In Situ Characterization Of Ligament Fracture In Nanoporous Gold During CH₄ Pyrolysis

During the participation of nanoporous metals as catalysts in gas-phase catalytic reactions,the surface atoms change more drastically compared to the internal ones due to the first contact between the reacting gas and the surface interface atoms,resulting in the structural reorganization of the structure and causing the evolution of the catalyst structure.This work observes the ligament fracture behavior of nanoporous gold（NPG）as a catalyst in the pyrolysis of methane（CH₄）using high spatial and temporal resolution in situ gas phase electron microscopy experiments.which can further elucidate the ligament morphology and microstructure evolution mechanism of nanoporous metal catalysts under the working conditions.We obtained the following conclusions by analyzing the experimental phenomena.（1）NPG with regular facets catalyzes the reaction of CH₄ gas pyrolysis,firstly,the atoms on the surface of NPG ligament migrate from the high-energy（200）crystalline surface to the more stable（111）crystalline surface due to the adsorption of CH₄ gas on the surface of NPG ligament,and then the NPG ligament gradually becomes thinner due to the continuous migration of surface atoms as the reaction continues,and finally the NPG ligament undergoes fracture.By observing the transient changes after ligament fracture,there is a phenomenon of liquid-like rebound at the moment of ligament fracture due to the tip surface pressure at both ends of the fracture.（2）During the reaction of catalytic pyrolysis of CH₄ gas,the NPG ligaments with different curvatures exhibited different structural evolution processes.In situ observations show that during the reaction process,the surface of the NPG ligament first evolves from an energy-unstable irregular high-index curved crystal surface to a lower energy regular（111）and（200）crystal surface,and in the subsequent process,the two reconfigured crystal surfaces with different coordination numbers of atoms selectively evolve to satisfy the energy minimum principle.The rate of atomic migration on both sides is different until the ligament breaks.Therefore,the change of interatomic binding energy at the surface is the main energy source for ligament rupture.