A versatile route to core-shell catalysts: synthesis of dispersible M@oxide (M=Pd, Pt; oxide=TiO2, ZrO2) nanostructures by self-assembly.

A method, based on self assembly, for preparing core-shell nanostructures that are dispersible in organic solvents is demonstrated for Pd and Pt cores with CeO(2), TiO(2), and ZrO(2) shells. Transmission electron microscopy (TEM) of these nanostructures confirmed the formation of distinct metal cores, approximately 2 nm in diameter, surrounded by amorphous oxide shells. Functional catalysts were prepared by dispersing the nanostructures onto an Al(2)O(3) support; and vibrational spectra of adsorbed CO, together with adsorption uptakes, were used to demonstrate the accessibility of the metal core to CO and the porous nature of the oxide shell. Measurements of water-gas-shift (WGS) rates demonstrated that these catalysts exhibit activities similar to that of conventional supported catalysts despite having lower metal dispersions. Pd-based CeO(2) and TiO(2) core-shell catalysts exhibit significant transient deactivation, which is probably caused by a decrease in the exposed metal surface area due to the ease of reduction of the shells. Alternatively, Pt-based analogous core-shell catalysts do not exhibit such a transient decrease. Both Pd- and Pt-based ZrO(2) core-shell catalysts deactivate at a significantly lower rate due to the less reducible nature of the ZrO(2) shell.