The effect of Ca 2+ and Mg 2+ ions on the formation of electron-deficient palladium-proton adducts in zeolite Y

Abstract

FTIR spectra of adsorbed CO on Pd particles in supercages of zeolite Y show that positively charged Pd n clusters are formed when the concentration of protons in the supercages is high and thereby confirm our hypothesis that [Pd n H] + adducts are the “electron-deficient” palladium clusters that are responsible for catalytic superactivity. The formation of such adducts is favored by a high concentration of divalent charge-compensating ions, for example, Mg 2+ or Ca 2+. Carbon monoxide displaces the protons from the [Pd n H] + adducts; this leads to an increase of the IR band due to supercage O ZH groups. Simultaneously the Pd n (CO) x entities migrate through the supercage channels and coalesce with each other. Temperature-programmed reduction (TPR) shows that substituting Na + by Mg 2+ ions results in a downward shift by 70°C of the TPR peak for Pd 2+ reduction. This suggests that the divalent ions preferentially populate sodalite cages and hexagonal prisms, thus preventing the migration of Pd 2+ ions into these positions. During reduction, both Pd n clusters and protons are formed in supercages, setting the stage for the formation of the [Pd n H] + adducts.