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Insight into interfacial coordination chemistry through EPR spectroscopy
Colloids and Surfaces A: Physicochemical and Engineering Aspects 158 (1999) 165 – 178 www.elsevier.nl/locate/colsurfa
Insight into interfacial coordination chemistry through EPR spectroscopy Zbigniew Sojka a,*, Michel Che b,c b
a Faculty of Chemistry, Jagiellonian Uni6ersity, ul. Ingardena 3, 30 -060 Cracow, Poland Laboratoire de Re´acti6ite´ de Surface, URA 1106, CNRS, Uni6ersite´ P. et M. Curie, 4, Place Jussieu, 75252 Paris Cedex 05, France c Institut Uni6ersitaire de France, Paris, France
Abstract The application of EPR techniques to investigate the structure of surface complexes of selected transition metal ions and their interaction with small molecules are reviewed using the concepts of interfacial coordination chemistry. Such complexes act as catalytically active sites on surfaces and exhibit distinct features and reactivity. The bonding and activation pathways of small molecules include molecular adsorption, activation via non-dissociative and dissociative electron transfer as well as electroprotic chemical transformation. Complications arising from the low symmetry of surface complexes are also discussed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: EPR; Interfacial coordination chemistry; Surface; Complex; Activation; Small molecules; Low symmetry; Adsorption; Catalysis; Catalyst; Oxide; Transition metal
1. Introduction The concepts of solution coordination chemistry have been extended to heterogeneous systems consisting of transition metal ions dispersed on solid surfaces or incorporated into solid matrices [1–4]. Despite the close analogies between the chemical status of isolated transition metal ions (TMI) at fluid-oxide interfaces and their homogeneous analogues, surface complexes exhibit many distinct features. The unusual oxidation and coordination states of TMI, the presence of surface * Corresponding author. Tel.: +48-12-633-63-77; fax: + 48-12-634-05-15. E-mail address: [email protected] (Z. Sojka)
functional groups in the inner-sphere along with low symmetry and higher thermal stability of surface complexes give rise to new types of reactivity not encountered in homogeneous chemistry [1]. Investigations of the structure of surface complexes, constituting most often active centers, and of the elementary reactions occurring within their coordination sphere are important to elucidate the dynamic phenomena involved in catalysis and the molecular aspects of surface reaction mechanisms. It is particularly fortunate that TMI involved in surface complexes turn out to function as the most suitable probes to follow their own interaction with the oxide surface in the course of the
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preparation and interaction with various reagents during the catalytic reactions. Because of their partly filled d orbitals, any change in the inner and outer coordination spheres is reflected in their optical and magnetic properties and can thus be revealed by suitable spectroscopy [1,2]. In the present paper, we consider the applications of EPR techniques to interfacial coordination chemistry of oxide surfaces in relation to adsorption and catalysis. The purpose is to show how both field benefit from EPR spectroscopy with emphasis on those aspects where greatest advance has been made. The main goals are to reveal the structure of surface complexes and understand the mechanism of their interactions with small molecules. As many surface TMI possess a heteroleptic nature, the ways of probing such structures with EPR and the impact of low symmetry on appearance of the spectra are also briefly discussed using typical examples.
cesses which are important for catalysis on supported TMI. Therefore, the structure of the surface complexes and the nature of their interactions with reactants need to be determined. A TMI can associate with the surface to form ‘outer-sphere’ or ‘inner-sphere complex’ (Fig. 1) depending on whether the surface is acting as a mere charge balancing supramolecular counter ion (electrostatic adsorption) or if the chemical bonds between the metal and surface oxygen donor ligands are formed (grafting) [4]. Due to the heterogeneous nature of the inner-sphere complexes (Fig. 1(b)) they usually exhibit a low point symmetry. The link between the inner- and outersphere complexes can be rationalized in terms of potential energy curves (Fig. 1(a)). The picture shows that the latter acts as a precursor state for the former, providing a favorable pathway of lower energy barrier for grafting. Direct process would require the release of two ligands prior to
2. Interfacial coordination chemistry The principles of interfacial coordination chemistry (ICC) have been established in relation to the preparation of catalysts composed essentially of TMI and oxide supports and their catalytic properties [1–4]. The main results of ICC include: (a) the demonstration that the surface of oxide supports may be studied at the molecular level using TMI as probes; (b) the evidence that oxide surfaces can play the role of a solvent, a counter anion and a ligand, and can thus be inserted into the spectrochemical series (Cl− BAlO− BZO− B SiO− B H2OB NH3 Ben, where ZO− stands for the framework of Y zeolite) [1,3]; and (c) the classification of the different possible interfacial coordination chemistries.
2.1. Surface complexes A surface complex can be defined simply as the stable molecular entity formed by reaction between a transition metal complex in fluid phase and functional groups exposed at the surface of a solid [5]. The reactivity of these molecular species determines the mechanism of many surface pro-
Fig. 1. Postulated potential energy curves (a) for outer-sphere and inner-sphere surface complexes (b) and the corresponding Boltzmann energy distribution for the population of complexes in solution (c) (Adapted from M. Che, Y.G. Shul, Sci. Technol. Catalysis, 1994, 21).
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Table 1 Relationships between g and A tensors, the point symmetry and EPR symmetry of paramagnetic center (adapted for surfaces from [7]) EPR symmetry
Restrictions on g and A tensors
Coincidence of principal axes of g and A tensors
Molecular point symmetry
Surface point symmetry
Isotropic
gxx = gyy = gzz Axx = Ayy = Azz gxx = gyy " gzz Axx = Ayy " Azz gxx " gyy " gzz Axx " Ayy " Azz gxx = gyy " gzz Axx = Ayy " Azz gxx " gyy " gzz Axx " Ayy " Azz gxx " gyy " gzz Axx " Ayy " Azz
All coincident a =b= g =0°
Oh, Td, O, Th, T
–
All coincident a =b =g = 0°
D4h, C46, D4, D2d, D6h, C66, D6, D3h, D3d, C36, D3 D2h, C26, D2
C46, C66, C36
Axial Rhombic Axial noncollinear Monoclinic Triclinic
All coincident a =b= g =0° Only gzz and Azz coincident a "0°, b= g =0° One axis of g and A coincident a" 0°, b =g =0° All non-coincident a "b "g "0°
grafting, and if the barrier is high it involves only a small fraction of the bulk complex population (given by a Boltzmann distribution, Fig. 1(c)). The intersection of both potential curves determines the activation energy Ea for this transformation. If the crossing point is below the line of zero potential energy (dotted curve), the overall process is non-activated, if above (solid curve) it requires activation. The coordination sphere of TMI can be followed directly by EPR and/or with the aid of appropriate probe molecules [1,2].
3. Determination of the symmetry of surface complexes from powder EPR spectra The surface imposes some restrictions on the existence of permissible symmetry elements. The analysis has shown that for flat and uniform crystalline surfaces the inner-sphere complexes can rigorously belong to one of the following point groups: C1, Cs, C26, C4, C46, C3, C36, C6 and C66 [6]. In the case of microporous and layered materials due to the particular geometry of the surface, the cage and interlayer complexes can exhibit higher symmetries. The point symmetry at the transition metal complexes determines whether the principal values of the g or hyperfine (hf) A tensors are required to
C26
C3, S6, C4, S4, C4h, C6, C3h, C6h C3, C4, C6 C2h, Cs, C2
Cs, C2
C1, Ci
C1
be equal to each other or not [7]. Also it determines whether the principal axes of A and g coincide or not. As a result, each type of the EPR signal is associated with a restricted number of molecular or surface point symmetries (Table 1). Thus for a system of unknown structure, the EPR symmetry, if correctly determined, can provide valuable structural information. However, it should be noted that the degree to which EPR parameters can reflect the symmetry of paramagnetic species will essentially depend on the strengths of the metal-ligand interactions. The typically low symmetry of the surface complexes implies low symmetry of the corresponding EPR spectra, which can be recognized only if both g and A tensors are available. Although at first glance, it would appear that in powder EPR spectra information about the relative orientations of the principal axes of the g and A tensors is lost, it is often possible to find their relative orientation providing that the metal nuclear spin I be greater than 1. In general, the effects of axis non-coincidence will be most pronounced if both tensors have anisotropies large and comparable in magnitude. Then, for large values of mI the field extrema will occur at angles close to the hyperfine tensor axes, while for small mI , the extrema will correspond to angles close to the g tensor axes [8]. The result of such a competition produces a series of lines with distinctly uneven spacings, which can
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not be accounted for basing on the second order effects only. Another simple indication of axes non-coincidence is the apparent presence of too many features in the spectrum, lines appearing at non-expected positions or lacking of expected lines while extrapolating hfs starting from either side of the spectrum. An important criterion of low symmetry is the impossibility to simulate spectra assuming coincidence [7]. The appearance of a resolved superhyperfine structure (shfs) due to coupling of the unpaired electron with magnetic nuclei (I " 0) of ligands provides an additional means of identification of the coordination sphere. The above considerations underlines the importance of using isotopically enriched species and computer simulation in extracting the maximum information from powder EPR spectra.
