Tin oxide surfaces. Part 17.—An infrared and thermogravimetric analysis of the thermal dehydration of tin(IV) oxide gel

Abstract

The thermal dehydration of high-surface-area (ca. 180 m² g^–1) tin(IV) oxide gel has been studied by thermogravimetric analysis and infrared spectroscopy in the temperature range 293–673 K. The thermogravimetricanalysis data show that two types of physisorbed water are present, one which is largely removed by evacuation at ambient temperature and a more strongly held type which is removed by evacuation at 373–423 K. Further mass loss occurring at higher temperatures is due to the condensation of hydroxyl groups on the surface of the oxide particles. The amount of physisorbed molecular water present on the freshly prepared gel corresponds to a ca. three-monolayer coverage, reducing to ca. 0.74 of a monolayer after evacuation at 295 K, and is totally removed after evacuation at 423 K. The surface concentration of hydroxyl groups is determined to be 19.8 nm^–2 at 423–473 K, falling to 16.2 nm^–2 at 573 K and 13.0 nm^–2 at 673 K. Infrared spectra confirm that physisorbed molecular water [v(OH) 3800–2000 cm^–1, δ(OH) 1640 cm^–1] is removed by 423 K. Isolated and hydrogen-bonded surface hydroxyl groups give rise to stretching modes at 3640 cm^–1(sharp) and 3500 cm^–1(very broad), respectively. Surface hydroxyl deformation bands are found at 870, 940, 1175 and 1245 cm^–1. The bands at 870 and 950 cm^–1 are removed on evacuation by 473 and 523 K, respectively. The deformation bands at 1175 and 1245 cm^–1 which remain at 523 K are assigned to isolated surface hydroxyl groups. The band at 770 cm^–1, which increases in intensity during dehydration, is assigned to surface Sn—O—Sn linkages formed by the dehydroxylation process. The results are interpreted in terms of hydroxylated exposed [100], [101](both of which contain clusters of three hydroxyl groups attached to each surface tin atom) and [110](which contains both isolated and geminal pairs of hydroxyl groups attached to surface tin atoms) crystal planes of the rutile structure, the former predominating.