X-ray absorption spectroscopy of the continuous change from Ce(IV) to Ce(III)

J. Phys. Chem. Sohds Vol. 53, No. 2, pp. 233-237, Printed in Great Britain.

S5.00 + 0.00 OC22-3697/92 0 1992 Pergamon Press pk

1992

X-RAY ABSORPTION SPECTROSCOPY OF THE CONTINUOUS CHANGE FROM Ce(IV) TO Ce(III) C.

PRIETO,~

P. LAGARDE, H. DEXPERT, V. BRIOIS, F. VILLAIN and M . VERDAGUER

Laboratoire pour IUtilization du Rayonnement Electromagnetique, Universitt de Paris-Sud (Bat. 209D), F-91405 Orsay Cedex, France (Received 3 April 1991; accepted 30 May 1991)

Abstract-The fast acquisition method for X-ray Absorption Spectroscopy was used to obtain the Extended X-ray Absorption Fine Structure (EXAFS) and the X-ray Absorption Near Edge Structure (XANES) of the dynamical redox process in the Ce(IV)/Ce(III) chemical solutions system. From the XANES data, the relative concentration of both species was determined as a function of time and from the EXAFS data two different distances and two coordination numbers were found for the initial and final states, respectively. The analysis of the consecutive steps of the reaction indicates the absence of intermediate valence states of the cerium ions. Keywords: Mixed valence compounds,

cerium oxides, X-ray absorption spectroscopy during a chemical

reaction.

1. INTRODUCTION The existence of the fluorite related RO, (R = Ce, Pr and Tb; 1.S < x < 2.0) systems [l] is interesting from the point of view of valence anomalies in rare earth ions. These systems offer suitable examples for a comparative study of 4f level hybridization in Ce materials. The X-ray absorption spectra of the stable intermediate oxides show features which are clearly identified as contributions from trivalent and tetravalent sites [2, 31. In the present work we are interested in the continuous change from the Ce(IV) to the Ce(II1) oxidation states. This process can be carried out in solution during a chemical redox reaction. X-ray absorption spectroscopy has become a very useful technique for determining both the electronic structure and the structural environment of the absorbing atom. By tuning the X-ray energy it is possible to determine the fine structure at and above the absorption edges of the selected atom. The preedge structure may give information about site geometry and valence state, the XANES (X-ray Absorption Near Edge Structure) region contains stereo chemical information (e.g. bond angles) and EXAFS (Extended X-ray Absorption Fine Structure) gives accurate atomic distances and coordination numbers [4]. t Permanent address: Instituto de Ciencia de Materiales de Madrid (sede B), Consejo Superior de Investigaciones Cientificas, Facultad de Ciencias (C-4), Universidad Autonoma, E-28049 Madrid, Spain.

The test of the existence of intermediary valence states may be applied, in general, to oxides with different oxidation stoichiometry. 2. EXPERIMENTAL

We used synchrotron radiation emitted by the L.U.R.E. DC1 storage ring running at 1.85 GeV with an average current of 250 mA. At the EXAFS-III beam station the X-ray were monochromatized using a Si(311) two crystal spectrometer and the fast acquisition method [5,6] was used in order to obtain during the reaction time the absorption &-edges spectra needed to study the kinetics of the reduction of the Ce ions in aqueous solution. The Ce4+ ion in nitric medium becomes reduced to Ce3+ by adding a small quantity of alcohol. The procedure has been inspired by the one described by M. P. Doyle [7]. We used solutions of 0.1 M ceric ammonium nitrate in 2 M nitric acid. An 0.1 ml ethanol solution was added to 5 ml of the ceric solution. The resulting solution was well mixed in the flask before transfer to the cell. The reaction was carried out in a cell of about 0.5 mm thick with kapton windows and kept at a temperature of 45°C in order to well thermalize the reaction. At this temperature, and for such a Ce(IV) concentration and such an ethanol volume, the redox reaction is completed in about 20min. From a macroscopic point of view, the solution’s characteristic orange color of the Ce(IV) in nitric medium instantly changes to dark red when the 233

234

1.2

3.4 0.8 3.0

2.8 0.4

2.:7L

-0

8’690

5710

I

’

5730 ENERGY

5750

7

‘0

(eV)

Fig. I. Cerium L,,,-edge XANES spectra (atomic absorption has been removed) taken during the kinetic reduction process. The arrows show the time evolution of the white line. The inset shows the fitting of the intermediary spectrum.

ethanol is added. This color change is due to the formation of a complex between Ce(IV) and the ethanol. After that a slow decrease of the red color intensity can be. observed with time until colorless solution is reached, which is characteristic of the Ce(II1) ion in solution. In this work we characterized, by fast EXAFS only, the kinetic reaction of the electron transfer to cerium within the cerium+thanol complex: [Ce(IV)-ethanol],,,,,-+Ce(III)

+ products.

The first step (complex formation) is not investigated here. 3. RESULTS AND DISCUSSION Both oxidation states (IV and III) of the cerium atoms are easily distinguished by the X-ray absorption spectra near the L,,,-edge. The XANES shows

Fig. 2. Time evolution of the Ce(IV) and Ce(lII) relative concentration (star points). The relative coordination number (open points) for the two different shells obtained by EXAFS analysis are shown for comparison.

ENERGY (eV) Fig. 3. EXAFS spectra of the Ce La,-edge. The reduction of the Ce(IV) to Ce(II1) is followed in real time. The spectrum corresponding to the initial solution is the first top one and the time sequence is from top to bottom. One spectrum is taken each 65 s, one every other measured spectrum is represented.

two big bands for the Ce4+ ion [2,3] with an energy difference of about 7 eV. Their final states have been described in the literature [8] as follows: the first band corresponds to the final state 2p5-4f-Sd* and the second one to 2p5+‘-5d*. On the other side, the XANES of the Ce3+ ions has a single band [2,3] shifted to lower energy from those appearing in the Ce4+ spectrum. The final state is related to 2pS-4f’-5d* and the energy difference from the low energy band of Ce4+ is about 2 eV. Figure 1 shows the X-ray absorption spectra evolution from the initial to the final solution, one spectrum has been taken each 84s but only every other spectrum is represented in the picture. Data are presented here after removing the atomic contribution taking into account a Victoreen law extrapolated from about 150 eV before the edge. The initial and the final spectra are in accordance with those previously reported for nominally tetravalent and trivalent states of cerium ions, and a continuous evolution from Ce4+ to Ce3+ can be observed. Each spectrum of the kinetic reaction can be fitted as a linear combination of the initial and the final solutions spectra, the inset of Fig. 1 shows a comparison between the experimental data and the calculated one from the linear combination of the intermediate solution. The relative concentration for both species was determined to be equal to the linear coefficient as has been proved in other similar preparations [9]. It must be noted that the sum of the two coefficients is directly equal to one at every step of the solutions. The time evolution of the concentration for both species (Ce4+ and Ce3+) is represented in Fig. 2, over an interval of 4500s after the reaction had started. EXAFS spectra were obtained each 65 s, for the same reaction. The energy range was selected over

X-ray absorption spectroscopy during a chemical reaction

235

but the magnitude of the first peak is considerably bigger at the end of the kinetic reaction, going through a minimum at the fourth spectrum which corresponds to 450 s, as can be observed in the inset of Fig. 4. This effect can be explained as an interference from two different distances which should correspond to cerium atoms with different coordination spheres. In order to prove the hypothesis above, we carried out a quantitative analysis. The well-known EXAFS expression [1] were used in order to calculate the contribution to the EXAFS signal x(k) from the first coordination shell. Fig. 4. Fourier transform of the key weighted EXAFS signal at several steps of the reduction from Ce(IV) to Ce(II1). I-Initial solution. 2-After 245 s. 3-After 375 s. 4-After 570 s. 5-After 765 s. &-Final solution. The inset shows the evolution of the magnitude of the big peaks.

