SiO2 Catalysts via

14776

J. Phys. Chem. 1996, 100, 14776-14785

EPR Investigation of the Activation of N2O on Mo/SiO2 Catalysts via Electron Transfer: From N2O as a Ligand to Adsorbed O- Ion Zbigniew Sojka*,† and Michel Che Laboratoire de Re´ actiVite´ de Surface, URA 1106, CNRS, UniVersite´ P. et M. Curie, 4 Place Jussieu, 75232 Paris Cedex-05, France ReceiVed: March 22, 1996X

EPR spectroscopy has been used to monitor changes occurring at the molecular level upon adsorption of N2O on grafted Mo/SiO2 catalysts using naturally abundant and 95Mo-enriched molybdenum. The tetracoordinated Mo4c5+ surface species produced upon reduction with dx2-y2 SOMO coordinates N2O at 298 K and rearranges into a pentacoordinated complex of apparent C4V symmetry with dxy ground state indicating attachment of the N2O ligand in equatorial position. Upon temperature-induced metal to ligand electron transfer (MLET) between 3π* and dxy redox orbitals, O- radicals are produced with an activation energy of 20 ( 4 kJ/mol measured in the 323-393 K range. The mechanism of this process and the hindrance to electron transfer are discussed in terms of intrinsic barriers and within the framework of potential energy surfaces. From the spin Hamiltonian parameters of O-, the unpaired electron density on molybdenum and the splitting of energy levels of O- species were calculated to be 4.3% and ∆Ex,y-z ) 1.5 eV, respectively. The crystal field approach was used to discuss the O--Mo6+ bonding.

Introduction Investigations of the elementary reactions in the coordination sphere of surface complexes constituting active centers are of great importance in heterogeneous catalysis. One of the fundamental objectives of such studies is to understand the ways by which reactants can be activated. The variety in the shapes and spacial accessibility of d-orbitals of appropriate energy and the multiplicity of oxidation and spin states allow transition metal ions to activate reagents following symmetry or spin conservation rules.1 Chemical activation resulting from coordination may in some cases be related to the vibronic mixing of the electronic ground state with close-lying excited states giving rise to the lowering of the potential barrier of the chemical reaction.2 As a case study of activation via electron transfer (ET), the mechanism of dissociative activation of N2O within the coordination sphere of Mo5+ ions dispersed on silica is considered in the present paper. The importance of nitrous oxide as an oxidant in catalysis arises from its propensity to yield highly reactive O- species upon dissociative adsorption on the catalyst surface, providing electrons are available. Experimental evidence for the role of O- ions in numerous reactions including oxidative dehydrogenation of ethane by nitrous oxide3 and partial oxidation of methane over MoO3 supported on silica gel,4-6 and over Lipromoted MgO catalyst,7 low-temperature oxidation of CO with N2O over supported molybdenum oxide,8 and exchange reactions between oxygen-containing gases and metal oxides9 as well as oxidative hydroxylation of aromatics over zeolites10 was reported. The reactivity of this radical ion has been also studied in surface stoichiometric model reactions of catalytic importance including among others the oxidative dehydrogenation of various hydrocarbons11 or methanol.12 On the other hand, the catalytic decomposition of N2O alone has been used to evaluate the catalytic activity of various oxide surfaces. The advantage is that it follows an electronic mechanism which is very suitable for correlating catalytic activity with electronic properties. The † Present address: Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Cracow, Poland. X Abstract published in AdVance ACS Abstracts, July 15, 1996.

S0022-3654(96)00871-4 CCC: $12.00

reaction has been studied thoroughly and there is a large amount of experimental data concerning the activation energy and major mechanistic steps. This subject has been reviewed earlier.13,14 Moreover, nitrous oxide has been also extensively used as an indicator of electron availability at semiconductor surfaces15 and for the determination of the surface area of highly dispersed supported metal particles, using adsorption measurements.16 There is likewise an increasing interest for studies of the chemistry of N2O in relation to environmental problems since it provides the principal control of the O3 concentration in the stratospheric ozone layers.17 Despite the obvious role of redox orbitals in electron transfer processes, few studies have probed the way by which such orbitals are involved and revealed the associated structural rearrangements. The purpose of this paper is to investigate the activation of nitrous oxide on isolated surface Mo centers and to identify the elementary steps of this process. In particular, the identification of the particular orbitals involved in the coordination and the rationalization of surface electron transfer in terms of intrinsic factors involved in changes of the molecular structure of both the ligand and active site have been established. This work represents a first attempt to study electron transfer processes occurring at oxide surfaces. The choice of grafted Mo/SiO2 catalysts containing isolated Mo5+ species was dictated by the fact that the latter acts as an EPR self-probe because its relatively large spin-orbit coupling constant produces g-tensor shifts significantly large to study changes in the Mo coordination. In the case of 95Mo-labeled species, additional information can be drawn from the superhyperfine structure. All these factors provide a unique possibility for monitoring changes occurring within the coordination sphere of Mo5+ ions by EPR spectroscopy. Experimental Section Grafted Mo/SiO2 samples have been prepared by reacting partially dehydrated silica (type Spherosil XOA, 400 m2/g) with molybdenum pentachloride in cyclohexane as described elsewhere.18 Analysis of the grafted catalysts gave 0.15 ( 0.02 and 0.32 ( 0.02 wt % of molybdenum, all practically in the © 1996 American Chemical Society

