OXYGEN REDUCTION ON THE SURFACE OF Fe3O4&Au NANOCOMPOSITES

Functional materials based on ferromagnetics and noble metals are attractive due to their unique optical, catalytic and magnetic properties. Nowadays these materials find their application in various fields of science and technology: medicine, biotechnology, chemistry, physics and energy sector. The aim of the present work is to study the electrochemical characteristics of the oxygen reduction on the surface of Fe3O4&Au nanocomposites. The formation of the composite particles took place on the surface of a rotating steel disk which was in contact with HAuCl4 aqueous solution and air. Initial gold concentration was varied from 0.2 to 10.0 mg/l. Oxygen reduction was studied on a floating gas diffusion electrode (pressed carbon black P803 + 30 % PTFE) with surface, modified by prepared Fe3O4&Au nanocomposites, in 1 M KOH solution at 20 ºC. Electrochemical measurements were carried out using a PI-50-1.1 potentiostat. Tafel slopes of the stationary polarization curves ∂E/∂lgj of oxygen reduction on the electrode modified by composites formed at с(Au3+) from 0.2 to 5.0 mg/l lay in the ranges: b1 = 0.048 – 0.060 V, b2 = 0.119 – 0.131 V. When nanocomposites were obtained at с(Au3+) from 7.0 to 10 mg/l these values were: b1 = 0.042 –0.061 V, b2 = 0.079 – 0.105 V. It was concluded that oxygen reduction mechanism is multistep and includes formation of the hydrogen peroxide as intermediate. Obtained b1 and b2 values indicate that electrode process me-chanism changes when current density grows. The largest exchange current (8.51∙10–3 A/g) was achieved when electrode was modified by composite formed at с(Au3+) = 1.0 mg/l, whereas when the composite formed at с(Au3+) = 10.0 mg/l was used, the j0 value was smallest. Perhaps, the increase in the initial Au3+ concentration during nanocomposite formation led to an increase in the number of separate gold clusters on the surface of Fe3O4 nanoparticles and to raise of composite catalytic activity. But when с(Au3+) exceeded 1.0 mg/l the core-shell Fe3O4&Au composites with compact gold shell were formed. Such coreshall composites had lower catalytic activity in the oxygen reduction than composites with separate gold clusters on the surface of Fe3O4 particles.

Why nitrogen favors oxygen reduction on graphitic materials

The roles played by the different nitrogen dopants in the oxygen reduction mechanism are elucidated, determining active sites, selectivity and the effect of the solvation and the hydrogenation of pyridinic nitrogen-dopants.