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; MeOH, 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 de~ ? ~are highly reactive composition of NzO on a reduced c a t a l y ~ tand (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
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2931
Reaction of CH, on M o o 3 Surface
cez%G@
tu
CH-0 u*
I? I' I ' 013
0'4 - M c d 411 jl0,-O7 0-Mo-S l1
l 2
B
B
Top view
Side view Figure 1. Model of (100) edge of a layer of MOO,. O,,Ole, and Old are
CH-0
@Km=B Figure 2. Schematic representation of the closed-shell repulsive interaction between a CH a-orbital and 02-.
basal plane (010) atoms.
even at 77 K.5,6 It has been postulated that the 0- ions initiate the catalytic process by abstracting a hydrogen atom from methane,'-2*6forming OH- groups and methyl radicals. The formation of methyl radicals has recently been confirmed by Lunsford and co-workers by passing methane gas over MgO at approximately 500 OC, trapping the methyl radicals formed on the surface of MgO in a solid argon matrix, and analyzing them with EPR spectroscopy.' The methyl radicals thus formed on the surface are assumed to react with the catalyst forming methoxide ions, which on subsequent reaction with water, yield methanol. Formation of formaldehyde would require further dehydrogenation of the methoxide ion or the oxidation of methanol. Except for a recent paper from this laboratory? which explains the photon-assisted C H bond activation in methoxy coordinated in a molybdenum oxyanion,8b we are not aware of any other theoretical effort to understand CH bond activation by 0- on oxide surfaces. The focus of the present study is to gain an understanding of the role of 0- ions toward C H bond activation of methane on a molybdenum trioxide surface. We use atom superposition and electron delocalization molecular orbital (ASED-MO) theory9 for all our calculations in this paper. In this theory the electronic charge density of a molecule is partitioned into free-atom parts and an electron delocalization bond formation component. As the atoms bond together forming a molecule, the electrostatic forces on the nuclei are integrated to yield a repulsive energy due to rigid-atom densities and an attractive energy due to electron delocalization. The sum is the exact molecular binding energy. In the ASED-MO method the atom superposition energy is calculated from actual atomic densities and the electron delocalization energy is approximated by a one-electron molecular orbital energy obtained by using a Hamiltonian which has some features of the extended Huckel Hamiltonian. Numerous studies of molecular and surface structures and reactions have been published using the ASED-MO technique. Some of the recent studies using transition metals and metal oxides include the photoactivation of C H bonds in a coordinated methoxy on MOO3$ the electronic properties of crystalline Bi,03 and MoO3,Io ahydrogen abstraction of propylene on a Pt(1 l l ) surface," the structures of defect clusters in wustite,I2 the selective oxidation of propylene to acrolein on a-bismuth molybdate,I3 and the activation of methane and propylene a C H bonds by Bi203in oxidized and reduced forms.14 The atomic parameters for Mo, 0, ( 6 ) Lipatkina, N. I.; Shvets, V. A,; Kazansky, V. B. Kinet. Katal. 1978, 19,919 and references therein. (7) Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H. J. Am. Chem. Sor. 1985, 107, 58. (8) (a) Anderson, A. B.; Ray, N. K. J. Am. Chem. SOC.1985,107,253. Ib) McCarron, E. M., 111; Harlow, R. L. I. J. Am. Chem. Sor. 1983, 205, 6179. (9) a. Anderson, A. B. J. Chem. Phys. 1974, 60, 2477. b. Anderson, A. B. J . Chem. Phys. 1975, 62, 1187. (IO) Anderson, A. B.; Kim, Y.; Ewing, D.W.; Grasselli, R. K.; Tenhover, M.Surf.Sei. 1983, 134, 237. (11) Anderson, A. B.; Kang. D. B.; Kim, Y. J . Am. Chem. Soc. 1984,106, 6591. ..
Figure 3. Transition state for H transfer from methane to a basal plane 0- a t an edge site.
-101
I Figure 4, Electronic structure a t the transition state for H transfer from methane to a basal plane 0- at an edge site. The second column of levels shows how a u-orbital is destabilized on stretching to the transition state in the absence of the surface cluster.
