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J . Pkys. Ckem. 1984, 88, 2522-2525
Phase Diagram of N, Determined by Raman Spectroscopy from 15 to 300 K at Pressures to 52 GPa Steven Buchsbaum, Robert L. Mills,* and David Schiferl University of California, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received: October 27, 1983)
Diamond-anvil cells were used to study the Raman scattering of solid nitrogen from 15 to 300 K at pressures up to 52 GPa. By associating characteristic features in the vibration and phonon spectra of N2 with each of the known /3,y, and 6 crystal forms over regions where the structures have been determined by X-ray diffraction, we found it possible to extend the phase diagram through optical measurements alone. The 6-Nzphase exists above a few gigapascals at all experimental temperatures. Over wide regions of this phase, however, the solid appears to distort from its earlier assigned space group Pm3n. It is not clear whether the distortion is continuous or whether various discrete low-symmetry structures are formed.
Introduction In principle, one should be able to predict the structures of molecular crystals at different pressures and temperatures from detailed intermolecular potentials. Although great strides have been made’ in describing theoretically the interactions between molecules, the phase diagrams of only a few simple diatomics can presently be derived from a priori considerations. Nitrogen, a first-row closed-shell molecule, is an ideal subject for study because its strong intramolecular triple bond should remain stable up to relatively high pressures and temperatures. Also, much experimental and theoretical work has already been carried out on condensed nitrogen. It is known that solid nitrogen exists in four phases: an ordered cubic a phase2 (space group Pa3) at low temperature and pressure; a disordered hexagonal p phase3 (P63lmmc) adjacent to the melting curve; an ordered tetragonal y phase4v5(P4z/mnm)above about 0.4 GPa at low temperature; and a new, apparently disordered, cubic 6 phase6 (“Pm3n”)above about 4.5 GPa at room temperature. While this new solid form of Nz appears to be cubic within the resolution of previous X-ray6 and Raman’ measurements, the work presented here indicates that there are P-T regions where lower symmetry distortions of the Pm3n space group exist. To indicate our uncertainty in the structure, we use the characterization “Pm3n”. The present study was carried out to establish the P-T phase boundaries of the various crystal forms of solid nitrogen from about 15 to 300 K and 0.6 to 52 GPa, using Raman scattering in diamond cells. A comparison of these experimental data with current theory may help point the way to improved models for molecular interactions. Experimental Section We employed two types of diamond-anvil cells to apply pressure to the nitrogen samples. A Merrill-Bassett6 cell was used to achieve pressures up to 27 GPa in the temperature range 15-300 K, and a Mao-Bellg cell was used for pressures from 20 to 52 GPa and temperatures from 85 to 300 K. Both cells were loaded by the liquid-immersion technique.l0 ( I ) R. LeSar and R. G. Gordon, Phys. Rea. B, 25, 7221 (1982). (2) J. A. Venables and C. A. English, Acta Crystallogr., Sect. B, 30, 929 (1974). (3) W. E.Streib, T. H. Jordan, and W. N. Lipscomb, J . Chem. Phys., 37, 2962 (1962). (4) R. L. Mills and A. F. Schuch, Phys. Reu. Lett., 23, 1154 (1969). (5) A. F. Schuch and R. L. Mills, J . Chem. Phys., 52, 6000 (1970). ( 6 ) D. T. Cromer, R. L. Mills, D. Schiferl, and L. A. Schwalbe, Acta Crystallogr., Sect. B, 37, 8 (1981). (7) R. LeSar, S.A. Ekberg, L. H. Jones, R. L. Mills, L.A. Schwalbe, and D.Schiferl, Solid State Commun., 32, 131 (1979). (8) L. Merrill and W. A. Bassett, Rev. Sci. Instrum., 45, 290 (1974). (9) H. P. Mao and P. M. Bell in “Carnegie Institution of Washington Year Book”, 1975, p 399. (10)D. Schiferl, D.T. Cromer, and R. L. Mills, High Temp.-High Press., 10, 493 (1978).
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The Merrill-Bassett cell had diamonds with 16 facets, culets 600 pm in diameter, and tables supported by hardened beryllium-copper backing plates. After the cell was loaded, it was mounted on the cold finger of a Janus Model S/T helium-flow cryostat. Temperatures in the range 15-300 K could be controlled to within f5 K by varying the flow of liquid helium and by regulating the power to an electrical heater on the cold finger. We were unable, however, to manipulate the pressure of the cell while it was inside the cryostat. Cooling the cell from room temperature to 15 K caused the pressure to increase or decrease by as much as 2.0 GPa, depending on whether the pressure had been, respectively, raised or lowered just before cooling. The magnitude of this pressure change was quite unpredictable. Diamonds used in the Mao-Bell cell had 16 facets with culet tips 300 bm in diameter. The gasket was T301 stainless steel, full hard, 250 bm thick, and preindented to 30 bm with a hole diameter of 100 pm. Because of its larger size, the Mao-Bell cell required a different cryostat which, unfortunately, could be cooled to only 85 K. In this cryostat, however, the cell pressure could be varied at low temperature. We used the ruby-fluorescence method11s12to measure pressures, assuming the relation”
P = 3 8 0 . 8 ( [ v o ( T ) / ~ p ( i-“ )1]) ~
(1)
where P is the pressure in GPa, vp( T ) is the frequency of the ruby R, line at pressure P and temperature T, and vo(T) is the corresponding frequency at the same T and zero pressure. Equation 1 has been checked at low temperature^'^ up to only about 1 GPa. If a different relationship should be found at higher pressures, the values of P reported here can easily be corrected. The zero-pressure frequency vo( T) was measured in fluorescence from 15 to 300 K for a sample of ruby containing 0.16 wt % Crz03. We fitted the data to within experimental error (*0.4 cm-I) to the empirical expression v o ( T ) = 14422.0
- 36.612(T/300)3”2+ 169.77(T/300)4/2-
264.54(T/300)5/2+ 112.14(T/300)6/2( 2 )
in which 0 < T < 300 K and vo(T=O) = 14422.0 cm-’ for our ruby specimen. Equation 2 is plotted as yo( T=O)- uo( r ) in Figure 1 where good agreement is shown with the earlier results of McCumber and Sturge15 who made their measurements in ab( I 1) R. A. Forman, G. J. Piermarini, J. D. Barnett, and S. Block, Science, 176, 284 (1972). (12) J. D. Barnett, S. Block, and G. J. Piermarini, Reu. Sci. Instrum., 44, 1 (i973). (13) H.K.Mao, P. M. Bell, J. W. Shaner, and D. J. Steinberg, J . Appl. Phys., 49, 3276 (1978). (14) R. A. Noack and W. B. Holzapfel in “High Pressure Science and Technology, Sixth AIRAPT Conference”, Boulder, CO, Vol. 1, K. D. Timmerhaus and M. S. Barber, Eds., Plenum, New York, 1979, p 748.
0 1984 American Chemical Society
N2 Phase Diagram
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 2523
3’1 ivl C
I
\
t
t
1
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3
-P
.VI C
(u
-P C
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1
0
I
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1
200
I
I
300
T (IO
Figure 1. Frequency shift in R1fluorescence line of present ruby sample as a function of temperature at P = 0 GPa: open circle, Kr’ laser excitation;closed circle, Ar’ laser excitation; cross, ref 15; solid line, eq 2.
sorption. Above room temperature eq 2 begins to fail. At high temperatures, however, the temperature shift of ruby has been described15in terms of Raman scattering of Debyemodel phonons. Below 20 K we observed that the R2 peak (not shown in Figure 1 ) became extremely weak for single-crystal ruby chips from zero pressure up to at least 6 GPa. Both the ruby-fluorescence pressure measurements and observations of the Raman spectra were made with a Spex 1403 spectrometer equipped with a periscope for viewing the sample image in the entrance slits. The 488-nm argon laser line was used in all studies except a series of vo( T ) measurements where krypton 568-11, excitation was used. We steered the laser spot, which had a diameter of 30 pm, around in the sample area so it was either focused on a ruby chip for pressure measurements or directed to a ruby-free area for Raman determinations. To avoid heating the sample, we kept the power incident on the cell below 30 mW for pressure measurements and below 200 mW for Raman studies.
Results Figure 2a-c shows the distinctive features in the stretching-mode Raman spectra of the P6,/mmc, P42/mnm,and “Pm3n”structures of N2,respectively. The well-separated stretching-mode doublet of the “Pm3n” structure easily distinguishes it from both the P6,lmmc solid, which exhibits a single stretching-mode peak, and from the P4,lmnm solid, which shows a single peak with a weak shoulder that appears only below about 50 K. Above this temperature we were unable to discriminate between the P6,lmmc and P4,lmnm forms on the basis of stretching-mode features in their Raman spectra. Each of the two peaks for the “Pm3n” structure appears to be symmetrical at low pressures, but the stronger of the two peaks begins to show a marked asymmetry by 20 GPa. Shoulders develop and move out continuously until at 52 GPa the original peak has transformed into two, well-resolved (15) D. E. McCumber and M. D. Sturge, J . Appl. Phys., 34, 1682 (1963).
2340.00
c
23513.00
Raman sh i f t
2360.00 [crn-ll
Figure 2. Raman vibrational spectra of solid nitrogen in three different crystal forms: (a) 8-N, (P6,lmmc) at T = 300 K, P = 2.8 GPa; (b) 7-N2 (P4,lmnm) at T = 1 5 K, P = 0.4 GPa; (c) 6-N2(“Pm3n”) at T = 15 K. P = 5.5 GPa.
peaks plus a third broad, asymmetric peak which is at least a doublet. Similar behavior was observed at both high and low temperatures. We present the low-energy Raman spectra for the P4,lrnnm and “Pm3n” structures in Figure 3 , a and b. The low-frequency spectra, or so-called phonon modes, of the Pa3, P63/mmc,and P42/mnmphases have been observed and found to be very different from each other.16 In the present work, the two modes of the P42/mnmstructure correspond to those identified by Medina and Daniels.16 We also found a distinct phonon spectrum consisting of seven peaks for the “Pm3n” structure below about 20 K. (16) F. D. Medina and W. B. Dainels, J. Chem. Phys., 64, 150 (1976). W. B. Daniels and F. D. Medina in “Physics of Solids under High Pressure”, J. S. Schilling and R. N. Shelton, Eds., North-Holland, Amsterdam, 1981, p 23.
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Buchsbaum et al.
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984
t
1 :
t
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Ill
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a n
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Raman s h i f t [crn-ll Figure 3. Raman phonon spectra of solid nitrogen in two different crystal forms: (a) y N 2 (P4,lrnnm) at 7‘ = 15 K, P = 0.4 GPa; : (b) 6-N2 (“Pm3n”) at T = 15 K, P = 4.4 GPa.
Unfortunately, we could not see any low-frequency modes above this temperature, which meant that there was no way of distinguishing between the P63/mmc and P4,lmnm phases by using Raman scattering. The phase diagram of N 2 is given in Figure 4 where present results are combined with those of previous worker^.^-'^-*^ The transition between the “Pm3n” and P63/mmc structures, represented by a line in Figure 4, actually showed a hysteresis of about 20 K.
Discussion One striking feature of the N 2 phase diagram is the extensive P-T region of stability of the “Pm3n” phase, shown in Figure 4 as existing above a few gigapascals over our entire temperature range. There are large domains in this phase where a multiplicity of observed Raman-active lines is not consistent with the point symmetry of space group Pm3n. The extra lines may arise from a distortion of the cubic Pm3n structure. At present, however, it is not clear whether such a distortion is continuous with pressure and temperature or whether various discrete low-symmetry structures are formed. It is quite surprising that the ordered P4,lmnm structure transforms with pressure into the “Pm3n” space group, in which the molecules are apparently disordered, at temperatures as low as 15 K. We emphasize that, while pressures could not be manipulated at low temperatures, the initial conditions could be set so the sample passed through the P4,/mnm-to-“Pm3n” transition as the pressure increased on cooling. At 2.1 GPa and 50 K, as shown in Figure 4, both phases were seen to coexist. We conclude, therefore, that even at low temperatures the “Pm3n” structure ~~
(17) C. A. Swenson, J . Chem. Phys., 23, 1963 (1955). (18) J. W. Stewart, J . Phys. Chem. Solids, 1, 146 (1956). (19) J. R. Brookeman and T. A. Scott, J . Low Temp. Phys., 12, 491 (1973). (20) R. L. Mills, D. H. Liebenberg, and J. C. Bronson, J . Chem. Phys., 63, 4026 (1975).
/ 100 150 200 TEMPERATURE ( K)
250
300
Figure 4. Phase diagram of condensed N,. Present spectral observations: 0, hexagonal @-N, (P6Jmmc) phase; 0,tetragonal 7-N, (P4,lrnnm) phase; 0 , ”cubic” 6-N2 (“Pm3n”) phase. Coincident points indicate coexisting phases. Lines incorporate results from ref 5 and 17-20. Measurements did not extend into the cubic a-Nz (Pa3) phase.
is thermodynamically more stable than the P42/mnm form at pressures above about 2.0 GPa. A true Pm3n structure is also observed in oxygen2’(7-0,) and fluorine22(6-F,) near their gas-liquid-solid triple points. There appear to be no high-pressure studies on F2, but Nicol and cow o r k e r ~have ~ ~ shown that the Pm3n phase in oxygen is contiguous with the melting curve and ends near room temperature. At 300 K, 6-0, (space group R3m) is the solid phase in equilibrium with f l ~ i d . ’We ~ ~are ~ not ~ able to explain why the “Pm3n” structure should be restricted to such a narrow range of P and Tin oxygen and be stable over such a wide range in nitrogen. From molecular-dynamics calculations on N2at 7 GPa, Nos6 and Klein24found that the Pm3n space group transforms to 12’3 at 230 K, triggering a further change to R3c at 140 K. Unfortunately we are unable to confirm these structures from phonon spectra, which extend only up to 20 K. The present Raman stretching-mode spectra, however, are not inconsistent with such transformations. Calculations for 0 K by LeSar and Gordon25 and Etters and co-workers26indicate that in nitrogen a transition from the P42/mnm to the R3m (p-0,) structure should take place at a few gigapascals a t 0 K, but this change is clearly in conflict with observed Raman spectra. Failure of theoretical models to explain the phase diagram of N 2 may occur because the “Pm3n” structure is not always truly cubic but is, sometimes at least, a lower symmetry distortion. Klein et al.27have pointed out that the Pm3n structure is closely related to the A-I5 structure in superconducting metal alloys, which is known to be subject to a number of distortions to lower symmetry forms. In nitrogen, the splitting of one of the stretching modes (21) T. H. Jordan, W. E. Streib, H. W. Smith, and W. N. Lipscomb, Acta Crystallogr., 17, 777 (1964). (22) T. H. Jordan, W. E. Streib, and W. N. Lipscomb, J . Chem. Phys., 41, 760 (1964). (23) H. d’Amour, W. B. Holzapfel, and M. Nicol, J . Phys. Chem., 85, 130 (1981). (24) S. Nos6 and M. L. Klein, Phys. Reu. Lett., 50, 1207 (1983). (25) R. LeSar and R. G. Gordon, J . Chem. Phys., 78, 4991 (1983). (26) K. Kobashi, A. A. Helmy, R. D. Etters, and I. L. Spain, Phys. Reu. B, 26, 5996 (1982). (27) M. L. Klein, D. Levesque, and J.-J. Weis, Phys. Reu. B, 21, 5785 (1980).
J. Phys. Chem. 1984,88, 2525-2530 at high pressure is consistent with a distortion to lower symmetry. A detailed description of the "Pm3n"structure(s), however, must await careful X-ray diffraction measurements in the 6-N2 phase.
Acknowledgment. We thank J. 0. Willis for use of the helium flow cryostat. L.C. Schmidt, R. D. Holmes, and L. E. Trimmer constructed the optical setup. Trimmer also suggested modifications for improving the reliability of diamond cells. We acknowledge useful discussions with R. C. Hanson, B. W. Olinger,
2525
and S. K. Satija. S.B. expresses appreciation for a Graduate Research Assistantship sponsored by the Los Alamos National Laboratory Center for Materials Science. Work was performed under the auspices of the US.Department of Energy and was supported in part by the Division of Materials Sciences of the Office of Basic Energy Sciences and by the Division of Military Applications. Registry No. Nitrogen, 7727-37-9.
Crystallite Size Effect in the Selective Oxidation of Butene to Butadiene on Iron Oxide. 1. Mossbauer, X-ray, and Magnetization Characterization of the Catalysts F. Hong, B. L. Yang, L. H. Schwartz,+and H. H. Kung* Chemical Engineering Department and the Ipatieff Laboratory, Northwestern University, Evanston, Illinois 60201 (Received: April 19, 1983; In Final Form: December 13, 1983)
A series of silica-supportedand unsupported iron oxide catalysts were characterized by X-ray diffraction, room-temperature Mossbauer spectroscopy, and low-temperature magnetization measurements. The average crystallite sizes were determined by the X-ray line-broadening technique. They ranged from 2.5 to 9.5 nm for the supported samples and from 14.5 to 61 nm for the unsupported samples. X-ray diffraction detected only the presence of a-Fe2O3. Fourier line-shape analyses showed that the samples were rather strain free, and the width of the crystallite size distribution increased with increasing average crystallite size. The Mossbauer patterns showed the presence of a six-line magnetic component and a superparamagnetic component. The Mossbauer parameters of both components are consistent with the assignment of a-Fe203. In particular, no component with zero quadrupole splitting assignable to y-Fe2O3 was observed. Magnetization measurements showed that the small crystallite samples possess magnetic moments higher than that of bulk a-Fe2O3 but much lower than that of 7-Fe2O3. The data was explained by the absence of Morin transition and incomplete cancellation of spins in very small crystallites of a-FezO3.
Introduction The study of the catalytic properties of small crystallites has been of great interest and intensity in the past two decades. According to the classification by Boudart, catalytic reactions are either structure sensitive or structure insensitive,' depending on whether the reaction characteristics vary or not with crystallite size and the nature of the surface crystal plane. With the exception of a few studies, practically all studies have been on metallic catalysts. While the available data are limited, studies on oxide catalysts have provided examples in which the chemical and catalytic properties of oxide catalysts depend on the crystallite size of the oxide or more generally on the structure of the surface crystal plane. For example, compared with large crystallites, small Fe304crystallites have been shown to react more readily with the silica support in the presence of water vapor and result in a catalyst of lower activity in the water-gas shift reaction.2 The activity and selectivity in methanol decomposition on Zn03 and in methanol oxidation on M o o t 4 have been shown to depend on the exposed crystal plane. On the other hand, the decomposition of 2-propanol was shown not to depend on the exposed crystal plane of ZnO.' The selectivity for butadiene in the oxidation of butene on iron oxide has been reported to depend on the specific surface area of the catalyst.* Results from our laboratory also showed that on a-Fe203the selectivity for butadiene varied from less than 50% to about 80%, depending on the preparation of the catalyst? This prompted the investigation on the possibility that the reaction is structure sensitive and that the selectivity depends on the crystallite size. In this and the following paper;results of this investigation are reported. The catalysts used include both silica-supported and unsupported iron oxide. Results of the characterization of the catalysts by Mossbauer spectroscopy, X-ray diffraction, and f
From the Materials Science and Engineering Department.
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magnetization are reported here. These techniques were used to identify the number and the nature of the iron oxide phases present and to determine the average crystallite sizes. Such information is important to understand the results of the reaction studies, which are reported in the following paper. Fully oxidized iron oxide exists in two crystallographic forms, corundum a-Fe203 and spinel y-Fe203. In the bulk, massive oxide, these two forms differ in their X-ray diffraction spectra, the magnetic properties, and the Mossbauer spectra. Specifically, y-Fe203is ferrimagnetic, while a-Fe2O3 is weakly ferromagnetic above and antiferromagnetic below the Morin transition (263 K). The Fe ions in y-Fe2O3 are in a cubic symmetric environment such that the Mossbauer spectrum does not show quadrupole splitting. On the other hand, the distorted octahedral coordination of the Fe in a-Fe203 results in quadrupole splitting. The physical properties of fine particles of Fez03 have also been rather extensively studied. Reduced magnetic hyperfine splitting in small crystallites has been observed by a number of workers on a-Fe2031w16and on y-Fe203.12J7218The reduction has been (1) M. Boudart, Adu. Catal., 20, 153 (1969). (2) C. Lund and J. A. Dumesic, J . Catal., 76, 93 (1982). (3) W. H. Cheng, S. Akhter, and H. H. Kung, J . Catal., 82,341 (1983). (4) J. M. Tatibouet and J. E. Germain, J. Catal., 72, 375 (1981); C. R. Hebd. Seances Acad. Sci., Ser. C, 290, 321 (1980). ( 5 ) J. C. Volta, M. Forissier, F. Theobald, and T. P. Pham, Faraday Discuss. Chem. SOC.,No. 72, 13 (1981). ( 6 ) J. M. Tatibouet, J. E. Germain, and J. C. Volta, J . Catal., 82, 240 (1983). (7) G. Djega-Mariadassou and L. Davignon, J . Chem. Soc., Faraday Trans. 1, 78, 2447 (1982). ( 8 ) Th. Simons, E. Verheijen, Ph. Batist, and G. Schuit, Adu. Chem. Ser., No. 76, 261 (1968). (9) B. L. Yang, F. Hong, and H. H. Kung, J. Phys. Chem., following paper in this issue. (10) G. B. Raupp and W. N. Delgass, J . Catal., 58, 337 (1979). (11) E. Unmuth, L.H. Schwartz, and J. B. Butt, J. Catal., 61,242 (1980). (12) C. Lund, and J. A. Dumesic, J . Phys. Chem., 85, 3175 (1981).
0 1984 American Chemical Society
