Atomic Structure of Cu Centers in Mordenite Formed by Interaction of Copper Chloride with H-MOR Zeolite and Temperature Treatment

Abstract

The effect of temperature on the formation of copper centers in Cu-mordenite (Cu-MOR), obtained by solid-state ion exchange between copper chloride and zeolite H-mordenite, was studied by combining Density Functional Theory (DFT) simulations of the local atomic structures of copper with the analysis of Cu K-edge XANES and EXAFS, measured “in situ” from room temperature to 400 °C. Cu K edge XANES and EXAFS spectra have a different level of sensitivity regarding the detection of the angular and radial distribution of atoms in the closest vicinity of copper. Based on this, a simultaneous theoretical description of XANES and Fourier transforms F(R) of EXAFS in the extended range of interatomic distances R (up to ∼6 Å) was used as an efficient filter for selecting suitable atomic models of copper species from the number of models given by DFT. It was revealed that, between RT and ∼200 °C, the local structure around copper in Cu-MOR did not change and was similar to that of bulk CuCl. By increasing the temperature to 300 °C and then to 400 °C, changes in both Cu K-edge XANES and EXAFS were observed, indicating the reconstruction of the local structure of copper. At 300 °C, copper-containing fragments, with a CuCl-like structure, decompose to form two different types of copper centers in the vicinity of the eight-member rings of the zeolite framework. The first type contains one copper atom, a monomer, coordinated to three O atoms of the framework as first neighbors and an O, an Al, and two Si atoms as the next neighbors. The second type contains two Cu atoms, which form a dimer, in which each Cu atom has two O atoms and one Cl atom as the first neighbors and one Al and two Si atoms as the next neighbors. At 400 °C, the Cu–Cu interaction in all the structural models, which describe the Cu K-edge XAS spectra, led to the assumption that the formation of dicopper centers dominated. Furthermore, the simultaneous description of both Cu XANES and EXAFS suggested the necessity of doing the structural analysis based on a single dicopper model of the Cu center and to also consider linear combinations of other such models. The suitable linear combination was found based on the three most plausible structural models of the Cu environment containing a Cu–Cl–Cu chain, in which each copper atom is characterized by a very similar pair radial distribution function of the neighboring atoms. However, the angular distributions of these neighboring atoms are different in all three Cu centers due to the difference in the nonequivalent crystallographic sites of the Al atoms, near which the Cu centers formed in the zeolite framework. In Cu-MOR at 400 °C another type of Cu center, containing a Cu–O–Cu dimer, can exist simultaneously with the above-mentioned Cu centers with Cu–Cl–Cu dimers.

Recent Progress of Diatomic Catalysts: General Design Fundamentals and Diversified Catalytic Applications

“…The atomic structure of the copper dimer was further resolved by combining the EXAFS analysis with the atomic model based on DFT calculations. [45,170] Comparing the Cu K-edge EXAFS spectra of Cu 2 @ C 3 N 4 , Cu foil, Cu 2 O, CuO, and Cu-TPP (see Figure 43c), the Cu 2 @C 3 N 4 catalyst exhibited the first shell scattering at R space 1.62 Å, which was similar to that of Cu-TPP (1.59 Å) and CuO (1.55 Å), but much different from that of Cu 2 O (1.50 Å) and Cu foil (2.30 Å). [171] Based on the above results, it could be speculated that the main peak at 1.62 Å was attributed to CuN or CuO coordination (see Figure 43d-f).…”