Rate Of Methane Formation Research Articles

Promotion of CO and CO2 Hydrogenation over Rh by Metal Oxides: The Influence of Oxide Lewis Acidity and Reducibility

An investigation has been carried out of CO and CO2 hydrogenation to methane over a Rh foil decorated with submonolayer quantities of AlOx, TiOx, VOx, FeOx, ZrOx, NbOx, TaOx, and WOx. The rate of methane formation was measured at 1 atm and the state of the working catalyst was characterized by XPS immediately after reaction. With the exception of AlOx, each of the oxides was found to enhance the rate of CO methanation relative to that observed over unpromoted Rh. The maximum degree of rate enhancement occurs at an oxide coverage of approximately half a monolayer. AlOx retards the formation of methane in direct proportion to the oxide coverage. FeOx behaves in a manner identical to that of AlOx in the case of CO2 hydrogenation, whereas all of the other oxides studied produce a maximum degree of methanation rate enhancement at an oxide coverage of half a monolayer. The enhancement of CO and CO2 hydrogenation is attributed to the formation of Lewis acid-base complexes between the oxygen end of adsorbed CO (or H2CO formed during the reaction) and anionic vacancies present at the edge of the oxide-metal boundary. The degree of enhancement of CO and CO2 hydrogenation is found to increase with the average oxidation state of the metal in the oxide overlayer. This trend is attributed to the direct relationship between Lewis acidity of an oxide and the oxidation state of the metal in the oxide. The degree of reduction of a given oxide is found to be higher during CO2 hydrogenation than CO hydrogenation.

A Transient Kinetic Study of the Co/H2 Reaction on Rh/Al2O3 Using FTIR and Mass Spectroscopy

The evolution of surface species (their chemical composition and surface coverage) formed during the CO/H2 reaction at 220°C on 1 wt% Rh/Al2O3 catalyst was studied by transient isotopic and various hydrogen titration techniques using both in situ mass spectrometry and FTIR. There is a very small amount of active carbon, CHx (θCHx < 0.03), and a large amount of surface linear and bridged CO species (θCO = 0.93) which participate in the formation of CH4 during reaction. Their amounts stay practically constant with reaction time in CO/H2 even after 1 h on stream. Also present on the Rh surface are some CxHy (alkyl chains) species which largely grow with reaction time (during the first hour on stream) but do not participate in the reaction mechanism of CH4 formation (spectator species). In addition, formate and carbonate (spectator species) build slowly on the alumina support surface even after 1 h of reaction. The surface coverage of hydrogen, θH, is found to be very small, a result consistent with the coverages of CO, CHx, and CxHy species. CO dissociation over the present 1 wt% Rh/Al2O3 (1.5-nm Rh particles) appears to largely control the overall rate of methane formation, a result similar to that previously reported over the 5 wt% Rh Al2O3 (9.0-nm Rh particles) catalyst.

Production and transport of methane in oceanic particulate organic matter

METHANE is an important component of the global carbon cycle1 and a potent greenhouse gas2,3. Surface ocean waters are typically supersaturated with dissolved methane relative to atmospheric equilibrium, presumably as a result of in situ microbial methane production4–8. Because methanogenic bacteria are strict anaerobes9and surface ocean waters are highly oxygenated, the observation of methane supersaturation has been termed the 'oceanic methane paradox'10. Although methanogenic bacteria have been isolated from oceanic particulate matter, faecal pellets and zooplankton11–14, no data are available on in situ rates of methane formation in these microenvironments. During a series of experiments in the North Pacific ocean, we have identified a previously unrecognized component of the oceanic methane cycle. We find that methane is associated with sinking particles, presumably as a dissolved constituent of the interstitial fluids of particulate biogenic materials, which exchanges with the water column as particles sink. This phenomenon provides a mechanism for the active transport in the water column of an otherwise passive, dissolved species. The particle-to-seawater methane flux that we measure is sufficient to replace all of the methane present in the upper water column in about 50 days and to produce the characteristic methane supersaturations in less than a month. We suggest that particulate production and transport may also be relevant to the redistribution and cycling of other bioreactive compounds in the marine environment.

Continuous culture of Methanococcus jannaschii, an extremely thermophilic methanogen

Methanococcus jannaschii, an extremely thermophilic methanogen isolated from a deep-sea hydrothermal vent, was grown at 80 degrees C in continuous culture on a mineral salts medium gassed with H(2) and CO(2) at three different flow rates. The maximum specific growth rate was 0.56 h(-1), and the maximum specific methane productivity was 0.32 (mol g(-1) h(-1)). Uncoupling of growth and methane production was evidenced by an increase in teh non-growth-associated rate of methane formation, beta, with increasing gaseous input. The specific hydrogenase activity exhibited growth-assiciated behaviour at low growth rates, but showed no dependence on growth at higher growth rates. The growth dependence of hydrogenase activity is consistent with the pressure dependence of hydrogenase activity measured in previous experiments. In contrast, the specific protease activity was independent of the growth rate over the entire range of dilution rates studied. (c) 1994 John Wiley & Sons, Inc.

An integrated modeling of stoichiometry and biokinetics of anaerobic processes

The objective of this paper was to present an integrated modeling approach to biokinetics and stoichiometry of anaerobic treatment processes. A software system has been developed to calculate bacterial yield coefficients from stoichiometry under different anaerobic bioprocess conditions. Kinetic constants, i.e., the reaction rate constant and Monod half-velocity coefficient, were determined from batch experimental data using Conreg, a software system for constrained nonlinear regression. Additional experiments were performed to evaluate the usefulness of the Inhibition Coefficient model to predict toxicity of ammonia and methylene chloride in anaerobic batch systems. The integrated approach to biokinetics and stoichiometry and the software developed for this purpose should help engineers quickly evaluate a wide variety of design options for anaerobic treatment systems under both toxic and non-toxic conditions. Other useful applications include prediction of natural rates of methane (a gas causing global warming) formation in rice paddies.

Gas Phase Synthesis of Molybdenum and/or Iron Nitrides, Carbides and Sulfides

ABSTRACTThe thermolytic decomposition of Mo(CO)6 with hydrogen sulfide or ammonia vapor (in a He carrier stream) at temperatures ranging from 300 to 1100 °C produces high surface area molybdenum sulfides (MoS2 or Mo2S3) or molybdenum carbides (hexagonal Mo2C) and carbonitrides, (hexagonal MoN(C) or cubic Mo2N(C)), respectively. The MoS2 surface areas range from 16.7 to 82.0 m2/g, while the surface areas of molybdenum carbides and carbonitrides vary from 14.9 to 21.1 m2/g. The maximum surface area for MoS2 is achieved at 500 °C and decreases with increasing or decreasing temperature. The surface area of the carbonitrides formed from 300 to 800 °C increases with increasing temperature up to 950 °C, where lower surface area Mo2C is formed. Crystallographically pure hexagonal MoN is prepared by decomposing Mo(CO) 6 in pure ammonia. Fe(CO) 5 decompositions in ammonia produce FexZ (where 5.8≥x≥1.6 and Z=C and N), and in some cases elemental Fe. Hexagonal Fe3 N(C) forms when Fe(CO) 5 is thermolyzed in ammonia from 300 to 600 °C, with surface areas ranging from 9.5 to 13.7 m2/g, whereas orthorhombic Fe3C and cubic Fe are produced at 700, 800, 900 and 1000 °C with surface areas of 6.7, 7.6, 2.2 and 2.0 m2/g, respectively. Within the same phase, the surface areas of the carbonitrides increase with increasing reaction temperature. These iron and molybdenum carbonitrides catalyze the conversion of CO/H2 to alkanes and methanol. Based on preliminary catalytic studies, the highest rate of methane (2850 g/kg/hr at 374 °C) and methanol (440 g/kg/hr at 284 °C) formation was accomplished with an FeMo carbonitride prepared by decomposing Mo(CO)6 and Fe(CO)5 in ammonia at 800 °C.

