Methane, direct oxidation

In this paper, we summarize results from a small scale methane direct oxidation reactor for residence times between lO and lO seconds. For this work, methane oxidation (using air or oxygen) was studied over Pt-10% Rh gauze catalysts and Pt- and Rh-coated foam and extruded monoliths at atmospheric pressure, and the reactor was operated autothermally rather than at thermostatically controlled catalyst temperatures. By comparing the steady-state performance of these different catalysts at such short contact times, tiie direct oxidation of methane to synthesis gas can be examined independent of the slower reforming reactions. 

At p. 307 of Williams (2002), MIT enters the direct hydrocarbon field presenting alternative anode structures, asserted to be an improvement on copper-based anodes, as immediately above. See Figure A.5 on methane direct oxidation in Appendix A. 

Fig. 2.4 Comparison in conversion efficiency (a) (1) Carnot efficiency after correction of selfheating of methane combustion, (2) methane direct oxidation, (3) oxidation of reformed gas, (4) oxidation of reformed gas after correction for conductive properties of YSZ in given thickness (b) comparison between YSZ and LSGM in the same thickness of 50 pm...

For direct oxidation of methane, it is shown [12] that the mechanism is ... 

Direct oxidation of methane to methanol is an obvious dream reaction ... 

The Active Oxygen for Direct Oxidation of Methane to Methanol in the Presence of Hydrogen... 

The large amounts of natural gas (mainly methane) found worldwide have led to extentive research programs in the area of the direct conversion of methane [1-3]. Ihe oxidative transformation of methane (OTM) is an important route for the effective utilization of the abundant natural gas resources. How to increase catalyst activity is a common problem on the activation of methane. The oxidation of methane over transition m al oxides is always high active, but its main product is CO2, namely the product of deep oxidation. It is because transition metal oxides have high oxidative activity. So, they were usually used as the main corrqtonent of catalysts for the conqilete oxidation of alkane[4]. The strong oxidative activity of CH4 over tran on metal oxides such as NiO indicates that the activation of C-H bond over transition metal oxides is much easier than that over alkaline earth metal oxides and rare earth metal oxides. Furthermore, the activation of C-H bond is the key step of OTM reaction. It is the reason that we use transition metal oxides as the mam conq>onent of the OTM catalysts. However, we have to reahze that the selectivity of OTM over transition metal oxides is poor. 

These results confirm that cobalt oxides particles (C03O4 and CoOx) have a very important role in the direct oxidation of methane to C02. On the other hand, at high temperature, cationic cobalt (Co2+) appears to be able to reduce NO to N2, even in the presence of an excess of oxygen. 

The oxidation of hydrocarbons involves the sequential formation of a number of similar reactions in which various intermediate radicals which are combinations of carbon, hydrogen and oxygen are formed. In the simplest case, the oxidation of methane, the methyl radical CH3 plays an important part both in direct oxidation to CO(g) and in indirect oxidation through the formation of higher hydrocarbons such as C2H6 before CO is formed. The chain reactions include... 

In the 1980s, the oxidative coupling of methane to give ethylene and ethane was reported by Keller and Bhasin (8), whose discovery prompted numerous attempts to convert methane directly—and not only to ethylene and ethane (8), but also to methanol and formaldehyde (9) (Table I). Research on oxidative coupling of methane was motivated by results showing that the methane was... 

Putna ES, Stubenrauch J, Vohs JM, and Gorte RJ. Ceria-based anodes for the direct oxidation of methane in solid oxide fuel cells. Langmuir 1995 11 4832-4837. 

Park S, Craciun R, Vohs JM, and Gorte RJ. Direct oxidation of hydrocarbons in a solid oxide fuel cell I. methane oxidation. J Electrochem Soc 1999 146 3603-3605. 

R. Remick, O. Spaldon-Stewart, K. Krist, Alternative mechanism for direct oxidation of dry methane on ceria-containing anodes, Proceedings Fuel Cell Seminar, San Antonio, Texas, 2004. Courtesy Associates, Washington DC, USA (2004). 

