CH4 Synthesis Research Articles

Pleurotus Ostreatus and Volvariella Volvacea Can Enhance the Quality of Purple Field Corn Stover and Modulate Ruminal Fermentation and Feed Utilization in Tropical Beef Cattle.

Simple SummaryThe use of purple field corn stover contains enriched anthocyanin. Moreover, the effect of anthocyanin in purple field corn could inhibit methane (CH4) synthesis by changing hydrogen from the CH4 route to form propionic acid. However, purple field corn was found to have poor digestibility owing to its fiber linkage structure. Pleurotus ostreatus and Volvarialla volvacea are species of white-rot fungal mushroom, famous and abundant in tropical regions. These fungi have potential to decrease indigestion of cell wall contents and increase cell wall digestibility of rice straw material by secondary metabolites namely lignin peroxidase, manganese peroxidase, and laccase enzyme, and showed the structural polymer found in rice straw and purple field corn residue. Accordingly, feeding purple field corn stover fermented with P. ostreatus or V. volvacea will increase feed utilization and reduce CH4 production in beef cattle.This objective is to elucidate the effect of purple field corn stover treated with Pleurotus ostreatus and Volvarialla volvacea on feed utilization, ruminal ecology, and CH4 synthesis in tropical beef cattle. Four male Thai native beef cattle (100 ± 30 kg of body weight (BW) were assigned randomly as a 2 × 2 factorial arrangement in a 4 × 4 Latin square design. Factor A (roughage sources) was rice straw and purple field corn stover and factor B was species of white-rot fungi (P. ostreatus and V. volvacea). After fermentation, crude protein (CP) was increased in rice straw and purple field corn stover fermented with P. ostreatus and V. volvacea. The unfermented purple field corn stover contained 11.8% dry matter (DM) of monomeric anthocyanin (MAC), whereas the MAC concentration decreased when purple field corn stover was fermented with white rot fungi. There were no changes (p > 0.05) in DM intake of body weight (BW) kg/d and g/kg BW0.75 among the four treatments. The organic matter (OM), CP, and acid detergent fiber (ADF) intake were different between rice straw and purple field corn stover and were the greatest in the purple field corn stover group. Moreover, the current study showed that neutral detergent fiber (NDF) and ADF digestion was higher in purple field corn stover than in rice straw, but there were no significant differences between P. ostreatus and V. volvacea. There were significant effects of roughage sources on ammonia nitrogen (NH3-N) at 4 h after feeding. Bacterial population was changed by feeding with purple field corn stover fermented with P. ostreatus and V. volvacea. On the other hand, the number of protozoa was reduced by approximately 33% at 4 h after feeding with purple field corn stover (p < 0.01). Propionic acid concentration was different between roughage sources (p < 0.01) enhanced with purple field corn stover fermented with P. ostreatus and V. volvacea. In addition, methane production decreased by 15% with purple field corn stover fermented with P. ostreatus and V. volvacea compared to rice straw. There were significant differences on all nitrogen balances parameters (p < 0.05), except the fecal N excretion (p > 0.05) were not changed. Furthermore, microbial crude protein and efficiency of microbial N synthesis were enhanced when purple field corn stover fermented with P. ostreatus and V. volvacea was fed compared to rice straw group. Base on this study, it could be summarized that P. ostreatus or V. volvacea can enhance the quality of purple field corn stover and modulate rumen fermentation and feed digestion in Thai native beef cattle.

A density functional theory study of CO methanation mechanism on Ni4/SiC catalyst

The reaction mechanism of CO methanation on the Ni4/3C-SiC(1 1 1) catalyst surface was explored by the density functional theory in this paper. The configuration of Ni4/3C-SiC(1 1 1) is built and the binding energy of Ni4 cluster on Ni4/3C-SiC(1 1 1) is higher than that on Ni4/t-ZrO2 and Ni4/Al2O3 surface, which may be contributed to improve the dispersion and stability of Ni-based catalyst. And the main pathway of CH4 formation on Ni4/3C-SiC(1 1 1) surface is CO + H → CHO + H → CH2O + H → CH3O → CH3 + H → CH4. The results show that the rate-determining step for CO methanation on Ni4/3C-SiC(1 1 1) is the formation of CH3 from CH3O dissociation with an energy barrier of 241.3 kJ/mol, which is lower than that on Ni4/t-ZrO2. In addition, the activation energy of CH3OH formation is higher than that of CH4 synthesis on Ni4/3C-SiC(1 1 1) surface (254.5 vs. 241.3 kJ/mol). Different from Ni(1 1 1), Ni4/γ-Al2O3, and Ni4/t-ZrO2 catalysts, the Ni4/3C-SiC(1 1 1) catalyst has a much higher selectivity for CH4 in the reaction of CO methanation, which may be attributed to the lower activation energy of CH3O dissociation to CH3 than that hydrogenation to CH3OH. Therefore, Ni4/3C-SiC(1 1 1) can be recommended preferentially for CH4 synthesis via CO hydrogenation, rather than CH3OH formation.