Isolation and Characterization of a Novel Thermophilic Methanosaeta Strain

A novel thermophilic acetotrophic Methanosaeta strain was isolated from a thermophilic anaerobic digest or by using acetate enrichment and serial dilution in the presence of vancomycin and neomycin. This isolate, designated Methanosaeta sp. strain PT, resembled Methanosaeta sp. strain CALS-1 morphologically; however, it occasionally formed filaments longer than 100 μm and exhibited autofluorescence. The content of coenzyme F420 was much higher than that of Methanosaeta reference strains, and coenzyme F420 with four glutamyl residues on the side chain was the predominant component. Furthermore, a comparative analysis of the antigenic fingerprint of strain PT with the fingerprints of reference organisms showed that this isolate was not related antigenically to the reference methanogens, including Methanosaeta sp. (“Methanothrix” sp.) strain CALS-1 and Methanosaeta concilii (“Methanothrix soehngenii”) Opfikon. Strain PT formed visible colonies in a deep agar medium when high cell concentrations were present. However, transfer of a colony into liquid medium resulted in no growth. Strain PT could utilize only acetate as a sole carbon and energy source. The optimum temperature and optimum pH for methanogenesis were near 55°C and 6.7, respectively. The specific methane formation rate μCH 4 under optimum conditions was 0.47 day−1, and the doubling time was 1.49 days. The DNA base composition was 52.7 mol% guanine plus cytosine.

Activities of formylmethanofuran dehydrogenase, methylenetetrahydromethanopterin dehydrogenase, methylenetetrahydromethanopterin reductase, and heterodisulfide reductase in methanogenic bacteria

The activities of formylmethanofuran dehydrogenase, methylenetetrahydromethanopterin dehydrogenase, methylenetetrahydromethanopterin reductase, and heterodisulfide reductase were tested in cell extracts of 10 different methanogenic bacteria grown on H2/CO2 or on other methanogenic substrates. The four activities were found in all the organisms investigated: Methanobacterium thermoautotrophicum,M. wolfei, Methanobrevibacter arboriphilus, Methanosphaera stadtmanae, Methanosarcina barkeri (strains Fusaro and MS), Methanothrix soehngenii, Methanospirillum hungatei, Methanogenium organophilum, and Methanococcus voltae. Cell extracts of H2/CO2 grown M. barkeri and of methanol grown M. barkeri showed the same specific activities suggesting that the four enzymes are of equal importance in CO2 reduction to methane and in methanol disproportionation to CO2 and CH4. In contrast, cell extracts of acetate grown M. barkeri exhibited much lower activities of formylmethanofuran dehydrogenase and methylenetetrahydromethanopterin dehydrogenase suggesting that these two enzymes are not involved in methanogenesis from acetate. In M. stadtmanae, which grows on H2 and methanol, only heterodisulfide reductase was detected in activities sufficient to account for the in vivo methane formation rate. This finding is consistent with the view that the three other oxidoreductases are not required for methanol reduction to methane with H2.

