Ceren Aydin

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

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Publications (25)173.34 Total impact

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    ABSTRACT: Agglomerative sintering of an atomically dispersed, zeolite Y-supported catalyst, Ir1/zeolite Y, formed initially from the well-characterized precatalyst [Ir(C2H4)2]/zeolite Y, and in the presence of liquid-phase reactants, was monitored over three cycles of 3800 turnovers (TTOs) of cyclohexene hydrogenation at 72 °C. The catalyst evolved and sintered during each cycle even at the relatively mild temperature of 72 °C in the presence of the cyclohexene plus H2 reactants and cyclohexane solvent. Post each of the three cycles of catalysis, the resultant sintered catalyst was characterized by extended X-ray absorption fine structure spectroscopy and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The results—the first quantitative investigation of sintering of an atomically dispersed catalyst—show that higher-nuclearity iridium species, Irn, are formed during each successive cycle. The progression from the starting mononuclear precursor, Ir1, is first to Ir~4-6, then, on average, Ir~40, and, finally, on average, Ir~70, the latter more accurately described as a bimodal dispersion of on-average Ir~40-50 and on-average Ir~1600 nanoparticles. The size distribution and other data disprove Ostwald Ripening during the initial and final stages of the observed catalyst sintering. Instead, the diameter-dispersion data plus quantitative fits to the cluster or nanoparticle diameter vs. time data provide compelling evidence for the underlying, pseudo-elementary steps of bimolecular agglomeration, B + B  C, and autocatalytic agglomeration, B + C  1.5C, where B represents the smaller, formally Ir(0) nanoparticles, and C is the larger (more highly agglomerated) nanoparticles (and where the 1.5 coefficient in the autocatalytic agglomeration of B + C necessarily follows from the definition, in the bimolecular agglomeration step), that 1 C contains the Ir from 2 B). These two specific, balanced chemical reactions are of considerable significance in going beyond the present state-of-the-art, but word-only, “mechanism”—that is, actually and instead, just a collection of phenomena—for catalyst sintering of “Particle Migration and Coalescence”. The steps of bimolecular plus autocatalytic agglomeration provide two specific, balanced chemical equations useful for fitting sintering kinetics data—as is done herein—thereby quantitatively testing proposed sintering mechanisms. These two pseudo-elementary reactions also define the specific words and concepts for sintering of bimolecular agglomeration and autocatalytic agglomeration. The results are also significant as the first quantitative investigation of the agglomeration and sintering of an initially atomically dispersed metal on a structurally well-defined (zeolite) support and in the presence of liquid reactants (cyclohexene substrate and cyclohexane solvent) plus H2. A list of additional specific conclusions is also provided in a summary section.
    ACS Catalysis 05/2015; 5(6):3514. DOI:10.1021/acscatal.5b00321 · 7.57 Impact Factor
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    ABSTRACT: Images of La-exchanged NaY zeolite obtained with aberration-corrected scanning transmission electron microscopy (STEM) show that about 80% of the La cations were present as site-isolated species, with the remainder in pair sites. The observed distances between La cations in the pair sites ranged from 1.44 to 3.84 Å, consistent with the presence of pairs of cations tilted at various angles with respect to the support surface. The actual distance between La cations in the pair sites is inferred to be approximately 3.84 Å. The results suggest the presence of dimeric structures of La cations bridged with O anions, and the presence of such species has been inferred previously on the basis of X-ray photoelectron spectroscopy (W. Grünert, U. Sauerlandt, R. Schlögl, H.G. Karge, J. Phys. Chem., 97 (1993) 1413).
    Microporous and Mesoporous Materials 04/2015; 213. DOI:10.1016/j.micromeso.2015.04.008 · 3.21 Impact Factor
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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; 9(6). DOI:10.1038/nnano.2014.72 · 33.27 Impact Factor
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    ABSTRACT: Oxidative treatment of single metal carbonyl complexes has been shown in literature to activate the metal complex for catalysis. We hypothesize that a similar in-situ oxidative treatment of supported metal carbonyl clusters or chemoselective oxidation of the Ir4 precursor will also create open sites, and that these treatments will result in a more active catalyst for hydrogenation reactions. Open sites consist of CO vacancies, which typically result in cluster instability during catalysis. We achieve stability and structural integrity via attachment of calixarene phosphine ligands to the Ir4 cluster core. We test this hypothesis of creating more catalytically active, yet stable, open metal clusters by using the structure insensitive C2H4 hydrogenation reaction.
    13 AIChE Annual Meeting; 11/2013
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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 09/2013; 5(9). DOI:10.1002/cctc.201200872 · 5.04 Impact Factor
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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 05/2013; 125(20). DOI:10.1002/anie.201300238 · 11.26 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.
    04/2013; 3(9):-. DOI:10.1039/C3CY00113J
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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 12/2012; 142(12). DOI:10.1007/s10562-012-0928-8 · 2.29 Impact Factor
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    ABSTRACT: Selective Hydrodeoxygenation of Guaiacol Catalyzed by Pt/MgO T. Nimmanwudipong, C. Aydin, J. Lu, R. C. Runnebaum, K. Brodwater, N. D. Browning, D. E. Block, and B. C. Gates 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, and others included methane, n-butane, n-pentane, butenes, and carbon monoxide. The predominant reactions were hydrodeoxygenation (with about 70% of the total products being reduced in oxygen), but when the catalyst incorporated an acidic support, Pt/g-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. Pt/MgO underwent deactivation slightly more slowly than Pt/g-Al2O3, consistent with the lower rate of coke formation and with the observations by transmission electron microscopy showing the lack of sintering of the platinum (the average platinum particle size (which was approximately 1–2 nm in each catalyst) did not change significantly during operation.
    12 AIChE Annual Meeting; 10/2012
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    ABSTRACT: Supported iridium catalysts, which are active for numerous hydrocarbon conversions, offer many advantages in characterization, because known organometallic chemistry guides the precise synthesis of essentially molecular iridium complexes and clusters and because the heavy Ir atoms on supports can be readily imaged by scanning transmission electron microscopy. Oxide- and zeolite-supported isostructural Ir(C2H4)2 complexes were prepared from Ir(C2H4)2(acac). The transformation of the ligands and of the iridium nuclearity were tracked with time-resolved IR, EXAFS, and XANES spectroscopies with the samples in reactive atmospheres and undergoing catalysis of reactions such as ethylene hydrogenation. The data indicate the movements of Ir atoms and the early stages of cluster formation and show how the ligands (including the supports) influence the catalytic properties and the tendency of the iridium to form clusters—thus, the basis for designing the catalysts.
    12 AIChE Annual Meeting; 10/2012
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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 10/2012; 142(10). DOI:10.1007/s10562-012-0884-3 · 2.29 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. DOI:10.1021/cs300366w · 7.57 Impact Factor
  • Jing Lu · Ceren Aydin · Nigel D Browning · Bruce C Gates
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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. DOI:10.1021/la302522a · 4.46 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 07/2012; 3(14). DOI:10.1021/jz300574u · 7.46 Impact Factor
  • C. Aydin · J. Lu · B. C. Gates · N. D. Browning
    Microscopy and Microanalysis 07/2012; 18(S2):1290-1291. DOI:10.1017/S1431927612008306 · 1.76 Impact Factor
  • Ceren Aydin · Jing Lu · Nigel D Browning · Bruce C Gates
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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 06/2012; 51(24):5929-34. DOI:10.1002/anie.201201726 · 11.26 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). DOI:10.1002/ange.201202061
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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). DOI:10.1002/anie.201202061 · 11.26 Impact Factor
  • Jing Lu · Ceren Aydin · Nigel D Browning · Bruce C Gates
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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 06/2012; 51(24):5842-6. DOI:10.1002/anie.201107391 · 11.26 Impact Factor
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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. DOI:10.1021/cs300139p · 7.57 Impact Factor