Nitrate Reduction to Ammonia (NITRR)

References

1. NonAqueous NITRR

2. Aqueous NITRR

2.1. References

Low-Coordination Rhodium Catalysts for an Efficient Electrochemical Nitrate Reduction to Ammonia ACS Catal. 2023, 13, 2, 1513–1521
Unveiling the Activity Origin of a Copper-based Electrocatalyst for Selective Nitrate Reduction to Ammonia Angewandte chemie international edition 59.13 (2020): 5350-5354
Structure Sensitivity of Pd Facets for Enhanced Electrochemical Nitrate Reduction to Ammonia ACS Catal. 2021, 11, 12, 7568–7577

2.2. Experiments

2.2.3. Preparing electrolyte

The electrocatalytic activities of WE for NITRR were tested in 0.1 M Na2SO4 (adjust to pH 11.5 using 1.0 M NaOH solution) with 0.1 M KNO3 under 25 °C and ambient pressure. Due to the presence of HER and NITRR during the test at negative potential, the H+ in the electrolyte can be constantly consumed, resulting in the pH change of unbuffered 0.1 M Na2SO4 with 0.1 M KNO3 electrolyte from neutral to ~11 during the NITRR test. Therefore, in order to ensure the pH stability of the electrolyte during NITRR tests, 0.1 M Na2SO4 solution (pH 11.5, adjusted by 1.0 M NaOH solution) was used as the electrolyte with 0.1 M nitrate-N.4 Pure Ar was continuously fed into the cathodic compartment in the process of potentiostatic testing, where ammonia synthesis occurs. Magnetic stirring with 500 rpm was used. The downstream gas was bubbling into a container filled with 1 mM H2SO4 solution at the end of the cell, which could absorb the vaporized NH3 during the test. After electrochemical reduction reaction, indophenol blue method was used to detect the produced NH3. The amount of NH3 produced by electrocatalytic NITRR was the sum of the NH3 from cathode compartment, anode compartment and the off gas absorber.^{[ACS Catal. 2023, 13, 2, 1513–152]}

The copper-based sample on Cu mesh, saturated calomel electrode (SCE) and platinum foil was used as the working electrode, reference electrode and counter electrode, respectively. The surface area of the working electrode was controlled with 1 cm2. 0.5 M Na2SO4 solution (80 mL) was evenly distributed to the cathode and anode compartment. NaNO3 was added into the cathode compartment for NO3- reduction (containing 200 ppm nitrate-N). All potentials were recorded against the reversible hydrogen electrode (RHE). Before nitrate electroreduction test, Linear sweep voltammetry (LSV) curves are performed until that the polarization curves achieve steady-state ones at a rate of 10 mV s-1 from 0.05 to -0.95 V. Then, the potentiostatic test was carried out at different potentials for 2 h with a stirring rate of 300 rpm^{[Angewandte chemie international edition 59.13 (2020): 5350-5354]}

The potential sweeps started at 0.2 VRHE and repeated in a potential range of −0.2 to 0.8 VRHE. In all testing, an Ar-saturated 0.1 M NaOH aqueous solution containing 20 mM NaNO3 or 2 mM NaNO2 was used as the electrolyte. Alkaline conditions were deemed necessary to reduce the proton concentration to investigate intrinsic NO3RR/NO2RR activity of Pd facets. Electrolysis testing was conducted with an H-type electrolytic cell separated by a Nafion 212 membrane, which is connected to an electrochemical workstation. The Nafion membrane was boiled in 3% H2O2 for 1 h, ultrapure water for 2 h, and then 0.5 M H2SO4 for 1 h, sequentially. After the pretreatment, the membrane was thoroughly rinsed with ultrapure water several times. The working electrode was used as Pd-based catalysts that were spray-coated on carbon papers as a working electrode and Ag/AgCl was used as a reference electrode in the cathode chamber. Pt wire was used as a counter electrode and placed in the anode chamber. The geometric surface area of the working electrode was controlled with 0.4 cm2. The electrolyte in the H-type cell is the same as our previous half-cell test solution. The 0.1 M NaOH electrolyte was evenly distributed to the cathode and anode compartment, and 400 ppm of NO3–-N was added into the only cathode compartment for NO3RR with an Ar-purged environment. Chronoamperometry (CA) was performed at a potential of −0.2 VRHE for 1, 2, 3, and 4 h, respectively, for NO3RR, detecting concentration variations of NO3–-N, NO2–-N, and NH3-N.^{[ACS Catal. 2021, 11, 12, 7568–7577]}

2.2.4. Equation

All potentials were adjusted to the reversible hydrogen electrode (RHE) using this equation, E (vs. RHE) = E (vs. SCE) + 0.059 × pH + 0.24^{[ACS Catal. 2023, 13, 2, 1513–1521]}

2.3. Determination

2.3.1. Determination of nitrate-N

Firstly, a certain amount of electrolyte was taken out from the electrolytic cell and diluted to 5 mL to detection range. Then, 0.1 mL 1 M HCl and 0.01 mL 0.8 wt% sulfamic acid solution were added into the aforementioned solution. The absorption spectrum was measured using an ultraviolet-visible spectrophotometer and the absorption intensities at a wavelength of 220 nm and 275 nm were recorded. The final absorbance value was calculated by this equation: A=A220nm-2A275nm. The concentration-absorbance curve was calibrated using a series of standard potassium nitrate solutions and the potassium nitrate crystal was dried at 105-110 oC for 2 h in advance.

2.3.2. Determination of nitrite-N

A mixture of p-aminobenzenesulfonamide (4 g), N-(1-Naphthyl) ethylenediamine dihydrochloride (0.2 g), ultrapure water (50 mL) and phosphoric acid (10 mL, ρ=1.70 g/mL) was used as a color reagent. A certain amount of electrolyte was taken out from the electrolytic cell and diluted to 5 mL to detection range. Next, 0.1 mL color reagent was added into the aforementioned 5 mL solution and mixed uniformity, and the absorption intensity at a wavelength of 540 nm was recorded after sitting for 20 min. The concentration-absorbance curve was calibrated using a series of standard sodium nitrite solutions.

2.3.3. Determination of ammonia-N

Ammonia-N was determined using Nessler’s reagent as the color reagent. First, a certain amount of electrolyte was taken out from the electrolytic cell and diluted to 5 mL to detection range. Next, 0.1 mL potassium sodium tartrate solution (ρ=500 g/L) was added and mixed thoroughly, then 0.1 mL Nessler’s reagent was put into the solution. The absorption intensity at a wavelength of 420 nm was recorded after sitting for 20 min. The concentration-absorbance curve was calibrated using a series of standard ammonium chloride solutions and the ammonium chloride crystal was dried at 105 oC for 2 h in advance.

2.3.4. Isotope Labeling Experiments

99.21% Na15NO3 was used as the feeding N-source to perform the isotopic labeling nitrate reduction experiments to clarify the source of ammonia. 0.5 M Na2SO4 was used as electrolyte and Na15NO3 with a concentration of 200 ppm 15NO3–15N was added into the cathode compartment as the reactant. After electroreduction, electrolyte with obtained 15NH4+-15N was taken out and the pH value was adjusted to be weak acid with 4 M H2SO4 for further quantification by 1H NMR (600 MHz) with external standards of maleic acid. The calibration curve was created as follows: First, a series of 15NH4+-15N solutions ( (15NH4)2SO4) ) with known concentration (50, 100, 150, 200, 250 ppm) were prepared in 0.5 M Na2SO4 as standards; Second, 50 mL of the 15NH4+-15N standard solution with different concentration was mixed with 0.02 g maleic acid; Third, 50 μL deuterium oxide (D2O) was added in 0.5 mL above mixed solution for the NMR detection; Fourth, the calibration was achieved using the peak area ratio between 15NH4+-15N and maleic acid because the 15NH4+-15N concentration and the area ratio were positively correlated. Similarly, the amount of 14NH4+-14N was quantified by this method when Na14NO3 was used as the feeding N-source