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)

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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+ (νCO at 2146 cm−1 and νNO 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.