Synthsis Of Co-MOFs-Derived Hollow Catalysts For The Transformation Of CO

Metal-organic frameworks?MOFs?are a new kind of porous materials which have gained great attention in recent years,due to their outstanding structural properties such as ultrahigh porosities and surface areas.The MOFs-derived materials could partly reserve the structural properties from their precursors and show high stability,exhibiting great potential for practical applications.In this thesis,a series of MOF-derived catalysts for the oxidation and hydrogenation of CO with novel hollow hierarchical structure were successfully prepared with ZIF-67 as both the source of Co element and template,by designing the calcination process and using the calcination atmosphere to modulate the structures of materials.The structure-activity relationship between the hollow-hierarchical structure and the catalytic performance in CO conversions were systemically investigated.The main contents and experimental results of this thesis are as follows:A composite material with hollow-hierarchical Co₃O₄/C?denoted as H-Co₃O₄@H-C?was prepared by a reduction-oxidation method using ZIF-67 as template.H-Co3O4@H-C exhibited higher activity for oxidation of carbon monoxide than other catalysts with only a single hollow structure.A complete conversion of carbon monoxide was achieved at 130°C.It was found that the large cavity in the H-Co₃O₄@H-C could optimize the diffusion of the reaction gas and reduce the internal diffusion resistance.The hollow Co₃O₄ could expose more active sites and capture more adsorption oxygen so that accelerate the reaction by generating more oxygen vacancies.The combination of the two kinds of hollow structures allowed the hollow-hierarchical H-Co3O4@H-C to afford comparable catalytic activity to those of noble-metal catalysts even using a higher space velocity(the full completed temperature of CO at 200000 mL h^–1 g^–1_cat.was 190°C).Furthermore,the H-Co₃O₄@H-C catalyst exhibited excellent stability under the investigated conditions.A reduction step was further introduced after the reduction-oxidation processes to realize the conversion of the hollow-hierarchical H-Co₃O₄@H-C material into hollow-hierarchical H-Co@H-C.The unique hollow-hierarchical structure allowed the catalyst to exhibit about twice of the catalytic activity and about twice of the selectivity to the C₅⁺products as compared with the solid materials in the FTS reaction.Among them,the selectivity of the C₅⁺products is up to 73%.Compared with the catalytic activity of a single hollow catalyst,it was found that the large cavity structure in the FTS reaction could improve the selectivity of the C₅⁺products.The small cavities could not only expose more active sites of the Co nanoparticles and increase the catalytic activity of the reaction,but also produce more primary products per unit time due to the increased reactivity,which could increase the collision probability of the feed gas and the primary products so that further increase the selectivity of the C₅⁺products.These results proved that the hollow-hierarchical structure had self-synergistic effects in the hydrogenation of carbon monoxide.