Selective Oxidation of Carbon Using Iron-Modified Cerium Oxide

Aug 5, 2009 - Faculty of Materials and Metallurgy Engineering, Kunming ... Kunming UniVersity of Science and Technology, Kunming 650093, China...
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J. Phys. Chem. C 2009, 113, 15288–15297

Selective Oxidation of Carbon Using Iron-Modified Cerium Oxide Kong Zhai Li,§ Hua Wang,* and Yong Gang Wei Faculty of Materials and Metallurgy Engineering, Kunming UniVersity of Science and Technology, Kunming 650093, China

Dong Xia Yan Faculty of Chemical Engineering, Kunming UniVersity of Science and Technology, Kunming 650093, China ReceiVed: April 18, 2009; ReVised Manuscript ReceiVed: July 14, 2009

This research investigated a solid-solid reaction between carbon and CeO2 and the selective oxidation of carbon to CO by air using a cerium iron catalyst. The carbon-containing materials were obtained by reaction of fresh CeO2 or CeO2-Fe2O3 compounds with methane. X-ray powder diffraction (XRD), Raman spectra, scanning electron microscopy (SEM), and temperature-programmed reduction (TPR) procedures were used to study the structure, morphology, and redox performance of fresh and reduced CeO2 or CeO2-Fe2O3 samples and their carbon deposits. XRD analysis showed no significant differences between reduced CeO2 (left overnight) and fresh samples with respect to their lattice parameters. However, some iron and cementite (Fe3C) did appear on the reduced CeO2-Fe2O3 sample. Raman results showed that the carbon formed on the surface of reduced CeO2 was amorphous, whereas a higher degree of graphitization was obtained using the reduced CeO2-Fe2O3 sample. SEM measurements showed filament conformation of the carbon of the reduced CeO2-Fe2O3 and a floc structure on the reduced CeO2 sample. The H2-TPR experiment showed that the redox performance of CeO2 was strongly enhanced by doping with Fe2O3. The temperature-programmed and isothermal reactions indicated that CeO2 could provide oxygen as the sole oxidant in the oxidization of carbon deposits to CO with high selectivity in the absence of gaseous oxygen at an appropriate temperature, such as 800 °C. Moreover, carbon conversion was significantly enhanced in the presence of Fe species; the in situ reoxidation of reduced CeO2-Fe2O3 with air produced mostly CO at 800 °C with a rapid reaction rate. Less CO was obtained using reduced Fe2O3-ZrO2, indicating that the reduced Ce species in association with the reduced Fe species contributed to CO formation during carbon oxidation by air. It also showed that carbon oxidation by air was strongly influenced by the reaction temperature; the selective oxidation of carbon to CO cannot occur once the temperature is too low (e.g., 400 °C). A mechanism describing oxygen adsorption and migration with a cerium iron catalyst and CO formation was proposed based on the selective oxidation of carbon by CeO2 and on the excellent oxygen adsorption capacity of cerium suboxides (anionic vacancies) and reduced Fe sites (e.g., Fe and Fe3C). This mechanism reveals that the carbothermic reduction of CeO2 (i.e., reduction of CeO2 by carbon) along with the active oxygen formed on the reduced Fe sites may play an important role in the selective oxidation of carbon by air at high temperature. Introduction Ceria has attracted increased attention in catalysis and materials science given its wide range of applications1 because the cerium atom in the CeO2 crystal is capable of adjusting its valence state to best fit its surroundings.2 Under reducing conditions, oxygen atoms can move away from their lattice positions, creating anionic vacancies (CeO2-x) correlated to high lattice oxygen mobility. Such oxygen vacancy defects have been proposed as reactive sites on the surfaces of metal oxides.3 Besides the suboxides having a strong tendency to remain in their fluorite structured lattice even after considerable oxygen loss, they also can easily be reoxidized to form stoichiometric CeO2.4,5 These properties are advantageous in various reactions, including CO2 reforming, steam reforming, the purification of exhaust gases in three-way automotive catalytic converters, and the production or purification of hydrogen.6-9 There is also * To whom correspondence should be addressed. Telephone: +86-8715153405. E-mail: [email protected]. § E-mail: [email protected].

increasing interest in using ceria as a promoter or active catalyst in the partial oxidation of methane and in the direct electrochemical oxidation of various hydrocarbons using solid oxide fuel cells (SOFC) where the potential for deactivation through carbon deposits is much greater.10-14 CeO2 has been shown to convert CH4 to H2 and CO using its lattice oxygen in the absence of a gaseous oxidant with almost no carbon deposits produced when the CeO2 reduction is less than 10%.15-17 While the reaction between methane and cerium oxide in the production of synthesis gas is strongly promoted by platinum, the promoter leads to some carbon deposition. Since methane decomposition can be catalyzed by the Pt promoter, some authors have suggested that methane oxidation may proceed in two steps;16,17 methane first decomposes to carbon and hydrogen, and the carbon then is oxidized to gaseous products by cerium oxide through solid-solid reactions. In addition, regeneration of the used samples (Pt-promoted CeO2) with oxygen removes carbon deposits almost entirely as CO.17 While the oxidation of methane by CeO2 in our study was enhanced in the presence of Fe species, carbon deposition

10.1021/jp903563v CCC: $40.75  2009 American Chemical Society Published on Web 08/05/2009

Selective Oxidation of Carbon

J. Phys. Chem. C, Vol. 113, No. 34, 2009 15289 In the present work, we report on the solid-solid reaction between cerium oxide and carbon (produced by methane decomposition). Our purpose was to verify the performance of carbon selective oxidation by CeO2. We then compare above results to those for cerium iron compounds to explore the role that Fe species may play in promoting the reaction between cerium oxide and carbon. The catalytic performance of the reduced CeO2-Fe2O3 sample for the oxidation of carbon with air was also investigated. As will be shown below, the results of this study indicate that selective oxidation of carbon to CO could occur over CeO2-containing materials in both the absence and presence of gaseous oxidant. Experimental Section

Figure 1. Variations in ∆Go with temperature for the reactions in reactions 1-4.

remained significant.18 Moreover, reoxidation with air produced CO rather than CO2, suggesting there may be some selective catalytic oxidation of carbon in these processes assisted by CeO2 or CeO2-x. A number of mechanisms associated with the catalytic oxidation of carbon materials have been described in the literature.19-22 McKee et al. stated that a metal oxide could catalyze the oxidation of carbon only if the carbothermic reduction (i.e., reduction of the metal oxide by carbon) was thermodynamically feasible.19 This suggests that the lattice oxygen of the catalyst plays an essential role in the carbon oxidation mechanism. However, some researchers proposed that the various surface oxygen complexes (SOC) formed through chemisorption of active oxygen (gaseous oxidant activated on the catalytic surface) on a carbon black surface are likely to be the most important active species for the catalytic oxidation of carbon.21,22 This would imply that the carbothermic reduction is not an essential parameter in determining the oxidation of carbon at low temperature. Obviously, the salient question is whether the thermodynamic feasibility of a solid-solid reaction between carbon and a catalyst is necessary for the catalytic oxidation of carbon to proceed. Reactions 1-4 summarize the possible reactions occurring during carbon oxidation by CeO2 in the absence of gaseous oxidant.

CeO2 + xC f xCO + CeO2-x

(1)

2CeO2 + xC f xCO2 + 2CeO2-x

(2)

CeO2 + xCO f xCO2 + CeO2-x

(3)

xCO2 + CeO2-x f xCO + CeO2

(4)

Figure 1 shows the standard Gibbs free energy changes (∆Go) for reactions 1-4 at different temperatures. The oxidation of carbon to CO and CO2 via CeO2 is thermodynamically viable at temperatures g750 (CO) and g775 °C (CO2) under standard atmospheric pressure. Reaction 3 occurs at temperatures greater than 840 °C, while reaction 4 is feasible below 840 °C. Collectively, the overall reaction temperature range for the four reactions is between 750 and 840 °C.

Fresh Sample Preparation. The CeO2-Fe2O3 compound was prepared by coprecipitation. The starting Ce(NO3)3 · 6H2O and Fe(NO3)3 · 9H2O materials were mixed in a 7:3 mol ratio, blended using a magnetic stirrer, and heated to 70 °C. A 10% ammonia solution was gradually added with stirring. When the pH value was increased to 7-8 and 10-11, the resulting solutions were maintained at 70 °C with continued stirring for 1 h, respectively. The resulting precipitate was filtered and washed with distilled water and ethanol after 2 h of settling. The precipitate was dried at 110 °C for 24 h following a preliminary overnight natural drying with some rufous massive objects obtained. These massive objects were subjected to decomposition at 300 °C for 2 h and ground to powder. The powders were calcined under ambient air at 800 °C for 6 h to produce the CeO2-Fe2O3 oxides. Pure CeO2 and ZrO2-Fe2O3 (Zr/Fe ) 7:3) powders were similarly prepared. The starting material for the zirconium oxides was Zr(NO3)4 · 5H2O. Reduced Samples Preparation (Materials Containing Carbon). In order to obtain carbon deposits for the study, 1.8 g of CeO2 or CeO2-Fe2O3 oxide was placed in the reactor and pretreated with methane (99.99% purity,10 mL/min) at 800 °C for 60 min after the reactor was purged with N2 (99.99% purity, 40 mL/min) for 30 min. This produced a black mixture containing carbon, which was removed and mixed evenly with an additional 1.0 g of CeO2 before letting it sit overnight. The amount of carbon formed during the reduction was quantified by combustion gas chromatography. The amounts of carbon on the surface of the reduced CeO2 and CeO2-Fe2O3 samples were 0.012 and 0.094 g, respectively. Reactions. The oxidation of carbon in the absence or in the presence of a gaseous oxidant was carried out in a fixed bed quartz reactor (inner diameter of 19 mm) under atmospheric pressure. (1) Temperature-Programmed and Isothermal Reactions. The solid-solid reaction between CeO2 and carbon was performed in a tubular reactor packed with 2.0 g of mixture. Nitrogen (99.99% purity) was used as a carrier gas at a flow rate of 30 mL/min. In the temperature-programmed experiments, the temperature was increased at 15 °C/min. The isothermal reaction between CeO2 containing Fe species and carbon was carried out at 800 °C. The product gas components were analyzed on line using a gas chromatograph (GC112A, produced by Shanghai Precision and Scientific Instrument Co., and 6890N, produced by Agilent Co.). (2) The Oxidation of Carbon by Air with a Cerium Iron Catalyst. Following the reactions of the CeO2-Fe2O3 and ZrO2-Fe2O3 oxides (1.8 g) with methane at 800 °C for 60 and 49 min (in order to obtain the same carbon content with the CeO2-Fe2O3 sample), respectively, the reactor was purged with N2 for 30 min before the pressed air (35 mL/min) was introduced

