The Evolution Of Nanoporous Gold Structure And Its Surface Graphene Structure Under CH₄ Pyrolysis

The process of gas-phase catalysis is usually accompanied by the evolution of the catalyst structure and the products' structure at the interface of the two phases.Further revealing the dynamic details of the reaction at the atomic scale is of great significance for understanding the internal mechanism of the catalytic process,exploring and optimizing the structure of the catalyst,improving the performance of the catalytic reaction,and accurately regulating the catalytic reaction products.In this paper,we used the in-situ transmission electron microscope technology with high spatial and temporal resolution to observe the methane pyrolysis process of nanoporous gold(NPG),and explored the dynamic reaction details of NPG as a catalyst substrate and related products on the surface of the substrate.We have obtained the following conclusions through the analysis of experimental data:(1)After pretreatment,the NPG sample exposed a large number of Au(111)crystal planes.During the reaction,the Au(111)crystal plane began to appear shallow trenches with a depth of several atomic layers,and then the shallow trenches expanded in the width and depth directions.Atomic steps appeared on the surface,which eventually evolved into a flat Au(111)crystal surface.Through simulation calculations,the charge distribution of Au atoms and surrounding C atoms showed that the adsorption of C atoms on the gold substrate could weaken the interaction between Au-Au atoms,and promote the exfoliation of Au atoms from the surface layer by C atoms.After the first layer of Au atoms was peeled off,it was easier to remove the Au atoms around the vacancies,and the expansion and deepening of the shallow trenches further occurred macroscopically.Finally,a flat Au(111)crystal plane was formed again.(2)The multilayer graphene grown on the surface of the NPG substrate interacted with amorphous carbon,which was the product of methane cracking.One is that amorphous carbon could have a destructive effect on the graphene.The second is to hinder the catalytic cracking of methane after the catalytic substrate was coated with graphene,reducing the reaction activity of the catalytic substrate,reducing surface atom migration rate and reducing the amount of amorphous carbon generated,since the damage of amorphous carbon to graphene is decreased.The third is that graphene could grow in a wide range of amorphous carbon regions on the surface.The growth of graphene was accompanied by the disappearance of the surrounding amorphous carbon regions.Therefore,the growth of graphene may be transformed by carbon atoms in the amorphous carbon region as raw materials.During the catalysis process,it was also discovered that a large number of H atoms from methane cracking could induce oscillations between adjacent graphene layers containing defects.