeN4 and MoPPc with MoN4, respectively. These new insights may open up opportunities for exploiting efficient NRR electrocatalysts by atomically regulating the spin state of metal centers.


Supplementary Tables 5
Chemicals and reagents. Pyromellitic dianhydride and Urea were purchased from Beijing InnoChem Science technology Co., Ltd. Molybdenum (Ⅴ) chloride and Ammonium molybdate (di) were purchased from Macklin. Iron (Ⅲ) chloride anhydrous was bought from Sinopharm Group Chemical Reagent Co., Ltd. Ammonium chloride was obtained from Tianjin Shengao Chemical Reagent Co., Ltd. Nafion (5.0 wt%) was purchased from Sigma-Aldrich. Ketjen Black (KB) was bought from Sinopharm Group Chemical Reagent Co., Ltd. All chemicals were used as received without any further purification.
Deionized water was used in all experiments. minutes. Then, the mixture was transferred into a crucible, covered with a lid, and placed in a muffle furnace, and heated at 220 °C for 3 hours with a ramp rate of 2 °C·min -1 . After cooling down to room temperature, the obtained product was washed with deionized water, acetone, and methanol several times. Finally, the product was dried under vacuum at 60 °C for 12 hours to obtain FeMoPPc. The   (2.516 mg,0.0074 mmol), pyromellitic dianhydride (220 mg, 1 mmol), and carbon black (50 mg) were mixed and ground uniformly in an agate mortar for 20 minutes. Then, the mixture was transferred into a crucible, covered with a lid, and placed in a muffle furnace, and heated at 220 °C for 3 hours with a ramp rate of 2 °C·min -1 . After cooling down to room temperature, the obtained product was washed with deionized water, acetone, and methanol several times. Finally, the product was dried under vacuum at 60 °C for 12 hours to obtain MoPPc. Diffractometer with copper Kα radiation (λ=1.5406 Å) at 40 kV, 40 mA. UV/Vis diffuse reflectance spectra was measured by using a U-4100 UV/Vis-NIR spectrometer (Hitachi). The X-ray photoelectron spectroscopy (XPS) measurements were acquired with an ESCA LAB 250 spectrometer on a focused monochromatic Al K α line (1486.6 eV) X-ray beam with a spot diameter of 200 μm. A micromeritics ASAP 2020 surface area analyzer was used to obtain the N 2 adsorption/desorption curve by BET measurements. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to measure element content on Aglient 5110. The Fe and Mo K-edge X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) were investigated at the SXRMB and Bio-XAS beamlines at the Canadian Light Source. References, such as Fe and Mo foils, are used to calibrate the beamlie energy and for comparison to samples. Fluorescence detection was performed using a 7-element Si drift detector for samples and the total electron yield was used for measurement of samples with high concentration, such as references. The EXAFS raw data were then background-subtracted, normalized and Fourier transformed by the standard procedures with the IFEFFIT package. A conventional spectrometer (Germany, Wissel MS-500), using a 57 Co (Rh) source with activity of 25 mCi, in transmission geometry with constant acceleration mode was used to perform the Mössbauer measurements. The velocity calibration was done with a room temperature α-Fe absorber.

Synthesis of FeMoPPc.
The spectra were fitted by the software Recoil using Lorentzian Site Analysis.
Preparation of Working Electrode. 1 mg catalyst were dispersed in 100μL of ethanol and 10μL of Nafion solution to form a homogeneous catalyst ink under sonication for 30min. Then, 50μL of catalyst ink were dropped evenly on carbon paper for catalytic area (1×1 cm 2 ), and dried at room temperature.
Electrocatalytic measurement. CHI760E electrochemical workstation (CH Instrument Co., Shanghai) was used to perform the electrochemical measurements in a H-type cell separated by a Nafion 115 membrane using a typical three-electrode setup (counter electrode: Pt mesh, 1×1 cm 2 ; reference electrode: Ag/AgCl, saturated KOH electrolyte). The electrolyte for electrochemical testing was 0.1M KOH solution, 0.1M HCl in absorption bottle was used to absorb NH 3 . All potentials were converted to the RHE reference scale by E (vs RHE) = E (vs Ag/AgCl) + 0.197 V + 0.059 × pH. Before experiment, the electrolyte in the cathode cell was bubbled with pretreated pure N 2 (99.999% purity) for 30 min to eliminate oxygen in solution. Pure N 2 was continuously supplied with a constant gas flow rate in the entire electrolytic process. LSV curves of samples were performed in N 2 -saturated and Ar-saturated 0.1 M KOH with the scan rate of 5 mV s -1 to examined the electrochemical activities of catalyst. A potentiostatic test was performed in a N 2 -saturated 0.1 M KOH solution. After electrolysis, a colorimetry was used to measure ammonia and hydrazine hydrate in the electrolyte and absorber, respectively. 15 N isotope labeling experiment. The 15 N isotopic labeled experiment were performed using the 15 N 2 isotope to determine the N source of ammonia. [1] First, Ar gas is continuously injected into the electrolyte for 1 hour to remove O 2 and N 2 , and then using 15 N 2 as the feeding gas. After electrolysis at concentrations were then measured, and the relationship curve between NH 4 + ion concentration and absorbance were drawn, which is a calibration curve. Through the calibration curve, the absorbance of the KOH electrolyte after the NRR reaction was been measured. The method of detecting NH 4+ and the calibration curves of NH 3 in HCl absorber were the same as above. By superimposing the NH 4 + ion concentration of KOH electrolyte and HCl absorber, the amount of NH 4 + ions produced in the NRR can be calculated.
Determination of hydrazine hydrate. The quantitative detection of N 2 H 4 in the electrolyte was carried out according to the Watt and Chrisp method. [3] The mixture of para-(dimethylamino) benzaldehyde (5.99 g), HCl (concentrated, 30 mL) and ethanol (300 mL) was used as a color reagent. After the electrolytic reaction, 2 mL of KOH electrolyte was put it in the reaction bottle, and 5 mL above prepared color reagent was added with stirring 15 min at room temperature. The solutions added into electrolyte were used as color reagent. Absorbance test was performed at a wavelength of 460 nm. The blank electrolyte was used for background determination. The corrected absorbance values of N 2 H 4 with different standard concentrations were then measured, and the relationship curve between NH 4 + ion concentration and absorbance were drawn. The remaining steps were the same as the method for detecting ammonia production.

Calculation method for the yield rate of ammonia and faradaic efficiency (FE). The FE of NH 3
production was calculated as follows [2] : N, the number of electrons transferred for product formation, which is 3 for NH 3 .
V, the volume of the electrolyte (0.1 M KOH).
Q, total electric charge.
M: the relative molecular mass of NH 4 + , which is 18 g mol -1 The Yield Rate of NH 3 was calculated as follows: ν NH 3 , the yield rate of NH 3 .
V, the volume of the electrolyte (0.1 M KOH).
m, the mass of the supported catalyst.

Calculation equation for the number of unpaired d electron (n) of Fe ion.
 , magnetic susceptibility. μ eff , the effective magnetic moment.
n, the number of unpaired d electron.