Hyperfine Interactions 90(1994)371-375
371
Oxidation states of Fe in LaNil_xFexO3
A.E. Goeta, G.F. Goya, R.C. Mercader, G. Punte l
Departamento de F[sica, Universidad Nacional de La Plata,
CC 67, 1900 La Plata, Argentina
H. Falc6n and R. Carbonio
INFIQC, Depto. de Ffsic-Qu[mica, Universidad Nacional de C6rdoba,
C6rdoba, Argentina
The distributoin of oxidation states in perovskites of the type LaA l_xBxO3 (A and
B transition metal ions) can be "tailored" by x variation. In paticular, in LaNiO 3 it has
been shown that Fe substitution for Ni foces some Ni 3+ into Ni 2§ while some Fe 3+
changes into the unusual Fe 4+ state. In addition, the existence of mixed oxidation states
of Fe and/or Ni in LaNil_xFexO3 has been related to its catalytic activity in hydrogen
peroxide decomposition. The Fe 4+ population, obtained using M0ssbauer spectroscopy,
was found to be constant for all the analyzed annealing temperatures for x = 0.25
concentration, where the isomer shift difference for both states is the highest and the
catalytic activity is maximum.
1.
Introduction
Transition metal oxides with perovskite structure display changes in their
properties when the metals are substituted. Some of them have shown electrocatalytic
activity that increases the rate of oxygen evolution, reduction and hydrogen peroxide
decomposition reactions [ 1]. In perovskites of the type LnAI_xBxO 3 (Ln - lanthanide,
A and B - transition metal ions), it is possible to modify the oxidation state of the
metal ions that may be directly involved in the catalytic site by an appropriate
choice of the cations. Previous studies on LaNil_xFexO3 have shown maximum
catalytic activity at x = 0.25. This has been associated with the appearance of mixed
oxidation states [2,3].
Many of the catalytic processes follow the Arrhenius equation
[km = k o e x p ( - E a / R T ) ] , where/Co is called the reaction frequency factor and Ea the
activation energy. Sometimes, for a series of catalysts a particular inter-relationship
between k0 and Ea, called the compensation effect [4], is found: In ko = A + BEa (A
and B constants). The net result is that the effective influence of one of the Arrhenius
I Also with Facultad de Ingenieffa, UNLP, La Plata, Argentina.
9 J.C. Baltzer AG, Science Publishers
372
A.E. Goeta et al. / Oxidation states of Ire in LaNil_xFex03
factors on the rate constant is partially compensated by the other. In this
paper, X-ray and MOssbaur studies of LaNi0.75Fe0.250 3 prepared at different temperatures
were used to relate the rate constants for HO2 decomposition with the compensation
effect of LaNiO 3 and LaNi0.TsFe0.2503.
2.
Experimental
Two series of LaNiO3 (SI) and LaNi0.75Fe0.2503 (SII) perovskite catalysts
were prepared at different temperatures by citrate amorphous precursor decomposition
as described in ref. [3]. The precursors were annealed in air at temperatures (Ta)
of 500, 550, 600, 650, 700, 750, 800, 900, 1000 and 1100 ~ X-ray powder
diffraction data were collected for each specimen of SII from 20 to 120 ~ 2 0 , with
a step width of 0.02 ~ and a count time of 10 s per step in a Phillips PW-1710
diffractometer with Cu Ktz radiation. M6ssbauer measurements were obtained in
transmission geometry with a 50 mCi 57Co in Rh matrix source in a standard multiscalar
of 512 channels. A nonlinear least-squares program was used to fit the spectra to
Lorentzian line shapes. Isomer shifts are referred to ot-Fe at 300 K. Rate constants
for hydrogen peroxide decomposition were measured according to the procedure
described in ref. [3]. The decomposition reaction rate constants were measured in
the range from 267 to 363 K; k0 and E a were obtained from the Arrhenius plot.
3.
