Evidence of stable hydroxyl radicals and other oxygen radical species

May 1, 1993 - Single Electron Traps at the Surface of Polycrystalline MgO: Assignment of the Main Trapping Sites. Mario Chiesa, Maria Cristina Paganin...
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J. Phys. Chem. 1993,97, 5135-5140

5735

Evidence of Stable Hydroxyl Radicals and Other Oxygen Radical Species Generated by Interaction of Hydrogen Peroxide with Magnesium Oxide Elio Giamello,' Luigi Calosso, Bice Fubini, and Francesco Geobaldo Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universitb di Torino, Via P. Giuria 9, 10125 Torino, Italy Received: December 15, 1992; In Final Form: February 19, 1993

Three distinct radical species are formed and stabilized in the solid matrix upon the treatment of magnesium oxide with hydrogen peroxide in aqueous solution leading to the transformation of the solid into magnesium peroxide. The species in question are the 02-superoxide radical ion, the 0- radical ion, and the OH hydroxyl radical. The latter radical is characterized by orthorhombic g and A tensors whose principal values, due to the ionicity of the trapping matrix, are slightly different from those previously observed for OH in irradiated ice or similar systems. In comparison to the O H radical trapped in ice, the hydroxyl radical observed in the present case (as well as the two companion radicals) is stable up to 473 K.

communication of the present findings was given in ref 6. The hydroxyl radical is of paramount importance in many chemical and biological processes, but when present in liquid media, it is detectable only by means of spin-trapping techniques.I9 Some direct spectroscopicobservations were instead performed by EPR in irradiated frozen aqueous systems such as H20r20-23 electrolyte glasses,24 and hydrated ~ a l t s . 2 ~In ~ ~all 6 cases the OH radical originates from radiolysis of the H2O molecule and is stable in a limited range of temperature (i.e., it decays rapidly above 100 KZ7). In the present case the OH radicals are formed by a very complex liquid-solid reaction and are stable in a large temperature range. Other oxygen radical species are simultaneously formed whose nature will be discussed in the following.

Iatroduction

Paramagnetic oxygen derivatives in solid-state materials have been the object of a great deal of investigation in recent years either when localized in the bulk of the material or at its surface. In the former case oxygen radical speciesare impuritiesor defects of the solid132sometimes generated by various kinds of irradiation of the solid itself. The relevance of oxygen radical species at solid surfaces is instead related to their role in surface chemistry and in heterogeneous catalysis since the catalytic oxidation of various substrates is usually performedvia dioxygen at the surface of metal oxides. The complex process of heterogeneousactivation of dioxygen has stimulated a large number of studies in the past which are reviewed in several papers.34 Due to the radical nature of many of the oxygen intermediates involved in such processes Experimental Section the EPR technique has played a fundamental role in this kind of investigation. The magnesium oxide samplesemployed in this work are highMore recently, an increasinglyimportant role has been assumed grade MgO (Puratronic) from Johnson Matthey. In all experby the processes of heterogeneous oxidation of various substrates iments the solid has been contacted at ambient temperature with on oxide or zeolitic catalysts by means of hydrogen p e r o ~ i d e . ~ ~ ~an excess of an aqueous solution of HzO2(Fluka, 30% by weight). However, despite the fact that some papers concerning both the Some experiments have been performed by employing a solution basic features of the HzO+xide interactiong-I5and the kinetics of D202 in DzO of the same nominal concentration from Icon of H202 decomposition on oxides1"Is have been published in the Services (NJ, USA). The decomposition of hydrogen peroxide, past, a well-defined description of the phenomena occurring at with parallel dioxygen evolution,was observed during the contact the interface of this system (comparable to that available for the between the solid and the liquid phase. After 60 min of contact O+xide interface) is far from being complete. We have therefore the mixture was filtered and the residual solid was gently dried recently tackled a systematic study of the H202/metal oxide at 343 K in air. Prior to recording the spectra the dried samples systems, with particular attention to the oxygen paramagnetic were transferred into a vacuum cell, holding an EPR tube, and species which are stabilized on the solid surface (or in the bulk) further pumped off at room temperature up to a vacuum of about upon interaction with H202. The present paper reports on the 1 x 10-21 x l0-3Pa. results obtained by studying the interaction between H202 and X-ray powder diffraction patterns were collected with a Philip MgO. The system H202/Mg0 was also recently investigated by diffractometer (CuKa). X- and Q-band spectra were recorded Evans and co-workers,l2 who reported an intenseand well-defined by means of a VARIAN E- 109 EPR spectrometer equipped with EPR spectrum, arising upon contact of the solid with an aqueous a dual cavity. Varian Pitch was used as a reference for the solution of hydrogen peroxide and assigned to 02-superoxide calibration of g values. Spectral simulations were obtained by radical ions. These radical ions are thought to be located in means of the SIM14A program from QCPE. subsurface layers since the spectra did not undergo dipolar broadening upon contact with oxygen at low temperature. Two Resulta and Discussion lines in the spectrum mentioned above (labeled C and F and 1. X-rayDiffraction. The interaction between MgO and H202 unassigned by the authors) attracted, however, our attention due causes both the catalytic decomposition of hydrogen peroxide to their particularly small line width. In the spectrum they lie and the simultaneous modscation of the compositionof the solid symmetricallywith respect to the intense and wider line assigned which is substantiallytransformed into the correspondingperoxide to the g,, and gyycomponents of the superoxide spectrum. (MgO2) according to the reaction We decided therefore to revisit the H202/Mg0system, and we will try todemonstrate, in the following, that those'anomalous" H,O, + MgO H 2 0 + MgO, (1) lines are components of a signal due to the OH hydroxyl radical This reaction easily occurs at ambient temperature as shown by trapped in the bulk of magnesium peroxide. A preliminary

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0022-3654/93/2091-5135S04.00/0

0 1993 American Chemical Society

Giamello et al.

5736 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993

Figure 1. X-ray diffraction pattern of the dried solid obtained upon treatment of M g 0 with hydrogen peroxide. The main lines are due to a phase of magnesium peroxide; the two narrow lines indicated by a star are due to unreacted MgO.

Figure 1 where the X-ray diffraction spectrum of the solid, taken after interaction with H202 and successive drying, is reported. All the lines, except the two indicated by a star, belong to the diffraction pattern of the Mg02 microcrystalline phase.28 The two lines evident in the figure are due to residual unreacted MgO. 2. EPR Spectra. The dried magnesium peroxide exhibits an intense EPR spectrum which is reported in Figure 2 together with its computer simulation. The relevant lines have been labeled with a letter (A-F). The observed spectrum is strictly similar to that previously reported by Amorelli et a1.12 and obtained by a similar experiment. The reproducibility of the spectra observed in the present case is excellent. All the spectra recorded in a series of some 15 experiments were similar, the only exception being related to the intensity ratio between the lines labeled A and B and the remaining lines that can vary within a limited range. The intensity ratio between lines A and B is constant in all experiments. The paramagnetic speciesgiving rise to the spectrum in Figure 2 are located in the bulk of the solid. This can be stated because the line broadening shown by surface radical species when the EPR spectrum is recorded at 77 Kin the presence of physisorbed oxygen is not observed in the present case. The observed spectrum is stable between 4 and 473 K the only modification observed for spectra recorded at temperatures higher than 77 K is a slight and reversible increase of the separation between the A and B lines. If the sample is heated at temperatures higher than 473 K the whole EPR spectrum progressively vanishes. The spectrum reported in Figure 2 is quite complex and will be discussed in two steps. In the first step we will consider the two lines labeled A and B, corresponding to lines C and F in the spectrum reported in ref 12. The two lines under discussion, centered at g = 2.0137 and separated by about 5.7 mT at 298 K, belong to one component of an orthorhombic g tensor split by hyperfine interaction. The other two components of the g tensor are identified (section 3) at g = 2.050 (line C) and at g = 2.0038 (buried in line F), respectively. The nature of the A-B pair was first demonstrated by recording the X-band spectrum at two different frequencies (9.23 and 9.55 GHz, respectively) and

I/ Figure 2. X-band EPR spectrum (upper trace) and corresponding simulation (lower trace) of the solid obtained upon treatment of MgO with an aqueous solution of H202. The spectrum has been recorded a t 298 K.

observing the variation in the separation of the lines due to each g component. The center of the two lines also shifts from the

expected values by changing the frequency, but the separation

Interaction of Hydrogen Peroxide with Magnesium Oxide g.2.0137

1 0.9 m P

The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 5737

-..Oj

,,,r

Figure 4. Q-band EPR spectrum (upper trace) and corresponding simulation (lower trace) of the solid obtained upon treatment of MgO with H202. The spectrum has been recorded at 298 K.

Figure 3. X-band EPR spectrum (upper trace) and corresponding simulation (lower trace) of the solid obtained upon treatment of MgO with an aqueous solution of D202, The spectrum has been recorded at 298

K.

between the two lines remains constant. This indicates that they are members of a hyperfine structure. The stronger intensity of B in cdmparison to A, as it will be demonstrated later on, is due to the fact that line B actually comprises the overlapping of two different lines. The hyperfine doublet constituted by theA and B lines iscaused by the presence in the system of a paramagnetic species containing one hydrogen atom interacting with the unpaired electron. Hydrogen is, in fact, the only nucleus with nuclear spin Z = '/z in the present system. This hypothesis has been confirmed by repeating the experiment with a solution of D202 in DzO (the nuclear spin of deuterium is Z = I) which gives rise to the spectrum reported in Figure 3 (upper trace). This spectrum is strictly similar to that shown in Figure 2 with respect to the C, D, E, and F lines: the only exception is due to the substitution of the A-B doublet (with 5.7 mT separation) by a triplet (about 0.9 mT separation) centered at the same g value. Two of the three components of the triplet are clearly visible in Figure 3,the third one being buried in the intense central line of the spectrum (line F) * The spectrum in Figure 3 demonstrates two basic facts: (i) the species giving rise to lines A and B in Figure 2 contains one hydrogen atom in interaction with the unpaired electron since the hyperfine doublet is transformed into a triplet by employing Dz02 instead of H202. The separation of the triplet is roughly nuclear magnetic that expected on passing from 'H (Z = moment (nmm) = 2.79 nuclear magnetons) to ZH(Z = 1, nmm = 0.86) and (ii) the other lines visible both in Figures 2 and 3 are due to one or more paramagnetic species which do not contain hydrogen (or deuterium) atoms interacting with the unpaired electron. Figure 4 reports the Q-band ( v = 35 GHz) spectrum obtained for the sample treated with H202 together with its computer

n Figure 5. Comparison of the computer-simulated X-band spectra corresponding to the treatments of MgO with hydrogen peroxide (left) and deuterium peroxide (right),respectively. The stick diagrams evidence the spectral structure resulting from simulation.

simulation. Two lines indicated in the figure are centered at g = 2.0137,are separated by 5.7 mT, and therefore correspond to the A-B doublet in Figure 2. 3. Computer Simulation and Spin Hamiltonian Parameters of the EPR Spectra. Figure 5 compares the computer simulations of the X-band experimental spectra already reported in Figures 2 and 3. The two simulated spectra reported in Figure 5 and that related to the Q-band experiment (Figure 4) have all been obtained by considering the presence of three distinct radical species (1, 11, and 111)in the system, one of which exhibitshyperfinecoupling with an H (I) or a D (1') atom. The set of spin Hamiltonian parameters that gave the best fit with the experimental spectra are collected in Table I together with the relative abundance of each species. All species exhibit an anisotropic g tensor except for species I11which has an axialg tensor. Species I is responsible for the A-B doublet visible in Figure 2 and due to the hyperfine coupling along the g2 direction. The species is also characterized by a second hyperfine doublet along the gl direction, one component of which is buried in the intense central F line while the second overlaps with line B. The standard deviations indicated in Table I correspond to modifications of gand A parameters causing appreciable changes

Giamello et al.

5738 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993

TABLE I: Components of the S in Hamiltonian for the Three Paramagnetic Species Identified in the System. The Parameters of Species I, II, and III Have Been for Simulations of Spectra Obtained by H202 Treatment Reported in Figure 2 (X-band, Relative Abundances in Column a) and in Figure 4 (Q-band, Abundances in Column c). The Parameters of Species I’, II, and III Have Been Employed for the Simulation of the Spectrum Obtained by D202Reported in Figure 3 (Relative Abundances in Column b)

$played

abundance of species in simulated spectra

spin Hamiltonian components 1

2

2.0038 f 0.0003 2.5 f 0.05

Species I 2.0137 f 0.0003 5.70 & 0.05

0 f 0.5

AdmT

2.0038 f 0.0003 0.38 f 0.02

Species I’ 2.0137 f 0.0003 0.88 0.02

2.050 f 0.005 0 f 0.5

8

2.003 f 0.001

Species I1 2.008 f 0.001

2.057

* 0.001

0.64

0.62

0.60

g

2.002 f 0.001

Species I11 2.043 f 0.001

2.043

0.001

0.16

0.27

0.24

g A(H,W

g

3

a

2.050 f 0.005

0.20

in the spectral shape with a definite decrease of the goodness of fit. In one case, however, the fitting between experimental and computed spectra remains excellent despite a severe change of the input parameters. This occurs when the very broad g3 component (g3 = 2.050, line width about 5 mT) for both species I and I’ is neglected (thus reducing the two species to axial symmetrywithgg =g11andg2= g,)and,inparallel,theabundance of these species with respect to that of species I1 and I11 is decreased. This alternative possibility (that also fits the experimental spectrum due to the scarce relevance for the whole signal of the very broad component at g = 2.053) must, however, be discarded because the resulting values of the isotropic and anisotropic hyperfine coupling constants are totally unreliable (vide infra). It must be outlined here that the three simulations reported in the present paper have not been obtained independently but have been calculated employing the same set of g and A values for each species (the hyperfine constants of the D-containing speciescorrespond in fact to those of the H-containing one taking into account the different values of the nuclear magnetic moment). The only differences in the parameters adopted for the three simulated spectra concern: (i) the relative abundances of the three paramagnetic species (also reported in Table I) which are not the same in all experiments and (ii) the line width which, in the case of the Q-band spectrum, was definitely higher than that observed in the case of X-band spectra, because of the inhomogeneity of the g tensor and, likely, because of the presence of some residual water in the sample. Even though the fit of each single simulation could be actually further improved by small independent adjustments of the corresponding spin Hamiltonian parameters, we conclude that the remarkable self-consistencyof the three simulationsindicates the accuracy of the parameters reported in Table I. 4. Nature of the H (or D) Containing Species (SpeciesI). The identificationof species I is not straightforward. Both theQ-band and the isotopic labelingexperimentshave clarified that the species contains at least one hydrogen (deuterium) atom. The relatively high values of two g tensor components (and thus of the average g value) suggest that the species also contains oxygen, which is characterized by a high value of the spin-orbit coupling constant. The presence of both H and 0 atoms in the species is indeed not surprising considering the system in question where hydrogen peroxide reacts with the solid undergoing, in parallel, decomposition. The decomposition of hydrogen peroxide,

-

0, + 2 H 2 0 (2) is catalyzed by many homogeneous and heterogeneous catalysts. Several reaction mechanisms have been proposed related to the nature of the catalyst itself.29 Some of the mechanisms proposed for the heterogeneous decomposition of hydrogen per0xide3~JI 2H202