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Figure 2. XRD patterns of CeO2 powders and the CeO2-Fe2O3 compound.

to reoxidize the reduced sample. The reactant and product components were analyzed on line by a gas chromatograph. Experiments conducted at different temperatures were operated similarly. Materials Characterization. The powder X-ray diffraction (XRD) experiments were performed on a Japan Science D/max-R diffractometer using Cu KR radiation (λ ) 0.15406 nm). The X-ray tube was operated at 40 kV and 40 mA. The X-ray diffractogram was recorded at 0.01° intervals with a scanning rate of 5°/min for scanning angles (2θ) from 10 to 80° and at 0.01° intervals with a scanning rate of 0.4°/min for the 2θ from 36 to 50°. Identification of the phase was made using JCPDS cards (Joint Committee on Powder Diffraction Standards). Raman spectra were acquired with a Renishaw Invia microscopic confocal Raman spectrometer. The emission line at 514.5 nm from the Ar ion laser was focused on the samples under a microscope. The power of the incident beam on the samples was 4 mW, and the scanning range was set between 100 and 1800 cm-1 with 5 cm-1 resolution. Scanning electron microscopy (SEM) was performed with a Holland Philips XL30ESEM-TEP. Temperature-programmed reduction (TPR) experiments were performed on TPR Win v 1.50 (produced by Quanta Chrome Instruments Co.) under a flow of a 10% H2/He mixture (75 mL/ min) using 100 mg catalysts using a heating rate of 10 °C/min. Results and Discussion Characterization of the Fresh Materials. The XRD results for the fresh CeO2 powders and the CeO2-Fe2O3 oxides are shown in Figure 2. The CeO2 diffractogram shows a strong peak at 2θ ) 28.5° and a number of weaker peaks characteristic of fluoritestructured CeO2. For the CeO2-Fe2O3 sample, characteristic peaks appear for both CeO2 and R-Fe2O3. The shift in the characteristic peak to higher 2θ positions for the ceria cubic phase was observed from the insert. In addition, the lattice parameter of the ceria cubic cell in the CeO2-Fe2O3 sample was calculated to be 0.5395 nm, which was only a little smaller than that of pure CeO2 (0.5411 nm). These observations indicate that only a small part of Fe3+ incorporated into the ceria lattice forms solid solutions, with the rest is left on the surface of the mixed oxides. Characterization of Reduced Materials. To see the effect of methane reduction on the material structure and components,

Figure 3. XRD patterns of reduced CeO2 powders and CeO2-Fe2O3 oxides by methane. (a) Data collected at 0.01°/step with the scanning rate of 5°/min. (b) Data collected at 0.01°/step with the scanning rate of 0.4°/min.

the XRD, Raman, and SEM measurements were performed for the reduced CeO2 and CeO2-Fe2O3 samples. All test samples were pretreated at room temperature for 24 h following their reactions with methane for 60 min at 800 °C. Figure 3a shows the XRD patterns for the reduced CeO2 and CeO2-Fe2O3 samples. The reduced CeO2 sample still presents a typical characteristic of cubic fluorite CeO2 with no carbon crystal. The XRD pattern for the reduced CeO2-Fe2O3 sample presents reflections due to Fe (JCPDS 06-0696) and Fe3C (cementite, JCPDS 35-0772) in addition to the characteristic peaks of CeO2 with no graphite phase observed. However, the colors of the fresh CeO2 (yellow) and CeO2-Fe2O3 (henna) samples changed to black after reaction with methane, which would suggest the presence of carbon deposits in the process. Given the XRD results, it is plausible that the carbon deposits on the surface of reduced CeO2 or CeO2-Fe2O3 samples were either well dispersed with very small particle size or existed in some amorphous state along with some cementite on the reduced CeO2-Fe2O3 sample. In order to better verify the shape and position of the Fe and Fe3C peaks, data were collected using a slow scanning speed for the reduced CeO2-Fe2O3 sample between 36 and 50° (Figure 3b). The characteristic Fe and Fe3C peaks became weak and broad, implying that the Fe and Fe3C particles were well dispersed on the reduced CeO2-Fe2O3 surface. The Fe3C and metallic Fe crystallite sizes calculated by the Scherrer equation were found to be 6.3 and 5.2 nm, respectively, which were much smaller than those for the Fe2O3 particles (33.1 nm) on the fresh CeO2-Fe2O3 surface. This indicates that the reduction of the CeO2-Fe2O3 sample by methane leads to fragmentation of the iron species particles.

