J. Phys. Chem. C 2008, 112, 6531-6537
6531
Decomposition of Carbon Dioxide over the Putative Cubic Spinel Nanophase Cobalt, Nickel, and Zinc Ferrites Camilla Nordhei,*,† Karina Mathisen,† Igor Bezverkhyy,‡ and David Nicholson† Department of Chemisty, Norwegian UniVersity of Science and Technology, N-7491 Trondheim, Norway, and Institut Carnot de Bourgogne UMR 6209 CNRS-UniVersite´ de Bourgogne, 9 aV A. SaVary, BP 47870, 21078 Dijon Cedex, France ReceiVed: NoVember 27, 2007; In Final Form: February 4, 2008
Nanophase (4-6 nm) cobalt, nickel, and zinc spinel ferrites were synthesized by coprecipitation, and the decomposition of carbon dioxide at 300 °C was studied. Oxygen deficiency was obtained by reducing the materials in hydrogen. The divalent metals strongly influence the extent of the reactions; oxygen-deficient nickel ferrite was found to reduce three and nine times more carbon dioxide than the zinc and cobalt analogues, respectively. Carbon monoxide rather than carbon is produced for all of these ferrites. The reducibility in hydrogen and the degrees of oxygen-deficiencies were studied by X-ray absorption spectroscopy. The X-ray absorption near edge structure reveals that removing oxygen from the spinels leads to the concomitant reduction of iron(III) to iron(II), while the valence state of the divalent metal is unchanged. Of the three ferrites, nickel ferrite exhibits the highest degree of oxygen-deficiency. Extended X-ray absorption fine structure of the nickel ferrite shows that, although minor amounts of metallic nickel are expelled through over-reduction, the material is oxygen-deficient in a putative spinel structure.
Introduction The dynamic alternation between stoichiometry and nonstoichiometry in spinel ferrites (A(II)Fe(III)2O4) is an important feature that can be exploited in gas reactions. These reactions involve lattice oxygen, either through the transfer of oxygen atoms from the ferrite to the gas (oxidation reaction) or through oxygen vacancies being replenished by oxygen from the gas (reduction reaction). The latter process requires the nonstoichiometric form which can be obtained by the use of a reducing agent such as hydrogen. These nonstoichiometric or oxygen-deficient ferrites can be used to decompose water to hydrogen and carbon dioxide to carbon.1 We envisage the reaction of ferrites with mixtures of H2O and CO2 as being especially attractive in this context. A part of our work on this system is to measure the efficacies of a series of cubic spinel ferrites in abstracting oxygen from carbon dioxide in the absence of water. Oxygen-deficient ferrites are particularly interesting because they exhibit high decomposition efficiencies for both water and carbon dioxide at temperatures as low as 300 °C. For example, Tamaura et al.1 report that oxygen-deficient magnetite (Fe3O4) decomposes carbon dioxide to carbon with efficiencies up to 100% at 290 °C with no or little formation of carbon monoxide. At this low temperature the spinel structure is stable and able to accommodate interchanges between stoichiometry and nonstoichiometry. Oxygen-deficient ferrites are expressed as A(II)Fe(III)2O4-δ, where δ is the degree of oxygen-deficiency. A high δ in magnetite leads to increased Fe(II) reduction potentials and enhanced decomposition efficiencies. The A metal influences the reactivities of reduced ferrites toward decomposing carbon dioxide by steering the maximum * To whom correspondence should be addressed. E-mail: Camilla.
[email protected]. † Norwegian University of Science and Technology. ‡ UMR 6209 CNRS-Universite ´ de Bourgogne.
degree of oxygen-deficiency and thereby the reduction potential of iron(II).2-5 Significantly, Kato et al.2 showed that both the rate of reduction and decomposition are greatly improved when the A metal is nickel(II) instead of iron(II), whereas Tabata et al.3,4 report that the opposite is true when the A metal is manganese(II). Decomposition efficiencies are improved on increasing surface areas because this positively affects the number of active sites per volume unit. Crystallites smaller than 5 nm differ significantly from their bulk counterparts because the surface dominates over the volume thereby modifying the properties.6 These dimensional effects are important in heterogeneous catalysis and in solid-state reactions because of improved availabilities of the active sites. Kormarneni et al.7 reported that nanoparticulate zinc ferrite (5-10 nm) not only showed better decomposition efficiency than the bulk material but also lessened the formation of carbon monoxide. Both the reduction rate and reactivity of nickel ferrite toward carbon dioxide decomposition is higher in the nanophase material (15-29 nm) compared to the bulk (100-200 nm).8 The way the metals are distributed between the single tetrahedral and two octahedral sites per spinel formula unit gives the normal, inverse, and partial inverse systems.9 This is also influenced by particle size. Thus, partially inverted structures are often observed for nanocrystalline materials.10,11 Recently, we reported a study on the crystallite-size dependency on metal distributions in the nanophase (4-25 nm) cobalt, nickel, and zinc ferrites synthesized by coprecipitation.6 Cobalt and nickel ferrite adopt the inverse structure in the bulk, whereas zinc is normal.9 However, for nanoparticles, partially inverted systems were found for cobalt ferrite and zinc ferrite. In nanoparticulate nickel ferrite, there is an over-population of iron in octahedral sites.6 We attribute this to the significant influence of the surface on the structure. It is the octahedral sites at the surface that are active in reactions with gases,12 and therefore, we expect that
10.1021/jp7112158 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/02/2008
6532 J. Phys. Chem. C, Vol. 112, No. 16, 2008
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TABLE 1: Summary of the Average Crystallite Size, Surface Area, and Metal Composition of Cobalt, Nickel, and Zinc Ferrite material
crystallite sizea/nm
surface areab/m2g-1
chemical formulac
CoFe2O4 NiFe2O4 ZnFe2O4
5 4 6
224(2) 285(2) 185(1)
Co0.97Fe2.03O4 Ni0.99Fe2.01O4 Zn0.83Fe2.17O4
a Estimated from the Scherrer method (XRD). b Measured by the BET method. The standard deviation in the last significant digit is given in parentheses. c Measured from ICP-MS with standard deviation in the range of 2-5%.
these materials are also interesting in the decomposition of carbon dioxide. In nanophase crystallites, interatomic interactions within the surface and near surface regions differ significantly from those within. This puts into question the designation “spinel” when entering the lower nanoregime. Accordingly, the term “putative spinel” structure is used here. The scope of this work is to study the reactivities of the three nanophase putative cubic ferrites with three different A metals (CoFe2O4 (5 nm), NiFe2O4 (4 nm), and ZnFe2O4 (6 nm)) in the reaction with carbon dioxide at 300 °C. The influence of the A metal on the reducibility of the ferrites with hydrogen was studied in situ using X-ray absorption spectroscopy (XAS) to bring forth electronic and structural changes. Experimental Section The nanophase cobalt, nickel, and zinc ferrites were synthesized by the coprecipitation method.6 Characterization6 was carried out using XAS, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), nitrogen sorption analyses (BET method), and inductively coupled plasma mass spectrometry (ICP-MS). The crystallite sizes, surface areas, and chemical formulas are listed in Table 1. The carbon dioxide reactions were performed in a continuous flow mode using a fixed bed U-tube reactor connected to a gasphase chromatograph equipped with a mass sensitive detector (Hewlett-Packard 6890). The ferrite (1.5 g) was heated from room temperature to 300 °C (rate of heating being 5°/minute) in a pure hydrogen flow (100 mL/min) and maintained at this temperature for 3 h. After flushing the reactor with helium (100 mL/min, 20 min), carbon dioxide (10% in He, 5 mL/min) was decomposed at 300 °C over the reduced spinel. Time zero is defined as being the time the mixture was brought to the reactor. The measurements in carbon dioxide were run until no further changes were detected in the gaseous output. After the carbon dioxide decomposition, the reactor was flushed with hydrogen while monitoring the outlet gas for hydrocarbons. The XAS was carried out at the Swiss-Norwegian Beamlines (SNBL, BM1B) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The beamline is equipped with a Si(111) monochromator and chromium mirrors to reject higher order harmonics (EXAFS). The data were collected in the transmission mode with an electron energy of 6 GeV and a maximum current of 200 mA. The reduction of cobalt, nickel, and zinc ferrite in hydrogen (5% in helium) was studied by in situ XANES (X-ray absorption near edge structure). The amount of material in the samples was calculated to give a maximum absorber optical thickness of 1 absorber length. The materials were mixed and ground with boron nitride and put into the in situ cell (thickness: 3 mm). The cell was enclosed by aluminum windows (Goodfellow, thickness: 0.020 mm, purity: 99.0% (cobalt and nickel ferrite) and 99.999% (zinc ferrite)). The samples were heated to 150 °C
Figure 1. CO2 consumption over time on cobalt ferrite (square), nickel ferrite (diamond), and zinc ferrite (triangle). (Standard deviation: (0.1 vol %.)
using a rate of 2°/min in pure helium (He, 15 mL/min) before switching to hydrogen (5% H2 in He, 15 mL/min) and heating to 300 °C while simultaneously measuring the XANES. The XANES of the reference compounds (Fe2O3, Fe3O4, NiO, CoO, and ZnO) and metals foils (Fe, Co, Ni, and Zn) were collected under ambient conditions. The energy-corrected XANES spectra were normalized from 45 to 135 eV above the edge using the ATHENA program in the iFeffit package.13 The EXAFS (extended X-ray absorption fine structure) measurements were performed on nickel ferrite in a pretreatment cell as described elsewhere.14 The samples were mixed and ground with boron nitride and pressed into a pellet. The amount of sample was empirically chosen. The cell was heated (5°/min) to 300 °C in helium (35 mL/min) and kept at this temperature for 30 min before cooling to room temperature. After EXAFS data collection, the cell was heated in helium to 300 °C (5°/min). The sample was then subjected to a hydrogen flow (5% in He) at 300 °C for 1 h and then cooled to room temperature in helium before the final EXAFS measurements. The EXAFS measurements were summed and background subtracted using the Daresbury programs EXCALIB and EXBACK.15 The EXAFS oscillations were extracted and converted to k-space. The least-squares curve fitting of the experimental k3 weighted EXAFS to the theoretical EXAFS function was carried out using the curved-wave theory and ab initio phaseshifts by EXCURV98.16 In accordance with guidelines,17,18 the spectra were not Fourier filtered to remove noise. The final data were treated according to refs 6 and 14 and references therein. The amplitude reduction parameter (AFAC) was refined for the reference compounds Fe3O4 19 and NiO20 and transferred to the unknowns. Mixed sites were refined as a single average site because the fit was not improved by refining several sites with the accompanying increased number of parameters. Mixed octahedral sites were refined as iron (nickel as backscatterer gave similar results). To reduce the coupling between correlated parameters, the EXAFS spectra were fitted using k1 and k3 weighted data and solutions common to both weighting schemes were chosen.21 For both the reference compounds and the samples, the interatomic distance (R) and the refined correction of Fermi energy (EF) were decorrelated. In addition, the same procedure was carried out for the AFAC and Debye-Waller factor (2σ2) on the reference data, and the multiplicity (N) and Debye-Waller factor for the samples. Results Results from the decomposition of carbon dioxide over all three reduced ferrites are shown in Figure 1. The consumption of carbon dioxide was estimated from that removed from the gas stream. In all of the materials, the highest activity was observed within the first 5 min of reaction and thereafter gradually decreases and stabilizes at relatively low levels. The
Decomposition of Carbon Dioxide
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6533
TABLE 2: Total Amount of CO2 Consumed during the Reactiona on Reduced Cobalt, Nickel, and Zinc Ferrite material
total consumed CO2 (mL)
total consumed CO2 (mol)
CoFe2O4 NiFe2O4 ZnFe2O4
4.7 41.2 14.3
2.1 × 10-4 1.8 × 10-3 6.4 × 10-4
a Reaction time: cobalt ferrite, 38 min; nickel ferrite, 124 min; and zinc ferrite, 120 min.
