Activation Energy For Methanation Research Articles

Determining the global activation energy of methane–air premixed flames

In this work, the determination of the apparent activation energy of a global chemical reaction mechanism of the methane–air flames is revisited. The one-step formulation allows to derive the theoretical background for the method to measure the activation energy within the burner stabilised flame setup. The validity of this approach is demonstrated by using the numerical simulations with the detailed reaction model and direct thin filament pyrometry measurements of the temperature distribution in flame. The combination of numerical and experimental approaches allows us to find the activation energy for various mixture compositions. The prediction of numerical simulations and measured values of the activation energy is found to be in good agreement with each other and the data known from the literature. It is demonstrated that two critical phenomena need to be taken into account to obtain a reliable estimate of the activation energy: the flame blow-off and the onset of the diffusive-thermal instabilities. The effect of these critical events on the accuracy of the measurements is discussed as well as prospects of further investigation.

Insight into the role of preparation method on the structure and size effect of Ni/MSS catalysts for dry reforming of methane

Dry reforming of methane (DRM) is considered a promising process to convert CH4 and CO2 into syngas for achieving carbon neutrality. However, sintering and carbon deposition of Ni pose significant challenges to the industrialization of DRM. The size of Ni particles significantly affects the generation of carbon deposits. In this study, a series of Ni/MSS catalysts were prepared using four different methods, including common impregnation, glycine-assisted impregnation, ethylene glycol-assisted impregnation and ammonia evaporation. The effects of preparation methods on the anti-sintering and anti-coking properties were explored through various characterization techniques. Results showed that glycine-assisted impregnation and ethylene glycol-assisted impregnation effectively improved the dispersion of Ni, resulting in small Ni particles while preserving the unique pore structure of MSS supports. Smaller Ni particles expose more active sites, which facilitate the resistance to carbon accumulation. These catalysts show the highest activity and excellent stability. The catalyst prepared by ammonia evaporation showed the best stability due to the synergistic effect of the strongest basicity and metal-support interaction. However, nickel phyllosilicate generation consumed a small amount of MSS, resulting in reduced specific surface area. Compared with the common impregnation method, the other three methods reduced the particle size of Ni and improved the interaction between support and metal, effectively enhancing the ability of the catalyst to resist coking and sintering. Kinetic studies also showed a significant decrease in the apparent activation energy of methane and carbon dioxide cracking due to the reduced particle size of Ni particles.

Effect of different valence metals doping on methane activation over La2O3(001) surface

La2O3 as a catalyst is used for oxidative coupling of methane (OCM) reactions due to its excellent stability and high C2 selectivity, but poor activity on methane dissociation limits its wide application. Different valence metals are doped on the La2O3(001) surface to improve the methane conversion activity, and the activation of methane on metal-doped La2O3(001) surfaces has been investigated via the density functional theory (DFT) calculations. The relationship between the valence states of doped metals and the methane conversion activities shows that doping low valence metals (Li, Na, K, Mg, Ca, Sr and Ba) and equivalent metals (Al, Ga, In) can significantly improve the conversion activity of methane. Among them, the activation energy of methane on the Li-La2O3(001) surface is the lowest, which is only 13.0 kJ/mol. However, doping of high valence metals (Zr, Nb, Re and W) cannot improve the CH4 dissociation activity. Furthermore, the relationships between surface oxygen vacancy formation energies, acid-base properties and the activation energies of CH4 have also been investigated. The results show that with the increase of metal valence state, the oxygen vacancy formation energy increases, while the dissociation activity of CH4 decreases. The introduction of alkali and alkaline earth metals increases the alkalinity of La2O3(001) surface, and the alkalinity of La2O3(001) doped with the alkali metal is stronger than that with the alkaline earth metal, exhibiting higher dissociation activity of CH4. Our research may provide a guide for improving methane conversion activity on La2O3 catalysts.

Nickel Hydroxide Nanosheets Prepared by a Direct Manual Grinding Strategy for High-Efficiency Catalytic Combustion of Methane.

Nickel hydroxide nanosheets were prepared by a very simple direct manual grinding strategy and then calcined at 200, 300, 400, and 500°. The synthesized samples were tested in lean methane (1.0% CH4, air balanced) catalytic combustion and subjected to a series of physical and chemical characterizations. The sample calcined at 200 °C (Ni(OH)2-200) presented a typical nanosheet structure and the best methane catalytic activity in all the samples, which can completely catalyze methane at 400 °C. The crystal structure changed from β-Ni(OH)2 to NiO at a calcination temperature of 300 °C. The β-Ni(OH)2 nanosheets began to partially agglomerate into nanoparticles at 400 °C and almost transformed into nanoparticles at 500 °C. Interestingly, the original nanosheet samples Ni(OH)2-200 and NiO-300 still maintained their morphology and structure although they all went through an activity test at 500 °C in a 1.0% CH4 atmosphere, which proves that the calcination of nanosheets in a CH4 atmosphere tended to maintain their nanosheet morphology compared with calcination in the air. Furthermore, through the activity test, X-ray photoelectron spectroscopy results, TPx, and in situ DRIFTS characterization, it was proved that the hydroxyl groups on the Ni(OH)2-200 and NiO nanosheets were beneficial to the dissociation of methane on the catalyst surface, and the nanosheet structure was also prone to generating more active adsorbed oxygen, so the activation energy of methane was lowered. A methane catalytic mechanism on the Ni(OH)2 nanosheets and NiO nanoparticles was proposed, which further proved the key role of hydroxyl groups in methane combustion.

Gas content of the D6 coal seam

The article deals with the issues of gas content of the most thick and stable D6 coal seam in the Tentek region. This complex structure seam is dangerous in underground mining for gas and dust outbursts, it consists of coal packs separated by interlayers of mudstones, while the lower layer 0.5-1.5 m thick is very soft, has a strong shear disturbance and is most saturated with methane. Extraction of coalbed methane is a necessary process to ensure the safety of mining operations, to reduce its emissions into the atmosphere, and to utilize it as a fuel and a product for obtaining synthetic materials. The regularity of changes in the particle size distribution of the upper thick pack and the lower thin pack indicates the difference in small coal particles in them, while there are much more of them in the lower layer therefore, the specific surface is larger, which is an important factor of the adsorption processes intensity in the accumulation of methane, and during the gas drainage from the seam. The activation energy of methane from carbohydrate has been determined. A quadratic relationship between methane gas evolution and its initial concentration has been shown. In carbohydrate, it depends on the energy of external forces. A complex relationship has been established between the gas pressure in the coal seam and its concentration and characteristics of the «coal-methanenatural moisture» system. Regularities of changing the methane content of the coal seam depending on its fracturing formed due to the effect of the energy of destruction and the energy appearing with increasing the area of a crack in the coal, have been obtained. The effect of the coal mineral composition on the gas content has been shown.

Efficient methane-to-acetylene conversion using low-current arcs.

The proliferation of natural gas production had led to increased utilization of methane as a raw material for chemicals. The most significant bottleneck in this process is the high activation energy of methane. This paper reports the direct conversion of methane to acetylene in a novel rotating arc driven by AC electrical power. By feeding a sufficiently high concentration of CH4 (greater than 43%) diluted in H2 (the discharge gas) through the arc column, a low specific energy requirement (SER) of 10.2 kW h kg−1 C2H2 was achieved. The use of hydrogen as the discharge gas strongly suppressed soot formation during the methane conversion process under high methane concentration conditions, resulting in a carbon balance of greater than 95% and a C2H2 selectivity of greater than 90% while maintaining a methane conversion rate of greater than 70%, depending on the conditions. The novel rotating arc enabled the elongation of the arc column itself, which controlled heat loss and improved the energy use for reaction. The ability to control the arc length based on low-current type arc generation has additional benefits for reaction enhancement. These results demonstrate that arc control, optimization of the reaction conditions, and a full understanding of reaction pathway are viable means for the energy-efficient direct conversion of methane to acetylene.

Equilibrium and Kinetics of Methane and Ethane Adsorption in Carbon Molecular Sieve

Abstract The comprehensive study involving uptake and kinetics behaviour of methane and ethane was carried out on a commercial carbon molecular sieve. The gravimetric technique was applied to measure the adsorption isotherms at different temperatures. The kinetic behaviours of methane and ethane were investigated by extracting the uptake data from isotherms at different pressure steps. Considering CMS as a homogenous microporous adsorbent, only two main resistances: surface barrier (at the pore mouth) and diffusion resistance (inside the pores) were considered in analysing the kinetic data. The kinetic analysis showed dominance of surface barrier resistance at all temperatures and pressures investigated in this study. Modified Darken equation was used to calculate the sorbates’ activation energies for adsorption in CMS pores. Compared to Darken equation, the modified Darken model predicted the concentration dependency of the surface rate constants more accurately, especially at higher sorbates’ concentrations. The activation energy of methane and ethane was greater than the heat of adsorption demonstrating a large impedance and barrier at the pore surface to the methane and ethane molecules. The kinetic selectivity of methane over ethane was calculated from the combined surface barrier/diffusion model parametric analysis. The results generally showed a greater selectivity to methane over ethane, i. e., the higher mass transfer rates.