b

C

0.16

0.13

+2.18

& = ai,,, + B

= -21.36

+

-29.54

(3) +27.36

Interaction of Hydrogen Peroxide with Magnesium Oxide aim value being, for a series of papers,2k26around -2.3 f 0.1 mT. Slight differencesin value are observed for the same components when the OH radical is observed in hydrated salts. The data reported in the present paper suggest an assignment of species I to an OH radical trapped in the bulk (as shown by the absence of spectral broadening under oxygen) of magnesium peroxide (Figure 1). The remarkable difference between the properties of the covalent matrix of ice and the ionic one of magnesium peroxide should be considered in order to account for the observed parameters. It has been shown in fact that the bonding properties of the surrounding matrix exert a pronounced influence on the parameters of a trapped r a d i ~ a 1 . ~ ~ , 3 ~ A further comment is devoted to the g,, feature of species I which is scarcely visible in the EPR spectra because of its large line width value and that (as mentioned above) could even be neglected in the spectral simulations obtaining therefore axial tensors for species I and I' instead of orthorhombic tensors. The extremely large line width of the g,, feature is reminescent of the findings of Riederer et al.24who showed that, when OH radicals are generated in the amorphous matrix of aqueous glasses containing high concentrations of various electrolytes, the gz, feature becomes very broad and practically detectable as an ascending baseline only, owing to the effect of the electrostatic field of the ions. This seems to be also the case for the OH radicals in the ionic MgOl, reported in the present paper. Any possibility of ignoring the g,, component, and so reducing the species to axial symmetry, is, moreover, in contradiction with the expected values of the proton hyperfine coupling for a *-radical. The isotropic ais,,value would, in that case, attain the unrealistic value of about 4.6 mT. 5. Nature of the Signah 11and HI. On the basis of the gvalues reported in Table I and of the chemistry of the system discussed in the previous section it is easily understandable that species I1 and I11 are oxygen radical species which do not contain hydrogen since their spectrum exhibits the same features when generated via contact of MgO with H202 or D202. We tentatively assign the orthorombic species I1 to superoxide 0 2 - ions and the axial species I11 to 0- ions. The gz and g3 values of species I1 (as well as the shape of the signal (line F)) are in fact in close agreement with the values expected for g,, and gx, respectively for a superoxide radical ions.4 As to the third value (g3 = g,, = 2.057, z being the direction of the 0-0 internuclear axis) it is well-known that it is themost sensitive to the electrostatic field felt by the negative radical ion, which is, obviously, always stabilized by positive centers. It has been shown by Kanzig and Cohen35that theg,, comwnent for 02radicals can be expressed (neglectingsecond-orderterms) as (4) where A is the spin-orbit coupling constant for oxygen and A is theenergy splitting between the two ?r antibonding orbitals of the species which is caused by an electrostatic positive field. When a superoxide ion is stabilized at the surface of ionic systems containing Mg2+ cation^^,^ the observed g,, values are in the range 2.06-2.09, i.e., slightly higher than that observed for species I1 (gZz= 2.057). In our case, however, the comparison with surface adsorbed 02-radicals is no longer valid since we are dealing with a bulk stabilized species. It is very likely that the superoxide radicals observed in the present case, being trapped in the ionic Mg02 matrix, occupy the site of the similar diatomic peroxide 0 2 2 - ion therefore constituting a sort of defective site. If this is true, each of the two oxygen atoms of the superoxide ion is thus surrounded by three nearest Mg2+ cations according to the structure of magnesium peroxide shown in Figure 6. Although the few examples of 0 2 - stabilized in the bulk of ionic system^^,^^^^^ exhibit g,,values higher than 2.1, we think that the effective electrostatic field exerted by the six Mg2+ cations

The Journul of Physical Chemistry, Vol. 97, No. 21, 1993 5739

Figure 4 The cell of magnesiumperoxide. The sticks linkeach ions with

the nearest neighboring ions. Small balls are for Mg2+ions; big balls are for individual oxygen atoms of 0z2-ions. (depending not only on the symmetry but also on the distance between the negative radical ion and the positive cations) cannot be evaluated a priori and should be the object of an "ad hoc" calculation for a final and unambiguous confirmation of our assignment. On the basis of both the values and the axial symmetry of the g tensor, species I11 is assigned to an 0- ion. This ion has been observed in several systems either at the surface or in the bulk. In particular, when formed in the bulk of magnesium oxide, 0displays an axial symmetry with gl = 2.0385 and gll = 2.00323 whereas, when stabilized at the surface of the same oxide, the values of gl = 2.042 and gll = 2.0013 have been found. Despite the fact that all the radical species studied in the present investigationarevery likely stabilized in the le99 known magnesium peroxide matrix, the gvalues observed for species I11 are in close agreement with those reported above and typical of the oxide. A second consideration is also in favor of the assignment of species I11 to 0- ions. This species is in fact the conjugated base of the OH radical and can be formed, under suitable conditions, by proton transfer from the hydroxyl radical. It was reported by Symons'g that the equilibrium,

H 2 0 + OH tH30++ 0-

(5) is established at very low temperature between hydroxyl radicals and water. In our case a corresponding reaction would occur in the highly basic medium characterized by the presence of the oxide ( 0 2 - ) and peroxide (OZ2-)basic ions typical of MgO and Mg02, respectively. The 0- ion would thus directly arise from a fractionof theOH radicals generated along the process according to the following reactions:

OH + 02--,0-+ OH-

(6)

OH + 0;- 0-+ HO,. )

6. Formation Mechanism of the Trapped bdicals. A last comment is devoted to the chemistry of the overall process that, despite the high complexity of the system, leads to very reproducibleresults with three distinct paramagnetic species &e., the OH hydroxyl radical, and the 02- and 0- radical ions) simultaneously formed and trapped in the bulk of magnesium peroxide. The decomposition mechanism of H202 in heterogeneous systems (though not completelyunderstood) dependsonthe nature of the solid. It has been shown, for a variety of solid catalysts contacted with hydrogen peroxide, that radicals are formed in the vapor phase during the decomposition of H202 vaporsloJIJs or in the liquid phase during the decomposition in aqueous solutions31and at thesurfaceof thesolid itself.9J4.30 Theseradicals have been identified as hydroperoxyl (H02) or, in other cases, superoxide ions ( 0 2 - ) .

