The effect of chemical composition and structure on XPS binding energies in zeolites

The effect of the composition and structure of zeolites on the XPS core level binding energies has been studied for a large class of zeolites, viz. FAU, MFI, MOR and LTA with Si:Al ratio chabging from 1 to 160. Also the effect of the difference in the counter ions (Na, K, Rb, Ca, Mg, Ba, La) was investigated. As absolute binding energies cannot be determined to any reasonable degree of accuracy we focuss in this work on relative peak positions, which can be found with high precision. The main effect on binding energy differences between silicon, aluminium and oxygen in the zeolites is caused by their aluminium content. Effects of the zeolite structure and the co-cation are far less important. The Si(2p) to Al(2p) XPS peak separation decreases regularly with increasing aluminium content. We performed quantum chemical calculations that enable us to rationalise these findings in terms of a non-homogeneous charge distribution in the zeolite framework.