Effects of rice cultivars on methane fluxes in a paddy soil

CH₄ emission and its relevant processes involved (i.e. CH₄ production, rhizospheric CH₄ oxidation and plant-mediated CH₄ transport) were studied simultaneously to comprehensively understand how rice cultivars (Yanxuan, 72031, and 9516) at growth stages (early and late tillering, panicle initiation, ripening, and harvest stage) affect CH₄ emission in a paddy soil. Over the entire rice-growing season, Yanxuan had the highest CH₄ emission flux with 5.98 g CH₄ m^–2 h^–1 followed by 72031 (4.48 g CH₄ m^–2 h^–1) and 9516 (3.41 g CH₄ m^–2 h^–1). The highest CH₄ production rate of paddy soils planted to Yanxuan was observed with 18.0 g CH₄ kg_{{ (d.w.soil)}} h^–1 followed by the soil planted to 9516 (17.5 g CH₄ kg_{{ (d.w.soil)}} h^–1). For each cultivar, both rhizospheric CH₄ oxidation ability and plant-mediated CH₄ transport efficiency varied widely with a range of 9.81–76.8% and 15.5–80.5% over the duration of crop growth, respectively. Multiple regression analyses showed that CH₄ emission flux was positively related with CH₄ production rate and rice plant-mediated CH₄ transport efficiency, but negatively with rhizospheric CH₄ oxidation (R ²=0.425 for Yanxuan, P<0.01; R ²=0.426 for 72031, P<0.01; R ²=0.564 for 9516, P<0.01). The contribution of rice plants to CH₄ production seems to be more important than to rhizospheric CH₄ oxidation and plant-mediated transport in impact of rice plants on CH₄ emission.     

CO2 reforming and partial oxidation of methane

CO₂ reforming and partial oxidation of CH₄ were investigated on different supported noble metal and Ni catalysts. A detailed thermodynamic analysis was performed for both reactions. The observed reaction behaviour can be predicted by thermodynamics. Product selectivity is catalyst independent, the role of the catalyst is to bring the reactants to approach equilibrium. The partial oxidation is a two-stage process, total oxidation of CH₄ is followed by CO₂ and H₂O reforming of the remaining CH₄. A staged addition of O₂ to the reactor is tested and recommended. TPSR show that the catalyst surface for CO₂ reforming was highly covered with carbonaceous species of four different types; two were identified as reactive intermediates.     

Nitrous oxide and methane emissions from soil–plant systems

The closed chamber method was used to measure the N₂O and CH₄ emissions from rice, maize, soybean and spring wheat fields in Northeast China. Rice field almost did not emit or deposit N₂O in total during flooding period, whereas N₂O was substantially emitted during non-flooding period. The annual emission amount of N₂O was 1.70 kg N₂O ha^-1, but that in flooding period was only 0.04 kg N₂O ha^-1. Daily average and seasonal total CH₄ emission in rice field were 0.07 and 7.40 g CH₄m^-2, respectively. A trade-off between N₂O and CH₄ emissions from rice field was found. The growth of Azolla in rice field greatly stimulated both N₂O and CH₄ emissions. Total N₂O emissions (270 days) from maize and soybean fields were 7.10 and 3.12 kg N₂O ha^-1, respectively. The sink function of the uplands monitored as the atmospheric CH₄ was not significant. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fluxes of methane and nitrous oxide in water-saving rice production in north China

Lowland rice production is currently facing serious water shortages in numerous Asian countries. In the North China Plain water limitations are severe. Water-saving rice production techniques are therefore increasingly searched for. Here we present the first study of trace gas emissions from a water-saving rice production system where soils are mulched and are kept close to field capacity in order to compare their contribution to global warming with traditional paddy rice. In a two-year field experiment close to Beijing, CH₄ and N₂O fluxes were monitored in two forms of the Ground Cover Rice Production System (GCRPS) and in traditional paddy fields using closed chambers. With paddy rice the observed CH₄ emissions were very low, about 0.3 g CH₄ m⁻² a⁻¹ in 2001 and about 1 g CH₄ m⁻² a⁻¹ in 2002. In GCRPS, the CH₄ emissions were negligible. N₂O fluxes in GCRPS were similar, 0.5 to 0.6 g N₂O m⁻² a⁻¹ in 2001 and 2002, and emission peaks mainly followed fertilizer applications. In paddy rice, N₂O fluxes were unexpectedly low throughout the year 2001 (0.03 g N₂O m⁻² a⁻¹), and in 2002 larger emissions occurred during the drainage period. So with 0.4 g N₂O m⁻² a⁻¹ the cumulative flux was similar to emissions in GCRPS. Total CO₂ equivalent fluxes calculated according to IPCC methodology were tenfold higher in GCRPS compared to paddy in 2001. In 2002, fluxes from both systems were similar with 175 and 141 g CO₂ equivalents m⁻² a⁻¹ from GCRPS and paddy. Burkhard Sattelmacher deceased.     

