Ir Au Research Articles

Outstanding synergistic effect of Au–Ir/Al2O3 catalysts on the total oxidation of propane

Bimetallic Au–Ir catalysts supported on ɣ-Al2O3 were synthesized by a sequential deposition-precipitation with urea (DPU) method. These materials displayed outstanding synergistic performance in the propane oxidation reaction at low reaction temperatures. A T50 of 150 °C was obtained for a bimetallic Au–Ir catalyst (Au loading of 3 wt % and Ir loading of 5 wt %), while for their gold and iridium monometallic counterparts it was of 370 and 190 °C, respectively. The synthesized materials were characterized by XRD, HRTEM-HAADF, H2-TPR, O2-TPD, UV–vis, and pyridine thermo-desorption, followed by FTIR and XPS, showing distinct characteristics that were correlated with their catalytic activity. Outcomes revealed a strong interaction among the gold and iridium particles when the catalysts were activated in H2 and that the thermal treatment atmosphere (hydrogen or air) modified the active sites; likewise, gold-iridium and monometallic iridium catalysts supported on ɣ-Al2O3 showed considerable endurance to activity loss because they hardly presented metal sintering at high reaction temperatures. Moreover, XPS results revealed electron transfer between Au–Ir (Au0 and Ir2+ species). Only the bimetallic system presented Lewis and Brønsted acidity (2.95 and 0.25 μ-mol m−2, respectively), which may have been responsible for enhancing the total oxidation of C3H8 at a low temperature. Likewise, the 3Au–5Ir/Al2O3 catalyst exhibited high stability and reusability during five reaction cycles and using a GHSV of 41,000 h−1.

Role of bimetallic Au-Ir subnanometer clusters mediating O2 adsorption and dissociation on anatase TiO2 (101).

A comprehensive computational study on the oxygen molecule (O2) adsorption and activation on bimetallic Au-Ir subnanometer clusters supported on TiO2(101)- up to five atoms in size-is performed. A global optimization density functional theory-based basin-hopping algorithm is used to determine putative global minima configurations of both mono- and bimetallic clusters supported on the metal oxide surface for all sizes and compositions. Our results indicate a strong cluster-oxide interaction for monometallic Ir clusters with calculated adsorption energy (Eads) values ranging from -3.11 to -5.91eV. Similar values are calculated for bimetallic Au-Ir clusters (-3.21 up to -5.69eV). However, weaker Eads values are calculated for Au clusters (ranging from -0.66 to -2.07eV). As a general trend, we demonstrate that for supported Au-Ir clusters on TiO2(101), those Ir atoms preferentially occupy cluster-oxide interface positions while acting as anchor sites for the Au atoms. The overall geometric arrangements of the putative global minima configurations define O2 adsorption and dissociation, particularly involving the monometallic Au5 and Ir5 as well as the bimetallic Au2Ir3 and Au3Ir2 supported clusters. Spontaneous O2 dissociation is observed on both Ir5 and on the Ir-metallic part of Au3Ir2 and Au2Ir3 supported clusters. This is in sharp contrast with supported Au5, where a large activation energy is needed (1.90eV). Interestingly, for Au5, we observe that molecular O2 adsorption is favorable at the cluster/oxide interface, followed by a smaller dissociation barrier (0.71eV). From a single cluster catalysis point of view, our results have strong implications in the ongoing understanding of oxide supported bimetallic while providing a useful first insight into the continuous in silico design of novel subnanometer catalysts.

