Cu- and Fe-speciation in a composite zeolite catalyst for selective catalytic reduction of NO_x: insights from operando XAS

Abstract

Cu-SAPO-34 (Cu-CZC) and Fe-Mordenite (Fe-MOR) and their mechanical mixture (50:50) have been exhaustively investigated by means of operando X-ray absorption spectroscopy under NH3-SCR conditions. Fe K-edge XANES and EXAFS analysis...

Mapping Vibrational Spectra to the Structures of Copper Species in Zeolites Based on Calculated Stretching Frequencies of Adsorbed Nitrogen and Carbon Monoxides

“…The observed NO-stretching frequencies in the case of dimeric copper-oxo species lie within the range of 1935–1960 cm –1 , with binding energies in the range of 1.50–1.60 eV. Experimentally, the bands with low vibration frequencies (∼1900 cm –1 ) are believed to be due to the copper monomeric species, while the bands at high frequencies starting from ∼1940 cm –1 belong to the oligomeric species interacting with NO. ,− Our theoretical data provided above, in general, match these experimental observations. NO on trimeric species exhibits higher stretching frequencies than dimers and larger copper species. …”

“…With copper reduced from Cu­(II) to Cu­(I), this process requires the participation of two copper atoms to produce one methanol molecule. A number of recent publications implicate the involvement of higher-nuclearity clusters in the process and propose that monomeric copper is inactive for partial methane oxidation. − At the same time, we have recently demonstrated the possibility of self-organization of mononuclear hydroxylated cations toward the activation of methane, in zeolites mordenite and omega. , As can be seen from the plethora of literature abovementioned, zeolite frameworks may stabilize a range of different copper species, making a clear identification of the exact geometrical configurations of the copper species challenging. As a result, suggestions to transform the discussion of a well-defined copper site toward the mixtures of sites or dynamic coexistence of species have emerged. − …”

Understanding C–H activation in light alkanes over Cu-MOR zeolites by coupling advanced spectroscopy and temperature-programmed reduction experiments

“…where A (cm −1 ) is the integrated absorbance dened in a specic spectral region, r (mg cm −2 ) is the pellet density, 3 (cm mmol −1 ) is the molar attenuation coefficient (11.5 cm mmol −1 ) obtained from Deplano et al, 40 and C (mmol g −1 ) is the concentration of CO. 11,18 Then, as expected, during the high-temperature activation step in O 2 , the Cu ions progressively turned into framework-coordinated Cu II species with three/four-fold coordination. 46,66 When cooling from 500 to 120 °C, the Cu II oxidation state is fully preserved. Small modications in the spectral shape are observed and are suggested to stem from temperature-dependent rearrangements in the coordination motif for a small fraction of framework-coordinated Cu ions.…”

Analogous Mechanistic Features of NH3-SCR over Vanadium Oxide and Copper Zeolite Catalysts

“…V- and Cu-based catalysts have been thoroughly investigated for NH 3 -SCR at the molecular level over the past four decades. − Although the mechanism of NH 3 -SCR with these catalysts has been separately discussed, a detailed literature survey suggests mechanistic similarities between V − and Cu − catalysts, as well as common unresolved issues regarding the NH 3 -SCR mechanism. The common features of the NH 3 -SCR mechanisms featuring V and Cu catalysts are depicted in Figure A,B.…”