Thermal Desorption of Water on Na+ and Mg2+-doped Tin(IV) Oxide

A thermal desorption study was performed to examine the influence of Na+ and Mg2+-doping on the adsorbent properties of SnO2 for water. Three discrete desorption peaks at 435, 490, and 620°C of hydroxyl groups on the pure SnO2 surface changed into a single peak on Na doping. With an increasing dose of Na+, the temperature of this peak lowered and the peak area decreased. The peak almost completely disappeared at 2.0 atom% Na+. At its expense, a broad and large peak appeared at ca.200°C, which was attributed to the desorption of molecular species adsorbed at the surface oxygens through hydrogen bonding. Similar results were obtained in the case of Mg2+-doping. However, the lowering of the desorption tem perature of surface hydroxyls was less remarkable. This is probably due to the difference in electronegativity between Na+ and Mg2+. The desorption spectra of surface hydroxyls from the doped SnO+ samples varied with the pretreatment atmosphere at 800°C. The pretreatment under helium or in vacuo resulted in a decrease of the peak area, lowering of the desorption temperature, or appearance of a small low-temperature peak. The surface area of the doped samples also decreased by the pretr eat ment under helium or in vacuo. This kind of variation of the desorption spectra was suggested to arise from some surface reconstruction associated with the sintering process of the SnO2 particles.