Abstract:

"Unprotected" metal and alloy nanoclusters prepared using the alkaline-ethylene glycol method (AEGM), stabilized by adsorbed solvent molecules and simple ions, have been widely applied in the development of high-performance heterogeneous catalysts and the exploration of the effects of metal particle size and composition, surface ligands of support, and modifiers on the catalytic properties of heterogeneous catalysts. The formation process and mechanism of such unprotected metal nanoclusters need to be further investigated. In this study, the formation process and mechanism of unprotected Pt and Ru nanoclusters prepared with AEGM were investigated by in situ quick X-ray absorption fine spectroscopy (QXAFS), in situ ultraviolet-visible (UV-Vis) absorption spectroscopy, transmission electron microscopy, and dynamic light scattering. It was discovered that during the formation of unprotected Pt nanoclusters, a portion of Pt(Ⅳ) species was reduced to Pt(Ⅱ) species at room temperature. With increasing temperature, Cl^- coordinated to Pt ions was gradually replaced with OH^- to form intermediate platinum complexes, which further condensated to form colloidal nanoparticles. Obvious scattering signals of the colloidal nanoparticles could be observed in the UV-Vis absorption spectra of the reaction system before the formation of Pt-Pt bonds, as revealed by QXAFS measurements. In situ QXAFS analysis revealed that Pt nanoclusters were derived from the reduction of Pt oxide nanoparticles. The average particle size of the nanoparticles obtained by heating the reaction mixture for 15 min at 80 ℃ was 3.7 nm. High resolution transmission electron microscopy (HRTEM) images showed that the spacing between the crystal planes of the nanoparticles was 0.249 nm, indicating that the intermediate nanoparticles were platinum oxide. As the reaction proceeded, the average size of the nanoparticles decreased to 2.4 nm, and two types of nanoparticles were observed having different contrasts, corresponding to Pt metal nanoclusters standing on the intermediate metal oxide nanoparticles as confirmed by HRTEM images. When the reaction time was further extended, the average size of nanoparticles decreased to 1.4 nm, and the observed lattice spacing of the nanoparticles was the same as that of Pt(111) crystal plane at 0.227 nm, indicating that the final products were Pt metal nanoclusters. In general, when metal oxides are reduced to metal nanoclusters, the density of the nanoparticles will increase, whereas the volume will decrease. Moreover, as shown in this study, the formation of multiple small metal nanoclusters standing on one metal oxide nanoparticle was also observed in TEM photographs. Thus, compared with the size of the initial nanoparticles, the average size of the final metal nanoclusters was significantly reduced. On the other hand, during the formation of unprotected Ru metal nanoclusters, Cl^- in RuCl₃ was first replaced with OH^- to form Ru(OH)₆^3-, which further condensated to form Ru oxide nanoparticles, and unprotected Ru metal nanoclusters were derived from the reduction of Ru oxide nanoparticles by ethylene glycol. Because of the formation of intermediate metal oxide nanoparticles in the reaction process, the subsequent rapid reduction reaction was confined to the nanoparticles, resulting in unprotected metal nanoclusters having a small size and narrow particle size distribution. This study is of significance to the development of high-performance energy conversion catalysts, fine chemical synthesis catalysts, sensors, and other functional systems.

Key words: Metal nanocluster, Unprotected metal nanoclusters, Formation mechanism, Pt nanolcuster, Ru nanocluster