Ind. Eng. Chem. Res. 2006, 45, 4163-4168
4163
Complementary Physicochemical Characterization by SAXS and 129Xe NMR Spectroscopy of Fe-ZSM-5: Influence of Morphology in the Selective Catalytic Reduction of NO Ariel Guzma´ n-Vargas,† Enrique Lima,*,‡,§ Gerard Delahay,| Bernard Coq,| and Victor Lara‡ Instituto Polite´ cnico Nacional - ESIQIE, AVenida IPN UPALM Edifice 7, Zacatenco, 07738 Me´ xico D.F., Me´ xico, UniVersidad Auto´ noma Metropolitana, Iztapalapa, AVenida San Rafael Atlixco No. 186, 09340 Me´ xico D.F., Me´ xico, Instituto de InVestigaciones en Materiales, UniVersidad Nacional Auto´ noma de Me´ xico, Ciudad UniVersitaria, 04510 Me´ xico, D.F. Me´ xico, and Laboratoire de Mate´ riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618-CNRS-ENSCM, Institut Charles Gerhardt FR 1878, 8, rue de l’Ecole Normale, Montpellier CEDEX 5, France
Fe-ZSM-5 catalysts were prepared by FeCl3 sublimation (CVD method) and impregnation in aqueous media. To describe the morphology of the iron species, the materials were characterized by XRD, SAXS, and 27Al and 129Xe NMR spectroscopy. In the catalysts prepared by impregnation, XRD detected large hematite particles at the external surface of the zeolite. In contrast, the Fe2O3 phase was not detectable by XRD in the materials prepared by CVD, indicating much smaller species. The 129Xe NMR results show that, in the CVD-prepared materials, the amount of free space inside the zeolite cavities was decreased appreciably by the presence of the Fe oxo species. Furthermore, 129Xe NMR spectroscopy reveals a sharp peak for the CVD-prepared material with Fe (3.8 wt %), indicating that this catalyst exhibits strong adsorption sites that are not detectable in the sample of similar Fe content that was prepared by impregnation. Morphology appears to be strongly affected by the method of preparation of the catalyst. The fractal dimension of the metal phase was associated with the catalytic activity in the selective catalytic reduction (SCR) of NO using ammonia and n-decane as reducing agents. Introduction In heterogeneous solid-gas catalysis, the activity of a catalyst is determined, on one hand, by the intrinsic activity of the active component and, on the other, by effects of mass transport. The latter are strongly associated with the diffusion (intra- or intercrystalline) of the reactants and products.1,2 The intrinsic activity depends, of course, on the chemical and physical surface properties of the involved species. However, little attention has been paid to linking the surface properties to the atomic arrangement in the bulk. Indeed, the surface of some materials can be visualized as the external layer on the disturbed edges where interatomic distances are different from those in the bulk. The bulk arrangement is highly ordered, and impurities or defects can be negligible. Often, the most active surfaces correspond to materials containing a less-ordered bulk structure. In the case of solid supported catalysts, the morphology of the dispersed phase is a crucial parameter for the catalytic activity.3 Evidently, surface properties of support are also important in performance. In this sense, molecular sieves, particularly zeolites, have been extensively applied as excellent industrial catalysts or catalyst supports for important reactions, e.g., cracking of paraffins and isomerization of aromatic compounds. The zeolites most widely used in the catalysis field are X, Y, and ZSM-5. The latter has made viable the production of “synthetic gasoline” by the catalytic conversion of methanol to a mixture of aromatic and aliphatic hydrocarbons.4-6 * To whom correspondence should be addressed. E-mail: lima@ xanum.uam.mx. Fax: (525) 58044666. Tel.: (525) 58044667. † Instituto Polite ´ cnico Nacional - ESIQIE. ‡ Universidad Auto ´ noma Metropolitana. § Universidad Nacional Auto ´ noma de Me´xico. | Institut Charles Gerhardt FR 1878.
