EUROCAT oxide: an European V2O5-WO3/TiO2 SCR standard catalyst study - Characterisation by electron microscopies (SEM, HRTEM, EDX) and by atomic force microscopy

The morphology and the homogeneity in chemical compositions of fresh and us ed V2O5-WO3/TiO2 EUROCAT SCR samples, in their original monolith form and a fter gentle grinding, have been investigated by means of electron microscop ies and EDX analyses. It appears clearly that the monoliths were constitute d of fibres rich in Si, Al and Ca embedded without preferential orientation in a nearly homogeneous oxide phase containing Ti, V, and W. This phase wa s in the form of small particles of homogeneous size of around 20-40 nm. Th e used catalyst was very similar to the fresh one, only the presence of S e lement and of more defects and more fibres were observed on the surface of the monolith. This observation was confirmed by a higher roughness detected using AFM technique. EDX-TEM studies on the powders obtained by gently grinding the monoliths ha ve shown that W and V species were well distributed in TiO2 support and tha t the repartition of the W species, very homogeneous in the fresh sample, b ecame somewhat slightly more heterogeneous in the used sample. V species we re not so well dispersed that W species and even, some particles rich in V were observed on the used sample. This may be due to the migration and aggl omeration of some of the V species. More particles, very rich in Si, were a lso observed for the used sample suggesting that the coating of the fibres by the active phase was partly deteriorated during SCR reaction, This obser vation was supported by an AFM analysis which showed a higher surface rough ness for the used sample. It was also observed by high resolution TEM that the first one or two atomi c layers at the surface of all crystallites appear amorphous, while the fur ther layers are well crystallised with the anatase structure. For the used sample this amorphous layer is slightly larger. This is an important featur e for electrical conductivity (mainly at the surface) and catalytic propert ies. (C) 2000 Elsevier Science B.V. All rights reserved.