600eV in order to observe both the L,,, and the L,, absorption Ce-edge. The available energy interval between the two edges is about 450eV, which we found large enough to allow a good resolution on the first coordination sphere parameters. Figure 3 shows some of the spectra obtained during the reaction: a continuous evolution of the EXAFS oscillations can be noted, their amplitudes being bigger for the last spectra. The analysis of the EXAFS signals was carried out following a standard method: the X-ray atomic absorption above the edge was removed by a multiiteration method, special attention has been paid to the energy threshold (E,,) because the reaction is a redox process which means a shift of the edge energy during the reaction. The E,, values were determined by the Lee and Beni method [lo], where E, is selected in order to give a coincidence matching of the maxima (within the same shell) of the modulus and of the imaginary part of the phase corrected Fourier transforms of the EXAFS signal. In this way, we find a difference of about 2eV between the initial solution edge energy and the final one (of the same order as the energy shifts in the white lines), as it can be observed in Table I: it corresponds to the change from Ce(IV) to the Ce(II1) species. Figure 4 shows the evolution of the uncorrected Fourier transform modulus of the k* weighted EXAFS signal, the peaks centered at about 2A correspond to the first coordination sphere, with their magnitude related to the number of neighbours. The Fourier transform was performed over an interval from 3.6 to 9.6 A-’ and a Hanning window was used for apodization. No large difference can be observed in the distance of the first oxygen coordination sphere

x(k)=C~exp(-2k*af)exp(-I’jRj/k) j kRI’ x $(k)sin(2kRj

+ 4(k)).

(1)

This expression describes the EXAFS oscillations for a Gaussian distribution of neighbors around the central atom, in the single scattering theory and in the plane-wave approximation. k is the wave vector of the photoelectron, which is related to the electron mass (m,) and to the threshold energy (E,,) by

k*=$(E-&,), Here, N, is the average coordination number of the Gaussian distribution of the distance centred at the R, contribution, value, 0, is the Debye-Waller 4j(k) = 26(k) + q(k), are the phase shifts with 6(k) and q(k) the central and backscattering atom phase shifts respectively, h(k) is the amplitude of the backscattering atoms and r, is related to the mean free path of the photoelectron. The phase and amplitude backscattering functions reported by McKale et al. [ 111 were used, the mean free path was taken r = 2 A -*, as that obtained for the CeO, compound using these backscattering functions. The EXAFS signal corresponding to the big maximum (the filter was 1.5 - 3.0 A) was fitted both in k and R spaces [12] for each step of the reaction. The initial state cerium ions (Ce(IV)) have a coordination of eight oxygen atoms at a distance of 2.48 A, and for the final solution a coordination of 12 at a distance of 2.57 8, was found as the best fit for the Ce(II1) ions. The intermediary step-solutions are not correctly fitted by assuming a single Ce-0 distance as in the precedent cases. In the fitting procedure we considered two different oxygen shells corresponding to the 2.48 8, and 2.51 A distances with their corresponding Debye-Waller factors (a* = 0.098 A*, 17~= 0.092 A*) as fixed parameters given from the

C. PRIETO

236

et

al.

Table 1 summarizes the parameters obtained from the best fit of several spectra of the kinetic reaction. The E parameter gives the deviation of the calculated EXAFS signal from the experimental data. Because of the same experimental error for all the spectra, we have defined

this merit parameter

as

c* = (l/N) 1 (data, - Modeli)*.