Selective Oxidation of Carbon

Figure 4. Raman spectra of carbon on CeO2 and CeO2-Fe2O3 samples after reaction with methane for 60 min at 800 °C.

It should be mentioned that no suboxides (CeO2-x) were detected in the XRD patterns of the two reduced samples. Important information on the reduced state of CeO2 can be derived by careful analysis of the variation of its unit cell parameter.23 The lattice parameters of the cerium oxides in the reduced CeO2 (0.5410 nm) and CeO2-Fe2O3 (0.5393 nm) samples were almost equal to those of the fresh samples (0.5411 nm for fresh CeO2 and 0.5395 nm for fresh CeO2-Fe2O3). This suggests that the cerium oxide in the reduced sample (mixed with an additional 1.0 g of fresh CeO2 with a long time grind) should be stoichiometric CeO2; hence, essentially all of the cerium oxide ceria in the subsequent solid-solid reaction between Ce-containing materials and carbon would be IV-valent ceria, which supports the proposition that cerium suboxides (CeO2-x) can be easily reoxidized under oxidizing conditions to form stoichiometric CeO2 at room temperature.4,5 The Raman spectrum provides an ideal method of measuring carbon materials.24 The Raman spectra for carbon on the CeO2 and CeO2-Fe2O3 samples after reacting with methane at 800 °C for 60 min are shown in Figure 4. Two bands are observed at around 1355 (D band) and 1590 cm-1 (G band) for both Raman spectra. The G band can be attributed to the in-plane carbon-carbon stretching vibrations of graphite layers, while the D band can be ascribed to defects in the hexagonal graphitic layers (such as amorphous carbon).25-27 Generally, the graphitic degree of carbon materials is evaluated using the intensity ratio of the G band and D band (IG /ID), with higher IG /ID values indicating higher degrees of graphitization.26 The IG /ID value for the reduced CeO2-Fe2O3 sample (IG /ID ) 1.47) was much larger than that for reduced CeO2 (IG /ID ) 0.50), confirming that the carbon deposits appearing on the reduced CeO2 surface should be amorphous. However, the presence of Fe species during methane decomposition could enhance the graphitic grade of carbon deposits. Takenaka et al.28 reported that Fe2O3/Al2O3 could produce multiwalled and chain-like carbon nanotubes with methane decomposition; the IG /ID (1.48), the Raman spectrum curve, and the position of the G and D bands are all in agreement with our results (for carbon on the reduced CeO2-Fe2O3 sample). Therefore, when methane decomposes on Fe2O3/CeO2, the disordered structure of the carbon deposits can be decreased and effectively form carbon nanotubes. SEM micrographs of the reduced CeO2 and CeO2-Fe2O3 samples are presented in Figure 5. Those for reduced CeO2, Figure 5a and b, show particles of different shapes with uneven

