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2009, 113, 16847–16849 Published on Web 09/09/2009
Imaging Gold Atoms in Site-Isolated MgO-Supported Mononuclear Gold Complexes Alper Uzun,† Volkan Ortalan,† Yalin Hao,† Nigel D. Browning,†,‡ and Bruce C. Gates*,† Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, 1 Shields AVenue, DaVis, California 95616, and Lawrence LiVermore National Laboratory, LiVermore, California 94550 ReceiVed: July 16, 2009; ReVised Manuscript ReceiVed: August 26, 2009
Metal oxide- and zeolite-supported mononuclear gold complexes have been inferred, on the basis of spectroscopic data, to be catalytically active for alkene hydrogenation and CO oxidation. However, direct evidence that the catalysts are isolated, individual gold complexes in the absence of gold clusters or particles has been missing. Now we report the first evidence of uniquely site-isolated gold complexes on high-area MgOsin the absence of gold clusters or particles; the images were obtained with aberration-corrected scanning transmission electron microscopy. The recent discoveries of unprecedented catalytic properties of gold complexes in solution1-3 and highly dispersed gold on oxide supports4,5 have led to intense efforts to find connections between the two classes of catalysts and to identify the catalytically active surface species. Presumed analogies between the solution and surface reactions have led to suggestions that the catalytically active species are mononuclear gold complexes on supports6 and extremely small supported gold clusters that are cationic (and act as Lewis acids);7 gold cations at the metal-support interface have also been implicated. Attempts to characterize such species on supports have led investigators to use scanning tunneling microscopy to image extremely small gold species. Researchers have attempted to characterize size-selected gold clusters soft-landed on model (planar) metal oxide surfaces,8-10 including TiO2(110)-supported clusters containing as few as two to eight Au atoms.9 Transmission electron microscopy11 has also been used12-17 together with spectroscopic methods to investigate gold on high-area porous supports. A disadvantage of the planar supports is that they do not capture all of the properties of real catalyst supports; for example, the planar supports typically lack OH groups9 and are flat, low-area, and electrically conducting. However, a disadvantage of the real (high-area) supports is that they are intrinsically nonuniform, with the consequence that the gold species on them are also typically nonuniform, so that spectra provide only average information. In an investigation of gold on a high-area support (FeOx), Herzing et al.12 used aberration-corrected scanning transmission electron microscopy (STEM) to image the gold species. The images indicated mixtures of individual Au atoms and clusters with nuclearities ranging from 3-4 atoms (ca. 0.2-0.3 nm in diameter) to ca. 5000 atoms (ca. 7 nm in diameter). Similarly, Allard et al.15 reported STEM images of FeOx-supported gold showing mixtures of individual Au atoms and clusters with diameters up to 2-4 nm. * Corresponding author. E-mail:
[email protected]. † University of California, Davis. ‡ Lawrence Livermore National Laboratory.
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These examples illustrate how aberration-corrected STEM allows resolution of individual Au atoms on supports, but there are still no reports that conclusively resolve which species are catalytically active, and there are no reports of images of samples with site-isolated mononuclear gold species or uniform gold clusters; such samples are challenging to prepare and to stabilize. On the basis of spectroscopic data (extended X-ray absorption fine structure, EXAFS; IR; and 13C NMR), metal oxide- and zeolite-supported mononuclear gold complexes were inferred to be catalytically active for alkene hydrogenation and CO oxidation.18-21 However, direct evidence of the site isolation of the Au atoms (the term “site isolation” implies that the metal sites are well separated and do not interact significantly with each other) in the absence of any clusters has been missing, and the lack is significant because clusters are often apparently active and as minority species might account for the catalysis.22 Now, we report the first evidence of uniquely site-isolated gold complexes on a high-area support (MgO)sin the absence of gold clusters. The images were obtained with aberrationcorrected STEM. The MgO was highly dehydroxylated by treatment in O2 at 973 K. The supported catalyst (0.2 wt % Au) was made by the reaction of Au(CH3)2(acac) (acac is CH3COCHCOCH3)23 (I) with MgO (EM Science) in an n-pentane slurry, as has been reported.24 Our spectroscopic results are broadly consistent with results characterizing similarly synthesized gold complexes on MgO, which was less fully dehydroxylated than ours, and shown to be catalytically active for hydrogenation of ethene.18,24 Specifically, IR spectra (Supporting Information) demonstrate that the methyl ligands on Au (2959 and 2909 cm-1) remained intact as I reacted with MgO. The peaks characterizing acac are consistent with Guzman’s data,24 indicating that they were dissociated from the Au(CH3)2(acac) to form Hacac and Mg(acac) species on the support. EXAFS spectra show that, within error and on average, each Au atom was bonded to two carbon atoms (one for each methyl ligand) and to two oxygen atoms (inferred to be of the support, evidently a bidentate ligand) with no detectable Au-Au contribution (Table 1), as described in previous work.24 The X-ray absorption edge position and the presence of a white line (not present in the spectrum of gold 2009 American Chemical Society
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TABLE 1: EXAFS Parametersa Characterizing MgO-Supported Gold Complex Synthesized by Reaction of Au(CH3)2(acac) with Highly Dehydroxylated MgO absorber-backscatterer pair
N
R/Å
103 × ∆σ2/Å2
∆E0/eV
Au-Au Au-C Au-O Au-Mg Au-Olc
b 2.1 2.2 0.6 1.1
b 2.06 2.15 2.91 3.52
b 5.9 9.1 3.5 6.9
b -7.9 2.5 8.3 2.1
a Goodness of fit parameters are provided in the Supporting Information. Error bounds (accuracies) characterizing the structural parameters obtained by EXAFS spectroscopy are estimated to be as follows: coordination number N, (20%; distance R, (0.02 Å; Debye-Waller factor ∆σ2, (20%; and inner potential correction ∆E0, (20%. b Contribution undetectable. c Au-Ol represents long gold-oxygen contributions, characterizing the interactions of gold with neighboring nonbonding oxygen atoms. This contribution is too small to allow a confident assignment. It was included to make the overall fitting complete.
Figure 1. High-angle annular dark-field (Z-contrast) image of the sample prepared by reaction of I with MgO (with a surface area of approximately 100 m2/g). The image shows individual Au atoms in bright contrast (black circles), present in the absence of gold clusters.
foil) confirm that the gold was cationic, as in I.23,24 Thus, we infer that Au atoms were present on the MgO surface as cationic Au(CH3)2 complexes, which are bonded to support oxygen atoms, as indicated by EXAFS and IR data. The images were obtained at Oak Ridge National Laboratory by high-angle annular dark field (HAADF) STEM with an FEI Titan microscope equipped with a high-brightness field emission gun and operated at 300 kV with a CEOS dodecapole probe aberration corrector. The sample handling to exclude air and moisture is described in the Supporting Information. The STEM images (Figures 1 and 2a are representative images obtained at different regions of the sample surface) clearly show individual, isolated Au atoms on the MgO; these are not evident in images of the MgO alone. The diameter of each Au atom is 1.7 ( 0.1 Å, including the blurring effects arising from the remaining higher-order aberrations and defocus.25-27 The lattice spacing of MgO was measured as 2.1 ( 0.1 Å, corresponding to the (200) lattice plane of MgO.28 The demonstration of isolated Au atoms (the ligands are too small to image) in the absence of clusters on MgO is consistent with the IR, EXAFS, and XANES spectra (Supporting Informa-
Figure 2. Two consecutive STEM images characterizing the same region of a sample prepared by the reaction of I with MgO. The region indicated by the white circle shows the migration of individual Au atoms in bright contrast (black circles) and the formation of gold clusters (white arrow in part b) under the influence of the electron beam.
tion), which indicate site-isolated cationic gold complexes but do not exclude the presence of clusters as minority species. The images (Figures 1 and 2a) show nearly planar parts of an MgO crystallite and the locations of the Au atoms, which are clearly evident, but only in parts of the images where the focus was optimized.26 The loading of the Au atoms calculated from the images (0.06 ( 0.01 Au atom/nm2) matches that of the sample as a whole determined by the preparation conditions and mass balance (approximately 0.06 Au atom/nm2 for 0.2 wt % Au loading). We emphasize that the images presented are only the first ones that were obtained in these regions of the samplesssubsequent images, in contrast, show effects of the electron beam, which caused migration of the Au atoms (possibly facilitated by modification of the ligands) and formation of gold clusters. Figure 2 (representing two consecutive images taken of the same region with an interval of 20 s) shows how the electron beam caused movement of the Au atoms and formation of a gold cluster (indicated by a white arrow in Figure 2b) consisting of two or three Au atoms. Cluster formation was evidenced by the disappearance of one small bright spot and the increase in size of another bright spot in the region indicated by a white circle in consecutive images. Other researchers29,30 have reported evidence of comparable effects of the electron beam on supported metals. In parts of
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the sample having higher loadings of Au atoms than that shown in Figure 1, the effects of the electron beam were more pronounced. Therefore, we would expect greater effects than we observed of the electron beam on individual metal atoms for samples with higher metal loadings,12,15 depending on the beam energy and exposure times. In summary, our images are the first to show site-isolated gold alone on a support; in combination with IR and EXAFS spectra, the images demonstrate mononuclear gold complexes in the absence of gold clusters on a high-area metal oxide support. Acknowledgment. This work was supported by DOE (A.U., Grant No. DE-FG02-04ER15600; Y.H., Grant No. DE-FG0204ER15513) and by the National Science Foundation (NSF) (V.O., Grant No. CTS-0500511); we acknowledge beam time and the support of the Stanford Synchrotron Radiation Laboratory, operated by Stanford University for the DOE, Office of Energy Research, Basic Energy Sciences, for access to beam time on beamline 2-3. The STEM images were acquired at Oak Ridge National Laboratory’s Shared Research Equipment User Facility, supported by the Division of Scientific User Facilities, Basic Energy Sciences, DOE. Supporting Information Available: EXAFS and IR characterization of the sample prepared by reaction of Au(CH3)2(acac) on highly dehydroxylated MgO and sample handling for STEM imaging. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) 6405. (2) (3) (4) (5) (6) 2127.
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