Heterostructured Au–Ir Catalysts for Enhanced Oxygen Evolution Reaction

Electrochemical water splitting operated under acidic conditions provides a clean approach to generate hydrogen fuels. Currently, the sluggish kinetics of oxygen evolution reaction (OER) at the anode is a bottleneck limiting the acidic water splitting. Here, we report that the OER activity of Ir, one of the most commonly used OER catalysts, can be boosted by forming phase boundaries with Au. Mixed Au and Ir catalysts were synthesized on carbon paper electrodes, which underwent structural evolution under the OER environment to form an Au-Ir interface-rich structure. Compared with Ir catalyst, which requires an overpotential of ∼393 mV to achieve a current density of 10 mA/cm2 in 0.1 M HClO4 electrolyte, the evolved Au-Ir catalyst shows a lower overpotential at ∼351 mV. X-ray photoelectronic spectrum (XPS) study, in conjunction with electrochemical analysis on the surface-site-normalized activity, reveals that the improved OER performance is due to the presence of Au/Ir interfaces in the catalysts. In addition to Ir, the strategy of accelerating OER via Au/Ir interfaces has been further applied to IrCo, IrNi, and IrCu alloy catalysts. With the broad applicability of the strategy demonstrated, this study opens a new route to design OER catalysts for efficient acidic water splitting.

A Narrowband Mid-infrared Absorber with Tunable Peak Absorbance and Nearly Unchanged Peak Wavelength Using a Phase Changing Material

A narrowband mid-infrared (MIR) absorber with tunable peak absorbance and nearly unchanged peak wavelength is demonstrated using a phase changing material. The absorber is formed by an Au IR reflector, a SiO2 dielectric cavity and a 1D photonic crystal that consists of SiO2 and GeSbTe (GST), which is a phase-transition material and thus controls the absorbance without static power consumption. The dimension of the Au/SiO2/GST/SiO2 are optimized for high peak absorbance, high quality factor and low background absorbance. The as-fabricated absorber with amorphous GST shows the peak absorbance of 0.704 at 5.636μm, quality factor of 46.19, and background absorbance less than 0.02. Without significantly changing the absorption peak wavelength and background absorbance, the peak absorbance of the absorber can be tuned up to 4.47 dB by the phase transition of GST. Such tunable narrowband MIR absorber is a promising candidate for low-cost thermal emitter for infrared applications.

A new ternary compound with the BGa8Ir4 structure type in the Al-Au-Ir system.

Following the recent determination of the Al3AuIr structure, a new ternary phase has been identified in the Al-Au-Ir phase diagram. It has a chemical composition Al9(Au;Ir)4 with an apparently low gold content. Its crystal structure has been determined with single-crystal X-ray diffraction. The new compound crystallizes in the tetragonal crystal system and has been successfully solved in space group I41/acd (Pearson symbol tI104) with lattice parameters a = 8.6339 (2)and c = 21.8874 (7) Å. Atomic environments are described as well as similarities with the BGa8Ir4 compound.

Reduced-Graphene-Oxide-Based Hybrid Supports for Platinum Catalysts Active at Low Loadings during Oxygen Reduction

Hybrid systems composed of the reduced-graphene-oxide supported gold, iridium or bimetallic (AuIr) nanoparticles (at loadings typically below 5 µg cm-2) have been considered as active matrices for platinum catalysts utilized at low loadings (<20 µg cm-2) during the reduction of oxygen in mostly acid but also alkaline media. Comparison is made to the analogous systems based on conventional Vulcan carbon carriers. Gold nanoparticles are prepared both by the electrochemical and chemical reductionapproaches in which the pre-reduced Keggin-type phosphomolybdate heteropolyblue species act as the reducing agent for the HAuCl4 precursor. Polyoxmetallate (PMo12O40 3-) adsorbates or “capping ligands” tend to stabilize gold (iridium or bimetallic) nanoparticle deposits, facilitate their dispersion and attachment to carbon supports. Indeed, it is apparent from the independent diagnostic voltammetric experiments (in 0.5 mol dm-3 H2SO4) that heteropolymolybdates form readily stable adsorbates on nanostructures of gold, irridium and carbon (reduced graphene oxide and Vulcan). It is reasonable to expect that the polyoxometallate-assisted nucleation of gold has occurred in the proximity of oxygenated defects existing on carbon substrates. Under conditions of the electrochemical experiments performed in 0.1 mol dm-3 KOH) the phosphomolybdate adsorbates are removed from the interface because they undergo dissolution and decomposition in alkaline medium. High electrocatalytic activity of the reduced-graphene oxide-supported catalytic systems toward the reduction of oxygen is demonstrated using chronoamperometry and gas-diffusion electrode, in addition to the conventional and rotating ring-disk electrode (RDE) voltammetry. Among important issues is the presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide (as evident from Raman spectroscopy). When using the reduced graphene oxide carriers for Au (Ir or AuIr) nanostructures together with Pt nanoparticles, the resulting catalytic systems have exhibited typically higher (certainly not lower) O2-reduction currents (relative to those recorded at conventional Vulcan-supported Pt at the same loading) in acid medium (0.5 M H2SO4). The RDE data are consistent with the lower formation of the hydrogen peroxide intermediate (<1% at potentials 0.6 V, or lower, vs RHE). Furthermore, the long-term durability of this family of catalysts should be appreciated.

