Reactive molecular dynamic simulations of hydrocarbon dissociations on Ni(111) surfaces

doi:10.1016/j.susc.2011.11.035

Surface Science

Volume 606, Issues 5–6, March 2012, Pages 615-623

https://doi.org/10.1016/j.susc.2011.11.035 Get rights and content

Abstract

Empirical potential parameters for H, C and Ni elements have been developed for the ReaxFF force field in order to study the decomposition of small hydrocarbon molecules on nickel using molecular dynamics simulations. These parameters were optimized using the geometrical and energetic information obtained from density functional (DFT) calculations on a subset of hydrogen and methane reactions with nickel (111) surfaces. The resulting force field was then used to obtain a molecular perspective of the dynamics of the methane dissociative adsorption on Ni(111) as well as two other small alkane molecules, ethane and n-butane. NVT simulations of dissociative adsorption of methane over a range of temperatures enabled the estimation of the sticking coefficient for the adsorption as well as the activation energy of the first C–H bond breaking. The rate constants of each elementary step (both forward and reverse) of CH_x dissociation on Ni(111) were obtained by monitoring the surface species and a microkinetic model was constructed as a result. Qualitative analyses of the simulations of ethane and n-butane decompositions on Ni(111) demonstrate that such reactive MD technique can also be used to obtain useful information on complex reaction networks.

Highlights

► Force field development for Ni/H/C systems based on first-principles data. ► MD simulations on CH₄, C₂H₆ and n-C₄H₁₀ dissociative chemisorptions on Ni(111). ► Use of microkinetic models for CH₄ dissociation and coke formation analyses.

Introduction

The quest for efficient use of fossil-based energy resources includes intense interest in the direct use of hydrocarbon feedstock by solid-oxide fuel cells (SOFCs) [1], [2]. This process can effectively avoid the limitation of storing and transporting hydrogen and is potentially of much higher efficiency than that can be achieved in a conventional energy use scenario. However, SOFCs are plagued by unwanted reactions on anodes such as soot formation [3] and sulfur poisoning [4], [5], [6], which must be resolved before larger scale application of this technology. Towards this end, it is of great value to extend the reach of computational inquiries so that they can provide both qualitatively and quantitatively accurate descriptions of the complex reactions associated with the decomposition of hydrocarbons and initiation of carbon coke formation. The current work takes a step in this direction.

Molecular Dynamics (MD) provides an explicit approach for studying chemical reactions. Molecular trajectories can be easily monitored in order to quantify the evolution of surface species. In addition, precise reaction conditions can be modeled and explored.

MD has the potential to generate macroscopic kinetic information for surface reactions. A semi-empirical potential energy surface constructed by Lee and DePristo[7] with many-body expansions (e.g. 3-body, 4-body and so on) approximated by the Morse potential to study hydrogen dissociation on Ni(111) and Ni(100) surfaces. However, the LEPS (London–Eyring–Polanyi–Sato potential energy surface) used does not provide a very accurate reactive potential energy surface. A theoretical model using reduced dimensions, i.e., Z (methane-surface distance) and D (methyl-H distance) presented by Harris and Luntz et al. [8], was used to explain the thermally assisted tunneling of methane dissociations on platinum surfaces. Nevertheless, possible steric hindering was eliminated and might affect the methane dissociation behavior. Based on Harris and Luntz's work, another reduced-dimension model was proposed by Weaver et al. [9] to extend its applications to other alkanes such as ethane, propane, and neo-pentane to investigate the steric factors in the C-H bond dissociation. The steric effect deficiency noted in the Harris’ model has been addressed in Weaver's work, but still in an empirical fashion. Some quantitative analyses were conducted using relatively computationally simple models. For example, Gislason and Sellers [10] used a modified bond order based Morse-type potential for C₂ hydrocarbons so that Arrhenius rate constants, pre-exponential factors and activation barriers could be obtained. It would be ideal that qualitative and quantitative analyses on hydrocarbon dissociation on solid surface can be studied using a more sophisticated and relatively assumption free theoretical model so that more detailed and accurate chemistry can be properly captured especially when bond breaking and formation are involved.

