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

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}

What do we already know about water adsorption on platinum surface? Water adsorbs intact on platinum surface and forms hydrogen bonded overlayers. 110K135K150K170K Laboured diffusion Crystalline ice (CI) Chemisorbed bilayer Water monomers Thermal mobility Amorphous solid water (ASW) Prevalence of forming ordered hydrogen bonds Desorption of multilayers Complete desorption Clean platinum surface The chemisorbed water bilayer on Pt{111} shows complicate LEED patterns characteristic for: at 0.47ML coverage at 0.67ML, saturation, coverage 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 7% compression of lattice constant

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 Mixed layer was found to be stable up to 35 K higher than an intact water bilayer One water molecule is necessary to stabilise OH hydrogen network: 3H 2 O ad + O ad  2(OH ad + H 2 O ad ) The presence of OH allows the structure to relax to a particular adsorption site, forming a commensurate layer with a (3  3) periodicity :

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

Changes in O1s during water adsorptionO1s at different adsorption temperatures and exposures Results: water interaction with clean Pt{111} at 115K Water adsorbs intact at 135K and 115K. O1s shifts towards higher BE w/r to chemisorbed bilayer. Shift depends on coverage

155K135K115K Results: water desorption from H 2 O/Pt{111}

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 structures have different BE’s, which probably correspondsand to different bonding with surface

Changes in O1s during oxygen and water adsorption Saturation (0.25 ML) coverage of oxygenHalf-saturation (0.13ML) coverage of oxygen Results: water adsorption on oxygen pre-covered Pt{111} Adsorbed atomic oxygen is characterized by single O1s peak at eV Water adsorbs intact at 90K on oxygen pre-covered platinum At 140K water interacts with oxygen and produces hydroxyl: H 2 O ad + O ad  OH ad Some of water remains on the surface after the reaction, however, it significantly changes BE from to eV. Mixed layer is stable up to 205K

Results: Reaction of water with 0.25ML O/Pt{111} Reaction starts at 120K – 130K. Mixed layer is stable up to 190K Ratio between initial O and “OH” is 1.4. Only 30-40% of oxygen take part in the reaction 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: 2OH ad → H 2 O + O ad 3H 2 O ad + O ad  2(OH ad + H 2 O ad )

Results: water desorption from (H 2 O ML 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: 3H 2 O ad + O ad  2(OH ad + H 2 O ad )

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

Some more interesting slides: Water NEXAFS

Conclusions Water interaction with clean and oxygen pre-covered Pt{111}. Conclusions Water is necessary to stabilise hydroxyl network Mixed layer is stable up to 190K Incompleteness of the reaction (for high coverage?). Only 40% of oxygen convert into hydroxyl. Water adsorption is fully reversible: OH converts into O and H 2 O due to thermal desorption Ratio between water and hydroxyl is 1.0. One H 2 O molecule for one “OH” molecule structures have different BE’s, which probably correspondsand to different bonding with surface

Acknowledgments To be continued...

Some more interesting slides: Why water and platinum? 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: H 2  2H + + 2e - 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. 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

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

What do we already know about water adsorption on platinum surface? Water adsorbs intact on platinum surface and forms hydrogen bonded overlayers. 110K135K150K170K Laboured diffusion Crystalline ice (CI) Chemisorbed bilayer Water monomers Thermal mobility Amorphous solid water (ASW) Prevalence of forming ordered hydrogen bonds Desorption of multilayers Complete desorption Clean platinum surface The chemisorbed water bilayer on Pt{111} shows LEED patterns characteristic for water on many close-packed surfaces of transitions metals: