They observed a normal isotope effect with regard to

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Unformatted text preview: M pulse experiments over Ni/SiO2 using CH4 and CD4 [101]. They observed a normal isotope effect with regard to methane conversion (higher CD4 than CH4 conversion) and no changes in the CO/CO2 ratio. No data were presented concerning H2 and water. From these results they concluded that the activation of CH4 is the rate-determining step in CPOM. They furthermore observed that when the CH4 flow was stopped and only O2 pulsed that CO and CO2 were evolved and their amount decreased with the number of O2 pulses [102]. They attributed this finding to the existence of surface carbon formed by the CH4 pyrolysis (CH4 → C + 2 H2). Surface carbon formed on Ni catalysts under the reaction conditions was also observed by Dissanayake et al. [107]. On this basis the authors advocated a so-called pyrolysis mechanism as the possible reaction scheme for the partial oxidation of CH4 over Ni catalysts (Fig. 5). According to this mechanism, carbon deposits on the catalyst surface are the active carbon source. CO and CO2 selectivities are mainly governed by the relative rates of the CO desorption and oxidation on the catalyst surface. A possible mechanism concerning the formation of hydrogen and water was not presented. CH4 CHx(s) CO(g) CO(s) O2(s) CO2(s) CO2(g) Fig. 5 Pyrolysis reaction scheme for the partial oxidation of CH4 (s = surface, g = gas phase) 1. Introduction 38 1.5.1.4. Parallel pathways Qin et al. investigated the partial oxidation, steam reforming, CO2 reforming and the mixed reforming of CH4 by steady state experiments over MgO supported Ru, Pd, Pt, Ir, Rh and to a lesser extent Ni catalysts [94]. Additionally they performed temperature programmed reactions over the Rh/MgO catalyst. In all steady state experiments the same GHSV was used, allowing a direct comparison of the results for the different reactions. In the steady state experiments they observed CH4 conversions near the equilibrium only for CPOM, whereas in the other reactions the CH4 conversions were considerably lower. They noted a significant temperature rise in CPOM, despite...
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