Annealing crystallization and catalytic performances of ultrafine NiB and NiP amorphous alloys

The structural evolution of ultrafine NiB and Nil? amorphous alloys prepare d by chemical reduction during the annealing process is investigated by X-r ay absorption fine structure (XAFS), X-ray diffraction (XRD) and differenti al thermal analysis (DTA) techniques. The XRD results show that the ultrafi ne NiB amorphous alloy is crystallized into nanocrystalline Ni3B at the ann ealing temperature of 300 degreesC. However, metal Ni and small amount of N i3P are formed at 300 degreesC for the ultrafine NiP amorphous alloy. When the annealing temperature increases to 500 degreesC, the majority of NiB an d NiP alloys is decomposed into metal Ni. The XAFS results quantitatively i ndicate that the average bond length R-j, static disorder sigma (S) and the rmal 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 d ecreases 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 5 00 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 N iP 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 tha t the active sites for the catalytic benzene hydrogenation are composed of Ni and B atoms.