Colloids and Surliwes A. Physicochemical and Engineerin~ Aspects, 72 (1993) 245 255 245 Elsevier Science Publishers B.V., Amsterdam

ESR determination of TiO2 and SnO 2 temperatures

Tammann

Anne Davidson, Bernard Morin and Michel Che Laboratoire de ROactivit~ de Surface et Structure ( U.R.A. l l06-CNRS), Universitf Pierre et Marie Curie, 4 place Jussieu, 75252 Paris C~dex 05, France

(Received 25 June 1992; accepted 23 October 1992)

Abstract

The thermal behaviour of vanadium(IV) paramagnetic ions deposited by impregnation on two rutile compounds (TiO2 and SnO2) has been investigated by ESR. Beyond a critical temperature, these probe V(IV) ions migrate from the surface into the bulk of the oxide. The most signiticant ESR features of the intralattice and extralattice V(IV) species are briefly analysed.

For the V/TiO 2 system, the amount of detectable bulk V(IV) ions depends strongly on the annealing temperature; their signal intensity drastically increases above 865 _+ 15 K. This sudden change indicates the temperature at which cations and cationic vacancies begin to move inside TiO2, namely the Tammann temperature of this oxide. This value agrees with the empirical range generally admitted for the Tummann temperature of oxides (between 0.3Tin and 0.5T,,, where T,n represents the oxide melting temperature, i.e. between 634 and 1057 K in the case of TiO2). It is also consistent with a previous experimental determination (870 K) obtained by specilic surface measurements.

A poor resolution hampers a similar analysis for the V/SnO 2 system. However, near 650 K, the ESR spectra are strongly moditied and indicate that diffusion mechanisms have begun. A 650_+ 50 K value appears therefore as a reasonable estimate of the SnO 2 Tammann temperature. Such a small value agrees with the low Tm of this oxide (1400 K) which leads to a 400 700 K empirical range.

Keywords. ESR spectra: SnO2; Tammann temperature; TiO2.

Introduction

The Tammann temperature [1,2] T~ of an oxide is the critical temperature above which a drastic increase in its plasticity and flexibility is observed. It corresponds to the temperature above which its constitutive cations become mobile so that their bulk diffusion is possible. This parameter is useful when studying mixed catalysts; for instance, much attention is devoted now to the V2Os/TiO 2 system which can be used in various oxidation reactions [3,4] (phthalic anhydride transformation to o-

Correspondence to." A. Davidson, Laboratoire de Raactivite de Surface et Structure (URA I I06-CNRS), Universit6 Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France.

xylene for instance), in ammoxidation reactions [5] (ammoxidation of benzene or toluene to the corresponding nitriles for instance) and in the selective reduction of nitrogen oxides [-6]. This catalyst is obtained by deposition of a small amount of active vanadium hemipentoxide on a titanium dioxide support. During the activation treatment of such a mixed catalyst, special care should be taken since its catalytic properties may drastically decrease. If a temperature greater than the T~ of TiO 2 is reached, diffusion mechanisms may induce a partial dilution of the surface vana- dium phase into its TiO2 support [7,8]. Any simple method to determine the T, parameter should therefore prove useful.

0927-7757/93/$06.00 ~" 1993 Elsevier Science Publishers B.V. All rights reserved.

2 4 6 A. Davidson et al./Colloids Surfaces A: Physicochem. Eng. Aspects 72 (1993) 245-255

It is generally admitted that, for a given com- pound of melting temperature Tin, T~ can be empiri- cally estimated to be within the range 0.3-0.5Tin [9]. TO our knowledge, a more precise value is difficult to obtain since few literature data are available on the experimental determination of T,. In the case of TiO2, for instance, the only reported technique [10] is based on specific surface area measurements. Near 870 K the microporous struc- ture of the oxide collapses and this sintering tem- perature can be regarded as its Tammann temperature. In order to check this result, another technique, based on electron spin resonance (ESR) spectroscopy and derived from a previous study [-7] on the diffusion mechanisms occurring inside a futile TiO2 lattice, is described here and extended to another oxide, SnO2, of similar structure. A paramagnetic probe is deposited at the surface of the oxide. Annealing treatments in air are then performed which induce the migration of this probe from the surface into the bulk of the solid. ESR which is directly sensitive to the local environment of the probe can be used to investigate this drastic evolution. The temperature at which the introduc- tion of the probe inside the selected lattice becomes noticeable, as inferred from the changes of the ESR spectra, is the Tammann temperature of the solid.

The vanadium ion V(IV) has been selected as a probe because of the large magnetic moment of the s~V nucleus (99.77% natural abundance with a nuclear spin I--7/2) which leads to a well-resolved hyperfine structure [11,12]. This paramagnetic ion has been widely studied and useful information about its coordination sphere can be obtained by directly comparing its spin-hamiltonian parameters to literature data [7,12,13]. The present study is limited to MO2 rutile-type compounds because the M ion of these lattices is in the same oxidation state as that of the vanadium probe and its intro- duction inside these matrices is therefore expected to be easier. We have extended the method described above (and reported earlier [7]) for titanium dioxide (TiO2) to tin dioxide (SnO2) for three main reasons.

(i) The Ti(IV) and Sn(IV) cations have almost

the same ionic radii as the V(IV) ions (ionic radius of a hexacoordinated (6c) V(IV) ( r v ( l v ) ) 6 c = 0.63 A; ( rT i ( lV) )6 e : 0.68 A and ( r s n ( i V ) ) 6 c : 0.71 A [ 14]).

(ii) Due to 16.19% of magnetically active tin nuclei (117Sn and 1198n, respectively 7.61 and 8.58%

natural abundance, both with I = 1/2 [15]) super- hyperfine couplings are expected for V(IV) ions introduced into the SnOz lattice. These features can give meaningful information about the sites of the intralattice vanadium ions.

(iii) Various solid solutions (single crystals and polycrystalline samples as well) of general formula VxM~-x)Oz with M = Ti [16-18] and Sn [19,20] have been previously described and their ESR spectra can be used as standards to detect intralat- tice V(IV) ions.

Experimental

Preparation of vanadium-supported materials V/MO 2 (M = Ti, Sn)

A TiO2 rutile support of 13.2 m 2 g-1 specific surface area has been prepared by air calcination (7 h at 1100 K) of a Degussa P25 powder. SnO 2 is a commercial (Merck) compound of high-grade purity (>99%) and has been used without any further treatment. Its specific surface area is 4.4 m 2

g 1. This support contains a small amount of paramagnetic impurities (mainly copper) which give the ESR background discussed below.

The impregnation has been performed with an aqueous solution of ammonium metavanadate NH4VO3 (0.01 M). The pH of the solution is 6.5. After 2 days of maturation, the paste obtained is dried for 14 h at 80°C. For the V/TiO 2 system, care was taken to avoid dipolar interactions between adjacent V(IV) ions and to improve the spectrum resolution, i.e. proportions are adjusted to obtain a 1% V/Ti atomic ratio. The ESR spectra obtained for the V/SnO 2 system are only poorly resolved. Indeed, the superhyperfine interaction between the unpaired electron and the tin nuclei induces the splitting of each hyperfine line into various multiplets and a large decrease in the

A. Davidson et al./Colloids Surjaces A: Physicochem. Eng. Aspects 72 (1993) 245 255 247

spectrum intensity is then observed. To balance

this trend, a greater vanadium content, correspond- ing to a 2% V/Sn atomic ratio, has been chosen.

Results

Thermal evolution of the V/Ti02 samples

ESR experiments

The ESR spectra have been recorded at liquid

nitrogen temperature on a Varian CSE-109 spec-

trometer (X-band; frequency 9.3 GHz). A 100 kHz

field modulation and a 10 Gauss standard modula-

tion width have been used. The g values were

measured using diphenylpicrylhydrazyl (DPPH;

grof = 2.0036) as a reference. The ESR signals evolution has been studied vs

the temperature of annealing in air. ESR quartz

tubes (4 mm in diameter) were filled with a constant

amount of powder (30 mg for V/TiO2; 80 mg for

V/SnO2). All these tubes were introduced into a

microfurnace and the temperature was slowly

increased (2 K rain 1). Every 100 K, a tube was

taken out and quenched to room temperature.

A first-order analysis is used to obtain a set of

approximate values for the diagonalized values of

d' and A tensors of each spectrum. The outermost

line of each spectrum (the first one at low field) is

then used to determine its other characteristics [-21]. The full width at half-maximum z]l/2 of this

line can be considered as a good estimate of the

spectrum linewidth and the same line, which is not

distorted by overlapping with the other compo-

nents, can be compared with the parameterized

values given by Poole [22] for a pure Gaussian

line and a pure Lorentzian one and this comparison

provides a convenient determination of the

spectrum line shape. All this information is used as first input in a

simulation program. In this program, calculations

are performed to second order assuming that both

Zeeman and hyperfine interactions act as perturb-

ations on the energy levels derived from the crystal

field. The spin-hamiltonian parameters are then

corrected until the best visual agreement between

calculated and experimental spectra is obtained.

In the absence of any unpaired electron, the V(V) (electronic configuration 3d °) surface ions deposited by impregnation on TiO 2 are ESR silent. Fortunately, some vanadium reduction is observed during this preparation and a small amount of

V(IV) (3d 1) is detected by ESR. Before any annealing treatment, the observed

spectrum (Fig. 1, signal la) results from the super-

position of two sets of 8 hyperfine lines due to the interaction of the unpaired electron with the 51V nucleus (I = 7/2) subjected to an axial crystal field. The best simulation, presented in Fig. 1, has been obtained with a Gaussian line shape, A1/2=75 Gauss and the set of spin-hamiltonian parameters reported in Table 1.

The spin-hamiltonian parameters of signal l a are characteristic of vanadyl VO 2+ ions. As pre-

viously discussed [-7], the coordination sphere of

2005 I

DPPH

1 I I [ I I I I

i 0 ; I I I I 1 J I

g±

Fig. l. ESR spectrum of a V/TiO2 sample before annealing (signal la). (a) Experimental; (b) simulation with the set of spin- hamiltonian parameters given in Table 1, a Gaussian line shape and a linewidth of 75 G.

248 A. Davidson et al./Colloids Surfaces A. Physicochem. Eng. Aspects 72 (1993) 245-255

TABLE 1

Comparison between the spin-hamiltonian parameters of surface vandayl species observed after impregnation of polycrystalline TiO2 and SnO2 supports and those previously reported for vanadyl impurities in an amorphous V205 matrix

Sample Signal A II A± gll g± fl~2 Ref. (G) (G)

V/TiOz la 175 54 1.937 1.961 0.71 This work V/SnO2 2a 195 75 1.929 1.978 0.75 This work Amorphous V205 199 75 1.932 1.977 0.71 39

these vanadyl ions is probably octahedral and completed either by some surface 0 2- anions of the rutile lattice or by water molecules in a similar way to that proposed for V/SiO2 catalysts (on the basis of electron nuclear double beam resonance (ENDOR) [23] and ESR results [24,25]) and for Mo/SiO2 catalysts (on the basis of ESR results [26]). A large parallel hyperfine constant indicates an appreciable delocalization of the unpaired electron on an axial oxygen [11,12]. The /~,2 parameter, given in Table 1, measures the fraction of the unpaired electron localized on the 3dxy orbital of the vanadium atom, mainly involved in the ground state. It has been calculated as follows:

fl~2 = (7/6)[Agll] - - (5/12)]Ag± [ - (7/6)(A II - A±)/P

where P = 184.5 G [27] and [Agrl [ and IAg±l repre- sent the deviation of the parallel and of the perpen- dicular ~ tensor components from the free electron value ge=2.0023. This formula has been demon- strated in detail elsewhere [11,28].

Annealing in air up to 673 K induces the disap- pearance of signal l a. This observation is not surprising since in the presence of oxygen and in this temperature range, the reoxidation of the vanadium surface precursors, leading to ESR-silent species, can be expected. A comparable behaviour was reported for vanadyl ions contained in an amorphous V2Os deposit (obtained by splat- cooling [29]); they are reoxidized and transformed into crystalline V205 at about 480 K in air.

Above 870K, a new signal designated lb is detected. Three sets of eight hyperfine lines indicate that the vanadium ion is being subjected to an orthorhombic crystal field. A convenient simula-

tion has been obtained with a Lorentzian line shape, a very sharp linewidth ( A 1 / 2 = 13.5 G) and the spin-hamiltonian parameters given in Table 2. These parameters are close to those previously reported for various VxTi~l_x)O 2 solid solutions [16-19] (Table 2). Signal lb is therefore assigned to some intralattice V(IV) ions. It can be noticed that the greatest hyperfine constant (156G) is associated with the largest ~ component (g~,= 1.965 compared to 1.913 for gx~ or gyy). The same trend was previously reported for V(1V) and Mo(V) ions introduced inside various rutile matrices (V/GeO2 [30], V/TiO2 [16-18], V/SnO2 [19,20], Mo/TiO2 [31,32], Mo/SnO2 [33,34] and Mo/GeO2 [35]) and was claimed to characterize ions in reticular sites [32,33]. By contrast, the greatest hyperfine constant is expected to be associ- ated with the smallest component of the ~ tensor for interstitial ions [36-38]. This observation sug- gests that the intralattice vanadium ions evidenced here by signal l b are localized in reticular sites.

For higher annealing temperatures, the reso- lution of signal lb is not modified while its intensity drastically changes. Since its linewidth remains constant, integration of the spectra is not necessary and the height of a chosen line can be used directly for quantitative purposes. We have selected the line corresponding mainly to the mzz = + 1 trans- ition (assuming that the vanadium hyperfine con- stants take negative values; only the magnitudes of the hyperfine constants are accessible experimen- tally but a negative sign is required to obtain positive values for the/~,2 parameter). This line is the strongest of spectrum lb and is indicated by the arrow on Fig. 2. Figure 3 presents the evolution

A. Davidson et al./Colloids Sur/aces A: Physicochem. Eng. Aspects 72 (1993) 245 255 249

TABLE 2

Comparison between the spin-hamiltonian parameters of the intralattice V(IV) ions generated by annealing treatments in the present work and those of various solid solutions of general formula VxM(~ _xtOz(M = Ti, Sn) reported in the literature