Aging behavior of near atmospheric N2 ambient sputtered/patterned Au IR absorber thin films

Near atmospheric N2 ambient sputtered Au thin films exhibit significant spectral absorptivity over medium to long wave infrared radiations. Thin films were found adequately robust for micropatterning using conventional photolithography and metal lift off processes. Since long term spectral absorptivity is major practical concern for Au blacks, this paper reports on aging behavior of near atmospheric Ar and Ar + N2 (1:1) ambient sputtered infrared absorber Au thin films. Comparative analysis on electrical, morphological and spectral absorption behavior of twenty-five weeks room temperature/vacuum aged Au infrared absorber thin films is performed. The Ar and Ar + N2 ambient sputtered Au thing films have shown anticipated consistency in their physical, electrical and spectral properties regardless the long term aging in this work.

Au–Ir nanoalloy nucleation during the gas-phase condensation: a comprehensive MD study including different effects

The number of formed clusters and their size increases with the increasing temperature and pressure, which is in good agreement with the experimental results.

Concentrations of platinum-group elements (PGE), Re and Au in arsenian pyrite and millerite from Mo–Ni–PGE-Au black shales (Zunyi region, Guizhou Province, China): results from LA-ICPMS study

Lower Cambrian Mo–Ni sulphidic black shales from the Huangjiawan mine (Guizhou Province, south China) have anomalous platinum-group element (PGE) concentrations (up to ∼1 ppm in total). We used LA-ICPMS to study the distribution of PGE in pyrite and Ni-sulphide (millerite) and FE-SEM/EDS for determination of As in pyrite. A sulphide concentrate was produced by innovative hydroseparation techniques from one representative sample, which contained 162 ppb Pt, 309 ppb Pd, 12.2 ppb Ru, 11.3 ppb Rh, 1.5 ppb Ir, 11 212 ppb Re and 343 ppb Au. Mineralogical analysis revealed that pyrite forms ∼12 vol%, which corresponds to a calculated ∼18.4 wt% of all mineral phases in mineralized black shale. We found that pyrite contains on average (144 analyses) 0.10 ppm Pt, 0.11 ppm Re and 1.40 ppm Au (Ru, Rh, Pd, Os and Ir were below detection limit). It also contains from ∼0.5 to ∼1.8 wt% As and can be therefore classified as arsenian pyrite. Millerite (77 analyses) showed PGE, Re and Au values below detection limit. We suggest that pyrite represents a dominant Au carrier, containing between 64 and 83% Au of the total Au mineralised rock budget. Conversely, pyrite does not bear any significant amount of Re and Pt, contributing up to ∼0.2% and ∼12.5% to their whole rock budgets, respectively. Time resolved LA-ICPMS spectra in pyrite indicate that Pt, Re and Au behave as typical lattice-bound elements, with only Re locally forming micro-inclusions. Arsenic is heterogeneously distributed in pyrite and the Au/As ratio (much lower than 0.02) is in support of Au to be structurally bound in solid solution.

Successful synthesis and thermal stability of immiscible metal Au–Rh, Au–Ir andAu–Ir–Rh nanoalloys

We successfully prepared face-centred cubic nanoalloys in systems of Au–Ir, Au–Rh and Au–Ir–Rh, with large bulk miscibility gaps, in one-run reactions under thermal decomposition of specially synthesised single-source precursors, namely, [AuEn2][Ir(NO2)6], [AuEn2][Ir(NO2)6]х[Rh(NO2)6]1−х and [AuEn2][Rh(NO2)6]. The precursors employed contain all desired metals ‘mixed’ at the atomic level, thus providing significant advantages for obtaining alloys. The observations using high-resolution transmission electron microscopy show that the nanoalloy structures are composed of well-dispersed aggregates of crystalline domains with a mean size of 5 ± 3 nm. Еnergy dispersive x-ray spectroscopy and x-ray powder diffraction (XRD) measurements confirm the formation of AuIr, AuRh, AuIr0.75Rh0.25, AuIr0.50Rh0.50 and AuIr0.25Rh0.75 metastable solid solutions. In situ high-temperature synchrotron XRD (HTXRD) was used to study the formation mechanism of nanoalloys. The observed transformations are described by the ‘conversion chemistry’ mechanism characterised by the primary development of particles comprising atoms of only one type, followed by a chemical reaction resulting in the final formation of a nanoalloy. The obtained metastable nanoalloys exhibit essential thermal stability. Exposure to 180 °C for 30 h does not cause any dealloying process.

Low temperature total oxidation of toluene by bimetallic Au–Ir catalysts

Intimate contact between gold and iridium nanoparticles supported on TiO2 provides a synergetic effect leading to low temperature VOC oxidation activity.

Double complex salts [Au(En)2][Ir(NO2)6] • nH2O (n = 0, 2), [Au(En)2][Ir(NO2)6] x [Rh(NO2)6]1–x • nH2O (x = 0.25, 0.5, 0.75): Synthesis, structure, thermal properties

The double complex salts [Au(En)2][Ir(NO2)6] • 2H2O (I) and [Au(En)2][Ir(NO2)6] (II) and solid solutions [Au(En)2][Ir(NO2)6]x[Rh(NO2)6]1–x • nH2O, where x = 0.25, 0.5, 0.75, were prepared and characterized by powder X-ray diffraction, IR spectroscopy, elemental analysis, and DTA. The crystal structures were determined by X-ray diffraction. The thermal behavior of compounds under reducing atmosphere was studied. The final products of thermolysis were found to be two-phase mixtures consisting of nano-sized pure gold particles (~55 nm) and particles of Au–Ir–Rh solid solutions (~3 nm).

Phase transition in crown-jewel structured Au-Ir nanoalloys with different shapes: a molecular dynamics study.

We have studied the melting process for crown-jewel structured Ir55, Ir54Au, Ir43Au12, Ir25Au30, Ir13Au42, and Au55 nanoclusters in the icosahedral, Ir55, Ir54Au, Ir43Au12, Ir19Au36, Ir13Au42, and Au55 nanoclusters in the cuboctahedral, and Ir54, Ir53Au, Ir47Au7, Ir17Au37, Ir7Au47, and Au54 nanoclusters in the decahedral morphologies. We have investigated the different thermodynamic, structural, and dynamical properties for the different nanoclusters in the different structures. Our thermodynamic results indicated that as the concentration of Au atoms in the nanoclusters increases, the absolute value of internal energy, and so the melting points, of the nanoclusters decrease. It is also shown that the Au atoms decrease the melting temperature of the pure cuboctahedral cluster more than that of the other structures. We have also found that the Au atoms were located in favorable positions at the surface sites of nanoalloys. Also, the doping of the Ir nanocluster by Au atoms makes the cluster more stable. It is also found that nanoclusters with different morphologies have almost the same stability. Our structural results indicated that after the melting process, the Au atoms generally tend to lie in the outer shell of the cluster, but the Ir atoms generally tend to lie in the core of the cluster (see the Ir13Au42 and Ir7Au47 nanoclusters, for example). We have also found the interesting result that the Ir7Au47 nanocluster shows a solid-solid transition from a decahedral structure to an icosahedral structure before melting. The Ir43Au12 nanocluster also shows a transformation from a cuboctahedral structure to an icosahedral-like structure before melting. Our dynamical results showed that doping of the Ir55 cluster with an Au atom sharply increases the self-diffusion coefficient in the initial state in the solid phase, especially in icosahedral and cuboctahedral structures. It is also shown that the Ir13Au42 cluster in icosahedral and cuboctahedral and the Ir7Au47 and Ir17Au37 clusters in decahedral morphologies have smaller values of self-diffusion coefficients than other clusters after the melting point and that this could be due to the formation of core-shell structures.

