Relationships Between Metal Dispersion, Surface Area, and Crystallite Size

5 Relationships Between Metal Dispersion, Surface Area, and Crystallite Size 

Metal dispersion has been defined earlier by Equation 3.1. The metal content is routinely determined by various analytical methods to obtain the weight percentage [3] and hence the total number of metal atoms, Nmi (mole g ), is known. Once a particular adsorption technique is chosen and used, the appropriate adsorption stoichiometry directly gives the number of surface metal atoms, Nm, (mole g ). Metal surface areas can then be determined 

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 

Mn304 catalysts used for N2O and NO decomposition [61,62] and for NO and O2 adsorption on La20s and other rare earth oxide catalysts used for the decomposition of NO and the reduction of NO with CH4 and other gases [63 66], 

In addition, in some reactions catalyzed by solid acids, such as zeolites, Haag and coworkers have shown a direct correlation between the concentration of surface acid sites and activity [67]. Consequently, it appears that each acid site constitutes an active site, and such sites can be measured by appropriate adsorption methods [68-70]. 

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