Supported platinum-group metals

Pt group metals can activate alcohols and molecular oxygen under close to ambient conditions, producing the corresponding carbonyl or carboxylic acids in high yields. Enhanced selectivity and activity has been obtained by the use of bi- and multimetallic catalysts.35-37 However, as was the case with selective hydrogenation, the optimum catalysts developed to date have 

The state of the bi- or multi-metallic catalyst is important as segregated single metal sites yield reduced selectivity. These may be formed either by single metal agglomerations on the surface or multilayer adsorption of the secondary component on the initial supported metal catalyst.32 It has been shown that selectivities and activities may be influenced by the presence 

Catalysts with Supported Platinum Group Metals 

Many groups have reported XPS data on this type of systems. Before any further discussion it is important to note that, although a number of effects may be induced by metal-ceria interactions, they can be also provoked, or at least enhanced, by the presence of chlorine introduced with the metal. The strong retention at the surface (or/and in the bulk) of ceria and related oxides, even after redox treatments, of chlorine coming from the transition metal salt used in preparing the catalyst is well documented [109, 138, 147, 148, 40, 149, 150, 151]. It has been verified that Cl 

Reverse influences (effect of Ce on the redox state of the supported metal) have been also observed. In some cases the reports claim that ceria specifically favors the retention of the metal in an oxidized state [88], although less than thoria in the case of Rh/M02/Si02 [126]. In any case, XPS is used regularly to check the metal redox state after catalyst preparation or diverse treatments. Oxidized states have been reported for Pd and Pt on Ce02 catalysts prepared by a combustion method leading mainly to (+2) and mixed (+2/+4) states respectively [161] on the 

Pt group metals can activate alcohols and molecular oxygen under close to ambient conditions, producing the corresponding carbonyl or carboxylic acids in high yields. Enhanced selectivity and activity has been obtained by the use of bi- and multimetallic catalysts. However, as was the case with selective hydrogenation, the optimum catalysts developed to date have involved a combination of supported metals and selectivity promoters. The most commonly used catalysts consist of either Pt or Pd as the active metal combined with Bi or Pb as promoters, commonly on carbon or alumina supports. Other promoters reported include Cd, Co, Cu, Se, ° Ce, Te, Sn, Au and Ru. The catalysts can be prepared by simultaneous deposition and reduction of the metal precursors onto a suitable support. However, more commonly, preparation involves a 

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Catalysts with Supported Platinum Group Metals ... 

Uemiya S, Kato W, Uyama A, et al.. Separation of hydrogen from gas mixtures using supported platinum-group metal membranes, Sep. Purif Technol. 2001 22—23 309-317. 

Particular interest has been shown in the catalytic oxidation of polyols with dioxygen using supported platinum-group metals as the catalysts. The most studied metals have been palladium and platinum which are, however, often affected by deactivation problems [2]. The introduction of cocatalysts such as bismuth or lead represents an enhancement in the use of these catalysts having the double effect of Increasing catalytic activity and improving catalyst life [3]. 

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... 

In practical terms this has constrained the development of only three basic control strategies (refs 9,10) in the context of stringent legislation. These are all based upon application of supported platinum group metal catalysts. The strategies are ... 

In the case of Haber-type catalysts the potassium is present in the unreduced catalyst, predominantly in a compound form associated with both the iron oxide and alumina phases, while in supported platinum group metal catalysts the alkali may be added in salt form (nitrate, carbonate, hydroxide, etc.) or as the metal. In single-crystal studies the alkali may be fired at the crystal surface from molecular sieve ion sources or evaporated onto the surface as the metals. 

The most active catalysts are derived from supported platinum group metals, or copper chromite when the VOC contains chlorine. The catalyst supports are produced as high surface area gamma-alumina spheres, extmdates or cylindrical pellets that can be used in dust free conditions. Alternatively, when treating large volumes of effluent gas, honeycomb monohths made from cordierite or metal sheets are coated with a surface washcoat of the same alumina are particularly usefiil. The supports are selected to be stable at the operating temperature of the reaction. 

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