Spectroscopic observation and ab initio simulation of copper clusters in zeolites

Abstract

Small copper clusters Cu n feasible in zeolites (in the range of sizes 5 < n < 9) were simulated with ab initio quantum chemical calculations, and results were compared with the experimental data on optical absorption of copper-exchanged and reduced zeolites. The short-wavelength absorption features in the spectra of Cu-mordenite, Cu-zeolite-beta, and Cu-ZSM-5 are interpreted as stabilization of few-atomic copper clusters with nuclearity in the range n = 6–8 according to the geometrical factor, while in the range with λ > 500 nm the familiar plasmon resonance band due to copper nanoparticles was observed. A comparison of the data for the three types of zeolites with theoretical results can argue on the occurrence of the clusters stabilized within channels under mild reduction conditions. The most probable candidates to fit the cavities of mordenite, zeolite-beta and ZSM-5 are positively charged clusters Cu 6 +, Cu 7 + and Cu 8 + with point groups C 2v(2), D 5h and C 2v(1), respectively.