Methylviologen mediated electrochemical synthesis of catalytically active ultrasmall bimetallic PdAg nanoparticles stabilized by CTAC

Abstract

Efficient electrosynthesis of the monodisperse ultrasmall spherical mono- and bimetallic nanoparticles (NP) of Ag and Pd stabilized in the CTAC shell in solution bulk was carried out by methylviologen mediated reduction of equimolar amounts of Pd(II) and Ag(I) (1.5 mM) in the presence of CTAC (75 mM) at controlled potentials of the MV2+/MV•+ redox couple in water/0.1 M NaCl medium at room temperature. Metal ions are quantitatively converted to NP upon consumption of the theoretical amount of electricity. Sizes of isolated PdNP and AgNP are 5 ± 1 nm and 18 ± 5 nm, respectively. Bimetallic PdAgNP were obtained in three ways: (i) by preliminary synthesis of PdNP and subsequent reduction of AgCl (Pd/AgNP); (ii) by preliminary synthesis of AgNP and subsequent reduction of PdCl2 (Ag/PdNP); (iii) by joint reduction of PdCl2 and AgCl (Ag-PdNP). In all cases, a solid-solution alloy PdAgNP is obtained. The driving force for the alloy formation is the energy gain of ∼33 kJ/mol of an alloy. In the first way, the NP size (8 ± 2 nm) and the Pd content in the alloy (76%) are slightly higher than in the other two methods (6 ± 1 nm, 50 and 68%), but the alloy crystallite size (2–5 nm) in NP is the same. PdNP and alloys NP exhibit a high catalytic activity toward the p-nitrophenol reduction and the Suzuki-Miyaura cross-coupling reactions. In the reduction reaction, an addition of silver to palladium decreases the catalytic activity of NP. The catalytic activity of alloys in the coupling reaction correlates with the Pd content in an alloy. And the catalytic activity of the alloy Pd/AgNP obtained in the first method of the NP preparation is higher than that of PdNP.