Showing papers on "Methane published in 2010"
TL;DR: An overview of the current status of clean energy applications of porous MOFs, including hydrogen storage, methane storage and carbon dioxide capture is provided.
1,192 citations
TL;DR: In this paper, a review of the development of metal or carbonaceous catalysts for enhanced methane conversion and on the improvement of long-term catalyst stability is presented, where the roles played by various parameters, such as temperature and flow rate, on the rate of hydrogen production and the characteristics of the carbon produced.
730 citations
TL;DR: This work presents an isoreticular series of (3,24)-connected MOFs made from dendritic hexacarboxylate ligands, one of which has a Langmuir surface area as high as 6033 m2 g-1.
Abstract: Metal-organic frameworks (MOFs) are newly emerging porous materials. Owing to their large surface area and tunable pore size and geometry, they have been studied for applications in gas storage and separation, especially in hydrogen and methane storage and carbon dioxide capture. It has been well established that the high-pressure gravimetric hydrogen-adsorption capacity of an MOF is directly proportional to its surface area. However, MOFs of high surface areas tend to decompose upon activation. In our previous work, we described an approach toward stable MOFs with high surface areas by incorporating mesocavities with microwindows. To extend this work, we now present an isoreticular series of (3,24)-connected MOFs made from dendritic hexacarboxylate ligands, one of which has a Langmuir surface area as high as 6033 m2 g-1. In addition, the gas-adsorption properties of this new isoreticular MOF series have been studied.
664 citations
TL;DR: A number of gas hydrates are known to form at moderate pressure, and nearly ten structures in the pressure range above 100MPa as discussed by the authors. But, these structures are not stable at high temperatures.
608 citations
TL;DR: It is shown that more than 5000 at-sea observations of dissolved methane demonstrates that greater than 80% of ESAS bottom waters and greater than 50% of surface waters are supersaturated with methane regarding to the atmosphere, and that the current atmospheric venting flux is on par with previous estimates of methane venting from the entire World Ocean.
Abstract: Remobilization to the atmosphere of only a small fraction of the methane held in East Siberian Arctic Shelf (ESAS) sediments could trigger abrupt climate warming, yet it is believed that sub-sea permafrost acts as a lid to keep this shallow methane reservoir in place. Here, we show that more than 5000 at-sea observations of dissolved methane demonstrates that greater than 80% of ESAS bottom waters and greater than 50% of surface waters are supersaturated with methane regarding to the atmosphere. The current atmospheric venting flux, which is composed of a diffusive component and a gradual ebullition component, is on par with previous estimates of methane venting from the entire World Ocean. Leakage of methane through shallow ESAS waters needs to be considered in interactions between the biogeosphere and a warming Arctic climate.
521 citations
01 Jan 2010
TL;DR: In this paper, the phase transition in organic matter using equilibrium molecular dynamics simulations involving methane in small carbon slit-pores of varying size and temperature is addressed. And the results suggest that a significant level of adjustment is necessary in volume calculations, especially for gas shales high in total organic content.