Activity of different zeolite-supported Ni catalysts for methane reforming with carbon dioxide

The catalytic performance of Ni based on various types of zeolites (zeolite A, zeolite X, zeolite Y, and ZSM-5) prepared by incipient wetness impregnation has been investigated for the catalytic carbon dioxide reforming of methane into synthesis gas at 700 °C, at atmospheric pressure, and at a CH₄/CO₂ ratio of 1. It was found that Ni/zeolite Y showed better catalytic performance than the other types of studied zeolites. In addition, the stability of the Ni/zeolite Y was greatly superior to that of the other catalysts. A weight of Ni loading at 7 wt.% showed the best catalytic activity on each zeolite support; however, the 7% Ni catalysts produced a higher amount of coke than that of two other Ni loadings, 3 and 5%.     

MCM-41 supported PdNi catalysts for dry reforming of methane

A series of bimetallic PdNi catalysts supported on mesoporous MCM-41 with different Ni content (Ni/Si ratio of 0.2–0.4) was synthesized. The effect of Pd addition to Ni-containing catalysts as well as the effect of the Ni content on the surface and catalytic properties of the catalysts was studied. The samples were characterized using various techniques, such as energy-dispersive X-ray spectroscopy, N₂ adsorption–desorption isotherms, X-ray diffraction, thermogravimetric and differential analyses, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy and temperature-programmed reduction. Reforming of methane with carbon dioxide was used as a test reaction. The results indicated that the addition of a small amount of Pd (0.5%) to Ni-containing catalysts leads to formation of small nano-sized, easy reducible NiO particles. Agglomeration of NiO as well as of metallic nickel phase over PdNi samples increased with increasing the Ni content. Formation of filamentous carbon over surface of spent monometallic Ni and bimetallic PdNi catalyst was observed. In spite of filamentous carbon deposition, the catalytic activity and stability of bimetallic PdNi catalysts are higher than those of monometallic Ni one. Within bimetallic system, the PdNi catalyst with Ni/Si ratio of 0.3 revealed the best performance and stability caused by presence of small nickel particles well dispersed on the catalyst surface.     

Quantitative dependence of methane emission on soil properties

To identify the key soil parameters influencing CH₄ emission from rice paddies, an outdoor pot experiment with a total of 18 paddy soils was conducted in Nanjing Agricultural University during the 2000 rice growing season. The seasonal average rate of CH₄ emission for all 18 soils was 6.42±2.70 mg m^–2 h^–1, with a range of 1.96 to 11.06 mg m^–2 h^–1. Correlation analysis indicated that the seasonal average of CH₄ emission was positively dependent on soil sand content and negatively on total N as well as NH₄ ⁺-N determined before rice transplanting. Copper content of soils had a significant negative impact on CH₄ emission. No clear relationship existed between CH₄ emission and soil carbon content. In addition, soil type cannot explain the variability in CH₄ emission. Soil parameters influencing CH₄ emission were different as rice growth and development proceeded. A further investigation suggested that the seasonal average rate of CH₄ emission could be quantitatively determined by a linear combination of soil NH₄ ⁺-N, available copper, the ratio of available to total sulphur, and the ratio of available to total iron. Moreover, the average rates of CH₄ emission in the vegetative, reproductive and ripening stages could be also respectively described by a linear combination of different soil variables.     

