Ir Clusters Research Articles

Size and Site Dependence of the Catalytic Activity of Iridium Clusters toward Ethane Dehydrogenation.

This research focuses on optimizing transition metal nanocatalyst immobilization and activity to enhance ethane dehydrogenation. Ethane dehydrogenation, catalyzed by thermally stable Irn (n = 8, 12, 18) atomic clusters that exhibit a cuboid structure, was studied using the B3LYP method with triple-ζ basis sets. Relativistic effects and dispersion corrections were included in the calculations. In the dehydrogenation reaction Irn + C2H6 → H-Irn-C2H5 → (H)2-Irn-C2H4, the first H-elimination is the rate-limiting step, primarily because the reaction releases sufficient heat to facilitate the second H-elimination. The catalytic activity of the Ir clusters strongly depends on the Ir cluster size and the specific catalytic site. Cubic Ir8 is the least reactive toward H-elimination in ethane: Ir8 + C2H6 → H-Ir8-C2H5 has a large (65 kJ/mol) energy barrier, whereas Ir12 (3 × 2 × 2 cuboid) and Ir18 (3 × 3 × 2 cuboid) lower this energy barrier to 22 and 3 kJ/mol, respectively. The site dependence is as prominent as the size effect. For example, the energy barrier for the Ir18 + C2H6 → H-Ir18-C2H5 reaction is 3, 48, and 71 kJ/mol at the corner, edge, or face-center sites of the Ir18 cuboid, respectively. Energy release due to Ir cluster insertion into an ethane C-H bond facilitates hydrogen migration on the Ir cluster surface, and the second H-elimination of ethane. In an oxygen-rich environment, oxygen molecules may be absorbed on the Ir cluster surface. The oxygen atoms bonded to the Ir cluster surface may slightly increase the energy barrier for H-elimination in ethane. However, the adsorption of oxygen and its reaction with H atoms on the Ir cluster releases sufficient heat to yield an overall thermodynamically favored reaction: Irn + C2H6 + 1/2O2 → Irn + C2H4 + H2O. These results will be useful toward reducing the energy cost of ethane dehydrogenation in industry.

Enhanced spectral histology in the colon using high-magnification benchtop FTIR imaging

Label-free imaging of cells and tissues is a promising tool to study the molecular alterations for improved cancer diagnosis. In this regard, the vibrational spectroscopic method of FTIR imaging has been employed to study the histo-pathological features and the alterations were correlated to a disease state. The aim of this study was to see if higher magnification images resulting in pixel sizes smaller than the diffraction limit of mid-IR wavelengths with two NA objectives could provide cellular and subcellular resolution of key diagnostic features. To this effect, FTIR spectroscopic imaging has been carried out directly on paraffinized colon tissue sections using a benchtop imaging system consisting of two different IR objectives with a NA of 0.81 and 0.62. The high NA objective (0.81) at a high magnification (63×) provided a pixel size of 0.63×0.63μm2 in comparison to the standard magnification (12×) where the pixel size is 3.3×3.3μm2. The second objective (0.62 NA) at a high magnification (36×) provided a pixel size of 1.1×1.1μm2 in comparison to the standard magnification (7×) pixel size of 5.5×5.5μm2. The spectral images were corrected for paraffin and other spectral interferences using a modified EMSC algorithm and subjected to cluster analysis in order to appreciate the histopathological details. Comparison of the IR cluster analysis results to adjacent haematoxylin and eosin stained tissue sections, which were used as the morphological controls, revealed various levels of histological and cellular organization. Together with large scale features such as the glandular and connective tissue regions, small scale features like goblet cells, interfaces between tissue types especially the peri-cryptal fibroblastic sheath were observed. Noticeable differences between the intracellular mucin and secreted mucin were also observed. However, it appears that high power mid-IR radiation sources would be beneficial, especially when measuring in rapid clinically relevant timescales.

Multibody expansion of particle interactions: How many-body is a particular element in a cluster

The particle-particle interaction potential of an $N\text{-atom}$ cluster is expanded in $n\text{-body}$ contributions. The expansion allows us to determine the magnitude of each one of the $n\text{-body}$ terms, and consequently quantifies how $n\text{-body}$ a potential really is. This way we obtain bounds for the relative error due to truncation, a feature that could be applicable in several contexts like the search of minimal energy cluster conformations, to obtain adequate seeds for further ab initio refinement, or to speed up molecular dynamics computations. We develop the formalism, and test the procedure numerically for the Lennard-Jones, Murrel-Motram, Gupta, and Sutton-Chen potentials. The contributions of the $n\text{-body}$ terms for Ag, Al, Au, Co, Cu, Fe, Ir, Ni, Pb, Pd, Pt, and Rh clusters are computed up to $n=9$; they show that the importance and magnitude of the $ng2$ interaction terms depend on the particular element. The relevance of the $n\text{-body}$ corrections as a function of cluster size is also explored for $N\ensuremath{\le}50$, and for a linear chain of $N\ensuremath{\le}1000$ atoms.

A DFT Study of CO2 Hydrogenation on Faujasite-Supported Ir4 Clusters: on the Role of Water for Selectivity Control.

Reaction mechanisms for the catalytic hydrogenation of CO2 by faujasite‐supported Ir4 clusters were studied by periodic DFT calculations. The reaction can proceed through two alternative paths. The thermodynamically favoured path results in the reduction of CO2 to CO, whereas the other, kinetically preferred channel involves CO2 hydrogenation to formic acid under water‐free conditions. Both paths are promoted by catalytic amounts of water confined inside the zeolite micropores with a stronger promotion effect for the reduction path. Co‐adsorbed water facilitates the cooperation between the zeolite Brønsted acid sites and Ir4 cluster by opening low‐energy reaction channels for CO2 conversion.

Platinum‐group Mineral (PGM) and Base‐metal Sulphide (BMS) Inclusions in Chromitites of the Zedang Ophiolite, Southern Tibet, China and their Petrogenetic Significance

AbstractVoluminous platinum‐group mineral (PGM) inclusions including erlichmanite (Os,Ru)S2, laurite (Ru,Os)S2, and irarsite (Ir,Os,Ru,Rh)AsS, as well as native osmium Os(Ir) and inclusions of base metal sulphides (BMS), including millerite (NiS), heazlewoodite (Ni3S2), covellite (CuS) and digenite (Cu3S2), accompanied by native iron, have been identified in chromitites of the Zedang ophiolite, Tibet. The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them; most are less than 10 μm in size and vary in shape from euhedral to anhedral. They occur either as single or composite (biphase or polyphase) grains composed solely of PGM, or PGM associated with silicate grains. Os‐, Ir‐, and Ru‐rich PGMs are the common species and Pt‐, Pd‐, and Rh‐rich varieties have not been identified. Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle. If the activity of chalcogenides (such as S, and As) is low, PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys. Under appropriate conditions of fS2 and fO2, PGE alloys might react with the melt to form sulfides‐sulfarsenides. Thus, we suggest that the Os, Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low fS2, followed by crystallization of sulphides of the laurite‐erlichmanite, solid‐solution series as the magma cooled and fS2 increased. The abundance of primary BMS in the chromitites suggests that fS2 reached relatively high values during the final stages of magnesiochromite crystallization. The diversity of the PGE minerals, in combination with differences in the petrological characteristics of the magnesiochromites, suggest different degrees of partial melting, perhaps at different depths in the mantle. The estimated parental magma composition suggests formation in a suprasubduction zone environment, perhaps in a forearc.

Manipulation of magnetic anisotropy in [formula omitted] clusters by Co atom

Based on the first principles calculations, we have investigated the magnetic properties of Irn+1 clusters modulated by Co atoms. The research conclusions show that the amplitude of magnetic anisotropy energy (MAE) and magnetization direction of the small Irn+1 can be manipulated by Co atom if we can control the size very precisely. Such regulatory mechanism of MAE is ascribed to the distributing variation of Ir-5d orbits around the Fermi level induced by Co atom. More importantly, the colossal MAE values, 67.4 meV/atom, 40.26 meV/atom and 91.37 meV/atom, can be obtained for Ir2, Ir4, and CoIr clusters, respectively. Such high values provide a promising avenue for developing high-density magnetic storage units at sub-nanometer size.

Partially oxidized iridium clusters within dendrimers: size-controlled synthesis and selective hydrogenation of 2-nitrobenzaldehyde.

Iridium clusters nominally composed of 15, 30 or 60 atoms were size-selectively synthesized within OH-terminated poly(amidoamine) dendrimers of generation 6. Spectroscopic characterization revealed that the Ir clusters were partially oxidized. All the Ir clusters efficiently converted 2-nitrobenzaldehyde to anthranil and 2-aminobenzaldehyde under atmospheric hydrogen at room temperature in toluene via selective hydrogenation of the NO2 group. The selectivity toward 2-aminobenzaldehyde over anthranil was improved with the reduction of the cluster size. The improved selectivity is ascribed to more efficient reduction than intramolecular heterocyclization of a hydroxylamine intermediate on smaller clusters that have a higher Ir(0)-phase population on the surface.

Stability and nucleation of Irn (n = 1–5) clusters on different γ-Al2O3 surfaces: A density functional theory study

Density functional theory (DFT) calculations with periodic slab models are employed to explore the stability and nucleation of Irn (n=1–5) clusters on three surfaces, namely, dehydrated γ-Al2O3(100), dehydrated γ-Al2O3(110), and hydrated γ-Al2O3(110) surfaces. The results show that the adsorption and nucleation of Irn (n=1–5) clusters preferentially occurs on the (110) surface than on the (100) surface. Moreover, (110) surface hydroxylation does not favor the adsorption of Ir clusters, while benefits the nucleation of Ir clusters. Meanwhile, the support can weaken the nucleation ability of Ir clusters and block the aggregation of clusters.

Electronic transport properties of Ir-decorated graphene.

Graphene decorated with 5d transitional metal atoms is predicted to exhibit many intriguing properties; for example iridium adatoms are proposed to induce a substantial topological gap in graphene. We extensively investigated the conductivity of single-layer graphene decorated with iridium deposited in ultra-high vacuum at low temperature (7 K) as a function of Ir concentration, carrier density, temperature, and annealing conditions. Our results are consistent with the formation of Ir clusters of ~100 atoms at low temperature, with each cluster donating a single electronic charge to graphene. Annealing graphene increases the cluster size, reducing the doping and increasing the mobility. We do not observe any sign of an energy gap induced by spin-orbit coupling, possibly due to the clustering of Ir.

Kinetic and Mechanistic Assessment of Alkanol/Alkanal Decarbonylation and Deoxygenation Pathways on Metal Catalysts.

This study combines theory and experiment to determine the kinetically relevant steps and site requirements for deoxygenation of alkanols and alkanals. These reactants deoxygenate predominantly via decarbonylation (C-C cleavage) instead of C-O hydrogenolysis on Ir, Pt, and Ru, leading to strong inhibition effects by chemisorbed CO (CO*). C-C cleavage occurs via unsaturated species formed in sequential quasi-equilibrated dehydrogenation steps, which replace C-H with C-metal bonds, resulting in strong inhibition by H2, also observed in alkane hydrogenolysis. C-C cleavage occurs in oxygenates only at locations vicinal to the C═O group in RCCO* intermediates, because such adjacency weakens C-C bonds, which also leads to much lower activation enthalpies for oxygenates than hydrocarbons. C-O hydrogenolysis rates are independent of H2 pressure and limited by H*-assisted C-O cleavage in RCHOH* intermediates on surfaces with significant coverages of CO* formed in decarbonylation events. The ratio of C-O hydrogenolysis to decarbonylation rates increased almost 100-fold as the Ir cluster size increased from 0.7 to 7 nm; these trends reflect C-O hydrogenolysis reactions favored on terrace sites, while C-C hydrogenolysis prefers sites with lower coordination, because of the relative size of their transition states and the crowded nature of CO*-covered surfaces. C-O hydrogenolysis becomes the preferred deoxygenation route on Cu-based catalysts, thus avoiding CO inhibition effects. The relative rates of C-O and C-C cleavage on these metals depend on their relative ability to bind C atoms, because C-C cleavage transitions states require an additional M-C attachment.

The influence of the composition of eight-atom Pt–Ir clusters on the magnetic properties

The energetic stability, electronic structure and magnetic properties of Pt8–nIrn clusters have been investigated by employing the spin-polarised generalised gradient approximation. The cubic structure is expected to be the effective building block in Ir-rich clusters after optimisation extensively. The average binding energy of all the clusters presents the linear increment trend with iridium atoms, due to the stronger interaction between Ir atoms than Pt atoms. Bader charge analysis shows how tiny charge transfers from iridium to platinum. The atomic moments of Ir are larger than that of Pt, and the Ir-rich clusters show greater moments than the Pt-rich cluster, with the exception of Ir8 and Ir7Pt. A unique magnetic property is found in the Pt4Ir4 cluster, where two Pt atoms show antiferromagnetic alignment and the other atoms are found to be aligned ferromagnetically.

N-acetyl-D-glucosamine kinase is a component of nuclear speckles and paraspeckles.

Protein O-GlcNAcylation, dictated by cellular UDP-N-acetylglucosamine (UDP-GlcNAc) levels, plays a crucial role in posttranslational modifications. The enzyme GlcNAc kinase (NAGK, E.C. 2.7.1.59) catalyzes the formation of GlcNAc-6-phosphate, which is a major substrate for the biosynthesis of UDP-GlcNAc. Recent studies have revealed the expression of NAGK in different types of cells especially in neuronal dendrites. Here, by immunocytochemistry (ICC) and immunonucleochemistry (INC) of cultured rat hippocampal neurons, HEK293T and GT1-7 cells, we have showed that NAGK immuno-reactive punctae being present in the nucleoplasm colocalized with small nuclear ribonucleoprotein-associated protein N (snRNPN) and p54NRB, which are speckle and paraspeckle markers, respectively. Furthermore, NAGK IR cluster was also found to be colocalized with GTF2H5 (general transcription factor IIH, polypeptide 5) immuno reactive punctae. In addition, relative localization to the ring of nuclear lamin matrix and to GlcNAc, which is highly enriched in nuclear pore complexes, showed that NAGK surrounds the nucleus at the cytoplasmic face of the nuclear outer membrane. By in situ proximity ligation assay (PLA) we confirmed the colocalization of NAGK with snRNPN in the nucleus and in dendrites, while we also verified the interactions of NAGK with p54NRB, and with GTF2H5 in the nucleus. These associations between NAGK with speckle, paraspeckle and general transcription factor suggest its regulatory roles in gene expression.

The role of Ir4 cluster in enhancing the adsorption of CO2 on selected zeolites – GCMC simulations

We have investigated the adsorption of CO2 molecules inside the EMT, SAO, SBS, SBT and IWS zeolites with respect to the influence of the Ir4 clusters on the adsorption capabilities of these materials. We have determined that the capabilities of CO2 adsorption depend on the combined effect of the framework topology and the position of the Ir4 cluster. Adsorption intensifies despite the fact that a fraction of the pore volume is occupied by the Ir4 cluster, and thus, the adsorption is more intense than that on empty zeolite. The pore topology however is also playing a crucial role in the effect, as in certain cases it allows the CO2 molecules to order in such a way they fill the most pore space.

Influence of the Rotational Domain in the Growth of Transition Metal Clusters on Graphene

The influence of the relative orientation between graphene monolayers and the underlying substrate on the growth of transition metal clusters over epitaxial graphene on Ir(111) surfaces is investigated by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). This experimental study has been carried out for W and Ir clusters and, in both cases, the results revealed the existence of noticeable differences in the size and distribution of the clusters formed over areas where graphene and substrate lattices are aligned with respect to those grown on regions where both lattices present a relative rotation angle. In particular, while over aligned domains, in a consistent way with previous findings, the formation of ordered arrays of monodisperse clusters exhibiting a great structural perfection is observed, on the rotated ones it takes place the formation of larger size isolated clusters scattered around the surface. Moreover, the boundaries between different rotational domains are found to be decorated by these larger clusters. This disparity observed in the growth of clusters is explained in terms of the differences in the graphene–substrate interaction existing on aligned and rotated domains.

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

Rates and locations of C–C cleavage during the hydrogenolysis of alkyl-cyclohexanes determine the isomeric products of ring opening and the yield losses from dealkylation. Kinetically relevant transition states for C–C rupture form by sequential quasi-equilibrated dehydrogenation steps that break C–H bonds, form C–metal bonds, and desorb chemisorbed H atoms (H*) from H*-covered surfaces. Activation enthalpies (ΔH⧧), entropies (ΔS⧧), and the number of H2(g) formed with transition states are larger for 3C–xC rupture than for 2C–2C or 2C–1C cleavage for all cycloalkane reactants and Ir cluster sizes. 3C–xC rupture transition states bind to surfaces through three or more C atoms, whereas those for less-substituted 2C–2C bonds cleave via α,β species bound by two C atoms. 3C–xC rupture involves larger ΔH⧧ than 2C–2C and 2C–1C because the former requires that more C–H bonds cleave and H* desorb than for the latter two. These endothermic steps are partially compensated by C–metal bond formation, whereas the forma...

