Pd/WO3-ZrO2催化剂的结构对乙烯直接氧化制乙酸的影响

利用浸渍法制备了一系列具有不同W/Zr比和不同WO3-ZrO2焙烧温度的1%pd/WO3-ZrO2催化剂.通过X射线衍射、N2吸附、氨程序升温吸附、吡啶吸附红外光谱和程序升温还原方法表征了催化剂的晶体结构、表面状态以及酸性.结果表明,制备条件对W和Pd的表面状态具有很人的影响.随着W/Zr比的增加和焙烧温度的提高,催化剂表而的WOx经历了从聚钨酸物种到聚钨酸/WO3晶体共存再到WO3品体颗粒的转化.W/Zr比为0.2且焙烧湍度为1073K的催化剂具有最多的聚钨酸物种,而该催化剂具有最好的活性.因此,聚钨酸物种的最决定了催化剂的活性.Pd的分散状态只依赖于WO3-ZrO2的焙烧温度.Pd物种分散性好,则乙酸选择性高,而分散性差的人颗粒Pd会导致乙烯的燃烧反应.

Effect of Structure of Pd/WO3-ZrO2 Catalyst on Its Activity for Direct Oxidation of Ethylene to Acetic Acid

A series of 1%Pd/WO3-ZrO2 catalysts with different W/Zr ratios and calcination temperatures of WO3-ZrO2were prepared by an impregnation method. Their crystal structure, surface state, and acidity were determined using X-ray diffraction, N2adsorption, NH3tem-perature-programmed desorption, pyridine infrared spectroscopy, and temperature-programmed reduction. Special attention was paid to the surface states of tungsten and palladium under different preparation conditions. The results revealed that WOxsurface species underwent a transformation from polytungstate species to coexistent polytungstate/crystalline WO3 and further to crystalline WO3particles with increase of W/Zr ratio and calcination temperature. The W/Zr=0.2 sample calcined at 1073 K showed the maximum amount of polytungstates, which were responsible for the excellent activity. Moreover, the state of palladium was only dependent on the calcination temperature. Well-dispersed Pd species were responsible for high selectivity to acetic acid, and large metallic Pd particles were favorable for ethylene combustion.