Abstract
Hydrogenation of CO with supported palladium catalysts was studied under atmospheric pressure and in the temperature range from 250°C to 400°C. Conversion of CO over the Pd-SiO2 catalyst was only ca. 10%, but was markedly increased by a ThO2-promoted catalyst (Fig. 1). Product distributions obtained over several catalysts are shown in Table 1. Hydrocarbon products obtained with the Pd-SiO2 catalyst contained essentially only methane, but the molecular weight distribution of hydrocarbons over alkali-promoted catalysts shifted to higher levels. Conversion of CO over the ThO2-promoted catalyst increased with Th/Pd atomic ratio, and when the ratio was 3, the conversion was maximum (Fig. 2).
The temperature programmed desorption of CO from catalysts was studied in a H2 stream to make clear the behavior of CO adsorbed on the palladium catalyst surface. In the Pd-SiO2 catalyst, the peak of methane desorption appeared at 320°C, and CO desorbed in the temperature range from 170°C to 350°C (Fig. 3). With increasing Th/Pd ratio, the peaks of methane in the desorption spectra shifted to lower temperatures and the peak areas increased (Fig. 4). There were two peaks in the desorption spectra of CO for each catalyst (Fig. 6). One appeared in a lower temperature range from 100°C to 250°C and the other in a higher temperature range from 200°C to 350°C. The peak in the latter range appeared at the same temperature as the peak of methane and its area increased with Th/Pd ratio, while the peak area in the former range decreased with Th/Pd ratio. This indicates that CO species desorbed at higher temperatures reacts with H2 to form methane. Addition of ThO2 enhances hydrogenation of CO over the palladium catalyst surface and increases the amount of adsorbed CO species which can be hydrogenated.
