Direct Visualization of 2-Butanol Adsorption and Dissociation on TiO2(110)

High-resolution scanning tunneling microscopy (STM) images of identical regions of a TiO 2 (110) surface were gathered before and after controlled doses of 2-butanol (CH 3 CH 2 CH(OH)CH 3 ) at ambient temperature (∼300 K). When dosing is initiated, 2-butanol preferentially adsorbs at bridge-bonded oxygen vacancy (BBO V ) sites and dissociates via O−H, not C−O, bond scission to form paired 2-butoxy and hydroxyl species evidenced by two local maxima in STM line profiles. As the dose increases, but before all the BBO V 's are occupied, there is direct STM evidence of hydrogen hopping of the hydroxyl to an adjacent oxygen anion row. This process is facilitated by species bound to 5-coordinate Ti 4+ rows, presumably undissociated 2-butanol, that hop slowly on the STM imaging time scale. The carbon backbones of these mobile species are centered over the Ti 4+ rows with a preference for lying parallel to these rows. On the other hand, the carbon backbones of the 2-butoxy species that fill BBO V 's are centered over the O 2- rows and prefer an orientation perpendicular to these rows. As the oxygen vacancy concentration increases from 0.4 to 11% and 2-butanol is dosed, the ratio of mobile Ti 4+ -bound 2-butanol to stationary BBO V -bound 2-butoxy species decreases for doses that do not fill all the BBO V 's.