? Nonthermal Plasma-enhanced Catalytic Methanation of CO over Ru/TiO2/Al2O3

Catalytic methanation of carbon monoxide (CO) was investigated in a nonthermal plasma (NTP) reactor packed with Ru/TiO2/Al2O3 catalyst pellets. With the NTP alone, no conversion of CO was observed over the entire temperature range of 240 300 ◦C, but the combination of plasma and catalyst was found to greatly improve the catalytic conversion of CO, particularly in the lower temperature region below 280 ◦C. Higher CO conversion efficiencies were obtained at higher applied voltages because of increased NTP action in the catalytic reactions. The increase in the voltage resulted in a decrease in the activation energy for methanation. Reaction products, including CH4 and CO2, were identified by using a Fourier transform infrared (FTIR) spectrometer. The increases in the applied voltage and in the temperature tended to slightly bring down the selectivity toward CH4, it being more than 0.93 under all experimental conditions explored.

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Abstract Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40% , laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.

Cracking conditions of crude oil under different geological environments

There are mainly 3 kinds of existing states of oil generating from source rocks, that is, dispersive liquid hydrocarbon inside of source rock, dispersive liquid hydrocarbon outside of source rock and concentrated liquid hydrocarbon outside of source rock. Because of the differences in thermal history and medium conditions around, and the interaction of organic and inorganic matter, the liquid hydrocarbon with 3 kinds of existing state has different cracking conditions. The gas generation dynamics experiments of crude oil matching different mediums indicate that the distribution of activation energy of methane changes a lot according to medium difference. The carbonate has a main influence on oil cracking conditions and can largely reduce its activation energy, which reflects the lower cracking temperature of crude oil. The mudstone takes a second place and the sandstone is the smallest. The catalytic cracking function to the oil of the carbonate, of the mudstone and of the sandstone changes weaken in turn. The corresponding Ro values of main gas generation period in different mediums are as follows: 1.5%–3.8% with pure crude oil, 1.2%–3.2% with dispersive crude oil in carbonate, 1.3%∼3.4% with dispersive crude oil in mudstone and 1.4%–3.6% with dispersive crude oil in sandstone. The influence of pressure to crude oil cracking is relatively complicated. In the low heating speed condition, pressure restrains the oil cracking and gas generation, but in the high heating speed condition, pressure has an indistinctive influence to the oil cracking and gas generation. Pressure also makes a different effort in different evolvement stage. Taking the middle and lower Cambrian source rocks in the Tarim Basin as an example, primary oil generating quantity is 2232.24×108t, residual oil and oil cracking gas quantity is 806.21×108t and 106.95×1012m3 respectively.

A Kinetic Study on the Adsorption and Reaction of Hydrogen over Silica-Supported Ruthenium and Silver–Ruthenium Catalysts during the Hydrogenation of Carbon Monoxide

The simultaneous adsorption and reaction of hydrogen with preadsorbed carbon monoxide was investigated over a series of silica-supported ruthenium and silver–ruthenium bimetallic catalysts to elucidate the role of site sensitivity on hydrogen adsorption and methane formation. The specific rate of methane synthesis, which is a direct measure of the catalyst activity toward the Fischer–Tropsch synthesis, measured at 460 Torr and temperatures from 400 to 500 K, varied from 0.01×10−3 to 3×10−3 s−1. As little as 3% Ag reduced the methanation rate of Ru by 80%. The temperature dependence of the turnover frequency showed that the apparent activation energy for methanation dropped from 24 kcal mol−1 for the monometallic ruthenium case to 18 kcal mol−1 for the bimetallic case. Since silver does not adsorb or react with either hydrogen or carbon monoxide, these results show that silver does not act merely as a dilutant in the bimetallic system. These observations are elucidated in terms of a new model for surface-sensitive hydrogen adsorption, termed “portal site mediated adsorption,” where low-coordination edge and corner sites on the catalyst surface act as sinks for rapid, dissociative adsorption of weakly bound, highly mobile surface hydrogen. In situ1H-NMR measurements of surface hydrogen coverages during reaction were used to confirm the kinetic model treatment of the reaction data.

