Revisiting Acidity of SnBEA Catalysts by Combined Application of FTIR Spectroscopy of Different Probe Molecules

The Lewis and Brønsted acid properties of SnBEA zeolites as well as SnO₂ supported on silica BEA have been examined by means of IR spectroscopy of adsorbed pyridine, 2,6-ditertbutylpyridine, and deuterated acetonitrile. Three types of surface sites have been detected and assigned to (i) framework Sn centers possessing Lewis acid properties; (ii) weak Brønsted acid sites associated with framework tin atoms; and (iii) nonframework SnO₂ particles which possess Lewis acidity. The total amount of Lewis sites can be determined using pyridine as a probe molecule, while the type of Lewis sites can be distinguished by FTIR of adsorbed acetonitrile. The band at 2316 cm^–1 is attributed to strong framework sites, whereas the band at 2287 cm^–1 is attributed to nonframework sites. Brønsted acid sites can be characterized using 2,6-ditertbutylpyridine (bands at 3363, 1613, and 1530 cm^–1) and deuterated acetonitrile (band at 2308 cm^–1). The relative amount of Lewis and Brønsted acid sites on SnBEA can be varied by treatment in hydrogen at different temperatures

Structural Evolution of Copper-Oxo Sites in Zeolites upon the Reaction with Methane Investigated by Means of Cu K-edge X-ray Absorption Spectroscopy

The structure of copper sites formedunder an oxidative environmentand their evolution in the course of the reaction with methane atelevated temperature was investigated by means of Cu K-edge X-rayabsorption spectroscopy for a series of copper-containing MFI, MOR,and FAU zeolites. The pretreatment in oxygen at 723 K leads to theformation of copper-(II)-oxo sites, whose nature depends on the frameworktype. Dimeric species are formed in CuMFI material, dimeric and monomericsites coexist in CuMOR, and agglomerated copper-oxo nanoclusters arefound in large-pore copper-containing faujasite (CuFAU). For all studiedmaterials, the reaction with methane resulted in the exclusive formationof copper-(I) species; no formation of metallic copper was detectedeven at 748 K. The nature of formed copper-(I) species is governedby the structure of corresponding copper-(II) centers. In particular,monomeric and dimeric copper-(II)-oxo sites hosted in CuMOR and CuMFIare transformed into isolated copper-(I) cations coordinated to ion-exchangepositions of the zeolite. Contrarily, copper-(II)-oxo clusters presentin CuFAU undergo restructuring with only a partial loss of extra-frameworkoxygen and form aggregated species with a structure similar to thatof bulk copper-(I) oxide.ISSN:1932-7455ISSN:1932-744

Structure of copper sites in zeolites examined by Fourier and wavelet transform analysis of EXAFS

Copper-exchanged zeolites are a class of redox-active materials that find application in the selective catalytic reduction of exhaust gases of diesel vehicles and, more recently, the selective oxidation of methane to methanol. However, the structure of the active copper-oxo species present in zeolites under oxidative environments is still a subject of debate. Herein, we make a comprehensive study of copper species in copper-exchanged zeolites with MOR, MFI, BEA, and FAU frameworks and for different Si/Al ratios and copper loadings using X-ray absorption spectroscopy. Only obtaining high quality EXAFS data, collected at large k-values and measured under cryogenic conditions, in combination with wavelet transform analysis enables the discrimination between the copper-oxo species having different structures. The zeolite topology strongly affects the copper speciation, ranging from monomeric copper species to copper-oxo clusters, hosted in zeolites of different topologies. In contrast, the variation of the Si/Al ratio or copper loading in mordenite does not lead to significant differences in XAS spectra, suggesting that a change, if any, in the structure of copper species in these materials is not distinguishable by EXAFS.ISSN:2041-6520ISSN:2041-653

Site-selective luminescence spectroscopy of bound excitons and local band structure of chlorine intercalated 2H-and 3R-MoS2 polytypes

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Excitonic Luminescence, X-ray Analysis and Local Band Structure of Chlorine Intercalated 2H- and 3R-MoS2 Polytypes

International audienc

Excitonic Luminescence, X-ray Analysis and Local Band Structure of Chlorine Intercalated 2H- and 3R-MoS2 Polytypes

International audienc