Spectroscopic Evidence of Extra-Framework Heterometallic Oxo

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Spectroscopic Evidence of Extra-Framework Heterometallic Oxo-Clusters in Fe/Ga-ZSM-5 Catalysts Haian Xia,†,|| Samuel D. Fleischman,‡,|| Can Li,*,† and Susannah L. Scott*,‡,§ †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China, ‡Department of Chemistry & Biochemistry, and §Department of Chemical Engineering, University of California, Santa Barbara, California 93106-9510, United States

ABSTRACT The effect of introducing extra-framework Ga on the local structure of the metal sites in Fe/ZSM-5, resulting in enhanced reactivity toward N2O, was investigated using a combination of Raman and X-ray absorption spectroscopies. The Raman spectra indicate an increased abundance of oxo- and/or hydroxobridged diiron sites, whereas the Fe K-edge XANES reveals more extensive reduction of Fe(III) to Fe(II). Curvefits of the EXAFS at both the Ga and Fe K-edges are consistent with heterometallic oxo-clusters containing both Ga-Fe and Fe-Fe paths. The spectroscopic evidence suggests a tetranuclear [Fe2Ga2O42þ] core, possessing an open dicubane structure. SECTION Surfaces, Interfaces, Catalysis

F

e/ZSM-5 catalysts have been widely studied because of their activity in direct N2O decomposition,1-3 selective reduction of NOx with hydrocarbons or ammonia,4,5 and the oxidative dehydrogenation of alkanes.6,7 In the presence of N2O at room temperature, Fe/ZSM-5 can even oxidize benzene to phenol and methane to methanol, which is analogous to the catalytic properties of methane monooxygenase in biological systems.8,9 Fe may be present inside the zeolite as mono-, bi-, or oligo-nuclear species, iron oxide clusters in varying degrees of agglomeration, mixed-oxide phases containing both Fe and Al, as well as bulk iron oxides on the external surfaces of the zeolite.9-13 The preparation and pretreatment of the catalyst can affect the distribution and even the nature of the Fe sites. As a result of this heterogeneity, the species responsible for the unique catalytic activity of Fe/ZSM-5 remains elusive. The zeolite framework heteroatom (e.g., Al) may play an important role in the formation of the active Fe sites.14-16 Binuclear Fe species bound to framework Al as well as isolated Fe species associated with framework or extra-framework Al have been proposed as the active sites for benzene hydroxylation.8,10,17 Raman spectroscopy is useful in the study of these materials because the vibrational modes of oxo/hydroxobridged metal sites are readily observed.18 XAFS has also proven to be a powerful tool for establishing the connectivity of the Fe species present in Fe/ZSM-5.3,11,13 Recently, the intentional incorporation of extra-framework Al or Ga was shown to enhance greatly the activity of Fe/ZSM-5 for N2O decomposition.14 Because the X-ray backscattering properties of Ga are significantly different from those of either Si or Al, XAFS can be used to seek structural information about extra-framework mixed metal sites in Fe/Ga-ZSM-5. In this work, XAFS analysis, in combination with Raman spectroscopy, provides the first evidence of the formation of extra-framework heterometallic oxo-clusters in Fe/Ga-ZSM-5.

r 2011 American Chemical Society

The materials were prepared by a solid-state ion-exchange method.12,17 HZSM-5 was heated first with FeCl3 in an inert atmosphere; then, a portion of the product was mixed and heated with GaCl3. Chloride was removed with water vapor; then, the materials were calcined. Finally, both materials were heated in He at 1173 K. Figure 1A-a shows the visible Raman spectrum of Fe/ZSM-5 (1 wt % Fe) after heating in He at 1173 K. The band at 380 cm-1 arises because of the five-membered rings of ZSM-5, whereas the band at 800 cm-1 has been assigned to symmetric T-O-T stretching.19 Figure 1A-b shows the Raman spectrum collected after treating the Fe/ZSM-5 with N2O at 523 K, that is, forming the reactive “R-oxygen” species. A new band at 867 cm-1 is associated with the appearance of this species. When the N2O-treated Fe/ZSM-5 was exposed to water vapor at 523 K, the band at 867 cm-1 disappeared, whereas an intense band at 743 cm-1 appeared (Figure 1A-c). Figure 1B shows the analogous visible Raman spectra for Fe/Ga-ZSM-5. A new band at 526 cm-1 appears upon incorporation of extra-framework Ga. This band shows resonance enhancement in the UV Raman spectrum in both the presence and absence of Ga (Figure S1 of the Supporting Information), although it is more intense for Fe/Ga-ZSM-5. Previously, its intensity in the UV-Raman spectrum of Fe/ ZSM-5 was shown to increase with Fe concentration.17 Because Fe2O3 was not detected by XRD, the band was assigned to a symmetric bending mode involving either Fe-O-Si(Al) or Fe-O-Fe sites. The incorporation of Ga into Fe/ZSM-5 appears to cause the number of sites responsible for this band Received Date: November 7, 2010 Accepted Date: January 4, 2011 Published on Web Date: January 11, 2011

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DOI: 10.1021/jz101507s |J. Phys. Chem. Lett. 2011, 2, 190–195

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Figure 2. Comparison of the XANES for Fe/ZSM-5 (1.0 wt % Fe, red), Fe/Ga-ZSM-5 (1.0 wt % Fe, Ga/Fe = 0.6, blue), and Fe2O3 (black), all normalized to an edge step height of 1.0. The inset shows an enlargement of the edge region.

The Raman spectra show that the introduction of extraframework Ga causes the number of reactive Fe species to increase. This explanation is consistent with the activity for N2O decomposition (Figure S2 of the Supporting Information). The amount of N2 formed was quantified after a step change from flowing He to N2O/He (5.0 vol %) over both Fe/ZSM-5 and Fe/Ga-ZSM-5 at 523 K. In the presence of extraframework Ga, the formation of N2 increased substantially from 0.042 and 0.063 mmol/g. If the N2 yield corresponds to the initial amount of Fe(II) present, then the fraction of Fe present as Fe(II) must have increased from 0.23 and 0.35. Regardless, the incorporation of extra-framework Ga caused the number of sites capable of forming the R-oxygen species to increase by ca. 50%. Both the UVand visible Raman spectra of Fe/ZSM-5 and Fe/ Ga-ZSM-5 are therefore consistent with the presence of oligonuclear iron sites possessing one or more oxo/hydroxo bridges, whereas their reactivity suggests the presence of an appreciable amount of Fe(II). Because vibrational spectroscopy does not establish the precise nuclearity or oxidation states of these metal sites, we investigated the X-ray absorption spectra of both the homo- and heterometallic systems. The Fe K-edge XANES for Fe/ZSM-5 and Fe-Ga/ZSM-5 are shown in Figure 2 as well as that of R-Fe2O3, which contains exclusively Fe(III). For Fe/ZSM-5, the edge position is shifted 0.25 eV lower in energy compared with Fe2O3, despite the more ionic environment of the former. Although this difference is at the limit of our spectral resolution, it is consistent with the presence of a small but significant amount of Fe(II) in Fe/ZSM-5. The introduction of Ga results in a further 0.50 eV red shift relative to Fe/ZSM-5. This is evidence of a much higher fraction of Fe(II) sites in Fe/Ga-ZSM-5, in agreement with the N2O decomposition experiments. There are also changes in the appearance of the XANES above the white line, suggesting subtle changes in the local coordination environment of Fe. The Fe K-edge EXAFS of Fe/ZSM-5 is shown in Figure 3a. There is no evidence of Fe-Cl paths in the EXAFS. The curvefit, shown in Figures S3 and S4 of the Supporting Information, is consistent with four-coordinate FeO4 sites.

Figure 1. Visible Raman spectra, recorded with 532 nm excitation, for (A) Fe/ZSM-5 (1.0 wt % Fe) and (B) Fe/Ga-ZSM-5 (1.0 wt % Fe; Ga/Fe = 0.6): (a) after pretreatment in He at 1173 K, (b) after exposing the sample in part a to N2O at 523 K, and (c) after exposing the sample in part b to water vapor at 523 K.

