PtSiO2: II. Characterization of the gel and the platinum particles by X-Ray diffraction
PtSiO2: II. Characterization of the gel and the platinum particles by X-Ray diffraction
复制标题
PtSiO2：II．
DOI：
10.1016/0021-9517(77)90060-4
复制
发表时间：
1977
影响因子：
7.3
通讯作者：
J. Butt
中科院分区：
文献类型：
--
作者：
S. Sashital;J. Cohen;R. L. Burwell;J. Butt
用现代X射线衍射图分析技术对第一部分所述的几种铂-硅胶催化剂进行了分析，以确定铂的平均微晶尺寸、暴露百分率、微晶形状、尺寸分布、晶格参数、残余应力和应变、是否有断层、以及振动的均方幅度。此外，还用小角散射(SAS)测定了凝胶的比表面积。第一部分用气体吸附测定的铂暴露百分率与本文报道的根据微晶尺寸计算的结果吻合得很好，表明微晶尺寸就是真实的铂颗粒尺寸。已经检查了小到≈25？的微晶。此外，晶体的形状是等轴的(绝对不是长方体)，并且没有应变和缺陷，除非平均尺寸接近凝胶的孔径大小。浸渍法制备的催化剂比离子交换法制备的催化剂粒度分布更均匀。粒度分布表明，一些表面物种在制备过程中发生了合并，而不是粗化(Ostwald熟化)。铂的平均振动幅度随着颗粒尺寸的减小而增加≈30%，从≈100减小到≈25？晶格参数没有大于≈0.1%的变化。用SAS测定的凝胶表面积与第一部分用氮气物理吸附测定的凝胶表面积一致。
Modern techniques for analysis of X-ray diffraction profiles have been applied to several of the platinum-silica gel catalysts described in Part I to determine average platinum crystallite size, percentage exposed, crystallite shape, size distribution, lattice parameter, residual stresses and strains, presence or absence of faulting, and mean-square amplitude of vibration. In addition the surface area of the gel was determined with small-angle scattering (SAS). There is good agreement between the percentage exposed of platinum measured by gas adsorption in Part I and the results reported here, calculated from crystallite sizes, indicating the crystallite size is the true platinum particle size. Crystallites to sizes as small as ≈25 Å have been examined. Furthermore, the crystallites are equiaxed in shape (and definitely not cuboidal) and are strain and defect free, except in the case where the average size is near the pore size of the gel. Size distributions are sharper when the catalyst preparation is by impregnation rather than by ion exchange. The size distributions indicate that there is coalescence of some surface species during preparation rather than coarsening (Ostwald ripening). The meansquare amplitude of vibration of platinum increases by ≈30% as the particles decrease in size from ≈100 to ≈25 Å. There is no change in the lattice parameter greater than ≈0.1%. The gel surface areas determined by SAS are in agreement with those determined in Part I by physisorbtion of nitrogen.