ESR STUDIES OF CuO SUPPORTED ON ALUMINA
attributable to the presence of the electronegative OH group, which amounts to 0.05 A. It is possibly apropos to note that the imposition of such a correction also results in the Si-C (methyl) bond being longer than the Si-C (benzene) bond by about 0.03 A. Such a difference would be expected by analogy to C-C bonds, for which the covalent radius of a trigonal carbon atom is approximately 0.02 ,4less than that of a tetrahedral carbon atom.
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Acknowledgments. The authors wish to thank Robert L. Merker of the Silicones Fellowship supported here by the Corning Glass Works and the Dow Corning Corporation for supplying the compound, and they are grateful to Robert F, Stewart for helpful and stimulating discussions. They also acknowledge indirect support in the form of NSF Grant G11309 awarded to the Computer Center of the University of Pittsburgh, where the crystallographic calculations were performed.
Copper Oxide Supported on Alumina. 11. Electron Spin Resonance Studies of Highly Dispersed Phases
by Pierre A. Berger and James F. Roth Central Research Department, Monsanto Company, St. Louis, Missouri 69166 (Received April $6. 1967)
Copper oxide dispersed on y-alumina contains a significant amount of Cu2+ ions that are present as atomically dispersed species and exhibit only weak magnetic interactions. The measured spin-Hamiltonian parameters indicate that the Cu2+ ions are sitting in open octahedral holes. When the sample is reduced, oxygen is removed from the surface and copper atoms migrate along the opened paths and form clusters. Therefore, the reduced state is diamagnetic. A new embryonic state of reoxidation has been discovered. I n this state the oxidized atoms are still close together and give rise to an esr spectrum which is exchange narrowed. With increasing temperature the Cu2+ ions move further apart, oxygen is inserted back into the surface, and finally the original structure is restored.
Introduction I n supported catalysts, the state of dispersion of the active phases has been recognized to be important. I n many instances a definite relationship between dispersion state and catalvtic activity has been observed. has been directed at the Of Some atomically dispersed species as active sites in catalysis, and one even finds elaborate theories based on this concept.’ It is, however, fallacious to conclude that low concentration of dispersed species necessarily implies “atomic” dispersion. For example, Kikuchi, et aLJ2have recently demonstrated that on silica gel the sites of lowest energy (ie., where the molecules
first adsorb) accommodated several molecules of copper acetylacetonate S~ultaneouslY. Magnetic susceptibility3and electron spin resonance ( e ~ r investigations )~ have been useful in distinguishing (1) For a review, see N. I. Kobozev, V. P. Lebedev, and A. N. Mal’tsev, 2.Physik. Chem. (Leipzig), 217, 1 (1961). (2) K. Kikuchi, H. W. Bernard, T. T. Fritz, and J. G. Aston, J. Phys.
699 3654 (1965).
(3) P. W. Selwood, “Magnetochemistry,” Interscience Publishers, Inc., New Yo&, N. Y., 1956; P. w. Selwood, “Adsorption and coilective Paramagnetism,” Academic Press Inc., New York, N. Y .,
1962. (4) V. Ya. Vol’fson and L. N. Ganyuk, Russ. Chem. Rev., 34, 701 (1965).
Volume 71, Number IS December 1967
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between different phases formed in supported catalysts. Several esr studies of copper dispersed on various media have appeared in the literature.5-* However, in none of these was the system studied in the reduced state. The original aim of the work reported here was to detect the atomic phase of copper in a reduced cupric oxide on y-alumina catalyst. This was prompted by the magnetic susceptibility studies by Selwood, et aL9 These authors measured the dependence of the paramagnetic susceptibility of Cu2+on the amount dispersed on high surface area y-alumina. They demonstrated that the magnetic moment of copper gradually rose to a limiting value of 1.7 Bohr magnetons @), (corresponding to the spin only value) with decreasing copper concentration. This and the observation of a Curie-Weiss law (103" < T < 298°K) with a small and negative paramagnetic Curie temperature was interpreted as indicative of the attainment of infinite magnetic dilution at low copper content. However, samples reduced to Cu" failed to exhibit the paramagnetic susceptibility expected to be of the order of x = 0.375/T. The original susceptibility was reversibly restored upon reoxidation. Migration of atoms during reduction was ruled out. Thus no clear picture of the dispersion state of copper on y-alumina emerged. A later esr study by Matsunaga6 suggested that several kinds of dispersion states might exist on the surface depending on substrate concentration, but was not very explicit. Our findings on the oxidized catalyst agree with work reported in the 1iterature;j-9 no atomic phase was detected in the reduced state, but under certain conditions a new oxide phase has been detected whose properties indicate short-range migration of copper atoms during the reduction. The esr results provide a consistent picture for the state of highly dispersed copper both when fully oxidized and reduced.
Experimental Section Apparatus. The esr measurements were performed a conventional varian v-4502 spectrometer in a TE104 dual cavity operating at a nominal frequency of 9.5 Gc/sec. The apparatus was equipped with a &in. magnet, the Varian Fieldial calibrated sweep system, and a dual-channel Atfoseley 2 FR-AM X-Y recorder, A Hewlett-Packard X-530 A frequency meter was attached to the 20-db coupler provided in the microwave bridge' Diphenylpicrylhydrazyl (DPPH) and and sium peroxylamine disulfonate served for sweep calibration purposes, and a single crystal of CuSO~.5H20embedded in paraffin as intensity standard. The conventional Varian quartz insert in conjunction with the v-4540 temperature permitted measurements to be made from about - 180 On
The Journal of Physical Chemistry
PIERRE A. BERGERAND JAMES F. ROTH
to 300'. The accuracy of the temperature scale was checked with DPPH, which is supposed to follow an exact Curie-Weiss law below room temperature. Samples. Spherical particles of a high surface area y-alumina, XA331, obtained from Kaiser Chemical Co. were impregnated in an excess aqueous solution of Cu(N03)2-3H20 for 1 hr. They were subsequently surface dried with towels, dried overnight a t 120', and finally calcined in air a t 500' for 12 hr. The catalysts thus formed were of green color. The specific surface area of the XA331 alumina support was found to be 301 m2/g. The esr spectrum of XA331 arising from impurities has been described previously. l o The broad line centered around g = 2.09 did not affect the detectability of the intense spectrum of Cu3+under the conditions of modulation used in this work. Samples with 3.35, 5.67, 7.46, and 8.8 wt % copper were obtained in the described manner. At none of these concentrations was a crystalline CuO phase detected by standard X-ray diffraction analysis. The limit of detection was estimated to be 0.5 wt % crystalline CuO. Particular care was applied in getting reliable quantitative esr data. The catalysts were ground to particle sizes between 105 and 207 p , then filled to a height exceeding the effective cavity length into quartz tubes of uniform diameter. These quartz tubes were themselves sealed to Vycor ground joints for attachment to a conventional glass vacuum manifold. The effective cavity volume was determined with a small crystal of DPPH which was then dissolved in benzene. Generally, the first experimental moment was used as intensity measure.
Experimental Results and Analysis The Esr Spectrum of Dispersed Divalent Copper. The esr spectrum of the oxidized catalyst containing 3.35% CuO is shown in Figure 1. The spectra for all the four samples were similar, but the one corresponding to the lowest copper concentration was the best resolved one. The observed line shape is typical for a polycrystalline sample containing Cu2+ in a site of symmetry* The line shape was performed according to the method of Lardon and Giint(5) R. J. Faber and M. T. Rogers, J . A m . Chem. SOC.,81, 1849 (1959). (6) Y. Matsunaga, Bull. Chem. SOC.Japan, 34, 1291 (1961). (7) E. V. Kavalerova, V. B. Golubev, and V. B. Evdokimov, Ruse. J . Phy.8. Chem., 37, 116 (1963). (8) A. Nicula, D. Stamires, and J. Turkevich, J . Chem. Phys., 42, 3684 (1965). (9) P. W. Selwood and N. s. Dallas, J . Am. Chem. SOC.,70, 2145 (1948); P. E. Jacobson and P. W. Selwood, ibid., 76, 2641 (1954). (io) P.A. Berger and J. F. Roth, J . Cata7ysis, 4, 717 (1965).
ESRSTUDIES OF CuO SUPPORTED ON ALUMINA
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Table I : Spin-Hamiltonian Parameters for Cupric Ion on Various Supports Ail,
Cu’ on
PI
91 I
Powdered zeolite Linde Y zeolite dehydrated a t 300” Ion exchangers, hydrated Charcoal Acetyl acetonate on aluminosilicate 3 . 3 5 % copper on XA331 7-alumina
2.083 2.0714
2.35 2.3462
-0.00164
2.079-2.099
2.19-2.40
(
