The interaction of H2 + CO2 has been investigated on Rh dispersed on MgO, TiO2, SiO2 and Al2O3 supports. The adsorption measurements revealed that, with the exception of Rh/SiO2, the presence of H2 greatly enhances the uptake of CO2 by Rh samples at 373 K. I.r. spectroscopic measurements showed that adsorbed CO and formate ion are formed in the surface interaction of H2 + CO2. On Rh/SiO2 there was no enhanced adsorption, and only adsorbed CO was identified by i.r. spectroscopy. No such phenomenon was observed in the absence of Rh, i.e. on the support alone. The spectra of the adsorbed CO formed differed from that observed during the adsorption of CO; the twin band was missing and the band due to linearly bonded CO was shifted to lower frequencies. The relation between the absorbance of the formate band (1600 cm-1) and the amount of surface formate on Rh/MgO was determined. The apparent activation energy for the formation of formate was calculated to be 22.9 kJ mol-1. In the interpretation of the results we conclude that the formate ion formed in the surface reaction is located on the support. Two possible routes of formation of formate ion are envisaged: (i) it is formed on Rh in the reaction between activated hydrogen and CO2, then migrates onto the support where it can stabilize and accumulate; (ii) the activated hydrogen migrates onto the support and reacts with hydrocarbonate to yield formate ion. The latter route is considered the more probable.
|Number of pages||13|
|Journal||Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases|
|Publication status||Published - Dec 1 1981|
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