Atomically well-defined gold clusters: Oxidation reactions catalized by ultra-small gold core
Résumé
Recently, atomically well defined thiolate-capped gold nanoclusters (denoted Aun(SR)m with (10,10) < (n,m) < (333,79)) have been successfully isolated and few structures have been solved. Thus, these monodispersed functionalized clusters, with precise atomic positions and gold core less than 2 nm, hold promises as a new generation of catalysts. More importantly, these nanoclusters permit in-depth studies on the subtle correlation of the structure, core diameter, thiolate ligands effect and electronic state of gold nanoparticles with their catalytic activity. New clusters made of 4-aminothiophenol (HSPhNH2) have been isolated, such as Au25(SPhNH2)17, and fully characterized by mass spectrometry, X-ray diffraction and XPS.3 Moreover these clusters exhibit absorption bands related to their molecular state. Those clusters have then be used to investigate in aerobic oxidations reactions and compared to the Au25(SC2H4Ph)18 cluster, having the same Au core but different thiolate ligand. They have been deposited on silica supports (hydrophobic 16 nm-SiO2 nanoparticles and SBA-15 mesoporous silica) and by properly selecting the support to maximize clusters/support interactions, it was possible to prepare materials with ultra-small gold particles (<2nm, fig. 1) and very narrow size distribution. We investigated the catalytic properties in aerobic oxidations of these sub-2 nm gold catalysts in (i) the aerobic epoxidation of trans-stilbene and (ii), the oxidative dehydrogenation of benzyl alcohol. By comparing the results for the different supported gold clusters, we pointed out that while still being functionalized, gold nanoclusters exhibit a good catalytic activity for the trans-stilbene epoxidation and that this catalytic activity is linked to the presence of the thiolate moiety.4 For the benzyl acohol oxidation, we showed that the presence of ligands is inhibiting the catalytic activity. This ligand effect observed with functionalized gold clusters makes possible to tune the overall catalytic selectivity and specificity to have chimio specific reactions on multi-modal sites. We showed here that thiolated gold nanoclusters compounds hold great interests and promises as a new class of catalysts thanks to their well-defined structure and to the ligands effect over the catalysis. [1] Qian et al., Acc. Chem. Res., 2012, 45, 1470 [2] R. Jin et al., Chem. Eur. J., 2011, 17, 6584. [3] C. Lavenn et al., Nanoscale, 2012, 4, 7334. [4] C. Lavenn et al., Cat. Tod., Accepted