Supported Rh catalysts for methane partial oxidation prepared by OM-CVD of Rh(acac)(CO)2

The organometallics chemical vapour deposition (OM-CVD) technique, using Rh(acac)(CO)₂ as a precursor, was employed for the preparation of heterogeneous Rh catalysts supported on low surface area refractory oxides (α-Al₂O₃, ZrO₂, MgO and La₂O₃). Prepared systems were tested in the methane catalytic partial oxidation (CH₄-CPO) reaction in a fixed bed reactor and compared to a reference catalyst prepared from impregnation of Rh₄(CO)₁₂.Catalysts supported on Al₂O₃, ZrO₂ and MgO show better or comparable performances with respect to the reference system.Complete decomposition of Rh precursor during formation of the metal phase under reductive conditions was investigated by TPRD and confirmed by infrared and mass spectrometry data.Supported Rh phase was characterized by CO and H₂ chemisorption, CO-DRIFT spectroscopy and HRTEM microscopy in fresh and aged selected samples. Rh(I) isolated sites and Rh(0) metal particles were found on fresh catalysts; after ageing an extensive reconstruction occurs mainly consisting in a sintering of Rh isolate sites to metal particles but without large increase in mean particles size.Catalytic performances and Rh species balance were found to be dependent on the support material.     

Isotopic study of the reaction of methane with the lattice oxygen of a NiO/MgO solid solution

The reaction between methane and the lattice oxygen of a NiO/MgO (1/1.85) solid solution was investigated by an isotopic pulse-GCMS method. During a single-pulse reaction of CH₄/CD₄ (1/1) with the lattice oxygen, besides CH₄, CD₄, carbon monoxide and water, very small amounts of CHD₃, CH₂D₂ and CH₃D were detected in the exit gas. Because the isotopic kinetic effect (K_H/K_D) was 1.01, the dissociation of CH₄ cannot constitute the rate-determining step. The rate-determining step is the reaction with the lattice oxygen, because of the high stability of the lattice oxygen in the NiO/MgO solid solutions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Conversion of methane to higher hydrocarbons in pulsed DC barrier discharge at atmospheric pressure

Conversion of methane to C₂/C₃ or higher hydrocarbons in a pulsed DC barrier discharge at atmospheric pressure was studied. Non-equilibrium plasma was generated in the barrier discharge reactor. In this plasma, electrons which had sufficient energy collided with the molecules of methane, which were then activated and coupled to C₂/C₃ or higher hydrocarbons. The effect of the change of applied voltage, pulse frequency and methane flow rate on methane conversion, selectivities and yields of products was studied. Methane conversion to higher hydrocarbons was about 25% as the maximum. Ethane, propane and ethylene were produced as primary products, including a small amount of unidentified C₄ hydrocarbons. The selectivity and yield of ethane as a main product came to about 80% and 17% as the highest, respectively. The selectivities of ethane and ethylene were influenced not by the change of pulse frequency but by the change of applied voltage and methane flow rate. However, in case of propane, the selectivity was independent of those condition changes. The effect of the packing materials such as glass and A1₂O₃ bead on methane conversion was also considered, showing that A1₂O₃ played a role in enhancing the selectivity of ethane remarkably as a catalyst.     

NO/CH4 Reaction over Nanodispersed Pt Particles

The catalytic behavior of the cubic (70%) Pt nanoparticles supported on alumina, with an average diameter of 132nm, was investigated for NO/CH₄ reaction. It was observed that the formation of reaction products (N₂O, CO and NH₃) is related to the size as well to the shape (facet) of the Pt nanoparticles.     

CH4/CD4 isotope effects in the carbon dioxide reforming of methane to syngas over SiO2-supported nickel catalysts

By performing the CH₄ + CO₂ and CD₄ + CO₂ reactions alternately over SiO₂-supported nickel catalysts in a pulse micro-reactor, normal deuterium isotope effects on both the methane conversion reaction and on the CO formation reaction have been observed in the process of CO₂ reforming of methane. Based on the observed CH₄/CD₄ isotope effects, the pathways for the formation of CO are discussed.     

