Decomposition of Large Cu Crystals into Ultrasmall Particles Using Chemical Vapor Deposition and Their Application in Selective Propylene Oxidation.

In this work, we report a novel application of chemical vapor deposition (CVD) in which the calcination and reduction of Cu(thd)2 deposited onto 4.9 wt % Cu/SiO2 induces significant decomposition of 28 nm crystalline Cu into ultrasmall ∼2 nm particles (5.1 wt % Cu/SiO2). The Cu loading slightly increased, but the particle size dramatically decreased. The deposition of Cu(thd)2 onto the Cu surface can initially affect the size reduction of the metallic Cu particles due to charge transfer between Cu(thd)2 and the Cu surface. Thermal treatments, including calcination in air and reduction in H2, can further influence the Cu particle decomposition. The mechanism of change in the Cu particle decomposition was investigated by a variety of experiments, such as X-ray diffraction and in situ X-ray absorption spectroscopy. CVD treatment of Cu/SiO2 can create Cu-rich sites, which effectively enhance the conversion and acrolein yield in selective propylene oxidation. The intermediate associated with propylene oxidation on the Cu catalysts was also examined by IR spectroscopy.