CCl 4 Sorption and Degradation on Fe-oxide surfaces
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Transcript of CCl 4 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 InstituteColumbia University, New York, NY
2Electrical Engineering
1Chemistry
3Environmental Engineering
4EMSL @ PNNL
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Organochlorine compounds: PCBs, DDT, CCl4...
• Extensive point source contamination and elevated global background levels.
Compounds readily react on certain metal oxide surfaces.
• CCl4: oxides of V, Cr, Ti and Fe
C-Cl CO+MeCl=MeOsurf+
•High levels observed in human tissue, blood and breast milk.
•Compounds are hormonally active, toxic and carcinogenic.
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Not all phases of Fe-(hydr)oxides react with CCl4
Fe oxide phase Fe oxidationstate
Coordination Aqueous
ReactivityNon-aqueous
(UHV)
FeOOH (goethite) 3+ Octahedral NR ?
w/ adsorbed Fe2+ 3+ ReactiveOctahedral ?
Fe2O3 (hematite) 3+ Octahedral NR ?
Fe3O4 (magnetite) 2+,3+ Oct. & Tet. Reactive ?
Fe1-xO (wustite) 2+ Oct. & Tet. ? ?
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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Heat
Surface preparation in ultrahigh vacuum (UHV)
in pO2 = 10-6-10-5 T
Heat
Chamber with base pressure = 3x10-10 T
•Ar+ ion sputtering
1-2kV, 3-10 A
•Annealing 950-1100K
in UHV
•Characterize surface phase with LEED
Ion gun
LN2Heat
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Surface preparation conditions control surface Fe-oxide phase
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Heat
Temperature Programmed Desorption (TPD) experiments
LN2Heat
QMS
CCl4 multi-layer desorption
CCl4 monolayer desorptionRecombinative desorption of CCl4 and C2Cl4
FeCl2 Dosing tube
OCCl2 (phosgene) desorption
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Proposed reactions of CCl4 on Fe3O4 (III) surface
Recombinative Desorption
C
Cl Cl
O
Fe3O4(III)
Cl
Fe
Fe3O4(III)
2 X
2Cl + CCl2 CCl4
2CCl2 C2Cl4
Surface Abstraction
FeCl2
OCCl2
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How much of the surface reacts with CCl4?A
mou
nt o
f CC
l 4 des
orbe
d (a
.u.)
No CCl4 desorbs after doses of < 1 ML
0.3 L dose contains enough CCl4 to cover entire surface (1 ML 20%)
•
CCl4 does not only react at minority site (e.g., defects).
•
CCl4 must react at site(s) that cover the entire surface.
•
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Initial dose
Where does the reaction occur?
• Between doses: anneal to 450 K No FeCl2 desorption
OCCl2 production
• Fe-Cl surface species inhibit CCl4 degradation reactions
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What did we learn from the TPD studies?
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?
• Fe-Cl surface species inhibit further reaction
• This reaction likely occurs at regular surface sites
• CCl4 readily reacts with the Fe3O4(111) surface to predominantly form OCCl2