Hydrogen Production from a Combination of the Water−Gas Shift and Redox Cycle Process of Methane Partial Oxidation via Lattice Oxygen over LaFeO 3 Perovskite Catalyst

Research Center for Eco-Environmental Science, Chinese Academy of Sciences, Beijing 100085, PR China.
The Journal of Physical Chemistry B (Impact Factor: 3.3). 01/2007; 110(51):25856-62. DOI: 10.1021/jp0654664
Source: PubMed


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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    • "Fathi et al. [11] observed that the carbon formed on the Pt/CeO 2 /c-Al 2 O 3 catalysts in the process of partial oxidation of methane could be oxidized selectively (almost 100 %) to carbon monoxide at 700 °C. It also showed that a large amount of CO was produced during pulsing with oxygen after repetitious pulses of methane over LaFeO 3 catalyst (carbon deposition formed during the methane pulses), while no CO 2 was detected [12]. Previously , we reported that carbon deposition could be selectively oxidized to CO with little CO 2 evolution by oxygen over the CeO 2 –Fe 2 O 3 catalysts [13] [14]. "
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    ABSTRACT: A series of Fe2O3/Al2O3, Fe2O3/CeO2, Ce0.7Zr0.3O2, and Fe2O3/Ce1−x Zrx O2 (x = 0.1–0.4) oxides was prepared and their physicochemical features were investigated by X-ray diffraction (XRD), transmission electron microscope (TEM), and H2-temperature-programmed reduction (H2-TPR) techniques. The gas–solid reactions between these oxides and methane for syngas generation as well as the catalytic performance for selective oxidation of carbon deposition in O2-enriched atmosphere were investigated in detail. The results show that the samples with the presence of Fe2O3 show much higher activity for methane oxidation compared with the Ce0.7Zr0.3O2 solid solution, while the CeO2-contained samples represent higher CO selectively in methane oxidation than the Fe2O3/Al2O3 sample. This suggests that the iron species should be the active sites for methane activation, and the cerium oxides provide the oxygen source for the selective oxidation of the activated methane to syngas during the reaction between methane and Fe2O3/Ce0.7Zr0.3O2. For the oxidation process of the carbon deposition, the CeO2-containing samples show much higher CO selectivity than the Fe2O3/Al2O3 sample, which indicates that the cerium species should play a very important role in catalyzing the carbon selective oxidation to CO. The presence of the Ce–Zr–O solid solution could induce the growth direction of the carbon filament, resulting in a loose contact between the carbon filament and the catalyst. This results in abundant exposed active sites for catalyzing carbon oxidation, strongly improving the oxidation rate of the carbon deposition over this sample. In addition, the Fe2O3/Ce0.7Zr0.3O2 also represents much higher selectivity (ca. 97 %) for the conversion of carbon to CO than the Fe2O3/CeO2 sample, which can be attributed to the higher concentration of reduced cerium sites on this sample. The increase of the Zr content in the Fe2O3/Ce1−x Zrx O2 samples could improve the reactivity of the materials for methane oxidation, but it also reduces the selectivity for CO formation.
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    ABSTRACT: The site requirements for the methane total combustion, partial oxidation, and decomposition on LaFeO3 perovskite catalysts during the chemical looping process were addressed by a kinetic study through varying the O content and the crystal size. The exchange between the surface oxygen and lattice oxygen is rapid, and the active sites of Fe associated with the surface oxygen are highly dynamic in nature, which depends on the amount of O in the perovskite during reaction and the O binding energy. Three types of active sites are identified where Fe atoms highly coordinated with O lead to total combustion of methane, Fe moderately coordinated with O leads to partial oxidation, and Fe highly coordinated with vacancy sites leads to carbon formation. The crystal size of LaFeO3 perovskite plays a significant role in determining the O binding energy and their catalytic properties on the partial oxidation of methane by the chemical looping process. Different crystal sizes of perovskites have been obtained using various preparation methods. Larger crystal has a high selectivity to synthesis gas and high capacity of O removal, due to a lower O–Fe bond energy, as indicated by a smaller band gap measured by UV–Vis absorption spectroscopy. Nearly pure synthesis gas was produced by methane partial oxidation using chemical looping on LaFeO3 by tuning the surface O coverage through controlling the oxidation degree in the oxidation step.
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