The microenvironment of iron in (Ba,Ca)(Fe,Co)03_~ catalyst system: A Miissbauer study

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1 Journal of Radioanalytical and Nuclear Chemistry, Vol. 239, No. 2 (1999) The microenvironment of iron in (Ba,Ca)(Fe,Co)03_~ catalyst system: A Miissbauer study Z. Homonnay, 1 K. Nomura, 2. G. Juhfisz, 1 A. V~rtes, 1 Y. Ujihira 3 1 Department of Nuclear Chemistry, i~tv6s Lordnd University, P.O. Box 32, H18 Budapest 112, Hungary e School of ngineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 113, Japan 3Research Center for Advanced Science and Technology, The University of Tokyo, Komaba 4-6-1, Meguro-ku, Tokyo 153, Japan (Received April 8, 1998) Perovskite type oxides of BaxCal_xF%sCo0.203_~ with x = 0.95, 0.8, 0.6 and 0.4 have been studied by 57Fe transmission M6ssbauer spectroscopy. The microenvironment of Fe was found to be sensitively dependent on the Ba to Ca ratio and, more importantly, on the oxygen content. At high Ba concentration, magnetic relaxation is indicated by the M6ssbauer spectra. Identification of the various sextets and doublets in the Mtissbauer spectra is attempted by the help of thermal treatments of the samples in air and methane as well as by quenching experiments and low temperature measurements. Introduction BaFeO 3 has an ABO 3 perovskite structure. 1,2 If the metal ions are partially substituted by other but chemically not very different elements, the mobility of oxygen will increase in the lattice while the original crystallographic structure is preserved. In certain composition ranges, these compounds can become good oxygen transferring catalysts, which is why these compounds have attracted considerable attention. It was found earlier 3 that substitution of a small amount of Ca at the A site and some amount of Co at the B site makes barium ferrate particularly suitable for oxidative coupling of methane to ethane at 550 to 750 ~ and also for deep oxidation of methane and higher hydrocarbons at 300 to 500~ These compounds also show remarkable reactivity with CO 2, which makes them promising absorbent materials to reduce industrial CO 2 output in order to prevent greenhouse effect. To improve the properties of these materials and to find better ones for special purposes, it is imperative to find out the mechanism of the reactions which are responsible for the catalytic and absorptive properties. To achieve this goal, the structure of the catalyst should be first of all studied. M6ssbauer spectroscopy is the ultimate tool for this purpose as it gives information about the microenvironment of Fe, the element which can have two valence states (III and IV) in these compounds, and this results in variable oxygen content - an obvious prerequisite for an oxygen transferring catalyst. lectronic density at the 57Fe nucleus, its asymmetry, and the internal magnetic field can be learned from the isomer shift (fi), quadrupole splitting (A) and magnetic splitting (B) of the M6ssbauer patterns, respectively. Barium ferrate with various Ca-substitutions at the Ba site was investigated earlier by M6ssbauer spectroscopy, 4 and it was found that the compound contained paramagnetic Fe(III) and Fe(IV) species (at room temperature) when the Ca-substitution was low. However, from about 20% Ca-substitution on, magnetic sextets appeared in the M6ssbauer spectra. At full Casubstitution, the spectrum practically corresponded to that of the brownmillerite, CaFeO2. 5. Appearance of the (antiferro) magnetic order is known to be due to increasing oxygen deficiency which, in turn, is caused by introducing Ca into the lattice. Decreasing oxygen content also caused transforming Fe(IV) into Fe(III), which was obvious from M6ssbauer isomer shifts. The Fe(III) atoms are normally coordinated in octahedral and tetrahedral sites. Interestingly, the spectra contained a new low intensity sextet, indicating a magnetic species, with internal magnetic field B = 47.0 T, and its relative intensity in the spectra was constant at all Ca contents except at full Ca substitution where it seemed to be absent. For catalytic activity, 5% Ca substitution proved to be ideal, and it was also found that Co substitution at the B site (Fe) can improve catalytic and absorptive properties. Co substitution at constant 5% Ca substitution at the Ba site has been studied by several methods including M6ssbauer spectroscopy. 5 The presence of Co in the lattice resulted in the appearance of magnetic sextets in the M6ssbauer spectra, and most interestingly, at 20% Co substitution, a rather complicated spectrum was obtained, showing either magnetic relaxation or a distribution of internal magnetic field at the Fe site in the lattice. * Author for correspondence /99/USD 17. O Akadgmiai Kiad6, Budapest All rights reserved lsevier Science B. V., Amsterdam Akadgmiai Kiad6, Budapest

2 Z. HOMONNAY et al.: TH MICRONVIRONMNT OF IRON IN (Ba, Ca)(Fe,Co)O3_ 8 CATALYST In our present work, we have selected BaxCal_xFeo.8Coo.203_ 8 with four different Ca substitution levels for a study. Our primary goal was to achieve a better understanding of the complex M6ssbauer spectra and find correlation with catalytic properties. For this, (1) we have re-synthesized Bao.95Cao.05Feo.2Coo.803_ 8 studied earlier by us, 5,6 a very promising composition for catalysis and CO 2 absorption, to see reproducibility of M6ssbauer spectra, (2) performed quenching experiments on the whole series of Ca-substituted materials, (3) applied treatments in various gas ambients, and (4) measured M6ssbauer spectra at low temperature. Sample characterization was helped by using X-ray diffraction, thermogravimetric analysis (TGA) and iodometric titration. xperimental The BaxCal_xFeo.sCo0.203_ 8 samples with x=0.95, 0.8, 0.6 and 0.4 were synthesized with the citrate method. 7 Stoichiometric amounts of the metal nitrates in aqueous solution (containing some nitric acid) were reacted with citric acid in 1:1 molar ratio and then 10 to 15 moles of ethylene glycol was added for each mole of citric acid. This solution was preheated at ~ until a jelly-like material formed. The jelly was then decomposed at ~ and the dark brown powder obtained was transferred into platinum crucibles and heated at 850 ~ for 10 hours and then at 1100 ~ for another 10 hours in air. The samples were cooled to room temperature in about 12 hours (uncontrolled furnace cooling). X-ray diffraction patterns indicated cubic structure for all four samples (Fig. 1). Only Bao.4Cao.6Fe0.8Co0.203_ 8 (lowest Ba-content) contained a noticeable amount of an unknown impurity phase. Its presence could not be seen in the M6ssbauer spectra, thus the iron content of this phase might have not exceeded about 5% of the total iron content of the sample. To increase the number of oxygen defects in the lattice, part of the samples were kept at 700 ~ for 15 min in air, and then cooled by simply removing them from the furnace. The temperature dropped to 50 ~ in about 2 minutes. We will quote these samples as "airquenched" ones. The M6ssbauer spectra were recorded with a constant acceleration type spectrometer using 57Co(Rh) source. All isomer shifts are given relative to c~-fe at room temperature. Results and discussion M6ssbauer spectra of the four compounds BaxCal_xFe0.8Co0.203_ 8 with x=0.95, 0.8, 0.6 and 0.4 recorded at room temperature are shown in Fig. 2. The evaluated M6ssbauer parameters are listed in Table 1. As can be seen, rather badly resolved spectral envelopes were obtained especially at high Ba-content. The upper two spectra resemble a shape typical to magnetic relaxation effects. To be able to evaluate these spectra, we applied the crude approximation of fitting the "relaxational background" by a very broad Lorentzian singlet. From the data listed in Table 1, it can be seen that this relaxational part of the spectra is very large. Although the M6ssbauer parameters of this singlet are not meaningful due to the approximation, it is common in the two spectra that there is a sharp sextet sitting on the relaxational background with internal magnetic field of approximately B =47 T. This sextet corresponds to the one observed earlier in substituted barium ferrates 8 and assigned to a 5-fold coordinated Fe in the lattice. Note