Figure 2. Formation of CO (peak area) in the reaction on cobalt ferrite (square), nickel ferrite (diamond), and zinc ferrite (triangle).
order of activity for the carbon dioxide decomposition is as follows: nickel ferrite > zinc ferrite > cobalt ferrite. For cobalt and zinc ferrite, the amount of adsorbed carbon dioxide diminishes strongly after 20 and 50 min, respectively. However, nickel ferrite remains active with a consumption rate of about 0.5 vol % throughout the course of the experiment (124 min). Table 2 shows the consumptions of carbon dioxide for the three ferrites under the given reaction conditions. The amount of consumed carbon dioxide was found by measuring the areas under the consumption profiles (Figure 1). Figure 2 shows the formation of carbon monoxide over time. Since French safety regulations precluded us from carrying out a carbon monoxide calibration, the amount of carbon monoxide produced in the experiment cannot be determined. However, information is available from the significantly different individual reaction profiles of the three ferrites (Figures 1 and 2). In the case of zinc ferrite, and to some extent cobalt ferrite, the carbon monoxide and carbon dioxide reaction profiles are similar. This shows that the formation of carbon monoxide and the concomitant consumption of carbon dioxide is initially relatively high followed by a progressive decay toward zero activity. Interestingly for nickel ferrite, the formation of carbon monoxide is minimal at the start when carbon dioxide consumption is at a maximum. After the first 5 min, the production of carbon monoxide increases and remains constant. Any carbon deposited on the surface during the decomposition might be envisaged as being removed as hydrocarbons (methane) by flushing the postreaction sample with hydrogen.22,23 Methane was not detected for any of the present samples. This may either be due to low carbon contents (below detector limit) or because carbon is in a polyatomic form that does not react with hydrogen under these conditions. The reduction of cobalt, nickel, and zinc ferrite by hydrogen at 300 °C was followed by XANES. The average valence states of the metals are reduced as oxygen is removed by hydrogen
from the system in the form of water. Thus, the degrees of oxygen deficiencies in the reduced ferrites are expressed by this decrease in the valence state. It is well-known that the position of the absorption edge is influenced by the valence state of the target metal atoms and by a contribution from the environment (chemical shift).24 Therefore, the valence states of A and iron metals in the ferrites were established by comparing their edge energies with those of reference compounds. Figure 3 shows the normalized A metal (Co, Ni, and Zn) K-edge XANES of the as-prepared and reduced cobalt, nickel, and zinc ferrites. The A metals remain in their initial divalent states after treatment in hydrogen at 300 °C as confirmed by comparison with the reference compounds cobalt(II), nickel(II), and zinc(II) oxides. However, for the cobalt and nickel ferrites, the spectral profiles change upon reduction. This is consistent with minor changes in the local bonding environments when oxygen is removed. There are no apparent changes in the Zn XANES of zinc ferrite following reduction in hydrogen. Figure 4 shows the normalized Fe K-edge XANES of the reduced ferrites and the references Fe2O3 and Fe3O4. Upon reduction in hydrogen, the iron absorption edge shifts to lower energies, consistent with a reduction in the average valence state of iron(III). Thus, removing oxygen mainly influences the valence state of iron with the shift being largest for nickel ferrite (Figure 4a). From the XANES, the order of reducibility of iron in these putative spinels is as follows: nickel ferrite > cobalt ferrite > zinc ferrite. In order to estimate the valence states of the reduced ferrites, the edge energies of the three references Fe2O3, Fe3O4, and iron metal were plotted against their average valence state (3, 2.67, and 0, respectively), shown in Figure 5. The edge energies ((0.3 eV) were defined as being half the edge height. The figure displays a linear relationship between the edge energy and valence state, which was used to establish the valence states of the ferrites as exemplified by the reduced zinc ferrite (Figure 5c). The local environments in the oxide references and the ferrites are similar, hence the contribution from the chemical shifts to the absorption edges is also similar. The spinel Fe3O4 is especially appropriate as a reference because the iron atoms are in both valence states two and three. Therefore, it is valid to use Figure 5 to ascertain the valence states of the reduced ferrites (Table 3). The decrease in the average valence state of iron was used to estimate the degree of oxygen-deficiency in the reduced materials. The oxygen-deficiency (δ) was obtained by taking into account charge neutrality assuming that the spinel structure is maintained and that the decrease in the valence state of iron reflects the loss of oxygen from the spinel structure (A(II)Fe(III-d)2O4-δ). The δ values for the reduced cobalt, nickel, and zinc ferrites are 0.26(3), 0.58(3), and 0.16(3), respectively. The corresponding chemical formulas are given in Table 3. For the given δ values, Table 3 also gives the estimated amounts of carbon dioxide required to reoxidise the ferrites (1.5 g). Two reaction routes were considered: reduction to carbon or carbon monoxide. The amount of carbon dioxide needed for complete
TABLE 3: Metal Valence State, Chemical Formula, Degree of Oxygen Deficiency, and Calculated CO2 Consumption of Reduced Cobalt, Nickel, and Zinc Ferrites at 300 °Ca material
average Fe valence state
average A metal valence state
degree of oxygen deficiency (δ)
chemical formula
CO2 required CO2 f C (mol)
CO2 required CO2 f CO (mol)
CoFe2O4 NiFe2O4 ZnFe2O4
2.74(3) 2.41(3) 2.84(3)
2 2 2
0.26(3) 0.58(3) 0.16(3)
CoFe2O3.74 NiFe2O3.41 ZnFe2O3.84
9.5 × 10-4 1.9 × 10-3 6.2 × 10-4
1.9 × 10-3 3.8 × 10-3 1.2 × 10-3
a
The estimated standard deviation in the last significant digit is given in parentheses.
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Figure 3. Normalized XANES of the (A) Co K-edge for (a) CoO, (b) reduced CoFe2O4, and (c) CoFe2O4; (B) Ni K-edge for (a) NiO, (b) reduced NiFe2O4, and (c) NiFe2O4; and (C) Zn K-edge for (a) ZnO, (b) reduced ZnFe2O4, and (c) ZnFe2O4.
Figure 4. Normalized Fe K-edge XANES for (a) reduced NiFe2O4, (b) Fe3O4, (c) reduced CoFe2O4, (d) reduced ZnFe2O4, and (e) Fe2O3. The reduced ferrites are presented by solid line and the reference materials by dotted lines.
Figure 6. Experimental (s) and calculated (---) Fe K-edge EXAFS (A) and their Fourier transform (B) for (a) reduced NiFe2O4 and (b) NiFe2O4.
TABLE 4: Results from the Fe K-Edge Curve Fitting for Nickel Ferrite and Reduced Nickel Ferritea material
shell
original Fe-O NiFe2O4 Feocta‚‚‚Feocta Fetetra‚‚‚Feocta reduced Fe-O NiFe2O4 Feocta‚‚‚Feocta Fetetra‚‚‚Feocta
Figure 5. Edge energies of (a) iron metal, (b) Fe3O4, (c) reduced zinc ferrite, and (d) Fe2O3 as a function of their average valence states.
reoxidation of the reduced materials (Table 3) is within the range of that consumed in the decomposition (Table 2). Nickel ferrite was studied further by EXAFS because of its aforementioned anomalous behavior in the decomposition of carbon dioxide. This region of the XAS spectrum gives insight into the effects of the reduction on the local environments about the targeted atoms. The EXAFS of nickel ferrite reduced in hydrogen at 300 °C was compared to that of the as-synthesized material that had been pretreated in helium at the same temperature. Figure 6 shows the k3 weighted and least-square fitted iron EXAFS and their Fourier transforms. Since the XANES shows significant changes, it is interesting to note that
N
r/Å
4.9(3) 3.9(3) 1.8(8) 4.7(3) 5.1(9) 2.1(6)
1.964(5) 2.999(8) 3.46(1) 1.978(6) 3.001(7) 3.481(7)
2σ2/Å2
EF/eV
R/%b
0.019(2) -3.0(5) 24.4 0.026(8) 0.017(6) 0.021(2) -3.8 26.8 0.027(3) 0.014(3)
a The EXAFS refinements give information about multiplicity (N), bonding energy (R), and thermal vibration (Debye-Waller factor, 2σ2). EF is the refined correction of Fermi energy in vacuum, compared to E0 in EXBACK. The standard deviation in the last significant digit as calculated by EXCURV98 is given in parentheses. These estimates, however, will in cases of high correlation between parameters lead to an overestimation of accuracy as the standard deviations for bonding distances are (0.01 Å for small r values and (0.04 Å for r values exceeding 3 Å. The deviation for 2σ2 is (20%. b The statistical R-factor th is defined as R ) ∑Ni [1/σi(|χexp i (k) - χi (k)|)] 100% and gives indication of the quality of fit in k-space.
