presented to the 2004 american nuclear society winter meeting washington, d.c. november 14–18,...

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


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 Surfactant extraction or calcination

Silica condensation

Rodlike micelle

Silicate encapsulated micelles

TEOS(C16H33)N(CH3)3 Br

+ NaOH / H2O

5


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


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


Photograph of ReactorUsed 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 ) 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


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


Temperature (C)

300 400 500 600 700

Tolu

ene

Con

vers

ion

(%)

0

20

40

60

80

100

U-MAS 5U3O8

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 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


U:Si

Mesoporous synthesis

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

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


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

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


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


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


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


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


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.

presented to the 2004 american nuclear society winter meeting washington, d.c. november 14–18,...

Documents

o slide

mlmin o

o ak r idge n ational

d epartment of e nergy

reactor slide

catalyst slide

project slide

o syringe flow meter