Activation of N2O on Mo/SiO2 Catalysts isolated state. 95Mo-supported silica sample (2 wt %) was prepared by addition of silica to a solution of labeled ammonium paramolybdate prepared by dissolving 95Mo enriched (97%) in MoO3 in concentrated ammonia solution.19 The labeled molybdena was supplied by A.E.R.E., Harwell. Grafted samples differ from impregnated ones in the better dispersion of molybdenum which is also in stronger interaction with the support surface. In the standard preparation procedure, catalysts were heated at 873 K successively in vacuum for 0.5 h, under 200 Torr of oxygen for 2 h, under 200 Torr of hydrogen for 2 h, and finally outgassed for 0.5 h. In the case of impregnated samples, the reduction time was shortened to 0.25 h while the final evacuation was prolonged to 0.75 h. The gases O2 and H2 (Air Liquide) were used without any further purification. N2O (Air Liquide) was purified by freezepump-thaw cycles prior to adsorption, which was carried out at room temperature under a pressure of 30-40 Torr. Referring to previous works,12,20 the O- ions were (except in the variabletemperature study) generated by heating the sample with preadsorbed N2O at 373 K for 15-20 min, followed by evacuation to 10-5 Torr. The EPR spectra were recorded at 77 K on a Bruker (Model ESP 300) or Varian (Model E-3) spectrometers, both working at 9.4 GHz (X-band) with 100 kHz field modulation and equipped with a variable-temperature unit. Data manipulation was performed with the software provided by Bruker. EPR spectra were simulated with the program SIM14A21 or analyzed by the least-squares method which focuses on the field positions of resolved spectral features.22 Results and Interpretation N2O Acting as a Ligand. Upon standard reduction, the Mo/ SiO2 samples exhibit a complex EPR spectrum at 77 K due to three superimposed signals of Mo5+ species with different coordinative numbers. The spectra of these species were discussed and assigned earlier.1,12,18,23 The predominant signal with g⊥ ) 1.944 and g| ) 1.89 corresponds to a hexacoordinated Mo6c5+ ion, a weaker one to the pentacoordinated Mo5c5+ ion (g⊥ ) 1.957, g| ) 1.87) while the signal with g⊥ ) 1.926, g| ) 1.76 corresponds to the tetracoordinated Mo4c5+ ion (Figure 1a). All three types of Mo5+ cations possess a molybdenyl character, which is preserved upon adsorption.18,23 The low-field features of weak intensity belong to the two outermost |mI| ) 5/2 and 3/2 perpendicular and parallel hyperfine lines, partially overlapping and arising from the interaction of the unpaired electron with nuclear spins I ) 5/2 of the 95Mo and 97Mo isotopes of natural abundance 15.78 and 9.6%, respectively. Since they have almost the same nuclear magnetic moment (-0.914 and -0.933 βN, respectively) the resulting hyperfine structures are largely superimposed. The other parallel and perpendicular hfs features remain unresolved but are clearly visible in the 95Moenriched sample (Figure 2a). The following parameters were found for this signal via computer simulation: g⊥ ) 1.943, g| ) 1.889, |MoA⊥| ) 44 G, and |MoA|| ) 93 G. Upon exposure of the samples to purified N2O (30 Torr) at room temperature, the signal due to the tetracoordinated Mo species completely disappears while that of pentacoordinated Mo5c5+ considerably increases in intensity becoming now dominant in the spectrum (Figure 1b). The intensity of the line corresponding to Mo6c5+ remains unchanged within experimental error. This indicates that the symmetry of the Mo4c5+ centers is altered upon coordination of N2O and obviously the tetracoordinated species becomes pentacoordinated with g⊥ ) 1.96 and g| ) 1.87. Moreover, this result indicates that each Mo4c5+

J. Phys. Chem., Vol. 100, No. 35, 1996 14777

Figure 1. EPR spectra (X-band, taken at 77 K) of grafted 0.33 wt % Mo5+/SiO2 catalyst (a) prior and (b) after adsorption of 30 Torr of N2O at 293 K; (c) spectrum obtained after heating at 373 K for 20 min.

site can accept only one N2O molecule. Thus, in agreement with earlier results,12,20 it appears that the penta- and hexacoordinated Mo5+ complexes play only a spectator role during N2O adsorption at room temperature. The same behavior was previously observed in the case of CO and Et3P adsorption onto Mo/SiO2 catalysts, contrasting with the ability of highly polar molecules like H2O or CH3OH to fill the coordination vacancies also of the pentacoordinated molybdenum. However, while the Mo4c5+ centers can adsorb two molecules of CO, H2O, and CH3OH1,12,18 and even Et3P,24 in the case of N2O, adsorbed under 30-40 Torr, only the molecule is coordinated. Although this phenomenon was tentatively related to the kinetic diameter of adsorbate molecules,20 on the basis of the analogy with CO adsorption,25 one could expect that the number of adsorbed N2O molecules can be controlled by the adsorbate pressure. Though the results of NO adsorption onto Cu-ZSM5 catalysts provide further precedence for this hypothesis,26 it was reported that increase of the pressure of nitrous oxide adsorption up to 300 Torr does not produce further changes in the EPR spectrum.20 It is likely that this phenomenon is due to the known poor ligand property of nitrous oxide. Dissociative Reductive Activation of N2O. Heating the sample with preadsorbed N2O at 373 K for 15-20 min produces dramatic changes in the EPR spectrum. The signal due to the Mo5+ decreases and at the same time a new sharp signal characteristic of O- ions12,20 appears at g⊥ ) 2.02 and g| ) 2.005 (Figure 1c). The monatomic nature of the oxygen ionradical was confirmed using 17O-enriched nitrous oxide.27 When the same reaction is repeated with an isotopically enriched molybdenum sample, additional reasonably well resolved superhyperfine (shf) structure (two sets of lines corresponding to perpendicular and parallel components), due to the coupling of the unpaired electron with the nucleus of 95Mo (I ) 5/2) can be additionally detected (Figure 2b). This structure is superimposed on the two much broader perpendicular hfs lines (corresponding to |mI| ) 5/2 and 3/2 transitions) of the EPR

14778 J. Phys. Chem., Vol. 100, No. 35, 1996

Sojka and Che TABLE 1: EPR Parameters for Various Signals Observed during the Interaction of N2O with Mo/SiO2 species 5+

Mo6c Mo5c5+ Mo4c5+ 95 Mo6c5+ N2O-Mo4c5+ O--Mo4c6+ O--95Mo4c6+

g⊥

g|

1.944 1.957 1.926 1.943b 1.96 2.02 2.021b,c

1.89 1.87 1.76 1.889b 1.87 2.005 2.0059b,c

|A⊥| (G)a

44b 6.7b,c

|A|| (G)

93b 7.8b,c

a 1 G ) 0.1 mT. b Parameters obtained from simulation. c The principal axes of g and shfs tensors are approximately perpendicular.

Figure 3. Kinetics of the formation of O- species as a function of temperature (]) 323 K, (*) 348 K, (4) 373 K and (0) 393 K. Figure 2. EPR spectra (X-band) of impregnated 2 wt % 95Mo5+/SiO2 catalyst (a) before and (b) after adsorption of 40 Torr of N2O at 293 K followed by heating at 373 K for 20 min. Experimental spectra: thin line, simulated spectra: thick line. Due to the larger loading, the Osignal is superimposed onto two strong outermost hf components coming from the EPR signal of unreacted Mo5+ species.

signal of the hexacoordinated 95Mo5+ species, which owing to its enrichment are now much more intense than in the case of the previous naturally abundant sample. The following EPR parameters g⊥ ) 2.021 and g| ) 2.0059 and |A⊥| ) 6.7 G and |A|| ) 7.8 G, were found by least-squares fitting of the experimental field positions of more than 10 unambiguously assigned spectral features (corresponding to various minima or maxima of the resonant field) to the theoretical resonance field values calculated for major (parallel) axes of the g and shfs matrices being approximately perpendicular to one another (β ≈ 90°). The least-squares spin-Hamiltonian parameters were next used as starting values to generate the ultimate simulated spectrum (refined by simplex method) shown in Figure 2b. The agreement between experimental and simulated spectra is quite satisfactory. Because the 95Mo shf tensor anisotropy is rather small, the orientations of its principal axes are difficult to pin down precisely (the least-squares parameters appear virtually superimposable for β ) 90 ( 15°). Thus, we can say only that both tensors are approximately perpendicular. The spinHamiltonian parameters for all seven species identified in the present work are collected in Table 1. The presence of distinct 95Mo shf features in the EPR spectrum of O- radical provides direct evidence that they are stabilized within the coordination sphere of molybdenum centers. The simultaneous decrease in the intensity of the molybdenum signal confirms that in this reaction electrons come from the oxidation of Mo5+ (ModO3+) to Mo6+ (ModO4+). Kinetic Measurements. Since the formation of O- species is an activated process, the temperature dependence of the rate of formation of this radical is used to determine the apparent

