Ammonia oxidation platinum catalysts

In 1922 Wohl [13] published his observation that the oxidation of naphthalene in the presence of ammonia over vanadia gives phthalimide but this result went unnoticed both by the scientific community and by industry. Andrussov [6] found, in 1935, a route to produce hydrogen cyanide effectively by conversion of methane in the presence of air and ammonia over platinum catalysts at ca 1273 K. Thus, the first steps towards the development of ammoxidation had been taken. The conversion of aliphatic olefins was first claimed by Cosby in the late forties [e. g. 14] and the conversion of toluene to benzonitrile was first performed by Cosby and Erchak in 1950 [15]. The term ammoxidation was introduced by Hadley in 1961 [16]. Since the fifties the fundamentals of the reaction and the reaction technique, for different aromatic compounds, have been reviewed [e.g. 9,16,17]. 

Dual-Pressure Process. Dual-pressure processes have a medium pressure (ca 0.3—0.6 MPa) front end for ammonia oxidation and a high pressure (1.1—1.5 MPa) tail end for absorption. Some older plants still use atmospheric pressure for ammonia conversion. Compared to high monopressure plants, the lower oxidation pressure improves ammonia yield and catalyst performance. Platinum losses are significantiy lower and production mns are extended by a longer catalyst life. Reduced pressure also results in weaker nitric acid condensate from the cooler condenser, which helps to improve absorber performance. Due to the spHt in operating conditions, the dual-pressure process requires a specialized stainless steel NO compressor. 

Conversion of Ammonia. Ammonia [7664 1-7] mixed with air and having an excess of oxygen, is passed over a platinum catalyst to form nitric oxide and water (eq. 10). The AH g = —226 kJ/mol of NH consumed (—54 kcal/mol). Heats of reaction have been derived from heats of... 

Hydroxylamine sulfate is produced by direct hydrogen reduction of nitric oxide over platinum catalyst in the presence of sulfuric acid. Only 0.9 kg ammonium sulfate is produced per kilogram of caprolactam, but at the expense of hydrogen consumption (11). A concentrated nitric oxide stream is obtained by catalytic oxidation of ammonia with oxygen. Steam is used as a diluent in order to avoid operating within the explosive limits for the system. The oxidation is followed by condensation of the steam. The net reaction is... 

The precious-metal platinum catalysts were primarily developed in the 1960s for operation at temperatures between about 200 and 300°C (1,38,44). However, because of sensitivity to poisons, these catalysts are unsuitable for many combustion apphcations. Variations in sulfur levels of as Httle as 0.4 ppm can shift the catalyst required temperature window completely out of a system s operating temperature range (44). Additionally, operation withHquid fuels is further compHcated by the potential for deposition of ammonium sulfate salts within the pores of the catalyst (44). These low temperature catalysts exhibit NO conversion that rises with increasing temperature, then rapidly drops off, as oxidation of ammonia to nitrogen oxides begins to dominate the reaction (see Fig. 7). 

By far the most important use of the platinum metals is for catalysis. The largest single use is in automobile catalytic converters. Platinum is the principal catalyst, but catalytic converters also contain rhodium and palladium. These elements also catalyze a wide variety of reactions in the chemical and petroleum industry. For example, platinum metal is the catalyst for ammonia oxidation in the production of nitric acid, as described in Pt gauze, 1200 K... 

In school demonstrations of the oxidation of ammonia to nitric acid over platinum catalysts, substitution of oxygen for air causes fairly vigorous explosions to occur [1], Practical details are given [2],... 

In the early 1900s, it was discovered that ammonia could be oxidized in the presence of a platinum catalyst (the Ostwald process). 

Currently, nitric acid is manufactured exclusively by catalytic oxidation of ammonia. Platinum or platinum-rhodium is an effective catalyst of this oxidation (Ostwald process). Three basic steps in such ammonia oxidation process are (1) oxidation of ammonia to form nitric oxide ... 