Concerning environmental catalysis, the selective catalytic reduction (SCR) process has been one of the most studied methods for reducing NOx emissions. Particularly, the use of nonammonia reductants, e.g., hydrocarbons, has attracted the interest of the scientific community, because of the harmful effects of NOx emissions from exhaust gases issued by heavy cars powered by diesel engines and emissions from stationary sources.7,8 In the past decade, zeolite-based catalysts containing transition metal ions have shown remarkable activity in DeNOx reactions in lean conditions (rich oxygen atmosphere), even in the presence of H2O and SO2. Indeed, NOx elimination is an urgent environmental protection goal because of its contribution as a precursor of photochemical clouds and acid rain and the generation of fine particles and ozone.9,10 NOx reduction by hydrocarbons using Fe-ZSM-5 materials has been the subject of many experimental and theoretical studies. It has been clearly shown that the method of preparation of the catalyst is a critical factor in obtaining active iron species. Thus, different species have been proposed: binuclear oxo ions [HO-Fe-O-Fe-OH]2+, mononuclear [Fe-O]+, nanoclusters Fe4O4, and FeOx nanoparticles.11-13 However, the largest number of studies related to these species are focused on the chemical structure, and hardly any attention has been paid to morphological aspects. Therefore, the influence of geometric aspects of the particles containing Fe on the catalytic activity remains unclear. We have focused this work on showing how the preparation method directly influences the structure and morphology of the catalysts and, thereby, their catalytic performance. A fractal geometry approach was employed to characterize the iron particles.
10.1021/ie0601596 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/05/2006
4164
Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006
Experimental Section Material Preparation. The various Fe-ZSM-5 samples were prepared by FeCl3 sublimation (CVD) and impregnation (IMP). The sample coding is Fe(x)-ZSM5-CVD or -IMP, where x is the molar Fe/Al ratio. The starting zeolite was NH4-ZSM5 (Zeolyst CBV3024E, Si/ Al ) 15, SBET ) 400 m2 g-1). The Fe precursors were FeCl3 (Aldrich, >97%) and Fe(NO3)3‚9H2O (Fluka, >99%). (i) Fe(0.5)-ZSM5-CVD, Fe(0.83)-ZSM5-CVD. These two catalysts were prepared according to the protocol reported by Chen and Sachtler.14,15 Briefly, zeolite H-ZSM-5 was obtained by calcination in air of NH4-ZSM-5 at 873 K. FeCl3 as a separated bed was then sublimed at 593 K onto the H-ZSM5 zeolite, where it reacted chemically with the protons according to the reaction
H+ + FeCl3 ) [FeCl2]+ + HClv The [FeCl2]+-loaded sample was washed with water and calcined in air at 823 K. (ii) Fe(0.65)-ZSM5-IMP, Fe(0.87)-ZSM5-IMP. The amount required of an aqueous Fe(NO3)3‚9H2O solution to obtain the Fe desired content was dropped onto NH4-ZSM-5 at 398 K. Then, the solid was dried at 353 K and calcined at 823 K. Characterization. The materials were characterized by X-ray diffraction, small-angle X-ray scattering, xenon adsorption, and 129Xe and 27Al nuclear magnetic resonance spectroscopy. The X-ray diffraction (XRD) powder patterns were obtained with a Siemens D-5000 diffractometer coupled to an X-ray copper anode. KR radiation was selectively monochromated using a Ni filter. Conventional identification of crystalline compounds was performed by comparing the difractograms with JCPDS (Joint Committee on Powder Diffraction Standards) files. The average size of crystallites was calculated from the DebyeScherrer equation. Corundum alumina was used as a reference. Small-angle X-ray scattering (SAXS) experiments were performed with a Kratky camera coupled to a copper anode tube. The distance between the sample and the linear proportional counter was 25 cm; a Ni filter selected the Cu KR radiation. To avoid any interference or absorption of the sample holder, the powdered sample was packed in the window of an aluminum sample holder. The SAXS data were processed with the ITP program,16-20 where the angular parameter (h) is defined as h ) 2π sin θ/λ; θ and λ are the X-ray scattering angle and the wavelength, respectively. The shape of the scattering objects was estimated from the Kratky plot, i.e., h2I(h) vs h. The shape of the particles is determined by the shape of the Kratky curve; for instance, if the curve presents a peak, the particles are globular.21 If a shape can be assumed,19 the size distribution function can be calculated. From the slope of the curve log I(h) vs log(h), the fractal dimension of the scattering objects was calculated.22,23 The h interval was 0.07 Å-1 < h < 0.18 Å-1. Note that, by the Babinet principle, the small-angle X-ray scattering might be due to either dense particles in a low-density environment or low-density inclusions in a continuous medium of high electron density. One-pulse solid-state 27Al MAS NMR spectra were obtained at a frequency of 78.15 MHz on a Bruker ASX 300 spectrometer using a 4-mm MAS probe with a spinning rate of 10 kHz. The calibrated π/2 pulse was 2 µs, and the repetition time was 0.5 s. The chemical shift reference was [Al(H2O)6]3+.
Table 1. Iron Contents of Different Fe-ZSM-5 Solids sample
Fe content (wt %)
Fe(0.65)-ZSM5-IMP Fe(0.87)-ZSM5-IMP Fe(0.5)-ZSM5-CVD Fe(0.83)-ZSM5-CVD
3.00 3.90 2.12 3.80
Xenon gas (Praxair, 99.999%) was used for the 129Xe NMR measurements. For these experiments, the sample powder was placed in an NMR tube equipped with J. Young valves, through which the xenon gas was equilibrated with the sample at 291 K under different pressures. Before xenon loading, the samples were dehydrated by gradual heating to 673 K in a vacuum (1.33 × 10-4 kPa). 129Xe NMR spectra were recorded at 291 K in a Bruker DMX-500 spectrometer operating at 138.34 MHz. Single excitation pulses were used, and at least 1000 scans were collected with a delay time of 2 s. The chemical shift was referenced to xenon gas extrapolated to zero pressure. Catalytic Tests. The selective catalytic reduction (SCR) of NO was studied in a flow reactor operating at atmospheric pressure. The catalytic tests were carried out in a temperatureprogrammed surface reaction protocol (TPSR) from 473 K to 823 K, heating at 10 K min-1. Catalyst aliquots (∼100 mg) were activated in situ at 823 K in air (O2/N2 ) 20/80) for 2 h and then cooled to 473 K. The reaction gas for SCR was varied as follows: (1) In the presence of n-decane, a mixture of NO/ n-decane/O2/He (0.1/0.03/9.0/90.87) was fed to the reactor. The gas hourly space velocity (GHSV) was 35 000 h-1. (2) In the presence of NH3, the feed conditions were NO/NH3/O2/He (0.2/ 0.2/3.0/96.4), which gave a GHSV of 332 000 h -1. The effluent composition was monitored continuously by online sampling using a quadruple mass spectrometer (Pfeiffer Omnistar) equipped with Channeltron and Faraday detectors (0-200 amu) and by IR spectroscopy using a Bruker Vectra 22 equipped with a gas cell. The concentrations of the reactants and products were derived from intensities using standardization procedures. Results and Discussion The iron contents in various samples, as determined by chemical analyses, are reported in Table 1. The low iron concentrations in the two series are not very different (3.0 and 2.12 wt %). The same is true for the samples with high amounts of iron (3.9 and 3.8 wt %). Figure 1 presents the XRD patterns of the catalysts. The samples are crystalline, and they all were indexed as the H-ZSM-5 phase (JCPDS file 44-0002); no amorphous material was detected. To facilitate the presentation and discussion of the results, the diffraction patterns were grouped in two series according to the preparation method. In catalysts obtained by impregnation, the IMP series, the Fe2O3 phase (hematite) was detected as the narrow peaks at 33.6°, 40.9°, 48.3°, and 53.4°; they were clearly resolved. In contrast, in solids obtained by the sublimation method (CVD series), the peaks due to Fe2O3 were not observed. Nonetheless, it should be emphasized that, by XRD, a compound is observed if its content is higher than 3 wt % and its crystal size is larger than 3 nm. Hence, taking into account the amount of iron in the CVD samples, the iron species are probably well-dispersed, forming crystallites smaller than 3 nm. It is clear that the two methods influence the distribution of the iron species in the host structure differently. For the IMP series, the diffraction peaks associated with Fe2O3 were narrow and clearly resolved, indicating the formation of very large
Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006 4165
Figure 1. X-ray diffraction patterns of powdered samples: (a) Fe(0.65)ZSM5-IMP, (b) Fe(0.87)-ZSM5-IMP, (c) Fe(0.5)-ZSM5-CVD, and (d) Fe(0.83)-ZSM5-CVD. Peaks labeled + correspond to the hematite phase, Fe2O3 (JCPDS file 2-0919). All other peaks correspond to ZSM-5 zeolite (JCPDS file 44-0002).
