Methanation reaction, Fischer-Tropsch catalysis

These observations regarding the stability of the methyl species are in line with conclusions about the frustration of methane formation in Fischer-Tropsch catalysis 40) and also the inference that the slow step in CO methanation is the reaction between methyl groups and hydrogen 41), as Fig. 8c illustrates a resistance of the methyl groups to hydrogenation. 

The adsorption of CO is probably the most extensively investigated surface process. CO is a reactant in many catalytic processes (methanol synthesis and methanation, Fischer-Tropsch synthesis, water gas shift, CO oxidation for pollution control, etc. (1,3-5,249,250)), and CO has long been used as a probe molecule to titrate the number of exposed metal atoms and determine the types of adsorption sites in catalysts (27,251). However, even for the simplest elementary step of these reactions, CO adsorption, the relevance of surface science results for heterogeneous catalysis has been questioned (43,44). Are CO adsorbate structures produced under typical UHV conditions (i.e., by exposure of a few Langmuirs (1 L = 10 Torrs) at 100—200 K) at all representative of CO structures present under reaction conditions How good are extrapolations over 10 or more orders of magnitude in pressure Such questions are justified, because there are several scenarios that may account for differences between UHV and high-pressure conditions. Apart from pressure, attention must also be paid to the temperature. 

Since carbon dioxide is a thermodynamically stable, highly oxidized compound, its synthetic utilization requires some kind of a reduction -reaction with molecular hydrogen is a distinct possibility. Stepwise reduction of C02 with H2 may yield formic acid, formaldehyde, methanol and finally methane, together with CO or Fischer-Tropsch-type derivatives as shown on Scheme 3.42. In aqueous organometallic catalysis the most common product of such a reduction is formic acid. Formation of carbon monoxide, formaldehyde, and methane has already been reported, however, methanol and Fischer-Tropsch type products were not observed. 

Due to the known limitations of the world oil reserves, methane oxidation under fuel rich conditions will become increasingly important for the production of synthesis gas. which through methanol synthesis and Fischer-Tropsch reactions is the basis of many important petrochemical synthesis routes. Therefore, catalytic oxidation of methane has again become the focus of much basic and applied catalysis research in recent years. In this context, Schmidt and coworkers were able to show recently, that catalytic direct oxidation of methane over noble metal coated monoliths can yield CO and H2 with very high conversions and selectivities at the desired 1 2 CO H2 ratio (Hickman and Schmidt. 1992 and 1993a Torniainen and Schmidt. 1994 Bharadwaj and Schmidt. 1995). 

Much of the justification for the extensive study of transition metal cluster chemistry is embedded in the assumption that reactions of metal clusters are realistic structural models for reactions at metal surfaces in such processes as heterogeneous catalysis (9,10,11). For example, the metal carbonyl clusters, Ir4(CO)i2 and Os3(CO)i2, were demonstrated to be effective homogeneous catalysts for methanation (12). Additionally, Demitras and Muetterties (13) have found Ir4(CO)i2 to be a homogeneous catalyst in the Fischer-Tropsch synthesis of aliphatic hydrocarbons. Homogeneous catalysis of the water gas shift reaction by metal carbonyl clusters (e.g., Ru3(CO)i2) in alkaline solution has been reported by Laine, Rinker, and Ford (14), and more recently by Pettit s group (15). Nevertheless, mononuclear metal carbonyls (e.g., Fe(CO)s and the group VIb metal hexacarbonyls) have been demonstrated to have considerable activity above 120°C as soluble catalysts for Reaction 2 (16),... 

This expression can indeed account for a positive, first order in hydrogen and a negative or close to zero order in CO as is experimentally observed. The expression is also valid for the Fischer-Tropsch synthesis of higher hydrocarbons. In this case the scheme of (3.8) has to be extended with chain-growth reactions, as discussed in Section 6.6.5. How to control the selectivity of this process is a key issue in CO hydrogenation catalysis. Methane and methanol are the only products that can be obtained with 100% selectivity. 

---