Solid-solution alloying of immiscible metals at the nanoscale: Ir and Au

A solid solution alloy of Ir–Au, theoretically predicted to be an excellent catalyst, equivalent to Pt, has been experimentally synthesized.

ChemInform Abstract: Al3AuIr: A New Compound in the Al—Au—Ir System.

AbstractAl3AuIr is found in the Al‐rich area of the Al—Au—Ir system by arc melting and annealing of the elements in nominal composition of Al65Au20Ir15 (Ta foil in evacuated quartz tubes, 950 °C, 72 h).

In-Situ DRIFT Study of Au–Ir/Ceria Catalysts: Activity and Stability for CO Oxidation

Monometallic gold and iridium, and bimetallic gold–iridium on ceria catalysts were synthesized by deposition–precipitation with urea. The activity of these ceria supported catalysts for CO oxidation was: Au–Ir ≫ Au > Ir. The bimetallic catalyst was highly active and stable during the carbon monoxide oxidation reaction, showing synergism between both metals. The catalysts were thoroughly characterized chemical and structurally. High-resolution electron microscopy coupled with energy dispersive spectroscopy and X-ray photoelectron spectroscopy showed evidences of iridium–gold intimacy. In situ infrared spectroscopy in the diffuse reflectance mode was used to investigate the reactivity of the active sites. Concentration-modulation excitation spectroscopy allowed selective identification of intermediates and ‘spectator’ species. CO chemisorbed onto iridium sites (2096, 2060 and 2012 cm−1) was found inactive, whereas carbon monoxide on metallic gold sites (Au0–CO, 2110 cm−1) exhibited reactivity for CO oxidation. In the bimetallic catalyst, new active sites (Auδ−–CO, 2095 and 2026 cm−1) together with Au0–CO species were observed, most likely produced by the isolation and stabilization of gold atoms by iridium.

The effect of metal composition on the performance of Ir–Au/TiO2 catalysts for citral hydrogenation.

The effect of gold addition to iridium catalysts and the nature of active sites for citral hydrogenation were investigated over Ir–Au/TiO2 catalysts. All samples (Au/TiO2, Ir/TiO2 and Ir–Au/TiO2) were prepared by deposition-precipitation with urea (DPU). Bimetallic catalysts were synthetized by co-deposition at different Ir/Au atomic ratios (3, 1, 0.3). The catalysts were characterized by ICP, BET, H2–TPR, H2-Chemisorption, TEM, DRIFTS and XPS techniques. A partial coverage of iridium sites by gold atoms takes place as the gold amount in the catalyst increases as shown by chemisorption and XPS measurements. The latter also evidenced a trend consistent with modification of the electronic environment of iridium due to interaction with gold atoms. Also, compared to Ir/TiO2, the amount of Irδ+ species was higher in the bimetallic catalysts reaching a maximum in Ir–Au (1) sample. Characterization of the catalytic surface using DRIFTS of adsorbed CO evidenced a shift towards higher wavenumbers as a function of the gold content indicating, as the XPS results, a modification of the adsorption site. The catalytic activity for citral hydrogenation increased as a function of the gold content. The selectivity to unsaturated alcohol is related to the amount of Irδ+ species which in turns depends on the catalyst composition. For the most active sample Ir–Au (1), a suitable Irδ+/Ir0 ratio is already obtained when the catalyst is reduced at 573K and does not vary with reduction temperature.

