This work demonstrates
the synthesis of an efficient photocatalyst,
Au25(PPh3)10Cl2(SC3H6SiO3)5/TiO2,
for selective oxidation of amines to imines. The photocatalyst is
prepared via hydrolysis of Au25(PPh3)10Cl2[(SC3H6Si(OC2H5)3]5 nanoclusters in the presence of
TiO2 support. The gold nanoclusters exhibit good photocatalytic
activity using visible light and under mild thermal conditions for
the selective oxidation with molecular oxygen (O2). The
turnover frequency (TOF) of 4-methylbenzylamine oxidation is found
to be 1522 h–1, which is considerably higher than
that conventional gold catalysts. The gold nanoclusters present good
recyclability and stability for the oxidation of a wide range of amines.
The superior activity of the photocatalyst is associated with its
unique electronic structure and framework. The catalytically active
sites are deemed to be the exposed gold atoms upon detaching protecting
ligands: i.e., PPh3. The Hammett parameter suggests that
the photocatalytic process involves the formation of carbocation intermediate
species. Further, Au–H species were confirmed by TEMPO (2,2,6,6-tetramethylpiperidinyloxy)
as a trapping agent.
We
here report a protocol for the synthesis of Au38S2(SAdm)20 nanoclusters (−SAdm = 1-adamantanethiolate)
with a higher production yield (10%) in comparison to previous reported
methods. The photosensitizing properties of the gold nanoclusters
are investigated for the formations of singlet oxygen (1O2) using visible light wavelengths of 532 and 650 nm.
The formation of 1O2 was detected by 1,3-diphenylisobenzofuran
as the chemical trapping probe as well as direct observation of the
characteristic 1O2 emission (ca. 1276 nm). The
efficiency of the 1O2 formation using the Au38S2(SAdm)20 nanoclusters is found to
be notably higher than that of Au25(SR)18 nanoclusters.
Finally, selective aerobic oxidations of sulfide to sulfoxide and
benzylamine to imine in the presence of oxygen (3O2) and photoexcited Au38S2(SAdm)20 are well studied. This work demonstrates the promise of
Au38S2(SAdm)20 nanoclusters in the
generation of activated singlet oxygen for selective catalytic reactions.
By direct video monitoring of dynamic colloidal self-assembly during solvent evaporation in a sessile drop, we investigated the effect of surface charge on the ordering of colloidal spheres. The in situ observations revealed that the interaction between charged colloidal spheres and substrates affects the mobility of colloidal spheres during convective self-assembly, playing an important role in the colloidal crystal growth process. Both ordered and disordered growth was observed depending on different chemical conditions mediated by surface charge and surfactant additions to the sessile drop system. These different self-assembly behaviors were explained by the Coulombic and hydrophobic interactions between surface-charged colloidal spheres and substrates.
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