Nearly Uniform Decaosmium Clusters Supported on MgO

May 27, 2009 - Supattra Khabuanchalad , Jatuporn Wittayakun , Rodrigo J. Lobo-Lapidus , Stefan Stoll , R. David Britt , and Bruce C. Gates. Langmuir 2...
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J. Phys. Chem. C 2009, 113, 13377–13385

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Nearly Uniform Decaosmium Clusters Supported on MgO: Characterization by X-ray Absorption Spectroscopy and Scanning Transmission Electron Microscopy Apoorva Kulkarni,† Shareghe Mehraeen,† Bryan W. Reed,‡ Norihiko L. Okamoto,† Nigel D. Browning,†,‡ and Bruce C. Gates*,† Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, One Shields AVenue, DaVis, California, 95616, and Condensed Matter and Materials DiVision, Lawrence LiVermore National Laboratory, LiVermore, California 94550 ReceiVed: April 9, 2009; ReVised Manuscript ReceiVed: May 1, 2009

Samples containing small, nearly uniform clusters of a heavy metal, Os, were prepared on a high-area porous support consisting of light atoms, MgO, to provide an opportunity for a critical assessment of estimates of cluster size determined by extended X-ray absorption fine structure (EXAFS) spectroscopy and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Supported carbonyl clusters approximated as decaosmium were prepared by reductive carbonylation of adsorbed Os3(CO)12 at 548 K and 1 bar. Infrared (IR) spectra of the clusters resemble those attributed in earlier work to supported clusters similar to [Os10C(CO)24]2-, consistent with the EXAFS data. The spectra indicate a molar yield of decaosmium carbonyl clusters of about 65-70%. As these clusters were treated in flowing H2, they were partially decarbonylated, as shown by IR and EXAFS spectra. The rms (root-mean-square) radii of the undecarbonylated and partially decarbonylated clusters were found by HAADF-STEM to be 3.11 ( 0.09 and 3.06 ( 0.05 Å, respectively, and the close agreement between these values is consistent with the inference that the cluster frame was essentially the same in each. The average rms radius of the undecarbonylated clusters, estimated on the basis of EXAFS data, was 2.94 ( 0.07 Å, calculated on the basis of the assumption that the osmium frame matched that of [Os10C(CO)24]2-. EXAFS analysis of the data characterizing the partially decarbonylated sample, aided by the STEM results, showed, consistent with the STEM results, that the partial decarbonylation did not lead to a significant change in the rms radius of the metal frame. Introduction Supported metal clusters, an important class of industrial catalyst,1 are typically nonuniform in structure, because the clusters are of various sizes and shapes and the support surfaces are intrinsically heterogeneous. This nonuniformity limits the determination of structures by spectroscopic methods and contributes to the lack of selectivity of supported catalysts. Therefore, there is a motive for the synthesis of structurally uniform supported catalysts, and researchers have used molecular metal clusters (such as metal carbonyls) as precursors in attempts to make catalysts in which the supported species retain the metal frame of the precursor.2 When the metals are heavy and the supports consist of light atoms, the opportunities for characterization by high-resolution transmission electron microscopy (TEM) are maximized, and thus researchers have turned to the family of osmium carbonyls (including Os3(CO)123 and [Os10C(CO)24]2-5) as precursors, with MgO as the support. TEM, and especially high resolution Z-contrast scanning transmission electron microscopy (STEM), provides some of the best opportunities for characterization of supported clusters, and this technique is complemented by extended X-ray absorption fine structure (EXAFS) spectroscopy (which provides average information integrated over a sample). Together, these are the dominant techniques for characterization of supported nanostructured catalysts, but there are only a few results that allow a quantitative comparison of them.6 The lack of such * Corresponding author. E-mail: [email protected]. † University of California, Davis. ‡ Lawrence Livermore National Laboratory.

examples is explained at least in part by the lack until recently of (a) STEM instruments with sufficient resolution; (b) the lack of facilities for handling air-sensitive samples without contamination in STEM experiments; and (c) methods for analysis of STEM results. Thus, our goal was to use STEM and EXAFS with MgOsupported osmium cluster catalysts prepared with such a high degree of uniformity that their structures could be determined with unprecedented accuracy by these complementary methods. When clusters on supports are small enough, they are sensitive to the beam in STEM and require low-dose techniques that invariably introduce noise into images. To address the noise, the Z-contrast technique (also known as high-angle annular dark field in scanning transmission electron microscopy (HAADFSTEM)) was applied with a novel method of nanocluster size measurement,6 allowing accurate determinations of the sizes of individual clusters, which were compared with the average cluster sizes determined by EXAFS spectroscopy. We report the first characterization of nearly uniform MgOsupported decaosmium and partially decarbonylated decaosmium clusters by both EXAFS spectroscopy and STEM and show how these techniques in combination provide essential details of the structures of supported clusters, including interactions of the clusters with the support. An earlier investigation of similar samples illustrated the value of using EXAFS and STEM in tandem for characterization of such samples,3,5,7 but the results were limited because of air exposure of the samples, leading to cluster fragmentation. Now

10.1021/jp903309d CCC: $40.75  2009 American Chemical Society Published on Web 05/27/2009

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we have used air exclusion techniques to avoid this complication and better STEM image analysis techniques. Experimental Section Sample Preparation and Handling. Methods and Materials. [Os10C(CO)24]2- was synthesized on the surface of MgO powder from the precursor Os3(CO)12.4 The synthesis and handling were performed with exclusion of air and moisture on a doublemanifold Schlenk line and in an N2-filled glovebox (AMO-2032, Vacuum Atmospheres). The treatment gases He (Praxair, 99.999%), CO (Airgas, 99.999%), and H2 (Whatman hydrogen generator, 99.999%) were purified by passage through traps containing particles of copper and activated zeolite to remove traces of O2 and moisture, respectively. n-Pentane (Aldrich), used as a solvent, was refluxed under N2 in the presence of sodium/benzophenone ketyl to remove traces of water and then deoxygenated by sparging of dry N2 prior to use. Os3(CO)12 (Strem, 99%) was used as received. The MgO powder (EM Science), a white solid with a high degree of crystallinity and presenting some well-defined crystal faces, was pretreated by calcination in flowing O2 (Airgas, 99.999%) at 673 K for 6 h and evacuation at 673 K for 14 h. The BET surface area of the resultant material was approximately 70 m2/g. Synthesis of Supported Decaosmium Carbonyl Clusters. The MgO-supported samples were prepared, as before,4 by slurrying Os3(CO)12 with the calcined MgO powder in n-pentane under N2 for 24 h at room temperature followed by overnight evacuation at 298 K to remove the solvent. A calculated amount of Os3(CO)12 was added to the support MgO to give samples containing 2.0 wt % Os when all of the added osmium remained on the support.4 This procedure was followed by treatment of the sample in flowing helium at 548 K for 2 h to form mononuclear osmium subcarbonyls followed by treatment in CO at 548 K and 1 bar for 4 h leading to reductive carbonylation and formation of decaosmium carbonyl clusters.4 Decarbonylation of Supported Decaosmium Carbonyl Clusters. The supported osmium carbonyl clusters were decarbonylated by treatment in flowing H2 at 1 bar as the temperature was ramped at 3 K/min from 25 to 473 K and then held at 473 K for 2 h. The treatment was done with the sample in a tubular flow reactor, and the effluent was characterized by mass spectrometry. IR spectra were recorded during the treatment and after the samples had been cooled to room temperature. Characterization Methods. IR Spectroscopy. Transmission spectra were recorded with a Bruker IFS-66v/s spectrometer with a spectral resolution of 2 cm-1. Samples were pressed into self-supporting wafers and mounted in the cell and sealed in the N2-filled glovebox. EXAFS Spectroscopy. EXAFS experiments were performed at X-ray beamlines X-10C and X-18B at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). The storage ring operated with an electron energy of 3 GeV; the ring current was in the range 60-100 mA. The supported osmium carbonyl clusters and the decarbonylated clusters were characterized by EXAFS spectroscopy. In an argon-filled glovebox at the synchrotron, each powder sample was pressed into a self-supporting wafer. The sample mass was chosen to give an X-ray absorbance of 2.5 at the Os LIII edge (10871 eV). The wafer was loaded into an EXAFS cell,8 sealed under a positive N2 pressure, and removed from the glovebox. The cell was then evacuated (10-5 mbar), and the sample was aligned in the X-ray beam and cooled to nearly liquid-nitrogen temperature. EXAFS spectra were then collected in transmission mode. Higher harmonics in the X-ray beam were minimized

Kulkarni et al. by detuning the Si (111) monochromator by 20-25% at the Os LIII edge when the data were recorded at beamline X-18B. In contrast, when the data were recorded at beamline X-10C, with a Si (220) monochromator, higher harmonics were rejected by use of a rhodium-coated cylindrically bent mirror. HAADF STEM. Images of the samples were acquired by the HAADF method in STEM mode with a 200-kV JEOL JEM2500SE TEM/STEM instrument with a probe size of 0.3 nm at full-width half-maximum (fwhm), a convergence semiangle of 12 mrad, and a collection semiangle in the range of 35-90 mrad. To protect the air-sensitive samples from exposure to the atmosphere, they were handled in an N2-filled glovebox. A 200mesh copper grid with a holey carbon film was dipped into the sample powder. The grids then were loaded onto a Fischione vacuum-transfer-holder (model 2020). This holder, which was designed for this work, has a sealed chamber that could be opened and closed in the glovebox for sample loading. It protected the sample from air and moisture until it was opened inside the microscope column under vacuum. Further details of the holder are given in the Supporting Information. For each field of view, a set of images with a focus series was taken so that sizes of the clusters could be determined at the optimum focus. Data Analysis EXAFS Data Analysis. The X-ray absorption edge energy was calibrated with the measured signal of a hafnium foil (scanned simultaneously with the sample) at the Hf LII edge. The Hf LII edge (10739 eV) was chosen because of its proximity to the Os LIII edge (10871 eV). Data reduction and analysis were carried out with the averages of all of the scans (typically four) taken for each sample, by use of the software XDAP.9 XDAP was used for edge calibration, deglitching, data normalization, and conversion of the data into an EXAFS (chi) file.10 Analysis was carried out with unfiltered data; iterative fitting was performed until optimum agreement was attained between the calculated k0-, k1-, k2-, and k3-weighted EXAFS data and each postulated model. The data were fitted in R space with the Fourier-transformed χ(k) data (R is distance from the absorbing Os atom and χ is the EXAFS function).10 The objective function used for the least-squares data fitting is reported elsewhere.9 Both the magnitude and the imaginary part of the Fourier-transformed data were fitted with k0, k1, k2, and k3 weightings of the data until the fit was optimized. The software FEFF711 was used to determine amplitude- and phaseshift functions by theoretical calculations for reference materials with known crystal structures, except that experimental EXAFS results were used as a reference for Os-O* contributions (O* is oxygen in carbonyl groups). The reference compounds used for each EXAFS contribution are summarized in the Supporting Information. XDAP allowed the efficient application of a difference-file technique10 for determination of optimized fit parameters. The postulated models used in the data fitting included Os-Os, Os-C, Os-O*, and Os-Osupport contributions. The Os-C and Os-O* contributions in each model are characterized by multiple scattering (as expected for linear Os-C-O moieties), as was evident in the fitting; to distinguish such contributions from single-scattering contributions, phase and amplitudecorrection was used. The EXAFS data were analyzed with a maximum of 20 free parameters. The number of free parameters used in the fitting was always less than the statistically justified number, computed with the Nyquist theorem,12 n ) (2∆k∆R/π) +2, where ∆k and

