Development and Application of the ReaxFF Potential for Heterogeneous Catalysis and Metal Oxidation: Toward the Dynamic Sampling of Large Free Energy Surface

Open Access
Author:
Zou, Chenyu
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 13, 2014
Committee Members:
  • Adrianus C Van Duin, Dissertation Advisor
  • Adrianus C Van Duin, Committee Chair
  • Michael John Janik, Committee Member
  • Donghai Wang, Committee Member
  • Lasse Jensen, Committee Member
Keywords:
  • ReaxFF
  • Heterogeneous Catalysis
  • Metal Oxidation and Diffusion
Abstract:
A ReaxFF force field has been developed to describe the complex catalytic chemical reactions on the surface of the iron/iron carbide Fischer-Tropsch (FT) catalysts. Based on fitting parameters against an extensive training set containing data obtained from both ab-initio calculations and experimental measurements, the ReaxFF potential can reproduce reasonably well the potential energy surface of a relevant Fe/C/H/O atomistic system. The force field is first validated by performing molecular dynamics (MD) simulations to describe the dissociative adsorption and desorption of hydrogen molecules on iron and iron carbide surfaces. It was found that the existence of carbon atoms at the subsurface sites tends to increase the hydrogen dissociation barrier on the surface, and also stabilizes the adsorbed surface hydrogen atom. We then used this force field to study the complex catalytic surface chemistry as one will typically encounter at the initial stage of the FT synthesis. By performing MD simulations using relevant atomistic systems, the carbon monoxide methanation and the hydrocarbon chain initiation processes were studied. It was found that the catalytic methanation initiates from the undissociated CO molecules absorbed on the surface of the catalyst. This process leads to the generation of surface absorbed CHx- groups which initiate the synthesis of methane and the hydrocarbon chain growth. Direct hydrogenation of the surface carbide was not observed in the simulation. Coordination analysis of the carbon atoms in the system shows that the surface carbon atoms tend to diffuse toward the subsurface sites. This diffusion indicates the tendency of the formation of iron carbide at elevated temperatures. Furthermore, MD simulations enable us to investigate the various reaction pathways of key intermediates under FT conditions. We found that the surface CH- groups can dissociate into surface carbon atoms or be further hydrogenated into CH2- groups. The latter is an important intermediate species in the synthesis of methane as well as the chain initiation. Results from the C-C coupling simulation suggested the preference of coupling between CH- and CH2- groups. These results agree with the available experimental observations and ab-initio based study. This study demonstrates that the ReaxFF reactive potential can efficiently probe the catalytic heterogeneous interface, generate complex reaction networks, and hence improve our mechanistic understanding of heterogeneous catalysis. During catalytic surface reactions, the metal materials will also experience oxidation and corrosion under realistic working conditions at high temperature. In order to study the metal oxidation phenomenon, we have developed a new ReaxFF potential for the Ni/O system. The force field was validated by performing MD simulations of self-diffusion of nickel and the interstitial diffusion of oxygen. The predicted diffusivity and the activation energy achieved quantitative agreement with their respective published values. Furthermore, this force field enables us to study the effects of vacancies on the diffusion of interstitial oxygen and the successive initiation of internal oxidation. A new oxygen diffusion mechanism is proposed in which the oxygen atom diffuses via the movement of the oxygen-vacancy pair. In addition, our MD simulation results suggest that the voids at the grain boundaries can induce local oxygen segregation due to the strong oxygen-vacancy binding effect. This segregation is responsible for the formation of nickel oxide particles in subsurface voids. These results demonstrate that the ReaxFF MD study can contribute to bridging the gap between the QM calculations and the experimental observations in the study of metal oxidation.