Water Molecules on Carbon Surfaces

George Darling

Surface Science Research CentreDepartment of Chemistry

The University of Liverpool

• Introduction

• Computational details

• Single water molecules on graphite

• Water overlayers on graphite

• Partial dissociation (H and OH adsorption)

• Adsorption on a defect site

• Proton impacts with ice surfaces

Outline

Why water on graphite?

The TMR network funding the research was working on atmospheric chemistry

Graphite is a model for soot particles in the atmosphere- these particles can form ice nucleation sites

In the upper atmosphere we can expect substantial amountsof photodissociation of the water molecules

Goal - to determine the structures of water overlayers on graphite

Note: water + graphite has been studied also for catalytic chemistry coal gasification: H2O + coal CO + H2

there is also interest from tribology

Computational Details

Compute total energies using standard density functional codes written for solid state physics (CASTEP and VASP).

• Periodic boundary conditions in all directions - for a surface need a vacuum gap

• Basis set for electrons is plane waves

• In principle only one parameter - maximum plane wave energy

• Core electrons replaced by pseudopotentials - this can affect the results

• Number of k-points can affect answer

Reaction barriers, adsorption energies etc. are strongly dependent on choice of exchange correlation functional

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Single Molecule Adsorption

Single molecules physisorb: molecule-surface distance > 3.5 Å water does not wet graphite

Adsorption energy: ~0.53 eV (expt. ~0.45 eV) (DFT not good for physisorption)

Water molecules interact with periodic images- gives spurious computational results- gives order in overlayers

En

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Unit cell dimension (Å)

Water Clusters and Overlayers

Water clusters formed above the graphite are identical to gas-phase clusters

Dimers form oriented H up or down - degenerate

Periodic boundary conditions produce ordered overlayers

Hexamers form many nearly degenerate structures- DFT does not do a great job of the energies

No registry between clusters and surface

- water does not wet graphite

Partial Dissociation of Water Overlayer

H - Chemisorption

Hydrogen chemisorbs on top of C atom, distorts bonding from sp2 to sp3

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Coverage = 1/32

Coverage = 1/8

H - surface distance (Å)

Chemisorption is activated

Barrier height depends on coverage

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H coverage

Echem (eV)

Spin

Non-Spin

2x2 unit cell (Spin)

2x2 unit cell (Non-Spin)

Chemisorption energy decreasesas coverage increases

OH - Chemisorption

OH also chemisorbs on top of C atom, distorts bonding from sp2 to sp3

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Potential Energy (eV) Energ. Diff. OHEnerg. Diff. H

Chemisorption of OH has negligible activation barrier

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d i s t . D i f f . ( O H )

d i s t . D i f f . ( H )

OH barrier

H chemisorpt ion

OH chemisorption

H barrier

ZH (Å) and ZOH (Å)

ZC (Å)

For both H and OH graphite has to be distorted for chemisorption

This leads to a barrier.

H shows little tendency to interact with a water overlayer

But OH clearly bonds to co-adsorbed water

OH should fix a water overlayer into some registry with substrate

Water Adsorption on a Vacancy Defect

Carbonaceous surfaces in the ISM are not going to be perfect

What happens to water molecules approaching defects?

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O-surface distance ( )

Energy (eV)

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close to C

far from C

1. The water will physisorb on the defect site

2. Push in hard enough and it will overcome a barrier (~0.4 eV) to chemisorption

3. The chemisorption energy > 4 eV!

4. Chemisorption is dissociative!

The O-H bonds are completely broken when the molecule dissociates

Reminder: coal gasification H2O + coal CO + H2

Can we get the H2 and CO to desorb back into the gas-phase?

physisorbed

dissociated

CO and desorbed

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general states

total energy (eV)

Unfortunately not.

The total energy is much higher when the products desorb - higher than the energy of the physisorbed molecule.

CO and H2 desorbed

What about desorbing just the CO?

As the CO pulls away from the surface the graphite distorts strongly as the neighbouring carbons are dragged after the CO

- 5 3 0 7

- 5 3 0 6

- 5 3 0 5

- 5 3 0 4

- 5 3 0 3

- 5 3 0 2

- 5 3 0 1

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Total Energy (eV)

P a t h w a y w h e r e C a r b o n

a t o m s p u s h e d d o w n

I n i t i a l p a t h w a y

zC (Å)

Physisorption Overall the reaction to produce 2 chemisorbed H’s and desorbed CO is favourable

But it is unlikely to happen - the desorbing CO would need to carry ~3.7 eV of the dissociation energy.

H2 can also desorb leaving the CO chemisorbed

But the H2 must carry almost the entire dissociation energy with it!

Also the barriers to desorption are very high(compared to the energy of a physisorbed molecule

Conclusions

• Water physisorbs on graphite - behaves almost exactly as in gas-phase

• H chemisorbs with chemisorption energy and barrier dependent on coverage - 1.2 eV and 0.06 eV with 1 H per 32 C 0.7 eV and 0.25 eV with 1 H per 8 C

• OH chemisorbs and interacts with a water overlayer

• H2O can dissociate to C-O and C-H at a vacancy site - E = 4.3 eV

• Although H2 and CO can desorb individually it is energetically unlikely

Molecular dynamics of H+- ice collisions

Protons from cosmic rays or photodissociation of H2O can restructure ice surface

Study with classical MD - H2O molecules rigid

At low energy, sticking

probability is not 1

Protons stick forming Zundel complex

Even at the lowest energies the impact can lead to desorption of H2O

The desorption is a very subtle process resulting from slight tugs on water molecules pulling them out of the hydrogen bonding network

QuickTime™ and aGIF decompressor

are needed to see this picture.

H2O / graphite Pepa CabreraKurt KolasinskiStephen Holloway

H+ / ice Pepa CabreraAyman Al RemawiStephen HollowayGeert-Jan Kroes

Acknowledgements

EU

EUEPSRC