Methane, adsorption

The role of oxygen and hydrogen solutions in the metal catalyst does not appear to be that of impeding the major reactions, but merely to provide a source of these reactants which is uniformly distributed diroughout the catalyst particles, without decreasing die number of surface sites available to methane adsorption. It is drerefore quite possible that a significatit fraction of the reaction takes place by the formation of products between species adsorbed on the surface, and dissolved atoms just below the surface, but in adjacent sites to the active surface sites. 

The models of Matranga, Myers and Glandt [22] and Tan and Gubbins [23] for supercritical methane adsorption on carbon using a slit shaped pore have shown the importance of pore width on adsorbate density. An estimate of the pore width distribution has been recognized as a valuable tool in evaluating adsorbents. Several methods have been reported for obtaining pore size distributions, (PSDs), some of which are discussed below. 

Chaffee, A.L., Loch, H.J. and Pandolfo, A.G., "Methane Adsorption on High Surface Area Carbons" CSIRO, Division of Fuel Technology, Investigation Report FT/IR 03IR (1989)... 

A CH4 pyrolysis mechanism appears to be consistent with our observation that preheating improves partial oxidation selectivity. First, higher feed temperatures increase the adiabatic surface temperature and consequently decrease the surface coverage of O adatoms, thus decreasing reactions lOa-d. Second, high surface temperatures also increase the rate of H atom recombination and desorption of H2, reaction 9b. Third, methane adsorption on Pt and Rh is known to be an activated process. From molecular beam experiments which examined methane chemisorption on Pt and Rh (79-27), it is known that CH4 must overcome an activation energy barrier for chemisorption to occur. Thus, the rate of reaction 9a is accelerated exponentially by hi er temperatures, which is consistent with the data in Figure 1. 

Several investigations have been reported on the adsorption of methane on coal (4,5,9,10,12,14,15). Methane adsorption at room temperature is usually highest for anthracite, passes through a minimum for bituminous coals... 

Logarithmic plots of the Freundlich equation, Q = kpn, where Q is the amount of methane adsorbed at a pressure p, and k and n are constants, for methane adsorption at 0°, 30° and 50°C. in Figures 6 and 7 indicate that the equation is valid up to at least 1000 torr. Equilibrium sorption points obtained on different samples in a manostatic adsorption apparatus are shown as solid points in Figures 6 and 7. The exponent n varied from 0.72 at 0° to 0.87 at 50°C. for the Pocahontas coal and from 0.78 at 0° to 0.94 at 50°C. for Pittsburgh coal (Table III). 

The two binders, the ENG and the thermoplastic polymer, are inert concerning the methane adsorption. The methane quantity delivered depends only on the activated carbon adsorption isotherm and its apparent density in the adsorbent composite block (Table 1). For pure methane, the quantity delivered at 298K between 3.5 and 0.1 MPa is equal to 89 (v/v). 

The stability of the adsorption of a hydrocarbon on the active site prior to C-H bond dissociation apparently strongly depends on the nature of the site and has not yet been unequivocally established. None of the studies on oxidative coupling of methane on non-transition metal oxides reported a stable methane preadsorption.34 Transition metal oxides, however, may reveal different behaviour and there exists already theoretical evidence for stable methane adsorption on transition metal atoms and complexes.35 Also the stability of the so-called encounter complexes has been qualitatively predicted to increase the reactivity of transition metal MO species... 

Kowalczyk P, Tanaka H, Kaneko K, Terzyk AP, and Do DD. Grand canonical Monte Carlo simulation study of methane adsorption at an open graphite surface and in slit like carbon pores at 273 K. Langmuir, 2005 21(12) 5639-5646. 

In relation to methane adsorption in active carbon, which as previously described is formed by slit pores (see Figure 2.21), several numerical simulations have revealed that the highest density of the adsorbed phase is achieved within slit pores of 1.12-1.14 nm of diameter [187,200], For slit pores of a width, L = 1.13 nm (see Figure 2.21), two facing methane molecule monolayers may be inserted between pore walls [203-205],... 

Indeed, if the methane molecule is supposed to act similar to a spherical molecule, its diameter is 0.381 nm hence, pores that have widths, L, of less than approximately 1 nm can house only one monolayer, while those narrower than around 0.4nm cannot store methane at all [186], If wider pores are considered, the attractive potential produced by the facing pore walls diminishes very fast with the value of the pore width, L, so that, if L > 3-4 nm, methane is weakly adsorbed, and its density is comparable to that of the gas phase in equilibrium with it. Subsequently, it is obvious that the maximum adsorption capacities are achieved with materials for which the volume of pores that have the pertinent width is the maximum [200,202], As a result, active carbons having slit-shaped pores may be the best material for methane adsorption [187],... 

The study of methane adsorption on activated carbon fibers has demonstrated, as was previously explained, that these carbonaceous materials, because of their cylindrical morphology and smaller diameter, have higher packing density than activated carbons with similar micropore volumes [191]. Subsequently, the higher adsorption capacity for the powdered activated carbons against the higher packing density for the fibers helps both kinds of materials reach similar, maximum adsorption values [191]. 

Kazansky VB, Serykh AI, Pidko EA (2004) DRIFT study of molecular and dissociative adsorption of light paraffins by HZSM-5 zeolite modified with zinc ions methane adsorption. Journal of Catalysis 225(2) 369-373... 

A neutron diffraction study was undertaken by Madih et al. (1989) alongside methane adsorption measurements on the MgO(lOO) powder. The well-defined stepwise character of the CH4 isotherm at 87.4 K in Figure 10.26 is again indicative of the layer-by-layer mode of adsorption. A somewhat similar isotherm was given by C2H6 on the MgO(l 00) surface at 119.68 K, although the higher steps were not as distinctive as those for CH4. 

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