Oxidative dimerisation of methane on a supported Pd-Ag catalyst using a diffusion controlled ceramic wall reactor

Supported palladium-silver oxides were used as catalysts for the partial oxidation of methane by molecular oxygen in a tubular reactor with ceramic wall separation. The ceramic wall controls the O₂ supply in the catalyst bed. The results indicate that the reactor configuration can play an important role in methane oxidation. C₂H₆, C₂H₄, CO₂ and H₂O were obtained at temperatures less than 300 °C. At this temperature any contribution from homogeneous gas phase reaction can be ruled out.     

NO x -Catalyzed Gas-Phase Activation of Methane: A Reaction Study

The catalytic effect of NO _x on methane oxidation in the absence of any solid catalyst has been investigated. The experimental results show that NO _x has very good catalytic activity in the partial oxidation of methane. The predominant products for reactions in a CH₄-O₂-NO _x co-feed mode are CO, CO₂, H₂O and H₂, CH₃OH, HCHO, and C₂H₄. Aromatics are also observed.     

Synthesis gas production in methane conversion using the P|yttria-stabilized zirconia|Ag electrochemical membrane system

The electrochemical membrane reactor of YSZ (yttria-stabilized zirconia) solid electrolyte coated with Pd and Ag as anode and cathode, respectively, has been applied to the partial oxidation of methane to synthesis gas (CO + H₂). The Pd|YSZ|Ag catalytic system has shown a remarkable activity for CO production at 773 K, and the selectivity to CO was quite high (96.3%) under oxygen pumping condition at 5 mA. The H₂ production strongly depended on the oxidation state of the Pd anode surface. Namely, the H₂ treatment of the Pd anode at 773 K for 1 h drastically reduced the rate of H₂ production, while air treatment enhanced the H₂ production rate. From the results of the partial oxidation of CH₄ with molecular oxygen, it is considered that the reaction site of the electrochemical oxidation of CH₄ to synthesis gas was the Pd–YSZ–gas-phase boundary (triple-phase boundary). In addition, it is found that the oxygen species pumped electrochemically over the Pd surface demonstrated similar activity to adsorbed oxygen over Pd, PdO_ad, for the selective oxidation of CH₄ to CO, when the Pd supported on YSZ was used as a fixed-bed catalyst for CH₄ oxidation with the adsorbed oxygen. The difference with respect to the H₂ formation between the electrochemical membrane system and the fixed-bed catalyst reactor results from differences in the average particle size of Pd and the way of the oxygen supply to the Pd surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Improving selectivity during methane partial oxidation by use of a membrane reactor

A reactant-swept catalytic membrane reactor for partial oxidation of methane to formaldehyde has been modeled. Kinetic parameters were taken from the literature for a V₂O₅/Sio₂ methane partial oxidation catalyst, and membrane parameters characteristic of commercially available materials were used. The models show that the selectivity for formaldehyde can be significantly improved by using a membrane reactor.     

Spatial and temporal profiles in millisecond partial oxidation processes

Methods are presented to measure axial species and temperature profiles within catalytic partial oxidation foam monoliths at atmospheric pressure with 0.3 mm spatial resolution using a capillary sampling technique with a quadrupole mass spectrometer. The system allows sampling within the catalyst with negligible interference in flow or temperature by using a 0.6 mm quartz capillary containing a thermocouple and possessing a 0.3 mm side orifice. The capillary tightly fills a concentric channel drilled within the 10 mm long ceramic foam minimizing gas bypass. This technique has been used to measure axial catalyst species profiles at temperatures up to 1300 °C for catalytic partial oxidation of methane and ethane to synthesis gas and ethylene, respectively. CH₄ and O₂ conversion are approximately twice as fast on Rh than on Pt. For C₂H₆ the reaction products at the catalyst entrance are H₂, H₂O, CO, and CO₂. Ethylene production begins only after ~4 mm into the catalyst after most of the O₂ has reacted. Transient operation where the feed composition is varied stepwise between different C/O ratios has also been used to characterize these systems. The capillary sampler has a time resolution of ~0.05 s, and C/O step changes within 0.5 s have been achieved using mass flow controllers. For switches from C/O = 0.6 to 1.4, sharp overshoots are observed for syngas (H₂ and CO) and similar undershoots for combustion products (H₂O and CO₂). By placing the sampling orifice at different positions and stepping the C/O ratio, spatio-temporal profiles can be obtained. Spatio-temporal profiles are extremely important in validating detailed reaction mechanisms because their information content is much higher compared to integral steady state measurements at the reactor outlet. The spatial profiles show where and how quickly different species are formed or consumed along the catalyst axis. Transient profiles provide additional diagnostics of mechanisms and surface coverages because they show how temperature and species concentrations follow a perturbation from steady state.     

Temperature profile in a two-stage fixed bed reactor for catalytic partial oxidation of methane to syngas

Detailed axial temperature distribution has been studied in a two-stage process for catalytic partial oxidation of methane to syngas, which consists of two consecutive fixed bed reactors with oxygen or air separately introduced. The first stage of the reactor, packed with a combustion catalyst, is used for catalytic combustion of methane at low initial temperature. While the second stage, filled with a partial oxidation catalyst, is used for the partial oxidation of methane to syngas. A pilot-scale reactor packed with up to 80 g combustion catalyst and 80 g partial oxidation catalyst was employed. The effects of oxygen distribution in the two sections, and gas hourly space velocity (GHSV) on the catalyst bed temperature profile, as well as conversion of methane and selectivities to syngas were investigated under atmospheric pressure. It is found that both oxygen splitting ratio and GHSV have significant influence on the temperature profile in the reactor, which can be explained by the synergetic effects of the fast exothermic oxidation reactions and the slow endothermic (steam and CO₂) reforming reactions. Almost no change in activity and selectivity was observed after a stability experiment for 300 h.     

Oxidative reforming of methane on structured Ni-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/cordierite catalysts

The catalytic properties of Ni/Al₂O₃ composites supported on ceramic cordierite honeycomb monoliths in oxidative methane reforming are reported. The prereduced catalyst has been tested in a flow reactor using reaction mixtures of the following compositions: in methane oxidation, 2–6% CH₄, 2–9% O₂, Ar; in carbon dioxide and oxidative carbon dioxide reforming of methane, 2–6% CH₄, 6–12% CO₂, and 0–4% O₂, and Ar. Physicochemical studies include the monitoring of the formation and oxidation of carbon, the strength of the Ni-O bond, and the phase composition of the catalyst. The structured Ni-Al₂O₃ catalysts are much more productive in the carbon dioxide reforming of methane than conventional granular catalysts. The catalysts performance is made more stable by regulating the acid-base properties of their surface via the introduction of alkali metal (Na, K) oxides to retard the coking of the surface. Rare-earth metal oxides with a low redox potential (La₂O₃, CeO₂) enhance the activity and stability of Ni-Al₂O₃/cordierite catalysts in the deep and partial oxidation and carbon dioxide reforming of methane. The carbon dioxide reforming of methane on the (NiO + La₂O₃ + Al₂O₃)/cordierite catalyst can be intensified by adding oxygen to the gas feed. This reduces the temperature necessary to reach a high methane conversion and does not exert any significant effect on the selectivity with respect to H₂.     

Experimental and numerical study on the transient behavior of partial oxidation of methane in a catalytic monolith

The objective of this investigation is a better understanding of transient processes in catalytic monoliths. As an example, the light-off of the partial oxidation of methane to synthesis gas (H₂ and CO) on a rhodium/alumina catalyst is studied experimentally and numerically.Methane/oxygen/argon mixtures are fed at room temperature and atmospheric pressure into a honeycomb monolith, which is preheated until ignition occurs. The exit gas-phase temperature and species concentrations are monitored by a thermocouple and mass spectroscopy, respectively. In the numerical study, the time-dependent temperature distribution of the entire solid monolith structure and the two-dimensional laminar reactive flow fields in the single monolith channels are simulated. A multi-step heterogeneous reaction mechanism is used, and the surface coverage with adsorbed species is calculated as function of the position in the monolith. During light-off, complete oxidation of methane to water and carbon dioxide occurs initially. Then, synthesis gas selectivity slowly increases with rising temperature.     

Partial Oxidation of Methane to Syngas in a Packed Bed Catalyst Membrane Reactor

The planar membrane reactor configuration was explored for partial oxidation of methane (POM) to syngas. A supported membrane composed of yttria‐stabilized zirconia and La_0.8Sr_0.2Cr_0.5Fe_0.5O_3‐δ was sealed to a stainless holder, and a Ni/Al₂O₃ catalyst bed was placed under the membrane plane with a small slit between them. This reactor configuration would facilitate the POM reaction via oxidation‐reforming mechanism: the oxidation reaction occurring at the membrane surface and the reforming reaction taking place in the catalyst bed. At 800°C and a methane feed rate of 32 mL min^?1, the reactor attained methane throughput conversion over 90%, CO and H₂ selectivity both over 95%, and an equivalent oxygen permeation rate 1.4 mL cm^?2 min^?1. The membrane and catalyst remained intact after the POM testing. The planar membrane reactor configuration explored in this study may lead to the development of a compact reactor for syngas production. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2170–2176, 2016     

Partial Oxidation of Methane to Synthesis Gas Over Ru-Loaded Y2O3 Catalyst

Ru-loaded Y₂O₃ catalyst was investigated for the partial oxidation of methane to synthesis gas. Ru(0.5 wt%)/Y₂O₃ catalyst afforded a high CH₄ conversion of 27% at a CH₄:O₂ ratio of 5 to give nearly a 1:2 ratio of CO and H₂ with a selectivity of 75% at 873 K. Ru(0.5 wt%)/Y₂O₃ catalyst maintained high catalytic activity over 10 h in the partial oxidation of methane. Carbon deposition of the catalyst surface in the reaction of CH₄ was examined by thermogravimetric analyses, and it was found that no carbon deposition occurred on the Ru(0.5 wt%)/Y₂O₃ catalyst. The synthesis-gas production proceeded basically via a two-step reaction consisting of methane combustion to give H₂O and CO₂, followed by the reforming of methane from CO₂ and steam.     

Performance of an oxygen-permeable membrane reactor for partial oxidation of methane in coke oven gas to syngas

Perovskite-type oxygen-permeable membrane reactors of BaCo_0.7Fe_0.2Nb_0.1O_3−δ packed with Ni-based catalyst had high oxygen permeability and could be used for syngas production by partial oxidation of methane in coke oven gas (COG). The BCFNO membrane itself had a poor catalytic activity to partial oxidation of CH₄ in COG. After the catalyst was packed on the membrane surface, 92% of methane conversion, 90% of H₂ selectivity, 104% of CO selectivity and as high as 15 ml/cm²/min of oxygen permeation flux were obtained at 1148 K. During continuously operating for 550 h at 1148 K, no degradation of performance of the BCFNO membrane reactor was observed under the condition of hydrogen-rich COG. The possible reaction pathways were proposed to be an oxidation-reforming process. The oxidation of H₂ in COG with the surface oxygen on the permeation side improves the oxygen flux through the membrane, and H₂O reacts with CH₄ by reforming reactions to form H₂ and CO.     

