Two-step electrosynthesis and catalytic activity of CoO−CoO • xH₂O-supported Ag, Au, and Pd nanoparticles

Abstract

Two-step electrosynthesis of CoO-CoO•xH₂O-supported metal nanoparticles (MNPs, M = Au, Ag, Pd) was carried out in N,N-dimethylformamide in the absence and in presence of poly(N-vinylpyrrolidone) (PVP) using atmospheric oxygen as both a reagent and a mediator at potentials of its reduction to a superoxide radical anion. In the first step, oxygen reduction in the presence of Co²⁺ ions added to the solution as a salt or generated by dissolving the Co-anode during electrolysis produces a mixture of cobalt oxide CoO and its hydrated form CoO-CoO • xH₂O (CoO_xH_y ). When Ag⁺, Au⁺, Pd²⁺ ions are added to the obtained solution of CoO_x H _y, a redox reaction between CoO and the metal ion gives the MNPs and CoO⁺. In the second step, oxygen-mediated electroreduction of CoO⁺ serving as the second mediator is carried out, and the redox reaction is completely shifted towards the formation of MNPs. In the absence of PVP, AgNPs (18±4 nm) bind and stabilize completely in the CoO_xH_y matrix, PdNPs (6±1 nm) stabilize only partially, and AuNPs (21±10 nm) do not bind and, therefore, only their agglomerates are obtained (~200 nm). In the presence of PVP, individual AgNPs (5±2 nm), AuNPs (13±5 nm), PdNPs (3±1 nm) are stabilized in the PVP shell and are bound by the matrix. The obtained nanocomposites M/CoO_x H _y and M@PVP/CoO_xH_y catalyze the reduction of p-nitrophenol with sodium borohydride in an aqueous medium. Their catalytic activity is due to MNPs; CoO_x H _y acts as an inert matrix.