氧化态Co/γ-Al_2O_3催化剂的结构与反应性能研Ⅰ.活性及体相、表相结构表征

Studies on the Structures and Catalytic Properties of Oxidized Co/γ- Al_2O_3 Catalysts Ⅰ. Activities and Characterizations of Surface and Bulk Structures

采用等量浸渍法 ,制得 5 0 0、 6 0 0、 75 0和 95 0℃焙烧的 Co/ γ- Al2 O3催化剂 ;考察了它们对 CO氧化和乙烯选择还原 NO的反应性能 ;用 XRD和 XPS方法表征了催化剂的体相与表相结构 .活性测试结果表明 ,随焙烧温度升高 ,样品对 CO的氧化活性呈下降趋势 ;对乙烯选择还原 NO反应 ,活性先上升 ,而后又有所下降 (当焙烧温度高于 75 0℃时 ) .从硝酸钴制得的样品 ,其氧化活性要高于从醋酸钴制得的样品 ,但对乙烯选择还原 NO,后者的活性更好 .结构表征结果表明 ,催化剂中钴物种的存在形式与原料盐类及焙烧温度密切相关 .在 Co/ γ- Al2 O3催化剂中主要存在两种钴相 ,即 Co3O4和非化学计量的 Cox Al( 8/ 3- 2 x/ 3) O4尖晶石相 ,前者为完全氧化活性中心 ,后者是 NO选择还原的活性中心 .在相同焙烧温度下 ,以醋酸钴为原料时 ,更容易形成 Cox Al( 8/ 3- 2 x/ 3) O4尖晶石相 .随着焙烧温度提高 ,活性组分与载体的相互作用加强 ,Co3O4相逐步向 Cox Al( 8/ 3- 2 x / 3) O4尖晶石相转化 ,这可能是样品氧化活性下降和选择还原活性升高的主要原因 .在更高焙烧温度下 ,随着 Cox Al( 8/ 3- 2 x / 3) O4尖晶石相颗粒度的增加和晶形改变 ,以及钴离子由表相向体相的迁移 ,使样品比表面积下降 ,表面氧空位及活性位减少 ,

Co/γ Al 2O 3 catalysts were prepared by wet impregnation method, and calcined at 500, 600, 750 and 950 ℃ for 2 h, respectively. The results of activity show that with the increase of calcination temperature, the activities for CO oxidation of the samples decrease, while the activities for NO selective reduction by C 2H 4 increase at first, then decrease to some extent. The samples prepared from cobalt nitrate possess higher CO oxidation activities than those from cobalt acetate, however, for NO selective reduction by C 2H 4, the samples from cobalt acetate exhibit higher activity. The results of XRD and XPS show that the form of cobalt species is related to the calcination temperatures and the starting salts. There are two cobalt phases in Co/ γ Al 2O 3 catalysts, that is Co 3O 4, the main active phase for CO oxidation, and spinel phase Co x Al (8/3 2 x /3) O 4, the main active phase for NO selective reduction by C 2H 4. At the same calcination temperature, Co x Al (8/3 2 x /3) O 4 spinel is easier to be formed when acetate is used as starting salt. With the increase of calcination temperature, the interaction between cobalt phase and support becomes stronger, and Co 3O 4 gradually changes to Co x Al (8/3 2 x /3) O 4 spinel. At higher calcination temperature, Co x Al (8/3 2 x /3) O 4 spinel tends to be changed to stoichiometric CoAl 2O 4, and the growth of particle size, the decrease of oxygen vacancies and the migration of Co 2+ ions from surface to bulk result in the decrease of the activities of the samples for NO selective reduction by C 2H 4.