J. Phys. Chem. C, Vol. 113, No. 34, 2009 15291 particle diameters and some flocks surrounding the larger particles. In Figure 5c and d, however, the grains were smaller and more even, and there was filament conformation on the reduced CeO2-Fe2O3 sample. When considered along with the Raman results, the floc and filament conformation on the pictures should represent the morphology of carbon on the reduced CeO2 and CeO2-Fe2O3 samples, respectively. There were also a number of catalyst particles tightly surrounded by carbon filament in Figure 5c and d but limited contact between carbon and cerium oxide particles in Figure 5a and b. Temperature-Programmed Reaction. Figure 6 shows the CO and CO2 products as a function of reaction temperature and time during the temperature-programmed reaction between carbon and CeO2. Since the current discussion is concerned with the volume fraction of CO and CO2, the volume fraction of N2 is not displayed. The reaction commences at 800 °C, with the CO concentration rising rapidly with the temperature increases. The main reaction product is CO, with only a little CO2 detected (the maximal volume fraction of CO2 is 2.1%). At the end of the test when the temperature is stabilized at 900 °C, the concentration of CO in the outlet gas decreases slowly within a range of 12-15%. Generally, there are two kinds of oxygen species on oxides, surface-adsorbed oxygen, which is highly active at low temperatures, and bulk lattice oxygen, which is active at high temperature.29 The TPR measurement (Figure 7) showed that the reduction of pure CeO2 by hydrogen took place sequentially with three peaks at ∼145, 340, and above 900 °C. The peaks at low temperature should be related to the reduction of surface adsorption oxygen, while the peak at high temperature should be attributed to the reduction of bulk lattice oxygen inside of CeO2. Dai et al.30 reported that two kinds of oxygen species exist in LaFeO3 oxides, allowing for both selective and complete oxidation of methane. We anticipated a similar situation in this research, where the CO2 formed during the early stages of the reaction was created by surface adsorption of oxygen on the CeO2. As the surface oxygen species were consumed, the CeO2 lattice oxygen then reacted selectively with carbon to form CO. The oxygen may have been transported by surface reactions or by a bulk ion conduction mechanism. In either case, the oxygen diffusion rate should have been related to temperature, with the rate becoming more rapid with temperature increases. Figure 8 describes the outlet gas composition for the CeO2-Fe2O3 samples (containing CeO2, Fe, Fe3C, and some other carbon species) while heating under a flow of pure N2. In Figure 8, both CO and CO2 simultaneously appeared at 650 °C and the volume fractions of both CO and CO2 were noticeably enhanced with increases in temperature until reaching maximum levels (31.6% and 4.9% at 800 and 700 °C, respectively) after which the volume fraction of CO2 declined swiftly to baseline. The volume fraction for CO, on the other hand, remained relatively high, although some decrease was observed between 800 and 850 °C. In comparison with the pure CeO2 sample (Figure 6), the initial carbon oxidation temperature for the reduced CeO2-Fe2O3 sample was lower by 150 °C with more carbon oxides formation (the maximal volume fractions of 31.6 and 4.9% for CO and CO2, respectively, for the Ce-Fe sample versus 14.7 and 2.1% for the pure CeO2). This suggests that the reaction in the Fe-modified sample was much quicker and more violent than that with pure CeO2. Possible Reasons for These Changes. First, the Fe species may take an active part in the carbon conversion. With a Ce Fe

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Figure 5. SEM picture of reduced CeO2 and CeO2-Fe2O3 samples; (a) reduced CeO2 (10000×), (b) reduced CeO2 (20000×), (c) reduced CeO2-Fe2O3 sample (10000×), and (d) reduced CeO2-Fe2O3 sample (20000×).

Figure 6. Byproduct composition during the reaction between carbon and CeO2 at different temperatures.

Figure 7. H2-TPR profiles of CeO2 and CeO2-Fe2O3 samples.

molar ratio of 7:3, there is a large quantity of Fe species in contact with the cerium oxide fractions. It is possible that the Fe species could affect the oxygen migration dynamics of the whole ceria phase. For the ceria reduction to take place, the

Figure 8. Byproduct composition from the reaction between carbon and CeO2 in the presence of Fe species at different temperatures.

lattice oxygen must migrate from the bulk of cerium oxide to the surface where it can react with carbon. Oxygen mobility may also be enhanced by the presence of Fe species. Kaneko et al.31 reported that the addition of Fe3+ to cerium dioxide (CeO2) could enhance the oxygen-release capability of CeO2. In addition, there appears an obvious peak at 800 °C on the CO volume fraction curve in Figure 8 that is not present in Figure 6. This would suggest that the oxygen diffusion rate from the CeO2 lattice to carbon was accelerated by the presence of Fe species. Both H2-TPR and BET experiments confirm this hypothesis. The TPR patterns (Figure 7) show that the CeO2-Fe2O3 sample presents much larger hydrogen consumption at low (800 °C) temperatures than that for pure CeO2. This is especially true of the hightemperature reduction peak for the CeO2-Fe2O3 sample that shifts to lower values in contrast to that of the pure CeO2 sample. Because the surface area of the CeO2-Fe2O3 sample (12.56 m2/ g) is much lower than that for pure CeO2 (20.42 m2/g), the enhanced reducibility of cerium oxides was not obtained by improving the textural features (e.g., BET surface area) of the material. This would suggest that it is the addition of iron spe-

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Figure 9. Byproduct composition during the reaction between carbon and CeO2 in the presence of Fe species at 800 °C.