Decaosmium Clusters Supported on MgO ∆R respectively are the k and R ranges used in the fitting. The fitting ranges, determined by the data quality, varied from sample to sample. To estimate the statistical error associated with the χ(k) values for each data set (used in the estimation of precisions), the averaged data were Fourier filtered by using a k-window larger than that used for the data fitting. The filtered data were then subtracted from the raw data to obtain an estimate of the error at each point. The root-mean-square error was calculated and used for the calculation of precisions and the goodness of fit. The approximate accuracies of the fit parameters characterizing the absorber-backscatterer pair contributions are estimated to be as follows: coordination number N, ( 20%; distance R, ( 2% Å; Debye-Waller factor ∆σ2, ( 20%; and inner potential correction ∆E0, ( 20%. The precisions reported for each of the parameters in the EXAFS models were calculated on the basis of the objective function of the fitting routine.9 The values of goodness of fit (recommended by the International XAFS Society13) are included with each fit. In some of the postulated models, the Os-Mg and Os-Osuppport contributions are of questionable physical meaning, and the errors in these contributions are the largest of any. STEM Data Analysis. An advantage of the HAADF technique in STEM mode relative to conventional TEM is that the incoherent contrast mechanism is less sensitive to the phase and diffraction effects that make size determination difficult in conventional TEM.14 Furthermore, the intensity in the image is approximately proportional to the square of the atomic number of the elements contributing to the image. Thus, excellent imaging of the heavy Os atoms of the clusters on the light oxide support is attainable. However, in all microscopy experiments the accuracy in the size determination of the supported clusters is limited by the size of the electron probe and the instability of the clusters under the electron beam (i.e., beam damage).15 Reduction of the electron dose can minimize the effects of the beam, but at the expense of signal-to-noise ratio, so the direct size measurement is characterized by substantial errors, because the clusters are extremely small and the images noisy. To address these limitations of the technique, we applied a method of measurement of the STEM images6 that gives highly precise results by eliminating the effect of the probe size on the cluster size measurements while minimizing the noise. Thus, the analysis was done by a procedure that involves smoothing of the data to minimize the effects of noise by artificially introducing a standard blurring function (such as a Gaussian function) to the individual images and obtaining size estimates by curve fitting, then extrapolating to determine an estimate of the cluster size that would be obtained with zero artificial and instrumental blurring. Details of this process are given elsewhere.6 Separate numerical calibrations of this fitting procedure suggested that, for our experimental conditions, this method tends to overestimate the size by approximately 6%. This overestimate seems to arise from the approximation of the projected atomic density of the cluster as a two-dimensional Gaussian function. The calculations leading to this conclusion are extensive and will be published separately.16 Results and Discussion Comparison with Literature of Similar Samples. Building on literature reports4,5 regarding the synthesis of decaosmium carbonyl clusters on MgO with yields of approximately 65-70% (Figure 1), we carried out similar syntheses to obtain predominantly decaosmium carbonyl clusters supported on MgO, as shown by the following results. MgO-supported osmium clusters

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Figure 1. Structure of [Os10C(CO)24]2- in a crystalline salt.17

provide a unique opportunity to use the STEM and EXAFS techniques in concert, because the osmium clusters can be synthesized selectively with controlled nuclearities, such as 3,3 5,7 and 10 atoms,4 and because osmium and MgO offer the advantage of high contrast in STEM. MgO-supported clusters with as few as 10 Os atoms each are just small enough to give minimal errors in the determination of coordination numbers of metal-metal shells by EXAFS spectroscopy, provided that the samples meet a high standard of uniformity.2 Because good EXAFS estimates of cluster size require such small clusters, complementary STEM measurements require instruments with the best resolution available. The EXAFS data presented here are of markedly higher quality than those reported,5 and the new data allow the determination of more shells than had been reported for such samples.5 Furthermore, the STEM data presented here were obtained with HAADF STEM (JEOL JEM2500), whereas only bright-field TEM (Phillips EM 400) data were reported earlier. In contrast to the earlier work, we carried out an error analysis of the data acquired from STEM imaging to determine sizes of the cluster frames, accounting for the systematic errors associated with the equipment and the presence of carbonyl ligands on the osmium. Thus, we have now endeavored to demonstrate how a sample chosen to maximize the uniformity and the data quality can be characterized in depth by taking advantage of the complementarities of EXAFS spectroscopy and STEM, augmented by IR spectroscopy and mass spectrometry of effluent gases formed in the synthesis. Formation of Decaosmium Carbonyl Clusters on MgO. Characterization of Decaosmium Carbido Carbonyl Clusters Supported on MgO by IR and EXAFS Spectroscopy. In previous investigations5 it was shown that MgO-supported triosmium carbonyl clusters could be converted predominantly to decaosmium carbonyl clusters in yields of approximately 65-70%. Earlier reports4 suggested a two-step process including the oxidative fragmentation of triosmium clusters in helium at 548 K and reductive carbonylation to form decaosmium species. We treated our sample under conditions nearly matching those reported for this conversion; thus, surface-mediated synthesis of supported decaosmium clusters was effected by reductive carbonylation of mononuclear osmium carbonyls in the presence of CO at 548 K for 4 h.4 The IR spectra (Figure 2) of the resulting species indicate the formation of decaosmium carbonyl clusters, consistent with earlier observations.5

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Figure 2. IR spectra in the νCO region of species formed from reductive carbonylation of Os3(CO)12 on MgO at 548 K.

The results of the analysis of the EXAFS data characterizing our sample incorporating the decaosmium carbonyl clusters are shown in Table 1. Plots of the EXAFS data showing the fits in k space and in R space are presented in Figure 3, panels A and B, respectively. A previously reported5 structural model based on an EXAFS analysis consisted of one Os-Os shell with a coordination number of 4.5. In contrast, the models used to fit our data include two Os-Os contributions, at distances of 2.80 and 2.98 Å. The new results are more consistent with the crystallographically determined structure of [Os10C(CO)24]2- than that reported,17 and the new results indicate the presence of Os atoms at two different distances from each other in the cluster structure. Here we discuss the model (referred as model I) that fits the data best; this was chosen from the two models identified as providing the best fits; the other (model II) is considered in the Supporting Information. The basis for the choice of model I is given in the Supporting Information. The isolated fits characterizing the first Os-Os and the second Os-Os shells in Model I are shown in Figures 3C and D, respectively. The Os-Os contributions at distances 2.80 and 2.98 Å indicate the presence of a decaosmium frame with interatomic distances matching (within error) those indicated by X-ray diffraction data characterizing [Os10C(CO)24]2- in the crystalline state.17 Coordination numbers of 2.8 and 2.7 were found for the Os-C and Os-O* contributions, respectively, at distances of 1.87 and 3.06 Å (Table 1), respectively, indicating CO ligands on the Os. The agreement of the Os-C and Os-O* coordination numbers (within error) with the data characterizing crystalline [Os10C(CO)24]2-17 (Table 1) points to clusters with a frame similar to that of [Os10C(CO)24]2- as the predominant supported species. The Os-Osupport contribution (Table 1) is characterized by a distance (2.15 Å, Table 1) consistent with bonding of Os atoms to support O atoms, which is not consistent with coordinative saturation of the osmium with CO, Os, and C ligands. Thus, we infer that CO ligands were distorted (and some may even have been missing) as a result of interactions between the clusters and the support. This suggestion is consistent with the peak broadening in the νCO region of the IR spectrum, as shown in an earlier report.4 In summary, the spectroscopic evidence, which represents an average integrated over the sample, supports the presence of clusters with the decaosmium frame with carbonyl ligands and strong metal-support interactions indicating direct Os-Osupport bonding. Thus, the approximation that the frames of the supported clusters are similar to that of [Os10C(CO)24]2is a simplification, at least in part because the Os-Osupport

Kulkarni et al. interaction evidently distorts the cluster structure; furthermore, the EXAFS model does not account for a carbido C atom within the metal frame (EXAFS spectroscopy lacks the sensitivity to determine whether this atom is present when there are numerous other Os-C contributions, as in the carbonyl ligands) and does not determine exactly how many CO ligands are present in a clustersand the evidence of direct Os-Osupport bonding indicated by the bonding distance of 2.15 Å (Table 1) points to a number of CO ligands less than 24 (corresponding to coordinative saturation of the cluster with these ligands) to allow this close Os-Osupport interaction. Estimation of AWerage Size of Decaosmium Carbonyl Clusters Supported on MgO by EXAFS Spectroscopy. The average interatomic distances (R) obtained according to model I for the Os-C, Os-O*, and two Os-Os contributions were used to model the structure of the supported clusters, represented for simplicity as decaosmium carbonyls assumed to have the Td symmetry of the osmium frame of [Os10C(CO)24]2-.17 Because the EXAFS data indicate that the number of CO ligands per cluster was less than 24, we assumed that all the CO ligands were terminal. The resulting three-dimensional model was used to determine the average rms radius of the supported clusters (including the Os-C-O distance), namely, 2.94 Å. The basis for this approximation has been discussed,6 and details of the calculation are as stated elsewhere.6 To be conservative in the estimate of the error in the rms radius, we used the upper error bound of the interatomic distances, namely, (2%. (Typically, the errors in the interatomic distance determined by EXAFS spectroscopy of samples such as ours are considered to be in the range of (1-2%.10) We estimated the error in the rms radius by using a standard error propagation method,18 finding a value of (0.07 Å. Characterization of Decaosmium Carbido Carbonyl Clusters Supported on MgO by STEM. Figure 4 shows a STEM image of the sample incorporating the supported clusters approximated as decaosmium carbonyls. An image of part of this sample was communicated earlier,6 with a discussion of the image analysis methodology. We now compare the structural information determined by STEM with that determined by EXAFS spectroscopy. Comparison of Size of Decaosmium Carbido Carbonyl Clusters on MgO Determined by EXAFS and STEM Data with XRD Data Characterizing Crystalline [Os10C(CO)24]2-. The rms radius of [Os10C(CO)24]2- in the crystalline state was calculated from the X-ray diffraction data17 by a method similar to that described above for the supported clusters. For comparison with the rms radius of the supported osmium carbonyl clusters calculated from the EXAFS data, 2.94 ( 0.07 Å, we used XRD data17 to calculate the rms radius of [Os10C(CO)24]2- in the crystalline state, finding a value of 2.91 ( 0.04 Å. Thus, these values agree within error and are to be compared with the STEM images. The STEM measurements shown earlier6 demonstrate that the samples contained not only clusters approximated as decaosmium, but also triosmium clusters. The presence of triosmium clusters with decaosmium clusters is expected, as the triosmium carbonyl clusters are precursors of the decaosmium carbonyl clusters in the synthesis; the conversion to the larger clusters was evidently not complete, as the literature indicates.5 The average radii attributed to triosmium and decaosmium carbonyl clusters were found to be 2.01 and 3.11 Å, respectively.6 The reported STEM results6 show that approximately 8-12% of the Os atoms were present in triosmium clusters;

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TABLE 1: Structural Parameters Corresponding to Structural Models for EXAFS Data Characterizing MgO-Supported Osmium Clustersa sample

model

species formed by reductive carbonylation of Os3(CO)12 on MgO at 548 K, approximated as [Os10C(CO)24]2-/MgO

I

species formed by treatment of decaosmium carbonyl clusters on MgO at 473 K for 2 h in H2

III

species formed by treatment of decaosmium carbonyl cluster on MgO at 473 K for 2 h in H2

IV

absorber-backscatterer pair

N

R (Å)

103 × ∆σ2 (Å2)

∆E0 (eV)

Os-Os Os-Os Os-C Os-O* Os-Osupport Os-Os Os-Os Os-C Os-O* Os-Osupport Os-Os Os-Os Os-C Os-O* Os-Osupport

2.3 ( 0.5 2.4 ( 0.4 2.8 ( 0.4 2.7 ( 0.1 0.6 ( 0.2 2.2 ( 0.4 2.5 ( 0.1 2.0 ( 0.3 1.8 ( 0.1 1.4 ( 0.1 2.2 ( 0.5 2.0 ( 0.6 1.5 ( 0.4 1.0 ( 0.2 1.4 ( 0.4

2.80 ( 0.02 2.98 ( 0.03 1.87 ( 0.03 3.06 ( 0.01 2.15 ( 0.04 2.68 ( 0.02 2.94 ( 0.01 1.95 ( 0.01 3.09 ( 0.01 2.09 ( 0.01 2.84 ( 0.02 2.94 ( 0.02 1.86 ( 0.01 3.10 ( 0.03 2.09 ( 0.03

7.4 ( 1.5 7.3 ( 2.6 5.4 ( 0.1 0.6 ( 1.3 1.3 ( 0.6 10 ( 2.2 6.8 ( 1.1 11 ( 1.9 9.3 ( 1.3 2.6 ( 1.3 0.8 ( 1.3 0.4 ( 1.3 6.0 ( 2.8 5.1 ( 2.6 4.4 ( 1.6

-1.8 ( 1.2 -6.5 ( 1.1 13 ( 4.7 -6.3 ( 0.6 9.5 ( 3.2 9.7 ( 1.7 -7.6 ( 0.4 3.7 ( 1.2 -9.9 ( 0.4 7.5 ( 1.1 -5.5 ( 1.9 5.5 ( 2.2 8.5 ( 0.5 -12 ( 3.3 -10 ( 2.8

a The errors given in the table correspond to the precisions of the parameters. Notation: N, coordination number; R, interatomic distance; ∆σ2, Debye-Waller parameter; ∆E0, inner potential correction; The estimated accuracies of the parameters are: N, ( 20%; R, ( 2% Å; ∆σ2, ( 20%; ∆E0, ( 20%.

Figure 3. EXAFS data (Model I) characterizing species formed from reductive carbonylation of Os3(CO)12 on MgO at 548 K: (A) k3-weighted EXAFS function, k3(χ) (solid line) and sum of the calculated contributions (dotted line); (B) k3-weighted imaginary part and magnitude of the Fourier transform of data (solid line) and sum of the calculated contributions (dotted line); (C) k3-weighted, phase and amplitude corrected, imaginary part and magnitude of the Fourier transform of data (solid line) and calculated contributions (dotted line) of Os-Os first shell; (D) k3-weighted, phase and amplitude corrected, imaginary part and magnitude of the Fourier transform of data (solid line) and calculated contributions (dotted line) of Os-Os second shell.

this small fraction had only a negligible effect on the EXAFS parameters (this result is in agreement with our estimate stated above and based on the IR spectra and earlier reports that the molar yield of decaosmium clusters was approximately 65-70%). In summary, the EXAFS data indicate a radius of the undecarbonylated clusters of 2.94 ( 0.07 Å and the STEM images a radius of 3.11 ( 0.09 Å. We might expect even more exact agreement between these numbers if it were not for the error introduced by the approximation that the fit function used

in the STEM analysis is a Gaussiansthis approximation results in a slight overestimation of the rms radius, as discussed in detail elsewhere.16 Partial Decarbonylation of Decaosmium Clusters Supported on MgO. The supported samples represented as predominantly decaosmium carbonyl clusters on MgO was treated at 473 K in flowing H2 for 2 h with the goal of partially decarbonylating the clusters while retaining the decaosmium frame. IR spectra were recorded during the decarbonylation,

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Figure 4. HAADF-STEM image of decaosmium carbonyl clusters supported on MgO.

Figure 5. IR spectra in the νCO region of MgO-supported decaosmium carbonyl cluster during decarbonylation in H2 at (A) room temperature; (B) 323 K; (C) 398 K; (D) 473 K for 1 h; and (E) 473 K for 2 h.

and the partially decarbonylated samples were characterized by IR and EXAFS spectroscopies and HAADF-STEM. Characterization of Partially Decarbonylated Decaosmium Clusters Supported on MgO by IR Spectroscopy. IR spectra in the νCO region were tracked as the temperature was ramped from 298 to 473 K at a rate of 2 K/min with the sample in flowing H2; subsequently, the sample in flowing H2 was held at a temperature of 473 K for 2 h as spectra were recorded. The νCO peak intensities decreased during the temperature ramp; the bands were not removed entirely (Figure 5), as the decarbonylation was incomplete. During the subsequent treatment at 473 K, the νCO peak intensities remained essentially unchanged for 2 h, whereupon the experiment was stopped; hence we infer that no further decarbonylation occurred during this period. Characterization of Partially Decarbonylated Decaosmium Clusters Supported on MgO by STEM. To determine the effect of decarbonylation on the metal frame, the partially decarbonylated sample after the treatment in H2 at 473 K for 2 h was characterized by STEM. A raw STEM image of the sample is shown in Figure 6A. Figure 6B shows the kernel density estimator (KDE) of the measured rms radii with all known systematic effects removed for 28 clusters indicated in the STEM image. These results show that the clusters can be divided into two groups, designated A and B in order of size. Two peaks in the KDE curve (rms kernel width of 0.23 Å) are evidently

Figure 6. HAADF-STEM data characterizing the MgO-supported decaosmium carbonyl clusters treated at 473 K and 1 bar for 2 h in H2: (A) HAADF-STEM image; (B) distributions of measured particle rms radii with all known systematic effects removed. Gaussian-kernel probability density estimator with rms kernel size of 0.23 Å was used to determine the number of subpopulations required to describe the population. The population distribution is estimated with a maximumlikelihood estimate assuming a sum of two Gaussians (as shown).

necessary and sufficient to describe the statistical distribution of the 28 clusters (Figure 6B). The KDE is a substitute for the better-known histogram, providing similar information while being less wasteful of information and less sensitive to bin size and centering artifacts that can plague histogram analysis for small data sets.19 A detailed explanation of how KDE was used here is given elsewhere.6 Parameters characterizing these peaks were derived from the maximum likelihood (ML) method by using a sum of Gaussian distributions as a model18 and varying the height, width, and position of each peak to maximize the likelihood of the complete set of 28 measured values. This procedure eliminates the need for assuming that any specific measurement belongs to a specific peak. We emphasize that similar results were obtained by making this assumption for each of the 28 values. The statistical distribution estimated from ML is shown in Figure 6B. Note that this curve is not a curve fit to the KDE curve but is instead derived directly from the list of 28 rms radii. Table 3 is a summary of the means and standard deviations of rms radii for each group determined by the ML calculation. The ML calculation indicates that group A contains 10 clusters drawn from a distribution with a mean radius of 2.28 Å; group B contains 18 clusters with a mean radius of 3.06 Å. Standard errors for these values are stated in Table 3. The mean radius of the partially decarbonylated clusters represented as group B (Table 3) is the same, within error, as

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TABLE 2: Fit Diagnostic Parameters for EXAFS Models Representing the MgO-Supported Osmium Speciesa Fit Variances model

∆k [Å-1]

∆R [Å]

εv2

I

3.60-13.22

0.75-4.00

1.7

III

3.62-13.84

0.75-4.00

2.7

IV

3.62-13.84

0.75-4.00

3.2

Abs 0

k : 0.04 k1: 0.05 k2: 0.09 k3: 0.28 k0: 0.02 k1: 0.06 k2: 0.21 k3: 0.57 k0: 0.28 k1: 0.31 k2: 0.50 k3: 0.68

Im 0

k : 0.10 k1: 0.12 k2: 0.23 k3: 0.50 k0: 0.05 k1: 0.11 k2: 0.33 k3: 0.78 k0: 0.48 k1: 0.57 k2: 0.84 k3: 1.15

a Model I, species formed by reductive carbonylation of Os3(CO)12 on MgO at 548 K approximated as [Os10C(CO)24]2-/MgO; models III and IV, species formed by treatment of decaosmium carbonyl cluster on MgO at 473 K for 2 h in H2; Abs, real part of the FT; Im, imaginary part of the FT; εv2, goodness-of-fit parameter; kn, weighting in the Fourier transformation.

that of the undecarbonylated clusters approximated as decaosmium, as reported earlier.6 Taking account of the results that the STEM images do not provide evidence of the light CO ligands and the IR spectra show that the removal of these ligands from the clusters was incomplete, we suggest that the partial decarbonylation process did not lead to any significant change in the decaosmium cluster frame. The STEM images provide information about the smaller clusters in the sample. The data of Table 3 show that the clusters identified as group A are characterized by a mean radius of 2.28 Å, and this value is significantly greater than that characterizing the triosmium carbonyl clusters in the undecarbonylated sample (2.01 Å).6 This comparison shows that there was some aggregation of the osmium during the decarbonylation taking place in H2 at 473 K. Evidently the triosmium carbonyl clusters on the support are less stable than the clusters approximated as decaosmium carbonyls at temperatures in the range of 450-573 K; these latter clusters are the ones of greatest interest in this work, because they are larger and more stable than triosmium and the clusters formed from triosmium in the initial stages of sample preparation. It is likely that decaosmium clusters formed from the precursors by a process involving cluster fragmentation and reassembly into the stable decaosmium frame.4 Characterization of Partially Decarbonylated Decaosmium Clusters Supported on MgO by EXAFS Spectroscopy. Analysis of the EXAFS data indicated two statistically valid models, III and IV (Tables 1 and 2), which are discussed in turn in the following paragraphs. Structural Model III. The isolated fits characterizing the first Os-Os and the second Os-Os shells in model III are shown in Figure 7. The Os-Os contributions at distances of 2.68 and 2.94 Å indicate a contraction of the decaosmium metal frame as a result of decarbonylation. The distance of the first Os-Os shell decreased by 0.12 Å and that of second by 0.04 Å relative to the undecarbonylated sample (Table 1). Coordination numbers of 2.0 and 1.8 were found for the Os-C and Os-O* contributions, respectively, at distances of 1.95 and 3.09 Å (Table 1), respectively, indicating removal of approximately 35-40% of the carbonyl ligands from the clusters, in agreement with the estimate of approximately 35% indicated by the decrease in the area under the appropriate peaks in the νCO region of the IR spectra.

The Os-Osupport contribution (Table 1) is characterized by a distance of 2.09 Å and a coordination number of 1.4. These results reinforce the suggestion that removal of CO ligands resulted in an increase in the strength of the metal-support interaction, consistent with replacement of carbonyl ligands by support ligands. Structural Model IV. The second structural model (model IV) characterizing the MgO-supported partially decarbonylated decaosmium carbonyl clusters also indicates retention of the decaosmium frame with minimal distortion resulting from decarbonylation. Model IV is characterized by first and second Os-Os contributions at 2.84 and 2.94 Å, respectively (Figure 8). The Os-Os distance of the first contribution increased by 0.04 Å relative to that of the undecarbonylated sample (Model I). Model IV shows a corresponding decrease of 0.04 Å in the second Os-Os contributions. Coordination numbers of 1.5 and 1.0 were found for the Os-C and Os-O* contributions, respectively (Table 1), confirming the partial removal of the CO ligands from the cluster. The Os-Osupport contribution (Table 1) is characterized by a distance of 2.09 Å and a coordination number of 1.4, which suggests a strengthening of the metal-support interaction associated with the removal of CO ligands. Of the two structural models, model IV agrees better with the STEM results. Both the structural parameters of model IV (Table 1) and the STEM images (Table 3) suggest negligible changes in the decaosmium frame during the process of partial decarbonylation. Thus, we suggest that, although both models III and IV are statistically sound, model IV should be preferred because it better agrees with the STEM results characterizing the partially decarbonylated decaosmium carbonyl clusters on MgO. This comparison illustrates how STEM results can help in the interpretation of EXAFS data. We emphasize that, because our samples were not perfectly uniform, statistical analysis of a sufficient number of cluster images was required for sufficient data quality and a tight comparison of the STEM and EXAFS data; we also emphasize that our samples are among the most nearly uniform of any reported supported metal clusters. The STEM data were important in guiding the choice of the best model to fit the EXAFS data characterizing the partially decarbonylated clusters, and, typically, even for nearly uniform supported clusters such as ours, it may be difficult to discriminate between candidate EXAFS models. Thus, this investigation exemplifies how the two techniques complement each other, as well as demonstrating the value of IR spectroscopy in elucidation of structure. Estimation on Basis of EXAFS Data of AWerage Radius of Osmium Frame of Partially Decarbonylated Decaosmium Clusters Supported on MgO. The average interatomic distances (R) obtained according to model IV for the two Os-Os contributions were used to model the structure of the decaosmium frame, which, again, was assumed to have the Td symmetry of the osmium frame of [Os10C(CO)24]2-.17 The resulting three-dimensional model was used to determine the average rms radius of the osmium frame of partially decarbonylated supported osmium clusters; the value was found to be 2.73 ( 0.05 Å, compared with the value of 2.71 ( 0.05 Å for the metal frame of the undecarbonylated clusters. The details of the calculation are as stated above.6 Because the degree of decarbonylation was not determined exactly by the data, we have not estimated the contribution of the CO ligands to the cluster radius (but we might expect that the this contribution would be almost the same as that in undecarbonylated clusters).

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Figure 7. EXAFS data (model III) characterizing MgO-supported decaosmium carbonyl clusters treated at 473 K and 1 bar for 6 h in H2: (A) k3-weighted EXAFS function, k3(χ) (solid line), and sum of the calculated contributions (dotted line); (B) k3-weighted imaginary part and magnitude of the Fourier transform of data (solid line) and sum of the calculated contributions (dotted line); (C) k3-weighted, phase- and amplitude-corrected, imaginary part and magnitude of the Fourier transform of data (solid line) and calculated contributions (dotted line) of the first Os-Os shell; (D) k3-weighted, phase- and amplitude-corrected, imaginary part and magnitude of the Fourier transform of the data (solid line) and calculated contributions (dotted line) of the second Os-Os shell.

Figure 8. EXAFS data (model IV) characterizing MgO-supported decaosmium carbonyl clusters treated at 473 K and 1 bar for 6 h in H2: (A) k3-weighted, phase- and amplitude-corrected, imaginary part and magnitude of the Fourier transform of data (solid line) and calculated contributions (dotted line) of the first Os-Os shell; (B) k3-weighted, phase- and amplitude-corrected, imaginary part and magnitude of the Fourier transform of the data (solid line) and calculated contributions (dotted line) of the second Os-Os shell.

TABLE 3: Means and Standard Deviations of rms Radii Obtained by the Maximum Likelihood Method19 from STEM Image Characterizing Partially Decarbonylated Decaosmium Clusters on MgO

Conclusions

EXAFS spectroscopy, respectively, indicate the rms radii of MgO supported clusters approximated as decaosmium carbonyls to be 3.11 ( 0.09 and 2.94 ( 0.07 Å. Partial decarbonylation of the supported clusters led to only minimal changes in the osmium cluster frame, with the rms radius found by STEM being 3.06 ( 0.05 Å. This is the most exact comparison reported of the size of supported metal clusters by these two methods. We infer that future characterizations of highly dispersed supported metal clusters and related samples will benefit strongly from the application of complementary techniques, and STEM and EXAFS spectroscopy seem to us to be essential.

MgO-supported carbonyl clusters that are approximated as decaosmium were prepared by reductive carbonylation of Os3(CO)12 adsorbed on MgO and characterized by STEM and EXAFS spectroscopy. Supported decaosmium carbonyl clusters were partially decarbonylated in H2 at 473 K. STEM and

Acknowledgment. We thank P. A. Stevens and M. Sansone of ExxonMobil Research and Engineering Co. and Larry Fareria of Brookhaven National Laboratory for helpful discussions and assistance in the acquisition of EXAFS data. This work was supported by the National Science Foundation, GOALI Grant

number of standard deviation sample standard clusters in mean rms of mean rms error of mean group group radius (Å) radius (Å) (Å) A B

10 18

2.28 3.06

0.21 0.12

0.07 0.05

Decaosmium Clusters Supported on MgO CTS-05-00511, and ExxonMobil. We acknowledge the National Synchrotron Light Source (NSLS), a national user facility operated by Brookhaven National laboratory on behalf of the U.S. Department of Energy, Office of Science, Basic Energy Sciences, for access to beam time at beamline X-18B. We thank ExxonMobil for providing access to beam time at beamline X-10C. Supporting Information Available: Detailed tables and graphs relating to various models tested in the fitting, an example of data fitting, and schematic diagram of microscope holder. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Gates, B. C. Chem. ReV. 1995, 95, 511. (2) Guzman, J.; Gates, B. C. J. Chem. Soc., Dalton Trans. 2003, 17, 3303. (3) Bhirud, V. A.; Iddir, H.; Browning, N. D.; Gates, B. C. J. Phys. Chem. B 2005, 109, 12738. (4) Lamb, H. H.; Fung, A. S.; Tooley, P. A.; Puga, J.; Krause, T. R.; Kelley, M. J.; Gates, B. C. J. Am. Chem. Soc. 1989, 111, 8367. (5) Lamb, H. H.; Gates, B. C. J. Phys. Chem. 1992, 96, 1099.

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13385 (6) Okamoto, N. L.; Reed, B. W.; Mehraeen, S.; Kulkarni, A.; Morgan, D. G.; Gates, B. C.; Browning, N. D. J. Phys. Chem. C 2008, 112, 1759. (7) Bhirud, V. A.; Panjabi, G.; Salvi, S. N.; Phillips, B. L.; Gates, B. C. Langmuir 2004, 20, 6173. (8) Jentoft, R. E.; Deutsch, S. E.; Gates, B. C. ReV. Sci. Instrum. 1996, 67, 2111. (9) Vaarkamp, M.; Linders, J. C.; Koningsberger, D. C. Phys B: Condens Matter 1995, 208, 159–209. (10) Koningsberger, D. C.; Mojet, B. L.; van Dorssen, G. E.; Ramaker, D. E. Top. Catal. 2000, 10, 143. (11) Ankudinov, A. L.; Rehr, J. J. Phys. ReV. B 1997, 56, R1712. (12) Stern, E. A. Phys. ReV. B 1993, 48, 9825. (13) In International XAFS Society, Error Reporting Recommendations: A Report of the Standards and Criteria Committee. http://fisica.unicam.it IXS/OLD/ subcommittee _reports/sc/err-rep.pdf (accessed Feb 2006). (14) Nellist, P. D.; Rodenburg, J. M. Acta Cryst. A 1998, 54, 49. (15) Batson, P. E.; Dellby, N.; Krivanek, O. L. Nature 2002, 418, 617. (16) Reed, B. W.; Morgan, D. G.; Okamoto, N. L.; Kulkarni, A.; Gates, B. C.; Browning, N. D. Ultramicroscopy 2009, submitted. (17) Jackson, P. F.; Johnson, B. F. G.; Lewis, J.; Nelson, W. J. H.; McPartlin, M. Dalton Trans. 1982, 2099. (18) Bevington, P. R. Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1969; Chapters 2 and 5. (19) Silverman, B. W. Density Estimation for Statistics and Data Analysis; Chapman and Hall: London, 1986.

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