6 research outputs found

    Oxide- and Zeolite-Supported Isostructural Ir(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> Complexes: Molecular-Level Observations of Electronic Effects of Supports as Ligands

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    Zeolite HĪ²- and Ī³-Al<sub>2</sub>O<sub>3</sub>-supported mononuclear iridium complexes were synthesized by the reaction of IrĀ­(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>(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 IrĀ­(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> complexes anchored to each support by two Irā€“O<sub>support</sub> 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 IrĀ­(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> 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 Ī½<sub>CO</sub> frequencies of these complexes and white line intensities in the X-ray absorption spectra at the Ir L<sub>III</sub> edge show that the electron density on iridium increases in the following order on these supports: zeolite HY < zeolite HĪ² < zeolite HSSZ-53 ā‰Ŗ Ī³-Al<sub>2</sub>O<sub>3</sub> < MgO. The IR spectra of the iridium carbonyl complexes treated in flowing C<sub>2</sub>H<sub>4</sub> show that the CO ligands were replaced by C<sub>2</sub>H<sub>4</sub>, with the average number of C<sub>2</sub>H<sub>4</sub> 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

    Hydrogen Activation and Metal Hydride Formation Trigger Cluster Formation from Supported Iridium Complexes

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    The formation of iridium clusters from supported mononuclear iridium complexes in H<sub>2</sub> 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 IrĀ­(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> complexes are triggered by the activation of H<sub>2</sub> and the formation of iridium hydride, accompanied by the breaking of iridiumā€“support bonds. This reactivity can be controlled by the choice of ligands on the iridium, which include the support

    Atomically Resolved Site-Isolated Catalyst on MgO: Mononuclear Osmium Dicarbonyls formed from Os<sub>3</sub>(CO)<sub>12</sub>

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    Supported triosmium clusters, formed from Os<sub>3</sub>(CO)<sub>12</sub> 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 OsĀ­(CO)<sub>2</sub>{O<sub>support</sub>}<sub><i>n</i></sub> (<i>n</i> = 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 OsĀ­(CO)<sub>2</sub> 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

    Ir<sub>6</sub> Clusters Compartmentalized in the Supercages of Zeolite NaY: Direct Imaging of a Catalyst with Aberration-Corrected Scanning Transmission Electron Microscopy

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    By use of the precursor Ir(CO)<sub>2</sub>(acac) (acac is acetylacetonate), a ship-in-a-bottle synthesis was used to prepare Ir<sub>6</sub>(CO)<sub>16</sub> and, by decarbonylation, clusters well approximated as Ir<sub>6</sub> 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 (āˆ¼10<sup>8</sup> e<sup>ā€“</sup>/ƅĢ<sup>2</sup>, 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 Ir<sub>6</sub>, 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 Ir<sub>6</sub> 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

    Mononuclear Zeolite-Supported Iridium: Kinetic, Spectroscopic, Electron Microscopic, and Size-Selective Poisoning Evidence for an Atomically Dispersed True Catalyst at 22 Ā°C

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    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, [IrĀ­(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>]/zeolite Y precatalyst retained, or are possible alternatives including Ir<sub>4</sub> subnanometer clusters or larger, Ir(0)<sub><i>n</i></sub>, 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 PĀ­(C<sub>6</sub>H<sub>11</sub>)<sub>3</sub> and PĀ­(OCH<sub>3</sub>)<sub>3</sub> determining the location of the catalyst in the zeolite pores. Also reported are quantitative poisoning experiments showing that each added PĀ­(OCH<sub>3</sub>)<sub>3</sub> 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 [IrĀ­(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>]/zeolite Y precatalyst allows a site-isolated [Ir<sub>1</sub>]/zeolite Y hydrogenation catalyst to be retained even when in contact with solution, at least at 22 Ā°C; (ii) allow a comparison of the solidā€“solution catalyst system with the equivalent one used in the solidā€“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

    Agglomerative Sintering of an Atomically Dispersed Ir<sub>1</sub>/Zeolite Y Catalyst: Compelling Evidence Against Ostwald Ripening but for Bimolecular and Autocatalytic Agglomeration Catalyst Sintering Steps

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    Agglomerative sintering of an atomically dispersed, zeolite Y-supported catalyst, Ir<sub>1</sub>/zeolite Y, formed initially from the well-characterized precatalyst [IrĀ­(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>]/zeolite Y and in the presence of liquid-phase reactants, was monitored over three cycles of 3800 turnovers (TTOs) of cyclohexene hydrogenation at 72 Ā°C. The catalyst evolved and sintered during each cycle, even at the relatively mild temperature of 72 Ā°C in the presence of the cyclohexene plus H<sub>2</sub> 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 show that higher-nuclearity iridium species, Ir<sub><i>n</i></sub>, are formed during each successive cycle. The progression from the starting mononuclear precursor, Ir<sub>1</sub>, is first to Ir<sub>āˆ¼4ā€“6</sub>; then, on average, Ir<sub>āˆ¼40</sub>; and finally, on average, Ir<sub>āˆ¼70</sub>, the latter more accurately described as a bimodal dispersion of on-average Ir<sub>āˆ¼40ā€“50</sub> and on-average Ir<sub>āˆ¼1600</sub> 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, pseudoelementary steps of bimolecular agglomeration, B + B ā†’ C, and autocatalytic agglomeration, B + C ā†’ 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 1C contains the Ir from 2B). These two specific, balanced chemical reactions are of considerable significance in going beyond the present state-of-the-art, but word-only, ā€œmechanismā€ī—øthat is, actually and instead, just a collection of phenomenaī—øfor catalyst sintering of ā€œParticle Migration and Coalescenceā€. 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 pseudoelementary 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 H<sub>2</sub>. A list of additional specific conclusions is provided in a summary section
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