Influence of surface hydroxyls on the adsorption and reaction of ethanol on polycrystalline titania

Previous studies have demonstrated that the binding and reactivity of aliphatic alcohols on TiO2 surfaces are controlled by local coordination environments at the binding sites. As a result, there is excellent correspondence between chemisorption/temperature-programmed desorption results on rutile single crystals and those on both rutile and anatase powders. In this paper we examine the sensitivity of that correspondence to surface hydroxyl content by studying the adsorption and reaction of ethanol on rutile TiO2 powder as a function of surface hydroxyl population. These studies have been carried out under high vacuum conditions using a combined temperature-programmed desorption, TPD, and thermal gravimetric analysis, TGA, technique. The mass spectrometer signal collected during TPD was calibrated against microbalance measurements, which provided an excellent method to determine quantitatively the population of adsorbates as a function of temperature. Surfaces were prepared at different surface ethoxide-to-hydroxyl ratios, and the complete C, H, O mass balance was followed throughout the TPD. We find that both the number of ethoxides that desorb via the high-temperature decomposition pathway and the product distribution in the high-temperature region are not sensitive to initial hydroxyl coverage. However, the water-to-ethanol ratio in the low-temperature desorption channel is very sensitive to the surface preparation conditions. Our results are in good agreement with similar experiments of Gamble et al.(1) on single-crystal rutile (110) surfaces. Both the single-crystal and polycrystalline powder experiments appear to be consistent with two separate ethoxide adsorption modes, perhaps bridging and terminal ethoxides as suggested by Gamble et al.(1)