(3)

The preceding results agree with those reported by Sham [13], where a distance of 2.50 rf: 0.02 A is given

Fig. 5. Distance space comparison between the calculated (continuous line) and experimental fitted data (points) of the modulus and imaginary part of the Fourier transform of the first coordination sphere EXAFS contribution. (a) Initial solution. (b) After 310s. (c) Final solution.

for the III oxidation state and a length difference of 0.07 f 0.02A between the Ce(IV) and Ce(III) ions and their oxygen neighbours in the nitric medium. The coordination numbers corresponding to the two distances are plotted in Fig. 4. In order to compare them with the relative concentration of both species, a relative concentration for each shell has been

initial

and

final

solutions,

adjustable parameters coordination numbers

respectively.

The

three

were the two corresponding and the change in the energy

threshold (A&). We have taken the same threshold for the two coordination shells because of the similar value, the difference must not be larger than 2eV. The comparison of the experimental data and the Fourier transform calculated contribution Fig. 5.

of several different

defined

where

N is the

distance

of 2.48 8, and Ce(II1) at about

The new experimental spectra

set-up

provided

8.0

R(A) 2.48

G(eV) 5721.5

d(A2)

0 180

1.3 0.4 5.2 3.1 5.1 3.4 4.5 3.9 4.2 4.6 3.9 4.9 3.8 6.0 3.0 8.9 2.1 10.0 1.7 11.0 0.3 11.7 12.0

2.48 2.57 2.48 2.57 2.48 2.57 2.48 2.57 2.48 2.57 2.48 2.57 2.48 2.57 2.48 2.57 2.48 2.57 2.48 2.57 2.48 2.57 2.57

5727.0

0.098 0.092 0.098 0.092 0.098 0.092 0.098 0.092 0.098 0.092 0.098 0.092 0.098 0.092 0.098 0.092 0.098 0.092 0.098 0.092 0.098 0.092 0.092

245 310 375 440 570 700 840 1035 1220 1490 4500

5727.0 5726.8 5726.5 5726.1 5726.0 5726.0 5726.0 5726.0 5726.0 5726.0 5726.0

0.098

&(W

XANES

which were used to follow the

Table 1. Set of values obtained as the best fit at several steps of the Ce4+ -+Ce3+ chemical reaction. N is the number of neighbors, R the distance, E, the energy threshold, D the Debye-Waller factor and A&, is the energy shift introduced in order to compensate for the errors between the energy threshold of the ions and the corresponding one for the atom where the theoretical phase and amplitude functions are calculated, and E is the merit parameter defined in the text N

2.57 A.

4. CONCLUSIONS

and EXAFS

Time

co-

and Ce(III), respectively). Their linearity assures that Ce(IV) has always one coordination sphere with a

filtered EXAFS

spectra are shown in

as: n = N/N,,

ordination and Nr is the maximum coordination expected for each ion (Nr = 8 and Nr = 12 for Ce(IV)

E (x 10-q

11.0 11.2

4.6 5.6

11.8

3.0

10.7

8.4

12.2

3.1

10.8

8.3

10.0

4.9

10.8

5.1

9.1

9.4

9.4

6.3

9.3

7.6

9.6

8.2

9.0

6.7

X-ray absorption spectroscopy during a chemical reaction chemical reduction from Ce(IV) to Ce(III). The concentration of the two oxidation states was determined as a function of time from the XANES spectra. EXAFS data gave two different Ce-0 distances of 2.49 and 2.58 8, for the IV and III oxidation states, respectively, and the EXAFS analysis of the intermediary steps of the reaction gave a mixture of the two CeeO distances provided by the initial and final solutions, resulting in the mixture of the two oxidation states. In this way, therefore, the existence of intermediary valence ions was not observed by EXAFS spectroscopy, in accordance with the linearity of the coordination numbers with the relative concentrations. This result could be generalized to the different stoichiometries of the cerium oxides CeO,, with 1.5 ,< x < 2.0, in order to understand the mixed valence states if the Ce ions have the possibility to be in some inequivalent sites. Acknowledgements-We

acknowledge the Spanish Ministry of Education for supporting the stay at L.U.R.E. of Dr C. Prieto with a grant. Further, we also thank the staff in charge of the D.C.I. machine.

237

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