Regeneration of S-poisoned Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts for the combustion of methane

This work investigates the regeneration of S-poisoned Pd/Al₂O₃ and Pd/CeO₂/Al₂O₃ catalysts under different CH₄ containing atmospheres. Under lean combustion conditions in the presence of excess O₂, partial regeneration took place for both systems only above 750 °C after decomposition of stable sulphate species adsorbed on the support. Under alternate lean combustion/CH₄-reducing pulse regeneration is markedly anticipated down to 550–600 °C. Experiments evidenced an effective role of ceria in preventing PdO from sulphation and in promoting regeneration via sulphates decomposition under reducing conditions.     

Partial oxidation of CH4 at low pressure over SiO2 prepared from Si

The oxidation of CH₄ with O₂ at low pressure was carried out over SiO₂ prepared from metal Si. The Si showed only total oxidation activity while the Si partly oxidized to SiO₂ showed high selectivities to CH₃OH and HCHO. The results on SiO₂ prepared from Si were compared with those over commercial silicas. The role of SiO₂ in the CH₄ oxidation was discussed.     

Selective reduction of nitric oxide by methane on cobalt-ion-exchanged synthetic clinoptilolite zeolite in oxygen-rich atmosphere

Selective reduction of NO by CH₄ in the presence of excess O₂ has been studied on cobalt-ion-exchanged synthetic clinoptilolite (Co-CLI) zeolite. The catalytic results are compared with those obtained from Co-ZSM-5 and Co-FER, the two most widely studied catalysts for this reaction. At T ≥ 500 oC, Co-CLI is much more active for NO reduction than Co-ZSM-5, but it is less active than Co-FER. However, it is found that the selectivity for CH₄ toward reacting with NO at high temperatures (T ≥ 450 oC) is higher on Co-CLI than on Co-ZSM-5 or Co-FER. On the basic of the overall catalytic results, the importance of zeolite pore size as a structural parameter influencing the activity of intrazeolitic Co²⁺ ions for NO reduction by CH₄ is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methane steam reforming for synthetic diesel fuel production from steam-hydrogasifier product gases

Steam-methane reforming (SMR) reaction was studied using a tubular reactor packed with NiO/γ-Al₂O₃ catalyst to obtain synthesis gases with H₂/CO ratios optimal for the production of synthetic diesel fuel from steam-hydrogasification of carbonaceous materials. Pure CH₄ and CH₄-CO₂ mixtures were used as reactants in the presence of steam. SMR runs were conducted at various operation parameters. Increasing temperature from 873 to 1,023 K decreased H₂/CO ratio from 20 to 12. H₂/CO ratio decreased from 16 to 12 with pressure decreasing from 12.8 to 1.7 bars. H₂/CO ratio also decreased from about 11 to 7 with steam/CH₄ ratio of feed decreasing from 5 to 2, the lowest limit to avoid severe coking. With pure CH₄ as the feed, H₂/CO ratio of synthesis gas could not be lowered to the optimal range of 4–5 by adjusting the operation parameters; however, the limitation in optimizing the H₂/CO ratio for synthetic diesel fuel production could be removed by introducing CO₂ to CH₄ feed to make CH₄-CO₂ mixtures. This effect can be primarily attributed to the contributions by CO₂ reforming of CH₄ as well as reverse water-gas shift reaction, which led to lower H₂/CO ratio for the synthesis gas. A simulation technique, ASPEN Plus, was applied to verify the consistency between experimental data and simulation results. The model satisfactorily simulated changes of H₂/CO ratio versus the operation parameters as well as the effect of CO₂ addition to CH₄ feed.     