Graphene-laminated architectures obtained by chemical vapor deposition: From graphene to graphite

Graphene and graphite are of great interest in materials science. Using chemical vapor deposition at various CH4:H2 ratios, we synthesized materials with graphene-laminated architectures, ranging from graphene to graphite. A lower proportion of CH4 and lower synthesis temperature produced fewer graphene layers. The transparent properties changed from transparent to semi-transparent, black, and silver as the number of graphene layers was increased. The sheet electrical resistivity ranged from 106 to 0.2Ω□−1, and the smaller resistivity was nearly equaled as the values of highly orientated pyrolytic graphite and glassy carbon. The graphene-laminated materials featured a wide range of transmittance, reflectance, and electrical conductance properties.

In vitro rumen gas and methane production of grass silages differing in plant maturity and nitrogen fertilisation, compared to in vivo enteric methane production

The potential of an in vitro gas production (GP) system to predict the in vivo enteric methane (CH4) production for various ryegrass-based silages was evaluated, using adapted rumen fluid from cows. Rumen fluid from 12 lactating rumen-cannulated Holstein-Friesian cows were used for in vitro incubations and compared with in vivo CH4 production data derived from the same cows fed the same grass silages. The cows consumed a total mixed ration consisting of six different grass silages and concentrate at an 80:20 ratio on a dry matter (DM) basis. The grass silages differed in plant maturity at harvest (28, 41 and 62days of regrowth) and N fertilisation (65 and 150kg of N/ha). Rumen fluid from cows consuming each of the six grass silages was used to determine the in vitro organic matter (OM) fermentation and in vitro CH4 synthesis, using an automated GP technique. In vitro GP decreased with increasing maturity of the grass. In vitro CH4 production, expressed either in ml/g of OM, in ml/g of degraded OM (DOM) or as a% of the total GP, increased with increased N fertilisation (P<0.05). Maturity of grass at harvest did not affect the CH4 synthesis expressed in ml/g of DOM and CH4 expressed as% of the total gas, whereas N fertilisation increased the in vitro CH4 synthesis, expressed in any unit. The in vitro data correlated poorly with the in vivo data. Across the six grass silages tested, the in vitro CH4 production, expressed in ml/g of OM after 8, 12, 24, and 72h of incubation did not correlate with the in vivo enteric CH4 production, expressed in g/kg of DM intake (R2=0.01–0.08). Stepwise multiple regression showed a weak, but positive correlation between the observed in vivo CH4 synthesis, expressed in g/kg FPCM and the predicted CH4 per kg FPCM, using the amount of in vitro organic matter degraded (R2=0.40; P=0.036). In vitro gas and CH4 parameters did not improve the accuracy of the prediction of the in vivo CH4 data.

Abiotic methane formation during experimental serpentinization of olivine

Fluids circulating through actively serpentinizing systems are often highly enriched in methane (CH4). In many cases, the CH4 in these fluids is thought to derive from abiotic reduction of inorganic carbon, but the conditions under which this process can occur in natural systems remain unclear. In recent years, several studies have reported abiotic formation of CH4 during experimental serpentinization of olivine at temperatures at or below 200 °C. However, these results seem to contradict studies conducted at higher temperatures (300 °C to 400 °C), where substantial kinetic barriers to CH4 synthesis have been observed. Here, the potential for abiotic formation of CH4 from dissolved inorganic carbon during olivine serpentinization is reevaluated in a series of laboratory experiments conducted at 200 °C to 320 °C. A 13C-labeled inorganic carbon source was used to unambiguously determine the origin of CH4 generated in the experiments. Consistent with previous high-temperature studies, the results indicate that abiotic formation of CH4 from reduction of dissolved inorganic carbon during the experiments is extremely limited, with nearly all of the observed CH4 derived from background sources. The results indicate that the potential for abiotic synthesis of CH4 in low-temperature serpentinizing environments may be much more limited than some recent studies have suggested. However, more extensive production of CH4 was observed in one experiment performed under conditions that allowed an H2-rich vapor phase to form, suggesting that shallow serpentinization environments where a separate gas phase is present may be more favorable for abiotic synthesis of CH4.

