April 2013 Archives

We utilize density functional theory to explore hydrogen mobility on doped graphene surfaces, and identify candidate materials that will meet both thermodynamic and kinetic constraints for room temperature hydrogen uptake via surface diffusion from a catalyst that dissociates molecular H2 into active surface species.[1]  The results help to explain recent spectroscopic evidence of reversible ambient temperature hydrogenation of oxidized carbon surfaces via hydrogen spillover from platinum nanoparticles[2] and clarify the hydrogen spillover mechanism. The identified kinetic and thermodynamic constraints demonstrate that significant mobility at room temperature will occur only via H diffusion in a chemisorbed state, and this requires heteroatoms or chemical dopants to simultaneously increase the binding energy and decrease the barrier for chemical diffusion. Despite prior assumptions in the literature, the binding energy of atomic hydrogen on a surface is not correlated to mobility when the surface does not directly dissociate the H2.  Beyond hydrogen storage, clarification of the mechanism by which hydrogen diffuses on carbon extends to basic surface science, catalysis, astrophysics, novel materials, energy storage devices, and electronics.  

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1.            Lueking, A.D., G. Psofogiannakis, and G.E. Froudakis, Atomic Hydrogen Diffusion on Doped and Chemically Modified Graphene. Journal of Physical Chemistry C, 117 (12), pp 6312-6319, 2013. http://pubs.acs.org/doi/abs/10.1021/jp4007763

2.            Liu, X.M., et al., Evidence for Ambient-Temperature Reversible Hydrogenation in Pt-doped Carbons. Nano Letters, 2013. 13: p. 137-141. http://pubs.acs.org/doi/abs/10.1021/nl303673z

 

We identify a molecular fingerprint to probe H mobility on catalyzed carbon surfaces and confirm a weak carbon-hydrogen chemical bond may form reversibly at ambient temperature.1  We elucidate surface properties that lead to high mobility, and demonstrate reversibility requires mobility back to the catalyst.  Our technique extends prior ex post facto evidence of hydrogen spillover to carbon materials, by probing the carbon-hydrogen bond in situ, at high pressure and ambient temperature.  The results clarify a mechanism that has been disputed in recent years, as experimental reports claiming combined ambient temperature reversibility and mobility seem to defy theoretical predictions of the nature and strength of the carbon-hydrogen bond and have not been easily substantiated. Beyond hydrogen storage, clarification of the mechanism by which hydrogen diffuses on carbon extends to basic surface science, catalysis, astrophysics, novel materials, energy storage devices, and electronics. 

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(1)  Liu, X. M.; Tang, Y.; Xu, E. S.; Fitzgibbons, T. C.; Larsen, G. S.; Gutierrez, H. R.; Tseng, H. H.; Yu, M. S.; Tsao, C. S.; Badding, J. V.; Crespi, V. H.; Lueking, A. D. (Corresponding Author) Nano Lett. 13, 137-141, 2013.DOI: 10.1021/nl303673z; http://pubs.acs.org/doi/abs/10.1021/nl303673z

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Lueking seminar at University of Crete

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Abstract: Hydrogen spillover involves addition of a catalyst to a high-surface area microporous support, such that the catalyst acts as a source for atomic hydrogen, the atomic hydrogen diffuses from the catalyst to the support, and ideally, the support provides a high number of tailored surface binding sites to maximize the number of atomic hydrogens interacting with the surface.  Hydrogen spillover has been proposed as a means to increase the operative adsorption temperature of nanoporous materials from cryogenic conditions to near ambient temperature.  However, this proposition has become highly controversial in the past few years, due largely to discrepancies between laboratories, and even variations of the magnitude of hydrogen uptake observed for materials prepared with near-identical techniques within the same laboratory.  These discrepancies have pointed to the fact that the hydrogen spillover mechanism is not understood on a molecular level.  Amidst this controversy, a combined approach of in situ spectroscopic techniques and theoretical multi-scale modelling calculations are being used to resolve the hydrogen spillover mechanism and illuminate the nature of the exact surface sites and structures responsible for the high uptake in select materials. The first direct spectroscopic evidence of a reversible room temperature carbon-hydrogen wag mode, and how this experimental data was used to modify model chemistry in density functional calculations, will be discussed. The ultimate goal of this project is to not only resolve the hydrogen spillover controversy, but to use the findings to design new materials for hydrogen storage and catalytic hydrogenation.

And just for fun.... I included the following picture of my non-research activities in Crete.View image

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