Ab-initio and molecular dynamics study of Cobalt nanoparticle catalyst for Fischer-Tropsch synthesis

 

Chandana Ghosh, Subramanian K.R.S. Sankaranarayanan, Venkat R. Bhethanabotla and Babu Joseph

Sensors Research Laboratory, Department of Chemical Engineering,

University of South Florida, Tampa, Florida, 33620, USA

 

Abstract

 

Cobalt particles are widely used as catalysts in the Fischer-Tropsch synthesis (FTS) which involves reaction of carbon monoxide and hydrogen to form hydrocarbons^1-3. It has been observed that particles in the size range between 6-8 nm have show maximum catalytic activity. This has been attributed to their surface area as well as their ability to stabilize steps on which the CO dissociation can occur. Cluster sizes below this range have shown a decrease in catalytic activity and it is still not understood as to why this happens. To obtain an in-depth understanding of the smaller Cobalt nanocluster morphologies we need to perform a detailed study on their stable configurations at the FTS temperature ranges. In this work, this is done using simulated annealing of Cobalt clusters in the size range 1-8 nm using molecular dynamics in a canonical ensemble, with metal-metal interactions modeled by tight-binding, second moment approximation (TB-SMA)⁴. The stable configurations obtained using MD simulations are utilized to derive starting configurations for density functional theory (DFT) calculations to study the adsorption and dissociation mechanism of CO on the Cobalt surfaces and their subsequent reconstruction. A comparison of the structures before and after reconstruction can yield useful information about  (1) initial mechanism of FTS and whether it is indeed particle size sensitive (2) the change in the surface structure due to reconstruction and how it relates to the bonding and charge transfer (3) whether the smaller particles can accommodate the required reconstruction in the presence of reagents and (4) whether the formation of CoO and reduction to Co and/or the formation of CoC at smaller particle sizes decreases or enhances the catalytic activity. Reference:

 

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4. F. Cleri and V. Rosato, Tight-binding potentials for transition metals and alloys  Phys. Rev. B  48, 22 (1993).