(461g) First-Principles Investigation of the Contribution of Electronic and Geometric Effects and Collision Dynamics to the Dissociation and Coupling of Methane on Small Copper Clusters

Small transition metal clusters like those of copper exhibit unique size and morphology dependent catalytic activity.^1-4 Abundant potentially exploitable reserves of methane exists on the earth and thus, its conversion to fuels, useful chemicals and materials is a pertinent challenge.⁵ The search for alternate minimum energy pathways and catalysts to transform methane to useful chemicals and carbon nanomaterials led us to investigate Collision Induced Dissociation (CID)^6,7 of methane on small Cu clusters. Car Parrinello molecular dynamics⁸ and metadynamics⁹ simulations were performed to investigate the collision induced dissociation and coupling of methane on small Cu clusters (Cu_n where n=2 to 12) and the free energy barriers were computed. The collision induced activation of the stretching and bending vibrations of methane significantly reduces the free energy barrier for its dissociation. Electron Localisation Function (ELF)¹⁰ topology analysis¹¹ suggests that increase in the copper cluster size and order of dimension, increases the delocalisation of electron density within the cluster and makes more number of sites available for the chemisorption of CH₃ and H upon dissociation. The formation of a collision complex, which we refer to as ‘precursor’, allows methane and the cluster to sample relevant vibrational states of methane and lattice configurations of the cluster. This enables higher probability of favourable alignment of the C-H stretching vibration of methane towards regions of high electron density within the cluster. These characteristics contribute in lowering the free energy barrier for dissociation of methane. Severe distortion of the cluster geometry and lattice reorganisation due to high temperature collision dynamics disturbs electron delocalisation within them and increases the barrier for dissociation. Complete dehydrogenation of methane to carbon on these clusters has extremely high barriers and the barrier increases with increase in cluster size. Coupling reactions of CH_x (x=1 to3) species and recombination of H with CH_x have free energy barriers significantly lower than complete dehydrogenation of methane. Thus, competition favours the coupling and recombination reactions at high hydrogen saturation on the clusters.

References

1. Kabir, M., Mookerjee, A. & Bhattacharya, A. K. Copper clusters: Electronic effect dominates over geometric effect. European Physical Journal D 31, 477-485 (2004).

2. Guvelioglu, G. H., Ma, P., He, X., Forrey, R. C. & Cheng, H. Evolution of small copper clusters and dissociative chemisorption of hydrogen. Physical Review Letters 94, 026103 (2005).

3. Forrey, R. C., Guvelioglu, G. H., Ma, P., He, X. & Cheng, H. Rate constants for dissociative chemisorption of hydrogen molecules on copper clusters. Physical Review B - Condensed Matter and Materials Physics 73, 155436 (2006).

4. Yuan, X. et al. Theoretical investigation of adsorption of molecular oxygen on small copper clusters. Journal of Physical Chemistry A 115, 8705-8712 (2011).

5. Lunsford, J. H. Catalytic conversion of methane to more useful chemicals and fuels: A challenge for the 21st century. Catalysis Today 63, 165-174 (2000).

6. Rettner, C. T., Pfn̈r, H. E. & Auerbach, D. J. Dissociative chemisorption of CH4 on W(110): Dramatic activation by initial kinetic energy. Physical Review Letters 54, 2716-2719 (1985).

7. Beckerle, J. D., Yang, Q. Y., Johnson, A. D. & Ceyer, S. T. Collision-induced dissociative chemisorption of adsorbates: Chemistry with a hammer. The Journal of Chemical Physics 86, 7236-7237 (1986).

8. Car, R. & Parrinello, M. Unified approach for molecular dynamics and density-functional theory. Physical Review Letters 55, 2471-2474 (1985).

9. Laio, A. & Parrinello, M. Escaping free-energy minima. Proceedings of the National Academy of Sciences of the United States of America 99, 12562-12566 (2002).

10. Becke, A. D. & Edgecombe, K. E. A simple measure of electron localization in atomic and molecular systems. The Journal of Chemical Physics 92, 5397-5403 (1990).

11. Silvi, B. & Savin, A. Classification of chemical bonds based on topological analysis of electron localization functions. Nature 371, 683-686, (1994).