23870
J. Phys. Chem. B 2006, 110, 23870-23880
Relevance in the Fischer-Tropsch Synthesis of the Formation of Fe-O-Ce Interactions on Iron-Cerium Mixed Oxide Systems F. J. Pe´ rez-Alonso,† M. Lo´ pez Granados,† M. Ojeda,† T. Herranz,† S. Rojas,† P. Terreros,† J. L. G. Fierro,*,† M. Gracia,‡ and J. R. Gancedo‡ Instituto de Cata´ lisis y Petroleoquı´mica, (CSIC), Marie Curie 2, 28049 Madrid, Spain, and Instituto de Quı´mica-Fı´sica Rocasolano (CSIC), Serrano 119, 28006 Madrid, Spain. ReceiVed: July 19, 2006; In Final Form: September 14, 2006
A series of Fe-Ce mixed oxides (95 atom % Fe-5 atom % Ce) has been prepared by different methods: coprecipitation, impregnation, and physical mixture of Ce and Fe oxides. These solids have been tested in the Fischer-Tropsch synthesis. The characterization of the catalytic precursors was carried out by X-ray diffraction (XRD), Raman, Mo¨ssbauer, and X-ray photoelectron (XPS) spectroscopic techniques. When the preparation method ensures a microscopic contact between Fe and Ce cations in the solid, several types of Fe-Ce interactions are present in the calcined solids. The interactions take the shape of Fe-O-Ce bridges that can exist either in the hematite-like solid solution or in the interphase between the Fe oxide covered by microcrystals of Ce oxide. In the case of the hematite-like solid solution, Ce(IV) cations are dissolved in the R-Fe2O3 network. The promotion by Ce of the catalytic properties observed in the final catalysts can be directly related with the detection of these Fe-O-Ce bridges in the calcined solids. The Ce promotion results in a larger yield to hydrocarbons, a higher production of olefins, and a higher selectivity to medium and large chain hydrocarbons (larger than six carbon atoms). It is proposed that the Ce promotion is due to the presence of Fe0-Ce(III) ensembles in the final catalysts arising from the initial Fe-O-Ce bridges developed in the parent calcined samples.
1. Introduction The synthesis of hydrocarbons by CO hydrogenation, the socalled Fischer-Tropsch synthesis (FTS), is primarily governed by a polymerization mechanism. The distribution of products can be, in a first approximation, described by the AndersonSchulz-Flory (ASF) model for polymerization processes. According to this model, the growth of hydrocarbon chains occurs by the successive incorporation of monomer units (methylene species). In principle, the chain growth probability (R) determines the composition of the pool of hydrocarbons.1-3 One of the challenges of the Fischer-Tropsch synthesis is to control the distribution of products. Although the operation conditions (pressure, contact time, reactor selection, or temperature) can be tuned for directing the FTS toward a desired target pool of products, in the practice the effect is limited. The promotion of conventional FT catalysts (based either on Fe or Co oxides) by certain additives has shown promising results. Much research work exists on the effect of some promoters such as K, Cu, Ru, Zn, and Al.3 However few studies exist with respect to the promotion of Fe based FT catalysts by rare earth (RE) oxides. In a previous study carried out on Fe-Ce catalysts,4 it was found that the existence of Fe-Ce interactions in the calcined precursor defines the final catalytic behavior of Fe-based catalysts. Higher CO conversion rates, higher hydrocarbon formation rates, and higher olefinicity degree were observed in the catalysts prepared from precursors exhibiting Fe-Ce interactions. These Fe-Ce interactions are the result of the * Corresponding author:
[email protected]. † Instituto de Cata ´ lisis y Petroleoquı´mica. ‡ Instituto de Quı´mica-Fı´sica Rocasolano (CSIC).
formation of hematite-like and/or ceria-like solid solutions. A coprecipitation method was required to form the solid solution, whereas the thermally driven solid-state reaction between hematite and ceria is unable to form such solid solutions. In the hematite-like solid solution, the Ce cations are dissolved in the hematite structure. On the other hand, Fe cations are dissolved in the cubic ceria-like solid solution of the cerium oxide structure. Therefore, the Fe-Ce interactions can occur through the formation of Fe-O-Ce bridges either in the hematite- or in the ceria-like solid solutions. The hematite-like solid solution predominates in the samples with less than 15 atom % of Ce. Below this threshold, the cubic ceria-like solid solution (fluorite-type structure) prevails. Thus, ceria-like mixed oxide was detected in a solid with 50 atom % of Fe and 50 atom % of Ce. The Fe-Ce catalysts display an induction period under the FTS reaction environment. It is well-known that with Fe-based catalysts, a reduction process and a partial carburization of the Fe phases takes place. Several types of Fe carbides have been reported to coexist under the reaction conditions,3 and as a matter of fact, it has been proposed that the active phase for the FTS is an Fe carbide-like site.5 Therefore, the exact nature (physical and/or chemical appearance) of the Fe-Ce interaction in the final catalyst cannot be described by the presence of Fe-OCe bridges in the solid solutions of the fresh samples. However, it is expected that such a Fe-O-Ce interaction in the fresh sample will determine the nature of the Fe-Ce interaction in the final state of the catalyst. In fact, the final catalytic properties are determined by the initial state of the calcined precursor.4 In this work, we have studied the catalytic properties of a series of Ce-promoted Fe catalysts for the FTS. The calcined precursors were prepared by different methods with the aim of
10.1021/jp064575f CCC: $33.50 © 2006 American Chemical Society Published on Web 10/27/2006
Fischer-Tropsch on Iron-Cerium Mixed Oxides controlling the type and/or the extension of the Fe-Ce interactions, that is, by modifying the nature of the Fe-O-Ce bridges present in the Fe-Ce oxide. We have observed that catalysts derived from ceria-like mixed oxides present a higher catalytic activity per gram of Fe. However, the dilution effect of the larger amount of Ce required to form the ceria-like solid-solution results in catalysts with a lower activity per gram of solid. Taking into account these facts, and considering the lower price of any Fe precursor with respect to any Ce precursor, the catalysts here studied were prepared with low Ce concentration (Fe/Ce atomic ratio) 95/5). Three different strategies of Ce incorporation have been studied: impregnation, coprecipitation, and solid-state reaction between Fe oxide and Ce oxide. The catalytic properties of the different systems have been evaluated not only to find out the best method to prepare Fe-Ce catalysts, but also to gain fundamental information on the effect of the Fe-Ce interactions developed in the calcined precursors on the catalytic properties of Fe systems. The characterization work was focused on the calcined phase, which is the solid loaded in the reactor, paying specific attention to the presence and to the chemical nature of the Fe-O-Ce bridges present in mixed oxides. 2. Experimental Section 2.1. Catalyst Preparation. A series of iron-cerium catalysts precursors (95 atom % Fe, 5 atom % Ce) has been prepared by different methods: coprecipitation, impregnation, and solid-state reaction. All precursors were calcined under ambient air at 573 K for 6 h to obtain the calcined precursor that will be later loaded in the reactor. Next, we will describe the preparation procedures. 2.1.1. Coprecipitation Methods. The control of the pH during the coprecipitation of Fe and Ce ion has been accomplished in two different ways: with a pH-state and with a bicarbonate/ carbonate buffer solution. Microemulsions have been used in an attempt to achieve a more homogeneous precipitation since this methodology prevents the presence of important gradients of pH during precipitation and constrains the precipitation to nanoambients. Coprecipitation Method with pH-State. The catalytic precursor was prepared by batchwise coprecipitation using an aqueous solution of Fe(NO3)3 and Ce(NO3)3 (1 M) with a NH4OH (5.6 M) solution at constant pH (8.0) at 343 K, as it has been already detailed.4 The pH was kept constant by the controlled addition of the NH4OH solution with a pH-state. The precipitate was filtered off, washed thoroughly with distilled water, dried at 323 K for 24 h and then treated in air at 573 K for 6 h. This catalytic precursor is referred as FeCe-P. Coprecipitation Method Using a Bicarbonate/Carbonate Buffer. The catalytic precursor was prepared by coprecipitation through dropwise addition of an aqueous solution of the corresponding Fe and Ce nitrates (0.5 M), to a solution of NaHCO3 (1 M) at 353 K under vigorous stirring. CO2 was bubbled through the solution in order to keep constant the pH (around 8.5) by the formation of a bicarbonate/carbonate buffer solution. The aqueous solutions were previously degassed with N2. The precipitate was filtered off, washed thoroughly with distilled water, dried at 323 K for 24 h and then treated in air at 573 K for 6 h. This catalytic precursor is referred as FeCe-C. Microemulsion Method. Two different iron-cerium precursors were prepared by this methodology.6 A first sample was prepared from two individual microemulsions with the appropiate amounts of Fe and Ce by addition of a corresponding aqueous solution of respective nitrates to a mixture of Tergitol