Figure 4. Arrhenius plot for the formation of O- via electron transfer process.

activation energy of the electron transfer. The experimental temperature window is rather narrow since below 300 K the reaction is too slow causing problems with the reproducibility, while above 420 K a new consecutive reaction complicates the issue. For this reason, the reaction was carried out in the temperature range 323-393 K. The semilogarithmic plots (ln(If - I)/If ) -kt, where If and I are the final and actual EPR signal intensity of O- species) of the reaction rates at four selected temperatures 323, 348, 373, and 393 K, are shown in Figure 3. Inspection of the figure indicates that the reaction can be described by a first-order kinetics. From the corresponding Arrhenius plot (Figure 4) the activation energy for the electron transfer was found to be Ea ) 20 ( 4 kJ/mol. The value of the activation energy 20 ( 4 kJ/mol determined from the EPR data can be further compared to 43.5 ( 1.7 kJ/ mol28 and 35.1 ( 8.4 kJ/mol,29 which were obtained for the gas phase thermal electron dissociative attachment to N2O molecules, revealing thereby a catalytic function of the surface Mo complex in ET process. On the other hand, it should be

Activation of N2O on Mo/SiO2 Catalysts

J. Phys. Chem., Vol. 100, No. 35, 1996 14779

Figure 5. General scheme for interaction of N2O with molybdenum grafted on silica and evolution of corresponding molecular orbital energy levels (a) Mo4c5+ surface species, (b) coordination of N2O to Mo4c5+, (c) deformation of N2O/N2O- species in the transition state, and (d) Ostabilized within the coordination sphere of Mo.

noted that this value is close to the minimal activation energy (18 kJ/mol) observed in the real catalytic decomposition of N2O over Cr2O3/Al2O3,13 indicating probably the same (C∞V f Cs deformation) rate-determining step (Vide infra) also in this case. Discussion The understanding of electron transfer occurring at catalytic surfaces is limited in comparison to what is known about ET occurring in solutions. The few literature reports on ET which deal with surfaces of oxides30 provide only a general phenomenological description. Thus in order to get more insight of reaction dynamics and to describe at a molecular level the phenomena accompanying the ET on catalytic surfaces, the tenets of well-established theories of ET developed initially in other fields of chemistry will be used. A knowledge of the N2O- potential energy surface and its relationship to that for N2O can help to understand ET induced dissociation processes28,31-33 and to identify factors contributing to the observed activation energy. An important step in this direction is the extension of the quadratic activation-driving force relationship (Marcus-type equation) to reactions in which bonds are broken and/or formed like dissociative ET, or proton or methyl transfer.32,33 This approach is based on the approximation of the potential energy of the reactant by a Morse function and the potential energy of the products by a purely repulsive portion of the corresponding Morse function.32 It follows then that the dynamics of the various reactions like outer-sphere ET, dissociative ET, and the cleavage of the bond in the intermediate (which appears as intramolecular dissociative ET) may be represented by this type of equation. In this connection, a simple model based on a Morse curve description of bond cleavage is applied to the ET-induced dissociation of N2O on the surface of Mo/SiO2 catalysts. The results are discussed and compared to those obtained in gas phase electron attachment.

N2O as Ligand Bonded to Mo4c5+ Centers. Electron transfer reaction consists in electron movement between relevant orbitals. It follows then that interaction between the occupied (donor) and unoccupied (acceptor) orbitals must play a vital role. Both energy and orientation (symmetry) are important for effective orbital overlap. In addition, the separation distance and spin conservation should be considered giving rise to symmetry restrictions associated both with the spin and the orbital motion of the electron being transferred.34 Adsorption of nitrous oxide onto the surface of reduced Mo/ SiO2 catalysts leads to thermally induced activation which produces O- species, according to the simple scheme Mo5+ + N2O f Mo6+ + O- + N2v, proposed earlier.20,27 In order to conduct this reaction, the Mo5+ active sites play the role of the source of electrons and a center providing orbitals of proper symmetry and energy. The changes occurring within the coordination sphere of molybdenum centers which transform N2O from a mere ligand into a O- reactant, capable of abstracting hydrogen even at 77 K,1,12,35 can be deduced from the corresponding EPR spectra. The signal of isolated molybdenum center is best described by distorted compressed tetrahedron of the actual symmetry reduced to Cs.24 The analysis of the g tensor1,18 indicates that the semioccupied molecular orbital (SOMO) should be identified mainly with the dx2-y2 orbital of pentavalent Mo (Figure 5a). Tetrahedral molybdenum species easily coordinates N2O at 298 K owing to the symmetry match and spatial accessibility of the relevant SOMO and LUMO, respectively. Upon coordination of N2O, the parent signal of Mo4c5+ transforms into a signal characteristic of molybdenum(V) species of square pyramid structure with a SOMO being predominantly dxy.1,18 This transformation is indicative for rearrangement of the complex structure and attachment of the N2O molecule at the equatorial position cis with respect to the ModO bonding (Figure 5b). The changes in the energy levels of Mo which accompany this process have been inferred from the corresponding g tensor shifts

14780 J. Phys. Chem., Vol. 100, No. 35, 1996 using classic expression of the type gi ) ge - nb2λMo/∆i, as described elsewhere in detail,1,18 and are depicted in Figure 5a,b. The coordination of N2O either via O or N atom can not be discriminated on the basis of the present EPR data, although experimental fact that O- remains attached to Mo after dissociation seems to favor N2O attached by oxygen. We cannot conclude either using analogies with other transition metal complexes of N2O since those compounds are usually unstable and the linkage is still controversial (36). Dipolar character of nitrous oxide (δ+)NNO(δ-) (µ ) 0.161 D37) would suggest coordination to molybdenum via negatively charged oxygen atom. On the other hand, theoretical considerations (extended Hu¨ckel) of Tuan and Hoffmann38 favor N-linked complex of N2O with M(NH3)5 (M ) Ru, Co, Os, and N2O coordinated at axial position). This mode of linkage is explained in terms of the spatial extension of the relevant orbitals of nitrous oxide. Because of the difference in electronegativities of N and O, the extensions of antibonding orbitals relevant for N-linkage of NNO are larger than those of the O side, leading to greater overlap with the corresponding d orbitals of the metal center. For this reason, the bonds in N-linked complexes should be stronger. However, other interpretation based on force constant calculations suggests that N2O is bonded via the O atom in Ru(NH3)5 complexes.39 Since the linkage of nitrous oxide remains unresolved, we confine our discussion only to the type of orbitals involved in the bonding without specifying which atom is actually directly linked. Among the interactions allowed by symmetry and the compatibility of the orbital energies, only a π-type overlap between frontier 3π*(N2O) and dxy (Mo) orbitals and a weaker σ-type interaction (due to the less favorable energy match between 7σ(N2O) and dx2-y2 (Mo) orbitals) are important. The σ-type interaction is, however, decisive to disfavor the side-on coordination of N2O ligand. Similar π-type overlap was observed in the case of several guest molecules like C2H2, CO, N2, or nitrate, coordinated in cis to terminal oxide in a variety of oxomolybdenum complexes including, e.g., those with dithiolene or dithiocarbamato ligands.40 It was pointed out by Garner et al.41 that if such molecules bind in cis to ModO, then the symmetry is correct for the metal dxy orbitals to overlap with ligand π* orbitals. The interaction between these redox orbitals provides a pathway for the electron transfer process, which in the present case consists in the flow of the unpaired electron density from the dxy donor orbital of molybdenum toward the antibonding 3π* acceptor orbital of N2O ligand. This leads to a severe bending of the transient N2O- intermediate which through asymmetric stretching oscillation dissociates into N2 and O- (Figure 5c,d), as is discussed below. The O- species is thus formed via thermally induced metal to ligand electron transfer (MLET) process. It is worth noting that this interaction requires a particular geometry of the host surface Mo complex and a proper guest-molecule orientation. The requirement is the square pyramidal structure of molybdenyl complex and equatorial binding of N2O. From N2O as a Ligand to Adsorbed O- Ion. Electron Transfer ActiVation of N2O. General Considerations for NegatiVe Ion Process. Numerous studies have established that Ois the negative ion formed upon low-energy electron attachment to N2O according to the reaction N2O(1Σ+) + e f N2(1Σg) + O-(2P) which involves transitory negative ion N2O- resonant state.29,42,43 This observation is consistent with the order of the dissociation energies D(N2-O-) ) 0.4 ( 0.1 eV and D(NNO-) ) 5.1 ( 0.1 eV, determined from beam-collision chamber experiments31 for these two possible fragmentation schemes. The existence of N2O- was first established by Paulson42 and

