Using Aberration-Corrected STEM Imaging to Explore Chemical and

Jun 10, 2008 - Using Aberration-Corrected STEM Imaging to Explore Chemical and Structural Variations in the M1 Phase of the MoVNbTeO Oxidation Catalys...
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J. Phys. Chem. C 2008, 112, 10043–10049

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Using Aberration-Corrected STEM Imaging to Explore Chemical and Structural Variations in the M1 Phase of the MoVNbTeO Oxidation Catalyst William D. Pyrz,† Douglas A. Blom,‡ N. Raveendran Shiju,§ Vadim V. Guliants,§ Thomas Vogt,| and Douglas J. Buttrey*,† Center for Catalytic Science and Technology, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716, NanoCenter and Electron Microscopy Center, UniVersity of South Carolina, Columbia, South Carolina 29208, Department of Chemical and Materials Engineering, UniVersity of Cincinnati, Cincinnati, Ohio 45221, NanoCenter and Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed: February 22, 2008; ReVised Manuscript ReceiVed: April 11, 2008

We report results of aberration-corrected STEM imaging of the orthorhombic M1 phase in the MoVNbTeO selective oxidation catalyst prepared using two different solution techniques. Atomic coordinates and cation site occupancies for both sample preparations are compared with a previously reported Rietveld-based model for this structure. The high angle annular dark-field (HAADF) images from the two preparations varied significantly only in the occupancy of the heptagonal channels. The M1 sample prepared under ambient conditions exhibited partial occupancy, whereas the hydrothermally prepared sample had heptagonal channels that were primarily vacant. Compared to the Rietveld model, both the atomic positions and the occupancies were found to be consistent, with the exception of the Mo4 and the two Te sites. The Mo4 site shows a lower HAADF intensity than expected, suggesting that appreciable V content may be present, whereas both Te sites had low Te occupancy as compared with the refined model, likely due to e-beam sublimation. Z-contrast imaging may become a valuable tool for rapidly obtaining initial model parameters for the Rietveld refinement of complex structures. Introduction Selective oxidation catalysis is of vital importance to industrialized societies, providing about one-quarter of all organic chemicals and intermediates used to make consumer goods and the industrial products that rely on them.1 The desire to use less-expensive feedstock materials has spurred the development of new and improved heterogeneous catalysts for C2-C4 conversions. In the selective oxidation and ammoxidation of C3 feedstocks, significant efforts are underway to use propane feeds to produce acrylic acid and acrylonitrile rather than processes that have traditionally relied on propene feeds catalyzed by multicomponent bismuth molybdates for the production of acrolein and acrylonitrile.1,2 The production of acrylic acid (currently from acrolein) and acrylonitrile has reached a scale that approaches the equivalent of nearly 1 kg for every human being on Earth each year. Several families of vanadium-containing complex oxide materials such as the Al-Sb-V-W-O,3 Mo-V-W-O,4,5 Mo-V-Nb-Sb-O,6–11 and Mo-V-Nb-Te-O systems11–25 have been identified as promising catalysts for the efficient and selective oxidation and ammoxidation of propane. Important attributes of these catalysts are the redox behavior associated with vanadium, the availability of lattice oxygen, and the structural isolation of active sites, which in some cases relies * Corresponding authors. E-mail: [email protected]. Phone: (302) 8312034. Fax: (302) 831-2085. † University of Delaware. ‡ NanoCenter and Electron Microscopy Center, University of South Carolina. § University of Cincinnati. | NanoCenter and Department of Chemistry and Biochemistry, University of South Carolina.

