CCl 4 Sorption and Degradation on Fe-oxide surfaces

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

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.

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

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.

Experimental Approach

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

Surface preparation conditions control surface Fe-oxide phase

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

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

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.

•

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

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