to increase substantially to the level where they can be observed without resonance enhancement. After treatment with N2O, followed by exposure to water vapor, the band at 743 cm-1 is more intense for Fe/Ga-ZSM-5 than for the corresponding Fe/ZSM-5 sample, and the band at 867 cm-1 disappears. The band at 867 cm-1 is tentatively assigned to a peroxodiiron site, Fe2(μ-1,2-O2). Analogous sites in both synthetic18,20 and biological Fe-containing catalysts (e.g., MMOHperoxo in sMMO20,21) show υ(O-O) frequencies in the range of 850-890 cm-1. This result as well as our previous experimental studies of Fe/ZSM-59,22 suggest that peroxodiiron sites are candidates for the R-oxygen species in Fe/ZSM-5. The frequency of the band at 743 cm-1 falls in the range expected for the asymmetric (M-O-M0 ) stretching in both coordination complexes and Fe-containing enzymes (725-850 cm-1).23 Consequently, the band at 743 cm-1 is attributed to M-O-M0 sites. However, because it does not shift upon exposure to water, we cannot rule out the possibility of M-OH-M0 sites.


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Table 1. Curvefit Parameters for the Fe K-edge EXAFS of Fe/ZSM-5, Obtained Using a Single-Scattering Model Based on the [Fe2O2] Corea path

N

R (Å)

σ2 (Å2)

Fe-O

2.02 ( 0.28

1.93 ( 0.05

0.004 ( 0.002

Fe-O Fe-Fe

2.06 ( 0.31 2.49 ( 0.47

2.03 ( 0.05 2.94 ( 0.05

0.004 ( 0.002 0.007 ( 0.002

a Global fit parameters: S02 = (0.86 ( 0.11), ΔE0 = (-4.1 ( 1.9) eV, residual = 31. The number of independent parameters used was 10 out of a total of 18 allowed by the Nyquist theorem.29

to 2.49 ( 0.47, which is consistent with small (i.e., di- or oligonuclear) clusters. In the literature, this peak has been modeled as an Fe-Fe path with 0.5 e N e 1.6.2,11,13,24-26,30 The curvefit distance at 2.94 Å is 0.23 Å longer than that reported for a simple di-μ-oxo bridged compound, L2Fe2(μ-O)2 (L is tris(2-pyridylmethyl)amine), at 2.71 Å;18 however, it is considerably shorter than the Fe-Fe distance in μ-oxo-bridged [(H2O)5Fe(μ-O)Fe(OH2)5]4þ at 3.55 Å.27 It is also somewhat shorter than the Fe-Fe separation reported for L2Fe2(μ-OH)2, 3.11 Å (L is 4-dimethylamino-2,6-decarboxylato-pyridine).18 The Fe-Fe distance in Fe/ZSM-5 is therefore most consistent with a dinuclear core containing, on average, one bridging oxo and one bridging hydroxo ligand, that is, Fe2(μ-O)(μ-OH).28 We were unable to obtain a satisfactory fit for the feature at 3.2 Å, which may represent contributions from several single- and multiple-scattering paths. It has been attributed to either a second Fe-Fe path24,26 or an Fe-Al path involving a framework cation.10 It may also reflect contributions from a variety of sites with different medium-range order. Although the changes in the Fe K-edge XANES upon incorporation of Ga are subtle (Figure 2) the Fe K-edge EXAFS undergoes a dramatic change (Figure 3b and Figure S5 of the Supporting Information). The feature in the FT magnitude at ca. 1.6 Å, which arises because of Fe-O scattering paths involving atoms bonded directly to Fe, is reduced in intensity by ca. 30%, whereas both features at 2.5 and 3.2 Å are severely attenuated. These observations confirm that the presence of Ga causes changes in the local environment of Fe. Modeling the feature at 2.5 Å in R-space using either Fe-O or Fe-Si/Al paths did not generate satisfactory fit parameters. (See Figure S6 and Table S1 of the Supporting Information.) Curvefits including a single-scattering Fe-M path generated using FEFF models with either a homometallic [Fe2O2] core or a heterometallic [FeGaO2] core (Figure S7 of the Supporting Information) do generate this feature (Figure S8a and Table S2 of the Supporting Information), although they do not predict the feature in R-space at 3.2 Å. The Ga K-edge EXAFS shows a broad feature centered at 2.7 Å in R-space (Figure 4a). It is consistent with a Ga-Fe path but not with Ga-O, Ga-Si, or Ga-Al paths (Figure S9 and Table S4 of the Supporting Information). However, the FEFF model based on the heterometallic [FeGaO2] core did not generate sufficient intensity beyond 3 Å in R-space (Figure S8b and Table S3 of the Supporting Information). Structural models must take into account the Ga-enhanced reactivity of the material toward N2O and the presence of

Figure 3. Comparison of the Fe K-edge EXAFS (FT magnitude: red; imaginary: black) for (a) Fe/ZSM-5 (1.0 wt % Fe) and (b) Fe/GaZSM-5 (1.0 wt % Fe; Ga/Fe = 0.6).

