| Abstract
| - Temperature-programmed desorption (TPD) and oxygen isotopiclabeling studies were used to probethe dissociation of water on the (100) and (110) surfaces ofTiO2 (rutile). Water TPD spectra fromthesetwo surfaces were distinctive. Three monolayer desorption stateswere observed for the (100) surface (at205, 250, and 305 K), while only a single desorption state was observedfor the (110) surface (at 270 K).TPD experiments on the surfaces enriched with 18Orevealed that water desorbing in the 305 K state fromTiO2(100) was isotopically scrambled with the latticeoxygen atoms, strongly suggesting that this TPDstate resulted from recombination of surface hydroxyl groups.Isotopic scrambling was not observed forany other desorption state on either surface (in the absence ofdefects). Since very little water desorptionoccurred from the (110) surface in the temperature range in whichexchange was observed on the (100)surface and since previous HREELS work (Henderson, M. A. Surf.Sci.1996, 355, 151) indicated thatverylittle water dissociation was detected for the (110) surface, the idealTiO2(110) surface appears to beinactive for water dissociation under ultrahigh vacuum (UHV)conditions. Comparison of the geometricarrangement of acidic and basic sites on these two surfaces suggeststhat the bridging two-coordinateO2-sites (basic sites) on TiO2(110) are too distant fromthe binding sites of water (five-coordinate Ti4+sites)to form hydrogen-bonding interactions with water that might facilitateO−H bond dissociation, whereasthe proximity of these sites on the TiO2(100) surfaceshould favor such a concerted interaction. TheTiO2(110) surface was active for dissociation of waterwhen structural defects such as steps or kinks werepresent. Defects created by annealing or by electron-beamirradiation were less active for water dissociation.
|