on the presence and cooperative reactivity of distinct phases.11,17,19,26 Among these various candidate systems, the Mo-V-Nb-Te-O one commonly referred to as the M1/M2 system may perhaps be the most viable for commercial implementation; this system is a composite in which two phases have been proposed as critical for the achievement of optimal selectivity during propane conversion.11,17,19 M1 is the majority phase present in this catalyst and has an orthorhombic polyhedral network-type molybdenum bronze structure with a framework similar to that of Mo5O14 and Mo17O47.14 This phase can be described with the generic formula {TeO}1-x · (Mo,V,Nb)10O28, where the {TeO} component is intercalated into framework channels. The M2 phase is present in amounts of roughly 20-30 mol % and is an orthorhombically distorted hexagonal tungsten bronze (HTB)-type structure.14 By analogy with the generic description of the M1 formula, the formula for M2 can be written as {TeO}2-x · (Mo,V,Nb)6O18. This M1/M2 composite is both active and selective, providing ammoxidation yields of up to 62% for the conversion of propane to acrylonitrile.11 In an earlier paper involving two of the present coauthors, the M1 structure was modeled using a combined Rietveld analysis of synchrotron X-ray and neutron powder data.13 An improved model was obtained later by accounting for trace impurity phases and indicated the composition as {TeO}0.94 · (Mo7.8V1.2Nb)O28.14 Unlike other model structures proposed, this Rietveld analysis provided crystallographic coordinates for all metal and oxygen sites in the structure, as well as site occupancies for the metals. Similar information was obtained from the Rietveld analysis of the M2 phase, and its composition is modeled as {TeO}1.81 · (Mo4.31V1.36Nb0.33)O18.14 A later study by Murayama et al. took this model and rerefined it using only synchrotron X-ray diffraction from their hydrothermally pre-

10.1021/jp801584m CCC: $40.75  2008 American Chemical Society Published on Web 06/10/2008

10044 J. Phys. Chem. C, Vol. 112, No. 27, 2008 pared M1 catalyst.18 Their refinement results suggest slight variations to the site occupancies, namely in the form of additional V to metal sites 1, 4, 5, and a reduction in V content in metal site 2.18 Recently, we have shown that aberrationcorrected STEM imaging provides a powerful tool for the characterization of complex materials; a model for the M1 structure was developed through direct interpretation of high angle annular dark-field (HAADF) images.27 This independently obtained model of atomic coordinates and elemental occupancies is consistent with the model developed by DeSanto et al.14 and confirms the presence of Te in both the hexagonal and heptagonal channels.27 Furthermore, the results suggested that additional improvements to the model may be obtained by combining HAADF image analysis with Rietveld analysis.27 Although we have atomistic models describing the two major phases, M1 and M2, we have not yet explored the structural variability that exists for these solid solution phases. The analysis of other M1 preparations using bulk inductively coupled plasma mass spectroscopy (ICP) has indicated a variability of about 50% in V content among samples.19 The extent of Te intercalation also seems to vary, and arguments persist over whether the Te is restricted to the hexagonal channels in the structure18 or is actually dispersed among both hexagonal and heptagonal channels.13,14,27 Further challenges present themselves when investigating this system because it is not only chemically complex but several coexisting impurities may also be present along with the desired phases. For an arbitrary composition in this oxide system with four metals, equilibrium coexistence allows for up to four phases when not at a phase transition point with T, P, and fO2. At low phase fractions, impurity phases are difficult to identify in diffraction data and require high-resolution measurements to identify and quantify their presence. The structural complexity of the M1 phase is such that trace impurity phases, including M2, are masked easily in a diffraction pattern; thereby giving the impression of phase purity. Minority phases that may be present include elemental Te,14 Mo5O14,14 TeMo5O16,14 and other phases may share equilibrium (Alkemade) joins here as well. In the presence of “hidden” impurities, the quantitative reliability of bulk characterization techniques is limited. Obtaining overall compositions from Rietveld refinements of site occupancies also has its drawbacks, especially in very complex structures such as M1 because of the need to use constraints to limit the range and number of refined parameters. In this work, an aberration-corrected28–30 JEOL2100F TEM27 operated in scanning transmission electron microscope (STEM) mode was used to explore the variations between two MoVNbTeO M1 phases prepared using different solution-based procedures. The spatial resolution provided by the TEM allows the analysis of microscopic single crystals, which eliminates contributions from trace impurities commonly encountered in multicomponent complex oxides. The enhanced resolution provided by aberration-corrected microscopes has been demonstrated in several structural studies, with some examples including the observation of single fullerene (C60) molecules attached to carbon nanotubes,31 sub-angstrom imaging of single gold atoms and dimers,28 and the atomic spacings in a Ge30Si70 alloy.28 For this study, HAADF imaging was used to estimate the chemical composition of individual atomic columns, atomic coordinates within the metal framework, and the occupancy of the intercalation species within the polyhedral rings. A major advantage of using HAADF imaging is that the scattered electrons used to generate the image are mainly incoherent and the HAADF signal, for a reasonable specimen thickness, is a monotonic function without contrast reversal.28,32,33 To a first

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Figure 1. Rendering of the model M1 structure from simultaneously refined powder X-ray and neutron data as proposed by DeSanto et al.14 The refined orthorhombic structure has space group Pba2, formula unit {TeO}0.94 · (Mo7.8V1.2Nb)O28 (or equivalently Mo7.8V1.2NbTe0.937O28.9 in ref 14), and lattice parameters a ) 21.134(2) Å, b ) 26.658(2) Å, and c ) 4.0146(3) Å with Z ) 4.14

approximation, the HAADF image contrast scales with the square of the atomic number (Z).32,33 Finally, the HAADF results for M1 samples from two different preparations will be compared to the Rietveld-based model of DeSanto et al. (Figure 1).14 Experimental Methods Two different synthetic approaches were used in the synthesis of the MoVNbTeO M1 specimens examined in this study. The first sample was prepared using slurry methods published previously14 at ambient pressure and yielded nearly phase pure M1. This sample was the same as that used for the M1 structure refinements described by DeSanto et al.13,14 This sample will be referred to as M1-Nb-Amb because it was developed using combinatorial solution chemistry under ambient pressure. The second M1 catalyst was made using hydrothermal synthesis, combining appropriate amounts of ammonium heptamolybdate hydrate ((NH4)6[Mo7O24] · 4H2O), vanadyl sulfate (VOSO4), niobium oxalate (Nb(HC2O4)5 · 6H2O/H2C2O4), and tellurium oxide (TeO2) at 175 °C for 72 h under autogenous pressure (∼8.9 bar). After the synthesis, the solid product was filtered, washed several times with deionized water, and dried at 80 °C overnight. The material was then calcined in a flow of ultrapure N2 at 873 K. This sample will be referred to as M1-Nb-Hydro because it was prepared using hydrothermal synthesis at elevated pressures. HR-STEM was used to image the materials with a JEOL 2100F equipped with a CEOS Cs-corrector on the illumination system. The geometrical aberrations were measured and controlled to provide less than a π/4 phase shift of the incoming electron wave over the probe-defining aperture of 14.5 mrad. HAADF STEM images were acquired on a Fischione Model 3000 HAADF detector with a camera length such that the inner cutoff angle of the detector was 65.6 mrad. The scanning acquisition was synchronized to the 60 Hz AC electrical power to minimize 60 Hz noise in the images, and a pixel dwell time of 32 µs was chosen. Samples were prepared for TEM by finely grinding the as-prepared catalyst and then dipping a holeycarbon-coated Cu grid into the powder. Results and Discussion Our Rietveld-based model for the orthorhombic MoVTeNbO M1 phase along with site occupancies is depicted in Figure 1.14

Using Aberration-Corrected STEM Imaging Along the [001j] direction, the structure is composed of a network of metal oxide polyhedra that form pentagonal (green), hexagonal (red), and heptagonal (yellow) rings. The pentagonal rings are assumed to have Nb centers that edge-share oxygens with adjacent Mo sites; these Mo sites connect to one another by corner-sharing oxygen of the octahedra. The hexagonal and heptagonal rings are comprised of Mo or have mixed occupancy of Mo with V; it is assumed that the Nb occupancy is essentially restricted to the pentagonal center sites and not mixed into the octahedral sites.14 Finally, the centers of both the hexagonal and heptagonal rings are partially filled with Te-O chains. The high-resolution HAADF image in Figure 2a shows an M1-Nb-Amb catalyst crystallite oriented along the [001j] direction. A careful examination of the intensities of the atomic columns shows distinct contrast variations roughly consistent with the anticipated Z2 contrast based on our Rietveld model. The bright-field (BF) STEM and HAADF images in Figure 2b show that all of the hexagonal channels and a fraction of the heptagonal channels are filled (indicated by the arrows). The fact that the heptagonal channels clearly show a low and statistical filling is consistent with our Rietveld model prediction of ca. 20% occupancy.14,27 To better illustrate the atomic column contrast variations in the HAADF images, a rendering of the unit cell based on our Rietveld model was scaled with a constant aspect ratio and superimposed on the image as shown in Figure 2c. Good agreement between the superimposed unit cell and the HAADF image is observed. Slight deviations in the atomic column coordinates are attributed to (i) distortions associated with the rastering of the electron beam and (ii) sample drift over the course of the image acquisition. The HAADF image in Figure 3a shows an M1-Nb-Hydro crystallite with [001j] orientation. The image of this crystallite obtained from a hydrothermal synthesis batch shows similar characteristics to those observed in Figure 2a for the M1-NbAmb sample. However, there is a noteworthy difference in the occupancy of the heptagonal channels: in M1-Nb-Hydro, the heptagonal channels were found to be largely unoccupied. A thorough analysis of numerous images of several M1-Nb-Hydro particles revealed that only a very small number of channels (,1%) reveal contrast that can be associated with an atomic column in the heptagonal channel. This contradicts our Rietveld model predictions, but is not surprising because that refinement was based on X-ray and neutron data taken from the M1-NbAmb sample where we do see evidence of heptagonal channel filling via HAADF STEM. This result clearly indicates that the synthesis procedure has an impact on the composition and structure of the product. Efforts are currently underway to test whether there is a direct correlation between the occupancy of the Te in heptagonal channels and enhanced catalytic activity. A rendering of the unit cell based on our Rietveld model was scaled with a constant aspect ratio and superimposed on the HAADF image of the M1-Nb-Hydro sample and shown in Figure 3b. Good agreement between the model and the HAADF image is observed with the exception of the Te position in the heptagonal channels. A quantitative comparison of the atomic coordinates derived from HAADF images for both the M1-Nb-Amb and M1-NbHydro samples with our Rietveld model is presented in Figures 4a and 5a, respectively. In the 〈001〉 projections, the M1 structure has no overlapping crystallographically distinct metal sites, which allows for the direct extraction of fractional x and y atomic coordinates. These coordinates were determined by placing an adjustable grid across the unit cell. This adjustable grid was necessary to compensate for the rastering and sample drift

