Characterisation of the acidity of well defined Cu-ZSM-5 catalysts using pyridine as a probe molecule

Abstract

The acidic sites in Cu/ZSM-5 catalysts active for the reduction of nitrogen oxides under lean conditions have been investigated by examination of the IR spectrum of adsorbed pyridine: proton and sodium forms of the zeolite have also been studied to provide reference results. As expected, H-ZSM-5 exhibited both Lewis and Brønsted acidity, with the Brønsted acid sites being both stronger and more numerous. Na-ZSM-5 contained only Lewis acidity, represented almost entirely by the sodium cations. Exchange of copper into H-ZSM-5 led to a large increase in the concentration of Lewis acid sites, largely represented by isolated Cu¹ cations; the concentration of Brønsted acid sites decreased by about an order of magnitude. The acid properties of the catalyst prepared using the sodium form of the zeolite were very similar to the Cu/H-ZSM-5 sample, the exchange process having introduced some Brønsted acid sites. The catalytic properties of these two materials were also very similar. The results are discussed in relation to the mechanism of reduction of nitrogen oxides. Heating either Cu-ZSM-5 sample to ca. 570 K caused the decomposition of some adsorbed pyridine and the formation of adsorbed ammonia, probably by the Hoffman degradation reaction.