Combined Effect of Pressure and Temperature on Nitrogen Reduction Reaction in Water

Abstract

The synthesis of ammonia starting from nitrogen and using electrochemical processes is considered an interesting strategy to produce ammonia in a sustainable way. However, it requires not only the development of efficient catalysts for nitrogen reduction but also the optimization of the operating conditions of the employed electrochemical devices. In this work, we optimize the kinetics and the thermodynamics of the electrocatalytic nitrogen reduction reaction in water by developing a pressurized H-cell that may operate at temperatures up to 80 °C. Ni foam with low Au loading (0.08 mg cm−2) has been adopted as a catalyst at the cathode. Ammonia has been produced during chronoamperometry experiments in a saturated N2 atmosphere and measured by the indophenol blue method. The effect of voltage, temperature, and pressure has been studied. The nitrogen reduction experiments have been repeated under saturated Ar. To remove contributions due to environmental contamination, we determined the net value as the difference between the produced ammonia in N2 and in Ar. The ammonia yield increases by increasing the temperature and the pressure. The best results have been obtained by using the combined effects of temperature and pressure. Operating at 5 bar of saturated N2 and 75 °C, a production rate of 6.73 μg h−1·cm−2 has been obtained, a value corresponding to a 5-fold enhancement, compared to that obtained under ambient conditions and room temperature.