The wet synthesis and quantification of ligand-free sub-nanometric Au clusters in solid matrices

Abstract

The synthesis of ligand-free sub-nanometric metal clusters on a large scale suffers typically from very low yields (<5% yield) and needs very high dilutions. Here we show that Au clusters can be prepared with ethylene-vinyl alcohol copolymers (EVOH), charcoal, and different metal oxides (CeO, AlO, TiO and ZnO) in >15% yields, as unambiguously determined using a very simple and extremely sensitive analytical reaction test.

Disassembling Metal Nanocrystallites into Sub‐nanometric Clusters and Low‐faceted Nanoparticles for Multisite Catalytic Reactions

“…In contrast, the filtrates of a sample of Au–nCeO 2 heated in DMF at 135 °C for 10 min, without any other added reagent, catalyze the Sonogashira coupling in the presence of supported Au NPs, and analysis of this solution by UV/Vis spectroscopy, electrospray ionization mass spectrometry (ESI‐MS) and matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) spectroscopy show the presence of Au 3–7 clusters, and not of Au NPs (Figure S3 in the Supporting Information). This solution contains 17 % of the original Au present in Au–nCeO 2 , as assessed by inductively coupled plasma atomic emission spectroscopy (ICP‐AES) after hot filtration, and this value correlates well with the total amount of Au clusters in Au–nCeO 2 . These results indicate that the Au NPs remain immobilized during the coupling whereas the Au 3–7 clusters leach into the solution, ready to interact with the reagents and furnish a bifunctional Au catalyst for the Sonogashira reaction if using DMF as a solvent.…”

“…Table shows that a combination of low‐faceted, commercially available metallic Au NPs of approximately 3 nm and sub‐nanometric Au clusters catalyzes the Sonogashira coupling between iodobenzene 1 and phenylacetylene 2 , in dimethylformamide (DMF) at 135 °C with K 3 PO 4 as a base, with much higher selectivity, initial rate, and final yield than any other Au catalyst tested, including both species used separately, Au NCs of approximately 14 nm particle size, and Au salts and complexes (see Figures S1–S2 and Table S1 in the Supporting Information). Different sources of Au clusters and NPs, including Au–nCeO 2 , Au@ethylene–vinyl alcohol (EVOH) copolymer, Au–C, and three Au‐supported inorganic oxides for clusters, and commercially available and in‐house prepared Au–TiO 2 and Au–Al 2 O 3 for NPs give similar catalytic results, which illustrates the robustness of the methodology.…”

“…The addition of Au clusters and NPs separately is clearly more advantageous than using a solid with both species together. For instance, the direct disassembling of Au NCs into clusters and NPs was carried out for Au–TiO 2 by the I 2 method, which transforms the Au NCs into 7 % clusters and some remaining NPs. The catalytic results (Table ) show that the catalytic activity improves but not as much as if the two Au species are independently added, as the relative population of Au catalytic species on TiO 2 after I 2 treatment is interconnected with and limited by the nominal amount of Au (1 wt %) on the solid.…”

“…Then, the hot supernatant was passed through a syringe filter and an aliquot was taken for analysis by ICP‐AES, as indicated above. The re‐dispersion of Au‐supported catalysts was carried out by treating Au–Al 2 O 3 or Au–TiO 2 (500 mg) with a solution of I 2 (50 mg) in 1,2‐dichloroethane (1,2‐DCE) at 60 °C for 24 h . After cooling to RT, the solids were filtered and washed with water and acetone.…”

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles

“…By a top-down approach, subnanometric Au clusters can be generated on solid support (such as Al 2 O 3 , TiO 2 , ZnO, CeO 2 , and carbon) after the treatment of supported Au nanoparticles with I 2 solution, and those Au clusters are efficient catalyst for ester-assisted hydration of the alkyne. 351 Because it has also been observed that the in situ formation of Au clusters (Au 3 –Au 6 ) play a key role in some other reactions, this may indicate that the transformation of mononuclear Au precursor into Au clusters can be a common phenomenon in Au-catalyzed organic reactions, including phenol synthesis from alkynylfurans rearrangement, w -bromination of phenylacetylene, bromination-hydration cascade reaction, and Conia–ene reaction of alkyne. 352 The in situ formation process of Au clusters can be affected by the reaction environment.…”