氧化态Co/γ-Al_2O_3催化剂的结构与反应性能研究.钴物种微观结构的XAFS表征

Studies on the Structures and Catalytic Properties of Oxidized Co/γ-Al_2O_3 Catalysts Ⅱ. XAFS Characterizations of the Micro-Structures of Co Species

采用 X-射线吸收近边结构 ( XANES)和扩展 X-射线吸收精细结构 ( EXAFS)技术 ,对用不同原料盐和不同焙烧温度制得的 Co/γ- Al2 O3催化剂中钴的微观结构进行了详细的表征 .XANES结果表明 ,以硝酸盐为原料盐于50 0℃焙烧的样品 Co( N) - 50 0 ,其 Co- K边的近边结构与标样 Co3O4 相似 ,而其它样品的近边结构则与标样Co Al2 O4 相似 .随焙烧温度提高 ,在吸收边前的弱吸收峰 ( 1 s→ 3d)逐渐增强 ,在吸收阈值处 ,主吸收峰 ( 1 s→ 4p)的分裂变得更明显 .950℃焙烧的样品 ,在主吸收峰上升过程中出现了肩峰 ( 1 s→ 4s) .这些特征表明 ,样品中钴主要以 Co2 + 离子形式存在 ,钴离子与载体作用的加强 ,使非化学计量的尖晶石相在结构上与化学计量的 Co Al2 O4 越来越接近 .EXAFS结果表明 ,样品 Co( N) - 50 0中 ,钴主要以 Co3O4 的形式存在 .其第一配位壳层 Co- O配位数明显低于标样 Co3O4 ,说明该相具有较高的分散性 .其它所有样品中 ,钴主要以非化学计量的尖晶石相存在 ,其第一配位壳层 Co- O配位数随焙烧温度从 50 0℃提高至 950℃ ,与标样 Co Al2 O4 越来越接近 ;相同焙烧温度下 ,从醋酸钴制得的 Co( A)系列样品更容易形成 Co- Al尖晶石相 .XANES和 EXAFS结果很好地说明了前文 [7] 中样品对 CO氧化和乙烯选?

X ray Absorption Near Edge Structure(XANES) and Extended X ray Absorption Fine Structure(EXAFS) were used to characterize the micro structures of Co species in Co/ γ Al 2O 3 catalysts prepared from different cobalt salts and calcined at different temperatures. The results of XANES show that the spectrum of Co(N) 500 (prepared from cobalt nitrate and calcined at 500 ℃) is similar to that of the reference compound Co 3O 4, while the spectra of all other samples are close to that of the reference compound CoAl 2O 4. As the calcination temperature increases, the pre edge absorption peak(1s→3d) becomes stronger, and the main absorption peak(1s→4p) splits more obviously. In XANES spectra of the samples calcined at 950 ℃, the shoulder peak (1s→4s) can be detected. All these characteristics imply that the cobalt species are becoming closer to the reference compound CoAl 2O 4 in structure. The results of EXAFS show that cobalt in sample Co(N) 500, which possesses high dispersion in the catalyst, exists mainly as Co 3O 4, while in all other samples the cobalt exists as Co Al spinel. From the view of the coordination distance and coordination number, the Co Al spinel becomes much closer to the reference compound CoAl 2O 4 with the increase of calcination temperature. The Co Al spinel is easier to be formed when cobalt acetate was used as the starting salt, but the effect of the starting salts is weakened by the increase of calcination temperature. The results of EXAFS and XANES are consistent with those of XRD and XPS. The activity performances for CO oxidation and NO selective reduction by C 2H 4 of the samples are well correlated with the above results.