Ni-B和Ni-Ce-B超细非晶态合金的退火晶化及其催化性能

Annealing Crystallization and Catalytic Performance of Ultrafine Ni-B and Ni-Ce-B Amorphous Alloys

采用XAFS ,XRD和DTA方法研究了Ni B和Ni Ce B超细非晶态合金在退火过程中的结构变化及其结构与催化性能的关系 .活性结果表明 ,在退火温度为 62 3K时 ,Ni B和Ni Ce B样品的苯加氢催化反应转化率最高 ,分别为 63 %和 81% ,0 3 %Ce的掺入提高了Ni Ce B的催化活性 .DTA结果表明 ,Ni B超细非晶态合金在 5 98和65 3K有两个晶化峰 ,而Ni Ce B样品有 5 4 8,60 3 ,696和 80 1K四个晶化峰 .XAFS和XRD结果进一步说明 ,在5 73K退火时 ,Ni B样品晶化生成晶态Ni3B和纳米晶Ni,此时Ni Ce B仅有少量晶态Ni3B生成 .在 673K退火时 ,Ni B样品中的Ni3B开始分解生成晶态Ni,同时纳米晶Ni聚集并形成大颗粒晶态Ni,而Ni Ce B样品晶化生成晶态Ni3B和纳米晶Ni.在 773K和更高的温度退火处理后 ,Ni B样品中Ni的局域环境结构与金属Ni箔基本一致 ,但Ni Ce B样品晶化生成的Ni晶格有较大畸变 ,同时Ni3B并未分解 .说明 0 3 %的Ce对提高Ni Ce B样品的稳定性有显著作用 .本文首次报道了Ni B和Ni Ce B超细非晶态合金中苯加氢催化活性中心为纳米晶Ni和类似于金属Ni的Ni B非晶态合金 .

The structural evolution of ultrafine Ni-B and Ni-Ce-B amorphous alloys annealed at different temperatures was studied by XAFS, DTA and XRD techniques. The benzene hydrogenation was used to characterize the catalytic activity of the Ni-B and Ni-Ce-B alloys. The results indicate that the benzene conversion over the Ni-B and Ni-Ce-B samples reaches their maximum of 63% and 81%, respectively, at the annealing temperature of 623 K. Addition of a small amount (0.3%) of Ce can greatly improve the catalytic activity of the Ni-Ce-B alloy. The DTA results show that the crystallization process of the Ni-B amorphous alloy includes two steps (at 598 K and 653 K), while the Ni-Ce-B alloy exhibits four exothermal peaks at 548, 603, 696 and 801 K. The XAFS and XRD results further reveal that the Ni-B amorphous alloy is crystallized to form nanocrystalline Ni and crystalline Ni3B after annealing at 573 K, but only a small amount of crystalline Ni3B is produced in the case of Ni-Ce-B alloy. At the annealing temperature of 673 K, the crystalline Ni3B in the Ni-B starts to decompose into crystalline Ni, and the nanocrystalline Ni aggregates to form crystalline Ni. At 773 K or higher annealing temperatures, the local structure of Ni atoms in the annealed Ni-B alloy is similar to that in Ni foil. The element Ce in the Ni-Ce-B alloy can retard the crystalline Ni3B to be decomposed and make the Ni lattice distorted greatly. We proposed that the catalytic activity of the ultrafine Ni-B and Ni-Ce-B amorphous alloys for the benzene hydrogenation is in the order: nanocrystalline Ni > ultrafine Ni-B amorphous alloy > crystalline Ni greater than or equal to crystalline Ni3B.