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采用X射线吸收精细结构(XAFS),X射线衍射(XRD)和差热分析(DTA)等方法研究了以化学还原法制备的NiB和NiP超细非晶态合金催化剂在退火过程中的结构变化.XRD结果表明,在300℃下退火时,NiB超细非晶态合金晶化生成纳米晶Ni3B亚稳物相,NiP超细非晶态合金则主要晶化生成金属Ni和部分晶态Ni3P的混合物相;在500℃退火且近于完全晶化的条件下,大部分超细非晶态合金都晶化为金属Ni.XAFS结果定量地说明,对于NiB和NiP初始样品,第一近邻Ni-Ni配位的平均键长Rj分别为0.274和0.271nm,其结构无序度σS很大,分别为0.033和0.028nm,其热无序度σT分别为0.0069和0.0060nm.300℃退火后,晶化生成的Ni3B的Ni-Ni配位的σS降低到初始样品的33%,仅为0.011nm.500℃退火后,NiB样品的结构参数与金属Ni基本一致,但NiP样品的Ni-Ni配位的σS还远大于σT,仍为0.0125nm,表明NiB和NiP超细非晶态合金的退火晶化行为有很大的差别.纳米晶Ni3B催化苯加氢反应的转化率比超细Ni-B非晶态合金或多晶金属Ni更高,表明纳米晶Ni3B中的Ni与B原子组成了苯加氢催化反应的活性中心.

The structural evolution of ultrafine NiB and NiP amorphousalloys prepared by c hemical reduction during the annealing process is investigated by X-ray absorpt ion fine structure (XAFS), X-ray diffraction (XRD) and differential thermal ana lysis (DTA) techniques. The XRD results show that the ultrafine NiB amorphous al loy is crystallized into nanocrystalline Ni3B at the annealing temperature of 300 ℃. However, metal Ni and small amount of Ni3P are formed at 300 ℃ for the ultrafine NiP amorphous alloy. When the annealing temperature increases to 500 ℃, the majority of NiB and NiP alloys is decomposed into metal Ni. The XAFS results quantitatively indicate that the average bond length Rj, static diso rder σS and thermal disorder σT are 0.274, 0.033 and 0.0069 nm respectively for the Ni-Ni first neighbor shell in the initial NiB sample, and 0.271, 0.028 and 0.0060 nm for the NiP sample. The σS of Ni-Ni shell significantly decreases from 0.033 to 0.011 nm when NiB is crystallized at the annealing temperature of 300 ℃. After annealing at the higher temperature of 500 ℃, the structural parameters of NiB sample are almost the same as that of Ni foil, while the σS is rather larger than σT for NiP sample. The results indicate that annealing crystallization is different between the ultrafine NiB and NiP amorphous alloys. The catalytic activity of nanocrystalline Ni3B for benzene hydrogenation is much higher than that of ultrafine NiB amorphous alloy and polycrystalline Ni. This suggests that the active sites for the catalyt ic benzene hydrogenation are composed of Ni and B atoms.