Abstract: Using FIB/SEM imaging technology, a series of 2-D and 3-D submicro-scale investigations are performed on the types of porous constituents inherent to gas shale. A finely-dispersed porous organic (kerogen) material is observed imbedded within an inorganic matrix. The latter may contain larger-size pores of varying geometries although it is the organic material that makes up the majority of gas pore volume, with pores and capillaries having characteristic lengths typically less than 100 nanometers. A significant portion of total gas in-place appears to be associated with inter-connected large nano-pores within the organic material. This observation has several implications on reservoir engineering of gas shales. Primarily, thermodynamics (phase behavior) of fluids in these pores are known to be quite different. Most importantly, gas residing in a small pore or capillary is rarefied under the influence of organic pore walls and shows a density profile across the pore with damped-oscillations. This raises the following serious questions related to gas-in-place calculations: under reservoir conditions, what fraction of the pore volume of the organic material can be considered available for the free gas phase and what fraction is taken up by the adsorbed phase? If a significant fraction of the organic pore volume is taken up by the adsorbed phase, how accurately is the shale gas storage capacity estimated using the conventional volumetric methods? And, finally, do average densities exist for the free and the adsorbed phases and how large would a typical density contrast be in an organic pore for an accurate gas reserve calculation? In order to answer these questions we combine the Langmuir equilibrium adsorption isotherm with the volumetrics for free gas and formulate a new gas-in-place equation accounting for the organic pore space taken up by the sorbed phase. The method yields a total gas-in-place prediction based on a corrected free gas pore volume that is obtained using an average adsorbed gas density. Next, we address the fundamental-level questions related to phase transition in organic matter using equilibrium molecular dynamics simulations involving methane in small carbon slit-pores of varying size and temperature. We predict methane density profiles across the pores and show that (i) an average total thickness for an adsorbed methane layer is typically 0.7 nm, which is roughly equivalent to 4% of a 100 nm diameter pore volume, and (ii) the adsorbed phase density is 1.8-2.0 times larger than that of the bulk methane, i.e., in the absence of pore wall effects. These findings suggest that a significant level of adjustment is necessary in volume calculations, especially for gas shales high in total organic content. Finally, using typical values for the parameters, we perform a series of calculations using the new volumetric method and show a 10-25% decrease in total gas storage capacity compared to that using the conventional approach. This additionally could have a larger impact in shales where the sorbed gas phase is a more significant portion of the total gas-inplace. The new methodology is recommended for estimating shale gas-in-place and the approach could be extended to other unconventional gas-in-place calculations where both sorbed and free gas phases are present.
469 citations
TL;DR: The Cassini-Huygens Probe Gas Chromatograph Mass Spectrometer (GCMS) determined the composition of the Titan atmosphere from ~140km altitude to the surface as mentioned in this paper.
Abstract: The Cassini-Huygens Probe Gas Chromatograph Mass Spectrometer (GCMS) determined the composition of the Titan atmosphere from ~140km altitude to the surface. After landing, it returned composition data of gases evaporated from the surface. Height profiles of molecular nitrogen (N2), methane (CH4) and molecular hydrogen (H2) were determined. Traces were detected on the surface of evaporating methane, ethane (C2H6), acetylene (C2H2), cyanogen (C2N2) and carbon dioxide (CO2). The methane data showed evidence that methane precipitation occurred recently. The methane mole fraction was (1.48+/-0.09) x 10(exp -2) in the lower stratosphere (139.8 km to 75.5 km) and (5.65+/-0.18) x 10(exp -2) near the surface (6.7 km to the surface). The molecular hydrogen mole fraction was (1.01+/-0.16) x 10(exp -3) in the atmosphere and (9.90+/-0.17) x 10(exp -4) on the surface. Isotope ratios were 167.7+/-0.6 for N-14/N-15 in molecular nitrogen, 91.1+/-1.4 for C-12/C-13 in methane and (1.35+/-0.30) x 10(exp -4) for D/H in molecular hydrogen. The mole fractions of Ar-36 and radiogenic Ar-40 are (2.1+/-0.8) x 10(exp -7) and (3.39 +/-0.12) x 10(exp -5) respectively. Ne-22 has been tentatively identified at a mole fraction of (2.8+/-2.1) x 10(exp -7) Krypton and xenon were below the detection threshold of 1 x 10(exp -8) mole fraction. Science data were not retrieved from the gas chromatograph subsystem as the abundance of the organic trace gases in the atmosphere and on the ground did not reach the detection threshold. Results previously published from the GCMS experiment are superseded by this publication.
403 citations
TL;DR: In this paper, an experimental study has been conducted to assess the potential for coalbed methane production and carbon dioxide storage in coals, carbonaceous shales and source rocks in the Parana Basin in Brazil.
346 citations
TL;DR: The role of the nitrogen cycle in repression of methane production is probably low, and concentrations of inorganic nitrogen compounds are low in the root zones in the growing season due to the nitrogen-consuming behaviour of the plant.
