58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Technical Progress Report

The research objective of this proposal is to understand how the local structure influences the oxidative hydrogenation of alkane related reactions on TiO₂ supported VO_x model oxide nanocatalysts. In the first year (1/01/2012 — 8/31/2013), we have worked on two correlated projects in parallel. One is the surface reaction of ketone and diols on rutile TiO₂(110). Variable temperature scanning tunneling microscopy (VT-STM) was used to ascertain the site-specific elemental reaction steps (adsorption, dissociation and rotation). The other project is the atomic structure study of anatase TiO₂(001) thin films. The goal is to adapt high quality anatase films ex situ for the molecular-level surface reaction mechanism study.

Project I: Surface Reactions on Rutile TiO₂(110)

·         Surface Reactions of Acetone on TiO₂(110)

Point defects are of fundamental importance for the chemistry and photochemistry of metal oxides. We have studied the dynamic relationship between acetone and bridge-bonded oxygen (O_b) vacancy (V_O) defect sites on the TiO₂(110)-1 x 1 surface using scanning tunneling microscopy (STM) and density function theory (DFT) calculations. We report an adsorbate-assisted V_O diffusion mechanism.  The STM images taken at 300 K show that acetone preferably adsorbs on the V_O site (Figure 1a) and is mobile. The sequential isothermal STM images directly show that the mobile acetone effectively migrates the position of V_O by a combination of two acetone diffusion channels: one is the diffusion along the O_b row and moving as an alkyl group (Figure 1b), which heals the initial V_O; another is the diffusion from O_b row to five coordinated Ti⁴⁺ row and then moving along the Ti⁴⁺ row as an acetone (Figure 1a, 1c), which leaves a V_O behind. The calculated acetone diffusion barriers for the two channels are comparable and agree with experimental results.

Figure 1. STM images recorded as a function of time (t = 0, 12, 18, 21 min) on the same area of acetone pre-adsorbed TiO₂(110). The corresponding ball models illustrate the diffusion of V_O mediated by acetone diffusion at 300 K.

·         Reactions of Acetone with Oxygen Adatoms on TiO₂(110)

Understanding the interaction of O₂ with ketones on metal oxide surfaces is important for the oxidation of organic molecules. The consecutive reaction steps of acetone molecules with oxygen adatoms (O_a's) on partially oxidized TiO₂(110) surfaces have been studied using high-resolution STM at 300 K. The sequential isothermal STM images reveal two types of acetone-O_a species as a result of reactions of acetone with an oxygen adatom and oxygen vacancy (Figure 2). One species is the Ti_5c-bound acetone-O_a diolate formed from Ti_5c-bound acetone reacting with O_a. The diolate is mobile at 300 K and can assist the diffusion of surface O_a by exchanging the acetone oxygen with the O_a. The second acetone-O_a species is V_O-bound acetone-O_a complex formed from a V_O-bound acetone reacting with an O_a located on the neighboring Ti row. The V_O-bound complex is stationary at 300 K. This species has not been reported previously.

Figure 2. STM images revealing the interaction of acetone with oxygen adatoms (O_a) and oxygen vacancies (V_O) on oxidized TiO₂(110).

  ·         Ethylene Glycol and Propylene Glycol on Rutile TiO₂(110)

We have employed variable-temperature STM and dispersion-corrected DFT to study the interactions of 1,2-ethylene and 1,3-propylene glycols with reduced TiO₂(110) surfaces. We found that at 300 K, both glycols dissociate via O-H bond scission of one of the OH groups on oxygen vacancy defects forming pairs of monoalkoxide (O_b-(CH₂)_n-OH, where n = 2 or 3) and bridge-bonded bonded hydroxyl (HO_b) species. The O_b-(CH₂)_n-OH species are observed to rotate around their O_b anchor, switching the position of the OH between the two adjacent Ti_5c rows (Figure 3). The rotating species are found to assist cross-O_b row HO_b hydrogen transfer. The OH group of the monoalkoxide species is further observed to dissociate forming a bidentate type dioxo (O_b-(CH₂)_n-) species and an additional HO_b. The reversible interconversion between the mono- and di-oxo species clearly shows the attainment of a dynamic equilibrium between these conjugate acid/base pairs.

Figure 3. STM images showing the rotation of monoalkoxide resulted from dissociation of 1,3-propylene glycols TiO₂(110).

Project II: Surface Reactions on anatase TiO₂(001)

·         Anatase TiO₂(001) thin films

The anatase polymorph of TiO₂ appears to be an even more potent catalyst than rutile. Understanding the structure of well-defined anatase TiO₂ surfaces is critical for deciphering site-specific thermal and photo- reaction mechanisms on anatase TiO₂. We have successfully resolved the surface structure of anatase TiO₂(001) thin films grown by oxygen plasma assisted molecular beam epitaxy. These well-defined surfaces are ideal to correlate the structure with the activity at the molecular level. High-resolution STM images taken from the same area at different bias voltages show that the structure of the bright row is originated from combinations of two basic atomic building blocks (Figure 4). We propose a modified added molecule model for the anatase TiO₂ (001) surface structure.

Figure 4. STM images showing that the atomic structure of the bright row of anatase TiO₂(001) (1×4) epitaxial thin films is originated from two atomic building blocks.

Impact of the Research on My Career and the Students  

This PRF new investigators grant enables me to establish an active research program at Baylor University in the critical third year of my tenure-track. The results from the supported research are published in four peer-reviewed publications in prestigious international journals. My group presented six oral/poster presentations at five international, national, and regional professional conferences. The support and publications build the solid foundation for my federal funding endeavors.

Two graduate students were partially supported by this grant. This support enables one student (Yaobiao Xia) conduct the major part of his thesis work. I am proud that he won a best poster prize at Southwest Catalysis Society 2012 Spring Symposium. He will graduate next year. Four undergraduate students are involved in the projects. Two of them (Amir Ali and Blake Birmingham) won the Outstanding Poster Presentations at university-wide Undergraduate Research Scholars Week.