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In the context of catalytic partial oxidation of methane to synthesis gas (CPO), the activation of methane is a critical rate determing step. A theoretic study of this step is performed, merging a two level approach.

First, this activation can be divided into a sequence of more elemental reaction steps, which are suitable for a study DFT as shown for example by Ciobica and co-workers [1]. This enables a quantification and ordering elemental in order to find the most relevant reaction steps, and increased insight into the influence of surface coverage as well as probabilities for (rhodium) surface reconstruction.

Transition metal surfaces show reconstruction under adsorption and roughening under reaction conditions. An extreme case, which is also found for the CPO process, is catalytic etching. This phenomenom was first described in detail by Schmidt in 1974 for platinum under conditions of ammonia oxidation [2]. A better understanding of these processes is essential to the development of practical catalysts as the issue is closely related to catalyst stability.

To obtain an understanding of this phenomena on a molecular level, and to asses the effects thereof on reactivity, we will use, as a second step, Molecular Dynamics simulations of (rhodium) surfaces in the presence of reactant ad-atom species (CH_x, C, H, CO). This requires us to work on a larger scale than possible using DFT, so that we must rely on an effective potential method. We use the Modified Embedded-Atom Method (MEAM) as developed by Baskes and co-workers [3,4] combined with MEAM potentials based on DFT calculations, as developed by van Beurden [5].

For the DFT calculations we use the VASP package [6], which solves the Kohn-Sham Hamiltonian with a plane-wave basis set and ultrasoft Vanderbilt pseudopotentials. The MEAM MD calculations are performed with the CAMELION package by Thijsse and co-workers [7].

References:

1. I.M. Ciobica, F. Frechard, R.A. van Santen, A.W. Kleyn, and J. Hafner, J. Phys. Chem. B 104, 3364-3369 (2000).
2. R.W. McCabe, T. Pignet, and L.D. Schmidt, J. Catal. 32, 114 (1974).
3. M.I. Baskes, Phys. Rev. B 46, 2727 (1992).
4. M.S. Daw and M.I. Baskes, Phys. Rev. Lett. 50, 1285 (1983); M.S. Daw and M.I. Baskes, Phys. Rev. B 29, 6443 (1984).
5. P. van Beurden and G.J. Kramer, Phys. Rev. B 63, 165106 (2001); Paul van Beurden.
6. G. Kresse and J. Furthm|ller, Comp. Mat. Sci. 6, 15 (1996); G. Kresse and J. Furthm|ller, Phys. Rev. B 54, 11169 (1996); also see the VASP description on our software page.
7. CAMELION is a joint product of ComMaS and CRI->SIS, both at Delft University of Technology, Faculty of Applied Sciences, Laboratory of Materials Science. Contact person is Dr. B.J. Thijsse (email: B.S.Thijsse@tnw.tudelft.nl, homepage: http://www.tm.tudelft.nl/secties/fcm/matphy/pp.html, see for software: http://www.tm.tudelft.nl/secties/fcm/matphy/software/software.htm).

These pages are maintained by Bouke Bunnik (B.S.Bunnik@tue.nl). Comments and suggestions are welcome.