J. Phys. Chem. 1992,96, 1095-1099 TABLE WI: SIlmmVy of the ExperiWaW rad Clllcuhted Moaowr-to-compkxFrequcsley Shifts for the N*O/HCN System.
expt 6-31G** D95** 6-31+G(2d,2p) NNGHCN -13.7 +4.0 +2.8 -3.2 ONN-HCN -18.9 -19.4 -21.3 bent NNO-HCN -21.6 HCN/ONN (11) -2.02 +2.8 -2.3 -2.8 HCN/NNO (IV) +0.4 -4.1 4.5 OHCN/ONN (11) and HCN/NNO (IV)are similar to structures I1 and IV in Figure 3. the basis sets used or if they reflect some interesting kinetics associated with the formation of these complexes. Table XI11 contains a summary of the experimental and calculated monomer-to-"plex frequency shifts for the two isomers. The results agree very well for the slipped-parallel isomer, while the agreement is relatively poor for the linear NNO-HCN complex. The latter fact could be a result of the very flat bending potential, which we have already noted makes the calculations for this isomer very basis set dependent. The encouraging aspect of the calculations for this isomer is that the frequency shift seems to be improving as the quality of the basis set is improved. su"rry
The optdermal method has been used to establish the existence of two stable isomers of the N20/HCN binary complex. In the case of the linear isomer, two different isotopic forms were studied to unambiguously determine that the HCN subunit is hydrogen bonded to the oxygen end of nitrow oxide. For the slipped-parallel isomer, a single isotopic substitution was not sflicient to establish
1095
an unambiguous structure. Further isotopic work on this system is needed, perhaps from a pulsed Fourier transform microwave spectrometer, which requires smaller quantities of gas. Nevertheless, arguments based on the electrostatics of this system, as well as the reported ab initio calculations, strongly favor a structure similar to that shown in Figure 311. Comparisons between the experimental and ab initio results are mixed. Although some of the trends look rather good and there is a great deal of quantitative agreement, the comparisons are somewhat less quantitative than we have found in a number of previously reported systems, such as C2H2/HCNI3and CO2-HCN.l0 Overall, we fmd that the agreement between experiment and ab initio theory improves as the size of the basis set is increased. Although some of the quantitative agreement is missing in the present study, we still feel that these comparisons are helpful in providing further insights into the nature of the intcrmolccular interactionsin this interesting system. For example, both the experimental and ab initio results imply that the linear NNO-HCN complex has a very flat intermolecular bending potential. Acknowledgment. This work was supported by the National Science Foundation under Grant CHE-89-00307 and by the donors of the Petroleum Research Fund, administered by the American Chemical Society. Supplementaty Material Available: Transition frequencies for both isotropic forms of the linear and slipped-parallelN20-HCN complexes available on microfilm or directly from the authors (9 pages). Ordering information is given on any current masthead page.
FTIR Spectra of the Hydrolysis of Uranium Hexafluoride Sumo A. Sberrow Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
and Rodney D. Hunt*?+ Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: July 15, 1991) The hydrolysis of uranium hexafluoride has been examined with infrared spectroscopy under three different reaction conditions. First, uF6 was deposited with water in excess argon at 12 K. The mIR spectra of UF6/H20mixtures revealed several product absorptions due to the perturbed vl, v2, and v3 modes of H20as well as the v3 mode of UF6. These new absorptions have been assigned to the anti-hydrogen-bonded 1:l complex, UF6-OH2. Photolysis of this weak complex produced a doublet at 868.2 and 857.1 cm-', which has been assigned to the UOF,. Next, solid films of UF&O mixtures were prepared at 12 K and slowly annealed at 242 K. The reaction profile of the uF6 hydrolysis from uF6 (or UF6-OH2)to UOF, to U02F2 was obtained. Finally, UF6 and H20 were reacted at low pressures and ambient temperatures in a new IR gas cell. While no gaseous uranium oxyfluorides were detected, the final product distribution inside the gas cell did indicate that the fluorinated nickel surface served as a catalyst for the UF6 hydrolysis.
