379663 Ab Initio Kinetic Monte Carlo Simulations of H2 Spillover Control on a Bimetallic Catalyst

Monday, November 17, 2014: 1:10 PM
307 (Hilton Atlanta)
Matthew Darby, Chemical Engineering, University College London, London, United Kingdom, Charles Sykes, Department of Chemistry, Tufts University, Medford, MA, Angelos Michaelides, London Centre for Nanotechnology, University College London, London, United Kingdom and Michail Stamatakis, Department of Chemical Engineering, University College London, London, United Kingdom

The heterogeneous catalysis of hydrogenation reactions plays a significant role within numerous areas such as the petrochemical, pharmaceutical and food industries. Precious metals including Pt, Pd and Ru are often employed as hydrogenation catalysts owing to their high reactivity towards hydrogen. Using such metals on a commercial scale can prove costly and so cheaper alternatives are being sought after.

Experiment has shown that doping a less reactive, cheaper host metal such as Cu, with a low concentration of a more reactive metal, such as Pd, increases the surface reactivity towards hydrogen with respect to the pure host metal [i]. The dopant metal provides a low energy pathway for hydrogen dissociation and facilitates spillover onto the host metal. Furthermore, control over the spillover, and hence the surface coverage, may be established through the selective binding of a spectator CO molecule at the minority metal site. The low energy pathway is selectively blocked, thereby inhibiting the dissociation or re-association of hydrogen, trapping the latter on the surface. This phenomenon has been termed the ‘molecular cork effect’ and may have implications in mediating the kinetics of hydrogen uptake in bimetallic catalysis. Though observed in experiment, the underlying cause of the molecular cork effect is still under investigation [ii].

First-principles kinetic Monte Carlo (KMC) was employed to elucidate the origin and underlying kinetics of the molecular cork effect. This multiscale theoretical approach entails a comprehensive molecular-level analysis at the nanoscale before extending to the catalyst level at the microscale. Density functional theory (DFT) has been used in order to calculate the ground state energies of a variety of hydrogen and CO binding configurations on the (111) surface of Pd, Cu and a Pd/Cu alloy. The binding energies computed support the observation that hydrogen binding at the minority Pd site within the Pd/Cu alloy is more stable than binding to pure Cu. Furthermore the affinity of CO for Pd during ‘corking’ is also investigated, with CO binding more strongly to Pd than Cu within the alloy. Additionally, Nudged Elastic Band (NEB) calculations were performed to compute the transition state energy for hydrogen dissociation and diffusion across the Pd/Cu alloy surface. The barriers calculated corroborate the experimental findings; diffusion barriers were found to be low, thereby promoting facile spillover. Moreover, the dissociation barriers specifically at the Pd minority site were found to be lower than on pure Cu or elsewhere on the alloy surface.

The binding and lateral interaction energies along with NEB barrier heights and gaseous formation energies are taken from our DFT calculations to parameterise KMC simulations. Subsequently, first-principles KMC was able to reproduce the observations seen in experiment, with the resulting data extracted from the simulations providing valuable insight into an unknown mechanism and also aiding in the design of improved hydrogenation catalysts.

[i] Tierney, H. L., Baber, A. E., Kitchin, J. R., Sykes, E. C. H., Phys. Rev. Lett., 2009, 103, 246102.

[ii] Marcinkowski, M. D., Jewell, A. D., Stamatakis, M., Boucher, M. B., Lewis, E. A., Murphy, C. J., Kyriakou, G.., Sykes, E. C. H., Nat. Mater., 2013, 12, 523.

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