EPR AND DRS INVESTIGATION OF REDUCTION OF NANOSTRUCTURED COPPER SPECIES IN Cu-ZSM-5 CATALYSTS BY CARBONACEOUS SPECIES

The Cu-ZSM-S is a system of large interest because to this family of materials belong active and selective catalysts for important NOx abatement reactions like the NO decomposition and the selective catalytic reduction (SCR) of NO* by hydrocarbons in the presence of Oz [1]. The reduction of nanostructured Cu2+ species to Cu+ seems to play a key role in the proposed catalytic mechanisms [2]. EPR and Diffuse Reflectance (DRS) spectroscopy, coupled to elemental and thermogravimetric analysis, have been used to investigate some Cu-ZSM-S catalysts (Si/Al = 25 and 80) at different copper loadings. It has been found that at high temperatures (673-773 K) in vacuum, or in inert gas flow, a quasi complete reduction of Cu2+ to Cu+ surprisingly occurs in the as prepared Cu-ZSM-5 samples. Evidence is given that such extensive reduction of Cu2+ species is caused by the carbonaceous deposits (CnH,n) left in the zeolite after destruction of the organic amine template used during the solid synthesis. For a complete removal of these carbonaceous deposits from the as prepared catalysts, prolonged treatments in air (or O2) at high temperature (673-773 K) are necessary. In absence of carbonaceous deposits, the reduction of Cu2+ species can be solely ascribed to a self-reduction process but it involves only a small fractíon of total copper. Our results support the self-reduction lwamoto-Hall mechanism via oxygen evolution from [Cu-O-Cu]2+ species. [1] M. Shelef, Chem. Rev. 95, 209 (1995). [2] G. Moretti, G. Ferraris, G. Fierro, M. Lo Jacono, S. Morpurgo and M. Faticanti, J. Catal. 232, 476 (2005).