Iron Based Catalysts for Nitrogen Reduction Reaction

Milazzo, Rachela

Ammonia (NH₃) is a fundamental feedstock for the global population not only for its wide use as fertilizer and chemical, but also for its potential as energy storage medium. Its synthesis at the industrial scale is based on the well-known Haber Bosch process, operating at high temperature (400-500°C) and high pressure (150-300atm), thus making it highly pollutant. Considering the global climate emergency, it is mandatory to develop a green ammonia synthesis that relies on milder conditions and may adopt renewable energy sources. The electrochemical synthesis, of NH₃ from N₂ and H₂O at ambient conditions and using renewable energy driven electricity could be a promising approach. An efficient electrochemical ammonia synthesis however, is currently still lacking. The main reasons are the absence of adequate catalysts capable of dissociating the N₂ triple bond and the competition with the hydrogen evolution reaction (HER) at the cathode, since the required potential is close to that of nitrogen reduction reaction (NRR), required for the ammonia formation.

There are some reaction models in the literature but it is commonly accepted that the dissociative adsorption of N₂ is the rate limiting step and extensive research has been done on nitrogen interaction with metal surfaces. Ru, Co, Bi, Au, Fe, Mo, etc based materials have been extensively tested for the green ammonia synthesis, but results are still conflicting also because a complete evaluation of environmental contaminations is still lacking.

In this work we adopted as NRR catalyst Fe based nanoparticles on carbon cloth substrate via a simple and fast electroless deposition technique. We prepared a FeCl₃ solutions with different concentrations in the range 1-10mM and deposited a drop on (3x1.5) cm² samples of carbon cloth, on a hot plate, at 80°C. After evaporation of the water, the as deposited substrates were dipped in NaBH₄ solution while stirring.

The catalyst morphology has been studied by Scanning Electron Microscopy (SEM), as a function of the concentration. Very small particles, with a size ranging from 10 to 70nm, were obtained with the most diluted solution. The cluster size increases with increasing FeCl₃ concentration in the solution, giving rise to strong coalescence effects and to the formation of a thicker and continuous layer in the case of the most concentrated solution. Scanning Transmission electron Microscopy (STEM) and Electron Energy Loss Spectroscopy (EELS) have been adopted to determine the composition. EELS spectra showed that the iron L-edge is that typical of Fe₃O₄ materials.

The electrochemical ability to reduce nitrogen, with the formation of ammonia, was evaluated in a standard two compartment cell, with a phosphate buffered solution (PBS 0.1M) adopting a Zirfon membrane as a gas separator. Ar or N₂ gas (both with a purity of 99.9999%) flowed in the cathode chamber after going through an acidic trap and a water trap, to be further purified. A rigorous protocol has been adopted to evaluate the ammonia production, including a two steps measurement of the environmental ammonia at the open circuit potential. The ammonia was measured by spectrophotometric analysis using the indophenol blue method. The electrochemical production of ammonia was obtained by chronoamperometry under constant voltage.

For the iron-based catalysts, a very efficient activation procedure has been developed, based on cyclic voltammetry under N₂ flow. The activation process allows an improvement up to 10 times in the ammonia generation rate. Moreover, a strong correlation has been found between the particle size of the catalyst and its activity for NRR, with the best results achieved with the sample covered with nanoparticles, exhibiting also the highest electrochemical active area. Iron based nanoparticles showed excellent activity for NRR, with a faradaic efficiency of 15% at -0.35 V vs RHE and a maximum ammonia production rate of 85µg mg^-1_cat h^-1. Acknowledgements:This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No101006941.