By use of a new PSRA (pulse surface reaction rate analysis) apparatus equipped with an emissionless infrared diffuse reflectance spectrometer (EDR) and a flame ionization detector (FID), the dynamics of both adsorbed CO and produced CH4 were simultaneously measured in the CO hydrogenation over Ru/Al2O3 catalyst. When a CO pulse was introduced to the catalyst via the H2 at temperatures above 405 K, an IR absorption band assignable to a linearly adsorbed CO appeared at 2040 cm-1. At the same time an FID response of CH4 quickly increased. Then the IR absorption of the linear CO gradually decreased with time accompanying the gradual decrease in the FID response of CH4, indicating that linear CO gradually reacts with H2 to form CH4. Since CH4 is considered to be produced through the hydrogenation of surface carbon species [(CHx)ad], viz. (CO)ad →kCO (CHx)ad →kCH CH4, the rate constants of the respective steps (kCO and kCHx) can be determined from the dynamics of (CO)ad and CH4 after the CO pulse. It was found that kCHx is more than 50 times larger than kCO, indicating that on the Ru/Al2O3 catalyst the formation of (CHx)ad from adsorbed CO is much slower than its hydrogenation to CH4. Furthermore, an H2-D2 inverse isotope effect was observed for kCO, suggesting that hydrogen atoms play an important role in the C-O bond dissociation in the CO hydrogenation. A mechanism of the C-O bond dissociation was also discussed on the basis of these results for kCO.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry