Ammonia oxidation is used in the production of nitric acid. The process is either run at high pressure or low pressure, with the latter requiring larger equipment. Platinum gauze is typically used as a catalyst operating at high temperatures (in the range of 810-940 OC). The platinum based catalyst is highly active and highly selective in producing the desired NOx products, with some formation of the undesired byproducts, i.e. N2 , N20 and N20 4. However, a significant amount of platinum is lost during the process due to platinum volatilisation resulting in plant operating times varying between 2-12 months. Furthermore, the loss of platinum is the 2nd largest expense of the operation. Platinum loss can only be minimised but not eliminated , thus a variety of metal oxide catalysts for oxidation of ammonia to nitrogen oxides have been studied. Cobalt oxide seems to be the most promising alternative for platinum exhibiting a high activity and selectivity towards NO. The aim of this study is to explore the use of a supported cobalt Co30 4 on silica catalyst for ammonia oxidation and compare some of the results with a commercial catalyst consisting of a pure, unsupported Co304. Both the synthesised and commercial catalyst showed a maximum conversion of ammonia at approximately 600 OC. A supported catalyst with a low cobalt loading and smaller crystallites yielded similar conversions of ammonia compared to the pure cobalt catalyst with much larger crystallites. However, the calculated intrinsic activity constant per m2 of Co30 4 revealed that the commercial catalyst was more active compared to the in-house prepared Co30JSi02 catalyst. Indicating that severe deactivation might have taken place on the synthesised Co30,JSi02 catalyst under ammonia oxidation conditions using an ammonia content of 7.1 vol.-% at 450- 800 OC. A high ammonia conversion can be achieved by adjusting the space time. The NO content as a fraction of NO plus N20, increases with increasing temperature before the catalyst is completely deactivated at temperatures above 800 OC. The inhouse prepared Co30,JSi02 catalyst displayed a higher relative NO selectivity compared to the commercial Co304 catalyst under industrially relevant conditions at complete conversion of ammonia. Applying the rate equation proposed by Saykov et al. (2000) and operating under a regime where Knudsen diffusion is the dominant diffusion mechanism, the in-house synthesised supported catalyst and the commercial catalyst showed severe mass transport limitations, indicating inefficient use of the catalyst. The heat transfer limitations were assumed to be negligible with minimal temperature gradients with the catalyst and boundary layer. The supported Co3041'Si02 catalyst showed severe mass transfer limitations with the effectiveness factor less than 0.5 for particles greater than 300 microns. The mass transfer parameters (Thiele modulus, effectiveness factor, rintrinsic and r observed) exhibit small changes over the catalyst bed at low conversions of NH3 and displayed major significant changes at more industrially relevant conditions where higher conversion of NH3 is achieved., The conversion of ammonia decreases rapidly at higher temperatures. It is deduced that sintering of the catalyst is not a major concern. Ammonia oxidation proceeds via the Mars and Van Krevelen mechanism. Therefore the deactivation of the catalyst might be caused by the reduction of active Co30 4 phase to the catalytically inactive CoO phase. Since the mechanism involves the lattice oxygen, the deactivation mechanism is thought to be reversible by utilising excess air. However for the supported catalyst, the CoO acts as an intermediate in the formation of cobalt silicate (Co2Si04) , resulting in an irreversible deactivation. In conclusion , a support material which can react with CoO under hydrothermal conditions, i.e. silica and alumina should be avoided in the preparation of cobalt catalysts for ammonia oxidation.

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