Rhodium-platinum alloys linings

As an attempt to simulate real operating conditions of automotive converters, a laboratory bench has been designed and ageing procedures determined to reproduce simultaneous chemical and thermal modifications encountered by catalysts in the exhaust line. Characterization of commercial samples after ageing according to different temperature cycles evidences formation of both platinum/rhodium alloys and cubic perovskite-type compound, CeA103. Simultaneously with the formation of cerium aluminate, a thermal stabilization of catalysts is observed, in terms of mean noble metal particles size and concentration of rhodium in alloyed phases. An interpretation based on the crystallographic adaptation of alumina, cerium aluminate and ceria is proposed. 

The need to minimize and control contact time to suppress unwanted reactions and to minimize catalyst requirements led to the early adoption of fine woven screens as a suitable catalyst form. These are usually circular in shape and are stacked in multiple array, because the use of several (5 to 45) screens permits the residence time and contact time to be easily varied to obtcun the maximum yield of nitric oxide. Usually a platinum-rhodium catalyst is used in the form of a fine mesh (standardized at 1,024 mesh/cm ), either of the fishbone or line cloth pattern. In an attempt to achieve higher efficiencies and smaller platinum losses [61, knitted gauzes were introduced not long ago. Chrome lickel alloy grids are used to support these fine screens. 

Platinum and rhodium-platinum and iridium-platinum alloys are frequently employed to line and sheath autoclaves, reactor vessels and tubes, and a wide range of equipment. Linings are generally 0-13 mm to 0- 38 mm thick, and for certain applications co-extruded platinum-lined Inconel or other metal reactor or cooling tubes are fabricated. In such cases the platinum is bonded to the base metal, but in all other instances platinum linings are of the loose type. 

Rhodium melts at 1907° C.4 and boils at about 2500° C. It is less volatile than platinum,5 and when alloyed with that metal not only stiffens it, but, unlike iridium, reduces its volatility at all temperatures above 900° C. It has been suggested,6 therefore, that a useful alloy for best quality crucibles would consist of platinum 95 to 97 per cent., and rhodium 3 to 5 per cent., and containing no other detectable impurities. Below 900° C. the presence of rhodium appears to exert a negligible effect. When cooled to — 80° C. rhodium appears to undergo a molecular transformation of some kind, analogous to that evidenced by copper. At this temperature the electrical resistance is considerably below the calculated value.7 The most intense lines in the spectrum of rhodium are as follow 8 ... 

I.A. Kovalev, L.V. Bogacheva, G.I. Tsysin, A.A. Formanovsky, and Y.A. Zolotov. FIA-FAAS system including on-line solid phase extraction for the determination of palladium, platinum and rhodium in alloys and ores. Talanta 52 39-50, 2000. 

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