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PubMed2004

Theoretical studies of the kinetics of methane hydrate crystallization in external electromagnetic fields.

Nonequilibrium molecular-dynamics (MD) simulations have been performed for the growth and dissolution of a spherical methane hydrate crystallite, surrounded by a saturated water-methane liquid phase, in both the absence and presence of external electromagnetic (e/m) fields in the microwave to far infrared range (5-7500 GHz) at root-mean square (rms) electric field intensities of up to 0.2 V/A. A rigid/polarizable potential was used to model water and a rigid/nonpolarizable model was utilized for methane. In the absence of a field, it was found that the average growth rate of the crystallite was approximately 0.32 water and 0.045 methane molecules per picosecond, evaluated over a 500 ps NPT simulation for three different initial geometries. Upon the application of an e/m field, it was found that no significant deviations from the zero-field crystal growth patterns were observed for rms electric field intensities of less than about 0.1 V/A, regardless of the field frequency. At, and above, this "threshold" intensity, it was found that dissolution took place. The mobility of the molecules in the system was enhanced by the e/m field, to the greatest extent for frequencies of 50-100 GHz. Furthermore, it was observed that there was a systematic frequency variation in the pattern of dipole alignment with the external field and this led to marked differences in the rate of dissolution.

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PubMed2004

Microcanonical unimolecular rate theory at surfaces. II. Vibrational state resolved dissociative chemisorption of methane on Ni(100).

A three-parameter microcanonical theory of gas-surface reactivity is used to investigate the dissociative chemisorption of methane impinging on a Ni(100) surface. Assuming an apparent threshold energy for dissociative chemisorption of E(0)=65 kJ/mol, contributions to the dissociative sticking coefficient from individual methane vibrational states are calculated: (i) as a function of molecular translational energy to model nonequilibrium molecular beam experiments and (ii) as a function of temperature to model thermal equilibrium mbar pressure bulb experiments. Under fairly typical molecular beam conditions (e.g., E(t)>/=25 kJ mol(-1), T(s)>/=475 K, T(n)</=400 K), sticking from methane in the ground vibrational state dominates the overall sticking. In contrast, under thermal equilibrium conditions at temperatures T>/=100 K the dissociative sticking is dominated by methane in vibrationally excited states, particularly those involving excitation of the nu(4) bending mode. Fractional energy uptakes f(j) defined as the fraction of the mean energy of the reacting gas-surface collision complexes that derives from specific degrees of freedom of the reactants (i.e., molecular translation, rotation, vibration, and surface) are calculated for thermal dissociative chemisorption. At 500 K, the fractional energy uptakes are calculated to be f(t)=14%, f(r)=21%, f(v)=40%, and f(s)=25%. Over the temperature range from 500 K to 1500 K relevant to thermal catalysis, the incident gas-phase molecules supply the preponderance of energy used to surmount the barrier to dissociative chemisorption, f(g)=f(t)+f(r)+f(v) approximately 75%, with the highest energy uptake always coming from the molecular vibrational degrees of freedom. The predictions of the statistical, mode-nonspecific microcanonical theory are compared to those of other dynamical theories and to recent experimental data.

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PubMed2005

Microcanonical unimolecular rate theory at surfaces. III. Thermal dissociative chemisorption of methane on Pt(111) and detailed balance.

