Czasopismo | INŻYNIERIA MATERIAŁOWA | Rocznik 2013 - zeszyt 4

Analysis of two catalytic systems PtRhPd-PdAu and PtRh-PdAu after long-term exploitation

nr katalogowy: 79733

Precious-metal catalytic gauzes are the major catalyst used in the oxidation process of ammonia. The most common material for this application is platinum-rhodium alloy [1, 2]. Due to the high costs of elements and to improve mechanical properties, alloying elements like palladium are also commonly used. Beside catalyticonly gauzes also PdAu or PdNi gauzes are also used in the catalytic systems [3÷5]. Gauzes of this type are installed in the reactor directly under PtRh gauzes. Role of these gauzes, besides catalysis, is to capture platinum drained from above located PtRh or PtRhPd ones. This phenomenon has been studied [3, 4, 7] and several mechanisms were introduced. In the aggressive environment and under extreme conditions elements are drained from the surface of the wires and transform to the gaseous species like PtO2 and RhO2 [8]. These oxides quickly decompose forming characteristic “cauliflower growths". Some part of these gaseous oxides (especially platinum) decomposes later on PdAu gauzes, which are located below. All these phenomena causes strong change in morphology and chemical composition of the wires. Rate of these changes is related to the exposure time and conditions in the reactor. These conditions may vary depending on the part of the reactor. This paper consists of study on differences in morphology and chemical composition of the gauzes from two different catalytic systems, also depending on their location in the reactor during long-term exploitation. MATERIALS AND METHODS All gauzes were provided by Precious Metal Mint in Radzymin, Poland. Gauzes were made of 80 μm diameter wires produced by standard method. At first, alloy was melted in an induction furnace, casted, hot forged, rolled and drawn to the specified diameter. After that, wires were woven or knitted into the gauze form. Both analysed catalytic systems were exploited in two different industrial reactors for period of 6 months. Firs[...]

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