Hydrogen production over Ni/ceria-supported catalysts by partial oxidation of methane

The catalytic performance of Ni dispersed on ceria-doped supports, (Ce_0.88La_0.12) O_2-x, (Ce_0.91Gd_0.09) O_2-x, (Ce_0.71Gd_0.29) O_2-x, (Ce_0.56Zr_0.44) O_2-x and pure ceria, was tested for the catalytic partial oxidation of Methane (CPOX). The catalysts were characterized by Brunauer Emmett Teller (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR) and temperature programmed oxidation (TPO). Ni/ (Ce_0.56Zr_0.44) O_2-x showed higher Hydrogen production than the Ni/Gadolinium-doped catalysts, which may be due to its higher reducibility and surface area. By enhancing the support reducibility in Ni/doped-ceria catalysts, their catalytic activity is promoted because the availability of surface lattice oxygen is increased, which can participate in the formation of CO and H₂. It was also found that Ni/(Ce_0.56Zr_0.44) O_2-x showed higher catalytic performance after redox pretreatments. Similarly, a higher amount of H₂ or O₂ was consumed during hydrogenation and oxidation pretreatments, respectively. This may be correlated to re-dispersion of metallic particles and changes on the metal-support interface. In addition, it was observed that the ionic conductivity of Ni/(Ce_0.56Zr_0.44) O_2-x had an effect on the amount of carbon formed during the CPOX reaction at oxygen concentrations lower than the stoichiometric required, O/C ratios lower than 0.6. Its high oxygen mobility may have accelerated the surface oxidation reactions of carbon by reactive oxygen species, thus, inhibiting carbon growth on the catalyst surface.     

Partial oxidation of methane over Rh/supported-ceria catalysts: Effect of catalyst reducibility and redox cycles

Partial oxidation of methane (POM) was studied over Rh/(Ce_0.56Zr_0.44)O_2−x, Rh/(Ce_0.91Gd_0.09)O_2−x, Rh/(Ce_0.71Gd_0.29)O_2−x and Rh/(Ce_0.88La_0.12)O_2−x. The effect of catalyst reducibility and redox cycles was investigated. It was found that the type of doped-ceria support and its reducibility played an important role in catalyst activity. It was also observed that redox cycles had a positive influence on H₂ production, which was enhanced as the number of redox cycle increased. Results of carbon formation are discussed as a function of ionic conductivity. Temperature programmed reduction (TPR) profiles, BET surface area, ionic conductivity and XRD patterns were determined to characterize catalysts. Catalytic tests revealed that of the materials tested, Rh/(Ce_0.56Zr_0.44)O_2−x was the most active material for the production of syngas, which correlates with its TPR profile. It was observed that doping CeO₂ with Zr, rather than with La or Gd caused an enhanced reducibility of Rh/supported-ceria catalysts.     

Confining Ni nanoparticles in honeycomb-like silica for coking and sintering resistant partial oxidation of methane

3D honeycomb-like silica surrounded by ZrO₂ layer (3HL-ZrO₂-SiO₂) was prepared via sol-gel coating method. The average cell diameter of prepared material is about 10 nm. The highly dispersed Ni nanoparticles were immobilized to cell of honeycomb by impregnation method. The synthesized catalysts were characterized by SEM, TEM, XRD, H₂-TPR/TPD, TGA and N₂ adsorption-desorption techniques. Contributed by confinement of honeycomb-like silica, the Ni/3HL-ZrO₂-SiO₂ catalyst showed superior anti-sintering and coking ability compared with conventional catalysts. Further, improving the oxygen storage capacity from ZrO₂ and highly dispersed active-sites afford excellent catalytic activity. CH₄ conversion up to 90%–92% was obtained. The blocking effect of honeycomb cell endowed outstanding sintering resistance referring to Ni based catalysts.     

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.     

Partial oxidation of methane on co-precipitated Ni–Mg/Al catalysts modified with copper or iron

Methane transformation to hydrogen and synthesis gas (CO + H₂) by heterogenous catalysts can play an important role to secure the supply of energy, chemicals and fuels in the future. Methane is the main constituent of natural gas and biogas and it is also found in crystalline hydrates at the continental slopes of many oceans. In view of this vast reserves and resources, the use of methane as chemical feedstock has to be intensified. In this present work, (NiMg)Al catalysts doped with Fe or Cu, prepared by co-precipitation method and characterized by different techniques, were studied in the partial oxidation of methane (T_reaction = 750 °C, CH₄/O₂ ratio = 2). The effect of catalyst composition and pre-treatment conditions of these catalysts were investigated. Also, these catalysts show a very high activity and selectivity in the partial oxidation reaction, which depends on the conditions of catalysts preparation. The obtained results indicated increasing of activity and selectivity with decreasing calcination temperature and increasing nickel and aluminium contents in the catalysts composition. The solid doped with iron constituted the best catalyst for the total oxidation of methane and for the water–gas shift reaction. On the other hand, the addition of copper was remarkably improved the catalytic performances of the (NiMg)Al solid. So, the presence of this element supported the partial oxidation of methane with production of syngas (CO + H₂). With the addition of iron or copper for the catalyst composition, we were observed (in our previous work) the possibility of formation of NiM (M = Fe or Cu) alloy which increased nickel particles dispersion. In the case of copper, the reducibility of NiO was also assisted (TPR results) which increased catalytic activity in partial oxidation of methane.     

