Reduced Step Edges on Rutile ${\mathrm{TiO}}_2(110)$ as Competing Defects to Oxygen Vacancies on the Terraces and Reactive Sites for Ethanol Dissociation

The rutile ${\mathrm{TiO}}_2(110)$ surface is the most studied surface of titania and considered as a prototype of transition metal oxide surfaces. Reactions on flat ${\mathrm{TiO}}_2(110)\mathrm{\text{−}}(1\mathbf{\times }1)$ surfaces are well studied, but the processes occurring on the step edges have barely been considered. Based on scanning tunneling microscopy studies, we here present experimental evidence for the existence of O vacancies along the $⟨1\overline{1}1{⟩}_{\mathrm{R}}$ step edges (${\mathrm{O}}_{\mathrm{S}}$ vac.’s) on rutile ${\mathrm{TiO}}_2(110)$. Both the distribution of bridging O vacancies on the terraces and temperature-programed reaction experiments of ethanol-covered ${\mathrm{TiO}}_2(110)$ point to the existence of the ${\mathrm{O}}_{\mathrm{S}}$ vac.’s. Based on experiments and density functional theory calculations, we show that ${\mathrm{O}}_{\mathrm{S}}$ vac.’s are reactive sites for ethanol dissociation via O-H bond scission. Implications of these findings are discussed.

Figure 1

(a) Stoichiometric ${\mathrm{TiO}}_2(110)$ surfaces with reconstructed $⟨1\overline{1}1{⟩}_{\mathrm{R}}$ step edges, i.e., with additional ${\mathrm{TiO}}_2$ units. (b) Reconstructed $⟨1\overline{1}1{⟩}_{\mathrm{R}}$ step edge on $r\mathrm{\text{−}}{\mathrm{TiO}}_2(110)$, characterized by ${\mathrm{O}}_{\mathrm{S}}$ vac.’s. Light gray (yellow) dots indicate additional ${\mathrm{TiO}}_2$ units. In (b), ${\mathrm{O}}_{\mathrm{br}}$ vac.’s and ${\mathrm{O}}_{\mathrm{S}}$ vac.’s are indicated by squares and circles, respectively.

Figure 2

(a)–(c) Selected STM images ($410\AA \times 410\AA$) showing areas of (a) low, (b) medium, and (c) high total step length, all acquired from the same $r\mathrm{\text{−}}{\mathrm{TiO}}_2(110)$ surface [20]. In (a), black and white arrows indicate $⟨1\overline{1}1{⟩}_{\mathrm{R}}$ and $⟨001⟩$ step edges, respectively. The inset in (b) shows a $82\AA \times 82\AA$ large zoom-in on a terrace. (d) Zoom-in STM image ($82\AA \times 82\AA$) of the area indicated in (b). Squares indicate ${\mathrm{O}}_{\mathrm{br}}$ vac.’s. (e) Number of ${\mathrm{O}}_{\mathrm{br}}$ vac.’s [gray (red) bars] and $⟨1\overline{1}1{⟩}_{\mathrm{R}}$ step sites [dark gray (blue) bars] of 17 STM images ($410\AA \times 410\AA$) acquired on different regions on $r\mathrm{\text{−}}{\mathrm{TiO}}_2(110)$. Bars corresponding to the STM images depicted in (a)–(c) are indicated. The hatched gray (red) bar on the left shows the number of ${\mathrm{O}}_{\mathrm{br}}$ vac.’s in ten $130\AA \times 130\AA$ areas located far from any step edges. The total area of these ten areas almost equals that of the 17 $410\AA \times 410\AA$ STM images. The dashed line indicates the average sum of ${\mathrm{O}}_{\mathrm{br}}$ vac.’s and $⟨1\overline{1}1{⟩}_{\mathrm{R}}$ step sites. (f) Number of ${\mathrm{O}}_{\mathrm{br}}$ vac.’s plotted against the number of $⟨1\overline{1}1{⟩}_{\mathrm{R}}$ step sites corresponding to the data shown in (e).

Figure 3

STM images ($390\AA \times 390\AA$) of $\mathrm{EtOH}/r\mathrm{\text{−}}{\mathrm{TiO}}_2(110)$ prepared by saturating $r\mathrm{\text{−}}{\mathrm{TiO}}_2(110)$ surfaces with EtOH at 270 K and flashing to (a) 380 K and (b) 570 K, respectively [20]. Insets (i) were acquired from other areas on the two surfaces. Insets (ii) are enlargements of the areas indicated in (a) and (b), respectively. All insets show areas of $72\AA \times 72\AA$. Symbols indicate ${\mathrm{EtO}}_{\mathrm{br}}$ ethoxides (dotted circles), ${\mathrm{H}}_{\mathrm{ad}}$ species (hexagons), and ${\mathrm{O}}_{\mathrm{br}}$ vac.’s (squares), respectively. (c) Corresponding TPD/TPR experiments with $m/e=26$ (${\mathrm{C}}_2{\mathrm{H}}_4$) and $m/e=31$ (EtOH) traces. EtOH cracking fragments to the $m/e=26$ signal were substracted. For “experiment I,” the $m/e=18$ signal (${\mathrm{H}}_2\mathrm{O}$) is also shown (scaled by 0.25). Black arrows in (c) indicate the flashing temperatures.

Figure 4

Computed most stable EtOH adsorption structures at reduced $⟨1\overline{1}1{⟩}_{\mathrm{R}}$ step edges. (a) Dissociatively adsorbed EtOH at the end of the Ti troughs, consisting of ${\mathrm{EtO}}_{\mathrm{S}\mathrm{\text{−}}\mathrm{Ti}}$ and ${\mathrm{H}}_{\mathrm{ad}}$ species. (b) Dissociatively adsorbed EtOH at the end of the ${\mathrm{O}}_{\mathrm{br}}$ rows, consisting of ${\mathrm{EtO}}_{\mathrm{S}}$ and ${\mathrm{H}}_{\mathrm{ad}}$ species. Atoms of the adsorbates are shown as follows: C atoms: dark gray (black) balls; O atoms: large light gray (pink) balls; H atoms: small light gray (yellow) balls. Circles indicate ${\mathrm{O}}_{\mathrm{S}}$ vac.’s. (c) Potential energy curves of EtOH at reduced $⟨1\overline{1}1{⟩}_{\mathrm{R}}$ step edges. The solid light gray curve describes the dissociation of ${\mathrm{EtOH}}_{\mathrm{S}\mathrm{\text{−}}\mathrm{Ti}}$ to ${\mathrm{EtO}}_{\mathrm{S}\mathrm{\text{−}}\mathrm{Ti}}$ and ${\mathrm{H}}_{\mathrm{ad}}$ species, with the two local minima corresponding to configurations where the ${\mathrm{H}}_{\mathrm{ad}}$ species are adsorbed on the O atoms indicated in (a). The dashed black curve describes the diffusion of ${\mathrm{EtOH}}_{\mathrm{S}\mathrm{\text{−}}\mathrm{Ti}}$ into an ${\mathrm{O}}_{\mathrm{S}}$ vac. and the solid black curve its subsequent dissociation. Potential energies are with respect to EtOH in the gas phase (${\mathrm{EtOH}}_{\mathrm{g}}$).