Microbial communities responsible for fixation of CO2 revealed by using mcrA, cbbM, cbbL, fthfs, fefe-hydrogenase genes as molecular biomarkers in petroleum reservoirs of different temperatures

Sequestration of CO2 in oil reservoir is one of the feasible options for mitigating atmospheric CO2 building up. The in situ bioconversion of sequestrated CO2 to methane by microorganisms inhabiting oil reservoirs is feasible. To evaluate the potential of in situ microbial fixation and conversion of CO2 into CH4 in oil reservoirs, a comprehensive molecular survey was performed to reveal microbial communities inhabiting four oil reservoirs with different temperatures by analysis of functional genes involved in the biochemical pathways of CO2 fixation and CH4 synthesis (cbbM, cbbL, fthfs, [FeFe]-hydrogenase encoding gene, and mcrA). A rich diversity of these functional genes was found in all the samples with both high and low temperatures and they were affiliated to members of the Proteobacteria (cbbL and cbbM, fthfs), Firmicutes and Actinobacteria (fthfs), uncultured bacteria ([FeFe]-hydrogenase), and Methanomirobiales, Methanobacteriales and Methanosarcinales (mcrA). The predominant methanogens were all identified to be hydrogenotrophic CO2-reducing physiological types. These results showed that functional microbial communities capable of microbial fixation and bioconversion of CO2 into methane inhabit widely in oil reservoirs, which is helpful to microbial recycling of sequestrated CO2 to further new energy in oil reservoirs.

Metamorphism of peritotites in the mantle wedge above the subduction zone: Hydration of the lithospheric mantle

Two areas with different types of hydration (serpentinization), which occurred in two settings distinct in temperatures, pressures, and stresses, are spatially individualized in the ophiolitic ultramafic massifs of the Polar Urals. The high-temperature hydration of ultramafic rocks occurred in the lithosphere of the mantle wedge directly above the subducted slab. The initial conditions of hydration are limited to 1.2–2 GPa and 650–700°C; a stable assemblage of olivine + antigorite + magnetite → amphibole → talc → chlorite was formed at 0.9–1.2 GPa and 550–600°C. The low-temperature mesh lizardite–chrysotile serpentinization occurred in the crustal, near-surface conditions. Both types of hydration were accompanied by release of hydrogen, which participates in abiogenic CH4 synthesis in the presence of CO2 dissolved in water.

Methane Synthesis Characteristics of H2O/CO2 Co-Electrolysis in Tubular Solid Oxide Electrolysis Cells

Producing methane by H2O/CO2 co-electrolysis through solid oxide electrolysis cell (SOEC) is a promising method to store unstable renewable power and reduce CO2 emission simultaneously. Theoretically, methanation reaction is possible to happen in standard condition when temperature is lower than 600oC. Tubular solid oxide electrolysis cell (TSOEC) has great potential for large-scale application due to easily sealing and high strength. Moreover, gradient distribution of temperature in flow direction is beneficial to CH4 production. In this paper, CH4 synthesis characteristics in TSOEC is studied in the temperature range of 500-650oC by combining experiment and simulation. Experiment demonstrates the addition of H2 increases the outlet molar fraction of CH4 from 0 to 2.85% at 600oC, and from 0.01% to 8.77% at 550oC. In addition, it’s found that electricity significantly promotes CH4 production, especially when H2 is added. The applied voltage of 1.5V just increases CH4 production ratio by <0.04% in the absence of H2. But when 20% H2 is fed in cathode, CH4 production ratio is improved by 3-4%. Finally, the optimal operating condition, thermal distribution and reaction active zones are simulated by a well validated thermal model. The thermal distribution can be changed by outer boundary condition. CH4production can be further improved by increasing the temperature of upstream zone and reducing that of downstream zone. Fig.1 Methane synthesis schematic in TSOEC Figure 1

