59th Annual Report on Research 2014

Narrative 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 second year (9/01/2013 — 8/31/2014), we have worked on the surface reaction of diols and aldehyde 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). Dehydration of Diols on TiO₂(110) Scanning tunneling microscopy is employed to follow elemental steps in conversion of ethylene glycol and 1,3-propylene glycol on partially reduced TiO2(110) as a function of temperature. Mechanistic details about the observed processes are corroborated by density functional theory calculations. The use of these two diol reactants allows us to compare and contrast the chemistries of two functionally similar molecules with different steric constraints, thereby allowing us to understand how molecular geometry may influence the observed chemical reactivity. We find that both glycols initially adsorb on Ti sites, where a dynamic equilibrium between molecularly bound and deprotonated species is observed. As the diols start to diffuse along the Ti rows above 230 K, they irreversibly dissociate upon encountering bridging oxygen vacancies. Surprisingly, two dissociation pathways, one via O-H and the other via C-O bond scission, are observed. Theoretical calculations suggest that the differences in the C-O/O-H bond breaking processes are the result of steric factors enforced upon the diols by the second Ti-bound OH group. Above ∼400 K, a new stable intermediate centered on the bridging oxygen (O_b) row is observed. Combined experimental and theoretical evidence shows that this intermediate is most likely a new dioxo species. Further annealing leads to sequential C-O_b bond cleavage and alkene desorption above ∼500 K. Simulations demonstrate that the sequential C-O_b bond breaking process follows a homolytic diradical pathway, with the first C-O_b bond breaking event accompanied with a nonadiabatic electron transfer within the TiO₂(110) substrate. Figure 1. Elementary reaction steps of ethylene glycol on TiO₂(110). Imaging of Formaldehyde Adsorption and Diffusion on TiO₂(110) Surface reactions of formaldehyde with reduced TiO₂(110) surfaces have been studied using variable-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). STM images taken from a same area at various temperatures clearly show that formaldehyde preferentially adsorbs on the bridge-bonded oxygen (O_b) vacancy (V_O) defect sites. Bias-dependent STM images show that the STM features corresponding to both the Ti-bound CH₂O and the V_O-bound CH₂O are positioned between the O_b row and the Ti row. While the V_O-bound formaldehyde rotates at 95 K, the Ti-bound CH₂O does not. The V_O-bound CH₂O starts to diffuse along the O_b row as –CH₂– at ~170 K and starts to diffuse along the Ti row as an intact molecule at ~215 K. However, the stabilities and the configurations of the Ti-bound and V_O-bound formaldehyde calculated using DFT are not in line with the experimental results. The discrepancy between the experiment and theory indicates the presence of a complex charge distribution related to the surface defects. Figure 2. STM images taken from the same area of reduced TiO₂ (110) at different temperatures, representing the adsorption of formaldehyde and the conversion of the Ti-bound formaldehyde to the V_O-bound formaldehyde 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. The results from the supported research are published in one peer-reviewed publications in prestigious international journals. My group presented five oral/poster presentations at five international, national, and regional professional conferences. One graduate student (Yaobiao Xia) was supported by this grant. This support enables him conduct the major part of his thesis work. He will graduate next year. Four undergraduate students are involved in the projects last year. One of them Blake Birmingham) won the Outstanding Poster Presentations at university-wide Undergraduate Research Scholars Week.