Introdaction At ambient temperatures, the hydrolysis of uranium hexafluoride with excess water is thought to p r d spontaneously in three stages,' which can be represented by the following equations: UF6 + H20
+
UOF4 + H2O
UOF4 + 2HF
(1)
U02F2 + 2HF
(2)
U 0 3 + 2HF
(3)
-
+
U02F2+ H 2 0
However, attemptsz3 to form the first hydrolysis product, UOF,, To whom compondenoc should be addread. 'Oak Ridge National Laboratory, Oak Ridge, TN 37831-6226.
0022-3654/92/2096-lO95$03.00/0
by reacting UF6 with H 2 0 in the gas phase have proven unsuccessful. Brooks et al.' reported that excess UF6 combined with water in a 1:3 ratio to @vea mixture of UO& and UO~F~(H~O)Zs u r l y , aeY et ala3formed U305F~by reacting UF6 with very small quantities of water vapor at 160-170 "C. Under these reaction conditions, Otey et aL3proposed two additional reaction intermediates, U2O3F6 and U305F8. In order to generate UOF,, Wilson? Jacob et and Paine et a1.6 reacted excess UF6 with (1) Weigcl. F. The Chemistry of the Actinide Elements, 2nd ad.;Chapman and Hall: N e w York, 1986; Vol. 1, Chapter 5. (2) Brooks, L. H.; Gamer, E. V.; Whitehead, E. U.K . A?. Energy Rep. 1959, I c R - T N / c ~ - 2 7 7 . (3) Otey, M. G.; LeDoux, R.A. J . Inorg. Nucl. Chem. 1967, 29, 2249. (4) Wilson, P. W. Chem. Commun. 1972, 1241; J . Inorg. Nucl. Chem. 1974, 36, 303. ( 5 ) Jacob, E.; Polligkeit, W. Z . Naturforsch., B 1973, 28, 120.
Ca 1992 American Chemical Society
1096 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
H 2 0 in a hydrogen fluoride medium. The difficulty in forming UOF4 indicates that the reaction mechanism for the UF6 hydrolysis may not be adequately described by the above three equations. Other uranium oxyfluorides, which could be formed during the UF, hydrolysis, have been synthesized by reaction processes which did not involve H20. Wilson' prepared U203F6, by the thermal reacted U02F2 decomposition of UOF4 at 563 K. Asada et with UF3at 673 K under a UF, pressure of 3 atm to form U202F,. With the exception of U305Fn,each of theae uranium oxyfluorides has been characterkd using infrared s-py. U30sF8,which is yellow, was identified by using several elemental analysis techniques including the pyrohydrolysis method. Since the hydrolysis of UF, is a reaction of recurring fundamental concem in actinide chemistry, further examination of this reaction mechanism is clearly warranted. The results from three complementary FTIR studies on the UF, hydrolysis will be presented. First, the matrix isolation technique was employed since the inert gas matrix can trap both weak complexes and reactive intermediates. Photolysis and annealing of the matrix samples permit the reaction to occur and be examined in a very controlled manner. While Anderson9conducted the initial matrix IR study on the hydrolysis of UF6, it was severely hampered by an unacceptable level of impurities. Unfortunately, Anderson was unable to perform isotopic substitutions so he could not determine that most of his product absorptions were actually impurities. Next, solid films of UF&O were formed by codepositing the reactants at 12 K. The reaction profile of the UF, hydrolysis from UF, to U02F2was obtained by slowly annealing the films in small increments. Finally, a new series of gas-phase experiments using UF6/H20 mixtures at low pressures and ambient temperatures were conducted in an effort to detect any gas-phase hydrolysis products other than HF. In a related gas-phase FTIR study, Armstrong et a1.I0 have produced HF and airborne, particulate U02F2from the release of hot, pressurized UF, into ambient air.
