Electrocatalytic properties of mixed-valence molybdenum oxide thin film modified microelectrodes

Abstract

A compact blue conducting mixed-valence Mo (VI,V) oxide film was grown on the surface of a carbon fibre (CF) microelectrode by cycling the potential between +0.20 and ∼ 0.70 V SCE in freshly prepared Na 2 MoO 4 solution in H 2 SO 4 (pH2). The thickness of the oxide film was controlled by the charge passed. X-ray photoelectron spectroscopy (XPS) examination confirmed that the components of the molybdenum oxide film on the CF electrode surface are mainly a mixture of Mo 6+ and Mo 5+ complex, together with a small amount of Mo 4+; no Mo 3+ peak was observed. Well-defined redox transitions have been attributed to the reductive formation of hydrogen molybdenum bronzes [MoO 3−x (OH) x] and re-oxidation in acidic media. The coating is easily permeable to ions. The molybdenum oxide film catalyses the electroreduction of chlorate to chloride and bromate to bromide in H 2SO 4. At a bare CF microelectrode, chlorate and bromate are not reduced prior to the onset of hydrogen evolution at about −1.0 V. At the modified surface, a chlorate or bromate reduction peak appears at about −0.4 V SCE. The peak current is linear with concentration from 10 −5 to 10 −3 M chlorate or from 10 −6 to 10 −4 M bromate, and is nearly diffusion controlled. The electrocatalytic activity of H 0.34MoO 3 towards chlorate or bromate ions is even stronger than that of H 0.93MoO 3. The reactivity sequence determined by comparing current magnitudes on the molybdenum oxide film modified CF microelectrode is BrO 3 −> ClO 3 −.