A local hot spot model of gas-surface reactivity is used to investigate the state-resolved dynamics of methane dissociative chemisorption on Pt(111) under thermal equilibrium conditions. Three Pt surface oscillators, and the molecular vibrations, rotations, and the translational energy directed along the surface normal are treated as active degrees of freedom in the 16-dimensional microcanonical kinetics. Several energy transfer models for coupling a local hot spot to the surrounding substrate are developed and evaluated within the context of a master equation kinetics approach. Bounds on the thermal dissociative sticking coefficient based on limiting energy transfer models are derived. The three-parameter physisorbed complex microcanonical unimolecular rate theory (PC-MURT) is shown to closely approximate the thermal sticking under any realistic energy transfer model. Assuming an apparent threshold energy for CH(4) dissociative chemisorption of E(0)=0.61 eV on clean Pt(111), the PC-MURT is used to predict angle-resolved yield, translational, vibrational, and rotational distributions for the reactive methane flux at thermal equilibrium at 500 K. By detailed balance, these same distributions should be observed for the methane product from methyl radical hydrogenation at 500 K in the zero coverage limit if the methyl radicals are not subject to side reactions. Given that methyl radical hydrogenation can only be experimentally observed when the CH(3) radicals are kinetically stabilized against decomposition by coadsorbed H, the PC-MURT was used to evaluate E(0) in the high coverage limit. A high coverage value of E(0)=2.3 eV adequately reproduced the experimentally observed methane angular and translational energy distributions from thermal hydrogenation of methyl radicals. Although rigorous application of detailed balance arguments to this reactive system cannot be made because thermal decomposition of the methyl radicals competes with hydrogenation, approximate applicability of detailed balance would argue for a strong coverage dependence of E(0) with H coverage--a dependence not seen for methyl radical hydrogenation on Ru(0001), but not yet experimentally explored on Pt(111).

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PubMed2006

Resonant infrared laser-induced desorption of methane condensed on NaCl(100): isotope mixture experiments.

Resonantly enhanced infrared laser-induced desorption of methane condensed on a single-crystal NaCl(100) surface is observed after excitation with the widely tunable infrared laser output of the free-electron laser at the free-electron laser for infrared experiments facility using mass spectroscopic detection and time-of-flight analysis. Desorption of methane is observed only when the exciting light is in resonance with an internal vibrational mode of the molecule. Different intramolecular modes of the three methane isotopologues under study--CH(4), CD(4), and CD(3)H--are excited; the degenerate deformation mode nu(4) is observed for CH(4) and CD(4) at 7.69 and 10.11 microm, respectively, as well as the nu(2) and nu(4) modes of CD(3)H at 7.79, 9.75, and 9.98 microm. The desorption signals for the pure layers of these different methane isotopologues as well as for different mixtures of two of these are investigated as a function of the infrared wavelength and the laser fluence. The desorption behavior for pure and mixed layers is compared and the underlying desorption mechanism is discussed.

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PubMed2006

Methane hydrate formation and decomposition: structural studies via neutron diffraction and empirical potential structure refinement.

Neutron diffraction studies with hydrogen/deuterium isotope substitution measurements are performed to investigate the water structure at the early, medium, and late periods of methane clathrate hydrate formation and decomposition. These measurements are coupled with simultaneous gas consumption measurements to track the formation of methane hydrate from a gas/water mixture, and then the complete decomposition of hydrate. Empirical potential structure refinement computer simulations are used to analyze the neutron diffraction data and extract from the data the water structure in the bulk methane hydrate solution. The results highlight the significant changes in the water structure of the remaining liquid at various stages of hydrate formation and decomposition, and give further insight into the way in which hydrates form. The results also have important implications on the memory effect, suggesting that the water structure in the presence of hydrate crystallites is significantly different at equivalent stages of forming compared to decomposing. These results are in sharp contrast to the previously reported cases when all remaining hydrate crystallites are absent from the solution. For these systems there is no detectable change in the water structure or the methane hydration shell before hydrate formation and after decomposition. Based on the new results presented in this paper, it is clear that the local water structure is affected by the presence of hydrate crystallites, which may in turn be responsible for the "history" or "memory" effect where the production of hydrate from a solution of formed and then subsequently melted hydrate is reportedly much quicker than producing hydrate from a fresh water/gas mixture.

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PubMed2006

Methane clathrate: CH(4) quantum rotor state dependent rattling potential.

In methane hydrate the dominant peak in the density of states above 3 meV represents a rattling mode of the guest molecule CH(4) in the large ice cages. This mode shifts from 6.7 meV at T=4.5 K to T=30 K to 7.14 meV with conversion of CH(4) guest molecules into the tunneling ground state. The less symmetric angular density distribution PsiPsi(*) in the excited rotational state compared to the ground state allows the methane to fit better in the orientation dependent cage potential surface. This leads to a larger average distance to the cage-forming molecules with a weaker potential and a reduced rattling energy. A two state single particle model with characteristic rattling energies of 5.20 meV for pure T-methane and 7.3 meV for pure A-methane weighted by the population factors can fit the data.

