A. Erdöhelyi

Methanation of CO2 on supported rhodium catalyst

Abstract The synthesis of methane from CO 2 and H 2 was investigated on supported Rh catalysts. The adsorbed species present on the catalysts were characterized by infrared spectroscopy. The hydrogenation of CO 2 on Rh Al 2 O 3 occurred at a measurable rate above 443 K, yielding exclusively methane. The rate of methane formation on Rh Al 2 O 3 is described by the expression N CH 4 = 2.69 × 10 6 exp ( −16.200 RT )P H 2 0.61 · P CO 2 0.26 . From a comparison of the specific activities of Rh Al 2 O 3 in the H 2 + CO 2 and H 2 + CO reactions it appears that the hydrogenation of CO 2 occurs much faster than that of CO. The support exerted a marked influence on the specific activity of Rh. The most effective support was TiO 2 and the least effective one SiO 2 . Infrared spectroscopic measurements revealed that linearly bonded CO (perturbed by hydrogen adsorbed on the same Rh atom) and adsorbed formate species were present on the catalyst surface during the reaction. Evidence is presented to show that the surface formate is located not on the Rh, but rather on the support. The formation of surface C was also detected. Its amount slightly increased during the conditioning period and also with temperature elevation. It is proposed that the important steps in CH 4 formation are the dissociation of CO 2 promoted by adsorbed hydrogen, the subsequent dissociation of CO into reactive surface carbon and hydrogenation of the latter. The possible reasons for the different methanation rates of CO 2 and CO are discussed.

Catalytic reaction of CH4 with CO2 over alumina-supported Pt metals

The dissociation of CH4 and CO2, as well as the reaction between CH4 and CO2 at 723–823 K have been studied over alumina supported Pt metals. In the high temperature interaction of CH4 with catalyst surface small amounts of C2H6 were detected. In the reaction of CH4+CO2, CO and H2 were produced with different ratios. The specific activities of the catalysts decreased in the order: Ru, Pd, Rh, Pt and Ir, which agreed with their activity order towards the dissociation of CO2.

Surface interaction between H2 and CO2 on RhAl2O3, studied by adsorption and infrared spectroscopic measurements

The low-temperature interaction between H2 + CO2 on RhAl2O3 was investigated by adsorption and infrared spectroscopic measurements. Adsorption studies indicated that the presence of H2 greatly enhances the uptake of CO2 by RhAl2O3 catalyst. Infrared spectroscopic studies revealed that adsorbed formate and CO are formed in the low-temperature interaction between H2 and CO2. On comparison of the spectrum of adsorbed CO with that obtained after coadsorption of the H2 + CO2 mixture, it appeared that (i) the doublet due to twin CO (RhCO2) was completely missing, (ii) the linearly bonded CO appeared at lower frequency, 2020–2039 cm−1, and (iii) no significant change was observed in the position of the bridged CO. The apparent activation energy of the formation of formate ion was calculated to be 4.2 kcal/mol, which is markedly lower than that determined for pure Al2O3, indicating the great promoting effect of Rh. Possible mechanisms of the H2 + CO2 interaction are discussed.

A comparative study on the activation and reactions of CH4 on supported metals

The catalytic effects of silica-supported Pt metals were tested and compared in the decomposition of methane and its conversion into higher hydrocarbons. The dissociation of methane is readily measurable at 473–673 K. The rate of initial decomposition at 523 K was the highest on Rh, but it dropped to a low value within a short contact time. The gaseous products were hydrogen and ethane in small, variable amounts. Hydrogen of the carbonaceous species formed in the decomposition led to the production of aliphatic hydrocarbons.

