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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
0
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0 2 4 6 8 10 12
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
erg
y d
iffe
ren
ce (
eV
)
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
0.0-0.5-1.01.01.52.02.53.0
Coverage = 1/32
Coverage = 1/8
H - surface distance (Å)
Chemisorption is activated
Barrier height depends on coverage
0.5
0.7
0.9
1.1
1.3
1.5
1.7
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0 1 2 3 4 5
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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1 1.5 2 2.5 3 3.5 4 4.5
Potential Energy (eV) Energ. Diff. OHEnerg. Diff. H
Chemisorption of OH has negligible activation barrier
- 0 . 1
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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?
-5.00
-4.00
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0 1 2 3 4 5
O-surface distance ( )
Energy (eV)
0.00
0.20
0.40
0.60
1 1.5 2 2.5 3
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
-5307
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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
0 . 5 1 1 . 5 2 2 . 5 3 3 . 5
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