Sojka and Che

Figure 6. Walsh type correlation diagram for NNO molecules (adapted from Simons, J. P. Photochemistry and Spectroscopy; Wiley: New York, 1971). The 23rd electron is indicated by the arrow; note the discontinuity of in LUMO and SOMO labeling.

then confirmed by Chantry29 and others,31 and the molecular anion has a lifetime of the order between 10-7 and 10-3 s in the gas phase.31,44 It is well-known that the geometrical configuration of many polyatomic molecules depends, to a reasonably good approximation, mainly on the number of valence electrons. Theoretical background for this observation has been provided by Mulliken45 and Walsh46 and is often referred to as the Walsh rules. It follows that the geometry of some polyatomic negative ions will differ significantly from that of the corresponding neutral molecule. A strong deformation of transient N2Ospecies results from the electronic structure of the 22- and 23electron molecules. For the triatomic neutral N2O (the nuclei arranged as linear N-N-O correspond to a C∞V symmetry) in its ground state configuration (1σ)2(2σ)2(3σ)2(4σ)2(5σ)2(6σ)2(7σ)2(1π)4(2π)4, the minimum of energy corresponds to the linear configuration of the nuclei (Figure 6). However, in the case of the 23-electron N2O- species (of Cs symmetry) with electronic configuration (1a′)2(2a′)2(3a′)2(4a′)2(5a′)2(6a′)2(7a′)2(1a′′)2(8a′)2(9a′)2(2a′′)2(10a′)1 the unpaired electron occupies the orbital 10a′ which is strongly stabilized upon bending as can be inferred from the corresponding Walsh diagram (Figure 6). According to the calculations of Hopper et al.,31 the bending angle is equal to 133 ( 2°. Since the formation of N2O- requires the change in geometry due to the C∞V f Cs deformation, there will be an energy condition for the electron transfer (bending of N2O has to precede the actual ET event) which will be explained later in detail. Thus, the mechanistic pathway for the surface dissociative ET process can be expressed as N2O + Mo4c5+

(N2O–Mo4c5+) "precursor complex" linear N2O

[N2O–Mo4c5+

N2O––Mo4c6+]#

bent N2O

(O-–Mo4c6+) + N2

(I)

"successor complex" dissociated N2O

A mechanistic subtlety of this reaction is that, in principle, the

Activation of N2O on Mo/SiO2 Catalysts

J. Phys. Chem., Vol. 100, No. 35, 1996 14781

electron transfer and the N-O bond breaking processes can occur simultaneously or consecutively, which as shown by Save´ant32,33 may have important consequences for the activation energy. The reaction, in contrast to the dissociation of neutral N2O molecule, is spin-allowed since the change in spin at each redox site is ∆S ) (1/2, i.e., as required by the condition that integration over spin wave functions must contain a nonzero component.47 Identification of the Intrinsic Barriers for Electron Transfer. Intrinsic electronic and nuclear barriers are rate-determining factors that determine at the molecular level the electron transfer process. In order to understand the origin of these barriers for reaction I, it is necessary to analyze the structural changes that accompany the ET in more detail. For electron transfer to occur between reactants, an electronic interaction must exist which determines the behavior of the system on reaching the crucial intersection at Franck-Condon region of diabatic potential energy surfaces. The primary factor influencing the degree of electronic coupling is the extent of spacial interaction between the redox dxy and 3π* orbitals. Due to the proximity of the donor and acceptor species and the correct symmetry of both orbitals we may reasonably assume that conditions for electronic coupling are satisfied.34 Therefore, we may now focus on the intrinsic energy barriers due to nuclear reorganization. Electron transfer obeys the Franck-Condon restriction which means that the nuclear positions remain frozen during ET activation. The N2O molecule must therefore acquire the energy, by appropriate deformation, necessary to reach the transition state. This indicates that the two resonance forms in Scheme I have identical arrangements of all nuclei but differ in the position of the electron to be transferred. As a consequence, they are identical in energy which implies that the process must be activated. The standard free energy of activation ∆Gq0, i.e., the free energy of activation at zero driving force, is determined by the internal reorganization factor λi which features the changes in bond length and angles of N2O and Mo active site accompanying electron transfer. Within the harmonic approximation, the internal reorganization energy can be related to the force constants of the bonds undergoing distortion during activation of N2O by the following formula48

λi ) ∑kjRkjP/(kjR + kjP)(qjR - qjP)2

(1)

j

where kjR and kjP are the normal-mode force constants of the j-th vibrational coordinate in the reactants and products respectively, and (qjR - qjP) are the changes in bond lengths and bond angles in going from reactant to product. Therefore, a severe bending of N-N-O expected from Walsh diagram (Figure 6) in passing from linear N2O molecule to the transient N2O- ion, substantially contributes to the observed activation energy. Numerical values of the stretching and bending force constants are available from experimental data for N2O (kNN ) 17.88, kNO ) 11.39 mdyn/Å, kNNO ) 0.001 245 eV/deg2, rNN ) 1.128 Å, rNO ) 1.184 Å, φNNO ) 180°) and from theoretical calculations for N2O- (kNN ) 11.56, kNO ) 3.94 mdyn/Å, kNNO ) 0.002 054 eV/deg2, rNN ) 1.222 Å, rNO ) 1.375 Å, φNNO ) 133°) enabling estimation of the N2O reorganization component of λi for the activation process.31 The obtained value of λNNO ) 2.7 eV (260 kJ/mol) is comparable to 2.3 eV (220 kJ/mol) obtained for isoelectronic NO2+ species.49 The calculation revealed that the major part of ligand reorganization energy is related to the bending of the N2O molecule (60%) and then to the stretching of its N-O bonding (25%).