Methanation of CO2 on supported Ru catalysts

The transformation of C in the form of CO2 into hydrocarbons was investigated on supported Ru catalysts. Special attention was paid to the surface processes occurring during the removal of the O atoms of CO2 by H2 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 H2+ CO2 at 373 K and also during the methanation of CO2 at higher temperatures. The CO formed produced a weak absorption band at lower frequencies (1990–2000 cm–1) than did the CO alone (2030–2040 cm–1). 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 of CO2.The hydrogenation of CO2 on Ru/Al2O3 occurred at a measurable rate above 443 K yielding almost exclusively CH4. The formation of surface carbon was detected during the reaction at a level ca. 1.5 orders of magnitude less than in the H2+ CO reaction. The rate of CH4 formation is described by the expression NCH4= 2.7 × 106 exp (–16.1/RT)×PH2× 0.47 PCO2. It is proposed that the synthesis of CH4 from H2+ CO2 occurs via the formation of surface C and its subsequent hydrogenation.

Partial oxidation of ethane over silica-supported alkali metal molybdate catalysts

Abstract The partial oxidation of ethane has been investigated on silica-supported M 2 MoO 4 catalysts ( M  Li, Na, K, Rb, Cs) in a fixed-bed continuous-flow reactor at 770–823 K using N 2 O as oxidant. Additional measurements included pulse experiments, temperature-programmed reduction of the catalysts, and a study of the catalytic decomposition of N 2 O and C 2 H 5 OH. The numbers of acidic and basic sites have also been determined. Temperature-programmed reduction of the alkali metal molybdates showed that the onset temperature of the reduction decreased from Li to Cs, while the extent of the reduction increased in this sequence. The main products of the oxidation reaction were ethylene, acetaldehyde, CO, and CO 2 . Small amounts of CH 4 and C 2 H 5 OH were also identified. The ethane conversion and the rate of the C 2 H 4 and CH 3 CHO formation all increased from Li to Cs. Detailed kinetic measurements were carried out on K 2 MoO 4 /SiO 2 . The activation energy of the ethane consumption was 71 kJ/mol. A possible mechanism for the oxidation reaction is discussed.

Catalysis for alternative energy generation

General overview.- Hydrogen energy by environmetally friendly fuels.- Hydrogen production.- Utilization of biomass.- Utilization of biogas and methane dry reforming.- Ethanol reforming .- Methanol reforming.- Biodiesel.- Catalysis beyond biodiesel including fine chemistry.- Low Temperature methane combustion.- Reaction in Membranes and Catalysts.- PEMFC.- DMFC.- Photocatalysis and solar cells.- Direct utilization of solar energy.- Concluding remarks and future perspectives.

Infrared study of the reaction of adsorbed formate ion with H2 on supported Rh catalysts

The reaction of H/sub 2/ with formate ion adsorbed on the catalyst or support was studied using Ru on Al/sub 2/O/sub 3/ and MgO. Infrared spectroscopy was used to monitor the catalyst surface. From the results, it was concluded that the formate ion formed in the hydrogenation of both CO and CO/sub 2/ is not an inactive as formerly had been thought. Even though the formate ions are located on the support rather than on the Ru, the ions were found to react with H/sub 2/ to yield CH/sub 4/. However, it was noted that this production of CH/sub 4/ did not take place unless the active metal catalyst was present to produce activated hydrogen. The question of significance of this reaction of formate ions with H/sub 2/ in the overall yield of CH/sub 4/ during the hydrogenation of CO and CO/sub 2/ remains unanswered. (BLM)

Infrared study of the surface interaction between H2 and CO2 over rhodium on various supports

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.

CO2 Hydrogenation on Rh/TiO2 Previously Reduced at Different Temperatures

Hydrogenation of CO2 was studied on 1% Rh/TiO2 reduced at different temperatures. The interaction of CO2 with the catalyst and that of the CO2+H2 mixture was also studied. FTIR and TPD measurements revealed that CO2 dissociation depends on the reduction temperature of the catalyst. In the surface reaction, besides Rh carbonyl hydride, formate groups and different carbonates and surface formyl species were also formed. The surface concentration of the formyl group depended on the reduction temperature. The initial rate of CO2 hydrogenation significantly increased with increasing reduction temperature but after some time it drastically decreased. The promotion effect of the reduction temperature was explained by the formation of oxygen vacancies on the perimeter of the Rh/TiO2 interface, which can be re-oxidized by the adsorption of CO2 and H2O.