J. Phys. Chem. 1988, 92, 1541-1547 *
* * *
* * ** * * *
* **’*
*
*
*
* e
*
*
I
a
U 8
*
*
*a
* *
*
t I
**
*
am
:*
u
*
*
*
* * * +
I
8
* *
* * *
* 8
I
a
*
l
*8 * t
*
8
I
*
*
*
** a
*
* 8
**
8
C
*
a
8
* *
* * * * W
m
*
R
** a
@
a*
*
*
=*
-
+
-
8
*
Summary New anomalies have been observed for steady-state kinetics on fractals and on disjoint islands. For the A + B 0 (A B AB?) reaction, a random location, steady-source, landing of A and B (NA= NB)leads to segregated steady-state distributions. Quantitatively, however, the large anomalies already observed for the A + A 0 reaction (where no segregation is possible) appear to dominate the reaction order. The size of the islands appears to be a very important factor for both the reaction order and the segregation phenomena. However, the shape of the islands seems to be of less importance. Direction bias (electric field) effects are very significant for the connected lattice (percolating cluster): there is a striking increase in reactant segregation as well as in steady-state densities. Acknowledgment. This work was supported by N S F Grant DMR-8303919 and NSF Special Computer Allocation at the San Diego Supercomputer Center.
-
8 .
*
1541
Figure 7. Same as Figure 5 , except with an external electric field (direction bias), after lo5 steps. Note lower density, compared to Figure 6. Q - 1.
ramifications for charge separation and recombination in specialized biological systemsz7 is especially intriguing.
(20) Clement, E.; Li, L.; Kopelman, R.,unpublished results. (21) Kopelman, R.;Lindenberg, K.; West, B., unpublished results. (22) Stauffer, D. Introduction to Percolation Theory;Taylor and Francis: London, 1985. (23) Kopelman, R. Science, in press. (24) Anacker, L. W.; Kopelman, R.J. Phys. Chem. 1987, 91, 5555. (25) Prasad, J.; Kopelman, R.J. Phys. Chem. 1987, 92,265. (26) Kuzovkov, V.; Kotomin, E. Czech. J. Phys., Sect. B 1985.35, 541. (27) Fauman, E.; Kopelman, R.Comments Mol. Cell. Biophys., in press.
Electronic Structure and Orientation of Dioxygen Species on the Surface of CoO-MgO Solid Solutions Zbigniew Sojka,*+Elio Giamello,*Michel Che, Laboratoire de Rgactivitg de Surface et Structure UA 1 1 06 CNRS, UniversitZ P. et M . Curie, 75252 Paris Cedex 05. France
Adriano Zecchina, Istituto di Chimica Fisica, Facolta di Scienze, M.F.N., Universita di Torino, Corso M. d’Azeglio 48, 10125 Torino, Italy
and Krystyna Dyrek Department of Chemistry, Jagiellonian University, Karasia 3, 30- 060 Cracow. Poland (Received: May 14, 1986)
The adsorption of oxygen on the surface of COO-MgO solid solutions was studied by EPR spectroscopy using 170-enriched oxygen. The spin Hamiltonian parameters, geometry, and electronic structure of adsorbed oxygen were discussed. The comparative analysis of the experimental and calculated hf structure evidenced two main forms of adsorbed dioxygen: one with nonequivalent nuclei (end-on bent structure), bonded reversibly at room temperature to surface cobalt ion, and another with equivalent nuclei (side-on structure) stabilized electrostatically and irreversibly on Mg2+at room temperature. The contribution of electron delocalization and spin polarization to the hf structure was discussed. Significant electron transfer indicates that the C0-02 system is best described as a Co3+-02- adduct in the case of CoO-MgO solid solutions. Introduction In recent years, the transition metal-dioxygen complexes have received a great deal of attention. Cobalt-oxygen complexes have been particularly useful in formulating some of the basic principles of metal-dioxygen chemistry. Several articles’-5 and reviewsb* have appeared in the literature concerning the fixation of dioxygen by cobalt complexes, and it was shown that it is possible to extract valuable structural information from EPR spectra.’” In contrast to the abundant EPR data on oxygen activation by homogeneous cobalt complexes, very little is known about the structure of dioxygen adducts with cobalt dispersed in solid oxide mat rice^.^ On leave of absence from the Department of Chemistry, Jagiellonian University, Cracow, Poland. *On leave of absence from the Istituto di Chimica Generale ed Inorganica, Facolta di Farmacia, Universita di Torino, Torino, Italy.
