Methanation compensation effect

McKee (21, 195) and McKee and Norton (219, 249, 250) have reported compensation effects in the exchange reactions of methane on several pairs of binary noble metal alloy catalysts. For each combination of elements kinetic measurements were made at a number of different compositions. Although the compensation behavior was generally very similar, there were perceptible differences in the values of B and e calculated for the various alloy combinations. The parameters found, by use of the formulas given in Appendix II, are summarized in Table IV, A-E, and are subject to the following comments. In consideration of data for the Pd-Rh alloys, the point for... 

In Fig. 2 the fluxes of several light hydrocarbons through a silicalite-1 membrane are shown as a function of their partial pressure on the feed side. The trend that can be deduced from this figure is that as the molecules get larger, their flux becomes lower. This decrea.se in flux is, however, smaller than expected on the basis of differences in diffusion coefficients [14]. The increase in the size of the molecule results in a lower mobility in the pores, but this effect is partly compensated by the higher concentration in the membrane, due to better adsorption of the larger molecules. This compensation effect is also the reason that at low partial pressures, ethane permeates faster through the membrane than does methane. 

Figure 7.4. Compensation effect for the methanation reaction. The logarithm of the preex-pontial factor is plotted againt the apparent activation energy, A , for this reaction over several transition-metal catalysts [18].

The reaction enthalpy and thus the RSE will be negative for all radicals, which are more stable than the methyl radical. Equation 1 describes nothing else but the difference in the bond dissociation energies (BDE) of CH3 - H and R - H, but avoids most of the technical complications involved in the determination of absolute BDEs. It can thus be expected that even moderately accurate theoretical methods give reasonable RSE values, while this is not so for the prediction of absolute BDEs. In principle, the isodesmic reaction described in Eq. 1 lends itself to all types of carbon-centered radicals. However, the error compensation responsible for the success of isodesmic equations becomes less effective with increasingly different electronic characteristics of the C - H bond in methane and the R - H bond. As a consequence the stability of a-radicals located at sp2 hybridized carbon atoms may best be described relative to the vinyl radical 3 and ethylene 4 ... 

Intrinsic to interpreting catalytic poisoning and promotion in terms of electronic effects is the inference that adsorption of an electropositive impurity should moderate or compensate for the effects of an electronegative impurity. Recent experiments have shown this to be true in the case of CO2 methanation where the adsorption of sulfur decreases the rate of methane formation significantly. The adsorption of potassium in the presence of sulfur indicates that the potassium can neutralize the effects of sulfur. 

A significant drop in catalytic activity for catalytic combustion of methane due to the above-mentioned PdO decomposition or the inability of metallic Pd to chemisorb oxygen above 650 C, however, can be effectively avoided by using a catalytically more active Mn-substituted hexa-aluminate (X = Mn) as a catalyst support [71]. The catalytic activity of this Mn-hexa-aluminate compensates for the drop in activity of Pd so that a stable combustion reaction can be attained in a whole temperature range. Thus, the use of catalytically active support materials is one possible solution to overcome the unstable... 

The lifetime of CH4 with respect to oxidation by OH radicals is around 10 years in today s atmosphere. In the early atmosphere OH levels would be so low that only the photolysis remains as a sink, resulting in a residence time of CH4 in the order of 10 years (Kasting and Siefert 2002). It is assumed that methane remained until formation of the oxygenic environment in air (2.2-2.7 Gyr ago) at relatively high concentration (ppm level) to maintain a warming potential (a greenhouse effect ). Therefore, CH4 must be produced from the crust at rates compensating its atmos-... 

---