Platinum as a Catalyst
Design of the gauzes
The use of platinum in catalysts presumably represents the most important application of platinum. In most catalysts, the platinum is applied to a ceramic or metallic substrate in the form of a fine dispersion. However, self-supporting platinum-rhodium catalyst gauzes are used in the Ostwald process for the manufacture of nitrogen monoxide by the combustion of ammonia and in the Andrussow process for the manufacture of hydrogen cyanide from ammonia, because of the high operating temperatures (850 and 1,050 respectively) and very high reactant loadings encountered in these processes.
For many decades, woven gauzes of fine Pt-Rh wires were used (typical wire diameters 76 µm or 60 µm), in which the preferred alloy was PtRh90/10. Since the mid 1990s, the woven gauzes have been almost completely replaced by warp knitted or weft knitted gauzes. Furthermore, in the Ostwald process, catalyst alloys with lower rhodium contents – in particular PtRh95/5 – have taken on the leading role because their long-term catalytic properties have proved more reliable in practice.
Under the strongly oxidizing conditions of the Ostwald process, considerable quantities of volatile platinum oxides, primarily platinum dioxide, are formed which are carried off by the gas flow and thus can lead to high precious metal losses during the course of typical process campaigns lasting several months. In the 1960s it was found that palladium gauzes installed beneath the platinum gauzes (gas stream from above) can catch, i.e. reclaim, a large proportion of the platinum. Initially, palladium-gold alloys were used for the catchment gauzes, in particular PdAu80/20. However, since the 1980s the alloy PdNi95/5 has been used almost exclusively. The catchment of platinum is presumably achieved via an exchange reaction between the platinum dioxide in the gas phase and metallic palladium resulting in the formation of palladium oxide. The value of the palladium lost in this way is normally considerably less than the value of the reclaimed platinum.
In more recent years, complex catalyst systems have been developed, in particular the FTC system from Heraeus, that combine the properties of both the catalyst and the catchment gauzes. This permits considerable savings in precious metals to be achieved. Further developments of the FTC1) system enable a reduction in the emission of the hazardous greenhouse gas dinitrogen monoxide (nitrous oxide or laughing gas) which forms in small quantities as a by-product of the Ostwald process. The laughing gas can be largely eliminated by means of specially developed secondary catalysts that are installed immediately downstream from the primary FTC2) system. The laughing gas catalyst consists of an extremely thin precious metal coating on an α-alumina support. Depending on the type of process reactor, the supports can be in the form of Raschig rings or small spherical pellets.