A typical case of nano-sized active site: the NO decomposition on Cu-ZSM-5 catalysts through pairs of Cu(I) species

Nitric oxide (NO), the major component of NOx, is one of the most dangerous air pollutants and many efforts are devoted to find an efficient catalytic way for its abatement. Different reactions are proposed, the most simple one is the direct NO decomposition to give N2 + O2. In the large family of zeolites containing transition metal ions, the Cu-ZSM-5 is attracting much attention because it is unique catalyst able to decompose NO significantly at temperatures between 673 and 773 K. Although it is widely accepted that reduced copper species were responsible for the NO decomposition and that the reaction proceed through a redox cycle, the nature of the active sites is still not completely clear, giving rise to a strong scientific debate on it. We provided some evidences that dimeric Cu(I) species, produced by self-reduction of Cu(II) during the thermal activation of a Cu-ZSM-5 (Si/Al=25) catalyst in He or under vacuum, are the sites where N2 is generated from two adsorbed NO molecules, and from which O2 desorbs from the catalyst at the steady state. Such dimeric Cu(I) species, located on the opposite sides of the ten-membered rings in the straight channels of the Cu-ZSM-5 framework, as suggested by DFT calculations, are able to adsorb irreversibly N2 at low temperature. Interestingly, the amount of irreversible N2 adsorbed at 273 K per total amount of copper as well as the turnover frequency (TOF) of the NO decomposition had both a typical volcano-like shape when they were plotted against the Cu exchange level, i.e. the copper content. Combining these data, a linear relationship was found between the TOF and the amount of irreversible N2 per total copper. The slope of such a linear relationship, i.e. the TOF calculated by taking into account only the copper concentration titrated by the irreversible N2 adsorption, is a constant value regardless the copper loading and the Si/Al ratio of the ZSM-5 framework. By contrast, the Cu-S-1 (MFI framework type without framework Al), Cu-Y and [Al]-MCM41 catalysts are unable to adsorb N2 irreversibly at 273 K and are inactive for the NO decomposition under the same experimental condition as used for Cu-ZSM-5 preparation. These findings, as a whole, strongly suggest that dimeric Cu(I) species are very likely the active sites for NO decomposition on Cu-ZSM-5 catalysts. [1] G. Moretti, G. Ferraris, G. Fierro, M. Lo Jacono, S. Morpurgo and M. Faticanti, J. Catal., 232, 476 (2005)