cies that must be responsible for the improved oxygen mobility and the contribution to the onset of CO formation at lower temperatures. Second, the degree of graphitization and the morphology of carbons on the reduced CeO2 and CeO2-Fe2O3 samples as characterized in Figures 4 and 5 are very different, and these differences may lead to changes in the carbon oxidation mechanism. Third, Issa et al.32 reported that the oxidation of carbon black by CeO2 was closely related to the mean diameter of the catalyst particles as well as the degree of contact between carbon and the catalyst. The presence of small cerium oxide particles in intimate contact with carbon black could lead to a significant catalytic effect and reduce the oxidation temperature.31 The SEM pictures of the reduced CeO2 and CeO2-Fe2O3 samples (Figure 5) show smaller catalyst particles and better contacts between carbon and catalyst particles for reduced CeO2-Fe2O3 samples, which suggests that carbon oxidation is enhanced by smaller catalyst particles in close contact with carbon on the reduced CeO2-Fe2O3 sample. There is also a shoulder peak on the CO volume fraction curve at 700 °C in Figure 8, which corresponds to the highest CO2 volume fraction. We found reaction 5 to be thermodynamically favored at temperatures g 600 °C. Our experiments introducing 10 mL/min pure CO2 (99.99%) to 1.0 g of Fe powders (99.9%) confirm that the reaction would always produce CO between 600 and 900 °C. On the other hand, for the Fe3C species, Pinheiro et al.33 reported that the formation of cementite (Fe3C) promoted the deactivation of an iron-cobalt catalyst during CO disproportionation, while the presence of CO2 in CO feed gas inhibited Fe3C generation. Because CO2 corrosion occurs easily on the Fe3C sites in carbon steel34 and because iron carbide (Fe3C) is an effective ethanol dry (CO2) reforming catalyst,35 Fe3C species should provide favorable adsorption ability for CO2. It is possible that CO2 amply adsorbs on the Fe or Fe3C sites and transforms to CO through reactions 5 and 6, which may cause the shoulder peak toward the CO volume fraction observed in Figure 8, with the Fe and Fe3C species acting as a bridge to transport oxygen from CO2 to carbon to form CO with high selectivity.

Fe + CO2 f CO + FeO

(5)

Fe3C + CO2 f 2CO + 3Fe

(6)

Isothermal Solid-Solid Reactions. Figure 9 shows the product evolution during the reaction of carbon with CeO2 in

Figure 10. Byproduct composition during the in situ oxidation of a CeO2-Fe2O3 sample after reaction with methane for 60 min at 800 °C.

the presence of Fe species in a N2 flow at 800 °C. Significant quantities of CO were produced during the preliminary stage, with only a little CO2 (e2.7%) detected. Once the oxygen in CeO2 was consumed, the CO2 and CO volume fractions declined rapidly. As the analysis above, CO2 is most likely produced by the reaction between carbon and the oxygen surface adsorbed on oxides, with the lattice oxygen reacting selectively with carbon deposits to form CO. The fact that CO and CO2 appear simultaneously during the early stage of the reaction in Figure 9 indicates that the reduction of the bulk lattice oxygen of CeO2 must occur simultaneously with the reduction in surface oxygen at 800 °C. The volume fractions of CO and CO2 were 8.0 and 1.5% at 650 °C (Figure 8), respectively, at the beginning of the temperature-programmed reaction but increased to 34.0% for CO and 2.7% for CO2 at 800 °C (Figure 9). Because the rate of oxygen migration from the bulk to the surface increases with rising temperature, more CO is formed through the selective oxidation of carbon by the lattice oxygen of CeO2 at the higher reaction temperature. There is also the possibility that the combined reactions 5 + 6 promote the transformation from CO2 to CO at 800 °C. We found no CO2 in the outlet gas, and the CO volume fraction declined smoothly through the 14th min, indicating that the deep-seated lattice oxygen had better selectivity but a slow migration rate. Isothermal Reactions in the Presence of Air. The experiments above show that cerium oxide containing Fe species exhibits good performance in the selective oxidation of carbon species to CO in the absence of gaseous oxygen at high temperature. The oxidation of carbon deposits on catalysts by gaseous oxygen in atmospheric air is an essential step in catalyst regeneration. Because CO is a more applicable byproduct of carbon removal than CO2, it is important to investigate the possibility of producing CO through oxidative regeneration of methane decomposition catalysts. The in situ regeneration results of a reduced CeO2-Fe2O3 sample (after treatment in pure methane (10 mL/min) for 60 min at 800 °C) are shown in Figure 10. It shows that the reaction is initiated smoothly and a little CO (14.9%) and CO2 (0.5%) are produced. The CO volume fraction increased rapidly from the 1st to 9th min and maintained a stable state until the 23rd min before declining sharply. The CO2 volume fraction shows, on the other hand, a more linear trend and stays at a relatively low level (e3.2%) throughout the experiment. No O2 was observed during the initial 37 min, and the gas composition gradually returned to that of air from the 38th to 44th min.

