Decomposition of Carbon Dioxide at 500 Â°C over Reduced Iron

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Decomposition of Carbon Dioxide at 500 °C over Reduced Iron, Cobalt, Nickel, and Zinc Ferrites: A Combined XANES-XRD Study Camilla Nordhei,*,† Karina Mathisen,† Olga Safonova,‡ Wouter van Beek,‡,§ and David G. Nicholson† Department of Chemistry, Norwegian UniVersity of Science and Technology, N-7491 Trondheim, Norway, Swiss Norwegian Beamlines, European Synchrotron Radiation Facility, F-38043 Grenoble, France, and Dipartimento di Scienze e Tecnologie AVanzate, UniVersita` del Piemonte Orientale “A. AVogadro”, Via V. Bellini 25/G, 15100 Alessandria, Italy ReceiVed: May 27, 2009; ReVised Manuscript ReceiVed: September 23, 2009

A series of ferrite spinels [AFe2O4, where A ) Fe(II), Co(II), Ni(II), or Zn(II)] synthesized by coprecipitation were reduced in hydrogen and subsequently studied in the decomposition of carbon dioxide at 500 °C by a combination of in situ XANES, XRD, and mass spectrometry methods. The reducibilities of the ferrites in hydrogen depend on the A metal. In the reduction to the metallic states, XANES data reveal that iron(II) oxide is formed as an intermediate product with A(II) being expelled to form A(II) oxides as the spinel structure breaks down. Carbon monoxide is produced in the decomposition of carbon dioxide through the oxidation of iron(II) or metallic iron. The highest activities in the reaction were observed for nickel and zinc ferrite. However, as opposed to cobalt ferrite and magnetite, the regeneration to the original ferrite structure in carbon dioxide was not complete for these two materials. For nickel ferrite, which exhibits high reducibility and carbon dioxide reactivity, magnetite is formed as the major reoxidation product. I. Introduction We have recently reported on the decomposition of carbon dioxide at 300 °C over cobalt, nickel, and zinc ferrites, which had been previously reduced by hydrogen to oxygen deficiency.1 In this reaction, carbon dioxide is decomposed by transferring oxygen to oxygen vacancies within the spinel lattice thereby producing carbon or carbon monoxide. This process is important in applications for removing carbon dioxide from the atmosphere (e.g., space shuttles).2 However, when ferrites are reduced beyond specific critical degrees of oxygen deficiency, the structures disintegrate into the constituent metals or their oxides. It is known that both phases contribute to the decomposition at 300 °C.3-6 Decomposition of carbon dioxide on spinel systems has also been previously studied at higher temperatures.7-10 Completely reduced nickel and copper ferrites7,8,10 differ from cobalt ferrite in that only metallic iron is active in the former, whereas in the latter both metals are active. The phase transitions following reduction and reoxidation depend on the type of ferrite, its particle size, and temperature. Ideally, reoxidation to the original phase is desirable in order to maintain cyclic activity. Any understanding of the processes involved is predicated on the acquisition of information relating to reduction and reoxidation. In the present work, we have studied the hydrogen reduction of iron, nickel, cobalt, and zinc ferrites and their subsequent reoxidation at 500 °C by carbon dioxide. The techniques used here include combined in situ X-ray absorption near-edge structure (XANES) and X-ray powder diffraction (XRD). Gaseous products of reaction were identified by leading the exhaust from the reaction cell into a mass spectrometer. * To whom correspondence should be addressed. [email protected]. † Norwegian University of Science and Technology. ‡ European Synchrotron Radiation Facility. § Universita` del Piemonte Orientale “A. Avogadro”.

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XANES is very useful in this type of study because the spectra contain information on the formal valence states of the target element. For elements in similar environments, edge energies correlate with valence states.11 However, in reality it is often difficult to extract full information because the observed XANES are an average of different phases. XRD is the key to obtaining a more complete structural and hence electronic picture because the reflections from constituent crystalline phases are separated. We have therefore combined XAS and XRD in a common setup under in situ conditions. II. Experimental Section The nanophase iron, cobalt, nickel, and zinc ferrites were synthesized by the coprecipitation method and characterized as reported previously.12 The XANES and XRD measurements were carried out at the Swiss-Norwegian Beamlines (SNBL, BM1B) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The data were collected using a Si(111) monochromator with an electron beam of 6 GeV and maximum current of 200 mA. The XANES were measured in transmission mode using ion chamber detectors [I0 (31 cm), 100% N2; It (31 cm), 60% N2 and 40% Ar]. The X-ray powder diffractograms were collected using a two-circle diffractometer equipped with six counting chains [Si(111) analyzer crystals and Na-I scintillation counter detectors]. The measurements were performed using an in situ cell for the combined XAS and XRD with a sample cell thickness of 3 mm. The cell was enclosed by aluminum windows [Goodfellow, thickness 0.020 mm, purities of 99.0% (for iron, cobalt, and nickel ferrite) and 99.999% (for zinc ferrite)]. The amount of sample was optimized for XANES and mixed with boron nitride to give an absorber optical thickness of 1 absorption length. The gaseous output from the in situ cell was connected to a mass spectrometer (Hiden HPR-20). The samples were heated

10.1021/jp9049473 CCC: $40.75  2009 American Chemical Society Published on Web 10/14/2009

Decomposition of CO2 over Fe, Co, Ni, and Zn Ferrites

J. Phys. Chem. C, Vol. 113, No. 45, 2009 19569

Figure 1. X-ray powder diffractogram of CoFe2O4 (a) in helium at 25 °C, (b) after reduction in hydrogen at 500 °C, (c) after reoxidation in carbon dioxide at 500 °C, and (d) in helium after cooled to 25 °C.

Figure 2. X-ray powder diffractogram of Fe3O4 (a) in helium at 25 °C, (b) after reduction in hydrogen at 500 °C, (c) after reoxidation in carbon dioxide at 500 °C, and (d) in helium after cooled to 25 °C. (Unfilled square is indentified peak.)

to 150 °C in helium (15 mL/min, ∼2°/min) and then reduced in hydrogen (5% in helium, 15 mL/min) while heating to 500 °C. During the reduction, the XANES at the iron K-edge were collected continuously except at selected temperatures (25, 300, 400, and 500 °C), where the A K-edge XANES (A ) Ni, Co, or Zn) were collected. The reduction of the ferrites by hydrogen (5% in He) was complete when no further changes were observed in the XANES. The X-ray powder diffractograms (9-26°in 2θ) were collected before and after reduction in hydrogen using a wavelength of 0.5 Å. After reduction in hydrogen, the cell was flushed with helium (15 mL/min) for approximately 1 h (until hydrogen was removed from the system as confirmed by the mass spectrometer). Carbon dioxide (10% in He, 15 mL/min) was then directed to the cell while collecting short XANES scans at the iron K-edge (-10 to 0 eV from the edge). These scans were sufficiently fast to monitor the reaction of carbon dioxide on the reduced material. After the reaction, XANES at the iron and A edges were collected. The XRDs were collected when no further changes were observed in the XANES. The gas was switched to helium, and the samples cooled to room temperature where the final XANES and XRDs were measured. The XANES of references

(Fe2O3, Fe3O4, CoO, NiO, and ZnO) and metal foils (Fe, Co, Ni, and Zn) were collected at room temperature. The output gas from the cell was monitored by a mass spectrometer. The consumption of carbon dioxide was measured as the gas was removed when passed through the sample. The increase in the carbon monoxide signal corresponds to generation of carbon monoxide by the reaction. In the calculations, the signal relative to pure helium was used. The mass spectrometer was calibrated at room temperature. The XANES were energy-corrected from the metal foils and normalized from 45 to 135 eV above the edge using the Athena program in the Ifeffit package.13 The spectra were baselinecorrected from -60 to -30 eV prior to the edge. The absorption edge energies were measured as the first inflection point in the derivative spectra. Linear combination analyses were performed in Athena using a fitting range of -20 to 60 eV relative to the absorption edge energy, and weighting was constrained to unity. The quality of fit is given by the R-factor, defined as R ) [sum(data-fit)2]/[sum(data2)]. The number of significant standards in the linear combination analyses was tested by monitoring the R-factor when adding another standard to the fit. The standard was considered descriptive to the XANES of the sample

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Figure 3. X-ray powder diffractogram of NiFe2O4 (a) in helium at 25 °C, (b) after reduction in hydrogen at 500 °C, (c) after reoxidation in carbon dioxide at 500 °C, and (d) in helium after cooled to 25 °C.

