Formation of copper nanoparticles in LTL nanosized zeolite: spectroscopic characterization

Abstract

The state of copper species stabilized in nanosized LTL zeolite subjected to various post-synthesis treatments was unveiled by a range of spectroscopic techniques. FTIR and UV-Vis studies demonstrated that the reduction process of copper in the LTL nanosized zeolite leads to the formation of different species including Cu²⁺, Cu⁺ and Cu nanoparticles (Cu NPs). The adsorption of probe molecules (NO and CO) was used to selectively monitor the copper species in the LTL nanosized zeolite upon oxidation and reduction post-synthesis treatments. Both the Cu²⁺ and Cu⁺ species were probed by NO and CO, respectively. The amount of Cu⁺ in the LTL zeolite nanocrystals was about 43% as determined by FTIR, while the amount of Cu NPs was about 55% determined by the UV-Vis spectroscopic characterization. These results were complemented by EPR, ²⁹Si and ⁶³Cu MAS NMR spectroscopic data. The EPR spectroscopy was further applied to monitor the effective reduction of the Cu²⁺ species and their re-oxidation, while the ⁶³Cu MAS NMR verified the presence of Cu NPs in the LTL nanosized zeolite crystals.