Surface methanation, effect

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

Since methane diffusion is much slower than nitrogen diffusion into clinoptilolite, it is reasonable to suspect that the presence of CH4 on the zeolite surface will effectively slow the uptake of N2. If true, then it would be much more appropriate to measure DN2 in the presence of CH4 rather than in the pure gas... 

In the presence of La203 on the surface of y-Al203, NiO would be reduced to Ni crystallites of smaller size and higher thermal stability, and the improved performance of the methanation catalyst may well be due to the modified surface. Similar effects of rare earth oxides have been observed on the catalyst Pt/y-Al203 61). 

Here n indicates an active surface site, and X— indicates die species X adsorbed on an active site. The first reaction allows for the possibility that methane may occupy more than one active site on adsorption. The dik d and fourth reactions show die observed retarding effects of steam and hydrogen... 

The temperature for methane and butane calculated with the isothermal model is a factor 1.4 times greater than the average temperature measured by Lihou and Maund (1982) in their small-scale tests, although higher local maximum temperatures were measured. In this model, combustion is stoichiometric, thus leading to very high fireball temperatures which, in turn, lead to high radiation emissions. Effective surface emissions measured experimentally were one-half the value calculated from this model, because combustion is not stoichiometric and emissivity is less than unity. 

Coping with the greenhouse effect is a vei-y difficult sociopolitical problem. A greenhouse effect existed on Earth long before the Industrial Revolution. Had it not. Earth s surface would be much colder than it is now. The introduction of gases absorbing infrared radiation only enhances the greenhouse effect. Carbon dioxide is not the only gas of importance water vapor and methane, for exam-... 

Tn the synthesis of methane from carbon monoxide and hydrogen, it is desired to operate the reactor or reactors in such a way as to avoid carbon deposition on catalyst surfaces and to produce high quality product gas. Since gas compositions entering the reactor may vary considerably because of the use of diluents and recycle gas in a technical operation, it is desirable to estimate the effects of initial gas composition on the subsequent operation. Pressure and temperature are additional variables. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

Methanatlon Studies. Because the most effective way to determine the existence of true bimetallic clusters having mixed metal surface sites Is to use a demanding catalytic reaction as a surface probe, the rate of the CO methanatlon reaction was studied over each series of supported bimetallic clusters. Turnover frequencies for methane formation are shown In Fig. 2. Pt, Ir and Rh are all poor CO methanatlon catalysts In comparison with Ru which Is, of course, an excellent methanatlon catalyst. Pt and Ir are completely inactive for methanatlon In the 493-498K temperature range, while Rh shows only moderate activity. 

Surface methyls have also been synthesized by the collision-induced dissociation of methane physisorbed on Ni(lll) surfaces.[7, 8] This approach avoids the effects of coadsorbates other than hydrogen, and a number of aspects of Ae reaction and decomposition of CH3 and CH fragments on Ni(lll) have been determined.[9] However, the method is relatively complex and best suited for study of low coverages. 

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