A good example of this approach is provided by the EPR study of a {SiO}2Mo5 + (OD)CO surface complex obtained by adsorption of carbon monoxide on a Mo/SiO2 catalyst activated by UV-irradiation under 30 Torr of D2 at 77 K [9]. Although the ‘orthorhombic’ g tensor (see Fig. 2(a) and Table 2) suggests a C2v (or lower) symmetry of the complex, the non-coincidence of g and A axes deduced from the 95Mo-labeled species (Fig. 2(b)) reveals the monoclinic nature of the spectrum which restricts the point group to Cs to which the Mo complex belongs. Further experiments using 13C enriched CO and OH ligands (Fig. 2(c and d)) allowed to identify a CO and a OH ligand within the molybdenum coordination sphere. As a result a structure of the complex (shown below) compatible with Cs symmetry was proposed (Scheme 1).
Fig. 2. EPR spectra (X band, 77 K) after adsorption of 12CO onto: (a) Mo/SiO2 catalyst irradiated by UV at 77 under D2 (b) onto 95 Mo enriched Mo/SiO2 UV-irradiated at the same conditions as in (a). The non-compatible stick diagrams obtained while extrapolating the 95Mo hfs (A3) starting from a low and a high field part of the spectrum indicate the non-coincidence of the g and A axes. The same as in (a) but after adsorption of (c) 13CO (top first and bottom third derivative spectrum) and (d) UV-irradiated at 77 K under H2.
Z. Sojka, M. Che / Colloids and Surfaces A: Physicochem. Eng. Aspects 158 (1999) 165–178 Table 2 EPR parameters of selected transition metal complexes supported on silica surface containing Sample
Surface complex
g tensor
13
V/SiO2 Mo/SiO2
O =VIV(OL)3(CO)2 O= MoV(OL)2OHCO
Mo/SiO2
O= MoV(OL)3(CO)2
Ni/SiO2
NiI(OL)2(CO)2
Ni/SiO2
NiI(OL)(CO)4
gÞ = 1.985, gW =1.931 g1 = 1.965, g2 =1.941, g3 = 1.891 g1 = 1.969, g2 =1.965, g3 — g1 = 2.191, g2 =2.086, g3 = 2.066 gÞ = 2.130, gW =2.009
C shfsa tensor (mT)
169
13
CO ligands
Ground state
Reference
AÞ :0.7, AW50.7 A1 =0.65
dp(xy) dp(xy)
[14] [9]
A1 =0.75, A2 =0.75, A3 — A1 =3.0, A2 =3.25, A3 =3.25 AÞ =5.15b, AÞ =2.5c AW =5.5b, AW =5.5c
dp(xy)
[11–13]
ds(x2–y2)
[26]
ds(z2)
[26]
a
shfs, superhyperfine splitting. Shf splitting of axial ligand. c Shf splitting of equatorial ligand. b
3.1. Use of probe molecules to assist EPR spectra assignment Often the interpretation of EPR spectra is complicated by overlapping signals due to more than one surface complex. In such cases, temperatureand pressure-controlled adsorption of probe molecules can help to assign spectra [2,10]. EPR has proven to be a powerful tool for investigating interfacial coordination chemistry by means of ligands with magnetic (I" 0), and non-magnetic (I = 0) nuclei and able to fill up coordination vacancies producing characteristic changes in EPR spectra. Typical probe molecules are 13CO [11], H2O [11,12], CH3OH [13] which form predominantly s-type bonding between the lone pair and 3d orbitals of the TMI of appropriate symmetry. While water and methanol probes can provide information on the coordinative unsaturation of the central cation, via the g tensor changes upon coordination, 13CO can produce a shf structure due to the interaction of the nuclear spin I =1/2 of 13C with the unpaired electron. The counting of vacancies in transition metal ions with 13 CO has been shown to be particularly suitable for TMI 3ds orbitals carrying the unpaired electron, which have lobes pointing along the metalligand bonds (e.g. the dz2 or dx2 − y2 orbitals of Ni+ in C36 or C26 symmetry) that are able to make bonding with s donor CO probe. The magnitude of coupling in such cases is usually suffi-
ciently large (2.0–2.5 mT, Table 2) to enable a straightforward analysis of the shf structure. However, for ions with 3dp orbitals pointing between ligands, like dxy of Mo5 + or V4 + in C46 symmetry, the superhyperfine coupling constant between TMI and 13CO is significantly smaller (0.4–0.75 mT, Table 2) due to the dismatch between the dp and ligand s-orbitals. However, they can produce a considerable overlap with p-bonding ligands. In order to improve the poor resolution often observed, first and third derivative EPR spectra may be employed after adsorption of 13 CO [14]. It was recently found [15] that ligands with 31P (I= 1/2, 100%) such as alkylphosphines, are better probes than those with 13C for the determination of coordination unsaturation, due to the much larger hyperfine values of 31P (Aiso =364 mT and Aaniso = 20.6 mT) in comparison to 13C (Aiso = 113 mT and Aaniso = 6.6 mT). This type of
Scheme 1.
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Fig. 3. EPR spectrum (X band, 77 K) of the molybdenyl bis(triethylphosphine) and its computer simulation (a), and schematic picture of the steric hindrance of phosphines (b).
probe was used to determine the number of coordination vacancies in tetracoordinated molybdenyl Mo54c+ species grafted onto silica, even in the presence of other more abundant hexa(Mo56c+ ) and pentacoordinated (Mo55c+ ) species. The number n = 2 of phosphine molecules filling up the coordination vacancies of Mo54c+ is directly deduced from the number N of the shf
lines N= 2nI+ 1 (Fig. 3(a)). Adsorption of bulky phosphines with different cone angles helped to determine the stereochemistry of molybdenum surface complexes and to distinguish between two possible structures of the tetracoordinated Mo54c+ of pseudo D2d and C3v symmetry, as both exhibit quite different spatial accessibility (Fig. 3(b)).
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4. Surface adducts of transition metal ions with CO, NO, O2, N2O CO, NO, O2, molecules can bind to surface mononuclear transition metal ions to form adducts with geometries ranging from the common linear top-on coordination h1 (a) through bent and kinked geometry (b) and (c), to side-on h2 structure (d) until complete dissociation of the bond (e) leading to oxidative addition [16] (Scheme 2). The chemical interactions established upon coordination involve redistribution of the electron density within the adduct (via s/p donor–acceptor interactions), rehybridization, and electron transfer. In the following, we will illustrate the role of surface metal complexes in the activation of small molecules on selected examples coming from our laboratories. We use the notation {MXY}r of Enemark and Feltham, where r stands for the number of electrons on d and p* orbitals of the metal M and the bound molecule XY, respectively [16,17]. We also will discuss electron transfer activation which refers to two different cases where the ligand coordinated to the metal center either does not dissociate or dissociates upon electron transfer.
4.1. Molecular adsorption 4.1.1. Acti6ation through coordination: binding of CO to surface TMI CO binds to mononuclear transition metal ions via the C atom to form a s-donor bonding, involving d orbitals. The bonding is reinforced by back donation of the metal dp electrons to the empty CO p* orbitals (Blyholder structure [18]). Such coordination is linear and only a partial net charge transfer between the metal and the ligand takes place. The number of coordinated CO
Scheme 2.
171
molecules and the nature of the bonding can be deduced from the shf structure observed providing that 13C-labeled carbon monoxide is used (Table 2). Interaction of CO with Mo/SiO2 catalysts at various temperatures and pressures may serve an example of molecular adsorption [10,13]. CO interacts only with the most reactive tetracoordinated Mo54c+ centers forming a monoh1{MoCO}1 and a dicarbonyl h1{Mo(CO)2}1 adduct, identified by the simultaneous use of 13C labeling and third derivative recording. Depending on the pCO and the temperature, dicarbonyl adduct can be formed directly or via monocarbonyl intermediate which reversibly transforms into dicarbonyl. From the EPR parameters, it is deduced that the bonding is basically s-donor in character as the back-donation inferred from the shf structure is small. This leads to the formation of a partial positive charge on the carbon atom which will favor nucleophilic attack at this position [19].
