Na-Rh/γ-Al_2O_3催化剂的表征及其对CO选择氧化的催化性能

Characterization of Na-Rh/γ-Al_2O_3 Catalyst and Its Catalytic Properties for Selective Oxidation of CO

用浸渍法制备了一种高效的Na-Rh/γ-Al2O3催化剂,应用X射线衍射、X射线光电子能谱和N2吸附-脱附等技术手段对催化剂进行了表征,考察了该催化剂的CO选择氧化性能,同时考察了不同预处理条件对催化剂活性的影响.在100℃,GHSV=2.25×104h-1,原料气组成2%CO,1.4%O2和70%H2的条件下,5%Na-0.5%Rh/γ-Al2O3催化剂上CO转化率和CO2选择性分别高达100%和70%.在饱和水蒸气存在时,CO转化率和CO2选择性可保持长达100h不变,说明该催化剂有很好的稳定性和抗水性.研究表明,Na原子可能通过向表面Rh提供电子,增强了Rh向CO的2π*分子轨道反馈电子的能力,使CO分子得到了活化.Na的添加在一定程度上提高了催化剂表面Rh的分散度,使其在高温条件下不易烧结.N2吸附-脱附结果表明,Na的存在虽然在一定程度上减小了催化剂的比表面积,但扩孔作用使催化剂平均孔径有明显的增大.

A novel active catalyst of Na-Rh/γ-Al2O3 was prepared by impregnation and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and N2 adsorption-desorption. The catalytic performance of Rh/ γ-Al2O3 and Na-Rh/γ-Al2O3 catalysts for the selective oxidation of CO in the presence of excess hydrogen was compared, and the effect of pretreatment on the catalytic activity was investigated. The 5% Na-0.5% Rh/γ-Al2O3 catalyst maintains its activity for CO oxidation for more than 100 h in the presence of hydrogen and moisture. The conversion of CO and CO2 selectivity up to 100% and 70%, respectively, were obtained at 100 ℃ , GHSV = 2.25 ×10^4 h^-1, 1.4 % O2, 2 % CO, and 70 % H2. XPS results indicated that the electronic interaction between Na and Rh causes a transfer of electron density from Na to Rh, which increases the back-donation of electrons from the filled d orbitals of Rh to the CO 2π＆＊ antibonding molecular orbitals and results in a weakening of the C=O bond. Highly dispersed Rh was found on the surface of the promoted catalysts. N2 adsorptiondesorption results revealed that the specific surface area decreases after the addition of Na, but the average pore size becomes larger, which provides more spaces for CO oxidation.