Gold, Copper, and Platinum Nanoparticles Dispersed on CeO_x/TiO_2(110) Surfaces: High Water-Gas Shift Activity and the Nature of the Mixed-Metal Oxide at the Nanometer Level

摘要

At small coverages of ceria on TiO_2(110), the CeO_x nanoparticles have an unusual coordination mode. Scanning tunneling microscopy and density-functional calculations point to the presence of Ce_2O_3 dimers, which form diagonal arrays that have specific orientations of 0, 24, and 42° with respect to the [1 -1 0] direction of the titania substrate. At high coverages of ceria on TiO_2(110), the surface exhibits two types of terraces. In one type, the morphology is not very different from that observed at low ceria coverage. However, in the second type of terrace, there is a compact array of ceria particles with structures that do not match the structures of CeO_2(111) or CeO_2(110). The titania substrate imposes on the ceria nanoparticles nontypical coordination modes, enhancing their chemical reactivity. This phenomenon leads to a larger dispersion of supported metal nanoparticles (M = Au, Cu, Pt) and makes possible the direct participation of the oxide in catalytic reactions. The M/CeO_x/TiO_2(110) surfaces display an extremely high catalytic activity for the water-gas shift reaction that follows the sequence Au/CeO/TiO_2(110) < Cu/CeO_x/TiO_2(110) < Pt/ CeO_x/TiO_2(110). For low coverages of Cu and CeO_x, Cu/CeO_x/TiO_2(110) is 8-12 times more active than Cu(111) or Cu/ZnO industrial catalysts. In the M/CeO_x/TiO_2(110) systems, there is a strong coupling of the chemical properties of the admetal and the mixed-metal oxide: The adsorption and dissociation of water probably take place on the oxide, CO adsorbs on the admetal nanoparticles, and all subsequent reaction steps occur at the oxide-admetal interface. The high catalytic activity of the M/CeO_x/TiO_2(110) surfaces reflects the unique properties of the mixed-metal oxide at the nanometer level.