Catalytic properties and structures of nano-amorphous Ni-B alloys affected by annealing temperatures

The catalytic activities of nano-amorphous Ni-B alloy catalysts prepared by chemical reduction and then annealed at various temperatures were investigated by using the benzene hydrogenation as a probe reaction. The results indicated that the benzene conversion (63%) and turnover frequency (0.67 s−1) reached a maximum for the nano-amorphous Ni-B alloy catalyst annealed at an optimized temperature of 623 K. Such values were about 110 and 70% higher than those of the as-prepared one. A high annealing temperature of 773 K led to a dramatic decrease in the catalytic activity of Ni-B alloy. From the XAFS and XRD results, we confirmed that the nano-amorphous Ni-B alloy annealed at 573 or 623 K was composed of nano-crystalline Ni and crystalline Ni3B. After being annealed at the higher temperature of 773 K, the Ni-B alloy may result in the decomposition of crystalline Ni3B into crystalline Ni and aggregation of nano-crystalline Ni into crystalline Ni with a large size. Our results indicated that the nano-crystalline Ni showed higher activity and turnover frequency than nano-amorphous Ni-B alloy did. We propose that the order of catalytic activity of benzene hydrogenation for various phases in the amorphous Ni-B alloy is as follows: nano-crystalline Ni > nano-amorphous Ni-B alloy > crystalline Ni ≈ crystalline Ni3B.