They found that the catalyst is transformed to its

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Unformatted text preview: t different CH4/O2 ratios [107]. They found that the catalyst is transformed to its low activity oxidised state at ratios ≤ 1.25, where the ratio of 2 corresponds to the stoichiometric value. At ratios ≥ 2 large amounts of carbon were formed that filled the catalysts pores and caused the disintegration of the catalyst granules. At a ratio of 1.78 they observed a steady-state contraction of surface carbon of about 1 monolayer, which caused no observable decrease in the activity of the catalyst during 50 h on stream. Schmidt et al. investigated the CPOM over Rh, Ni, Pt and Ir coated catalysts at CH4/O2 ratios of between 1.7 and 2 in autothermal reactors. Hence the reaction temperature could differ remarkably between the different catalysts by more than 300°C and between the CH4/O2 ratio over one catalyst by about 150°C [93]. The O2 conversion was almost complete in all experiments, whereas the CH4 conversion and the temperature decreased with increasing CH4/O2 ratio. CO and H2 were always the main products, regardless the CH4/O2 ratio. The influence of the gas hourly space velocity (GHSV), the inversion of the contact time, is more complicated to evaluate. This is firstly due to the fact that it can be expressed as volume(gases)· h-1· volume(catalyst)-1if the density of the catalyst is known or as volume(gases)· h-1· weight(catalyst)-1. The first definition gives normally higher 1. Introduction 43 ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff numbers as the density of solid catalysts is generally > 1. Also is it most often impossible to convert the first definition into the other, because in none of the publications referred to in this work the density of the catalysts was stated. Therefore no actual numbers may be stated in this section. Basile et al. observed that the reaction temperature increased with increasing GHSV, accounting for the observed higher activity of the catalyst [9]. Dissanayake et al. and Looij et al. showed that at sufficiently high GHSV t...
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This document was uploaded on 10/07/2013.

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