Effect of the annealing temperature of multi-elemental oxides (FeCoNiCuZn)yOx on the electrocatalytic hydrogenation of nitrobenzene at room temperature

Abstract

Nitrobenzene (NB) in effluent waters is a serious and complicated environmental problem due to its environmental toxicity and health hazard. In this study, the selective electrocatalytic hydrogenation of NB to aniline (AN) was studied using multi-elemental oxides (Fe0.3Co0.05Ni0.05Cu0.3Zn0.2)O, synthesized via urea hydrolysis method and heat-treated at different temperatures (300–1000 °C). The synthetic route chosen allowed the obtention of multi-phase materials and high entropy oxide (HEO) conditions, to be used as electrocatalysts, and to evaluate the effect of the crystal structure on the catalytic performance. The catalysts were characterized using physical methods scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. The electrochemical properties were evaluated using cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, and chronoamperometry. The polycrystalline structure obtained at 500 °C (HEO500) showed the highest selectivity (above 45%), while the more crystalline catalyst (HEO800) exhibited the highest conversion (30%) and kinetic constant (k = 9.4 × 10−3 s−1). The catalytic performance of the materials demonstrated a strong correlation with the microstructure and surface properties, like crystallite size, initial crystal phase, oxygen vacancies concentration, and redox properties. All the HEO materials exhibited a better catalytic performance than the commercial benchmark, Pt/C 5% wt catalyst, which showed no selectivity towards aniline and low conversion in 1.0 M KOH solution at -1.4 V vs Ag/AgCl. This improvement was attributed to the increase in the surface concentration of hydrogen found on the HEOs. These results highlight the importance of the surface defects on the activity and selectivity of multi-step surface-dependent catalytic hydrogenation reactions; and provide fundamental information on the importance of carefully choosing the synthesis and calcination conditions.