In this paper, the possibility of applying graph neural networks (NN) to study the structure of copper centers of zeolites is considered. The dataset used for NN training was prepared using the FDMNES software based on the finite difference method and included more than 2100 Cu K-XANES spectra for Cu-MOR. The performed study demonstrated the capability of graph neural networks to reproduce the Cu K-XANES spectrum corresponding to a particular model of the copper center in the zeolite framework.

Keywords: zeolite, mordenite, atomic structure, XANES, machine learning, graph neural networks

Today, catalysts based on zeolites containing transition metal ions are being intensively developed, ensuring the selectivity of the reaction of direct oxidation of methane to methanol. In current work, the X-ray absorption spectroscopy complimented with the structural models obtained in frames of the density functional theory were used to study the dependence of the local atomic structure of copper centers in mordenite type zeolites synthesized by solid-phase ion exchange using CuCl and H-MOR, upon the annealing temperature at different stages of synthesis. The typical Cu – O interatomic distances and the corresponding coordination numbers are determined. The dependence of the fraction of copper ions on the zeolite on the annealing temperature was established.

Keywords: EXAFS, DFT, zeolites, solid-phase synthesis, methanol synthesis

The X-ray absorption spectroscopy (XAS) is a powerful tool for local atomic structure investigation of materials with different origin including nanocomposites. The extended X-ray absorption fine structure spectroscopy (EXAFS) is very sensitive to the details of local environment of X-ray photon absorbing atom. In order to perform quantitative study of EXAFS data the preliminary calculations of scattering phases and amplitudes of photo-electron in the crystal are required. The quality of this calculation highly affects on precision of such results, such as inter-atomic distances and coordination numbers. An existing methods use iterative self-consisted approach for the estimation of interatomic potential which could be neglected using fixed potential value in interatomic region (“muffin-tin”-zero). The Hedin-Lundqvist exchange potential is commonly used. The paper presents the results of the development and testing of computer code HFampl for the first-principle calculation of scattering phases and amplitudes within “muffin-tin” approximation using exact Hartree-Fock (HF) exchange potential. The development using GNU compilers collection has allowed an execution of the codes in any compatible operating systems (Windows, Linux, Mac). The atomic configurations were prepared for the most elements of the periodic table. The program tests performed on metals, oxides and iron-containing aqueous solutions demonstrated good applicability of the phases and amplitudes for the precise structural analysis, which allows accuracy up to 1% in interatomic distance calculation. Such accuracy in distances determination is as good as accuracy of common program packages. But values of other parameters (such as reduction factor and Debye-Waller parameter) are more realistic in case of application scattering phases and amplitudes obtained with HFampl code.

Keywords: Photoelectron Scattering Phases And Amplitudes For The Determination Of Atomic Structure By EXAFS Spectroscopy

The technique of determination of local atomic structure in the vicinity of absorbing atom in the case of the presence of strong correlation between fitting parameters is proposed. This technique is applied to three test signals simulating the Pt L3-edge EXAFS spectra of Pt/Ag bimetallic nanoparticles. The test results demonstrates the improvements in precision of determined parameters comparing with common technique.

Keywords: Extented X-ray Absorbtion Fine Structure, EXAFS, multiparameter curve fitting, correlation between fitting parameters, bimetallic nanoparticles, local atomic structures, CUDA

The atomic structure of bimetallic nanoparticles of Pt-Ag composition, which are the part of metal-carbon electro-catalysts PtAg/C, is studied by means of EXAFS spectroscopy using the suggested technique. These materials have been prepared by methods of consistent and simultaneous deposition of Ag and Pt atoms from aqueous ethylene glycol solutions of their salts with subsequent acid treatment. The values ​​of the structural parameters of the near environment of platinum atoms are determined. The nature of the distribution of components in nanoparticles is established. Cluster models of nanoparticles are constructed. The fraction of Pt atoms in the working shell and its dependance on preparation methods in each case are determined.

Keywords: Metal-carbon electrocatalysts, bimetallic nanoparticles, Catalysis, Platinum, X-ray absorption spectroscopy, atomic structure