Identification of Surface Sites on Monoclinic WO3 Powders by Infrared Spectroscopy

Abstract

The dehydroxylation/dehydration and Lewis acidity of the surface of monoclinic tungsten oxide (m-WO3) powder as a function of evacuation temperature was investigated by infrared spectroscopy. At room temperature, the m-WO3 surface contains both isolated and hydrogen bonded hydroxyl groups along with equal amounts of strongly and weakly adsorbed layers of water. Most of the surface hydroxyl groups and the weakly adsorbed water layer are eliminated by evacuation at room temperature. The strongly adsorbed water is removed by evacuation above 200 °C. Adsorption of D2O shows that the surface hydroxyl groups and adsorbed water are accessible and easily exchanged. However, the removal of the strongly held water is not related to the number of Lewis or Brønsted acid sites on the surface. While there is little change in the amount of adsorbed water between a room temperature sample and a sample evacuated at 150 °C, pyridine adsorption shows that there is a corresponding 50% reduction in the number of Lewis acid sites. Furthermore, the strongly held water is eliminated by evacuation between 200 and 400 °C, whereas there is little change in the number of Brønsted or Lewis acid sites. The changes in Lewis/Brønsted acidity are not related to the dehydration but rather attributed to reduction of the oxide due to removal of lattice oxygen.