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PubMed1982

Methane: Interhemispheric concentration gradient and atmospheric residence time.

The ground level concentrations of methane in the atmosphere have been measured to be in the range from 1.45 to 1.62 parts per million by volume (ppmv) of dry air in remote locations between 62 degrees N and 54 degrees S latitudes during the time period from November 1977 to July 1979. The average (+/-rms) concentration for the northern hemisphere was 1.57 +/- 0.02 ppmv in January 1978 and 1.59 +/- 0.02 in July 1979. The average concentration in the southern hemisphere was lower-1.47 +/- 0.02 in January 1978 and 1.51 +/- 0.01 in July 1979. The ratio of concentrations between the two hemispheres was 1.068 +/- 0.016 in January 1978 and 1.055 +/- 0.013 in July 1979, for an average of 1.06 +/- 0.01. The higher concentrations in the northern hemisphere require either that the sources of methane lie preferentially in the northern hemisphere or that the removal processes operate more rapidly in the southern hemisphere or both. The primary removal process for CH(4) is reaction with tropospheric OH radicals and its estimated atmospheric lifetime is 10.5 +/- 1.8 yr. The observed interhemispheric gradient is consistent with this lifetime and preferential release of methane in the northern hemisphere. Measurements taken in the Amazon basin region indicate the presence of a substantial source of methane in that area.

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PubMed1997

Comparison of structures and energies of CH5(2+) with CH4(+) and their possible role in superacidic methane activation.

Contrary to previous theoretical studies at the UHF/6-31G* level, the methonium radical dication CH5(2+) is not a Cs symmetrical structure with a 2e-3c bond but a C2v symmetrical structure 1 with two 2e-3c bonds (at the UHF/6-31G**, UMP2/6-31G**, and UQCISD(T)/6-311G** levels). The Cs symmetrical structure is not even a minimum at the higher level of calculations. The four hydrogen atoms in 1 are bonded to the carbon atom by two 2e-3c bonds and the fifth hydrogen atom by a 2e-2c bond. The unpaired electron of 1 is located in a formal p-orbital (of the sp2-hybridized carbon atom) perpendicular to the plane of the molecule. Hydrogen scrambling in 1 is however extremely facile, as is in other C1 cations. It is found that the protonation of methane to CH5(+) decreases the energy for subsequent homolytic cleavage resulting in the exothermic (24.1 kcal/mol) formation of CH4(+*). Subsequent reaction with neutral methane while reforming CH5(+) gives the methyl radical enabling reaction with excess methane to ethane and H2. The overall reaction is endothermic by 11.4 kcal/mol, but offers under conditions of oxidative removal of H2 an alternative to the more energetic carbocationic conversion of methane.

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PubMed2004

Siberian peatlands a net carbon sink and global methane source since the early Holocene.

Interpolar methane gradient (IPG) data from ice cores suggest the "switching on" of a major Northern Hemisphere methane source in the early Holocene. Extensive data from Russia's West Siberian Lowland show (i) explosive, widespread peatland establishment between 11.5 and 9 thousand years ago, predating comparable development in North America and synchronous with increased atmospheric methane concentrations and IPGs, (ii) larger carbon stocks than previously thought (70.2 Petagrams, up to approximately 26% of all terrestrial carbon accumulated since the Last Glacial Maximum), and (iii) little evidence for catastrophic oxidation, suggesting the region represents a long-term carbon dioxide sink and global methane source since the early Holocene.

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PubMed2005

Unexpected changes to the global methane budget over the past 2000 years.

We report a 2000-year Antarctic ice-core record of stable carbon isotope measurements in atmospheric methane (delta13CH4). Large delta13CH4 variations indicate that the methane budget varied unexpectedly during the late preindustrial Holocene (circa 0 to 1700 A.D.). During the first thousand years (0 to 1000 A.D.), delta13CH4 was at least 2 per mil enriched compared to expected values, and during the following 700 years, an about 2 per mil depletion occurred. Our modeled methane source partitioning implies that biomass burning emissions were high from 0 to 1000 A.D. but reduced by almost approximately 40% over the next 700 years. We suggest that both human activities and natural climate change influenced preindustrial biomass burning emissions and that these emissions have been previously understated in late preindustrial Holocene methane budget research.