Some peculiarities in regeneration of zeolite-containing cracking catalysts by carbon dioxide

Catalytic properties of Mn and La oxides that are acceptors of CO2 in regeneration of zeolite-containing cracking catalysts in the presence of carbon dioxide have been studied. Regeneration rate of coked catalysts by carbon dioxide in the presence of Mn and La significantly increases. During regeneration, coke oxidation to Co and redistribution of hydrogen in carbonaceous fragments take place to produce methane and hydrogen. Activation energy of methane and hydrogen formation equals 46 kcal/mol. Variation of activation energy with temperature is typical for Co formation.

Methanation on exfoliated and supported MoS2

The methanation reaction on MoS2 exfoliated to a thickness of a few layers or less and adsorbed onto alumina is found to be very small. However, by calcining and resulfiding the exfoliated MoS2 catalysts, greatly improved performance is achieved that is at least equal to the commercial catalysts based on ammonium heptamolybdate. The creation of molybdenum oxysulflde surface species therefore appears to be a necessary step toward producing significant methanation rates with exfoliated and supported MoS2. The methanation products are almost exclusively CO2 and CH4, their mole ratios near unity, with otherwise only very much smaller amounts of longer chain hydrocarbons. The activation energy for methanation is generally observed to be near 100 kJ/mol, with the overall reaction being first order in the carbon monoxide concentration and third order in the hydrogen concentration. In contrast to the transition-metal catalysts, no water could be detected in the reaction products of the molybdenum based catalyst. Keywords: methanation reaction on MoS2, exfoliated and supported MoS2 as catalyst.

Solubility and diffusion of methane in coal strata

The authors study the solubility and diffusion of methane in coal strata and determine the absorption activation energy of methane experimentally between 133 and 323/sup 0/K. These experiments were made on samples of coal, degassed at 60/sup 0/C. The relation between initial and equilibrium swelling of anthracites in carbon dioxide is shown. The sorption of methane by superanthracites is also presented.

The effects of titania and alumina overlayers on the hydrogenation of CO over rhodium

Submonolayer quantities of TiOx and AlOx were deposited on a polycrystalline Rh foil. The samples were characterized by Auger electron spectroscopy and then enclosed in an atmospheric-pressure chamber to evaluate their activity for CO hydrogenation. AlOx deposition suppresses the rate of CO hydrogenation in direct proportion to the AlOx coverage, but the product distribution and kinetics are unaffected. These observations indicate that the only effect of AlOx is to block Rh sites. TiOxdeposition has a more complex effect. With increasing TiOx coverage, the methanation rate passes through a maximum at a TiOx coverage of 0.15 monolayer. The maximum rate is three times as large as the rate of clean Rh. The deposited TiOx also affects the catalyst selectivity as well as the kinetics of methane formation. The selectivity for methane reaches a minimum for a TiOx coverage of ca. 0.20 monolayer and in the same range the activation energy for methanation reaches a minimum and the H2 and CO partial pressure dependences reach a maximum. The unusual behaviour of TiOx-promoted Rh is attributed to the occurrence of highly active catalytic sites near the perimeter of the TiOx islands making up the overlayer.

Influence of sulfur on methanation over tungsten(110)

The effect of adsorbed sulfur on the rate of methanation over W(110) is reported for a range of sulfur coverages (0.03-0.55 monolayers) and temperatures (600-750 K) for 1 Torr of CO and 100 Torr of hydrogen. The rate drops sharply for low sulfur coverages (by 40% at 0.05 monolayer) but decreases less dramatically for higher coverages. This contrasts with the data of Goodman and Kiskinova for Ni, which exhibits an order of magnitude drop for 0.05 monolayer S. Postreaction Auger spectra reveal that the sulfur-free surface for this sample is covered with both carbidic carbon and oxygen and is oxygen-rich. Increasing sulfur coverages leads to little change in the postreaction carbon level but does cause the oxygen coverage to fall substantially, to zero by 0.40 monolayer S. The activation energy for methanation is unaffected by the presence of sulfur, suggesting that the decrease in rate is caused by sulfur limiting the saturation coverages that can be attained by the reactants. This possibility motivated a thermal desorption study of CO and hydrogen adsorption on sulfur precovered surfaces. Good agreement is obtained between the changes in these saturation coverages and the decrease in methanation rate due to the presence of sulfur whenmore » small coverages of coadsorbed oxygen are present. The coverage of coadsorbed oxygen used in the thermal desorption measurements was based on the postreaction Auger spectra. These results taken as a whole suggest that the inhibiting effect of sulfur appears to be suppressed by the formation of sulfur clusters above approx. 0.05 monolayer S.« less

CO methanation on low-weight loading [formula omitted]: Multiple reaction sites

Temperature-programmed reaction (TPR) with mass spectrometric detection was used to study the hydrogenation of adsorbed carbon monoxide on low-weight loading Ni Al 2O 3 catalysts. These catalysts are shown to differ from high-weight loading nickel/alumina catalysts and are shown to have two distinct pathways for carbon monoxide methanation. These pathways are attributed to two forms of adsorbed CO. The CO site which reacts at low temperature (446 K) appears to be from carbon monoxide adsorbed on nickel atoms that are bonded to other nickel atoms. It has an activation energy for methanation of 51 kJ/mol. The CO site that reacts at high temperature (546 K) has an activation energy of 145 kJ/mol. The high-temperature site, which is sensitive to the pretreatment temperature, results from CO adsorbed on nickel atoms that are interacting strongly with an oxide phase of the catalyst. Interconversion of CO between the two sites occurs and is influenced by hydrogen. Neither of the pathways appears limited by direct hydrogenation of surface carbon; this step is shown to be slightly faster under TPR conditions. The ability of TPR to measure specific reaction rates and to separate reaction pathways that might be obscured in steadystate kinetic measurements is demonstrated.

Theoretical investigation of the elimination and addition reactions of methane and ethane with nickel

Contracted CI calculations have been performed to study the mechanisms for the concerted elimination and addition reactions of ethane and methane with nickel. It is found that the /sup 1/A/sub 1/ state of nickel-ethane and the /sup 1/A' state of nickel-methane behave very similarly. The elimination barrier, for example, is computed to be 34 kcal/mol for methane and 37 kcal/mol for ethane. For the addition reaction a somewhat larger difference is obtained, the activation energy of methane being 54 kcal/mol and of ethane (breaking the C-C bond) 42 kcal/mol. This difference in activation energy can be explained by the difference in the bond strength of the C-H bond in methane and the C-C bond in ethane, the C-H bond being 16 kcal/mol stronger. The difference between these two systems on one side and H/sub 2/ on the other is large; H/sub 2/ has a computed addition barrier to nickel of only 3 kcal/mol. The hydrogen atoms can much more easily form bonds in several directions at the same time than methyl groups, and this difference leads to higher activation energies for reactions involving methyl groups. For all three systems the reaction takes place with nickel in a d/sup 9/ statemore » and with a large extent of d involvement in the bonding at the bent equilibrium geometry. 23 references, 3 figures, 5 tables.« less

A surface science investigation of the role of potassium promoters in nickel catalysts for CO hydrogenation

The role of potassium promoters in model Ni(100) catalysts for CO hydrogenation has been studied. High-pressure kinetic measurements of the H 2 + CO reaction on Ni(100) containing well-controlled submonolayer quantities of potassium adatoms have been combined with detailed surface analysis performed before and after reaction. Potassium addition decreases the steady-state rate of methane formation and increases that for higher hydrocarbons relative to clean Ni(100). These same results are reported for supported, high-surface-area Ni catalysts, indicating that metal/support interactions are not necessary in achieving the promoter effect. The activation energy for methanation ( ∼ 25 kcal mole −1) does not depend upon potassium coverage, suggesting that K changes neither the reaction mechanism nor the rate-limiting step. Surface carbide, a vital reaction intermediate, increases sharply in coverage upon the addition of 0.10 monolayer potassium. This is shown to result from a marked decrease in the activation energy for CO dissociation effected by potassium. The catalyst activity and selectivity are discussed in light of these results.

Sulfur deactivation of nickel methanation catalysts

Sulfur deactivation of supported Ni in CO hydrogenation was studied in an all-quartz internalrecycle reactor with a feed containing 4% CO in H 2. Thirteen ppb H 2S reduced the steady-state methanation activity of Ni γ- Al 2 O 3 about 200-fold at 661 K; 100 ppb H 2S reduced the activity 5000-fold. A dual site Langmuir-Hinshelwood rate expression predicts both the CO partial pressure dependence and the S poisoning. Poisoning and chemisorption data indicate formation of a stable two-dimensional, surface sulfide with a S: Ni surface atom ratio of 1: 2 for 13 ppb H 2S in H 2 at 661 K. The surface sulfide has a free energy of formation of at least −26 kcal/mole which is 15 kcal/mole more stable than bulk Ni 2S 3. Sulfur poisoning is due to geometric effects, i.e., site blockage, rather than electronic effects since the activation energy for methanation over S-poisoned Ni was the same as that over unpoisoned Ni, 24 kcal/mole.