Nitric oxide is commercially produced by the catalytic oxidation of ammonia using a platinum catalyst 4NH3(g) + 50 —> 4NO(g) + 6H20(g). Nitrous oxide is produced by the thermal decomposition of ammonium nitrate at approximately 240°C NH4N03(g) —> N O + 2H . ... 

Reduction. Benzene can be reduced to cyclohexane [110-82-7], C5H12, or cycloolefins. At room temperature and ordinary pressure, benzene, either alone or in hydrocarbon solvents, is quantitatively reduced to cyclohexane with hydrogen and nickel or cobalt (14) catalysts. Catalytic vapor-phase hydrogenation of benzene is readily accomplished at about 200°C with nickel catalysts. Nickel or platinum catalysts are deactivated by the presence of sulfur-containing impurities in the benzene and these metals should only be used with thiophene-free benzene. Catalysts less active and less sensitive to sulfur, such as molybdenum oxide or sulfide, can be used when benzene is contaminated with sulfur-containing impurities. Benzene is reduced to 1,4-cydohexadiene [628-41-1], C6HS, with alkali metals in liquid ammonia solution in the presence of alcohols (15). 

The situation is different in the case of ammonia oxidation. Both on platinum (156) and nonplatinum (157) catalysts under the conditions of a commercial process, the reaction occurs in the external diffusion region. Diffusion of ammonia rather than of oxygen is determining the rate since the reaction is conducted with oxygen in excess with respect to stoichiometry, as given by (397). Concentration of ammonia at the surface of the catalyst is so small as compared to its concentration in the gas flow that the difference of concentrations that determines the rate of diffusion virtually coincides with the ammonia content in the flow. 

Other workers (165) used X-ray photoelectron spectroscopy (XPS) to examine the influence of ammonia oxidation on the surface composition of alloy gauzes. After several months on stream, the surface was covered by the same types of highly faceted structures noted by others. As illustrated in Fig. 14, XPS analysis provides evidence that the top microns, and in particular the top 100 A of the surface, were enriched in rhodium. This enrichment was attributed to the preferential volatilization of platinum oxide. The rhodium in the surface layers was present in the oxide form. Other probes confirm the enrichment of the surface in rhodium after ammonia oxidation (166). Rhodium enrichment has been noted by others (164, 167), and it has been postulated that in some cases it leads to catalyst deactivation (168). 

Researchers returned to the oxidation of ammonia in air, (recorded as early as 1798) in an effort to improve production economics. In 1901 Wilhelm Ostwald had first achieved the catalytic oxidation of ammonia over a platinum catalyst. The gaseous nitrogen oxides produced could be easily cooled and dissolved in water to produce a solution of nitric acid. This achievement began the search for an economic process route. By 1908 the first commercial facility for production of nitric acid, using this new catalytic oxidation process, was commissioned near Bochum in Germany. The Haber-Bosch ammonia synthesis process came into operation in 1913, leading to the continued development and assured future of the ammonia oxidation process for the production of nitric acid. 

Recent developments in the ammonia oxidation process have included efforts to reduce catalyst losses in the process. Platinum recovery filters have been installed at various stages in the process. Gold/palladium gauze filter pads have been added on the exit side of the catalyst bed, inside the reactor/converter units. These filters have reportedly ensured a platinum recovery of 80% (Ref. PT4). Another trend has been for the use of additional filters in the downstream units. These filters are of alumino-silicate construction. 

Ohmic heating of catalyst is often used as a simple method of igniting the chemical reaction during reactor startup, for instance, in the oxidation of ammonia on platinum-rhodium gauze catalysts. Another application is the prevention of cold-start emissions from automotive catalysts responsible for much of the residual pollution still produced from this source (21). The startup times needed for the catalyst to attain its operating temperature can be cut by a factor of 5 or more by installing an electrically heated catalyst element with a metallic support upstream of the main catalyst unit. Direct electrical catalyst heating permits facile temperature control but requires a well-defined catalyst structure to function effectively. 

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