4.1.2. Acti6ation through rehybridization: adsorption of NO onto Cu/ZSM-5 Due to its electronic configuration, NO can bind to TMI to form a linear (hybridization sp), a bent (sp2) and an intermediate (spn, 1B nB2) species. Its chemical reactivity is thus strongly related to the type of bonding since change of the coordination mode involves both rehybridization and a change in the oxidation state in the NO ligand. An example of NO binding studies is provided by adsorption of NO on Cu-exchanged ZSM-5 zeolites [20]. NO contacted with Cu+/ZSM-5 under low pressures forms a {CuNO11} adduct of h1 geometry (Cs symmetry). Its EPR spectrum (gx = 1.999, gy = 2.003, gz = 1.889, CuAx = 16 mT, Cu Ay = 15.5 mT, CuAz = 20.5 mT, NAx = 3.0 mT, N Ay = 0.43 mT, NAz = 0.55 mT) with well-defined hyperfine coupling with 63,65Cu and 14N nuclei (Fig. 4(a)) was interpreted in terms of completely anisotropic g and A tensors with non-coincident axes (Fig. 4(b)). From the EPR parameters, it was inferred that {CuNO11} is a bent complex in which copper remains monovalent and NO carries most of the spin density. The bonding interaction
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Fig. 4. (a) Monoclinic EPR spectrum (X band, 77 K) of h1CuNO11 adduct (solid line) and its simulation (line with black points) and (b) the reference axes for g and Cu shf tensors. Both systems are non-coincident in xz plane.
involves essentially an overlap of Cu 3dz2 and 3dxz orbitals with the antibonding 2p p*x orbital and a lone pair n of NO ligand as well as an overlap of the Cu 3dyz orbital with a second orthogonal NO 2p p* y orbital. The molecular model developed for interpreting the magnetic parameters allowed to determine the principal bonding characteristics. The total spin density on copper was found to be equal to 0.2 and is shared among the 3dz2 (0.079), 3dxz (0.021) and 4s (0.1) orbitals. The remaining part of the spin density is localized on nitrogen (0.55) and oxygen (0.25) atoms. The Cu-N-O bending angle a = 20° is associated with the rehybridization of NO orbitals upon coordination. The implications of this coordination mode on the possible mechanism of the catalytic decomposition of NO over Cu/ZSM-5 were discussed. It was proposed that the redistribution of the electron density which results from the angular geometry of the adduct appears functional in the formation of a NN bond via an electrophilic attack by the gas phase NO at the nitrogen atom of Cu+-NO adduct and opens an outer-sphere pathway for the decomposition of NO (Scheme 3).
4.2. Electron transfer acti6ation 4.2.1. Non-dissociati6e electron transfer: interaction of dioxygen with Co/MgO Dioxygen can bind to TMI forming adducts which can have a bent h1 (Pauling structure), a h2 (Griffith structure) geometry, bridged m structure and involve combinations of those generic forms [16]. Extensive EPR studies using 16O2 and 17O2 (I= 5/2) have shown that Co/MgO solid solution provide a unique model system to investigate the multistep pathway of the adsorption process of dioxygen [21–23]. Dilute Co/MgO solid solutions contain mainly isolated Co2 + ions exposed predominantly at (100) planes. Those ions have been found to function as heterogeneous oxygen carriers since they are able to reversibly bind oxygen [21,23]. By adsorption of O2 at 77 K below 0.1 Torr, a complex spectrum with a distinct shf structure due to 59Co (I=7/2, 100%) is obtained showing two slightly different h1{CoO2}9 adducts (covalent complexes) localized at distinct Co3 + sites (species I and II in Fig. 5(a)). The spectrum corresponds to the initial stage of oxygen activation, since two species, stable at 77 K, undergo differ-
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ent evolution when the temperature is increased to 120–150 K under slight evacuation. This treatment leads to the spectrum of Fig. 5(b) composed of two superimposed signals. The first one belongs 2+ to a new complex Co3 + – m – h1:h1 – O− 2 –Mg (species III), where the superoxide ion adsorbed on Co3 + is simultaneously stabilized by interaction with an adjacent Mg2 + . The second one is due to side-on O− adsorbed on Mg2 + center 2 2 − 2+ complex (species IV). The forming h O2 – Mg cobalt superoxide adducts disappear upon evacuation at room temperature leading to species IV (Fig. 5(c)) but they can be reversibly restored by readsorption of dioxygen at low temperature. The nature of these various species has been confirmed with 17O-enriched oxygen [22]. Analysis of the 17O and 59Co hf and shf structures, respectively, indicated that species I, II and III have inequivalent oxygen nuclei corresponding to a bent CoOO geometry, while species IV exhibits oxygen nuclei equidistant from the Mg2 + surface cation. The data obtained revealed that the CoO2 bonding has a s – p character with the highest occupied molecular orbital (HOMO) formed by the overlap of the Co 3dz2 and 3dyz orbitals with while the singly occupied molecular the O p*(2p), z orbital (SOMO) is produced from the Co 3dxz and orbitals. The extent of the metal to the O p*(2p) x ligand electron transfer (MLET) was found to be 0.8 for the adducts I and II and increased further to 0.86 for adduct III, reaching the ultimate value of 0.96 for adduct IV.
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The mechanism of adsorption which has a ‘cooperative character’ consists of an initial adsorption of O2 on the Co2 + ion acting as electron donor site with formation of a covalent complex. The superoxide ion is then stabilized by bridging to an adjacent Mg2 + before it migrates towards matrix Mg2 + centers with the formation of an electrostatic complex. The mechanism can be written as follows: h1 O2− –Co3 + Co3 + –m–h1:h1O2− –Mg2 + h2 O2− –Mg2 + Along this activation pathway, the superoxide adducts undergoes a significant change in geometry and electronic structure. The process involves a gradual change of the orientation of superoxide species, which is accompanied by a shortening of the CoO bonding and an increase of its ionicity (the extent of electron transfer) [22]. This leads to a lengthening of the OO distance as the bond order decreases and a further activation of dioxygen (Fig. 5(d)).
4.2.2. Dissociati6e electron transfer: acti6ation of N2O on Mo/SiO2 The N2O molecule is the prototype of linear triatomic molecules, the activation of which requires vibrational excitation leading to bending before the MLET can occur. To initiate the reaction, the TMI of the surface complex acts as a source of electron and provides orbitals of proper energy and symmetry for coordination. The entire process consists of three steps involving: (a) N2O
Scheme 3.
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Fig. 5. EPR spectrum (X band, 77 K) of dioxygen adsorbed onto Co/MgO solid solutions. (a) species I and II observed immediately after adsorption of oxygen at 77 K, (b) species III and IV observed upon evacuation at 120 – 140 K, (c) species IV detected after evacuation at room temperature, (d) changes in the geometry of superoxide species during dioxygen activation.
coordination; (b) vibronic preactivation (C 6 Cs transformation); and (c) dissociative MLET. The changes in coordination and oxidation state of molybdenum occurring in this reaction can be best followed by EPR for the N2O-Mo5 + / SiO2 system using natural and 95Mo-enriched molybdenum [24]. The tetrahedral molybdenum centers Mo5 + /SiO2 readily coordinate N2O at 298 K owing to the symmetry match and spatial accessibility of the relevant metal and ligand orbitals. Consequently, the initial EPR signal (Fig. 6(a)) of tetracoordinated Mo5 + with gÞ = 1.926 and g ® =1.76 transforms into another one with
gÞ = 1.957 and g ® = 1.87 (Fig. 6(b)) due to pentacoordinated Mo5 + of apparent C46 symmetry, indicating rearrangement of the complex due to attachment of N2O in an equatorial position. The O− radical (gÞ = 2.020 and g ® = 2.005) is produced upon a temperature-induced MLET (Fig. 6(c)). The well pronounced 95Mo shfs shows that O− remains within the coordination sphere of molybdenum (Fig. 6(d)). The overlap between 3p* and 3dxy orbitals provides a pathway for electron transfer. The activation energy was found to be 20 4 kJ/mol in the temperature range 323– 393 K. The hindrance to MLET arises mainly
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from the inner reorganization of nitrous oxide upon bending from linear N2O (C6 ) to the transient bent N2O− (Cs ) species. N2O +Mo54c+ N2O-Mo54c+ (N2O-Mo54c+ l N2O − –Mo64c+ ) c O − – Mo64c+ +N 2 The use of an angle-dependent Morse potential has allowed to get some insight into the dynamics
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of the electron transfer at the surface. The MLET process requires a lower activation energy than in the gas phase and involves a less demanding transition state (r*NO = 0.123 nm and f*NNO = 160o). From the spin Hamiltonian parameters of the O− radical, the spin density on 95Mo (4.3%), the splitting of O− energy levels (DEx,y-z = 1.5 eV) and associated crystal field stabilization energy (CFSE= − 1 eV) were calculated.
