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Theoretical and experimental investigations on site occupancy for palladium oxidation states in mesoporous Al-MCM-41 materials

A Gannouni, Xavier Rozanska, B Albela, M Saïd Zina, F Delbecq, L Bonneviot, and A Ghorbel
Journal of Catalysis 289, no. C (May 1, 2012): 227–237

The site occupancy of the various oxidation state of palladium supported on mesostructured porous aluminosilica of Al-MCM-41 type was studied combining density functional theory (DFT), FTIR, and Raman investigations. The study focuses on the usual +2 oxidation state highly dispersed during palladium incorporation via template-ion exchange (TIE) but also on other oxidation states. The simulations of the IR spectra using silica clusters of different sizes confirm the Pd²⁺ preference for sites contained in 6- or 8-membered rings (band at 930 cm⁻¹). The calculations suggest that, after calcination, isolated oxidized palladium species such as [PdO]²⁺, [Pd(OH)₂]²⁺ for the +4 oxidation state, as well as [PdOH]²⁺ for the +3 EPR active oxidation state may coexist. The match between experimental and calculated vibrational frequencies relies on the two intense Raman bands at 270 and 630 cm⁻¹. All these oxidized species are preferentially located on 8-membered rings and linked to three framework oxygen atoms.