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PubMed1999

Abiogenic methane formation and isotopic fractionation under hydrothermal conditions

Recently, methane (CH(4)) of possible abiogenic origin has been reported from many localities within Earth's crust. However, little is known about the mechanisms of abiogenic methane formation, or about isotopic fractionation during such processes. Here, a hydrothermally formed nickel-iron alloy was shown to catalyze the otherwise prohibitively slow formation of abiogenic CH(4) from dissolved bicarbonate (HCO(3)-) under hydrothermal conditions. Isotopic fractionation by the catalyst resulted in delta(13)C values of the CH(4) formed that are as low as those typically observed for microbial methane, with similarly high CH(4)/(C(2)H(6) + C(3)H(8)) ratios. These results, combined with the increasing recognition of nickel-iron alloy occurrence in oceanic crusts, suggest that abiogenic methane may be more widespread than previously thought.

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PubMed1979

Measuring radioactive methane with the liquid scintillation counter.

Although a gas proportional counter is the most convenient method of measuring the radioactivity of fixed gases such as methane, it cannot be used when high nonradioactive concentrations of methane are present in the gas phase, due to quenching. If only methane and carbon dioxide are present in radioactive form in the gas phase, a liquid scintillation method for measuring these substances can be used. The procedure is described in detail, and the solubility of methane in liquid scintillation cocktails is determined.

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PubMed1980

Comparison of in situ and in vitro rates of methane release in freshwater sediments.

Anaerobic lake sediment incubated in vitro was investigated for its ability to mimic natural in situ sediment activities, using rate of methane production for the comparison. Two lakes with different rates and seasonal patterns of methanogenic activity were compared. There was good agreement (at the 97.5% confidence level) between rates of in situ methane release and initial (lasting an average of 120 h) rates of production measured in vitro in surface (0- to 3-cm) sediment. Evidence from this study, and others, indicated that it is the in situ surface sediment methane production which is primarily responsible for maintaining in situ methane release, and thus the above agreement was what was expected if surface in situ activity was maintained in vitro. When deeper sediment was investigated, however, the sum of in vitro rates from 0 to 20 cm (measured in 1.5- to 3-cm intervals) was much higher than in situ release rates and would have resulted in an impossibly high volume of gas. The extra gas could not have been stored within the sediments. We conclude that the in situ methanogenic activity of the 0- to 3-cm anaerobic surface sediments could be preserved during removal and laboratory incubation. However, similar treatment of deeper sediment appeared to stimulate methanogenic activity.

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PubMed1981

Fate of immediate methane precursors in low-sulfate, hot-spring algal-bacterial mats.

The fates of acetate and carbon dioxide were examined in several experiments designed to indicate their relative contributions to methane production at various temperatures in two low-sulfate, hot-spring algal-bacterial mats. [2-C]acetate was predominantly incorporated into cell material, although some CH(4) and CO(2) was produced. Acetate incorporation was reduced by dark incubation in short-term experiments and severely depressed by a 2-day preincubation in darkness. Autoradiograms showed that acetate was incorporated by long filaments resembling phototrophic microorganisms of the mat communities. [H]acetate was not converted to CH(4) in samples from Octopus Spring collected at the optimum temperature for methanogenesis. NaHCO(3) was readily converted to CH(4) at temperatures at which methanogenesis was active in both mats. Comparisons of the specific activities of methane and carbon dioxide suggested that of the methane produced, 80 +/- 6% in Octopus Spring and 71 +/- 21% in Wiegert Channel were derived from carbon dioxide. Addition of acetate to 1 mM did not reduce the relative importance of carbon dioxide as a methane precursor in samples from Octopus Spring. Experiments with pure cultures of Methanobacterium thermoautotrophicum suggested that the measured ratio of specific activities might underestimate the true contribution of carbon dioxide in methanogenesis.

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PubMed1983

Substrates for sulfate reduction and methane production in intertidal sediments.

