(320f) Dehydrogenation of Water Species on Ni Catalysts from First Principles: Investigations of Electric Field Influence

Solid oxide fuel cells will no doubt find more widespread use in the new millennium in the conversion of chemical to electrical energy offering, as they do, very high efficiencies and a unique scalability in electricity-generation applications [1]. To control the ongoing reactions in such a device one can modify the selectivity of the catalyst itself, which oxidizes the fuel at the anode and dissociates O₂ at the cathode. The selectivity of the catalyst in a solid oxide fuel cell will be altered by the presence of a huge electric field of the order of 1 Volts per Angstrom, which occurs near the electrode-electrolyte interface [2]. Experimentally, Sekine et al. [3-5] studied the electrocatalytic steam reforming of methane in the presence of an electric field. They found that in the presence of an electric field the conversion of methane over Pd/CeO₂, Rh/CeO₂ and Pt/CeO₂ increased. In particular, they found that the conversion of methane occurred at 423K with an external electric field, which is a temperature where a conventional catalytic reaction would hardly proceed. Given this background, it is surprising that the role of the electric field is often ignored in the optimization of such devices.

In order to provide insight into the electrocatalytic mechanisms of the hydrocarbon steam reforming reaction on Ni surfaces, an investigation of how an external electric field influences on the dehydrogenation of water species on Ni surfaces was carried out using density functional theory calculations. We found that an external electric field significantly changed the adsorption structures and charge distribution of water species on the Ni(111) and (211) surfaces. As a result, the electric fields greatly influenced the adsorption energies of water species and reaction barriers of dehydrogenation processes. The electric field effect on the energy barriers is much larger than that of our previous work on the methane dehydrogenation [6, 7]. Particularly for Ni(111), when we applied a positive electric field, the energy barrier of the dehydrogenation of OH increases by 0.3 eV as compared to when an electric field is absent. Our results suggest that a positive electric field significantly suppresses the dehydrogenation of water species on Ni catalysts.

1.         Nikolla, E., J. Schwank, and S. Linic, Direct Electrochemical Oxidation of Hydrocarbon Fuels on SOFCs: Improved Carbon Tolerance of Ni Alloy Anodes. J. Electrochem. Soc., 2009. 156(11): p. B1312-B1316.

2.         Stuve, E.M., Ionization of water in interfacial electric fields: An electrochemical view. Chem. Phys. Lett., 2012. 519-520: p. 1-17.

3.         Sekine, Y., M. Haraguchi, M. Tomioka, M. Matsukata, and E. Kikuchi, Low-Temperature Hydrogen Production by Highly Efficient Catalytic System Assisted by an Electric Field. J. Phys. Chem. A, 2009. 114(11): p. 3824-3833.

4.         Sekine, Y., M. Tomioka, M. Matsukata, and E. Kikuchi, Catalytic degradation of ethanol in an electric field. Catal. Today, 2009. 146(1–2): p. 183-187.

5.         Sekine, Y., M. Haraguchi, M. Matsukata, and E. Kikuchi, Low temperature steam reforming of methane over metal catalyst supported on CexZr1−xO2 in an electric field. Catal. Today, 2011. 171(1): p. 116-125.

6.         Che, F., R. Zhang, A.J. Hensley, S. Ha, and J.-S. McEwen, Density functional theory studies of methyl dissociation on a Ni(111) surface in the presence of an external electric field. Phys. Chem. Chem. Phys., 2014. 16(6): p. 2399-2410.

7.         Che, F., A. Hensley, S. Ha, and J.-S. McEwen, Decomposition of Methyl Species on a Ni(211) surface: Investigations of the Electric Field Influence.Catal. Sci. and Technol. (submitted), 2014.