The molecular probe approach assessment of acid-base properties

The Molecular Probe Approach Assessment of Acid-Base Properties 

This molecular probe XPS approach has been extended successfully to the assessment of the acid-base properties of a completely different class of material, i.e., polymers. Workers in this field have used various molecular probes, e.g.. trichloromethane, dimethyl sulfoxide and hexafluoroisopropanol, in which the key atoms whose core-level shifts were determined following adsorption were Cl. S, and F, respectively [52], 

It is clear that there is further potential for the molecular probe approach. For instance, it is attractive to apply another type of molecular probe, developed for the quantitative determination of hydroxyl groups on polymer surfaces by XPS [5. 1, to the characterization of catalyst supports such as silica-gel or alumina. The authors found that the derivatization of surface OH groups by tri-fiuoroacidanhydride (TFAA) was specific and quantitative, thus allowing the total number of these groups on the surface to be estimated. However, it should be emphasized that such XPS analysis is rather sophisticated and should not be regarded as a routine method. 

A powerful but expensive tool for tackling this problem is the technique of EXAFS. but it requires that the constituents have high scattering cross sections [56j. From an analysis of EXAFS data the number of atoms of a particular kind at a particular distance from a chosen absorber atom. i.e.. those in coordination spheres, can be evaluated. An EXAFS determination for each alloy constituent can provide information on the individual environment of atoms of that constituent on the support, thus indicating whether alloy formation has occurred. 

For highly dispersed metals on high surface-area supports, EXAFS, which is a bulk analysis method, actually monitors phenomena at the catalyst surface. EXAFS is also a very powerful method for investigating promoter-support interactions (cf. Sharpe et al. [561). 

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