The activity of and potential substrates for methane-producing bacteria and sulfate-reducing bacteria were examined in marsh, estuary, and beach intertidal sediments. Slow rates of methane production were detected in all sediments, although rates of sulfate reduction were 100- to 1,000-fold higher. After sulfate was depleted in sediments, the rates of methane production sharply increased. The addition of methylamine stimulated methanogenesis in the presence of sulfate, and [C]methylamine was rapidly converted to CH(4) and CO(2) in freshly collected marsh sediment. Acetate, hydrogen, or methionine additions did not stimulate methanogenesis. [methyl-C]methionine and [2-C]acetate were converted to CO(2) and not to CH(4) in fresh sediment. No reduction of CO(2) to CH(4) occurred in fresh sediment. Molybdate, an inhibitor of sulfate reduction, inhibited [2-C]acetate metabolism by 98.5%. Fluoracetate, an inhibitor of acetate metabolism, inhibited sulfate reduction by 61%. These results suggest that acetate is a major electron donor for sulfate reduction in marine sediments. In the presence of high concentrations of sulfate, methane may be derived from novel substrates such as methylamine.

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PubMed1984

Variants of the Obligate Methanotroph Isolate 761M Capable of Growth on Glucose in the Absence of Methane.

Isolate 761M is an unusual type I methanotroph that possesses a complete tricarboxylic acid cycle. Variants of this methanotroph that were capable of growth with methanol (isolate 761AR) or glucose (isolate 761H) have been isolated. Cultures of isolate 761H grown with glucose and casein hydrolysate as the sole carbon and energy sources retained the ability to grow on methane, contained methane monooxygenase and 3-hexulose phosphate synthase, and possessed intracytoplasmic membranes similar to those found in thin sections of isolate 761M grown on methane. Methane monooxygenase was also present in cultures of isolate 761AR grown on methanol and casein hydrolysate.

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PubMed1988

Trichloroethylene biodegradation by a methane-oxidizing bacterium.

Trichloroethylene (TCE), a common groundwater contaminant, is a suspected carcinogen that is highly resistant to aerobic biodegradation. An aerobic, methane-oxidizing bacterium was isolated that degrades TCE in pure culture at concentrations commonly observed in contaminated groundwater. Strain 46-1, a type I methanotrophic bacterium, degraded TCE if grown on methane or methanol, producing CO(2) and water-soluble products. Gas chromatography and C radiotracer techniques were used to determine the rate, methane dependence, and mechanism of TCE biodegradation. TCE biodegradation by strain 46-1 appears to be a cometabolic process that occurs when the organism is actively metabolizing a suitable growth substrate such as methane or methanol. It is proposed that TCE biodegradation by methanotrophs occurs by formation of TCE epoxide, which breaks down spontaneously in water to form dichloroacetic and glyoxylic acids and one-carbon products.

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PubMed1990

Liquid-to-Gas Mass Transfer in Anaerobic Processes: Inevitable Transfer Limitations of Methane and Hydrogen in the Biomethanation Process.

Liquid-to-gas mass transfer in anaerobic processes was investigated theoretically and experimentally. By using the classical definition of k(L)a, the global volumetric mass transfer coefficient, theoretical development of mass balances in such processes demonstrates that the mass transfer of highly soluble gases is not limited in the usual conditions occurring in anaerobic fermentors (low-intensity mixing). Conversely, the limitation is important for poorly soluble gases, such as methane and hydrogen. The latter could be overconcentrated to as much as 80 times the value at thermodynamic equilibrium. Such overconcentrations bring into question the biological interpretations that have been deduced solely from gaseous measurements. Experimental results obtained in three different methanogenic reactors for a wide range of conditions of mixing and gas production confirmed the general existence of low mass transfer coefficients and consequently of large overconcentrations of dissolved methane and hydrogen (up to 12 and 70 times the equilibrium values, respectively). Hydrogen mass transfer coefficients were obtained from the direct measurements of dissolved and gaseous concentrations, while carbon dioxide coefficients were calculated from gas phase composition and calculation of related dissolved concentration. Methane transfer coefficients were based on calculations from the carbon dioxide coefficients. From mass balances performed on a gas bubble during its simulated growth and ascent to the surface of the liquid, the methane and carbon dioxide contents in the gas bubble appeared to be controlled by the bubble growth process, while the bubble ascent was largely responsible for a slight enrichment in hydrogen.

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