Hydrogen gasification of HNO 3-oxidized carbons

The oxidation of carbon prior to hydrogen gasification as a means of enhancing methane formation rate is investigated. Carbon is oxidized by HNO 3 and gasified in hydrogen in a high-pressure differential reactor. Samples are analyzed by x-ray photoelectron spectroscopy (XPS) for surface oxygen content before and after pretreatment and reaction. Results for uncatalyzed gasification show a correlation between initial surface oxygen content and hydrogasification rate, but XPS results reveal that essentially no oxygen is present on the carbon surface during hydrogen gasification. This indicates that desorption of oxygen groups from the carbon surface generates reactive sites at which hydrogen gasification occurs. These nascent sites arise from acidic oxygen groups both fixed during oxidation and from oxygen in bulk carbon. In potassium carbonate-catalyzed hydrogen gasification, oxidation enhances the catalyzed rate as much as threefold. The catalyst interacts with basic oxygen groups on carbon to form reactive sites which are formed and regenerated continuously during gasification. Analysis by XPS shows that substantial oxygen and potassium are present on the carbon surface during hydrogen gasification; at high catalyst loadings and 725°C the Cls peak shows both carbonate groups and singly bound oxygen-carbon groups tentatively assigned as M—O—C.

Enhanced cytochrome formation and stimulate methanogenesis rate by the increased ferrous concentrations in Methanosarcina barkeri culture

The concentration of Fe 2+ in the methanol minimum medium was found to be a stimulating factor on the formation of cytochromes, the excretion of hemes and the rate of methane formation by Methanosarcino barkeri cells. Overproduced hemes due to the high Fe 2+ concentration were released into the medium as free forms. When Fe 2+ concentration was employed more than 30 μM for the medium, the contents of cytochrome b and c in the cells seemed to be saturated at 40 nmol/g cell. Under the same condition, not only membrane-bound but also soluble cytochromes were detected in the ultracentrifuged fractions from the cells grown on 35 μM of Fe 2+

Enhanced cytochrome formation and stimulate methanogenesis rate by the increased ferrous concentrations inMethanosarcina barkericulture

The concentration of Fe2+ in the methanol minimum medium was found to be a stimulating factor on the formation of cytochromes, the excretion of hemes and the rate of methane formation by Methanosarcino barkeri cells. Overproduced hemes due to the high Fe2+ concentration were released into the medium as free forms. When Fe2+ concentration was employed more than 30 μM for the medium, the contents of cytochrome b and c in the cells seemed to be saturated at 40 nmol/g cell. Under the same condition, not only membrane-bound but also soluble cytochromes were detected in the ultracentrifuged fractions from the cells grown on 35 μM of Fe2+

Some strategies for the kinetic study of complex heterogeneous catalytic reactions

AbstractThis paper presents a method of transforming the non‐liner regression problem in the kinetic study of complex heterogeneous catalytic reactions into a linear regression problem. Application of this method reduced the number of parameters to be estimated by n‐1, where n is the number of independent reactions. In addition, a stepwise model discrimination strategy in introduced to reduce the number of equation sets ad equations in the set undergoing parameter estimation. These two new approaches are very advantageous in reducing the computation effort, especially when the number of independent reactions is large. The linear regression method and the stepwise model discrimination strategy are successfully applied in the kinetic study of the methanol synthesis system in which the formation rates of methanol, methane, ethanol and ethane are considered.

ChemInform Abstract: The Mechanism and Kinetics of Methane Formation by Decomposition of Methanol on a Ni/SiO2 Catalyst

The rate of methane formation by decomposition of methanol is studied at temperatures between 473 and 583 K and methanol pressures between 0.2 and 10.4 kPa. Coverages of adsorbed species are measured using temperature-programmed hydrogenation (TPH), temperature-programmed desorption (TPD), and magnetic measurements. The reaction system can be adequately described assuming the decomposition of methanol to proceed in two steps, viz. a decomposition into CO and H2 followed by methanation of CO. The former reaction is much more rapid than the latter. Assuming CH hydrogenation to be rate-determining, the rate of methane formation could be described by the LHHW rate equation TOF = 1.2 × 105e-−68RTp1.5(1 + 1.8 × 10−1e15RTp0.5 + 3.1 × 10−16 e144RTp)2 TOF is the turnover frequency in s−1, p is the methanol pressure in kPa, T is the temperature in K, and R = 8.314 × 10−3kJ mol−1 K−1. Coverages of adsorbed species predicted by this equation differ strongly from results obtained by measurements of total surface coverages using TPH, TPD, and magnetic measurements. This has been interpreted as an indication that the number of surface sites actively participating in the methanation reaction composes only a small fraction of the total number of sites.