System Sample Signal Axx A .~. A =: gxx g>,~, g=: Ref. (G) (G) (G)

V/TiO2 Polycrystalline I b V/SnO 2 Polycrystalline 2d V:,Ti(~ xlO2 Single crystal Reticular

VxSntl _x)02

Polycrystalline Reticular Single crystal Interstitial Single crystal Reticular

< 31 45 156 1.913 1.913 1.956 This work 23 47 154 1.939 1.903 1.943 This work 35 49 155 1.914 1.912 1.956 16 35 48 153 1.915 1.913 1.956 17 35 49 156 1.913 1.913 1.955 18 75 75 192 1.967 1.967 1.906 36 23 47 154 1.939 1.904 1.943 19,20

2006 I I

i t i J

gzz

gyy . . . . . . .

V,~(---¢--,, . . . . . .

i I I

Fig. 2. ESR spectrum of a V/TiO2 sample after air annealing at 1120 K (constant heating rate 2 K min 1 until I120 K then quenching to room temperature; signal lb). (a) Experimental; (b) simulation with the set of spin-hamiltonian parameters given in Table 2, a Lorentzian line shape and a 13.5 G linewidth.

of its height vs the annealing temperature and shows that the amount of bulk V(IV) ions drasti- cally increases near 865 K.

Thermal evolution o/'the V/SnO 2 samples

The thermal behaviour of the V/SnO2 system displays almost the same general trend as that observed with V/TiO2. Before any thermal treat- ment, an axially distorted ESR spectrum (signal 2a, Fig. 4) due to some surface VO 2 + species with a six-fold coordination of the vanadium atom is observed. The spin-hamiltonian parameters of this

M(T)'

100

865K

z j~ar" B z • I I I I I I

370 570 770 970 1170 ~T(K)

Fig. 3. Thermal evolution of the height of signal lb. A small and rather constant height of signal lb is observed for the non- calcined sample and for samples treated at annealing temper- atures lower than 840 K. The observed values (M(T)= 1-1.2 in arbitrary units) are within the admissible error range ( _+ 12 in the same units). This point is discussed in Ref. [7].

2a signal, summarized in Table 1, are similar to those of vanadyl ions included inside an amor- phous V20 5 matrix [39] and differ slightly from those previously observed for vanadyl ions dis- persed on the TiO2 surface (signal l a).

250 A. Davidson et al./Colloids Surfaces A. Physicochem. Eng. Aspects 72 (1993) 245-255

DPPH

I I I I I

I I I I I 1 I I !

gi

Fig. 4. ESR spectrum of a V/SnO 2 sample before annealing (signal 2a). (a) Experimental; (b) simulation with the spin- hamiltonian parameters given in Table 1, a Gaussian line shape and a 80 G linewidth.

The larger All constant of signal 2a (195G compared to 175 G for signal la) indicates a more pronounced axial character for the vanadium species deposited on a SnO2 rather than on a TiO2 support. A smaller (1 -//72) parameter is observed for the V/SnO2 system (0.25 compared to 0.29 for the V/TiO2 system) and shows that the unpaired electron is less delocalized on the equatorial ligands of the vanadium ion.

The deviation IAg±] of the perpendicular compo- nent of the ~ tensor from the free electron ge value (2.0023) is greater for the V/TiO2 sample than for the V/SnO2 one (0.0413 for signal la and 0.0243 for signal 2a). This parameter is known to be very sensitive to the tetragonal distortion of the octahe- dron of oxygen ligands surrounding the vanadium ion [-28].

The amounts of vanadium present in the V/TiO2 and V/SnO2 samples used here are different and explain all these observations. Indeed, a 1% V/Ti atomic ratio corresponds to a 0.5 monolayer of V20 5 on TiO2 assuming that 1.44 mg of V205 are necessary for a monolayer coverage of 1 m 2 of the support [40]. A strong interaction of the vanadium species with their TiO2 support is therefore

expected. A long V=O bond, a poorly pronounced distortion of the coordination polyhedron and a large ]Ag±l value are then observed. By contrast, a 2% V/Sn atomic ratio corresponds approximately to three theoretical monolayers of V205 deposited on Sn O 2. The formation of amorphous domains of V205, characterized by short vanadyl bonds and hence greater//.2 and All parameters associ- ated with a smaller IAg±] value, are then observed.

The influence of the heat treatment (see Fig. 5) on the ESR spectra can be summarized as follows.

At 470 K, the hyperfine singularities of signal 2a appear as shoulders on a broad signal designated 2b.