Experimental Section For the low-temperature experiments, the cryostat and the photolysis arrangement have been previously described in detail.' All FTIR spectra were recorded with the Bomem DA 3.01 FTIR spectrometer from 4450 to 450 cm-'. A single-beam spectrum of the CsI window at 12 K was recorded and ratioed as a background to each subsequent singlebeam spectrum of the samples in order to produce a simulated double-beam spectrum. FTIR spectra at a resolution of 0.25 or 0.5 cm-I were recorded after each deposition, photolysis, and annealing. In the matrix isolation study, an excess of argon (AirProducts) or xenon (Matheson) was condensed on a CsI window at 12 K. Siultaneously, UF, and H2'60 (Oak Ridge National Laboratory) were admitted through metering valves into a 5-cm-long inlet which terminated 5 cm from the CsI window. Deposition rates of the inert gas ranged from 1 to 7 mmol/h. Additional UF6 experiments were conducted with D20, H2180,and Hi60/H2180 (50%/5096) (Oak Ridge National Laboratory). The UF6 and water concentrations were estimated by comparisons with spectra of matrices formed from standard mixtures. The typical inert gas/reagent ratios ranged from 300/1 to 1000/1. Temperature of sample window was maintained at 12 K except during 30 K annealings. In the solid films of UF6 with H2160or H2180,the deposition conditions of UF6 and water were not changed from the matrix isolation experiments. After the UF6 and H2I60were deposited without an inert gas, the first film was photolyzed for 1 h. Then, the temperature of this film was raised at intervals of 10 K for 10 min until the temperature reached 242 K. In the other UF, (6) Paine, R. T.; Ryan, R. R.; Asprey, L. B. Inwg. Chem. 1975,14,1113. ( 7 ) Wilson, P. W. J. Inorg. Nucl. Chem. 1974, 36, 1783. (8) Asada, K.; Ema, K.; Iwai, T.Bull. Chem. Soc. Jpn. 1987.60, 3189. (9) Anderson, S.P. M.S. Thesis,University of Tennessee, 1982. (10) Armstrong, D. P.; Bostick, W. D.; Fletcher, W. H. Appl. Spectrosc. 1991, 45, 1008. (11) Sherrow, S. A. J . Chem. Phys. 1989. 90, 5886.
Sherrow and Hunt
8 2 a
m
a
8m a
3800
3720
3640
WAVENUMBER (cm-')
Figure 1. (a) Infrared spectrum in the 3800-3600-m-' region after the deposition of argon-diluted samples of water and uranium hexafluoride at 12 K for 2.5 h. (b) Infrared spectrum of H 2 0 and UF, after the 1-h photolysis at 12 K. (c) Difference spectrum with the vertical scale expanded by 3 showing the effects of the 1-h photolysis.
solid films with H 2 W or H2180,the main spectral and chemical transitions were examined more closely. The temperature of these solid films was raised in 5 K intervals and maintained at the new temperature for 30 min in order to allow the hydrolysis reaction an opportunity to proceed. For the gas-phase investigation, a variable path length gas cell (Spectra-Tech) with a nickel-plated interior was equipped with Teflon-coated elastomer seals to m i n k e potential interference from H F reactions with the cell materials. Prior to these experiments, the cell was conditioned with UF6 until the level of UF, would remain constant for several hours. At the start of each experiment, the gas cell was evacuated and the IR beam was set at a path length of 10 m. A single-beam FTIR spectrum at a resolution of 0.1 or 0.5 cm-l was obtained and used as a background for each subsequent spectrum. Typically, 0.06-0.12m o l of UF6 was added to the cell, and a s p t x t " was recorded. Then, 0.014.16mmol of H 2 0 was added through an inlet at the center of the gas cell's cover plate. This inlet penetrated 0.5 in. into the gas cell. The length time for the reaction was varied from 30 s to 30 min. During the reaction period, groups of 20-40 interferrograms at a resolution of 0.5 cm-' were madded to provide ''snapshots* of the hydrolysis in progress. After the completion of the reaction, a final spectrum was recorded. When the series of experiments had been completed, the gas cell was placed in a glovebag which was purged with dry nitrogen. Then, the interior of the cell was examined. Results
Xe and AI Matrices. Oridinarly, the choice of the inert gas for the matrix does not have a pronounced effect on the chemistry of the system of interest. However, in the case of the UF6/H20 mixtures, the spectra in xenon matrice were sisnificantlydifferent from their argon counterparts. After the Xe-diluted samples were deposited, no interaction between UF6 and H 2 0 was detected. Subsequent photolysis and annealings to 60 K failed to induce any direct reaction between UF, and H20. However, the photolysis did produce UF6-, which was generated by a charge transfer between the UF, and its Xe host. The results and conclusions based on these Xe experiments have been earlier discussed in detail." In sharp contrast to the study with Xe, the spectra of the 6 and H20in Ar revealed several new product reaction between m absorptions. Based on the differences between the Ar and Xe results, the polarizability of the inert gas, which may lead to competing interactions, should be taken into account when very
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1097
FTIR Spectra of the Hydrolysis of UF6
TABLE I: Absorptiom (em-') Roduced on the Codeposition and Photolysis of Uranium Hexafluoride and Water with Excess A m at 12 K
UF6 + H2160
uF6 + DzO
UF6 + HzI8O
3717.9 3624.7 3623.4 1588.5 1585.6 868.2 857.1 608.8 587.8
2752 2645.1 2644.6 1 174.6
3701 3617.0
861.8 856.7 604.1 582.0
825.0 814.8 608.9 586.9
assignment v3"
VI"
VIcw
1579.5
VZS
v2"
U 4 (UOFJ U 4 (UOF,) vjCu
bQ
UOz F P
650
570
e10
WAVENUMBER (cm-')
Figure 2. Infrared spectrum in 650-550-~m-~region after the codeposition of Ar/UF6/H20 samplcs at 12 K for 2.5 h. UOFA PREPARED FROM :
Y
z m
w+.-bJ
181 K
(d)
P
8z
sa
s:m
1000
800
000
WAVENUMBER (cm-')
U
--
9 1
860
820
WAVENUMBER (cm-')
Figure 3. Infrared spectra in the 900-800-~m-~region of isotopic water and UF6samples h solid argon: (a) after the 1-h photolysis of the HZI60 and UF6 sample; (b) after the 1-h photolysis of the HzL80and UF6 sample; (c) after the 1-h photolysis of the DzOand UF6 sample.
weak complexes are involved. After UF6 and H 2 0 were codeposited with excess Ar,the spectra in Figures la and 2a exhibited two new product doublets at 3624.7 and 3623.4 cm-' (labeled v~")~'and at 608.8 and 587.8 cm-'(labeled v 3 9 . Another product doublet at 1588.5 and 1585.6 cm-' (labeled uts) was observed. The intensities of these new absorptions appeared to be directly related to the UF6 and H 2 0 gas-phase interaction prior to the deposition on the cold window since annealing the matrices after deposition failed to produce any significant change in the product bands. However, further reaction could be induced by irradiating the samples with 254-nm photons of approximately 1 mW from a low-pressure mercury lamp. Ultraviolet photolysis is known to dissociate UF6 into UFS.12 In addition to the expected UF5 absorption at 583.4 an-', the photolysis produced a new doublet at 868.2 and 857.1 cm- (labeled UOF,), which is displayed in F i i 3a. The intensities of the prephotolysis product absorptions were greatly reduced. The difference spectrum in Figure IC shows the net effect of the photolysis, and the spectrum revealed the presence of another prephotolysis product band at 3717.9 cm-I (labeled v 3 9 A complete photolytic reaction as indicated by the ( 1 2 ) McDowell, R. S.; Asprey, L. B.; Paine, R. T. J. Chcm. Phys. 1974, 61, 3571.
Figure 4. Infrared spectra in the 1000-500-cm-' region: (a) after the deposition of H2l60and UF6without argon at 12 K for 1.5 h; (b) after annealing to 46 K; (c) after warming to 151 K; (d) after annealing to 161 K, (e) after warming to 172 K.
destruction of the vEw and vN was usually achieved in less than 1 h. Similar UF6 experiments were conducted with D20, H2180,and H2160/H2180(50%/50%). The growth pattern of the product absorptions was not affected by the isotopic substitution, and the frequencies of these product bands are listed in Table I. In the D20/UF6 study, the codeposition product absorptions in the u3" absorptions were slightly red-shifted (4-5 cm-I) from their H 2 0 counterparts while the deuteration had very little effect on the frequencies of the photolysis products. In sharp contrast to the D 2 0 experiments, the l80substitution caused a significant red shift ( 4 2 4 3 cm-') in the UOF, doublet while the Y~~ doublets with H2160and H2180were very comparable. Finally, the codeposition and photolysis of UF6 with the H2'60/H2180mixture produced only the bands that were described earlier in the isotopically pure experiments. Solid Eplmp of m6 and H20. Figure 4 is typical of the spectra observed in the experiments conducted with UF6 and H 2 0 only. After the deposition of the film was completed, the spectrum in Figure 4a displayed new product bands at 854,817 (labeled UOF,) and 550 cm-' (labeled v3"). In addition, an absorption at 610 cm-I (labeled UF6) was due to the v3 mode of UF6. It must be noted that these frequencies of the product absorptions are affected by both the temperature of the film and the relative concentration of the reagents. The initial annealings, which are represented by the spectra in Figure 4b,c, produced an increase of the UOF, doublet and a corresponding decrease in the v3" absorption until this absorption was completely destroyed. By 151 K, a very weak absorption at 925 cm-l (labeled U02F2)appeared in the spectrum, which is displayed in Figure 4c. In Figure 4d, the annealing to 161 K caused the UOzF2absorption to grow at the expense of the UOF, doublet while the UF6 absorption was unchanged. The annealing to 172 K,which is shown in Figure 4e,produced sharp
1098 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
decreases in the both the UF6 absorption and the UOF, doublet. The U02F2band continued to increase in intensity. In the study with UF6 and H21a0,a similar growth pattern during the stepwise annealings was observed. The major product absorptions at 925, 854, and 817 cm-' were red-shifted to 876, 821, and 778 cm-I, respectively. In contrast, the 550-cm-' band in the normal H 2 0 spectra was slightly blueshifted to 560 cm-'. However, this last frequency shift appears to be a function of the reagent concentrations rather than an isotope effect. Cas P b . Several experiments with the gas cell performed in order to examine the hydrolytic behavior of UF6 at room temperature. The FTIR spectra after the addition of 0.06-0.12 mmol of UF6 to the gas cell exhibited a very strong UF6 band at 626.0 an-".In addition, several combination bands of UF6 were observed at 1290.4 (vl + u3), 1157.0 (u, + v3), 820.8 (u3 + v s ) , and 676.8,668.1 cm-' (u, v6). These UF6 bands are in close agreement with those found in an earlier gas-phase low-resolution infrared study by McDowell et al.', Impurity bands were observed at 2984.4, 1272.7, 1031.0 (SiF,), 948.9,910.2, and 822.3 cm-I. Similar impurities, which are quite volatile, had been detected earlier in the condensed-phase experiments. T h w impurities were eliminated by removing Viton from the part of the gas handling system that was exposed to HF. The intensities of the impurity absorptions were dramatically reduced as the cell surfaces were further conditioned with UF,. The spectra that were taken during the hydrolysis of UF6 displayed two common features regardless of the experimental conditions. The intensities of the UF6 bands were sharply reduced while the absorptions of gaseous HF appeared. Due to the excess UF6 and the fluorinated walls of the cell, the HzO absorptions were either weak or nonexistent. No absorptions of gas-phase uranium oxyfluorides or oxides were detected at any point during these experiments. However, the spectra which examined the cumulative effects of these reactions with the gas cell display two new bands at 976.2 and 918.5 cm-'. Finally, an examination of the interior of the cell was conducted at the conclusion of this study. The majority of the pale yellow dustlike coating was found on the cover plate near.the H 2 0 inlet. No further analysis of this nonvolatile product was performed.
