Catalytic effect of molybdenum oxo-species on reduction of allyl alcohol at a Pt electrode in strongly acidic solutions

摘要

Kinetics of the electroreduction of molybdenum(VI) species and allyl alcohol (AA) alone as well as of AA after the addition of Na_2MoO_4 to perchloric acid solutions (HClO_4) on a polycrystalline Pt electrode was characterized under the conditions of cyclic voltammetry (CV). Using the XPS technique evidence was provided that mixed valent Mo(V/VI) layer is formed upon reduction of Mo(VI) moieties present in 1 M HClO_4 solution, while the cationic Mo(VI) species existing in strongly acidified solutions are reduced successively to relevant Mo(V) and Mo(III) species at E < 0.25 V. It was ascertained that a relatively fast adsorption of AA precedes its hydrogenation to propene and propane in the absence of Mo(VI) in the electrolyte solution. However, the Pt surface is partially poisoned with CO-like species due to a rupture of the C_3-carbon backbone in the potential range of hydrogen adsorption-desorption. A considerable enhancement of the cathodic current corresponding to the AA electroreduction was discovered in the presence of cationic Mo(VI) species in 4 M HClO_4 solutions. Simultaneously, the rate of electroreduction of this species was effectively increased on the addition of AA in comparison with that in the absence of AA. According to the proposed reaction pathways, AA is reduced via a catalytic chemical reaction with Mo(III) and/or Mo(V) cationic species. The cyclic regeneration of the electroactive Mo(VI) and/or Mo(V) species accounts for the observed catalytic phenomenon. Furthermore, the decomposition of the C_3-carbon skeleton of AA molecules and formation of strongly adsorbed CO-like species is eliminated in the presence of Mo(VI). The optimal conditions for monitoring of AA and/or Mo(VI) in electrochemical devices with a Pt electrode are specified.