Investigation of SCR Catalysts. Main and Side Reactions, Characterization and Kinetics

摘要

In the project we have studied different aspects of SCR such as activity, kinetics and selectivity of the reaction. This has been done in relation to the type of catalyst and preparation or modification of it. Characterization of the support and the catalyst is therefore very important and a number of different techniques have been used. For the support we have studied the properties of coprecipitated SiO(sub 2)-TiO(sub 2) mixtures. The gain of using this support is that the anatase grains are separated by a matrix of amorphous SiO(sub 2). This improves the resistance of the support towards sintering. The method of preparation also introduces a macropore volume in the support with a maximum around equimolar mixtures of Si and Ti. The catalysts were prepared with different V(sub 2)O(sub 5) loading, ranging from 1 to 50 wt% on a SiO(sub 2)-TiO(sub 2) (1:1) carrier by impregnation with ammonium metavanadate solutions. The investigation shows that a monolayer is gradually built up and is best developed at 5% V(sub 2)O(sub 5) loading. The formation of N(sub 2)O during the SCR reaction is depending on the V(sub 2)O(sub 5) loading. At sufficiently high loading 'super monolayer' and crystalline V(sub 2)O(sub 5) phases are formed. By washing the catalyst with aqueous NH(sub 3), those phases can be removed. This procedure enhances the selectivity towards N(sub 2). The presence of high partial pressures of H(sub 2)O in the flue gas also suppresses the formation of N(sub 2)O over the catalyst during the SCR reaction. By the use of H-Mordenite, a zeolitic catalyst, decomposition of N(sub 2)O into N(sub 2) and O(sub 2) can be achieved at 500 C. The rate of this reaction is greatly enhanced by additions of small amounts of NO. The decomposition rate is not affected by the oxygen partial pressure. The kinetics of the SCR reaction was modelled by the use of a Langmuir-Hinselwood rate expression. 26 refs, 25 figs, 7 tabs