The adsorption of CO2 and its surface interaction with H2 were investigated on Pd dispersed on SiO2, MgO, TiO2, and Al2O3 supports. No evidence was found for the dissociation of CO2 at 300 K on any Pd samples. The occurrence of this process, however, was observed at 423 K. The adsorption measurements revealed that, with the exception of Pd SiO2, the presence of H2 greatly enhances the uptake of CO2 by Pd samples. The extent of the enhanced adsorption increased with rising temperature up to 423 K, and also with increase in the H2 + CO2 pressure. Infrared spectroscopic measurements showed that adsorbed CO and formate ion are formed in the surface interaction of H2 + CO2. The apparent activation energy for the formation of formate on Pd Al2O3 was calculated to be 16.2 kJ/mol. On Pd SiO2 there was no enhanced adsorption, and only adsorbed CO was identified. No such phenomenon was observed in the absence of Pd i.e., on the support alone. The development of the absorption bands due to the adsorbed CO formed differed from that observed following CO adsorption. It appeared that in the presence of hydrogen, Pd carbonylhydride is formed and the production of adsorbed CO in the surface reaction requires an ensemble of Pd atoms. It is concluded that the formate ion formed in the surface reaction is located on the support: the hydrogen activated on the Pd spills over onto the support and reacts with hydrocarbonate to yield formate ion.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry