Methane conversion to syngas and hydrogen in a dual phase Ce0.8Sm0.2O2-δ-Sr2Fe1.5Mo0.5O5+δ membrane reactor with improved stability

Coupling of partial oxidation of methane (POM) with water dissociation in an oxygen transport membrane is a promising technology for methane utilization. However, cobalt-based membrane materials show poor stability under the above harsh conditions. In this work, a nominal 60 wt % Ce_0.8Sm_0.2O_2-δ-40 wt % Sr₂Fe_1.5Mo_0.5O_5+δ (CSO-SFMO) dual phase membrane is reported, which was synthesized by using a one-pot EDTA-citric acid complexing method. The phase structure and morphology of the CSO-SFMO membrane were characterized by XRD, SEM and EDXS. It was found that a uniform distribution of CSO phase with a fluorite structure and SFMO phase with a perovskite structure was achieved in the dual phase membrane. The CSO-SFMO membrane exhibited an improved stability compared with cobalt based perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ, (BSCF) membrane under CO₂ or reductive gas atmospheres. The oxygen permeation flux of the dual phase membrane was investigated under different oxygen partial pressure gradients: air/He, air/CO₂, air/POM, and H₂O/POM. At 950 °C, the oxygen permeation fluxes of the CSO-SFMO membrane under air/POM and H₂O/POM gradients were 2.7 cm³ (STP) min^?1 cm^?2 and 0.75 cm³ (STP) min^?1 cm^?2, respectively, which were much higher than the oxygen flux of 0.1 cm³ (STP) min^?1 cm^?2 under air/He. Moreover, a CO selectivity of 98%, a CH₄ conversion of 97% on the POM side and a H₂ production of 1.5 cm³ (STP) min^?1 cm^?2 on the H₂O splitting side were achieved in CSO-SFMO membrane reactor under the oxygen partial pressure gradient of H₂O/POM, which was steadily run for 100 h before the measurement was intentionally stopped.     

Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition

Bi-reforming of methane (BRM) was evaluated for Ni catalyst dispersed on SBA-15 support prepared by hydrothermal technique. BRM reactions were conducted under atmospheric condition with varying reactant partial pressure in the range of 10–45 kPa and 1073 K in fixed-bed reactor. The ordered hexagonal mesoporous SBA-15 support possessing large specific surface area of 669.5 m² g^?1 was well preserved with NiO addition during incipient wetness impregnation. Additionally, NiO species with mean crystallite dimension of 14.5 nm were randomly distributed over SBA-15 support surface and inside its mesoporous channels. Thus, these particles were reduced at various temperatures depending on different degrees of metal-support interaction. At stoichiometric condition and 1073 K, CH₄ and CO₂ conversions were about 61.6% and 58.9%, respectively whilst H₂/CO ratio of 2.14 slightly superior to theoretical value for BRM would suggest the predominance of methane steam reforming. H₂ and CO yields were significantly enhanced with increasing CO₂/(CH₄ + H₂O) ratio due to growing CO₂ gasification rate of partially dehydrogenated species from CH₄ decomposition. Additionally, a considerable decline of H₂ to CO ratio from 2.14 to 1.83 was detected with reducing H₂O/(CH₄ + CO₂) ratio due to dominant reverse water-gas shift side reaction at H₂O-deficient feedstock. Interestingly, 10%Ni/SBA-15 catalyst was resistant to graphitic carbon formation in the co-occurrence of H₂O and CO₂ oxidizing agents and the mesoporous catalyst structure was still maintained after BRM. A strong correlation between formation of carbonaceous species and catalytic activity was observed.     

Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation

Landfill leachate was gasified in supercritical water (SCW) in a batch reactor made of 316 SS. The effects of temperature, pressure, reaction time and oxidation coefficient (OC) on the pollutant removal efficiencies and gasification characteristics were investigated. To observe the formation of tar and char visually, a capillary quartz reactor was also used. Results indicated that CO₂, H₂ and CH₄ were the most abundant gaseous products. Temperature has an appreciable effect on the gasification process. Increasing temperature enhanced the H₂ yield (GY_H2) and TOC removal efficiency (TRE) significantly. Although the influence of reaction time on the fractions of gaseous products was negligible at time above 300 s, the yields of H₂, CH₄, and CO₂ increased with reaction time whereas the CO, C₂H₄ and C₂H₆ yields decreased. Tar and char formation was evident on the interior surface of capillary quartz reactor. Adding a little oxidant could increase H₂ and CH₄ yields and decrease tar and char formation. GY_H2 reached up to the maximum of 231.3 mmol L^?1 leachate at 500 °C, 25 MPa, 600 s and 0.2 OC, which was 2.4 times of that without oxidant.     

A novel tubular oxygen-permeable membrane reactor for partial oxidation of CH4 in coke oven gas to syngas

Dense BaCo_0.7Fe_0.2Nb_0.1O_3−δ (BCFNO) membrane tubes were prepared by slip casting and readily brazed to 310S stainless steel supports using a silver-based alloy. A novel tubular membrane reactor was constructed by placing a cylindrical Ni-based monolithic catalyst coaxially around the tubular membrane and a conventional Ni-based catalyst-bed apart from the membrane tube. The novel membrane reactor was successfully applied to partial oxidation of CH₄ in coke oven gas (COG). At 850 °C, 94% of CH₄ conversion, 93% of H₂ and as high as 11.3 cm³ cm⁻² min⁻¹ of oxygen permeation flux were obtained. The experimental H₂ and CO selectivity and CH₄ conversion were close to the thermodynamically predicated ones. There was a good match in the coefficient of thermal expansion (CTE) among BCFNO membrane, Ag-based alloy and 310S metal support. Long-term operation test results indicate that the novel tubular BCFNO membrane reactor exhibited not only high activity but also good stability for the partial oxidation of CH₄ in COG to syngas.     

Catalytic reduction of nitrogen oxide by carbon monoxide,methane and hydrogen over transition metals supported on BEA zeolites

Different metals were impregnated onto BEA zeolites supports by ion-exchanging method to prepare catalysts for NO reduction. Activities of the prepared catalysts were then examed in a fixed bed reactor for NO reduction by CO, H₂, and CH₄. Experimental results indicated that Cu-BEA shows high catalytic activity for NO reduction by CO and H₂ at 300 °C–500 °C, while Co-BEA shows high catalytic activity of NO reduction by CH₄ at 400 °C–500 °C. Activities of all the catalysts increase as the temperature rises. Then characterization of H₂-TPR, XED, XPS and BET were conducted used to investigate the physical and chemical properties of these BEA catalysts. In-situ Fourier transform-infrared spectroscopy was also used to study the mechanism of the reaction of NO reduction by CO, CH₄ and H₂. The reason of different activities over different BEA catalysts was then carefully explored.     

Effects of Co-substitution on the reactivity of double perovskite oxides LaSrFe2-xCoxO6 for the chemical-looping steam methane reforming

The double perovskite oxides (DPOs) LaSrFe_2-xCo_xO₆ (x = 0, 0.2, 0.4, 0.6, 0.8) were investigated as oxygen carriers for the chemical looping steam methane reforming (CL-SMR). The fresh oxides were prepared by micro-emulsion method and their physical and chemical properties were characterized by X-ray diffraction, H₂-temperature programmed reduction and X-ray photoelectron spectroscopy technologies. Meanwhile, isothermal reactions for methane reforming and steam splitting were carried out in a fixed-bed reactor to determine the influences of Co-substitution on the reactivity of LaSrFe_2-xCo_xO₆. The substitution of metal Co has no obvious effect on the crystal structure of double perovskite, but induces a certain degree of Fe/Co disorder generating oxygen vacancies and/or higher oxidation states of metal cations. Synergistic interaction between surface metal ions, such as (Fe⁴⁺/Fe⁵⁺-O^2--Co²⁺) and (Fe³⁺-O^2--Co³⁺), plays a positive effect for the dissociation of methane. The activity may be more likely to be associated with the active oxygen species in connection with Co species on the DPOs surface and abundant of syngas was generated due to the concordant of methane dissociation with the lattice oxygen diffusion. Comprehensively considered, an optimal range of the degree of Co substitution is x = 0.4–0.6 for LaSrFe_2-xCo_xO₆, probably converting 70% of CH₄ into CO and H₂ with molar ratio around 2:1. At the reduced states, the ability of DPOs for steam splitting is primarily associated with the oxygen vacancies after oxygen consumption. The substitution of metal Co slightly enhances the hydrogen production capacity and resistance to carbon formation, achieving the average hydrogen yields at 2.89–3.33 mmol/g oxygen carrier and 1.46–1.61 wt% of carbon depositions.     

Influence of impregnated copper and zinc on the pyrolysis of rice husk in a micro-fluidized bed reactor: Characterization and kinetics

Catalytic performance of Cu and Zn catalysts was investigated during rice husk (RH) high-temperature pyrolysis under isothermal conditions in a micro-fluidized bed reactor. The results showed that the presence of Cu and Zn evidently influenced the release characteristics and conversion of the gas components. The impregnated Cu promoted the conversion of H₂, CH₄, CO and CO₂, while Zn showed positive catalytic effect on the conversion of H₂, CH₄ and CO₂ and negative effect on the conversion of CO. The X-ray diffraction patterns of the residue chars revealed that metallic copper nanoparticles (Cu⁰) were formed during Cu impregnated biomass pyrolysis. Textural characterization and SEM images showed that the impregnation of Cu and Zn, particularly Zn, promoted the generation of micropores and mesopores, with the pore sizes predominantly at around 1.3 nm and 3.9 nm. Reaction kinetics for generating these gases was studied based on model fitting method, and the most probable reaction mechanism was obtained based on the relative error between experimental and calculated conversion data. The resulting apparent activation energies were 85.08, 12.56, 49.72 and 38.37 kJ/mol for the formation of H₂, CO, CH₄ and CO₂ from pure RH pyrolysis. The presence of Cu decreased the forming activation energies of the four gases, and Zn decrease the forming activation energies of H₂, CH₄ and CO₂ while increased the value for the formation of CO.     

Methane steam reforming using a membrane reactor equipped with a Pd-based composite membrane for effective hydrogen production

Herein, a methane steam reforming (MSR) reaction was carried out using a Pd composite membrane reactor packed with a commercial Ru/Al₂O₃ catalyst under mild operating conditions, to produce hydrogen with CO₂ capture. The Pd composite membrane was fabricated on a tubular stainless steel support by the electroless plating (ELP) method. The membrane exhibited a hydrogen permeance of 2.26 × 10^?3 mol m² s^?1 Pa^?0.5, H₂/N₂ selectivity of 145 at 773 K, and pressure difference of 20.3 kPa. The MSR reaction, which was carried out at steam to carbon ratio (S/C) = 3.0, gas hourly space velocity (GHSV) = 1700 h^?1, and 773 K, showed that methane conversion increased with the pressure difference and reached 79.5% at ΔP = 506 kPa. This value was ～1.9 time higher than the equilibrium value at 773 K and 101 kPa. Comparing with the previous studies which introduced sweeping gas for low hydrogen partial pressure in the permeate stream, very high pressure difference (2500–2900 kPa) for increase of hydrogen recovery and very low GHSV (<150) for increase hydraulic retention time (HRT), our result was worthy of notice. The gas composition monitored during the long-term stability test showed that the permeate side was composed of 97.8 vol% H₂, and the retentate side contained 67.8 vol% CO₂ with 22.2 vol% CH₄. When energy was recovered by CH₄ combustion in the retentate streams, pre-combustion carbon capture was accomplished using the Pd-based composite membrane reactor.     

Application of CuNi–CeO2 fuel electrode in oxygen electrode supported reversible solid oxide cell

The oxygen electrode-supported reversible solid oxide cell (RSOC) has demonstrated distinguishing advantages of fuel flexibility, shorter gas diffusion path and more choices for fuel electrode materials. However, there are serious drawbacks including the difficulty of co-firing the oxygen electrode and electrolyte, and the inefficient electrochemical performance. In this study, a (La_0.8Sr_0.2)_0.95MnO_3-δ (LSM) supported RSOC with the configuration of La_0.6Sr_0.4Fe_0.9Sc_0.1O_3-δ (LSFSc)-YSZ/YSZ/CuNi–CeO₂-YSZ is fabricated by tape casting, co-sintering and impregnation technologies. The single cell is evaluated at both fuel cell (FC) and electrolysis cell (EC) mode. Significant maximum power density of 436.0 and 377 mW cm^?2 is obtained at 750 °C in H₂ and CH₄ fuel atmospheres, respectively. At electrolysis voltage of 1.3 V and 50% steam content, current density of ?0.718, ?0.397, ?0.198 and ?0.081 A cm^?2 is obtained at 750, 700, 650 and 600 °C respectively. Much higher electrolysis performance than FC mode is exhibited probably due to the optimized electrodes with increased triple phase boundary (TPB) area and faster gas diffusion (oxygen and steam) and electrochemical reactions for water splitting. Additionally, the short-term stability of single cell in H₂ and CH₄ are also studied.     

Novel twin reactor for separate evolution of hydrogen and oxygen in photocatalytic water splitting

Photocatalytic water splitting with separate H₂ and O₂ evolution is crucial because it eliminates the explosion potential and hydrogen-purification cost. A novel twin reactor was designed to separate the evolution of hydrogen and oxygen in photocatalytic water splitting under visible light. A modified Nafion membrane was employed to segregate the two photocatalysts in the twin reactor so that hydrogen and oxygen can be evolved separately. Conventional Z-scheme catalysts, Pt/SrTiO₃:Rh and WO₃, were used as hydrogen-photocatalyst and oxygen-photocatalyst, respectively. Fe²⁺ and Fe³⁺ were added in the reaction solution as electron-transfer mediator. The ratio of evolved H₂ and O₂ was in agreement with the stoichiometric ratio (2:1) of hydrogen and oxygen of water. An average hydrogen generation rate of 1.59 μmol/g-h was achieved in the twin-reactor system, which was twice as much as that in the conventional Z-scheme system. The improved H₂ yield was due to the prevention of the water-splitting backward reaction in the twin reactor.     

Numerical study of rich catalytic combustion of syngas

The rich catalytic combustion of syngas/air mixtures over platinum has been investigated numerically in a two-dimensional circular channel using steady simulations and detailed hetero-homogeneous chemistry. The channel dimensions are representative of a catalytic monolith. Simulations have been conducted in the pressure range of 1–10 bar and Φ = 3–5 with varying inlet velocities, residence time, H₂/CO ratio and CH₄ percentage. Detailed kinetic studies including the reaction path diagram (RPD) in a plug flow reactor have also been conducted to understand the kinetic interactions between H₂, CO, and CH₄. It has been observed that the homogeneous reaction rates are significant at higher pressures and cannot be neglected, although they were highly localized. The channel temperature significantly affected the relative conversion of H₂ and CO. The kinetic coupling between H₂ and CO oxidation was studied and the reason for the differential consumption of O₂ by the reactants was addressed by analyzing the reaction pathways. The residence time in the channel affected the species oxidation and four operation regimes were identified. Both the water-gas shift reaction and the reverse water-gas shift reaction were observed under varying conditions of pressure and equivalence ratio. The effect of H₂/CO ratio has also been investigated. The present study shows that rich catalytic combustion of syngas is fundamentally different from lean combustion.     

Syngas production by chemical looping gasification of rice husk using Fe-based oxygen carrier

The chemical looping gasification (CLG) of rice husk was conducted in a fixed bed reactor to analyze the effects of the ratio of oxygen carrier to rice husk (O/C), temperature, residence time and preparation methods of Fe-based oxygen carriers. The yield of gas, H₂/CO, lower heating value of syngas (LHV), conversion efficiency and performance parameters were analyzed to obtain CLG reaction characterization and optimal reaction conditions. Results showed that when O/C increased from 0.5 to 3.0, the gas production, H₂/CO, CO₂ yield and carbon conversion efficiency gradually increased, while the yield of H₂, CO and CH₄ and LHV gradually decreased. At the same time, a highest gasification efficiency was obtained when O/C was 1.5. As increasing temperature, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, while the yield of H₂, CH₄ and CO₂, H₂/CO and LHV gradually decreased. Sintering and agglomeration was obvious when the temperature was higher than 850 °C. When the reaction time increased from 10 min to 60 min, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, but the yield of H₂, H₂/CO and LHV decreased, among which 30 min was the best reaction residence time. In addition, coprecipitation was the best preparation method among several preparation methods of oxygen carrier. Finally, O/C of 1.5, 800 °C, 30 min and coprecipitation preparation method of oxygen carrier were the optimal parameters to obtain a gasification efficiency of 26.88%, H₂ content of 35.64%, syngas content of 56.40%, H₂/CO ratio of 1.72 and LHV of 12.25 MJ/Nm³.     

Effect of steam reforming on methane-fueled chemical looping combustion with Cu-based oxygen carrier

The reduction characteristics of Cu-based oxygen carrier with H₂, CO and CH₄ were investigated using a fixed bed reactor, TPR and TGA. Results showed that temperatures for the complete reduction of Cu-based oxygen carrier with H₂ and CO are 300 °C and 225 °C, respectively, while the corresponding temperature with CH₄ is 650 °C. The carbon deposition from CH₄ occurred at over 550 °C. CO-chemisorption experiments were also conducted on the oxygen carrier, and it was indicated that Cu-based oxygen carrier sinter seriously at 700 °C. In order to lower the required reduction temperature of oxygen carriers, a new chemical looping combustion (CLC) process with CH₄ steam reforming has been presented in this paper. The basic feasibility of the process was illustrated using CuO–SiO₂. The new CLC process has the potential to replace the conventional gas-fired middle- and low-pressure steam and hot water boilers.     

Research on the release of gases during the bituminous coal combustion under low oxygen atmosphere by TG–FTIR

The recirculation of boiler tail gas with a low oxygen concentration can reduce NO_x emissions. Experiments on bitumite combustion were carried out using simultaneous TG/FTIR dynamic runs with different atmospheric compositions, 2%, 6%, 10%, and 12% O₂. Reducing oxygen concentrations led to the burnout temperature shifting to higher temperature and coal combustion becoming more challenging. The reducing gas (CO, CH₄) emissions were abundant between 330 °C and 690 °C. However, along with the reduction in oxygen, CH₄ intensity increased, while the CO precipitation peak lowered. Kinetic parameters were defined using the Coats-Redfern model. According to the data obtained, bitumite combustion activation energy increased as oxygen concentration increased.     

Effects of anodic gas conditions on performance and resistance of a PBI/H3PO4 proton exchange membrane fuel cell with metallic bipolar plates

In this study, polybenzimidazole (PBI) is used as membrane material of the high-temperature membrane electrode assembly which has the features of high-performance stability and high CO tolerance. Moreover, compared to graphite bipolar plates, metallic bipolar plates have better mechanical properties and seismic capacity, as well as lighter weight. We thus use metallic bipolar plates and a PBI-based membrane electrode assembly to setup a single cell and examine its performance. The experimental results show that the cell temperature has a significant effect on the cell performance. When the temperature increases from 120 °C to 180 °C, the performance is significantly enhanced. Moreover, the CO tolerance of the fuel cell increases along with the temperature. At the same time, methane is fed in the anode stream to assess the performance of the cell under different simulated methane reformate gases. The test of various CH₄/H₂ mixtures reveals the residual methane in the reformate gases only decreases fuel cell performance slightly due to the dilution effect. We also examined H₂/CO/N₂/CH₄ mixtures in this study, and these had only a small effect on the fuel cell performance at cell temperatures higher than 160 °C. As such, it is recommended that the cell temperature should be kept higher than 160 °C.     

La0.6Sr0.4Co0.8Ni0.2O3−δ hollow fiber membrane reactor: Integrated oxygen separation – CO2 reforming of methane reaction for hydrogen production

An integrated reactor system which combines oxygen permeable La_0.6Sr_0.4Co_0.8Ni_0.2O_3−δ (LSCN) perovskite ceramic hollow fiber membrane with Ni based catalyst has been successfully developed to produce hydrogen through oxy-CO₂ reforming of methane (OCRM). Dense La_0.6Sr_0.4Co_0.8Ni_0.2O_3−δ hollow fiber membrane was prepared using phase inversion-sintering method. OCRM reaction was tested from 650 °C to 800 °C with a quartz reactor packed with 0.5 g Ni/Al₂O₃ catalyst around the LSCN hollow fiber membrane. CH₄ and CO₂ were used as reactants and air as the oxygen source was fed through the bore side of the hollow fiber membrane. In order to gauge the effectiveness of this membrane reactor system, air flow was closed at 800 °C and dry reforming of methane (DRM) was tested for comparison. The results show that the oxygen fluxes of LSCN membrane swept by helium are nearly 3 times less than those swept by OCRM reactants. With increasing temperature and oxygen supply, methane conversion in the OCRM reactor reaches 100%, but CO₂ conversion decreases from 87% to 72% due to the competition reaction with POM. CO selectivity is as high as nearly 100% at reaction temperatures of 700 °C–800 °C while H₂ selectivity reaches a maximum of 88% at 700 °C. At 800 °C, when air supply was closed and DRM was conducted for comparison, CO selectivity decreased to 91%, resulting in carbon deposition which was around 4 times more than those obtained under OCRM reaction and H₂/CO ratio decreased from 0.93 to 0.74, showing better carbon resistance and higher H₂ selectivity of the Ni-based catalyst over the integrated oxygen separation-OCRM reaction across the LSCN hollow fiber membrane reactor.     