Sequential production of H2 and CO over supported Ni catalysts

Methane decomposition into H₂ and carbon nanofibers at 823 K and subsequent gasification of the carbon nanofibers with CO₂ into CO at 923 K were performed over supported Ni catalysts (Ni/SiO₂, Ni/TiO₂ and Ni/Al₂O₃). Supported Ni catalysts were deactivated for CH₄ decomposition with time on stream due to deposition of a large amount of carbon nanofibers. Subsequent contact of CO₂ with carbon nanofibers on the deactivated catalysts resulted in the formation of CO with a conversion of the carbons higher than 95%. In addition, gasification with CO₂ regenerated the activity of supported Ni catalysts for CH₄ decomposition, indicating that H₂ formation through CH₄ decomposition and CO formation through gasification with CO₂ could be carried out repeatedly. Conversions of carbon nanofibers into CO were kept higher than 95% in the repeated gasification over all the catalysts, while change in the catalytic activity for CH₄ decomposition with the repeated cycles depended on the kind of catalytic supports. Catalytic activity of Ni/SiO₂ for CH₄ decomposition was high at early cycles, however, the activity decreased gradually with the repeated cycles. On the other hand, Ni/TiO₂ and Ni/Al₂O₃ showed high activity for CH₄ decomposition and the activity was kept high during the repeated cycles. These changes of catalytic activities for CH₄ decomposition could be explained by changes in particle sizes of Ni metal, i.e. Ni metal particles in Ni/SiO₂ aggregated into ones larger than 150 nm with the repeated cycles, while the particle sizes of Ni metal in Ni/TiO₂ and Ni/Al₂O₃ remained at an effective range for CH₄ decomposition (60-100 nm).     

The electronic effects of Pr on La1−xPrxNiAl11O19 for CO2 reforming of methane

The CO₂ reforming of CH₄ to synthesis gas by using praseodymium modified hexaaluminate La_1−xPr_xNiAl₁₁O₁₉ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) as catalysts was studied. The modifier Pr improved the reducibility and catalytic activity of Ni ions as active component in the hexaaluminate lattices, especially the conversion of CH₄ and CO₂ reached 89.62% and 92.94%, respectively over La_0.8Pr_0.2NiAl₁₁O₁₉. It was found that the addition of Pr can promote the electronic transformation between the Ni ions and the La ions to maintain Ni at a lower valence, which promotes the activation of CH₄.     

Methane production from anaerobic soil amended with rice straw and nitrogen fertilizers

Laboratory experiments were conducted on the effects of rice straw application and N fertilization on methane (CH₄) production from a flooded Louisiana, USA, rice soil incubated under anaerobic conditions. Rice straw application significantly increased CH₄ production; CH₄ production increased in proportion to the application rate. Urea fertilization also enhanced CH₄ production. The maximum production rate was 17% higher, and occurred 1 week earlier, than that of soil samples which did not receive urea, possibly due to the increase in soil pH following urea hydrolysis. The increase in soil pH following urea hydrolysis may have stimulated CH₄-generating bacteria by providing more optimal soil pH conditions or contributed to the drop in redox potential (Eh). The significant decrease in both the production rate and the total amount of CH₄ by application of NH₄NO₃ was associated with increases in soil Eh after addition of this oxidant. Addition of 300 mg. kg^–1 NO ₃ ^- -N increased soil Eh by 220 mV and almost completely inhibited CH₄ production. However, this inhibitory effect was short-termed. Soon after the applied NO ₃ ^- -N was reduced through denitrification, CH₄ production increased. When (NH₄)₂SO₄ was applied, the inhibition of CH₄ production was not associated with an increase in soil Eh which did not change significantly. A direct inhibitory effect of sulphate on methanogenesis might have been more important.     

Methanol interaction with NO2: An attempt to identify intermediate compounds in CH4-SCR of NO with Co/Pd-HFER catalyst

The objective of this work is the study of fundamental common aspects of NO_x catalytic reduction over a Co/Pd-HFER zeolite catalyst, using methanol or methane as reducing agent. Temperature Programmed Surface Reaction (TPSR) studies were performed with reactant mixtures comprising NO₂ and one of the reducing agents.The formation of formaldehyde was detected in both studied reactions (NO₂–CH₄ and NO₂–CH₃OH) in the temperature range between 100 and 220 °C. At higher temperature, when the NO_x reduction process effectively begins, formaldehyde starts to be consumed.Using methanol as reducing agent, nitromethane and nitrosomethane, are detected. At 300 °C these species are consumed and cyanides and iso-cyanides formation occurs. On the contrary, with methane, these last species were not detected; however, there are strong evidences for CH₃NO and CH₃NO₂ formation.Thus, using methanol or methane, similar phenomena were detected. In both cases, common intermediary species seem to play an important role in the NO_x reduction process to N₂.These results suggest that methanol can be considered as a reaction intermediate species in the mechanism of the reduction of NO₂ with methane, over cobalt/palladium-based ferrierite catalysts.