Identification of Alcohol Dehydration Sites on an Oxide Surface by Scanning Tunneling Microscopy

Abstract

High-speed scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), Auger electron spectroscopy and low-energy electron diffraction (LEED) were used to characterize the (001) surface of monoclinic γ-WO3, and to determine the structure and reactivity of active sites on this surface. Cleavage along the WO3(001) plane followed by heating in O2 to remove carbonaceous impurities and subsequent reduction above 775 K in ∼10-5 Torr of O2 resulted in a c(2 × 2) reconstruction that is visible in LEED and atomic resolution STM images. The catalytic activity of the c(2 × 2) surface was probed through exposure to a series of alcohols. No difference in the sticking coefficient for 1-propanol, 2-propanol and 2-methyl-2-propanol (tert-butyl alcohol) was detectable. As the surface temperature was increased during TPD, desorption of unreacted alcohol and water was seen at temperatures less than 600 K, independent of the alcohol. The water desorption is attributed to deprotonation of the adsorbed alcohols to form alkoxides; at this stage STM images show terraces covered with adsorbates with no preference for the adsorbates to occupy step or other defect sites. At higher temperatures the alkoxide all desorbs as alkenes: no dehydrogenation products were observed, indicating that the c(2 × 2) surface displays only dehydration activity under these conditions. The alkene desorption peak temperature decreases from primary to tertiary alcohol (1-propanol → tert-butyl alcohol), indicating that desorption is limited by the rate of C−O bond scission of the adsorbed alkoxide. STM images are presented in which the alkoxide intermediates are resolved simultaneously with atomic resolution of the WO3(001)-c(2 × 2) substrate. These demonstrate that the sites for oxidative dehydration of the alcohol molecules are the exposed 5-fold coordinated W6+ ions. Further, it is demonstrated that the alkoxide can be removed from the surface with the STM tip to reveal the structure of the underlying adsorption sites.