F. Solymosi

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.

The bonding, structure and reactions of CO2 adsorbed on clean and promoted metal surfaces

Abstract The characteristics of the adsorption and reactions of CO 2 on Rh, Pd, Pt, Ni, Fe, Cu, Re, Al, Mg and Ag metals are discussed with particular emphasis on the activation of the CO 2 molecule. Strong spectroscopie evidence is presented for the formation of negatively charged CO 2 − , which - depending on the nature of the metal - may dissociate into CO and O, or transform into CO 3 + CO. The presence of surface adatoms dramatically influences the adsorption and reactivity of CO 2 . Alkali adatoms increase the binding energy of adsorbed CO 2 , promote the dissociation and/or the transformation of CO 2 into CO 3 + O. In the presence of preadsorbed oxygen the formation of carbonate of different structures predominates.

Hydrogenation of CO on supported Rh catalysts

The hydrogenation of CO on supported Rh was investigated in a flow technique. Special attention was focused on the identification of surface species formed during the reaction. The reaction occurred at measurable rate above 473 K. The product distribution sensitively depended on the support; while on RhSiO2 mainly CH4 was formed, on RhTiO2 a number of C2C5 compounds were also produced. In situ infrared spectroscopic measurements showed that only the linearly bonded CO exists with a detectable concentration on the surface during the reaction. It appeared, however, at lower frequencies than that corresponding to the RhCO species. In addition, the absorption bands characteristic for the formate ion and CHx compounds were also identified. The formation of surface C was also detected. Its amount increased during the conditioning period and also with temperature elevation. The specific rate of CH4 formation on RhTiO2 was more than one order of magnitude higher than that of the less effective RhMgO and RhSiO2 catalysts. From the behaviors of surface formate under different conditions it was inferred that it does not play an important role in hydrocarbon synthesis on Rh catalysts. It is proposed that the important steps in CH4 formation are the dissociation of CO promoted by adsorbed hydrogen and the subsequent hydrogenation of surface carbon. As regards the high activity of RhTiO2 it is assumed that an electronic interaction operates between the TiO2 and Rh influencing the bonding and reactivity of chemisorbed species.

Dehydrogenation of methane on supported molybdenum oxides. Formation of benzene from methane

The dehydrogenation of methane on MoO3 supported on various oxides has been investigated under non-oxidizing conditions in a fixed bed, continuous flow reactor. Detailed measurements were performed with MoC3/SiO2. The reaction of methane was observed above 923 K after a significant time lag, when a partial reduction of Mo6+ occurred, the reduced phase being characterized by X-ray photoelectron spectroscopy (XPS). The initial gaseous products are CO2, H2O, H2 and CO. But this stage is followed by the dehydrogenation of methane and coupling of hydrocarbon fragments to various hydrocarbons. A possible pathway of the formation of benzene, the main product of reaction with selectivities ranging from 26 to 56%, is suggested.

Conversion of methane to benzene over Mo2C and Mo2C/ZSM-5 catalysts

The activation and dehydrogenation of CH2 on Mo2C and MO2C/ZSM-5 have been investigated under non-oxidizing conditions. Unsupported Mo2C exhibited very little activity towards methane decomposition at 973 K. The main reaction pathway was the decomposition of methane to give hydrogen and carbon with a trace amount of ethane. Mixing Mo2C with ZSM-5 support somewhat enhanced its catalytic activity, but did not change the products of the reaction. A dramatic change in the product formation occurred on partially oxidized Mo2C/ZSM-5 catalyst; besides some hydrocarbons benzene was produced with a selectivity of 70–80% at a conversion of 5–7%. Carburization of highly dispersed MoO3 on ZSM-5 also led to a very active catalyst: the conversion of methane at the steady state was 5–6% and the selectivity of benzene formation was 85%.

Catalytic reaction of methane with carbon dioxide over supported palladium

The reforming of methane with carbon dioxide has been investigated at 673–773 K on supported palladium catalysts in a fixed-bed continuous-flow reactor. In addition, the dissociation of carbon dioxide and methane, and the reactivity of the surface carbon formed have also been examined. The dissociation of carbon dioxide, detected by infrared spectroscopy, occurred at the lowest temperature, 373 K, on Pd/TiO2. It was greatly promoted by the presence of methane. The decomposition of methane at the temperature of the CH4 + CO2 reaction (ca. 773 K) proceeded initially at a high rate yielding hydrogen and small amounts of ethane and ethene. The deposition of surface carbon was also observed, which was hydrogenated only above 720 K. The reaction between carbon dioxide and methane occurred rapidly above 673 K to give carbon monoxide and hydrogen with a ratio of 1.3–1.7. Very little carbon was deposited during the reaction of a stoichiometric gas mixture. Kinetic parameters of the reaction were determined and a possible reaction mechanism is proposed. kw|carbon dioxide hydrogenation; kinetics; methane reforming; palladium

Catalytic hydrogenation of CO2 over supported palladium

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, PdTiO2 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 PdTiO2, 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.

Importance of the Electric Properties of Supports in the Carrier Effect

Abstract It has long been recognized in heterogeneous catalysis that the efficiency of the catalyst can be markedly increased when supported by certain solids of large surface. Although this observation has been applied very fruitfully in the preparation of catalysts with technical importance, the reason for the carrier effect is still not fully understood. In earlier investigations the effect of support was explained mainly by saying that it stabilizes the state of the active component or that it increases the degree of dispersion and the surface area of the catalyst. There were, however, some observations stating that, besides the above-mentioned factors, there is a chemical interaction between the catalyst and the support which may also play an important role in producing the carrier effect. Therefore Adadurov and co-workers [1] have pointed out that, depending on its atomic radius and valence, the carrier polarizes the molecules of the catalyst, thus considerably altering the properties of the latter....

Interactions of methyl halides (Cl, Br and I) with Ag(111)

The adsorption of methyl halides (Cl, Br and I) on a Ag(111) surface has been investigated by temperature programmed desorption (TPD), work function change (ΔΦ), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). All three compounds adsorb with high sticking probability at 100 K, even up to multilayer coverages. The absolute coverage at one monolayer of CH3X is independent of X and is (4.6 ± 0.3) × 1014 molecules/cm2. UPS and XPS spectra suggest that the adsorption of methyl halides is molecular at 100 K and with little distortion of the corresponding gas-phase molecular electronic structures. Submonolayer adsorption is accompanied by a significant work function decrease (ΔΦ between −0.83 and −1.25 eV) indicating a dipole with the positive end pointed away from the surface. At high exposures, multilayers form and desorb at 113 (Cl), 121 (Br), and 136 K (I). Whereas chemisorbed monolayer coverages of CH3C1 and CH3Br desorb, with Tp = 126 and 142 K, without detectable decomposition, a significant fraction (≅ 35%) of monolayer CH3I dissociates between 130 and 190 K to give adsorbed CH3 and I. The dissociation of CH3I is accompanied by a decrease in the binding energy of the I(3d52) core electrons. The adsorbed CH3 does not dehydrogenate on Ag(111), but recombines above 190 K to yield C2H6 which immediately desorbs.