Ag Clusters as Active Species for HC-SCR Over Ag-Zeolites

Abstract

The addition of hydrogen in the reaction atmosphere is effective in promoting the activity of Ag/alumina and Ag-zeolites on the selective reduction of NO by hydrocarbons (HC-SCR) at low temperatures. The increment of NO conversion over Ag-MFI corresponds to the periodic addition of hydrogen into C 3 H 8 -SCR conditions. The UV-VIS spectra of Ag-MFI have revealed that the addition of hydrogen results in the formation of Ag n d+ clusters due to partial reduction and agglomeration of Ag species . The coincidence of the formation of the Ag n d+ clusters and the increment of NO conversion suggests that Ag n d+ clusters are the highly active species for HC-SCR. From analysis by H 2 -TPR, UV-VIS, and EXAFS, the structure of Ag n d+ clusters on Ag-MFI is identified as being Ag 4 2+ on average. The formation of Ag clusters was strongly affected by the type of zeolites: The major Ag species are Ag + ions for MOR, Ag n d+ clusters for MFI and BEA, and relatively large metallic Ag m particles for Y. The sequence of Ag agglomeration (MOR < MFI < BEA < Y) is in accordance with the strength of the acid sites of zeolites. It can be expected that the interaction between the positive charge of Ag n d+ clusters and acid sites, i.e., the ion-exchange site of zeolites, stabilizes Ag n d+ clusters. The type of Ag species under HC-SCR conditions depends on the concentration of gas-phase oxidants (NO, O 2 ) and reductants (H 2 , HC), and also on the number and strength of the zeolite acid sites.

Identification of the silver state in the framework of Ag-containing zeolite by XRD, FTIR, photoluminescence,109Ag NMR, EPR, DR UV-vis, TEM and XPS investigations

“…8), in line with earlier reports [41][42][43][44][45][46][47][48]. The band at 245 nm for as prepared Ag 6.0 SiBEA is more pronounced than that for as prepared Ag 3.0 SiBEA.…”

“…8) and EPR signal of CAg 6.0 SiBEA at g = 2.00159 and Red-C-Ag 6.0 SiBEA at g = 2.00155 characteristic for silver nanoparticles (Fig. 7) [42,45,47]. The metallic nanoparticles are also formed in Red-CAg 3.0 SiBEA as shown by the DR UV-vis band at 375 nm ( Fig.…”

Functional Ag‐Exchanged Zeolites as Biocide Agents

“…This makes easier the silver liberation to the media, improving the bactericidal properties of the FAU zeolite. Regarding the Ag speciation, different silver species have been described in diverse zeolites; i) well‐dispersed silver oxides on the surface of the zeolite; ii) isolated and non‐isolated monovalent silver cations (Ag I ) placed in non‐framework positions as compensating cations; and iii) cationic silver clusters . The zeolite with the highest biocidal effect is the sample FAU‐2.4, which possesses a Si/Al ratio of 2.4.…”

Simultaneous selective catalytic reduction (SCR) of NOx and N2O over Ag/ZSM-5 – Catalytic studies and mechanistic implications

“…H 2 -TPR was applied to determine the reduction behaviour of the catalyst samples, specifically the behaviour of their metal component. As was previously shown by Shibata [16,21] silver ions incorporated into zeolite matrices are reduced in two steps: Ag + Ag dþ n Ag 0 . Accordingly, at least a bimodal TPR profile consisting of a low temperature (LT) signal for the reduction of Ag + to ionic silver clusters and a high temperature (HT) signal for their further reduction to metallic silver is meant to be observed.…”

“…In located at ion exchange positions of the zeolite network [17][18][19][20], while the signal at 270 nm is caused by silver clusters, assumingly with an ionic character [21]. The intensity of the aforementioned absorption bands increases when the amount of incorporated silver is raised.…”