5740 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993

There is no doubt therefore that, whatever the details of the mechanism acting in the case of a given solid, the heterogeneous decomposition of hydrogen peroxide OCCUTS via radical intermediates. The crucial point of any possible mechanism is the generation of the primary radical species which can subsequently initiate the series of reaction steps. In the case of the insulating magnesium oxide, a mechanism based on an initial electron transfer from the catalyst to the peroxide,

H,02 + e-

-

(7) which seems to be well-established in the case of metal catalysts31 as well as in many homogeneous decomposition p r o c e s ~ e s , ~is~ J ~ not very likely except if due to microtraces of transition metal impurities present in the solid. The initial step proposed in Other cases, in particular in the case of heterogeneous H202 decomposition on a series of metal oxides,II,30is the homolytic rupture of the 0-0 peroxidic bond at a suitable surface site.

-

20H

(8)

The hydroperoxylradical HO2 (i.e., the most commonly observed intermediate of the heterogeneous decomposition of H202) is readily formed from OH according to

OH + H202 H,O

+ HO,

(9) Once the H02 radicals are generated in the system the final step of the reaction is

HO, + OH

nonaqueous matrix (including in the term aqueous matrix also the water of crystallization of hydrated salts) and generated by a chemical and nonradiolytic method. Acknowledgment. The authors wish to thank Professor Michel Che of P. et M. Curie University (Paris, France) for allowing the use of the Q-band facility. References and Notes (1) Wertz, J. E.; Griffiths, J. H. E.; Orton, J. W. Discuss. FaradaySoc.

OH + OH-

H202

Giamello et al.

H,O + 0,

(10) which is a well-documented and widely investigated reaction.38 The mechanism illustratedaboveis compatiblewith the findings of the present work, Le., the presence of OH, 0-,and 02-radicals trapped in the solid phase because (i) the hydroxyl OH radical is the species initiating the radical reaction, (ii) 0- is formed from OH according to reactions 6, and (iii) 02-, the conjugated base of HO2, derives from the ionization of this latter radical in the basic medium according to

-

HO, + 0,- OH-+ 0; This step is favored by the relatively high acidity of the hydropcroxyl radical (pK, = 4.839). Conclusions

The interaction of hydrogen peroxide with magnesium oxide at ambient temperature leads to thedecomposition of themolecule and to the parallel transformation of the oxide into the corresponding peroxide. During the process three radical species are generated and subsequently trapped into the solid peroxide matrix (probably asdefectivesitessubstitutingthe peroxide anions) giving rise to an intense and reproducible EPR spectrum. The observed radical species are the hydroxyl OH radical, the 0-,and the 02radical ions. As to the first radical it is noticeable that, being isolated in the ionic Mg02 matrix, it displaysa remarkablethermal stabilitybeingobservedupto473K. To thebestofourknowledge this is the first observation of a stable hydroxyl radical in a

1956, 28, 136.

