Keywords: Catalysts, Nitric acid, Platinum, Noble metals, Tail-gas treatment.
In this brief review, it has only been possible to consider a few of the many and complex factors which govern the choice of catalyst, the operating parameters and reaction mechanisms during the catalytic oxidation of ammonia to nitric acid. Platinum alloy gauze catalysts are almost universally used for this purpose, and it appears extremely unlikely that this material will be superseded by base metal catalysts in the foreseeable future.
Although so widely used, many of the basic features of the processes occurring at the gauze surface remain to be explained. For example the precise nature of any transient intermediate species formed during the oxidation of ammonia needs to be identified with certainty before a full understanding of the process will be possible. Also the detailed mechanism of platinum and rhodium losses is still a matter of debate, as is the question of whether these volatilised elements participate catalytically in the vapour phase. That very rapid restructuring of the gauze surface takes place during ammonia oxidation cannot be questioned, but no good correlation exists between the variety of surface structures observed and ambient operating conditions.
In spite of this lack of basic understanding of detailed mechanisms, the catalytic process for the oxidation of ammonia to nitric acid is operated at very high efficiency for long campaign periods. Most plant operators are now fully aware of the deleterious effects of contaminants on catalyst performance, and in recent years considerable success has been achieved by improved filtration or other methods of cleaning up the gas and air input to the plant. Contamination from the gas stream by elements, such as iron, forms an inert barrier to the reaction and it is difficult to envisage any inherent catalyst improvement to combat this type of poisoning. No filtration system is perfect, and some progressive deterioration inevitably occurs in gauze performance. Experimental pilot-plant work can determine the catalyst requirement for a particular conversion efficiency and loading, but in practice most plant operators will have established the correct number of gauzes to cater for poisoning by empirical methods, or perhaps on the basis of sound technical advice from enlightened gauze suppliers. The extra gauzes needed to maintain high efficiency inevitably leads to higher losses from the pack, but these may be effectively and economically reduced by the use of the catchment system based on palladium-gold alloy. The latter is a gauze system with dimensions similar to the rhodium-platinum oxidation gauzes, and hence can be fitted to most burners with virtually no plant modification. Also the recovered catchment material, being a mixture of noble metals only, poses no problems in the reclamation of metal content by chemical processing.
It is not intended to enter into a discussion of economic factors in this review, but in practice it is often these considerations in individual plants that dictate the precise system used. High ammonia costs for example mean that it is viable to use an excess of gauzes to maintain high conversion efficiency, or vice versa.
The catalytic treatment of tail-gas to minimise pollution is also now a well established technology and systems based on platinum metal catalyst can effectively and economically reduce emissions to a low level. The incorporation of tail-gas nitrogen oxides emission control systems need not necessarily lead to increased nitric acid production costs and indeed if such a system is an integral part of the overall plant design, costs are generally lower than for a similar plant not incorporating tail gas treatment.
J A Busby PhD, CChem, MRIC, and A G Knapton, BSc, PhD, FIM, Johnson Matthey Research Centre, UK.
A E R Budd, Johnson Matthey Chemicals Ltd, UK.
39 Pages, 11 Figures, 2 Tables, 47 References.