Unfortunately, analogous calculation of the molecular reorganization energy of the surface Mo complex can be only approximately assessed, since not all required force constants and bonds distortions are available for both relevant Mo oxidation states. Nevertheless, on the basis of EXAFS studies of Iwasawa,50 we can infer that during the reduction of the silicagrafted ModO4+ into ModO3+ species, only the surfacebridging Mo-Ob bonds change considerably their length (from 2.11 to 1.94 Å) while the extension of the terminal ModOt bonds remains practically the same. Thus assuming a dominant contribution of Mo-Ob stretching to molybdenum surface complex reorganization energy and using the force constants 5.92 and 5.6 mdyn/Å for ModO4+ and ModO3+, respectively,51 the reorganization energy of the Mo center can be estimated to be ≈0.9 eV. Thus the resultant reorganization energy of the redox couple is equal to ∆Gq0 ) λi/4 ) (2.7 + 0.9)/4 ≈ 0.9 eV. This quantity is the measure of the intrinsic resistance to ET. The actual activation energy results from the competition between the driving force ∆G° for the ET reaction and the ∆Gq0 value opposing this process (Vide infra). If the electron transfer and NN-O bond breaking are synchronous, the dynamics is governed (except of the internal changes in lengths and angles of the bonds that are not broken) also by a contribution from bond breaking itself which is equal to a fourth of the bond dissociation energy.32 However, the recognition that ET follows a stepwise or concerted mechanism may be arduous when the N2O- intermediate is so short-lived that kinetic control occurs by ET, as in the present case. Potential Energy CurVes. Often the methods used for predicting the threshold for dissociative electron attachment to N2O make use of potential energy surfaces.28,29,31 Following the approach of Gagarin et al.52 and Larsson13 who interpreted surface reactions in terms of potential energy curves of the activated molecules, we may discuss the qualitative molecular aspects of electron transfer activation of N2O in terms of the potential surface of the N2O/N2O- system. Then, the modifications caused by the interaction of N2O with the surface Mo complex can be further considered. As already stated, the thermal activation energy is mainly associated with the difference in N2O and N2O- equilibrium geometry. In order to substantiate this idea, potential energy curves are calculated as a function of the N-O separation for both N2O and N2O- species, using Morse potential expressions28,31,32,53 of the type

GN-O(N2O) ) -2D°NO exp[-β(r-re)](1 - g(φ)/2D°)nq + D°NO exp[(-2β(r-re)] + D°NO + ∆G° (2) where D°NO ) DNO + 1/2hν0 and β ) ν0(2π2µ/D°NO)1/2, ν0 is the fundamental vibration of N2-O stretching while re is the N2-O equilibrium distance, ∆G° ) AEA(N2O) - (N2O) + (N2O-) positions the potential minimum of the neutral surface with respect to the negative, AEA stands for the adiabatic electron affinity while µ and  for the reduced mass and zero point energy respectively. The function g(φ) ) 1/2kNNO(φe φ)2 with φe the equilibrium NNO angle in neutral or negative ion molecule. The term nq where n is a fractional bond order and parameter q ≈ 0.5 was introduced, following Bu¨rgi and Dunitz,53 in order to account for deformation of the Morse function induced upon coordination. Rationalizations for the placement of the angular dependence in only attractive terms have been given elsewhere.28 For the gas phase electron attachment to nitrous oxide, the value of ∆G° parameter may be set to zero for N2O-, while for neutral N2O molecule it is determined from its AEA value corrected for difference in the

14782 J. Phys. Chem., Vol. 100, No. 35, 1996

Sojka and Che

Figure 7. Approximate potential energy curves (plotted in the vicinity of the intersection) vs internuclear rNO distance for N2O and N2O- species calculated at selected N2O and N2O- bond angles for (a) gas phase and (b) surface ET reaction. The solid curves labeled 1 and 2 correspond to equilibrium geomtry of N2O and N2O-, respectively, while dashed curves labeled 1′ and 2′ correspond to the geometry of these at the postulated transition state deduced from Franck-Condon (FC) restriction.

zero-point energies.31 This approach enables calculation of the potential energy in the function of NN-O bond distance for arbitrary values of the angle φ of NNO bending with additional formal allowance for coordination. In the case of surface electron transfer both the potential shape and the parameter ∆G° should be modified due to the interaction of N2O/N2O- with the adsorption site. Using the parameters reported by Hopper et al.,31 the explicit angle-dependent potential energy for neutral nitrous oxide and N2O- can be obtained as a function of the N-O bond length for equilibrium value of the N-N distance.

GN-O(N2O) ) -7.516 exp[-3.076(r - 1.184)][1 - 8.28 × 10-5(180 - φ)2]n0.5 + 3.758 exp[-6.152(r - 1.184)] + 3.92 GN-O(N2O) ) -0.958 exp[-4.993(r - 1.375)][1 - 1.07 × 10-3(133 - φ)2]n0.5 + 0.479 exp[-9.986(r - 1.375)] + 0.479 (3) Both functions are plotted in Figure 7a for the gas phase reaction, i.e., for the driving force determined by the adiabatic electron affinity of N2O. The solid line curves 1 and 2 correspond to N2O and N2O- potential energy curves in equilibrium geometry, respectively, while dashed curves 1′ and 2′ correspond to the potential functions in transition state deformations which obey the Franck-Condon restriction. Electron transfer takes place at the crossing locus (r*NO ) 1.28Å, φ*NNO ) 153°) between the potential energy curves 1′ and 2′ describing each species. The activation energy Ea ) 0.5 ( 0.1 eV (48 kJ/mol) for electron attachment was obtained from the energy at curves intersection with respect to the N2O equilibrium corrected for zero-point difference31 by minimizing the activation free energies. This value is quite close to that determined experimentally (0.45 eV) by Wenworth28 showing that the electron transfer is not synchronous with N-O bond breaking, since otherwise it would require an increase of the activation energy by one-fourth of the N-O dissociation energy, which contradicts the experiment. Because of the fair agreement between the experimental and calculated activation energies, this approach can be considered as satisfactory. As implied by Figure 7a owing to the shallow minimum in N2O- potential energy curve 2′, this species can appear as a temporal intermediate28,43 which then spontaneously dissociates since the crossing point is situated above the N2O- dissociation limit (Figure 7a). This means that the bond-breaking process

