The main factors controlling generation of synthetic natural gas by methanation of synthesis gas in the presence of sulfur-resistant Mo-based catalysts

The Co-Mo-Al and Co-Mo-Ce-Al catalysts were prepared and tested for their activity in the methanation of synthesis gas in the presence of hydrogen sulfide. The results showed that the Co-Mo-Ce-Al series was superior to the Co-Mo-Al system in terms of CO conversion. The former system was used to examine the main factors controlling the methanation behavior. Among these are: H₂S concentration in the reaction mixture, reaction temperature and pressure, concentrations of CO₂, CH₄, and H₂O, H₂/CO ratio, and gaseous hourly space velocity. The methanation activity increased with increasing temperature, pressure and H₂/CO ratio. The reason why adding CO₂ or H₂O decreases the methanation activity is discussed.     

The oxidative stream reforming of methane to syngas in a thin tubular mixed-conducting membrane reactor

The oxidative stream reforming of methane (OSRM) to syngas, involving coupling of exothermic partial oxidation of methane (POM) and endothermic steam reforming of methane (SRM) processes, was studied in a thin tubular Al₂O₃-doped SrCo_0.8Fe_0.2O_3−δ membrane reactor packed with a Ni/γ-Al₂O₃ catalyst. The influences of the temperature and feed concentration on the membrane reaction performances were investigated in detail. The methane and steam conversions increased with increasing the temperature and high conversions were obtained in 850–900 °C. Different from the POM reaction, in the OSRM reaction the temperature and H₂O/CH₄ profoundly influenced the CO selectivity, H₂/CO and heat of the reaction. The CO selectivity increased with increasing the temperature or decreasing the H₂O/CH₄ ratio in the feed owing to the water gas shift reaction (H₂O + CO → CO₂ + H₂). And the H₂ selectivity based on methane conversion was always 100% because the net steam conversion was greater than zero. The H₂/CO in product could be tuned from 1.9 to 2.8 by adjusting the reaction temperature or H₂O/CH₄. Depending on the temperature or H₂O/CH₄, furthermore, the OSRM process could be performed auto-thermally with idealized reaction condition.     

Conversion of CH4 Catalyzed by Gas Phase Ions Containing Metals

Methane is an abundant and cheap feedstock to produce valuable chemicals. The catalytic reaction of methane conversion generally requires the participation of multiple molecules (such as two or three CH₄ molecules, O₂, CO₂, etc.). Such complex process includes the cleavage of original chemical bonds, formation of new chemical bonds, and desorption of products. The gas phase study provides a unique arena to gain molecular-level insights into the detailed mechanisms of bond-breaking and bond-forming involved in complicated catalytic reactions. In this Review, we introduce the methane conversion catalyzed by gas phase ions containing metals and three topics will be discussed: (1) the direct coupling of methane molecules, (2) the conversion of CH₄ with O₂, O₃ and N₂O, and (3) the conversion of CH₄ with CO₂ and H₂O. The obtained mechanistic aspects may provide new clues for rational design of better-performing catalysts for conversion of methane to value-added products.     

The Mechanistic Study of Methane Reforming with Carbon Dioxide on Ni/α-Al2O3

The reactions of oxidized and reduced 6 wt % NiO/-Al₂O₃ with H₂, CH₄, CO₂, O₂, and their mixtures are studied in flow and pulse regimes using a setup equipped with a differential scanning calorimeter DSC-111 and a system for chromatographic analysis. It is shown that treatment with hydrogen at 700° results in the partial reduction of NiO to Ni. Methane practically does not react with oxidized Ni/-Al₂O₃ but it does react actively with the reduced catalyst to form H₂ and surface carbon. The latter is capable of reacting with lattice oxygen of Ni/-Al₂O₃ (slowly) and with adsorbed oxygen (rapidly). Carbon dioxide also reacts with surface carbon to form CO (rapidly) and with metallic Ni to yield CO and NiO (slowly). Thus, the main route of methane reforming with carbon dioxide on Ni/-Al₂O₃ is the dissociative adsorption of CH₄ to form surface carbon and H₂ and the reaction of this carbon with CO₂ resulting in the formation of CO by the reverse Boudouard reaction. Side routes are the interaction of the products of methane chemisorption with catalyst oxygen and the dissociative adsorption of CO₂ on metallic nickel. A competitive reaction of surface carbon with adsorbed oxygen results in a decrease in the CO₂ conversion in methane reforming with carbon dioxide. Therefore, the presence of gaseous oxygen in the reacting mixture decelerates methane reforming (catalyst poisoning by oxygen).     

Optimizing the parameters of the steam-carbon dioxide conversion of methane by Gibbs energy minimization

The thermodynamic equilibrium for the steam-carbon dioxide conversion of methane was studied by Gibbs energy minimization. The degree of coke formation, the content of methane and carbon dioxide in the synthesis gas, and the synthesis gas H₂/CO ratio were plotted as functions of the molar ratios of CO₂/CH₄ and H₂O/CH₄ in the initial mixture at different temperatures and pressures. The regions of the optimum CH₄/CO₂/H₂O molar ratios for steam-carbon dioxide conversion were discovered, with no coke formation taking place in these regions. The optimized CH₄/CO₂/H₂O molar fractions characterized by the minimum content of methane and carbon dioxide in the synthesis gas were found for each region.     

Methane Reforming with Carbon Dioxide on the Co/α-Al2O3 Catalyst: The Formation, State, and Transformations of Surface Carbon

The interactions of oxidized and reduced Co/-Al₂O₃ (4 wt % CoO) with H₂, CH₄, CO₂, and O₂ and their mixtures are studied in flow and pulse regimes using a setup involving a DSC-111 differential scanning calorimeter and a system for chromatographic analyses. It is shown that treatment with hydrogen at 700°C results in the partial reduction of cobalt oxide to Co. Methane poorly reacts with the oxidized catalyst but readily reacts with the reduced catalyst to form H₂ and surface carbon. The initial surface carbon transforms into other forms, which block the cobalt surface to different extents and differ in the heats of reaction with CO₂. Carbon dioxide may react with the surface carbon to form CO (rapidly) and with metallic Co to form CO and CoO (slowly). Thus, the main route of methane reforming with carbon dioxide on Co/-Al₂O₃ is the dissociative adsorption of CH₄ to form surface carbon and H₂ and the reaction of surface carbon with CO₂ to form CO via the reverse Boudouard reaction.     

Novel Ni/CeO2-Al2O3 composite catalysts synthesized by one-step citric acid complex and their performance in catalytic partial oxidation of methane

A series of novel Ni/CeO2-Al2O3composite catalysts were synthesized by one-step citric acid complex method. The as-synthesized catalysts were characterized by N2physical adsorption/desorption, X-ray diffraction(XRD), Fourier transform infrared(FT-IR) spectroscopy, hydrogen temperature-programmed reduction(H2-TPR), X-ray photoelectron spectroscopy(XPS) and thermogravimetry analysis(TGA). The effects of nickel content, calcination and reaction temperatures, gas hourly space velocity(GHSV) and inert gas dilution of N2on their performance of catalytic partial oxidation of methane(CPOM) were investigated. Catalytic activity test results show that the highest methane conversion(85%), the best selectivities to carbon monoxide(87%) and to hydrogen(95%), the excellent stability and perfect H2/CO ratio(2.0) can be obtained over Ni/CeO2-Al2O3with 8 wt% Ni content calcined at 700 ℃ under the reaction condition of 750 ℃, CH4/O2ratio of 2 : 1 and gas hourly space velocity of 12000 mL h-1 g-1. Characterization results show that the good catalytic performance of this composite catalyst can be contributed to its large specific surface area(～108 m2 g-1), small crystallite size, easy reducibility and low coking rate.     

Syngas production in a novel perovskite membrane reactor with co-feed of CO2

Partial oxidation of methane（POM） co-fed with CO₂ to syngas in a novel catalytic BaCo_0.6Fe_0.2Ta_0.2O_3-δ oxygen permeable membrane reactor was successfully reported.Adding CO₂ to the partial oxidation of methane reaction not only alters the ratio of CO/H₂,but also increases the oxygen permeation flux and CH₄ conversion.Around 96%CH₄ conversion with more than 93%CO₂ conversion and 100%CO selectivity is achieved,which shows an excellent reaction performance.A steady oxygen permeation flux of 15 mL/（cm² min） is obtained during the 100-h operation,which shows good stability as well.     

