Molecular sieve manufacturer explains the properties of metal clusters on molecular sieves
Among the most important industrial applications of molecular sieve catalysts, some have both B-acid centers and noble metal atoms or atomic clusters. For example, these bifunctional molecular sieves serve as catalysts for shape selective hydrogenation, hydroalkylation, hydroisomerization, hydrocracking, catalytic reforming, and CO hydrogenation processes. Among the metals that may be used, platinum, palladium, and rhodium are the most important hydrogenation/dehydrogenation components in bifunctional molecular sieves.
The above metals form nanoscale atomic clusters within the pores of the molecular sieve. The density functional theory (DFT) calculations of electrically neutral, negatively charged, and positively charged Pdn clusters indicate that stable structures are often independent of the charge of the cluster. For clusters with atomic numbers less than 6 (n<6), the stable structures of anionic and neutral clusters follow the basic concept of maximizing the number of coordination atoms. However, cationic clusters tend to have planar structures. For clusters with atomic numbers 7-13, whether charged or neutral, their stable structure is convergent. When the number of atoms is 13, it tends to form a heteromorphic cubic octahedron. In the case of platinum and rhodium, the smallest stable cluster is a 13 atom icosahedron, with one atom at the center and 12 atoms at equivalent positions surrounding the central atom; Or a heteromorphic cubic octahedral structure. When a 13 atom platinum cluster is located in a molecular sieve cage, it can be expected that the interaction between the platinum atom and the cage wall is much weaker than the Pt-Pt bond.
When the size of a metal cluster becomes smaller than the average free travel of conductive electrons or the size of ferromagnetic domains, the collective electronic characteristics, such as conductivity, thermal conductivity, and ferromagnetic susceptibility, change significantly. These behaviors are related to the electron band structure, which varies with cluster size. For example, the energy required to remove an electron from a small cluster is greater than the escape work of bulk metals, but smaller than the ionization potential of metal atoms.
The chemical characteristics corresponding to catalytic performance are less sensitive to cluster size than electronic effects. However, when isolated atoms are on the support or when the size of the metal cluster reaches a limit, drastic changes in catalytic activity must be expected. In catalysis, the metal clusters on the carrier are treated as very small pieces of metal, but highly unsaturated atoms are exposed. The atoms participating in chemical adsorption on macroscopic metal sheets may be coordination unsaturated, but the coordination unsaturated degree of small clusters is significantly higher. Hunan Providence New Materials Co., Ltd is a professional molecular sieve manufacturer
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