RELACIÓN DOPPLER CEREBROPLACENTARIA COMO PREDICTOR DE RESULTADOS PERINATALES ADVERSOS EN GESTANTES DE RIESGO ALTO

OBJETIVOS: evaluar la utilidad y la capacidad predictiva de la relación cerebroplacentaria (RCP) como prueba diagnóstica de resultados perinatales adversos (RPA) en gestantes de riesgo alto. DISEÑO: Se realizó un estudio longitudinal prospectivo con velocimetría Doppler pulsado en arterias umbilicales y cerebrales fetales. MATERIAL Y MÉTODOS: Se seleccionó 72 gestantes de riesgo alto, entre 28 y 41 semanas, excluyéndose a pacientes con embarazo múltiple y anomalía congénita fetal. En todas se determinó los índices de resistencia (IR) de la arteria umbilical (AU) y arteria cerebral media (ACM) fetal y se calculó la relación cerebro placentaria que consiste en el cociente de IR ACM/IR AU, considerándose como valor anormal =&lt; 1. Los resultados perinatales fueron categorizados y correlacionados con los previos índices Doppler. RESULTADOS: La incidencia de RPA fue 59,7%. El flujo de la ACM fue anormal en 25%, de AU en 38,9% encontrándose RCP anormal en 51,4% del total. En el grupo con RCP =&lt; 1, se encontró con alta significancia (p&lt; 0,01) incremento de riesgo de partos prematuros de &lt; 34 semanas. RCIU y peso bajo al nacer y significativamente (p &lt; 0,05) acidosis neonatal precoz, principalmente. La RCP anormal alcanzó mayor exactitud (88,9%), con valor predictivo positivo de 97,3% comparado con los IR anormales de AU y ACM solas, que alcanzaron 68,1% y 57% en exactitud de predicción, respectivamente. CONCLUSIONES: La relación cerebroplacentaria anormal posee alta capacidad predictiva de resultados perinatales adversos en gestantes de riesgo alto, siendo mucho más sensible que los índices de resistencia de las arterias umbilical y cerebral media, individualmente.

Ir promotion of TiO2 supported Au catalysts for selective hydrogenation of cinnamaldehyde

We synthesized Ir-on-Au (Au–Ir) nanoparticles (NPs) using 8–9nm Au NPs, which had a partial coverage of Ir. Both the studied systems allowed the obtainment of cinnamyl alcohol with high selectivities (>83%). However, Au–Ir/TiO2 delivered a hydrogenation rate 5 times higher than that of Au/TiO2. The deposition of Ir onto the surface of Au and the presence of surface Au–Ir alloy were confirmed by UV–vis and HRTEM respectively. Moreover. Moreover, the strongly shifted XPS binding energy implyed the electron transfer from Ir to Au, which was believed to be responsible for the enhanced H2 activation capacity of Au.

Kinetic evaluation of direct NO decomposition and NO–CO reaction over dendrimer-derived bimetallic Ir–Au/Al2O3 catalysts

The effects of Au on the catalytic performance of Ir metal towards NO reduction by CO and direct NO decomposition have been investigated over dendrimer-derived Ir–Au bimetallic catalyst. Results were also compared to a catalyst prepared by conventional incipient wetness impregnation of metal salts. Characterization of Ir–Au bimetallic samples using scanning transmission electron microscopy, H2 chemisorption and temperature programmed desorption of NO reveals that highly dispersed Ir–Au particles can be obtained by dendrimer templating method. In-situ transmission Fourier transform infrared spectroscopy following adsorption of probe molecules (i.e., CO and NO) on the dendrimer-derived Ir–Au bimetallic sample indicated the presence of electron transfer from Ir to Au sites. In contrast, a geometric effect of Au on Ir was observed over the conventional-derived sample, consistent with X-ray photoelectron spectroscopy measurements. These properties influence the catalytic performance, with higher selectivity towards N2O and a better intrinsic catalytic turnover frequency for reduction of NO by CO and NO decomposition observed for the dendrimer-derived sample. Overall, the results suggest that both NO–CO and NO decomposition reaction are structure-sensitive reactions.