On the origin of the changes in color of Ag/Al2O3 catalysts during storage - Sorbonne Université Accéder directement au contenu
Article Dans Une Revue Research on Chemical Intermediates Année : 2019

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

Résumé

Despite the large number of catalytic studies involving Ag/Al2O3 catalysts, especially in deNOx reactions, the phenomenon of ageing 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 to 4 wt% Ag), and secondly the gradual changes of color to grey/black and the associated UVvisible 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 physico-chemical changes in the silver species during ageing were explained thanks to a comprehensive characterization study of the Ag/Al2O3 catalysts after calcination and after ageing. 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 Agn 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 ageing 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 physico-chemical phenomena that can influence the color of various silver-containing materials, such as Ag0 particle size, aggregation/redispersion and surface alteration. Consequently, the ageing process of the calcined Ag/Al2O3 catalysts, associated to sample darkening and to 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), followed by the growth of Ag0 particles, It is proposed that the Agn clusters may act as nucleation sites for the formation of larger Ag0 particles and that the formation of aggregates is favored by an easy migration of Ag on alumina.

Domaines

Chimie
Fichier principal
Vignette du fichier
Chaieb et al. - 2019 - On the origin of the changes in color of AgAl2O3 .pdf (3.15 Mo) Télécharger le fichier
Origine : Fichiers produits par l'(les) auteur(s)
Loading...

Dates et versions

hal-02407541 , version 1 (12-12-2019)

Identifiants

Citer

Tesnim Chaieb, Dalil Brouri, Sandra Casale, Jean-Marc Krafft, Tiago da Silva, et al.. On the origin of the changes in color of Ag/Al2O3 catalysts during storage. Research on Chemical Intermediates, 2019, 45 (12), pp.5877 - 5905. ⟨10.1007/s11164-019-04007-8⟩. ⟨hal-02407541⟩
89 Consultations
219 Téléchargements

Altmetric

Partager

Gmail Facebook X LinkedIn More