Surface segregation of lanthanum and cerium ions in ceria/lanthana solid solutions: comparison between experimental results and a statistical–mechanical model

In correlating the composition of the mixed oxides of ceria and lanthana, Ce1−xLaxO2−x/2, with their catalytic activities, it is advantageous to model quantitatively the composition of the surface in terms of that of the bulk. An earlier paper presented the empirical results in terms of a Freundlich-style adsorption isotherm. Here, those results are shown to fit a statistical–mechanical model that includes the change in segregation energy with surface coverage due to solute–solute interactions. Lanthanum and cerium exhibit preferential segregation to the surface of their solid solutions in ceria and lanthana, respectively. From the most dilute solutions examined here the free energies of segregation are 34 and 11 kcals/mol, for lanthanum and cerium as solutes, respectively. Both surface segregation energies are coverage-dependent and obey a reciprocal dependence on the ratio of segregated solute/solvent ionic fractions falling asymptotically to 7 and 2 kcals/mol for lanthanum and cerium as solutes, respectively.