(699d) Probing the Catalytically Active Phase of Cobalt Fischer-Tropsch Catalysts from First Principles: Predicting High Coverage Surface Hydroxyl Conformations Under Reaction Conditions

Authors: 
Collinge, G., Washington State University
Stampfl, C., University of Sydney
Kruse, N., Washington State University
McEwen, J. S., Washington State University

The reaction mechanism of
Fischer-Tropsch (FT) synthesis is a continual source of intrigue and debate.
While two classes of mechanisms have been proposed (namely, carbide vs.
CO-insertion), the composition and configuration of the catalytically active
phase has received a comparatively small amount of attention. Chemical transient
kinetic (CTK) experiments performed in our group on model Co FT catalysts have
yielded evidence for a CO-insertion mechanism, but just as importantly, these
experiments have also provided evidence for a "crowded" reactive
surface, where oxygen, hydrogen, and carbon bind to the surface of the catalyst
in quantities exceeding the monolayer limit [1]. Because of the considerable
delay in water formation in CTK experiments it is considered likely that the
catalytically active phase contains substantial amounts of surface hydroxyl
(OH). Such OH may therefore be considered a candidate precursor for
constructing the  "most abundant surface intermediate" ("masi")
necessary for inducing hydrocarbon chain lengthening. Irrespective of what the
mechanistic pathway for constructing the "masi" looks like, we
present here a DFT-based investigation of an OH-rich surface, anticipating that
such information will provide guidance for more complete mechanistic studies addressing
chain lengthening initiation and growth.  

By utilizing the Alloy Theoretic
Automated Toolkit (ATAT) [2,3] to generate a large variety of extended Co (111)
surfaces with different coverages and configurations of OH, we constructed a
large library of OH/Co(111) structures. The face centered cubic facet is chosen
because we wish to model smaller particle size Co, which typically transitions
from its native hcp phase to the fcc phase. From this library we find that
proximal OH have a large tendency to hydrogen bond as seen in Figure 1. These
structures are quite common in the calculations, which suggests they are likely
relevant to their stable surface phases. However, to actually test this notion,
a corresponding lattice gas (LG) model using these structures will be
constructed and subsequent grand canonical Monte Carlo simulations will be
performed. In this way, we will elucidate the nature of OH patches on Co
catalysts under FT synthesis conditions and their susceptibility to an
interaction with CO molecules. A prevalence of surface OH was previously
considered key in the CO dissociation on Co catalysts during FT [4], but may
also react with CO molecules to provide an oxygen- and hydrogen containing C1 "masi".
Perhaps more importantly, however, this result, combined with our previous CTK
evidence [1], suggests that modeling the FT reaction over purely metallic
Co—a common practice—is ill-advised if one wishes to properly test
the mechanism of the reaction

Figure
1. Example of predicted ground state structures of OH on Co(111) at 0.5 ML
(left panel) and 0.75 ML (right panel). All ground state structures below 0.5
ML do not exhibit hydrogen bonding and compression like that seen above.
However, all ground state structures above 0.5 ML do. Blue spheres are Co, red spheres
are O, and white spheres are H.

CITATIONS

1. Schweicher,
J., A. Bundhoo, and N. Kruse, Hydrocarbon Chain Lengthening in Catalytic CO
Hydrogenation: Evidence for a CO-Insertion Mechanism. Journal of the American
Chemical Society, 2012. 134(39): p. 16135-16138.

2. van
de Walle, A., M. Asta, and G. Ceder, The Alloy Theoretic Automated Toolkit: A
User Guide. Calphad - Computer Coupling of Phase Diagrams and Thermochemistry,
2002. 26: p. 539-553.

3. van
de Walle, A. and G. Ceder, Automating First-Principles Phase Diagram
Calculations. Journal of Phase Equilibria, 2002. 23: p. 348.

4. Gunasooriya,
G.T.K.K., et al., Key Role of Surface Hydroxyl Groups in C–O Activation
during Fischer–Tropsch Synthesis. ACS Catalysis, 2016. 6: p. 3660-3664.