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Figure 2. (a) High-resolution HAADF STEM images showing the [001j] projection of the M1 phase prepared using combinatorial methods at ambient pressure; (b) high-resolution HAADF STEM image and BF STEM image showing the same catalyst particle with occupied heptagonal channels as indicated by the white arrows; (c) magnified portion of the [001j] projection showing a scaled model unit-cell superimposed on the high-resolution HAADF STEM image (Te13 sites left empty for clarity). Note that all of the images shown are raw data and have not been processed or altered.

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Figure 3. (a) High-resolution HAADF STEM images showing the [001j] projection of the M1 phase prepared using hydrothermal synthesis. Note that the heptagonal channels are empty in this sample (white arrows point to the empty channels); (b) magnified portion of the [001j] projection showing a scaled model unit-cell superimposed on the highresolution HAADF STEM image. Note that all of the images shown are raw data and have not been processed or altered.

distortions. To minimize the influence of these distortions, each metal coordinate site within the first quadrant of a chosen unit cell was measured and averaged for four different reference points [(0,0), (0,0.5), (0.5,0), (0.5,0.5)]. For both samples (excluding Te13 in the M1-Nb-Hydro sample), the HAADFbased x,y coordinates and those from our Rietveld model agree to within 1% with respect to the fractional coordinates, which is graphically evidenced by the small deviations from the 45° line. The differences observed between our Rietveld model and the two HAADF models are shown in Figures 4b and 5b. The maximum deviation in real space between the atomic coordinates of either of the two HAADF models and the metal positions determined by our Rietveld refinement was 0.3 Å for the Te sites, but the majority of the coordinates determined using HAADF images had deviations below 0.1 Å. The coordinate variations for Te sites, especially for Te13 (the heptagonal channel site) suggest that there is some local variation in the coordinates for these intercalated sites. This is not revealed in our Rietveld analysis because it provides only average information. The fact that the metal coordinates that were directly measured from HAADF images of both samples were consistent with the Rietveld model suggests that the M1 phase is appropriately described by the orthorhombic model proposed by DeSanto et al.14 It appears that the variations between the

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Figure 4. (a) Plot showing the fractional coordinates of the refined model plotted against the experimentally determined coordinates measured directly from raw high-resolution HAADF STEM images of the M1 catalyst prepared using combinatorial methods at ambient pressures. Note that the standard errors fall within the size of the plot symbols. (b) Plot showing the difference between the coordinates from the Rietveld model and the coordinates measured from the HAADF image from the M1-Nb-Amb sample after conversion to real space distances. * indicates the coordinates for the sparsely occupied heptagonal site.

