Reparation, Structure And Hydrogenation Properties Of Highly Dispersed Supported Metal Catalysts

Supported metal catalysts have been widely used in the hydrogenation, dehydrogenation and reforming catalytic reactions because of their high activity, selectivity and stability. The conventional preparation method is solution impregnation method. However, supported metal catalysts prepared by this method have an inhomogeneity in metal distribution over the support, due to the surface tension of the impregnating solution and other solvent effects. Futhermore, the weak interactions between the support and metal ion species lead to the migration and aggregation of metal ions during subsequent calcinations and reduction processes. Thus, the catalysts prepared by this method possess low metal dispersion, active surface and catalytic properties.On the purpose of increasing Pd dispersion, Ag was introduced onto the surface of catalyst during the preparation of Pd catalyst. The obtained Pd-Ag/Al2O3 was then used as a catalyst for the selective hydrogenation of acetylene. The results indicated that the addition of Ag has no significant effect on increasing Pd dispersion but alters the electronic state of Pd supported on the surface of alumina. The formation of Pd-Ag alloy or sosolid strengthened the interaction between Pd atom and H atom and decreased the amount of adsorbed hydrogen. In comparsion of Pd/Al2O3 catalyst, Pd-Ag/Al2O3 exhibited higher ethylene sectivity but lower acetylene conversion in the selective hydrogenation of acetylene.The layered double hydroxides (LDHs) are a group of anionic clays with layered structure. Metal cations are uniformly dispersed within the layers without the formation of'lakes'of like cations. Moreover, the anions in the interlays are uniformly dispersed due to the electrostatic balance of the layer and the electrostatic repulsion among ions. Thus, these materials were used as the precursor to make high dispersed metal catalysts in this work.PdCl42-/MgAl-CO3-LDH crystallites have been synthesized in-situ on the surface of micro-spherical A12O3 using urea as the precipitant. After calcination and reduction, Pd/MgO-Al2O3 catalyst was obtained. SEM and TPR results indicated that Pd/MgO-Al2O3 prepared by in-situ method has higher Pd dispersion, smaller particle size and larger surface area than Pd/Al2O3 prepared by impergantion method. In addition, MgO microcrystallites decreased the surface acidity of alumina support. The obtained Pd/MgO-Al2O3 exhibited higher catalytic activity and selectivity than Pd/Al2O3 prepared by the impregnation method over the temperature range 50-100℃in the selective hydrogenation of actelene.Pd/MgAl-LDHs crystallites were synthesized on the surface of A12O3 by in-situ precipitation-reduction method. The obtained Pd/MgAl-LDHs/Al2O3, with high specific surface area, uniform Pd particle size, high dispersion and more active sites, can be used as a catalyst for the selective hydrogenation of actelene. After calcination and reduction, another catalyst Pd/MgO-Al2O3 was obtained. Due to the existence of LDHs crystallites grown on the surface of A12O3, the congregation of Pd2+ during calcination process was prevented. Thus, the calcinated product Pd/MgO-Al2O3 still has high dispersion and surface area. In addition, the LDHs and MgO microcrystallines decreased the surface acidity of alumina. Pd/MgO-Al2O3 and Pd/MgAl-LDHs-Al2O3 exhibited much higher activity and stability than Pd/Al2O3 prepared by the impregnation method in the selective hydrogenation of actelene.Moreover, micro-sphericalγ-Al2O3 was used as a support for the synthesis of NiAl-CO3-LDHs in the pores of the material. Formation of the LDH resulted from decomposition of urea dissolved in an aqueous solution of Ni2+ impregnated into theγ-Al2O3. After calcination and reduction, Ni/γ-Al2O3 catalyst was obtained. Due to the existence of LDHs and strong interaction between Ni and alumina, the migration and aggregation of Ni species during calcination was suspressed. In comparison of the metal dispersion of Ni catalyst prepared by impregation method, that of Ni/γ-Al2O3 prepared by in-situ method increase by 28%. Thus, Ni/γ-Al2O3 prepared by in-situ method exhibited much higher acitivity and stability than the catalyst obtained by impregation method in the hydrodechlorination of chlorobenzene. In this work, the influence of procress conditions, including reaction temperature, pressure and velocity on the catalytic properties, was also studied in the selective hydrogenation of actelene and hydrodechlorination of chlorobenzene.