Nano gold particles supported on titanium silicalite-1 catalysts (Au/TS-1) have shown high propylene oxide (PO) rate (similar to 160 g(PO) h(-1) kg(cat)(-1)) at similar to 200 degrees C and 1 atm for gas-phase epoxidation of propylene in the presence of hydrogen and oxygen. In this work, we address the issue of whether gold clusters inside TS-1 can be the active sites for the PO reaction. Coating TS-1 seeds with silicalite-1 produced a material (S-1/TS-1), which possessed a well-defined MFI structure, as shown by XRD and selected area electron diffraction, and Ti on the external surface could not be detected by XPS after addition of gold by deposition precipitation. The Au/S-1/TS-1 catalyst had at least 20 times higher PO rate (per gram of catalyst) at similar to 200 degrees C compared to that of a Au/S-1 sample with similar gold loading at similar to 0.05 wt%. Since neither Au nor Ti alone gives any significant PO activity, this is the first direct experimental evidence showing that gold clusters inside the TS-1 are active for the PO reaction at H-2/C3H6/O-2/N-2 = 3.5/3.5/3.5/24.5 mL min(-1) and a space velocity of 14,000 mL h(-1) g(cat)(-1). Furthermore, a much higher PO rate per gram of TS-1 for the Au/S-1/TS-1 catalyst (similar to 240g(PO) h(-1) kg(TS-1)(-1)) than expected for a Au/TS-1 with similar gold loading at similar to 0.05 wt% and Si/Ti molar ratio from 65 to 100 (similar to 60-140 g(PO) h(-1) kg(TS-1)(-1)) implies that Au clusters inside the TS-1 nanoporous channels can be the dominant active sites for the PO reaction. (C) 2012 Elsevier Inc. All rights reserved.