Adding ammonia during Fischer-Tropsch Synthesis: Pathways to the formation of N-containing compounds.

Abstract

The Fischer-Tropsch synthesis (FTS) process, better known for its ability to produce synthetic

fuel via the hydrogenation of CO, has shown potential to produce valuable chemicals when

ammonia is added to the feed. In this work certain aspects of the pathway to the formation of

N-containing compounds that form when NH3 is added during FTS, using mostly iron based

catalysts is investigated. In addition, the effect this has on the FTS reaction itself is evaluated.

To achieve this goal, both theoretical and experimental techniques are used in this study.

The CO adsorption and dissociation reactions are assumed to be important elementary reactions

for many proposed FTS pathways. In the theoretical part of this thesis, spin-polarized periodic

density functional theory (DFT) calculations are employed to study aspects of the initial stage

of the pathway on a model Fe(100) surface. Considering the formation of N-containing hydrocarbons,

one would assume that NH3 initially adsorbs and dissociates on the catalyst surface,

which could take place in the presence of CO. The surface chemistry of these adsorbates is well

studied both experimentally and theoretically, but their co-existence has not yet been evaluated

on model Fe surfaces. Initially a platform is generated by calculating the individual potential

energy surfaces (PES) for the decomposition of CO and NH3 on Fe(100) at a coverage of θ

= 0.25 ML. These calculations provided the basis for comparing the adsorption and dissociation

profiles of CO and NH3 on the Fe(100) surface via the use of the same computational

methodology, and importantly making use of the same exchange correlation functional (RPBE)

for both adsorbates. Furthermore, it was desired to evaluate the kinetics and thermodynamics

of the NH3 decomposition on the Fe(100) surface at relevant temperatures and pressures (by

combining the DFT results with statistical thermodynamics) to better understand the role of

NHx surface species involved in the pathway to the formation of the N-containing compounds

on a model catalyst surface. The DFT results that are reported for the individual decomposition

PES for CO and NH3 were generally found to be in close agreement with what has been

reported in previous DFT studies and deduced experimentally for the relevant adsorption and

decomposition pathways. The resulting Gibbs free energies for the PES suggests that NH2 may

be kinetically trapped on the Fe(100) surface at a coverage of θ = 0.25 ML and the reaction