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
3
2
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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2
1
0
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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(