Estabilização e migração de elementos metálicos suportados em magadiita

Abstract

The concept of single-atom catalysts originated in nanoscience, where the refinement of the synthesis techniques of these devices reduce the dimensions of the active sites to a single atom dispersed on the surface of the support material. In addition, the catalytic activity is critically related to the dimensions of the sites, whose order of magnitude is on the nanometer scale. The behavior of many materials at this scale, is different from that presented when the structure becomes larger. In this context, magadiite, a lamellar silicate widely used in adsorption processes, ion exchange and heterogeneous catalysis, becomes interesting as a possible support in single atom catalysts. Therefore, this work aimed to simulate single-atom models of noble metals supported on acid magadiite surfaces, called here M/H-Magadiite (M = Ag, Au, Pt, Pd), to evaluate their migration and stabilization capacity in different sites and their respective catalytic potentials. Ab initio computational calculations were performed using the QUANTUM-ESPRESSO package. The calculations were based on density functional theory (DFT), and uses a basis set given by plane waves and periodic boundary conditions. The exchange and correlation functional was of the GGA-PBE type, and pseudopotentials of the Vanderbilt Ultrasoft type. The vacuum, the shear kinetic energy, and k-point sampling were 19 Å, 55 Ry, and 2x2x1, respectively. Metal elements were analyzed at different at the deposition sites in H-Magadiite. An energy survey identified two sites for the Au/H-Magadiite and Ag/H-Magadiite systems: on the surface and in the cavity, with energy difference of 36.08 and 38.54 kcal mol−1 respectively. On the other hand, three sites (cavity, surface, and edge) were observed for Pd/H-Magadiite and Pt/H-Magadiite, and energy variation of 9.10 and 15.15 kcal mol−1 . As for the reaction path calculation, the migration energy barrier of the Ag/H-Magadiite, Au/H-Magadiite, and Pd/H-Magadiite models were less than 5.00 kcal mol−1 , giving indications of metal sintering. The Pt/H-magadiite cavity structures, however, showed a migration barrier of 51.00 kcal mol−1 , becoming the most promising among the models studied.