(520g) Developing Multi-Scale Models of Bimetallic Catalysts for the Hydrodeoxygenation of Bio-Oil Compounds

            With
the ever-increasing need to find a sustainable source of renewable energy,
bio-oil produced via the fast pyrolysis of biomass is a promising source of
liquid fuel; however, the resulting bio-oil contains oxygenated products that contribute
to poor fuel quality [1]. Hydrodeoxygenation (HDO) is used to refine the
bio-oil by reducing the oxygen content, ideally using a minimal amount of H₂
in order to form H₂O. Bimetallic catalysts such as Pd on Fe have
demonstrated synergistic behavior that contributes to a more cost-effective and
longer-lasting catalyst [2-4].  Similarly, Pt/Sn catalysts have also exhibited
traits favorable for the HDO of phenolic compounds, namely a propensity to
promote lower temperature desorption of aromatic compounds like benzene [5]. Identifying the cause of the different synergetic
effects of these systems can provide insight into the synthesis of a superior
HDO bimetallic catalyst. However, the behavior of bimetallic catalysts is
complex and necessitates a thorough understanding of the nanoscale behavior of
these systems in order to develop a truly predictive model on relevant time
scales. We present two DFT studies examining first, the interactions between a
surface and its adsorbates and second, the interactions between vicinal
adsorbates on a surface.

            While
the addition of Pd to Fe catalysts exhibits a synergistic interaction [2-4], further investigation is needed in order to conclude
with greater certainty the role of Pd. We have developed a preliminary
microkinetic model of the dissociation of H₂O on Fe (100) and on two
surface alloys with different concentrations of Pd. We found that increasing
the surface concentration of Pd from 0% to 50% caused a ~0.7 eV decrease in the
adsorption strength of O, while only a ~0.1 eV decrease in the adsorption of H.
This trend is consistent with other studies that investigated the influence of
noble metal dopants on the adsorption of benzene and phenol [6, 7]. Moreover, after determining the activation energies
for each reaction step from linear scaling relationships [8], it was found that Pd increases the barriers for both
the dissociation of H₂O in OH and H and the subsequent dissociation
of OH. These results show that the primary function of Pd is the disruption of
the formation of an oxide, due to the aversion O demonstrates for Pd in the
Pd/Fe surfaces. Consequently, the risk of catalyst deactivation due to the
formation of a surface oxide decreases with the addition of a Pd dopant on an
Fe surface, thereby promoting the HDO with minimal deactivation of the active
Fe surface.

            Quantifying
the lateral interactions between adspecies is also necessary in the development
of truly predictive, theoretical models for bimetallic catalysts as they will
significantly affect the coverage and diffusion of adspecies and therefore the
heterogeneous catalytic environment. However, such a characterization of
adspecies’ lateral interactions for the highly complex HDO reaction has not yet
been attempted. To such end, we have characterized the benzene-benzene lateral
interactions on Pt (111) and PtSn (111) surfaces. These systems have been well-characterized
experimentally using temperature-programmed desorption (TPD) [5]. Here, we studied the effect of surface coverage on
the adsorption energy of benzene on Pt (111) and Pt₃Sn (111) at
different adsorption sites by modeling the TPD of benzene. We found that a mean
field model is sufficient in describing the interactions between vicinal
benzene molecules (Figure 1). And while it was previously speculated that the
broad desorption peak of benzene on Pt (111) was the result of desorption from
two adsorption sites of different binding strengths [5], our results indicate that the behavior is more likely
due to coverage effects on the surface. Furthermore, we found that benzene’s
adsorption was significantly weakened on the Pt₃Sn (111) as compared
to Pt (111), which is consistent with experimental TPD [5]. Our investigation of modeling the lateral
interactions of an aromatic molecule on both the Pt (111) and the Pt₃Sn
(111) surfaces justifies future application of our method onto systems less
experimentally characterized, such as doped Fe surfaces.

Figure 1. The adsorption energy for benzene
on Pt (111) as a function of the benzene coverage.

References

[1] H.
Wang, J. Male, Y. Wang, ACS Catalysis 3 (2013) 1047-1070.

[2] Y.
Hong, H. Zhang, J. Sun, K. M. Ayman, A. J. R. Hensley, M. Gu, M. H. Engelhard,
J.-S. McEwen, Y. Wang, ACS Catalysis 4 (2014) 3335-3345.

[3] J. Sun,
A. M. Karim, H. Zhang, L. Kovarik, X. S. Li, A. J. Hensley, J.-S. McEwen, Y.
Wang, Journal of Catalysis 306 (2013) 47-57.

[4] A. J.
R. Hensley, Y. Hong, R. Zhang, H. Zhang, J. Sun, Y. Wang, J.-S. McEwen, ACS
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[5] C. Xu,
Y. L. Tsai, B. E. Koel, The Journal of Physical Chemistry 98 (1994) 585-593.

[6] A. J.
R. Hensley, R. Zhang, Y. Wang, J.-S. McEwen, The Journal of Physical Chemistry
C 117 (2013) 24317-24328.

[7] A. J.
R. Hensley, S. Schneider, Y. Wang, J.-S. McEwen, RSC Advances 5 (2015)
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[8] C.
Tsai, K. Lee, J. S. Yoo, X. Liu, H. Aljama, L. D. Chen, C. F. Dickens, T. S.
Geisler, C. J. Guido, T. M. Joseph, C. S. Kirk, A. A. Latimer, B. Loong, R. J.
McCarty, J. H. Montoya, L. Power, A. R. Singh, J. J. Willis, M. M. Winterkorn,
M. Yuan, Z.-J. Zhao, J. Wilcox, J. K. Nørskov, Catalysis Letters  (2016) 1-7.