+
Discussion AI Matrices. The product bands that are listed in Table I were not observed in the argon matrix sample of uranium hexafluoride or water alone. However, these absorptions were produced when the reagents were dep@ed and later photolyzed. Two different groups of product absorptions can be identified and assigned on the basis of their frequencies and growth patterns during the deposition and photolysis. In the first group, the uIW, uZW, u , ~ and , vjcu absorptions exhibited the same growth pattern; they were produced during the deposition and destroyed by the UV photolysis. This growth pattern is indicative of a 1:l complex which is photochemically active. The uZW absorptions at 1588.5 and 1585.6 cm-'strongly indicated that the UF6 interacts with water through the oxygen atom based on comparisons of other H 2 0complexes in which H20 serves as a Bronsted acid or a Lewis base. In the (CH3),&H0Hl3 and H,0-HOH4 complexes, the v c bands were observed at 1615.3 and 1611.3 cm-I, respectively. In the ClH-OH2,13 HOH-0H2,l4 and F2-OHz15complexes, the u$ modes were detected at 1590.4, 1593.6, and 1590.0 cm-I, respectively. Therefore, these four product absorptions have been assigned to a 1:1 complex with an anti-hydrogen-bondedI5structure (UF6-OH2). This proposed structure was anticipated due to the high affinity of U(V1) for oxygen ligands and the Lewis acidity of UF6.16 All three uoy absorptions exhibited small red shifts from the perturbed H 2 0 modes in the van der Waals complex F2-OH215whose structure has been confirmed by theoretical calculations. For the antihydrogen-bonded complex, these red shifts would be expected since (13) Ayen, G. P.; Pullin, A. D. E. Specirochim. Acta 1976,32A, 1641. (14) Ayers, G. P.; Pullin, A. D. E. Specirochim. Acta 1976, 32A, 1695. (15) McInnis, T. C.; Andrews, L. J . Phys. Chem., submitted for publication. (16) Wilson, P. W. Reo. Pure Appl. Chem. 1972, 22, 1.
Sherrow and Hunt UF6 is a stronger Lewis acid than F2based on HF complexes with UFJ7 and F2.'* While the '*O substitution had very little effect on the ujCu absorptions, the u3N bands in the D20 experiments exhibited a 4-5-cm-' red shift from their normal water counterparts. The larger perturbation by D20 is probably due to the smaller librational amplitude of D20. A similar rationale17has been employed to explain small frequency shifts in HF and DF complexes. With the anti-hydrogen-bonded complexes, the time-average position of the electron density, which is directly related to the average position of the hydrogens, determines the average binding interactions of these complexes. Therefore, the UF,&D2 complexes should be slightly more stable than their HzO counterparts. In the second group, the UOF, bands appeared only after the photolysis of the UF6-OH2complex. The UOF, doublets have been assigned to the U - 0 stretch of UOF,. As shown in Figure 3, deuterium substitution had no effect on the UOF, doublet. Since the UF6 experiments with equal parts of H2160and H2180 did not result in any additional product absorptions, the product species consists of a single oxygen atom. Finally, the U160/U1a0 frequency ratio of 1.052 is quite reasonable for the U=O stretch with little or no mode coupling. Solid Films of u F 6 and HzO. Absorptions of three different product species can be identified and assigned on the basis of their frequencies, growth patterns during the annealing, and a comparison of the products that were formed in the Ar matrim. First, the 550-cm-' vjcu absorption in Figure 4 was formed during the codeposition and destroyed during the initial annealings. The observed growth pattern strongly suggests that this product is the initial complex of the UF6 hydrolysis. Therefore, the 550-cm-I absorption has been assigned to the UF6 u3 modes of the UF6-n(OH,) (n 2 1) complexes. In comparison with ujS absorptions in solid Ar, this ujCuabsorption is considerably broader and redshifted. These observations can be easily rationalized by multiple H 2 0 interactions with a single UF6. Second, the UOF, doublet was the initial product formed during the annealings, and this doublet was destroyed by subsequent annealings. This growth