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Qixiu Li Prof. Derek Elsworth EGEE 520 04-29-2008

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Introduction Hydrogen spillover is defined as the dissociative chemi- sorption of hydrogen on the metal and the subsequent migration of atomic hydrogen onto the surface of the support such as alumina, carbon and so on that would not typically adsorb active species under the same conditions. Three steps: 1)Hydrogen molecules dissociate on metal and then diffuse from metal to metal. 2)Hydrogen atoms diffuse from metal to carbon surface (spillover). 3)Hydrogen atoms diffuse from carbon to carbon and be adsorbed. H 2 molecules Metal Support 1 2 3 ???

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Governing equation and formulation A: time-scaling coefficient c: concentration D: diffusion coefficient R: adsorption rate The fluid flow and diffusion equation (Fick’s law) Arrehenius expression for surface diffusion D o : surface diffusivity at zero loading E a : activation energy for surface diffusion k: Boltzmann constant T: absolute temperature

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Comsol model metal Graphite Setting: Graphite: surface cell containing 50 x 50 carbon atoms ( Diameter =12.2 nm) Metal: one single Pt particle (Diameter = 2nm)

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Parametric study ParametersEquation used Calculation result D on metal surface 3.6 x 10 -10 m 2 /s D on carbon surface 7.58 x 10 -20 m 2 /s Adsorption rate mol/m 3 ·s AmTE a (on metal)E a (on carbon) 13298.15 K0.40 eV0.78 eV Constants

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Comsol solution 1x10 -4 s 10 s 1x10 -1 s 200 s

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Comsol solution (cont.) Hydrogen spillover diffusion coefficient : 1.1 x 10 -16 m 2 /s t =200 s t =0 s

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Validation Example Grid for the Monte Carlo simulations at steady state Comsol sulotion for the infinite element analysis at steady state Literature

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Conclusion Infinite element analysis is an useful method for the analysis of hydrogen spillover process. Comsol diffusion model (transient analysis) can be applied to this process. In our modeling, the whole hydrogen diffusion process takes about 150 seconds. Hydrogen diffusion (from metal to carbon) coefficient has been calculated to be 1.1 x 10 -16 m 2 /s which is reasonable. The modeling result agrees well with the literature result.

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