КИНЕТИКА И КАТАЛИЗ, 2013, том 54, № 3, с. 340-347
UDC 542.943.7:547.211:546.655.3'723-31
SYNGAS PRODUCTION FROM METHANE OVER CeO2-Fe2O3 MIXED OXIDES
USING A CHEMICAL-LOOPING METHOD © 2013 Zhenhua Gu1, Kongzhai Li2, *, Hua W&ng2, Yonggang Wei2, Dongxia Yan2, Tianqiang Qiao2
1Oxbridge College, Kunming University of Science and Technology, Kunming University of Science and Technology, Kunming 650093, China 2Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China
E-mail: lkz_514@yahoo.com.cn Received 21.06.2012
A series of mixed oxides Cei _ xFexO2 was prepared by a hydrothermal method. XRD and Raman spectra were measured to study the structure of the prepared materials. The temperature-programmed reduction was undertaken to estimate reducibility of the oxides. Syngas generation from methane using these materials as oxygen carriers/catalysts via a chemical-looping procedure was investigated in detail. This procedure includes catalytic oxidation and decomposition of methane to produce H2-rich gas at the first step followed by the production of the CO-rich gas by oxidizing the carbon deposited on deactivated catalysts. The results showed that all iron ions were incorporated into the ceria lattice with the formation of oxygen vacancies in the Ce0.9Fe0.iO2 sample, while isolated Fe2O3 particles were distributed on the surface of the Ce08Fe02O2 sample. TPR measurements and the analysis of the two-step chemical-looping reactions indicated a strong interaction between the Ce and Fe species which accounts for an increased activity of the mixed oxides in the syngas generation compared to that of individual oxides. Among the several samples, the Ce0 8Fe0 2O2 catalyst showed the highest activity for methane partial oxidation due to the synergetic effects caused by the interaction of surface iron entities and Ce—Fe solid solution. In addition, selective oxidation of carbon by oxygen to CO can also be found over this material since gaseous products are formed at the carbon oxidation step with the selectivity to CO reaching 91.2%. Evidence is presented that syngas can be feasibly produced from methane with high selectivity via the chemical-looping procedure over the CeO2—Fe2O3 mixed oxides.
DOI: 10.7868/S0453881113030040
Partial oxidation of methane (POM) using the lattice oxygen of solid oxides instead of gaseous oxygen is an attractive technology for syngas generation [1, 2]. In this process, syngas with a H2/CO ratio of 2.0 is produced through gas—solid reaction between methane and a suitable solid oxide (oxygen carrier), and then the reduced oxygen carrier can be re-oxidized by oxygen to restore its initial state. Two interconnected reactors (a methane reactor and an air reactor) are used in this process to transfer heat from the oxidizing/exothermic step to the reducing/endothermic step, making it possible to produce syngas under an autothermic condition. An advantage over the traditional POM technology is a possibility to separate the fuel stream from the gaseous oxidant thus avoiding the risk of explosion.
However, one of the very serious problems with this method is that a rapid consumption of the lattice oxygen in the oxygen carrier during the gas—solid reaction could easily cause the formation of carbon deposition [3], leading to the H2/CO values in the syngas over 2.0. To resolve this problem, the reaction between methane and oxygen carrier must be controlled in a very short period of time, which makes it uneconomical.
Recently, we proposed a new chemical looping process for the catalytic production of syngas from
methane [4, 5]. In this method, methane is initially decomposed over a catalyst to produce H2 and carbon, and then the carbon deposition on the reduced catalyst is selectively oxidized by oxygen to generate CO as the target product. The syngas with a H2/CO ratio of 2 is obtained by mixing the gas produced through the two steps. Since catalytic decomposition of methane can proceed for a relatively longer time without a pronounced deactivation of catalysts caused by the formation carbon deposition [6, 7], successive production of syngas can be performed for a longer cycle time. The key of this process is the preparation of a two-way catalyst, which has a dual catalytic activity for both methane decomposition and carbon selective oxidation.
Many catalysts (e.g., Fe-, Co- and Ni-based catalysts) were reported to have high activity for methane decomposition, but the investigations on the selective oxidation of carbon to CO are rare. Several authors confirmed that deposited carbons could be gasified by steam or CO2 to syngas or CO [7—11]. Unfortunately, the steam or CO2 gasification cannot completely remove all the carbon deposition from the catalyst surface and the rate of carbon removal is very low [7, 8, 11]. It is well known that the carbon deposition on cat-
alyst could be removed efficiently by O2, but, generally, the presence of oxygen leads to the formation of CO2 rather than CO [11, 12].