Results and discussion
Rietveld refinements performed on SII X-ray data using, DBWS-9006PC [5],
have shown that from 600 ~ < Ta-< 1100 ~ compounds were single phase, with
the LaNiO3 structure, and with only small changes in the cell parameters. Figure 1
displays a typical room-temperature M6ssbauer spectrum for SII. It could be fitted
with two single lines of IS = (0.16 :t: 0.04) and (0.39 + 0.05) mm/s, assigned to Fe 4§
and Fe 3§ respectively. The hyperfine parameters and Fe 4§ to Fe 3§ populations
(52(3)% and 48(3)%) do not change with Ta. Figure 2 shows the In k0 versus Ea plot
for the hydrogen peroxide decomposition reaction in the presence of La.NiO 3 and
LaNi0.75Fe0.2503. For both compounds, the results can be fitted with two straight
lines of different slopes. This can be explained if we assume that two types of
catalysts are formed for each compound in different Ta ranges.
The small difference between the two slopes for LaNi0.75Fe0.2503 would
indicate that the two types of catalysts are very similar. In LaNiO3, one type would
form for 600 ~ < Ta < 800 ~ and another one for 900 ~ < Ta < 1100 ~ This
interpretation is consistent with the known decomposition of LaNiO3 to La2NiO4
and NiO above 800 ~ Our present M6ssbauer and X-ray study results indicate that
the upper limit of stability for LaNi0.75Fe0.2503 is over 1100 ~ This different
behaviour is compatible with the fact that LaFeO3 is a highly stoichiometric perovskite
oxide [6], while LaNiO3 easily forms oxygen vacancies [10, 11].
373
A.E. Goeta et al. / Oxidation states of Fe in LaNit_xFexO 3
I
.
I
~
I
I
I
I
,o
~176
I
o
o
o
|
~176
.
o
~176176176
o
"q'.
~6
v
o
~n
I.
[-.
I
I
I
I
I
I
I
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
velocity (mm/s)
Fig. 1. Room-temperature 57Fe M6ssbauer spectrum of LaNio.75Feo.zsO3
sintered at 800 ~ The dotted curves are the Fe3*and Fe4+single-line components.
The solid curve is the result of the least-squares fit described in the text.
LaNil_~FexO 3 perovskite oxides proved to be good catalysts for oxygen
cathodes [2,3,7]. Also the x = 0 compound (SI) has been shown to be a suitable
candidate for oxygen reduction [8,12-14]. However, contrary to LaNiO3 [8],
LaNil_xFe~O3 is stable at the potentials where oxygen is reduced [9]. Therefore, for
its use in oxygen cathodes, the introduction of 25% of iron into the structure of
LaNiO3 allows the compound to be synthesized at higher temperatures without
decomposition, and that also increases its stability to reduction conditions.
It is important to remark that the population ratio Fe4§247 is independent
of Ta, since the former is responsible for the enhancement of the electron
transfer process operating during the heterogeneous decomposition of hydrogen
peroxide [3].
From the present results, we may conclude:
(1)
(2)
The introduction of 25% of iron into the structure of LaNiO3 allows the
compound to be synthesized at higher temperatures without decomposition.
The proportion of the highly oxidized Fe 4§ in LaNit_xFe~O3 is independent
of Ta.
374
A.E. Goeta et al.
/ Oxidation
states of Fe in LaNil_xFexO 3
2.5
6 5 0 y
700
750
2.0
600
/~
I000
C
1.5
LaNiOa
1.0
3
I
I
4
5
activation
I
6
energy
[Kcal/mol]
(a)
3.0
v
6OO
700 /
2.5
6 5 0 ~
/
I000/900
~500
800
J
2.0
1100
LaNio.vsFeo.zsOa
1.5
4
5
I
i
I
I
6
7
8
9
activation energy
10
[Kcal/mol]
(b)
Fig. 2. Ln/co versus activation energy. Numbers indicate temperature of synthesis (Ta). (a) LaNiO3. (b) LaNio.7sFeo.~O 3.
A.E. Goeta et al. / Oxidation states of Fe in LaNi I_xFex03
375
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