Insights on finite size effects in ab initio study of CO adsorption and dissociation on Fe 110 surface

Abstract

Adsorption and dissociation of hydrocarbons on metallic surfaces represent crucial steps on the path to carburization, eventually leading to dusting corrosion. While adsorption of CO molecules on Fe surface is a barrier-less exothermic process, this is not the case for the dissociation of CO into C and O adatoms and the diffusion of C beneath the surface, that are found to be associated with large energy barriers. In practice, these barriers can be affected by numerous factors that combine to favour the CO-Fe reaction such as the abundance of CO and other hydrocarbons as well as the presence of structural defects. From a numerical point of view, studying these factors is challenging and a step-by-step approach is necessary to assess, in particular, the influence of the finite box size on the reaction parameters for adsorption and dissociation of CO on metal surfaces. Here, we use density functional theory (DFT) total energy calculations with the climbing-image nudged elastic band (CI-NEB) method to estimate the adsorption energies and dissociation barriers for different CO coverages with surface supercells of different sizes. We further compute the effect of periodic boundary condition for DFT calculations and find that the contribution from van der Waals interaction in the computation of adsorption parameters is important as they contribute to correcting the finite-size error in small systems. The dissociation process involves carbon insertion into the Fe surface causing a lattice deformation that requires a larger surface system for unrestricted relaxation. We show that, in the larger surface systems associated with dilute CO-coverages, Cinsertion is energetically more favourable, leading to a significant decrease in the dissociation barrier. This observation suggests that a large surface system with dilute coverage is necessary for all similar metal-hydrocarbon reactions in order to study their fundamental electronic mechanisms, as an isolated phenomenon, free from finite-size effects.

Influence of surface vacancy defects on the carburisation of Fe 110 surface by carbon monoxide

“…The C-O bond is stretched The adsorption energies for the clean surface are slightly larger in comparison with the ones calculated in previous work which are in the range of 1.7-2.0 eV. 14,15,17 We have shown in an earlier work 28 that this small advantage in adsorption is facilitated by a dilute coverage with negligible van der Waals repulsion between the neighbouring CO molecules. As for the surface with vacancy defect, an adsorption site in a close vicinity of the vacancy such as the one right into the vacancy and the bridge site next to it have thermodynamically less favourable adsorption energies and are unstable.…”

“…This topic has been revisited by a number of recent works. [23][24][25][26][27] While a dense CO-coverage (≤0.25 monolayer) is commonly considered, we have shown that the dissociation barrier is greatly reduced at a dilute coverage (up to 0.0625 monolayer), 28 due to the unrestricted relaxation of the Fe lattice. However, the presence of intrinsic defects is a likely candidate that can significantly affect the carburisation process and has not been considered yet.…”

“…Previous work with PAW-PBE pseudo-potential method confirmed that a vacuum of 10 Å is adequate as the interaction between slabs due to the periodic boundary condition is negligible. 28,45,46 The top four layers are free to relax in all 3 dimensions. The next two layers are free to relax only in the direction perpendicular to the surface while the remaining layers are kept fixed with atoms in their ideal positions.…”

“…In an earlier work we have shown that the 110 surface is highly stable with minimum reconstruction associated. 28 Only the atoms in the top 3 layers are displaced. Therefore for computationally demanding calculations such as dissociation barriers, we select a 4 × 4, 7-layer thick supercell with 112 atoms and 16 Å of vacuum.…”

Carbon adsorption on and diffusion through the Fe(110) surface and in bulk: Developing a new strategy for the use of empirical potentials in complex material set‐ups

“…The remaining layers were kept frozen to imitate an infinite bulk. It is shown in reference that the structure relaxation for the reconstruction of the surface converges over a depth of less than five layers. This can be seen in Table .…”