2926
J . Phys. Chem. 1987, 91, 2926-2930
different from the values obtained without added water. The role of the water in this order-disorder transition is intriguing. The water might be temporarily incorporated into an unstable intermediate phase which forms nonuniformily throughout the DSC sample. The exotherm and endotherm at 114 "C may represent the leading and trailing edges of much larger heats of adsorption and desorption which mostly cancel. Another speculative explanation of the role of the water is that it facilitates a very slow transition that can always occur to some degree during melting but is usually not observed. Several recent papers2*have demonstrated the importance of thermal desorption in disrupting the structure of heated LB films. It is conceivable that the extra exotherm and endotherm are related to generation and desorption of the free acid. The weight change expected, a fraction of a monolayer, may not be detected in the low surface area powder. No matter what the underlying explanation, it is apparent that LB multilayers of cadmium arachidate will not be stable to moisture near or above T,,, (1 10 "C). Furthermore, past studies of the order-disorder transition of LB films of cadmium arachidate may have been misled by the inclusion of small amounts of water in the film. Because some water does not desorb rapidly from the powder, it is likely that a submolar amount of water is trapped during the preparation of the LB film and is still present during the measurement of an order-disorder transition. The amount of water will depend on the method of film preparation, the thermal history of the sample, and the humidity of the atmosphere during the measurement. The question of partial reversibility of
multilayer^^^-^^ vs. irreversibility of monolayers2' now must be reanalyzed very carefully with respect to the influence of included water. The data from different experiments may not be able to be compared28aif the water levels in the preparation or the measurement chamber are dissimilar. Very small amounts of moisture can affect the transition, Le., 1.1% relative humidity at 110 "C and, thereby, our understanding of the stability of the film. Conclusion The heating of bulk cadmium arachidate in the presence of water vapor shows an unexpected exotherm and endotherm near 110 OC which is not apparent with the anhydrous powder. Although it is likely that the adsorption and desorption of water is responsible for the thermal changes measured, there is no weight change corresponding to the heat change during the melting of either the hydrated or the anhydrous powder. Because previous studies have shown that water vapor affects the structure of the Langmuir-Blodgett multilayers after melting and recooling, it is likely that rigorous control of the level of water in all orderdisorder studies of cadmium arachidate and other fatty acid salts is necessary to compare the results from different experiments and laboratories. Acknowledgment. I wish to thank Dr. John Rabolt for this encouragement, insights, and helpful discussions. I also wish to thank Dr. Jerry Swalen for his support and for providing the powder sample used in this study.
Study of Cu2+-Doped Zeolites Na-rho, K-rho, and Ca-rho by Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopies Michael W. Anderson and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: October 27, 1986)
The location and adsorbate interaction of Cu2+in Na-, K-, and Ca-rho zeolites doped with Cuz+ has been studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. It is found that the activation process varies very little with the type of countercation and that the major hydrated species in zeolite rho is a square-coplanar or square-pyramidal Cu2+bound to two water molecules. Adsorption of polar molecules, such as ammonia, methanol, and ethanol, results in complexing with the Cu2+although in the case of methanol the number of ligands is less for Na-, K-, and Ca-rho than for H-rho studied previously. Nonpolar molecules, such as ethylene, are essentially excluded from the zeolite by the Na, K, and Ca cations which are located in the octagonal prisms blocking entrance to the zeolite interior.
Introduction Owing to their microporous nature and ion-exchange properties, zeolites provide a unique substrate on which to perform catalytic chemistry. By introduction of the requisite cation, the catalyst may be made either acidic or nonacidic and is usually highly selective because of the molecular-sieving characteristic of the micropores. When the catalytic center is paramagnetic, such as Cu2+,much information regarding cation location and adsorbate interaction may be gleaned from a combination of ESR and ESEM spectroscopic technique^.^-^ The stereochemistry of the Cu2+ complex may be surmised from the ESR spectrum while more quantitative information regarding the number and orientation of ligand molecules may be obtained from ESEM. The A, X,and Y zeolite systems have been studied in some detail by these techniques. This work describes the application of these techniques to Cu2+-doped zeolites Na-, K-, and Ca-rho and (1) Kevan, L.; Narayana, M. ACS Symp. Ser. 1983, No. 218, 283. ( 2 ) Narayana, M.; Kevan, L. J . Chem. SOC., Faraday Trans. I 1986,82,
213. (3) Ichikawa, T.; Kevan, L. J . Am. Chem. SOC.1981, 103, 5355. (4) Ichikawa, T.; Kevan, L. J . Phys. Chem. 1983, 87, 4433. (5) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1986, 90, 3206.
0022-365418712091-2926$01SO10
further develops a recent study of zeolites CuCsH-rho and CuH-rho.6 It is found that the counteraction has very little effect on the type of hydrated Cu2+ species present in zeolite rho, which i s markedly different from the case in zeolites A, X, and Y. However, because of the blocking effect of the cocations, which occupy cation sites in the octagonal prism entrances to the a-cage of the zeolites, only polar molecules which can circumvent these cations by solvation may complex with the Cu2+. Smaller complexes with methanol are observed in the zeolites under consideration here compared with those in zeolite H-rho: presumably owing to some steric hindrance.
Experimental Section The zeolite rho used in this study was prepared according to the methods of Robson et al.7*8and has been described in detail elsewhere.6 Commercial atomic absorption analysis of this material gave a Si/A1 ratio of 3.4, which amounts to a cation-ex(6) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1986, 90, 6452. ( 7 ) Robson, H. E.; Shoemaker, D. P.; Ogilvie, R. E.; Manor, P. C. Adu. Chem. Ser. 1973, No. 121, 106. (8) Robson, H. E. U S . Patent 3904738, 1975.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2921
Cu2+-Doped Zeolites Na-, K-, and Ca-rho
TABLE I: ESR Parameters at 77 K of Cuz+ in Na-, K-, and Ca-rho Zeolites All,' lo4
sample treatmentQ CuNa-rho fresh
x4
evac RT evac80 "C
b
x4
evac400 OC sat. H 2 0 sat. CH30H sat. CH3CH20H
-
+90 Torr of
A CuK-rho
g,,=2.314
yoG
*
I
v
NH3 fresh evac 80 OC CUO
CuNa-rho
evac 400 OC sat. HzO sat. CH30H sat. CH,CHZOH
77K
Figure 1. ESR spectra of CuNa-rho recorded at 77 K (a) fresh sample; (b) evacuated at room temperature; (c) dehydrated at 80 OC; (d) dehydrated at 400 "C.
change capacity of 10.9 monovalent equivalents per unit cell (based on a unit cell comprising 48 AlO4+ and Si04+ tetrahedra). The "as prepared" zeolite was in the Na, Cs cationic form. First, this was converted to the protonic form by exchanging repeatedly with a 20% solution of N H 4 N 0 3followed by calcination in air at 300 O C . The Na, K,and Ca cationic forms were then obtained by repeated exchange with 0.1 M NaOH, KOH, and CaOH solutions at 80 OC, respectively. All the zeolites were doped with Cuz+ by M Cu(NO& and 100 mL of HzO exchanging with 10 mL of per 1 g of zeolite. This amounted to an exchange of approximately 1 Cuz+ cation for every 40 a-cages. Finally, the zeolites were washed with hot, triply distilled water and dried in air at room temperature. a zeolite prepared in such a manner is termed "fresh". Zeolite samples were loaded into 3-mm-0.d. Suprasil quartz sample tubes which could be connected to a vacuum and gashandling line. The zeolites were activated by first evacuating to Torr, then heating to 400 OC over an 8-h a pressure of 1 X period, and finally, oxidizing with 400 Torr of dry, high-purity oxygen. Samples could be sealed at various stages of dehydration in order to record their ESR spectrum. After activation, adsorbates as both vapors and gases were admitted at room temperature to the sample tubes and left for 2 h to equilibrate. Except in the case of ethylene, where 100 Torr was admitted, exposure to adsorbates was at the saturated vapor pressure. Deuteriated C D 3 0 H , C H 3 0 D , and CzD4 were obadsorbates such as DzO, tained from Aldrich and Stohler Isotope Chemicals and were used without further purification. ESR spectra were recorded at both room temperature and 77 K on a Varian E-4 spectrometer. ESE spectra were recorded at 4 K on a home-built spectrometer, described elsewhere:JO linked to a Nicolet 1280 computer. In all cases the threepulse, stimulated echoes were recorded by using a 90°-90°-90° pulse sequence, where the echo is measured as a function of time T between the second and third pulses. In order to maximize the modulation depth from deuterium and eliminate the modulation from zeolitic 27Al,the time between the first two pulses, 7,was kept between 0.28 and 0.30 ws. The method of phase cycling" was employed to eradicate the two-pulse glitches appearing at times T = T and T = 27. The following phase cycle was used: [(000) (nn0)
+
(9) Ichikawa, T.; Kevan, L; Narayana, P. A. J . Phys. Chem. 1979, 83, 3378. (10) (a) Narayana, P. A.; Kevan, L. Photochem. Phorobiol. 1983.37, 105. (b) Narayana, P. A.; Kevan, L. Magn. Reson. Rev. 1983, 1, 234. (1 1) Fauth, J. M.; Schweiger, A.; Braunschweiler, L; Forrer, J.; Ernst, R. R. J . Magn. Reson. 1986, 66, 14.
+90 Torr of
CuCa-rho
NH3 fresh evac 80 OC evac 400 OC sat. H 2 0 sat. CH30H sat. CHSCH20H +90 Torr of
glib
cm-'
probable configuration 2.078 Cull gLb
1.996 2.371 2.314 2.355 2.314 2.361 2.367 2.356
87 142 175 148 170 154 136 135
2.251 2.076 Cull
2.309 2.239
180 175
2.381 1.993d 2.307
150 82 175
2.078 Cull
2.365 2.314 2.370 2.359 2.355
147 170 150 135 137
cull CUO 2.074 Cui1 2.074 Cul
2.310 2.239
180 175
2.365 2.333 2.318 2.363 2.320 2.361 2.310 2.322
147 182 181 141 170 149 180 176
2.360 2.239
146 175
CUO CUI1 2.067 Cuo 2.075 Cull 2.077 Cul CUO
2.051 Culll or CuIv
CUO
2.051 CuIIlor Culv
cull CUO cull 2.057 Cuo 2.075 Cull CUO CUO
2.051 Culll or Culv NH3 'evac = evacuated at; sat. = saturated with - after activation; RT = room temperature. *Estimated uncertainty is f0.005. 'Estimated uncertainty is f 5 X lo4 cm-I. dVery small concentration of this species.
+
(nnn) (OOn)]. Both the theory and methods used for simulation of the data are described in detail elsewhere.I2
-
Results The samples studied are designated in the text as CuNa-rho, CuK-rho, and CuCa-rho where the Cu indicates Cu2+ doping to about 1 per 40 a-cages. Table I contains a list of the ESR parameters at 77 K for the three samples after various pretreatments. The hyperfine splitting is only given in the gllregion as generally the hyperfine splitting in the g, region is unresolved. Where more than one species exists, it is usually possible to determine the value of g, for each. Assignment of the species is usually based on the hyperfine "fingerprint" in the gll region of the spectrum. Figure 1 shows a series of ESR spectra monitoring the activation of CuNa-rho. In the fresh sample (Figure l a ) there are two major species. One species has g, = 2.078 where the values of g,,and A,,are obscured by other species. The latter species has reversed g values g, > gll, which is less usual for Cu2+,with g, = 2.251, gll = 1.996, All = 0.0087 cm-I. Evacuation at room temperature (Figure lb) is sufficient to destroy this species with reversed g values, indicating its instability. Evacuation a t the relatively low temperature of 80 "C produces another species with ESR parameters gll = 2.314 and All = 0.175 cm-I. The concen(12) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1979; Chapter 8.
2928
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987
Anderson and Kevan
-
CuK-rho t methanol 77K
CuNa-rho t ammonia 77K
200 G
/r
J
x4
-
-50 G
A=12.7G
Figure 4. ESR spectrum of CuNa-rho with adsorbed ammonia recorded
at I 1 K.
200G
Figure 2. ESR spectrum of CuK-rho with adsorbed methanol recorded E L 17 K.
CuCa-rho t ammonio 77 K
CuNo-rho +ethanol 77 K
4IDWH
xw "I
f
I/ I
-
9,,=2.309 I
g,, = 2.356
200 G
Figure 5. ESR spectrum of CuCa-rho with adsorbed ammonia recorded at 77 K. CuNa- rho t D20
Figure 3. ESR spectrum of CuNa-rho with adsorbed ethanol recorded at I 1 K.
tration of this species grows with increased temperature of evacuation until at 400 "C it is the only species present (seeFigure Id). The ESR parameters aregll = 2.314 and All = 0.0170 cm-'. Rehydration by saturation with water vapor at room temperature does not generate the species with reversed gvalues seen in Figure l a but only the species with normal gvalues which are g, = 2.075, gll= 2.361, and All = 0.0154 cm-'. A similar pattern is seen upon dehydration of both CuK-rho and CuCa-rho although in fresh CuK-rho there is very little of the species with reversed g values. Also in fresh CuCa-rho there is no species with reversed g values, but there is an additional species with ESR parameters g = 2.333 and All= 0.0182 cm-I. This species is destroyed by evacuation at room temperature. Rehydration in all instances produces the same species as seen in rehydrated CuNa-rho. Adsorption of methanol at room temperature on all these zeolite samples produces a species with ESR parameters of g, = 2.074, gll= 2.359, and All = 0.0135 cm-I. This is illustrated in Figure 2, which shows the ESR spectrum after adsorption of methanol on CuK-rho. On CuCa-rho, however, even after a lengthy equilibration period with methanol vapor, an ESR signal remains from uncomplexed copper(I1). Adsorption of ethanol produces a species with ESR parameters of g, = 2.356 and Ail = 0.0135 cm-' in all samples (see Figure 3). However, in all cases some of the copper(I1) remains uncomplexed. Adsorption of ammonia (Figure 4) produces the same species in all three zeolites with ESR parameters of g, = 2.05 1, g,, = 2.239, and All = 0.0175 cm-I. In the g, region hyperfine splitting due to nitrogen is observed with a characteristic 12.7-G
N=4 R = 0.28nm A.0.2 MHz
0
I
4
I
I
2
3
I
4
5
T, P
and simulated (- - -) three-pulse ESE modulation spectrum of CuNa-rho with adsorbed D,O,recorded at 4 K. Figure 6. experimental (-)
splitting. Between nine and eleven hyperfine components may be discerned from the spectrum. Such hyperfine splitting is indicative of direct Cu2+-N coordination. In the case of adsorption of ammonia on CuCa-rho an additional species is observed (see Figure 5), but it is difficult to measure g values for this species. Adsorption of ethylene has no effect on the ESR spectrum, even after prolonged exposure. Also the three-pulse ESE spectrum, not shown, exhibits no modulation indicating an absence of ethylene in close proximity to the Cu2+. Figure 6 shows the three-pulse ESE spectrum and associated simulation for CaNa-rho rehydrated with D20. The modulation is due to the deuterium, and the best fit to the data corresponds to interaction with four deuteriums, Le., two molecules of water, with a Cu2+-D distance of 0.28 nm. Similar spectra for adsorption
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987
Cu2+-Doped Zeolites Na-, K-, and Ca-rho
-
CuNa-rho
-F2a>
N.3, R=0.36nm, A.0.1 MHz N.3, R=0.28nm, A.0.4MHz
v,
g
t U zJ W I-
z
0 8 W V I
I
Figure 8. Crystal structure of zeolite rho showing cation positions. Site S2 is at the center of a six-ring face with site S2* displaced slightly along the triad axis into the a-cage. Sites S1' and S12 are adjacent to fourrings in the octagonal prism, and site S5 is at the center of the octagonal
prism. of C D 3 0 H and C D 3 0 D on CuNa-rho are shown in Figure 7. The best-fit simulation for C H 3 0 D is one deuterium at 0.26 nm. However, for adsorption of C D 3 0 H it is necessary to invoke a two-shell simulation model where the Cu2+interacts with three deuteriums at 0.36 nm and three deuteriums at 0.28 nm.
Discussion The structure of zeolite rho is shown in Figure 8. It consists of cy-cages, the same 26-hedra which exist in zeolite A, linked together via double eight-rings, or octagonal prisms. This yields two identical interpenetrating, three-dimensional systems of cages and prisms. Access from one cage system to the other is via the six-rings of the a-cages. In a previous study on Cu2+-dopedzeolite Cs-rho and H-rho,6 it was concluded that the Cu2+ had a very strong affinity for cation sites in the cy-cage rather than in the octagonal prisms, that is, at sites S2, S2*, or S3 as shown in Figure 8. There is no crystallographic data in the literature at present which gives the cation siting of Na, K, and Ca in these cationexchanged forms of zeolite rho. The only data in this respect pertain to Cs+-exchanged zeolite rho.I3 In all the hydrated forms of zeolites CuNa-, CuK-, and CuCa-rho, both fresh and rehydrated, there is a species with similar g values which are typically gll N 2.365, g, N 2.075, and Ail N 0.01 50 cm-'. From the ESE spectrum of CuNa-rho rehydrated with DzO shown in Figure 6, it is apparent that this species corresponds to Cuz+ coordinated to two molecules of water. (13) Parise, J . B.; Price, E. Marer. Res. Bull. 1983, 18, 841.
2929
The Cu-D distance of 0.28 nm is consistent with the water being directly coordinated to the Cu2+ through the water oxygen. A similar species was observed in both zeolites CuCsH-rho and CuH-rho6 coordinated to two water molecules and with ESR parameters g, = 2.062, gll = 2.384, and All = 0.0145 cm-'. The line shapes in the g, region are also very similar between this study and that on CuCs-rho and CuH-rho6. It would appear, therefore, that the major hydrated Cu2+ species in rho-type zeolites is independent of the type of cocation. This is very different from the case of the zeolites A, X, and Y doped with Cu2+where the type of hydrated Cu2+species is strongly dependent on the nature of the coca ti or^.^*^ In keeping with previous work: the hydrated Cuz+ species in this work is designated Cull where the Roman numeral subscript indicates the number of water ligands. The g and A parameters for this species are consistent with either square-coplanar or square-pyramidal stereochemistries2of which the former could be located at site S3 and the later a t S2*. In zeolites CuNa-rho and to a much lesser extent in CuK-rho, there is an additional Cu2+ species in the fresh sample which exhibits reversed g values. The assignment of this species was not possible by ESE as complete D 2 0 / H 2 0exchange by slurrying the zeolite in DzO proved unsuccessful. However, the g values are very similar to those reported in alkali metal cation exchanged zeolite A.2 The hydrated species in this zeolite was assigned to a trigonal-bipyramidal complex with the Cu2+ located at site S2 with axial ligands to two water molecules and equatorial ligands to three lattice oxygens in a six-ring. The possibility of forming such a species in zeolite rho exists as there is a similar site S2. In all samples adsorption of methanol produced a species with ESR parameters g, = 2.077, gll = 2.367, and A,, = 0.0136 cm-', which are different from the parameters previously reported for adsorption of methanol on CuH-rhoa6 In CuH-rho the Cu2+was found to be coordinated to two methanol molecules. Simulation of the ESE spectra for adsorption of CD30H and C H 3 0 D (Figure 7) proved not to be straightforward. For adsorption of C D 3 0 H the ESE spectrum exhibits a strong modulation at short T indicative of close deuteriums, but the modulation is also persistent to longer values of T indicative of further deuteriums. A similar problem was encountered before in CuH-rho.6 As in that work, the problem was tackled by first simulating the data from adsorption of C H 3 0 D (Figure 7b). Here, the best fit is with N = 1 and R = 0.26 nm, indicating interaction with one molecule of methanol through the hydroxyl oxygen. For adsorption of CD30H it is known from previous work that the simulation parameters for one molecule of methanol are R = 0.36 nm and A = 0.1 MHz and also we know N to be three. Using this as the first shell of deuteriums, a best fit is performed with a second shell of deuteriums. A good simulation is obtained with a second shell consisting of N = 3, R = 0.28 nm, and A = 0.4 MHz. These deuteriums are very close to the Cu2+,suggesting that the methyl end of another methanol molecule is directed toward the Cu2+. This is possible if the Cu2+ is located at site S2* of one cy-cage with this extra methanol molecule in the adjacent a-cage with the methyl protons hydrogen bonded to the oxygens of the zeolite six-ring. The one methanol molecule which is directly coordinated to the Cu2+ would project into the middle of the same a-cage containing the Cu2+cation. The reason why in CuH-rho the Cu2+ interacts directly with two methanol molecules6whereas in zeolites CuNa-rho, CuK-rho, and CuCa-rho it only interacts directly with only one methanol is most likely due to steric hindrance caused by the bulkier cations. It is not possible to make an assignment to the species formed by adsorption of ethanol using ESE spectroscopy as there is always more than one Cu2+species present. However, as with methanol, the species formed with ethanol is different in CuH-rho than in the bulkier cation forms. All cation forms of the zeolite rho form the same complex upon adsorption of ammonia. The nitrogen hyperfine splitting observed in the g, region of the spectrum (Figures 4 and 5) is indicative of direct Cuz+-N ligands. Also, the presence of between nine and eleven hyperfine lines suggests that the complex contains either three or four ammonia molecules. Verification of the number of
J. Phys. Chem. 1987, 91, 2930-2934
2930
interacting ammonia molecules was attempted by adsorbing ND, and monitoring the ESE modulation pattern. However, it was not possible to adequately simulate the data, possibly because of strong quadrupole interactions from nitrogen and/or the presence of more than one shell of strongly interacting deuteriums. All the absorbed molecules described above are rather polar. However, ethylene, which is much less polar, showed no detectable interaction with Cu2+. It was concluded by Barrer and RosenblatI4 in a sorption study of Na-, K-, and Ca-rho that the cations essentially block the double eight-ring entrances to the zeolite by occupying cation sites near S5. A polar molecule which can partially solvate these cations may subsequently gain access to the zeolite cages. A molecule such as ethylene, however, is excluded from the zeolite interior because of its poor solvating ability. Our work would seem to corroborate this conclusion. Ethylene did complex with Cuz+ in H-rho6 where there are no cations present at site S5 to restrict ethylene migration into the zeolite cages. (14) Barrer, R. M.; Rosenblat, M. A. In Proceedings ofrhe Sixth rnrernational Zeolite Conference; Bissio, A., Olson, D. H., Eds.; Butterworth Scientific: London, 1984; p 226.
Conclusions The effect of the cocation on the type of hydrated Cu2+species formed in zeolite rho is far less pronounced than in zeolites A, X, and Y . However, the presence of the cations Na, K, and Ca results in the formation of smaller complexes with methanol and ethanol than may be formed in zeolite CuH-rho. The major hydrated species in CuNa-rho, CuK-rho, and CuCa-rho is a Cu2+ species coordinated to two water molecules located either in site S2* or S3, giving a square-coplanar or square-pyramidal stereochemistry. Also, in fresh samples of CuNa-rho and to a much lesser extent in CuK-rho an unstable Cuz+ species is found consisting of direct coordination to water molecules in a trigonal-bipyramidal arrangement. Adsorption of methanol forms direct coordination of Cu2+ to one methanol molecule with the Cu2+located at site S2*, while adsorption of ammonia forms direct coordination of Cu2+ with three or four ammonia molecules.
Acknowledgment. This research was supported by the Robert A. Welch Foundation, the National Science Foundation, and the Texas Advanced Technology Research Program. Registry No. Cu, 7440-50-8; NH,, 7664-41-7; M e O H , 67-56-1; EtOH, 64-17-5.
Role of 0- Surface Radicals for Methane CH Bond Activation and Subsequent Reactions on MOO,: Molecular Orbital Theory S. P. Mehandru, Alfred B. Anderson,* Chemistry 'Department, Case Western Reserve University, Cleveland, Ohio 441 06
James F. Brazdil, and Robert K. Grasselli Department of Research and Development, The Standard Oil Company, Warrensville Heights, Ohio 441 28 (Received: October 29, 1986)
-
A molecular orbital study is made of the reaction of methane with 0- hole centers on the surface of MOO,. The following predictions are made. When the 0- is created by a UV 0 2p Mo 4d charge-transfer excitation, heterolytic products, OH- and MoV.CH3,form readily at the edges of the crystals where unsaturated molybdenum sites are present. Deexcitation to OH- and MoV1:CH3-is expected to proceed rapidly and a side reaction to slightly less stable homolytic products, OH-, 2MoV,and OCH3-, may also take place, preventing deexcitation so that MoVis not oxidized to MoV1,so that the excitation energy may be said to be chemically stored. The CH bond activation barrier is calculated to be 0.7 eV. Activation is a consequence of a stabilizing 3-centered CH-0- cr-donation interaction, the antibonding counterpart of which takes the hole and is occupied by only one electron in the transition state. If a second electron-hole pair is formed, the methyl radical can shift to the 0- center with a slight loss in stability, and the surface is reduced by two electrons. The methyl cation which is formed can be viewed as a methyl radical which has promoted an electron to a nearby MeV'. Once the methyl radical moves to a basal plane of the crystal, which is covered entirely by oxygen anions, it diffuses with a low -0.4-eV barrier. Methyl radicals thus formed on the basal plane by either route can combine, yielding ethane and two MoVcenters or, in the presence of additional UV-created 0- centers, can transfer a hydrogen atom to them to form formaldehyde and two MoV centers. Homolytic adsorption at electron-hole pair sites on the basal planes is possible, and the surface is reduced by two electrons. These methyl radicals can undergo the above reactions. When 0-sites are present as a result of cation vacancy nonstoichiometry, dissociative methane chemisorption is activated as described above and heterolytic (OH- and Mo"CH3) and homolytic (OH-, MoV,and OCH,-) products are comparable in stability. Mobile methyl radicals can combine to form ethane and two MoV. Alternatively, a methyl radical can be trapped at a second 0- site, yielding immobile methoxy coordinated to MoV1. The methoxy can be activated by adjacent 0- to form formaldehyde and MoV.
Introduction The activation of C H bonds in methane for its selective oxidation to methanol and formaldehydre on supported oxide catalysts is a subject of current interest. Kazansky andco-workers' have reported that methane and ethane react with 0- hole centers generated by y-irradiation of V5+and P5+ions deposited on silica gel and also by UV irradiation of V5+/SiOz and Ti02. Lunsford and c o - w o r k e r ~have ~ * ~ recently found that molybdenum supported on silica is a catalyst for the selective oxidation of methane, with *Address inquiries to this author.
0022-3654/87/2091-2930$01.50/0
nitrous oxide as the oxidant, to methanol and formaldehyde in the presence of water. It has been shown by using EPR spectroscopy that the 0-anion radicals are formed by surface decomposition of NzO on a reduced c a t a l y ~ tand ~ ? ~are highly reactive (1) Kaliaguine, S. L.; Shelimov, B. N.; Kazansky, V. B. J . Catal. 1978, 55, 384. (2) Liu, R.-S.; Iwamoto, M.; Lunsford, J. H. J . Chem. SOC.,Chem. Commun. 1982, 78.
(3) Liu, H.-F.; Liu, R.-S.; Liew, K. Y.; Johnson, R. E.; Lunsford, J. H. J. Am. Chem.Soc. 1984, 106, 4117. (4) Shvets, V. A,; Kazansky, V. B. J. Cutal. 1972, 25, 123. ( 5 ) Taarit, Y. B.; Lunsford, J. H. Chem. Phys. Letr. 1973, 19, 348.
0 1987 American Chemical Society
