CCl4 Sorption and Degradation on Fe-oxide surfaces

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CCl4 Sorption and Degradation on Fe-oxide surfaces
Jeff Fitts, Kwang Rim1, George Flynn1, Kaveh
Adib2, Nick Camillone III2, Richard Osgood2,
Peter Schlosser3, Steve Joyce4
Environmental Molecular Sciences
Institute Columbia University, New York, NY
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Organochlorine compounds PCBs, DDT, CCl4...

• Extensive point source contamination and
  elevated global background levels.

• Compounds are hormonally active, toxic and
  carcinogenic.

• High levels observed in human tissue, blood and
  breast milk.

Compounds readily react on certain metal oxide
surfaces.

• CCl4 oxides of V, Cr, Ti and Fe

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Not all phases of Fe-(hydr)oxides react with CCl4
FeOOH (goethite)
3
Octahedral
NR
?
w/ adsorbed Fe2
3
Reactive
Octahedral
?
Fe2O3 (hematite)
3
Octahedral
NR
Fe3O4 (magnetite)
2,3
Oct. Tet.
Reactive
Fe1-xO (wustite)
Oct. Tet.
?
2
-
Fe metal (ZVI)
0,2,3
Reactive
Reactive
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Project goals

• Determine whether CCl4 reacts at hematite,
  magnetite and wustite surfaces in UHV.

• Identify unique characteristics of Fe-oxides
  surfaces responsible for observed reactivity.

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Experimental Approach
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Surface preparation in ultrahigh vacuum (UHV)

• Chamber with base pressure 3x10-10 T

Heat
Heat
LN2
Heat

• Ar ion sputtering

1-2kV, 3-10 mA

• Annealing 950-1100K

Ion gun
in UHV
in pO2 10-6-10-5 T

• Characterize surface phase with LEED

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Surface preparation conditions control surface
Fe-oxide phase
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Temperature Programmed Desorption (TPD)
experiments
Heat
LN2
Heat
CCl4 multi-layer desorption
CCl4 monolayer desorption
Recombinative desorption of CCl4 and C2Cl4
OCCl2 (phosgene) desorption
Dosing tube
FeCl2
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Proposed reactions of CCl4 on Fe3O4 (III) surface
Recombinative Desorption
2 X
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How much of the surface reacts with CCl4?
No CCl4 desorbs after doses of lt 1 ML
Amount of CCl4 desorbed (a.u.)
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Where does the reaction occur?
OCCl2 production
Fe-Cl surface species inhibit CCl4 degradation
reactions
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What did we learn from the TPD studies?

• CCl4 readily reacts with the Fe3O4(111) surface
  to
• predominantly form OCCl2

• This reaction likely occurs at regular surface
  sites

• Fe-Cl surface species inhibit further reaction

What do we want to know about these reactions?

• What type of surface site reacts with CCl4?

• How do Fe-Cl species inhibit CCl4 degradation?