Uranium-Based Catalyst

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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 1418,
2004
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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 oxidationmethane 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

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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

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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

TEOS
(C16H33)N(CH3)3 Br NaOH / H2O
Silicate encapsulated micelles
Rodlike micelle
Surfactant extraction or calcination
Silica condensation
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Nonpowder Forms of DU Catalysts

• 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
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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 )
vent
bypass
bypass
Adjusting Valve
bubbler
bubbler
Pressure Gauge
GC/MS
R
R
Heating Zone
Thermocouple
21 ml/min (He)
O2
He
To Mass Spec/ G.C
Flow Regulator
He O2 He
77
21
42
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
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Photograph of Reactor Used in DU Project
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Light-Off Curves to Compare ActivityU3O8

• 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 (lt0.1
  m2/g )

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U impregnated in Mesoporous Support

• U-MAS-5
• UO2 (NO3)2 impregnated into solid mesoporous
  silica
• silica contains 5 Al
• USi 110
• improved light-off compared to pure U3O8

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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 USi ratio
• 50 conversion above 450?C
• Activity higher than U3O8 although lower U
  concentration
• Gave poorly ordered mesopores
• Broad BJH pore distribution
• BET SA 225300 m2/g

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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

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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
• USi 120
• Mesoporous structure shows as parallel walls
• Pore spacing 10.3 nm
• Uranium oxide particles are uniformly sized
• lt1015 nm

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X-ray Diffraction of DU Catalysts

• XRD permits identification of phases present in
  catalyst before or after reaction
• U-meso-8
• USi 110
• Poor activity
• UO2 and U3O8 present
• U-meso-6
• USi 120
• Good activity
• Only U3O8 present
• XRD shows that U3O8 is the most active phase
• Cause of UO2 growth in U-meso-8 not clear

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Promotion of Uranium CatalystsEffects 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
• USi120
• UK 11
• 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

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Promotion of Uranium CatalystsEffects 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

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Effect of Uranium Loading in TiO2 Based
Mesoporous Catalysts

• Get optimal activity at 5 mole U (UTi120)
• Surface area (and activity) affected by
  calcination temperatures

Toluene oxidation
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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
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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 50C lower for U-mTiO2
  catalyst than for Pt catalyst

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Effect of Water Addition

• In most applications water is present (e.g. soil
  vapor extraction wells for groundwater clean-up)
• Water does not interfereeven enhances activity
  for TCE oxidation
• Water permits higher HClCl2 ratios of byproducts
  (good for most applications)
• HCl by-product can be trapped

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Conclusions

• Many DU based catalysts have been prepared and
  tested
• A catalyst formulation based upon a
  titania-uranium (Ti-U) oxide (TiU 120) 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.