different M1 formulations found throughout the literature are simply different elemental arrangements of this structural backbone, and in some cases it is possible to synthesize the M1 phase with different occupancies in selected sites, for example, the Te13 site as shown in this report. The composition of each atomic column was calculated for each sample using the scattered intensity observed in the HAADF images. The following assumptions were made: (i) the observed scattering follows the Z2 Rutherford relationship;27,32,33 (ii) within a single unit cell, the thickness of the crystal was constant; (iii) scattering contributions from oxygen were only considered at positions in the 〈001〉 projections, which are superimposed with metal site columns; (iv) a constant integration area was appropriate for all metal framework sites; and (v) the background was constant throughout the portion of the image being analyzed. Using these assumptions, the total intensity for each atomic column was measured by integrating the individual pixel intensity from a constant region of interest for each framework metal site. The background from several empty heptagonal channels near the unit cell was averaged and then subtracted from the integrated intensity of each site. The raw intensities were then normalized to the Nb9 site assuming complete occupancy by Nb. Any partial occupancy of Mo in this site would have minimal influence on the results because they are

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Figure 6. Comparison of occupancies derived from HAADF STEM images for both M1 samples (combinatorial and hydrothermal preparation) with those obtained by Rietveld refinement.11 Normalization is achieved with reference to complete occupancy of Nb in the Nb9 site. The bar to the right displays the expected ratio for various occupancies of Mo and V in a single site. Discrepancies for the Te occupancies are expected because of sublimation under the electron beam. Note that Te occupancy was not observed in the heptagonal channels for the sample prepared using hydrothermal methods (Te13).

Figure 5. Plot showing the fractional coordinates of the refined model plotted against the experimentally determined coordinates measured directly from raw high-resolution HAADF STEM images of the M1 catalyst prepared by hydrothermal methods. Note that the standard errors fall within the size of the plot symbols. (b) Plot showing the difference between the coordinates from the Rietveld model and the coordinates measured from the HAADF image from the M1-Nb-Hydro sample after conversion to real space distances.

neighbors in the periodic table. The contrast ratios based on our Rietveld model,14 and measured from the M1-Nb-Amb and the M1-Nb-Hydro samples are presented in Figure 6. The contrast ratios of the three data sets were found to be in agreement, and the maximum deviations in the site occupancies between the models were on the order of 10-15%. Comparing first the HAADF results for the two samples, the only deviations observed are for the Te12, Te13, and Mo3/V3 sites. These deviations suggest that the M1-Nb-Hydro sample has a deficiency of Te and is slightly richer in V compared to the M1Nb-Amb sample. The lack of Te in the M1-Nb-Hydro sample was expected because the vast majority of the heptagonal channels were unoccupied. However, one must be careful in quantifying Te in electron microscopy because it is quite volatile under the electron beam. In our previous work, bright-field high resolution imaging on the M1-Nb-Amb sample resulted in images that showed completely vacant heptagonal and even hexagonal channels.14 In our experience, a scanning STEM beam results in less beam damage, but we expect a small rate of electron-beam-induced sublimation. In both samples, the STEM contrast for the Mo4 site is also somewhat low compared to the Rietveld model, suggesting that this site does have some vanadium occupancy. The Rietveld results from the hydrothermally prepared M1 sample of Murayama et al. showed evidence of slight V occupancy at metal position 4;18 however, our STEM results suggest that this V

occupancy is significantly higher. The Mo1/V1 site and the pentagonal ring sites (Mo5, Mo6, Mo8, Mo10, and Mo11) also have a STEM contrast slightly lower than expected. These deviations are sufficiently small that they may be associated with limitations from the imaging assumptions mentioned above, or they might indicate low-level occupancy by vanadium in these sites that was not accounted for in the Rietveld studies. Efforts are currently underway to simulate these HAADF STEM images to better understand the various contributions that lead to the observed variations in the model occupancies. On the basis of these STEM results, the sites exhibiting systematic deviations (Mo4 and the Te12) will be reassessed in our Rietveld model and rerefined to test whether improvements in the current structural model are necessary. In studying complex oxide systems, such as the M1/M2 system, the use of a single characterization tool proves to be inadequate. Bulk scattering techniques such as high-resolution X-ray and neutron diffraction, as well as bulk analytical tools, such as ICP, only provide average information for the sample as a whole. In the M1/M2 system, the Gibbs’ phase rule allows for the coexistence of up to four distinct phases at equilibrium (away from thermodynamic transitions). Because of the complexity of the M1 phase, the identification and quantification of mixed complex phases requires very high-resolution scattering data, and in some cases these impurities remain “invisible” because of severe peak overlap. The use of high-resolution TEM and STEM allows the analysis of single crystallites within a sample and assists in the identification of the individual phases present, provided they are observed, because the TEM samples sizes are so limited. Once an impurity is identified using TEM, it can then be searched for in the scattering data and the relative phase fractions can be determined. The ICP results can be modified using those impurity phase fractions, and the necessary information can be incorporated back into a Rietveld refinement and improvements can be applied to a structural model. In this study, high-resolution HAADF imaging generated a model that was consistent with our original Rietveld model with few exceptions (namely, the site occupancies of the Mo4, Te12, and Te13 sites). Iterating back to our Rietveld model with these adjustments and completing careful simulations of the HAADF images should provide further improvements to the structural

10048 J. Phys. Chem. C, Vol. 112, No. 27, 2008 model for M1. Integration of both local (TEM, EDS, STEM) and bulk (ICP, synchrotron X-ray diffraction, and neutron scattering) characterization techniques may provide insight into the structural and compositional variations between samples prepared using various techniques and starting precursor compositions. High-resolution scattering studies are currently underway for the M1-Hydro sample to better quantify the differences observed between the two samples from the HAADF studies. Fully understanding the bulk structure and composition may allow for better prediction or modeling of the active components (ordered or disordered) on the catalyst surface and improve the understanding of the catalytic functions of this multicomponent complex oxide. Additionally, a thorough understanding of the bulk structure and composition provides valuable information that can be used to further develop our understanding of the phase diagram for this five-component complex oxide system. We propose that analyzing complex structures such as those present in the M1/M2 system by starting with aberrationcorrected HAADF STEM images, and selected-area electron diffraction with elemental analysis using EDS will provide us with high-quality starting models for subsequent Rietveld analysis and eliminate the currently prevalent coarse “trial-anderror” approaches. Such an approach would generate preliminary estimates of lattice parameters, symmetry, and atomic coordinates for the heavy atoms, and would give indications of likely site occupancies. Additionally, the low-Z sites (oxygen in this case) can be modeled by starting from expected bond lengths, valences, and coordination environments. Furthermore, bulk composition tools (ICP being one example) can assist in determining the phase purity by providing a consistency check against the single-phase elemental composition estimated using EDS analysis. Such a high-quality model combined with bulk characterization of composition should increase the speed of subsequent Rietveld refinements significantly. Conclusions For the most part, the x,y coordinates and occupancies are in good agreement with our Rietveld model. Differences in Te site occupancies between the Rietveld model and STEM images from the same sample are consistent with electron-beam-induced Te sublimation. Deviations in x,y coordinates for Te provide evidence of slight disorder in the intercalation of Te-O chains, especially into the heptagonal channels. Rietveld analysis provides only average information, but the STEM images provide direct evidence for the disorder. We intend to explore this disorder further in future studies. The low relative contrast observed for the Mo4 site suggests a systematic difference between the two HAADF models and the Rietveld model. We will revisit the Rietveld analysis to test possible variations, with particular focus on including partial occupancy by V in this site. There may also be small amounts of vanadium in other sites modeled as exclusively occupied by molybdenum, but the contrast variations are too small to be considered significant at this stage of the analysis. Efforts are underway to simulate the HAADF images and provide a more quantitative analysis of the contrast observed. The results of this study indicate that the M1 structure is described well by the proposed orthorhombic model and that the differences in the observed catalytic performance are likely a result of small variations in the framework composition as well as the distribution of coexisting phases (M1, M2, Te°, Mo5O14, etc.) in each composite catalyst. The good agreement between our structural model refined using X-ray and neutron scattering data and those obtained from

