The oxidation of ammonia using air to form nitric oxide for nitric acid production, for nitrate fertilisers or explosives, has been established since the early 20th Century (the Ostwald process), and is in widespread use across the world. The established process uses a platinum or platinum alloy gauze as catalyst for the reaction of ammonia with the oxygen in air. The Andrussow process for the production of hydrogen cyanide uses essentially similar technology, and should be considered as being within the scope of the present invention. The Pt gauze is usually supplied as a “pack” of several gauzes mounted one above the other. Successive gauzes may vary in filament thickness, alloy composition, and in other ways.
During operation, platinum is lost physically or by volatilisation from the gauzes, to a greater or lesser extent depending upon the process conditions, e.g. pressure. Whilst the platinum may collect in cooler parts of the plant, recovery of the platinum can prove difficult. It is known to include one or more “catchment” or “getter” gauzes, which are palladium (Pd)-based, which are mounted closely downstream of the catalyst gauzes. The catchment gauzes collect a high proportion of the Pt lost, although a proportion of the Pd is itself simultaneously displaced from the catchment gauze. Instead of having a separate Pd catchment gauze, it is possible to incorporate Pd filaments in a second or third gauze. It has been suggested that in such a case, the Pt recovered continues to be available to act as a catalyst. Recent research work by the Applicants indicates strongly, however, that Pd gauzes may increase N2 and/or N2O levels compared to a Pt gauze, up to several times. One mechanism may be:

It is not presently known whether N2 and N2O generation is the result of incomplete oxidation of residual ammonia or by reaction of residual ammonia with NOx, but indications are that residual ammonia is undesirable. Also undesirable, because of the effect on overall yield of NO and NO2, is either loss of NO, for example by the above reaction, or conversion of ammonia or NOx to N2.
There have been few advances in the Pt gauze-based technology over the last 20 years, apart from the development and introduction of knitted gauzes, in place of woven gauzes, by the present Applicants.
The substantial capital cost of Pt gauzes has led to some exploration of base metal catalysts for ammonia oxidation. These catalysts are generally based upon cobalt compounds such as Co oxides or perovskites. However, although the best Co perovskite catalysts offer some technical advantages over Pt-based gauze catalysts in addition to the lower capital cost, it appears that sufficient disadvantages remain, primarily a significant sensitivity to poisoning from atmospheric sulphur compounds, so that there has been only very limited commercial scale applications, in the manner of trials, as far as is presently in the public domain.
There have been a few proposals to combine platinum gauze with a Co-containing material. For example, Chemical Abstracts 114:188430 describes the combination of 1-2 Pt grids with a downstream Co3O4 catalyst. Such a catalyst is stated to show maximum selectivity to NO formation at 350° C., which is very much lower than the operating temperature of a conventional Pt gauze catalyst, and hence lower than the normal operating temperature of ammonia oxidation plants. Low temperatures are necessary for Co3O4 catalysts because of a phase change they are known to undergo above 850° C. resulting in the formation of CoO which has lower selectivity for NO in the ammonia oxidation process. Furthermore, no reference was made to the resulting N2O levels. WO 99/64352 describes a platinum gauze catalyst followed by a Co-containing catalyst, where a reduction in side reactions; such as nitrous oxide formation, is claimed. However the combined catalyst system only demonstrated at best N2O levels of 700 ppm in laboratory apparatus. This patent application was abandoned, and we are not aware of any commercialisation of the technology. Indeed, platinum or platinum alloy gauzes remain the only commercially available and technically acceptable technology in the marketplace.
The gauze manufacturers design gauze packs individually for each plant, or for each burner on a plant. It is state of the art practice that pack design aims for maximum conversion possible under the plant operating conditions, since there is a gradual fall-off of conversion efficiency as the platinum gauze deteriorates in use. Eventually, when conversion drops to unacceptable levels, the plant campaign is terminated and the catalyst is replaced. It would therefore be counter-intuitive to design a catalyst pack or catalyst charge where the conversion over a first platinum group metal catalyst element is deliberately chosen to be less than complete. Since incomplete conversion results in residual ammonia in the gas stream, the opportunity to make N2O by side reactions is likely to be increased. Indeed, loss of yield is established in the art as being very important; ammonia slip can cause the side reaction yielding N2, which clearly causes loss of yield, and ammonia slip also raises the possibility of forming explosive ammonium nitrate downstream of the catalyst.