Optical appearance of copper clusters and nanoparticles in zeolites

Abstract

Copper incorporation into zeolites by the ion-exchange from Cu(II) solutions followed by different heat treatments results in a production of a number of species. Redistribution among different sites after dehydration, spontaneous and forced reduction, cluster and particle aggregation, etc. can occur, and a final copper state depends on type of zeolite, SiO2/Al2O3 molar ratio and processing conditions. Various species where observed: copper ions Cu2+ and Cu+, small particles and clusters Cun. We concentrate on the appearance of small copper clusters feasible in zeolites with size of cavities those match the cluster size. The clusters were simulated with ab initio quantum chemical calculations in the range of sizes 5 < n < 10 those are probable within zeolites cavities. Experimental data available on optical absorption of the reduced copper in the three types of zeolites can argue on the occurrence of the clusters stabilized within channels under mild reduction conditions while the larger copper nanoparticles appear under the harder reduction. The model calculation proposes some few-atomic copper clusters (Cun) as the candidates to fit the zeolite cavities with correspondence of the calculated absorption bands with the experimental spectra.