Fig. 6. EPR spectra (X band, 77 K) of grafted 0.33 wt.% Mo/SiO2 catalyst (a) prior to and (b) after adsorption of 30 Torr of N2O at 293 K. Spectra obtained upon heating at 373 K on (c) natural and (d) 95Mo-enriched molybdenum.
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Analysis of the data shows that the reaction consists of two processes: (a) an activation of the C-H(sp3) bonding of CH3OH occurring through ligand to ligand hydrogen transfer (LLHT) to form a hydroxymethyl radical; and (b) the conversion of transient ·CH2OH into final CH2O with concomitant reduction of Mo. This latter ‘electroprotic’ step involves a ligand to metal electron transfer (LMET) coupled with a ligand to ligand proton transfer (LLPT) (CH3OH+ O − )-Mo6 + (·CH2OH + OH − )-Mo6 + Fig. 7. Kinetics of the reaction between O− and CH3OH ligands within the coordination sphere of molybdenum at 210 K; [O−] and [Mo5 + ] refer to change of the intensity of the EPR signal of these ions with time while ln[O−] line represents the semi-logarithmic plot of O− decay.
5. Electroprotic transformation: oxidative dehydrogenation of CH3OH on Mo/SiO2 catalysts The dehydrogenation of methanol to formaldehyde involves the extraction of two chemically different hydrogens coming from the methyl and hydroxyl groups and formally requires the elimination of two protons and two electrons. In an attempt to explain the mechanism of this process, cycles of model reactions between CH3OH and in situ-generated O−, both adsorbed on grafted Mo/ SiO2 catalysts, have been studied by EPR [25]. The use of O− has allowed a bypass the rate-determining step, i.e. the activation of the CH(sp3) bond and to reveal the subsequent steps of the reaction. The finding that each step started at a different threshold temperature allowed to isolate the different steps and to study them in detail. The interaction between adsorbed O− (produced from N2O) and methanol on oxidized molybdenum was first investigated. The sequence of events observed includes coordination of N2O, formation of O− intermediate, adsorption of methanol and the inner-sphere reaction between O− and CH3OH ligands. The reaction is accompanied by the reduction of the Mo center (the Mo5 + EPR signal intensity increases while that of the O− signal simultaneously decreases, Fig. 7).
(·CH2OH + OH − )-Mo6 + + O2 − CH2Oads + OH − -Mo5 + + OH − The actual reductant ·CH2OH intermediate was detected in a separate experiment where methanol was adsorbed on isolated Mo centers before the O− species were generated at 77 K by UV-irradiation of the MoO bond (Fig. 8 top). Its appearance was accompanied by simultaneous reduction of molybdenum due to the LLHT process (socalled ‘initial reduction’ step)
Fig. 8. EPR spectra (X band, 77 K) obtained after 10 min of UV irradiation at 77 K with methanol preadsorbed on grafted (top) and impregnated Mo/SiO2 (bottom) catalysts. The doublet line corresponds to (H radical, the triplet one to (CH2OH while the broader signal at the high field side corresponds to Mo5 + species
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Fig. 9. (a) EPR spectra taken at 77 K (solid line, initial reduction) and after exposure to temperatures above 140 K (dotted line, terminal reduction); (b) reduction of molybdenum by hydroxymethyl radicals at 140 K. UV
Mo6 + =O2- OH(Mo5 + —O−) +CH3OH CH3
·CH2OHads +OH−Mo5 + The hydroxymethyl radicals, stable below 140 K, decay above this temperature, reducing the remaining Mo6 + ions via LMET coupled with LLPT (terminal reduction) electroprotic step: − ·CH2OH +Mo6 + + O2surf − CH2Oads + OHsurf +Mo5 +
as shown by the increase of the Mo5 + and the disappearance of the hydroxymethyl radical EPR signals (Fig. 9). Since the Mo species were isolated and reduced in the first step of the reaction, the hydroxymethyl radicals have to spillover onto silica searching for Mo6 + sites for their ultimate transformation. Thus the resultant electron transfer has a ‘non-complementary’ character, as isolated molybdenum sites cannot accept all the electrons supplied by CH3OH molecule during the reaction. The whole process thus involves three steps: (a) initial reduction; (b) migration; and (c) terminal reduction. However, in the case of clustered Mo sites migration of hydroxymethyl is not necessary because such sites can accommodate more than one electron and the whole process involves a ‘complementary’ electron transfer. This is indeed the case of impregnated samples (i.e.
possessing predominantly clustered Mo sites). In analogous experiment, hydroxymethyl radicals were not detected by EPR and only the signal of reduced molybdenum was present (Fig. 8 bottom). The results obtained have allowed to unravel the elementary steps of the oxidative dehydrogenation of methanol: electron, proton and hydrogen atom transfers (LMET, LLPT, LLHT) and to ascertain the sequence and conditions of their appearance. It was shown that the molybdenum active site consists of a metal redox center and coacting O− and O2 − ligands. The surface complex acts also as a center for molecular rearrangement of both reactants (CH3OH and N2O). The metal redox couple Mo6 + /Mo5 + functions as a trap and source of electrons while O− activates the CH(sp3) bond and O2 − acts as a trap of protons.
6. Conclusions EPR spectroscopy appears very useful in studying phenomena occurring at oxide surfaces and can allow to distinguish various types of surface complexes, to identify them and to follow their transformations upon contact with reactants.
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Changes occurring at the molecular level during binding and activation of small molecules by surface transition metal ions can be directly monitored as the latter can act as EPR self-probes. The surface complexes act as centers providing orbitals of suitable energy and proper symmetry for coordination of molecules and electron transfer processes. Many aspects of interfacial coordination chemistry can be resolved and understood at the molecular level, by means of EPR. However, being very sensitive but limited to paramagnetic species only, EPR spectroscopy may provide a selective and fragmentary picture of phenomena involved in interfacial coordination chemistry, and is more effective when employed in concert with other techniques such as UV-VIS, Raman, IR, EXAFS or photoluminescence.
Acknowledgements Z. Sojka is grateful to the Universite´ Pierre et Marie Curie for invited Professorship in 1998 and for KBN (Poland) for financial support (Grant No 3T09A07314)
References [1] M. Che, Stud. Surf. Sci. Catal. 75A (1993) 31. [2] K. Dyrek, M. Che, Chem. Rev. 97 (1997) 305.
.
[3] J.F. Lambert, M. Hoogland, M. Che, J. Phys. Chem. 101 (1997) 10347. [4] Ch. Lepetit, M. Che, J. Mol. Catal. A 100 (1995) 147. [5] G. Sposito, Chimia 43 (1989) 169. [6] A.M. Bradshaw, Z. Phys. Chem. N. F. 112 (1978) 33. [7] F.E. Mabbs, Chem. Soc. Rev. (1993) 313. [8] Ph.H. Rieger, in: W.C. Trogler (Ed.), Organometallic Radical Processes, Elsevier, Amsterdam, 1990, Chapter 8, p. 270. [9] B. Canosa, A.R. Gonzalez-Elipe, M. Che, J. Catal. 131 (1991) 300. [10] Z. Sojka, Catal. Rev.-Sci. Eng. 37 (1995) 461. [11] C. Louis, M. Che, J. Phys. Chem. 91 (1987) 2875. [12] M. Che, C. Louis, Z. Sojka, J. Chem. Soc. Faraday Trans. I 85 (1985) 3939. [13] Z. Sojka, K. Dyrek, P. Roberge, M. Che, Pol. J. Chem. 65 (1991) 637. [14] M. Che, B. Canosa, A.R. Gonzalez-Elipe, J. Phys. Chem. 90 (1986) 618. [15] Z. Sojka, A. Adamski, M. Che, J. Mol. Catal. A 112 (1996) 469. [16] R. Hoffmann, M.M.-L. Chen, D.L. Thorn, Inorg. Chem. 16 (1977) 503. [17] J.H. Enemark, R.D. Feltham, Coord. Chem. Rev. 13 (1974) 339. [18] G. Blyholder, J. Phys. Chem. 68 (1964) 2772. [19] Z. Sojka, D. Sc. Dissertation, Cracow, 1997. [20] Z. Sojka, M. Che, E. Giamello, J. Phys. Chem. B 101 (1997) 4831. [21] E. Giamello, Z. Sojka, M. Che, A. Zecchina, J. Phys. Chem. 90 (1986) 6084. [22] Z. Sojka, E. Giamello, M. Che, A. Zecchina, K. Dyrek, J. Phys. Chem. 92 (1988) 1541. [23] M. Che, K. Dyrek, E. Giamello, Z. Sojka, Z. Phys. Chem. N. F. 152 (1987) 139. [24] Z. Sojka, M. Che, J. Phys. Chem. 100 (1996) 14776. [25] Z. Sojka, M. Che, J. Phys. Chem. 99 (1995) 5418. [26] L. Bonneviot, D. Olivier, M. Che, J. Mol. Catal. 21 (1983) 415.
Insight into interfacial coordination chemistry through EPR spectroscopy Zbigniew Sojka a,*, Michel Che b,c b
a Faculty of Chemistry, Jagiellonian Uni6ersity, ul. Ingardena 3, 30 -060 Cracow, Poland Laboratoire de Re´acti6ite´ de Surface, URA 1106, CNRS, Uni6ersite´ P. et M. Curie, 4, Place Jussieu, 75252 Paris Cedex 05, France c Institut Uni6ersitaire de France, Paris, France
Abstract The application of EPR techniques to investigate the structure of surface complexes of selected transition metal ions and their interaction with small molecules are reviewed using the concepts of interfacial coordination chemistry. Such complexes act as catalytically active sites on surfaces and exhibit distinct features and reactivity. The bonding and activation pathways of small molecules include molecular adsorption, activation via non-dissociative and dissociative electron transfer as well as electroprotic chemical transformation. Complications arising from the low symmetry of surface complexes are also discussed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: EPR; Interfacial coordination chemistry; Surface; Complex; Activation; Small molecules; Low symmetry; Adsorption; Catalysis; Catalyst; Oxide; Transition metal
1. Introduction The concepts of solution coordination chemistry have been extended to heterogeneous systems consisting of transition metal ions dispersed on solid surfaces or incorporated into solid matrices [1–4]. Despite the close analogies between the chemical status of isolated transition metal ions (TMI) at fluid-oxide interfaces and their homogeneous analogues, surface complexes exhibit many distinct features. The unusual oxidation and coordination states of TMI, the presence of surface * Corresponding author. Tel.: +48-12-633-63-77; fax: + 48-12-634-05-15. E-mail address: [email protected] (Z. Sojka)
functional groups in the inner-sphere along with low symmetry and higher thermal stability of surface complexes give rise to new types of reactivity not encountered in homogeneous chemistry [1]. Investigations of the structure of surface complexes, constituting most often active centers, and of the elementary reactions occurring within their coordination sphere are important to elucidate the dynamic phenomena involved in catalysis and the molecular aspects of surface reaction mechanisms. It is particularly fortunate that TMI involved in surface complexes turn out to function as the most suitable probes to follow their own interaction with the oxide surface in the course of the
0927-7757/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 1 4 4 - 2
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preparation and interaction with various reagents during the catalytic reactions. Because of their partly filled d orbitals, any change in the inner and outer coordination spheres is reflected in their optical and magnetic properties and can thus be revealed by suitable spectroscopy [1,2]. In the present paper, we consider the applications of EPR techniques to interfacial coordination chemistry of oxide surfaces in relation to adsorption and catalysis. The purpose is to show how both field benefit from EPR spectroscopy with emphasis on those aspects where greatest advance has been made. The main goals are to reveal the structure of surface complexes and understand the mechanism of their interactions with small molecules. As many surface TMI possess a heteroleptic nature, the ways of probing such structures with EPR and the impact of low symmetry on appearance of the spectra are also briefly discussed using typical examples.
cesses which are important for catalysis on supported TMI. Therefore, the structure of the surface complexes and the nature of their interactions with reactants need to be determined. A TMI can associate with the surface to form ‘outer-sphere’ or ‘inner-sphere complex’ (Fig. 1) depending on whether the surface is acting as a mere charge balancing supramolecular counter ion (electrostatic adsorption) or if the chemical bonds between the metal and surface oxygen donor ligands are formed (grafting) [4]. Due to the heterogeneous nature of the inner-sphere complexes (Fig. 1(b)) they usually exhibit a low point symmetry. The link between the inner- and outersphere complexes can be rationalized in terms of potential energy curves (Fig. 1(a)). The picture shows that the latter acts as a precursor state for the former, providing a favorable pathway of lower energy barrier for grafting. Direct process would require the release of two ligands prior to
2. Interfacial coordination chemistry The principles of interfacial coordination chemistry (ICC) have been established in relation to the preparation of catalysts composed essentially of TMI and oxide supports and their catalytic properties [1–4]. The main results of ICC include: (a) the demonstration that the surface of oxide supports may be studied at the molecular level using TMI as probes; (b) the evidence that oxide surfaces can play the role of a solvent, a counter anion and a ligand, and can thus be inserted into the spectrochemical series (Cl− BAlO− BZO− B SiO− B H2OB NH3 Ben, where ZO− stands for the framework of Y zeolite) [1,3]; and (c) the classification of the different possible interfacial coordination chemistries.
2.1. Surface complexes A surface complex can be defined simply as the stable molecular entity formed by reaction between a transition metal complex in fluid phase and functional groups exposed at the surface of a solid [5]. The reactivity of these molecular species determines the mechanism of many surface pro-
Fig. 1. Postulated potential energy curves (a) for outer-sphere and inner-sphere surface complexes (b) and the corresponding Boltzmann energy distribution for the population of complexes in solution (c) (Adapted from M. Che, Y.G. Shul, Sci. Technol. Catalysis, 1994, 21).
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167
Table 1 Relationships between g and A tensors, the point symmetry and EPR symmetry of paramagnetic center (adapted for surfaces from [7]) EPR symmetry
Restrictions on g and A tensors
Coincidence of principal axes of g and A tensors
Molecular point symmetry
Surface point symmetry
Isotropic
gxx = gyy = gzz Axx = Ayy = Azz gxx = gyy " gzz Axx = Ayy " Azz gxx " gyy " gzz Axx " Ayy " Azz gxx = gyy " gzz Axx = Ayy " Azz gxx " gyy " gzz Axx " Ayy " Azz gxx " gyy " gzz Axx " Ayy " Azz
All coincident a =b= g =0°
Oh, Td, O, Th, T
–
All coincident a =b =g = 0°
D4h, C46, D4, D2d, D6h, C66, D6, D3h, D3d, C36, D3 D2h, C26, D2
C46, C66, C36
Axial Rhombic Axial noncollinear Monoclinic Triclinic
All coincident a =b= g =0° Only gzz and Azz coincident a "0°, b= g =0° One axis of g and A coincident a" 0°, b =g =0° All non-coincident a "b "g "0°
grafting, and if the barrier is high it involves only a small fraction of the bulk complex population (given by a Boltzmann distribution, Fig. 1(c)). The intersection of both potential curves determines the activation energy Ea for this transformation. If the crossing point is below the line of zero potential energy (dotted curve), the overall process is non-activated, if above (solid curve) it requires activation. The coordination sphere of TMI can be followed directly by EPR and/or with the aid of appropriate probe molecules [1,2].
