Hydrogen production from a combination of the water-gas shift and redox cycle process of methane partial oxidation via lattice oxygen over LaFeO3 perovskite catalyst.
ABSTRACT A redox cycle process, in which CH4 and air are periodically brought into contact with a solid oxide packed in a fixed-bed reactor, combined with the water-gas shift (WGS) reaction, is proposed for hydrogen production. The sole oxidant for partial oxidation of methane (POM) is found to be lattice oxygen instead of gaseous oxygen. A perovskite-type LaFeO3 oxide was prepared by a sol-gel method and employed as an oxygen storage material in this process. The results indicate that, under appropriate reaction conditions, methane can be oxidized to CO and H2 by the lattice oxygen of LaFeO3 perovskite oxide with a selectivity higher than 95% and the consumed lattice oxygen can be replenished in a reoxidation procedure by a redox operation. It is suggested that the POM to H2/CO by using the lattice oxygen of the oxygen storage materials instead of gaseous oxygen should be possibly applicable. The LaFeO3 perovskite oxide maintained relatively high catalytic activity and structural stability, while the carbonaceous deposits, which come from the dissociation of CH4 in the pulse reaction, occurred due to the low migration rate of lattice oxygen from the bulk toward the surface. A new dissociation-oxidation mechanism for this POM without gaseous oxygen is proposed based on the transient responses of the products checked at different surface states via both pulse reaction and switch reaction over the LaFeO3 catalyst. In the absence of gaseous-phase oxygen, the rate-determining step of methane conversion is the migration rate of lattice oxygen, but the process can be carried out in optimized cycles. The product distribution for POM over LaFeO3 catalyst in the absence of gaseous oxygen was determined by the concentration of surface oxygen, which is relevant with the migration rate of lattice oxygen from the bulk toward the surface. This process of hydrogen production via selective oxidation of methane by lattice oxygen is better in avoiding the deep oxidation (to CO2) and enhancing the selectivity. Therefore, this new route is superior to general POM in stability (resistance to carbonaceous deposition), safety (effectively avoiding accidental explosion), ease of operation and optimization, and low cost (making use of air not oxygen).
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ABSTRACT: The perovskite type oxides (PTO) supported Ni catalysts were prepared by one step citrate complexing method and were applied to steam reforming of ethanol (SRE). The catalysts were characterized by X-ray diffraction (XRD), oxygen temperature-programmed desorption (O2-TPD), temperature programmed reduction (TPR), thermal analysis (TG), mass spectrometer (MS), physical adsorption for specific surface areas and hydrogen chemical adsorption for metal surface areas. The perovskite oxide without substitution is LaFe1−yNiyO3. For the samples substituted by Sr or Ca, as indicated by the XRD results, the calcium and strontium were successfully introduced into the La site of the LaFe1−yNiyO3. The Ca substitution in LaFeyNi1−yO3 perovskite leads to the enrichment of oxygen vacancies, and some of released oxygen species is resulted from the reduction of the Fe4+ into Fe3+ in the perovskite. Although the enrichment of oxygen vacancies was also observed for the samples with Sr substitution, the insertion of Sr into the perovskite lowers the dispersion of metallic Ni, leading to a poor SRE activity. The correlation between the oxygen vacancies and the stability for SRE indicates that the surface oxygen vacancies and the promoted bulk oxygen species, as the results of the La site substitution, restrain the carbon formation and facilitate the carbon elimination. The surface oxygen vacancies as well as lattice oxygen vacancies are beneficial for the reaction between water and hydrocarbon species on the catalyst surface, reducing carbon containing intermediates and accelerating eliminating reaction of the deposited carbon. In sum, the A site doped perovskite La1−xCaxFe1−yNiyO3 supported nickel catalysts exhibit very good stability for SRE, due to the surface and bulk oxygen vacancies.International Journal of Hydrogen Energy. 01/2009;
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ABSTRACT: Hierarchical LaFeO3 fibers were prepared by a sol-gel nanocasting method using a cotton cloth as the template. The resulting LaFeO3 fibers inherited the initial network morphology of the template very well and showed enhanced catalytic CO oxidation activity and satisfactory stability compared to the counterpart particles prepared by the conventional sol-gel method.Nanoscale 01/2011; 3(3):974-6. · 6.23 Impact Factor
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ABSTRACT: This work reports on the preparation and characterization of perovskitic materials with the general formula La1−xSrxFeO3 (x = 0, 0.3, 0.7, 1) for application in a dense mixed conducting membrane reactor process for simultaneous production of synthesis gas and pure hydrogen. Thermogravimetric experiments indicated that the materials are able to loose and uptake reversibly oxygen from their lattice up to 0.2 oxygen atoms per “mole” for SrFeO3 with x = 1 at 1000 °C. The capability of the prepared powders to convert CH4 during the reduction step, in order to produce synthesis gas, as well as their capability to dissociate water during the oxidation step, in order to produce hydrogen were evaluated by pulse reaction experiments in a fixed bed pulse reactor. The high sintering temperatures (1100–1300 °C) required for the densification of the membrane materials result in decreased methane conversion and H2 yields during the reduction step compared to the corresponding values obtained with the perovskite powders calcined at 1000 °C. Addition of small quantities of NiO, by simple mechanical mixing, to the perovskites after their sintering at high temperatures, increases substantially both their methane decomposition reactivity, their selectivity towards CO and H2 and their water splitting activity. Maximum H2 yield during the reduction step is achieved with the La0.7Sr0.3FeO3 sample mixed with 5% NiO and is 80% of the theoretically expected H2, based on complete methane decomposition. In the oxidation – water splitting step, 912 μmol H2 per gr solid are produced with the La0.3Sr0.7FeO3 sample mixed with 5% NiO. The experimental results of this work can be equally well applied for the “chemical-looping reforming” process since they concern using the lattice oxygen of the perovskite oxides for methane partial oxidation to syngas, in the absence of molecular oxygen, and subsequent oxidation of the solid.Fuel. 06/2010;