The evolution of self-sustained reaction-rate oscillations in the catalytic oxidation of propane over a nickel foil has been studied in situ using X-ray photoelectron spectroscopy coupled with on line mass spectrometry and gas chromatography. Changes in the effective surface area and in the catalyst morphology under reaction conditions have been examined by scanning electron microscopy and a krypton adsorption technique. It is shown that the regular kinetic oscillations arise under oxygen-lean conditions. CO, CO 2 , H 2 , H 2 O, and propylene are detected as products.The conversion of propane oscillates in a range from 1% to 23%. During the half-periods with high activity, the main reaction pathway is the partial oxidation of propane: selectivity toward CO achieves 98%. In contrast, during the half-periods with low activity, the reaction proceeds through three competitive pathways: the partial oxidation of propane, the total oxidation of propane, and the dehydrogenation of propane to propylene. The driving force for the selfsustained kinetic oscillations is the periodic reoxidation of nickel. According to the Ni2p and O1s core-level spectra measured in situ, the high-active catalyst surface is represented by metallic nickel, whereas during the inactive half-periods the catalyst surface is covered with a thick layer of NiO. The intensity of O1s spectra follows the oscillations of O 2 in the gas phase during the oxidation of propane. It is found that during the induction period before the regular oscillations appear, a rough and porous structure develops because of strong reconstruction of the catalyst surface. The thickness of the reconstructed layer is approximately 10-20 µm. This process is accompanied with at least an 80-fold increase in the effective surface area compared with a clean, non-treated nickel foil, which undoubtedly leads to a drastic increase in the number of active sites. We believe that it is the main reason for the induction period always being  Corresponding author.E-mail address: vvk@catalysis.ru (V.V. Kaichev) 2 observed before the appearance of self-sustained oscillations in the catalytic oxidation of light hydrocarbons over catalysts with a low specific surface area (single crystals, foils, or wires).Moreover, without such reconstruction, the oscillations cannot arise due to low activity of such catalysts.

Photoemission from GaAs cathodes with negative electron affinity (NEA) is applied for producing electron beams with very low longitudinal and transverse velocity spread. GaAs transmission-mode cathodes were activated with Cs and either O2 or NF3 in an extremely high vacuum setup (base pressure below 10−12 mbar). Quantum efficiencies of 20%–25% (at 670 nm) and long dark lifetimes (about 1000 h) could be achieved for both types of activation in a reproducible way. Using a method based on the adiabatic transverse expansion of an electron beam in a spatially decreasing magnetic field, the mean transverse energy (MTE) of the photoemitted electrons was measured systematically, recording longitudinal energy distribution curves. Both the MTE and the longitudinal energy spread strongly depend on the value of NEA and the position of the extracted distribution relative to the bulk conduction band minimum. Electrons with energies above the conduction band minimum are thermalized with the lattice temperature of the cathode, while electrons with energies below this level show a non-Maxwellian distribution with enhanced transverse energies. Thus, when extracting all electrons in a current limited emission mode, the MTE increases with the absolute value of NEA and reaches values up to ≈100 meV. By cutting off the low energy electrons with an external potential barrier, the longitudinal as well as transverse energy spread of the extracted electron ensemble are reduced. The MTE could be reduced down to about 28 meV at room temperature and to about 14 meV at liquid nitrogen temperature. The behavior of the MTE was found equivalent for (Cs, O) and for (Cs, F) activation layers on the same cathode. Conclusions about energy loss and scattering in the emission of photoelectrons from NEA GaAs cathodes are discussed.

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