Palladium single crystals have been found to be active for the C–H bond dissociation of methane in the temperature range 400–600 K, and the activities of the presently investigated Pd(111) and Pd(311) surfaces are compared with previously studied Pd(679). Structure sensitivity is reported that spans an order of magnitude in terms of the rates in the order Pd(111)<Pd(311)<Pd(679), while the effective activation energies range from 32–34 kJ/mol for Pd(111) and Pd(311) to 44 kJ/mol for Pd(679). These data are analyzed with a model that involves unsymmetrical barriers, first by constructing a potential-energy surface for Pd(111), in which the reaction pathway is well-simulated by the Eckart barrier. The Eckart barrier is then shown to obey exactly the Marcus rule for exothermic or endothermic processes. This property is used in comparing the H–CH3 dissociation on the different crystal faces, with the result that the Pd(679) surface provides a driving force of some 26 kJ/mol due to the role of defects compared to smooth planar Pd(111) surface and 22 kJ/mol compared to the Pd(311) surface.
Chlorohydrocarbons chemisorb dissociatively on Pd surfaces at g200 K and g10 -8 Torr, as shown by high-resolution electron energy loss spectroscopy (HREELS) and C 1s, Cl 2p, and Pd 3d surface core level shifts (SCLS). From CH2Cl2-generated overlayers on Pd(100), hydrogen is removed thermally and carbon is removed by oxidation as CO and CO2, leaving voids between the chlorine (Cl) ensembles that are accessible to other adsorbates. The resulting Cl overlayers are partially ordered depending on initial conditions. The concomitant low-energy electron diffraction (LEED) patterns show that the Cl ensembles are stable to high-temperature reaction cycles. The order-disorder phenomena observed in the temperature range 300-900 K include the generation of domains consisting of Cl only that surround reactive sites of the metal. With CH2Cl2/Pd(100), these domains are formed by lateral packing of 16 Pd/CCl2 units that restrict the supply of O(a) for oxidation of C. Selectivity is switched from CO2 to CO with increasing Cl concentration. Lateral interactions are of two types: mobile O-immobile Cl and mobile O-mobile O. This is reflected in a lowering of the O2 temperature programmed desorption (TPD) maxima with increasing Cl concentration. A statistical-mechanical model is presented for the effects of Cl(a) with phase-equilibration between a dense and a rare phase of O(a).
[CpCr(mu-Cl)Cl](2) reacted with dioxygen (O(2)) to produce CpCr(O)Cl(2) (1), which has been structurally characterized. Although 1 oxidized PPh(3) and 1,4-cyclohexadiene catalytically, it did not epoxidize olefins. DFT calculations have been performed on the system to characterize the potential energy surface for the epoxidation of ethylene and, in particular, the consequences of the crossing from the doublet surface of the starting materials to the quartet surface of the product (i.e. a chromium(III) epoxide adduct). These calculations suggested that "spin-blocking" was not a significant problem and that the reaction of CpCr(O)Cl(2) (3) with ethylene should have a lower activation barrier. On the basis of this computational prediction, 3 was prepared; it was found to epoxidize olefins stoichiometrically.
The lowest electronic excited state on the Si(100) surface and its coupling to the ground electronic state have been investigated using first-principles theory. The energy difference between the optimal geometry in the two states is small enough for a significant equilibrium population of the excited state to exist under common reaction conditions. The kinetics of crossing between spin states have been determined by explicit calculation of the minimum-energy crossing point and the spin−orbit coupling between them. The predicted excited-state lifetime is very short, except at low temperature.
The angle dependence of the valence-band photoemission from the trigonal prismatic layered MoS 2 shows both the forward-scattering features normally observed in core-level photoelectron diffraction and, in addition, the initial-state orbital character associated with partially occupied, nonbonding Mo IV (4d z 2ϩ 4d x 2 Ϫy 2 ϩ4d xy ) orbitals near the top of the valence band. The difference in forward scattering between the Mo and S emitters is also used to assess relative contributions from the Mo and S atomic orbitals at specific binding energies within the valence band. Deposition of cesium ͑0.23 ML Cs with 1 ML equal to the Cs saturation coverage͒ onto the basal plane of MoS 2 introduces a density of states at 1.25 eV above the top of the valence-band maximum. The intensity anisotropy for this Cs-induced valence level is interpreted via the angle dependence of the electric dipole matrix element as due to the initial-state orbital character at the bottom of the conduction band of the Cs/MoS 2 heterostructure.
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