Methane complete oxidation

An increase in the activity of alumina supported Pd catalysts under conditions of methane complete oxidation was once again observed in the present work. Even catalysts which were treated under the reaction mkture at 900°C, an elevated temperature even for a combustion reaction, were activated. The samples can only be activated under reaction conditions and at high temperatures. Furthermore, the increase in activity was accompanied by an increase in the particle size. However, the catalytic activity could not be correlated directly with the particle size as already observed by others (4, 5). The absence of a correlation is not surprising since it was observed that the active catalyst contained both metallic and oxidized palladium. 

Magnesium oxide has been reported to be active in methane complete oxidation by Berg and Jaras. The activity was somewhat lower compared with a Ba-substituted hexa-aluminate. The difference between the two catalysts decreased after calcination to 1500 °C. 

By far the majority of the million or so known compounds of carbon also contain hydrogen and oxygen. There are several important types of oxygen-containing organic compounds and they can be studied as an oxidation series. For instance, the compound methanol, CH3OH, is very closely related to methane, as their structural formulas show. Methanol can be regarded as the first step in the complete oxidation of methane to carbon dioxide and water. 

Using a temperature-programmed surface reaction (TPSR) technique, Li et al. (154) showed that this complete oxidation of methane took place on the NiO catalyst during the CHfOi reaction. Weng et al. (145) used in situ microprobe Raman and in situ time-resolved IR spectroscopies to obtain a relationship between the state of the catalyst and the reaction mechanism. These authors showed that RuC>2 in the Ru/SiC>2 catalyst formed easily at 873 K in the presence of a CH4/02/Ar (2/1/45, molar) mixture and that the dominant pathway to synthesis gas was by the sequence of total oxidation of CH4 followed by reforming of the unconverted CH4 by C02 and H20. Thus, these results indicate that the oxidation of methane takes place principally by the combustion mechanism on the oxidized form of this catalyst. 

Application to Methane Oxidation. This selection of an appropriate initial model can be accomplished as shown here for the complete oxidation of methane. A general representation of the surface reaction model is (K12)... 

Problem 5 How many liters of oxygen (Oj) are required for the complete oxidation of 1 gram of methane ... 

Co and Fe catalysts have also been studied for the partial oxidation of methane to synthesis gas. Their potential relies on the fact that Co and Fe have higher melting and vaporizing points than Ni. Lower performances were mostly observed, however, which is probably related to the higher activity of CoO and FC2O3 for the complete oxidation of methane [121, 132, 133]. The recognized order of reactivity for partial oxidation is in fact Ni Co > Fe. However, it was observed that the performance of Co improves when a promoter is added. An extensive study of the catalytic partial oxidation of methane over CO/AI2O3 catalysts with different metals (0.1 wt% of Ni, Pt,... 

Concerning the operation of catalysts under adiabatic conditions, Basini et al. [156] reported the results of methane partial oxidation runs in a pilot-scale reactor operating at high pressure and short contact times, showing stable activity (almost complete conversion of methane and over 90% selectivity to CO and H2) during more than 500 h on-stream. In addition, operability for 20 000 h bench-scale testing has been claimed recently by the same group [157]. 

We also note that the oxidation of methane completely to C02 requires 2 moles of 02 for every mole of methane consumed. Thus, the degradation of 450 /xmol methane L-1 can be accomplished with the 02 present (ca. 1000 /xM). 

In combustion systems it is generally desirable to minimize the concentration of intermediates, since it is important to obtain complete oxidation of the fuel. Figure 13.5 shows modeling predictions for oxidation of methane in a batch reactor maintained at constant temperature and pressure. After an induction time the rate of CH4 consumption increases as a radical pool develops. The formaldehyde intermediate builds up at reaction times below 100 ms, but then reaches a pseudo-steady state, where CH2O formed is rapidly oxidized further to CO. Carbon monoxide oxidation is slow as long as CH4 is still present in the reaction system once CH4 is depleted, CO (and the remaining CH2O) is rapidly oxidized to CO2. 

Figure 14.3 shows results from flow reactor experiments on NO sensitized oxidation of methane in the 800 to 1200 K temperature range. In the absence of NO, temperatures of about 1100 K are required to initiate rapid oxidation of CH4 [31], but in the presence of NO, reaction occurs at temperatures as low as 850 K. The results indicate three different temperature regimes a low-temperature region (900-1000 K) with partial oxidation of methane, an intermediate-temperature regime with little reaction (1000-1150 K), and a high-temperature regime (>1150 K) with complete oxidation. 

As for the complete oxidation of propene, propane and methane, Nieuwenhuys and coworkers studied the influence of metal oxides additives on the catalytic activity of Au/Al203 [109-115], The addition of 3d transition metal oxides (MnOx, CoOx or FeOx), which were active by themselves, or ceria that was poorly active by itself promoted the catalytic activity of Au/Al203 in the total oxidation of propene [112]. The most active catalyst was Au/Ce0x/Al203, with a T95 at 497 K and with a high stability. In these cases, ceria and the transition metal oxides may act as co-catalysts and the role is twofold it stabilizes the Au NPs against sintering (ceria)... 

The former is a volume-decreasing reaction, while the latter is not. Both reactions are exothermic. Methanation is a deep hydrogenation reaction for carbon monoxide and WGSR is a complete oxidation reaction in which carbon monoxide is oxidized into carbon dioxide and water is reduced with the formation of hydrogen. As in the preparation of methane, other hydrocarbons, low alcohols and particularly, carbon dioxide and water are formed. Because of the presence of water, WGSR always occurs in the methanation process, which reduces the selectivity and yield of the desired product. 

P. Gelin and M. Primet, Complete oxidation of methane at low temperature over noble metal based catalysts, a review, Appl. Catal. B 39, 1-37 (2002). 

Also, methane is considered a viable alternative to currently used long-chain hydrocarbons as a source of combustible fuel. However, it is also a more harmful greenhouse gas than CO2. Therefore, if methane is to be used as a fuel source, it must either be completely oxidized on its first pass through the system or recycled back to ensure that no unreacted methane is released. 

Vinogradov has pointed out that with the appearance of the biosphere somewhere on the verge of 3-10 yr ago, there was a major upheaval in the evolution of the Earth. Oxidizing processes were abruptly accelerated, a nitrogen atmosphere arose in which carbon dioxide predominated over methane, and free carbon was oxidized to CO2. After the carbon was oxidized or at the same time as that process, there began oxidation of divalent iron (at — 10 ), which led to subsequent wholesale deposition of the sediments of the Precambrian BIF. Free carbon in equilibrium with the atmosphere appeared only after complete oxidation of ferrous iron compounds in the hydrosphere and on the land surface. 

F.H. Ribeiro, M. Chow, and R.A. Dalla Betta, Kinetics of the complete oxidation of methane over supported palladium catalysts, J, CataL 146 531 (1994). 

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