IN-SITU EXAFS STUDY OF THE NUCLEATION AND CRYSTAL-GROWTH OF NI PARTICLES ON SIO2 SUPPORT

Ni/SiO2 catalysts were prepared by wetness impregnation method (Wi) an d by a new two-step method (ENien(2.5/600)+ INi2.5). The two-step meth od consists of formation of chemical glue, ''nickel nuclei'', having s trong metal support interaction with SiO2 and preparation of the ''nic kel reservoir'' which has weak interaction with support. The nucleatio n and growth behaviors of such Ni/SiO2 catalysts in the preparation st age were continuously monitored by in situ EXAFS spectroscopy. The in situ EXAFS spectra of Wi catalyst began to change after 320 degrees C and bulk NiO like structure was formed at 500 degrees C heat treatment . The Ni/SiO2 prepared with the two-step method had larger coordinatio n number of Ni with silica support in the initial stage of in situ mon itoring than that of Wi catalyst, which showed the coexistence of Ni n uclei and Ni reservoir, The ENien(2.5/600)+INi2.5 had smaller particle s of Ni compared with Wi after high temperature calcination. The forma tion of small Ni particles could be attributed to the strong ion-suppo rt interaction of nickel nuclei with nickel reservoir in ENien(2.5/600 )+ INi2.5 catalyst. Slight particle growth occurred after 370 degrees C calcination in Ni/SiO2 prepared by two-step method. (C) 1998 Elsevie r Science B.V. All rights reserved.