Ceren Aydin

University of California, Davis, Davis, CA, United States

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Publications (18)151.46 Total impact

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    ABSTRACT: The active sites of enzymes are contained within nanoscale environments that exhibit exquisite levels of specificity to particular molecules. The development of such nanoscale environments on synthetic surfaces, which would be capable of discriminating between molecules that would nominally bind in a similar way to the surface, could be of use in nanosensing, selective catalysis and gas separation. However, mimicking such subtle behaviour, even crudely, with a synthetic system remains a significant challenge. Here, we show that the reactive sites on the surface of a tetrairidium cluster can be controlled by using three calixarene-phosphine ligands to create a selective nanoscale environment at the metal surface. Each ligand is 1.4 nm in length and envelopes the cluster core in a manner that discriminates between the reactivities of the basal-plane and apical iridium atoms. CO ligands are initially present on the clusters and can be selectively removed from the basal-plane sites by thermal dissociation and from the apical sites by reactive decarbonylation with the bulky reactant trimethylamine-N-oxide. Both steps lead to the creation of metal sites that can bind CO molecules, but only the reactive decarbonylation step creates vacancies that are also able to bond to ethylene, and catalyse its hydrogenation.
    Nature Nanotechnology 04/2014; · 31.17 Impact Factor
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    ABSTRACT: Rh(C2H4)2 complexes supported on HY zeolite selectively catalyze ethylene dimerization in the presence of H2, but iridium complexes anchored near the rhodium alter the selectivity by spilling over hydrogen that limits the adsorption of ethylene on Al–OH sites that act in concert with the rhodium sites, thereby triggering the rhodium complexes to operate as hydrogenation rather than dimerization catalysts.
    Catal. Sci. Technol. 04/2013;
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    ABSTRACT: Size, shape, nuclearity: Aberration-corrected scanning transmission electron microscopy was used to determine the 3D structures of MgO-supported Os3 , Os4 , Os5 , and Os10 clusters, which have structures nearly matching those of osmium carbonyl compounds with known crystal structures. The samples are among the best-defined supported catalysts.
    Angewandte Chemie International Edition 04/2013; · 13.73 Impact Factor
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    ABSTRACT: AbstractZ‐contrast imaging in an aberration‐corrected scanning transmission electron microscope can be used to observe and quantify the sizes, shapes, and compositions of the metal frames in supported mono‐, bi‐, and multimetallic metal clusters and can even detect the metal atoms in single‐metal‐atom complexes, as well as providing direct structural information characterizing the metal–support interface. Herein, we assess the major experimental challenges associated with obtaining atomic resolution Z‐contrast images of the materials that are highly beam‐sensitive, that is, the clusters readily migrate and sinter on support surfaces, and the support itself can drastically change in structure if the experiment is not properly controlled. Calibrated and quantified Z‐contrast images are used in conjunction with ex situ analytical measurements and larger‐scale characterization methods such as extended X‐ray absorption fine structure spectroscopy to generate an atomic‐scale understanding of supported catalysts and their function. Examples of the application of these methods include the characterization of a wide range of sizes and compositions of supported clusters, primarily those incorporating Ir, Os, and Au, on highly crystalline supports (zeolites and MgO).
    ChemCatChem 01/2013; 5(9). · 5.18 Impact Factor
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    ABSTRACT: This work addresses the question of what is the true catalyst when beginning with a site-isolated, atomically dispersed precatalyst for the prototype catalytic reaction of cyclohexene hydrogenation in the presence of cyclohexane solvent: is the atomically dispersed nature of the zeolite-supported, [Ir(C2H4)2]/zeolite Y precatalyst retained, or are possible alternatives including Ir4 subnanometer clusters or larger, Ir(0)n, nanoparticles the actual catalyst? Herein we report the (a) kinetics of the reaction; (b) physical characterizations of the used catalyst, including extended X-ray absorption fine structure spectra plus images obtained by high-angle annular dark-field scanning transmission electron microscopy, demonstrating the mononuclearity and site-isolation of the catalyst; and the (c) results of poisoning experiments, including those with the size-selective poisons P(C6H11)3 and P(OCH3)3 determining the location of the catalyst in the zeolite pores. Also reported are quantitative poisoning experiments showing that each added P(OCH3)3 molecule poisons one catalytic site, confirming the single-metal-atom nature of the catalyst and the lack of leaching of catalyst into the reactant solution. The results (i) provide strong evidence that the use of a site-isolated [Ir(C2H4)2]/zeolite Y precatalyst allows a site-isolated [Ir1]/zeolite Y hydrogenation catalyst to be retained even when in contact with solution, at least at 22 °C; (ii) allow a comparison of the solid–solution catalyst system with the equivalent one used in the solid–gas ethylene hydrogenation reaction at room temperature; and (iii) illustrate a methodology by which multiple, complementary physical methods, combined with kinetic, size-selective poisoning, and quantitative kinetic poisoning experiments, help to identify the catalyst. The results, to our knowledge, are the first identifying an atomically dispersed, supported transition-metal species as the catalyst of a reaction taking place in contact with solution.
    ACS-Catalysis. 08/2012; 2(9):1947.
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    ABSTRACT: Zeolite Hβ- and γ-Al(2)O(3)-supported mononuclear iridium complexes were synthesized by the reaction of Ir(C(2)H(4))(2)(acac) (acac is acetylacetonate) with each of the supports. The characterization of the surface species by extended X-ray absorption fine structure (EXAFS) and infrared (IR) spectroscopies demonstrated the removal of acac ligands during chemisorption, leading to the formation of essentially isostructural Ir(C(2)H(4))(2) complexes anchored to each support by two Ir-O(support) bonds. Atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM) images confirm the spectra, showing only isolated Ir atoms on the supports with no evidence of iridium clusters. These samples, together with previously reported Ir(C(2)H(4))(2) complexes on zeolite HY, zeolite HSSZ-53, and MgO supports, constitute a family of isostructural supported iridium complexes. Treatment with CO led to the replacement of the ethylene ligands on iridium with CO ligands, and the ν(CO) frequencies of these complexes and white line intensities in the X-ray absorption spectra at the Ir L(III) edge show that the electron density on iridium increases in the following order on these supports: zeolite HY < zeolite Hβ < zeolite HSSZ-53 ≪ γ-Al(2)O(3) < MgO. The IR spectra of the iridium carbonyl complexes treated in flowing C(2)H(4) show that the CO ligands were replaced by C(2)H(4), with the average number of C(2)H(4) groups per Ir atom increasing as the amount of iridium was increasingly electron-deficient. In contrast to the typical supported catalysts incorporating metal clusters or particles that are highly nonuniform, the samples reported here, incorporating uniform isostructural iridium complexes, provide unprecedented opportunities for a molecular-level understanding of how supports affect the electronic properties, reactivities, and catalytic properties of supported metal species.
    Langmuir 08/2012; 28(35):12806-15. · 4.19 Impact Factor
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    ABSTRACT: Dispersed gold complexes anchored in defined positions of a zeolite NaY are catalytically active sites for the oxidation of carbon monoxide at room temperature. In their Communication on page 5842 ff., B. C. Gates, et al. show images of these sites, which were obtained by aberration‐corrected scanning transmission electron microscopy and show changes in the catalytic activity associated with changes in the location of the gold atoms in the zeolite.
    Angewandte Chemie International Edition 06/2012; 51(24). · 13.73 Impact Factor
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    ABSTRACT: Dispergierte Gold‐Komplexe in definierten Positionen eines Zeoliths NaY sind die katalytisch aktiven Zentren für die Oxidation von Kohlenmonoxid bei Raumtemperatur. In der Zuschrift auf S. 5944 ff. zeigen B. C. Gates et al. Bilder dieser Zentren, die mithilfe von aberrationskorrigierter Rasterelektronenmikroskopie erhalten wurden und die Veränderungen in der katalytischen Aktivität, die mit Veränderungen in der Zeolithumgebung der Goldatome einhergehen, veranschaulichen.
    Angewandte Chemie 06/2012; 124(24).