pattern would be anticipated for the first hydrolysis product, UOF,. Since the UOF, doublet is similar to the UOF, doublet in the Ar matrices and exhibits comparable l8Oshifts, this UOF, doublet has also been assigned to the U s 0 stretch of UOF,. Third, the U02F2absorption was initially detected at 925 cm-' at 151 K and was blueshifted to 950 cm-'at 242 K. Since this absorption grew at the expense of the first hydrolysis product, the new product species is due to the further hydrolysis of UOF,. Infrared studies have shown that both potential hydrolysis products, UOzF2l9and U03,,0 have an infrared absorption in this spectral region. However, UO, should also have another strong IR absorption, which should be found in the 850-825-cm-I region. On the basis of the absence of the second absorption, the 925-cm-' band has been assigned to U02F2. In addition, several observations about the thermal barriers of the UF6 hydrolysis can be made. The UF6-OH2complex in the presence of excess water reacts spontaneously to form UOF,, even at very low temperatures. However, this complex must first be formed in the gas phase. The thermal barriers for the formation of U02F, are approximately 150 K for UOF, and H 2 0 and 165 K for UF6 and 2H20. The formation of uranium oxide from which must start at a temperature above 242 U02Fz and H20, K,is probably the rate-determining step for the hydrolysis of UF6 to uranium oxide under these experimental conditions. Gas F"se. The most sisnifcant finding from this study involves the distribution of the pale yellow dustlike coating. If the initial steps of the UF6 hydrolysis occurred solely in the gas phase, the (17) Hunt, R. D.; Andrews, L.; Toth, L. M. J . Phys. Chem. 1991, 95, 1183. (18) Hunt, R. D.; Andrews, L. J . Phys. Chem. 1988,92, 3769. (19) Hoekstra, H. R. Inorg. Chem. 1963, 2, 492. (20) Fodor, M.; Poko, Z.; Mink, J. Mikrochim. Acia 1966, 4-5, 865. (21) It will be seen in the Discussion that the P and uw symbols refer to the absorptions arising from the uranium and water modes, respectively, of the UF6-OH, complex.
J. Phys. Chem. 1992,96, 1099-1105 vast majority of the hydrolysis product, which was formed outside of the H 2 0inlet, should have been observed at the bottom of the gas cell. However, the majority of the product was found on the side wall near the H 2 0 inlet. Therefore, the metal surface apparently served as a catalyst in these gas-phase experiments. Finally, the yellow dustlike product is probably the yellow compound that Otey et al.3 formulated as U305FB.Other possible candidates for this product can be eliminated by their IR absorptions and/or their colors. Except for moderately different reaction temperatures, the experimental conditions in the Otey et al. study3 and this gas-phase investigation were quite comparable. In sharp contrast to the Ar matrices and the solid films, the considerably higher UF6/H20 ratios in the gas-phase study produced the same deviation from the reaction mechanism, which is given in eqs 1-3, as observed by Otey et al.3 COnClUSiOM
UF6 and H 2 0ctrtainly do not react as spontaneouslyat ambient temperatures as originally thought. The first step in the UF6
1099
hydrolysis is apparently the formation of an anti-hydrogen-bonded complex, UF6-OH2. When the UF6-OH2 complex is trapped without an inert gas, the complex reacts spontaneously to form UOF, even at temperatures below 30 K. In solid argon, no thermal reaction of the u F 6 4 H 2complex was detected during any of the annealings while UV photolysis of thii weak complex did produce UOF,. Complete reaction in the gas phase appears unlikely since UOF, cannot be generated by a single gas-phase encounter of the reactants. In addition, the collision of the UF6-OH2complex with additional H 2 0 molecules is just as likely to lead to dissociation rather than further reaction. In order for the gas-phase hydrolysis to proceed, a catalyst such as a fluorinated nickel surface is probably needed. Acknowledgment. This research was sponsored by the Division of Chemical Sciences, U.S. Department of Energy, under Contract DE-AC05-840R21400 with the Martin Marietta Energy Systems, Inc. R-try NO. UF6, 7783-81-5.
