Selective Oxidation and Combustion Sites

A common method for determining the existence and the densities of different active sites is selective chemisorption. However, ignorance of the nature of the active sites makes it impossible to choose suitable probe molecules. Instead, a variation of the selective chemisorption technique can be used that makes use of the reactivity of the active sites. 

The technique is temperature programmed desorption and reaction, or more generally, thermal desorption and reaction. The principle of the technique is simple. A reactant molecule is adsorbed at low temperatures on an active site. The temperature is then raised in a controlled manner and the evolution of the reaction products is monitored. If the various products are evolved independent of each other, they must be formed on independent sites. 

The technique was applied to a-Fe203 (5). cis-2-Butene was introduced to the catalyst at 25°C for equilibration with the active site. After equilibration and removal of the gas-phase butene, temperature programmed desorption 

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 

Adsorption sequence Total C4 Hydrocarbon adsorbed c Amount of C02 normally desorbed 8 

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