A TEM and UV–visible study of silver reduction by ethanol in Ag–alumina catalysts

A transmission electron microscopy (TEM) study dedicated to the understanding of dispersion and location of Ag nanospecies in silver–alumina catalysts is reported. A special attention is put on the conditions of powder catalyst deposition on the grids for TEM analysis. When the deposition is conducted through conventional suspension in ethanol (EtOH) then drying, numerous Ag nanoparticles with sizes between 2 and 10 nm are observed in the micrographs of calcined Ag/Al2O3. This drastically contrasts with the absence of particle after dry deposition, silver being then well dispersed all over the alumina support, as can be seen by combined STEM/EDS mapping. These distinct observations are straightforwardly attributable to the reducing effect of ethanol towards Ag species that is also identified by UV–visible spectroscopy. The results highlight the importance, for correctness of TEM interpretations, of avoiding any contact between an alcohol solvent and Ag–oxide catalysts in the course of TEM studies, in contrast to experimental conditions that are still often applied nowadays. Finally, the reducing effect of ethanol on silver active sites that is likely to occur during SCR (selective catalytic reduction) of NOx in the presence of EtOH is envisaged and discussed as well.

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► State and dispersion of silver supported on alumina catalysts.
► Reduction of the highly dispersed cationic silver species by ethanol.
► Active sites transformation in the course of reaction (SCR of NOx).
► Importance of the conditions of grids preparation for transmission electron microscopy (TEM) study of metal supported catalysts.