Initial Reaction Mechanism of Platinum Nanoparticle in Methanol-Water System and the Anomalous Catalytic Effect of Water

Understanding the detailed reaction mechanism in the early stage of noble metal nanoparticles is very critical for controlling the final crystal's size, morphology, and properties. Here, we report a systematic study on the initial reaction mechanism of Pt nanoparticles in methanol-water system and demonstrate an anomalous catalytic effect of H2O on the reduction of H2PtCl6 to Pt nanoparticles using a combination of UV-vis, X-ray absorption spectroscopy (XAS), liquid chromatography mass spectrometry (LCMS), and first-principles calculation methods. The observations reveal the transformation route [PtCl6](2-) → [PtCl5(CH3O)](2-) → [PtCl4](2-) → [PtCl3(CH3O)](2-) → [PtCl2](2-) and finally to form Pt nanoparticles in a pure CH3OH solution. With 10 vol % water adding in the CH3OH solution, a new and distinct chemical reduction pathway is found in which the precursors change from [PtCl6](2-) to [PtCl5(CH3O)(H2O)](2-) to [PtCl4](2-) to [PtCl3(CH3O)(H2O)](2-) to [PtCl2](2-) and to Pt nanoparticles. Notably, the supernumerary water molecular can significantly accelerate the rate of chemical reduction and greatly shorten the reaction time. This work not only elucidates the initial reaction mechanism of Pt nanoparticles but also highlights the pronounced influence of H2O on the reaction pathway, which will provide useful insights for understanding the formation mechanism of noble metal nanoparticles and open up a high efficient way to synthesize new functional nanomaterial.