Structure of active Ag clusters in Ag zeolites for SCR of NO by propane in the presence of hydrogen

Abstract

The detailed structure of Ag clusters formed in H zeolites during H 2 reduction as well as under NO + C 3H 8 + O 2 + H 2 reaction condition is examined with H 2-TPR, XRD, UV–vis, and Ag K-edge XAFS. H 2-TPR results suggest a quantitative conversion of the exchanged Ag + ion to Ag 2 p p + cluster in Ag-MFI during H 2 reduction in a temperature range from 373 to 573 K. Combined with the UV–vis spectrum of the cluster, exhibiting bands at 255 and 305 nm, and Ag K-edge EXAFS result, the structure of the cluster is identified as Ag 4 2 + . UV–vis and Ag K-edge XANES/EXAFS results reveal that, during NO + C 3H 8 + O 2 + H 2 reaction at 573 K, part of the Ag + ions are converted to Ag n δ + clusters, whose average structure can be close to Ag 4 2 + . The amount of the clusters increases with the Ag/Al ratio of Ag zeolites. The NO reduction rate of Ag-MFI for the NO + C 3H 8 + O 2 reaction is significantly improved by an addition of 0.5% H 2, and the rate increases with Ag/Al ratio of Ag-MFI. This cluster does not form on Ag-MOR under the same conditions, and the NO reduction rate does not increase by H 2 addition. The structure–activity relationship shows that the Ag n δ + , probably the Ag 4 2 + cluster, in Ag-MFI catalysts is responsible for the selective reduction of NO by C 3H 8.