There are two Fe-O distances, at 1.93 and 2.03 Å, with coordination numbers of 2.03 and 2.06, respectively (Table 1). This result suggests that there are two kinds of Fe-O interactions, for example, involving both framework and extraframework oxygen atoms. In previous studies of Fe/ZSM-5, the first feature in R-space was modeled using either two or three shells of Fe-O paths (each with N = 2), for a total coordination number ranging from 3.8 to 6.1.2,5,11,13,24-26 The lack of consensus about the Fe coordination number may be due to sample differences in the ZSM-5 used (e.g., different Si and Al sources or Si/Al ratios) or variations in the method of synthesis of Fe/ZSM-5. One group reported that the coordination of the Fe sites depends on the postcalcination thermal treatment: materials heated in He at 423 K showed coordination numbers close to 6 compared with 4 for materials treated in He at 573 K.11,13 The EXAFS of Fe/ZSM-5 described here is consistent with the lower coordination, as expected because it was treated in He at 1173 K. This EXAFS resembles previously reported spectra that were shown to be consistent with di- or oligonuclear Fe sites.13,24 The curvefit in Figure S3 also contains an Fe-Fe path giving rise to a peak at ca. 2.5 Å in R-space. Its coordination number is refined


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Figure 5. Tetranuclear, open dicubane structures for: (a) a crystallographically characterized anionic coordination complex, [Fe4(OAc)(3,5-DBCat)4(L)2]- (Fe: blue, O: red, C: black, N: green),31 and (b) the proposed heterometallic sites in Fe/Ga-ZSM-5. 3,5DBCatH2 = 3,5-di-tert-butylcatechol, LH2 = 2-[[(2-hydroxyethyl)imino]phenyl-methyl]phenol. Table 2. Curvefit Parameters for the Ga K-edge EXAFS of Fe/GaZSM-5 Using a Single-Scattering Model Based on the Heterometallic [Fe2Ga2O4] Corea Nb

R (Å)

Ga-O

4

1.91 ( 0.03

0.005 ( 0.002

Ga-Fe

0.81 ( 0.21

3.03 ( 0.05

0.008 ( 0.003

Ga-Fe

0.88 ( 0.26

3.36 ( 0.03

0.008 ( 0.003

path

σ2 (Å2)

Global fit parameters: = (0.83 ( 0.05), ΔE0 = (-4.7 ( 2.2) eV, residual = 51. The number of independent parameters used was 11 out of a total of 22 allowed by the Nyquist theorem.29 b Coordination number (N) was not refined for the Ga-O path. S02

a

Table 3. Curvefit Parameters for the Fe K-edge EXAFS of Fe/GaZSM-5 Using a Single-Scattering Model Based on the Heterometallic [Fe2Ga2O4] Corea model

Figure 4. EXAFS (FT magnitude: red; imaginary: black) for Fe/GaZSM-5 (1.0 wt % Fe; Ga/Fe = 0.6), at (a) the Ga K-edge and (b) the Fe K-edge. Curvefits (blue) are shown for the heterometallic [Fe2Ga2O4] model.

R (Å)

σ2 (Å2)

Fe-O short Fe-Fe modela

2

1.94 ( 0.04 0.005 ( 0.001

Fe-O

2

2.07 ( 0.04

Fe-Fe

0.82 ( 0.24

3.02 ( 0.05 0.008 ( 0.002

Fe-Ga 0.78 ( 0.29

dinuclear Fe sites, which allow the formation of the peroxobridged sites, [Fe2(μ-1,2-O2)], suggested by Raman spectroscopy. This evidence led us to consider tetranuclear model structures. Several well-characterized molecular compounds containing [Fe4O4] or [Mn4O4] cores are known to have open dicubane structures.31,32 A typical cluster is shown in Figure 5a, and a proposed heterometallic analog [Fe2Ga2O4] in Fe/ Ga-ZSM-5 is shown in Figure 5b. This model was investigated by analysis of the EXAFS at both metal edges. At the Ga K-edge, there should be two, inequivalent Ga-Fe paths; the Ga-Ga path is too long (>4 Å) to be relevant. At the Fe K-edge, the model requires one Fe-Fe path and two inequivalent Fe-Ga paths. Curvefits of the Fe/ Ga-ZSM-5 EXAFS to the heterometallic [Fe2Ga2O4] model are shown in Figure 4, and the curvefit parameters are summarized in Tables 2 and 3. For the Ga K-edge EXAFS, the first coordination sphere is well-described by a single Ga-O shell (N = 4). Separating this path into two shells (both with N = 2) did not improve the fit residual, nor did the resulting two Ga-O distances differ appreciably. A satisfactory fit to the


Nb

path

long Fe-Fe modelc

0.005 ( 0.001

3.15 ( 0.05

0.008 ( 0.002

Fe-Ga 0.85 ( 0.34 3.33 ( 0.04 Fe-O 2 1.94 ( 0.04

0.008 ( 0.002 0.005 ( 0.001

2.07 ( 0.04

0.005 ( 0.001

Fe-Ga 0.77 ( 0.19 2.96 ( 0.05

0.009 ( 0.004

Fe-O

2

0.58 ( 0.28 3.11 ( 0.05

0.009 ( 0.004

Fe-Ga 0.62 ( 0.23 3.38 ( 0.04

0.009 ( 0.004

Fe-Fe

a Global fit parameters: S02 = (0.83 ( 0.06), ΔE0 = (-2.2 ( 0.5) eV, residual =31. The number of independent parameters used was 13, out of a total of 20 allowed by the Nyquist theorem.29 b Coordination numbers (N) were not refined for the Fe-O paths. c Global fit parameters: S02 = (0.83 ( 0.06), ΔE0 = (-2.2 ( 0.5) eV, residual = 38.

long-range features was also obtained using this tetranuclear model. Both Ga-Fe distances, (3.03 ( 0.05) and (3.36 ( 0.05) Å, have fitted values of N close to one, implying that the majority of sites have similar structural features. For the Fe K-edge EXAFS, there are two fitted Fe-O distances at 1.94 and 2.07 Å, very similar to those for Fe/ ZSM-5 (Table 1). For comparison, the Fe-O (μ2-OR) distances

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in the [Fe4O4] cluster shown in Figure 5a range from 1.935 to 2.093 Å, whereas the Fe-O (μ3-OR) distances range from 2.051 to 2.238 Å.31 The Fe-O distances in Fe/Ga-ZSM-5 are therefore consistent with two types of bridging ligands, that is, doubly and triply bridging. Initially, the fitted Fe-Fe distance was refined at (3.02 ( 0.05) Å. This is at the lower end of the range of Fe-Fe distances found in the [Fe4O4] cores of open dicubane structures (3.05 to 3.26 Å).31,33 The shorter of the two Fe-Ga distances, (3.15 ( 0.05) Å, also falls in this range, although the longer Fe-Ga distance, at (3.33 ( 0.04) Å, is slightly greater than the longest Fe-Fe distance in the [Fe4O4] cluster. For this curvefit (denoted the “short Fe-Fe model” in Table 3), the fitted values of N for each of the Fe-Fe and Fe-Ga paths are close to one. The open dicubane model is therefore consistent with the EXAFS at the Fe and K-edge. For this model, the longer of the two Fe-Ga distances in the Fe K-edge EXAFS curvefit agrees well with the longer Ga-Fe distance obtained from the curvefit to the Ga K-edge EXAFS. The agreement is not as good for the shorter Fe-Ga distance; however, because Fe and Ga differ by only Z = 5, it is challenging to discriminate between Fe-Fe and Fe-Ga paths. We explored the effect of interchanging the starting distances of the Fe-Fe path and the shorter Fe-Ga path in the curvefit of the Fe K-edge EXAFS. The results (denoted as the “long Fe-Fe model”) are shown in Figures S10 and S11 of the Supporting Information; the parameters are given in Table 3. For this fit, the refined Fe-Ga distances are (2.96 ( 0.05) and (3.38 ( 0.04) Å, whereas the Fe-Fe distance is (3.11 ( 0.05) Å. These distances are in slightly better agreement with the results of the curvefit to the Ga K-edge EXAFS, although the uncertainties in N for the Fe-Fe path and in σ2 for both Fe-Ga paths are larger. This may be a consequence of heterogeneity in the metal site occupancy of the heterometallic clusters or of heterogeneity in the structures of the metal oxo clusters. The active sites of Fe/ZSM-5 and Fe/Ga-ZSM-5 represent only a fraction of all Fe sites. The Raman spectra are ambiguous on the question of the protonation state of the clusters; however, the quality of the EXAFS curvefits for Fe/Ga-ZSM-5 and the consistency in the M-M0 distances refined at both the Fe and Ga K-edges suggests that the various sites are structurally similar. A simple explanation consistent with all of the available evidence is the coexistence of structurally related heterometallic clusters with different Fe(II)/Fe(III) contents, viz. [(FeII)2(GaIII)2O42þ], [FeIIFeIII(GaIII)2O3(OH)2þ], and [(FeIII)2(GaIII)2(OH)42þ]. The EXAFS would not readily discriminate between them, and curvefitting would return an Fe-Fe distance characteristic of mixed oxo-hydroxo bridging (see above). Presumably, the [(FeII)2(GaIII)2O42þ] sites are the most readily oxidized by N2O, to [(FeIII)2(μ-O2)(GaIII)2O32þ]. The open dicubane structures proposed for Fe/Ga-ZSM-5 may also exist in Fe/ZSM-5, with extra-framework Al atoms taking the place of Ga (i.e., [Fe2Al2O42þ]).34 The limited availability of extra-framework Al could limit the number of these sites formed. However, incorporation of Al or Ga favors these sites;14 there is a linear relationship between the addition of extra-framework Al to Fe/ZSM-5 and the number of active sites.16 In summary, we have identified structural changes that occur in Fe/ZSM-5 upon incorporation of extra-framework Ga.


Raman spectra show that this leads to more abundant active Fe sites and suggest that these sites contain two Fe centers capable of reacting with N2O to give a bridged peroxide site. EXAFS curvefits using data at both Fe and Ga K-edges are consistent with tetranuclear, heterometallic oxo-clusters organized in an open dicubane structure.

EXPERIMENTAL SECTION The reaction of H/ZSM-5 with FeCl3 was achieved by heating the two solids together at 593 K under N2, to give a solid containing ca. 1 wt % Fe. A portion of the parent material was mixed with the appropriate amount of GaCl3 and heated in flowing Ar at 473 K for 2 h. M-Cl bonds were hydrolyzed in a stream of wet Ar (0.5 vol % H2O) at 473 K. Elemental analysis (Columbia Analytical Services) confirmed that the amount of Cl remaining after hydrolysis was below the detection limit. Both materials were then calcined at 823 K for 2 h to remove water and subsequently treated in flowing He at 1173 K for 2 h to obtain Fe/ZSM-5 and Fe/Ga-ZSM-5 (Ga/ Fe = 0.6).

SUPPORTING INFORMATION AVAILABLE Detailed description of spectra acquisition, N2O decomposition, as well as data processing; k-space EXAFS, additional EXAFS curvefits, and parameter tables. This material is available free of charge via the Internet at http:// pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. (C.L.) Tel: 86411-84379070. Fax 86-411-84694447. E-mail: [email protected]. (S.L.S.) Tel: 1-805-893-5606. Fax: 1-805-893-4732. E-mail: sscott@ engineering.ucsb.edu

Author Contributions: ||

These authors contributed equally to this work.

ACKNOWLEDGMENT This work was supported financially by the Partnership for International Research and Education in Electron Chemistry and Catalysis, funded by the National Science Foundation under grant no. OISE-0530268, as well as the U.S. Department of Energy, Basic Energy Sciences, under Catalysis Science grant no. DE-FG01-03ER15647. This work was also supported financially by the National Basic Research Program of China (grant no. 2005CB221407) and the National Natural Science Foundation of China (grant nos. 20773118, 20673115, and 20620120428). A portion of this work was performed at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by the Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.

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