Heterogeneous catalysis Fischer-Tropsch reaction

In this paper, we will review the chemical behaviour of transition metal oxides which is of crucial importance for heterogeneous catalysis, adhesion and many technological applications. Among them, MgO(lOO) is the simplest surface, with a square unit-cell containing two ions with opposite charges titanium oxides represent another important class of systems used for their catalytic properties either directly as catalyst or indirectly as support for other catalysts (metals such as Ni, Rh for the Fischer-Tropsch reaction or V2O5 for the reduction of NOx) or as promotors[l]. The most stable surface for rutile is the (110) face. 

Mechanism originally proposed by Fischer and Tropsch for catalysis of the heterogeneous Fischer-Tropsch reaction CO + H2 — CH3(CH2)nCH3 +. .. 

Most organic commodity chemicals are currently made commercially from ethylene, a product of oil refining. In the next several decades, we may see a shift toward other carbon sources for these chemicals. Either coal or natural gas (CH4) can be converted with steam into CO/H2 mixtures called water-gas or synthesis gas and then on to methanol or to alkane fuels with various heterogeneous catalysts (Eq. 12.19). In particular, the Fischer-Tropsch reaction converts synthesis gas to a mixture of long-chain alkanes and alcohols using heterogeneous catalysis. 

Selective Oxidation and Ammoxidation of Propylene by Heterogeneous Catalysis Robert K. Grasselli and James D. Burrington Mechanism of Hydrocarbon Synthesis over Fischer-Tropsch Catalysts P. Biloen and W. M. H. Sachtler Surface Reactions and Selectivity in Electrocatalysis... 

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. 

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),... 

Recent reviews (31-34,36,37,51) provide a comprehensive survey of the types of heterogeneous catalytic reactions investigated at supercritical conditions including alkylation, amination, cracking, disproportionation, esterification, Fischer-Tropsch synthesis, hydrogenation, isomerization, and oxidation. Table 2 summarizes reported investigations under these classes of reaction. Some of these examples are described here to show how to systematically exploit supercritical media in heterogeneous catalysis. 

Both surface and bulk properties are relevant to catalytic reactivity. Although heterogeneous reactions by definition occur at the interface between a catalyst and reactant/product phase, the process of catalysis actually includes activation of an as-synthesized catalyst, catalytic reaction, and adverse processes leading to the deactivation of a working catalyst. Activation may involve chemical transformations of both the catalyst surface and bulk. For example, the iron oxide Fe Oj is chemically transformed into the active iron carbide during activation for the Fischer-Tropsch synthesis (FTS) from CO and [32, 33]. There are numerous other examples of reduction of a metal oxide to an active metal or oxidation of a metal to an active oxide, carbide, sulfide, or similar. Characterization of chemistry and structure of the surface and bulk of a catalyst nanoparticle using representative techniques are presented in Chapter 4. 

In spite of such catalytic effects of the metal cluster, the rate of this reaction remains very low. Nevertheless, this reaction is an example of a very interesting type of homogeneous catalysis. Here the activation of carbon monoxide appears to be achieved by interaction of both the carbon and oxygen atoms with the metal cluster atoms in a similar way to the CO-chemisorption on metals in heterogeneous Fischer-Tropsch processes. 

The Fischer-Tropsch synthesis, i.e., the formation of hydrocarbons from CO and H2, represents an oligomerization reaction in heterogeneous catalysis. 

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