The mechanism and kinetics of methane formation by decomposition of methanol on a [formula omitted] catalyst

The rate of methane formation by decomposition of methanol is studied at temperatures between 473 and 583 K and methanol pressures between 0.2 and 10.4 kPa. Coverages of adsorbed species are measured using temperature-programmed hydrogenation (TPH), temperature-programmed desorption (TPD), and magnetic measurements. The reaction system can be adequately described assuming the decomposition of methanol to proceed in two steps, viz. a decomposition into CO and H 2 followed by methanation of CO. The former reaction is much more rapid than the latter. Assuming CH hydrogenation to be rate-determining, the rate of methane formation could be described by the LHHW rate equation TOF = 1.2 × 10 5e- −68 RT p 1.5 (1 + 1.8 × 10 −1e 15 RT p 0.5 + 3.1 × 10 −16 e 144 RT p) 2 TOF is the turnover frequency in s −1, p is the methanol pressure in kPa, T is the temperature in K, and R = 8.314 × 10 −3kJ mol −1 K −1. Coverages of adsorbed species predicted by this equation differ strongly from results obtained by measurements of total surface coverages using TPH, TPD, and magnetic measurements. This has been interpreted as an indication that the number of surface sites actively participating in the methanation reaction composes only a small fraction of the total number of sites.

In situ FTIR and surface analysis of the reaction of trimethylgallium and ammonia

Using a combination of in situ FTIR spectroscopy and detailed surface analysis, we find that TMGa decomposes at the same rate in either hydrogen or nitrogen forT < 300° C. Although ammonia does not decompose under these conditions, mixing TMGa with ammonia increases the rate of methane formation. Reacting perdeutroammonia with TMGa shows that hydrogen from the ammonia is incorporated into the product methane (whereas deuterium in the gas phase is not incorporated into the gaseous product). TMGa and ND3 do react; however, nitrogen incorporation in the growing film is temperature dependent. Further, although the decomposition of TMGa occurs in the gas phase, the last steps of the decomposition/reaction occur on the substrate surface.

Evidence for multiple carbon monoxide hydrogenation pathways on platinum alumina

The reactivity of CO toward hydrogen over 12% Pt/Al{sub 2}O{sub 3} is examined by temperature-programmed reduction of adsorbed CO (CO TPR) and infrared spectroscopy. The CO TPR spectra show two methane formation rate maxima near 490 and 613 K. The relative intensities of these peaks depend on sample pretreatment temperature with the 490 K peak dominating with higher reduction temperatures. Infrared spectra show the CO chemisorbs molecularly on Pt at 300 K, but some bound CO is converted to alumina-bound methoxy species as the sample is heated to 400 K in H{sub 2}. IR spectra recorded during CO TPR show the 490 K peak reflects hydrogenation of the surface methoxy to methane. The second CO TPR peak is due to methanation of the remaining CO on Pt.

Methanogenesis and ATP synthesis in methanogenic bacteria at low electrochemical proton potentials. An explanation for the apparent uncoupler insensitivity of ATP synthesis.

