Structure and Reactivity of Methanol Adsorbed on Rutile TiO₂(011) Surface

Abstract

The rutile TiO 2 (011) surface exhibits a (2 × 1) reconstruction when prepared by standard techniques in ultrahigh vacuum. However, the reconstructed surface is incomplete and discussed controversially for understanding the enhanced photoactivity occurred in most photocatalytic experiments. Herein, we investigated the reactivity of unreconstructed TiO 2 (011)-(1 × 1) and reconstructed TiO 2 (011)-(2 × 1) surfaces toward methanol adsorption and oxidation using first-principles calculations. The TiO 2 ( 011)-(1 × 1) surface is preferred energetically over the TiO 2 (011)-(2 × 1) surface at high methanol coverage, confirming that the interaction with methanol leads to an inversion of the stabilities of the reconstructed and unreconstructed surfaces. Then, we intensively investigated methanol oxidation into formaldehyde on the TiO 2 (011)-(1 × 1) surface and found that the methanol oxidation underwent the same pathways as those on the TiO 2 (011)-(2 × 1) and TiO 2 (110) surfaces. By means of the energy barriers of the rate-determining step (i.e., C−H cleavage), it is demonstrated that the unreconstructed TiO 2 (011) surface exhibits higher reactivity toward methanol oxidation than the reconstructed TiO 2 (011) and TiO 2 (110) surfaces. In addition, the calculated density of states further indicate that the TiO 2 (011) surface has stronger oxidative ability than TiO 2 (110) for photocatalysis. Thus, our findings suggest that the synergistic effect of surface atomic and electronic structures may play an important role in the enhanced photocatalytic activity of TiO 2 (011) observed in aqueous environments typical of most photocatalytic application of TiO 2 .

Effect of methanol and photoinduced surface oxygen vacancies on the charge carrier dynamics in TiO2

“…Nevertheless, TiO 2 has emerged as a focal point in photochemical studies. 12,[15][16][17] The effectiveness of TiO 2 for applications relies on the interaction between its surface and the adsorbate, which is influenced by its electronic structures. 18,19 To this end, recent work has investigated the charge carrier dynamics of TiO 2 .…”

“…[24][25][26] Being the simplest form of organic alcohol, methanol is utilized in various applications in photocatalytic systems, e.g., hole scavenger, hydrogen production, and reduction of CO 2 . [27][28][29] Even though the interaction between TiO 2 and methanol has been heavily investigated to reveal its effect on photocatalytic activity, 16,24,[30][31][32][33] a fundamental understanding of the dynamics of the charge carriers is still lacking. Likewise, the efficiency of photochemical processes is ARTICLE pubs.aip.org/aip/apm determined by the fraction of photogenerated charge carriers that reach the surface of the catalyst to react with adsorbed electron acceptors and donors.…”

“…14,34 Nevertheless, both methoxy and formaldehyde can be produced due to the photocatalytic reaction of methanol. 16 Their formation kinetics is based on the methanol's tendency to react with photogenerated agents, i.e., electrons and holes. 35,36 More specifically, the adsorption of methanol molecules on the TiO 2 surface can result in their reduction to methoxy anions through interaction with photogenerated electrons, 16 while holes act as oxidizing agents, leading to the formation of formaldehyde molecules under irradiation.…”

“…16 Their formation kinetics is based on the methanol's tendency to react with photogenerated agents, i.e., electrons and holes. 35,36 More specifically, the adsorption of methanol molecules on the TiO 2 surface can result in their reduction to methoxy anions through interaction with photogenerated electrons, 16 while holes act as oxidizing agents, leading to the formation of formaldehyde molecules under irradiation. 30,32 Therefore, it is essential to understand the dynamics of the charge carrier variations under representative conditions, i.e., at a wide range of operating temperatures and under high-energy ultraviolet (UV) irradiation, which leads to photoinduced surface oxygen vacancies (PI-SOVs).…”

“…The stability and mechanism of methanol adsorption are affected by the surface orientation, termination, defect density, and temperature. 16,30,61 Methanol can be adsorbed both molecularly and dissociatively on a TiO 2 (001) surface at 200 K. 62 However, at relatively higher temperatures, e.g., room temperature, molecular methanol is desorbed from the surface. 19,63 Nevertheless, it was shown that surface defects increase the coverage of methanol on the (100) and (110) TiO 2 surfaces.…”