Inductive Effect of Alkyl Chains on Alcohol Dehydration at Bridge-bonded Oxygen Vacancies of TiO 2 (110)
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Inductive Effect of Alkyl Chains on Alcohol Dehydration at Bridge-bonded Oxygen Vacancies of TiO2(110) Yu Kwon Kim Æ Bruce D. Kay Æ J. M. White Æ Zdenek Dohna´lek
Received: 16 May 2007 / Accepted: 2 July 2007 / Published online: 25 July 2007 Springer Science+Business Media, LLC 2007
Abstract The activation energies for alkene formation via dehydration of alcohols on bridge-bonded oxygen (BBO) vacancy sites of TiO2(110) is found to correlate with the inductive electron donating effect of alcohol alkyl groups as measured by the Taft parameter. Based on this correlation we conclude that the reaction involves a single transition state that undergoes concerted rupture of the C–O bond of the alkoxide and a C–H bond of the alkyl group attached to the b-carbon. Keywords Alcohols Heterogeneous catalysis TiO2(110) Dehydration Oxygen vacancies
Y. K. Kim J. M. White Department of Chemistry and Biochemistry, Center for Materials Chemistry, University of Texas at Austin, Austin, TX 78712, USA Y. K. Kim e-mail: [email protected] J. M. White e-mail: [email protected] B. D. Kay J. M. White Z. Dohna´lek (&) Pacific Northwest National Laboratory, Fundamental Sciences Directorate and Institute for Interfacial Catalysis, Richland, WA 99352, USA e-mail: [email protected]
1 Introduction Reactivity of adsorbates with titania is typically influenced by the presence of surface oxygen vacancy defects [1–3]. For alcohols, it is well-established that their dissociation on bridge-bonded oxygen vacancies (BBOV’s) of a prototypical TiO2(110) surface proceeds via heterolytic cleavage of the RO–H bond [4, 5]. The resulting intermediates are alkoxide (RO) that fills the BBOV and a proton that binds to a neighboring bridge-bonded oxygen (BBO) anion [6–10]. Except for methoxide, these alkoxides dehydrate at elevated temperatures (well above 300 K) via b-hydride elimination to form alkenes [6, 9, 10]. In some cases, the dehydration is accompanied by a minority dehydrogenation channel yielding aldehydes [6–9]. In homogeneous reactions, the reaction rates for a series of organic species with a common functionality, have, historically, often been correlated using empirical Taft parameters [11]. Here, we show that this correlation can be extended to a heterogeneous system, namely, a series of alkoxides, primary, secondary and tertiary, bound to a single crystal rutile surface, TiO2(110). From the linear correlation of the experimental activation energies for alkene formation with tabulated values of the Taft induction parameter, r* [11], we conclude that the reaction transition state connecting the adsorbed reactant, alkoxide, and the gas phase product, alkene, is a structure involving concerted elongation of the C–O bond of the alkoxide and a C–H bond of the R group attached to the b-carbon.
B. D. Kay e-mail: [email protected]
2 Experimental
J. M. White e-mail: [email protected]
The experiments were performed in an ultra-high vacuum (UHV) molecular beam scattering chamber (5 · 10–11
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