Co-ordination chemistry of palladium cations in Pd-H-ZSM-5 as revealed by FTIR spectra of adsorbed and co-adsorbed probe molecules (CO and NO)

Abstract

Low temperature CO adsorption on a Pd-H-ZSM-5 sample results in formation of Pd³⁺(CO)₂ species (2221 and 2200 cm⁻¹). The dicarbonyl structure has been proven by coadsorption of ¹²CO and ¹³CO: mixed Pd³⁺(¹³CO)(¹²CO) species absorb at 2210 and 2158 cm⁻¹. Evacuation of the sample at increasing temperatures leads to reduction of palladium and formation of different (poly)carbonyls of Pd²⁺, followed by Pd⁺–CO (2126 cm⁻¹) and Pd⁰–CO (2105 cm⁻¹) species. Adsorption of NO on the activated sample leads mainly to formation of Pd²⁺–NO linear nitrosyls (1881 cm⁻¹), a small amount of NO⁺ (2137 cm⁻¹), and Pd²⁺(H₂O)(NO) species (1839 cm⁻¹). Coadsorption of NO and O₂ results in formation of (i) species typical of the H-ZSM-5 support (NO⁺, N₂O₃, NO₂ and N₂O₄) and (ii) nitrates formed with the participation of palladium. Coadsorption of NO and excess CO results in appearance of mixed carbonyl–nitrosyl species of Pd²⁺ (ν_CO at 2146 cm⁻¹ and ν_NO at 1813–1791 cm⁻¹). The latter are decomposed easily losing their CO ligand. The results suggest a high coordinative unsaturation of the cationic palladium sites in Pd-H-ZSM-5.