J. Phys. Chem. B, Vol. 110, No. 47, 2006 23871 15-S-5 (surfactant) and isooctane (oil phase). A microemulsion containing NH4OH was also prepared. The composition (weight percentage) of the microemulsions was 70, 30, and 10% for the surfactant, oil, and aqueous phases, respectively. A transparent orange colored mixture (because of the Fe(III)) was obtained by mixing the Fe and Ce microemulsions, suggesting the formation of stable nanodroplets in oil. After this, the NH4OH microemulsion was rapidly added to the Fe-Ce microemulsion under vigorous stirring, and a brown precipitate was observed immediately. The final mixture was aged under vigorous stirring for 2 h and left to decant overnight. The precipitate was filtered off, washed with water and ethanol, dried overnight at 383 K, and then treated in air at 573 K for 6 h. This catalytic precursor is referred as FeCe-M1. A second sample was prepared by dropwise addition of the Fe-Ce microemulsion to the NH4OH microemulsion under strong stirring. A brown gel was formed that was aged under stirring for 2 h. In this case, it was necessary to add tetrahydrofurane (THF) in order destabilize the gel. The solid was left to decant overnight, filtered off, washed thoroughly with ethanol, dried overnight at 383 K, and then treated in air at 573 K for 6 h. The so obtained calcined precursor is referred as FeCe-M2. 2.1.2. Impregnation Methods. Cerium Impregnation on Iron Oxyhydroxide (FeOOH). First, iron oxyhydroxide was prepared by precipitation by fast addition of a 5.6 M NH4OH solution to a 1 M aqueous solution of Fe(NO3)3 solution at constant temperature (353 K) and without controlling the pH. The oxyhydroxide precipitate was filtered off and washed with distilled water. Two different iron-cerium precursors were prepared by wet impregnation. A first sample was obtained by wet impregnation of the iron oxyhydroxide precipitate (without calcination) with an aqueous solution of Ce(NO3)3 (1.5 M) in a rotary evaporator at 333 K and 1 kPa. The solid was dried at 323 K for 24 h and then treated in air at 573 K for 6 h. The catalytic precursor obtained is referred as FeCe-I. Cerium Impregnation on Iron Oxide (R-Fe2O3): This catalyst precursor was obtained by wet impregnation of iron oxide (RFe2O3), which was obtained by treatment of the iron oxyhydroxide in air at 573 K for 2 h. Therefore, the Ce incorporation is performed over a hematite solid, in contrast to the previous preparation, in which Ce was deposited over the iron oxyhydroxide. The solid thus obtained was dried at 323 K for 24 h and then treated in air at 573 K for 4 h. The obtained catalytic precursor is named as FeCe-IC. 2.1.3. Physical Mixing. The catalyst precursor was prepared by physically mixing the pure Fe and Ce oxide precursors obtained by batchwise precipitation under the conditions described in section 2.1.1. The pure precipitates were mixed in the required ratio in order to obtain a mixed oxide (95 atom % Fe, 5 atom % Ce). The thermogravimetric analyses of the pure Fe and Ce precursors were used for calculating the weight loss during the decomposition of the pure precursors to the pure oxides. These precursors were finely ground before treatment in air at 573 K for 6 h. The calcined precursor is referred to as FeCe-PM. For the sake of comparison, pure iron oxide (R-Fe2O3) and cerium oxide (c-CeO2) samples were also prepared by precipitation at constant pH (8.0) and temperature (343 K) (as in the preparation of FeCe-P). The solids are named as Fe and Ce, respectively. 2.2. Characterization Techniques. The SEM-EDS analyses were obtained on a Philips XL30 scanning electron microscope equipped with an energy-dispersive X-ray analyzer (EDAX DX4i) at 20 keV to obtain quantitative information on the
23872 J. Phys. Chem. B, Vol. 110, No. 47, 2006 distribution of Fe and Ce atoms. To avoid charging phenomena, powder samples were sputter coated with gold with a Sputter Coater SC502. Powder X-ray diffraction (XRD) patterns were recorded in the 5-80° 2θ range in scan mode (0.02°, 2 s) using a Seifert 3000 XRD diffractometer equipped with a PW goniometer with Bragg-Brentano θ/2θ geometry, an automatic slit, and a bent graphite monochromator. The unit cell parameters were obtained by refining the peak positions of the XRD patterns with a least squares refinement method using the CELREF program (CELREF unit-cell refinement software for Windows by Laugier and Bochu, http://www.ccp14.ac.uk/). To determine peak positions, the peak profiles were fitted with the commercially available ANALYZE program (pseudo-Voigt function). Raman spectra were recorded with a Renishaw 1000 spectrophotometer equipped with a cooled CCD detector (200 K) and a holographic Notch filter that removes the elastic scattering. The samples were excited with the 633 nm line of a He-Ne laser. The band at 520 cm-1 of a silicon wafer was used for the frequency calibration. Acquisition of spectra consisted of 10 accumulations of 30 s collected at room temperature under a N2 flow (ca. 100 mL min-1) in an in situ Linkam cell. X-ray photoelectron spectroscopy (XPS) studies were performed on a VG Escalab 200 R spectrometer equipped with a hemispherical electron analyzer and a Mg KR (1253.6 eV) X-ray source. The sample was first placed in a copper holder mounted on a sample rod in the pretreatment chamber of the spectrometer and then degassed at room temperature for 1 h before being transferred to the analysis chamber for the recording of the spectra of the calcined samples. Some selected samples were treated in situ in the cell of the spectrophotometer. First, a pulse of H2 (50 kPa) was admitted in the cell, and then the sample was heated to 673 K for 16 h. Subsequently, it was cooled to room temperature, then the H2 was evacuated, and a pulse (around 50 kPa) of a mixture of H2/CO/N2 (31/62/7 molar ratio) was admitted. The sample was under this mixture at 673 K for 16 h and then cooled to room temperature before being transferred to the analysis chamber for the recording of the XPS spectra. A certain energy region of the XPS spectrum was scanned a number of times in order to obtain a good signalto-noise ratio. The binding energies (BE) were referenced by using the C 1s peak (284.6 eV) of spurious carbon as an internal standard. The areas of the peaks were computed by fitting the experimental spectra to Gaussian/Lorentzian curves after removal of the background (Shirley function). Surface atomic ratios were calculated from peak area ratios normalized by the corresponding atomic sensitivity factors.7 Nitrogen adsorption isotherms were recorded at the temperature of liquid nitrogen (77 K), using a Micromeritics ASAP 2000 apparatus. Samples were degassed at 413 K for 12 h prior to the determination of the adsorption isotherm. Transmission 57Fe-Mo¨ssbauer spectra were recorded at room temperature using a conventional constant-acceleration spectrometer equipped with a 57Co source. Finely ground powdered absorbers were prepared with a thickness of about 10 mg cm-2 of natural iron. The spectra were computer-fitted to a sum of Lorentzian-shaped lines by applying the constraints of equal line width and area for the two peaks of doublets, and equal line width and areas in the ratio 3:2:1:1:2:3 for the six peaks of sextets. A fitting procedure based on hyperfine magnetic field (H) distribution was used to fit spectra with magnetic broad components. The relative concentrations of the different Fe species were calculated from their spectral area ratios assuming equal f factors (probability of recoilless absorption) for all the
Pe´rez-Alonso et al. Fe species present in the spectrum. Isomer shifts were referred to the center of the R-Fe sextet at room temperature. 2.3. Catalytic Measurements. The catalysts were tested in the CO hydrogenation reaction using a fixed-bed microreactor (stainless steel 316, 9 mm i.d.). The reactor temperature was measured with a K-type thermocouple buried in the catalytic bed. All pipes after the reactor outlet were kept at 443 K. The reaction system was equipped with a stainless steel hot trap set at 423 K in order to collect the heavier products (hydrocarbons higher than C16). Flow rates were controlled using a Brooks 5850 TR series mass flow controller. To facilitate the heat transfer and to prevent the presence of hot spots owing to the exothermal character of the reaction, the calcined precursors (calcined samples) (200 mg, 0.25-0.30 mm particle size) were diluted with SiC (ca. 2 g, 0.25-0.30 mm particle size) following recommendations described by Perez-Ramirez et al.8 to prevent bypass because of dilution. First, the catalysts were activated in situ at 673 K (heating rate of 10 K min-1) for 16 h in a mixture 33 vol % H2/N2 (5.9 L h-1 gcat-1) at atmospheric pressure. The reactor was then cooled under N2 to the reaction temperature (573 K) and the system was pressurized to 1.01 MPa. Then, the flow was switched to synthesis gas (H2/CO ) 2; GHSV ) 0.0043 L s-1 gcat-1). This moment was considered as the initial time of the reaction. It was estimated that at this pressure and with this flow rate the N2 is flushed out the reaction system (including the reactor and downstream lines connecting the reactor with the GC) in 45-50 min. The catalytic activity was monitored for 120 h of time on stream (TOS). This period was considered to be long enough to achieve the building-up of the active phase. After this period, the catalytic properties were also tested at lower temperatures (493-523 K). Product analysis was performed on line with a gas chromatograph (HP 6890 Plus). A Porapak Q (1/8” x 3 m)-packed column connected to a thermal conductivity detector was used to analyze the inorganic gases (H2, N2, CO, CO2) and water. Hydrocarbons and oxygenated compounds were analyzed with a DB-1 capillary column (60 m × 0.25 mm × 0.25 µm) connected to a flame ionization detector. The equipment configuration allowed the analysis of C1-C16 hydrocarbons, C1-C10 alcohols and other oxygenated compounds. The carbon balance was always higher than 95%, indicating that the yield to hydrocarbons larger than C16+ is small (
