Crystallization deactivation of Ni-P/SiO2 amorphous catalyst and the stabilizing effect of silica support on the Ni-P amorphous structure

A supported Ni-P amorphous catalyst (Ni-P/SiO2) was prepared by electroless plating. When the sample was treated at elevated temperatures from 363 to 973 K in N-2 flow for 2.0 h, the XRD patterns and SEM morphologies revealed that the Ni-P amorphous alloy crystallized stepwise from 673 to 873 K. The stabilizing effect of the support on the Ni-P amorphous alloy was observed by DSC, TPO, and TPR, which was attributed to the high dispersion of the Ni-P alloy particles in the support matrix, the affinity of SiO2 support for the Ni-P alloy particles, and the transfer of the heat produced during the crystallization from the surface to the bulk of the support. The change in the activity of the as-prepared Ni-P/SiO2 amorphous catalyst during heat treatment was measured using the liquid phase benzaldehyde hydrogenation as a probe and by comparing to that of the corresponding Ni/SiO2 crystalline catalyst. On one hand, the activity of both two catalysts decreased with the increase of the treating temperature owing to the decrease in surface active Ni atoms caused by the coalescence of small Ni particles at high temperature. On the other hand, the two catalysts exhibited considerably different catalytic behaviors during the heating treatment. (1) Only a smooth decrease in the activity of the Ni/SiO2 catalyst was observed with the increase of the treating temperature. While an impressive decrease in the activity of the Ni-PISiOz amorphous catalyst was observed in the temperature range from 673 to 873 K corresponding to its crystallization; (2) no significant change in the apparent activation energy (Ea) and the TOF value was observed during the heating pretreatment of the Ni/SiO2 catalyst. However, a considerable increase in Ea and a decrease in the TOF value were observed after the crystallization of the Ni-P/SiO2 amorphous catalyst. These results demonstrated a crystallization deactivation of the Ni-P/SiO2 amorphous catalyst. According to various characterizations, such as EXAFS, XRD, SEM, TEM, XPS, DSC, ICP, TPR, TPO, TPD, and hydrogen chemisorption, the decrease in the number of surface active Ni atoms and the surface P content in the Ni-P alloy the change in the structural characteristics, and the adsorbing properties of the Ni active sites were the main factors responsible for the deactivation of Ni-P/SiO2 amorphous catalyst during the crystallization. (C) 2000 Academic Press.