| Isotopic transient methods have been utilized in this thesis to study the mechanism of Fischer-Tropsch synthesis. Two different methods of preparing Co powder showed completely dissimilar transient behavior, although their steady state behavior is basically the same. In the first method, cobalt nitrate was precipitated by a concentrated ammonia hydoxide. The catalyst was calcined. In the second method, cobalt nitrate was precipitated by Na(,2)(CO)(,3) and the catalyst was not calcined. Calcination might cause different orientation of crystal planes which, in turn, might change the transient behavior of Co powder.; Over Co powder with the first method, all evidence pointed to the CH(,x) insertion mechanism. Because ('12)CO/('13)CO switch showed no time difference between ('13)C in C(,1), C(,2), and C(,3), it was concluded that chain growth proceeds very fast (>1 s('-1)). It was also found that the transient response of ('13)CO is much faster than that of ('13)C in C(,1), C(,2), and C(,3), suggesting that C(,1) and higher hydrocarbons originate from a surface intermediate which may not be CO. Further analysis of this finding lead to the conclusion that the C(,1)-building monomer is probably CH(,4)(x = 0, 3). Other evidence (H/D fast exchange in methane for H(,2)/D(,2) switch) indicated that hydrogenation of CH(,x) proceeds very rapidly and the scission of C-Co or C-C bond is a relatively slower step than the hydrogenation of CH(,x).; Over Co powder prepared with the second method, the CO insertion mechanism might be predominant, based on a measurable time difference between ('13)C in C(,1), C(,2) and C(,3) and a very short lifetime for C(,1)-building monomers. Chain growth proceeds with a measurable rate ((TURN)0.1 C-C bond/s) where hydrogenation proceeds very quickly. The rate-determining step might be the C-O dissociation. This catalyst also shows a more heterogeneous surface than the cobalt powder prepared with the first method does.; A Co/SiO(,2) catalyst was also studied. It was found that the lifetime for C(,1) surface intermediates is very small for supported catalysts, compared to that found with Co powder catalysts. This difference suggests the possibility of a support effect. The rate of chain growth over Co/SiO(,2) catalyst were observed to be very fast. C-O dissociation, hydrogenation and the breaking of C-M (or C-C) bond may be very rapid as well. |