the Fe EXAFS (unlike the Ni EXAFS, see below) of the two samples are virtually identical. Whereas the XANES shows that iron is reduced, the electronic changes about the iron atoms are not apparent from the structural information in the EXAFS (Table 4). This is a statistical effect (oxygen deficient coordinations versus stoichiometric environments), the structural changes being overwhelmed by the stoichiometric contribution. This means that in spite of removing a small number of oxygen atoms from the lattice the overall structure is maintained but with the electronic changes being highlighted in the XANES. In contrast to the Fe K-edge, the Ni K-edge Fourier transforms (Figure 7) of the as-synthesized and reduced nickel ferrite do
Decomposition of Carbon Dioxide
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6535 indicates that the overall treatment leads to crystallites growing from 4 to approximately 10 nm. Discussion There are two reaction routes for decomposing or reducing carbon dioxide by oxygen-deficient ferrites. One is the decomposition to carbon (eq 1) and the other the reduction to carbon monoxide (eq 2)
Figure 7. Experimental (s) and calculated (---) Ni K-edge EXAFS (A) and their Fourier transform (B) for (a) reduced NiFe2O4 and (b) NiFe2O4.
Figure 8. Ni K-edge difference Fourier transform spectrum of the experimental and theoretical ferrite spectrum.
TABLE 5: Results from the Ni K-Edge Curve Fitting for Nickel Ferrite and Reduced Nickel Ferritea material original NiFe2O4 reduced NiFe2O4 a
shell
N
r/Å
2σ2/Å2
EF/eV
R/%
Ni-O 5.8(4) 2.065(6) 0.017(2) 0.6(3) 33.1 Niocta‚‚‚Feocta 8(1) 3.002(7) 0.030(8) Ni-O 5.1(5) 2.066(9) 0.017(2) 0.5(5) 35.3 Ni-Ni 0.9(6) 2.51(1) 0.014(8) Niocta‚‚‚Feocta 10(2) 2.988(8) 0.028(4)
See footnote a in Table 4.
show significant differences. Upon reduction in hydrogen, the difference spectra (Figure 8) of the experimental and theoretical EXAFS show that minor amounts of nickel metal are formed (nickel-nickel shell at 2.5 Å (Table 5)). Moreover, the multiplicity of the Ni-O shell decreases while the Ni-O bond length remains unchanged. The increased amplitude of the Fe EXAFS oscillations at high k values and the increased intensities of the Fe‚‚‚Me shells at 3 and 3.5 Å (Figure 6) suggest that sintering occurs at 300 °C. Crystallite growth is further supported by the increased Fe‚‚‚ Me multiplicities in accordance with previous EXAFS results on nanophase ferrites.6 Indeed, comparison with previous EXAFS for nickel ferrite measured at various crystallite sizes
δ δ CO2 + AFe2O4-δ ) C(s) + AFe2O4 2 2
(1)
δCO2 + AFe2O4-δ ) δCO + AFe2O4
(2)
Both reactions which take place simultaneously are reported for a number of similar oxygen-deficient ferrites.4,5,8,25-28 In principle, oxygen (as O2-) from carbon dioxide is transferred to the oxygen lattice vacancies in the oxygen-deficient ferrite thereby restoring the ferrite to stoichiometry. At the same time, in the overall reduction, electrons are donated from the oxygendeficient ferrite to produce carbon or carbon monoxide. The activities of the ferrites depend on their relative abilities to donate electrons. As reported by Tabata et al.,3 the divalent iron which is formed on reduction acts as the electron donor. We can expect that a higher degree of oxygen-deficiency, or equivalently a higher iron(II) content, leads to increased reduction of carbon dioxide. Of the three nanophase ferrites, nickel ferrite consumes three and nine times more carbon dioxide than zinc and cobalt ferrite, respectively (Table 2). We have shown that nickel ferrite has the highest degree of reducibility of iron(III) of the ferrites in the this study (Table 3), which can explain the exceptional behavior of nickel ferrite in the decomposition reaction. Clearly, the A metal influences the reactivities of the nanophase ferrites in decomposing carbon dioxide. When the degree of oxygen-deficiency extends beyond the maximum tolerated by the spinel, the structure disintegrates into the constituent metals or metal oxides.25,29 The critical δ value of a particular ferrite depends on the A atom. For example, critical δ values of 0.19 and 0.06 are reported for nickel and zinc ferrite, respectively.5 In this study, we report δ values of 0.26(3), 0.58(3), and 0.16(3) for the reduced nanophase cobalt, nickel, and zinc ferrites, respectively (Table 3). Differences in the critical δ values for the different A metals