CO Adsorption Behavior of Cu/SiO2, Co/SiO2, and CuCo/SiO2 Catalysts Studied by in Situ DRIFTS

The mode of CO adsorption is an important factor in directing the selectivity of CO hydrogenation toward methanol, hydrocarbons, or higher alcohols. Here, we report results on three catalysts: Cu/SiO2, Co/SiO2, and CuCo/SiO2. Infrared spectroscopic studies on Cu/SiO2 and CuCo/SiO2 show similar CO adsorption behavior on copper sites, in terms of adsorption frequency and stability, regardless of the presence or absence of cobalt. The CO species adsorbing on copper is not thermally stable at typical CO hydrogenation reaction temperatures. For CO to adsorb on cobalt sites on either Co/SiO2 or CuCo/SiO2, temperatures above ambient are required. In general, linearly adsorbed CO binds more strongly to the cobalt sites of CuCo/SiO2 than of Co/SiO2. Thus, on CuCo/SiO2, CO is more likely to dissociate at the same sites where it linearly adsorbs, leading to increased probability of CO insertion and ethanol formation. A second type of site on Co/SiO2 is responsible for direct CO dissociation, leading to high activity and hydrocarbon selectivity on this catalyst, as well as faster deactivation.