The transformation of C in the form of CO, into hydrocarbons was investigated on supported Ru catalysts. Special attention was paid to the surface processes occurring during the removal of the 0 atoms of CO, by H, and on the identification of surface species formed during the reaction. Infrared spectroscopic measurements revealed that chemisorbed CO and formate ion are formed during the coadsorption of H,+CO, at 373 K and also during the methanation of CO, at higher temperatures. The CO formed produced a weak absorption band at lower frequencies (1990-2000 crn-l) than did the CO alone (2030-2040 cm-l). This shift was attributed to the effect of hydrogen adsorbed on the same Ru atoms and to that of surface C formed during the reaction. Evidence is presented to show that formate ion forms on the Ru but migrates rapidly onto the supports. It is considered as an inactive species in the methanation The hydrogenation of CO, on Ru/Al,O, occurred at a measurable rate above 443 K yielding almost exclusively CH,. The formation of surface carbon was detected during the reaction at a level ca. 1.5 orders of magnitude less than in the H, + CO reaction. The rate of CH, formation is described by the expression NCH, = 2 . 7 ~ lo* exp(-l6.l/RT)x PH, xO.47 Pco,.It is proposed that the synthesis of CH, from H,+CO, occurs via the formation of surface C and its subsequent hydrogenation. of co,.
The dissociation of CH4 and CO,, as well as the reaction between CH4 and CO,, has been investigated over supported Rh in a fixed-bed continuous-flow reactor. The decomposition of methane on rhodium occurred above 423 K, when transient evolution of hydrogen and ethane were observed. The deposition of different kinds of carbon species was established, which led to the termination of the decomposition. The reactivity of surface carbon towards hydrogen exhibited a great variance, and sensitively depended on the conditions of its formation. The dissociation of CO, was detected by infrared spectroscopy only above 523 K, and it was promoted by the presence of CH4. The reaction between CO, and CH4 proceeded rapidly above 673 K to give CO and H, with different ratios. No decay in the activity of Rh catalysts was experienced, and only very little, if any, carbon deposition was observed. The effects of different supports on all of the above processes have been examined.
The interaction of H,+CO, has been investigated on Rh dispersed on MgO, TiO,, SO, and Al,O, supports. The adsorption measurements revealed that, with the exception of Rh/SiO,, the presence of H, greatly enhances the uptake of CO, by Rh samples at 373 K. 1.r. spectroscopic measurements showed that adsorbed CO and formate ion are formed in the surface interaction of H,+CO,. On Rh/SiO, 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-') 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-'.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 CO,, 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.
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