does not necessarily occur simultaneously with ET and the reaction can be viewed as a kinetically unresolved two-step process. The particular interesting feature of this reaction is that the nuclear reorganization energy of the bonds (mainly N2O bending) which are not broken exceeds that required for breaking the N-O bond in the N2O- intermediate but is below the dissociative ET limit of the N2O curve determined by its crossing with the repulsive N2 + O- potential.32 This indicates that if the N2O molecule is sufficiently deformed to reach the transition state for ET to occur then the dissociation of the N-O bond of the negative temporal N2O- ion will be spontaneous. Indeed, the theoretical calculation of the activation energy according to Save´ant’s model32,33 for synchronous dissociative ET yielded a value of the activation energy which was 3 times larger than that actually observed. The determination of the ET intersection locus provides useful information concerning the molecular details of this process.31 Thus identified deviation of 25° from the linearity and stretching of the N-O bond by 0.1 Å requires, within the harmonic approximation, the bending mode v2 in N2O to be excited higher than to the 4-th vibrational level and stretching mode v3 to at least 1-st level, respectively. The latter asymmetric oscillation bringing two nitrogen atoms closer, and at the same time displacing away the remaining oxygen atom facilitates the process of transfer of the quantum of this vibration into the N-O dissociation and is expected to be more efficient than the symmetric breathing v1 mode. The value obtained from this model (for gas phase ET) is about twice greater than that experimentally determined in the present study, reflecting principally the difference in the driving force for electron transfer in both cases. An insight into the driving force (the value of ∆G°) for surface ET can be obtained from the experimental activation energy and using tentatively, as a first approach, a quadratic activation-driving force relationship ∆Gq ) ∆Gq0(1 + ∆G°/4∆Gq0)2.32 This equation devised for an external (or outersphere donor) may be not strictly applicable in the present case due to the coordination of N2O, but if the magnitude of the electronic coupling does not significantly alter the diabatic activation barrier, we may expect that it at least coarsely holds. Such attempt can be supported by EPR evidence that the N2O ligand preserves its integrity upon coordination and interacts with molybdenum rather weakly since no presence of any ligand shfs coupling was observed. As a result, the value of ∆G° ) -1.86 eV was obtained indicating so-called normal region since |∆G°| < λ (the second possible solution was rejected because of the earlier experimental data on ET activation of dioxygen on the same catalysts.54

Activation of N2O on Mo/SiO2 Catalysts The driving force for the surface reaction can be assigned to the combined effect of an increase of the effective A.E.A of N2O, due to the stabilizing interaction of its LUMO with the dxy orbital of Mo, the ionization potential of ModO3+ surface center and an extra electrostatic stabilization term due to the interaction of the corresponding negative ion with the positively charged molybdenyl complex. The latter effect may be regarded as a heterogeneous counterpart of the decrease of the activation energy of ET by solvation of the ion pair, which is a phenomenon well known in solution chemistry.55 Postulated potential energy curves intersection corresponding to this value of ∆G° are shown in Figure 7b, where the curves are labeled as in Figure 7a. The minimal activation energy Ea ) 0.17 eV was obtained for r*NO ) 1.23 Å and φ*NNO ) 160° indicating a less demanding transition state in this case than in the gas phase. Thus in order to reach the Frank-Condon (FC) region (Figure 7b), the N2O molecule needs to be excited only to the 3rd vibrational level. Another important feature is the lack of the minimum in 160° N2O- curve, which indicates an essentially dissociative character of the electron transfer (without apparent involvement of the N2O- intermediate species) in this case. We do not, however, presume that both events of ET and N-O bond breaking are necessarily synchronous. We rather prefer to regard this process as a two-stage transformation, introduced originally by Dewar,56 where in the first part of the reaction changes in bonding induced by ET take place, followed by the rest of the changes (bond splitting) in the second part of the reaction. Such mechanism should be favored over the transformation in unison by lower activation energy requirement. Finally, the value of the activation energy (0.17 eV) evaluated by this procedure, which is smaller by ca. 20% than that observed experimentally (0.21 eV), can be better reconciled with experimental value by taking into account changes in potential curves induced by the fractional bond order n caused by the coordination. The calculations have revealed that in the area of the intersection, only the potential curve of N2O is significantly modified. It is found further that the decrease of the fractional bond order (due to the cooperative action of the donative σ-bonding and π-back bonding to N2O 3π* orbital) results in an increase of the ET activation energy. The best agreement with the experimental value was obtained for 2-3% decrease in N2O bond order upon coordination to surface molybdenum center. The use of the simple Morse potential model has allowed to get some insight into the dynamics of the electron transfer at surfaces. The ET process requires less activation energy on the surface (0.21 eV) than in the gas phase (0.45 eV) and involves a less demanding transition state due to an extra driving force provided by the interaction of N2O with the Mo active center. However, because of the numerous approximations involved, the present approach can only give semiquantitative estimates of parameters concerning ET process and should in no way be regarded as definitive. Electronic Structure of O- Bound to Mo6+ Center. g Tensor. The ground state 2P of O- species is split by virtue of the crystal field of the molybdenum complex. In order to account for g⊥ > ge, the 2pz′ orbital in O- is assumed to be at higher energy than the 2px′ and 2py′ levels and the separation is equal to ∆Ex,y-z (Figure 5d). The observed effective axial symmetry of O- signal points that 2px′ and 2py′ orbitals are nearly degenerate while interacting with the Mo center. This is caused by the fact that the g tensor is basically confined to the O- moiety and senses less significantly the molybdenum part of the surface adduct which in turn imparts the shfs. As a result, the principal axes of both tensors do not coincide (the g

J. Phys. Chem., Vol. 100, No. 35, 1996 14783 tensor is defined in the x′, y′, z′ framework while the shf tensor in the x, y, z one) and the spectrum may be understood in terms of “parallel” shf axis corresponding to one of the “perpendicular” g tensor axes.57 Thus the six low-field shf features are interpreted as coming from g⊥, A| components while the remaining high-field features are attributed to g|, A⊥. In the second-order perturbation theory, the shift of the components of the g tensor from the free electron value is given by ∆gij ) 2λOΣn 〈n|Li|0〉〈0|Lj|n〉/(E0 - En), where λO is the spin-orbit coupling constant of oxygen and Li, Lj are components of the angular momentum operator. Because of symmetry restrictions, the matrix elements of Lz between the ground state and other states in Figure 5d vanish. On the other hand, Lx and Ly connect the ground state with px′〉 and py′〉 states giving rise to larger g shift in these directions. Thus the g tensor components expected from these considerations for λ , ∆E can be expressed as follows:

g| ) gz′ = ge g⊥ ) gx′ = gy′ = ge + (2λOb2/∆Ex,y-z)

(4)

Using the spin-orbit coupling constant λO for O- equal to 0.014 eV35 and covalency factor b2 ) 0.96 (determined from the analysis of the 95Mo shf structure, Vide infra) the separation of the energy levels ∆Ex,y-z ) 1.5 eV was calculated from eq 4. This value remains in good agreement with that estimated from the surface crystal field approach using the formula developed by Kollrack58

E(2pz) ) Σj1.02nje(cos2 θj - 1/3)(1/rj3) E(2px,y) ) -Σj0.51nje(cos2 θj - 1/3)(1/rj3)