Decomposition of methane on polycrystalline thick films of Ga2O3 investigated by thermal desorption spectroscopy with a mass spectrometer

Methane in air can be detected by the conductivity increase of Ga₂O₃ films. Films (200 μm) of β-Ga₂O₃ were prepared by depositing a suspension of β-Ga₂O₃ powder (Johnson Matthey; 32102; 99,99%) on alumina substrates. The films were exposed to 20 kPa O₂ for 15 min at 934 K. In thermal desorption spectroscopy (TDS, β = 4,6 K/s, UHV conditions) only O₂ occured at temperatures above 934 K. On reduction in 100 Pa H₂ for 5 min at 800 K, only a suboxide, Ga₂O (above 880 K), indicating a destabilisation of the lattice [1], a broad hydrogen peak (440–930 K) and the formation of water (700–900 K) were observed. No Ga₂O₃ and O₂ were found in desorption. At temperatures between 260 K and 934 K the film was exposed to methane (100 Pa, 5 min). For exposure temperatures between 630 K and 934 K, CO, CO₂, H₂, and small amounts of CH₄ and the suboxide Ga₂O appeared in desorption. A reaction scheme for the decomposition of methane is proposed. It includes the adsorption of CH₄, the dissociation of CH₄, the desorption of H₂O and the formation of oxygen vacancies. These vacancies and the adsorbed hydrogen both acting as donors may explain the conductance increase on exposure to methane observed by other authors.     

Reaction of methane and sulfur: Oxidative coupling and carbon disulfide formation

Reaction between methane and sulfur vapor (mainly S₂) has been studied at 773–1073 K in the presence of possible catalysts and in the absence of a catalyst. Reaction was carried out in a silica tube reactor at a total pressure of 1 atm with excess methane, with prereaction sulfidation using H₂S/H₂. Sm₂O₃ had significant catalytic effect on CH₄ conversion, but all other potential catalysts (including Li/MgO) gave CH₄ conversion close to that in the absence of catalyst. CS₂ was the major product, accompanied by C₂ coupling product which in most cases was predominantly C₂H₄ with some C₂H₆. In some cases, small amounts of C₃, C₄ hydrocarbons and CH₃SH, (CH₃)₂S, (CH₃)₂S₂ were also found. Experiments with varying space velocity indicated that C₂H₆ was the primary coupling product. Possible reaction pathways are discussed in terms of homogeneous CH₃·generation, augmented by CH₃. generated heterogeneously at the surface of sulfided Sm₂O₃. Sulfidation of Li/MgO is considered to be the reason for the inactivity of this potential catalyst.     

Surface Reaction Kinetics of the Oxidation and Reforming of CH4 over Rh/Al2O3 Catalysts

An 48‐step sur face reaction mechanism with thermodynamic consistent kinetic data is presented for the catalytic conversion of the gaseous chemical system H₂/O₂/H₂O/CO/CO₂/CH₄ over Rh/Al₂O₃ catalysts. Total and partial oxidation as well as steam reforming and dry reforming of methane over Rh catalysts is studied experimentally and numerically at varying temperature and composition. The results are used to extend the kinetic schemes we developed for H₂ oxidation, CO oxidation kinetics, and the water‐gas‐shift reactions in former studies. Aside from the experiments in a stagnation‐flow reactor presented here, we modeled a number of experiments from the literature to test the newly established kinetic scheme.     

Shock‐tube and modeling study of acetaldehyde pyrolysis and oxidation

Pyrolysis and oxidation of acetaldehyde were studied behind reflected shock waves in the temperature range 1000–1700 K at total pressures between 1.2 and 2.8 atm. The study was carried out using the following methods, (1) time‐resolved IR‐laser absorption at 3.39 μm for acetaldehyde decay and CH‐compound formation rates, (2) time‐resolved UV absorption at 200 nm for CH₂CO and C₂H₄ product formation rates, (3) time‐resolved UV absorption at 216 nm for CH₃ formation rates, (4) time‐resolved UV absorption at 306.7 nm for OH radical formation rate, (5) time‐resolved IR emission at 4.24 μm for the CO₂ formation rate, (6) time‐resolved IR emission at 4.68 μm for the CO and CH₂CO formation rate, and (7) a single‐pulse technique for product yields. From a computer‐simulation study, a 178‐reaction mechanism that could satisfactorily model all of our data was constructed using new reactions, CH₃CHO (+M) → CH₄ + CO (+M), CH₃CHO (+M) → CH₂CO + H₂(+M), H + CH₃CHO → CH₂CHO + H₂, CH₃ + CH₃CHO → CH₂CHO + CH₄, O₂ + CH₃CHO → CH₂CHO + HO₂, O + CH₃CHO → CH₂CHO + OH, OH + CH₃CHO → CH₂CHO + H₂O, HO₂ + CH₃CHO → CH₂CHO + H₂O₂, having assumed or evaluated rate constants. The submechanisms of methane, ethylene, ethane, formaldehyde, and ketene were found to play an important role in acetaldehyde oxidation. © 2007 Wiley Periodicals, Inc. 40: 73–102, 2008     

