Iridium-based Catalysts Research Articles

Water Oxidation at Hematite Photoelectrodes with an Iridium-Based Catalyst

The iridium complex [Cp*Ir(H2O)3](SO4) was used as an organometallic source for the electrodeposition of iridium oxide onto Fe2O3. The new iridium-containing electrode allowed us to study the coupling between the photocatalytic properties of hematite and the electrocatalytic properties of the iridium-based material. A cathodic shift of the photocurrent for water oxidation upon electrodeposition of the iridium complex was observed, which increased with increasing surface concentration of IrOx on Fe2O3. The shift for the highest surface concentration of iridium tested amounts to 300 mV at 200 μA·cm–2 current density. The catalytic mechanism of the IrOx layer was unveiled by impedance spectroscopy measurements fitted to a physical model and can be explained on the basis of a highly capacitive layer, which enhances charge separation and stores photogenerated holes at Fe2O3, subsequently oxidizing water. These findings improve our understanding of the mechanism of water oxidation by heterogeneous Ir-based cata...

ChemInform Abstract: Development of Iridium‐Catalyzed Asymmetric Hydrogenation: New Catalysts, New Substrate Scope

AbstractReview: ca. 70 refs.

Heterogeneous Transformation of Glycerol to Lactic Acid

Significant yields of lactic acid (LA) are obtained during the treatment of glycerol solution under inert gas with supported metallic catalysts under basic conditions. The nature of the atmosphere and the metal affected the activity and the selectivity of the reaction. Iridium-based catalysts are efficient for LA synthesis.

On the role of the atmosphere in the catalytic glycerol transformation over iridium-based catalysts

Here, we report the transformation of glycerol alkaline solutions in the presence of supported iridium catalysts under either reductive or inert atmosphere at 453 K. It was shown that under H2, the expected 1,2-PDO issued from hydrogenolysis of the triol was the main product while under He, lactic acid was formed predominantly via Cannizzaro rearrangement.

Alkenyl-functionalized NHC iridium-based catalysts for hydrosilylation

A family of alkenyl-functionalized N-heterocyclic-carbene–iridium(I) complexes has been synthesized, providing a series of mono-coordinated, bis-chelate and pinceralkenyl-NHC species. Olefin coordination is highly influenced by the nature of the substituents on the NHC ring, and on the length of the alkenyl branch. A fluxional process involving coordination/decoordination of the olefin in bis-allyl-NHC complexes has been studied, and the activation parameters have been determined by means of VT-NMR spectroscopy. The mono-coordinated complexes are highly active in the hydrosilylation of terminal alkynes, showing high selectivity for the Z-isomers, with no α-isomers or dehydrogenative silylation processes being observed. The molecular structures reported that are representative of the species have been determined by means of X-ray crystallography.

The Selective Catalytic Reduction of NO by Hydrocarbons over Pt- and Ir-Based Catalysts

Platinum- and iridium-based catalysts are the most active ones among noble metals for the selective catalytic reduction of NO by hydrocarbons (HC-SCR). Yet, the level of our understanding of the behavior of these two metals varies greatly. From the early stages of the research efforts in the HC-SCR field Pt-based catalysts were identified as “promising” and have been studied extensively for this application. Subsequently, there is a substantial body of literature focusing on the catalytic performance of such systems, as well as on the fundamental chemistry taking place on their surface under HC-SCR conditions. A fairly good understanding of the behavior of such catalysts has been developed and different reaction mechanisms have been proposed and debated extensively. In contrast, limited information is available regarding the properties and HC-SCR behavior of Ir-based catalysts. Recent reports have demonstrated advantages in the HC-SCR performance of such catalysts and have documented substantial iridium particle size, support and promoter effects.

Evaluation of the health risk of platinum group metals emitted from automotive catalytic converters

A health risk assessment of platinum (Pt) emitted from automotive catalytic converters is presented. Following a stepwise approach, the relevant literature is discussed in order to characterize Pt emissions as well as the toxic potential of Pt and its compounds. In an exposure assessment, ambient Pt concentrations in air are predicted to range from approximately 4 pg/m 3 (street canyon, typical conditions) up to approximately 112 pg/m 3 (express motorway, severe conditions). These values agree well with the few measured concentrations, which are also in the low pg/m 3 range. Pt is emitted from catalytic converters in very small amounts (ng/km range), mainly in the (0)-oxidation state (elemental Pt). The nanocrystalline Pt particles are attached to μm-sized aluminum oxide particles. Whether free ultrafine Pt particles may be emitted and result in biological effects has not been studied sufficiently. Hence, risk assessment can only be based on the respiratory sensitizing potential of halogenated Pt salts. The presence of such compounds in automotive Pt emissions cannot definitely be excluded. From recent occupational studies conducted in catalytic converter production, a conservative no-effect level (NOEL) of 1.5 ng/m 3 can be derived for the sensitizing effect of halogenated Pt salts. In a (reasonable) worst case approach, it is assumed that such compounds comprise 1% (0.1%) of the total Pt emissions. Applying a safety factor of 10 to account for interindividual variability, a guidance value of 15 (150) ng/m 3 is derived for catalyst-borne Pt. The exposure to Pt in ambient air as measured or predicted is at least two orders of magnitude below this guidance range. Rhodium is also contained in automotive catalysts, palladium has increasingly substituted Pt, and iridium-based catalysts have recently been introduced. Although the database on these platinum group metals is rather small, there is no evidence that they pose a health risk to the general population.

Parallel chemistry investigations of ortho-directed hydrogen isotope exchange between substituted aromatics and isotopic water: novel catalysis by cyclooctadienyliridium(I)pentan-1,3-dionates

Novel iridium-based catalysts which promote ortho-directed hydrogen isotope exchange between substituted aromatics and isotopic water have been identified via a combination of screening and subsequent ligand optimisation. The catalysts are more active, operate at lower temperature and are applicable to a wider variety of substrates than previously known systems.

Harnessing the Unique Properties of Iridium

With the exception of osmium, iridium is the least abundant of the six platinum group metals. It is of crucial importance in a number of high technology applications and, at least in the past, has been regarded as a strategic material and stockpiled by major governments. In recent years it has become more readily available, prompting renewed interest by researchers, including those seeking to develop improved catalysts. In this paper we discuss the results of our investigations on the use of iridium as an hydrogenation catalyst. Data are presented on the characteristics of an iridium-based catalyst developed here for the effective and selective synthesis of substituted N-aryl-hydroxylamines and chloro-substituted anilines, symmetric and asymmetric azoxybenzenes, and unsaturated alcohols. These demonstrate that iridium is unique among the platinum metals.

Nitride and carbide of molybdenum and tungsten as substitutes of iridium for the catalysts used for space communication

Satellites are equipped with microthrusters that control their orbit and attitude. The thrust is achieved by the catalytic decomposition of hydrazine by iridium supported on alumina. As nitrides and carbides of molybdenum and tungsten behave like noble metals in many catalytic reactions, they were tried in a 2 newton hydrazine microthruster. Their performance was similar to that of the iridium catalyst, with respect to ignition delay and thrust. Their mechanical resistance appears higher than that of iridium-based catalyst. This application is the first practical one for nitrides and carbides of early transition metals as substitutes of noble metals, a possibility first reported in 1973.

An iridium-based catalyst system for metathesis/isomerization of acyclic olefins, including methyl oleate

Reaction between [Ir(coe)2Cl]2(coe = cyclooctene) and AgO2CCF3(4 equiv.) produces a highly active system for metathesis/isomerization of acyclic olefins, including methyl oleate.

Alcohol synthesis from syngas on alumina-supported iridium-based catalysts: Effects of transition-metal compound additives

The influence of additives on the performances of alumina-supported iridium-based catalysts for the conversion of syngas has been examined. The alcohol synthesis activity of the catalyst was affected dramatically by additives and increased in the following order; Ir < Ir–La < Ir–Zr < Ir–W < Ir–Mn < Ir–Re < Ir–Cr < Ir–V < Ir–Mo. The greatest alcohol synthesis activity, achieved by an Ir–Mo catalyst, was 280 times higher than the activity of an unpromoted Ir catalyst, while the highest selectivity for alcohols, 61%, was attained by an Ir–Cr catalyst. The Ir–Cr catalyst system displayed a unique performance which was distinct from other promoted Ir catalysts, i.e. an excellent selectivity for alcohols, an iso-rich alcohol distribution and a larger methanol yield than that expected on the basis of the yield of higher alcohols.

EXAFS characterization of Pt, Ir and Pt-Ir on gamma alumina catalysts prepared with new precursors : influence of the nature of the precursor and of the acidic pretreatment of the support

The use of K2 PtCl6 or (NH4 )3 IrCl6 as precursors instead of H2 PtCl6 or H2 IrCl6 allows us to study the influence of the acidic pretreatment of the alumina on the aqueous impregnation reaction. In the case of Pt/Al2 O3 catalysts, the maximum of adsorption is reached after a neutral or weakly acidic pretreatment, whereas a more acidic one is needed for the adsorption of iridium. The determination by EXAFS of the local environment of the adsorbed metal after filtration and drying leads to the following adsorption schemes where adsn means adsorption over n sites and ads H+ adsorption neutralized by H+ :[math]The difference between the two precursors can be explained by the non saturation of the adsorption sites in the case of platinum. For the iridium-based catalysts, taking into account the EXAFS analysis, it has been possible to estimate the number of the alumina adsorption sites about 400 µmol. g-1 .Bimetallic Pt-Ir/Al2 O3 reforming catalysts were then prepared from two new bimetallic compounds by formation of the precursor in the porosity of the support : [Ir(NH3 )5 Cl] [PtCl4 ] and [Pt (NH3 )4 ]3 [Ir(Cl)6 ]2 . The EXAFS study at the iridium absorption edge shows : (i) the same iridium coordination after the calcination step for both precursors : Ir Cl1 O6.5 ; (ii) the absence of Ir-metal bonds after the reduction step and air handling of the sample, just as (iii) a lowering of the number of oxygen atoms bonded to iridium.

Propriétés catalytiques des complexes des métaux précieux: carbonylation du méthanol en acide acétique en presence de composes de l'iridium (I)

No major difference was found between rhodium- and iridium-catalysed homogeneous carbonylation of methanol to acetic acid, using methyl iodide as promoter. The dependence of the activity on the nature of the iridium-based catalyst is greater than that of the rhodium-based catalyst, and the maximum activity occurs at different conditions. We succeeded in isolating the postulated true catalytic species and we were able to refine the previously described mechanism. From the survey of the catalytic activities of platinum metal complexes (in their optimum conditions) we conclude that the best catalysts are the iridium complexes.