DYNAMIC EFFECTS IN CO ADSORPTION ON RU/ZSM-5 .2. THERMAL-STABILITY AND REACTIVITY OF CO ADSORBED SPECIES

The stability and reactivity of CO species adsorbed on Ru/ZSM-5 have been studied by in situ diffuse reflectance infrared spectroscopy (DRIFT). Three species were detected: a linearly adsorbed CO (2045 +/- 10 cm-1) on metallic Ru and two multicarbonyl species (2138-2080 cm-1 and 2150-2086 cm-1) on oxidized Ru. The most strongly held species was the linearly adsorbed CO. Among the two multicarbonyl species, the one showing a doublet at 2150 and 2086 cm-1 was more stable than the other. Because it is believed that the formation of the 2150-2086 cm-1 and the 2138-2080 cm-1 species involved silanol and Bronsted OH groups, respectively, it is inferred that the species with a low-frequency doublet (2138-2080) is predominantly located within the pore network of the HZSM-5 zeolite, while the other multicarbonyl species is mainly sited on the external surface of the zeolite. The latter species was found to be more stable than the other multicarbonyl species.