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manganese; zirconia; UV–vis spectroscopy; solid solution; crystal field theory
Abstract:
The optical absorption spectra of manganesepromoted
sulfated zirconia, a highly active alkane isomerization
catalyst, were found to be characterized by oxygen-to-manganese
charge-transfer transitions at 300-320 nm and d-d
transitions of manganese ions at 580 and 680 nm. The latter
were attributed toMn4þ and Mn3þ ions, which are known to be
incorporated in the zirconia lattice. The oxygen surroundings of
these ions were modeled assuming a substitutional solid solution.
The crystal field splittings, vibronic coupling constants, and
oscillator strengths of the manganese ions were calculated on the basis of a cluster model that considers the manganese center as a
complex with the adjacent ions of the lattice as ligands. The ratio of Mn3þ to Mn4þ ions was determined using the spectra and the
model, and the relative concentrations of Mn2þ, Mn3þ, and Mn4þ ions were determined with the help of the average valence known
from X-ray absorption data in the literature. The redox behavior of manganese-promoted sulfated zirconia in oxidizing and inert
atmosphere was elucidated at temperatures ranging from 323 to 773 K.