327 citations
TL;DR: Purified MCR from Methanothermobacter marburgensis converts methane into methyl-coenzyme M under equilibrium conditions with apparent Vmax and Km values consistent with the observed in vivo kinetics of the anaerobic oxidation of methane with sulphate, which supports the hypothesis of ‘reverse methanogenesis’.
Abstract: Large amounts (estimates range from 70 Tg per year to 300 Tg per year) of the potent greenhouse gas methane are oxidized to carbon dioxide in marine sediments by communities of methanotrophic archaea and sulphate-reducing bacteria, and thus are prevented from escaping into the atmosphere. Indirect evidence indicates that the anaerobic oxidation of methane might proceed as the reverse of archaeal methanogenesis from carbon dioxide with the nickel-containing methyl-coenzyme M reductase (MCR) as the methane-activating enzyme. However, experiments showing that MCR can catalyse the endergonic back reaction have been lacking. Here we report that purified MCR from Methanothermobacter marburgensis converts methane into methyl-coenzyme M under equilibrium conditions with apparent V(max) (maximum rate) and K(m) (Michaelis constant) values consistent with the observed in vivo kinetics of the anaerobic oxidation of methane with sulphate. This result supports the hypothesis of 'reverse methanogenesis' and is paramount to understanding the still-unknown mechanism of the last step of methanogenesis. The ability of MCR to cleave the particularly strong C-H bond of methane without the involvement of highly reactive oxygen-derived intermediates is directly relevant to catalytic C-H activation, currently an area of great interest in chemistry.
326 citations
TL;DR: In this article, a method for biogas scrubbing and CH 4 enrichment is presented, where a packed column of liquid solvents are circulated through the column, contacting the Biogas in countercurrent flow, and test results revealed that the aqueous solutions used were effective in reacting with CO 2 in BiOGas, creating CH 4 enriched fuel.
TL;DR: In this article, a model that simulates methane emissions from northern peatlands is incorporated directly into a dynamic global vegetation model, and a sensitivity test was conducted to identify the most important factors influencing methane emissions, followed by a parameter fitting exercise to find the best combination of parameter values for individual sites and over all sites.
Abstract: . For the first time, a model that simulates methane emissions from northern peatlands is incorporated directly into a dynamic global vegetation model. The model, LPJ-WHyMe (LPJ Wetland Hydrology and Methane), was previously modified in order to simulate peatland hydrology, permafrost dynamics and peatland vegetation. LPJ-WHyMe simulates methane emissions using a mechanistic approach, although the use of some empirical relationships and parameters is unavoidable. The model simulates methane production, three pathways of methane transport (diffusion, plant-mediated transport and ebullition) and methane oxidation. A sensitivity test was conducted to identify the most important factors influencing methane emissions, followed by a parameter fitting exercise to find the best combination of parameter values for individual sites and over all sites. A comparison of model results to observations from seven sites resulted in normalised root mean square errors (NRMSE) of 0.40 to 1.15 when using the best site parameter combinations and 0.68 to 1.42 when using the best overall parameter combination.
TL;DR: In this paper, it was shown that the reaction that drives Rayleigh fractionation of C 2 and C 3 involves redox reactions with transition metals and water at late stages of catagenesis at temperatures on the order of 250-300°C.
TL;DR: The substantial temperature-dependent methane emissions discovered in this 90-year-old reservoir indicate that temperate water bodies can be an important but overlooked methane source.
Abstract: Methane emission pathways and their importance were quantified during a yearlong survey of a temperate hydropower reservoir. Measurements using gas traps indicated very high ebullition rates, but due to the stochastic nature of ebullition a mass balance approach was crucial to deduce system-wide methane sources and losses. Methane diffusion from the sediment was generally low and seasonally stable and did not account for the high concentration of dissolved methane measured in the reservoir discharge. A strong positive correlation between water temperature and the observed dissolved methane concentration enabled us to quantify the dissolved methane addition from bubble dissolution using a system-wide mass balance. Finally, knowing the contribution due to bubble dissolution, we used a bubble model to estimate bubble emission directly to the atmosphere. Our results indicated that the total methane emission from Lake Wohlen was on average >150 mg CH4 m−2 d−1, which is the highest ever documented for a midlati...
