R.M. Iyer

Effect of. gamma. -irradiation on methanation of carbon dioxide over supported Ru catalysts

In situ γ-irradiation has been found to enhance the activity of Ru/molecular sieve and Ru/alumina catalysts for the CO2 methanation reaction in the temperature range 400–600 K. The extent of radiation enhancement in catalytic activity was inversely related to temperature. The activation energy for the formation of CH4 from 2% CO2 in H2 was reduced in the presence of radiation from 13.8 to 7.7 kcal mole−1 for Ru/alumina and from 7.3 to 4.2 kcal mole−1 for Ru/molecular sieve. The results indicate that γ-irradiation gives rise to energy storage in support materials. It is suggested that the energy released on thermal stimulation weakens the metal-CO2 bonds resulting in an accelerated rate of reduction of CO2 to active carbon and its subsequent methanation.

Co adsorption-desorption properties of cation-exchanged NaX zeolite and supported ruthenium

The binding states of carbon monoxide over cation-exchanged NaX zeolites and over corresponding Ru-containing samples have been investigated using thermal desorption spectroscopy. Exchange of sodium with cations such as Li+, Ca2+, Mg2+, and La3+ gave rise to additional CO adsorption states, a higher isosteric heat of CO adsorption, an increased density of acid sites, and an increased amount of adsorbed CO, depending on the nature and ionic radius of the chargebalancing cation. The charge-balancing cations at the zeolite surface (e.g., Ca2+, La3+) function as additional CO adsorption sites in conjunction with surface acid centers (e.g., the A13+ center) and metal sites. In addition to surface sites, CO is also found to be held in structural cavities and macropores of the zeolite matrix. The programmed heating of both the metal-free and Ru-containing zeolites subsequent to room temperature CO adsorption gave desorption peaks due to release of CO at temperatures less than ~500 K while the higher temperature peaks were constituted mainly of CO2. Electron spectroscopy results have revealed that the exposure of these samples to CO and subsequent thermal treatment resulted in the formation of surface carbonaceous species. The nature of CO adsorption states giving rise to CO2 formation is discussed.

Effect of γ-irradiation on methanation of carbon monoxide over Ru/molecular sieve catalyst

Abstract The effect of in situ γ-irradiation on methanation of carbon monoxide has been studied at different temperature in the range 400–575 K using a molecular sieve supported Ru catalyst. At temperatures less than 500 K, irradiation has been found to result in a large increase in catalytic activity. The enhancement in methane yields was dependent on the temperature of the catalyst, CO/H2 ratio and γ-dose. Thermoluminescence studies have shown that on γ-irradiation, energy is stored in molecular sieves in the form of trapped centres which get released on thermal stimulation. Enhanced catalytic activity has been attributed to the transfer of energy from the molecular sieve support to ruthenium which leads to an accelerated rate of reaction of H2 with active carbon and CO2, which are produced in the disproportionation reaction of carbon monoxide on the catalyst surface.

Carbon monoxide adsorption/desorption processes over NaX zeolite and supported ruthenium catalyst

The binding states of CO on NaX zeolite and RuNaX were investigated by thermal desorption spectroscopy. Desorption peaks centred at around 390, 430, 490, and 520 K were observed from NaX following room-temperature adsorption of CO. The activation energy values corresponding to these peaks were calculated to be 41.4, 45.7, 53.8, and 57.7 kJ mol−1, respectively. These peaks were also observed in desorption profiles from RuNaX although their temperatures were higher by 10 to 20 K. In addition, the desorption spectra from RuNaX also comprised two high-temperature peaks at around 575 and 640 K. With both the RuNaX and the NaX samples, the temperature and relative intensities of the desorption peaks depended on pretreatment conditions and on the lapsed time between CO exposure to the sample and the commencement of programmed heating. Mass spectral analysis revealed that the gas desorbed at 300–500 K consisted mainly of CO while at higher temperatures CO2 was the main component. The desorption peaks below 500 K are attributed to the release of carbon monoxide from structural and intragranular or intergranular zeolitic pores. Lewis sites on the zeolite surface are found to facilitate activation of CO, resulting in its transformation to CO2. The initial adsorption of carbon monoxide in zeolite pores and subsequent diffusion to metal sites leading to its disproportionation/oxidation is found to play an important role in the CO adsorption/desorption process on RuNaX.

Carbon monoxide methanation over FeTi1+x intermetallics

Abstract FeTi1+x intermetallics (0 ≤ x ≤ 0.15), well-known hydrogen absorption materials, have been found to be active catalysts for CO methanation. The initial activity for H2 absorption, CO disproportionation, and CO hydrogenation increased significantly with increasing Ti content. However, the catalysts rapidly lost their activity because of carbon layer deposition at the surface. Mossbauer and X-ray diffraction studies indicate that no surface or bulk carbides are formed during CO H 2 reaction. Auger electron spectroscopy data have revealed that for all the titanium compositions, the surface is rich in iron, and conversion electron Mossbauer results showed that the surface becomes enriched with iron metal clusters during catalytic reaction. Thus, the catalytic activity is attributed to α-iron centers at the surface which are responsible for the formation of different carbonaceous precursor species. Excess Ti concentration results in the formation of secondary iron titanium suboxide phases which help in the generation of additional active sites during the activation process. The activity of surface carbon species and the reaction routes involved in CO methanation are discussed in detail.

