Two models are presented that can simulate two peaks in a temperature-programmed desorption (TPD) spectrum from a high surface area catalyst at atmospheric pressure: a multisite model and a subsurface diffusion model. The multisite model assumes that the two peaks arise from two distinct adsorption sites on the catalyst surface with different activation energies for desorption. The subsurface diffusion model assumes that the high-temperature peak is produced by adsorbate that diffuses into the subsurface region of the catalyst during heating, and then back to the surface when it becomes depleted by the desorption process. It is shown that one can distinguish between the two models experimentally by measuring the effect of carrier gas flow rate and heating rate on the TPD spectrum.

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Crystals of [XeFs1 ]2[PdF62"] are yellow needles. The unit cell is orthorhombic with a = 9.346 (6), b = 12.786 (7), c = 9.397 (6) A, V= 1122.9 A3,Z = 4, and dc = 3.91 g cm"3. Structure refinement has proceeded satisfactorily in space group Pca2l (No. 29) using three-dimensional graphite-monochromatized X-ray Mo Ka data. With anisotropic temperature factors for all atoms, a final conventional R factor of 0.0256 (including zero weight data) for 1464 independent reflections was obtained. The asymmetric structural unit contains two crystallographically distinct XeFs+ ions and a PdF62" ion. The PdF6 2" ion is almost octahedral and only one Pd-F interatomic distance (1.860 (6) A) departs significantly from the average Pd-F distance of 1.893 A. The XeFs+ ions each approximate to Cw symmetry and are not significantly different in size, shape, or coordination. The cations are characterized by Fax-Xe-Feq angles of ~79°and average axial and equatorial bond lengths of 1.81 and 1.84 A, respectively. The individual Xe-F distances of the same type (axial or equatorial) do not depart significantly from the average values for the type. The coordination of the XeFs+ ions (by the F atoms of anions) in this structure is similar to the cation coordination observed in [XeF5+][AsF6"].

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