Russian Chemical Bulletin, International Edition, Vol. 69, No. 2, pp. 241—254, February, 2020 241 Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 0241—0254, February, 2020. 1066-5285/20/6902-0241 © 2020 Springer Science+Business Media LLC Two-step electrosynthesis and catalytic activity of СоО—СоО•xН 2 О-supported Ag, Au, and Pd nanoparticles R. R. Fazleeva, а G. R. Nasretdinova, а Yu. N. Osin, b A. Yu. Ziganshina, а and V. V. Yanilkin а a Arbuzov Institute of Organic and Physical Chemistry, Federal Research Center "Kazan Scientific Center of the Russian Academy of Sciences," 8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation. Fax: +7 (843) 275 2253. E-mail: yanilkin@iopc.ru b Kazan (Volga Region) Federal University, Interdisciplinary Center for Analytical Microscopy, 18 ul. Kremlevskaya, 420018 Kazan, Russian Federation Two-step electrosynthesis of СоО—СоО•хН 2 О-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 Со 2+ 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 СоО— СоО•хН 2 О (CoO x H y ). When Ag + , Au + , Pd 2+ 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 СоО + . In the sec- ond step, oxygen-mediated electroreduction of СоО + 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 x H 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 М/CoO x H y and М@PVP/CoO x H 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. Key words: electrochemical synthesis, oxygen-mediated electroreduction, cobalt(II) oxide, nanocomposite. Pseudo-homogeneous metal nanocatalysts, namely metal nanoparticles (MNPs), which are unstabilized or stabilized in solution by binding in the stabilizer shell or on the surface of stable carrier nanoparticles, are of undoubted interest in the development of energy- and resource-saving ecologically safe technologies for obtain- ing substances and materials for practical use. Unlike classical heterogeneous catalysts, pseudo-homogeneous nanocatalysts have a more developed surface. Due to their high polydispersity, nanosized catalysts are evenly distrib- uted in the medium and, therefore, the catalytic reaction proceeds throughout the entire bulk of the reaction solu- tion. As a result, these catalysts have a higher specific catalytic activity and lack most of the difusion restrictions characteristic of heterogeneous reactions. A positive aspect of immobilizing MNPs on the surface of larger carrier particles is the ease of separation of the catalyst after the completion of the catalytic reaction by filtration or cen- trifugation, as well as by application of external magnetic field in the case of magnetically active carriers. Various NPs can be used as carriers of MNPs, in particular, nano-sized metal oxides. 1—13 Metal oxides catalyze many reactions (oxidation, reduction, coupling with the formation of С—С and С—О bonds, condensa- tion, etc.), 5,14—20 therefore, they can serve not only as carriers and stabilizers of MNPs, but also as active co-cat- alysts, granting the composite bifunctionality or a syner- gistic efect. These nanocomposites are obtained mainly by chemical reduction of metal ions (complexes) (M 1 ) with various reducing agents in the presence of other metal oxides 5—7 (M 2 ) that gives a stoichiometric amount of the oxidized form of the reducing agent as waste. An electro- chemical method of producing nanocomposites М 1 /М 2 O х is also known, which consists in dispersing the metal of the М 1 electrode during electrolysis under an alternating current in the presence of introduced or electrochemically