J. Phys. Chem. 1987, 91, 2684-2686
2684 100
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CONVERSION
Figure 3. Calculated and observed selectivity, C2H4/C2H6 ratio, and C3+ selectivity vs. conversion using full model (reactions 1-12) at 1 and 3 atm.
between calculated and observed selectivity, C2H4/C2H6ratio, and C3+ yield, as functions of both conversion and pressure, is obtained (Figure 3). The relative rates for the various hydrocarbon species with M O correspond to those found for gas-phase radical reactions. The rate constant for the reaction of ethane with MO, relative to that
-
(10) In the final version, reaction 5 (with experimentally determined rate constant) was replaced by C,H, 2 CHI' (with literature rate ~ n s t a n t ' ~ ) , the first step in thermal ethane dehydrogenation. This substitution caused virtually no change in calculated results (although leaving out this step altogether did produce a substantial change).
of methane (defined as l.O), is 1.9. This is close to the relative rates for H' abstraction from the two hydrocarbons by CH30' in the gas phase (2.8, extrapolated from data7ato 825 "C). Other relative rates obtained from kinetic measurements and/or fitting the model are as follows: C2H4,1.4; C2H2,1100;C3Hs, 3.0; CH3', 2700. This observation, coupled with the successful model employing known gas-phase rate constants, implies a Rideal-type mechanism with minimal involvement of adsorption and surface chemistry. We suggest, somewhat paradoxically, that good selectivity is achieved because oJ not despite, the facts that this metal oxide is not very typical of known selective partial oxidation catalysts'' and the temperature of operation is quite high. Under these conditions, where hydrogen-atom abstraction determines rates, the reactivity of methane can come fairly close to that of the higher hydrocarbons. Any catalyst "improvement" is likely to magnify reactivity differences (either better adsorption, as the heavier hydrocarbons will absorb more strongly, or lower temperature, as some of the secondary reactions have lower Ea's) and thus result in higher CO, make via the consecutive pathway. While there is undoubtedly room for some improvement of selectivity for catalysts operating by this mechanism, very substantial improvements (e.g., conversions above 50% with selectivities above 90%) will probably require that the reactivity of methane be made higher than that of its products. This can probably be achieved only in molecular systems where steric effects may become important.12
Acknowledgment. We thank T. Shepard, D. Faragalli, B. Bacon, D. Grimmett, and A. Goldman for technical assistance, Drs. C. E. McBride and J. W. Sibert for valuable discussions, and Drs. C. A. Jones and J. A. Sofranko for providing metal oxide samples. ( 1 1) See, for
example, Hucknall, D. F. Selective Oxidation of Hydro-
carbons, Academic: New York, 1974.
(12) See, for example, Wenzel, T. T.; Bergman, R. G.J . Am. Chem. SOC. 1986, 108, 4856-67.
Electron Spin Echo Envelope Modulation of Powder Samples Exhibiting Axially Symmetric g Tensors Michael W. Anderson* and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: January 28, 1987)
Recent simulation procedures for electron spin echo modulation (ESEM) patterns for powder samples with axially symmetric g tensors are tested at arbitrary magnetic field positions by comparison with experimental data from a zeolite doped with
cupric ion. The good agreement supports the validity of the simulation procedures and makes it possible to record and simulate ESEM at arbitrary magnetic field so as to minimize complications from spectrally overlapping paramagnetic species.
Introduction Recently, the simulation of electron spin echo modulation (ESEM) patterns for powder samples with axially symmetric g tensors has been a d d r e ~ s e d l -due ~ to its importance for current applications in catalytic systems. The theory has been developed Anderson, M. W.; Kevan, L. J . Phys. Chem., 1986, 90, 6452. Anderson, M. W.; Kevan. L. J . Phys. Chem., 1987, 91, 1850. Anderson, M. W.; Kevan, L . J . Chem. Phys., in press. Dikanov, S. A.; Yadanov, V. F.; Tsvetkov, Yu. D. J . Magn. Reson. 1979, 34, 631. ( 5 ) Reijerse, E. J.; Van Aerle, N. A. J. M.; Keijzers, C. P.; Bottcher, R.; Kirsme, R.; Stach, J. J Mugn. Reson. 1986, 67, 114. (1) (2) (3) (4)
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to simulate spectra at all values of 80, the angle between g,,and the magnetic field Ho.However, in most of the reported cases ESEM patterns have only been recorded at glland g , where 80 equals 0 and 90°, respectively. The aim of this work is to compare experimental three-pulse ESEM data at many values of 80 on a well-defined powder system with an axially symmetric g tensor by using the theories developed.
