Thermal desorption spectroscopy

Abstract: Methods for determining the activation energy, rate constant and order of reaction from “flash-filament” desorption experiments are examined. Two heating schedules are considered: (a) a linear variation of sample temperature with time (T = T0+st), and (b) a reciprocal temperature variation ( 1 T = 1 T 0 −αt) . The activation energy of desorption is estimated from the temperature (Tp) at which the desorption rate is a maximum (for first-order reactions) and from the change of Tp with surface coverage (for second-order reactions). The order of the reaction is determined from the shape of the desorption rate versus time curve, or the variation of Tp with coverage. Applications of these methods to experiments on chemisorption and the desorption of ionically trapped gases are discussed. The effect of finite pumping speed in the system is analysed and the necessary corrections derived.

Abstract: The methods for the determination of various types of oxygen surface functions on carbon materials are briefly described, and their relative advantages and problems that may arise are discussed. Acidimetric titration techniques, IR spectroscopy, XPS, thermal desorption spectroscopy, and electrokinetic measurements are described.

Abstract: We have studied the interaction of polyaromatic hydrocarbons ~PAHs! with the basal plane of graphite using thermal desorption spectroscopy. Desorption kinetics of benzene, naphthalene, coronene, and ovalene at submonolayer coverages yield activation energies of 0.50 eV, 0.85 eV, 1.40 eV, and 2.1 eV, respectively. Benzene and naphthalene follow simple first order desorption kinetics while coronene and ovalene exhibit fractional order kinetics owing to the stability of two-dimensional adsorbate islands up to the desorption temperature. Preexponential frequency factors are found to be in the range 10 14 ‐10 21 s 21 as obtained from both FalconerMadix ~isothermal desorption! analysis and Antoine’s fit to vapor pressure data. The resulting binding energy per carbon atom of the PAH is 5265 meV and can be identified with the interlayer cohesive energy of graphite. The resulting cleavage energy of graphite is 6165 meV/atom, which is considerably larger than previously reported experimental values.

Abstract: Interactions of tin oxide surface with oxygen, water vapor, and hydrogen have been investigated. Temperature programmed desorption (TPD) chromatograms of oxygen showed the formation of four kinds of oxygen species on SnO2 which were desorbed around 80°C (α1), 150°C (α2), 560°C (β), and above 600°C (γ), respectively, on heating at 20°C/min. Based on electric conductivity and ESR measurements, these species were assigned as O2 (α1), O2− (α2). O− or O2− (β), and a part of lattice oxygen (γ), respectively. Two types of water adsorption were indicated by broad TPD peaks centered at 100°C (ga) and 400° C (β), for which molecular and dissociative (surface hydroxyls) adsorbates were assigned respectively. Of these, β caused an increase in electric conductivity of SnO2. The interactions of H2 with SnO2 surface were complicated by the reduction of SnO2 taking place at higher temperature. Exposure of evacuated SnO2 to h2 at room temperature gave a large TPD peak centered at 100°C (α). After cooling in H2 from 200°C to room temperature, however, large amounts of H2O desorption were observed, in addition to the H2 desorptions around 100°C (α) and 220°C (β). The type α was shown to be of depletive type. These phenomena are discussed briefly in relation to the actual case of gas detection.

Abstract: The surface structure and chemistry of various surface oxidized HT carbon fibers, an IM and a HM carbon fiber were studied by SEM, STM, CAM (contact angle measurement), XPS and TPD with special reference to adsorbed oxidation products and adsorbed water. It is shown that a real image of surface structure and surface chemistry is only obtained after removal of adsorbed oxidation products by extraction with water and careful drying of the fibers without changing the surface chemistry. A comparison on the usefulness of the various surface analytical methods will be given in Parts II–IV of this paper.