Oxidation over platinum

The mechanism for CO oxidation over platinum group metals has been established from a wealth of data, the analysis of which is beyond the scope of this chapter. It is quite evident that surface science provided the foundation for this mechanism by directly showing that CO adsorbs molecularly and O2 adsorbs... 

Hydroxylamine sulfate is produced by direct hydrogen reduction of nitric oxide over platinum catalyst in the presence of sulfuric acid. Only 0.9 kg ammonium sulfate is produced per kilogram of caprolactam, but at the expense of hydrogen consumption (11). A concentrated nitric oxide stream is obtained by catalytic oxidation of ammonia with oxygen. Steam is used as a diluent in order to avoid operating within the explosive limits for the system. The oxidation is followed by condensation of the steam. The net reaction is... 

The kinetics of CO oxidation over platinum and palladium has been investigated by numerous scientists, beginning with the pioneer work of Irving Langmuir in 1922 (81). Langmuir studied platinum wires at 230 to 450°C, where the oxygen and CO pressures were at 0.05 Torr. He found that the kinetics can be represented by an expression... 

Fig. 17. Transition between kinetic regimes of CO oxidation over platinum, where...

Many elements of a mathematical model of the catalytic converter are available in the classical chemical reactor engineering literature. There are also many novel features in the automotive catalytic converter that need further analysis or even new formulations the transient analysis of catalytic beds, the shallow pellet bed, the monolith and the stacked and rolled screens, the negative order kinetics of CO oxidation over platinum,... 

Ethylene oxidation over Platinum In situ electrochemical promotion using (V -A C and studies with aPt(l 1 1)/Namodel catalyst,/. Catal. 160, 19-26 (1996). 

Figure 3.30 Ignition/extinction loops for ammonia oxidation over platinum performed in micro reactors with different membranes [19],...

In typical diesel exhaust gas, the N02/N0x fraction is only 5-10%. This fraction may be increased by passing the gas over a strong oxidation catalyst containing platinum as the active component. However, it is difficult to obtain useful fractions of N02 at temperatures below 200°C at high space velocities due to the strong temperature dependency of the NO oxidation over platinum [42], As expected the NO conversion rises exponentially with temperature, but declines at higher temperatures due to the thermodynamic limit, of the reaction (Figure 9.13). 

Matsushima, T. Kinetic studies on the CO oxidation over platinum by means of carbon 13 tracer. Surface Sci. 1979. 

Cyan-cat A process for destroying gaseous hydrogen cyanide by catalytic oxidation over platinum metal at approximately 300°C. 

Zhang, B Yamaguehi, T Kawakami, H Suzuki, T. Seleetive reduction of nitric oxide over platinum eatalysts in the presenee of sulfur dioxide and excess oxygen, Appl Catal, B Environmental, 1992, Volume 1, Issue 3, L15-L20. 

Some metallic catalysts in the form of gauzes are employed in certain processes, e.g. the oxidation of ammonia to nitrogen oxides over platinum, and for these heat transfer data have been published by London et al. [41,42]. 

Sulfur dioxide oxidation. The first use of SEP to study catalytic processes was made by Vayenas and Saltsburg. Vayenas and Saltsburg studied sulfur dioxide oxidation over platinum, silver and gold electrodes.39 The oxygen activity... 

However, the analysis of the data was carried out in such a way as to cast doubt on the validity of these conclusions (Vayenas54). Okamoto, Kawamura and Kudo48 went on to use the e.m.f. interpretation from the above work49 to further investigate the mechanism of CO oxidation over platinum by using the cell as a probe of the surface coverage of carbon monoxide. 

Metcalfe and Sundaresan15 went on to confirm that e.m.f. s obtained in the case of CO oxidation over platinum cannot be explained if reaction (3.3) is the only electrochemical reaction to occur. Even under oxygen rich conditions, carbon monoxide was involved in the electrochemical reactions, the dominant electrochemical reactions being,15... 

Figure 7 Rate and e.m.f oscillations for CO oxidation over platinum at 337°C and inlet P02 = 5.43 x 10 2 bar (from 58). (a) Oscillations obtained on a preoxidised surface (b) oscillations obtained on a pre-reduced surface. (Reproduced from J. Catal, 1988,11, 152, by permission from Academic Press Inc.)...

Effect of Oxygen Concentration on Catalytic Oxidation and Ionization. A number of the hydrocarbons studied were oxidized over platinum by the usual procedure using a mixture of 60% oxygen and 40% nitrogen... 

Goodman, M. G., Kenney, C. N., Morton, W., Cutlip, M. B. and Mukesh, D., 1982, Transient studies of carbon monoxide oxidation over platinum catalyst. Surf. ScL 120, L453-460. 

The dynamic behavior of fixed-bed reactors has not been extensively investigated in the literature. Apparently the only reaction which has received close attention is the oxidation over platinum catalysts. The investigations reveal interesting and complex dynamic behavior and show the occurrence of oscillatory and chaotic behavior [88-90], It is easy to speculate that further studies of the dynamic behavior of catalytic/biocatalytic reactions will reveal similar complex dynamics like those discovered for the CO oxidation over a Pt catalyst, since most of these phenomena are due to nonmonotonicity of the rate process which is widespread in catalytic systems. 

This is the only mechanism from mechanisms (7) in which the total kinetic order with respect to intermediates for every step is no greater than two. Scheme (8) enters as a component into the detailed mechanisms of oxidation reactions over metals. In particular, many researchers describe CO oxidation over platinum metals by the type (8) mechanism. 

The dynamics of critical phenomena in catalytic oxidation over platinum, in particular of CO oxidation, has been examined comprehensively [156-160]. The authors [156-160] obtained a variety of interesting data concerning the "ignition of this reaction and its dependence on the catalyst state. These data were interpreted on the basis of the above branched-chain model. It will be of interest to analyze the same data using our model. 

Now possibilities of the MC simulation allow to consider complex surface processes that include various stages with adsorption and desorption, surface reaction and diffusion, surface reconstruction, and new phase formation, etc. Such investigations become today as natural analysis of the experimental studying. The following papers [282-285] can be referred to as corresponding examples. Authors consider the application of the lattice models to the analysis of oscillatory and autowave processes in the reaction of carbon monoxide oxidation over platinum and palladium surfaces, the turbulent and stripes wave patterns caused by limited COads diffusion during CO oxidation over Pd(110) surface, catalytic processes over supported nanoparticles as well as crystallization during catalytic processes. 

Sometimes reactions have complex rate expressions that cannot be separated into solely temperature-dependent and concentration-dependent portions. In the decomposition of nitrous oxide over platinum,... 

Gavril, D. Katsanos, N.A. Karaiskakis, G. Gas chromatographic kinetic study of carbon monoxide oxidation over platinum-rhodium catalysts. J. Chromatogr., A 1999, 852, 507-523. 

A review on the CO oxidation over platinum has been made by Engel and Ertl [4]. In this review they list the following relevant reaction steps ... 

F.H.M. Dekker, J.G. Nazloomian, A. Bliek, F. Kapteijn, J.A. Moulijn, D.R. Coulson, P.L. Mills, and J.J. Lerou, Carbon Monoxide Oxidation over Platinum powder A comparison of TAP and Step-Response Experiments, Appl. Catal. A, accepted for publication (1996). 

Below 300°C, NO oxidation over platinum is very sensitive to the space velocity. Above that temperature the conversion is limited by the thermodynamic equilibrium (Fig. 2). 

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