The hydrogenation of CO2 over Pd supported by A12O3, TiO2, SiO2, and MgO has been investigated in a flow technique at 1 and at 9.5 atm. For comparison the hydrogenation of CO was examined under the same experimental conditions. Attention was focused on the identification of surface species formed during the reaction. The hydrogenation of CO2 occurred at a measurable rate above 520 K. It appears that the dispersion of Pd plays a governing role in determining the direction of the H2 + CO2 reaction. On highly dispersed Pd, the main product of the reaction was methane at both pressures while on poorly dispersed Pd the reverse water-gas shift reaction (at 1 atm) or methanol formation (at 9.5 atm) occurred. In situ infrared spectroscopic measurements revealed that multiply bonded CO and formate species were present on the catalyst surface during the reaction at 1 atm. The formation of surface carbon was also detected. From the behavior of surface formate under different conditions it was inferred that it does not play a significant role in hydrocarbon synthesis on Pd catalysts. On the basis of the specific activities, Pd TiO2 proved to be the most effective catalyst for the hydrogenation of CO2. It is proposed that the important step in the methanation of CO2 is the dissociation of adsorbed CO. With respect to the high activity of Pd TiO2, it is assumed that an electronic interaction operates between TiO2 and Pd, influencing the bonding and reactivity of chemisorbed species. As concerns methanol synthesis at 9.5 atm, the results obtained failed to support the idea that methanol is produced in a direct reaction of CO2 and not through formation of CO and its consecutive hydrogenation.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry