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
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 IrāO<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
āŖ 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
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 iridiumāsupport 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>
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
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 (ā¼10<sup>8</sup> e<sup>ā</sup>/Ć
Ģ<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
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 solidāsolution catalyst system
with the equivalent one used in the solidāgas 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
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>ā¼4ā6</sub>; then, on average, Ir<sub>ā¼40</sub>; and finally, on average, Ir<sub>ā¼70</sub>, the latter more
accurately described as a bimodal dispersion of on-average Ir<sub>ā¼40ā50</sub> and on-average Ir<sub>ā¼1600</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, āmechanismāīøthat
is, actually and instead, just a collection of phenomenaīøfor
catalyst sintering of āParticle 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