(352c) Protecting the Fe Active Phase from Oxidation Under Hydrodeoxygenation Conditions: Evaluating the Influence of Promoters and External Electric Fields

Authors: 
Bray, J., Washington State University
Hensley, A., Washington State University
Collinge, G., Washington State University
McEwen, J. S., Washington State University

Bimetallic, promoted
iron catalysts have demonstrated synergistic behavior that contributes to more
cost-effective and longer-lasting hydrodeoxygenation (HDO) catalysts.[1-4]
While precious metal promoters exhibit an overall synergistic interaction with
Fe catalysts, further investigation into the behavior of oxygen on a promoted
Fe-based catalytic grain is required to minimize the use of costly precious
metals while maximizing the synergistic properties.[1, 2, 5] Furthermore, the
application of externally applied electric fields has demonstrated favorable
effects on unpromoted iron catalytic grains[6, 7] along with other catalytic
processes, such as methane steam reforming.[8-10] Here, we use density
functional theory and multi-scale modeling techniques to elucidate the effect
of precious metal promoters and external electric fields on the kinetic
behavior of oxygen over a full, multi-faceted catalytic Fe grain upon exposure
to O2 (Figure 1). Such a realistic, multi-faceted model of a
promoted Fe grain is an effective tool for optimizing the use of precious metal
promoters like Pd and Rh, taking advantage of an electric field as a means to
fine-tune catalytic performance. These realistic, predictive models help to
bridge the gap between experiment and theory, streamlining research efforts and
accelerating progress.

In order to build the
multi-faceted, promoted catalytic Fe grain model, the interaction of oxygen is
calculated on both Pd and Rh promoted Fe(100), Fe(110), and Fe(111) over a
range of oxygen surface coverages and electric field strengths (Figure 1). The
data from the three Fe-promoted facets is then used to construct a model of a
multi-faceted grain surface, incorporating the anisotropic effects of the
surface orientation on its kinetic behavior.[11] Such a model accounts for the
effect of pressure, temperature, electric field, promoters, and surface
orientation on the adsorption/desorption and surface coverage behavior of
oxygen. Preliminary results indicate that an Fe grain promoted with ¼ monolayer
(ML) of either Pd or Rh experiences a massive reduction in oxygen surface
coverage compared to the unpromoted system (Figure 1c). Moreover, the
application of an electric field further reduces oxygen coverage but to a
lesser degree than the promoter, consistent with adsorption energy trends in
Figure 1b. The promoted Fe catalytic grain model can be used as an
experimentally predictive, useful tool for optimizing methods in which HDO
catalysts are designed to reduce oxidative deactivation while minimizing the
use of costly precious metals.

Figure 1: The
effect on ¼ ML Pd, ¼ ML Rh, and an applied electric field on oxygen’s coverage
distribution over a multi-faceted Fe grain, where (a) is a side view of the
multi-faceted grain, (b) details the electric field effect of Pd and Rh on the
adsorption energy of oxygen on Fe(110), and (c) is a top-down view of the
oxygen coverage distribution over the multi-faceted grain surface, modeled at
650 K and 1×10-21 Pa.

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