0022-3654/88/2092-1541$01.50/0
TABLE I Experimental Spin Hamiltonian Parameters of Co3+-O
I+ 62 a"
+ b'*
(X2b:
62)]''z
where a' and b' are the M O coefficients of a2in eq 3, X is the spin-orbit coupling constant of oxygen, and I represents a correction to the angular momentum. The values of 6 derived for the three species lie around 0.1 eV. The character of cobaltdioxygen bonding depends on the donor-acceptor behavior of all the orbitals contributing to both ~ ( 0and ~ 7r(a2) ) MO's, which are directly responsible for the extent of electron transfer. The EPR results, because of the unknown value of Ucdpo, may be related only to the coefficients of the a2orbital containing the unpaired electron and 6' = (1 - u ' ~ ) ' /were ~ already estimated from the analysis of the spectra. However, some indications concerning the a and b coefficients of a1may also be obtained. (21) Ruzic, I. M.; Smith, T. D. J . Chem. SOC.,Dalton Trans. 1982, 373. (22) Raynor, J. B. Inorg. Nucl. Chem. Lett. 1974, 10, 867. (23) Hunter, T. F.; Symons, M. C. R. J . Chem. SOC.A 1967, 1770. (24) Kinzig, W.; Cohen, M. H. Phys. Reo. Left. 1959, 3, 509.
The Journal of Physical Chemistry, Vol. 92, No. 6, 1988 1545 Neglecting the small contribution of d, to al, the a and b values are related to the corresponding energy levels by the following formula:25
bz =
(
Ed,2 - E+, )az ET*,( - E+,
(9)
The small value of 6 and the large one of b' indicate that both a1and a2orbitals are close in energy to the ~ * ( 2 p orbitals ) of oxygen (Figure 4b). Thus, for the energy difference Ex*;- E*, in the denominator of eq 9, the inequality EA*^ -E*, < 6 holds. The small 6 value implies b2 being much greater than a2;Le., a significant E T takes place. Such an electronic structure of the Co3+-0; species requires strong axial distortion of surface Co2+ ions, which are involved in oxygen linking (Figure 4b). This is in good accordance with recent results of Indovina et a1.,26who report that Ni3+ ions (3d7) isoelectronic to Co2+ions incorporated into MgO undergo strong axial distortion at the surface of solid solutions. This phenomenon may be connected to the anisotropy of the Madelung energy at the surface. The coordination number of the axial and equatorial Oz- ligands of cobalt ions located at the surface are different. This involves different values of the corresponding Madelung energies,27giving rise to a strong distortion of the surface Co2+ environment. The correlation between the extent of ET and distortion, Le., d,z - (d,,, d,,,) separation, was also confirmed by Ochiai in the case of homogeneous complexes.z8 But in contrast to homogeneous cobalt-superoxo complexes, the ET for Coo-MgO ss is almost always complete due to the additional influence of cobalt energy levels which are split by the anisotropy of the surface Madelung potential. Moreover, the increased charge of cobalt ion may be readily stabilized by a negative 02-ion surrounding (vide infra). The superoxide formulation of the cobalt-dioxygen adduct finds strong support from the transfer of oxygen from cobalt to the MgO matrix. The heterolytic decomposition of the Co-0,- adduct, confirmed by the 170hf pattern of the spectrum in Figure 3, and the calculated spin density on the 7r* M O of Mg-Oz-, equal to 17&/170B = 1-50 G/-52 GI = 0.96 (where I7OT is the experimental anisotropic part of the hf I7O tensor and 170B refers to the corresponding theoretical value9), demonstrate that dioxygen is transferred as the negatively charged superoxide ion. This indicates a heteropolar character of the cobalt-oxygen bondingz9where the oxygen molecule keeps the larger part of the bonding electrons. Such a negatively charged dioxygen may be very sensitive to interaction with the highly ionic MgO matrix (especially with Mg2+ cations13), resulting in its isolation on the matrix. Moreover, the negative 02-ligands on cobalt may easily stabilize the higher (3+) oxidation degree of cobalt as already proposed by Ochiai for C O ~ + ( C N - ) ~ - O TThese . ~ ~ facts are thus in accordance with the large effective E T which takes place from cobalt to oxygen upon adduct formation. This result would be very difficult to understand solely on the basis of the spin-pairing model. Moreover, accurate a b initio calculations recently performed by Newton and HalP1 indicate that the Co-0, system should be described as a superoxide 0, forming a dative u bond by electron donation to an empty Co3+ orbital. A comparison between the magnetic parameters of the dioxygen adducts in homogeneous systems and at the surface of COO-MgO ss is given elsewhere.32 (25) Gokbiewski, A. Introduction to Quantum Mechanics and Chemistry; PWN: Warsaw. 1982: u 109. (26) Indovina, V.; Cordischi, D.; Febbraro, S.; Occhiuzzi, M. J. Chem. SOC.,Faraday Trans. I 1985, 81, 37. (27) Dyrek, K.; Sojka, Z. In Adsorption and Catalysis on Oxide Surfaces; Che, M., Bond, G. C., Eds.; Elsevier: Amsterdam, 1985; p 195. (28) Ochiai, E. Inorg. Nucl. Chem. 1973, 35, 1727. (29) Fleming, I. Frontier Orbitals and Organic Chemical Reactions; Wiley: New York, 1978. (30) Ochiai, E. Inorg. Nucl. Chem. 1973, 35, 3375. (31) Newton, J. E.; Hall, M. B. Inorg. Chem. 1984, 23, 4627. (32) Che, M.; Dyrek, K.; Giamello, E.; Sojka, 2. 2.Phys. Chem. Neue Folge 1987, 152, 139.
1546 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
-i/i
+1/2
-1/2
+7/2
-7/2
0
+7/2
-7/2
-2
1
+7/2
-7/2
*7/2
-3
"U2 c o - i%ji
+7/2
- L
Figure 5. Stick diagram of the 59C0and "0 hfs for dioxygen adsorbed in the form of superoxide on Coo-MgO ss with two equivalent oxygen nuclei
Sojka et al. oxygen and the cobalt donor center and (ii) another with equivalent nuclei (species of type IV), equidistant from the Mg2+ surface cation. The nature of the hfs was discussed. It is shown that the cobalt hf structure of species I, 11, and 111arises mainly from the unpaired electron delocalization from the M O 7r* aIII> aIV 0. On the other hand, the decrease of a is related to an increase of ionicity of the oxygen-CoO-MgO surface bond. Moreover, as shown unambiguously by Bo&: there is a good correlation between the Co-0 and 0-0 bond length and the angle a. The lower the value of a,the shorter the Cc-0 distance and the longer the 0-0 bond length. These results indicate that the stability of dioxygen on the surface of CoO-MgO ss is achieved by decreasing the value LY (gradual change of orientation of dioxygen species from nonequivalent to equivalent oxygen nuclei) and shortening the Co-0 bond. This is accompanied by an increase of ionicity, leading to the further activation of oxygen as indicated by the lengthening of the internuclear distance of 02.This latter condition is fulfilled by increasing the electron density on dioxygen until almost complete ET (0.96) and parallel orientation (a= 0) will be obtained (species IV). The whole process of dioxygen evolution on the surface of CoO-MgO ss m a y be thus schematically represented a s follows:
I,!I
Ill
IV
Conclusions Interaction of oxygen on the surface of CoO-MgO ss gives rise to various forms of adsorbed oxygen. The analysis of the hfs based on the comparison of calculated stick diagrams with experimental spectra allows to distinguish two main types of dioxygen: (i) one with nonequivalent nuclei corresponding to a bent Co3+-02structure (species I, 11, and 111) and a covalent bonding between
Appendix Usually, after the magnetic parameters have been estimated from an analysis of the spectra, it is necessary to confirm or correct these parameters by computer s i m ~ l a t i o n .For ~ ~ superoxide ions, this has often been performed in the simple cases, Le., for I6O2ions adsorbed on cations with I = 034or I # 0.5*7J541Although a good fit can be obtained between experimental and simulated spectra, this does not mean that the parameters are unique. The example of 0, ions adsorbed on GaAs surfaces can illustrate this point. From the published data, it appears that two laboratories have obtained the same experimental spectrum for 02-ions adsorbed on Ga sites and produced by adsorption of oxygen on GaAs surfaces under very similar conditions. However, two sets of magnetic parameters were obtained which substantially differ: g,, = 2.004, gyy = 2.007, g,, = 2.035, A,, = 2.5 G, A, = 25.0 G, and A,, = 15.0 G36and g,, = 2.006, gyy = 2.009, g,, = 2.046, A,, = 10 G, A, = 39 G, and A,, = 24.5 Ge3' In contrast to the appreciable number of simulations performed for 1602spectra, there are, to our knowledge, very few cases reported for 1702spectra. There is only one such example for I7O2ions adsorbed on Ti4+ sites with I = 0.42 Simulations of the spectra have been carried out by Shiotani et ai.@ using a rigid limit simulation program, including second-order corrections, on two different assumptions. One is based on a different line width for each line (belonging to either 170180or I8OI7O-)of the doublet, the larger line width being associated with the larger hyperfine splitting. The other is that the signal intensity (or radical concentration) of the 0; giving the smaller hyperfine splitting is twice as large as that of the 02-with the larger hyperfine splitting. The simulation carried out by Shiotani et a1.,42in better agreement with the experimental spectra obtained by using the first assumption of different linewidths, led to two hyperfine spiittings (74.8 and 80.3 G) in the spectra of 170'80ions adsorbed on Ti-supported surfaces. Earlier results obtained for "0-labeled 0,- ions adsorbed on various oxide systems had shown that two explanations were possible:9s45one is that these two splittings refer (33) Che, M.; Ben Taarit, Y. Adv. Colloid Interface Sci. 1985, 23, 179. (34) Kodratoff, Y.; Mdriaudeau, P.; Imelik, B. J . Chim. Phys. 1971, 68, 1085. (35) Miller, D. J.; Haneman, D. Phys. Rev. B: SolidState 1971, 3, 2918. (36) Stauss, G . H.; Krebs,J. J. Phys. Lett. A 1974, 50, 49. (37) Miller, D. J.; Haneman, D. Phys. Lett. A 1977, 60, 355. (38) Howe, R. F.;Lunsford, J. H. J. Am. Chem. Soc. 1975, 97, 5156. (39) Losee, D. B. J. Caral. 1977, SO, 545. (40) Davis, S. M.; Howe,R. F.; Lunsford, J. H. J . Inorg. Nucl. Chem. 1977, 39, 1069. (41) Wang, J. X.; Lunsford, J. H. J . Phys. Chem. 1986, 90, 3890. (42) Shiotani, M.; Moro, G . ;Freed,J. H.J . Chem. Phys. 1981, 74, 2616.
Oxygen on the Surface of COO-MgO Solid Solutions to two 170nuclei in the same 02-ion and the other arises from I7Onuclei in two 02-ions in different surface sites. Despite the computer simulation, Shiotani et al.42 could not unequivocally demonstrate that one of those two explanations was much better than the other. However, they found it reasonable to attribute the nonequivalent I7O hyperfine splittings to two I7O nuclei in the same 02-being n o n e q u i ~ a l e n t . ' ~ * ~ ~ Recently this problem has been reinvestigated? and high-resolution spectra were obtained exhibiting hyperfine lines due to doubly labeled ions l7OI7O-which showed unambiguously that, for the Mo03/Si02 system, the previous analysis was correct, Le., two nuclei in the same 02-being n o n e q u i ~ a l e n t . ~ ~ * ~ ~ The other example of the simulation of 1702spectra concerns I7O2-ions adsorbed in a CoNa-Y zeolite.43 Two types of 1702ions were observed: one did not show any 59C0hyperfine structure and possessed two equivalent oxygen nuclei, while the other was adsorbed on Co3+ ions and was similar to adducts formed in solution. The spectrum of the former was simulated, and only the positions of the maxima and minima were correctly predicted. The lack of complete agreement was believed to be associated with anisotropic variations of the individual line width. By contrast, it was concluded for the latter 1701species adsorbed on Co3+ ions that the complexity of the spectrum prevented computer simulation. The authors did not report any I7O hyperfine splittings but suggested the inequivalent oxygen nuclei.43 The computer simulation of spectra of I7O2-ions adsorbed on COO-MgO solid solutions is complicated by a number of factors: (i) There are several types of 02-spectra corresponding to species I, 11,111, and IV13 which overlap. (ii) We are dealing with powder EPR spectra which are usually less resolved than single-crystal or frozen solutions spectra. (iii) The g and A tensors are anisotropic, and their axes do not coincide. (iv) There are two types of nuclei with nonzero and high values of I (100% naturally abundant 59C0nuclei with I = 7/2 and 58% artificially abundant I7O nuclei with I = 5/2). (v) Nuclei with I 2 1 all lead to quadrupole interactions. (vi) Each type of superoxide ion is in fact composed of four species: 59C03+-1602-, s9C03+-160170-, with one (s9C0),two (59C0, 170), sgCo3+-170160-,s9C03+-170170or three (one 59C0and two 170) nuclei with nonzero nuclear spins, respectively. Assuming that I7O hyperfine splittings appear only along the lowest g tensor component and that the other components are not split, the total number of lines is 384 for inequivalent oxygen spread over -750 G. This explains, as stated earlier by Wowe and Lun~ford,"~ why these complicating factors prevented computer simulation. This has led us to use stick diagrams to confirm the inequivalency of the oxygen nuclei and to estimate the I7O hyperfine splittings. Construction of the Stick Diagrams. The construction of the stick diagrams was performed on the basis of the following rules (1-5) and assumptions (a-c). A . Rules. (1) Because of the positive magnetic moment (4.6163)44 of cobalt, the low-field line corresponds to the -7/2 transition and the high-field line to the 7 / 2 transition. (2) The opposite holds for oxygen because of its negative magnetic moment (-1.8930).44 (3) The intensity Jik(OmI)of the hf lines was cal(43) Howe, R. F.; Lunsford, J. H.J . Phys. Chem. 1975, 79, 1836. (44) Expressed in multiples of the nuclear magneton ehl(4rMc). Handbook ofChemisrry and Physics, 61st ed.; CRC: Boca Raton, FL, 1980-81; pp E-71, E-72. (45) Che, M.; Tench, A. J.; Naccache, C. J . Chem. Soc., Faraday Trans. 1 1974, 70, 263. (46) Sojka, Z., et al., to be published.
"he Journal of Physical Chemistry, Vol. 92, No. 6, 1988
1547
culated from the known enrichment X, using the following formula f l ( 2 4 + 1)
i=O
where k = 0 and I refer to the case of nonequivalent and equivalent oxygen nuclei, respectively; i = 0, 1, 2 correspond to C0-1602-, CO-~~O~~O-, and C O - ~ ~ O ~species, ~ O - respectively; Io = 7 / 2 and I , = Z2 = 5 / 2 refer to the nuclear spin of 59C0(100% naturally abundant) and I7O (58% enrichment), respectively; and -2Z1 G OmI 6 +2Z1. The case of transition-metal isotopes which are not 100% naturally abundant can also be considered, but the above formula then require some changes.& (4) In the diagrams (Figures 5 and 6) and in the text, for practical reasons, the intensity of the weakest line(s) was taken as unity. (5) The intensity of the hf lines due to the oxygen species I6O2-, l7OI6O-, l6OI7O-,and I7Ol7O-,which are not in interaction with cobalt and have no physical meaning for the cobalt-dioxygen adducts, are not drawn to scale. B. Assumptions. (a) For equivalent oxygen nuclei JCoA,I= 15 G and IoAI,III = 70 G (see Results Section). (b) For nonequivalent oxygen nuclei lCoAxl= 15 G, loAIl = 65 G, and loArIl = 75 G (see Results section); the subscripts I and I1 indicate the inner and terminal oxygen, respectively. (c) Triply labeled Co170117011 species in the case of nonequivalent nuclei were not taken into account because of their too low intensity in comparison to the other lines. The stick diagrams for equivalent (Figure 5) and nonequivalent (Figure 6) oxygen nuclei were built up by addition of partial subdiagrams arising from the interaction of the unpaired electron of 0, with C O ( I = ~ and / ~ ) 170(Z=5/2) nuclei of different CO-'~OF, CO-'~O~~O-, CG-'~O~~O-, and C O - ' ~ O ~ ~species. OTaking into account the enrichment of oxygen, the intensities of the hf lines of the differently labeled cobalt-oxygen adducts were calculated via eq A l . Then starting from the initial gxt,z, line of l 6 O ~the , interaction of I6Oywith 59C0 results in 8 hf lines of equal intensity (0.022) corresponding to the C O - ~ species ~ O ~ (Figure 5 ) . On the other hand, the singly labeled I7Ol6O-species should give a 6-line pattern which, however, does not appear in the real spectrum due to the additional interaction with s9C0 resulting in further splitting giving 48 lines of intensity 0.0102. A fragment of this pattern which appears around gxt,zlis shown in Figure 5. For convenience, only the relevant part of the I7Ol6Ofeatures (lines between + 1 / 2 6 OmI 6 - 5 / 2 ) has been given. The doubly labeled 170170dioxygen ions in the case of equivalent nuclei should give rise to 11 lines of different intensity varying as 0.0096 (6 - IomIl). The position of some of these lines (between 0 G Oml G -3) is shown in Figure 5. Each of these lines is in fact, as in the previous case, additionally split by 59C0,and this gives rise to the CO-~~O~~Ohf structure (Figure 5). In the case of nonequivalent oxygen nuclei (Figure 6), besides the already discussed Co-l6O2- splitting of 0.022 intensity, two patterns for C0-'6011701f and C0-17011601,are expected, both with the same 59C0splitting but with different 170splittings. These lines have the same intensity (0.005 08); however, because of coincidence, some of the C O - ~ ~ O ~lines ~ ~are O ~degenerate ~ (Figure 6).
Finally, on the top of both figures the sum of all contributions was represented (designated as CCo-Oy), giving the actual stick diagrams for equivalent (Figure 5) and nonequivalent (Figure 6) oxygen nuclei. Registry No. 02, 7782-44-7; Coo-MgO ss, 112621-00-8.
