AVS 47th International Symposium
    Surface Science Monday Sessions
       Session SS1-MoM

Paper SS1-MoM8
Reactive Intermediate Species on WO@sub 3@ Resolved at the Atomic Scale during Alcohol Dehydration

Monday, October 2, 2000, 10:40 am, Room 208

Session: Atomic-Scale Studies of Model Catalysts
Presenter: R.E. Tanner, Yale University
Authors: R.E. Tanner, Yale University
P. Meethunkij, Yale University
E.I. Altman, Yale University
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In order to better relate structure and reactivity on monoclinic @gamma@-WO@sub 3@(001), we have studied reactions on the surface using scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), Auger electron spectroscopy and low-energy electron diffraction. The ordered c(2 x 2) reconstruction may be reproducibly generated by heating in O@sub 2@. The termination of the bulk structure is seen in LEED and STM to correspond to a WO@sub 2@ plane in which half the W@super 6+@ cations are covered by terminal O. The catalytic activity was probed by exposure of the c(2 x 2) surface to a series of alcohols. Methanol, 1-propanol, 2-propanol, and 2-methyl-propan-2-ol (t-butanol) all adsorbed on the surface. The sticking coefficient is similar for all alcohols indicating the ease of deprotonation and formation of alkoxide. TPD showed desorption of water and unreacted alcohol at temperatures less than 600 K, independent of the alcohol. The alkoxide all desorbed as alkenes at higher temperatures, so the c(2 x 2) surface displays only dehydration activity. The peak of the alkene desorption trace decreases from primary to tertiary alcohol (1-propanol --> t-butanol), indicating that desorption is limited by the rate of C-O bond scission of the adsorbed alkoxide, and is independent of the deprotonation rate. STM images revealed terraces covered with adsorbates that have no preference for attachment at steps or other defect sites. Further images are presented where the WO@sub 3@(001)-c(2 x 2) substrate atomic periodicity is seen simultaneously with the alkoxide intermediates. The sites for oxidative dehydration of the alcohol molecules are identified as the exposed five-fold coordinated W@super 6+@ ions on clean terraces, in agreement with the theory of enhanced activity at coordinatively unsaturated sites. The STM tip can be used to remove the alkoxide adsorbates from the surface to reveal the structure of the underlying, reactive adsorption sites.