Catalyst palladium-zinc alloy

Palladium is the precious metal most frequently apphed for methanol steam reforming [176-178]. Despite its higher price compared with the copper-based systems, it is an attractive alternative owing to the potential for higher activity and greater robustness, which are key features for small scale reformers. The combination of palladium and zinc showed superior performance and soon the formation of a palladium-zinc alloy was identified as a critical issue for optimum catalyst performance [179]. Besides palladium/zinc oxide, palladium/ceria/zinc oxide may well be another favourable catalyst formulation [177]. However, precious metal based catalysts have a tendency to show higher carbon monoxide selectivity than copper-zinc oxide catalysts, because it is a primary product of the reforming reaction over precious metals. 

An exception in terms of catalysts is the catalytic partial oxidation or OSR of methanol due to the low reaction temperature required. Copper [25, 32-36] and palladium-zinc ahoy [36-38] have been proven to give high selectivities and space-time yields. For the latter system, the palladium forms an alloy with the zinc oxide support under reducing conditions above 300 °C and is stable under the reaction conditions of methanol steam reforming [39]. However, the stability of the ahoy under CPO has not been proven so far by X-ray diflraction after exposure to reaction conditions. 

In a later study, Pfeifer et al. [30] prepared Pd/Zn catalysts by both pre- and postimpregnation of wash-coated zinc oxide particles with palladium and compared their performance in methanol steam reforming. The catalytic performance of the samples was tested at a 250 °C reaction temperature, 3 bar pressure, a S/C ratio of two and 250 ms residence time. The WHSV amounted as 0.3 Ndm3 (min gcat) 1. The thickness of the coatings was calculated to 20 pm. The formation of the PdZn alloy was proven to occur at temperatures exceeding 200 °C by XRD measurements. 

Some of the materials that have been examined as catalysts include Pure Platinum, Platinum-Iridium Alloys, Various Compositions of Platinum-Rhodium Alloys, Platinum-Palladium Alloys, Platinum-Ruthenium Alloys, Platinum-Rhenium Alloys, Platinum-Tungsten Alloys, FejOj-M CVI Oj (Braun Oxide), CoO-Bi20j, CoO with AI2O3, Thorium, Cerium, Zinc and Cadmium. 

Following the development of sponge-metal nickel catalysts by alkali leaching of Ni-Al alloys by Raney, other alloy systems were considered. These include iron [4], cobalt [5], copper [6], platinum [7], ruthenium [8], and palladium [9]. Small amounts of a third metal such as chromium [10], molybdenum [11], or zinc [12] have been added to the binary alloy to promote catalyst activity. The two most common skeletal metal catalysts currently in use are nickel and copper in unpromoted or promoted forms. Skeletal copper is less active and more selective than skeletal nickel in hydrogenation reactions. It also finds use in the selective hydrolysis of nitriles [13]. This chapter is therefore mainly concerned with the preparation, properties and applications of promoted and unpromoted skeletal nickel and skeletal copper catalysts which are produced by the selective leaching of aluminum from binary or ternary alloys. 

Enones are reduced to saturated ketones by catalytic hydrogenation provided the reaction is stopped following the absorption of 1 mol of hydrogen. " A number of catalysts were found useful for this, including platinum, platinum oxide,Pt/C, " Pd/C, - Rh/C, " tris(triphenylphosphine)rhodium chloride, - nickel-aluminum alloy in 10% aqueous NaOH, and zinc-reduced nickel in an aqueous medium. Mesityl oxide is formed from acetone and reduced in a single pot to methyl isobutyl ketone using a bifunctional catalyst which comprised palladium and zirconium phosphate (Scheme 20). 

In Raney s method a catalytically active metal is alloyed with a catalytically inactive one and then treated with a reagent that dissolves out the inactive metal. The catalytically inactive component that is to be dissolved out may be aluminum, silicon, magnesium, or zinc. The catalytically active metal is usually nickel, cobalt, copper, or iron. Noble-metal catalysts can, however, also be produced by Raney s method if an aluminum-platinum alloy (40% of platinum) or a zinc-palladium alloy (40% of palladium) is decomposed by hydrochloric acid.153... 

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