Ag–Al2O3 catalysts for lean NOx reduction—Influence of preparation method and reductant

Alumina supported silver catalysts for selective catalytic reduction of NOx with hydrocarbons under excess oxygen (HC-SCR) were prepared according to three different methods. These are impregnation (I) and two different sol–gel routes, via thermal drying (II) or freeze-drying (III). The latter method has not previously been reported for preparation of Ag–Al2O3 catalysts. The prepared samples were characterized by N2-sorption, TEM, XRD and XPS, and the catalytic properties were further investigated in flow reactor experiments. Two different hydrocarbons (propene and n-octane) were used separately as reductants for the SCR reaction. The TEM, XRD and XPS analyses confirmed that the sol–gel samples contain more dispersed silver of higher oxidation state than the corresponding impregnated sample. Further, it was shown that the freeze-dried sol–gel samples most likely contain well dispersed silver throughout the alumina matrix, mainly as oxidized silver. The XPS results also strongly indicate the presence of silver clusters on the surface of samples prepared by routes (I) and (II). The results from the flow reactor study imply that the ratio between silver clusters and oxidized silver species is crucial to achieve high NOx reduction.

Ag–Al2O3 samples were prepared via impregnation (I) and two sol–gel routes, including thermal drying (II) or freeze-drying (III, not previously reported). Characterization confirmed sol–gel samples containing more dispersed silver than corresponding impregnated samples. Silver clusters were indicated in samples by routes (I) and (II). A crucial ratio between oxidized silver species and clusters for high NOx reduction is implied.Figure optionsDownload as PowerPoint slide