Gel-assisted crystallization of [Ir4(IMe)7(CO)H10](2+) and [Ir4(IMe)8H9](3+) clusters derived from catalytic glycerol dehydrogenation.

The two title clusters were formed during iridium-catalyzed glycerol dehydrogenation and display a remarkably high NHC content. They were crystallized in either agarose or polyethylene oxide gel matrices, while more conventional crystallization techniques proved unsuccessful. Cluster [Ir4(IMe)8H9](3+), with a net charge of +3, was only crystallizable with a polyoxometalate Keggin trianion. The crystal packing of this intercluster compound is discussed. Computational studies position the iridium hydrides and provide insights into the bonding.

A density functional theory study of ethylene hydrogenation on MgO- and γ-Al2O3-supported carbon-containing Ir4 clusters.

Density functional theory was used to investigate the reaction mechanisms of ethylene hydrogenation on MgO(100)- and γ-Al2O3(110)-supported carbon-containing Ir4 clusters. The cluster supported on γ-Al2O3(110) is more active than that on MgO(100), which is consistent with experimental observations. The present calculations show that the binding energies of reactants on the carbon-containing Ir4 cluster are weaker on the γ-Al2O3 supported catalysts compared to the MgO supported Ir cluster. This relatively weak adsorption energy of ethylene on the γ-Al2O3 surface means that ethylene desorption is easier, hence a higher catalytic activity is achieved. To gain further understanding, the energy decomposition method and micro-kinetic analysis are also introduced.

The Birmingham parallel genetic algorithm and its application to the direct DFT global optimisation of IrN(N = 10–20) clusters

A new open-source parallel genetic algorithm, the Birmingham parallel genetic algorithm, is introduced for the direct density functional theory global optimisation of metallic nanoparticles. The program utilises a pool genetic algorithm methodology for the efficient use of massively parallel computational resources. The scaling capability of the Birmingham parallel genetic algorithm is demonstrated through its application to the global optimisation of iridium clusters with 10 to 20 atoms, a catalytically important system with interesting size-specific effects. This is the first study of its type on Iridium clusters of this size and the parallel algorithm is shown to be capable of scaling beyond previous size restrictions and accurately characterising the structures of these larger system sizes. By globally optimising the system directly at the density functional level of theory, the code captures the cubic structures commonly found in sub-nanometre sized Ir clusters.

The properties of Irn (n = 2-10) clusters and their nucleation on γ-Al₂O₃ and MgO surfaces: from ab initio studies.

The structural stability and magnetic properties of iridium clusters Irn (n = 2-10) and their interaction on γ-Al2O3(001) and MgO(100) surfaces have been investigated from first principles calculations. It is found that the adsorption energy of Irn (n = 2-10)/γ-Al2O3(001) is lower than that of Irn/MgO(100); meanwhile, the strongest adsorption energy cluster for γ-Al2O3(001) is the tetrahedral Ir4 cluster, while for MgO(100) is a cubic Ir8 cluster. On the other hand, the nucleation of Irn (n = 2-10) clusters on both surfaces is thermodynamically favorable and exothermic. Moreover, the nucleation energy of Irn (n = 2-10) clusters on the γ-Al2O3(001) surface is close to the corresponding energy on the MgO(100) surface, except for Ir3, Ir4 and Ir6 clusters. Interestingly, the nucleation of Ir3 and Ir6 clusters on the MgO(100) surface is more favorable than that on the γ-Al2O3(001) surface, while the nucleation of the Ir4 cluster is reverse and very close to the gas phase Ir4 cluster. More importantly, deformation energy and charge density analysis show that the adsorbed Ir4 cluster on the γ-Al2O3(001) surface has obviously charge transfer between Ir atoms and surface Al, O atoms with negligible deformation. However, for the MgO(100) surface, the Ir4 cluster connects directly to three surface O atoms with severe distortion, which inhibits the activity of the tetrahedral Ir4 cluster as a catalyst.

Adsorption of the Ir4 cluster on single-wall carbon nanotubes: the zigzag types are more suitable

The zigzag-type SWNTs not only stabilize the Ir clusters, but also enhance their catalytic performance.