Asymmetric dual-phase MIEC membrane reactor for energy-efficient coproduction of two kinds of synthesis gases

An asymmetric 75 wt% Sm_0.15Ce_0.85O_1.925-25 wt% Sm_0.6Sr_0.4Al_0.3Fe_0.7O_3-δ (SDC-SSAF) dual-phase mixed ionic-electronic conducting (MIEC) oxygen-permeable membrane reactor was applied to coproduce ammonia synthesis gas (ASG, H₂/N₂ = 3) and liquid fuels synthesis gas (LFSG, H₂/CO = 2). The effects of CH₄ concentration, CH₄ flow rate, steam flow rate and temperature on the performance of the membrane reactor were studied. The SDC-SSAF membrane reactor showed an excellent performance for the coproduction of ASG and LFSG. An ASG production rate of 20.7 mL cm⁻² min⁻¹, a LFSG production rate of 51.0 mL cm⁻² min⁻¹ and an oxygen permeation rate of 9.1 mL cm⁻² min⁻¹ were achieved at 925 °C. Compared with traditional industrial processes, the energy saving of this membrane reactor process is expected as high as 66.5%. The post-mortem of the membrane reactor using scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) characterization revealed that the membrane has an excellent structural stability under operation condition.     

585 divacancy-defective germanene as a hydrogen separation membrane: A DFT study

As sustainable and clean energy, hydrogen is the most attractive and promising energy source in the future. Membrane separation is attractive due to its high hydrogen separation performance and low energy consumption. Van-der-Waals-corrected density functional theory (DFT) calculations are performed to investigate the hydrogen separation performance of 585 divacancy-defective germanene (585 germanene). It is found that the 585 germanene presents a surmountable energy barrier (0.34 eV) for hydrogen molecule passing through the membrane, and that membrane exhibits extremely high selectivity for H₂ molecules over CO, CO₂, N₂, CH₄ and H₂S molecules in a wide range of temperatures. Meanwhile, the hydrogen permeance of 585 germanene can reach 1.94 × 10^?7 mol s^?1 m^?2 Pa^?1 at the low limit temperature of methane reforming (at 450 K), which is higher than the industrially acceptable gas permeance. With high selectivity and permeance, the 585 germanene is a promising candidate for hydrogen separation.     

Chemical stability in H2S and creep characterization of the mixed protonic conductor Nd5.5WO11.25-δ

The integration of hydrogen permeable membranes in catalytic membrane reactors for thermodynamically limited reactions such as steam methane reforming can improve the per-pass yield and simultaneously produce a high-purity H₂ stream. Mixed protonic-electronic materials based membranes are potential candidates for these applications due to their elevated temperature operation, good stability and potentially low cost. However, a specific mechanical behavior and stability under harsh atmospheres is needed to guarantee sufficient lifetime in real-world processes. This work presents the mechanical characterization and a study of the chemical stability under H₂S containing atmospheres for the compound Nd_5.5WO_11.25-δ. Mechanical characterization was performed by micro-indentation and creep measurements in air. Chemical stability was evaluated by XRD and SEM and the effect of the H₂S on the transport properties was evaluated by impedance spectroscopy. Under H₂S atmospheres, the total conductivity increases at 600 °C and 700 °C. The conductivity increase is attributed to the incorporation of S^2? ions in oxide-ion sublattice.     

Well-dispersed Rh nanoparticles with high activity for the dry reforming of methane

Rh catalysts with low Rh content were prepared by incipient wetness impregnation using [NH₄]₃[RhCl₆]·3H₂O or RhCl₃·3H₂O as precursor salts, on CaO–SiO₂ supports. All solids showed a high stability after 48 h on stream for the dry reforming of methane with low carbon content, which made them suitable for obtaining ultrapure hydrogen in a membrane reactor. The methane conversion and hydrogen recovery were measured increasing the sweep gas flow rates to rise the driving force for hydrogen permeation. The catalyst with 0.36 wt.% of Rh showed a slight deactivation. However, the Rh(0.6)/CaO–SiO₂ solid, in which the Rh impregnation was performed using [NH₄]₃[RhCl₆]·3H₂O, exhibited an increase on CH₄ conversion of 77% and a hydrogen recovery equal to 84%.Nanoparticles of about 1.4–1.7 nm surface average diameter were detected for the reduced and used solids indicating that Rh is well dispersed and sintering was not produced after the catalytic tests. Rh particle sizes calculated by CO chemisorption were coincident with those measured by Transmission Electron Microscopy. Characterization by this technique and Laser Raman Spectroscopy of the solids used in membrane reactor revealed the formation of scarce carbon filaments. However, a surface re-oxidation was detected in the low loading catalyst used in the membrane reactor suggesting that it is the main cause for the decrease in the activity of the highly dispersed catalyst.