NiB和NiP超细非晶态合金的退火晶化行为及催化性能

Annealing Crystallization and Catalytic Performances of Ultrafine NiB and NiP Amorphous Alloys

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 30 0℃下退火时 ,NiB超细非晶态合金晶化生成纳米晶Ni3 B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Ni3 P的混合物相 ;在 5 0 0℃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 2 74和 0 2 71nm ,其结构无序度σS 很大 ,分别为 0 0 33和 0 0 2 8nm ,其热无序度σT 分别为 0 0 0 6 9和 0 0 0 6 0nm .30 0℃退火后 ,晶化生成的Ni3 B的Ni Ni配位的σS 降低到初始样品的 33% ,仅为 0 0 11nm .5 0 0℃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 0 12 5nm ,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Ni3 B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Ni3 B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 Nil? amorphous alloys prepared by chemical reduction during the annealing process is investigated by X-ray absorption fine structure (XAFS), X-ray diffraction (XRD) and differential thermal analysis (DTA) techniques. The XRD results show that the ultrafine NiB amorphous alloy is crystallized into nanocrystalline Ni3B at the annealing temperature of 300 degreesC. However, metal Ni and small amount of Ni3P are formed at 300 degreesC for the ultrafine NiP amorphous alloy. When the annealing temperature increases to 500 degreesC, the majority of NiB and NiP alloys is decomposed into metal Ni. The XAFS results quantitatively indicate that the average bond length R-j, static disorder sigma (S) and thermal disorder sigma (T) 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 sigma (S) of NI-Ni shell significantly decreases from 0.033 to 0.011 rim when NiB is crystallized at the annealing temperature of 300 degreesC. After annealing at the higher temperature of 500 degreesC, the structural parameters of NiB sample are almost the same as that of Ni foil, while the sigma (S) is rather larger than sigma (T) 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 catalytic benzene hydrogenation are composed of Ni and B atoms.