Insight into the Formation Mechanism of "Unprotected" Metal Nanoclusters

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 Xray 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 C 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 met...

Highly selective synthesis of multicarbon compounds by carbon dioxide hydrogenation over Pt nanocrystals anchoring Ru clusters

“…In a typical experiment for preparing Ru-co-Pt/C (see ESI † for details), a colloidal solution of "unprotected" Ru NCs 24,25 was added to an acetone solution of 4,4′-bipyridine (BIPY) under stirring to form BIPY-linked Ru NCs, which could be separated as a precipitate by centrifugation. Subsequently, a colloidal solution of "unprotected" Pt NCs was added into a dispersion of BIPY-linked Ru NCs in acetone to produce nanoaggregates (Ru-bi-Pt).…”

Copper Single-Atom-Covered Pt Nanoparticles for Selective Hydrogenation of Phenylacetylene

“…The Pt nanoparticles in ethylene glycol are protected with simple ions, which can be adsorbed on the support, and the simple ions on the surface of Pt nanoparticles can be readily removed. 30,31 The advantage of Cu(NH 3 ) 4 (OH) 2 over copper salts is that after removing the NH S1.…”

“…The Pt–Cu bimetallic catalysts were prepared from the “unprotected” Pt nanoparticles and Cu­(NH 3 ) 4 (OH) 2 . The Pt nanoparticles in ethylene glycol are protected with simple ions, which can be adsorbed on the support, and the simple ions on the surface of Pt nanoparticles can be readily removed. , The advantage of Cu­(NH 3 ) 4 (OH) 2 over copper salts is that after removing the NH 3 ligands the deposited Cu species on the support at a relatively low temperature is Cu­(OH) 2 or CuO, which can be directly reduced to metallic Cu, and the conversion from copper salts to CuO at high temperature can be avoided. The Pt/C–Cu and Cu/C–Pt catalysts were prepared by changing the order of deposited Pt and Cu­(OH) 2 (or CuO) (Scheme ), and the order of Pt and Cu in the catalysts’ names represents the deposition order of Pt and Cu.…”

Metal Nanoclusters Synthesized in Alkaline Ethylene Glycol: Mechanism and Application

“…In a typical experiment for the preparation of unprotected metal nanoclusters by using the alkaline EG method, the pH of a glycol solution of a metal compound such as H 2 PtCl 6 ·6H 2 O, RhCl 3 ·3H 2 O or RuCl 3 ·3H 2 O was adjusted to a value that is higher than 12 by adding a glycol or water solution of NaOH at room temperature. Then, the mixture was heated at a temperature in the range from 80 to 198 °C, with a N 2 flow passing through the reaction system to carry away the water and by-products to produce a stable colloidal solution of unprotected metal clusters stabilized with simple ions and solvent molecules with an average diameter of 1.1–2.4 nm [ 10 , 102 ].…”

“…A main difference between the different views is whether metal ion compound nanoparticles form first or the metal ions are reduced to zero valence metal atoms first [ 10 , 97 , 98 , 99 , 100 , 101 ]. Recently, the evolution of metal species in the alkaline EG method was intensively studied by using the combined technologies of in situ time-resolved X-ray absorption fine structures (XAFS) and ultraviolet-visible absorption spectroscopy (UV-vis) [ 102 ], which provided convincing evidence for the former mechanism.…”