(5)

where θj is the angle formed by the main axis and the shortest connection between the O- ion and the jth ligand and rj the distance between the jth ligand and the O- ion. Using the values of Mo-O distance in the range 0.18-0.22 nm50 and the effective charge of molybdenum between +4 and +3,59 the value of ∆Ex,y-z ) 1.5 ( 0.16 eV can be obtained by adjusting the O-sModO distance to 0.20 ( 0.02 nm. This latter value was found to remain within this range of error whatever the charge assumed on the oxygen ligand (-2 or -1). The experimental ∆Ex,y-z value can be then used for assessment of the surface crystal field stabilization energy (CFSE) of Ospecies within the coordination sphere of the (OdMo)3+ ion. The appropriate calculations for O- with the electronic configuration given in Figure 5 lead to a CFSE ) [4 × (-1/3) + 1 × (2/3)] × 1.5 eV ) -1 eV. 95Mo Superhyperfine Tensor. The observed 95Mo shf structure clearly indicates that the simple O-pz′ orbital is not entirely adequate to account for the spin Hamiltonian parameters and that the bonding between O- and molybdenum exhibits slight covalent component. The clue to the interpretation is found in the noncoincidence of the g and hfs tensor axes, because otherwise the calculations which yield the correct spin distribution lead to the wrong directions of g and hfs tensors. The spin density on Mo center can be estimated from the analysis of the shf structure following the standard treatment to find the isotropic and anisotropic contributions.60 The 95Mo shf structure tensor consists of three different components:

A ) aisoE + T + dipA

Mo

(6)

where aiso ) (8π/3)geβegMoβNψ|(0)|2 represents the Fermi contact term (we assume that aiso ) 1/3(Ax + Ay + Az)), E is a

14784 J. Phys. Chem., Vol. 100, No. 35, 1996

Sojka and Che

unit matrix, T ) geβegMoβN〈r-3〉| + 2/7, + 2/7, -4/7| is the anisotropic shf, and the dipolar tensor dipA is defined as follows:

A⊥ ) geβegMoβN/r3

dip

(7)

A| ) -2geβegMoβN/r3

dip

The estimation of the dipolar term correction was performed for a Mo-O- distance equal to 0.2 nm (estimated earlier from crystal field analysis of the experimental g tensor) and the nuclear magnetic moment of 95Mo (-0.914βN). Since the unpaired electron is mainly localized on O- species (as revealed by the g tensor) and the hyperfine results from coupling with the nucleus of molybdenum, the dipolar tensor (through space interaction) must be subtracted to give the real anisotropic term arising from electron delocalization (through bond interaction). Further analysis of the shf structure is possible only if the signs of the MoA tensor components are known. The only acceptable combination is that all signs must be the same (positive or negative) since other combinations lead to unreasonably high (30%) spin density on molybdenum while there is good evidence from g tensor and also from 17O-labeled studies27 that the spin density is largely on O-. Since the magnetic moment of 95Mo is negative and the dz2 orbital is excluded from the bonding (Vide infra), the components of the MoA tensor should also be taken as negative quantities (if the isotropic coupling constant is negative) or positive quantities when aiso is positive.60 Only in the last case can consistent results be obtained, which lead to the following decomposition of the experimental shfs tensor

[6.7 6.7 7.8] ) 7.1[1 1 1] + [-0.65 -0.65 +1.3] + [+0.25 +0.25 -0.5] The magnitude of the isotropic coupling constant remains in reasonable agreement with the predictions inferred from its predominantly spin polarization origin, i.e., aiso ) QF3d.61 In the hexacoordinated surface Mo complex in C4V symmetry, the unpaired electron occupies a b2 orbital with no s character. In such a case, the observed aiso must arise entirely from polarization, and we may then use hyperfine data obtained for this species for assessment of the Q value (the required F3d can be estimated from the anisotropic part of the coupling). Following a method described elsewhere62 we obtained Q = 0.014 cm-1. Thus in the present case by taking F3d ) 0.043 (see below), we obtain aiso 0.043 × 0.014 cm-1 = 6.4 G, in fair agreement with the 7.1 G experimental value. The spin density in 4d Mo orbital is assessed using theoretical anisotropic hfs value of 30.6 G for molybdenum tabulated by Morton and Preston.63 The estimated spin density equal to (1.3 G/30.6 G) × 100% = 4.3% was obtained. This result consistent with the g tensor, is reasonably close to the 7% of covalent character of the Mo-O- bonding, obtained from 17O hfs analysis.27 However, it should be noted that the latter value was calculated taking the A⊥ component as zero since it could be not resolved. Usually, the hyperfine structure can be used to deduce the composition of the SOMO. Thus, the effective axial shf tensor can be interpreted in terms of coupling to basically a single d-orbital. The positive principal component of T tensor precludes the involvement of dz2 orbital in SOMO, as earlier reported,64 because for the negative magnetic moment this value should also be negative. The fact that N2O molecule is coordinated in equatorial position and that the principal value of T is positive for negative gMo would rather suggest the involvement of dx2-y2 (or dxy) in the SOMO. For these two

