Iridium oxide-supported metal catalysts

Determination of Coke Location. The TPO technique allows the determination of the coke location on supported metal catalysts, such as naphtha reforming. Since the metal, typically platinum promoted with rhenium, iridium, tin, or germanium, has a catalytic effect for coke burning, the TPO profile displays two main peaks. The low temperature peak is due to the oxidation of the coke directly deposited on the metal particle, or in its vicinity . In this way, it is possible to study the effect of catalyst formulation and operational conditions on the formation of coke on the metal and on the support. 

These heterogeneous catalysts consist of muitimetallic clusters, containing metals, such as platinum, iridium, or rhenium, supported on porous acidic oxide supports, such as alumina. The catalysts are said to be bifunctional because both the metal and the oxide play a part in the reactions. The metal is believed to carry out reversible dehydrogenation of paraffins to olefins, while the oxide is believed to carry out isomerization. 

Most of the catalysis using iridium compounds has typically exploited iridium with neutral, soft donor ligands such as phosphines, arsines, olefins, and carbon monoxide. Recent investigations into the chemistry of iridium in atypical environments has shown that oxygen ligand environments can support some very active iridium catalysts and may actually be more akin to the environment that exists around a metal when it is supported on an oxide support (see Water 0-donor Ligands and Oxides Solid-state Chemistry). 

Iridium was supported on different types of metal oxides by deposition precipitation (DP) and liquid phase grafting (LG) methods and was examined for the oxidation of CO and Hz. From these experiments, it was found that iridium supported on TiOz prepared by DP was much more active for CO oxidation than Ir/AIzOj and Ir/FezOy, and furthermore was active below room temperature. TEM observations showed that Ir was spread over the TiOz surface as a thin layer of 2 nm thickness, the structure of which was completely different from those of other noble metal catalysts. 

Iridium catalysts were prepared by DP using the following procedure. The metal oxide support (2 g) was dispersed in an appropriate amount of an aqueous solution of IrCU, the pH of which was adjusted to 7 (except where stated otherwise). The content of Ir in the starting solution was 1.8wt% with respect to the weight of the support. The dispersion was aged at room temperature for 1 h and was washed with distilled water several times. The solid material was vacuum-dried at 0.4 Pa for 12 h and calcined in air at 673 K for 4 h. 

Fig. 2 shows the conversion vs. temperature curves for H2 oxidation over Iridium catalysts pretreated by hydrogen reduction. Comparing the obtained conversions in H2 oxidation with those in CO oxidation, it was found that the temperatures for 50% conversion of H2 oxidation over Ir catalysts, except for IrA i02-DP, was similar to those in CO oxidation. Over Ir/Al203-DP and Ir/Fe203-DP catalysts, H2 oxidation proceeds at lower temperatures than CO oxidation. This feature is similar to those of other typical noble metal catalysts. It should be noted that CO oxidation over the Ir/Ti02 catalyst takes place at temperatures even below room temperature and at much lower temperature than H2 oxidation. This feature is the same as that of highly dispersed Au catalysts [1-4]. These results indicated that the support effect for the CO oxidation was much larger over Ir catalysts than that over Au catalysts. 

This new single-step synthesis unites the simplicity of preparation and lower production costs, with the outstanding properties of the final catalysts. By the single-step procedure proposed here, deposition of dispersed nanoparticles of noble metals on ceramic supports with customised textural properties and shape was achieved. Noble metals including platinum, palladium, rhodium, ruthenium, iridium, etc. and metal oxides including copper, iron, nickel, chromimn, cerium oxides, etc on sepiolite or its mixtures with alumina, titania, zirconia or other refractory oxides have been also studied. 

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

The literature cites several other oxidation catalysts that have shown selectivity in the presence of hydrogen. These include Au, Ir, Pt, Pt/Ru, Ru, Rh, and Cu as the active metals, dispersed on various supports. Iridium on cerium oxide has also been... 

Procedures for recovering ruthenium [1864d], rhodium [1864f], and iridium [1864e] from waste metal oxide catalysts have been patented. The key step in the procedure is the conversion of the oxides (usually supported on TiO or AIJO3) to the chlorides by treating with phosgene in the presence of powdered carbon. 

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