Local structure and catalytic performance of ultrafine NiB amorphous alloy

Ultrafine NiB amorphous alloy prepared by chemical reduction was characterized by X-ray absorption fine structure (XAFS), X-ray diffraction (XRD), and Differential thermal analysis (DTA). The DTA result indicated that the crystallization of ultrafine NiB amorphous alloy was carried out in two steps, as indicated by two strongly exothermic peaks located at 598 and 652 K, respectively. Two metastable intermediate states, i.e., crystalline Ni3B and a nanocrystalline Ni phase, were formed in the first exothermic process of ultrafine NiB amorphous alloy. Annealing at the temperature higher than 652 K may result in the decomposition of crystalline Ni3B into crystalline Ni and aggregation of nanocrystalline Ni. The activities of benzene hydrogenation for NiB samples showed a maximum at the optimized annealing temperature of 623 K and a minimum at the higher annealing temperature. The results showed the catalytic activity for the benzene hydrogenation of the NiB alloy sample was in this order: nanocrystalline Ni>ultrafine NiB amorphous alloy>crystalline Ni≥crystalline Ni3B. It is considered that the active sites are nanocrystalline Ni and amorphous NiB with fcc Ni-like structure for ultrafine NiB amorphous alloy