TL;DR: In this article, an important question is to what extent increased temperatures will affect the performance of methane formation (MF) and methane oxidation (MO) rates in aquatic environments, and the answer is that it depends on methane formation and MO rates.
Abstract: Methane emissions from aquatic environments depend on methane formation (MF) and methane oxidation (MO) rates. One important question is to what extent increased temperatures will affect the balanc ...
TL;DR: It is indicated that methane production could be stimulated at the nonproductive field site and that low microbial biomass may be limiting in situ methane generation, and the microcosm study suggests that the pathway for generating methane from coal involves complex microbial partnerships.
Abstract: Biogenic formation of methane from coal is of great interest as an underexploited source of clean energy. The goal of some coal bed producers is to extend coal bed methane productivity and to utilize hydrocarbon wastes such as coal slurry to generate new methane. However, the process and factors controlling the process, and thus ways to stimulate it, are poorly understood. Subbituminous coal from a nonproductive well in south Texas was stimulated to produce methane in microcosms when the native population was supplemented with nutrients (biostimulation) or when nutrients and a consortium of bacteria and methanogens enriched from wetland sediment were added (bioaugmentation). The native population enriched by nutrient addition included Pseudomonas spp., Veillonellaceae, and Methanosarcina barkeri. The bioaugmented microcosm generated methane more rapidly and to a higher concentration than the biostimulated microcosm. Dissolved organics, including long-chain fatty acids, single-ring aromatics, and long-chain alkanes accumulated in the first 39 days of the bioaugmented microcosm and were then degraded, accompanied by generation of methane. The bioaugmented microcosm was dominated by Geobacter sp., and most of the methane generation was associated with growth of Methanosaeta concilii. The ability of the bioaugmentation culture to produce methane from coal intermediates was confirmed in incubations of culture with representative organic compounds. This study indicates that methane production could be stimulated at the nonproductive field site and that low microbial biomass may be limiting in situ methane generation. In addition, the microcosm study suggests that the pathway for generating methane from coal involves complex microbial partnerships.
TL;DR: A review of the available scientific literature on how natural sources and the atmospheric fate of methane may be affected by future climate change is presented in this paper, where the authors discuss how processes governing methane wetland emissions, permafrost thawing, and destabilization of marine hydrates may affect the climate system.
Abstract: We have reviewed the available scientific literature on how natural sources and the atmospheric fate of methane may be affected by future climate change. We discuss how processes governing methane wetland emissions, permafrost thawing, and destabilization of marine hydrates may affect the climate system. It is likely that methane wetland emissions will increase over the next century. Uncertainties arise from the temperature dependence of emissions and changes in the geographical distribution of wetland areas. Another major concern is the possible degradation or thaw of terrestrial permafrost due to climate change. The amount of carbon stored in permafrost, the rate at which it will thaw, and the ratio of methane to carbon dioxide emissions upon decomposition form the main uncertainties. Large amounts of methane are also stored in marine hydrates, and they could be responsible for large emissions in the future. The time scales for destabilization of marine hydrates are not well understood and are likely to be very long for hydrates found in deep sediments but much shorter for hydrates below shallow waters, such as in the Arctic Ocean. Uncertainties are dominated by the sizes and locations of the methane hydrate inventories, the time scales associated with heat penetration in the ocean and sediments, and the fate of methane released in the seawater. Overall, uncertainties are large, and it is difficult to be conclusive about the time scales and magnitudes of methane feedbacks, but significant increases in methane emissions are likely, and catastrophic emissions cannot be ruled out. We also identify gaps in our scientific knowledge and make recommendations for future research and development in the context of Earth system modeling.
TL;DR: It is found that methane uptake takes place primarily at two types of strong adsorption site: the open Cu coordination sites, which exhibit enhanced Coulomb attraction toward methane, and the van der Waals potential pocket sites, in which the total dispersive interactions are enhanced due to the molecule being in contact with multiple "surfaces".
Abstract: Metal-organic frameworks (MOFs) are a novel family of physi- sorptive materials that have exhibited great promise for methane storage. So far, a detailed understanding of their methane adsorption mechanism is still scarce. Herein, we report a comprehen- sive mechanistic study of methane stor- age in three milestone MOF com- pounds (HKUST-1, PCN-11, and PCN- 14) the CH4 storage capacities of which are among the highest reported so far among all porous materials. The three MOFs consist of the same dicopper paddlewheel secondary building units, but contain different organic linkers, leading to cagelike pores with various sizes and geometries. From neutron powder diffraction experiments and ac- curate data analysis, assisted by grand canonical Monte Carlo (GCMC) simu- lations and DFT calculations, we un- ambiguously revealed the exact loca- tions of the stored methane molecules in these MOF materials. We found that methane uptake takes place primarily at two types of strong adsorption site: 1) the open Cu coordination sites, which exhibit enhanced Coulomb at- traction toward methane, and 2) the van der Waals potential pocket sites, in which the total dispersive interactions are enhanced due to the molecule being in contact with multiple "surfa- ces". Interestingly, the enhanced van der Waals sites are present exclusively in small cages and at the windows to these cages, whereas large cages with relatively flat pore surfaces bind very little methane. Our results suggest that further, rational development of new MOF compounds for methane storage applications should focus on enriching open metal sites, increasing the volume percentage of accessible small cages and channels, and minimizing the frac- tion of large pores.
TL;DR: The destabilization of gas hydrate reserves in permafrost areas is more certain as climate models predict that high-latitude regions will be disproportionately affected by global warming with temperature increases of over 12°C predicted for much of North America and Northern Asia.
Abstract: Gas hydrates are ice-like deposits containing a mixture of water and gas; the most common gas is methane. Gas hydrates are stable under high pressures and relatively low temperatures and are found ...
TL;DR: In this article, a series of permafrost thaw ponds in the Canadian Subarctic and Arctic and further investigated how optical properties of the carbon pool, the type of microbial assemblages, and light and mixing regimes influenced the rate of gas release.
Abstract: Arctic climate change is leading to accelerated melting of permafrost and the mobilization of soil organic carbon pools that have accumulated over thousands of years. Photochemical and microbial transformation will liberate a fraction of this carbon to the atmosphere in the form of CO2 and CH4. We quantified these fluxes in a series of permafrost thaw ponds in the Canadian Subarctic and Arctic and further investigated how optical properties of the carbon pool, the type of microbial assemblages, and light and mixing regimes influenced the rate of gas release. Most ponds were supersaturated in CO2 and all of them in CH4. Gas fluxes as estimated from dissolved gas concentrations using a wind-based model varied from 220.5 to 114.4 mmol CO2 m22 d21, with negative fluxes recorded in arctic ponds colonized by benthic microbial mats, and from 0.03 to 5.62 mmol CH4 m22 d21. From a time series set of measurements in a subarctic pond over 8 d, calculated gas fluxes were on average 40% higher when using a newly derived equation for the gas transfer coefficient developed from eddy covariance measurements. The daily variation in gas fluxes was highly dependent on mixed layer dynamics. At the seasonal timescale, persistent thermal stratification and gas buildup at depth indicated that autumnal overturn is a critically important period for greenhouse gas emissions from subarctic ponds. These results underscore the increasingly important contribution of permafrost thaw ponds to greenhouse gas emissions and the need to account for local and regional variability in their limnological properties for global estimates.
TL;DR: In this article, the authors investigated the effect of pre-treatments: thermal, thermo-chemical and chemical pre-treatment on the biogas and methane potential of dewatered pig manure.
TL;DR: There is increasing evidence that this gas has both physical and biological effects on gut function, and it is now often associated with functional constipation and may have an active role here.
