Thermal stability of vanadia SCR catalysts for the use in diesel applications

The effect of reaction temperature, water content, and vanadia content on the sintering of a model vanadia on silica-titania SCR catalysts has been investigated. A comparison is also made to a monolithic vanadia/titania catalyst submitted to conditions simulating at least 500,000 km driving on a diesel truck. The variation of the surface area, the pore structure and the crystallite size of the catalyst on thermal treatment in dry and wet air was investigated. The support stabilises after 5 h at 450 C in wet and dry conditions. The vanadia catalysts sinter to a much higher degree than the support. The vanadia crystallite size increases to fill the mesopores of the support in wet conditions at 450 C. It is finally transported into the macropore region of the catalyst. The effect of vanadia is to increase the rate of the conversion of the anatase phase of the support to rutile and to lower the transition temperature. The decrease in surface area could be modelled with existing sintering models. The importance of many data points, especially at long sintering times, is stressed. The surface area of the catalyst treated in the deactivation rig behaves in a similar way as the model catalyst. It is possible to predict the decrease of the surface area of real catalysts from measurements of sintering on the same material in laboratory studies. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved