Adsorption and Reaction of Methanol on Anatase TiO₂(101) Single Crystals and Faceted Nanoparticles

Abstract

Anatase TiO2 is used extensively in a wide range of catalytic and photocatalytic processes. As such, understanding its interactions with prototypical organics such as methanol is of high importance. In this study, we employ a cross-disciplinary approach and study adsorption and dissociation of methanol on the (101) surfaces of anatase TiO2 single crystals (SCs) and faceted nanoparticles (F-NPs). While scanning tunneling microscopy was used to follow the spatial distribution of adsorbed methanol molecules and methoxy and hydroxy species on SC TiO2(101), comparative temperature-programmed desorption and infrared reflection absorption spectroscopy measurements were performed on both SC and F-NP TiO2(101). We find that adsorption of molecular methanol at 80 K on SC TiO2(101) leads to the formation of chains along the titania rows. These chains are metastable and fall apart upon annealing due to the repulsion of neighboring molecules. In addition, we find that on SC TiO2(101) methanol deprotonates to produce neighboring methoxy and hydroxy groups following the annealing of high coverages to room temperature (RT). The coverage of methoxy and hydroxy groups can be increased with repeated adsorption and annealing cycles, and the annealing above room temperature leads to the recombinative desorption of methanol. On F-NP TiO2(101) surfaces, we find that methanol adsorbs both molecularly and dissociatively at RT. With increasing temperature, molecularly adsorbed methanol gradually converts to methoxy. Therefore, parallel behavior in adsorption and surface reaction is found on both SC and F-NP TiO2(101).

Molybdenum Trioxide on Anatase TiO2(101) - Formation of Monodispersed (MoO3)1 Monomers from Oligomeric (MoO3)n Clusters

“…This suggests that clustering likely occurs transiently when the gas phase (MoO3)n clusters land on TiO2(101). Similar clustering has been observed on TiO2(101) previously for water and methanol at 80 K. [51][52] There, we concluded that the molecules have to be mobile in some sort of precursor state before they chemisorb and become immobile at such low temperatures.…”

Molybdenum Trioxide on Anatase TiO2(101) - Formation of Monodispersed (MoO3)1 Monomers from Oligomeric (MoO3)n Clusters

“…This suggests that clustering likely occurs transiently when the gas phase (MoO3)n clusters land on TiO2(101). Similar clustering has been observed on TiO2(101) previously for water and methanol at 80 K. [52][53] There, we concluded that the molecules have to be mobile in some sort of precursor state before they chemisorb and become immobile at such low temperatures.…”

“…2,54 This is true for all measured coverages shown in Figure 10b and no changes are observed upon annealing to 450 K. After annealing to 550 K, the Mo 3d spectra reveal an onset of reduction with 9% and 6% contributions from (5+) and (4+) oxidation states at all coverages (observed at 231.0 and 229.6 eV), respectively. [53][54] Further reduction is observed after annealing at 650 K and the extent being slightly higher at low coverages. Interestingly, despite the observed reduction, the annealing did not lead to a decrease in the overall Mo coverage on the surface.…”

Observation of Molecular Hydrogen Produced from Bridging Hydroxyls on Anatase TiO2(101)

“…The experiments were performed in an ultrahigh vacuum (UHV) system that has been described previously. , The system, which has a base pressure of ∼1 × 10 –10 Torr, is equipped with a molecular beam source, a closed-cycle helium cryostat for sample cooling, a low-energy electron gun (Kimball Physics, Model ELG-2), a quadrupole mass spectrometer (Extrel, Model EXM720), and a Fourier-transform infrared (FTIR) spectrometer (Bruker, Model Vertex 80) for IRAS measurements performed in external reflection mode. The molecular beam was used to dose water or oxygen.…”