Characterization of Decaosmium Carbldo Carbonyl Clusters Supported on MgO H.Henry Lamb***and Bruce C.Gatest Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, and Department of Chemical Engineering, North Carolina State University, Box 7905, Raleigh, North Carolina 27695-7905 (Received: July 25, 1991; In Final Form: September 30, 1991)
Supporteddecaosmium carbido carbonyl clusters were prepared by surfacemediated organometallicsynthesis from chlormmium onto thermally activated MgO from THF solution. The supported complexes on MgO and by adsorption of [OsloC(CO)24]2clusters have been characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy,ultraviolet-visible diffuse reflectance (W-vis DR) spectroscopy, and infrared spectroscopy. Transmissionelectron microscope (TEM) images of clusters prepared by surfacemediated synthesis have been obtained. Analpis of the Os Lm EXAFS data indicates that each preparation ) ~ ~ ] with structures closely similar to that of the dianion in crystalline yields molecularly dispersed [ O S ~ ~ C ( C Oclusters CO [Et4N]2[OsloC(CO)24].The principal optical absorption of the supported clusters can be assigned to an Os-Os* u27r*" transition by comparison to the UV-vis spectrum of [PPN]2[OsloC(CO),] in solution. Infrared spectra provide evidence of '[OS~~C(CO)~~]" clusters in oxidized and reduced states (relative to the dianion) on the MgO surfaces. Apparently, n = 0, 1, and 2; and adsorption of [ O S ~ ~ C ( C Oonto ) ~ ~MgO ] ~ - yields surface-mediated synthesis yields [OS~~C(CO)~~]", [Osl,-,C(CO)u]*, n = 2,3, and 4. TEM images of supported clusters prepared by surfacemediated synthesis contain highcontrast scattering centers about 10 A in size that are inferred to be decaosmium species.
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Introduction Supported metal catalysts prepared by conventional techniques typically contain metal aggregates 10-30 A in size that expose surface metal atoms in many different geometric and electronic environments.' Aggregate size and morphology are important, as some metal-catalyzed reactions, e.g., alkane hydrogenolysis, are structure sensitive, exhibiting structure-dependent turnover frequencies (reaction rates per exposed metal atom).2 Experiments on single crystals have demonstrated that turnover frequencies can vary with surface crystallographic orientation by as much as several orders of magnitude? but further elucidation of structure sensitivity in supported metal catalysis has been problematic, as each catalyst incorporates a range of surface structures. Our goal was to prepare supported metal aggregates with nearly unique structures by using metal carbonyl clusters as precursors. In prospect, molecular clusters, unique metal assemblies stabilized by ligands, can be adsorbed on a metal oxide and converted into singlesized supported metal aggregates by removal of the ligands. 'To whom correspondence should be addressed, at North Carolina State University. University of Delaware. *North Carolina State University.
Unfortunately, fragmentation and/or aggregation of the clusters often w u r concomitant with adsorption on the support and/or thermal activation to remove the ligand^."^ The former difficulty can be overcome by synthesizing metal carbonyl clusters on the support via reductive carbonylation of a metal salt precurs0r.~*6 Metal carbonyl clusters that are likely to resist oxidative fragmentation by surface hydroxyl groups offer the best prospects of remaining intact as the ligands are removed by thermal activation. Robust osmium clusters with frameworks stabilized by encapsulated ligands (e.g., C or N) are promising candidates as they are especially resistant to oxidation and fragmentation in solution. ) ~ ~ ] with ~ - NO+ and I+, leaving For example, [ O S ~ ~ C ( C Oreacts the octahedral Os6Ccore intact, although bonds to capping Os(CO),, groups are opened.' We recently have developed a ( 1 ) Anderson, J. R. Structure of Metallic Curulys?s;Academic Rea: New York, 1975. (2) Boudart, M. Ado. Coral. 1969, 20, 153. (3) Somorjai, G. A.; Carrazzi, J. Ind. Eng. Chem. Fundum. 1986,25,63. (4) Fro,R.;Ugo, R.In Metal Clusters in Catalysis; Gates, B. C., Guczi, L., Knijzlnger, H., Eds.; Elsevier: New York, 1986. ( 5 ) Lamb, H. H.; Knazinger, H.; Gates, B. C. Angew. Chem., In?. Ed. Engl.' 1988, 27, 1127. (6) Lamb, H. H.; Fung, A. S.;Tooley, P. A.; Puga, J.; Krause, T. R.; Kelley, M. J.; Gates, B. C. J. Am. Chem. Soc. 1989, 111, 8367.
0022-3654/92/2096-1099$03.00/0Q 1992 American Chemical Society
