(629b) Developing Multi-Scale Models of Deoxygenation Reactions on Iron Carbide Catalysts: Hydrodeoxygenation of Phenol on Fe3C (001)

ABSTRACT

Developing
Multi-Scale Models of Deoxygenation Reactions on Iron Carbide Catalysts: Hydrodeoxygenation
of Phenol on Fe₃C (001)

Neeru
Chaudhary, Jake Bray, Renqin Zhang, Yong Wang, and Jean-Sabin McEwen

While
the efficient catalytic conversion of biomass to fuels and value-added products
is an attractive means of addressing the demands for sustainable energy
resources,[1] a major roadblock in the production of high-quality fuel is governed
by the presence of oxygenates in bio-oil that impart several of its deleterious
properties.[2-4]
Catalytic hydrodeoxygenation (HDO) has proved to be a promising method in upgrading
bio-oils to high-quality fuels.[5-8] Iron-based catalysts demonstrate exceptional
selectivity in cleaving C-O bonds: a necessary step in upgrading bio-oil to
fuels. However, iron catalysts easily deactivate via oxidation under HDO
conditions.[11, 12] Precious metal promoted iron catalysts exhibit a synergistic behavior with
improved catalytic activity and selectivity, but with an increase in catalyst cost. Iron carbide-based systems
are an attractive alternative to precious-metal-promoted catalysts since a small
amount of carbon atoms have been shown to impart chemical stability under an oxidizing
environment.[11,
13-15]
Metal carbide catalysts are a cost-effective alternative to expensive precious-metal
promoted catalysts and have the potential to lower biofuel production costs.

In this work, we elucidate from first
principles the vapor phase HDO reaction mechanism of phenol over an iron
carbide surface, Fe₃C(001). Our density functional theory based
calculations suggest that Fe₃C(001) retains its HDO activity similar
to metallic iron surfaces. Furthermore, carburization of iron slightly
decreases the selectivity for C-O bond
cleavage without significantly affecting the other reaction steps as compared
to phenol HDO over a pure iron surface (Figure 1a).[16,
17]
The decreased selectivity toward deoxygenation associated with the iron carbide
surface indicates a reduction in surface oxophilicity, thus minimizing
oxidative deactivation under an oxidizing, HDO-like environment.

The
lateral interactions between carbon adatoms on iron-based catalyst leads to the
formation of iron carbides. As such, it is important that one takes them into
account through a lattice gas Hamiltonian so as enable the development of
realistic multi-scale models of such complex systems. Preliminary lattice gas
cluster expansion models have characterized these lateral interactions for carbon
on Fe(100) and suggest some important ground state structures including the c(2×2)
ordered structure that occurs at ½ ML through the presence of repulsive nearest
neighbor and attractive second nearest neighbor lateral interactions (Figure
1b). This correlates well with previous studies for carbon adsorption on
iron surfaces and matches the experimentally-observed LEED structure.[18,
19]
These preliminary results set the stage for developing further insights into the
effect of carburization on iron surfaces. Overall, our current work is expected
to elucidate the synergistic interactions between iron and carbon providing the
first ingredients necessary to design robust bifunctional HDO catalysts while
minimizing catalyst cost using earth abundant materials.

Figure 1. Overview of the vapor phase HDO modeling work of phenol on Fe₃C(001).
(a) Comparison between the activation energies of phenol for several
possible HDO mechanisms over Fe₃C(001) and Fe(110), indicating that an
Fe carbide catalyst will decrease the selectivity toward C-O bond cleavage.
(DHOx = Dehydroxylation, TH and DO = Transhydrogenation & Deoxygenation,
Taut and DHO = Tautomerization and Dehydroxylation, DDO = Direct Deoxygenation).
(b) Bottom: Cluster expansion of carbon on Fe(100); Top: c(2×2) ground
state structure that occurs at 0.5 ML. 

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