# Presentation: 477272

## Uranium-Based Catalyst

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

**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**

## 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** **

**Notes:**
Relationship to cerium oxide
Selective and total oxidation are best chance

## 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**

## 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 SiO****2**** or TiO****2****, or other oxides**

- Surfactant extraction or calcination

- Silica condensation

- Rodlike micelle

- Silicate encapsulated micelles

- TEOS

- (C16H33)N(CH3)3 Br
- + NaOH  / H2O

## 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

**Notes:**
We have developed synthetic techniques which achieve some of the previously desired properties.  Picture shows a monolithic material, a hardened gel, composed of silica with U co-assembled inside it.  It has high surface area, (even as a chunk)and the uranium is available and catalytically active 

## 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**

**From O2 Tank**

**From He Tank**

**21 ml/min (He)**

**H****2****O**** Syringe Flow Meter**

**He**

**O****2**

**( 77 ml/min He + 42 ml/min O****2 ****)**

**Mixing  Point 140 ml/min**

**Pressure Gauge**

**Adjusting Valve**

**Bypass Flow**

**Bypass Flow**

**Line to Bypass the Bubbler**

**Line to Bypass the Reactor**

## Photograph of Reactor

- Used in DU Project

## 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 cm****3****/min**
**O****2**** flow 40 cm****3****/min**
**toluene 500 ppm**
**GHSV = 72000 hr****-1**
**Mesoporous silica (MCM-41) without DU is inactive**
**U****3****O****8**** obtained by calcination of UO****2****(NO****3****)****2**
**Pure U****3****O****8**** is active but low surface area (<0.1 m****2****/g )**

**Notes:**
This is the first data shown of activity.  Explain that use lightoff curves to compare activity.  Measure conversion of toluene (combustion to carbon dioxide) as function of temperature.  The lower the temperature at which conversion is achieved (for example 50% conversion, the “T50” )t, the better the catalysts.  We first tried supporting DU in silica mesoporous supports.  Reaction test shows that pure silica is quite inactive.  Pure U3O8 is active as shown by the T50 of about 500 C.  However, we want to improve this.  Earlier published work by Hutchings gave higher activity showing 100% conversion at around 400 C.  He created his samples by impregnation of fumed silica with uranyl nitrate and used slightly different reaction conditions..

## U impregnated in Mesoporous Support

**U-MAS-5**
**UO****2**** (NO****3****)****2 ****impregnated into solid mesoporous silica**
**silica contains 5% Al**
**U:Si = 1:10**
**improved light-off compared to pure U****3****O****8**

**Notes:**
Next we tried impregnating (essentially dripping uranium nitrate solution) into mesoporous silica.    This provides a more active catalyst (the “U-MAS-5” sample) compared to the pure U3O8,   especially since there is only a fraction of the sample is uranium oxide and the bulk is the silica support.  Silica gives high surface area, necessary for good activity.  

## 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 U****3****O****8**** although lower U concentration**
**Gave poorly ordered mesopores**
**Broad BJH pore distribution**
**BET SA 225–300  m****2****/g**

**Notes:**
Next we tried catalysts prepared by co-synthesis techniques.   This method, described in the slides above, puts the uranium into the synthesis mixture and used a micelle template (for example Pluronic 123) which distributes the U throughout the silica support.  Potentially it can give higher surface area and smaller Urania particle sizes. This figure shows the results of varying the amount of uranium in the catalysts.  The best is the 1:20 molar ratio, which means that atom ratio of Si to U is 1:20.  Therefore even dilute U gives good performance.  Now the T50 is down around 450 C, actually not much better than the impregnated sample U-MAS-5.  This may be because the U-MAS-5 had a bit of Al in it.

## TEM Characterization of DU Catalysts

**Catalyst particle of U-MAS-5**
**Al****3+**** doped silica mesoporous support impregnated with uranyl nitrate**
**Calcined 900****o****C**
**High resolution TEM using HD-2000 at ORNL**
**Uranium oxide particles located within pores**

**Notes:**
It is of interest to know a bit about the catalyst structure.  We used electron microsopy to show the morphology of the catalyst. The picture is of the U-MAS-5 and shows that the mesoporous structure may be fairly convoluted, rather than straight.  The dark patches are the uranium particles.   Since samples were calcined at high temperatures, the uranium is present as some sort of an oxide.  Particles appear to be located within pores, but it is actually difficult to say absolutely if most are on the external surface or in the pores.

## STEM Micrograph of DU Catalyst

**Catalyst U-SiF127**
**UO****2**** (NO****3****)****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**

**Notes:**
This micrograph is recorded on a STEM, a different type of microscope.  It is about the same magnification.  The scale shows the distance across the 10 tick marks.  The dark spots can be seen to be about 10 nm to 15 nm in diameter.  This size is actually larger than the mesopores of the silica.   This may mean that the particles are on the outside surface, or that the particles broke through the pore walls as they grew.  

## 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**
**UO****2**** and U****3****O****8**** present**
**U-meso-6**
**U:Si = 1:20**
**Good activity**
**Only U****3****O****8**** present**
**XRD shows that U****3****O****8**** is the most active phase**
**Cause of UO****2**** growth in   U-meso-8 not clear**

**Notes:**
X-ray diffraction was used to see what phases of uranium oxide are present.  The activity of these two catalysts were compared in an earlier viewgraph.  XRD was used to look for differences between them.  We find that the U3O8 appears to be the more active phase of the uranium oxide, since the U-meso-8 contains lots of UO2 but is less active than the U-meso-6 which has no UO2.  

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

**Notes:**
Next ride to promote the activity of the DU catalyst by adding other components.  One attempt was to use K

## Promotion of Uranium Catalysts:Effects of K, Ca Fe Oxide Additions

***Try other components for urania catalysts ***
**Co-assembly with FeNO****3**** and Mg acetate (Ca nitrate)**
**Surface area remains high**
**Pore structure good**
**But, no enhancement of activity**

**Notes:**
Addition of other possible promoters did not improve the activity compared to uranium oxide in silica.

## 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

**Notes:**
On change in the catalyst that did work well was to use mesoporous titanium dioxide instead of sicon oxide as a support.  TiO2 can be made into mesoporous supports similar to SiO2.  We did this by co-synthesis.  As with the SiO2 we tried various ratios of Si to Ti, and got good results with 1:20.  Now the T50 is down below 350 C.  
 These catalyst can not be heated as high as the silica, because crystallization of the titanium oxide causes a decrease in surface area.  Therefore, kept below 600 c.

## Activity for Oxidation of Other VOCs

**Chlorinated VOCs are common pollutants at industrial and DOE sites **
**Uranium loaded TiO****2**** 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 CO****2**** and water mostly ****–**** but small amounts of benzaldehyde from Cl-benzene **
**Cl products are both HCl and Cl****2**

*results of VOC combustion in absence of added water*

**Notes:**
We tried these catalyst for other volatile organic components. because this is probably the the most likely application.

## Comparison with Commercial Pt Catalysts

**Uranium oxide in mesoporous support outperforms a Pt catalyst (0.1 wt % Pt on alumina) for comparable reaction conditions **
**T****50**** for TCE is more than 50****°****C lower for U-mTiO****2**** catalyst than for Pt catalyst **

## 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:Cl****2**** ratios of byproducts (good for most applications)**
**HCl by-product can be trapped**

**Notes:**
For TCE there is not enough H in the molecule for Cl to evolve as HCl instead of Cl2.  Addition of water helps provide H available for reaction with the Cl.  Addition of H2o helps to increase the ratio of HCl / Cl2.  This is important because it is easy to trap the HCl using a simple water trap, thereby preventing release of more hazardous  byproduct Cl2.  Catalyst was run for long periods of times (a week or more) without loss of activity

## 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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