Figure 4. X-ray powder diffractogram of ZnFe2O4 (a) in helium at 25 °C, (b) after reduction in hydrogen at 500 °C, (c) after reoxidation in carbon dioxide at 500 °C, and (d) in helium after cooled to 25 °C.

for significantly improved fits (a reduction of the R-factor by a factor of minimum two). The peaks in the X-ray powder diffractograms were identified using references (card references, International Centre of Diffraction Data) where the reflections for the latter are calculated from tabulated d-spacings and a X-ray wavelength of 0.5 Å. The average crystallite size was estimated from the full width at half-maximum using the Scherrer equation. Implicit in the use of this equation is a number of assumptions. Other than instrumental contributions12 to line widths effects such as disorder and strain have not been considered because the necessary information is not available. III. Results and Discussion In the following, we discuss the reduction of the ferrites by hydrogen, reaction of carbon dioxide at 500 °C over the hydrogen-reduced ferrites, and reoxidation of the reduced ferrites in carbon dioxide. The as-synthesized cobalt, nickel, and zinc ferrites have previously been characterized by XAS (EXAFS and XANES) and conventional powder XRD.12 A. Reduction in Hydrogen. The X-ray powder diffractograms of the as-synthesized cobalt, iron, nickel, and zinc ferrites

TABLE 1: Average Crystallite Size Estimated from XRD of the Original,12 Hydrogen-Reduced and Reoxidized Ferrites material

original (nm)a,b

reduced (nm)c

reoxidized (nm)b,c

NiFe2O4 CoFe2O4 ZnFe2O4 Fe3O4

4 5 6 15

70d 95e 65f 130g

75 50 30 80

a Conventional XRD using a Cu KR radiation source. b Measured for spinel peaks. c XRD using a synchrotron source. d Measured for Ni-Fe alloy peaks. e Measured for Co-Fe alloy peaks. f Measured for ZnO peaks. g Measured for FeO peaks.

are shown in the diffractogram labeled a of Figures 1-4, respectively. The diffractograms are truncated so as to emphasize the reflections from the sample rather than the strong reflections from the diluent, boron nitride, and aluminum windows. There is a thermal shift in the positions of the reflections due to lattice expansion because the diffractograms were measured at different temperatures. The nanoparticulate nature of the ferrites (Table 1) is manifested as broadened diffraction peaks in the diffractograms; for magnetite and zinc ferrite only the major reflections are distinguishable from the background. The crystallite sizes of cobalt and nickel ferrite are too small for their peaks to be



Figure 5. Fe K-edge XANES of zinc ferrite, magnetite, nickel ferrite, and cobalt ferrite in the reduction in hydrogen and reoxidation in carbon dioxide at different temperatures and reaction length. For each material, only scans showing changes are included.

Figure 6. Zn, Ni, and Co K-edge XANES of zinc, nickel, and cobalt ferrite, respectively, in the reduction in hydrogen and reoxidation in carbon dioxide.

observed in the diffractograms. The individual ferrite phases are confirmed by the XANES (Figures 5 and 6) of the as-synthesized cobalt, iron, nickel, and zinc ferrite, which show iron and A metals (Co, Fe, Ni, and Zn) in the trivalent and divalent valence states, respectively.

The XANES of the iron, cobalt, nickel, and zinc ferrites were progressively measured up to 500 °C at the iron and A () Co, Ni, and Zn) K-edges under flowing hydrogen and helium (Figures 5 and 6). Already between 230-300 °C, the iron edges shift to lower energies consistent with decreased average valence

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states. In addition, other than the energy shift, the XANES is virtually unchanged. It would appear that at temperatures around 300 °C, negative edge shifts reflect moves from stoichiometry to nonstoichiometry of the spinel structure. The modifications are manifested by lower averaged iron valence states. These accompany the removal of lattice oxygen atoms in the reduction process but with the structural integrity of the spinel still being maintained.1 At higher reduction temperatures (300-500 °C), more dramatic changes are exhibited by all of the metal edge XANES. From the XANES (Figures 5 and 6) and the XRD (Figures 1-4b), it is apparent that the spinel structures break down. However, the reducibilities vary within the series. This variation reflects the differing influences that the A metals exert on ferrite reducibility toward hydrogen. After reduction in hydrogen at 500 °C, XANES at both edges and XRD show that iron and nickel in nickel ferrite are completely reduced to the metallic state, whereas the metals in magnetite and cobalt ferrite are only partially reduced under the same conditions to a mixture of oxides and metals. From the diffractogram of the reduced zinc ferrite (Figure 4b), it is also evident that iron(II) oxide and zinc oxide represent major fractions of the reduced products together with minor fractions of metallic iron. The XANES at the zinc edge (Figure 6) clearly shows that zinc is not reduced, although the spectra do change progressively with increasing temperature in a manner consistent with the expulsion of that metal from the spinel structure in the form of zinc oxide. Kodama et al.5 also report that zinc ferrite is reduced by hydrogen to zinc oxide and iron(II) oxide at 300 °C, which is apparent in this study at higher temperatures. Relative to magnetite (A ) Fe), reducibility under these conditions is enhanced by cobalt(II) and nickel(II), especially the latter, while the opposite is true for zinc. In the reduction of nickel ferrite at 500 °C, the diffractogram (Figure 3b) shows that metallic nickel is the sole product of reduction, although the XANES show that there is complete reduction of iron and nickel to the metallic states. This is explained if the two metals are alloyed because the reflections of Fe0.64Ni0.36 alloy14 and nickel metal15 have similar 2θ values. Therefore, the absence of iron metal peaks in the diffractogram is consistent with alloy formation. It is noteworthy that the XANES (Figures 5 and 6) of completely reduced nickel ferrite is not the same as the iron and nickel metal reference foils. We ascribe the differences to changes in the electronic configurations of the metals on forming the alloy. Similarly, for cobalt ferrite the diffractogram of the reduced material at 500 °C (Figure 1b) supports alloying of cobalt and iron. Only iron metal is apparent from the diffractogram, and the d-spacings of the cobalt-iron alloy16 and iron metal17 are similar. The reduction of cobalt and nickel ferrites by hydrogen into metallic alloys is supported by the literature.3,4,7,18,19 The structural degradation of the ferrites by hydrogen reduction leads to composite XANES for both edges (Fe and A). Therefore, linear combination analyses of the spectra using XANES of model materials were used to extract information about individual components. These analyses identified the materials produced from the hydrogen reaction (see below). The XRD shows that iron(II) oxide is formed on reduction. The XANES of an iron(II) oxide reference is not available because of the instability of the compound. In the case of zinc ferrite that has undergone reduction at 500 °C, the XRD supports iron(II) oxide being the main iron product of reduction. The shapes and relative energies of the features in the XANES of zinc ferrite reduced at 500 °C agree well with the reported XANES on iron(II) oxide.20,21 For this reason, the XANES of

