Iron carbide films as model catalysts for fischer-tropsch synthesis
Daniel Garcia Rodriguez defended his PhD thesis at the Department of Applied Physics and Science Education on February 28th.
For his PhD research, Daniel Garcia Rodriguez explored the world of iron-based catalysts, which are crucial players in turning syngas into valuable chemicals and fuels through Fischer-Tropsch synthesis (FTS). The focus of his work is on understanding the interaction between FTS precursors and a model iron carbide catalyst surface. Iron was evaporated on copper substrates using an electron beam evaporator, and ethylene was employed to synthesize iron carbide.
To start, Daniel Garcia Rodriguez looked at the structure and growth of iron and iron carbide films. Initial iron film growth follows FCC(100) crystallography, transitioning to coexisting FCC(100) and BCC(110) iron at higher thickness. Ethylene post-treatment forms a pure surface carbide with a p4g(2x2) clock reconstruction. Carbon atoms enforce a tetracoordinate square planar arrangement of iron atoms. Introducing carbon into subsurface layers via ethylene post-treatment is unsuccessful, but evaporation of iron in an ethylene atmosphere results in a p4g(2x2) surface structure in subsurface layers.
Interaction
Garcia Rodriguez then explored the interaction of iron and iron carbide films with CO and H2. On iron, CO and H2 dissociate independently of film thickness. A fully saturated Fe2C film inhibits dissociation, displaying weaker interaction with molecular CO. W filament-assisted H2 adsorption on Fe2C films overcomes the dissociative barrier.
After this, he studied different iron and iron carbide film thicknesses on Cu(111). Below 2.6 ML, the iron film consists of multilayer islands with an FCC-Fe structure. Ethylene exposure results in graphitic carbon on these islands. Between 2.6 and 16 ML, a mixed BCC-FCC film forms, with pure carbide after ethylene dissociation. Films thicker than 16 ML exhibit BCC(110) surfaces, and ethylene dissociation reveals on-surface and bulk carbide.
Closed iron film
For the next stage of his research, Garcia Rodriguez examined the formation of a closed iron film on Cu(111), requiring ~8.6 ML of Fe. Pure iron films allow a maximum of 0.2 ML CO dissociation, while films above ~16 ML form a pure BCC(110)-Fe phase, allowing up to 0.3 ML CO dissociation.
Fully saturated iron carbide films inhibit CO dissociation, with decreased desorption temperature attributed to weaker CO bonding in the presence of surface carbon. CO dissociation on an 8 ML iron film with varying pre-adsorbed carbon concentrations shows a linear decrease in dissociated amount with increasing carbon coverage.
Primary goal
The primary goal of Garcia Rodriguez’s research was to create an iron carbide model catalyst for FTS, revealing a stable and inert Fe2C surface carbide with a p4g(2x2) structure. A carbon-saturated iron surface exhibits no reactivity towards CO and H2 dissociation, while a non-saturated surface displays dissociative reactivity.
Despite experimental differences from real-life catalyst conditions, thick iron films with only BCC(110) present could serve as a model catalyst for studying reactive iron carbide structures.
In conclusion, his research provides valuable insights into the structural characteristics and reactivity of iron and iron carbide films, laying the foundation for understanding their catalytic behavior in Fischer-Tropsch synthesis.
Title of PhD thesis: Fabrication and reactivity of iron carbide films on copper substrates as model catalysts for fischer-tropsch synthesis. Supervisors: Richard van de Sanden and C. Weststrate (external).