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youpengxuan木虫 (正式写手)
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Platinum Particle Size Distribution from TEM, EXAFS,and Hydrogen Chemisorption The experimental results obtained with hydrogen chemisorption, EXAFS, and TEM analysis (see Table 4) and the modeling results for A(size) (Figs. 7A and 7B), NPt–Pt(size) (Fig. 8), and D(size) (Fig. 9) were used to calculate the average particle size and the average number of atoms per particle for a few selected samples. For the catalysts for which TEM did not reveal or hardly revealed the presence of large platinum particles (4–9 nm)(Pt/NaY), this calculation was done by combining the experimental EXAFS NPt–Pt numbers (Table 4) with the NPt–Pt (size) function (Fig. 8) and the A(size) function (Fig. 7A). The results are presented in Table 5. For the zeolite catalysts calcined at 1±C/min TEM revealed the presence of large particles. The mass fraction TEM of the particles in the size distribution as determined by TEM is calculated by estimating the mass of the particles using their size and the modeled atom size function, A(size). From the particle size distributions (Figs. 5A, 5B), applied loading, and relation between the first-shell coordination number and particle size (Fig. 8), the average coordination number of the particles observed by TEM (NPt–Pt TEM) for both catalysts is calculated to be 11.3. With this number and using Eq. [3], the average coordination number of the mass fraction of platinum particles of catalyst Pt/NaY (1.1–IE–1) <2 nm (NPt–Pt not detected with TEM) is found to be 6.7 [.1=0:95/ ￡ .6:9–0:05 ￡ 11:3/], and for catalyst Pt/HUSY( 1.05–IE–1) this coordination number is found to be 7.5 [.1=0:92/￡.7:8-0:08 ￡ 11:3/]. The corresponding average size and number of atoms per particle as given in Table 5 were calculated by combining these calculated coordination numbers with the NPt–Pt (size) function (Fig. 8) and the A(size) function (Fig. 7A) derived from the modeled particles. The accuracy of the EXAFS coordination numbers used to calculate the average particle size is 10%. This statistical error is included in the sizes listed in Table 5. The weight factor in the averaging of the coordination number over all particles is the fraction of platinum atoms of a given coordination. Because the number of atoms per particle increases with approximately r 3 and the coordination number of the particles strongly depends on their size (Fig. 8), the larger particles in a size distribution dominate the average EXAFS coordination number. The deduced average particle size from EXAFS analysis of a (narrow) particle size distribution is therefore a volume average of the particle size distribution and an overestimation of the diameter average of the particle size distribution. On the other hand TEM cannot be used to detect Pt particles smaller than 2 nm. A combination of TEM and EXAFS is a powerful method to detect bimodal particle size distributions. A schematic presentation is given in Fig. 10 of the size range (nm/number of atoms per particle) where EXAFS and TEM have to be used to determine particle size accurately. |
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2楼2014-10-30 21:42:42
lihl910807
木虫 (著名写手)
真强必胜: 屏蔽内容, 禁止Google等机器翻译，下次违规罚。。。 2014-10-31 14:55:14
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3楼2014-10-30 23:13:56
chenbaishan
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通过TEM（透射电镜）、EXAFS（扩展x射线吸收精细结构）以及氢原子吸附得到的Pt颗粒的粒度分布 通过氢原子吸附、EXAFS以及TEM分析（见表4）得到的实验结果以及A（尺寸）（图7A和7B），NPt-Pt（尺寸）（图8）和D（尺寸）（图9）被用来计算平均粒度以及选定的几个样品的每个颗粒的平均原子数。 因为TEM并没有反映或很难反映出大的pt颗粒（4-9nm）（Pt/NaY）催化剂，这个估算是通过结合实验EXAFS NPt-Pt数量（表4）和NPt-Pt（尺寸）函数（图8）以及A（尺寸）函数（图7A）来完成的。结果在表5中呈现了。 TEM反映出了在1±C/min下煅烧的沸石催化剂有大颗粒的存在。通过TEM测定的颗粒的尺寸分布的质量分数是用它们的尺寸和原子尺寸函数A（尺寸）建模来估算颗粒质量来判定的。从颗粒尺寸分布（图5A，5B），外加载荷以及第一外形同等数量和颗粒尺寸（图8）可以得到，两种催化剂通过TEM（NPt-Pt TEM）观察到的同等颗粒平均数量计算结果是11.3。这个数据结合使用Eq，催化剂Pt/NaY(1.1–IE–1) <2 nm (TEM下NPt–Pt没探测到)中的Pt颗粒的质量分数的同等数目被发现是6.4[.1=0:95/ ￡ .6:9–0:05 ￡ 11:3/]，对催化剂Pt/HUSY( 1.05–IE–1)来说同等数目是7.5 [.1=0:92/￡.7:8-0:08 ￡ 11:3/]。通过结合NPt-Pt（尺寸）函数（图8）以及源于粒子建模得到的A（尺寸）函数（图7A）估算的相当的数量得到相当的平均尺寸和颗粒平均原子数如表5所示。 用来估算平均颗粒尺寸的EXAFS相当数量的准度是10%.这个错误统计在表5中的尺寸数据中包含了。在所有颗粒的相当平均重量系数是给定相当的Pt原子的一部分。因为每个颗粒的原子数大约随着r的增加呈3次关系，并且颗粒的相当数量与尺寸密切相关（图8）,粒度分布中大的颗粒支配着EXAFS的平均相当数量。通过EXAFS分析的（窄的）颗粒尺寸分布推断出的平均颗粒尺寸因此是颗粒尺寸分布的平均体积，而且是对颗粒尺寸分布的平均直径的过高估计。另一方面，TEM不能被用来探测尺寸小于2nm的Pt颗粒。TEM和EXAFS的结合是用来探测颗粒尺寸分布的双峰曲线的强力手段。图10的图示给出了在EXAFS和TEM不得不被用来准确确定颗粒尺寸的情况下的粒度范围（nm/每个颗粒的原子数）。 我试着翻译的哈，非常粗糙，一方面急着吃饭，另一方面水平有限，等等看别人有没有更靠谱的翻译咯~  |
4楼2014-11-21 17:44:35
