Photoelectron transfer catalysts

Bauld and coworkers studied the [2+2] cycloaddition of A-vinyl carbazoles 86a and electron-rich styrenes 86b catalyzed by iron(III) catalysts A or B in the presence of 2,2 -bipyridine as a ligand, which was reported originally by Ledwith and coworkers (Fig. 21) [142, 143]. Deuterium-labeling studies provided support for the stepwise nature of the process, consisting of reversible SET oxidation of the electron-rich olefin to a radical cation 86 A. Nucleophilic addition of excess 86 leads to distonic radical cation 86B, which cyclizes to cyclobutane radical cation 86C. Back electron transfer affords cyclobutanes 87 and regenerates the catalyst. Photoelectron transfer catalysis gave essentially the same result, thus supporting the pathway. 

Ion cyclotron resonance studies confirm earlier conclusions based on photoelectron spectroscopic data that pyridine-4-thione exists mainly as the thiol tautomer in the gas phase. The potential of this technique for measuring tautomeric equilibrium constants in the gas phase is discussed. Alkylation of ambident anions of the type [N—C—S] , e.g. pyridine-2-thione, in the presence of a phase-transfer catalyst (Bu4N Br") leads exclusively to the, S-alkylated products. Yields are generally superior to those obtained by more conventional methods, and easier experimental work-up procedures are claimed for a variety of systems, including pyridine-2-thione, pyrimidine-2-thione, and benz-oxazoline-2-thione. Energies and Alkylations of Tautomeric Heterocyclic... 

The XPS spectra of the freshly sulfided Co-Mo/NaY catalysts were measured on an XPS-7000 photoelectron spectrometer (Rigaku, A1 anode 1486.6 eV). The sample mounted on a holder was transferred from a glove bag into a pretreatment chamber attached to the spectrometer as possible as carefully not to be contacted with air. The binding energies (BE) were referenced to the Si2p band at 103.0 eV for the NaY zeolite, which had teen determined by the Cls reference level at 285.0 eV due to adventitious carbon. 

An explanation for this difference in selectivity of the Ni catalysts is suggested by the studies of Okamoto et al. who correlated the difference in the X-ray photoelectron spectra of various nickel catalysts with their activity and selectivity in hydrogenations (ref. 28,29). They find that in individual as well as competitive hydrogenations of cyclohexene and cyclooctene on Ni-B, cyclooctene is the more reactive while the reverse situation occurs on nickel prepared by the decomposition of nickel formate (D-Ni). On all the nickel catalysts the kinetically derived relative association constant favors cyclooctene (ref. 29). The boron of Brown s P-2 nickel donates electrons to the nickel metal relative to the metal in D-Ni. The association of the alkene with the metal is diminished which indicates that, in these hydrocarbons, the electron donation from the HOMO of the alkene to an empty orbital of the metal is more important than the reverse transfer of electron density from an occupied d-orbital of the metal into the alkene s pi orbital. 

Diffuse reflectance spectroscopy (DRS) of VO-porphyrins on reduced and sulfided catalysts exhibit shifts in the porphyrinic electronic spectra (Soret, a, (3 bands) to higher frequencies. Adsorption results in modification of the delocalized electronic resonance structure not observed on the oxide form of the catalyst. X-ray photoelectron spectroscopy reveals shifts to higher Mo binding energies on reduced and sulfided catalysts following VO-porphyrin adsorption, consistent with transfer of electrons from Mo electron donor sites to the V02+ ion. Interaction at the electron donor sites is stronger than interaction at electron acceptor sites typical of the oxide catalyst. This gives rise to the possibility of lower VO-porphyrin diffusion rates on sulfided catalysts, but this effect has not been experimentally demonstrated. 

Fig. 7 X-ray photoelectron spectrometer. Left schematic view of a SSX 100/206 (Surface Science Instruments). Right, photographs of a Kratos Axis Ultra (Kratos Analytical) with the introduction and intermediate chambers (top) and analysis chamber (bottom), a, Turbomolecular pump b, cryogenic pump c, introduction chamber d, sample analysis chamber (SAC) e, transfer probe f, automatized X, Y, Z manipulator g, X-ray monochromator h, electrostatic lens i, hemispherical analyzer (HSA) j, ion gun k, aluminum anode (with monochromator) 1, aluminum-magnesium twin anode m, detector. Left channel plate. Right 8 channeltrons (Spectroscopy mode), phosphor screen behind a channel plate with a video camera (Imaging mode) n, spherical mirror analyzer (SMA) o, parking facility in the sample transfer chamber p, sample cooling device for the introduction chamber q, sample transfer chamber r, monitor interconnected with the video camera viewing samples in the SAC s, video camera in the SAC t, high temperature gas ceU (catalyst pretreatment)...

In a carbon-supported metal electrocatalyst, the electronic interaction between metal and carbon support has a significant effect on its electrochemical performance [4], For carbon-supported Pt electrocatalyst, carbon could accelerate the electron transfer at the electrode-electrolyte interface, leading to an accelerated electrode process. Typically, the electrons are transferred from platinum clusters to the oxygen species on the surfece of a carbon support material and the chemical bond formation or the charge transfer process occurs at the contacting phase, which is considered to be beneficial to the enhancement of the catalytic properties in terms of activity and stability of the electrocatalysts. Experimentally, the investigation into the electron interaction between metal catalyst and support materials could be realized by various physical, spectroscopic, and electrochemical approaches. The electron donation behavior of Pt to carbon support materials has been demonstrated by the electron spin resonance (ESR) X-ray photoelectron spectroscopy (XPS) studies, with the conclusion that the electron interaction between Pt and carbon support depends on their Fermi level of electrons. It is considered that the electronic structure change of Pt on carbon support induced by the electron interaction has positive effect toward the enhancement of the catalytic properties and the improvement of the stability of the electrocatalyst system. However, the exact quantitative relationship between electronic interaction of carbon-supported catalyst and its electrocatalytic performance is still not yet fully established [4]. 

---