can be rationalized in terms of the electronic requirements of the metals. For example, zinc is 3d10 and therefore cannot deviate from Zn(II), whereas cobalt and nickel, being 3d7 and 3d8, respectively, can tolerate a higher number of oxygen vacancies. The relatively high oxygen deficiencies for these materials indicate overreduction. This is supported by the formation of nickel metal as revealed by EXAFS (see below), although not visible in the XANES for any of the samples. However, whereas the reactivities of the bulk ferrites originate from the electronic configurations of the various A metals, particle size is an additional factor in the nanoregime. Generally, catalytic activity is enhanced in small nanoparticles since surface areas are much larger. In addition, for the present materials the proportion of tetrahedral versus octahedral sites at the surface is also relevant because it is the latter that are active.12 For all these nanophase materials, the distributions of atoms in the tetrahedral and octahedral sites differ from the distributions in the respective bulk materials as we have previously reported.6 These results indicate that small particles with large surface areas are easily reduced apparently beyond the critical oxygen-deficiency which means that these putative spinels have a higher oxygen-
6536 J. Phys. Chem. C, Vol. 112, No. 16, 2008 deficiency than the corresponding bulk spinels. Over-population of iron in octahedral sites, as in nickel ferrite, appears to enhance the activity toward reduction and reoxidation by carbon dioxide. However, there is some sintering at 300 °C, and the particle sizes are therefore larger than in the as-synthesized sample. The crystallite growth is similar for all three ferrites,6 and we can expect that all of the materials have grown to approximately 10 nm during the reaction. Still, a significant fraction of the metals are in different bonding environments at the surface where truncation effects apply. Whereas cobalt ferrite exhibits a larger oxygen-deficiency than zinc ferrite, the former actually shows less reactivity in reducing carbon dioxide than the latter. Comparing the amount of carbon dioxide consumed in the reaction (Table 2) with the estimated amount required to reoxidise the materials for the given oxygen deficiencies (Table 3) makes clear that there are differences in the reoxidation efficiencies for the three different ferrites. For cobalt ferrite, the carbon dioxide consumed is significantly less than that expected for complete reoxidation, suggesting that the transition between the reduced and stoichiometric ferrite is not fully reversible. An explanation is that carbon deposited on the surface may inhibit further reaction with carbon dioxide, leading to incomplete reoxidation. For zinc ferrite, the significant amount of carbon monoxide produced in the reaction (Figure 2) suggests that the reaction route in eq 2 is important thereby facilitating the reoxidation process. Structural changes resulting from reducing nickel ferrite by hydrogen were studied by EXAFS. At the nickel K-edge (Table 5), the multiplicity and bond length of the Ni-O shell in the unreduced material accords with nickel being octahedrally coordinated in the ferrite.6 This is consistent with the inverse structure. However, the EXAFS confirms that over-reduction gives metallic nickel with a refined Ni-Ni bond length of 2.51 Å. The very low multiplicity of this shell in the composite spectrum shows that the reduction is only partial. Still, the majority of nickel is in octahedral sites in the ferrite, as seen by the unchanged Ni-O bond length. The mixed nickel environments account for the reduced multiplicity of the Ni-O shell. At the iron edge, the local structures around the iron atoms are virtually the same before and after reduction (Table 4). The EXAFS refinements put iron in mixed tetrahedral and octahedral sites (inverse) in which the Fe-O multiplicity of ∼5 reflects the average of the two coordination environments. The lack of any structural changes confirms that the spinel structure is maintained after reduction, even though a minor amount of nickel metal is formed. Hence, the EXAFS suggests that overreduction does not lead to a complete disintegration of the structure but rather to ejection of metal from the oxygendeficient putative spinel. These results are similar to those reported by Zhang et al.30 for reduced magnetite, where the oxygen-deficiency is constant regardless of the quantity of metal formed. Importantly, this means that, in spite of over-reduction, the material is still active in the reaction with carbon dioxide. In several respects, as mentioned above, nickel ferrite stands out from the other ferrites. For example, the graphs of the concentrations of consumed carbon dioxide and produced carbon monoxide have different profiles (Figure 1 and 2). The consumption of carbon dioxide is accompanied by the production of carbon monoxide and/or carbon. Deviations of the consumption profile (CO2) and production profile (CO) are related to the formation of carbon. In the case of nickel ferrite, in the beginning the consumption of carbon dioxide is accompanied by a low production of carbon monoxide. Over a short period of time, the relative amount of carbon monoxide
Nordhei et al. increases significantly. Of the other two ferrites, the zinc ferrite is the most extreme and deviates from the behavior of nickel ferrite. In zinc ferrite, the consumption of carbon dioxide is lower than in nickel ferrite and the profile is followed by the production of carbon monoxide. The cobalt ferrite shows a similar but damped profile of carbon monoxide compared to zinc. From the above, we suggest that the putative spinel nickel ferrite initially decomposes carbon dioxide to carbon and oxygen with the latter entering the structure by filling vacant sites and hence lowering the deficiency. At lower degrees of deficiencies, the parallel reaction with carbon now dominates in which carbon acquires oxygen from the lattice to form carbon monoxide. The decomposition of carbon dioxide on zinc ferrite initially produces a significant proportion of carbon monoxide which decreases rapidly with time. Consistent with the lowest degree of deficiency, zinc produces a relatively higher fraction of carbon monoxide. That is, as carbon dioxide loses oxygen to the lattice, carbon monoxide is produced. Then the oxygen-deficiency is lowered sufficiently so that the decomposition of carbon dioxide ceases. In short, whereas the putative spinel nickel ferrite behaves as a catalyst the other two materials simply take part in solid-state reactions. Conclusion In the decomposition of carbon dioxide on nanophase oxygendeficient ferrites, we have shown that their activity and behavior depend on the divalent A metal. One of the factors governing the reactivities of cobalt, nickel, and zinc ferrites is the degree of oxygen-deficiency. This is influenced by the electronic properties of the A metal. Another factor is particle size because within the lower reaches of the nanoregime the degree of nonstoichiometry or oxygen deficiency is increased relative to the bulk. Significantly, nickel ferrite, which exhibits the largest oxygen deficiency when reduced in hydrogen, also consumes the largest amount of carbon dioxide. The excellent behavior of nickel ferrite compared to cobalt and zinc is ascribed to the increased fraction of octahedrally coordinated iron. The degree of oxygen deficiency also affects the reaction mechanism; low oxygen deficiencies appear to favor carbon dioxide reduction to carbon monoxide. Acknowledgment. The financial support from the Norwegian Research Council and the Faculty of Natural Science and Technology, Norwegian University of Science and Technology (NTNU) is much appreciated. Many thanks are due to Olga Safonova and Wouter van Beek at the Swiss-Norwegian Beamlines at the European Synchrotron Radiation Facility, Grenoble, France for invaluable assistance during XAS measurements. References and Notes (1) Tamaura, Y.; Tabata, M. Nature 1990, 346, 255. (2) Kato, H.; Kodama, T.; Tsuji, M.; Tamaura, Y.; Chang, S. G. J. Mater. Sci. 1994, 29, 5689. (3) Tabata, M.; Nishida, Y.; Kodama, T.; Mimori, K.; Yoshida, T.; Tamaura, Y. J. Mater. Sci. 1993, 28, 971. (4) Tabata, M.; Akanuma, K.; Nishizawa, K.; Mimori, K.; Yoshida, T.; Tsuji, M.; Tamaura, Y. J. Mater. Sci. 1993, 28, 6753. (5) Kodama, T.; Tabata, M.; Sano, T.; Tsuji, M.; Tamaura, Y. J. Solid State Chem. 1995, 120, 64. (6) Nordhei, C.; Ramstad, A. L.; Nicholson, D. G. Phys. Chem. Chem. Phys. 2008, 10, 1053. (7) Komarneni, S.; Tsuji, M.; Wada, Y.; Tamaura, Y. J. Mater. Chem. 1997, 7, 2339. (8) Kodama, T.; Wada, Y.; Yamamoto, T.; Tsuji, M.; Tamaura, Y. Mater. Res. Bull. 1995, 30, 1039.
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