Isotope effect, surface reaction, point defects, H2O dissociation on surface, solar nebula, cosmochemistry

1. New Experimental
Evidence of Silicate
Formation with Meteorite
Like Oxygen Isotopes on a
Dust Surface Analog
Subrata Chakraborty
Haiyang Kehoe and M. H. Thiemens
University of California, San Diego
Department of Chemistry and
Biochemistry, 9500 Gilman Drive,
La Jolla, CA 92093-0356

2. •Dust particles are obiquitous throughout the universe
•Dying stars are the ‘dust factories’
•Constitutes ~1 mass % in the ISM
•As high as 10 % in planetary nebula
•Dust particles are mainly of two types: carbonaceous
and silicates
•Long lifetime journey– stellar region through diffuse,
dense or molecular clouds to the new born star or its
planetary system
•Significantly processed during this journey
Background

3. Rational
Condensation of oxides in gas phase were theoretically
calculated
Ferrarotti and Gail,AA, 2002
Mg + H2O → MgO(s) + H2
Fe + H2O → FeO(s) + H2
2Fe + 3H2O → Fe2O3(s) + 3H2
3Fe + 4H2O → Fe3O4(s) + 4H2

4. Rational
 Oxidation Reaction on the surface of an existing oxide:
SiO, FeO, Al2O3 etc….
 With absorbed water molecule
 Specifically in colder environments

5. Experimental

6. Experimental
Sample: Si, SiO, Al2O3…..

7. Experimental

8. Experimental

9. Experimental
Four different sets of experiments:
1. SiO vapor deposit + H2O2 vapor deposit
2. SiO vapor deposit + H2O vapor deposit
3. (Al2O3 + Si) vapor deposit + H2O vapor deposit
4. (Al2O3 + SiO) vapor deposit + H2O vapor deposit

10. 1 mm
1. SiOVapor Deposit + H2O2@ 150 K

11. 1. SiOVapor Deposit + H2O2
Collection Plate
Side Wall- Foil
Collection Plate- Foil
Element Atomic (%)
Si 44.2
O 55.8
SiOx (<2)
Element Atomic (%)
Si 38.9
O 61.1
Element Atomic (%)
Si 34.9
O 65.1
SiO2
SiO2
@ 150 K

12. SiO2
1. SiOVapor Deposit + H2O2@ 150 K

13. 2. SiOVapor Deposit + H2O @ 150 K
2 mm

14. Element Atomic
(%)
Si 45.5
O 54.5
Stoichiometry: SiO1.2
2. SiOVapor Deposit + H2O @ 150 K

15. Element Atomic
(%)
Si 41.8
O 58.2
Stoichiometry: SiO1.4
2. SiOVapor Deposit + H2O @ 150 K

16. 2. SiOVapor Deposit + H2O @ 150 K
Oxygen Isotopes

17. 2. SiOVapor Deposit + H2O
Time Progression
27 Hrs19 Hrs5 Hrs
2.5 Hrs
1 Hr
@ 150 K
@ 300 K

18. 3. (Al2O3 + Si)Vapor Deposit + H2O @150 K
2 mm

19. 3. (Al2O3 + Si)Vapor Deposit + H2O @150 K

20. 3. (Al2O3 + Si)Vapor Deposit + H2O @150 K

21. 3. (Al2O3 + Si)Vapor Deposit + H2O @150 K

22. 3. (Al2O3 + Si)Vapor Deposit + H2O @150 K

23. 3. (Al2O3 + Si)Vapor Deposit + H2O @150 K
Element Atomic
(%)
Si 63.8
Al 9.4
O 26.8
Stoichiometry: AlSi6.7O2.9

24. 4. (Al2O3 + SiO)Vapor Deposit + H2O @150 K
2 mm

25. 4. (Al2O3 + SiO)Vapor Deposit + H2O @150 K

26. 4. (Al2O3 + SiO)Vapor Deposit + H2O @150 K

27. 4. (Al2O3 + SiO)Vapor Deposit + H2O @150 K

28. 4. (Al2O3 + SiO)Vapor Deposit + H2O @150 K
Element Atomic
(%)
Si 38.6
Al 16.9
O 44.5
Stoichiometry: AlSi2.3O2.6

29. 4. (Al2O3 + SiO)Vapor deposit + H2O vapor deposit
Element Atomic
(%)
Si 42.3
Al 13.3
O 44.4
~ 10 micron particle
Stoichiometry: AlSi3.2O3.3

30. Mechanism
Duley and Millar, 1978…..
 Model (theoretical) for the reaction of +ve ions with –ve
charged species on oxide grain surfaces
 Recombination of gas phase ions with surface O- and OH-
 Explains selective depletion of Al, Ca, Cr,Ti, Mn etc…..

31. Mechanism
Duley and Millar, 1978…..
 Model (theoretical) for the reaction of +ve ions with –ve
charged species on oxide grain surfaces
 Recombination of gas phase ions with surface O- and OH-
 Explains selective depletion of Al, Ca, Cr,Ti, Mn etc…..
Other way round….
 Reaction of adsorbed species (H2O) with the host species
(Si, Al etc…)

32. Bikondoa et al., Nature 2006
Mechanism
Steps:
 Water adsorbs into O vacancies
 Dissociates into OH and H fragments.
 The OH fragment is pinned at the
position of the O vacancy
 The H fragment attaches to another
oxygen from the bridging-O rows
forming a second OH group
Hypothesis:
XO + OH → (XOOH)* → XOO + H

33. Mechanism
Based on Marcus, 2004

34. Comparison
Gas Phase Reactions
Chakraborty et al., Science, 2013
Group-2
H2 + O → OH + H
SiO + OH → SiO2 + H
OH + OH → H2O + H
OH + O2 → HO2 + O
SiO + HO2 → SiO2 + OH
Group 1
SiO (g) + O2 → SiO2 + O
SiO (g) + O → SiO2

35. Comparison
Gas Phase Reactions
Chakraborty et al., Science, 2013
Group 1
SiO (g) + O2 → SiO2 + O
SiO (g) + O → SiO2
Group-2
H2 + O → OH + H
SiO + OH → SiO2 + H
OH + OH → H2O + H
OH + O2 → HO2 + O
SiO + HO2 → SiO2 + OH
Surface Reactions

36. Conclusions
Surface assisted oxidation reation is a possible
mechanism
Effective in the dark and cold molecular clouds and
colder parts of the solar nebula
Observed: Growth of new particles through
reactions
Observed: Mass-independent oxygen isotopic
compositions in the newly formed SiOx particles
Multicle element oxide grains do form in this
process
Need more studies in this direction

38. 3. (Al2O3 + Si)Vapor Deposit + H2O @150 K