The Effect of Chemical Composition and Structure on XPS Binding Energies in Zeolites

The effect of zeolite composition and structure on XPS core level binding energies has been studied for a large class of zeolites (FAU, MFI, MOR, LTA) differing in their counterions (Na, K, Rb, Ca, Mg, Ba) and Si:Al ratio (1 to 160). Whereas absolute binding energies cannot be determined to any reasonable degree of accuracy, relative peak positions can be found with high precision. In this paper, we show that the main effect on binding energy differences between silicon, aluminum, and oxygen is caused by the aluminum content of the zeolite. The effect of the zeolite structure and the co-cation is only minor. It was found that the Si(2p) to Al(2p) XPS peak separation decreases with increasing aluminum content. Quantum chemical calculations have been performed that rationalize these findings in terms of a nonhomogeneous charge distribution in the zeolite framework.