Water Molecules on Carbon Surfaces
George Darling
Surface Science Research Centre
Department of Chemistry
The University of Liverpool
Outline
• 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
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 amounts
of 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
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)
Energy difference (eV)
Water molecules interact with periodic images
- gives spurious computational results
- gives order in overlayers
0.6
0.5
0.4
0.3
0.2
0.1
0
0
2
4
6
8
Unit cell dimension (Å)
10
12
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
Chemisorption is activated
Chemisorption energy decreases
as coverage increases
Barrier height depends on coverage
2.1
1.9
Echem (eV)
1.7
1.5
1.3
Spin
Non-Spin
1.1
2x2 unit cell (Spin)
2x2 unit cell (Non-Spin)
0.9
0.7
0.5
0
1
2
3
H coverage
4
5
OH - Chemisorption
OH also chemisorbs on top of C atom, distorts bonding from sp2 to sp3
0.5
Chemisorption of OH has
negligible activation barrier
Potential Energy (eV)
0.3
0.1
-0.1
1
1.5
2
2.5
3
3.5
4
-0.3
-0.5
-0.7
-0.9
-1.1
-1.3
-1.5
Energ. Diff. OH
Energ. Diff. H
4.5
For both H and OH graphite has to be distorted for chemisorption
This leads to a barrier.
0.7
H chemisorption
H barrier
0.6
OH barrier
ZC (Å)
0.5
0.4
dist. Diff.(OH)
0.3
dist. Diff.(H)
0.2
0.1
OH chemisorption
0
-0.1
0
1
2
3
ZH (Å) and ZOH (Å)
4
5
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?
1.00
Energy (eV)
0.00
-1.00
0.60
close to C
-2.00
far from C
0.40
-3.00
0.20
-4.00
0.00
1
-5.00
0
1
2
1.5
3
2
2.5
4
3
5
O-surface distance ( )
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?
-5297
Unfortunately not.
CO
and H2
CO and
des
orbed
desorbed
-5298
The total energy is much
higher when the products
desorb - higher than the
energy of the physisorbed
molecule.
total energy (eV)
-5299
-5300
-5301
-5302
phys is orbed
-5303
-5304
-5305
-5306
dis s ociated
-5307
general states
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
But it is unlikely to happen
- the desorbing CO would
need to carry ~3.7 eV of
the dissociation energy.
-5301
Physisorption
-5302
Total Energy (eV)
Overall the reaction to
produce 2 chemisorbed
H’s and desorbed CO is
favourable
-5303
Pathway where Carbon
atoms pushed down
-5304
-5305
Initial pathway
-5306
-5307
0.5
1
1.5
zC (Å)
2
2.5
3
3.5
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 - DE = 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 a
GIF decompressor
are needed to see this picture.
Acknowledgements
EU
EU
EPSRC
H2O / graphite
Pepa Cabrera
Kurt Kolasinski
Stephen Holloway
H+ / ice
Pepa Cabrera
Ayman Al Remawi
Stephen Holloway
Geert-Jan Kroes