A First-Principle Study of Chain Propagation Steps in the Fischer−Tropsch Synthesis on Fe(100)

Abstract

We presented density functional theory based calculations on the formation and reactivity of C 3 species produced from CO and H 2 over Fe(100). The results indicated that the conversion of syngas to propylene and propane is overall exothermic, and the most stable C 3 surface species are CCCH 2 and CCCH 3 . It was found that the chain propagation on Fe (100) is initiated by the recombination of C and CCCH 2 /CCCH 3 rather than by the coupling reaction of surface vinyl and methylene. The potential energy surface associated with such an alkynyl mechanism has been computed and validated. Furthermore, this alkynyl mechanism has been compared to the conventional alkenyl mechanism in explaining the experimental observations of the 13 C 2 H 4 -probed polymerization over Fe. It was finally found that the two propylene isotopomers observed experimentally in the product mixture are produced from surface ethylene along different reaction paths rather than the isomerization of surface allyl.

Advanced fundamental modeling of the kinetics of Fischer–Tropsch synthesis

“…Quantum chemical calculations suggest that the alkynyl CCCH 2 and CCCH 3 intermediates would be more stable than alkyl and alkenyl. 22,23 Even in this case, the symmetry number of the transition states is independent of the chain length and similar to that of the alkyl intermediates. Indeed, according to the Hammond-Leffler postulate 24,25 in a highly exothermic reaction the structure of the transition-state is close to that of the reactant.…”

“…where The rate equations (22)(23)(24)(25)(26), derived in the above sections in terms of the elementary steps likely to occur in FT synthesis, are complex because of the number and variety of the elementary steps (8 types in total-viz. Table 1) operating in one or more homolog series of components.…”

HERFD-XAS and valence-to-core-XES: new tools to push the limits in research with hard X-rays?

“…As a consequence, it is not possible to distinguish, for example, carbon, nitrogen and oxygen atoms in the proximity of the metal center. Especially in catalysis, this is a serious drawback: most heterogeneous systems are composed of oxidic materials or catalysts on an oxidic support that are used for the conversion of small molecules consisting mainly of C, H, N, like in Fischer-Tropsch synthesis [24][25][26][27][28] or alkyne hydrogenation. [29][30][31][32] Usually homogeneous catalysts are transition metal complexes composed of H, C, N, O containing ligands that undergo only slight changes in catalytic transformations, which are not detectable by EXAFS.…”

Structure sensitivity of the Fischer–Tropsch reaction; molecular kinetics simulations

“…Obviously all these different possibilities can be computationally investigated and several important studies have recently appeared. 26,[28][29][30][31][32][33] There are still many options to be investigated, especially how the longer chains are formed. What has become quite clear is that which chain growth path actually occurs depends sensitively on the reaction site structure as well as on the type of metal one studies.…”