Figure 2. 27Al MAS NMR spectra of (a) H-ZSM-5, (b) Fe(0.65)-ZSM5IMP, and (c) Fe(0.5)-ZSM5-CVD. Peaks labeled * correspond to spinning sidebands (samples spun at 10 kHz).
particles containing iron on the zeolite. Indeed, the mean sizes of the crystallites, determined by the Debye-Scherrer equation,24 were 66 and 86 nm for Fe(0.65)-ZSM5-IMP and Fe(0.87)-ZSM5-IMP, respectively. As mentioned above, diffraction peaks associated with the Fe2O3 phase were not observed in the CVD series. The CVD and IMP series contain similar amounts of Fe, but the results show that particles containing Fe are better dispersed in the CVD series. Note that the term “particles containing iron” used in this work implies different iron species, most probably iron oxide and iron oxo species. The latter are preferentially formed by the CVD method.13,25,26 The large particles containing iron detected in the IMP samples could promote local destruction of the zeolite network. Nevertheless, the 27Al MAS NMR spectra (Figure 2) show that the aluminum coordination remains unchanged after and before iron incorporation. In fact, the spectra were dominated by an intense signal assigned to tetrahedral aluminum, AlIV (53 ppm).
Figure 3. Uptake of xenon in (9) H-ZSM-5 zeolite, (b) Fe(0.65)-ZSM5IMP, and (2) Fe(0.83)-ZSM5-CVD.
In addition, a low-intensity peak associated with octahedral aluminum, AlVI (0.5 ppm), was also present.27 The AlIV/AlVI ratios for spectra b and c, which correspond to samples loaded with iron, were similar and comparable to that of the free iron sample, spectrum a. Nevertheless, the AlIV signal for the ironcontaining samples was slightly wider at the base, when compared to that of the free iron solid. This feature was expected, as the iron species could promote paramagnetic effects. Note that the AlVI signal was not affected by the presence of iron because these aluminum species are not coordinated unsaturated sites and they are definitely due to detrital alumina commonly present in the calcined zeolites. This result, combined with the XRD data, confirms that no significant structural changes in the zeolite take place upon loading with iron. Furthermore, it should be remarked that XRD does not provide any information about the incorporation of Fe into the channels. Therefore, 129Xe NMR spectra of adsorbed xenon on samples provide more information, as the chemical shift of xenon is very sensitive to the local chemical environment.28,29 Figure 3 presents xenon adsorption isotherms. At low pressures, a linear correlation between the xenon amount and the pressure was observed. The uptake trend toward saturation started at around 133 kPa. Samples Fe(0.65)-ZSM5-IMP and Fe(0.83)ZSM5-CVD had lower xenon adsorption capacities compared to that of the iron-free sample. This was expected because of the decrease of the free space in zeolite channels that are occupied by particles containing iron. However, this result cannot discriminate between whether the channels are partially or completely filled or the entry of the channels is blocked. This point is clarified in Figure 4, where the 129Xe NMR chemical shift variation is plotted as a function of xenon concentration. It is worth mentioning that all 129Xe NMR spectra showed a single and symmetrical peak. At high xenon concentration, as Figure 4 shows, the samples without iron and Fe(0.65)-ZSM5-IMP follow the same tendency. On the contrary, the curve corresponding to sample Fe(0.83)-ZSM5-CVD differs from the others. In fact, higher chemical shifts were observed, confirming the presence of strong adsorption sites, i.e., some iron particles are present inside the zeolite channels. Furthermore, the curves behave differently at low xenon concentration, indicating that the local densities were different in the three samples.
4166
Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006
Figure 5. Kratky plots of samples (a) Fe(0.87)-ZSM5-IMP, (b) Fe(0.65)ZSM5-IMP, (c) Fe(0.5)-ZSM5-CVD, and (d) Fe(0.83)-ZSM5-CVD. Figure 4. 129Xe chemical shift as a function of concentration of xenon in (9) H-ZSM-5 zeolite, (b) Fe(0.65)-ZSM5-IMP, and (2) Fe(0.83)-ZSM5CVD.
The classical expression for the chemical shift of xenon in contact with porous media is the equation proposed by Fraissard30
δ ) δ0 + δnf0 + δXe-Xe + δE + δM This states the interaction of xenon with the zeolitic matrix. The observed chemical shift, δ, includes contributions of all interacting factors: δ0 is the chemical shift of the reference and was fixed at zero, δnf0 is the parameter due to collisions between xenon atoms and zeolite walls, δXe-Xe corresponds to xenon-xenon collisions, δE arises from interactions of xenon with exchangeable cations, and δM is the contribution of iron species (in this case, particles containing iron). The chemical shift close to zero xenon concentration was calculated by extrapolation using low-concentration data; the part of the chemical shift arising from collisions between xenon atoms and “zeolite walls” (δnf0) was more noticeable in the CVD sample, supporting our hypothesis that, in Fe(0.83)-ZSM5CVD, Fe diffuses into the channels and the incorporated iron reduces the free space. Note that the chemical shifts plotted in the curves for the Fe zeolites should also include the contribution of some paramagnetic effects of the iron species, as suggested by the 27Al NMR results. In summary, the XRD and NMR results show that the preparation method determines the nature and the localization of the Fe particles on the zeolite surface. We now discuss the morphological aspects. The XRD and NMR results have shown that zeolites do not undergo structural changes as a consequence of iron incorporation. It can therefore be reasonably assumed that only inhomogeneities scattered by SAXS are associated with the particles containing iron. In addition, zeolites are homogeneous materials composed of smooth crystals.31 Figure 5 displays the Kratky plots obtained for the catalysts. The Kratky plot, h2I(h) versus h2, is a useful tool for describing structural characteristics. Briefly, the scattering curve for the globular conformation follows the Porod law, where I(h) is proportional to h-4 for large h values and to h-2 for moderate h values. Hence, the Kratky plot exhibits a clear peak in the case of spherical particles. For fibrous particles, the scattering profiles have a region at moderate angles where the intensity is related to h-2, and at higher angles, the scattering intensity tends
Figure 6. Particle size distributions, determined by SAXS: (a) Fe(0.65)ZSM5-IMP, (b) Fe(0.87)-ZSM5-IMP, (c) Fe(0.5)-ZSM5-CVD, and (d) Fe(0.83)-ZSM5-CVD. Table 2. Fractal Dimension Values Determined from the SAXS Data sample
fractal dimension
Fe(0.65)-ZSM5-IMP Fe(0.87)-ZSM5-IMP Fe(0.5)-ZSM5-CVD Fe(0.83)-ZSM5-CVD
2.5 2.9 2.1 2.1
to be proportional to h-1. The corresponding Kratky plots show a monotonically increasing curve.21,32 In our study, the catalysts prepared by impregnation show the typical profile of spherical particles, and for the catalysts prepared by sublimation, fibrous particle profiles were observed. Therefore, the preparation method not only determined the size of the particles containing iron but also defined their shape. Assuming such shapes, the size distributions of the particles were calculated (Figure 6). The two samples in the IMP series appeared to have the same size distribution: a multimodal distribution with maxima at 1.8, 5.2, 11.8, and 15.8 nm, with the majority of the particles sized between 12 and 16 nm. The CVD series showed a similar multimodal size distribution with maxima at 1.3, 4.4, and 7.3 nm and contained small fibers with radii smaller than 5 nm. In contrast, samples of the IMP
Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006 4167
Figure 7. Schematic representation of particles containing iron formed by the different preparation methods: (a) FeCl3 sublimation (CVD), (b) impregnation in aqueous media.
