Propene epoxidation over TiO2-supported Au-Cu alloy catalysts prepared from thiol-capped nanoparticles

Propene oxidation to propene oxide (PO) was performed with N2O in the temperature range of 473-673 K using TiO2-supported Au and Au-Cu alloy nanoparticles synthesized from pre-formed thiol-capped nanoparticles of controlled composition and size. Catalysts were activated by calcination at different temperatures in the range 573-873 K and characterized by HRTEM, XPS, and TPR. Among a series of catalysts with different Au/Cu ratio and metal loading, the Au-1 Cu-3/TiO2 with 12 wt% exhibited the best catalytic performance for epoxidation, both in terms of propene conversion rate and selectivity towards PO (0.25 mol PO g(M)(-1) h(-1) at 573 K). The highest TOF was obtained over a catalyst calcined at 673 K. At this calcination temperature, HRTEM revealed a large perimeter interface between the nanoparticles and the support, which was accompanied by an intense TPR hydrogen uptake at low temperature. At increasing calcination temperature, the surface of Au-Cu alloy nanoparticles was progressively decorated with oxidized Cu species, which were detrimental for epoxidation and favored allylic oxidation products. Isolation effects and control of the extent of Cu oxidation by Au in the alloy nanoparticles as well as the perimeter interface between Au-Cu alloy nanoparticles and TiO2 are imagined to play pivotal roles in the epoxidation of propene. (C) 2008 Elsevier Inc. All rights reserved.