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Dynamic CuII/CuI speciation in Cu-CHA catalysts by in situ Diffuse Reflectance UV–vis-NIR spectroscopy.

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Title: Dynamic CuII/CuI speciation in Cu-CHA catalysts by in situ Diffuse Reflectance UV–vis-NIR spectroscopy.
Authors: Negri, Chiara1 (AUTHOR), Signorile, Matteo1 (AUTHOR), Porcaro, Natale G.1 (AUTHOR), Borfecchia, Elisa1 (AUTHOR), Berlier, Gloria1 (AUTHOR) gloria.berlier@unito.it, Janssens, Ton V.W.2 (AUTHOR), Bordiga, Silvia1 (AUTHOR) silvia.bordiga@unito.it
Source: Applied Catalysis A: General. May2019, Vol. 578, p1-9. 9p.
Subjects: Reflectance spectroscopy, Chemical speciation, Charge transfer, Oxidation states, High temperatures, Silicon isotopes
Abstract: Evolution of copper environment in Cu-CHA (Si/Al = 15 and Cu/Al = 0.5) followed by in situ DRS-UV–vis-NIR spectroscopy. • LMCT transitions of hydrated CuII ions in Cu-CHA are strongly affected by Al distribution. • Z[CuII(OH)] and Z 2 CuII sites are characterized by different d-d transitions. • DRS-UV-Vis alone is not able to discern among different Cu-oxo cores formed in O 2 at high temperature. • Resonant Raman shows that [CuII-(μ-O)-CuII]2+ cores are formed irrespective of the starting Cu oxidation state. • [CuII- trans -(μ-1,2-O-O)-CuII]2+ peroxo are favored during the O 2 -dehydration process. Copper exchanged CHA zeolites were investigated by in situ Diffuse Reflectance UV–vis-NIR spectroscopy in order to study the evolution of the copper environment in respect to the sample composition (2 samples differing in Si/Al and Cu content) and the activation procedure (in O 2 or N 2 from room temperature to 400 °C). In situ measurements allowed showing the different behavior of the two samples, both in the d-d and Ligand to Metal Charge Transfer regions, confirming a strong effect of the lattice composition to determine the copper environment and its reactivity. One of the two sample (Si/Al = 15 and Cu/Al = 0.5) was further investigated, in order to clarify the nature of the Cu-oxo species, formed along activation in oxygen. To shed light on this point, in situ measurements starting from a reduced form of the sample, obtained by activation in inert atmosphere at high temperature, were also considered. Additional suggestions on the nature of these species were obtained by resonant Raman measurements conducted in controlled atmosphere. [ABSTRACT FROM AUTHOR]
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Abstract:Evolution of copper environment in Cu-CHA (Si/Al = 15 and Cu/Al = 0.5) followed by in situ DRS-UV–vis-NIR spectroscopy. • LMCT transitions of hydrated CuII ions in Cu-CHA are strongly affected by Al distribution. • Z[CuII(OH)] and Z 2 CuII sites are characterized by different d-d transitions. • DRS-UV-Vis alone is not able to discern among different Cu-oxo cores formed in O 2 at high temperature. • Resonant Raman shows that [CuII-(μ-O)-CuII]2+ cores are formed irrespective of the starting Cu oxidation state. • [CuII- trans -(μ-1,2-O-O)-CuII]2+ peroxo are favored during the O 2 -dehydration process. Copper exchanged CHA zeolites were investigated by in situ Diffuse Reflectance UV–vis-NIR spectroscopy in order to study the evolution of the copper environment in respect to the sample composition (2 samples differing in Si/Al and Cu content) and the activation procedure (in O 2 or N 2 from room temperature to 400 °C). In situ measurements allowed showing the different behavior of the two samples, both in the d-d and Ligand to Metal Charge Transfer regions, confirming a strong effect of the lattice composition to determine the copper environment and its reactivity. One of the two sample (Si/Al = 15 and Cu/Al = 0.5) was further investigated, in order to clarify the nature of the Cu-oxo species, formed along activation in oxygen. To shed light on this point, in situ measurements starting from a reduced form of the sample, obtained by activation in inert atmosphere at high temperature, were also considered. Additional suggestions on the nature of these species were obtained by resonant Raman measurements conducted in controlled atmosphere. [ABSTRACT FROM AUTHOR]
ISSN:0926860X
DOI:10.1016/j.apcata.2019.03.018