C, and H are the same as used earlier10s13except that the Slater orbital exponents for the oxygen anions on which the adsorption of H and CH3 is considered are increased 0.2 au from the values for the other oxygen anions to prevent the calculations from overestimating H-0 and C-0 bond lengths. For equilibrium structures, the bond lengths are optimized by the variational theorem to the nearest 0.01 8, and bond angles to the nearest degree for all reported results. For transition-state structures the C H and O H bond lengths are optimized to the nearest 0.05 A and the HOMOangle to the nearest 5'. Our calculations typically overestimate C H bond lengths by about 0.10 8,. For simplicity, we do not relax the positions of any atoms in the surface cluster model. Results Bulk M o o 3 has a layered structure with basal planes covered with 02-anions. Mo6+ cations have distorted octahedral coordination with the six MOO bond lengths ranging from 1.84 to 2.34 A.I5 We have employed a bulk superimposable Mo3011bcluster model (see Figure l), which models the (100) surface of M o o 3 which is formed by cleavage perpendicular to the layers, and presents unsaturated MoV1sites. The four electrons assigned to the cluster, making it formally bear a charge of -4, serve to make all oxygen anions in the -2 oxidation state. The -4 charge does not affect the calculation of the Hamiltonian matrix elements. W h e n one hydrogen atom of the CHI molecule is brought near the surface 0'- and the C H bond is stretched, the resulting closed-shell repulsion (see Figure 2) is expected to lead to a high bond scission barrier. A high activation barrier is also present for the same reason in the case of methoxy coordinated in a Mo"'
(12) Anderson, A. B.; Grimes, R. W.; Heuer, A. H. J . Solid State Chem. 1984. 53. 353. ~-
(14) Mehandru, S. P.;Anderson, A. B.; Brazdil, J. F. J . Chem. Sor., Faraday Trans. 1, 1987, 83, 463. (15) Wyckoff, R. W. G. CrystalStrucrures, 2nd ed.; Wiley: New York,
J. D.; Brazdil, J. F. J. Caral. 1985, 96, 222.
1964; Vol 11.
--. (13) Anderson, A. B.; Ewing, D. W.; Kim, Y.;Grasselli, R. K.; Burrington, 1
2932 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987
Mehandru et al. TABLE I: Calculated Results for the Structures' of the HomolyticaUy Adsorbed H and CH3 Fragments on the Cluster Model, Their Stability Relative to the Gas-Wnse CH,, and the Chargesb no hole in the 1 hole in the 2 holes in the 0 2p band 0 2p band 0 2p band
Figure 5. Calculated structures for H' and CH,' adsorbed homolytically at basal plane 02-edge sites.
1.01 1.51
Ron Roc RCH
oxyanion and is responsible for its stability toward formaldehyde and water formation at temperatures below 500 0C.8 We can study the effect of the UV-induced 0 2p Mo 4d charge-transfer excitation occupying the cluster orbitals so that there is an electron at the bottom of the Mo 4d band and a hole at the top of the 0 2p band. When this is done, we find that the transition state is reached when the bond is stretched only by 0.05 A from its equilibrium value. The calculated activation barrier is 0.7 eV. This prediction of a low C H bond scission barrier is similar to that of the earlier theoretical studys8 which explained the observation of formaldehyde formation in the oxyanion case, which occurred at room temperature in the presence of W The structure of the transition state with an electron-hole pair is shown in Figure 3. Figure 4 shows the orbital correlation of CH, levels at the transition state. It is evident that the electron-hole is transferred to the antibonding CH-0 level as soon as it reaches the top of the 0 2p band, and this results in bonding between H and 0 in the transition state. The CH-O antibonding orbital does not correlate with the high-lying OH a*-orbital because of an avoided crossing due to mixing with the C H u*-orbital. It may be noted that the MoV does not affect the energy or structure of the transition state because the bonding is between hydrogen and oxygen. Therefore the above results also apply to methane activation on the defective oxide with 0- present on the surface. This explains the results of Lunsford et a1.293v5and Kazansky et aL6 for 0- on Moo3 surfaces and almost certainly explains the results of Kazansky and c o - ~ o r k e r s ' 'similar ~ ~ ~ ~ observations for Ti4+, V5+, and P5+oxide surfaces. As the C H bond is broken, the OH- group and CH,' radical are formed:
-
MoVO- + CH4(g)
-
MoVOH-
+ CH,'(g)
(1)
LMoOC LOCH stability H charge CH3 charge
1.01
1.01
1.58
105
1.58 1.21 180 104
0.30
0.30 0.30
0.30
0.35
0.39
0.42
1.21
1.22
130 106 -1.76
140
2.42
"See Figure 5 . Bond lengths are in angstroms, angles in degrees, and the energies in electronvolts.
Figure 6. Electronic structure for homolytic adsorption with structure given in Figure 5. The formation of holes in the 0 2p band results in the elimination of electrons from the Mo 4d band, as discussed in the text.
1.21
H
L
Two CH3'(g) radicals could combine and form ethane: 2CH3'(g)
-
C2H6(g)
(2)
Small amounts of ethane are always observed to form when methane reacts with the 0- hole centers.' A likely pathway is one where the methyl radicals react with the surface MOV1,02-, or possibly 0- ions. If the reaction takes place with the oxygen ions, the H and CH3 fragments are homolytically adsorbed, and if the reaction takes place with the MoV1and oxygen ions, the fragments are heterolytically adsorbed on the surface. We have calculated the structures and stabilities for the four possibilities using the cluster model of Figure 1. The calculations are performed by first taking the stoichiometric cluster with molybdenum and oxygen in their respective +6 and -2 oxidation states so that the 0 2p band is completely filled and the Mo 4d band is empty (that is, no hole in the 0 2p band). The H and CH3 fragments are bonded to two 02-ions of the cluster for the homolytic absorption: H on 02-and CH3 on MoV1,and H on MoV' and CH3 on 02-for the heterolytic adsorption. Homolytic adsorption on MoV1is relatively unstable and is not considered further. The calculations were repeated with one and then two electrons (corresponding to one and two holes, respectively, in the 0 2p band) removed from the cluster. The cluster with holes in the 0 2p band simulates cation vacancies or 0- ions on the surface. Figure 5 shows the structure of the homolytic products with no holes in the 0 2p band. The calculated results in the absence and in the presence of holes are given in Table I. It may be seen that the stability of the homolytically adsorbed fragments is 1.8 eV less than the cluster plus gas-phase methane. However, the presence of holes in the 0 2p band increases the stability of the fragments relative to CH4 (0.3 eV more stable with one hole and 2.4 eV more stable with two holes). The cause of this increased
Figure 7. Calculated structures for heterolytic products of methane adsorption on basal plane'0 edge sites and MoV' edge sites.
stability can be understood by focusing on the energy-level correlation diagram of Figure 6 . In the absence of the two holes the H 1s orbital is stabilized by the 0 2p band by forming an OH u bond, and the CH3 radical porbital is also stabilized by the 0 2p band by forming an OC u bond. The antibonding counterparts for both these u bonds lie above the empty MO 4d band. Thus the two electrons are transferred to the lowest Mo 4d level. This indicates the bonding of H+ and CH3+species to the two surface 02-ions forming OH- and O C H r groups and reducing the metal at the same time. Because of the gap between the 0 2p and the Mo 4d bands, the reduction of the metal is a high-energy process which affects the C H bond scission barrier and the stability of the homolytic products. This means that the homolytic products should not form on a stoichiometric defect-free surface in the absence of UV excited electron-hole pairs. However, with the holes in the 0 2p band, the two electrons are not destabilized by promotion to the Mo 4d band but instead reduce the two low-lying holes. The presence of 0-on the surface is therefore necessary for the homolytic formation of hydroxy and methoxy groups on a M o o 3 surface. The optimized structures for the heterolytic products on a stoichiometric cluster are shown in Figure 7. The calculated results with and without a hole in the 0 2p band are given in Table 11. In case I, where H is on the 02and CH, is on the MoV' ion, the fragments are slightly more stable than the gas-phase CHI
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2933
Reaction of CH4 on MOO, Surface
TABLE II: Calculated Results for the Structures"of the Heterdytically Adsorbed H and CH, Fragments on the Cluster Model, Their Stability Relative to the Cas-Phase CH* and the Chargesb H on MeV', C H 3 on 021 hole in the no hole in the
H on 02-,C H 3 on MoV1 1 hole in the no hole in the 0 2p band 0 2p band
Ron RMoC Rcn LMoCH
stability H charge CH3 charge
1.01 2.12 1.21 109 0.12 0.30 -0.33
1.01 2.26 1.20 100 0.64 0.30 0.24
RM~H Roc Rcn LOCH stability H charge CH, charge
0 2p band
0 2p band
1.70 1.51 1.21 106 -0.06 -0.34 0.39
1.69 1.54 1.22 105 -0.04 -0.33 0.40
"See. Figure 7. bBond lengths are in angstroms, angles in degrees, and energies in electronvolts.
Figure 8. Electronic structures for the two types of heterolyticmethane
adsorption in the structures of Figure 7. even for the stoichiometric cluster. This stability increases in the presence of a hole in the 0 2p band. In case 11, where H is on the MoV1and CH3 is on the 02-ion, the fragments are always a little less stable than the gas-phase CH4, irrespective of the presence or absence of a hole in the 0 2p band. The energy-level correlation diagram for the heterolytically adsorbed fragments when there is no hole in the 0 2p band is shown in Figure 8. Case I is shown on the left side of this figure and case I1 on the right side. The H 1s orbital is stabilized by the 0 2p band forming a OH orbital which lies about 1 eV below the 0 2p band. The CH, radical orbital is stabilized by the Mo 4d band and gives a MoC orbital which lies in the band gap region. The OH a*- and MoC a*-orbitals are high-lying and empty. The result is that one electron is transferred from H to CH3 so that in a formal sense H+is bonded to 02-and CH3- is bonded to Mow. The calculated charges are 0.3 on H and -0.3 on CH,. When there is one hole in the 0 2p band, it is transferred to the highest occupied MoC a-level, which corresponds to a neutral CH3 radical bonded to MoV1. The calculated charge on CH3 is 0.2. In case 11, MoH a- and O C a-orbitals lie near the bottom of the 0 2p band, and their antibonding counterparts are empty. This corresponds to H- bonded to MoV1and CH3+ bonded to 02-.The calculated charges are -0.3 on H and 0.4 on CH3. The Occurrence of a hole in the 0 2p band does not affect the stability of the fragments because neither a-bonding level lies in the band gap region. On comparing the stabilities of the fragments in Tables I and 11, one can conclude that on a stoichiometric cluster the heterolytically adsorbed fragments with H on 02-and CH3 on MoV1 are favored. With one surface 0-present the formation of homolytic products is possible and with two adjacent 0- on the surface homolytic adsorption is favored. Hydroxide and methoxide groups have indeed been seen by I R on reduced Moo3 treated with N 2 0 to form surface O-., In the case of homolytic methane adsorption the methyl radicals bond weakly with 02-and the unpaired electron is promoted to the Mo 4d band. In order to study their surface mobility, we have
calculated the energy barriers for moving a methyl radical from one 02-to the neighboring one. The saddle point of the energy surface in each case is assumed to be located at the middle point of the two 0" ions between which the motion takes place. The calculated barrier to the mobility from O4to 0 1 2 ,Le., from one edge-oxygen anion to another (see Figure 1 for the numbering of the oxygen atoms), is only 0.01 eV, and that from edge 0, to basal plane Ol0 is 0.4 eV. The last case represents the barrier to the mobility of the CH3radical on the basal plane which consists of a layer of oxygen ions alone. In this case, since the distance between the 02-ions is larger compared to the distance on the (100) face of MOO,, the methyl radicals are essentially desorbed while moving to the adjacent 02-ion. As the methyl radical diffuses away from the edge of the basal plane the electron must move from the surface state dangling bond orbital to a bulk molybdenum band state. By saturating the three molybdenum surface state dangling bond orbitals with oxygen anions, it is found that the bottom of the Mo 4d conduction band lies 0.32 eV above the surface states, and so there is an additional 0.3 eV barrier to methyl radical diffusion away from the edge sites on the basal planes. It is interesting that our Mo 4d defect states lie only 0.32 eV below the bottom of the bulk 4d band. Photoemission work suggests they are lower in the band gap. Two peaks 0.9 and 2.0 eV above the 0 2p valence band16 may owe these positions in part to restructuring ( M e 0 bonds will stretch some because the Mo 4d band orbitals are antibonding to 0 2p band orbitals and other restructuring is possible) but it is probable that final state relaxations increase the ionization potential from what is suggested by our initial state energy-level calculations. Although the calculated barriers are approximate, they indicate that the methyl radicals can move freely on the surface of MOO,, especially at the high temperatures (-550-750 "C) usually employed for studying methane ~ x i d a t i o n . ~ Discussion and Conclusions
Our results provide a theoretical framework for understanding the dissociative chemisorption of methane on MOO, surfaces in the presence of 0-. First of all, 0- strongly activates C H bonds. It accomplishes this by providing a low-lying state for taking one of the electrons from the 3-centered C H - 0 antibonding counterpart to the C H a orbital which is donating to oxygen. The resulting activation energy is calculated to be 0.7 eV. The barrier is low whether the 0- are created by UV 0 2p Mo 4d charge-transfer excitation or whether they are present due to the nonstoichiometry resulting from cation vacancies. When the 0- are created by electron-hole pair excitation, heterolytic adsorption is most likely at edge sites and most of the excitation energy is lost by deexcitation of the MoV electron to the carbon orbital, forming CH3- which forms a donation bond to MoV1. If a homolytic pathway is followed at an edge site, the excitation energy is stored as Mov, but such products are less stable and the system will probably revert to the heterolytic structure. Some methyl radicals may escape to the basal planes and undergo subsequent ethane or, in the presence of additional electron-hole pair excitations, formaldehyde formation reactions, as in ref 8,
-
(16) Firment, L. E.; Ferretti, A. SurJ Sci. 1983, 129, 155.
2934
J . Phys. Chem. 1987, 91, 2934-2938
with a two-electron reduction of the surface in either case. The heterolytic species may be photoexcited to the homolytic mode and undergo the same subsequent reactions. The barrier to methyl radical diffusion over 02-sites is approximately the same as the desorption energy, -0.4 eV. Homolytic adsorption at 0-sites on the basal planes will take place with the electron-hole pair excitation energy stored as MoV and the same catalyst reducing reactions forming ethane and formaldehyde will occur. Unless H2C0 is desorbed, additional electron-hole pair excitations should lead to further dehydrogenation and surface reduction with the formation of CO and C 0 2 as seen in ref 1. When 0-is present at the surface as a result of cation vacancies, homolytic and heterolytic products are comparable in stability at the edge sites, with the latter slightly favored. Homolytic adsorption will also take place at 0-sites on the basal planes. The methyl radical will be mobile over the 02-basal plane sites but when it comes to an 0-it will bind as methoxy, being trapped by -2.5 eV, the electron-hole pair recombination energy. In the
presence of an adjacent 0-, activated hydrogen loss from methoxy as in ref 8 is possible, resulting in the formation of formaldehyde, OH-, and MoV. Additional 0-sites should activate the C H bonds in formaldehyde to yield more highly oxidized products. These conclusions may be expected to apply to other systems such as the V5+, P5+,and Ti4+supported oxide catalysts studied by Kazansky and co-workers.’ The presence of 0-due to nonstoichiometry or UV charge-transfer excitation resulted in the reaction of methane to form various amounts of ethane, formaldehyde, carbon monoxide, and carbon dioxide. The variations in products for these catalysts and Moo3 suggest their is much to learn about the effects of surface structure on the various reactions. Reference 1 states “It will be impossible to understand the photocatalytic reactions of paraffins, without a thorough study of their specific interactions with hole centers of the 0-type”. We feel that the electronic aspects are now clear from the perspective of molecular orbital theory and that future characterization using surface science techniques will help clarify the structural effects.
Photochemical Methods for Characterlzing the Nature of Polymer Aggrdgates in Aqueous Solutions and on a Silica Surface Ping-Lin Kuo, Mas” Okamoto,+and Nicholas J. Turro* Chemistry Department, Columbia University, New York, New York 10027 (Received: November 4, 1986) The nature and structure of the aggregates formed by a water-soluble poly(ethy1erie oxide-propylene oxide-ethylene oxide) block copolymer adsorbed on silica particles have been investigated by photoluminescence probe methods. The micropolarity and the aggregation number of the polymer aggregates adsorbed on silica particles were determined by fluorescence methods using pyrene as a probe and were found to be significantly smaller than those of the aggregates in aqueous solution. The aggregates adsorbed on silica have a higher solubilization ability and a higher ability of protecting pyrene from quenching by Cu2+. The decay curves of pyrene in the aggregates on silica are similar to those observed in surfactant micelles. These results suggest that, relative to the solution phase, the polymer aggregates on the silica surface are smaller and more compact and possess properties similar to surfactant micelles. The entrance and exit rates for Cu2+ in the polymer aggregates on silica and in the solution phase are determined from the decay of pyrene fluorescence in the presence of Cu2+. The values of these kinetic parameters are compared to those of pyrene in SDS micelles and are interpreted in terms of the size and water content of the polymer aggregates in two phases and in terms of the interaction between Cu2+and the2ilica surface.
Introduction The poly(ethy1ene oxide-propylene oxide-ethylene oxide) block copolymers (EPE)are composed of hydrophobic poly(propy1ene oxide) (PPO)and hydrophilic poly(ethy1ene oxide) (PEO) segments. Ordinary hydrophilic (water-soluble) polymers are extended by water and assume structures possessing random-coil chains. An increase in polymer concentration increases the mutual interaction of polymer chains. Due to the different solubilities in water of the PPO and PEO segments, the EPE block copolymer forms polymer aggregates with internal PPO segments surrounded by PEO segments.’ Upon a further increase of the concentration of the polymer, the structure of thee aggregates changes from monomolecular aggregates to polymolecular aggregates.2 Thus, the structure and the properties of EPE polymer aggregates in aqueous solutions changes as a function of polymer concentration. Both polyoxyethylenated nonionic surfactants and PEO are adsorbed on negatively charged silica as a result of hydrogen bonding between the -SOH groups of the silica surface and the oxygen atoms of the oxyethylene group.3 A similar interaction between EPE polymer and silica is expected, but the resulting structure and properties of EPE polymers adsorbed on silica may be quite different from those in the solution phase. Various methods have been employed to measure the thickness of the adsorption layer^,^ but the size of the aggregate formed by an adsorbed surfactant on a solid surface cannot be readily Present address: Technical College, Kyoto Institute of Technology, Matsugasaki, Sakyo-Ku, Kyoto 606, Japan.
0022-3654/87/2091-2934$01.50/0
SCHEME I: Sketch of the Aggregates of EPE Block Copolymer on the Silica Surface and in Solution Phase IlGCRECIllES IN SOLUllON PlllEE d(LAICE RSIKSRTION W M D E R I
/RGCYGllESON S l l l C l t S M L L RGERECArlON NUMDER)
Silica determined by conventional methods such as light scattering. On the other hand, photochemical methods have been widely employed to study the structures and the properties of surfactant and/or polymer aggregates in aqueous solution containing surfactant and (1) (a) Sandron, C. Angew. Chem., Inr. Ed. Engl. 1963.2.248. (b) Tuzar, Z.; Kratochvil, P. Adv. Colloid Interface Sci. 1976, 6,201. (c) Ikemi, M.; Odagiri, N.; Tanaka, S.; Shinohara, I.; Chiba, A. Macromolecules 1982, IS, 281.
(2) Prasad, K. N.; Luong, R. T.; Fluorence, A. T.; Paris, J.; Vaution, C.; Puisieux, F. J. Colloid Interface Sci. 1979, 69,225. (3) Rupprecht, H.; Liebl, H. Kofloid. Z. Z . Polymn. 1982, 250, 719. (4) (a) Garvey, M. J.; Tadros, Th. F.; Vincent, 9. J. J. Colloid Interface Sci. 1976, 55,440. (b) Schentjens, J. N. H. M.; Fleer, G. J. J. Phys. Chem. 1979, 83, 1619.
0 1987 American Chemical Society
