Putting Depleted Uranium to Use: A New Class of Uranium-Based Catalysts

Background

    The US Department of Energy has more than a billion lbs of depleted uranium (DU) in stockpiles.  The DU is mostly in the form of UF6 stored in barrels in Oak Ridge, TN and Portsmouth OH.  UF6 is volatile and corrosive and must be converted to a more stable form in the future.   The costs of conversion are immense. Technologies which utilize uranium in large quantities or in a high value application are required to finance the expense of stockpile conversion.  

    Uranium oxide has shown promise as a catalyst for oxidation of volatile organic compounds (VOCs) and halogenated VOCs. (Hutchings 1996).  U.S. industries and the U.S. Department of Energy must manage a variety of off-gas wastes consisting of complex volatile organic compounds.  Development of uranium catalysts for combustion of VOC and  HVOCs offer the possibility of utilizing uranium in a high value application which solves two problems 

    Our group, in collaboration with Sheng Dai of the Chemical Technology Division, is investigating a new class of mesoporous uranium oxide and mesoporous sol-gel catalysts doped with uranium oxides for destruction of a range of volatile organic contaminants, including alkanes, aromatics, and chlorinated organic compounds.  This research will contribute to a fundamental understanding of the mechanism and rate-limiting processes that control the reactivity and selectivity between uranium-based catalysts and the organic substrates (or pollutants).

Synthesis of catalysts

    Mesoporous materials, first synthesized by researchers at Mobil Research Labs (Beck, 1992) can be grown to have very well ordered and uniform size distributions.  The micrograph in Fig. 1 shows a mesoporous silica, MCM-41, we synthesized using cetyltrimethylammonium bromide (CTAB) as a sufactant template molecule. Various mesoporous catalysts have been synthesized using CTAB and other surfactants which contain uranium  (uranyl nitrate) introduced directly into the silica based sol-gel synthesis.  A mesoporous MCM-41 support impregnated with uranium nitrate has also been produced for comparison.  The materials have been characterized by BET surface area measurements to determine surface area and total pore volume. 

Figure 1 - TEM of MCM-41

Catalytic properties of DU

    Catalytic measurements were made using a plug-flow reactor. "Light-off" curves for toluene destruction (total toluene conversion vs. temperature) were obtained on various uranium containing catalysts.  Typical light-off curves are shown in Figure 2.  The conversions were measured at a constant flow rate of 55 cc/min (0.6 % toluene, 18% O2, He balance) and using 0.1 g catalyst (W/F=0.11 g/ml/s; space velocity = 17,400 hr-1) .  Catalysts were synthesized various concentrations of alcohol solvent ,  surfactant and U:Si ratio.  Initially samples showed low activity following calcination at 375-400 C (UCT7B, UCT1B).  It was postulated that low activity was related to high oxidation state of uranium, since it is known that the active phase, U3O8 contains reduced U 4+ .  Therefore the samples were then calcined to 800 C, a procedure that is expected to decrease the oxidation state due to thermal decomposition of UO3.  This high temperature treatment resulted in an increase in catalytic activity.  The data shown in Fig. 2 labeled UCT2B was calcined to 800 C.

Figure 2 - Conversion of Toluene vs Reaction Temperature

    The best catalyst in terms of high toluene conversions at low temperatures was a sample prepared by solution impregnation of uranyl nitrate into a previously prepared silica mesoporous material, MCM-41 (indicated as U-MCM-41 in Fig. 2), followed by calcination to 800 C.  This sample was prepared to mimic those used by Hutchings et al. [Nature 384 1996, 341-343] except that mesoporous silica was used instead of fumed silica.

Oxidation States

Methods for determination of urania oxidation state are being investigated.  A first attempt for determination of oxidation state was to use UV-visible reflectance spectroscopy.  The absorption spectra by urania is known to depend upon the oxidation state.  Figure 3 shows an absorption spectrum of the active sample UCT2B.  The spectroscopic features associated with U4+ are quite weak and are barely visible while the band for U6+ is prominent.  It is planned to apply other techniques to determine the oxidation state of the calcined samples including diffuse reflectance Fourier transform infra-red spectroscopy (DRIFTS) and x-ray photoelectron spectroscopy.

Figure 3 - Optical absorption spectrum obtained in reflectance mode for UCT2B.

References:

Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olsen, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L., J. Am. Chem. Soc. 114 (1992) 10834.

Hutchings, G. J.; Heneghan, C. S.; Hudson, I. D.; Taylor, S. H., Nature 384 (1996) 341.

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[ Surface Chemistry Group I Oak Ridge National Laboratory I Chemical Sciences Division I Disclaimers]

Revised: 8 - August - 2002 by David R. Mullins