Reactant chemisorption, method

The poison method has been used more often than the reactant chemisorption method. Especially since the 1950 s many Russian workers, such as V.P, Lebedev and coworkers (73-76), have used this method to determine the site density. 

In 1970 Amenomiya and Cvetanovic (7 ) were able to use the reactant chemisorption method in the dimerization and hydrogenation of ethylene over silica-alumina catalysts for et Y ch iso ption they found site densities of the order of 10 to cm for the catalysts. 

In all above mentioned applications, the surface properties of group IIIA elements based solids are of primary importance in governing the thermodynamics of the adsorption, reaction, and desorption steps, which represent the core of a catalytic process. The method often used to clarify the mechanism of catalytic action is to search for correlations between the catalyst activity and selectivity and some other properties of its surface as, for instance, surface composition and surface acidity and basicity [58-60]. Also, since contact catalysis involves the adsorption of at least one of the reactants as a step of the reaction mechanism, the correlation of quantities related to the reactant chemisorption with the catalytic activity is necessary. The magnitude of the bonds between reactants and catalysts is obviously a relevant parameter. It has been quantitatively confirmed that only a fraction of the surface sites is active during catalysis, the more reactive sites being inhibited by strongly adsorbed species and the less reactive sites not allowing the formation of active species [61]. 

Other indirect methods, those not involving poisoning or reactant chemisorption, have been used to determine site densities. Perhaps the most common of these methods is to count the surface metal atoms, with either pure metal crystals or metal supported on oxides, with the counting carried out by determination of the amount of O2 or CO2 which chemisorbs ( ) the assumption has then sometimes been made that all such atoms are catalytically active. 

A main task of chemisorption kinetics theory is prediction of activation parameters on the basis of physicochemical properties of reactant and surface active sites. One of the most widespread methods used for homogeneous media is application of the linear models... 

The mechanism of a catalytic reaction is not likely to change with changes in catalyst composition or method of preparation, as long as the chemical nature of the catalyst is not changed. After all, a catalytic reaction mechanism involves chemisorption of the reactants and/or their fragments on active sites that depend largely on the composition of the catalyst. This means that the mechanism and therefore the overall rate expression can be expected to remain the same from sample to sample in a homologous series of catalysts. 

In other Pt-doped monolithic carbon aerogels, prepared by adding the Pt pre-cnrsor to the initial R/F mixture [41], the Pt particle size determined by H2 chemisorption was mnch higher than that determined by high-resolution transmission electron microscopy (HRTEM). This indicates that some Pt particles were encapsnlated by the carbon matrix and were consequently inaccessible to H2. This can be the main problem of this preparation method when the metal-doped carbon gel is to be used as catalyst, because part of the metal will not be accessible to the reactant molecules. 

In this paper, I review the recent advances and developments of first-principle quantum chemical methods and discuss their application to modelling chemisorption, surface reactivity of reactants/intermediates, and the catalytic behavior for a series of relevant commercial chemistries. We focus primarily on the static representation of the surface. 

The toxicity of a catalyst poison is defined as the number of active sites which are blocked by one poison molecule. Toxicity of acetylene was determined using the Maxted method described by Barbier [16], A mixture containing all reactants except acetylene was passed over the catalyst at 152°C, so that CO conversion was 21 %. A rapid step change in acetylene concentration from 0 to 7 ppmC was done. From the slope of the conversion decrease versus time, the molar flow of acetylene, and the metal dispersion of the catalyst (40 % from CO chemisorption) C2H2 toxicity was determined to be 3.6. This clearly confirms that C2H2 is a very strong poison, since toxicity values of 3 are only reported for a few sulfur and lead compounds [16]. 

Probably the most popular indirect methods of determining site density are the poison method and the method in which the amount of chemisorption of reactant is measured. The number of either the poison or the reactant molecules which chemisorb per unit area is then taken to be the site density. Some examples will be discussed. 

In summary, there are readily available and easy to use XRD and chemisorption techniques that are applicable to metal catalysts, in particular, and to other catalysts, in general. If available, TEM can add additional information. Chemisorption is the most sensitive method, and all kinetic studies of metal catalysts should be accompanied by a measurement of the metal surface area and dispersion via a standard adsorption procedure. For non-metallic catalysts, adsorption sites can still be counted in many situations by finding the appropriate region of temperature and pressure to measure adsorption of one of the reactants, and some (or all) of these sites would be expected to be active sites under reaction conditions. Examples of such efforts have been reported for N2O, NO and O2 adsorption on Mn203 and... 

---