Influence of a reaction mixture streamline on partial oxidation of methane in an asymmetric microchannel reactor

Partial oxidation of methane (POM) has been tested in an asymmetric microchannel reactor with different inlet configurations. One inlet of the reactor provided successive splitting of an inlet flow into parallel channels, whereas the opposite inlet allowed the inlet flow to enter the parallel channels simultaneously. It was found that concentrations of carbon monoxide and carbon dioxide changed by 20–30% and the conversion of methane changed by 5–20%, depending on the rate and direction of the inlet flow. The hydrogen production rate practically did not depend on the inlet configuration and equaled 15 l/h at the inlet flow rates from 600 to 1400 cm³/min and at the methane conversion of 80%. The data obtained demonstrated that the use of different operating modes of the asymmetric microreactor allows changing the composition of produced syngas.     

Preparation of supported Ni catalysts with a core/shell structure and their catalytic tests of partial oxidation of methane

Synthesis of supported Ni catalysts with a core/shell structure at the multibubble sonoluminescence (MBSL) condition and their catalytic tests for methane decomposition by partial oxidation were performed in this study. The catalysts prepared were analyzed by XRD, TEM and XPS. Without doping the third components, the supported catalyst of core/shell structure made with 10% Ni loading on Al₂O₃ yields 96% conversion efficiency of methane at reaction temperature of 800 °C and shows excellent thermal stability for the first 40 h. It turns out that coexistence of NiO and NiOx species on the surface of the catalysts play a very important role in the partial oxidation of methane. In addition, the uniform layer of Ni particles on the surface of support material hindered coke formation and sintering process, which enhances thermal stability for the catalysts.     

Partial oxidation of methane over Ni/Mg/Al/La mixed oxides prepared from layered double hydrotalcites

A series of Ni/Mg/Al/La mixed oxides prepared by thermal decomposition of layered double hydrotalcites (HT) were characterized by XRD, ICP, EXAFS, TGA, TPR-H₂, SEM, and N₂ adsorption/desorption technique. The results revealed the formation of periclase-type catalysts with mesoporous structure, and the addition of La³⁺ lowered the phase crystallization with the formation of small oxide particles. Such catalysts had both high activities and stabilities toward partial oxidation of methane (POM). The catalyst containing 6.5 mol.% La³⁺ showed the highest performance at 1053 K with CH₄ conversion of 99%, CO selectivity of 93% and H₂ selectivity of 96%, which could be attributed to the presence of highly dispersed nickel and then the resistance to coke formation due to the promotion effect of lanthanum.     

Role of lattice oxygen in the partial oxidation of methane over Rh/zirconia-doped ceria. Isotopic studies

Isotopic tracer and nuclear reaction analysis (NRA) are used to probe the identity of oxygen for CO formation during the catalytic partial oxidation (CPOX) of methane to synthesis gas on ¹⁸O₂ labeled Rh (1 wt.%)/(Ce_0.56Zr_0.44)O_2−x. Results reveal that methane is selectively oxidized by lattice oxygen ions from the catalyst to form carbon monoxide. ¹⁸O₂ isotopic exchange experiments, as a function of temperature in the 0–850 °C range, were performed on Rh (1 wt.%)/(Ce_0.56Zr_0.44)O_2−x, and (Ce_0.56Zr_0.44)O_2−x. It was observed that the presence of rhodium considerably accelerates the oxygen exchange with the support; the maximal exchange rates could be observed at lower temperatures, 250 °C. This may be due to oxygen spillover from the metal particles to the oxide. Comparing results from the isotopic exchange experiments on Rh/γ-alumina and Rh (1 wt.%)/(Ce_0.56Zr_0.44)O_2−x. It was revealed that oxygen conducting materials have a much higher oxygen storage capacity and isotopic exchange rate than non-oxygen conducting materials.     

Hydrogen production through partial oxidation of methane in a new reactor configuration

Performance of the side feeding (SF) air injection in the process of partial oxidation of methane (POM) has been investigated by means of developing a one-dimensional steady-state non-isothermal model. A fixed bed reactor (FBR), a one-side feeding reactor (One-SF), and a membrane reactor (MR) has been compared for the conversion of methane, selectivity of hydrogen and reactor temperature. The results of the model revealed that the One-SF can operate within FBR and MR, and increasing the number of air injections of SF could achieve to the performance of the MR. The performance of the two to five-SF was studied according to the hydrogen selectivity, methane conversion, temperature profile and H₂/CH₄ ratio. It was observed that increasing the number of injections up to the three, increased the selectivity of hydrogen from 0.496 to 0.530 and decreased the outlet temperature from 1269 K to 1078 K. These results lead to creating of a process with controllable operating temperature and enhancing the selectivity of hydrogen. Consequently, decreasing the problems of high operating temperature in FBR and reduction of the process cost compared with MR.     

Simulation of methane conversion to syngas in a membrane reactor. Part II: Model predictions

A one-dimensional non-isothermal model was employed in the simulation of partial oxidation of methane to syngas in a dense oxygen permeation membrane reactor. The model predicts that if methane is consumed completely in the reactor, a temperature runaway occurs. The reactor inlet temperature is chosen as a major factor to demonstrate the correlativity of the reactor performance and this phenomenon. A borderline inlet temperature (BIT) is defined. Simulation results showed that when the reactor inlet temperature approaches this value, an optimized reactor performance is achieved. This temperature increases with the increase of the air flow rate and carbon space velocity. The surface exchange kinetics at the oxygen-rich side has a small effect on this temperature, while that at the oxygen-lean side has a significant effect.