The rate of methane formation from H2 and CO2, the intracellular ATP content and the electrochemical proton potential (delta mu H+) were determined in cell suspensions of Methanobacterium thermoautotrophicum, which were permeabilized for K+ with valinomycin (1.2 mumol/mg protein). In the absence of extracellular K+ the cells formed methane at a rate of 4 mumol min-1 (mg protein)-1, the intracellular ATP content was 20 nmol/mg protein and the delta mu H+ was 200 mV (inside negative). When K+ was added to the suspensions the measured delta mu H+ decreased to the value calculated from the [K+]in/[K+]out ratio. Using this method of delta mu H+ adjustment, it was found that lowering delta mu H+ from 200 mV ([K+]in/[K+]out = 1000) to 100 mV ([K+]in/[K+]out = 40) had no effect on the rate of methane formation and on the intracellular ATP content. At delta mu H+ values below 100 mV ([K+]in/[K+]out less than 40) both the rate of methanogenesis and the ATP content decreased. Methanogenesis completely ceased and the ATP content was 2 nmol/mg when delta mu H+ was adjusted to values lower 50 mV ([K+]in/[K+]out less than 7). The data show that methanogenesis from H2 and CO2 and ATP synthesis in M. thermoautotrophicum are possible at relatively low electrochemical proton potentials. Similar results were obtained with Methanosarcina barkeri. Protonophoric uncouplers like 3,5,3',4'-tetrachlorosalicylanilide (TCS) or 3,5-di-tert-butyl-4-hydroxy-benzylidenemalononitrile (SF 6847) were found not to dissipate delta mu H+ below 100 mV in M. thermoautotrophicum even when used at high concentrations (400 nmol/mg protein). This finding explains the observed uncoupler insensitivity of methanogenesis and ATP synthesis in this organism.

Reduction of CO 2 and CO to methane on Cu foil electrodes

The electrochemical reduction of CO 2 has been studied on Cu foil electrodes in 0.5 M KHCO 3, pH 7.6. The highest methane formation rates at 22 and 0°C were 8×10 −5 (17 mA cm −2) and 1.1×10 −4 (23 mA cm −2) mol cm −2 h −1 at −2.0 and −2.3 V vs. SCE respectively. The onset potentials for both methane and ethylene were −1.5 to −1.6 V vs. SCE. It was demonstrated that CH 4 is formed from CO at a 50 times lower rate than from CO 2 with an onset potential of −1.5 to −1.6 V vs. SCE. The methane formation rate is higher if the surface is prepared by cleaning with HCl rather than HNO 3 or oxidation in air. Tafel slopes for the methane partial current were 174 and 110 mV/ decade for the HNO 3 and HCl pretreatment respectively. Electrochemically assisted dissociation of adsorbed CO by electron transfer is suggested as the rate determining step.

Methane formation from acetyl phosphate in cell extracts of Methanosarcina barkeri Dependence of the reaction on coenzyme A

Cell extracts (100 000 × g supernatant) of acetate grown Methanosarcina barkeri (strain MS) catalyzed the formation of methane from acetyl phosphate (apparent K m ∼ 2 mM) with specific activities up to 50 nmol·min −1·mg protein −1, which is 25% of the specific rate of methane formation from acetate by intact cells. Methane formation from acetyl phosphate was strictly dependent on coenzyme A (CoA) (apparent K m ∼ 0.1 mM), which was demonstrated with extracts pretreated with charcoal to remove endogenous CoA. The extracts contained high specific activities of acetate kinase (16 μmol·min −1·mg protein −1) and of phosphotransacetylase (26 μmol·min −1·mg protein −1). The findings indicate that methanogenesis from acetate proceeds via acetyl phosphate and acetyl-CoA as intermediates. Almost identical results were obtained with cell extracts of acetate grown M. barkeri (strain Fusaro).

Model discrimination and parameter estimation in the kinetic study of methanol synthesis

AbstractThe new model discrimination method is based on the correlation coefficient test of experimental data sets for different reactions, without requiring a prior parameter estimation. The new parameter estimation method reduces by n the number of dimensions to be tested, compared to the classical method where n is the number of independent reactions in the system. This has reduced the computation time for most complex reactions to a level, comparable to that needed for single reactions. The example used is the kinetic study of methanol synthesis in which the formation rates of methanol, methane, ethanol and ethane are considered. Two adequately accurate models were obtained from an extensive range of plausible models by using the new model discrimination and parameter estimation method with a relatively small computation effort.