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    ABSTRACT: Zeolite HSSZ-53, which has 1-dimensional channels with 14-ring extra-large pores, was used as a support for a molecular iridium complex synthesized from Ir(C2H4)2(C5H7O2) and characterized with infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM). The spectra show that Ir(C2H4)2(C5H7O2) reacted readily with the bridging OH groups of the zeolite, leading to the removal of C5H7O2 ligands and the formation of mononuclear Ir(C2H4)2 complexes bonded to the zeolite by Ir–O bonds at the framework aluminum sites. STEM images confirm the spectra, showing site-isolated iridium centers within the zeolite channels, with no evidence of iridium clusters. The samples constitute a highly uniform, well-defined array of essentially molecular catalytic species in a highly uniform, confined environment, allowing precise investigations of the chemistry of the iridium complex in the absence of solvents. IR spectra show that the supported Ir(C2H4)2 complexes were converted to Ir(C2H5)2, Ir(CO)2, Ir(CO)(C2H4), and Ir(CO)(C2H4)2 as various mixtures of H2, CO, and C2H4 reacted with the sample. The sample was tested as a catalyst for ethylene hydrogenation and for H–D exchange in the reaction of H2 + D2. The data, combined with results reported for isostructural iridium complexes bonded to zeolite HY and to MgO, demonstrate how the catalytic activity can be tuned by choice of the support, with the support being characterized as a ligand with electron-donating or electron-withdrawing properties. The results demonstrate that the rate of ethylene hydrogenation catalyzed by the supported iridium complexes is limited by H2 activation when the iridium is electron rich (on the MgO support), whereas the rate-limiting step is C2H4 adsorption when the iridium is electron deficient (on either zeolite support).
    ACS Catalysis 04/2012; 2(6):1002–1012. · 5.27 Impact Factor
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    ABSTRACT: Like billiard balls: Atomic-scale observations by electron microscopy of supported iridium nanoclusters show that the nanoclusters aggregate to reach a critical diameter of approximately 1 nm and then resist further aggregation. The observations highlight the potential for this catalyst to assemble into clusters that may be nearly optimum for catalytic activity.
    Angewandte Chemie International Edition 04/2012; 51(24):5929-34. · 13.73 Impact Factor
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    ABSTRACT: As good as atomic gold: Aberration-corrected scanning transmission electron microscopy images of zeolite NaY-supported mononuclear gold complexes, obtained with atomic resolution of the gold atoms, showed the locations of the gold complexes in the zeolite framework and identified them as the catalytically active species for CO oxidation at 298 K and 1 bar.
    Angewandte Chemie International Edition 03/2012; 51(24):5842-6. · 13.73 Impact Factor
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    ABSTRACT: The formation of iridium clusters from supported mononuclear iridium complexes in H(2) at 300 K and 1 bar was investigated by spectroscopy and atomic-resolution scanning transmission electron microscopy. The first steps of cluster formation from zeolite-supported Ir(C(2)H(4))(2) complexes are triggered by the activation of H(2) and the formation of iridium hydride, accompanied by the breaking of iridium-support bonds. This reactivity can be controlled by the choice of ligands on the iridium, which include the support.
    Journal of the American Chemical Society 03/2012; 134(11):5022-5. · 10.68 Impact Factor
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    ABSTRACT: Supported triosmium clusters, formed from Os{sub 3}(CO){sub 12} on MgO, were treated in helium at 548 K for 2 h, causing fragmentation of the cluster frame and the formation of mononuclear osmium dicarbonyls. The cluster breakup and the resultant fragmented species were characterized by infrared and X-ray absorption spectroscopies, and the fragmented species were imaged by scanning transmission electron microscopy. The spectra identify the surface osmium complexes as Os(CO){sub 2}{l_brace}O{sub support}{r_brace}{sub n} (n = 3 or 4) (where the braces denote support surface atoms). The images show site-isolated Os atoms in mononuclear osmium species on MgO. The intensity analysis on the images of the MgO(110) face showed that the Os atoms were located atop Mg columns. This information led to a model of the Os(CO){sub 2} on MgO(110), with the distances approximated as those determined by EXAFS spectroscopy, which are an average over the whole MgO surface; the results imply that these complexes were located at Mg vacancies.
    Journal of Physical Chemistry Letters 01/2012; · 6.59 Impact Factor
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    ABSTRACT: Using aberration-corrected scanning transmission electron microscopy (STEM), we imaged iridium atoms in isolated iridium complexes in the one-dimensional nonintersecting 14-ring channels of zeolite SSZ-53. STEM allows tracking of the movement of atoms in the channels, demonstrating the interaction of iridium with the zeolite framework (channel confinement) and providing a direct visualization of the initial steps of metal nanocluster formation. The results demonstrate how STEM can be used to help design improved catalysts by identifying the catalytic sites and observing how they change in reactive atmospheres.
    Nano Letters 11/2011; 11(12):5537-41. · 13.03 Impact Factor
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    ABSTRACT: By use of the precursor Ir(CO)2(acac) (acac is acetylacetonate), a ship-in-a-bottle synthesis was used to prepare Ir6(CO)16 and, by decarbonylation, clusters well approximated as Ir6 in the supercages of zeolite NaY. The samples were characterized by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies and imaged by aberration-corrected scanning transmission electron microscopy with a high-dose electron beam (108 e–/Ǻ2, 200 kV), and the catalyst performance was characterized by turnover frequencies for ethene hydrogenation at 298 K and atmospheric pressure. The images characterizing a sample with about 17% of the supercages occupied by decarbonylated nanoclusters indicated clusters well approximated as Ir6, with diameters consistent with such clusters, and some of the images show the clusters with atomic resolution and indicating each of the 6 Ir atoms. The cluster size was confirmed by EXAFS spectra. Two bonding positions of the Ir6 clusters in the supercages were distinguished; 25% of the clusters were present at T5 sites and 75% at T6 sites. The results represent the first example of the application of high-dose electron beam conditions to image metal nanoclusters in a nanoporous material; the data are characterized by a high signal-to-noise ratio, and their interpretation does not require any image processing or simulations. These statements are based on images determined in the first 5 s of exposure of the catalyst to the electron beam; thereafter, the electron beam caused measurable deterioration of the zeolite framework and thereupon aggregation of the iridium clusters.
    ACS Catalysis 10/2011; 1(11):1613–1620. · 5.27 Impact Factor
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    ABSTRACT: The performance of a supported catalyst is influenced by the size and structure of the metal species, the ligands bonded to the metal, and the support. Resolution of these effects has been lacking because of the lack of investigations of catalysts with uniform and systematically varied catalytic sites. We now demonstrate that the performance for ethene hydrogenation of isostructural iridium species on supports with contrasting properties as ligands (electron-donating MgO and electron-withdrawing HY zeolite) can be elucidated on the basis of molecular concepts. Spectra of the working catalysts show that the catalytic reaction rate is determined by the dissociation of H(2) when the iridium, either as mono- or tetra-nuclear species, is supported on MgO and is not when the support is the zeolite. The neighboring iridium sites in clusters are crucial for activation of both H(2) and C(2)H(4) when the support is MgO but not when it is the zeolite, because the electron-withdrawing properties of the zeolite support enable even single site-isolated Ir atoms to bond to both C(2)H(4) and H(2) and facilitate the catalysis.
    Journal of the American Chemical Society 08/2011; 133(40):16186-95. · 10.68 Impact Factor
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    ABSTRACT: The conversion of guaiacol catalyzed by Pt/MgO in the presence of H2 was investigated with a flow reactor at 573 K and 140 kPa. Among the dozens of reaction products identified by gas chromatography (GC) and GC/mass spectrometry, the predominant ones were phenol, catechol, and (surprisingly) cyclopentanone, with others including methane, n-butane, butenes, n-pentane, and carbon monoxide. The predominant reactions were hydrodeoxygenation (with about 70 % of the guaiacol that was converted forming products that were reduced in oxygen). In contrast, when the catalyst incorporated an acidic support, Pt/γ-Al2O3, other reactions became kinetically significant, exemplified by transalkylation, and the selectivity to deoxygenated products was reduced to about half the value observed with Pt/MgO at guaiacol conversions in the range of about 6–20 %. Pt/MgO underwent deactivation less rapidly than Pt/γ-Al2O3, consistent with a lower rate of coke formation and with observations by scanning transmission electron microscopy showing that the average platinum cluster diameter, approximately 1–2 nm in each catalyst, did not change significantly during operation. The results point to the advantages of basic supports for noble metal hydrodeoxygenation catalysts. Graphical Abstract .
    Catalysis Letters 142(10). · 2.24 Impact Factor
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    ABSTRACT: Results from a wide selection of literature sources reviewed here show that treatments of iridium complexes on various supports under harsh reductive conditions (e.g., 873 K in H2) lead to the formation of only uniform iridium clusters limited to a critical diameter of ~1 nm. The observations have been explained by the results of calculations at the level of density functional theory ory showing that cubic structures of this size are resistant to aggregation because coalescence of two such clusters would require energetically unfavorable rearrangements of their surface atoms, and this point has been reinforced by scanning transmission electron microscopy images demonstrating the non-coalescence behavior of iridium clusters of the critical size—which instead bounce off each other. Here we consider supported iridium catalysts in light of the literature, aiming to (1) demonstrate the generality of the sinter-resistant property of iridium nanoclusters (in contrast to those of other noble metals) and (2) summarize information regarding sample synthesis and preparation methods that lead to supported iridium catalyst with sinter-resistant properties. Graphical Abstract
    Catalysis Letters 142(12). · 2.24 Impact Factor