Interaction of hydrogen chloride and water with oxide surfaces: III. Titanium dioxide

Abstract

Adsorption of hydrogen chloride and water vapors on five TiO 2 powders in both the anatase and rutile crystalline forms have been investigated as a function of temperature, pressure, and outgas conditions. Characterization of the adsorbents was performed using X-ray powder diffraction, scanning electron microscopy (SEM/EDAX), surface area analysis, indicator method, microelectrophoresis, XPS, and infrared spectroscopy. Water vapor adsorption isotherms at 30, 40, and 50°C and heats of immersion in water were measured on TiO 2 after outgassing at 100, 200, 300, and 400°C. Both outgas temperature and adsorption temperature influenced the adsorption of water vapor on TiO 2. Water vapor adsorption on TiO 2 was completely reversible. Differences in the adsorption capacity of water on the different titanium dioxides were explained on the basis of the number of hydroxyl groups present on the surface. Isosteric heats of adsorption of water on TiO 2 were determined as a function of surface coverage. Heats of immersion in water were affected significantly by outgas temperature. Hydrogen chloride adsorption isotherms at 30°C were measured on TiO 2 after outgassing at 100–400°C. A part of the total HCl adsorbed was irreversibly adsorbed. Anatase showed the highest HCl adsorption capacity per unit area while pure rutile showed the lowest adsorption capacity. XPS, indicator analysis, and infrared analysis after HCl adsorption further confirmed the conclusions based on adsorption. Heats of immersion of TiO 2 were higher in 0.1 N HCl (aq) than in water and the difference was greater for the alumina-coated rutiles.