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Quantum and Classical Dynamics of Methane ScatteringThis project was finished with a set of publications, and a successful Ph.D. thesis defence.The dissociation of methane on transition metals is an important reaction in catalysis. It is the rate limiting step in steam reforming to produce syngas. Molecular beam experiments have shown that the energy in the internal vibrations are about as effective as the translational energy in inducing dissociation.The published wavepacket simulations on the methane dissociation reaction on transition metals have treated the methane molecule always as a diatomic up to now. Besides the C-H bond and molecule surface distance, a combination of other coordinates were included, like (multiple) rotations and some lattice motion. None of them have looked at the role of the internal vibrations. We were not able yet to simulate the dissociation including all internal vibrations. Instead we simulated the scattering of methane in fixed orientations, for which all internal vibrations can be included, and used the results to deduce consequences for the dissociation. We have been using the multiconfigurational time-dependent Hartree (MCTDH) method for our wavepacket simulation, because it can deal with a large number of degrees of freedom and with large grids. We have started with a study on different model potential energy surfaces (PESs) that have been developed with Ni(111) in mind. We found that the scattering of CH4 is almost completely elastic for all model PESs. Vibrational excitations when the molecule hits the surface and the corresponding deformation show that for methane to dissociate the interaction of the molecule with the surface should lead to an elongated equilibrium C-H bond length close to the surface. We studied the isotope effects with CD4 in the same way, and found an elastic scattering somewhat less than for CH4. Energy distribution analysis at the surface of the expectation values of the kinetic energy operators and terms potential energy terms gives enhanced insight in the scattering process. Our simulations give an indications that the isotope effect in the methane dissociation is caused mostly by the difference in the scattering behaviour of the molecule in the orientation with three bonds pointing towards the surface. Next we looked at the role of single vibrational excitations at the different orientations. A high increase of vibrational kinetic energy results in higher inelastic scattering. The highest increase of vibrational kinetic energy and of the accessibility of the entrance channel for dissociation are found for the v3 asymmetrical stretch mode, and especially for the v1 symmetrical stretch mode. This indicates that the v1 will give the highest enhancement of the dissociation probability. We ended with classical trajectory calculations of the rotational vibrational scattering of a non-rigid methane molecule from a Ni(111) surface. Energy dissipation and scattering angles have been studied as a function of the translational kinetic energy, the incident angle, the (rotational) nozzle temperature, and the surface temperature. Scattering angles are some degrees towards the surface for the incident angles of 30o, 45o, and 60o at a translational energy of 96 kJ/mol. Energy loss is primarily from the normal component of the translational energy and transfered for somewhat more than half to the surface and the rest mostly to rotational motion. List of publications
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These pages are maintained by Bouke Bunnik (B.S.Bunnik@tue.nl). Comments and suggestions are welcome. |
