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Optimization of Hydrogen Storage via Spillover through a Combined Experimental and Modeling Approach

Final Report Summary - HSPILL-CEMA (Optimization of Hydrogen Storage via Spillover through a Combined Experimental and Modeling Approach)

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 hydrogen to nanoporous materials from cryogenic conditions to near ambient temperature, to help enable mobile fuel cell applications at temperature and pressures of interest. 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. Froudakis et al. at the University of Crete have performed extensive density functional calculations of idealized surfaces to arrive at a proposed mechanism for hydrogen spillover from transition metal catalysts to both carbon-based and metal-organic-framework based surfaces. Meanwhile, Lueking et al. have recently published the first direct spectroscopic evidence of a reversible room temperature carbon-hydrogen wag mode to provide direct experimental evidence of the hydrogen spillover process. Certain aspects of these experimental studies were at odds with the mechanism proposed by Froudakis et al., suggesting the theoretical models of idealized surfaces are not yet sufficient to match the ‘real’ experimental surfaces. Likewise, the validated theoretical studies are needed to better design an experimental surface and improve reliability and reproducibility. This project pairs the theoretical expertise of Froudakis et al. with the experimental experience of Lueking, to take a combined experimental-theoretical approach 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 ultimate objectives of this project are not only to resolve the hydrogen spillover controversy and validate theory with experiment, but to use the findings to design new materials for hydrogen storage and catalytic hydrogenation, including identification of new structures that are likely to maximize hydrogen uptake via the hydrogen spillover mechanism. To this end, the role of heteroatoms on ready mobility to/from a carbon-supported catalyst was explored, with candidate structures identified. These theoretical results were generally consistent with the spectroscopic trends, but were insufficient to explain the relative stability and reproducibility observed for the experimental studies. Other potential roles of characteristic structures found in the experimental synthesis procedure were then explored, and new candidate catalytic-carbon interfaces were identified that enabled ready diffusion to/from the catalyst. In this process, new carbon structures were also identified that had the potential to ‘seed’ the hydrogen spillover process. Theoretical studies for hydrogenation of metal-organic frameworks helped to identify potential structures that could explain experimental trends. The work is on-going, but a consistent mechanism that explains both theoretical and experimental trends is beginning to emerge. The results are applicable to hydrogen storage, and extend to catalytic hydrogenation for fuel upgrading, adsorption and catalysis, and graphene-based devices.