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Presented toThe 2004 American Nuclear Society Winter Meeting
Washington, D.C.November 14–18, 2004
Uranium-Based Catalyst
M. J. HaireNuclear Science and Technology Division
S. H. Overbury, C. K. Riahi-Nezhad, and S. DaiChemical Sciences Division
Oak Ridge National Laboratory
2
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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 Surfactant extraction or calcination
Silica condensation
Rodlike micelle
Silicate encapsulated micelles
TEOS(C16H33)N(CH3)3 Br
+ NaOH / H2O
5
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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 MembranesWavelength (nm)
350 400 450 500 550
Ab
so
rba
nc
e0.2
0.3
0.4
0.5
0.6
0.7
0.8
Optical AbsorptionSpectrum
Monolithic U-SiO2
6
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Reactor Set-up for Catalytic Testing
77 21
vent
GC/MS
Reactor (Temp. Controlled)
w/ quartz tube & sample
To Mass Spec/ G.C
bypass bypass
bubbler bubbler
bypass bypass
R R
21.0
Thermocouple
Bubbler and Ice Bath
Heating Zone
42
He O2 He
He gas for bubbler
Flow Regulator
Fro
m O
2 T
ank
Fro
m H
e T
ank
21 ml/min (He)
H2O
Syringe
Flow
Meter
He O
2(
77 m
l/m
in H
e +
42
ml/
min
O2 )
Mixing Point 140 ml/min
Pressure Gauge
Adjusting Valve
Bypass Flow Bypass Flow
Line to Bypass the Bubbler Line to Bypass the Reactor
7
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Photograph of ReactorUsed in DU Project
8
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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 ) Temperature (C)
300 400 500 600 700To
luen
e co
nver
sion
(%)
0
20
40
60
80
100
Light-off curvesToluene oxidation
MCM-41 (no U)
U3O8
Hutchings et al.U / SiO2
9
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
Light-off curvesToluene oxidation
Temperature (C)
300 400 500 600 700
Tolu
ene
Con
vers
ion
(%)
0
20
40
60
80
100
U-MAS 5U3O8
10
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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 450C
Activity higher than U3O8 although lower U concentration
Gave poorly ordered mesopores Broad BJH pore distribution BET SA 225–300 m2/g
Temperature (C)
300 400 500 600 700To
luen
e co
nver
sion
(%)
0
20
40
60
80
100
1:301:201:10
Light-off curvesToluene oxidation
U:Si
Mesoporous synthesis
11
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
XRD for phase identification
Angle (deg)
10 20 30 40 50 60 70
Inte
nsity
(au)
0
50
100
150
200
250
300U-meso-6U-meso-8*
** *
* = UO2
14
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
0
20
40
60
80
100
200 300 400 500 600
Temperature (C)
Per
cent
Con
vers
ion
Fe and Mg doped
Fe and Ca doped
meso U (U:Si=1:20)
Chlorobenzene conversion
16
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
Temperature ( C)
0
20
40
60
80
100
200 300 400 500 600
To
luen
e C
on
vers
ion
(%
)
U:Ti=1:10
U:Ti=1:20
U:Ti=1:30
U:Ti=1:40
U:Ti mole ratio
Surface area m2/g
Calcination temperature (°C)
1:10 196 400 1:20 216 350 1:30 139 400 1:40 162 400
Toluene oxidation
17
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
0
20
40
60
80
100
100 200 300 400 500Temperature (C)
Con
vers
ion
(%
)
toluene
TCE
chlorobenzene
U-meso TiO2
results of VOC combustion in absence of added water
18
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
0
20
40
60
80
100
100 200 300 400 500 600Temperature (C)
Con
vers
ion
(%)
toluene
TCE
chlorobenzene
0.1 wt% Pt / Al2O3
0
20
40
60
80
100
100 200 300 400 500 600Temperature (C)
Con
vers
ion
(%)
toluene
TCE
chlorobenzene
U-meso TiO2
19
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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
0
20
40
60
80
100
250 350 450 550Temperature ( C)
TC
E C
onve
rsio
n (%
)
dry
7 %
9 %
15 %
TCE oxidation by U-mesoTiO2
[H2O]
20
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
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.