Chemisorption of gas mixtures

In distinction from the more refined, and thus much more complicated lattice-gas model, the form of the model of the surface electronic gas provides possibilities for its application to chemisorption of gas mixtures and thus to modelling of kinetics of complex reactions. Derivation of multicomponent chemisorption isotherms based on thermodynamic approach was presented in the previous chapter. Within the framework of this model the following generalized elementary reaction A+ZI+Z=>S is considered. This reaction is written as a three-body collision, which is highly improbable, but is presented here only for illustrative purposes of how to express the reaction rate... 

The adsorption of the several gases, which participate in the catalysis, onto the catalyst surface has been measured separately, and the thermodynamics and kinetics of chemisorption have been studied in various systems by many investigators. Chemisorption from gas mixtures, however, has been studied only in a limited number of cases and no measurements of adsorption on the catalyst in its working state has been carried out until recent years. 

With hydrogen this has been deduced from the rapid equilibration of deuterium and hydrogen gas mixture. With water the extent of dissociative chemisorption is not so clearly established. The formation of... 

The character of the chemisorption of nitrogen can be also judged from the results of studies of ammonia synthesis kinetics at the reversible poisoning of the catalyst with water vapor (102,103). If a gas mixture contains water vapor, an adsorption-chemical equilibrium of adsorbed oxygen, hydrogen gas, and water vapor sets in on the iron catalyst. 

The TPR and chemisorption experiments were carried out in an apparatus described elsewhere [3] equipped with a thermal conductivity detector. The experiments were performed using a gas mixture of 7 vol% H2 in Ar and a heating rate of 10 G/min. Separate TPR experiments (not shown here) indicate that the degree of reduction of tin, based on the reaction Sn02 + 2 - Sn + 2H20, was about 50 % for the Pt-Sn catalyst. This indicates that most of the tin is in the Sn2+ state. 

Effect of H20 and/or SOx. According to Salker and Weisweiler (98), water vapor (5%) enhances NO conversion on Cu-MFI catalyst. Using gas mixture containing 3 or 9% H20 Sullivan et al. (95) observed an increase of the NO conversion to N2 over Cu-MFI in the temperature range 373-823 K. Cu-MOR catalysts also showed a pronounced positive effect of having moisture in the gas mixture (83). From TPD studies, Salker and Weisweiler (98) have shown that water makes the strong chemisorption of ammonia easier and may explain its beneficial role on the reaction. 

Ceramic membranes with adsorptive properties is a new field of application for multifunctional membranes. Single or mixed reactive oxides can be used for removing toxic gas from gas mixtures by selective chemisorption. For instance, sulfur compounds like hydrogen sulfide, H2S, or SO2 can be chemisorbed as sulfide or... 

The in sim characterization of catalysts was earned out in an apparatus which included a quadiupole mass-spectrometer and a gas chromatograph for TPO and H2 chemisorption measurements. In situ coking was performed by injecting a mixture of He and n-hexane vapor over the reduced catalysts at 500 C, In TPO experiments, ihe coked sample was heated at a rate of 8 C/min in a stream of 2 voL% O2 + 98% He. The amount of CO2 produced was recorded. The chemisorption of H2 was carried out in the same appanitus by a flow method after reduction or caking. The flow rate of carrier gas (Ar) was maintained at 25 ml/min and the volume of H2 injected was 0.062 ml/pulse. Since the partial piessiire of H2 was very low in this system, the hydrogenation of coke was never observed. Isobaric H2 chemisorption measurements with fresh catalysts were carried out in a static adsorption apparatus. Dehydrogenation of n-butane was carried out in a flow micro-rcactor in H2 atmosphere at LHSV = 3 h-l and H2/HC=1. Reaction products were... 

The only important exception, in which kc < ki has to be assumed, is the chemisorption of a gas component from a gas mixture in flue gas scrubbing, since here depletion of the gas to be adsorbed is involved and in addition the concentration of the gas is already zero at the interface due to the extremely fast chemical reaction (chemisorption). Since gas scrubbing is not normally carried out in stirred tanks, this exception will not be considered here. 

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