Transformation of methane formation sites into methanol formation ones during CO---H2 reaction over Pd/CeO2 in its SMSI state

The phenomena of induction period of methanol formation, observed in CO---H₂ reaction over Pd/CeO₂ catalysts under SMSI state, was investigated in depth. The magnitude of the induction period was dependent on the extent of SMSI, and higher temperature H₂ reduction lengthened it accompanied with the increase of the number of active sites for methane formation. On the contrary, by the pretreatment of SMSI surface with water vapor, this induction period almost disappeared with the drastic decrease of methane formation rate. These results indicate that methane formation sites would be transformed into methanol formation sites by the oxidation of water vapor formed during CO---H₂ reaction. Infrared spectroscopic investigation of adsorbed CO after various pretreatments indicated that during the induction period thin layers of reduced ceria, which preferentially covered Pd(1 1 1) plane under SMSI state, were removed from the Pd(1 1 1) plane by formed water vapor during CO---H₂ reaction. It was concluded that Pd(1 1 1) plane adjacent to ceria would be the efficient active sites for methanol formation.