1. Presented to The 2004 American Nuclear Society Winter Meeting Washington, D.C. November 14–18, 2004 Uranium-Based Catalyst M. J. Haire Nuclear Science and Technology Division S. H. Overbury, C. K. Riahi-Nezhad, and S. Dai Chemical Sciences Division Oak Ridge National Laboratory

2. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 2 Depleted Uranium (DU) as Catalysts  DU has proven active for many catalytic reactions  Volatile organic compounds (VOCs) and chlorinated VOC oxidation  Selective oxidation and ammoxidation (patented mixed U-Sb oxide)  Partial oxidation—methane to methanol (patented mixed U-Mo oxide)  Oxidative coupling (C chain lengthening)  Selective catalytic reduction (SCR) of NO  Many other catalytic applications are possible (but unproven)  These reactions are important for many environmental applications and chemical production

3. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 3 New Synthetic Approaches  New techniques to improve catalyst performance and handling  nanoporous supports by templating techniques  co-assembly of U into nanoporous supports  complexing U onto Si cubes  Techniques lead to high surface areas  higher catalytic activity  more efficient use of uranium  dilutes specific radioactivity (dpm per gm material)  Convenient solid form  sol-gel approach leads to monoliths  easier handling before and after application  reduced risk of loss of powder blow-out  stabilize catalyst

4. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 4 Synthesis of Nanoporous Materials  Micelles of variable sizes used as template molecules  TEOS produces Si gel around template molecules. Dope with uranium nitrate  alignment (crystallization) of micelles leads to ordered arrays  surfactant “burned out” or removed by solvent extraction  approach can be used to make mesoporous SiO 2 or TiO 2, or other oxides Surfactant extraction or calcination Silica condensation Rodlike micelle Silicate encapsulated micelles TEOS(C 16 H 33 )N(CH 3 ) 3 Br + NaOH / H 2 O

5. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 5 Nonpowder Forms of DU Catalysts  High Surface Area  250 m 2 /g  monolithic catalysts simplifies handling  uranium oxide is not co- precipitated; it is on/in the pore walls  transparency, possible photochemical processes  Reactive Membranes Optical Absorption Spectrum Monolithic U-SiO 2

6. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 6 Reactor Set-up for Catalytic Testing 7721 vent GC/MS Reactor (Temp. Controlled) w/ quartz tube & sample To Mass Spec/ G.C bypass bubbler bypass RR 21.0 Thermocouple Bubbler and Ice Bath Heating Zone 42 He O 2 He He gas for bubbler Flow Regulator From O2 Tank From He Tank 21 ml/min (He) H 2 O Syringe Flow Meter He O2O2 ( 77 ml/min He + 42 ml/min O 2 ) Mixing Point 140 ml/min Pressure Gauge Adjusting Valve Bypass Flow Line to Bypass the BubblerLine to Bypass the Reactor

7. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 7 Photograph of Reactor Used in DU Project

8. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 8 Light-Off Curves to Compare Activity: U 3 O 8 measure light-off curve to compare activity for toluene oxidation  Reactor conditions  25 mg catalyst  He flow 150 cm 3 /min  O 2 flow 40 cm 3 /min  toluene 500 ppm  GHSV = 72000 hr -1  Mesoporous silica (MCM-41) without DU is inactive  U 3 O 8 obtained by calcination of UO 2 (NO 3 ) 2  Pure U 3 O 8 is active but low surface area (<0.1 m 2 /g )

9. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 9 U impregnated in Mesoporous Support  U-MAS-5  UO 2 (NO 3 ) 2 impregnated into solid mesoporous silica  silica contains 5% Al  U:Si = 1:10  improved light-off compared to pure U 3 O 8

10. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 10 Catalysts Synthesized by Co-Synthesis Techniques  U-SiP123 catalysts  Uranium nitrate put into synthesis mixture  Pluronic P123 (EO-PO-EO triblock co-polymer)  Acid conditions  Vary U:Si ratio  50% conversion above 450  C  Activity higher than U 3 O 8 although lower U concentration  Gave poorly ordered mesopores  Broad BJH pore distribution  BET SA 225–300 m 2 /g

11. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 11 TEM Characterization of DU Catalysts  Catalyst particle of U- MAS-5  Al 3+ doped silica mesoporous support impregnated with uranyl nitrate  Calcined 900ºC  High resolution TEM using HD-2000 at ORNL  Uranium oxide particles located within pores

12. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 12 STEM Micrograph of DU Catalyst  Catalyst U-SiF127  UO 2 (NO 3 ) 2 mixed in with TEOS  Pluronic F127 (EO-PO-EO triblock co-polymer)  Acid conditions  U part of the Si walls  U:Si = 1:20  Mesoporous structure shows as parallel walls  Pore spacing 10.3 nm  Uranium oxide particles are uniformly sized  <10–15 nm

13. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 13 X-ray Diffraction of DU Catalysts  XRD permits identification of phases present in catalyst before or after reaction  U-meso-8  U:Si = 1:10  Poor activity  UO 2 and U 3 O 8 present  U-meso-6  U:Si = 1:20  Good activity  Only U 3 O 8 present  XRD shows that U 3 O 8 is the most active phase  Cause of UO 2 growth in U-meso-8 not clear

14. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 14 Promotion of Uranium Catalysts: Effects of Potassium Addition Potassium is frequently used as promoter in many catalysts Idea: Promote Cl-C bond cleavage by K addition  Method 1: co-assembly including K salts  Br, Nitrate or oxalate salts  U:Si=1:20  U:K = 1:1  Surface area and pore structure collapses  Surface area drops from 190 m 2 /g to 1-5 m 2 /g loss of activity  Method 2: sequential impregnation of MCM-41 with uranyl nitrate and K salts  Surface area drops from 760 to 26 m 2 /g loss of activity

15. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 15 Promotion of Uranium Catalysts: Effects of K, Ca Fe Oxide Additions  Try other components for urania catalysts  Co-assembly with FeNO 3 and Mg acetate (Ca nitrate)  Surface area remains high  Pore structure good  But, no enhancement of activity

16. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 16 Effect of Uranium Loading in TiO 2 Based Mesoporous Catalysts  Get optimal activity at 5 mole% U (U:Ti=1:20)  Surface area (and activity) affected by calcination temperatures Toluene oxidation

17. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 17 Activity for Oxidation of Other VOCs  Chlorinated VOCs are common pollutants at industrial and DOE sites  Uranium loaded TiO 2 catalysts were active for destruction of chlorinated VOCs such as chlorobenzene and trichloroethylene (TCE)  TCE and Cl-benzene are more difficult to destroy  By-products are CO 2 and water mostly – but small amounts of benzaldehyde from Cl-benzene  Cl products are both HCl and Cl 2 results of VOC combustion in absence of added water

18. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 18 Comparison with Commercial Pt Catalysts Uranium oxide in mesoporous support outperforms a Pt catalyst (0.1 wt % Pt on alumina) for comparable reaction conditions  T 50 for TCE is more than 50°C lower for U-mTiO 2 catalyst than for Pt catalyst

19. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 19 Effect of Water Addition In most applications water is present (e.g. soil vapor extraction wells for groundwater clean-up)  Water does not interfere— even enhances activity for TCE oxidation  Water permits higher HCl:Cl 2 ratios of byproducts (good for most applications)  HCl by-product can be trapped

20. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 20 Conclusions  Many DU based catalysts have been prepared and tested  A catalyst formulation based upon a titania-uranium (Ti-U) oxide (Ti:U = 1:20) was found to be competitive with noble metal catalysts for the oxidation of VOCs and chlorinated VOCs, e.g., toluene, Cl-benzene, TCE  The catalyst is stable to deactivation by Cl  The catalyst operates effectively in the presence of large amounts of water  Catalyst is suitable for destruction of VOCs emitted from soil vapor extraction wells, etc.