By increasing annealing temperatures, signal 2a vanishes, indicating that the surface vanadyl species are progressively reoxidized. At 570 K, signal 2b is clearly observable. This structureless Lorentzian line has a 100 G linewidth and is centered in giso = 1.965. A similar signal has been observed in silica mixed-gel catalysts and has been assigned to V(IV) defects trapped into V2Os microcrystallites. In a similar way, we assume that the air-annealing treatments used here have induced the crystalliza- tion of V205 microdomains on the SnO2 surface.

At 650 K, a complete transformation of the spectrum is observed and signal 2b is replaced by a spectrum of a behaviour too complex to be analysed fully between 650 K and 1000 K.

At 1020 K, the characteristic signal of V(IV) ions in an orthorhombic crystal field (signal 2c) can be distinguished as seen on Fig. 6. A convenient simu- lation can be obtained with a Lorentzian line shape, a 50 G linewidth and by using the hyperfine spin-hamiltonian parameters given by Kikuchi and co-workers [19,20] for Sn O 2 single crystals doped with V(IV) ions (Table 2). Signal 2c can then be assigned to some intralattice V(IV) ions.

Superhyperfine features are helpful to check the exact localization of these intralattice ions. When a V(IV) ion is introduced inside the bulk of SnO2, its unpaired electron may be delocalized on the neighbouring tin ions and some superhyperfine (shf) coupling between the electronic spin (S = ½) and the nuclear spin I of tin centres can occur.

A. Davidson et al./Colloids Sur[aces A." Physicochem. Eng. Aspects 72 (1993) 245 255 251

_5 c_.. /l,

20off

e _ . ~ xl

Fig. 5. ESR spectra of the V/SnO 2 samples annealed in air with a constant heating rate 12 K min- 1) to increasing temper- atures (b, 470 K; c, 570 K; d, 670 K; e, 1020 K) and then quenched to room temperature. For comparison, the back- ground due to the paramagnetic impurities present in the S n O 2

support is presented in curve a.

Actually, tin may be found as three odd isotopes, all present ing a I = ~ value [15] (ltSSn, 11VSn and 119Sn with magnet ic momen t s # respectively equal

to - 0.9132, - 0.9949 and - 1.0409 Bohr magnet-

ons). Due to its low natural abundance (0.35% com pared to 7.61% for 117Sn and 8.58% for 119Sn)

b

y--

I I I I~ l~ '~X~ I I I

~zz

Fig. 6. ESR spectrum of a V/SnO2 sample after air annealing at 1020 K (constant heating rate 2 K rain 1) until 1120 K then quenching to room temperature, signal 2c). (a) Experimental; (b) simulation using a Lorentzian line shape, a 50 G linewidth and the set of hyperfine parameters previously reported for a single crystal of SnO z doped with vanadium [19,20].

115Sn can be neglected. The other two isotopes

have similar nuclear momen t s and can be consid-

ered equivalent. The global isotopic abundance of

magnet ical ly active tin is therefore 16.19% (approx-

imately 1/6). Accordingly, Kikuchi and co-workers

[ 19,20] have observed clear superhyperf ine features

for their oriented single crystal experiments. It can

be seen on Fig. 7 that the powder spect rum

obta ined after a 1220 K air-anneal ing t rea tment (signal 2d) presents more observable lines than the

previous one (signal 2c). A greater anneal ing tem- pera ture increases the signal intensity and, as a

consequence, superhyperf ine splittings can be detected on each hyperfine line. The small Axx and

Axy hyperfine constants (respectively 23 G and

50 G) and the close values of gx~ and gyy, lead to impor tan t overlaps for magnet ic fields within the

range 3000 < H < 3600 G, prevent ing any further

analysis of the middle par t of the spect rum

(Fig. 7(a)). On the contrary, the large A= hyperfine constant leads to isolated and well-resolved

multiplets below 3000 or above 3600 G and two

kinds of multiplets can be distinguished in these

parts of the spectra (Fig. 7(b)); namely triplets with

252 A. Davidson et al./Colloids Surfaces A: Physicochem. Eng. Aspects 72 (1993) 245-255

2~oo (a)

IOZZ J i i i i i i i

2800 ' 3200 ' 3600 ' 4000 H{Gi

~Q(a) ~ Q[a)

3 oo 3700 3800

(b)

I I Q(b)

s¢00 d00 Hlfif

Fig. 7. ESR spectrum of a V/SnO2 sample after air annealing at 1220 K (constant heating rate 2 K min 1 until 1120 K then quenching to room temperature; signal 2d). (a) Spectrum recorded for fields of 2300-4300 G; (b) enlargement of the high- field part (for fields 3600-4100 G).

relative line intensities 20/100/20 separated by 166 G and quintuplets with relative lines intensities 6/40/100/40/6 separated by approximately 20 G.

The first-neighbour tin ions around a given vanadium ion are very different if the vanadium ion is introduced at a substitutional or interstitial position and are, therefore, expected to give distinct shf features: the observed shf structure can be used to locate unambiguously the probe ions inside their rutile host lattice. The vanadium-tin distances necessary for this discussion have been derived

from the rutile unit-cell parameters given by Kikuchi et al. [19].

Let us first describe the theoretical spectrum of a V(IV) ion located in a substitutional site. This ion occupies the centre of the rutile unit cell presented in Fig. 8(a). In this position, it is sur- rounded by a slightly distorted oxygen octahedron and by eight tin nuclei at the corners of the rutile unit cell. The four tins lying in the (xOy) plane, which also contains the four first-neighbour oxygen atoms, are called Sn(b ) (average vanadium-tin dis- tance dv Sncu~ = 3.708 A). Two tin atoms lying along the y axis are also very near the vanadium atom and are labelled Snia ~ (dv s.~.~ = 3.185 A). Theoreti-

Oy axis

O c = 3.185 A

a = 4.737 A

Oz axis

O vanadium ion

e a tinions

Q b tin ions

Q ESR inactive tin neighbours

Fig. 8. Rutile-type structure: (a) tin first neighbours around a reticular vanadium ion located at the centre of the unit cell; (b) tin first neighbours around an interstitial vanadium ion located at the interface between two adjacent unit cells.

A. Davidson et al./Colloids Sur[aces A: Physicochem. Eng. Aspects 72 (1993) 245 255 253

cal calculations have shown that in this geometry, the unpaired electron of a V(IV) ion is mainly located in its dlx2 y2~ atomic orbital [16]. Therefore, only the four Sn(b ) tin nuclei located in the (xOy) plane and the two Snt,~ ones are expected to give observable shf structures. Since each tin nucleus has one chance in six to be magnetically active with a nuclear spin 1/2, three observable lines of 20/100/20 relative intensities are expected for the interaction with the two structurally equivalent Snt~ ~ atoms. Similarly, the four structurally equiva- lent Sn(b t nuclei lead to five observable lines with 6/40/100/40/6 relative intensities. This analysis agrees with previous observations concerning the Mo/SnO2 [34] and W/SnOz [41] systems (see Table 3). It also agrees well with our experimental data and explains the simultaneous observation of triplets and quintuplets on spectrum 2d. Further- more, the shf constants measured on our polycrys- talline samples (166 G and 20 G) are close to those reported for V/SnO2 single crystals [19] (respec- tively 168G and 28G). The large (166G) shf constant should then be attributed to the inter- action with the two Sn~ tin nuclei and is therefore labelled al~) hereafter and the small one (20 G) due to the interaction with the four b atoms is labelled albl.

To complete this discussion, we should also consider the theoretical spectrum of a V(IV) ion located at an interstitial site. Some general trends can be derived from a purely geometrical analysis. Figure 8(b) presents a V(IV) ion in an interstitial

position between two adjacent rutile unit cells. Depending on the orientation of the vanadium atom principal axes, two distinct situations can occur.

If its d(x2_y2) atomic orbital lies in the (b,c) plane of the unit cell, four equivalent b' nuclei are available to give important shf couplings and only quintuplets are expected.

If this orbital is oriented perpendicularly, only two structurally equivalent a' ions, lying in the a direction can give important shf structures and only triplets are expected.

Moreover, the vanadium tin distances measured here are considerably smaller than those previously given for a reticular vanadium atom: dv_s.(b, = 2.8528 A compared to dv s.(u)=3.708 A and dv_sn,~, =2.368 A, compared to dr_s.c, =3.185 •. Shf constants a(b') and ala'l larger than 20 G and 166 G respectively should, therefore, be expected. In the absence of any such features on spectrum 2d, we assume that the V(IV) ions observed in the present study are mainly located in substitutional sites inside their host SnO2 lattice.

The annealing temperature T a affects the spectrum intensity; a greater 77, is associated with a greater quantity of vanadium ions localized inside the SnO2 matrix. Unfortunately, poor resolution prevents the analysis of the spectra obtained between 650 and 1000 K. A plot representing the intensity of signal 2d vs T~, similar to Fig. 3 given for the V/TiO2 system, cannot therefore be pre- sented for V/SnO2.

