Hydrocarbon catalysis

T. Inui, and T. Takeguchi, Effective conversion of carbon dioxide and hydrogen to hydrocarbons, Catalysis Today 10, 95-106(1991). 

Other techniques such as low-energy electron diffraction (LEED) are also used for surface analysis, primarily for large single crystals. Single crystal metal surfaces have been used to study hydrocarbon catalysis on platinum (Anderson 1975). Techniques such as x-ray photoelectron spectroscopy (XPS) are also used for surface analysis but normally the reports describe mostly idealized single-crystal surfaces in high vacuum as opposed to using real-life (practical) catalyst systems under reaction environments. 

IX. A Descriptive Model of Hydrocarbon Catalysis on Platinum Surfaces. . 58... 

Using this approach to study heterogeneous catalysis on the atomic scale, we have investigated the mechanism of hydrocarbon catalysis by platinum surfaces. We shall describe in detail the results of these studies, which are pertinent in determining the nature of the active sites on the surface of this metal. We shall show how the results obtained for platinum may be extrapolated to other catalyst systems. Finally, we shall present a model of metal catalysis that has been emerging from our studies of platinum surfaces. 

Fie. 29. Relationship between homogeneous, heterogeneous, and enzyme catalysis as inferred from the experimental studies of hydrocarbon catalysis on platinum surfaces. 

Similar reaction sequences have been identified in other chemically reacting systems, specifically catalytic combustion (52, 53), solid-fuel combustion (54), transport and reaction in high-temperature incandescent lamps (55), and heterogeneous catalysis (56 and references within). The elementary reactions in hydrocarbon combustion are better understood than most CVD gas-phase reactions are. Similarly, the surface reaction mechanisms underlying hydrocarbon catalysis are better known than CVD surface reactions. 

In addition to the prolonged lifetime of the complex catalysts, such enhanced yields are also made possible by improved catalyst-hydrocarbon contact. Aluminium chloride is insoluble in hydrocarbons catalysis occurs in this case by surface contact with the solid. While the complex catalysts are also insoluble in hydrocarbons, they are liquids the ease of mixing is therefore greatly facilitated. 

The CFC replacements need to be nontoxic, nonflammable and have significantly lower, or zero ozone depletion potentials. Many organic- and aqueous-based systems, that do not contain chlorine or fluorine, have been developed for some applications while others use hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). Unlike hydrocarbon catalysis, the presence of hydrogen, chlorine and fluorine in the same molecule creates a very large number of... 

S. M. Davis, Hydrocarbon catalysis over platinum single crystal surfaces The role of adsorbed carbon deposits and oAer chemical additives. Report, LBL-13051 Order No. DE8201Q296,512 pp. Avail. NTIS From Energy Res. Abstr. 1982.7(13, Abstr. No. 34375. 

Early higher pressure reaction smdies over Pt-Sn model catalysts by Paffett [62,63] and Somorjai [64, 65] and their coworkers revealed new insights into hydrocarbon catalysis in such systems. Szanyi et al. [62] showed that n-butane hydrogenolysis under moderate pressures (1-200 Torr H3/butane=20) and temperatures (up to 650 K) could be carried out without disruption of the ordered Sn/Pt(lll) surface alloys. This established that such catalytic reactions could be studied while maintaining the composition and geometric structure of these alloys under reducing reaction conditions (but not catalytic oxidation due to the aggressive interaction of O3 with Sn). These ordered Sn/Pt surfaces are qualitatively different from those in many studies of promoters and poisons, or disordered alloys, e.g., Au-Pt, in which the quantitative information on ensemble sizes available for reactions is difficult to determine. 

Reddy, K.M. and Song, C.S. Synthesis of mesoporous zeolites and their application for catalytic conversion of polycyclic aromatic hydrocarbons. Catalysis Today, 1996, 31, 137. 

However, it is uncertain how far radical cations play a significant part in hydrocarbon catalysis over zeolites. 

Stabilization ( 900 kJ/mol) with the negative charge that develops on the zeolite framework when the proton is transferred. The major difference between superacids and zeolites is that in superacids cations and anions become solvated by the polar solvent molecules. No such stabilization occurs in zeolites. The chemistry in zeolites, when applied in hydrocarbon catalysis, is much closer to that in a vacuum (dielectric constant s 4) than to that in a polar medium such as a polar superacid (dielectric constant e 80-100). 

Reddy, K. M., and C. Song. Synthesis of Mesoporous Zeolites and Their Apphcation for Catalytic Conversion of Polycyclic Aromatic Hydrocarbons. Catalysis Today, 1996, 31 (1), 137-144. 

Given the constant presence of these carbonaceous deposits, it is somewhat surprising that any structural sensitivity is ever seen in hydrocarbon catalysis over Pt. The fact that some is seen is probably related to the fact that the underlying Pt structure controls the structure of this adlayer and the concentration of the few free sites found within this adlayer, which, in turn, can control the catalytic reaction rates. It is useful in this respect to point out that reaction probabilities (per collision of hydrocarbon molecule with the surface) are many orders of magnitude lower at high-pressure reaction conditions than in UHV where the surface is partially clean (64). Therefore, only a tiny (immeasurably small) fraction of free Pt sites are necessary to explain the overall observed rates of catalysis at high pressures. 

The potential for carrying out dinuclear hydrocarbon catalysis should be recognized. One catalytic sequence leading to selective hydrogenation is already suggested by the ability of Cp2M2(CO)4 compounds to coordinate unsaturated molecules that are four- but not two-electron donors ... 

Parteriheimer, W. Metodology and scope of metal/bromide autoxidation of hydrocarbon, Catalysis Today, 1991, 23, 69. 

The adsorption characteristics of organic molecules on solid surfaces are important in several areas of surface science. The nature of the chemical bonds between the substrate and the adsorbate and the ordering and orientation of the adsorbed organic molecules play important roles in adhesion, lubrication, and hydrocarbon catalysis. 

Cant, N. W, Angove, D. E. J. Patterson, M. The effects of residual chlorine on the behaviour of platinum group metals for oxidation of different hydrocarbons. Catalysis Today 44, 93-99 (1998). 

P. M. Henry, Palladium Catalyzed Oxidation of Hydrocarbons (Catalysis by Metal Complexes, Vol. 2), Kluwer, Dordrecht, the Netherlands, 1980. 

Ramirez-Cabrera, E., Atkinson, A., and Chadwick, D. The influence of point defects on the resistance of ceria to carbon deposition in hydrocarbon catalysis. Solid State Ionics 2000,136, 825-831. 

The following example of hydrocarbon catalysis illustrates the consequences of competitive adsorption for the order of the reaction, and the optimum conditions under which the reaction is carried out. The selectivity of the transition-metal catalyzed conversion of n-hexane to i-hexane versus the hydrogenolysis to smaller hydrocarbons depends strongly on the structure of the surface. Hydrogenolysis requires an ensemble of several atoms. When sites become blocked by inert atoms, such as carbon or sulfur, the selectivity for reactions maintaining chain length is significantly enhanced. 

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