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Uranium-Based Catalyst

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. Presented to The 2004 American Nuclear Society Winter Meeting Washington, D.C. November 14–18, 2004.

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Uranium-Based Catalyst

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  1. 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 Presented to The 2004 American Nuclear Society Winter Meeting Washington, D.C. November 14–18, 2004

  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. 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. 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 SiO2 or TiO2, or other oxides (C16H33)N(CH3)3 Br + NaOH / H2O TEOS Silicate encapsulated micelles Rodlike micelle Surfactant extraction or calcination Silica condensation

  5. Nonpowder Forms of DU Catalysts Optical Absorption Spectrum • High Surface Area • 250 m2/g • monolithic catalysts simplifies handling • uranium oxide is not co-precipitated; it is on/in the pore walls • transparency, possible photochemical processes • Reactive Membranes Monolithic U-SiO2

  6. Reactor Set-up for Catalytic Testing Line to Bypass the Bubbler Line to Bypass the Reactor Bypass Flow Bypass Flow Mixing Point 140 ml/min bypass bypass ( 77 ml/min He + 42 ml/min O2 ) bypass bypass vent Adjusting Valve bubbler bubbler Pressure Gauge R R GC/MS Heating Zone Thermocouple 21 ml/min (He) O2 He To Mass Spec/ G.C Flow Regulator He O2 He 77 42 21 H2O Syringe Flow Meter 21.0 From O2 Tank From He Tank Bubbler and Ice Bath Reactor (Temp. Controlled) w/ quartz tube & sample He gas for bubbler

  7. Photograph of Reactor Used in DU Project

  8. Light-Off Curves to Compare Activity:U3O8 measure light-off curve to compare activity for toluene oxidation • Reactor conditions • 25 mg catalyst • He flow 150 cm3/min • O2 flow 40 cm3/min • toluene 500 ppm • GHSV = 72000 hr-1 • Mesoporous silica (MCM-41) without DU is inactive • U3O8 obtained by calcination of UO2(NO3)2 • Pure U3O8 is active but low surface area (<0.1 m2/g )

  9. U impregnated in Mesoporous Support • U-MAS-5 • UO2 (NO3)2 impregnated into solid mesoporous silica • silica contains 5% Al • U:Si = 1:10 • improved light-off compared to pure U3O8

  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 U3O8 although lower U concentration • Gave poorly ordered mesopores • Broad BJH pore distribution • BET SA 225–300 m2/g

  11. TEM Characterization of DU Catalysts • Catalyst particle of U-MAS-5 • Al3+ 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. STEM Micrograph of DU Catalyst • Catalyst U-SiF127 • UO2 (NO3)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. 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 • UO2 and U3O8 present • U-meso-6 • U:Si = 1:20 • Good activity • Only U3O8 present • XRD shows that U3O8 is the most active phase • Cause of UO2 growth in U-meso-8 not clear

  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 m2/g to 1-5 m2/g loss of activity • Method 2: sequential impregnation of MCM-41 with uranyl nitrate and K salts • Surface area drops from 760 to 26 m2/g loss of activity

  15. Promotion of Uranium Catalysts:Effects of K, Ca Fe Oxide Additions • Try other components for urania catalysts • Co-assembly with FeNO3 and Mg acetate (Ca nitrate) • Surface area remains high • Pore structure good • But, no enhancement of activity

  16. Effect of Uranium Loading in TiO2 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. Activity for Oxidation of Other VOCs • Chlorinated VOCs are common pollutants at industrial and DOE sites • Uranium loaded TiO2 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 CO2 and water mostly – but small amounts of benzaldehyde from Cl-benzene • Cl products are both HCl and Cl2 results of VOC combustion in absence of added water

  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 T50 for TCE is more than 50°C lower for U-mTiO2 catalyst than for Pt catalyst

  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:Cl2 ratios of byproducts (good for most applications) • HCl by-product can be trapped

  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.

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