Hydrogen reactivity on highly-hydroxylated TiO2(110) surfaces prepared viacarboxylic acid adsorption and photolysis

Abstract

Combined scanning tunneling microscopy, temperature programmed desorption, photo stimulated desorption, and density functional theory studies have probed the formation and reactivity of highly-hydroxylated rutile TiO(2)(110) surfaces, which were prepared via a novel, photochemical route using trimethyl acetic acid (TMAA) dissociative adsorption and subsequent photolysis at 300 K. Deprotonation of TMAA molecules upon adsorption produces both surface bridging hydroxyls (OH(b)) and bidentate trimethyl acetate (TMA) species with a saturation coverage of nearly 0.5 monolayers (ML). Ultra-violet light irradiation selectively removes TMA species, producing a highly-hydroxylated surface with up to ~0.5 ML OH(b) coverage. At high coverages, the OH(b) species typically occupy second-nearest neighbor sites along the bridging oxygen row locally forming linear (2 × 1) structures of different lengths, although the surface is less ordered on a long scale. The annealing of the highly-hydroxylated surface leads to hydroxyl recombination and H(2)O desorption with ~100% yield, thus ruling out the diffusion of H into the bulk that has been suggested in the literature. In agreement with experimental data, theoretical results show that the recombinative H(2)O desorption is preferred over both H bulk diffusion and H(2) desorption processes.

Molecular Hydrogen Formation from Photocatalysis of Methanol on TiO2(110)

“…This result is fortuitously consistent with previous theoretical calculations, which show that the barrier for H 2 recombinative desorption from BBO sites on TiO 2 (110) is ∼1.6 eV, which is considerably higher than the barrier (1.10 eV) for H 2 O desorption from BBO sites. 42 In ref 42, no H 2 product was detected from the highly hydroxylated TiO 2 (110) surface and was attributed to this high energy barrier. From these results, they reached a conclusion that hydrogen recombination is not possible on TiO 2 (110), whereas our result clearly indicates that hydrogen recombination on TiO 2 (110) can happen.…”

Ability ofTiO2(110)surface to be fully hydroxylated and fully reduced

“…We have recently introduced the novel photochemical route for preparation of highlyhydroxylated (up to 0.5 ML OH coverage) TiO 2 (110) surface, which is based on deprotonation and subsequent photolysis of the trimethyl acetic acid (TMAA) [19]. Lately, this technique has been adopted by using photo-splitting of methanol for TiO 2 (110) hydroxylation [20].…”

“…Therefore, for the saturation TMA coverage of 0.5 ML, an additional 0.5 ML of OH b 's is generated concurrently. The subsequent UV irradiation selectively removes all TMA species while leaving OH b species intact [19,23]. As a result, Step II produces a highly-hydroxylated surface with accumulated Θ(OH b ) above 0.5 ML, which is also TMA-free.…”

“…It adds still more observation while residing beside much larger TMA species [24,25] After prolonged UV irradiation in the second segment of Step II, practically all TMA's are photodepleted, making the non-photoreactive OH b species visible in Fig. 2(d) [19,21]. While the surface is somewhat less ordered on a long-range scale, locally, the onset of the across-row alignment of the OH b "chains" Hence, we speculate that this process is likely responsible for a limiting maximum OH b coverage of the hydroxylated TiO 2 (110) surface to a little less than 1 ML.…”

Molecular Hydrogen Formation from Proximal Glycol Pairs on TiO2(110)

“…15−21 In line with these observations, theoretical calculations demonstrate that the kinetic barrier for H 2 recombinative desorption from HO b sites on TiO 2 (110) is considerably higher than the barrier for water formation. 20,21 However, in a recent temperature-programmed desorption (TPD) study, 22 Xu et al reported that D 2 is formed as a minor product via thermal recombination of the D atoms on O b sites from photocatalysis of methanol. Interestingly, hydrogen desorption between 375 and 500 K was also observed in our recent study of ethylene glycol (EG) reactions on TiO 2 (110).…”