This journal is © the Owner Societies 2014 Phys. Chem. Chem. Phys., 2014, 16, 2399--2410 | 2399 Cite this: Phys. Chem. Chem. Phys., 2014, 16, 2399 Density functional theory studies of methyl dissociation on a Ni(111) surface in the presence of an external electric field Fanglin Che, a Renqin Zhang, a Alyssa J. Hensley, a Su Ha a and Jean-Sabin McEwen* ab To provide a basis for understanding the reactive processes on nickel surfaces at fuel cell anodes, we investigate the influence of an external electric field on the dehydrogenation of methyl species on a Ni(111) surface using density functional theory calculations. The structures, adsorption energies and reaction barriers for all methyl species dissociation on the Ni(111) surface are identified. Our results show that the presence of an external electric field does not affect the structures and favorable adsorption sites of the adsorbed species, but causes the adsorption energies of the CH x species at the stable site to fluctuate around 0.2 eV. Calculations give an energy barrier of 0.692 eV for CH 3 * - CH 2 * + H*, 0.323 eV for CH 2 * - CH* + H* and 1.373 eV for CH* - C* + H*. Finally, we conclude that the presence of a large positive electric field significantly increases the energy barrier of the CH* - C* + H* reaction more than the other two reactions, suggesting that the presence of pure C atoms on Ni(111) are impeded in the presence of an external positive electric field. 1. Introduction Solid oxide fuel cells (SOFC) constitute an attractive power- generation technology that converts the chemical energy of various logistic fuels directly into electricity, while maintaining a high electrical efficiency and causing little pollution through electro- chemical processes. 1,2 The SOFC system is a widely studied system that uses methane as its fuel, mainly because it is abundantly available in natural gas and is a simple compound to model. 3 Recently, shale gas has received a tremendous amount of interest as a new future energy source. 4 In the future, this shale gas can be directly fed into the SOFC system and it can be more efficiently converted into electrical energy than the conventional gas turbine rote. 5,6 Such a technology can contribute to the additional energy required for our future society with a ‘‘more electrical’’ platform. There are two main reactions that take place in a SOFC system, as shown below: 7,8 CH 4 +H 2 O 2 CO + 3H 2 (1) CO + 3H 2 + 4O 2À - CO 2 +H 2 O + 8e À (2) At the anode, the steam can be either co-fed with the fuel to impede the carbon deposition rate or produced as the byproduct from the electrochemical oxidation reaction. Under the presence of this steam, the steam-reforming reaction takes place to produce syngas. The syngas is then electrochemically oxidized by reacting with the lattice oxygen from the cathode, producing CO 2 ,H 2 O, and electrons as products. These electrons flow through the external circuit to produce the net electrical power, and combine with gas phase oxygen at the cathode to produce the lattice oxygen and complete the electrical circuit. The rate-limiting step for this reforming reaction over the anode surface is the dissociative chemisorption of methane. 9 Nickel (Ni) is the most commonly used catalyst in industrial steam reforming processes today, due to its economic advantage. 10 Currently, many researchers are investigating the sequential dehydrogenation of methane (CH 4 ) on a Ni catalyst at a mole- cular level in order to have a better understanding of the overall reaction mechanism. However, the main disadvantage of using Ni as a catalyst for this reaction is that the carbon–carbon bonds are easily formed even under the presence of steam on the Ni active surface and decreases the activity of the catalyst over the long operation time. 11 In order to control the ongoing reactions in the SOFC, one can modify the selectivity of the Ni catalyst. Since CH 4 physisorbs on Ni, 12,13 various theoretical methods have focused more on the dissociative chemisorption of methyl species, providing important data concerning the thermodynamics of CH x inter- conversion. Early studies show that the Ni(111) surface is the least reactive of the low index surfaces; 14 however, as the most a The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, WA, 99164, USA. E-mail: js.mcewen@wsu.edu b Department of Physics and Astronomy, Washington State University, WA, 99164, USA Received 1st October 2013, Accepted 28th November 2013 DOI: 10.1039/c3cp54135e www.rsc.org/pccp PCCP PAPER View Article Online View Journal | View Issue