Nordhei et al. hydrogen-reduced zinc ferrite at 500 °C (Figure 5, time ) 0) was used as the iron(II) model in the linear combination at the iron edge for all of the ferrites studied here. Although using the sampled XANES as a model can introduce some uncertainties into the analyses, it can give invaluable insight into the structural changes accompanying the reduction process. At 500 °C, nickel ferrite is reduced to alloyed Fe-Ni. The linear combination analysis using the individual metal foils to model the XANES failed to give a satisfactory fit. This in itself leads to the supposition that an alloy is formed. Therefore, the XANES of the putative alloy (the sampled XANES at the iron and nickel edges after 2 h at 500 °C in hydrogen, Figure 5) was used in the analyses. In the case of cobalt ferrite, the metal references were used as standards in the linear combination analyses. Although XRD supports alloying of cobalt and iron, the XANES of cobalt ferrite reduced at 500 °C cannot be used to model the alloy because the metals are contained in a mix of oxides, and therefore, the use of metal foils in the analyses does not fit well in the XANES modeling. However, useful information is provided concerning the gradual reduction of cobalt ferrite to the metallic state. The linear combinations at the iron edge (Figure 7) show that reduction at these higher temperatures breaks down the spinel structure. The common product is iron(II) oxide. With the exception of the zinc ferrite system, iron(II) oxide is further reduced to the metal at higher reduction temperatures. These are 350, 400, and 500 °C for nickel ferrite, cobalt ferrite, and magnetite, respectively. There are clear differences in the reducibilities of iron(III). Structural breakdown to give iron(II) oxide and iron metal occurs at the lowest temperatures for nickel ferrite, followed by cobalt ferrite, magnetite, and zinc ferrite. Thus, the order of reducibility of iron(III) in terms of the A metal is Ni(II) > Co(II) > Fe(II) > Zn(II). Linear combination analyses at the A () Co, Ni, and Zn) K-edges (Figure 8) show that when iron(III) is reduced to iron(II) oxide, the A metals are expelled in the form of A(II) oxides. The analysis at the zinc edge (Figure 8A) shows that only 40% of zinc ferrite is reduced to the oxide after reduction at 500 °C for 2.5 h. However, the XRD (Figure 4b) shows no spinel reflections thereby confirming complete breakdown of that structure on reduction. Experimental compromises are made when combining the XAS and XRD setups which lower the XANES resolution. There are also mismatches between the temperatures at which the standards and ferrites were measured. These factors give rise to a relatively high degree of misfit in the linear combination at the zinc edge. For this reason, the transition from zinc ferrite to zinc oxide was also studied by measuring the energy position of the first inflection point in the derivative spectra of the zinc XANES. In zinc ferrite (normal) and zinc oxide, the metal is situated on the tetrahedral sites in the oxide lattice. However, the XANES of the two materials are not similar, therefore, the energy positions of the first inflection points differ (Figure 9). The energy position of this feature in the derivative spectrum of the reduced material relative to the two standards was used to estimate the fractions of zinc ferrite and zinc oxide produced on reduction (Figure 8A). The results are in accordance with XRD in showing that zinc is exclusively in the form of zinc oxide after reduction. Linear combination of the XANES at the nickel K-edge in the reduction of nickel ferrite (Figure 8B) show that at low temperatures (∼300 °C) nickel(II) oxide is formed and partially reduced to the metallic state. The reduction of nickel(II) to metal at 300 °C was also apparent from EXAFS as previously reported.1 At 400 °C, the XANES analysis at the nickel edge



Figure 8. (A) Fraction of zinc(II) in ZnFe2O4 and ZnO estimated from linear combination of the Zn XANES (2) and the energy position of the first inflection point in the derivative spectra ([) in the reduction of zinc ferrite in hydrogen and reoxidation in carbon dioxide. (B) The fraction of nickel in NiFe2O4 (2), NiO (9), and Ni-Fe alloy (b) estimated from linear combination of the Ni K-edge XANES in the reduction of nickel ferrite in hydrogen and reoxidation in carbon dioxide. (C) Fraction of cobalt in CoFe2O4 (2), CoO (9), and metallic cobalt (b) estimated from the linear combination of the Co K-edge XANES in the reduction of cobalt ferrite in hydrogen and reoxidation in carbon dioxide. Figure 7. Fraction of iron in the original ferrite (2), iron(II) oxide (9), metallic state (b) and magnetite ([) estimated from linear combination of the iron XANES in the reduction in hydrogen and reoxidation in carbon dioxide for zinc ferrite, magnetite, nickel ferrite, and cobalt ferrite.

indicate that complete structural breakdown of the spinel occurs at the same temperature as shown from the iron edge modeling

of nickel ferrite. At 500 °C, nickel is completely reduced to the metallic state as an alloy with iron. In the reduction of cobalt ferrite, the linear combination of the cobalt K-edge XANES (Figure 8C) shows that above 300 °C, the structure breaks down as iron(III) is reduced to iron(II). This is accompanied by the ejection of cobalt(II) oxide from

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Figure 9. First derivative XANES spectra of ZnFe2O4 and ZnO.