Abstract: Several gases are produced through enteric fermentation in the intestinal tract. Carbon dioxide, hydrogen, hydrogen sulfide, and methane are thought to be the most common of these. Recent evidence suggests that methane may not be inert. In this review article, we summarize the findings with methane. This is a review article discussing the various component gases in the gastrointestinal tract and their relevance to health and disease. Specific attention was paid to understanding methane. The majority of these gases are eliminated via flatus or absorbed into systemic circulation and expelled from the lungs. Excessive gas evacuation or retention causes gastrointestinal functional symptoms such as belching, flatulence, bloating, and pain. Between 30 and 62% of healthy subjects produce methane. Methane is produced exclusively through anaerobic fermentation of both endogenous and exogenous carbohydrates by enteric microflora in humans. Methane is not utilized by humans, and analysis of respiratory methane can serve as an indirect measure of methane production. Recent literature suggests that gases such as hydrogen sulfide and methane may have active effects on gut function. In the case of hydrogen sulfide, evidence demonstrates that this gaseous product may be produced by human eukaryotic cells. However, in the case of methane, there is increasing evidence that this gas has both physical and biological effects on gut function. It is now often associated with functional constipation and may have an active role here. This review of the literature discusses the significance of enteric flora, the biogenesis of methane, and its clinical associations. Furthermore, we examine the evidence for an active role of methane in gastrointestinal motility and the potential applications to future therapeutics.
TL;DR: In this article, it was shown that methylated compounds may serve as precursors for methane and thermodynamic calculations show that methylotrophic methanogenesis can provide energy in aerobic environments.
Abstract: . A methane surplus relative to the atmospheric equilibrium is a frequently observed feature of ocean surface water. Despite the common fact that biological processes are responsible for its origin, the formation of methane in aerobic surface water is still poorly understood. We report on methane production in the central Arctic Ocean, which was exclusively detected in Pacific derived water but not nearby in Atlantic derived water. The two water masses are distinguished by their different nitrate to phosphate ratios. We show that methane production occurs if nitrate is depleted but phosphate is available as a P source. Apparently the low N:P ratio enhances the ability of bacteria to compete for phosphate while the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) is utilized as a C source. This was verified by experimentally induced methane production in DMSP spiked Arctic sea water. Accordingly we propose that methylated compounds may serve as precursors for methane and thermodynamic calculations show that methylotrophic methanogenesis can provide energy in aerobic environments.
TL;DR: In this article, colorless distributed combustion (CDC) is characterized by distributed reaction zone of combustion which leads to uniform thermal field and avoidance of hot spot regions to provide significant improvement in pattern factor, lower sound levels and reduced NOx emission.
TL;DR: In this article, a numerical simulator (MH21-HYDRES: MH21 Hydrate Reservoir Simulator) was used to study the performance of gas production from oceanic hydrate (MH) deposits.
Abstract: Oceanic methane hydrate (MH) deposits have been found at high saturations within reservoir-quality sands in the Eastern Nankai Trough and the Gulf of Mexico. This study investigates the key factors for the success of depressurization-induced gas production from such oceanic MH deposits. A numerical simulator (MH21-HYDRES: MH21 Hydrate Reservoir Simulator) was used to study the performance of gas production from MH deposits. We calculated the hydrate dissociation behavior and gas/water production performance during depressurization for a hypothetical MH well. Simulation runs were conducted under various initial reservoir conditions of MH saturation, temperature, and absolute permeability. A productivity function (PF) was introduced as an indicator of gas productivity, which is a function of gas production rate, water production rate, and discount rate. The simulations showed that recovery factors over 36% and maximum gas production rates over 450 000 Sm3/d were expected for the most suitable conditions of ...
TL;DR: In this paper, the bimetallic catalyst was used for carbon dioxide reforming of methane using a quartz tube microreactor at a CH4/CO2 feed ratio of 1, 750 ÂC, 1 Â atm with a gas hourly space velocity of 125,000 ÂmL/g/h.
