X-ray diffraction study of nickel oxide reduction by hydrogen
X-ray diffraction study of nickel oxide reduction by hydrogen
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DOI：
10.1016/s0926-860x(02)00669-5
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发表时间：
2003-06-25
影响因子：
5.5
通讯作者：
Twigg, MV
中科院分区：
文献类型：
--
作者：
Richardson, JT;Scates, R;Twigg, MV
在 175-300℃ 的温度范围内，利用原位热级 X 射线衍射 (XRD) 研究了多孔块体 NiO 颗粒的氢还原。该技术能够同时测量 NiO 消失和 Ni 出现，以及各自的微晶尺寸。由于样品是非常薄的 50 微米厚的分散的直径为 20 微米的颗粒板，因此不存在 NiO 颗粒固定床研究中常见的结构和形态特征，测量结果仅反映了化学机理和动力学。结果表明，在还原气体中不添加水的情况下，还原遵循一系列步骤：（1）与 NiO 初始还原和 Ni 金属簇出现相关的诱导期； (2)随着团簇尺寸的增加，减少速度加快； (3)准一级（过量H-2）过程，其中NiO消失，Ni同时出现，直到还原速度在约0.8的转化率处减慢。晶粒尺寸测量显示，尺寸约为 3 nm 的 NiO 晶粒转变为大于 20 nm 的 Ni 晶粒，这意味着由于 NiO 晶粒非常接近被还原，因此还原后的 Ni-0 离子传输非常快。当将 2.2 x 10(-2) atm 的 H2O 添加到还原气体中时，诱导时间增加了大约两倍，还原率降低（在较低温度下增加），表观活化能为126 +/- 27 kJ mol(-1) 相比之下，不加水时为 85 +/- 6 kJ mol(-1)。之前的研究中没有观察到 NiO 还原和 Ni 生长之间的滞后，这表明结构和形态因素对于确定水蒸气在还原过程中的作用非常重要。 (C) 2003 Elsevier Science B.V 保留所有权利。
Hydrogen reduction of porous bulk NiO particles has been studied with in situ hot-stage X-ray diffraction (XRD) in the temperature range 175-300degreesC. This technique has the ability to measure NiO disappearance and Ni appearance simultaneously, together with the crystallite size of each. Since the sample was a very thin, 50-mum slab of dispersed 20-mum diameter grains, textural and morphological features normally encountered during studies with fixed beds of NiO particles were absent and measurements reflected only the chemical mechanism and kinetics.The results indicated that reduction in the absence of water added to the reducing gas followed a series of steps: (1) an induction period associated with the initial reduction of NiO and the appearance of Ni metal clusters; (2) acceleration of the reduction rate as the size of the clusters increase; and (3) a pseudo-first-order (excess H-2) process in which NiO disappeared and Ni appeared in concert until reduction slowed at a fractional conversion of about 0.8. Crystallite size measurements showed NiO crystallites of about 3 nm in size were transformed into Ni crystallites of more than 20 nm, implying that Ni-0 ion transport following reduction was very fast due to the close proximity of the NiO crystallites being reduced.When 2.2 x 10(-2) atm of H2O was added to the reducing gas, induction times increased by approximately a factor of two and reduction rates decreased (increasingly at lower temperatures) with an apparent activation energy of 126 +/- 27 kJ mol(-1) compared to 85 +/- 6 kJ mol(-1) without added water. The lag between NiO reduction and Ni growth observed in previous studies was not seen, indicating that textural and morphological factors are very important in establishing the role of water vapor in the reduction process. (C) 2003 Elsevier Science B.V All rights reserved.