Results of combined experimental and theoretical investigations of elementary chemical reaction processes of CO-CO 2 gas mixtures at nickel/yttria-stabilized zirconia ͑Ni/YSZ͒ solid oxide fuel cell ͑SOFC͒ model anode systems are presented. Temperature-programmed desorption and reaction measurements were performed in order to determine adsorption/desorption kinetic data as well as thermodynamic parameters for the CO/CO 2 /Ni and CO/CO 2 /yttria-stabilized zirconia ͑YSZ͒ heterogeneous reaction systems. From these data, an elementary kinetic reaction mechanism of the electrochemical CO oxidation at Ni/YSZ anodes was developed. Numerical simulations were performed for three different spillover mechanisms. Steady-state polarization curves and electrochemical impedance spectra were calculated, allowing for a direct comparison with experiments performed by Lauvstad et al. ͓J. Electrochem. Soc., 149, E506 ͑2002͔͒. Best agreement with the experimental data was obtained when assuming two consecutive charge-transfer steps from YSZ-O 2− via YSZ-O − to Ni-O, the second step being accompanied by oxygen spillover over the three-phase boundary.
Using the laser photolysis/vacuum−ultraviolet laser-induced fluorescence (LP/VUV−LIF) “pump-and-probe” technique the dynamics of H atom formation after photoexcitation of chloromethanes at 193.3 nm were studied in the gas phase at room temperature under collision-free conditions. For all chloromethanes, H atoms were detected by (2p2P ← 1s2S)-LIF using tunable narrow-band Lyman-α laser radiation (λLα ≈ 121.6 nm) generated by resonant third-order sum−difference frequency conversion of pulsed-dye-laser radiation. However, only in the cases of CH3Cl and CH2Cl2 were the H atoms found to originate from a primary photodissociation step. Absolute quantum yields for the formation of primary H atoms were measured by means of a calibration method to be φ H(CH3Cl) = (1.2 ± 0.6) × 10-2 and φ H(CH2Cl2) = (0.2 ± 0.1) × 10-2. From H atom Doppler profiles measured under single-collision conditions, the average translational energy released to the H + CH2Cl and H + CHCl2 products in the center-of-mass system was determined to be: E T(H−CH2Cl) = (86.6 ± 14.2) kJ/mol and E T(H−CHCl2) = (84.3 ± 8.9) kJ/mol. On the basis of available thermochemical data, the corresponding fraction of the available energy released as product translational energy was determined to be f T(H−CH2Cl) = (0.44 ± 0.07) and f T(H−CHCl2) = (0.41 ± 0.04). In the CHCl3 photodissociation, primary H atom formation was not observed. The H atoms detectable after laser irradiation of CHCl3 at 193.3 nm were found to originate from secondary photodissociation of the CHCl2 radical.
The dynamics of the gas-phase reaction of H atoms with HCl has been studied experimentally employing the laser photolysis/vacuum-UV laser-induced fluorescence (LP/VUV-LIF) "pump-and-probe" technique and theoretically by means of quasiclassical trajectory (QCT) calculations performed on two versions of the new potential energy surface of Bian and Werner [Bian, W.; Werner, H.-J. J. Chem. Phys. 2000, 112, 220]. In the experimental studies translationally energetic H atoms with average collision energies of E col ) 1.4 and 1.7 eV were generated by pulsed laser photolysis of H 2 S and HBr at 222 nm, respectively. Ground-state Cl( 2 P 3/2 ) and spin-orbit excited Cl*( 2 P 1/2 ) atoms produced in the reactive collision of the H atoms with room-temperature HCl were detected under single collision conditions by VUV-LIF. The measurements of the Cl* formation spin-orbit branching ratio φ Cl* (1.4 eV) ) [Cl*]/[Cl + Cl*] ) 0.07 ( 0.01 and φ Cl* (1.7 eV) ) 0.19 ( 0.02 revealed the increasing importance of the nonadiabatic reaction channel H + HCl f H 2 + Cl* with increasing collision energy. To allow for comparison with the QCT calculations, total absolute reaction cross sections for chlorine atom formation, σ R (1.4 eV) ) (0.35 ( 0.16) Å 2 and σ R (1.7 eV) ) (0.13 ( 0.06) Å 2 , have been measured using a photolytic calibration method. In addition, further QCT calculations have been carried out for the H + DCl isotope reaction which can be compared with the results of previous reaction dynamics experiments of Barclay et al. [
The H + O2 → O + OH chain-branching reaction, one of the most important elementary reactions in combustion chemistry, represents a challenging benchmark system for testing dynamical theories against experiments. The translational energy dependence of the total reaction cross section of the H + O2 (vibrational quantum number ν = 0) reaction was investigated experimentally employing a pulsed laser pump−probe technique and theoretically by means of quantum mechanical scattering calculations. The present results indicate that there is no sharp increase in reactivity for translational energies E tr ≥ 1.4 eV as was suggested by earlier experiments and approximate dynamical calculations. Furthermore, our results indicate that the potential energy surface needs to be improved to achieve quantitative agreement between experiment and theory.
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