Abstract
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.
Highlights
IntroductionCopper-exchanged zeolites have been in the focus of comprehensive studies for decades.[1,2,3] From a material's point of view, they are excellent adsorbents for various nitrogen- and sulfurcontaining compounds and can be used for waste water treatment.[4,5] The catalysis community extensively exploits copper zeolites for the selective catalytic reduction (SCR) of NOx in exhaust gases and for direct decomposition of NO and N2O.6–9 It has been recently shown that a er pre-oxidation these systems can be used as highly efficient stoichiometric oxidants in the selective conversion of methane to methanol.[10,11,12,13,14,15] In the light of these opportunities for application, the understanding of the structure of the copper species present within zeolites is of paramount importance.The high activity of copper-containing zeolites in oxidation and reduction reactions is typically associated with the redox
We report the analysis of EXAFS of CuII-oxo species hosted in a series of oxygen-activated copper-exchanged zeolites with MOR, MFI, BEA, and FAU frameworks with different Si/Al ratios and copper loadings
We show that the contributions from Cu–Al(Si) and Cu–Cu scattering in the second coordination shell can be resolved by wavelet transform of the EXAFS spectrum
Summary
Copper-exchanged zeolites have been in the focus of comprehensive studies for decades.[1,2,3] From a material's point of view, they are excellent adsorbents for various nitrogen- and sulfurcontaining compounds and can be used for waste water treatment.[4,5] The catalysis community extensively exploits copper zeolites for the selective catalytic reduction (SCR) of NOx in exhaust gases and for direct decomposition of NO and N2O.6–9 It has been recently shown that a er pre-oxidation these systems can be used as highly efficient stoichiometric oxidants in the selective conversion of methane to methanol.[10,11,12,13,14,15] In the light of these opportunities for application, the understanding of the structure of the copper species present within zeolites is of paramount importance.The high activity of copper-containing zeolites in oxidation and reduction reactions is typically associated with the redox. Copper-exchanged zeolites have been in the focus of comprehensive studies for decades.[1,2,3] From a material's point of view, they are excellent adsorbents for various nitrogen- and sulfurcontaining compounds and can be used for waste water treatment.[4,5] The catalysis community extensively exploits copper zeolites for the selective catalytic reduction (SCR) of NOx in exhaust gases and for direct decomposition of NO and N2O.6–9. It has been recently shown that a er pre-oxidation these systems can be used as highly efficient stoichiometric oxidants in the selective conversion of methane to methanol.[10,11,12,13,14,15] In the light of these opportunities for application, the understanding of the structure of the copper species present within zeolites is of paramount importance. The high activity of copper-containing zeolites in oxidation and reduction reactions is typically associated with the redox
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