Ostwald process porous catalysts

High reaction temperatures in catalytic processes can lead to loss of active components by evaporation. This does not only occur with compounds that are known to be volatile (e. g., P2O5 in H3PO4, silica gel, HgCl2/activated carbon), but also by reaction of metals to give volatile oxides, chlorides, or carbonyls. In the oxidation of ammonia on Pt/Rh net catalysts (Ostwald nitric acid process), the catalyst reacts with the gas phase to form volatile Pt02- Furthermore, porous platinum growths are observed on the surface. This can be prevented by addition of rare earth oxides. 

Loss via gas phase occurs only if at least one of the catalyst species on the surface shows a relevant vapor pressure under the reaction conditions (Figure 2.3.6d). Note that during a catalytic operation, for example, in the presence of impurities, volatile catalyst species may form that are not part of the catalytic cycle. Typical volatile complexes that can form under catalytic conditions in technical reactors are metal carbonyls. Another important example of catalyst loss via the gas phase is the catalytic oxidation of ammonia in the Ostwald process (Section 6.4). Here the catalyst, a non-porous Pt/Rh net, reacts at the very high reaction temperatures of 900 °C vhth O2 to give Pt02 which is volatile enough under these harsh conditions to leave the reactor via the gas phase (Hagen, 1999). By installation of Pd/Au wires behind the Pt/Rh net, up to 60% of the volatile precious metal vapor can be recondensed. 

---