Title: Uranium-Based Catalyst
1Uranium-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
2Depleted 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
3New 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
4Synthesis 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
5Nonpowder 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
6Reactor 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
7Photograph of Reactor Used in DU Project
8Light-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 )
9U 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
10Catalysts 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
11TEM Characterization of DU Catalysts
- Catalyst particle of U-MAS-5
- Al3 doped silica mesoporous support impregnated
with uranyl nitrate - Calcined 900ÂșC
- High resolution TEM using HD-2000 at ORNL
- Uranium oxide particles located within pores
12STEM 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
13X-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
14Promotion 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
15Promotion 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
16Effect 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
17Activity 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
18Comparison 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
19Effect 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
20Conclusions
- 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. -