the spinel. Cobalt(II) is reduced at 500 °C to the metallic phase with approximately 60-40% distribution of cobalt over cobalt(II) oxide and metal in the final reduced state The chemical compositions of the reduced ferrites derived from XRD and XANES are listed in Table 2. The results from the two complementary techniques are largely consistent, although XRD fails to detect the oxide phases in the reduced cobalt ferrite. This may be due to the high dilution in boron nitride and that the oxide crystallites are very small. For zinc ferrite, partial reduction of iron to the metal is shown by XRD but is not apparent from XANES. In the linear combination of the iron XANES for zinc ferrite reduced by hydrogen, the reduced ferrite itself was used as the standard, and a small weighted fraction of metallic iron may therefore not be descriptive to the XANES. Relative to magnetite, the results support the conclusion that reduction of the A metals alongside iron increases the reducibility of the ferrite in hydrogen. Nickel(II) is reduced to the metallic state ahead of iron and therefore facilitates the breakdown of the spinel followed by further reduction. For zinc ferrite, the zinc valence is constrained to Zn(II), which stabilizes iron thereby decreasing the reducibility. B. Carbon Dioxide Reaction. The effects of the reactions of carbon dioxide with the reduced (500 °C) ferrites were followed by XANES and mass spectrometry on the exhaust gases. The reaction times are defined relative to the mass spectrometer signal as shown in Figure 10 for reduced nickel ferrite. Similar reaction signals observed for the other reduced ferrites show that the reaction mechanisms are similar. The consumption of carbon dioxide was registered by the corresponding fall in exhaust concentration. Reaction times are short with no further consumption being detected after a few minutes (Figure 10A,b). Incomplete reduction of carbon dioxide to carbon is clearly evident from the generation of carbon monoxide as shown by the increase in the carbon monoxide signal (Figure 10B,a) relative to the baseline (Figure 10B,b). The carbon dioxide consumed and carbon monoxide produced during the reactions is summarized in Table 3. The reduction of carbon dioxide to carbon monoxide can be represented by the following reaction equation

Figure 10. Mass spectroscopy responses for (A) carbon dioxide and (B) carbon monoxide when passing carbon dioxide over reduced nickel ferrite at 500 °C.

xCO2 + RED ) xCO + OX

(1)

in which RED and OX represent the reduced and reoxidized materials, and oxygen is transferred from carbon dioxide to the reduced material thereby producing either carbon (complete reaction) or carbon monoxide (incomplete reaction). Quick XANES were measured over the iron pre-edge region in order to follow changes in the environments (Figure 11 shows the scans before, during, and after reaction). Because the reactions were very fast, collecting quick XANES within the time span of the reactions were not possible for all of the systems. After the reactions were complete (scan c in Figure 11), quick XANES could be measured but showed no changes and are therefore not shown. Prior to the carbon dioxide treatments, hydrogen was flushed out of the system with helium. During the helium flush, it was noted that some slight oxidation of iron occurred although the mass spectra show no evidence of air leaking into the system. This may arise from rearrangement of oxygen atoms within the structure or impurities in the helium gas. With the exception of reduced cobalt ferrite, quick XANES scans show that the consumption of carbon dioxide manifests itself by the significant oxidation of iron (Figure 11). The XANES scans of reduced magnetite and zinc ferrite before the reaction resemble that of iron(II) oxide. The carbon dioxide reaction leads to edge shifts consistent with oxidation of iron(II) oxide in the reduced ferrites to magnetite and zinc ferrite, respectively. From the XAS spectrum of reduced nickel ferrite (Figure 11C), it is evident that metallic iron is oxidized by carbon dioxide and also indicates a mix of valence states two and three for the oxidized material because the average valence state of oxidized iron is between these values. The XANES of

TABLE 2: Chemical Composition of Original, Reduced, and Reoxidized Ferrites Obtained from XANES and XRD material

original (XANES) original (XRD)

reduced (XANES)

reduced (XRD)

cobalt ferrite CoFe2O4 magnetite Fe3O4 nickel ferrite NiFe2O4

Fe3O4 -

FeO, Co-Fe alloy, CoO Co-Fe alloy FeO, Fe metal FeO, Fe metal Ni-Fe alloy Ni-Fe alloy

zinc ferrite

ZnFe2O4

FeO, ZnO

ZnFe2O4

reoxidized (XANES) reoxidized (XRD)

CoFe2O4 Fe3O4 Fe3O4, NiFe2O4, NiO, Ni-Fe alloy FeO, ZnO, Fe metal ZnFe2O4, ZnO

CoFe2O4 Fe3O4 Fe3O4, NiFe2O4, Ni-Fe alloy ZnFe2O4, ZnO



TABLE 3: Carbon Dioxide Reaction Time, Amount of Consumed Carbon Dioxide, and Produced Carbon Monoxide Measured from Mass Spectroscopy material NiFe2O4 CoFe2O4 ZnFe2O4 Fe3O4