Figure 8. Catalytic activity in NOx reduction. SCR with n-decane in TPSR mode over (A) Fe(0.5)-ZSM5-CVD and (B) Fe(0.87)-ZSM5-IMP: (b) NO conversion to N2, (O) n-decane conversion to CO + CO2. Conditions: NO/ n-decane/O2/He ) 0.1/0.03/9/90.87, GHSV ) 35 000 h-1. (C) SCR with NH3: (]) Fe(0.5)-ZSM5-CVD, (4) Fe(0.87)-ZSM5-IMP. Conditions: NO/ NH3/O2/He ) 0.2/0.2/3/96.6, GHSV ) 332 000 h-1, ramp ) 7.5 K min-1.
series presented spherical particles with radii larger than 10 nm. The SAXS and XRD results suggest that smaller iron particles are formed by using sublimation method (CVD), instead of the impregnation (IMP) method in aqueous media. The heterogeneity of the particles containing iron is shown in the fractal dimension values reported in Table 2, which were determined from the plot log h versus log I(h). Samples Fe(0.87)-ZSM5-IMP and Fe(0.65)-ZSM5-IMP have fractal dimensions of 2.9 and 2.5, respectively, indicating high irregularity on their surfaces. In contrast, catalysts from the CVD series have a fractal dimension of 2.1, which is a typical value for surfaces with minor defects.31 Note that the fractal dimension of the particles in the CVD series represents a typical case of cluster-cluster agglomerates for the vapor aggregation of iron; here, the density of particles containing iron is very low.22,33 This result strongly suggests that the iron particles follow a dendritic growth pattern. Furthermore, the high activity observed for the CVD catalyst should be attributed also to the formation of oxo Fe species, which appear to exhibit good performance in the SCR process, as shown previously.9 In Figure 7, we propose two simple models describing the results discussed above. From Figure 8, it seems that surface irregularities could be correlated with the catalytic performance in SCR DeNOx reaction. We should mention that our group has worked extensively on this reaction, and we previously reported the catalytic results under different conditions, even in the presence of H2O and SO2.15,34 Therefore, in this work, we selected the results of only two representative catalysts from two series.
In NO reduction by n-decane, typical volcano NO conversion profiles were observed, with maxima at 673 K (NO conversion ) 53%) and 703 K (NO conversion ) 47%) for Fe(0.5)-ZSM5CVD (Figure 8A) and Fe(0.87)-ZSM5-IMP (Figure 8B), respectively. In both cases, the n-decane oxidation profiles were similar, starting at around 523 K and reaching total hydrocarbon oxidation above 703 K. The selectivities to CO2, however, were completely different (see ref 34 for more details). In the case of Fe(0.5)-ZSM5-CVD, the selectivity was around 60%, whereas for Fe(0.87)-ZSM5-IMP, it was only 40%. In reference to NO conversion, the difference observed for Fe(0.5)-ZSM5-CVD can be explained by the presence of welldispersed small iron particles (5 nm) forming a uniform phase with a low density, as indicated by the characterization results. It should be emphasized that n-decane cannot enter inside the channels of ZSM-5 zeolite.35 Thus, some iron particles present in Fe(0.5)-ZSM5-CVD are not accessible to the reactant mixture, which can explain the small difference of activity compared to Fe(0.87)-ZSM5-IMP. Indeed, the SAXS results suggest that CVD samples contain iron species forming dendrites, at the external and inner surfaces. Of course, with this arrangement of Fe species, the active surface area is accessible to reactants at the external surface. In this sense, functionality support is reduced to stabilize and disperse the oxo Fe species. In the case of the catalytic tests with ammonia (Figure 8C) instead of n-decane, the performances of the two catalysts were remarkably different, as ammonia and the other gases in the reaction mixture can diffuse into the channels hosting the Fe species. For example, 50% NO conversion is reached at 580 K for Fe(0.5)-ZSM5-CVD but at 655 K for Fe(0.87)-ZSM5-IMP. Thus, the best performance of Fe(0.5)-ZSM5-CVD at low temperature can be associated with the contribution of the iron species located in the channels and on the external surface, which are in contact with the reactant mixture. Furthermore, the 129Xe NMR and SAXS experiments showed that solids of the CVD series include particles containing iron inside and outside the channels. Conclusion The preparation method not only determines the size of the particles containing iron but also defines their shape. According to SAXS and 129Xe NMR analysis, well-dispersed fibrous nanoparticles sized at 5 nm were obtained when the sublimation method was employed. They were located on the surface and in the channels of the ZSM-5 zeolite. 129Xe NMR results support the conclusion that CVD-prepared catalysts exhibit strong adsorption sites that are not detectable in materials of similar Fe content that were prepared by impregnation. The fractal dimension depends on the preparation method. It is a sensitive parameter associated with the morphological measurement of heterogeneities related to the particles containing iron and is directly correlated with the catalytic performance in SCR DeNOx reaction. Acknowledgment A.G.-V. thanks CONACYT (Mexico) for a Ph.D. scholarship. Dr. Pedro Bosch is gratefully acknowledged for fruitful discussions. Literature Cited (1) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press; London, 1999. (2) Karger, J.; Ruthven, D. Difussion in Zeolites and Other Microporous Solids; J. Wiley & Sons: New York, 1992.
4168
Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006
(3) Morales, R.; Melo, L.; Brito, J.; Llanos, A.; Moranta, D.; Albornoz, L.; Rodriguez, E. Acetone transformation over PtSn/H[Al]ZSM5 catalysts. J. Mol. Catal. A: Chem. 2003, 203, 277. (4) Csicsery, C. M. Zeolites 1984, 4, 202. (5) Breck, D. W. Zeolite Molecular SieVes; J. Wiley & Sons: New York, 1974. (6) Fyfe, C. A.; Strobl, H.; Kokotailo, G. T.; Kennedy, G. J.; Barlow, G. E. Ultra-High-Resolution 29Si Solid-State MAS NMR Investigation of Sorbate and Temperature-Induced Changes in the Lattice Structure of Zeolite ZSM-5. J. Am. Chem. Soc. 1988, 110, 3373. (7) Traa, Y.; Burger, B.; Weitkamp, J. Zeolite-based materials for the selective catalytic reduction of NOx with hydrocarbons. Microporous Mesoporous Mater. 1999, 30, 3. (8) Paˆrvulescu, V. I.; Grange, P.; Delmon, B. Catalytic removal of NO. Catal. Today 1998, 46, 233. (9) Chen, H. Y.; Sachtler, W. M. H. Activity and durability of Fe/ZSM-5 catalysts for lean burn NOx reduction in the presence of water vapor. Catal. Today 1998, 42, 73. (10) Delahay, G.; Berthomieu, D.; Goursot, A.; Coq, B. Zeolite-based catalysts for the abatement of NOx and N2O emissions from man-made activities. In Interfacial Applications in EnVironmental Engineering; Keane, M. A., Ed.; Marcel Dekker: New York, 2002; p 1. (11) Lobree, L. J.; Hwang, i.-C.; Reimer, J. A.; Bell, A. T. Investigations of the state of Fe in H-ZSM-5. J. Catal. 1999, 186, 242. (12) Joyner, Richard W.; Stockenhuber, Michael. Unusual structure and stability of iron-oxygen nanoclusters in Fe-ZSM-5 catalysts. Catal. Lett. 1997, 45, 15. (13) Malki, El-M. El; van Santen, R. A.; Sachtler, W. M. H. Introduction of Zn, Ga, and Fe into HZSM-5 Cavities by Sublimation: Identification of Acid Sites. J. Phys. Chem. B 1999, 103, 4611. (14) Chen, H. Y.; Sachtler, W. M. H. Promoted Fe/ZSM-5 catalysts prepared by sublimation: de-NOx activity and durability in H2O-rich streams. Catal. Lett. 1998, 50, 125. (15) Delahay, G.; Valade, D.; Guzma´n-Vargas, A.; Coq, B. Selective catalytic reduction of nitric oxide with ammonia on Fe-ZSM-5 catalysts prepared by different methods. Appl. Catal., B 2005, 55, 149. (16) Glatter, O. Convolution square root of band-limited symmetrical functions and its application to small-angle X-ray scattering. J. Appl. Crystallogr. 1981, 14, 101. (17) Glatter, O.; Hainisch, B. Improvements in real-space deconvolution of small angle scattering data. J. Appl. Crystallogr. 1984, 17, 435. (18) Glatter, O. Comparison of two different methods for direct structure analysis from small-angle X-ray scattering data. J. Appl. Crystallogr. 1988, 21, 886. (19) Glatter, O. O. Scattering studies on colloids of biological interest (amphiphilic systems). Prog. Colloid Polym. Sci. 1991, 84, 46. (20) Glatter, O.; Gruber, K. Indirect transformation in reciprocal space: desmearing of small-angle scattering data from partially ordered systems. J. Appl. Crystallogr. 1993, 26, 512. (21) Kataoka, M.; Flanagan, J. M.; Tokunaga, F.; Engelman, D. M. Use of X-ray solution scattering for protein folding study. In Synchrotron