Electrochemical promotion of catalytic reactions with Pt/C (or Pt/Ru/C)//PBI catalysts

The paper is an overview of the results of the investigation on electrochemical promotion of three catalytic reactions: methane oxidation with oxygen, NO reduction with hydrogen at 135 °C and Fischer–Tropsch synthesis (FTS) at 170 °C in the [CH₄/O₂(or NO/H₂ or CO/H₂)/Ar//Pt (or Pt/Ru)//PBI(H₃PO₄)/H₂, Ar] fuel cell. It has been shown that the partial methane oxidation to C₂H₂ and the C₂ selectivity were electrochemically promoted by the negative catalyst polarization. This was also the case in NO reduction with hydrogen for low NO and H₂ partial pressures. In both cases the catalytic reactions have been promoted by the electrochemically produced hydrogen. It has been found that the NO reduction with hydrogen on the Pt/PBI strongly depends on NO and hydrogen partial pressures in the working gas mixture. At higher NO and H₂ partial pressures the catalysis is promoted by the electrochemical pumping of H⁺ from the catalyst, i.e. at positive polarization. FTS demonstrated the highest methane production rate (11% of CO conversion) at zero fuel cell voltage.     

Post‐Plasma Catalysis for Methane Partial Oxidation to Methanol: Role of the Copper‐Promoted Iron Oxide Catalyst

Methane‐air partial oxidation to methanol over a ceramic‐supported Fe₂O₃‐CuO catalyst was investigated in a post‐plasma catalytic reactor at ambient conditions. The multicomponent catalyst exerted a better catalytic performance than the monocomponent Fe₂O₃ catalyst. Characterization of the catalysts by XPS showed that incorporation of the CuO additive to a Fe₂O₃‐based catalyst resulted in an increase of lattice oxygen in the surface of the catalyst which facilitated selective methane oxidation. Hydrogen temperature‐programmed reduction revealed that addition of the CuO promoter could improve the reduction performance of the catalyst. Moreover, this catalyst showed excellent stability and resistance against carbon deposition in the extended reactions while maintaining catalytic activity. A post‐plasma catalytic mechanism is proposed with three main pathways to methanol synthesis.     

Effects of MgO promoter on properties of Ni/Al2O3 catalysts for partial oxidation of methane to syngas

The effects of MgO promoter on the physicochemical properties and catalytic performance of Ni/Al₂O₃ catalysts for the partial oxidation of methane to syngas were studied by means of BET, XRD, H₂-TPR, TEM and performance evaluation. It was found that the MgO promoter benefited from the uniformity of nickel species in the catalysts, inhibited the formation of NiAl₂O₄ spinel and improved the interaction between nickel species and support. These results were related to the formation of NiO-MgO solid solution and MgAl₂O₄ spinel. Moreover, for the catalysts with a proper amount of MgO promoter, the nickel dispersiveness was enhanced, therefore making their catalytic performance in methane partial oxidation improved. However, the excessive MgO promoter exerted a negative effect on the catalytic performance. Meanwhile, the basicity of MgO promoted the reversed water-gas shift reaction, which led to an increase in CO selectivity and a decrease in H₂ selectivity. The suitable content of MgO promoter in Ni/Al₂O₃ catalyst was ∼7 wt-%. Translated from Journal of Fuel Chemistry and Technology, 2006, 34(4): 450–455 [译自: 燃料化学学报]     

Partial oxidation of methane to synthesis gas on a Rh/TiO2 catalyst in a fast flow porous membrane reactor

The partial oxidation of methane to synthesis gas has been studied over a 3% Rh/TiO₂ catalyst in a fixed bed and a novel membrane reactor under autothermal conditions using O₂ as oxidant. The membrane reactor allows the partial oxidation reaction to be performed without premixing the reactants reducing the risk of explosion even at low methane/oxygen ratios. The membrane reactor can operate autothermally and at millisecond residence time. Methane conversions of up to 65% with CO and H₂ selectivities of 90 and 82% respectively have been achieved. The low methane oxygen ratio and the high flow rates are the key factors to attain autothermal behavior. The most sensitive factor to attain high conversion and selectivities appears to be short contact time but high temperature. A kinetic model was used to interpret the experimental results. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Heat Exchangeability of a Catalytic Plate Reactor and Analysis of the Reactivity of Steam and CO2 in Methane Reforming

An assemble‐type plate reactor was developed and its intensified heat transfer compared to that of a conventional tubular reactor in methane reforming was confirmed. This characteristic enables accurate reaction kinetic analysis because of quasi‐isothermal operation with mild pressure loss. Reduced experiment cost is one of the features of the assemble‐type reactor. Simple thermal design equations applicable to plate reactors were also assessed. From experiments and accurate reaction analysis using the plate reactor it is suggested that H₂O and CO₂ have similar reactivity for a commercial Ni/α‐Al₂O₃ catalyst. The partial pressure of the oxidizing agent had much more influence on the reactivity of methane reforming than the species of this agent.     

