hide
Free keywords:
UV–vis–NIR; malachite; rosasite; aurichalcite; Cu/ZnO catalysts
Abstract:
In the present paper we report combined experimental and theoretical studies of the UV-vis-NIR spectra of the mineral
compounds malachite, rosasite, and aurichalcite and of the precursor compounds for Cu/ZnO catalysts. For the copper species in
the minerals the crystal field splitting and the vibronic coupling constants are estimated using the exchange charge model of the
crystal field accounting for the exchange and covalence effects. On this basis the transitions responsible for the formation of the
optical bands arising from the copper centers in minerals are determined and the profiles of the absorption bands corresponding to
these centers are calculated. The profiles of the absorption bands calculated as a sum of bands of their respective Cu species are in
quite good agreement with the experimental data. In agreement with crystal chemical considerations, the Zn ions were found to be
preferentially located on the more regular, i.e., less distorted, octahedral sites in zincian malachite and rosasite, suggesting a high
degree of metal ordering in these phases. This concept also applies for the mineral aurichalcite, but not for synthetic aurichalcite,
which seems to exhibit a lower degree of metal ordering. The catalyst precursor was found to be a mixture of zincian malachite and a
minor amount of aurichalcite. The best fit of the optical spectrum is obtained assuming a mixture of contributions from malachite
(0% Zn) and rosasite (38% Zn of [Zn + Cu]), which is probably due to the intermediate Zn content of the precursor (30%).