3. Determination of the symmetry of surface complexes from powder EPR spectra The surface imposes some restrictions on the existence of permissible symmetry elements. The analysis has shown that for flat and uniform crystalline surfaces the inner-sphere complexes can rigorously belong to one of the following point groups: C1, Cs, C26, C4, C46, C3, C36, C6 and C66 [6]. In the case of microporous and layered materials due to the particular geometry of the surface, the cage and interlayer complexes can exhibit higher symmetries. The point symmetry at the transition metal complexes determines whether the principal values of the g or hyperfine (hf) A tensors are required to
C26
C3, S6, C4, S4, C4h, C6, C3h, C6h C3, C4, C6 C2h, Cs, C2
Cs, C2
C1, Ci
C1
be equal to each other or not [7]. Also it determines whether the principal axes of A and g coincide or not. As a result, each type of the EPR signal is associated with a restricted number of molecular or surface point symmetries (Table 1). Thus for a system of unknown structure, the EPR symmetry, if correctly determined, can provide valuable structural information. However, it should be noted that the degree to which EPR parameters can reflect the symmetry of paramagnetic species will essentially depend on the strengths of the metal-ligand interactions. The typically low symmetry of the surface complexes implies low symmetry of the corresponding EPR spectra, which can be recognized only if both g and A tensors are available. Although at first glance, it would appear that in powder EPR spectra information about the relative orientations of the principal axes of the g and A tensors is lost, it is often possible to find their relative orientation providing that the metal nuclear spin I be greater than 1. In general, the effects of axis non-coincidence will be most pronounced if both tensors have anisotropies large and comparable in magnitude. Then, for large values of mI the field extrema will occur at angles close to the hyperfine tensor axes, while for small mI , the extrema will correspond to angles close to the g tensor axes [8]. The result of such a competition produces a series of lines with distinctly uneven spacings, which can
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not be accounted for basing on the second order effects only. Another simple indication of axes non-coincidence is the apparent presence of too many features in the spectrum, lines appearing at non-expected positions or lacking of expected lines while extrapolating hfs starting from either side of the spectrum. An important criterion of low symmetry is the impossibility to simulate spectra assuming coincidence [7]. The appearance of a resolved superhyperfine structure (shfs) due to coupling of the unpaired electron with magnetic nuclei (I " 0) of ligands provides an additional means of identification of the coordination sphere. The above considerations underlines the importance of using isotopically enriched species and computer simulation in extracting the maximum information from powder EPR spectra.
A good example of this approach is provided by the EPR study of a {SiO}2Mo5 + (OD)CO surface complex obtained by adsorption of carbon monoxide on a Mo/SiO2 catalyst activated by UV-irradiation under 30 Torr of D2 at 77 K [9]. Although the ‘orthorhombic’ g tensor (see Fig. 2(a) and Table 2) suggests a C2v (or lower) symmetry of the complex, the non-coincidence of g and A axes deduced from the 95Mo-labeled species (Fig. 2(b)) reveals the monoclinic nature of the spectrum which restricts the point group to Cs to which the Mo complex belongs. Further experiments using 13C enriched CO and OH ligands (Fig. 2(c and d)) allowed to identify a CO and a OH ligand within the molybdenum coordination sphere. As a result a structure of the complex (shown below) compatible with Cs symmetry was proposed (Scheme 1).
Fig. 2. EPR spectra (X band, 77 K) after adsorption of 12CO onto: (a) Mo/SiO2 catalyst irradiated by UV at 77 under D2 (b) onto 95 Mo enriched Mo/SiO2 UV-irradiated at the same conditions as in (a). The non-compatible stick diagrams obtained while extrapolating the 95Mo hfs (A3) starting from a low and a high field part of the spectrum indicate the non-coincidence of the g and A axes. The same as in (a) but after adsorption of (c) 13CO (top first and bottom third derivative spectrum) and (d) UV-irradiated at 77 K under H2.
Z. Sojka, M. Che / Colloids and Surfaces A: Physicochem. Eng. Aspects 158 (1999) 165–178 Table 2 EPR parameters of selected transition metal complexes supported on silica surface containing Sample
Surface complex
g tensor
13
V/SiO2 Mo/SiO2
O =VIV(OL)3(CO)2 O= MoV(OL)2OHCO
Mo/SiO2
O= MoV(OL)3(CO)2
Ni/SiO2
NiI(OL)2(CO)2
Ni/SiO2
NiI(OL)(CO)4
gÞ = 1.985, gW =1.931 g1 = 1.965, g2 =1.941, g3 = 1.891 g1 = 1.969, g2 =1.965, g3 — g1 = 2.191, g2 =2.086, g3 = 2.066 gÞ = 2.130, gW =2.009
C shfsa tensor (mT)
169
13
CO ligands
Ground state
Reference
AÞ :0.7, AW50.7 A1 =0.65
dp(xy) dp(xy)
[14] [9]
A1 =0.75, A2 =0.75, A3 — A1 =3.0, A2 =3.25, A3 =3.25 AÞ =5.15b, AÞ =2.5c AW =5.5b, AW =5.5c
dp(xy)
[11–13]
ds(x2–y2)
[26]
ds(z2)
[26]
a
shfs, superhyperfine splitting. Shf splitting of axial ligand. c Shf splitting of equatorial ligand. b
3.1. Use of probe molecules to assist EPR spectra assignment Often the interpretation of EPR spectra is complicated by overlapping signals due to more than one surface complex. In such cases, temperatureand pressure-controlled adsorption of probe molecules can help to assign spectra [2,10]. EPR has proven to be a powerful tool for investigating interfacial coordination chemistry by means of ligands with magnetic (I" 0), and non-magnetic (I = 0) nuclei and able to fill up coordination vacancies producing characteristic changes in EPR spectra. Typical probe molecules are 13CO [11], H2O [11,12], CH3OH [13] which form predominantly s-type bonding between the lone pair and 3d orbitals of the TMI of appropriate symmetry. While water and methanol probes can provide information on the coordinative unsaturation of the central cation, via the g tensor changes upon coordination, 13CO can produce a shf structure due to the interaction of the nuclear spin I =1/2 of 13C with the unpaired electron. The counting of vacancies in transition metal ions with 13 CO has been shown to be particularly suitable for TMI 3ds orbitals carrying the unpaired electron, which have lobes pointing along the metalligand bonds (e.g. the dz2 or dx2 − y2 orbitals of Ni+ in C36 or C26 symmetry) that are able to make bonding with s donor CO probe. The magnitude of coupling in such cases is usually suffi-
ciently large (2.0–2.5 mT, Table 2) to enable a straightforward analysis of the shf structure. However, for ions with 3dp orbitals pointing between ligands, like dxy of Mo5 + or V4 + in C46 symmetry, the superhyperfine coupling constant between TMI and 13CO is significantly smaller (0.4–0.75 mT, Table 2) due to the dismatch between the dp and ligand s-orbitals. However, they can produce a considerable overlap with p-bonding ligands. In order to improve the poor resolution often observed, first and third derivative EPR spectra may be employed after adsorption of 13 CO [14]. It was recently found [15] that ligands with 31P (I= 1/2, 100%) such as alkylphosphines, are better probes than those with 13C for the determination of coordination unsaturation, due to the much larger hyperfine values of 31P (Aiso =364 mT and Aaniso = 20.6 mT) in comparison to 13C (Aiso = 113 mT and Aaniso = 6.6 mT). This type of
Scheme 1.
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Fig. 3. EPR spectrum (X band, 77 K) of the molybdenyl bis(triethylphosphine) and its computer simulation (a), and schematic picture of the steric hindrance of phosphines (b).
probe was used to determine the number of coordination vacancies in tetracoordinated molybdenyl Mo54c+ species grafted onto silica, even in the presence of other more abundant hexa(Mo56c+ ) and pentacoordinated (Mo55c+ ) species. The number n = 2 of phosphine molecules filling up the coordination vacancies of Mo54c+ is directly deduced from the number N of the shf
lines N= 2nI+ 1 (Fig. 3(a)). Adsorption of bulky phosphines with different cone angles helped to determine the stereochemistry of molybdenum surface complexes and to distinguish between two possible structures of the tetracoordinated Mo54c+ of pseudo D2d and C3v symmetry, as both exhibit quite different spatial accessibility (Fig. 3(b)).
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4. Surface adducts of transition metal ions with CO, NO, O2, N2O CO, NO, O2, molecules can bind to surface mononuclear transition metal ions to form adducts with geometries ranging from the common linear top-on coordination h1 (a) through bent and kinked geometry (b) and (c), to side-on h2 structure (d) until complete dissociation of the bond (e) leading to oxidative addition [16] (Scheme 2). The chemical interactions established upon coordination involve redistribution of the electron density within the adduct (via s/p donor–acceptor interactions), rehybridization, and electron transfer. In the following, we will illustrate the role of surface metal complexes in the activation of small molecules on selected examples coming from our laboratories. We use the notation {MXY}r of Enemark and Feltham, where r stands for the number of electrons on d and p* orbitals of the metal M and the bound molecule XY, respectively [16,17]. We also will discuss electron transfer activation which refers to two different cases where the ligand coordinated to the metal center either does not dissociate or dissociates upon electron transfer.
4.1. Molecular adsorption 4.1.1. Acti6ation through coordination: binding of CO to surface TMI CO binds to mononuclear transition metal ions via the C atom to form a s-donor bonding, involving d orbitals. The bonding is reinforced by back donation of the metal dp electrons to the empty CO p* orbitals (Blyholder structure [18]). Such coordination is linear and only a partial net charge transfer between the metal and the ligand takes place. The number of coordinated CO
Scheme 2.
171
molecules and the nature of the bonding can be deduced from the shf structure observed providing that 13C-labeled carbon monoxide is used (Table 2). Interaction of CO with Mo/SiO2 catalysts at various temperatures and pressures may serve an example of molecular adsorption [10,13]. CO interacts only with the most reactive tetracoordinated Mo54c+ centers forming a monoh1{MoCO}1 and a dicarbonyl h1{Mo(CO)2}1 adduct, identified by the simultaneous use of 13C labeling and third derivative recording. Depending on the pCO and the temperature, dicarbonyl adduct can be formed directly or via monocarbonyl intermediate which reversibly transforms into dicarbonyl. From the EPR parameters, it is deduced that the bonding is basically s-donor in character as the back-donation inferred from the shf structure is small. This leads to the formation of a partial positive charge on the carbon atom which will favor nucleophilic attack at this position [19].
4.1.2. Acti6ation through rehybridization: adsorption of NO onto Cu/ZSM-5 Due to its electronic configuration, NO can bind to TMI to form a linear (hybridization sp), a bent (sp2) and an intermediate (spn, 1B nB2) species. Its chemical reactivity is thus strongly related to the type of bonding since change of the coordination mode involves both rehybridization and a change in the oxidation state in the NO ligand. An example of NO binding studies is provided by adsorption of NO on Cu-exchanged ZSM-5 zeolites [20]. NO contacted with Cu+/ZSM-5 under low pressures forms a {CuNO11} adduct of h1 geometry (Cs symmetry). Its EPR spectrum (gx = 1.999, gy = 2.003, gz = 1.889, CuAx = 16 mT, Cu Ay = 15.5 mT, CuAz = 20.5 mT, NAx = 3.0 mT, N Ay = 0.43 mT, NAz = 0.55 mT) with well-defined hyperfine coupling with 63,65Cu and 14N nuclei (Fig. 4(a)) was interpreted in terms of completely anisotropic g and A tensors with non-coincident axes (Fig. 4(b)). From the EPR parameters, it was inferred that {CuNO11} is a bent complex in which copper remains monovalent and NO carries most of the spin density. The bonding interaction
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Fig. 4. (a) Monoclinic EPR spectrum (X band, 77 K) of h1CuNO11 adduct (solid line) and its simulation (line with black points) and (b) the reference axes for g and Cu shf tensors. Both systems are non-coincident in xz plane.
involves essentially an overlap of Cu 3dz2 and 3dxz orbitals with the antibonding 2p p*x orbital and a lone pair n of NO ligand as well as an overlap of the Cu 3dyz orbital with a second orthogonal NO 2p p* y orbital. The molecular model developed for interpreting the magnetic parameters allowed to determine the principal bonding characteristics. The total spin density on copper was found to be equal to 0.2 and is shared among the 3dz2 (0.079), 3dxz (0.021) and 4s (0.1) orbitals. The remaining part of the spin density is localized on nitrogen (0.55) and oxygen (0.25) atoms. The Cu-N-O bending angle a = 20° is associated with the rehybridization of NO orbitals upon coordination. The implications of this coordination mode on the possible mechanism of the catalytic decomposition of NO over Cu/ZSM-5 were discussed. It was proposed that the redistribution of the electron density which results from the angular geometry of the adduct appears functional in the formation of a NN bond via an electrophilic attack by the gas phase NO at the nitrogen atom of Cu+-NO adduct and opens an outer-sphere pathway for the decomposition of NO (Scheme 3).
4.2. Electron transfer acti6ation 4.2.1. Non-dissociati6e electron transfer: interaction of dioxygen with Co/MgO Dioxygen can bind to TMI forming adducts which can have a bent h1 (Pauling structure), a h2 (Griffith structure) geometry, bridged m structure and involve combinations of those generic forms [16]. Extensive EPR studies using 16O2 and 17O2 (I= 5/2) have shown that Co/MgO solid solution provide a unique model system to investigate the multistep pathway of the adsorption process of dioxygen [21–23]. Dilute Co/MgO solid solutions contain mainly isolated Co2 + ions exposed predominantly at (100) planes. Those ions have been found to function as heterogeneous oxygen carriers since they are able to reversibly bind oxygen [21,23]. By adsorption of O2 at 77 K below 0.1 Torr, a complex spectrum with a distinct shf structure due to 59Co (I=7/2, 100%) is obtained showing two slightly different h1{CoO2}9 adducts (covalent complexes) localized at distinct Co3 + sites (species I and II in Fig. 5(a)). The spectrum corresponds to the initial stage of oxygen activation, since two species, stable at 77 K, undergo differ-
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ent evolution when the temperature is increased to 120–150 K under slight evacuation. This treatment leads to the spectrum of Fig. 5(b) composed of two superimposed signals. The first one belongs 2+ to a new complex Co3 + – m – h1:h1 – O− 2 –Mg (species III), where the superoxide ion adsorbed on Co3 + is simultaneously stabilized by interaction with an adjacent Mg2 + . The second one is due to side-on O− adsorbed on Mg2 + center 2 2 − 2+ complex (species IV). The forming h O2 – Mg cobalt superoxide adducts disappear upon evacuation at room temperature leading to species IV (Fig. 5(c)) but they can be reversibly restored by readsorption of dioxygen at low temperature. The nature of these various species has been confirmed with 17O-enriched oxygen [22]. Analysis of the 17O and 59Co hf and shf structures, respectively, indicated that species I, II and III have inequivalent oxygen nuclei corresponding to a bent CoOO geometry, while species IV exhibits oxygen nuclei equidistant from the Mg2 + surface cation. The data obtained revealed that the CoO2 bonding has a s – p character with the highest occupied molecular orbital (HOMO) formed by the overlap of the Co 3dz2 and 3dyz orbitals with while the singly occupied molecular the O p*(2p), z orbital (SOMO) is produced from the Co 3dxz and orbitals. The extent of the metal to the O p*(2p) x ligand electron transfer (MLET) was found to be 0.8 for the adducts I and II and increased further to 0.86 for adduct III, reaching the ultimate value of 0.96 for adduct IV.
173
The mechanism of adsorption which has a ‘cooperative character’ consists of an initial adsorption of O2 on the Co2 + ion acting as electron donor site with formation of a covalent complex. The superoxide ion is then stabilized by bridging to an adjacent Mg2 + before it migrates towards matrix Mg2 + centers with the formation of an electrostatic complex. The mechanism can be written as follows: h1 O2− –Co3 + Co3 + –m–h1:h1O2− –Mg2 + h2 O2− –Mg2 + Along this activation pathway, the superoxide adducts undergoes a significant change in geometry and electronic structure. The process involves a gradual change of the orientation of superoxide species, which is accompanied by a shortening of the CoO bonding and an increase of its ionicity (the extent of electron transfer) [22]. This leads to a lengthening of the OO distance as the bond order decreases and a further activation of dioxygen (Fig. 5(d)).
4.2.2. Dissociati6e electron transfer: acti6ation of N2O on Mo/SiO2 The N2O molecule is the prototype of linear triatomic molecules, the activation of which requires vibrational excitation leading to bending before the MLET can occur. To initiate the reaction, the TMI of the surface complex acts as a source of electron and provides orbitals of proper energy and symmetry for coordination. The entire process consists of three steps involving: (a) N2O
Scheme 3.
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Fig. 5. EPR spectrum (X band, 77 K) of dioxygen adsorbed onto Co/MgO solid solutions. (a) species I and II observed immediately after adsorption of oxygen at 77 K, (b) species III and IV observed upon evacuation at 120 – 140 K, (c) species IV detected after evacuation at room temperature, (d) changes in the geometry of superoxide species during dioxygen activation.
coordination; (b) vibronic preactivation (C 6 Cs transformation); and (c) dissociative MLET. The changes in coordination and oxidation state of molybdenum occurring in this reaction can be best followed by EPR for the N2O-Mo5 + / SiO2 system using natural and 95Mo-enriched molybdenum [24]. The tetrahedral molybdenum centers Mo5 + /SiO2 readily coordinate N2O at 298 K owing to the symmetry match and spatial accessibility of the relevant metal and ligand orbitals. Consequently, the initial EPR signal (Fig. 6(a)) of tetracoordinated Mo5 + with gÞ = 1.926 and g ® =1.76 transforms into another one with
gÞ = 1.957 and g ® = 1.87 (Fig. 6(b)) due to pentacoordinated Mo5 + of apparent C46 symmetry, indicating rearrangement of the complex due to attachment of N2O in an equatorial position. The O− radical (gÞ = 2.020 and g ® = 2.005) is produced upon a temperature-induced MLET (Fig. 6(c)). The well pronounced 95Mo shfs shows that O− remains within the coordination sphere of molybdenum (Fig. 6(d)). The overlap between 3p* and 3dxy orbitals provides a pathway for electron transfer. The activation energy was found to be 20 4 kJ/mol in the temperature range 323– 393 K. The hindrance to MLET arises mainly
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from the inner reorganization of nitrous oxide upon bending from linear N2O (C6 ) to the transient bent N2O− (Cs ) species. N2O +Mo54c+ N2O-Mo54c+ (N2O-Mo54c+ l N2O − –Mo64c+ ) c O − – Mo64c+ +N 2 The use of an angle-dependent Morse potential has allowed to get some insight into the dynamics
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of the electron transfer at the surface. The MLET process requires a lower activation energy than in the gas phase and involves a less demanding transition state (r*NO = 0.123 nm and f*NNO = 160o). From the spin Hamiltonian parameters of the O− radical, the spin density on 95Mo (4.3%), the splitting of O− energy levels (DEx,y-z = 1.5 eV) and associated crystal field stabilization energy (CFSE= − 1 eV) were calculated.
Fig. 6. EPR spectra (X band, 77 K) of grafted 0.33 wt.% Mo/SiO2 catalyst (a) prior to and (b) after adsorption of 30 Torr of N2O at 293 K. Spectra obtained upon heating at 373 K on (c) natural and (d) 95Mo-enriched molybdenum.
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Analysis of the data shows that the reaction consists of two processes: (a) an activation of the C-H(sp3) bonding of CH3OH occurring through ligand to ligand hydrogen transfer (LLHT) to form a hydroxymethyl radical; and (b) the conversion of transient ·CH2OH into final CH2O with concomitant reduction of Mo. This latter ‘electroprotic’ step involves a ligand to metal electron transfer (LMET) coupled with a ligand to ligand proton transfer (LLPT) (CH3OH+ O − )-Mo6 + (·CH2OH + OH − )-Mo6 + Fig. 7. Kinetics of the reaction between O− and CH3OH ligands within the coordination sphere of molybdenum at 210 K; [O−] and [Mo5 + ] refer to change of the intensity of the EPR signal of these ions with time while ln[O−] line represents the semi-logarithmic plot of O− decay.
5. Electroprotic transformation: oxidative dehydrogenation of CH3OH on Mo/SiO2 catalysts The dehydrogenation of methanol to formaldehyde involves the extraction of two chemically different hydrogens coming from the methyl and hydroxyl groups and formally requires the elimination of two protons and two electrons. In an attempt to explain the mechanism of this process, cycles of model reactions between CH3OH and in situ-generated O−, both adsorbed on grafted Mo/ SiO2 catalysts, have been studied by EPR [25]. The use of O− has allowed a bypass the rate-determining step, i.e. the activation of the CH(sp3) bond and to reveal the subsequent steps of the reaction. The finding that each step started at a different threshold temperature allowed to isolate the different steps and to study them in detail. The interaction between adsorbed O− (produced from N2O) and methanol on oxidized molybdenum was first investigated. The sequence of events observed includes coordination of N2O, formation of O− intermediate, adsorption of methanol and the inner-sphere reaction between O− and CH3OH ligands. The reaction is accompanied by the reduction of the Mo center (the Mo5 + EPR signal intensity increases while that of the O− signal simultaneously decreases, Fig. 7).
(·CH2OH + OH − )-Mo6 + + O2 − CH2Oads + OH − -Mo5 + + OH − The actual reductant ·CH2OH intermediate was detected in a separate experiment where methanol was adsorbed on isolated Mo centers before the O− species were generated at 77 K by UV-irradiation of the MoO bond (Fig. 8 top). Its appearance was accompanied by simultaneous reduction of molybdenum due to the LLHT process (socalled ‘initial reduction’ step)
Fig. 8. EPR spectra (X band, 77 K) obtained after 10 min of UV irradiation at 77 K with methanol preadsorbed on grafted (top) and impregnated Mo/SiO2 (bottom) catalysts. The doublet line corresponds to (H radical, the triplet one to (CH2OH while the broader signal at the high field side corresponds to Mo5 + species
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Fig. 9. (a) EPR spectra taken at 77 K (solid line, initial reduction) and after exposure to temperatures above 140 K (dotted line, terminal reduction); (b) reduction of molybdenum by hydroxymethyl radicals at 140 K. UV
Mo6 + =O2- OH(Mo5 + —O−) +CH3OH CH3
·CH2OHads +OH−Mo5 + The hydroxymethyl radicals, stable below 140 K, decay above this temperature, reducing the remaining Mo6 + ions via LMET coupled with LLPT (terminal reduction) electroprotic step: − ·CH2OH +Mo6 + + O2surf − CH2Oads + OHsurf +Mo5 +
as shown by the increase of the Mo5 + and the disappearance of the hydroxymethyl radical EPR signals (Fig. 9). Since the Mo species were isolated and reduced in the first step of the reaction, the hydroxymethyl radicals have to spillover onto silica searching for Mo6 + sites for their ultimate transformation. Thus the resultant electron transfer has a ‘non-complementary’ character, as isolated molybdenum sites cannot accept all the electrons supplied by CH3OH molecule during the reaction. The whole process thus involves three steps: (a) initial reduction; (b) migration; and (c) terminal reduction. However, in the case of clustered Mo sites migration of hydroxymethyl is not necessary because such sites can accommodate more than one electron and the whole process involves a ‘complementary’ electron transfer. This is indeed the case of impregnated samples (i.e.
possessing predominantly clustered Mo sites). In analogous experiment, hydroxymethyl radicals were not detected by EPR and only the signal of reduced molybdenum was present (Fig. 8 bottom). The results obtained have allowed to unravel the elementary steps of the oxidative dehydrogenation of methanol: electron, proton and hydrogen atom transfers (LMET, LLPT, LLHT) and to ascertain the sequence and conditions of their appearance. It was shown that the molybdenum active site consists of a metal redox center and coacting O− and O2 − ligands. The surface complex acts also as a center for molecular rearrangement of both reactants (CH3OH and N2O). The metal redox couple Mo6 + /Mo5 + functions as a trap and source of electrons while O− activates the CH(sp3) bond and O2 − acts as a trap of protons.
6. Conclusions EPR spectroscopy appears very useful in studying phenomena occurring at oxide surfaces and can allow to distinguish various types of surface complexes, to identify them and to follow their transformations upon contact with reactants.
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Changes occurring at the molecular level during binding and activation of small molecules by surface transition metal ions can be directly monitored as the latter can act as EPR self-probes. The surface complexes act as centers providing orbitals of suitable energy and proper symmetry for coordination of molecules and electron transfer processes. Many aspects of interfacial coordination chemistry can be resolved and understood at the molecular level, by means of EPR. However, being very sensitive but limited to paramagnetic species only, EPR spectroscopy may provide a selective and fragmentary picture of phenomena involved in interfacial coordination chemistry, and is more effective when employed in concert with other techniques such as UV-VIS, Raman, IR, EXAFS or photoluminescence.
Acknowledgements Z. Sojka is grateful to the Universite´ Pierre et Marie Curie for invited Professorship in 1998 and for KBN (Poland) for financial support (Grant No 3T09A07314)
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