Abstract
Catalytic properties of Au-Ag/Al2O3 catalysts, prepared by successive impregnation, have been investigated for the hydrocarbon selective catalytic reduction (HC-SCR) of NOx. The performances of pre-reduced and aged catalysts were evaluated on synthetic reaction mixtures with gas compositions representative of the exhaust of Diesel powered engines. An optimal 2wt.% Ag loading was chosen and the order of introduction during sequential impregnation of Ag or Au on respectively pre-impregnated Au/Al2O3 and Ag/Al2O3 substrates was investigated revealing drastic changes on the catalytic properties after aging. XPS analysis shows a significant silver enrichment irrespective of the preparation route and aging conditions whereas ex situ UV–vis DRS analysis reveals significant electronic disturbances which reflect different interactions between Au and Ag in fresh and aged Au-Ag/Al2O3 and Ag-Au/Al2O3 catalysts. As a consequence, significant rate enhancements in NOx conversion were observed with the superior performance of Au-Ag/Al2O3 but a lower resistance to deactivation at high temperature explained by different behaviour towards redispersion/agglomeration processes highlighted from TEM observations.
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