Direct visualization of 2-butanol adsorption and dissociation on TiO2(110)

High-resolution scanning tunneling microscopy (STM) images of identical regions of a TiO2(010) surface were gathered before and after controlled doses of 2-butanol (CH3CH2CH(OH)CH3) at ambient temperature (similar to 300 K). When dosing is initiated, 2-butanol preferentially adsorbs at bridge-bonded oxygen vacancy (BBOV) 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 BBOV'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 Ti4+ 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 Ti4+ rows with a preference for lying parallel to these rows. On the other hand, the carbon backbones of the 2-butoxy species that fill BBOV's are centered over the O2- 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 Ti4+-bound 2-butanol to stationary BBOV-bound 2-butoxy species decreases for doses that do not fill all the BBOV's.