Radiation in the Biosciences; Chanse, B., Deisenhofer, J., Ebashi, S., Goodhead, D. T., Huxley, H. E., Eds.; Clarendon Press: Oxford, U.K., 1994; Vol. 4, p 87. (22) Harrison, A. Fractals in Chemistry; Oxford University Press: New York, 1995. (23) Martin, J. E.; Hurd, A. J. Scattering from fractals. J. Appl. Crystallogr. 1987, 20, 61. (24) West, A. R. Solid-State Chemistry and Its Application; J. Wiley & Sons: New York, 1990. (25) Battiston, A. A.; Bitter, J. H.; de Groot, F. M. F.; Overweg, A. R.; Stephan, O.; van Bokhoven, J. A.; Kooyman, P. J.; van der Speck, C.; Vanko´, G.; Koningsberger, D. C. Evolution of Fe species during the synthesis of over-exchanged Fe/ZSM-5 obtained by chemical vapor deposition of FeCl3. J. Catal. 2003, 213, 251. (26) Marturano, P.; Drozdova´, L.; Kogelbauer, A.; Prins, R. Fe/ZSM-5 prepared by sublimation of FeCl3: The structure of the Fe species as determined by IR, 27Al MAS NMR, and EXAFS spectroscopy. J. Catal. 2000, 192, 236. (27) Lippmaa, E.; Magi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A. R. Structural studies of silicates by solid-state high-resolution siliconNMR. J. Am. Chem. Soc. 1980, 102, 4889. (28) Fraissard, J.; Ito, T. 129Xe NMR study of adsorbed xenon: a new method for studying zeolites and metal-zeolites. Zeolites 1988, 8, 350. (29) Springuel-Huet, M. A.; Bonardet, J. L.; Fraissard, J. 129Xe-NMR of physisorbed xenon used as a probe for the study of microporous solids. J. Appl. Magn. Reson. 1995, 8, 427. (30) Ito, T.; Fraissard, J. 129Xe NMR study of xenon adsorbed on Y zeolites. J. Chem. Phys. 1982, 76, 5225. (31) Avnir, D.; Farin, D.; Pfeifer, P. Molecular fractal surfaces. Nature 1984, 308, 261. (32) Flanagan, J. M.; Kataoka, M.; Fugisawa, M.; Engelman, D. M. Mutation can cause large changes in the conformation of a denatured protein. Biochemistry 1993, 32, 10359. (33) Thamm, H. Adsorption site heterogeneity in silicalite: A calorimetric study. Zeolites 1987, 7, 341. (34) Guzma´n-Vargas, A.; Delahay, G.; Coq, B.; Lima, E.; Bosch, P.; Jumas, J. C. Influence of the preparation method on the properties of FeZSM-5 for the selective catalytic reduction of NO by n-decane. Catal. Today 2005, 107-108, 94. (35) Boulard, S.; Gilot, P.; Brosius, R.; Habermacher, D.; Martens, J. A. Perturbation and tracer chromatography study of hydrocarbon adsorption from simulated exhaust gas on Fe-MFI and MOR type zeolite catalysts. Top. Catal. 2004, 30-31, 49.
ReceiVed for reView February 7, 2006 ReVised manuscript receiVed March 30, 2006 Accepted April 6, 2006 IE0601596
