AVS 60th International Symposium and Exhibition
    Surface Science Monday Sessions
       Session SS-MoA

Paper SS-MoA6
Elemental Steps in Dehydration of Diols on TiO2(110)

Monday, October 28, 2013, 3:40 pm, Room 201 A

Session: Metal Oxides: Reactivity and Catalysis
Presenter: Z. Dohnalek, Pacific Northwest National Laboratory
Authors: D. Acharya, Pacific Northwest National Laboratory
Y. Yoon, Pacific Northwest National Laboratory
Z. Zhang, Baylor University
Z. Li, Pacific Northwest National Laboratory
X. Lin, Pacific Northwest National Laboratory
L. Chen, Pacific Northwest National Laboratory
R. Mu, Pacific Northwest National Laboratory
B.D. Kay, Pacific Northwest National Laboratory
R. Rousseau, Pacific Northwest National Laboratory
Z. Dohnalek, Pacific Northwest National Laboratory
Correspondent: Click to Email
Simple diols (ethylene and propylene glycols) are employed as models for deoxygenation reactions of biomass. TiO2(110) is selected as a prototypical oxide catalyst to attain detailed mechanistic understanding of such reactions. In these studies atomically resolved imaging by scanning tunneling microscopy (STM) is combined with ensemble-averaging spectroscopic techniques such as temperature programmed desorption (TPD) and theoretical investigations via density functional theory (DFT). STM studies reveal that at low temperatures (140 K), diols adsorb on five-fold coordinated Ti4+ sites. The molecules readily dissociate via O-H bond scission forming Ti-bound hydroxyalkoxy and bridging hydroxy (HOb) species. The reverse reaction leading to molecularly bound diols is also observed indicating the attainment of a dynamic equilibrium between these conjugate acid/base pairs. Above 250 K, diols readily diffuse to oxygen vacancies and irreversibly dissociate via O-H bond scission of one of the OH groups forming geminate pairs of hydroxyalkoxy and hydroxyl species, both anchored on bridging oxygen (Ob) rows. The hydroxyalkoxy species rotate around their Ob anchor, switching the position of their OH between the two adjacent Ti rows. The rotating species are also found to assist cross-Ob row HOb hydrogen transfer. The OH group of the hydroxyalkoxy species is further observed to dissociate forming a dioxy species and an additional HOb. Annealing to ~450 K results in the formation of new dioxy intermediates that are centered on top of the Ob rows. Alkenes as final products are observed to desorb between 600 and 700 K. For ethylene glycol, our coverage dependent TPD studies further show acetaldehyde as a second carbon containing product at high coverages. Detailed theoretical calculations yield a deep insight into the mechanism and energetics of the observed reaction steps.