(656h) Multi-Scale Models of Oxygen on Iron-Based Hydrodeoxygenation Catalysts: Elucidating the Effect of External Electric Fields and Surface Dopants

Authors: 
Bray, J., Washington State University
Collinge, G., Washington State University
Wang, Y., Pacific Northwest National Laboratory
McEwen, J. S., Washington State University
Stampfl, C., University of Sydney

Fe catalysts are of great
interest as they have been shown to be highly selective for the catalytic
upgrading, via hydrodeoxygenation (HDO), of bio-oil compounds. Unfortunately, these
catalysts alone are unacceptably prone to oxidative deactivation. One method of
improving the oxidation resistance of Fe catalysts has been to dope the Fe with
a second metal (e.g. Pd, Pt, etc.). However, the addition of the second metal
increases the catalyst cost and complexity, thereby making it harder to
experimentally optimize such systems to both increase activity and prevent
oxidative deactivation. Previous work has shown that externally applied
electric fields can dramatically improve the coke resistance of a Ni catalyst
used in methane steam reforming (MSR).6-9 These results suggest that external electric fields are
another adjustable parameter that can be used to increase activity, tune
selectivity, or prevent catalyst deactivation. Due to the highly polar nature
of oxygen-containing compounds, it is expected that electric fields will have a
large effect on HDO and could potentially be used in conjunction with doping to
create a highly stable and selective Fe-based HDO catalyst. To this end, we
must develop multi-scale models that can guide the selection of dopants in a
rational manner—with both surface coverage and electric fields explicitly
accounted for.

Here, we develop density
functional theory (DFT) parameterized lattice gas models of oxygen on both undoped
and doped Fe(100) surfaces to show the direct effect of an applied electric field
on oxygen adsorption. Typical DFT models consider a uniform coverage
distribution of adsorbates on the surface, when in reality, this is not the
case.2-4 Our cluster expansions of oxygen on Fe(100) surfaces
both with and without an externally applied electric field accounts for the more
realistic non-uniform adsorbate distributions and characterizes the lateral
interactions between oxygen adatoms.5 The apparent sensitivity of the lateral interactions
on an applied external electric field reveals their influence on the oxidative
deactivation of Fe catalysts.

Our preliminary results for
O/Fe(100) indicate that an applied electric field has a significant effect on
the attractive/repulsive interactions between adsorbed oxygen atoms. As seen in
Figure 1, the clusters that are most important to the predictiveness of the
model also change after the application of an electric field. These changes suggests
that electric fields will have major effects on the Fe oxidation process as the
electric field has a significant effect on the dynamic surface behavior of
oxygen. Using such a lattice gas model for oxygen on both undoped and doped
Fe(100), we will characterize the synergy between surface dopants and external
electric fields in order to minimize the oxidation potential of the Fe surface.
Our preliminary results suggest that one can expect significant deviations in
catalytic activity after the application of external electric fields, but also,
they suggest that mean-field models will not be sufficiently predictive for the
work needed moving forward. Overall, the work presented here provides the first
ingredients necessary for the selection of potential dopants for optimizing
Fe-based HDO catalysts in the presence of external electric fields.

Figure
1: Effect of an electric
field on oxygen cluster lateral interactions on the Fe(100) surface.

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