Hydrocarbons, chemisorption

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

Collectors ndFrothers. Collectors play a critical role ia flotation (41). These are heteropolar organic molecules characterized by a polar functional group that has a high affinity for the desired mineral, and a hydrocarbon group, usually a simple 2—18 carbon atom hydrocarbon chain, that imparts hydrophobicity to the minerals surface after the molecule has adsorbed. Most collectors are weak acids or bases or their salts, and are either ionic or neutral. The mode of iateraction between the functional group and the mineral surface may iavolve a chemical reaction, for example, chemisorption, or a physical iateraction such as electrostatic attraction. 

Fischer-Tropsch ohgomerization of CO -1- H9 to make hydrocarbons and oxygenated compounds was originally catalyzed by cobalt, which forms the active carbonyl, but now iron promoted by potassium is favored. Dissociative chemisorption of CO has been observed in this process. 

Pot charging Particulates (dust), CO, HF, SO, CF4, and hydrocarbons High-efficiency cyclone, baghouse, spray towers, floating-bed scrubber, electrostatic precipitators, chemisorption, wet electrostatic precipitators... 

The mode of chemisorption of CO is a key-factor concerning selectivity to various products. Hydrocarbons can only be produced if the carbon-oxygen bond is broken, whereas this bond must stay intact for the formation of oxygenates. It is obvious that catalysts favoring the production of hydrocarbons must chemisorb carbon monoxide dissociatively (e.g. Fe) while those favoring the formation of oxygenates must be able to chemisorb carbon monoxide molecularly (e.g. Rh). 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The opposite change in electric conductivity of adsorbent occurs during adsorption of such active particles as atoms of hydrogen and atoms of metals [115, 124,125]. The similar result is obtained during radiolysis of hydrocarbons [126] due to formation and chemisorption of H-atoms. Both the rate of adsorption caused change in electric conductivity and the value of its stationary values are determined in this cases by all the processes accompanying chemisorption [127],... 

Recently a novel experimental approach using Schottky diodes with ultra-thin metal films (see Fig. 11) makes direct measurement of reaction-induced hot electrons and holes possible. See for example Refs. 64 and 65. The chemical reaction creates hot charge carriers which travel ballistically from the metal film towards the Schottky interface and are detected as a chemicurrent in the diode. By now, such currents have been observed during adsorption of atomic hydrogen and deuterium on Ag, Cu and Fe surfaces as well as chemisorption of atomic and molecular oxygen, of NO and N02 molecules and of certain hydrocarbons on Ag. Similar results have been found with metal-insulator-metal (MIM) devices, which also show chemi-currents for many exothermic surface reactions.64-68... 

The articles by J. R. Anderson, J. H. Sinfelt, and R. B. Moyes and P. B. Wells, on the other hand, deal with a classical field, namely hydrocarbons on metals. The pattern of modem wTork here still very much reflects the important role in the academic studies of deuterium exchange reactions and the mechanisms advanced by pioneers like Horiuti and Polanyi, the Farkas brothers, Rideal, Tw igg, H. S. Taylor, and Turkevich. Using this method, Anderson takes ultrathin metal films with their separated crystallites as idealized models for supported metal catalysts. Sinfelt is concerned with hydrogcnolysis on supported metals and relates the activity to the percentage d character of the metallic bond. Moyes and Wells deal with the modes of chemisorption of benzene, drawing on the results of physical techniques and the ideas of the organometallic chemists in their discussions. 

Kineti cs. To date only addition reactions have been reported. These reactions produce products or adducts that are the result of complete addition, or addition and subsequent elimination. An example of the later reaction is dehydrogenation of hydrocarbons on platinum clusters. These addition reactions are in many ways analogs to the chemisorption process on metal surfaces. 

Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

Several authors reporting cluster ion reactions use the terms physisorption and chemisorption for reaction types 2 and 3, respectively (198,225). The reactions of the Nb clusters with several hydrocarbons have been studied but the reactions... 

To avoid chemisorption problems, an alternative and more universal adsorption method was proposed [89,90], The method is based on the differences in potentials of physical adsorption on the different components. The background of this method is discussed in Problem 8. It advances a CM, and its efficiency was tested on the model mechanical mixtures and PS type of C/SiOz, C/A1203, C/MgO, Si02 + A1203, etc. by adsorption of COz or hydrocarbons. The main requirement of the adsorbate for such measurements is the existence of some specificity of adsorption on different components. 

C complexes, 32 185-186 CFjHCFjH, 39 340 chemisorption complexes, 32 170-172 CjH, enthalpies, 37 141, 143 "C-labeling studies, 25 166-172 commercial, 6 197 complex molecules, 30 58-72 medium-sized rings, 30 68-72 polymethylcycloalkanes, 30 59-65 substituted aromatics, 30 65-68 cyclic-acyclic product ratio, 30 8-9 cycloalkanes, 30 68-69 function, hydrogen pressure, 30 12, 15-16 hydrocarbon reaction models, 32 202-205 hydrogenolysis and, 23 93, 103 interconversion, 30 81-82 isopentane, 30 17 label scrambling, 30 7, 12-13 mechanism, 30 5-16 bifunctional, 30 4 catalyst particle size and, 30 72-85 concerted, 30 20... 

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