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

AAndrey Shavorskiyndrey ShavorskiyReading groupReading group

Berlin, 2007

Aims and main points03.25)3737( R 03.25)3737( R

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.

110K 135K 150K 170KLaboured diffusion

Crystalline ice (CI) Chemisorbed bilayerWater monomers

Thermal mobility

Amorphous solid water (ASW)

Prevalence of forming ordered hydrogen bonds Desorption of multilayers

Completedesorption

Clean platinum surface

The chemisorbed water bilayer on Pt{111} shows complicate LEED patterns characteristic for:

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0)R25.337×37(

at 0.47ML coverage

0)R16.139×39(

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?

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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:

3H2Oad + Oad 2(OHad + H2Oad)

The presence of OH allows the structure to relax to a particular adsorption site, forming a commensurate layer with a (33) periodicity :

Changes in O1s during water adsorption Change of SCLS in Pt4f during water adsorption

73 72 71 70 69 68

BE, eV

Clean platinum surface Water, adsorbed at 165K

Pt 4f

15

10

5

0

Tim

e, m

in

538 536 534 532 530 528

BE, eV

O1s

Results:water interaction with clean Pt{111} at 155K

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At 155K it forms chemisorbed bilayer.

Water adsorbs intact at temperatures lower than 165K

BE O1s= 532.0 eV; change in Pt4f7/2 shape;

Changes in O1s during water adsorption

12

9

6

3

0

Exp

osur

e tim

e, m

in

538 536 534 532 530 528BE, eV

O1s

O1s at different adsorption temperatures and exposures

538 536 534 532 530 528

BE, eV

Adsorption at 115K 5L Adsorption at 135K 1.5L Adsorption at 155K 3L

O 1s

Results:water interaction with clean Pt{111} at 115K

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Water adsorbs intact at 135K and 115K.

O1s shifts towards higher BE w/r to chemisorbed bilayer. Shift depends on coverage

538 536 534 532 530 528BE, eV

155K 170K 175K

155K135K

165

160

155

150

145

140

135

130

125

Te

mp

era

ture

, K

540 538 536 534 532 530 528 526BE, eV

C

180

175

170

165

160

155

150

145

140

135

130

125

Te

mp

era

ture

, K

538 536 534 532 530 528BE, eV

180

175

170

165

160

155

Tem

pera

tute

, K

538 536 534 532 530 528BE, eV

115K

Results:water desorption from H2O/Pt{111}

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540 538 536 534 532 530 528 526BE, eV

135K 152K 171K 180K

540 538 536 534 532 530 528 526BE, eV

147K 151K 155K 159K 163K

Water interaction with clean Pt{111}Conclusions

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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 corresponds39 37andto different bonding with surface

Changes in O1s during oxygen and water adsorption

Saturation (0.25 ML) coverage of oxygen Half-saturation (0.13ML) coverage of oxygen

538 536 534 532 530 528BE, eV

529.8

530.0

531.5

532.3

anneal to 202Kanneal to 140Kadd H2O at 90K ~0.4L0.25ML O/Pt{111}

538 536 534 532 530 528BE, eV

529.8532.1

531.5 530.05

anneal to 156K add 1L H2O at 90K

0.13ML O/Pt{111}

Results:water adsorption on oxygen pre-covered Pt{111}

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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

Results:Reaction of water with 0.25ML O/Pt{111}

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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:2OHad → H2O + Oad

3H2Oad + Oad 2(OHad + H2Oad)

Results:water desorption from (H2O + 0.13ML O) / Pt{111}

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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)

Results:Water uptake

0.45

0.40

0.35

0.30

0.25

0.20

0.15

wat

er u

ptak

e, M

L

0.240.220.200.180.160.140.120.10O coverage, ML

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

4

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1

0

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R271_214 0 1/2 sat O/Pt R274_210 65 1/2 sat O/Pt R333_214 0 H2O + 1/2 sat O/Pt R335_210 65 H2O + 1/2 sat O/Pt R345_214 0 H2O + sat O/Pt R346_210 65 H2O + sat O/Pt

ConclusionsWater 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 H2O due to thermal desorption

Ratio between water and hydroxyl is 1.0. One H2O molecule for one “OH” molecule

structures have different BE’s, which probably corresponds39 37andto 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:

H2 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

4

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R232_214 0 O2 @ 100K -> 300K; 60L R235_210 65 O2 @ 100K -> 300K; 60L R241_214 0 + H2O @ 100K 0.2L R244_210 65 + H2O @ 100K 0.2L R253_210 65 anneal 140K R250_214 0 anneal 140K R435_210 65 pure water 140K R430_214 0 pure water 140K

What do we already know about water adsorption on platinum surface?

Water adsorbs intact on platinum surface and forms hydrogen bonded overlayers.

110K 135K 150K 170KLaboured diffusion

Crystalline ice (CI) Chemisorbed bilayerWater monomers

Thermal mobility

Amorphous solid water (ASW)

Prevalence of forming ordered hydrogen bonds Desorption of multilayers

Completedesorption

Clean platinum surface

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The chemisorbed water bilayer on Pt{111} shows LEED patterns characteristic for water on many close-packed surfaces of transitions metals:

0)R303×3(