Electrosynthesis of Nanocomposites of Catalytic Active Pd–Cu and Pd–Au Bimetallic Nanoparticles with Poly(N-vinylpyrrolidone) and Nanocellulose

The work considers the preparation in an undivided cell of Pd–Cu and Pd–Au bimetallic nanoparticles (NPs) by methylviologen (MV2+)-mediated electrochemical reduction of equimolar amounts of Cu(II), Pd(II), and Au(I) in the presence of poly(N-vinylpyrrolidone) (PVP) and nanocellulose (NC) under controlled potential of the generating MV•+ cation radical in aqueous medium at room temperature. Electrosyntheses are performed by sequential or joint reduction of metal ions by passing a theoretical charge. When Pd(II) ions are added to CuNPs, as well as when Au(I) ions are added to PdNPs, a galvanic replacement process is observed, namely oxidation of Cu0 by Pd(II) and Pd0 Au(I) ions. The results of the complete reduction are nanocomposites of mainly spherical MNPs, dispersed in the solution bulk and stabilized by PVP on the surface of the NC. In the sequential synthesis of CuNPs and then PdNPs, the nanocomposite is represented as Cu2O nanoroses coated with fine PdNPs. Pd nanocomposites with Cu2O or Au are represented mainly by spherical particles with size of 4–50 nm depending on the production method. X-ray powder diffraction (XRPD) data of the nanocomposites confirm the formation of a mixture of PdNPs (0.8–10 nm) with large gold crystallites (up to 24 nm), as well as the oxidation of CuNPs to cuprite (Cu2O). The size of the metal crystallites and copper oxide varies in the range from 0.8 to 24 nm. In the test reaction of p‑nitrophenol reduction with sodium borohydride in aqueous medium, all of the tested nanocomposites show time-increasing catalytic activity. When Cu is added to Pd, the catalytic reduction reaction is maintained, while the addition of Au to Pd decreases the catalytic activity of PdNPs by an order of magnitude.