Role of Catalyst Support
The catalyst support can also greatly affect the activity and selectivity of the CO reduction reaction. The majority of earlier studies used silica (SiO2), alumina (Al2O3) or titania (TiO2) as the catalyst support in Rh catalyzed CO hydrogenation to oxygenates. In addition to Rh various promoters are added in these studies as well. There are relatively few studies of unpromoted Rh, because unpromoted Rh seems to produce hydrocarbons only, essentially independent of the support [39,23,40].
The effect of calcination the alumina support at different temperatures on ethanol production from CO hydrogenation over Rh/Fe/Al2O3 catalysts has been reported by Chen and coworkers [29]. In this study a series of Rh/Fe/Al2O3 catalysts, were prepared by a sequential impregnation method and calcined at different temperatures, during the catalyst preparation. These catalysts were tested by CO hydrogenation and characterized by N2 adsorption-desorption, X-ray diffraction (XRD), CO pulse chemisorption, temperature programmed surface reaction (TPSR) and temperature programmed reduction (TPR) techniques. They found that the activity of ethanol formation was highest when the catalyst support was calcined at 800°C, while that of methanol formation increased continuously with the calcination temperature of the support. According to the commonly accepted mechanism of C2-oxygenates formation, CO conversion followed three separate pathways after CO dissociation, and their results suggested
that the activity towards CO insertion and dissociation increased gradually with the calcination temperature, but began to decrease at 900°C. On the other hand, direct hydrogenation of CO to methanol was still increasing at 900°C. These observations were in agreement with TPSR results. Dispersion of the Rh or Fe species was not impaired, and even improved, with the declining of the surface area of the support due to high temperature calcination. TPR results revealed that Rh-Fe interaction was strengthened after calcination, due to a lowering in surface hydroxyl reactivity of the support and an increase of the Rh-Fe interface area. As a result, the amount of Rh-Fe-O sites for CO dissociation and insertion increased with the calcination temperature, giving rise to the increase in ethanol formation activity. However, an over-strong Rh-Fe interaction would be resulted when the catalyst support was calcined at 900°C, and this would cause more Fe species to be reduced, which would then cover the Rh sites. Consequently, CO uptake as well as dissociation and insertion of CO would decrease, leading to more CO molecules being hydrogenated directly to methanol, thus causing a decrease in the selectivity of ethanol formation [29].