Methane Synthesis Characteristics of H2O/CO2 Co-Electrolysis in Tubular Solid Oxide Electrolysis Cells

Producing methane by H2O/CO2 co-electrolysis through solid oxide electrolysis cell (SOEC) is a promising method to store unstable renewable power and reduce CO2 emission simultaneously. In this paper, CH4 synthesis characteristics in tubular SOEC (TSOEC) is studied in the temperature range of 550-650oC by combining experiment and simulation. Experiments demonstrates adding H2 increases the CH4 production ratio from 0 to 4.01% at 600oC. Besides, electricity significantly promotes CH4 production, especially when H2 is added. The voltage of 1.5 V just increases CH4 production ratio by <0.04% in the absence of H2. But when 20% H2 is fed in cathode, a voltage of 1.5 V can increase CH4 production ratio by 3-4%. Finally, chemical reactions, thermal distribution and optimal operating condition are simulated by a well validated thermal model. A reasonable thermal distribution created by operating TSOEC in counter-flow mode and lower inlet steam partial pressure can dramatically improve CH4 production.

Experimental Characterization and Theoretical Modeling of Methane Production by H2O/CO2 Co-Electrolysis in a Tubular Solid Oxide Electrolysis Cell

Methane production by H2O/CO2 co-electrolysis in solid oxide electrolysis cells (SOECs) is a promising method to store unstable renewable power and to reduce CO2 emission. In this paper, the CH4 synthesis characteristics in tubular SOECs (TSOECs) is studied by experiments and numerical simulation in the temperature range of 550–650°C. The experimental results show that the addition of H2 into the cathode inlet gas flow and the increase of electrolysis current could significantly promote CH4 production rate. In the case of the experiments without H2, the CH4 production ratio increased by less than 0.04% when the operating voltage varied from OCV to 1.5 V. After feeding the H2 into the cathode atmosphere, the CH4 production ratio increases by 3–4%. A CH4 yield of 9.94% and a CH4 production ratio of 12.34% were achieved at 550°C with 20% H2 fed into cathode and an applied voltage of 1.5 V. Finally, a theoretical model is developed by considering the chemical/electrochemical reaction and transport processes, and the thermal distribution and optimal operating condition of the TSOEC are studied. An designed thermal distribution in counter-flow mode with lower inlet steam partial pressure could dramatically improve the CH4 production.

Interactions between microbial consortia in biofilms: a paradigm shift in rumen microbial ecology and enteric methane mitigation

Minimising enteric CH4 emissions from ruminants is a current research priority because CH4 contributes to global warming. The most effective mitigation strategy is to adjust the animal’s diet to complement locally available feed resources so that optimal production is gained from a minimum of animals. This essay concentrates on a second strategy – the use of feed additives that are toxic to methanogens or that redirect H2 (and electrons) to inhibit enteric CH4 emissions from individual animals. Much of the published research in this area is contradictory and may be explained when the microbial ecology of the rumen is considered. Rumen microbes mostly exist in organised consortia within biofilms composed of self-secreted extracellular polymeric substances attached to or within feed particles. In these biofilms, individual colonies are positioned to optimise their use of preferred intermediates from an overall process of organic matter fermentation that generates end-products the animal can utilise. Synthesis of CH4 within biofilms prevents a rise in the partial pressure of H2 (pH2) to levels that inhibit bacterial dehydrogenases, and so reduce fermentation rate, feed intake and digestibility. In this context, hypotheses are advanced to explain changes in hydrogen disposal from the biofilms in the rumen resulting from use of anti-methanogenic feed additives as follows. Nitrate acts as an alternative electron sink when it is reduced via NO2– to NH3 and CH4 synthesis is reduced. However, efficiency of CH4 mitigation is always lower than that predicted and decreases as NO3– ingestion increases. Suggested reasons include (1) variable levels of absorption of NO3–or NO2– from the rumen and (2) increases in H2 production. One suggestion is that NO3– reduction may lower pH2 at the surface of biofilms, thereby creating an ecological niche for growth of syntrophic bacteria that oxidise propionate and/or butyrate to acetate with release of H2. Chlorinated hydrocarbons also inhibit CH4 synthesis and increase H2 and formate production by some rumen methanogens. Formate diffuses from the biofilm and is converted to HCO3– and H2 in rumen fluid and is then excreted via the breath. Short-chain nitro-compounds inhibit both CH4 and formate synthesis when added to ruminal fluid but have little or no effect in redirecting H2 to other sinks, so the pH2 within biofilms may increase to levels that support reductive acetogenesis. Biochar or activated charcoal may also alter biofilm activity and reduce net CH4 synthesis; direct electron transfer between microbes within biofilms may also be involved. A final suggestion is that, during their sessile life stage, protozoa interact with biofilm communities and help maintain pH2 in the biofilm, supporting methanogenesis.

