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

Adsorption and dissociation of hydrocarbons on metallic surfaces represent crucial steps to carburization of metal. Here, we use density functional theory total energy calculations with the climbing-image nudged elastic band method to estimate the adsorption energies and dissociation barriers for different CO coverages with surface supercells of different sizes. For the absorption of CO, the contribution from van der Waals interaction in the computation of adsorption parameters is found important in small systems with high CO-coverages. 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 larger surface systems associated with dilute CO-coverages, the dissociation barrier is significantly decreased. The elastic deformation of the surface is generic and can potentially applicable for all similar metal-hydrocarbon reactions and therefore a dilute coverage is necessary for the simulation of these reactions as isolated processes.Comment: 12 pages, 6 figures. Submitted to Journal of Applied Physic

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

Oil and gas infrastructures are submitted to extreme conditions and off-shore rigs and petrochemical installations require expensive high-quality materials to limit damaging failures. Yet, due to a lack of microscopic understanding, most of these materials are developed and selected based on empirical evidence leading to over-qualified infrastructures. Computational efforts are necessary, therefore, to identify the link between atomistic and macroscopic scales and support the development of better targeted materials for this and other energy industry. As a first step towards understanding carburization and metal dusting, we assess the capabilities of an embedded atom method (EAM) empirical force field as well as those of a ReaxFF force field using two different parameter sets to describe carbon diffusion at the surface of Fe, comparing the adsorption and diffusion of carbon into the 110 surface and in bulk of a-iron with equivalent results produced by density functional theory (DFT). The EAM potential has been previously used successfully for bulk Fe-C systems. Our study indicates that preference for C adsorption site, the surface to subsurface diffusion of C atoms and their migration paths over the 110 surface are in good agreement with DFT. The ReaxFF potential is more suited for simulating the hydrocarbon reaction at the surface while the subsequent diffusion to subsurface and bulk is better captured with the EAM potential. This result opens the door to a new approach for using empirical potentials in the study of complex material set-up

Electronic structure study of defects and impurities in oxide semiconductors

THESIS 9608In this work, the crystal and electronic structures of defects and impurities in ZnO and Fe3O4 are studied using first principles calculations. B3LYP hybrid density-functional theory calculations were used with supercell method to evaluate electronic structures and formation energies of intrinsic vacancy defects oxygen(Vo), zinc (Vzn) and zincoxygen pair (Vzno) vacancies. The magnetic exchange couplings of well-separated, singly negatively charged defects were also calculated and were found to be induced by a conduction band electron when the defect levels are partially filled, more than half-filling

Defect-trapped electrons and ferromagnetic exchange in ZnO

A model for ferromagnetism observed at ambient temperature in films of oxides such as ZnO is proposed and evaluated. The ferromagnetic moment in the model arises from electrons trapped at negatively charged vacancies in an n-type oxide. These vacancies are capable of trapping either one or two electrons. Trapped electrons are described by a one-band Hubbard Hamiltonian where the Hubbard U is the effective electron-electron repulsion for a pair of electrons in a vacancy. Ferromagnetism is known to exist in the Hubbard model applied to periodic three-dimensional (3D) lattices, provided the Hubbard U parameter exceeds the defect bandwidth W and the filling is away from half or complete filling. Hybrid and local-density approximation density-functional theory calculations are used to evaluate Hubbard model parameters for electrons trapped in defects in ZnO. They are also used to calculate magnetic exchange couplings of well-separated, singly negatively charged defects, which are induced by a conduction band electron. Strong ferromagnetic coupling between defects is found in these total-energy calculations over a range exceeding 10 angstrom when the defects have a large, positive Hubbard U value. Hubbard U values for oxygen (V(O)), zinc (V(Zn)), and zinc-oxygen complex (V(ZnO)) vacancies in various charge states are estimated from defect transition levels. V(ZnO)(-), the negatively charged ZnO pair vacancy, and V(Zn)(-) are proposed as possible sources of magnetic moment in ferromagnetic ZnO films. These vacancies can trap one or two electrons and their charge transition levels lie in the band gap. Some literature values of U and those obtained here for unrelaxed vacancies are large enough to support a Hubbard model for ferromagnetism; however, U values obtained depend strongly on lattice relaxation. The relaxed vacancies considered here have U/W values which are not large enough for ferromagnetism using the simple criterion U/W > 1

Elucidating the role of extended surface defects at Fe surfaces on CO adsorption and dissociation

The adsorption and dissociation of hydrocarbons on metallic surfaces during catalytic reactions in a steam reforming furnace often lead to the carburization of the catalysts and metallic surfaces involved. This process is greatly accelerated by the presence of intrinsic defects like vacancies and grain boundaries and is succeeded by surface to subsurface diffusion of C. We employ both density functional theory and reactive force field molecular dynamics simulations to investigate the effect of surface defects on CO dissociation rate directly related to metaldusting corrosion. We demonstrate that stable surface vacancy clusters with large binding energies accelerate the adsorption of CO molecules by decreasing the corresponding dissociation energies. In addition, we demonstrate that the appearance of multiple GBs at the surface leads to an enhancement of the CO dissociation rate. Furthermore, we demonstrate that the increase in surface roughness by emerging GBs leads to an increase in CO dissociation rate