Platinum group metal systems

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 

Differences in NOx conversion level are observed depending upon the type of hydrocarbon employed.101 The selective reduction of NOx appears more effective as the chain length or the degree of saturation of the hydrocarbon is increased, although deactivation phenomena (attributed to generation of carbonaceous deposits) may appear for relatively long chain hydrocarbons.101,111,116 Support effects are apparent in this respect when 

With respect to particle size effects, the catalytic activity apparently increases with decreasing Pt dispersion.108,121 A similar result has been observed for Ir containing systems, with the peculiarity that for Ir/AI2O3 the active state is only achieved following certain preconditioning of the sample (apparently the presence of CO + O2 are required).105 On the other hand, particle size effects on N2 selectivity are less certain.101 

As treated in detail in the review work by Burch et a/.,101 three distinct mechanisms can be differentiated for the SCR of NOx by hydrocarbons over Pt catalysts. The first one attributes a role of cyanide or isocyanate species in the formation of N2 or N2O. In this respect, although formation of such species under relatively mild (and dry) reaction conditions is well addressed in the literature (based on infrared experiments), it is not at all clear whether such species are real intermediates or mere spectators since there is no conclusive kinetic evidence correlating formation of N2 and N2O with the evolution of -CN or -NCO species. The absence of formation 

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The application of these methods is described in some detail for recovery of base metals and platinum group metals in Sections 9.17.5-9.17.6 focusing mainly on solution-based hydrometal-lurgical operations, largely those involving solvent extraction, because the nature of the metal complexes formed is usually best understood in such systems. NB. Extraction of lanthanides and actinides is not included as this subject is treated separately in Chapters 3.2 and 3.3. 

Carbonylation of Methyl Acetate on Ni/A.C. Catalysts. Table II shows the catalytic activities of nickel and platinum group metals supported on activated carbon for the carbonylation of methyl acetate. Ruthenium, palladium, or iridium catalysts showed much lower activity for the synthesis of acetic anhydride than the nickel catalyst. In contrast, the rhodium catalyst, which has been known to exhibit an excellent carbonylation activity in the homogeneous system (1-13), showed nearly the same activity as the nickel catalyst but gave a large amount of acetic acid. 

Investigations performed by Dupont on the chemical stability of Teflon reveal that Teflon immersed in a 20% HC1 solution at 200°C for extended periods will not absorb any chlorine within detectability limits. In addition, Teflon does not absorb any detectable sulfur from sulfuric acid or sulfur vapor. Therefore, a Teflon H, membrane may prove to be superior to a platinum-group metal membrane for use in sulfur- or chlorinebearing systems. Investigations are currently being performed to determine the feasibility of employing a Teflon membrane as an H2 monitor/controller in hydrothermal test apparatus. 

Even with these limitations, nuclear magnetic resonance has made significant contributions to four areas of the chemistry of the platinum group metals bonding problems, molecular stereochemistry, solvation and solvent effects, and dynamic systems—reaction rates. Selected examples in each of these areas are discussed in turn. Because of space limitations, this review is not meant to be comprehensive. 

The first applications of NMR to the study of dynamic systems of the platinum group metals appear to have been studies on the rotation about the metal-olefin bond of coordinated olefins, and this process has been investigated by many workers. There are two reasonable orientations of an olefin with respect to the rest of a square planar complex, XXIV and XXV. 

The six platinum group metals, platinum, palladium, ruthenium, osmium, rhodium, and iridium, usually occur together in nature. These metals are not often found in artifacts. These metals are rare and have only been widely used in industry and for ornaments since the early twentieth century. Most platinum used today is as a catalyst in the systems used to control car exhaust emissions, in dentistry, and to make surgical tools, jewelry, and electrical equipment. 

Commercial SCR catalyst used in connection with coal-based power stations are generally composed of base metals, since platinum-group metal catalysts are too readily poisoned and have too narrow an operating temperature window for this application. Favored compositions are titania-based together with active components, normally oxides of vanadium, tungsten, or molybdenum. For these systems the optimum reaction temperature is usually in the range 3(XM00°C. 

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