Unravelling Site-Specific Photo-Reactions of Ethanol on Rutile TiO2(110)

Abstract

Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental understanding and the development of efficient catalytic systems. Here we have studied the photo-activated dehydrogenation of ethanol on reduced and oxidized rutile TiO2(110) in ultrahigh vacuum conditions. Utilizing scanning tunnelling microscopy, various spectroscopic techniques and theoretical calculations we found that the photo-reaction proceeds most efficiently when the reactants are adsorbed on regular Ti surface sites, whereas species that are strongly adsorbed at surface defects such as O vacancies and step edges show little reaction under reducing conditions. We propose that regular Ti surface sites are the most active sites in photo-reactions on TiO2.

Figure 1. STM study of EtOH photo-reactions on r- and o-TiO₂(110) surfaces.

(a) STM image (420 Å × 420 Å) obtained after exposing an r-TiO₂(110) surface to EtOH at 300 K. (b) Zoom-in STM image (130 Å × 130 Å) corresponding to the data in (a) that allows to distinguish between EtOH_Ti/EtO_Ti and EtO_br species. (c) Zoom-in STM image (130 Å × 130 Å) obtained after exposing an o-TiO₂(110) surface to EtOH at 300 K. Insets in (b,c) display (40 Å × 40 Å) areas on the pristine r- and o-TiO₂(110) surfaces. (d–f) Corresponding STM images obtained after illumination of the EtOH-covered surfaces with UV-light for 11 min at 290 K in UHV. Insets in (a,d) show the indicated areas (40 Å × 40 Å) enlarged. (g–i) Corresponding STM images obtained after subsequent annealing of the TiO₂(110) crystals for 2 min at ~550 K (g,h) and ~630 K (i), respectively. Symbols indicate O_br vacancies (squares), an O_ot adatom (circle), EtO_br ethoxides (dotted circles), EtO_S ethoxides (arrows), isolated H_ad species (hexagons), and rows of H_ad species (rectangles), respectively. The Ti troughs are indicated in (b,c) and in the insets of (a,d) by thin white lines. STM images were collected with a tunnelling current ≤0.1 nA and a tunnelling voltage of ~1.2 V. The STM images are shown enlarged in the Supplementary Information (Figs S1–S9).

Figure 2. ISOMS data acquired during EtOH photo-reaction at 290 K.

Black curve: r-TiO₂(110); red curve: o-TiO₂(110). Prior to the experiments both TiO₂(110) surfaces were saturated with EtOH at 300 K. The inset displays the experimental setup. The quadrupole mass spectrometer (QMS), the TiO₂(110) crystal (light brown block) and the direction of the incident light are indicated.

Figure 3. STM study of the photo-reaction on EtOH/r-TiO₂(110) in oxygen (fate of EtO_br ethoxides).

(a,b) STM images (240 Å × 240 Å and 68 Å × 68 Å, respectively) of an illuminated EtOH/r-TiO₂(110) surface. The UV-light illumination was accomplished at 290 K in UHV. (c,d) STM images of the same sample after illumination in 5 × 10⁻⁸ mbar O₂ at 290 K for 25 min. (e,f) STM images of the same sample after additional illumination in 5 × 10⁻⁸ mbar O₂ at 290 K for 20 min. Symbols indicate H_ad species (hexagons), EtO_S ethoxides (white arrows) and EtO_br ethoxides (black crosses). Occupied adsorption sites of unidentified products “A” (dotted black circle) and “B” (black circle) are indicated by green and pink dots, respectively. Lattice grids in (b,d,f) are centred on-top of 5f-Ti sites.

Figure 4. STM study of the photo-reaction on EtOH/r-TiO₂(110) in oxygen (fate of EtO_S ethoxides).

(a) STM image (220 Å × 220 Å) of an illuminated EtOH/r-TiO₂(110) surface. The UV-light illumination was accomplished at 290 K in UHV. Few EtO_S ethoxides are indicated by white arrows. (b) STM image of the same sample after illumination in 5 × 10⁻⁸ mbar O₂ at 290 K for 45 min. White lines are superimposed on the Ti troughs. These STM images are shown with higher contrast than the images in the other figures.

Figure 5. Photo-reactions on EtOH/o-TiO₂(110) and EtOH/h-TiO₂(110) surfaces studied by PES.

(a) C1s PES data (dots) collected with photon energy (hν) of 350 eV on EtOH/o-TiO₂(110) following UV light illumination at 290 K for 0 and 11 min, respectively. Spectra are offset for clarity. Gaussian fits to the data are shown as red curves. (b) Logarithmic plot of the normalized integrated areas of the C1s spectra as function of illumination time (filled green and red dots) and X-rays (open green and red dots), respectively. Starting points in these experiments were h- (green) and o-TiO₂(110) surfaces (red) that were saturated with EtOH at 290 K. Full lines are exponential fits to the data points, and the shaded areas display the standard deviations of these fits. Contributions of the non-reactive EtO_S ethoxides (I_S) have been subtracted from the integrated intensity I_m of the measured C1s spectra. Accordingly, the plot shows the contributions to the C1s signals arising exclusively from the EtOH_Ti and EtO_Ti species, I_Ti (C1s).

Figure 6. DFT modelling of the acetaldehyde formation on TiO₂(110).

Energy profiles for EtOH_Ti adsorbed on (a) and [TiO₂(110)]⁰ (d) supercells. The formation of EtO_Ti ethoxides is considered in (b,e), and the formation of acetaldehyde species (CH₃CHO_Ti) is addressed in (c,f). All adsorption energies are given in eV. Small red-brown balls represent 5f-Ti atoms, large dark-grey balls O_br atoms and large light-grey balls in-plane O atoms. Atoms of the adsorbates are displayed as follows: C atoms: black balls; H_ad species: small yellow balls; O atoms: large pink balls. Corresponding band schemes, each consisting of the conduction (CB) and valence band (VB), are shown in the insets of (b,c,e,f). Filling of the bands is indicated by dark yellow colour, and positions of the Fermi level (E_F) are indicated by dashed lines.