Electrocatalytic hydrogenation of Nitrobenzene on high entropy materials (HEMs)

Global warming and environmental pollution are two of the major challenges currently in the spotlight of the research community. Nitrobenzene electrocatalytic hydrogenation could simultaneously tackle two problems. In the first place, as an option to replace the resource-inefficient and non-environmentally friendly processes for aniline production used nowadays. Also, as a viable and applicable process to enhance the degradation efficiency for wastewater treatment rising technologies, using nitrobenzene as a model pollutant. Herein, We present a comprehensive study of the nitrobenzene catalytic hydrogenation using high entropy materials (HEMs) as electrode materials. Different synthesis methods were used to produce high entropy alloys (HEAs) and high entropy oxides (HEOs), and extensive physical and electrochemical characterization was performed to establish correlations between the catalyst’s properties and the hydrogenation catalytic activity. The effect of the applied potential (0.0 - -2.0 V) was evaluated in different solution pH (5 and 14) using a concentration of 400 μM, and the selectivity of the reaction was found to be dependent on the surface concentration of hydrogen produced. A study on the effect of the annealing temperature revealed different morphological properties which were reflected in the hydrogenation performance of the HEOs, showing a clear correlation between the oxygen vacancies concentration, the redox properties of the catalysts, and the hydrogenation activity. Finally, a selected material heat treated at 500 °C (HEO500) was used to evaluate the nitrobenzene degradation in aqueous media, with concentrations ranging from 100 to 1000 μM, with outstanding results above 90% degradation (Eapp = 1.7 V vs Ag/AgCl in aerated condition), showing the important role played by the superoxide radical (O2*-) in the extensive mineralization mechanism. Several deactivation mechanisms were observed where the mechanical failure of the electrode was the most important one, followed by the potential-promoted surface transformations.