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Cheng-Yu Wang presents at AIChE annual meetingPrebridge Technique to Incorporate Transition 

Co-Authors:  Qihan Gong, Jing Li, Angela Lueking

 

Abstract:  Metal-organic frameworks, high surface area materials with the characteristics of adjustable structure and surface chemistry, are becoming promising candidates for hydrogen storage, gas separation, and catalyst supports.  In this paper, we explore MOF stability after various catalytic doping methods for IRMOF-8, Cu-BTC, and Cu-TDPAT.  After identifying a catalytic doping technique that maximizes stability and retains the surface area of the MOF precursor, we measure hydrogen isotherms at 300K up to 80 bar, and demonstrate catalyst addition significantly increases hydrogen storage via the hydrogen spillover effect.  Adsorption enhancement is most pronounced at low pressure, and kinetic limitations and MOF instability effects inhibit high-pressure adsorption via the spillover effect.  Novel hydrogen chemisorption sites are identified using spectroscopic techniques, for both undoped and doped MOFs.  An improved mechanistic understanding of the hydrogen spillover effect is developed by tracking of hydrogenation of N groups via spectroscopy, density functional theory, and comparison of defected versus pristine MOF structures.  Implications for hydrogen storage and use of MOFs as a catalyst support are discussed.

 

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

Press Release, RE: Nano Letter study

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Professor Angela Lueking has been awarded a highly competitive Marie Curie International Incoming Fellowship to partner with researchers at the University of Crete to study basics of hydrogen adsorption and diffusion on surfaces.  The work will include design of novel materials.  The abstract for the successful proposal is included below.

Objectives and Overview:  The objective of the proposed work is to synthesize catalyzed nanoporous materials that have superior hydrogen uptake between 300K and 400K and moderate pressures (20-100 bar) via the hydrogen spillover mechanism. 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.  The proposed work will provide a means to explore an extended collaboration to combine in situ spectroscopic techniques and theoretical multi-scale modelling calculations. Both carbon-based and microporous metal-organic framework (MMOF) materials with added hydrogen dissociated catalysts will be drawn from past and on-going projects, in order to identify specific binding sites that lead to appreciable uptake.  First, preliminary spectroscopic data will be used to validate and extend existing theoretical models. In situ characterization of materials with systematic variations in structure and/or synthesis will be used to identify properties that lead to high uptake, including effect of structure, geometry, surface chemistry, and catalyst-support interface. Resulting spectroscopic data will be analyzed with theoretical models to conclusively identify the nature of the binding site. Validated models will be used to direct future synthesis of novel materials.  The overall goal will be to identify tailored surface sites that reversibly bind atomic hydrogen between 300 K and 400K.

The work is incredibly timely, as the hydrogen spillover mechanism has become highly controversial in the past two years, due largely to discrepancies between laboratories, and even variations of the magnitude of uptake observed for materials prepared with near-identical techniques within the same laboratory. Amidst this controversy, a combined approach of in situ spectroscopic techniques and theoretical multi-scale modelling calculations will resolve the hydrogen spillover mechanism and illuminate the nature of the exact surface sites and structure responsible for the high uptake in select materials. The proposed work extends previous work of Professor Angela Lueking, who as a graduate student, was first author on the first papers identifying hydrogen spillover as a means to achieve appreciable uptake at room temperature. Subsequently, Lueking has studied hydrogen uptake and adsorption in other materials, and furthered her experience in material characterization. She has recently returned to the field of hydrogen spillover, employing in situ spectroscopic techniques,5, 6 as outlined below.  Lueking will pair with George Froudakis of the University of Crete, whose theoretical calculations (with George Psofogiannakis, a current Marie Curie fellow) provided the first multi-scale modelling of the hydrogen spillover mechanism. The proposed work will provide a means to explore an extended collaboration to combine their respective work in experiment and theory.  The combined approach is expected to not only resolve what has become a highly controversial issue in the literature, but ultimately, identification of the key sites responsible for high uptake in select materials is expected to lead to a significant increase in capacity and reproducibility in hydrogen spillover materials that are optimized for near-ambient temperature adsorption.

 

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