Enhanced performance in the direct electrocatalytic synthesis of ammonia from N2 and H2O by an in-situ electrochemical activation of CNT-supported iron oxide nanoparticles

The direct electrocatalytic synthesis of ammonia from N2 and H2O by using renewable energy sources and ambient pressure/temperature operations is a breakthrough technology, which can reduce by over 90% the greenhouse gas emissions of this chemical and energy storage process. We report here an in-situ electrochemical activation method to prepare Fe2O3-CNT (iron oxide on carbon nanotubes) electrocatalysts for the direct ammonia synthesis from N2 and H2O. The in-situ electrochemical activation leads to a large increase of the ammonia formation rate and Faradaic efficiency which reach the surprising high values of 41.6 µg mgcat−1 h − 1 and 17%, respectively, for an in-situ activation of 3 h, among the highest values reported so far for non-precious metal catalysts that use a continuous-flow polymer-electrolyte-membrane cell and gas-phase operations for the ammonia synthesis hemicell. The electrocatalyst was stable at least 12 h at the working conditions. Tests by switching N2 to Ar evidence that ammonia was formed from the gas-phase nitrogen. The analysis of the changes of reactivity and of the electrocatalyst characteristics as a function of the time of activation indicates a linear relationship between the ammonia formation rate and a specific XPS (X-ray-photoelectron spectroscopy) oxygen signal related to O2− in iron-oxide species. This results together with characterization data by TEM and XRD suggest that the iron species active in the direct and selective synthesis of ammonia is a maghemite-type iron oxide, and this transformation from the initial hematite is responsible for the in-situ enhancement of 3-–4 times of the TOF (turnover frequency) and NH3 Faradaic efficiency. This transformation is likely related to the stabilization of the maghemite species at CNT defect sites, although for longer times of preactivation a sintering occurs with a loss of performances.