Carbon precursors of methane synthesis in the rumen of sheep dosed with ionophores

Rates of methane (CH4) production and the sources of carbon (C) for its synthesis were studied in four mature ewes when dosed with a CH4-mitigating ionophore ICI-111075, or monensin, or when untreated. The sheep were given 700 g/day of chaffed lucerne hay in equal portions every hour, before and during experiments in which 14C-labelled NaHCO3–, acetate, propionate, lactate and formate were infused intraruminally over 12 h and the specific radioactivity of C (SR) in each of these substrates was determined. During these infusions, the SR of material in the primary pool (the tracer infusion site) and in secondary metabolites of this material (secondary pools) approached asymptotic or ‘plateau’ values. The rate of infusion of 14CH4 (kBq/day) divided by the plateau SR value (kBq/g C) gave estimates of the rate of irreversible loss of CH4 (g C/day). These calculations indicated that CH4 production rate was reduced by 72% when sheep were dosed with ICI-111075 and by 58% when dosed with monensin. With monensin, the reduction in CH4 production was not associated with hydrogen (H2) accumulation in the rumen headspace gases whereas with ICI-111075, the decrease in CH4 production was associated with marked H2 accumulation in the headspace gases. When plateau SR were attained during the tracer infusions, the percentage ratio, (SR in any secondary pool: SR in the primary pool) gave an estimate of the fraction of C in that secondary pool derived from material of the primary pool. Calculated in this way, the percentage of CH4-C derived from rumen fluid carbon dioxide (CO2) averaged 59% in untreated sheep, and when sheep were dosed with ICI-111075, the corresponding percentage averaged 12%. These findings indicate there are sources of C for rumen CH4 synthesis other than rumen fluid CO2. However, there was no evidence that C from acetate, propionate, lactate or formate in rumen fluid were direct sources of the unidentified CH4-C. One plausible explanation for these findings is that CH4 is synthesised within naturally occurring microbial biofilms (attached to feed particles or the rumen wall) from CO2 that is produced locally by fermentation of unlabelled substrates within the biofilms. It is postulated that such pools of CO2 would be kinetically distinct and, during the infusion of 14C-labelled substrates, would exhibit a lower SR than the CO2 in the surrounding rumen fluid.

Abiotic Methane in the Hyperalkaline Springs of Genova, Italy

Serpentinization of ultramafic rocks is considered a major process of production of abiotic methane (CH4) and hydrogen (H2) on Earth, and it may be responsible for CH4 occurrence on other planets. While serpentinization and CH4 synthesis have been widely studied and modeled in high temperature hydrothermal conditions, such as on submarine mid-ocean ridges, the increasing number of discoveries of abiotic CH4 in ophiolites on continents shows the importance of present-day (meteoric water driven) serpentinization in low temperature (<100°C) gas synthesis. As a new case, we report compositional, isotopic, and flux data of gas dissolved in hyperalkaline Ca-OH waters issuing from serpentinized peridotites at Genova (Italy). CH4 is dominantly abiotic (δ13C: -9 ‰ VPDB; δ2H: -168 to - 225 ‰ VSMOW), similar to that released by ophiolites in Oman, Turkey, the Philippines, and by the submarine Lost City serpentinization system. While the absence of CO2 was expected in this kind of fluids, the absence of H2 is unusual. This could be due to hydration of olivine and pyroxene by CO2-rich fluids, eventually associated with high silica activity, which inhibits H2 formation and produces CH4 directly. Thermodynamic modeling and H2O-CH4 isotope equilibrium confirm the low temperatures (<60°C) of the serpentinization system, and thus the abiotic methane synthesis.

