Iridium zeolite-supported catalyst

Catalytic activities of the zeolite-supported clusters (Table 4) are reported as turnover frequencies these are rates per total iridium atom for such small clusters. Rates were also reported for conventional (structurally nonuniform) supported catalysts consisting of aggregates of metallic iridium on supports, these rates, per unit of metal surface area, are markedly greater than those observed for the supported clusters [15]. Changing the support from zeolite NaY to MgO had little effect on the activities of the decarbonylated clusters. 

Iridium clusters in zeolite KLTL, like the platinum clusters, consisting of 4 to 6 atoms on average, have also been prepared by hydrogen reduction of [Ir(NH3)5Cl]Cl2 in the pores at temperatures >300°C [26]. Even though the iridium clusters were as small as the selective platinum clusters in the same basic zeolite support, they were found to be unselective catalysts, being similar to other iridium catalysts for conversion of n-hexane and hydrogen principally into hydrogenolysis products. It is inferred that the combination of cluster size, electronic... 

Ammonia decomposition was investigated by Choudhary et al. [283] over nickel, iridium and ruthenium catalysts supported by various carrier materials such as ZSM-5 and Y-zeolites, alumina and silica. Ruthenium on silica was most active, followed by iridium and nickel [283]. 

One of the most active and selective catalysts in this kind of reaction is undoubtedly Ir/support, as recently demonstrated by Jacobs and coworkers [276], Therefore, by combining the carbonyl affinity of metallic iridium with the promotion effect of the H-fl zeolite, which is a strong Bronsted acid, one can reduce a large variety of unsaturated ketones and aldehydes to allylic alcohols with high conversions, selectivities, and diasteieoselectivities. 

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. 

For the first and the second type of lean bum engines, therefore, catalytic after-treatment systems are considered that include such a Cu/Zeolite catalyst to convert NO c under the lean burn conditions, and a conventional three-way catalyst to convert CO and HC as well as, with the second engine type, NO t under stoichiometric operation conditions. Some applications actually use a supported Iridium catalyst instead of the Cu/Zeolite catalyst. 

Various supported platinum group metal systems have been tested for the SCR process.101 Among them, supported platinum systems appear to be the most active when jointly considering the NOx reduction level achieved and the temperature range at which the catalyst is active, while palladium, rhodium and iridium also show catalytic activity for the process and Rh and Ir apparently present higher selectivity to N2.101>i03-i07 Support effects are observed which generally depend on the type of hydrocarbon employed, the presence or absence of SO2 in the reactant mixture or the type of impurities present in the support.101 In this respect, a variety of materials like SiC>2, AI2O3, ZrC>2, sulphated alumina, zeolitic materials and activated carbons have been employed as supports of the metals and tested for the process.101-112... 

The used NaY zeolite catalyst containing iridium carbonyl clusters was yellow when removed from the flow reactor after three days of continuous operation at 250 °C and 20 bar, consistent with the presence of the [Ir((CO)i6] isomer with face bridging ligands. [5] The catalyst used under the same conditions but supported in the NaX zeolite was reddish brown after a day of operation, consistent with the presence of [Ir6(CO)is]. [4, 60]... 

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