Methane steam reforming in large pore catalyst

In this work we have studied the performance of catalyst extrudates of Ni-Al₂O₃ promoted with potassium for steam methane reforming. The most interesting property of this catalyst is the presence of large pores (average diameter of 8×10^?4 m) to reduce diffusional limitations. We have determined the true kinetics using catalyst powder in the temperature range covering 757–804 K. Furthermore, experiments using a fixed bed filled with extrudates were performed in the temperature range covering 701–800 K at constant methane/steam ratio for different feed flowrates.In the true kinetic experiments using catalyst powder it was observed that this catalyst has a very high CO₂ selectivity against CO. The conversion of the catalyst is smaller than other commercial materials due to the smaller content of Ni (10%).Experiments using catalyst extrudates showed that the reaction suffers from strong mass and heat limitations: diffusion of reactants/products and heat transfer in the gas/solid interface. The presence of large pores has an important contribution in decreasing the resistance to mass transfer in particles with 1.1×10^?2 m diameter. At 800 K and 2 bar the effectiveness factor was about 0.43 for the steam methane reforming reaction and 0.41 for the global reaction.     

In situ synchrotron powder diffraction study of the setting reaction kinetics of magnesium-potassium phosphate cements

This work reports a kinetic study of the formation of magnesium-potassium phosphate cements accomplished using in-situ synchrotron powder diffraction. The reaction: MgO + KH₂PO₄ + 5H₂O → MgKPO₄ · 6H₂O was followed in situ in the attempt of contributing to explain the overall mechanism and assess the influence of periclase (MgO) grain size and calcination temperature (1400-1600 °C) on the reaction kinetics. Numerical kinetic parameters for the setting reaction have been provided for the first time. The best fit to the kinetic data was obtained using a weighted nonlinear model fitting method with two kinetic equations, representing two consecutive, partially overlapping processes. MgO decomposition could be described by a first order (F1) model followed by a Jander diffusion (D3) controlled model. Crystallization of the product of reaction was modelled using an Avrami model (A_n) followed by a first order (F1) chemical reaction. A reaction mechanism accounting for such results has been proposed.     

Hydrogenation of carbon monoxide to methane over mesoporous nickel-M-alumina (M = Fe,Ni, Co,Ce, and La) xerogel catalysts

Mesoporous nickel(30 wt%)-M(10 wt%)-alumina xerogel (30Ni10MAX) catalysts with different second metal (M = Fe, Ni, Co, Ce, and La) were prepared by a single-step sol–gel method for use in the methane production from carbon monoxide and hydrogen. In the methanation reaction, yield for CH₄ decreased in the order of 30Ni10FeAX > 30Ni10NiAX > 30Ni10CoAX > 30Ni10CeAX > 30Ni10LaAX. Experimental results revealed that CO dissociation energy of the catalyst and H₂ adsorption ability of the catalyst played a key role in determining the catalytic performance of 30Ni10MAX catalyst in the methanation reaction. Optimal CO dissociation energy of the catalyst and large H₂ adsorption ability of the catalyst were favorable for methane production. Among the catalysts tested, 30Ni10FeAX catalyst with the most optimal CO dissociation energy and the largest H₂ adsorption ability exhibited the best catalytic performance in terms of conversion of CO and yield for CH₄ in the methanation reaction. The enhanced catalytic performance of 30Ni10FeAX was also due to a formation of nickel–iron alloy and a facile reduction.     

Modeling spatially resolved data of methane catalytic partial oxidation on Rh foam catalyst at different inlet compositions and flowrates

Spatially resolved species and temperature profiles measured for a wide range of inlet stoichiometries and flowrates are compared with microkinetic numerical simulations to investigate the effect of transport phenomena on the catalytic partial oxidation of methane on Rh foam catalysts. In agreement with the experimental data, the species profiles calculated at different C/O inlet stoichiometries show that both partial oxidation products (H₂, CO) and total oxidation products (H₂O, CO₂) are formed in the presence of oxygen. At the leaner stoichiometries, both oxygen and methane react in the diffusive regime at the catalyst entrance. At the richest methane stoichiometry (high C/O), surface temperatures are lower and methane consumption is only partly determined by transport. For all stoichiometries, a kinetically controlled regime prevails in the downstream reforming zone after O₂ is fully consumed. The effect of increasing the flowrate shifts all species profiles downstream and also slightly modifies the shapes of the axial profiles, due to the different effectiveness of heat and mass transfer. Despite enhanced mass transfer and increased surface temperature, the shortened contact time causes a reduced CH₄ conversion at high flowrates. The effect of flowrate on the dominant regime is investigated, for both reactants, comparing the resistances calculated in the pure transport regime and in the pure kinetic regime. From a chemical point of view, the model allows for the analysis of the reaction path leading to hydrogen. Due to inhibition of H₂O re-adsorption, it can be proven that H₂ can be a primary product even in the presence of gas phase O₂. The analysis of the surface coverages shows analogous effects on the profiles when decreasing C/O or increasing flow, because in both cases the surface temperature is increased. Syngas selectivity was also evaluated, both from measured and calculated profiles. S_H2 is well described by the model at each stoichiometry and flowrate, while S_CO is underestimated in every case. From this work, it is also indicated that the Rh catalyst works with CO (measured) selectivities higher than equilibrium. Carbon dioxide only forms in the oxidation zone, for C/O = 1 and 1.3, but in the rest of the catalyst zone, there is no further production despite what would be expected from equilibrium. This confirms Rh does not catalyze the water gas shift reaction. On the other hand, at C/O = 0.8, this reaction becomes active, due to the higher temperature, and the CO₂ is also produced in the reforming zone. This suggests that CO₂ will not rise after the oxidation section if the surface temperature is kept sufficiently low. Sensitivity analyses to the active catalytic surface and to the kinetic parameters are provided.     

Kinetic study of CO hydrogenation on the MgO supported Fe–Co–Mn sol–gel catalyst

The kinetic of the Fischer–Tropsch synthesis over the MgO supported Fe–Co–Mn catalyst prepared using sol–gel procedure, was investigated in a fixed bed micro-reactor. Experimental conditions were varied as follow: reaction pressure 5–20 bar, reaction temperature 220–250 °C, H₂/CO feed molar ratio of 0.67–2 and space velocity range of 2400–3600 h^?1. 18 models according to the Langmuir–Hinshelwood–Hougen–Watson (LHHW) type rate equation were derived, and the reaction rate is fitted fairly well by one kinetic expressions based on LHWW mechanism. The kinetic parameters were estimated with non-linear regression method. The activation energy was obtained 110.9 kJ/mol for the best-fitted model.     

Hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst

Supercritical water is a promising medium for the reforming of hydrocarbons and alcohols for the production of hydrogen at high pressures in a short reaction time. Water serves both as a dense solvent as well as a reactant. In this work, hydrogen is produced from glycerol by supercritical water reforming over a Ru/Al₂O₃ catalyst with low methane and carbon monoxide formation. Experiments were conducted in a tubular fixed-bed flow reactor over a temperature range of 700–800 °C, feed concentrations up to 40 wt% glycerol, all at short reaction time of less than 5 s. Glycerol was completely gasified to hydrogen, carbon dioxide, and methane along with small amounts of carbon monoxide. At dilute feed concentrations, near-theoretical yield of 7 mol of hydrogen/mol of glycerol was obtained, which decreases with an increase in the feed concentration. Based on a kinetic model for glycerol reforming, an activation energy of 55.9 kJ/mol was observed.     

Compact hollow fibre reactors for efficient methane conversion

In this study, a micro-structured catalytic hollow fiber membrane reactor (CHFMR) has been prepared, characterized and evaluated for performing steam methane reforming (SMR) reaction, using Rh/CeO₂ as the catalyst and a palladium membrane for separating hydrogen from the reaction. Preliminary studies on a catalytic hollow fiber (CHF), a porous membrane reactor configuration without the palladium membrane, revealed that stable methane conversions reaching equilibrium values can be achieved, using approximately 36 mg of 2 wt.%Rh/CeO₂ catalyst incorporated inside the micro-channels of alumina hollow fibre substrates (around 7 cm long in the reaction zone). This proves the advantages of efficiently utilizing catalysts in such a way, such as significantly reduced external mass transfer resistance when compared with conventional packed bed reactors. It is interesting to observe catalyst deactivation in CHF when the quantity of catalyst incorporated is less than 36 mg, although the Rh/CeO₂ catalyst supposes to be quite resistant against carbon formation. The “shift” phenomenon expected in CHFMR was not observed by using 100 mg of 2 wt.%Rh/CeO₂ catalyst, mainly due to the less desired catalyst packing at the presence of the dense Pd separating layer. Problems of this type were solved by using 100 mg of 4 wt.% Rh/CeO₂ as the catalyst in CHFMR, resulting in methane conversion surpassing the equilibrium conversions and no detectable deactivation of the catalyst. As a result, the improved methodology of incorporating catalyst into the micro-channels of CHFMR is the key to a more efficient membrane reactor design of this type, for both the SMR in this study and the other catalytic reforming reactions.     

Methane decomposition over ceria modified iron catalysts

The catalytic behaviour of ceria supported iron catalysts (Fe–CeO₂) was investigated for methane decomposition. The Fe–CeO₂ catalysts were found to be more active than catalysts based on iron alone. A catalyst composed of 60 wt.% Fe₂O₃ and 40 wt.% CeO₂ gave optimal catalytic activity, and the highest iron metal surface area. The well-dispersed Fe state helped to maintain the active surface area for the reaction. Methane conversion increased when the reaction temperature was increased from 600 to 650 °C. Continuous formation of trace amounts of carbon monoxide was observed during the reaction due to the oxidation of carbonaceous species by high mobility lattice oxygen in the solid solution formed within the catalyst. This could minimise catalyst deactivation caused by carbon deposits and maintain catalyst activity over a longer period of time. The catalyst also produced filamentous carbon that helped to extend the catalyst life.     

Ce-Doped La2O3 based catalyst for the oxidative coupling of methane

The effect of ceria was studied on the oxidative coupling of methane (OCM) with Ce-doped La₂O₃ in a La:Ce molar ratio of 75:25 using two preparation methods. The characterisation techniques used were XRD and XPS. The results revealed high concentration of oxygen vacancies. Different types of ions (Ce^3 + + Ce^4 +) were detected. More surface Ce^3 + and higher ratio [(O₂^2― + O^—)/ O^2―] were obtained in the oxide synthesised by the solvothermal method, affecting the OCM reaction in terms of higher C₂ hydrocarbons selectivity. This was ascribed to the higher relative amount of O^― species on the catalyst surface.     

Mechanism of NO reduction by CO over Pt/SBA-15

The mechanism of the CO + NO reaction catalyzed by Pt/SBA-15 was studied via independent investigations of CO oxidation and NO disproportionation. Below 400 °C, both CO + O₂ and CO + NO reactions approach 100 % conversion, while the catalyst shows negligible activity for NO disproportionation. These results suggest that CO oxidation by atomic oxygen arising from NO dissociation is not a major route for CO₂ formation in the CO + NO reaction. In situ IR spectra reveal the formation of isocyanates (NCO⁻) adsorbed on silica. Their surface concentration changes with the extent of the CO + NO reaction. A mechanism is proposed in which isocyanates are reaction intermediates.     

Production of hydrogen and MWCNTs by methane decomposition over catalysts originated from LaNiO3 perovskite

LaNiO₃ type perovskite was prepared by the “self-combustion” method and was used as catalyst precursor for the methane decomposition reaction at 600 and 700 °C. CH₄ conversion reaches 80% at 700 °C and 65% at 600 °C using pure CH₄. The yield of CNT and H₂ were 2.2 g_CNT g^?1 h^?1 and 8.2 L g^?1 h^?1 at 700 °C respectively after 4 h of reaction. When the reaction is prolonged to 22 h the catalytic activity decreases but the catalyst is still active, the production of hydrogen reaches 63.5 L (STP) per gram of catalyst and the production of MWCNT was equal to 17 g per gram of catalyst.Multi-wall carbon nanotubes were characterized by X-ray diffraction (XRD), surface area (BET), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and Raman spectroscopy. TEM micrographs showed that MWCNT longer than 20 μm were formed with inner diameters ranging from 5 to 16 nm and outer diameters up to about 40 nm.The results obtained here clearly show that the use of the perovskite LaNiO₃ as catalytic precursor is very effective for the simultaneous production of carbon nanotubes and hydrogen.     

Complementary effect of plasma–catalysis hybrid system on methane complete oxidation over non-PGM catalysts

Methane complete oxidation reaction was carried out in a non-thermal plasma (dielectric barrier discharge) quartz tube reactor where both plasma and catalyst were combined into one in-plasma catalysis system. In plasma only condition, the CO selectivity was maintained at high value (~ 50%) until the temperature reached about 200 °C. In the presence of both plasma and catalyst, however, methane was oxidized even at room temperature mostly to CO₂ with low CO selectivity over certain non-PGM catalyst like Co₁Ni₁O_x. Hence, methane complete oxidation reaction proceeded at much lower temperature similar to PGM catalyst such as Pd/Al₂O₃, while maintaining low CO selectivity.     

COx free hydrogen by methane decomposition over activated carbons

Catalytic characteristics of carbon catalysts for methane decomposition were examined in a fixed bed, quartz-tube continuous flow reactor. The reactions were carried out under atmospheric pressure at 800–900 °C. The kinetics of methane decomposition over carbon catalysts and surface changes before and after the reaction were investigated. In all the kinds of carbon catalysts tested, no discernible trend was observed between the initial activity and the surface area. Highest initial methane conversion rate of 1.27 mmols/min g-cat, and sustainability factor (R₁/R₀) of 0.40 for a high surface area carbon catalyst yielded carbon of 890 mg/g-cat at 900 °C. The activation energies are in the range from 170 to 175 kJ mol⁻¹. The physicochemical properties like surface area, pore size distribution, SEM and carbon content generated from the methane decomposition for the used and fresh catalysts are presented in this work.     

Kinetic study of CO hydrogenation over co-precipitated iron–nickel catalyst

The kinetic of the Fischer–Tropsch synthesis over a Fe–Ni/Al₂O₃ catalyst was investigated in a fixed bed micro reactor. Experimental conditions were varied as follow: reaction pressure 2–10 bar, H₂/CO feed ratio of 2/1 and space velocity of 96–450 cm³(STP)/h/gram_catalyst at the temperature range 523–573 K. On the basis of carbide-enol mechanism and Langmuir–Hinshelwood–Hougen–Watson (LHHW) type rate equations, seventeen kinetic expressions for CO consumption were tested and interaction between adsorption HCO and dissociated adsorption hydrogen as the controlling step gave the most plausible kinetic model. The activation energy was 46.5 kJ/mole for optimal kinetic model.     

Rational design of hierarchically structured porous catalysts for autothermal reforming of methane

It is shown that the performance of a commercial, meso-macroporous catalyst for the autothermal reforming of methane on Ni/Al₂O₃ could be improved significantly by optimizing the macroporosity and the size of the macropores. The commercial catalyst, taken as a base case, contains macropores with an average diameter of 2 μm and mesopores with an average diameter of 20 nm. The kinetics of Xu and Froment (1989a) for steam reforming and the water gas shift reaction were employed, in combination with kinetics for the total oxidation of methane. Multicomponent molecular diffusion, Knudsen diffusion and viscous flow were accounted for in the modeling of transport in the macropores. At typical reaction conditions, Knudsen diffusion dominates transport in the mesopores; the effect of pore surface roughness on Knudsen diffusion was included in the simulations. Both the macropore size and the macroporosity influence the overall conversion; increases of up to 40–300% with respect to a commercial catalyst are possible. A larger macroporosity typically favors a lower CO/H₂ ratio, that is, a higher selectivity toward hydrogen, when the reverse reaction of the water gas shift reaction dominates, and vice versa. Temperature gradients in the catalyst increase with macroporosity, as a result of the lower thermal conductivity of the solid porous material, but the maximum temperature in the catalyst was around 10 K above that at the outer surface at the investigated operating conditions.     

Hydrogen production from steam reforming of kerosene over Ni–La and Ni–La–K/cordierite catalysts

Ni–La and Ni–La–K catalysts supported on cordierite were prepared for steam reforming of kerosene to produce hydrogen. All these catalysts were tested in a fixed-bed reactor under different conditions. The catalysts obtained under different calcination temperatures and different reaction temperatures were characterized by TG–DTG and XRD techniques respectively. The influence of NiO and La₂O₃ contents on the activity of catalysts for steam reforming of kerosene to produce hydrogen was also investigated in our experiments. The experimental results indicate that the calcination temperature has much more influence on catalyst activity. The catalyst supported the promoter 5 wt% K₂O, 25 wt% NiO and 10 wt% La₂O₃, is the optimal catalyst under 773 K of reaction temperature and 2300 h⁻¹ of space velocity. Composition of Ni is highly dispersed on the catalyst surface. And through the duration test, the catalyst activity and stability are very satisfactory at 873 K of the reaction temperature.     

