Methane oxidation promotion

The lithium oxide-promoted barium oxide also functions as a catalyst for the methane coupling reaction, but the mechanism is not clearly understood at the present time. The only comment that might be offered here is that the presence of ions on the surface of this material might etdrance the formation of methyl radicals drrough the formation of hydroxyl groups thus... 

The influence of Zn-deposition on Cu(lll) surfaces on methanol synthesis by hydrogenation of CO2 shows that Zn creates sites stabilizing the formate intermediate and thus promotes the hydrogenation process [2.44]. Further publications deal with methane oxidation by various layered rock-salt-type oxides [2.45], poisoning of vana-dia in VOx/Ti02 by K2O, leading to lower reduction capability of the vanadia, because of the formation of [2.46], and interaction of SO2 with Cu, CU2O, and CuO to show the temperature-dependence of SO2 absorption or sulfide formation [2.47]. 

Methane oxidation and partial oxidation, electrochemical promotion of, 308 dimerization, 470 reforming, 410 Methanol dehydrogenation electrochemical promotion of, 403 selectivity modification, 404 Methanol oxidation electrochemical promotion of 398 selectivity modification, 400 Microscopy... 

Figure 2.8 The surface reaction between adsorbed carbon monoxide and hydrogen to methane over rhodium catalysts occurs at lower temperatures in the presence of a vanadium oxide promoter, which is known to enhance the rate of CO dissociation (from Koerts el al. 113]). 

Doping Tb into CeC>2 can move the oxygen releasing peaks into lower temperature (see Section 5.1) and that may promote the oxidation methane. Figure 44 demonstrates the catalytic result of the methane oxidation by Ceo.sTbo.202-5 oxide. The Ceo.8Tbo.202-5 oxide has better catalytic behavior than pure CeC>2. 

With adjustment of the steam/methane ratio, the reactor can produce a synthesis gas with CO/H2 = 1/2, the stoichiometric proportions needed for methanol production. This mixture at approximately 200 atm pressure is fed to the methanol unit where the reaction then proceeds at 350°C. Per pass conversions range from 30 to 50 over the catalyst— typically a supported copper oxide with a zinc, chromium, or manganese oxide promoter 3... 

Figure 9 Schematic representation of the reaction pathway for methane oxidative dimerization on alkali (A) promoted oxide catalyst (BO). The pathway represents the most probable routes, including the coexistence of various phases in equilibrium with the reaction environment.

Where the Fischer-Tropsch process has been used on an industrial scale, iron or cobalt are the essential catalyst components. Technical catalysts also contain oxidic promoters, such as alumina and potassium oxide. Ruthenium and nickel are most attractive for academic research since they produce the simplest product packages. Nickel is used for methanation (production of substitute natural gas and removal of carbon monoxide impurities from hydrogen). 

Two different cerium oxide promoted zirconias were prepared and tested as supports for Pd catalysts for the catalytic oxidation of methane, alone and in presence of a strong catalyst poison (SO2). The introduction of cerium oxide was carried out by incipient wetness of zirconium hydroxide or zirconium oxide, followed by calcination. Both catalysts present very different properties, the first method producing a catalyst with better performance, and thermal stability markedly higher than the unmodified zirconia support. However, the addition of cerium does not lead to any enhancement of the catalyst performance in presence ofSC>2,... 

Another very important feature of such models is their openness they can be built on and on to any direction from any given level. On the one hand, this in principle allows a very detailed elaboration of a given reaction. On the other hand, such openness is very helpful if more and more complicated systems must be analyzed. For instance, any action of additives (promoters, inhibitors, etc.) on hydrocarbon oxidation can be analyzed. On the other hand, a kinetic scheme describing the reaction of a certain compound can be spread on similar reactions of related compounds with gradually increasing complexity (e.g., from methane oxidation to analogous reactions of higher alkanes). 

OXIDATIVE COUPLING OF METHANE OVER PROMOTED MAGNESIUM OXIDE CATALYSTS RELATION BETWEEN ACTIVITY AND SPECIFIC SURFACE AREA... 