Low temperature production and exhalation of methane from serpentinized rocks on Earth: A potential analog for methane production on Mars

We evaluate, based on terrestrial analogs, the potential flux, origin and isotopic signature of methane (CH4) from serpentinized or serpentinizing rocks on Mars. The Tekirova ophiolites, in Turkey, have been shown to release, either via focused vents or through diffuse microseepage, substantial amounts of CH4 which could be produced via catalyzed abiotic methanation (Sabatier reaction) at low temperatures (<50°C). Serpentinized ultramafic rocks on Mars are likely to have necessary chemical constituents for methane production and fractures for release of gas to the atmosphere, similar to those on Earth. A simple, first-order estimation gas-advection model suggests that methane fluxes on the order of several mgm−2d−1, similar to microseepage observed in terrestrial ophiolites, could occur in martian rocks. High temperature, hydrothermal conditions may not be necessary for abiotic CH4 synthesis on Mars: low temperature (<50°C) methanation is possible in the presence of catalysts like ruthenium, rhodium or, more commonly, chromium minerals, which occur in terrestrial ophiolites as in martian mantle meteorites. The terrestrial analog environment of abiotic microseepage may thus explain production of methane on Mars in the ancient past or at present. The wide range of martian 12C/13C and D/H ratios and the potential secondary alteration of CH4 by abiotic oxidation, as observed on Earth, could result in large isotope variations of methane on Mars. CH4 isotopic composition alone may not allow definitive determination of biotic vs. abiotic gas origin. Using our terrestrial vs. martian analysis as guide to future Mars exploration we propose that direct methane and ethane gas detection and isotopic measurements on the ground over serpentinized/serpentinizing rocks should be considered in developing future strategies for unraveling the source and origin of methane on Mars.

Exergy analysis of synthetic natural gas production method from biomass

The paper presents the results of exergy analysis for a biomass-to-synthetic natural gas (SNG) conversion process. The presented study is based on wood gasification, which is analysed for different gasification conditions like temperature and/or pressure. The analysed temperature was varied in the range from 650 to 800 °C and the pressure range was from 1 to 15 bar. The main process units of biomass-to-SNG conversion technology are gasifier, gas cleaning, synthesis gas compression, CH4 synthesis and final SNG conditioning. The results showed that the largest exergy losses take place in the biomass gasifier, CH4 synthesis part and CO2 capture unit. The overall exergetic efficiency of the biomass-to-SNG process was estimated in the range of about 69.5–71.8%.

Reforming of CO2-Containing Natural Gas Using an AC Gliding Arc System: Effects of Operational Parameters and Oxygen Addition in Feed

In this research, the reforming of simulated natural gas containing a high CO2 content under AC non-thermal gliding arc discharge with partial oxidation was conducted at ambient temperature and atmospheric pressure, with specific regards to the concept of the direct utilization of natural gas. This work aimed at investigating the effects of applied voltage and input frequency, as well as the effect of adding oxygen on the reaction performance and discharge stability in the reforming of the simulated natural gas having a CH4:C2H6:C3H8:CO2 molar ratio of 70:5:5:20. The results showed marked increases in both CH4 conversion and product yield with increasing applied voltage and decreasing input frequency. The selectivities for H2, C2H6, C2H4, C4H10, and CO were observed to be enhanced at a higher applied voltage and at a lower frequency, whereas the selectivity for C2H2 showed an opposite trend. The use of oxygen was found to provide a great enhancement of the plasma reforming of the simulated natural gas. For the combined plasma and partial oxidation in the reforming of CO2-containing natural gas, air was found to be superior to pure oxygen in terms of reactant conversions, product selectivities, and specific energy consumption. The optimum conditions were found to be a hydrocarbons-to-oxygen feed molar ratio of 2/1 using air as an oxygen source, an applied voltage of 17.5 kV, and a frequency of 300 Hz, in providing the highest CH4 conversion and synthesis gas selectivity, as well as extremely low specific energy consumption. The energy consumption was as low as 2.73 × 10−18 W s (17.02 eV) per molecule of converted reactant and 2.49 × 10−18 W s (16.60 eV) per molecule of produced hydrogen.