that its intensity in our spectra is very low. In the sample with x=0.95 another, although very broad, sextet and a doublet also appeared. At x= 0.8, two broad doublets could be evaluated besides the sharp sextet and the relaxational background. We deliberately avoid hasty conclusions about the assignments of these subspectra due to the uncertainties introduced by the relaxation. Table 1. Room temperature M6ssbauer data of BaxCal_xFeo.sCoo.203_ ~ after "furnace cooling" the as prepared sample in air; average cooling rate -1 K/min. (Isomer shifts refer to cc-fe; signs are indicated only for the quadrupole splittings, e) d, mm/s A or, mm/s B (T) F, mm/s Area, % , O x = 0.95 x=0.8 x=0.6 x=

3 Z. HOMONNAY et al.: TH MICRONVIRONMNT OF IRON IN (Ba,Ca)(Fe,Co)O3_ 8 CATALYST 4,0K[ Ba, Ca i-,, Feo a Coo. z On-, 2.5K 8" 2.0K 1.5K 1.0K (c) II II ^ 0.5K (I 4d.0 50.(1 ' 60.0 ' 70.0 ' 20 Fig. 1. X-ray diffraction patterns of BaxCat~Feo.sCoo.203_~ with x = 0.95 (a), x = 0.80 (b), x = 0.60 (c) and x = 0.40 (d), furnace cooled after synthesis q~ (- o F-..,,..,,.., I [ I I 30 -s o ;o 1; -'% i '" - ~... 9 "*.-. ;2. ~. ~a, -10 ' -5' 0 ' 5 ' 1 '0 1 ; J Velocity, rnm/s Fig. 2. Room temperature M6ssbauer spectra of BaxCa 1 xfeo.sco0203_8 with x = 0.95 (a) x = 0.80 (b), x = 0.60 (c) and x = 0.40 (d), after "furnace cooling". (The velocity scale refers to c~-fe) In the lower two spectra representing 40 and 60% Ca substitutions, the relaxational background is significantly lower, and well developed magnetic sextets show up. These sextets correspond well to the observations on Cadoped barium ferrate: 4 as the composition of sample approaches that of the brownmillerite, the intensity of two magnetic sextets, representing Fe 3+ in octahedrally coordinated lattice site (higher magnetic field and higher isomer shift) and Fe 3 in tetrahedral coordination, increases at the expense of the paramagnetic doublets. The sextet with B =47 T is either absent in these spectra or hidden because of the intense brownmillerite-type sextets. The isomer shift of the relatively well resolved central doublet in the spectra indicates most probably a tetrahedral Fe 3+ species. The appearance of the browmillerite-like sextets in the spectra can be readily explained by the increased oxygen deficiency caused by the presence of Ca, just as it was argued in the case of the Co-free compound. 4 We note here that we have attempted to determine the oxygen stoichiometry of the samples by hydrogen reduction at 1000 ~ but the results were not conclusive on the absolute oxygen content. The Ca and Co-content modified substantially the ABO2. 5 final stoichiometry normally expected after such a treatment. However, a decrease of about 0.1 to 0.3 stoichiometry unit of oxygen as one proceeds from x=0.95 to x=0.4 could be estimated fi'om the results. Iodometric titration of the samples indicated also decreasing oxygen content with increasing Ca substitution within the surprisingly narrow stoichiometry 293

4 z. HOMONNAY et al.: TH MICRONVIRONMNT OF IRON IN (Ba,Ca)(Fe,Co)O3_ ~ CATALYST range of to This, however, is in agreement with our M6ssbauer data, namely that nowhere in our spectra could be find high intensity (>10%) well resolved subspectra which could be unambiguously attributed to Fe(IV) species. The anomalously low "isomer shift" of the broad singlet used to account for the relaxation part resulted most probably from a combination of the quadrupole and magnetic interaction during relaxation. In order to see the evolution of different species, with special attention to relaxational one(s), we have recorded M6ssbauer spectra on the air-quenched samples, which have somewhat higher degree of oxygen deficiency than the furnace cooled ones. The spectra of these airquenched samples (Fig. 3) indicate that the relaxational part started to convert into more or less resolved broad sextets (as compared to the furnace cooled materials), especially in the cases of x=0.95 and x=0.8. This change is much less pronounced at x=0.6, and in the case of the highest Ca substitution, the effect of quenching was negligible. This indicates that a high