Results and Discussion The system chosen is zeolite Na-A that has been exchanged with 2.2 Cs+ cations per unit cell and doped with Cu2+ cations to a concentration of one Cu2+cation per 40 unit cells. A complete description of the preparation of this sample is given elsewhere2 0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2685
Letters
(a) A
DPPH
n
I (b) g II =2.396 A,, = 0.0I32 cm-' Figure 1. Structure of dehydrated zeolite C U C S ~ , ~ Nshowing ~ - A cation positions of Cu2+and Cs+. The Cu2+is located at site S2 in the center of the zeolite 6-ring and Cs+ is located at site S5 in the center of the zeolite 8-ring.
200 G
+
gL=2.064 AL=0.0014 cm-' Figure 3. (a) Experimental and (b) simulated ESR spectra of dehydrated zeolite CuCszzNa-A recorded at 77 K.
Y 91
'\( Figure 2. Vector diagram showing the relationship between the external field Ho and the internuclear vector r for an ESE experiment. The diagram is drawn in the g-tensor reference frame.
and in keeping with that work the sample will be referred to as CuCs2,Na-A. The zeolite was loaded into a 3-mm-0.d. Suprasil quartz tube, heated under vacuum to 400 O C to dehydrate it, and then sealed. In this dehydrated zeolite the Cs+ cations reside in sites S5 (see Figure 1) located at the center of the single zeolite 8-rings while the Cu2+ resides amost exactly at site S2, the center of a zeolite 6-ring. This results in a trigonal Cu2+species coordinated to three lattice oxygens of the 6-ring with g,,directed perpendicular to the plane of the 6-ring. In order to calculate the ESEM patterns it is necessary to define three angles, Bo, L$, and &, which are depicted in Figure 2. Choosing g,, to lie along the z direction and r, the electron-nuclear vector, to lie in the z plane, Bo is the angle between z and Ho, 0 , is the angle between z and r, and is the azimuthal angle about z between the z, Hoplane and the I, r plane. With these angles defined, the average modulation (V,,,,) is given by using eq 3-7 or ref 2 combined with eq 3-1 1 of ref 1. These equations are too long to repeat here in this short communication; however, the angles are treated as follows. 19,is known from the relative crystallographic locations of Cu2+and Cs+ and in this example is 58O. Bo will depend on the field at which the experiment is performed. If Hocorresponds to the gll resonance then Bo = 0; if Hocorresponds to the g, resonance than Bo = 90'. For any intermediate fields the value of Bo may be determined by simulating the ESR spectrum. Finally 4, is integrated over all, possible orientations from 0 to 2?r as described in ref 1 and 2.
0
I
2
3 4 5 TI PS Figure 4. (a) Field swept ESE spectrum of CuCsz,Na-A recorded at 4 K. Other spectra are all three-pulse ESEM spectra of CuCs22Na-A recorded at 4 K showing both experimental (-) and simulated (---) data. The spectra were recorded at the following fields: (b) 2698 G, (c) 2802 G, (d) 2918 G, (e) 3102 G, (f) 3150 G, (9) 3178 G, and (h) 3190 G.
The ESEM simulations proceed as follows. It is first necessary to determine the value of Bo at each field setting. This is done by simulating the electron spin resonance (ESR) spectrum ac-
J. Phys. Chem. 1987, 91, 2686-2688
2686
TABLE I: Data for Simulations of the ESEM Patterns' HOIG 2698 2802 2918 3102 3150 3178 3190
mi = -312 33.510.064 44.710.067 55.910.076 76.210.147 90.0/0.208
(On/deg)/(relint)b m, = -112 m, = +1/2 15.1/0.067 34,310.075 48.710.084 71,710,147 8 1.910.284 90.0/0.057
0.0/0.002 11.2/0.073 37.810.095 66.0/0.154 75.9/0.242 90.010.332
intensity of each hyperfine component is given by Im, then the total will be given by echo intensity ( Vmod)tot mr = 312
mi= +3/2 0.0/0.002 11.7/0.092 57.510.170 68,710,239 77.810.375 84,910,462
Table I gives a list of Bo values and hyperfine intensities at all values of Ho used to record the ESEM spectra. All values are given for the electron spin echo spectrometer frequency of 9.159 GHz. Details of both the general theory of electron spin echoes and of the experimental conditions are given e l ~ e w h e r e . ' , ~ In -~ this work all experiments performed were three-pulse stimulat'The values of Bo and the relative intensities of all four parallel ed-echo experiments recorded with 7 = 0.3 p s where r is the time copper hyperfine components at the fields used in the ESEM experiments are shown. Values are calculated at Y = 9.159 GHz with g,, = between the first two pulses. This value of r is chosen so as to 2.396; g, = 2.064; All = 0.0132 cm-'; A , = 0.0014 cm-'; Lorentzian suppress any modulation from the *'A1 in the zeolite l a t t i ~ e . ~ line width = 7.0 G. *Relative intensity is the intensity of each hyperFigure 4 shows the three-pulse ESEM spectra recorded at seven fine component relative to the maximum ESR intensity near g, redifferent field values along with the respective simulations. All sulting from contributions from all four hyperfine components. seven simulations were possible with the position of the Cu2+cation fixed to within f0.01 nm of site S2. Such agreement leads cording to the methods of Siderer and L u z . ~ Figure 3 shows the ESR spectrum recorded a t 77 K of dehydrated c ~ ( 2 s ~ ~ N a - A credence to the validity of the approach outlined previously.'s2 The importance of being able to use such an approach is that zeolite. Also shown is the simulation which gave the following often ESR spectra consist of overlapping series. With the ESEM ESR parameters: gll= 2.396; g , = 2.064; All= 0.0132 cm-I; A , technique it is possible to tune the spectrometer to a part of the = 0.0014 cm-l, Lorentzian line width = 7.0 G. There is clearly spectrum where the overlap is minimal. If determination of 8, some discrepancy in the line profile in the g , region. However, at this field setting is possible then simulation of the ESEM this is probably due to the superposition of an ESR line from a spectrum should also be possible yielding hyperfine interaction small concentration of a minor Cu2+species which will have no parameters. effect on the ESEM results. With these ESR parameters the values of 8, for each of the four hyperfine components of Cu2+, Acknowledgment. This research was supported by the Robert ml = -312 to +3/2, and their respective intensities may be deA. Welch Foundation, the National Science Foundation, and the termined at all field settings. The relative intensities of each Texas Advanced Technology Research Program. hyperfine component are required so that the contribution from each component to the ESEM pattern may be given a weighting. If the echo modulation from each hyperfine component is given (7) Rowan, L. G.;Hahn, E. L.; Mims, W. B. Phys. Reo. 1965, 137, A61. (8) Mims, W. B. In Electron Paramagnetic Resonance, Geschwind, S., Ed.; where m, = -312, -112, 112, 312, and the relative by ( Vmod)m, Plenum: New York.. 1972: ChaDter 4. I
(6) Siderer, Y . ; Luz, 2.J . Magn. Reson. 1980, 37, 449.
1
(9) Kevan, L. In Time Domain Electron Spin Resonance, Kevan, L., Schwartz, R . N., Eds.; Wiley-Interscience: New York, 1979; Chapter 8.
The Photoexclted Triplet State of Porphycene, A Novel Porphin Isomer. Time-Resolved Electron Paramagnetic Resonance Spectroscopy Hanna Ofir, Ayelet Regev, Haim Levanon,* Department of Physical Chemistry, and The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University, Jerusalem 91904, Israel
Emanuel Vogel,* Mathias Kocher, and Metin Balci Institute fur Organische Chemie, der Universitat, 0-5000 Koln 41, West Germany (Received: February 4, 1987)
The photoexcited triplet state of free-base porphycene and its tetra-n-propyl derivatives, oriented in a uniaxial liquid crystal, was investigated by laser photoexcitation-EPR diode detection. By monitoring the transient behavior of the magnetization, time-dependent CW spectra were obtained and examined. The results yield (a) positive values for the zero-field splitting parameter D ;(b) the location of the fine structure tensor axes in the molecular frame (x and y lie in-plane and cross the bridges between the pyrrole rings, and z is perpendicular to the porphycene plane).
Introduction The participation of the porphyrin moiety in essential biological and photobiological processes led in recent years to an intensive synthetic research in the field of porphyrin and chlorophyll sysThe very recent report on the synthesis and basic ( 1 ) Johnson, A. W. Porphyrins and Metlalloporphyrins; Smith, K . M., Ed.; Elsevier: Amsterdam, 1975; p 729.
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photophysical properties of the novel porphyrin isomer, porphycene?l9 by Vogel and his co-workers prompted us to investigate (2) Callot. H. J.; Schaeffer, E. J . Urg. Chem. 1977, 42, 1567. (3) Bauer, V. J.; Clive, D. L. J.; Dolphin, D.; Paine 111, J. B.; Harris, F. L.; King, M. M.; Loder, J.; Chien Wang, S. W.; Woodward, R . B. J . A m . Chem. Sor. 1983, 105, 6429. (4) Rexhausen, H.; Gossauer, A . J . Chem. Soc., Chem. Commun. 1983, 275.
0 1987 American Chemical Society
