负载型贵金属催化剂的制备化学－载体等电点对Pd／A1_2O_3催化剂结构与催化性能的影响

本文通过化学修饰的方法改变了γ－Ａｌ_２Ｏ_３载体等电点（ＩＥＰＳ），并用具有不同等电点的Ａｌ_２Ｏ_３作载体，以（ＮＨ_４）_２ＰｄＣｌ４为活性组分前身制备了一系列Ｐｄ／Ａｌ_２Ｏ_３催化剂，在固定床微反色谱装置上考察了其对ＣＯ氧化的催化活性，用Ｈｉ—Ｏ_２滴定、紫外漫反射光谱（ＤＲＳ）、透射电子显微镜（ＴＥＭ）中的能量色散谱（ＥＤＸＳ）等方法对上述催化剂进行了表征，结果表明：Ａｌ_２Ｏ_３的等电点（ＩＥＰＳ）对所得催化剂中金属分散度和浓度分布都有明显影响，ＩＥＰＳ越高。活性组分越靠近外表面，在相同担载量下，其分散度也越高。这主要是由于载体ＩＥＰＳ的改变，导致了（ＮＨ_４）_２ＰｄＣｌ_４在γ－Ａｌ_２Ｏ_３上吸附机理的变化，并使所得催化剂对ＣＯ氧化的催化活性发生了规律性的变化。

Preparation Chemistry of Supported Noble Metal Catalyst the Effect of Isoelectric Point of Support(IEPS) on the Structure and Catalytic Property of Pd/Al_2O_3 Catalysts

?Al_2O_3 with various isoelectric point adjusted by chemical decoration was used as a support, and (NH_4)PdCl_4 as the precursor of active component Pd.A series of Pd/Al_2O_3 catalysts was prepared by ion-exchange method, The IEPS of the support, dispersion and concentration distribution of the noble metal, and the structure of the adsorption intermediate were studied by potentiometer titration, H_2O_2 titration, and energy dispersion of X-ray spectroscopy(EDXS) in TEM as well as UV-Visible diffuse reflectance spoctroscopy(DRS), respectively. The results show that the adsorption mechanism of(NH_4)_2PdCl_4 in aqueous solution on Al_2O_3 was different upon variation of IEPS. It markedly affects the dispersion and concentration distribution of palladium in the Pd/Al2O_3 catalysts. With the in creas of IEPS in the experimental range from 5.98 to 10.07, the Pd shell layer on the catalyst pellet become gradually thinner, and the palladium concentration in the top surface layger of the pellet as well as the palladium dispersion on the catalyst rise correspondingly, the catalytic activity of the catalysts with a constant palladium loading of 0.5 wt. for CO oxidation has also increased.