Effect of Nonthermal Plasma on the Methanation of Carbon Monoxide over Nickel Catalyst

Reduction of carbon monoxide to methane by hydrogen was investigated with a nonthermal plasma reactor in which Ni/alumina catalyst pellets was filled. The effect of reaction temperature, pressure and voltage on the conversion of CO was examined. It was found that the nonthermal plasma significantly enhanced the catalytic conversion of CO. The effect of the nonthermal plasma was especially remarkable at lower temperatures and pressures. At high temperatures, the catalyst itself exhibited very high catalytic activity for the conversion of CO. Since high pressure is unfavorable for creating electrical discharge plasma, the increase in pressure lowered the discharge power, thereby weakening the effect of the nonthermal plasma. With the nonthermal plasma alone, there was no conversion of CO. The reaction products identified by FTIR spectra were CH₄, CO₂ and H₂O. FTIR spectra also showed that CO was converted primarily into CH₄ with high selectivity above 90% at most experimental conditions.     

Oscillations during partial oxidation of methane to synthesis gas over Ru/Al2O3 catalyst (Special column of methane transformation, Article)

Oscillations in temperatures of catalyst bed as well as concentrations of gas phase species at the exit of reactor were observed during the partial oxidation of methane to synthesis gas over Ru/Al₂O₃ in the temperature range of 600 to 850 °C. XRD, H₂-TPR and in situ Raman techniques was used to characterize the catalyst. Two types of ruthenium species, i.e. the ruthenium species weakly interacted with Al₂O₃ and that strongly interacted with the support, were identified by H₂-TPR experiment. These species are responsible for two types of oscillation profiles observed during the reaction. The oscillations were the result of these ruthenium species switching cyclically between the oxidized state and the reduced state under the reaction condition. These cyclic transformations, in turn, were the result of temperature variations caused by the varying levels of the strongly exothermic CH₄ combustion and the highly endothermic CH₄ reforming (with H₂O and CO₂) reactions (or the less exothermic direct partial oxidation of methane to CO and H₂), which were favored by the oxidized and the metallic sites, respectively. The major pathway of synthesis gas formation over the catalyst was via the combustion-reforming mechanism.     

Nickel-copper-chromium catalyst for selective methane oxidation to synthesis gas at short residence times

Data on the selective oxidation of methane to synthesis gas on a 9% NiCuCr/2% Ce/(ϑ + α)-Al₂O₃ catalyst in dilute mixtures with Ar at short residence times (2–3 ms) are presented. The composition, structure, morphology, and adsorption properties of the catalyst with respect to oxygen and hydrogen before and after reaction were studied using XRD, BET, electron microscopy with electron microdiffraction, TPR, TPO, and TPD of oxygen and hydrogen. The following optimum conditions for the preparation and pretreatment of the catalyst for selective methane reduction were found: the incipient wetness impregnation of a support with aqueous nitrate solutions; drying; and heating in air at 873 and then at 1173 K (for 1 h at either temperature) followed by reduction with an H₂-Ar mixture at 1173 K for 1 h. At a residence time of 2–3 ms (space velocity to 1.5 × 10⁶ h⁻¹) and 1073–1173 K, the resulting catalyst afforded an 80–100% CH₄ conversion in mixtures with O₂ (CH₄/O₂ = 2: 1) diluted with argon (97.2–98.0%) to synthesis gas with H₂/CO = 2: 1. The selectivity of CO and H₂ formation was 99.6–100 and 99–100%, respectively; CO₂ was almost absent from the reaction products. The catalyst activity did not decrease for 56 h; carbon deposition was not observed. A possible mechanism of the direct oxidation of CH₄ to synthesis gas is considered.     

Catalytic Removal of NOx from Gas Mixtures Simulating Emissions from the Combustion of Biofuels

The activity of knitted silica-fibre supported Pd, Pt, Pt-Ni, Pd-Ni and Pd-Pt-Ni catalysts as well as Pd based H-ZSM-5 and H-ZSM-35 catalysts was studied in the conversion of gas mixtures containing 200 ppm CH₄, 2500 ppm CO, 500 ppm pyridine (or 500 ppm NO), 10 vol.% O₂ (or 0.155 vol.% O₂), 12 vol.% CO₂, 12 vol.% H₂O, balanced with He at GHSV of 60000 h^–1. Pyridine was found to inhibit both CO and CH₄ oxidation. IR studies indicated that NO adsorbed on Pd²⁺ is the principal adsorbed species on the Pd/HZSM-5 catalyst.     