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Carbon monoxide is the primary byproduct during carbon oxidation, while carbon dioxide forms from both the direct oxidation of carbon and the consecutive oxidation of CO,22 which supports the premise that the CO is derived directly from the selective oxidation of carbon. Since the carbon deposits may initially cover the active centers to such a degree, the oxidation of surface carbon by O2 is therefore slow, and little CO forms. As the reaction proceeds, the active centers become more exposed, and carbon conversion is strongly enhanced. Further, with increased consumption of carbon deposits, more CO is sequentially oxidized to CO2 by the excess air. We found CO still coexisting with O2 from the 38th to 39th min during the oxidation reaction, which would suggest that the selective oxidation of carbon to CO could still occur in the presence of excess oxygen, a result we attribute to the presence of the cerium iron catalyst in the experiment. It is common to introduce O2 to remove deposited carbon in the regeneration of methane decomposition catalysts, although this produces CO2 as the primary byproduct rather than CO.36 Our results, however, found CO as the main carbon oxidation byproduct from the cerium iron catalyst, which suggests that the cerium iron species must be playing a special role in the carbon oxidation reaction at 800 °C. As seen in Figure 10, only a little CO2 appears when the volume fraction of CO decreases sharply from the 23rd to 39th min. Given the oxygen balance, other reduced solid species must have been oxidized in this process, which is confirmed by the fact that the ratio of n(O2)/n(N2) was always smaller than that of air, even from the 40th to 44th min without other oxidation products. Since only cerium and iron species were in this system, the states of cerium and iron species should have always been cerium suboxides (CeO2-x) and reduced iron during the carbon oxidation process. The descriptions in Figure 8 indicate that the deep-seated lattice oxygen in CeO2 was utilized to form CO with better selectivity. Furthermore, reactions 5 and 6 are feasible under the reaction conditions shown in Figure 10, implying that the cerium suboxides (CeO2-x) such as Ce3+ and reduced iron species (e.g., Fe, Fe3C) must be taking an active part in the selective carbon conversion. Reactions 7-12 describe the oxidation of the cerium suboxides, reduced iron species, and carbon by air

O2 + C f CO2

(7)

O2 + 2C f 2CO

(8)

xO2 + 2CeO2-x f 2CeO2

(9)

xO2 + 2Fe f 2FeOx

(10)

(3x + 1)O2 + 2Fe3C f 6FeOx + 2CO

(11)

(3x + 2)O2 + 2Fe3C f 6FeOx + 2CO2

(12)

The oxidation process contains eliminating carbon from the surface of the catalyst and recruits the lattice oxygen of the cerium and iron oxides with oxygen. All of the reactions 7-12 need oxygen, and there must be some form of competition between these processes. To improve the understanding of the roles of cerium and iron species, an in situ regeneration experiment with iron on zirconia

Figure 11. Byproduct composition during the in situ oxidation of ZrO2-Fe2O3 after reaction with methane for 48 min at 800 °C.

as the inert carrier was performed under the same experimental conditions (Figure 11). Figure 11 presents result profiles for the volume fractions of gaseous effluent similar to those for CeO2-Fe2O3 (Figure 10). The carbon deposits are initially converted to CO by air, with most of the CO2 formed in the later stages of the oxidation process. CO2 formation from the ZrO2-Fe2O3 sample was much greater than that for CeO2-Fe2O3. For example, in the 29th minute, the volume fraction of CO2 attained 10.3% for ZrO2-Fe2O3 sample, while it was only 3.2% for the CeO2-Fe2O3 sample. Dai et al.37 found CO produced during the pulsing with oxygen after applying pulses of methane using a LaFeO3 catalyst (carbon deposits formed during the methane pulses), with only a very small amount of CO2 detected. The results shown in Figures 10 and 11 suggest that reduced iron species can enhance CO formation during the oxidation of carbon deposits by O2, especially during the early stages of the reaction, while the cerium species should be active for CO formation in the later stages of the reaction. Also, the better CO selectivity obtained from the reduced CeO2-Fe2O3 sample may confirm the presence of an interaction between cerium and iron species for the selective oxidation of carbon deposits. Three detailed reaction mechanisms have been proposed in the oxidation of carbon black on metal oxide catalysts, a redox mechanism, a “push-pull” mechanism, and a spillover mechanism.38 The redox mechanism consists of carbon oxidation by oxygen (either surface or bulk oxygen) from the catalyst and reoxidation and thus regeneration of the catalyst by gas-phase oxygen. The “push-pull” mechanism is derived from the redox mechanism but involves the adsorption of both the reactant and oxygen on the active site of a catalyst with the reactant adsorbed on the catalyst surface followed by carbon oxide formation (with the incorporation of lattice oxygen) and its release, in combination with a simultaneous reoxidation of the active site. This mechanism is suitable for oxidation of carbon on oxides that are easily reduced. The spillover mechanism is often proposed to explain synergetic effects of two metal oxides in selective oxidation reactions. Oxygen is adsorbed and activated on one metal oxide (the donor) and transferred to the second metal oxide (the acceptor), on which carbon reacts with the adsorbed oxygen. There is also the possibility that the three mechanisms may occur simultaneously. In our experiments, the catalysts were cerium suboxides or reduced iron species rather than stoichiometric CeO2 or Fe2O3. Since the reduced iron species enhanced the decomposition of methane, carbon was most likely present on or near the metal surface instead of on the ceria or zirconia surface during the