TABLE 3

Comparison between the superhyperfine parameters of signal 2d and those of various solid solutions of general formula XxSnll xlO2(X = V(IV), Mo(V) and W(V)) reported in the literature

System Sample Superhyperfine a(~) tensor Superhyperfine a(b) tensor Ref.

a(a)x x a(a)y), a(a)~: a(a)i~o a(b)x x a(b)),), a{b)== a(h)is o (G) (G) (G) (G) (G) (G) (G) (G)

V/SnO2 Polycrystalline - - 166 20 V/SnO2 Single crystal 166 173 165 168 28 28 28 28 Mo/SnO 2 Polycrystalline 280 308 282 290 58 48 48 51 W/Sn02 Polycrystalline 430 520 450 466 78 80 64 74

This work 19,20 34 41

254 A. Davidson et al./Colloids Surfaces A: Physicochem. Eng. Aspects 72 (1993) 245-255

Discussion

The ESR spectra described above clearly show that during the thermal treatment in air of the V / M O 2 samples (M = Ti, Sn), some surface V(IV) ions are progressively introduced into the MO2 rutile sublattice. In order to explain this introduc- tion, two different mechanisms may be considered: either a bulk diffusion which may transform a surface V(IV) ion into a bulk V(IV) defect or a sintering process which may trap surface V(IV) ions (and the remaining V(V) ions which are ESR silent and cannot be localized in the present study) inside some grain boundaries. However, the specific surface area of the TiO2 used in this work is small (13.2 m 2 g-l) and such a small value is not much affected when slow heating rates are used [7]. The SnO2 we used exhibits a smaller specific surface area (4.4m 2 g-l) and is therefore expected to behave similarly. Sintering processes can thus be neglected here.

Diffusion can occur directly either via the empty interstitial sites or via the reticular positions of the rutile lattice. In the latter case, the introduction of the V(IV) ions inside the rutile matrix can be due either to a reaction between surface V(IV) ions and cationic vacancies of the lattice (such defects are always present inside a rutile compound due to intrinsic Schottky or Frenkel disorder [42]) or to the substitution of a surface M(IV) cation of the lattice by a neighbouring V(IV) ion.

A detailed analysis of the path involved is diffi- cult but some general trends can be given. The shf structure observed for the V/SnO2 system unam- biguously indicates that the V(IV) ions detected by ESR are localized in substitutional sites. In the case of the V/TiO2 system, this meaningful infor- mation is absent but our ESR parameters are similar to those previously reported for reticular vanadium ions [16-18] and largely differ from those attributed to interstitial ones [36]. The V(IV) ions are, therefore, supposed to be mainly reticular inside the TiO/ matrix too. These results suggest that the diffusion mechanisms observed here essen- tially involve cationic jumps between adjacent

reticular sites in SnO 2 and in TiOz. This phenome- non is expected to start only when the thermal activation energy given to the lattice is large enough to promote cationic jumps, i.e. when its Tammann temperature is reached.

In the case of the V/TiO 2 system, an important intensity increase of signal 2a is observed above 870 K; the height measured at 1170 K is 500 times larger than that obtained at 870 K, indicating that a 500 times larger amount of vanadium ions is introduced into the TiOz matrix. The extrapolation of the curve presented in Fig. 3 leads to Tt= 865 _+ 15 K (the _+ 15 K error range has been calcu- lated assuming a 10% accuracy of the ESR signal heights). This value agrees well with the first experi- mental evaluation of Asher and Greg [10] (870 K) and is also consistent with the empirical range derived from Huttig (634-1057 K) [9].

Unfortunately, the poor resolution of the ESR spectra hampers a similar analysis in the case of V/SnO 2. However, the new ESR features observed above 650 K can be mainly interpreted as a poorly defined spectrum similar to spectrum 2c. An approximate value of 650 K for the Tammann temperature of SnO2 is therefore suggested. This value is consistent with the low melting temper- ature of this oxide (Tm=I400K) leading to a 420-700 K empirical range.

Conclusion

The environment of a paramagnetic probe deposited at the surface of a rutile-type oxide appears to be strongly dependent upon annealing treatments in air. When the thermal energy given to the solid is large enough, its cations and its cationic vacancies begin to move and a paramag- netic probe can migrate easily into its bulk. Since this diffusion process implies cationic jumps inside the host lattice, the critical temperature above which this mechanism becomes clearly noticeable can be defined as the Tammann temperature of the oxide. The surface vanadyl species generated by impregnation of the oxide and the intralattice V(IV) dopant ions give very distinct ESR spectra;

A. Davidson et al./Colloids Surfaces A: Physicochem. Eng. Aspects 72 (1993) 245-255 255

the ESR technique therefore provides a convenient and very sensitive tool to determine the critical temperature above which the probe migration begins, i.e. the Tammann temperature of the oxide support.

This important concept can directly apply to solid-solid reactions, in particular to the inter- diffusion mechanisms which may occur in mixed catalysts like VzOs/TiO2. In that case, the pro- gressive dilution of the active V205 phase inside its TiO 2 support may drastically modify the cata- lytic performance of the system. ESR can be used to characterize the VTi~ _x)O2 solid solution gener- ated during annealing treatments. In addition, a test reaction [-8] like methanol oxidation can give useful information about the state of the vanadium phase remaining at the surface.