Alkali-Metal-Promoted Metal Oxides. Researchers at Texas A M University studied methane oxidative coupling using MgO promoted with Oxygen cofeeding was used in all... 

In methane oxidation, a positive effect of methane partial pressure was measured in a wide range of feed composition the reaction order is 0.7 in CH4 and 0.1 in O2. In propane oxidation, the rise of fuel partial pressure promotes the reaction only at low reactant/oxygen ratio at higher values, surface saturation seems to take place (Figure 9). The oxygen dependence is 0.8, suggesting stronger competition by propane for surface sites, compared to methane. 

It turned out that the formation of alkane complexes and oxidative addition were favored as the system became negatively charged, suggesting that an electronic transfer from the metal to the methane molecule promotes the C-H bond activation. An increase in the p character of the metal center favors the C-H bond splitting. 

Later it was shown that sequential introduction of CH4 and O2 was not necessary to promote methane oxidative dimerization but this could be achieved directly by passing CH4/O2 mixtures over a metal oxide catalyst [57]. Since these early reports, work directed towards investigating the chemical oxidative dimerization of methane has increased with a significant number of papers [58-88] and reviews [89, 90] being published. 

Non-metallic catalysts — MgO, Li/MgO and La203 — known to produce methyl radicals during the methane oxidative coupling reaction have been shown to be active for NO reduction by CH4. Li-promoted MgO in the absence of O2 produces N2 and N2O with a (N2/N2O) selectivity below 2 at low temperature but which increases to 3 or more at higher temperatures. Unpromoted MgO is less active but produces almost 100% N2 at high temperatures. La203 is more active and selective for NO reduction to N2 by CH4 than MgO and Li/MgO catalysts. The activity of La203 continuously increases with temperature, at least up to 973 K, and selectivity for N2 rather than N2O... 

Cobalt-based low temperature Fischer—Tropsch catalysts, appHed at approximately 220 °C and 30 atm, are usually supported on high-surface-area Y-AI2O3 (150—200 m g ) and typically contain 15—30% weight of cobalt. To stabihze them and decrease selectivity to methane, these catalysts may contain small amounts of noble metal promoters (typically 0.05—0.1 wt% of ruthenium, rhodium, platinum, or palladium) or an oxide promoter (e.g., zir-conia, lanthana, cerium oxide, in concentrations of 1—10 wt%) (409). 

The structural promoter functions to provide a stable, high-area catalyst, while the chemical promoter alters the selectivity of the process. The effectiveness of the alkali metal oxide promoter increases with increasing basicity. Increasing the basicity of the catalyst shifts the selectivity of the reaction toward the heavier or longer-chain hydrocarbon products (Dry and Ferreira, 1967). By the proper choice of catalyst basicity and ratio, the product selectivity in the Fischer-Tropsch process can be adjnsted to yield from 5% to 75% methane. Likewise, the proportion of hydrocarbons in the gasoline range ronghly can be adjnsted to produce 0%-40% of the total hydrocarbon yield. 

Clean tungsten carbides, a-WC and a-W C, form essentially only hydrocarbons from CO—H2 reactions. At 673 K and atmospheric pressure, the main products on WC, W2C, and W are methane, CO2, and H2O (121). Ethane and propane are also formed at lower temperatures. WC was substantially more active than W2C and W. The nature of the products can be modified by oxide promoters, as for the case of Rh or Pt, or by the carbon vacancies at the surface (122). At 573 K and 5 MPa with 2H2/CO, turnover rates (based on sites titrated by CO chemisorption) of 0.25-0.85 s were reported for hydrocarbon synthesis over bulk and Ti02-supported tungsten carbides. In addition, WC and WC/Ti02 produced alcohols and other oxygenates with 20-50% selectivity. However, W2C of more metallic character did not produce any oxygenates. Coexistence of carbidic and oxidic components on the catalyst surface appeared to be responsible for alcohol formation. 

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