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CCl 4 Sorption and Degradation on Fe-oxide surfaces

2 Electrical Engineering. 1 Chemistry. 3 Environmental Engineering. 4 EMSL @ PNNL. CCl 4 Sorption and Degradation on Fe-oxide surfaces. Jeff Fitts, Kwang Rim 1 , George Flynn 1 , Kaveh Adib 2 , Nick Camillone III 2 , Richard Osgood 2 , Peter Schlosser 3 , Steve Joyce 4.

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CCl 4 Sorption and Degradation on Fe-oxide surfaces

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  1. 2Electrical Engineering 1Chemistry 3Environmental Engineering 4EMSL @ PNNL 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

  2. C-Cl + MeOsurf = MeCl + CO 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

  3. Reactivity Fe oxidation state Non-aqueous (UHV) Fe oxide phase Coordination Aqueous ? ? ? Not all phases of Fe-(hydr)oxides react with CCl4 FeOOH (goethite) 3+ Octahedral NR ? w/ adsorbed Fe2+ 3+ Octahedral Reactive ? 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

  4. 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.

  5. Experimental Approach

  6. Surface preparation in ultrahigh vacuum (UHV) • Chamber with base pressure = 3x10-10 T Heat Heat Heat LN2 • 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

  7. Surface preparation conditions control surface Fe-oxide phase

  8. QMS Temperature Programmed Desorption (TPD) experiments Heat Heat LN2 CCl4 multi-layer desorption CCl4 monolayer desorption Recombinative desorption of CCl4 and C2Cl4 OCCl2 (phosgene) desorption Dosing tube FeCl2

  9. Surface Abstraction Cl Cl 2CCl2 C2Cl4 C OCCl2 O 2Cl + CCl2 CCl4 Fe3O4(III) Cl FeCl2 Fe Fe3O4(III) Proposed reactions of CCl4 on Fe3O4 (III) surface Recombinative Desorption 2 X

  10. • 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. How much of the surface reacts with CCl4? No CCl4 desorbs after doses of < 1 ML Amount of CCl4 desorbed (a.u.)

  11. Between doses: anneal to 450 K No FeCl2 desorption Initial dose Where does the reaction occur? OCCl2 production • Fe-Cl surface species inhibit CCl4 degradation reactions

  12. 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?

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