(269h) Coverage Dependent Adsorption of Phenol on Pt (111): Estimating the Lateral Interactions Exhibited By Bio-Oil Model Compounds Under Hydrodeoxygenation Reaction Conditions

Authors: 
Chaudhary, N., Washington State University
Wang, Y., Pacific Northwest National Laboratory
Hensley, A., Washington State University
McEwen, J. S., Washington State University

ABSTRACT

Coverage-Dependent Adsorption of
Phenol on Pt (111): Estimating the Lateral Interactions Exhibited by Bio-Oil Model
Compounds under Hydro-deoxygenation Reaction Conditions

Neeru Chaudhary, Alyssa J.R.
Hensley, Yong Wang, and Jean-Sabin McEwen

Biofuels have gained attention as a
long-term, environmentally friendly alternative energy source that has the
potential to meet the increasing energy demands from a growing world
population. Lignocellulosic biomass can be sufficiently converted to liquid fuels
with high yields of about 70-80% through fast pyrolysis.[1] However, the
composition of biomass-derived oils are quite different as compared to
petroleum oils mostly due to the presence of water and oxygen in the form of
phenolic, aldehyde, ketone, and carboxylic compounds, which results in
undesired properties that hinder its direct utilization as a transportation
fuel.[1, 2] Bio-oil can be upgraded to fuels by catalytic hydro-deoxygenation (HDO) under high pressures and moderate
temperatures in the presence of hydrogen,[3] but requires the use of expensive metal
catalysts such as Pt, Pd, Ru, or Rh.[2] One way of mitigating this economic
drawback is to use inexpensive base metals like Fe and Cu with small amounts of
a noble metal, which improves the stability and activity of the base metal.[4,
5] Currently, the HDO mechanism is usually studied at low to medium coverages,
for which there are no adsorbate-adsorbate interactions. However, such
interactions can affect the chemical kinetics to a great extent.[6, 7]
Therefore, it is important to understand not only the effect of the substrate on
key HDO ad-species, but also the lateral interaction between ad-species in
order to get a clear picture of the surface under realistic reaction conditions.

In this work, we use density
functional theory (DFT) to model the coverage dependent adsorption energetics
of phenol on Pt (111). Several adsorption sites were tested that varied both
the ring position and the functional group position over the surface, as shown
in Figure 1a. Over the entire coverage range tested, the bridge
30° ring position followed by HCP 0° ring position were the most favorable,
with the most favorable functional group position being above surface bridge
sites. In general, the change in the ring position has a much larger energetic
effect than the functional group position.

Our results show that the phenol adsorption
energy varies fairly linearly with the coverage for all the adsorption configurations
(Figure 1b); however, near saturation the adsorption of phenol
slightly deviates from its linear behavior. This could be the result of
stronger through-space phenol-phenol repulsions at higher coverages as compared
to low coverages where such interactions are almost negligible due to the high
separation between ad-species. These through-space repulsions are manifested
through a decrease in the dihedral CO bond angle, which decreases from 111° to
107° as the nearest distance between the adsorbates decreases from 8.8 Å to 1.9
Å, From these adsorption energies, we then compute the average differential
heat of adsorption, which agrees well with the corresponding experimental data,
as shown in Figure 1c.[8]  This comparison shows that we have accurately
captured the coverage dependent adsorption energetics of phenol on Pt (111)
within a mean-field model. Since a mean field model can also applied for the
adsorption of benzene on Pt(111),[9] this also indicates that such a mean field
model can be extended to other aromatic molecules of interest. As such, it
provides a simple method for including coverage effects into micro-kinetic
models, thereby making such models significantly more realistic. Overall, this
work will have a large influence on the current state-of-the-art in modeling in
HDO surface reactions by allowing the use of simple models to incorporate
coverage effects for large, aromatic compounds.

Figure
1.

Adsorption of phenol on Pt (111). (a) Most favorable adsorption sites. (b)
Adsorption energy of phenol at each site as a function of coverage. (c) Comparison
between the differential heat of adsorption as obtained in a mean field model
and the experimental data.

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