Reaction over supported platinum catalyst

Fig. 33. The reaction path obtained with pure benzene initial composition for the reaction over supported platinum catalyst at 399°C., 20 atm of hydrogen and 5 atm of hydrocarbon.

Reactor System. A general purpose reactor system shown in Figure 1 was used in this work to study catalytic oxidation reactions over supported platinum catalyst pellets. This equipment allowed up to six precise gas mixtures to be prepared and made available for feed to the reactor. The switching valve directed a desired gas mixture flow to the reactor while another gas mixture flow was precisely measured by the bubble flow meter. 

Hanson FV, Boudart M. 1978. The reaction between H2 and O2 over supported platinum catalysts. J Catal 53 56-67. 

To examine the catalyst deterioration under the present superheated liquid-film conditions, a long-term tetralin dehydrogenation over the carbon-supported platinum catalyst (Pt/C) was carried out under the superheated liquid-film conditions (heating temperature 240°C) at the amount ratio of 1.1 g/0.5 mL/min [13]. Figure 13.21 shows the time courses of reaction rate and conversion in the tetralin dehydrogenation. High conversion (around 50%) was maintained for longer than 5 h. 

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenyl-ethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Ptj ratio the yield increased from 18 to 99 %. 

Table 1 presents several examples of unsteady-state kinetics models. These models are presented in the form of rate dependencies for catalytic reaction stages and side processes. The parameters of the models, such as reaction rate constants and activation energies, are given in references (Table 1) and were determined mainly from experimental data using transient response techniques. For the reaction of CO oxidation over a supported platinum catalyst, the kinetic gas theory was applied for estimating the adsorption constants. 

A simultaneous countercurrent movement of solid and gaseous phases makes it possible to enhance the efficiency of an equilibrium limited reaction with only one product (Fig. 4(a)) [34]. A positive effect can be obtained for the reaction A B if the catalyst has a higher adsorption capacity for B than for A. In this case, the product B will be collected mainly in the upper part of the reactor, while some fraction of the reactant A will move down with the catalyst. Better performance is achieved when the reactants are fed at some side port of the column inert carrier gas comes to the bottom and the component B is stripped off the catalyst leaving the column (Fig 4(a)). The technique was verified experimentally for the hydrogenation of 1,3,5-trimethylbenzene to 1,3,5-trimethylcyclohexane over a supported platinum catalyst [34]. High purity product can be extracted after the catalytic reactor, and overequilibrium conversion can be obtained at certain operating conditions. 

Cutlip, M. B., and C. N. Kenney, Limit cycle phenomena during catalytic oxidation reactions over a supported platinum catalyst, ACS Symp. Sen, 65, 475-486 (1978). 

Hudgins 17) has used a sinusoidally varying input in concentration to analyze the frequency response of a reactor for the catalytic dehydrogenation of ethanol using a conventional tubular reactor. Leder and Butt 18) have successfully used frequency response to analyze the dynamic behavior of a fixed-bed catalytic reactor used for the hydrogen-oxygen reaction over a supported platinum catalyst. Hydrogen inlet concentration was varied sinusoidally over frequencies from 2 cycles/ hour to 120 cycles/hour. 

One such alternative system comprises platinum-group metals supported on metal oxides. A number of groups have recently reported that such materials are active for NOx reduction under strongly oxidising conditions [4-6]. However, none of these studies have addressed the fimdamental questions pertaining to the mechanism of the selective reduction of NO on such catalysts, or to the nature of the catalytically active surface. In this paper, we describe the main features of the lean NOx reaction over a series of Pt/Al203 catalysts. We also present an extensive study of the mechanism of this reaction over Pt-based catalysts. 

The process took place on Ni/Cr203 between 373 and 525 K and must require the dissociative chemisorption of the methanes followed by random recombination not surprisingly, deactivation was found at all temperatures." The reaction between normal and deuterated n-butane has also been studied on a number of supported platinum catalysts." Little or no exchange was seen between light methylcyclo-hexane and deuterated n-octane (or the reverse) over Pt/Si02 at 755 K either in the presence or absence of hydrogen or deuterium because of rapid conversion to other molecules reaction did however take place between them at lower temperatures, and on Pt/Al203 it occurred both on the metal and the support. ... 

P. Granger, P. Malfoy, P. Esteves, L. Leclercq, and G. Leclercq [/ Catal., 187, 321-331 (1999)] studied the kinetics of the reaction between CO and NjO over a supported platinum catalyst. These authors examined the ability of several Hougen-Watson rate expressions to fit their kinetic data. 

Limit Cycle Phenomena during Catalytic Oxidation Reactions over a Supported Platinum Catalyst... 

Gupta NM, Mishra K, Belapurkar AD, et al Deuterium isotope exchange reaction between hydrogen and water over polyester-supported platinum catalysts, J Catal 121 (2) 386-395, 1990. 

To summarize, transient kinetic experiments are an estabHshed and valuable tool in the investigation of heterogeneously catalyzed gas-phase reactions. For liquid-phase systems, transient studies are much rarer than for gas-phase systems. It is probably related to slower dynamics and the fact that the intrinsic kinetic phenomena can be obscured by mass transfer effects and catalyst deactivation. As an illustration, we will consider three-phase continuous enantioselective hydrogenation of ethylbenzoylformate (Fig. 7.13) leading to two products over a platinum catalyst on an alumina support [1]. 

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