(2) Zeller, H. R.; Kanzig, W. Helu. Phys. Acta 1967, 40, 845. (3) Che, M.; Tench, A. J. Adu. Catal. 1982, 31, 77. (4) Che, M.; Tench, A. J. Adu. Catal. 1983, 32, 1. (5) Che, M.; Giamello, E.; Tench, A. J. Colloids Surf. 1985, 13, 231. (6) Giamello, E.; Che, M. In Electron Magnetic Resonanceof Disordered Systems (EMARDIS’91);Yor&nov,N. D., Ed.; WorldScientific: Singapore, 1991; p 112. (7) Notari, B. In Studies in Surface Science and Catalysis, Proceedings of the International Symposium on Innovation in Zeolite Material Science, Nieuwport, Belgium, 1987; Grobet, P. J., Eds. Elsevier: Amsterdam, 1988; Vol. 37, p 413. (8) Elchyan, A. M.; Grigoryan, G. L. Kinet. Katal. 1989, 30, 466. (9) Iyengar, R. D.; Subba Rao, V. V. J . Phys. Chem. 1971, 75, 3089. (10) Arutyunyan, A. Zh.; Grigoryan, G. L.; Nalbandyan, A. B. Kinet. Katal. 1985, 26, 785. (1 1) Arutyunyan, A. Zh.; Gazaryan, K. G.; Garibyan, T. A.; Grigoryan, G. L.; Nalbandyan, A. B. Kinet. Katal. 1988,29, 754. (12) Amorelli, A.; Evans, C. J.; Rowlands, C. C. J . Chem. SOC.,Faraday Trans. 1 1988. 84. 1723. (1 3) Klissursky, D.; Hadjiivanov,K.; Kantcheva, M.; Gyurova, L. J. Chem. Soc., Faraday Trans. 1990.86, 385. (14) Giamello, E.; Fubini, B.;Volante, M.; Costa, D. ColloidsSurf.1990, I~~

45, 155. (1 5 ) Bellussi, G.; Carati, A.; Clerici, M. G.; Maddinelli, G.; Millini, R. J. Catal. 1992, 133, 220. (16) Hart, A. B.; McFayden, J.; Ross, R. A. Trans. Faraday Soc. 1963, 59, 1458. (17) Murphy, B. J.; Ross, R. A. J . Chem. Soc. A 1968, 2044.

(18) Arutyunyan, A. Zh.; Grigoryan, G. L.; Nalbandyan, A. B. Kinet. Katal. 1986, 27, 1352. (19) Symons, M. C. R. Philos. Trans. R. Soc. London, B 1985,311,451. (20) Brivati, J. A.; Symons, M. C. R.; Tinling, D. J. A.; Wardale, H. q.; Williams, D. 0. Trans. Faraday Soc. 1967, 63, 21 12. (21) Brivati, J. A.; Symons, M. C. R.; Tinling, D. J. A.; Williams, D. A. J. Chem. SOC.A 1969,719. (22) Dibdin, G. H. Trans. Faraday Soc. 1967,63, 2098. (23) Box, H. C.; Budzinski, E. E.; Lilga, K. T.; Freund, H. G. J . Chem. Phys. 1970, 53, 1059. (24) Riederer, H.; Hiittermann, J.; Boon,P. J.; Symons, M. C. R. J. Magn. Reson. 1983, 54, 54. (25) Gunter, T. E. J . Chem. Phys. 1967, 46, 3818. (26) Toriyama, K.; Iwasaki, M. J. Chem. Phys. 1971.55, 1890. (27) Siegel, S.;Baum, L. H.; Skolnik, S.;Flournoy, J. M. J. Chem. Phys. 1960, 32, 1249. (28) X-Ray Powder Data File, A.S.T.M., Philadelphia, Card 11.19-771. (29) Baxendale, J. H. Adu. Catal. 1952, 4, 31. (30) Arutyunyan, A. Zh.; Grigoryan, G. L.; Nalbandyan, A. B. Kinet. Katal. 1987, 28, 1121. (31) Ono,Y.; Matsumura,T.;Kitajima,N.;Fukuzumi,S.J. Phys. Chem. 1917,81, 1307. (32) Ward, S.J.; Smith, R. C.; Adrian, F. J. J . Phys. Chem. 1968, 49, 2780. (33) Catton, R. C.; Symons, M. C. R. J. Chem. SOC.A . 1969, 1393. (34) Symons, M. C. R. J. Chem. Soc., Faraday Trans. I 1982,78, 1953. (35) Kanzig, W.; Cohen, M. H. Phys. Reu. Lett. 1959, 3, 509. (36) Bennett, J. F.; Ingram, D. J. E.; Schonland, D. Proc. Phys. SOC.1956, 69A, 556. (37) Haber, F.; Weiss, J. Proc. R . Soc. London 1934, A147, 332. (38) Schwab, J. J.; Brune, W. H.; Anderson, J. G. J. Phys. Chem. 1989, 93. ro30. (39) Bielsky, B. H. J.; Cabelli, D. E.; Arudi, R. L.; Ross, A. B. J . Phys. Chem. Ref. Data 1985,14, 1041.