Abstract
The zeolite topology directs the formation of different silver species with catalytic implications for the NH3-SCO reaction.
Highlights
IntroductionWithin the pores and cavities of microporous materials, as demonstrated for Pt, Pd, Au, Ag, and metal alloys.[3,4,5,6,7,8,9,10] The stabilization of metal clusters in zeolite hosts has been used to exploit their unusual structural, physical, electronic, magnetic, optical and biocidal properties, motivating several studies on their growth and stability under different conditions.[11,12,13,14] The diffusion and coalescence of metal moieties depend on the heating temperature and the gases present, and may occur under reaction conditions provoking catalyst deactivation
The development of supported sub-nanometric metal moieties, mainly clusters of few atoms and isolated atomic species, has been the focus of many studies as they can be advantageous from different points of view.[1]
The diffraction peaks of the Ag0 metal are considerably less intense for the AgRHO(4)R when compared with the AgCHA(4)-R with similar silver loading, indicating that at least in sample AgRHO(4)-R, there must be smaller NPs or other species such as metal clusters, not observable by X-ray diffraction (XRD)
Summary
Within the pores and cavities of microporous materials, as demonstrated for Pt, Pd, Au, Ag, and metal alloys.[3,4,5,6,7,8,9,10] The stabilization of metal clusters in zeolite hosts has been used to exploit their unusual structural, physical, electronic, magnetic, optical and biocidal properties, motivating several studies on their growth and stability under different conditions.[11,12,13,14] The diffusion and coalescence of metal moieties depend on the heating temperature and the gases present, and may occur under reaction conditions provoking catalyst deactivation. The catalysts must be optimized to enhance N2 selectivity In this context, determining the silver species can shed light on the active and selective sites and the reaction pathway, which is assumed to follow the internal SCR (i-SCR) mechanism.[19,29,30] In general, the activity of Ag-supported catalysts increases with the metal dispersion, while the selectivity to N2 is higher on larger silver particles.[31,32] A previous study of our group indicates that Ag+ sites in Ag-zeolites are practically inactive for the NH3-SCO reaction.[33] there are still controversies about the role played by charged (Agmd+) and neutral (Agn0) silver clusters, which are usually present in supported Ag catalysts. The results reported here prove that besides the activation treatment and the catalysts' chemical composition, the framework topology plays a key role in the formation of silver species and in determining their redox properties and catalytic performance in the NH3-SCO reaction. The reduced zeolites were treated in a diluted O2 ow (100 mL minÀ1, 10% O2/He) at 400 C for 40 min at a heating rate of 10 C minÀ1 and named AgCHA(2)-R-Ox, AgCHA(4)-R-Ox and AgRHO(4)-R-Ox
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