Pyrz et al. high-resolution HAADF images suggests that aberration-corrected STEM may become an important complimentary tool for determining unknown complex crystal structures, provided appropriate simple projections with little or no overlap of atoms can be found. The ability to quickly and directly interpret HAADF images and determine the fractional atomic coordinates to within 1% uncertainties and atomic site occupancies to within 15% could provide a dramatic improvement in the quality of initial structural models required for Rietveld refinements. Coupling both bulk and local techniques should significantly reduce the time and effort required to converge toward a structural solution and possibly increase the complexity of real crystalline systems, such as composites, that can be solved based on powder data alone. Such a reduction in time of analysis may parallel what was accomplished in protein crystallography in recent years. Acknowledgment. We acknowledge Claus G. Lugmair and Anthony F. Volpe Jr. of Symyx Technologies, Inc. for providing some of the M1 specimens used in this study. V.V.G. acknowledges the financial support from the Chemical Sciences, Geosciences and Biosciences Division, Offices of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Grant No. DE-FG02-04ER15604. D.A.B. and T.V. thank the State of South Carolina and the Vice President of Research & Health Sciences at the University of South Carolina for generous support. References and Notes (1) Grasselli, R. K. Top. Catal. 2002, 21, 79. (2) Grasselli, R. K. Catal. Today 1999, 49, 141. (3) Nilsson, J.; Landa-Canovas, A. R.; Hansen, S. A. J. Catal. 1999, 186, 442. (4) Hibst, H.; Tenten, A.; Marosi, L. BASF Aktiengesellschaft, Patent EP 774 297, publ. 21.06.1997 (5) Hibst, H.; Rosowski, F.; Cox, G. Catal. Today 2006, 117, 234. (6) Ueda, W.; Endo, Y.; Watanabe, N. Top. Catal. 2006, 38, 261. (7) Safonova, O. V.; Deniau, B.; Millet, J. M. M. J. Phys. Chem. B 2006, 110, 23962. (8) Guerrero-Perez, M. O.; Al-Saeedi, J. N.; Guliants, V. V.; Banares, M. A. Appl. Catal., A 2004, 260, 93. (9) Ueda, W.; Oshihara, K.; Vitry, D.; Hisano, T.; Kayashima, Y. Catal. SurV. Jpn. 2002, 6, 33. (10) Guliants, V. V.; Bhandari, R.; Swaminathan, B.; Vasudevan, V. K.; Brongersma, H. H.; Knoester, A.; Gaffney, A. M.; Han, S. J. Phys. Chem. B 2005, 109, 10234. (11) Grasselli, R. K.; Burrington, J. D.; Buttrey, D. J.; DeSanto, P.; Lugmair, C. G.; Volpe, A. F.; Weingand, T. Top. Catal. 2003, 23, 5. (12) Ushikubo, T.; Oshima, K.; Kayou, A.; Vaarkamp, M.; Hatano, M. J. Catal. 1997, 169, 394. (13) DeSanto, P.; Buttrey, D. J.; Grasselli, R. K.; Lugmair, C. G.; Volpe, A. F.; Toby, B. H.; Vogt, T. Top. Catal. 2003, 23, 23. (14) DeSanto, P.; Buttrey, D. J.; Grasselli, R. K.; Lugmair, C. G.; Volpe, A. F.; Toby, B. H.; Vogt, T. Z. Kristallogr. 2004, 219, 152. (15) Lopez Nieto, J. M.; Botella, P.; Solsona, B.; Oliver, J. M. Catal. Today 2003, 81, 87. (16) Shiju, N. R.; Guliants, V. V. Chemphyschem 2007, 8, 1615. (17) Grasselli, R. K.; Buttrey, D. J.; Burrington, J. D.; Andersson, A.; Holmberg, J.; Ueda, W.; Kubo, J.; Lugmair, C. G.; Volpe, A. F. Top. Catal. 2006, 38, 7. (18) Murayama, H.; Vitry, D.; Ueda, W.; Fuchs, G.; Anne, M.; Dubois, J. L. A. Appl. Catal., A 2007, 318, 137. (19) Holmberg, J.; Grasselli, R. K.; Andersson, A. Appl. Catal., A 2004, 270, 121. (20) Grasselli, R. K. Catal. Today 2005, 99, 23. (21) Grasselli, R. K.; Buttrey, D. J.; DeSanto, P.; Burrington, J. D.; Lugmair, C. G.; Volpe, A. F.; WeingandT., Catal. Today 2004, 91-92, 251. (22) Wagner, J. B.; Timpe, O.; Hamid, F. A.; Trunschke, A.; Wild, U.; Su, D. S.; Widi, R. K.; Abd Hamid, S. B.; Schlogl, R. Top. Catal. 2006, 38, 51. (23) Guliants, V. V.; Brongersma, H. H.; Knoester, A.; Gaffney, A. M.; Han, S. Top. Catal. 2006, 38, 50.

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