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W ater interaction with clean and oxygen pre-covered Pt{111}

W ater interaction with clean and oxygen pre-covered Pt{111}. A ndrey Shavorskiy Reading group. Berlin, 2007. Aims and main points. To study platinum surfaces with different roughness - roughness can affect on surface activity.

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W ater interaction with clean and oxygen pre-covered Pt{111}

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  1. Water interaction with clean and oxygen pre-covered Pt{111} Andrey Shavorskiy Reading group Berlin, 2007

  2. Aims and main points To study platinum surfaces with different roughness - roughness can affect on surface activity. We have to have the set of comparable data for the all kind of surfaces: {111}, {110}, {531}

  3. Prevalence of forming ordered hydrogen bonds Complete desorption Laboured diffusion Thermal mobility Desorption of multilayers Water monomers Crystalline ice (CI) Amorphous solid water (ASW) Chemisorbed bilayer Clean platinum surface 7% compression of lattice constant 110K 135K 150K 170K at 0.47ML coverage at 0.67ML, saturation, coverage What do we already know about water adsorption on platinum surface? Water adsorbs intact on platinum surface and forms hydrogen bonded overlayers. The chemisorbed water bilayer on Pt{111} shows complicate LEED patterns characteristic for: Prevalence of forming ordered hydrogen bonds on bonding to platinum – mismatch between the metal lattice and the distances of the hydrogen bonds in a bilayer. H-down structure

  4. What do we already know about water adsorption on oxygen-covered platinum surface? The presence even of a small amount of chemisorbed oxygen on Pt{111} leads water to react with it to form OH above 120 K One water molecule is necessary to stabilise OH hydrogen network: 3H2Oad + Oad 2(OHad + H2Oad) Mixed layer was found to be stable up to 35 K higher than an intact water bilayer The presence of OH allows the structure to relax to a particular adsorption site, forming a commensurate layer with a (33) periodicity :

  5. Results: water interaction with clean Pt{111} at 155K Changes in O1s during water adsorption Change of SCLS in Pt4f during water adsorption Water adsorbs intact at temperatures lower than 165K At 155K it forms chemisorbed bilayer. BE O1s= 532.0 eV; change in Pt4f7/2 shape;

  6. Results: water interaction with clean Pt{111} at 115K Changes in O1s during water adsorption O1s at different adsorption temperatures and exposures Water adsorbs intact at 135K and 115K. O1s shifts towards higher BE w/r to chemisorbed bilayer. Shift depends on coverage

  7. Results: water desorption from H2O/Pt{111} 115K 135K 155K

  8. and structures have different BE’s, which probably corresponds to different bonding with surface Water interaction with clean Pt{111} Conclusions Water adsorbs intact on Pt{111} at all temperatures. And adsorption is fully reversible Water desorbs at 165K - 170K Water multilayer peak shifts towards higher BE w/r chemisorbed bilayer

  9. Results: water adsorption on oxygen pre-covered Pt{111} Saturation (0.25 ML) coverage of oxygen Half-saturation (0.13ML) coverage of oxygen Changes in O1s during oxygen and water adsorption Adsorbed atomic oxygen is characterized by single O1s peak at 529.8 eV Water adsorbs intact at 90K on oxygen pre-covered platinum At 140K water interacts with oxygen and produces hydroxyl: H2Oad + Oad OHad Some of water remains on the surface after the reaction, however, it significantly changes BE from 532.2 to 531.5 eV. Mixed layer is stable up to 205K

  10. Results: Reaction of water with 0.25ML O/Pt{111} Reaction starts at 120K – 130K. 3H2Oad + Oad 2(OHad + H2Oad) Mixed layer is stable up to 190K Same amount of water as “OH” is necessary to stabilise hydrogen network. Water adsorption is fully reversible: water desorbs by the thermal decomposition of OH: 2OHad→ H2O + Oad Ratio between initial O and “OH” is 1.4. Only 30-40% of oxygen take part in the reaction

  11. Results: water desorption from (H2O + 0.13ML O) / Pt{111} Same behaviour as for full-saturation coverage. Ratio between initial O and “OH” is 1.8, which is more characterized for the reaction stoichiometry: 3H2Oad + Oad 2(OHad + H2Oad)

  12. Results: Water uptake Two possibilities for fitting: straight line with slope 1.7 and “saturated” curve Straight line is more truly for fitting the set of the dots The other data (NEXAFS) are saying for low coverages uptake is more close to 2, whereas for high – 1.4

  13. Some more interesting slides: Water NEXAFS

  14. and structures have different BE’s, which probably corresponds to different bonding with surface Water interaction with clean and oxygen pre-covered Pt{111}. Conclusions Conclusions Incompleteness of the reaction (for high coverage?). Only 40% of oxygen convert into hydroxyl. Water is necessary to stabilise hydroxyl network Ratio between water and hydroxyl is 1.0. One H2O molecule for one “OH” molecule Mixed layer is stable up to 190K Water adsorption is fully reversible: OH converts into O and H2O due to thermal desorption

  15. Acknowledgments To be continued...

  16. Water covers 2/3 parts of the Earth. Due to its abundance water plays an important role in fields as diverse as biology, atmospheric chemistry and astrophysics. It significantly influences many processes occurring in the earth’s biosphere Water covers most real solid surfaces. Water – surface interactions play a central role in many areas (electrochemistry, catalysis, corrosion, rock efflorescing…) and has many important applications e.g. fuel cells, hydrogen production, biological sensors and the heterogeneous catalysis. Platinum – in one of the best material for the electrodes in Proton exchange membrane fuel cell (PEMFC). Due to relatively easy splitting of hydrogen on platinum, electrode catalyses reaction of hydrogen oxidation: H2 2H+ + 2e- Some more interesting slides: Why water and platinum?

  17. Some more interesting slides: Water + saturation O/Pt{111} NEXAFS

  18. Prevalence of forming ordered hydrogen bonds Complete desorption Laboured diffusion Thermal mobility Desorption of multilayers Water monomers Crystalline ice (CI) Amorphous solid water (ASW) Chemisorbed bilayer Clean platinum surface The chemisorbed water bilayer on Pt{111} shows LEED patterns characteristic for water on many close-packed surfaces of transitions metals: 110K 135K 150K 170K What do we already know about water adsorption on platinum surface? Water adsorbs intact on platinum surface and forms hydrogen bonded overlayers.

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