Measurement of nickel surface area

The activity of a nickel catalyst is related to the nickel surface area. It is important to refer reaction rates to unit surface area in terms of the 

XRD provides a quick and easy method to determine the volume-averaged rrickel particle size, but generally smaller nickel particle sizes are indicated by XRD than by other methods. One reason is that nickel particles are polycrystalline. The presence of twin planes in the TEM images (Figtrre 4.4) of rrickel particles further supports this conclusion. 

TEM provides size distributions of the nickel particles and information about the stmctirre of the catalyst that can hardly be obtained by any other technique, but only a very small part of the sample is analysed. 

The most widely used method for measurements of the nickel surface area is chemisorption. This method requires [381]  

Chemisorption of various gases has been proposed for determination of the surface area of nickel [389]. Oxygen could involve a severe reconstruction of the nickel surface and the evaluation of results obtained with carbon monoxide could be impeded by the different ways in which carbon monoxide may chemisorb on nickel. 

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The dramatic increase in irreversible CO adsorption on presulfided supported nickel catalysts at moderate pressures (162) has significant, practical implications in regard to the use of CO chemisorption to measure nickel dispersion. For example, it is often desirable to determine nickel surface areas for catalysts used in a process where sulfur impurities are present in the reactants. Substantial differences in the measurements of nickel surface area by H2 or CO adsorption are possible depending upon the catalyst history and choice of adsorption conditions. In view of the ease with which catalysts may be poisoned by sulfur contaminants at extremely low concentrations in almost any catalytic process, and since large CO uptakes may be observed on supported Ni not necessarily representative of the unpoisoned nickel surface area, the use of CO adsorption to measure nickel surface areas is highly questionable under almost any circumstance. 

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