They did neither record it nor decrease the heating

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Unformatted text preview: the dilution of the feed stream by N2 (57 vol%). They did neither record it nor decrease the heating power of the furnace to compensate it. As the CH4 conversion was highest in CPOM they excluded a reaction mechanism of the form total oxidation followed by reforming as the main reaction pathway. In the temperature programmed reaction they observed that partial oxidation and the mixed reforming reactions are starting at very similar temperatures. Therefore they assumed that these reactions take place simultaneously and proceed by the same oxygen reaction intermediate, which they assumed to be adsorbed atomic oxygen Oad bound to the metal surface in the form M-O. They regarded the strength of the M-O bond to be the key factor for the activity and the selectivity of the catalysts. First, the formation of M-O assists the decomposition of O2, H2O and CO2, generating adsorbed atomic oxygen. Hence M-O bond strength is favourable. Weaker bond strength is required to allow M-O bond cleavage without high activation energy, crucial to form CO with adsorbed carbon species formed upon the adsorption of CH4. However, a too weak M-O bond enhances atomic oxygen desorption. Desorbed atomic oxygen is a very strong and non-selective oxidation reagent thus increasing the amount of the products of the total oxidation. The correlation of the M-O bond strength with the CO formation rate revealed that metals 1. Introduction 39 ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff with a low or high M-O bond strength, e.g. Pt and Ru respectively, showed a lower CO formation rate than those with a rather intermediate M-O bond strength such as Ni. This trend is also in quite good agreement with the results of the group of Schmidt et al. [93, 98, 100]. The same correlation made for steam reforming revealed that the CO formation rate increased with the M-O bond strength. They explained this with the need for a higher M-O bond strength in the steam reforming process, as H2O is more difficult t...
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This document was uploaded on 10/07/2013.

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