orbitals, the maximum of the anisotropic hfs is directed along the z direction which is the direction of the g⊥ and fits well with experimental observations, providing yet another evidence for the self-consistency of the analysis. Conclusions Changes occurring at the molecular level during electron transfer activation of N2O over reduced grafted Mo/SiO2 catalysts have been monitored by EPR spectroscopy. The surface complex of Mo4c5+ acts as a center providing orbitals of suitable energy and proper symmetry for N2O coordination and subsequent electron transfer process. The Mo4c5+ species coordinating N2O at 298 K rearranges into a pentacoordinated complex of apparent C4V symmetry with a dxy ground state, indicating attachment of the N2O ligand in equatorial position. The overlap between 3π* and dxy redox orbitals provides a pathway for ET. The O- radicals are produced via metal to ligand electron transfer (MLET) thermally induced with an activation energy of 20 ( 4 kJ/mol measured in the 323-393 K range. The N-O bond cleavage is discussed on the basis of simple and approximate model of the dynamics of ET based on Morse potential curves. The hindrance to electron transfer arises mainly from a substantial deformation of the transient N2O- from equilibrium geometry of the corresponding neutral species. This mechanism is supported by data coming from gas phase dissociative electron attachment to nitrous oxide. From the spin Hamiltonian parameters of the O- radical, the unpaired electron density on 95Mo (4.3%), the splitting of O- energy levels (∆Ex,y-z ) 1.5 eV) and associated crystal field stabilization energy (CFSE ) -1 eV) were calculated. Acknowledgment. Z.S. is grateful to the Ministe`re de l’Enseignement Supe´rieur et de la Recherche and the Universite´ P. et M. Curie for an invited Professorship (PAST programme). We are grateful to Prof. J. M. Save´ant for fruitful discussions. References and Notes (1) Sojka, Z. Catal. ReV. Sci. Eng. 1995, 37, 461. (2) Bersuker, I. B. The Jahn-Teller Effect and Vibronic Interactions in Modern Chemistry; Fackler, J. P., Jr., Ed.; Plenum: New York, 1984; p 251. (3) Ward, M. B.; Lin, M. J.; Lunsford, J. H. J. Catal. 1977, 50, 306. (4) Barbaux, Y.; Elamrani, A.; Bonnelle, J. P. Catal. Today 1987, 1, 147. (5) Liu, H.-F.; Liu, R.-S.; Liew, K. Y.; Johnson, R. E.; Lunsford, J. H. J. Am. Chem. Soc. 1984, 106, 4117. (6) Yang, T.-J.; Lunsford, J. H. J. Catal. 1987, 103, 55. (7) Lunsford, J. H. Catal. Today 1990, 6, 235. (8) Kazusaka, A.; Lunsford, J. H. J. Catal. 1976, 45, 25. (9) Shelimov, B. N.; Che, M. J. Catal. 1978, 51, 143. (10) Panov, G. I.; Kharitonov, A. S.; Sobolev, V. I. Appl. Catal A 1993, 98, 1. (11) Aika, K. I.; Lunsford, J. H. J. Phys. Chem. 1977, 81, 1393. (12) Sojka, Z.; Che, M. J. Phys. Chem. 1995, 99, 5418. (13) Larsson, R. Catal. Today 1989, 4, 235. (14) Swamy, C. S.; Christopher, J. Catal. ReV. Sci. Eng. 1992, 34, 409. (15) Cunningham, J.; Kelly, J. J.; Penny, A. J. Phys. Chem. 1970, 74, 1992. (16) Chinchen, G. C.; Hay, C. M.; Vandervell, H. D.; Waugh, K. C. J. Catal. 1987, 103, 79. (17) Seiler, W.; Conrad, R. J. Air Pollut. Control. 1981, 31, 767. (18) Louis, C.; M. Che, J. Phys. Chem. 1987, 91, 2875. (19) Che, M.; Mc Ateer, J. C.; Tench, A. J. J. Chem. Soc., Faraday Trans. 1 1978, 4, 2378. (20) Che, M.; Dyrek, K.; Louis, C. J. Phys. Chem. 1985, 89, 4526. (21) Lozos, G. P.; Hoffman, B. M. QCHPE No 265. (22) De Gray, J. A.; Rieger, Ph. H. Bull. Magn. Reson. 1987, 8, 95. (23) Che, M.; Fournier, M.; Launay, J. P. J. Chem. Phys. 1979, 71, 1954. (24) Sojka, Z.; Adamski, A.; Che, M. J. Mol. Catal., submitted for publication. (25) Sojka, Z.; Dyrek, K.; Roberge, P. C.; Che, M. Pol. J. Chem. 1991, 65, 637.

Activation of N2O on Mo/SiO2 Catalysts (26) Giamello, E.; Murphy, D.; Magnacca, G.; Morterra, C.; Shioya, Y.; Nomura, T.; Anpo, M. J. Catal. 1992, 136, 510. (27) Ben Taarit, Y.; Lunsford, J. H. Chem. Phys. Lett. 1973, 19, 348. (28) Wentworth, W. E.; Chen, E.; Freeman, R. J. Chem. Phys. 1971, 55, 2075. (29) Chantry, P. J. J. Chem. Phys. 1969, 51, 3380 and 3369. (30) Flockhart, B. D. Surf. Defect Properties Solids 1973, 2, 69. (31) Hopper, D. G.; Wahl, A. C.; Wu, R. L. C.; Tiernan, T. O. J. Chem. Phys. 1976, 65, 5474. (32) Save´ant, J.-M. Acc. Chem. Res. 1993, 26, 455. (33) Save´ant, J.-M. J. Phys. Chem. 1994, 98, 3716. (34) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401. (35) Che, M.; Tench, A. J. AdV. Catal. 1982, 31, 77. (36) Bottomley, F.; Brooks, W. V. F. Inorg. Chem. 1977, 16, 501. (37) Jalink, H.; Parker, D. H.; Stolte, S. J. Mol. Struct. 1987, 121, 236. (38) Tuan, D. F.-T.; Hoffmann, R. Inorg. Chem. 1985, 24, 871. (39) Diamantis, A. A. Sparrow, G. J.; Snow, M. R.; Norman, T. R. Aust. J. Chem. 1975, 28, 1231. (40) Mitchell, Ph. C. H. Proceedings of 4th International Conference on Chemistry and Uses of Molybdenum; Climax Molybdenum: Ann Arbor, MI, 1992; p 336. (41) Garner, C. D.; Hyde, M. R.; Mabbs, F. E.; Routledge, V. I. Nature 1974, 252, 579. (42) Paulson, J. F. AdV. Chem. Ser. 1966, 58, 28. (43) Schulz, G. J. J. Chem. Phys. 1961, 34, 1779. (44) Porter, G.; Windsor, M. W. Proc. R. Soc. A 1958, 245, 238. (45) Mulliken, R. S. Can. J. Chem. 1958, 36, 10. (46) Walsh, A. D. J. Chem. Soc. 1953, 2260. (47) Mayer, J. T.; Taube, H. In ComprehensiVe Coordination Chemistry; Wilkinson, G., Ed.; Pergamon: Oxford, U.K., 1987; Vol. 1, p 331.

J. Phys. Chem., Vol. 100, No. 35, 1996 14785 (48) Sutin, N. Prog. Inorg. Chem. 1983, 30, 441. (49) Eberson, L. Electron Transfer Reactions in Organic Chemistry: Springer: Berlin, 1987. (50) Iwasawa, Y. AdV. Catal. 1987, 35, 187. (51) Gonzalez-Vilchez, F.; Griffith, W. P. J. Chem. Soc., Dalton Trans. 1972, 1416; the value of the force constant for pentavalent Mo was estimated by interpolation between force constants of hexa- and tetravalent molybdenum given in Table 1. (52) Gagarin, S. G.; Rozovskii, M. O. Kinet. Katal. 1983, 24, 1327. (53) Bu¨rgi, H.-B.; Dunitz, J. D. J. Am. Chem. Soc. 1987, 109, 2924. (54) Sojka, Z.; Che, M., unpublished data. (55) Julliard, M.; Chanon, M. Chem. ReV. 1983, 83, 425. (56) Dewar, M. J. S. J. Am. Chem. Soc. 1984, 106, 209. (57) Geiger, W. E.; Rieger, Ph.H.; Tulyathan, B.; Rausch, M. D. J. Am. Chem. Soc. 1984, 106, 7000. (58) Kollrack, R. J. Catal. 1968, 12, 321. (59) Mc Garvey, B. R. J. Phys. Chem. 1967, 71, 51. (60) Goodman, B. A.; Raynor, R. B. AdV. Inorg. Chem. Radiochem. 1970, 13, 135. (61) Peake, B. M.; Riger, Ph. H.; Robinson, B. H.; Simpson, J. J. Am. Chem. Soc. 1980, 102, 156. (62) Sojka, Z.; Giamello, E.; Che, M. J. Phys. Chem., submitted for publication. (63) Morton, J. R.; Preston, K. F. J. Magn. Reson. 1978, 30, 577. (64) Kolosov, A. K.; Shvets, V. A.; Kazansky, V. B. Chem. Phys. Lett. 1975, 34, 360.

JP960871O