In this article, we report the development of a reactive force field (ReaxFF) for the nickel/hydrocarbon system since nickel is a common and effective reforming catalyst, an active ingredient in the fabrication of SOFC anodes, but nickel is also susceptible to deactivation due to carbon formation. The Ni-H-C reactive force field parameters will be obtained by fitting to the ab initio data and applied within a classical molecular dynamics (MD) setting to investigate the decompositions of alkanes and elucidate the origins of carbon soot formation. The simulations were performed to provide some insights in the dissociation mechanisms and estimate the kinetic data needed for macroscopic analyses. We obtained an estimate for the sticking coefficient of methane dissociative adsorption on Ni(111) at the zero coverage limit as well as the activation energy. The MD analyses were then coupled with a microkinetic modeling technique to quantify the step dissociations of methane on Ni(111). Similar methodology was then applied to provide a preliminary understanding of the complex reaction sequences associated with the decomposition of ethane and n-butane on Ni(111).

Section snippets

Force field

The reactive molecular dynamics potential, known as ReaxFF [11], has been found to accurately capture the reaction dynamics of a variety of molecular systems in both catalytic [12] and non-catalytic [13] environments. The total energy is expressed as a functional of the bond-order, which depends on inter-atomic distance. Bonded inter-atomic interactions are implemented in terms of bond stretching, angle bending, and torsional rotation within a molecule. Special molecular structures, such as,

Reactive force field development

The development of the ReaxFF for hydrocarbon, and hydrocarbon/transition metal (e.g., Ni, Cu) in the gas phase have been reported by van Duin et al. for a variety of hydrocarbon molecules [11] and catalyzed carbon fullerenes formations [12]. Mueller and coworkers have reported a recent development for the nickel/hydrocarbon system [25]. In this work, we make a separate attempt at extending ReaxFF to Ni/C/H systems. The parameters for C–H interactions developed in [11] are used. Therefore, the

Methane dissociations on Ni(111)

Experimental and theoretical studies report the energy barrier associated with methane dissociative adsorption on Ni(111) in the range of 12–20 kcal/mol [7], [27]. One of the major challenges to perform MD to simulate this reaction is that it should be treated as a rare event on the classical MD simulation time scale, which is supported by the extremely low reaction probability (< 10^− 9 on Ni(111) between 523 and 618 K) observed experimentally at low pressure [28].

The simulations of methane

Conclusions

We have generated a new reactive force field potential tailored for the interactions of hydrocarbons with nickel systems. The force field was applied within a classical MD-NVT setting to study the catalytic decomposition of several small hydrocarbon molecules, i.e., methane, ethane and n-butane on Ni(111). Quantum mechanical calculations were performed on nickel complexes molecules, nickel crystals as well as the (111) surface so that hydrocarbon–nickel interactions, more importantly

Acknowledgement

We would like to thank Dr. William A Goddard III and Dr. Adri C. T. van Duin for their valuable comments and suggestions on ReaxFF force filed and its parameter training during the preparation of this work. This work was financially sponsored by the U.S. Department of Energy, Office of Science, Grant No. DE-ER9542165. Computations were performed on the supercomputing cluster located at Golden, CO, which is managed by the Golden Energy Computing Organization (GECO) that uses sources acquired

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Reactive molecular dynamic simulations of hydrocarbon dissociations on Ni(111) surfaces

Abstract

Highlights

Introduction

Section snippets

Force field

Reactive force field development

Methane dissociations on Ni(111)

Conclusions

Acknowledgement

J. Mol. Catal. A: Chem.

J. Power Sources

Appl. Catal. A: Gen.

Surf. Sci.

Comput. Mater. Sci.

J. Catal.

J. Catal.

J. Catal.

J. Catal.

Appl. Catal A: Gen.

Nature

Chemical Review

J. Electrochem. Soc.

J. Chem. Phys.

Phys. Rev. Lett.

J. Chem. Phys.

J. Phys. Chem. A