Hydrogen production via CO2-reforming of methane over Cu and Co doped Ni/Al2O3 nanocatalyst: impregnation versus sol–gel method and effect of process conditions and promoter

In this study, Cu and Co doped Ni/Al₂O₃ nanocatalyst was synthesized via impregnation and sol–gel methods. The physiochemical properties of nanocatalyst were characterized by XRD, field emission scanning electron microscopy (FESEM), particle size distribution, BET, fourier transform infrared spectroscopy (FTIR), TG–DTA and energy dispersive X-ray (EDX) analysis. The samples were employed for CO₂-reforming of methane in atmospheric pressure, temperature range from 550 to 850 °C, under various mixture of CH₄/CO₂ and different gas hourly space velocity. XRD patterns besides indicating the decline of the peaks intensity in sol–gel method, proved the potential of this procedure in diminishing the crystal size and preventing the NiAl₂O₄ spinel formation. Moreover, high surface area might derive of smaller particle size and uniform morphology of sol–gel prepared ones, confirmed by FESEM and BET analysis. TG–DTG analysis as well supported the higher surface area for sol–gel made ones, represented the proper calcination temperature (approximately 600 °C). Also, presence of the active phases and elemental composition of nanocatalysts determine via EDX analysis. Promoting the basicity and the adsorption rate of CO₂, is attributed to the higher amount of OH groups for sol–gel prepared samples, proved by FTIR. Ni–Co/Al₂O₃ due to the synergetic effect of sol–gel method and cobalt addition depicted excellent characterization such as higher surface area, smaller particle size, supplying more stable support and enhanced morphology. Therefore, this nanocatalyst represented the best products yield (H₂ = 98.21 and CO = 95.64), H₂/CO close to unit (0.92–1.05) and stable conversion during 1,440 min stability test. So, Ni–Co/Al₂O₃ among all of the prepared nanocatalysts demonstrated the best catalytic performance and presented it as a highly efficient catalyst for dry reforming of methane. Despite of the stable yield of Ni–Cu/Al₂O₃, it depicted the lower catalytic activity and H₂/CO ratio than the unprompted nanocatalysts.     

Photocatalytic Dissociation of CH3OH on ZnO(0001) Surface

Photocatalysis of CH₃OH on the ZnO(0001) surface has been investigated by using temperature-programmed desorption (TPD) method with a 266 nm laser light. TPD results show that part of the CH₃OH adsorbed on ZnO(0001) surface are in molecular form, while others are dissociated. The thermal reaction products of H₂, CH₃^·, H₂O, CO, CH₂O, CO₂ and CH₃OH have been detected. Experiments with the UV laser light indicate that the irradiation can promote the dissociation of CH₃OH/CH₃O^· to form CH₂O, which can be future converted to HCOO^- during heating or illumination. The reaction between CH₃OH_Znand OH_ad can form the H₂O molecule at the Zn site. Both temperature and illumination promote the desorption of CH₃^· from CH₃O^·. The research provides a new insight into the photocatalytic reaction mechanism of CH3OH on ZnO(0001).     

Gas-chromatographic determination of impurities of organic and chlorinated organic compounds and carbon monoxide and dioxide in high-purity hydrogen chloride

A gas-chromatographic procedure was developed for determining impurities (CH₄, C₂H₆, C₃H₈, C₄H₁₀, iso-C₄H₁₀, C₅H₁₂, iso-C₅H₁₂, neo-C₅H₁₂, CH₃Cl, C₂H₅Cl, CH₂Cl₂, CHCl₃, CO, and CO₂) in hydrogen chloride using two columns and a column switching technique in an isothermal mode with a flame ionization detector; the detection limits were 0.01–0.1 ppm. The matrix was separated in a precolumn packed with urea. CO and CO₂ were determined by reaction gas chromatography with their conversion into methane.     

Methane oxidation by green oxidant to methanol over zeolite-based catalysts

To reduce greenhouse gas emission from oil and gas production,it is essential to better convert methane to useful chemicals（rather） than to flare it.Conversion of methane to liquid oxygenates（mainly methanol） has attracted extensive attention and countless efforts have been made;however,running this reaction in a green,efficient,and practical way has remained elusive.The novel catalyst and oxidants play a critical role in activating methane and converting it to oxygenates（methanol）.In this revie...