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)

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 (CO)-C-12 and (CO)-C-13: mixed Pd3+((CO)-C-13)((CO)-C-12) 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 Pd-0-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 O-2 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+ (nu(CO) at 2146 cm(-1) and nu(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.