Abstract: MgO–ZrO2 mesoporous support (Zr/Mg molar ratio = 9) impregnated with 6 wt% Ni, 6 wt% Co and 3 wt% of both Ni and Co were prepared using a novel surfactant assisted-impregnation method. Carbon dioxide reforming of methane using these catalysts was studied in a quartz tube microreactor at a CH4/CO2 feed ratio of 1, 750 °C, 1 atm with a gas hourly space velocity of 125,000 mL/g/h. Based on reactant's conversion and syngas production, the bimetallic catalyst was the most suitable catalyst for the reaction. The catalyst exhibited high and constant activity during 40 h reaction time with methane and carbon dioxide conversions of 80% and 84%, respectively with a syngas ratio close to unity without significant deactivation as compared to the respective monometallic catalysts. For longer time on stream, the catalyst showed constant activity up to 60 h after which it gradually decreased. The bimetallic catalyst also exhibited excellent regenerability by restoring its initial catalytic activity after 1 h of regeneration in air. The catalysts were also characterized by XRD, XPS, N2-physisorption, H2-chemisorption, TGA-DTA, HRTEM, H2-TPR, TPH and SEM. The high performance of the bimetallic catalyst was due to the stabilization of t-phase in zirconia, better metal dispersion, small metal particle size and synergetic effect between Ni and Co particles. The XPS results showed that bimetallic catalyst had the ability to hinder metal oxidation and exhibited presence of higher surface basicity which were responsible to maintain the stability of the catalyst.
TL;DR: It is shown that areas locally richer in methane will nucleate much more readily, and no 'memory' of the crystal is required for fast re-crystallization, and it is anticipated that much richer polycrystallinity and novel methane hydrate phases could be possible.
Abstract: In this work we show that homogeneous nucleation of methane hydrate can, under appropriate conditions, be a very rapid process, achieved within tens of nanoseconds. In agreement with recent experimental results on different systems, we find that the nucleation of a gas hydrate crystal appears as a two-step process. It starts with the formation of disordered solid-like structures, which will then spontaneously evolve to more recognizable crystalline forms. This previously elusive first-stage state is confirmed to be post-critical in the nucleation process, and is characterized as processing reasonable short-range structure but essentially no long-range order. Its energy, molecular diffusion and local structure reflect a solid-like character, although it does exhibit mobility over longer (tens of ns) timescales. We provide insights into the controversial issue of memory effects in methane hydrates. We show that areas locally richer in methane will nucleate much more readily, and no ‘memory’ of the crystal is required for fast re-crystallization. We anticipate that much richer polycrystallinity and novel methane hydrate phases could be possible.
TL;DR: In this paper, the physicochemical and catalytic properties of a Ni/La2O3 catalyst obtained by reduction of a lanthanum nickelite, LaNiO3, with perovskite structure were investigated.
Abstract: The objective of the present work has been the study of the physicochemical and catalytic properties of a Ni/La2O3 catalyst obtained by reduction of a lanthanum nickelite, LaNiO3, with perovskite structure. The perovskite, obtained by means of a spray pyrolysis method, provides a Ni/La2O3 system active in different methane reforming reactions. The catalyst was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-Ray photoemission spectroscopy (XPS), temperature-programmed reduction and oxidation (TPR, TPO) and catalytic activity tests. Although not evidenced by XRD data, XAS and TPR measurements show the presence of an amorphous NiO phase in the original sample, together with the crystalline LaNiO3 phase. Upon reoxidation treatment of the reduced Ni/La2O3 catalyst, the LaNiO3 structure is partly recovered which provides a convenient way to regenerate a waste catalyst (reoxidation and new reduction in hydrogen). The catalyst is active in several reactions of methane with oxygen, water and CO2, showing a remarkable stability specially under dry reforming of methane (DRM) reaction conditions. This quite great catalytic performance has been explained by the high resistance of the nickel particles to be oxidized, as detected by in situ XAS. In the presence of water, as in steam reforming of methane (SRM) reaction conditions, these metallic particles are gradually oxidized, which explains the linear decreasing of the catalytic performance observed for the SRM reaction.
TL;DR: In this article, the catalytic activity of Pt-doped CeO2 for the oxidation and the dry reforming of methane was studied and it was found that the presence of Pt activates the oxygen atoms next to it in the surface layer and this decreases the activation energy for dissociative adsorption of methane.