The observed ch4 conversions were still much higher

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Unformatted text preview: s CO and H2 as predicted by thermodynamics. The observed CH4 conversions were still much higher over these at least partially reduced catalysts than over the non-reduced ones. Upon the further decrease of the temperature down to about 400°C the catalysts lost their enhanced activity and the former low active state was restored. Similar results were also obtained by Au et al. [101, 102], who performed pulse experiments over NiO/SiO2 and Ni/SiO2 catalysts. 1.5.1.6. State of the active catalyst To gain information about the state of the active catalyst in CPOM several authors investigated the state of catalysts after a certain time on stream and under simulated reaction conditions. Looji et al. [110] conducted in-situ high temperature XRD and semi-in-situ XPS on an Ni/SiO2 catalyst under different reaction conditions and performed additional temperature programmed surface reactions (TPSR) experiments. They assumed a total combustion followed by reforming mechanism. According to this mechanism the CH4/O2 ratio should increase along the catalyst bed as first all O2 is consumed in the total oxidation of CH4, before the formed H2O and CO2 react with the remaining CH4 yielding syngas as discussed above. Therefore they investigated their catalyst under 1. Introduction 41 ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff different CH4/O2 inlet ratios. They observed that at low CH4/O2 inlet ratios, corresponding to O2 conversion levels of 0 and 25%, the reaction mixture is net oxidising for the reduced Ni catalyst resulting in the formation of NiO. For higher ratios, 60-85% O2 conversion, no oxidation of bulk Ni was observed, but partial oxidation of the surface Ni atoms, which was greater at 60% O2 conversion than at 85%. They showed by the TPSR experiments with Ni catalysts of different oxidation degrees, that the catalysts only mildly passivated were more active in the total combustion of CH4 than the f...
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