The effect of SiO2/Al2O3 molar ratio in mordenite upon the optical appearance of reduced copper

A series of protonated copper-containing mordenites with variable SiO2/Al2O3 molar ratio (MR) in the range 10⩽MR⩽206 was prepared by ion exchange in copper nitrate aqueous solution. The reduction of copper ions incorporated into the mordenites by hydrogen was shown to lead to different reduced copper species including small metallic particles inter alia. The optical appearance of the copper particles depends strongly on the value of MR, however, in a non-monotonic manner in line with the variation of acidity of this series of mordenites. Copper particles formation in the mordenite with MR=15 is almost non-existent, while an efficient reduction proceeds for MR=10 and 20⩽MR⩽206. The optical appearance of copper particles is different and consists of a contribution of the pronounced plasmon resonance maximum at 550–600 nm for MR=10 and MR=206, while for MR=20 and MR=31, it has the shape of shoulder in the same range. These features are simulated using Mie theory, taking into account the size-dependence of the imaginary part of the dielectric constant and the properties of the host medium. The differences in size and particle localization are considered as the main reasons for the difference of the optical features in the visible range.