The stability and reactivity of CO species adsorbed on Ru/ZSM-5 have been studied by <i>in situ</i> diffuse reflectance infrared spectroscopy (DRIFT). Three species were detected: a linearly adsorbed CO (2045 ± 10 cm<sup>−1</sup>) on metallic Ru and two multicarbonyl species (2138-2080 cm<sup>−1</sup> and 2150-2086 cm<sup>−1</sup>) on oxidized Ru. The most strongly held species was the linearly adsorbed CO. Among the two multicarbonyl species, the one shoving a doublet at 2150 and 2086 cm<sup>−1</sup> was more stable than the other. Because it is believed that the formation of the 2150-2086 cm<sup>−1</sup> and the 2138-2080 cm<sup>−1</sup> species involved silanol and Brøsted 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.

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