4684 The maximum epr intensity was attained at about oxygen atom into line in order to link the tetrahedra 10 coverage on r-A1203 as compared to about 40 % through a corner and form a siloxane linkage. This on Cabosil. The surface concentration of vanadium deformation could conceivably produce islands of geminal hydroxyls which remain after annealing. atoms is nearly the same at 10 coverage on alumina as In conclusion, the results reported here for both the that at 30 coverage on rehydrated Cabosil. All the paramagnetic probes are doubly attached to annealed and rehydrated Cabosils tend to support the the alumina surface as shown by the stoichiometry of model postulated by Peri and Henseley. However, HC1 evolution. Therefore, the vanadium atoms in this their picture for the former surface is probably an oversurface are immobile. In consonance with this conclusimplified view of the real structure. Alumina. The unit cell of y-Al2O3can be represented sion, heating the samples to 450" causes no decrease in epr intensities and line broadening of the spectra is a as a cube of side 7.84 A, presenting eight oxide ions at each face. It has a defect spinel s t r u c t ~ r e , with ~ ~ , ~ ~ function of coverage only and is independent of temperature. 21.33 A1 atoms arranged at random in the 16 octahedral That the surface of boehmite is different from those of and 8 tetrahedral vacancies. Chemisorption of water gibbsite and y-Al2O3has already been demonstrated by molecules saturates all the available oxide ions, giving a surface equivalent to that of gibbsite. This is consisadsorption experiments. 2 2 Both oxide and hydroxide ions are found on the surface of boehmite having laytent with the X-ray findings.25 There are 17 hyered patterns. The epr spectra indicate that the paradroxylsZ6per 100 A 2 of surface; the average distance separating the hydroxyls here is particularly favorable magnetic probe in this case has rhombic symmetry. It is possible that the presence of oxide ions in the surface for reaction of two neighboring groups with VC1,. The provides weak additional ligand-field perturbations of actual observed hydroxyl content of the Alon sample is 2.8 mmol g-l. The theoretical value is 2.17 mmol g-l the vanadium atoms. for 100 m 2 g-' of surface area ( n = 13. l), assuming each Acknowledgment. The author acknowledges numeroxide ion at the surface reacts with HzO to form a hyous discussion with Drs. H. G. Tennent, W. P. Long, droxide ion. Therefore, nonsurface hydroxyls may be and R. D. Mair, and the capable assistance of R. Croes. present in the sample. He is indebted to Dr. Mair for the samples of boehmite (23) G. Hagg and G. Soderholm, 2. Phys. Chem., Abt. B, 29, 88 and gibbsite. Acknowledgment is made to the donors (1935). (24) E. I
