Hydrocarbons, adsorbed temperature-programmed desorption

Fig. 4 (a) Stepwise dehydrocyclization of -hexane (21, 62). (b) Temperature programmed desorption of benzene originating from various adsorbates over Pt-AljOs. Temperature of adsorption 25°C. Rate of heating 23°C per minute. Detector monopolar mass spectrometer, the ordinate corresponds to the I intensity of mass number 78, in arbitrary units. For clarity, the thermodesorption curves for other compounds (starting hydrocarbon) hexene from hexa-dienes and hydrogen have not been shown (62c). 

The temperature programmed desorption profile for the adsorption of butadiene in place of cis-2-butene is shown in Fig. 1, curve c. Two sets of products are observed. The product below 210°C is unreacted butadiene, and the products above 210°C are carbon dioxide and water. The similarity in the evolution of the combustion products of butene and butadiene is an indication that their combustion proceeds via similar reaction mechanisms. The similarity in the desorption of butadiene suggests that in butene adsorption, butadiene desorption is desorption limited. Indeed, that both butene and butadiene adsorb on the same type of sites has been confirmed by sequential adsorption experiments. The results are shown in Table III. It was found that if the C4 hydrocarbons are adsorbed sequentially without thermal desorption between adsorptions, the amounts of the final desorption products are the same as those in experiments where only the first hydrocarbon... 

Until now, in order to select a hydrocarbon adsorber with higher hydrocarbon trapping and conversion efficiency, experimental tests using vehicle and engine dynamometer has been employed. A model. Temperature Programmed Adsorption (TPA), was proposed by Kim, et al. [4-S] to save cost and time. The model has an advantage to analysis adsorption, desorption and conversion of hydrocarbons simultaneously. 

A kinetic description of these reactions is difficult to give, due to the complicated decomposition pathways of the hydrocarbons on noble metal surfaces. The temperature programmed reaction between adsorbed ethylene and NO on rhodium in Fig. 5.16 illustrates some of the many reactions that may occur [58]. As seen before, the NO molecule starts to dissociate aroimd room temperature. Ethylene decomposes in several steps at different temperatures as evidenced by the release of H2 and H2O. The formation of CO and some CO2 between 500 and 600 K is well above the respective desorption temperatures of these gases, and suggests that the C-C bond of the hydrocarbon breaks in this temperature range and limits the rate of the oxidation on rhodium surfaces. Formation of HCN is observed as well. Note that a large reservoir of surface CN species forms at temperatures of 500 K and remains on the surface until 700-800 K, where it decomposes and is followed by the instantaneous desorption of N2. 

An advantage of the subtraction method, which, however, can be used in only a limited number of instances, is the possibility of selective determination of the adsorbed compounds after their desorption. The adsorbed compounds are regenerated either by increasing the temperature or by the action of a new reagent, which destroys the compounds formed or displaces the components absorbed. In this instance the selectivity of determination increases considerably, and so do the reliability and sensitivity. An example of the utilization of this method is the determination of n-alkanes in a mixture with other hydrocarbons n-alkanes absorbed at 350 C can be eluted at 550°C, whereupon their individual composition can be determined with temperature programming of the chromatographic column, which is located after the subtraction reactor [48]. 

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