A SIMPLIFIED KERKHOF-MOULIJN MODEL FOR DISPERSION QUANTIFICATION FROMXPS ATOMIC CONCENTRATIONS

Kerkhof-Moulijn model rigorously predicts XPS relative intensity betwe en ''promoter'' and support, for a catalyst with a homogeneous support ed phase. The model has been used mainly to demonstrate the maximum co ncentration of metal (or any deposit phase called ''promoter'' in the model) at which a deviation from monolayer deposition occurs. We demon strate that the central result of the model can be rewritten as: P-p/P -s = (p/s)(b)BC, where P-p and P-s are the XPS atomic concentrations o f ''promoter'' and support respectively, p and s are the corresponding bulk atomic concentrations. B is the function: B = (beta/2)1 + e(-bet a)(1 - e(-beta)), introduced by Kerkhof-Moulijn as a special case when ''promoter'' and support have close kinetic energies, but in our case is valid for the general case, without any restriction in the kinetic energies. beta depends only on the support through its real density a nd specific surface and lambda(ps), the mean free path of promoter ele ctrons passing through the support C depends only on the average ''cry stal'' size of the supported phase and lambda(pp) the mean free path o f promoter electrons passing through the promoter. Applications of the above relationship to different cases are presented together with a d iscussion of special cases of general interest.