Langmuir 1998, 14, 4217-4221
4217
Multiple Features of a Porous Structure as Assessed from the Hysteresis of Nitrogen Adsorption-Desorption: Case Study of the Formation of UO3 from UO2(NO3)2‚6H2O S. Borde`re,† P. L. Llewellyn,*,‡ F. Rouquerol,‡ and J. Rouquerol‡ ICMCB, Universite´ de Bordeaux I, 87 Ave. Dr. A. Schweitzer, 33608 Pessac Cedex, France, and CTM du CNRS, 26 rue du 141 RIA, 13331 Marseilles Cedex 3, France Received December 22, 1997. In Final Form: April 14, 1998 The porous structure of intermediates in the thermolysis of hexahydrated uranyl nitrate was found to provide an interesting case study for the use of nitrogen adsorption-desorption isotherms: nonrigid sheetlike pore structures, rigid mesopores, interconnected structures, and clearly bimodal pore size distributions were the situations encountered. The use of sample controlled thermal analysis (SCTA) made it possible to compare two highly reproducible and well-defined thermolysis routes (under residual pressures of 10-1 and 20 mbar, respectively) leading to very different porous UO3. Adsorption studies, with the help of SEM investigations and prior X-ray studies, permit a proposition for the mechanism of the formation of the different porous structures.
Introduction When trying to model furnaces on an industrial scale, the effects of temperature and pressure gradients have to be taken into consideration. To understand how these gradients affect the products, it is desirable to reproduce the local furnace conditions by using thermal techniques where the local pressure and temperature of the sample are well-controlled. The reactivity of the final products is often dependent on the surface area and porosity developed during thermal preparation. The present communication results from such a study of different conditions of temperature and pressure where samples of unusual porosity are obtained. This work presents the final piece in the jigsaw puzzle of understanding the mechanism of formation of divided uranium oxide (UO3) via the thermolysis of uranyl nitrate hexahydrate. The previous pieces were an analytical study,1 a kinetic study,2 and a study of the influence of the residual pressure on the specific surface area of the final product.3 Here, we now focus our attention on the porous structure of the final oxide and on its mechanism of formation during the thermolysis. We shall mainly rely on the interpretation of the nitrogen adsorption-desorption isotherms and on the highly reproducible and homogeneous thermolysis provided by sample controlled thermal analysis (SCTA). Experimental Section A. Sample Preparation. Like that of most hydroxides or nitrates leading to the formation of a divided oxide, the thermolysis of hexahydrated uranyl nitrate is strongly influenced by the residual vapor pressure above the sample: a change in this residual vapor drastically modifies the thermal analysis curve1 and the specific surface.3 This is why the thermal treatments on fresh hexahydrated uranyl nitrate (Merck) used in the present study were carried out under carefully selected † ‡
Universite´ de Bordeaux I. CTM du CNRS.
(1) Borde`re, S.; Fourcade, R.; Rouquerol, F.; Floreancig, A.; Rouquerol, J. J. Chim. Phys. 1990, 87, 1233. (2) Borde`re, S.; Rouquerol, F.; Rouquerol, J.; Estienne, J.; Floreancig, A. J. Therm. Anal. 1990, 36, 1651. (3) Borde`re, S.; Floreancig, A.; Rouquerol, F.; Rouquerol, J. J. Solid State Ionics 1993, 63-65, 229.
conditions of controlled rate-evolved gas detection (CR-EGD), which is a special case of SCTA where both the residual pressure above the sample and the rate of transformation are controlled, by an appropriate control loop.4,5 The CR-EGD setup used is described elsewhere.5 The rate of transformation can be selected at any value considered to be low enough to ensure negligible temperature and pressure differences within the sample bed. B. Gas Adsorption Measurements. Nitrogen adsorptiondesorption isotherms at 77 K were determined with a highaccuracy automatic apparatus for adsorption manometry, designed and built in house (CTM, Marseilles).6 To ensure the completion of the adsorption branch and thus the correct desorption branch (i.e. to avoid a scanning of the hysteresis) of the isotherm, conditions of filling to a relative pressure were carefully selected, so that the adsorption and desorption branches had a small portion in common at the upper end.
Experimental Results Figure 1 shows the thermal analysis curves of hexahydrated uranyl nitrate corresponding to two experiments carried out under a controlled residual pressure of 10-1 mbar (continuous line) and 20 mbar (dotted line). Repeat runs were carried out to isolate products at each of the points marked on Figure 1. Points C1, D1, and F1 (on the 10-1 mbar curve), together with F2 (on the 20 mbar curve), are those which will be discussed in this work where the thermal treatment was stopped to allow for the determination of the full N2 adsorption-desorption isotherm. In the case of a more standard heat treatment, a final temperature plateau would be needed to achieve some reproducibility, so that the representative point of the sample studied by adsorption would no longer be found on the general thermal analysis curve. It would follow that the samples obtained at various temperatures would not proceed exactly from each other. Here, the SCTA treatment is slow enough (100 h for the experiments reported in Figure 1) to provide at any time a reproducible and homogeneous sample which is immediately cooled, without any further transformation, as soon as its representative point reaches the preselected point on the thermal analysis. (4) Rouquerol, J. Thermochim. Acta 1989, 144, 209. (5) Rouquerol, J.; Borde`re, S.; Rouquerol, F. Thermochim. Acta 1992, 203, 193. (6) Boudellal, M. Thesis, University of Provence, Marseilles, 1979.
S0743-7463(97)01406-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/20/1998
4218 Langmuir, Vol. 14, No. 15, 1998
Figure 1. SCTA curve obtained for the thermolysis of UO2(NO3)2‚6H2O to form UO3 under water vapor pressures of 10-1 and 20 mbar (experiments lasted approximately 100 h).
The corresponding adsorption-desorption isotherms are plotted in Figure 2, whereas Table 1 provides the specific surface areas calculated for samples C1, D1, and F1 as well as for samples C2, D2, and F2. The surface area is determined either by application of the BET equation or from the pore size calculation using the desorption branch of the isotherm, using a form of the Barrett, Joyner, and Halenda analysis7 with the assumptions of slit-shaped or cylindrical pores, respectively. One sees that the major development of surface area takes place on decomposition of the nitrate and that the slit-shaped model generally allows a better agreement with the BET specific surface area. General Discussion A. General Significance of the Adsorption Isotherms. By comparison with previously published isotherms and with the help of a few extra pieces of information which will be commented on, the following interpretation can be given of the various isotherms of Figure 2. Samples C1 and D1. The samples give isotherms indicating a sheetlike, nonrigid, porous structure. The three main features of the hysteresis loop of isotherms 2-C1 and 2-D1 are (i) the total absence of any saturation plateau (type II in the IUPAC classification8) generally obtained with nonporous or macroporous solids, (ii) a monotonic desorption branch showing a significant type H3 hysteresis loop,8 and (iii) a lower closure point above p/p0 ) 0.42 (i.e. the minimum pressure at which capillary condensation of N2 at 77 K is known to give rise to an hysteresis). The shape of theses two isotherms is interpreted by the existence of slit-shaped pores8-10 and has also been observed in the case of montmorillonite,11,12 beryllium (7) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (8) Sing, K. S. W.; Everett, D. H.; Hall, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 58, 967. (9) de Boer, J. H. In Pore Structure & Properties of Materials; Everett, D. H.; Stone, F. S., Eds; Butterworth: London, 1958; p 68. (10) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area & Porosity, 2nd ed.; Academic Press: London, 1982. (11) Rouquerol, F. Thesis, University of Paris, 1965. (12) Rouquerol, F.; Rouquerol, J.; Imelik, B. Bull. Soc. Chim. 1970, 10, 3816.
Borde` re et al.
hydroxide,11-13 and aluminum hydroxide.14,15 De Boer showed that Acum ) ABET with the rigid, slit-shaped pore structure of aluminum hydroxide treated to 750 °C.9 In Table 1, the values of Acum > ABET, indicating a sheetlike, nonrigid porous structure, as was evidenced with beryllium hydroxide.12 On adsorption, the sheets are separated from each other by the condensing adsorptive so that there is no final saturation. On desorption, they are maintained at the distance which, for a given equilibrium pressure, leaves a cylindrical meniscus whose curvature is governed by the Kelvin equation. This gives rise to a smooth desorption curve down to the relative pressure of p/p0 ) 0.5, indicating a minimum slit width of 3 nm. Sample F1. This sample is characterized by a rigid, slit-shaped mesoporous structure. The isotherm in Figure 2-F1 is characterized by (i) a type IV adsorption isotherm representative of a mesoporous structure and (ii) a sloped desorption branch giving a clear H1 type hysteresis,8 which indicates the presence of a relatively narrow mesopore size distribution (centered around 8 nm from BJH calculations with the assumption of slit-shaped pores). The tail in this desorption between p/p0 ) 0.5 and 0.8 indicates the slight persistence of the slitlike pore structure. Sample F2. This sample is the same as sample F1, but now with a bimodal, interconnected porous structure (instead of a single one). This type of adsorption isotherm (shown in Figure 2-F2 and Figure 3d), although quite uncommon, is not unknown: Figure 3 shows comparable isotherms obtained with specific samples of controlled pore glass,16 alumina,11 and beryllia.13 In all these isotherms break A indicates the point where a given category of mesopores, highly homogeneous in size, is just filled. For the porous glass (Figure 3a) which was specially prepared with two different pore sizes,16 this break is due to this bimodal pore size distribution: above point A, a second category of pores is being filled. Now for the alumina (Figure 3b) and beryllia (Figure 3c) samples, which are made of thin, flat crystals, the shape of the isotherm results from the overlapping of two adsorption phenomena, that is, (i) the filling of rigid mesopores of similar size and, (ii) especially, but not only, above point A, the intersheet condensation in a nonrigid structure. In these three examples, the break observed on adsorption is also observed on desorption at the same Vads values. The case of UO3 is closer to that of the porous glass: the fact that the lower closure point of the hysteresis is far above 0.42 tells us that we are no longer in the presence of intersheet condensation. It is more likely that two sizes of rigid pores are present, the filling of the smallest ones (mesopores) being completed at point A. Between B and C the larger category of pores is then filled. As a second clear plateau region is not distinguished, then these pores may be considered as macropores. The difference with the porous glass is that (excepting that the desorption branch is not two stepped), on desorption, the plateau regions do not coincide with the one on adsorption. This can be interpreted by an interconnection of each category of pores. Between C and D, the macropores open to the external surface empty. At point D, desorption from the largest category of pores inside the solid should be possible; however, due to a bottleneck type phenomenon, these large (13) Rouquerol, F.; Rouquerol, J.; Imelik, B. In Principles and Applications of Pore Structural Characterization; Haynes, J. M.; RossiDoria, P., Eds.; J. W. Arrowsmith Ltd.: Bristol, 1985; p 213. (14) Ganteaume, M. Thesis, University of Provence, Marseilles, 1973. (15) de Boer, J. H.; Lippens, B. C. J. Catal. 1964, 3, 38. (16) Renou, J. Thesis, University of Paris, 1960.
Formation of UO3 from UO2(NO3)2‚6H2O
Langmuir, Vol. 14, No. 15, 1998 4219
Figure 2. Isotherms obtained with nitrogen at 77 K for samples C1, D1, F1, and F2: C1, uranyl nitrate monohydrate obtained under 10-1 mbar; D1, uranyl nitrate anhydrate obtained under 10-1 mbar; F1, uranium oxide obtained under 10-1 mbar; F2, uranium oxide obtained under 20 mbar. Table 1. BET and Cumulative Surface Areas Calculated from the Adsorption Isotherms of Nitrogen at 77 K on the Various Intermediate and Final Products Isolated ACUM/m2‚g-1 10-1 mbar ABET/m2‚g-1 C D F
uranyl nitrate monohydrate uranyl nitrate uranium oxide
20 mbar
10-1 mbar
20 mbar
slit-shaped model
cylindrical model
3