Redox features in the catalytic mechanism of the “standard” and “fast” NH3-SCR of NOx over a V-based catalyst investigated by dynamic methods

The redox mechanism governing the selective catalytic reduction (SCR) of NO/NO₂ by ammonia at low temperature was investigated by transient reactive experiments over a commercial V₂O₅/WO₃/TiO₂ catalyst for diesel exhaust aftertreatment. NO + NH₃ temperature-programmed reaction runs over reduced catalyst samples pretreated with various oxidizing species showed that both NO₂ and HNO₃ were able to reoxidize the V catalyst at much lower temperature than gaseous O₂: furthermore, they significantly enhanced the NO + NH₃ reactivity below 250 °C via the buildup of adsorbed nitrates, which act as a surface pool of oxidizing agents but are decomposed above that temperature. Both such features, which were not observed in comparative experiments over a V-free WO₃/TiO₂ catalyst, point out a key catalytic role of the vanadium redox properties and can explain the greater deNO_x efficiency of the “fast” SCR (NO + NH₃ + NO₂) compared with the “standard” SCR (NO + NH₃ + O₂) reaction. A unifying redox approach is proposed to interpret the overall NO/NO₂–NH₃ SCR chemistry over V-based catalysts, in which vanadium sites are reduced by the reaction between NO and NH₃ and are reoxidized either by oxygen (standard SCR) or by nitrates (fast SCR), with the latter formed via NO₂ disproportion over other nonreducible oxide catalyst components.     

Kinetics studies of nano-structured cobalt–manganese oxide catalysts in Fischer–Tropsch synthesis

The nano-structured cobalt/manganese oxide catalyst was prepared by thermal decomposition of [Co(NH₃)₄CO₃]MnO₄ precursor, and was tested for the Fischer–Tropsch reaction (hydrocarbon forming) in a fixed-bed micro-reactor. Experimental conditions were varied as follow: reaction pressure 1–10 bar, H₂/CO feed ratio of 1–2 and space velocity of 3600 h^?1 at the temperature range of 463.15–523.15 K. On the basis of carbide and/or enolic mechanisms and Langmuir–Hinshelwood–Hougen–Watson (LHHW) type rate equations, 30 kinetic expressions for CO consumption were tested and interaction between adsorption HCO and dissociated adsorption hydrogen as the controlling step gave the most plausible kinetic model. The kinetic parameters were estimated with non-linear regression method and the activation energy was 80.63 kJ/mol for optimal kinetic model. Kinetic results indicated that in Fischer–Tropsch synthesis (FTS) rate expression, the rate constant (k) has been increased by decreasing the catalyst particle size. The catalyst characterization was carried out using different methods including powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) surface area measurements.     

Synthesizing carbon nanotubes and carbon nanofibers over supported-nickel oxide catalysts via catalytic decomposition of methane

Supported-NiO catalysts were tested in the synthesis of carbon nanotubes and carbon nanofibers by catalytic decomposition of methane at 550 °C and 700 °C. Catalytic activity was characterized by the conversion levels of methane and the amount of carbons accumulated on the catalysts. Selectivity of carbon nanotubes and carbon nanofiber formation were determined using transmission electron microscopy (TEM). The catalytic performance of the supported-NiO catalysts and the types of filamentous carbons produced were discussed based on the X-ray diffraction (XRD) results and the TEM images of the used catalysts. The experimental results show that the catalytic performance of supported-NiO catalysts decreased in the order of NiO/SiO₂ > NiO/HZSM-5 > NiO/CeO₂ > NiO/Al₂O₃ at both reaction temperatures. The structures of the carbons formed by decomposition of methane were dependent on the types of catalyst supports used and the reaction temperatures conducted. It was found that Al₂O₃ was crucial to the dispersion of smaller NiO crystallites, which gave rise to the formation of multi-walled carbon nanotubes at the reaction temperature of 550 °C and a mixture of multi-walled carbon nanotubes and single-walled carbon nanotubes at 700 °C. Other than NiO/Al₂O₃ catalyst, all the tested supported-NiO catalysts formed carbon nanofibers at 550 °C and multi-walled carbon nanotubes at 700 °C except for NiO/HZSM-5 catalyst, which grew carbon nanofibers at both 550 °C and 700 °C.     

Self-oscillations of methane oxidation rate over Pd/Al2O3 catalysts: Role of Pd particle size

Oscillations of the methane oxidation rate were studied under methane-rich conditions on Pd/Al₂O₃ catalysts differing in Pd particle size. It was demonstrated that the temperature interval where oscillations occur narrows from 300–360 °C for the catalyst with Pd particle aggregates from 50–100 nm to 345–355 °C for the catalyst with isolated Pd particles of ~ 5 nm in size. At the same time, the period of oscillations showed ~ 6-fold increase. Structural transformations of Pd in the oscillation cycle were similar to those observed on bulk Pd used as a catalyst in the same reaction.