X-ray photoelectron spectroscopy investigation of Pd-Beta zeolite catalysts with different acidities

Three different H-Beta zeolites with SiO₂/ Al₂O ₃ molar ratios of 300, 150 and 25 were loaded with palladium by evaporation-impregnation method using Pd(NO₃)₂ as a precursor. Dependence between the electronic states of Pd as a function of the acidity of Beta zeolite was studied by XPS for calcined and reduced samples. The dependence of the electronic states of Pd on the support was detected to have a complicated character and to be influenced by various factors such as metal loading, density of the imperfections in the zeolitic framework and proton localization. In calcined zeolites mainly agglomerated PdO and AlPd_xO crystallites were detected referring to interaction of palladium with the support. Reduction of PdO led to formation of metallic Pd_n clusters, which further reduced to [Pd_n-H_m]^m+ complexes and AlPd_xO complexes. The increase of the reduction temperature seems to enhance interactions of Pd_n clusters with imperfect [AlO ^-_x] components instead of protons, especially in less acidic samples due to enhanced agglomeration of palladium particles outside the pores of the zeolite. The fraction, size and shape of the formed species were detected to be sensitive to the acidity of the support.