Reactivity of 1-propanol on p(n x 2) reconstructed WO3(100) thin films

The reactivity of active sites on reduced WO3(100) thin film surfaces was studied using temperature-programmed desorption (TPD), Auger electron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy (STM). When films were annealed in either O-2 or NO2 (similar to1 X 10(-6) Torr) above 700 K, the surfaces were predominantly covered by strands in a p(n x 2) periodicity with n ranging from 3 to 5. The catalytic activity of the p(n x 2) surfaces was probed using 1-propanol exposure at room temperature. During dosing, STM showed the accumulation of features associated with adsorption solely on the tops of the strands, unlike the stoichiometric c(2 x 2) WO3(001) surface which required annealing to desorb water before strongly localized features could be resolved. Thus the features in the images were attributed to 1-propoxide formed by deprotonation of the alcohol on adsorption at room temperature. High-resolution images revealed that the positions of the alkoxides on the strands were consistent with an added row model. Similar to other WO3 surfaces, in TPD experiments only unreacted 1-propanol, water, and propene were observed, indicating that p(n x 2) surfaces are only active for dehydration. The propene, however, desorbed at much lower temperatures than from other WO3 surfaces: around 400 K with much weaker peaks at 540 and 670 K. The 670 K desorption peak corresponded to that on other WO3 surfaces and was attributed to desorption from a low coverage of higher oxygen concentration domains on the surface seen with STM. Thus the surface structure did not affect the reaction pathway but did affect the kinetics with the p(n x 2) strands active for dehydration of adsorbed alkoxides to form alkenes at much lower temperatures.