Conditioning of Rh/α-Al2O3 catalysts for H2 production via CH4 partial oxidation at high space velocity

Experimental evidence and literature indications suggest that the process of methane partial oxidation over Rh catalysts is structure sensitive. Crystal phases and Rh cluster size are thus expected to affect the final catalytic performance. In this work, it is observed that outstanding performances are obtained when the as-prepared catalysts are conditioned through repeated runs at increasing temperature and O2/CH4=0.56. Catalysts slowly activate, that is CH4 conversion and synthesis gas selectivity progressively grow with time on stream. On the basis of TPO and CH4 decomposition measurements, this phenomenon is herein explained as the result of a surface reconstruction driven by the repeated exposition to the reaction at high temperature; it is thought that such reconstruction tends to eliminate defect sites and disfavors C-deposition reactions (extremely fast over steps and kinks). Conditioning with O2-enriched feed streams makes conditioning faster, since the accumulation of surface C-species is suppressed; however, the catalyst is eventually less active than a catalyst conditioned with standard feed mixtures. As an alternative, accumulation of carbon can be suppressed and surface reconstruction proceeds faster if the catalyst is directly exposed to the reaction at high temperature for several hours.

Methanogenesis and the K+ transport system are activated by divalent cations in ammonia-treated cells of Methanospirillum hungatei.

We describe a K+ transport system in Methanospirillum hungatei cells depleted of cytoplasmic K+ via an ammonia/K+ exchange reaction (Sprott, G. D., Shaw, K. M., and Jarrell, K. F. (1984) J. Biol. Chem. 259, 12602-12608). Ammonia-treated cells contained low concentrations of ATP and were unable to make CH4 or to transport 86Rb+. All of these properties were restored by CaCl2, MgCl2, or MnCl2, and not by CoCl2 or NiCl2. The Rb+ transport system had a Km of 0.42 and Vmax of 29 nmol/min X mg; K+ inhibited competitively. Both H2 and CO2 were required for appreciable transport, whereas air, valinomycin, or nigericin were potent inhibitors. The influx of Rb+ was electrogenic and associated with proton efflux, producing a delta pH (alkaline inside) in acidic media. In the absence of K+ (or Rb+), the activation of CH4 synthesis by Mg2+ produced little change in the cytoplasmic pH, showing that methanogenesis did not elicit a net efflux of protons. The pH optimum for transport was in the range 6.0-7.3 where the transmembrane pH gradient would contribute minimally to the proton motive force. Protonophores at pH 6.3 caused a partial decline in CH4 synthesis and the ATP content and dramatically collapsed Rb+ transport. These and other inhibitor experiments, coupled with the fact that the Rb+ gradient was too large to be in equilibrium with the proton motive force alone, suggest a role for both ATP and the proton motive force in Rb+ transport. Also, a role for K+ in osmoregulation is indicated.

Evidence for an internal electrochemical proton gradient in Methanobacterium thermoautotrophicum.

At pH > 7.5 cells of Methanobacterium thermoautotrophicum, when synthesizing CH4 from COZ and Hz, are interior acid with respect to the buffer. If the buffer pH is lowered below pH 6.5 with HC1, the pH gradient has the usual orientation, i.e. the cells become interior alkaline. As measured with the lipophilic anion, thiocyanate, the cells did not generate a measurable electrical potential (4). Flow dialysis experiments carried out at pH > 7.5 with permeant base methylamine indicated that these cells establish an inverse pH gradient, ie., interior acid of +145 mV. With pH > 7.5 the polarity of the protonmotive force, @H+, across the cytoplasmic membrane therefore is reversed and would not serve to generate cellular ATP. Results with different uncoupling reagents suggest that in M. thermoautotrophicum the synthesis of ATP and CH, is carried out by intracellular vesicles and that the cytoplasmic membrane probably is not the site of energy transduction. Chloroform or 2,4-dinitrophenol are powerful inhibitors of both CH4 and ATP synthesis in these cells but, when used alone, do not discharge the pH gradient. Carbonyl cyanide m-chlorophenylhydrazone, on the other hand, partially dissipates the pH gradient and at low concentrations partially inhibits CHs or ATP synthesis. Chloroform or 2,4-dinitrophenol, when added after carbonyl cyanide m-chlorophenylhydrazone, gives an additional and significant release of acid from these cells and thereby completes the discharge of the pH gradient.