A highly active silica(silicon)-supported vanadia catalyst for C1 oxygenates and hydrocarbon production from partial oxidation of methane

A very low surface area silica-silicon substrate has been used as a support for vanadium oxide and has been tested in the partial oxidation of methane. Use of a reactor with variable dead volume ahead of the bed of the catalyst allows determining the relevance of gas phase reactions in initiating methane conversion. Experimental evidence supports that at atmospheric pressure C₁ oxygenates are essentially produced on the catalyst surface rather than in the gas phase. Comparison with a high surface area silica-supported vanadium oxide catalyst clearly highlights the double role of surface area in promoting catalytic activity, but also in promoting non-selective further oxidation of reaction products. It is shown that a reaction system combining dead volume upstream the bed of the catalyst and a very low surface area is very promising to activate methane conversion to C₁ oxygenates and C₂₊ hydrocarbons at remarkable TOF number preventing further non-selective oxidation. In addition, production of C₂₊ hydrocarbons is observed at temperatures as low as 750 K.     

Influence of PdO content and pathway of its formation on methane combustion activity

The methane oxidation reaction is known to induce changes in the surface structure and composition of Pd catalysts; making it extremely arduous to relate the methane oxidation activity to specific catalyst properties by conventional methods (continuous flow reactor studies). To circumvent this, methodical pulse reactor studies have been undertaken to obtain correlations between the initial methane combustion activity and the catalyst properties (Pd⁰/PdO content and path of PdO formation). While the initial methane combustion activity (at 160–280 °C) continuously increased with increasing PdO concentration (0–100%) in the catalyst, it continuously decreased with increasing Pd⁰ content (0–100%). Controlled studies were undertaken to obtain catalysts with identical PdO content by two pathways: (i) by controlled partial oxidization of Pd⁰/Al₂O₃ and (ii) by controlled partial reduction of PdO/Al₂O₃. Interestingly, for a given PdO content, the catalysts obtained by partial oxidation of Pd⁰/Al₂O₃ showed a significantly superior performance to the catalyst obtained by partial reduction of PdO/Al₂O₃ for all the temperatures investigated. These studies unambiguously show that along with the relative concentration of PdO, the PdO formation pathway is also critical in deciding the methane combustion activity of the catalyst.     

Partial oxidation of methane to synthesis gas over Rh/α-Al2O3 at high temperatures

The partial oxidation of methane to synthesis gas has been studied in a continuous flow reactor using a Rh/α-Al₂O₃ catalyst under conditions as close as possible to those industrially relevant: pressures up to 800 kPa and temperatures higher than 1274 K in order to avoid the formation of carbon and to obtain high equilibrium selectivities to CO and H₂. Intrinsic kinetic data were obtained when the feed was diluted with helium. Gas-phase reactions were found to occur at 500 kPa when the feed was not diluted. A reaction network has been derived from experimental results in which oxygen conversions range from 0 to 1. CO₂, C₂H₆ and H₂O are the primary products. C₂H₄ is formed by oxidative dehydrogenation of C₂H₆. CO and H₂ are formed by reforming of CH₄ by CO₂ and H₂O; an additional direct route to CO and H₂ at low oxygen conversions cannot be excluded. The catalyst appears to be present in two states, the transition being at an oxygen conversion of 0.4 under the conditions used. The support probably enhances oxidation reactions by reverse spillover of oxygen or hydroxyl species onto rhodium. The support as such behaves similarly to the catalyst at low oxygen conversions, but shows no reforming activity. This revised version was published online in July 2006 with corrections to the Cover Date.