References

1 G. Tammann and Q.A.Z. Mansuri, Z. Anorg. Allg. Chem., 2 (1923) 126.

2 G. Tammann, Nathr. Ges. Wiss. Gottingen Math. Physik Klasse, (1930) 227.

3 D.J. Hucknall, Selective Oxidation of Hydrocarbons, Aca- demic Press, New York, 1974.

4 G.C. Bond and S.F. Tahir, Appl. Catal., I (1991) 71. 5 G. Busca, L. Tarchetti, G. Centi and F. Trifiro, J. Chem.

Soc., Faraday Trans. 1, 81 (1985) 1003. 6 H. Bosch and F. Janssen, Catal. Today, 2 (1988) 369. 7 A. Davidson and M. Che, J. Phys. Chem., 96 (1992) 9909. 8 J.P. Balikdjian, A. Davidson, J.M. Tatibou~t and M. Che,

in preparation. 9 G.F. Huttig, Kolloid-Z., 99 (1942) 262.

10 R.C. Asher and S.J. Greg, J. Chem. Soc., (1960) 5057. 11 D. Kivelson and S.K. Lee, J. Chem. Phys., 41 (1964) 1896. 12 B.A. Goodman and J.B. Raynor, Adv. Inorg. Chem.

Radiochem., 13 (1970) 188. 13 G. Busca and E. Giamello, Mater. Chem. Phys., 25 (1990)

475. 14 R.D. Shannon and C.T. Prewitt, Acta Crystallogr. Sect. B,

25 (1969) 925.

15 R.C. Weast (Ed.), Handbook of Chemistry and Physics, 46th. edn, The Chemical Rubber Co., CRC Press, Boca Raton, FL, 1966.

16 R. Gallay, J.J. van der Klinck and J. Moser, Phys. Rev. B, 34 (1986) 3060.

17 E. Yamaka and R.G. Barnes, Phys. Rev., 135 (1964) 1144. 18 P. M6riaudeau and J.C. V6drine, Nouv. J. Chim., 2 (1977)

133. 19 C. Kikuchi, I. Chen, W.H. From and P.B. Dorain, J. Chem.

Phys., 42 (1965) 181. 20 I. Chen, C. Kikuchi and H. Watanabe, J. Chem. Phys., 42

(1965) 189. 21 J.A. Weil and H.G. Hecht, J. Chem. Phys., 38 (1963) 281. 22 C.P. Poole, Electron Spin Magnetic Resonance, Wiley

lnterscience, New York, 1983, p. 479. 23 M. Narayana, C.S. Narisimham and L. Kevan, J. Catal.,

79 (1983) 237. 24 M. Narayama and L. Kevan, J. Phys. C, 16 (1983) 863. 25 M. Che, B. Canosa and A.R. Gonzalez-Elipe, J. Phys.

Chem., 90 (1986) 618. 26 M. Che, C. Louis and Z. Sojka, J. Chem. Soc., Faraday

Trans., 85 (1989) 3939. 27 J.A. McMillan and T.J. Halpern, J. Chem. Phys., 55

(1971) 33. 28 K.L. Walther, A. Wokaun and A. Baiker, J. Chem. Soc.,

Faraday Trans., 87 (1991) 1217. 29 M. Nabavi, C. Sanchez and J. Livage, Philos. Mag. B, 63

(1991) 941. 30 D.P. Madacsi, R.H. Bartham and O.R. Gilliam, Phys. Rev.

B, 7 (1973) 1817. 31 R.T. Kyi, Phys. Rev., 128 (1962) 151. 32 M. Che, G. Fichelle and P. M6riaudeau, Chem. Phys. Lett.,

17 (1972) 66. 33 P. De Montgolfier, P. M6riaudeau, Y. Boudeville and

M. Che, Phys. Rev. B, 14 (1976) 1788. 34 P. M6riaudeau, M. Che and A.J. Tench, J. Chem. Phys.

Lett., 31 (1975) 547. 35 T. Purcell and R.A. Weeks, Phys. Lett. A, 38 (1972) 473. 36 F. Kubec and F. Sroubek, J. Chem. Phys., 57 (1972) 1660. 37 Y. Boudeville and P. De Montgolfier, Chem. Phys. Lett.,

30 (1975) 469. 38 P. M6riaudeau, Chem. Phys. Lett., 72 (1980) 551. 39 P. Tougne, A.P. Legrand, C. Sanchez and J. Livage, J. Phys.

Chem. Solids, 42 (1981) 101. 40 J.M. Tatibou~t, Bull. Soc. Chim. Fr., 1 (1986) 18. 41 P. M6riaudeau, Y. Boudeville, P. De Montgolfier and

M. Che, Phys. Rev. B, 16 (1977) 30. 42 C.R. Catlow, R. James and M.J. Norgett, J. Phys. Colloid

Suppl., 1 (1976) C7-443, 37.