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) • Water molecules interact with periodic images • gives spurious computational results • gives order in overlayers Energy difference (eV) Unit cell dimension (Å)
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 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
Chemisorption is activated Barrier height depends on coverage Chemisorption energy decreases as coverage increases Partial Dissociation of Water Overlayer H - Chemisorption Hydrogen chemisorbs on top of C atom, distorts bonding from sp2 to sp3
OH - Chemisorption OH also chemisorbs on top of C atom, distorts bonding from sp2 to sp3 Chemisorption of OH has negligible activation barrier
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?
The water will physisorb on the defect site Push in hard enough and it will overcome a barrier (~0.4 eV) to chemisorption The chemisorption energy > 4 eV! Chemisorption is dissociative!
Unfortunately not. The total energy is much higher when the products desorb - higher than the energy of the physisorbed molecule. CO and H2 desorbed Reminder: coal gasification H2O + coal CO + H2 Can we get the H2 and CO to desorb back into the gas-phase?
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. 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
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
Acknowledgements EU H2O / graphite Pepa Cabrera Kurt Kolasinski Stephen Holloway H+ / ice Pepa Cabrera Ayman Al Remawi Stephen Holloway Geert-Jan Kroes EU EPSRC