Selective Oxidation of Carbon reaction between methane and the CeO2-Fe2O3 or ZrO2-Fe2O3 samples. Moreover, the adsorption of the oxidants on reduced metallic sites is generally much faster than that on carbon sites.39,40 The XRD measurement (Figure 2b) showed that metallic Fe and Fe3C species were well dispersed over the surface of the reduced CeO2-Fe2O3 sample, and both particle types were small, which would provide desirable sites for the adsorption of oxygen. In addition, reactions 5 and 6 may restrain the formation of CO2. It is reasonable to assume that the oxygen adsorbed on the reduced iron species is active and reacts first with carbon deposits to produce CO. Mul et al.21,38 found no oxygen spillover for an Fe2O3 catalyst during the complete oxidation of carbon black, in which only surface oxygen participated. The adsorption of oxygen on the surface of the reduced Fe species should only form a surface-layer oxygen species (active oxygen) rather than proceed to a complete oxidation state (i.e., Fe2O3). With the consumption of carbon deposits, more and more reduced iron species can be oxidized to higher oxidation states, thus allowing increased CO2 formation from the Fe2O3 catalyst. Several authors have shown the ease with which cerium suboxides can be reoxidized and restore their oxygen deficiency to form stoichiometric CeO2.4,5 Zhao et al.41 investigated CO oxidation using Ce0.67Zr0.33O2 by means of alternate dynamic pulses of CO-O2 measurements. His findings indicated that O2 was first adsorbed onto Ce3+ sites (oxygen vacancies) where the oxygen had been consumed by the reduction of CO. Oxygen migration at high temperatures was therefore considered to be the rate-limiting step for CO oxidation. Moreover, since the surface of cerium suboxides is quite large, the occurrence of adsorption and activation of oxygen on the cerium suboxides to form CeO2 was highly likely. Hofstad et al.42 reported that oxygen was transported not only through bulk diffusion exchange between two solids but also through surface spillover. Surface spillover is most likely when the surface is rich in oxygen. Moreover, the spillover of oxygen from CeO2 to the soot surface and its subsequent adsorption on the active carbon sites is considered an important intermediate step in soot oxidation in the presence of CeO2- and CeO2-containing catalysts.43 The possibility that CO forms through the carbothermic reduction of ceria (reaction 1) is therefore very likely, although there may exist a loose connecting condition between the cerium oxides and the carbon deposits. The remarkable oxidation activity and synergetic effects were observed for the cerium-iron catalysts in our TPR experiment (Figure 7) as well as in literature.18,31,44,45 It is proved that the Fe species could act as an O2 dissociative adsorption site that enhances the surface reaction between reactants on the neighboring sites.46 On the other hand, the excellent adsorption ability of oxygen from cerium suboxides may defer the oxidation of reduced Fe species (i.e., Fe and Fe3C), which is beneficial to the formation of active oxygen. It may be that there are synergistic effects between the Ce and Fe species during the selective oxidation of carbon, an assumption that agrees with the overwhelming CO generation during the reaction between CeO2 and carbon deposits in the presence of reduced Fe species (Figure 9). On the basis of these trends, we propose a reverse “push-pull” mechanism whereby oxygen is adsorbed and activated on the surface of CeO2-x or reduced Fe species (i.e., Fe and Fe3C) (“pull” step) and forms CeO2 or active oxygen (activated by Fe or Fe3C species), followed by the oxidation of carbon by CeO2 or active oxygen (carbon oxide formation and release-“push” step) in combination with a simultaneous formation of the active site.

J. Phys. Chem. C, Vol. 113, No. 34, 2009 15295 It should be mentioned that Fathi et al.17 pulsed oxygen using Pt/CeO2/γ-Al2O3 after treatments in methane, and the carbon formed by methane decomposition was oxidized selectively (almost 100%) to CO at 700 °C. Cerium oxide seems to be responsible for the formation of CO. However, Odier et al.47 observed that carbon deposits extracted from methane using Pt/ CeO2 were released essentially as CO2 during the oxidation step at 400 °C. Moreover, Gemmi et al.,23 in a redox reaction between iron oxide/CeO2 solid and either methane or air performed at 470 °C, found only CO2 in the outlet gas rather than highly selective CO formation in our present experiment at 800 °C. This suggests that reaction temperature affects the oxidation mechanism of carbon deposits. Figure 12 shows CO and CO2 formation during the in situ oxidation of a reduced CeO2-Fe2O3 sample (obtained after reacting with methane for 60 min at 800 °C) by air at 400, 600, and 700 °C. CO is the main oxidation byproduct at both 600 and 700 °C, while the amount of CO2 at 600 °C (7.7% for the max) is a little larger than that at 700 °C (5.3% for the max). When the oxidation temperature was 400 °C, only a little CO (