Annealing crystallization and catalytic performance of ultrafine Ni-B and Ni-Ce-B amorphous alloys

The structural evolution of ultrafine Ni-B and Ni-Ce-B amorphous alloys ann ealed 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 conv ersion over the Ni-B and Ni-Ce-B samples reaches their maximum of 63% and 8 1%, respectively, at the annealing temperature of 623 K. Addition of a smal l 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 XAF S and XRD results further reveal that the Ni-B amorphous alloy is crystalli zed 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 N i-Ce-B alloy. At the annealing temperature of 673 K, the crystalline Ni3B i n the Ni-B starts to decompose into crystalline Ni, and the nanocrystalline Ni aggregates to form crystalline Ni. At 773 K or higher annealing tempera tures, the local structure of Ni atoms in the annealed Ni-B alloy is simila r to that in Ni foil. The element Ce in the Ni-Ce-B alloy can retard the cr ystalline 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 a morphous alloys for the benzene hydrogenation is in the order: nanocrystall ine Ni > ultrafine Ni-B amorphous alloy > crystalline Ni greater than or eq ual to crystalline Ni3B.