J. Sfeir, P A. Buffat, P. Mockli, N. Xanthopoulos, R. Vasquez, H. J. Mathieu, J. Van herle, and K. Ravindranathan Thampi, Lanthanum chromite based catalysts for oxidation of methane directly on SOFC anode, J. Catal. 202, 229-244 (2001). 

Carbon monoxide (CO) and hydrocarbons such as methane (CH4) can be used as fuels in SOFCs. It is feasible that the water gas shift involving CO (CO + H2O H2 + CO2) and the steam reforming of CH4 (CH4 + H2O 3H2 + CO) occur at the high temperature environment of SOFCs to produce H2 that is easily oxidized at the anode. The direct oxidation of CO in fuel cells also is well established. It appears that the reforming of CH4 to hydrogen predominates in... 

There has been some controversy in the literature over precisely what should be called direct oxidation or direct utilization of hydrocarbons in an SOFC. As pointed out by Marina and Mogensen and Park et al., direct, electrochemical oxidation of complex hydrocarbons is unlikely to occur in one step. Even in the case of methane, the reaction produces eight electrons and must almost certainly occur in multiple steps. 

First, we will refer to the direct use of hydrocarbon fuels in an SOFC as direct utilization rather than direct oxidation. Second, we recognize that the broadest definition of direct utilization, exclusive from mechanistic considerations, should include rather conventional use of fuel by internal reforming, with steam being cofed to the fuel cell with the hydrocarbon. Indeed, this nomenclature has been used for many years with molten-carbonate fuel cells. However, because internal reforming is essentially limited to methane and because the addition of steam with the fuel adds significant system complexity, we will focus primarily on systems and materials in which the hydrocarbons are fed to the fuel cell directly without significant amounts of water or oxygen. 

The direct reaction of methane partial oxidation always competes with total oxidation reactions, which are also responsible for O2 consumption, whereas steam and dry reforming and C-forming reactions are also to be considered. All reactions are catalyzed by the materials which are active in partial oxidation, but different scales of reactivity for the catalysts can be estimated from the experimental data. Total oxidation prevails at the light-off of the fuel-rich stream over most catalysts, but precious metals are more active than transition metals. 

The stronger acidity of 4%Mo03/Si02 and 5%V2 l8i02 catalysts with respect to the unpromoted Si02 support seems to exert no direct influence on the reaction pathway of the methane partial oxidation. 

Synthesis Gas Formation by Direct Oxidation of Methane over Monoliths... 

Much recent research (7-5) has been devoted to converting methane to products that are more easily transported and more valuable. Such more valuable products include higher hydrocarbons and the partial oxidation products of methane which are formed by either direct routes such as oxidative coupling reactions or indirect methods via synthesis gas as an intermediate. The topic of syngas formation by oxidation of CH4 has been considered primarily from an engineering perspective (7-5). Most fundamental studies of the direct oxidation of CH4 have dealt with the CH4 + O2 reaction system in excess O2 and at lower temperatures (6-10). 

Tliese experiments demonstrated that hydrogen must be a primary product of the direct oxidation of methane. With only three layers of gauze (giving a contact time of about 10" seconds), Sh2 was --40% with -90% conversion of CH4. 

An example of the first situation is methafiol, which is made today from the three main chemical raw material sources plant matter (by wood distillation) coal (via carbon monoxide and hydrogen in water gas) and natural gas hydrocarbons (both by direct oxidation and through CO-H2 synthesis, where the synthesis gas is made from methane). 

It is also significant that the direct oxidative condensation of methane to higher hydrocarbons takes place in the presence of S8 over superacidic catalysts, such as TaF5 and the like.357... 

The main problems associated with the direct oxidation of methane are the higher reactivity of the products (methanol and formaldehyde) compared to methane, and the thermodynamically more favorable complete combustion of methane to carbon oxides and water ... 

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