Our early study showed that selective oxidation of carbon deposition by introducing O2 is achievable over iron-modified ceria catalyst [5]. In addition, the cerium iron mixed oxides also showed good performance in the partial oxidation of methane to syngas via the gas—solid reaction method [13—16]. Therefore, in the present paper, we combined the two chemical looping processes, focusing on the continuous process of methane selective oxidation, methane decomposition and consequently carbon selective oxidation over CeO2—Fe2O3 mixed oxide catalysts. Based on the previous research [14], a hydrothermal method, which can be used to obtain active Ce—Fe mixed oxides, was chosen to prepare the catalysts. The structure and re-dox properties of the CeO2—Fe2O3 system were also investigated.
EXPERIMENTAL
Material Preparation
The required amounts of Ce(NO3)3 • 6H2O and Fe(NO3)3 • 9H2O were dissolved in deionized water and well mixed to yield solutions with a total concentration of Ce and Fe nitrate of 0.25 mol/L. Brown slurry was precipitated by adding this solution dropwise to an ammonia solution with stirring while maintaining the pH at ca. 10. Then, 65 mL of slurry was transferred to 100 mL Teflon-lined stainless steel autoclaves and allowed to react at 220°C for 48 h. After cooling, the samples were washed with distilled water and ethanol three times and dried in air at 110°C for 12 h.
Material Characterization
The X-ray powders diffraction (XRD) experiments were performed using a Japan Science D/max-R dif-fractometer (Ka radiation, X = 0.15406 nm, 40 kV and 40 mA). The samples were scanned at a rate of 0.01° per step over the 10° < 29 < 80° range with a scanning time of 3 s per step. The diffractograms were compared with JCPDS references for identification purposes.
Raman spectra were acquired with a Renishaw Invia microscopic confocal Raman spectrometer. The emission line at 514.5 nm from an Ar-ion laser was focused on the samples under microscope. The power of the incident beam on samples was 4 mW and the scanning range was 100—1800 cm-1 with a resolution of 5 cm-1.
Temperature-programmed reduction (TPR) was performed using a TPR Win v 1.50 instrument ("Quantachrome Instruments Co."). The catalyst samples (0.1 g) were heated at 10°C/min in a 10% H2/He flow (75 mL/min) flow.
The BET specific surface area was determined from the N2 adsorption isotherm at -196°C, using a Quantachrome NOVA 2000e instrument.
Reactions
Reactions were carried out in a quartz fixed bed reactor (i.d. = 14 mm) under atmospheric pressure. Prior to the reactions, the samples (1 g) were heated in air at 300°C for 1 h, and then flushed with pure N2 for 15 min to clean the sample surface. The temperature program of the experiments is as follow. The samples were first heated form 300 to 900°C (10°C/min), then maintained at 900°C for 1 h, cooled to 850°C, maintained at 850°C for 1 h and cooled to room temperature. The reactions between methane (10 vol % CH4/N2 at a flow rate of 20 mL/min) and solid oxides were performed during the first two steps (temperature increase from 300°C to 900°C and plateau at 900°C), and the decomposition of methane occurred in the later stage of this period. The re-oxidation of the reduced oxides (catalysts) and carbon deposition was performed at the fourth step (plateau at 850°C) using 10 vol % O2/N2 at a flow rate of 200 mL/min. Between reaction and re-oxidation the samples were purged by pure N2 (60 mL/min) for 10 min to avoid mixing methane with oxygen.
The effluent gas from the methane reactions was analyzed by an Agilent 7890 gas chromatograph using a TCD detector and two capillary columns (HP-PLOT 5A and HP-PLOT-Q). A HP-PLOT 5A column was used to separate H2, CH4, O2, N2, and CO, while a HP-PLOT-Q capillary column separated CO2 from the other gases. Ar was used as a carrier gas. For the re-oxidation step, a nondispersive IR (NDIR) gas analyzer C600 ("Shanghai Baoying Technology Co.") was used to continuously monitor the concentration of CO and CO2.
RESULTS AND DISCUSSION
Material Characterization
The XRD patterns of the synthesized materials are shown in Fig. 1. The peaks from pure CeO2 correspond to a cubic fluorite structure [17]. The a-Fe2O3 (hematite) was the only crystalline phase detected in the diffractogram for pure Fe2O3 [18]. XRD for the Ce09Fe01O2 sample showed reflections from CeO2 and contained no peaks typical of Fe2O3. Very weak peaks characteristic a-Fe2O3 could be observed for the Ce08Fe02O2 sample. The lattice constant of the cubic cell of ceria calculated from the (311) spacing was found to decrease with iron oxide content as indicated in table (0.5411, 0.5388 and 0.5380 nm for pure CeO2, Ce0.9Fe0.1O2 and Ce0.8Fe0.2O2, respectively). This lattice contraction is probably due to the incorporation of smaller iron cations into the CeO2 lattice indicating the formation of Ce-Fe solid solution. It appears that
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