Ca-concentration tends to preserve a brownmillerite structure, and oxygen mobility is substantially diminished Table 2. Room temperature M6ssbauer data of BaxCal_xFe0.sCo0.203_ ~ after quenching in air; cooling rate -300 K/min. (Isomer shifts refer to o~-fe; signs are indicated only for the quadrupole splittings, e) d, mm/s A or e, mm/s B (T) F, mm/s Area, % x = x= x= x= == "g.9 ~, r- i-- 1 I I i I i -4o o 1'o 1; ' I l ' I ~ ' 9 c) - 0 ' -5,,,, 0,, 5 1 '0 ' 15, = I I I = I I I o 5 1'0 1; Velocity, mm/s Fig. 3. Room temperature M6ssbauer spectra of BaxCal_xFe0.sCoo.203_ ~ with x = 0.95 (a), x = 0.80 (b),x = 0.60 (c) and x = 0.40 (d), after "air quench". (The velocity scale refers to c~-fe) ) Let us note that the relative intensity of the sextet with B =47 T seems to be increased from about 2.7% to about 5.5% as an affect of the higher cooling rate (Table 2). As our samples did not have a starting oxygen stoichiometry much higher than 2.5, according to iodometric titration, we believe that this increase is not significant. Therefore, the appearance of 5-fold coordinated species with B =47 T is not by all means directly related to the concentration of the oxygen defects in the lattice. Further evolution of the species showing relaxation can be tracked by applying faster quench or reduction treatment. Figure 4 shows spectra of Bao.95Cao.05 Fe0.8Co0.203_ 5 as the most interesting member of this series after furnace cooling, methane treatment, and after quenching in liquid nitrogen. It is important to note that this sample was synthesized separately, in order to check reproducibility. As far as the effect of the quenching and the reducing treatment (with methane) is concerned, a further "cleaning up" of the spectrum occurred: the broad sextets showing relaxation became more resolved with narrower lines. This is in agreement with more oxygen deficiency which results in less perturbation of the magnetic coupling between iron spins. 294

5 Z. HOMONNAY et al.: TH MICRONVIRONMNT OF IRON IN (Ba,Ca)(Fe, Co)O3_ 5 CATALYST Conclusions == -.o I a, ~ o-'" I I 1 I I l I I I So -; ; ; 1'0 1; Velocity, mm/s Fig. 4. Room temperature M6ssbauer spectra of BaxCal_xFeo.sC%.203_8 with x = 0.95 from a separate synthesis after furnace cooling (a), after treatment in CH4He (4:96) atmosphere at 750 ~ for 15 min (b), and after quenching the as-prepared sample to liquid N 2 temperature (c). (The velocity scale refers to ~-Fe at RT) A substantial difference between the spectra regarding the two separate syntheses is the relative amount of the sextet of 47 T: 3-5% in the former and 15-18% in the latter case (Table 3). This is a significant difference. On the other hand, it is again confirmed that the amount of this sextet does not depend firmly on the oxygen deficiency, as its relative intensity is constant within experimental error in all three cases (furnace cooling, quenching and methane reduction). The most important proof on the relaxational origin of the spectral envelopes and on the assignment of the sextet of 47 T was obtained from low temperature measurements and CO 2 treatment of the Ba0.95Ca0.05Fe0.sCo0.203_ 8 sample (Fig. 5). As can be seen, either cooling the sample to 80 K or treating it with CO 2 results in well resolved sextets without any relaxational background left. By comparing the evaluated M6ssbauer parameters (Table 3) one can conclude that the sextet of 47 T is identical to the one observed after CO 2 treatment. Moreover, the same sextet forms in large quantity when the sample is cooled to 80 K, proved by the comparison of the two 80 K spectra. In conclusion, the following picture emerges from the analysis of the M6ssbauer spectra regarding catalytic properties of these perovskites. BaxCal_xFeyCol_yO3_ ~ shows the highest catalytic activity when relatively small amount of Ca and large amount of Co are substituted at the A and B sites of the lattice, respectively. At high substitution, the M6ssbauer spectra contain well resolved M6ssbauer patterns which indicates a rigid structure. The spectra tend to be very similar to that of brownmillerite which is not as good catalyst or CO 2 absorber as the mixed compounds. At low substitution level, the M6ssbauer spectra show magnetic relaxation which is due to fluctuating magnetic field at a frequency of about 108 s -1 (Larmor frequency of the 57Fe nucleus). As perturbation of the lattice by substitution is expected to decrease rigidity of the lattice due to the differing ionic sizes, it is reasonable to assume that the origin of the fluctuation of the magnetic field (relaxation) is vibration of lattice oxygens at a frequency close to the Larmor frequency. This shows that the lattice oxygens are highly mobile, and this gives a straightforward explanation for the good oxygen transferring catalytic properties of these materials. Vibration of the oxygens can be damped by either cooling the sample to low temperature, or by absorption of CO 2. CO 2 seems to have an indirect role in the formation of the 5-fold coordinated species with B =47 T. This species is stabilized by a nearby absorbed CO 2 molecule in the lattice. Now it is logical that its amount in the samples does not depend exclusively on the oxygen deficiency. The presence of the 5-fold coordinated species even in the freshly prepared samples can be explained by oxygen vacancies produced in the distorted interface between the Ba sublattice and the Ca sublattice. The difference in the amount of this species in the two separate syntheses is probably due to the possibly different lattice defect structure of the materials. The role of some absorbed CO 2 during cooling of the samples may also be considered9 The presence of CO 2 is not a pre-requisite for the formation of large amount of the 5-fold coordinated species. In a highly oxygen deficient lattice, it may be there but camouflaged by relaxation at room temperature. If the sample is cooled slowly, the vibrating oxygens find their equilibrium position in the lattice and the 5-fold coordinated species shows up. In case of quenching, the oxygens get frozen at non-equilibrium positions, and generate various subspectra in the Mgssbauer spectrum. 295

6 Z. HOMONNAY et al.: TH MICRONVIRONMNT OF IRON IN (Ba, Ca)(Fe,Co)O3_ 8 CATALYST 9 ~.._~.. Table 3. Room temperature Mrssbauer data of Ba0.95Ca0.05Feo.sCo0.203_ ~ obtained under various conditions. This material is a result of a synthesis independent from that of x=0.95 in Tables 1 and 2 (Isomer shifts refer to cc-fe; signs are given only for the quadrupole splittings, e) 5, mm/s A or e, mm/s B, T F, mm/s Area, % c,.i.o == F ~. ; ;.,.-. I! f -;o -5 o 5 1'o 1; Velocity, mm/s Fig. 5. Mrssbauer spectra of BaxCal_xFe0.sCoo.203_ ~ with x = 0.95 from a separate synthesis after furnace cooling, measured at RT (a) and at 80 K (b), as well as after treating in CO 2 atmosphere at 750 ~ for 15 min, measured at RT (c) and at 80 K (d). (The velocity scale refers to ~-Fe at RT) Finally, the environment of iron, which induced the sextet with 47 T by incorporation of Ca into Ba perovskite, is very similar (practically identical) to that found after CO 2 absorption. We conclude that the former is due to oxygen deficiency induced by sublattice distortion of A sites, whereas the latter is due to the strong interaction of CO 2 with one of the octahedrally coordinated oxygens in the perovskite. Furnace cooled, room temperature Furnace cooled, measured at 80 K Quenched to liquid N 2 temperature Treated in CH4-He (4:96) at 750 ~ 15 min Treated in CO 2 at 750 ~ 15 min, measured at RT Treated in CO 2 at 750 ~ 15 rain, measured at 80 K References 1. P. K. GALLAGHR, J. B. MACCHSNY, D. N.. BUCHANAN, J. Chem. Phys., 45 (1966) M. TAKANO, T. OKITA, N. NAKAYAMA, Y. BANDO, J. Solid State Chem., 73 (1988) K. NOMURA, T. HAYAKAWA, K. TAKHIRA, Y. UJIHIRA, Appl. Catal. A, 101 (1993) K. NOMURA, Y. SUGAWARA, HONG LING LI, Y. UJIHIRA, Hyperfine Interactions, 69 (1991) K. NOMURA, Y. UJIHIRA, T. HAYAKAWA, K. TAKHIRA, Appl. Catal. A, 137 (1996) K. NOMURA, Z. HOMONNAY, G. VANKO, A. VRTS, L. POPPL, A. NATH, Y. UJ1HIRA, T. HAYAKAWA, K. TAKHIRA, Hyperfine Interactions, 112 (1998) M. GAL, private communication. 8. K. NOMURA, T. GODA, Y. UJIHIRA, T. HAYAKAWA, K. TAKHIRA, Hyperfine Interactions, 69 ( 199 l)