On the origin of the changes in color of Ag/Al2O3 catalysts during storage

摘要

Despite the large number of catalytic studies involving Ag/Al2O3 catalysts, especially in deNO(x) reactions, the phenomenon of aging observed during catalysts storage after preparation by impregnation followed by a calcination treatment has not been reported earlier and rises questions. The present paper highlights firstly the colors of the calcined Ag/Al2O3 catalysts (white to yellowish) and the associated UV-visible spectra as a function of the Ag loading (0.5-4 wt% Ag) and secondly the gradual changes of color to gray/black and the associated UV-visible spectra when the samples are stored in ambient air or in vacuum in a desiccator in dark. These changes were found to be reversible when the samples are re-calcined. The physicochemical changes in the silver species during aging were explained thanks to a comprehensive characterization study of the Ag/Al2O3 catalysts after calcination and after aging. Several techniques such as UV-visible spectroscopy, XRD, electron microscopy, XAS and photoluminescence were used. After calcination, in addition to highly dispersed Ag+ species on alumina, the presence of Ag-n clusters of size smaller than 1 nm was found to be responsible for the yellowish color of the Ag/Al2O3 catalysts with Ag loadings higher than 2 wt% Ag and for the associated plasmon band at 350 nm. The aging process was explained based on characterizations, coupled to a close examination of the literature data on the mechanisms of reduction of Ag+ species and on physicochemical phenomena that can influence the color of various silver-containing materials, such as Ag-0 particle size, aggregation/redispersion and surface alteration. Consequently, the aging process of the calcined Ag/Al2O3 catalysts, associated with sample darkening and with Ag plasmon band shift and broadening in the entire visible range, was proposed to result from the modification of the nature of the supported Ag phase: the reduction of Ag+ species (by auto-reduction and/or photoreduction), fo