Towards a designed synthesis of metallic nanoparticles in polyols - elucidation of the redox scheme in a cobalt-ethylene glycol system

The polyol process has been used to synthesize metal and alloy nanoparticles over a couple of decades. Its potential has been demonstrated through the synthesis of metallic nanoparticles with various sizes and shapes. However, although one of the roles of polyol is to act as a reducing agent, research studies related to the investigation of the redox reaction mechanism have been scarce. In this study, we report the results of a detailed study undertaken to investigate the polyol oxidation and metal reduction for the cobalt(II)-ethylene glycol system with the possible addition of a base (Na) to the system using several physico-chemical techniques such as NMR, FT-IR, ESI-TOFMS and XRD. The results suggested that in the reduction reaction, ethylene glycol first reacts with the base to produce the ethylene glycol monoanion, hereinafter denoted as EG(-). This reaction not only occurs between ethylene glycol and the base, but also occurs with Co2+ species. In such a case, the formation of Co alkoxide and the subsequent reduction could progress even in the presence of a weak base such as acetate ion or any other solvent that has a lone pair of electrons such as oxygen in ether. On the other hand, in the case of oxidation of ethylene glycol, first the base attacks one of the two a-protons in Co alkoxide and along with the electron transfer to Co2+, ethylene glycol gets oxidized to aldehyde. Then, the aldehyde thus formed undergoes bond exchange with the neighboring coordinated ethylene glycol and an ester is formed. The above steps are repeated and the hydrogen atoms coordinated with Co get detached with the evolution of hydrogen gas, while ethylene glycol is oxidized to polyglycolic acid. The analytical techniques and the results obtained in this study could be used to enhance the properties of metals and alloy nanoparticles synthesized using the polyol process.