qixiu li prof. derek elsworth egee 520 04-29-2008

Qixiu LiProf. Derek Elsworth

EGEE 52004-29-2008

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.

H2 molecules

Metal

Support

1

2

3

???

Governing equation and formulation

( )c

A D c Rt

A: time-scaling coefficientc: concentrationD: diffusion coefficientR: adsorption rate

The fluid flow and diffusion equation (Fick’s law)

Arrehenius expression for surface diffusion

Do: surface diffusivity at zero loadingEa: activation energy for surface diffusionk: Boltzmann constant T: absolute temperature

exp( )ao

ED D

kT

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)

Parametric studyParameters Equation used

Calculation result

D on metal surface

3.6 x 10-10 m2/s

D on carbon surface

7.58 x 10-20 m2/s

Adsorption rate mol/m3·s-0.0015 c flc1hs(100-t,0)/100R

-2D=1.6 10 exp( 0.45 )q

mRT

0.9D=1.2 exp( )

eV

kT

A m T Ea (on metal) Ea(on carbon)

1 3 298.15 K 0.40 eV 0.78 eVConstants

Comsol solution

1x10-4 s

10 s

1x10-1 s

200 s

Comsol solution (cont.)

Hydrogen spillover diffusion coefficient :1.1 x 10-16 m2/s

t =200 s

t =0 s

Validation

Example Grid for the Monte Carlo simulations at steady state

Comsol sulotion for the infinite element analysis at steady state

Literature

ConclusionInfinite 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 be1.1 x 10-16 m2/s which is reasonable.

The modeling result agrees well with the literature result.