Platinum catalytic surface

From the study of the influencing of single reactions by products and by other added substances and from the analysis of mutual influencing of reactions in coupled systems, the following conclusions can be drawn concerning adsorption of the reaction components. (1) With the exception of crotyl alcohol on the platinum-iron-silica gel catalyst, all the substances present in the coupled system, i.e. reactants, intermediate products, and final products, always adsorbed on the same sites of the catalytic surface (competitive adsorption). This nonspecificity was established also in our other studies (see Section IV.F.2) and was stated also by, for example, Smith and Prater (32), (2) The adsorption of starting reactants and the desorption of the intermediate and final products appeared in our studies always as faster, relative to the rate of chemical transformations of adsorbed substances on the surface of the catalyst. 

Attempts have also been made to obtain the radicals (CF3)3C and CeFs as products of vacuum pyrolysis of (CF3)3CI and CeFsI (Butler and Snelson, 1980b). However, only perfluoroisobutene was observed in an IR spectrum of pyrolysis products of (CF3)3CI. Thermolysis of CeFsl led to formation of CF4, CF3 and CF2 as a result of decomposition of the aromatic ring. This behaviour was explained as due to catalytic effects which take place on the platinum reactor surface. 

Two new technologies have reduced the cost of alkali fuel cells to the point where a European company markets taxis that use them. One is the use of CO2 scrubbers to purify the air supply, making it possible to use atmospheric O2 rather than purified oxygen. The other is the development of ultrathin films of platinum so that a tiny mass of this expensive metal can provide the catalytic surface area needed for efficient fuel-cell operation. 

The platinum concentrations in the platinized carbon blacks are reported to be between 10 and 40% (by mass), sometimes even higher. At low concentrations the specific surface area of the platinum on carbon is as high as lOOm /g, whereas unsupported disperse platinum has surface areas not higher than 10 to 15m /g. However, at low platinum concentrations, thicker catalyst layers must be applied, which makes reactant transport to reaction sites more difficult. The degree of dispersion and catalytic activity of the platinum depend not only on its concentration on the carrier but also on the chemical or electrochemical method used to deposit it. 

Pellistors are used to detect flammable gases like CO, NH3, CH4 or natural gas. Some flammable gases, their upper and lower explosion limits and the corresponding self-ignition temperatures are listed in Tab. 5.1. This kind of gas sensor uses the exothermicity of gas combustion on a catalytic surface. As the combustion process is activated at higher temperatures, a pellistor is equipped with a heater coil which heats up the active catalytic surface to an operative temperature of about 500 °C. Usually a Platinum coil is used as heater, embedded in an inert support structure which itself is covered by the active catalyst (see Fig. 5.33). The most frequently used catalysts are platinum, palladium, iridium and rhodium. 

Ortho-para deuterium, 27 25, 50 Ortho-para hydrogen conversion, 27 23 Oscillatory catalytic reactions, 37 213-215, 271-272 see also Platinum catalytic CO oxidation on Pt(l 11) and Pt(llO) surfaces COj formation, 37 216-217 kinetic oscillation mechanism, 37 220-228... 

The chapter Chiral Modification of Catalytic Surfaces [84] in Design of Heterogeneous Catalysts New Approaches based on Synthesis, Characterization and Modelling summarizes the fundamental research related to the chiral hydrogenation of a-ketoesters on cinchona-modified platinum catalysts and that of [3-ketoesters on tartaric acid-modified nickel catalysts. Emphasis is placed on the adsorption of chiral modifiers as well as on the interaction of the modifier and the organic reactant on catalytic surfaces. 

To minimize overpotential effects, cathodes axe usually made of finely divided platinum on a porous support, for aqueous electrolytes. The catalytic surfaces of the anodes are particularly susceptible to poisoning by CO, olefins, sulfur compounds, and other impurities in the fuel. These lie above H2 in the chemisorption series (Eq. 6.3). 

As an illustration, consider the stagnation flow over a catalytic surface during an ignition event. The inlet flow is steady, but the surface temperature increases as power through the platinum-foil surface increases. At a certain temperature the catalytic ignition occurs very rapidly. The flow configuration and conditions, which are taken from Deutschmann [101], are u n — 8 cm/s, Tln = 300 K, with an inlet mixture of 3% CH4, 3% O2, and 94% N2. The inlet-to-surface separation is L = 5 cm, and the surface Pt sites are initially covered... 

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

Although the action of most additives on a catalytic surface is somewhat obscure, it seems safe to assume that the action of platinic chloride is a promoter action through the formation of metallic platinum under these reductive conditions. The platinum is plated out on the nickel surface. Promoter action has been observed when platinic chloride in amount sufficient to provide only 0.4 mg. of platinum is mixed with as much as 3 g. of Raney nickel catalyst (41). 

Zeolite-based hydrogenation catalysts containing platinum and palladium have increased resistance toward sulfur poisoning (101-104), and a higher activity (95, 105) than many other supports. In recent years there has been some effort devoted to attempt to explain this phenomenon. Although there is general agreement that the catalytic surface of the zeolites most probably... 

Brundege, J. A., and Panavano, G., The distribution of reaction rates and activatione energies on catalytic surfaces Exchange reactions between gaseous benzene and benzene adsorbed on platinum. J. Catal. 2,380 (1963). 

Cutlip, M. B., Concentration forcing of catalytic surface rate processes. I Isothermal carbon monoxide oxidation over supported platinum, AIChE J., 25, 502-508 (1979). 

A conceptual model depicting the sequences of steps in this platinum-catalyzed reaction is shown in Figure 10-10. Figure lO-lOis only a schematic representation of the adsorption of cumene a more realistic model is the formation of a complex of the -it orbitals of benzene with the catalytic surface, as shown in Figure 10-11. 

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