Identity and reactions of metastable species formed in γ-radiolysis and photolysis of CH3Br, CH2Br2, CHBr3, CBr4 and CHBr2CHBr2 in 3-methylpentane glass at 77 K

Abstract Metastable intermediate species formed on γ-radiolysis of solutions of monobromo-, dibromo-, tribromo-, tetrabromo methane, and 1, 1, 2, 2 tetrabromoethane in 3-methylpentane glass at 77 K, were characterized from a study of their absorption spectra and the changes thereof induced by proton acceptors, electron and hole scavengers, matrix viscosity and optical bleaching. Metastable species formed on photolysis of the same systems at 77K were evaluated and compared with the radiolytic data. These studies have indicated that the 385 nm, 435 nm and 490 nm optical absorption bands, observed both on γ-radiolysis and photolysis of CH2Br2, CHBr3 and CBr4, respectively, and the 400 nm band, observed on γ-radiolysis of CH3Br, are all due to the formation of a charge transfer complex between the solute molecule and bromine atom. In the case of 1, 1, 2, 2 tetrabromoethane, the radiolytically produced 375 nm band has been attributed to Br2- while the photolytically produced 400 nm band has been identified as due to molecular bromine.

Nature and reactivity of carbonaceous species deposited by ethylene on supported ruthenium catalysts

Abstract The species formed in the interaction of C2H4 with supported Ru catalysts have been investigated in the temperature range 296–650 K using a sequential pulse injection method. The time- and temperature-dependent reactivity of these species with hydrogen has been evaluated. The relative yields of the hydrogen-deficient dicarbon (C2) and monocarbon (C1) species formed at the catalyst surface depended on the temperature and the support material used, viz., molecular sieves, alumina, or glass beads. It was found that while the H2 reacted with these species to give C2H6 and CH4, in its absence, the C2 species transformed with time to the C1 form. Also, both the C2 and C1 species converted on standing to a form that was no more reactive to H2, the rate of transformation depending on the catalyst temperature and the nature of support material. The kinetics of these transformations has been evaluated.

Sulphur poisoning of ruthenium/alumina methanation catalyst and hydrogenation of CS2

Abstract Different species formed in the interaction of CS 2 with Ru/γ-alumina catalyst in the temperature range 425–625 K were examined using a pulse injection method and a flow microcatalytic reactor. In the presence of H 2 carrier flow, CS 2 reacts over the catalyst to form primarily CH 4 and small amounts of higher hydrocarbons. Prolonged treatment with CS 2 produced new products CO and H 2 S from CS 2 and reduced drastically the catalytic methanation of CO. Under the inert atmosphere of He, CS 2 interaction over catalyst generated CO 2 in the initial stage and subsequent H 2 injections gave rise to CH 4 . Further CS 2 H 2 injections gave progressively reduced yields of CH 4 . At the stage when no methane was formed on H 2 injections following a CS 2 pulse, further injections of CS 2 pulses produced new products, CO, SO 2 , COS, and H 2 S and subsequent H 2 injections again resulted in H 2 S formation. The effect of a time gap between CS 2 and H 2 injections on the yields of CH 4 and H 2 S has been studied. XPS studies indicated that CS 2 decomposes on the catalyst surface to give elemental carbon and sulphur. It is suggested that S atoms progressively block those Ru sites where carbon or carbon oxides are methanated. It has also been found that the nascent sulphur formed on decomposition of CS 2 loses its chemical reactivity with increasing temperature and time.

Deuterium isotope exchange reaction between hydrogen and water over polyester-supported platinum catalysts

Deuterium transfer between hydrogen and water over polyester-supported platinum has been found to follow a two-step process, namely, an initial fast step followed by a slower one. The faster exchange step was more susceptible to water poisoning while the other step continued to be active for H/D exchange reaction during prolonged contact with liquid water. The results are in agreement with the existence of heterogeneous metal sites with varying degrees of catalytic activity and support the mechanism whereby the chemisorption of both the water and hydrogen molecules at metal sites leads to the deuterium exchange process.

Hydrogen adstates in substituted Fe-Ti intermetallics; A TDS study

Hydrogen adstates in FeTi intermetallics have been investigated using thermal desorption spectroscopy. The number of adstates and the amount of adsorbed hydrogen was found to increase considerably with increase in titanium content and on substituting iron with manganese while the substitution of iron with nickel had a negligible effect. The desorption peaks observed in the temperature range 300–750 K are attributed to hydrogen adsorbed over Fe/TiOx or Fe/MnOx type surface species while the higher temperature peaks are ascribed to H2 release from FeTi bulk.