reaction (time/min) 1.63 1.49 1.96 1.34

CO2 (consumed/mL) 0.61 0.36 0.60 0.18

the reoxidized materials (see below) show that the major constituent of reoxidation by carbon dioxide is magnetite, which has average iron valence state of 2.6 and explains the smaller energy shift in the spectra. In the reaction over reduced cobalt ferrite, the quick XANES scans (Figure 11D) indicate almost no change in the average valence state of iron. Still, changes in the gas composition during the reaction clearly show that carbon dioxide is reduced to carbon monoxide. XANES at the iron edge show that reduced cobalt ferrite is a mixture of iron metal and iron(II) oxide (Figure 5, Table 2). It is evident from the quick XANES that the contribution from the metallic fraction dominates over that of the iron(II) oxide. Linear combination of the XANES, keeping the metallic fraction constant at 50%, show that any oxidation of iron(II) to iron(III) does not result in significant changes in this energy region of the XANES. Therefore, the absence of

Figure 11. XANES for the reaction of carbon dioxide on reduced (A) zinc ferrite, (B) magnetite, (C) nickel ferrite, and (D) cobalt ferrite. Quick XANES scans collected (a) before, (b) during, and (c) after the reaction. All spectra are collected at 500 °C

CO2 (consumed/mol) -5

2.7 × 10 1.6 × 10-5 2.7 × 10-5 8.0 × 10-6

CO (produced/mL)

CO (produced/mol) 1.5 × 10-5 8.1 × 10-6 1.4 × 10-5 5.1 × 10-6

0.34 0.18 0.32 0.11

TABLE 4: Reactant (RED) and Product (OX) in the Initial Stage of the Carbon Dioxide Reaction material

RED

OX

cobalt ferrite magnetite nickel ferrite zinc ferrite

FeO FeO Fe-Ni alloy FeO

CoFe2O4 Fe3O4 Fe3O4 ZnFe2O4

changes in the iron XANES scans supports the notion that iron(II) donates electrons during the reduction of carbon dioxide, whereas iron metal is inactive. The XANES at the cobalt edge collected after 30 min in carbon dioxide (Figure 6) shows only insignificant oxidation thereby ruling out cobalt as the main active material during the reaction. In Table 4, the RED and OX of eq 1 that are identified from the quick XANES scans are listed for each ferrite. The results suggest that when iron(II) oxide is present in the reduced material divalent iron is the active electron donor in the reaction whether or not iron is present in the metallic state. For all of the reduced ferrites, the ratio of consumed carbon dioxide and produced carbon monoxide is approximately 2:1 (Table 3). According to eq 1, only half of the consumed carbon dioxide is accounted for which would indicate simultaneous decomposition of carbon dioxide to carbon. The formation of carbide phases in the decomposition of carbon dioxide on reduced ferrites has been reported3,4,18 although at lower reaction temperatures. However, any carbide being formed cannot be observed by XRD because the reflections corresponding to carbide are positioned in the same 2θ region (∼13-15 2θ) as the high intensity peaks of boron nitride and aluminum (cell windows). Alternatively, the CO2/CO ratio might also reflect surface carbonate formation because this also would be manifested by the consumption of carbon dioxide. Of the four ferrites, nickel and zinc ferrite equally stand out as the largest consumers of carbon dioxide. Magnetite consumes significantly less than the other materials (Table 3). Interestingly, the former two are the ones that exhibit the best and poorest reducibility in hydrogen, respectively. This shows that extent of reduction alone does not govern the efficiency of the material in decomposing carbon dioxide. The crystallite sizes of the metal or oxide phases in the reduced ferrites are estimated from the line broadening of the reflections in the powder diffractograms (Table 1). The particle sizes of the ferrites before gas treatment are within the nanoregion. At higher temperatures, there is significant sintering. Reduced nickel (nickel-iron alloy) and zinc ferrite (ZnO) have average crystallite sizes of approximately 70 nm, whereas reduced magnetite (FeO) grows to twice that size. The smaller particles of the two former materials suggest that the increased surface areas relative to those of reduced cobalt ferrite and magnetite means that there is a more efficient exposure to carbon dioxide. Hence, the consumption of carbon dioxide correlates with particle size. C. Reoxidation of Ferrites. The consumption of carbon dioxide occurs only during the first initial minutes of reaction of the gas with the reduced ferrites. After this period, the mass spectrometer does not register signals that indicate continued

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drops in carbon dioxide concentrations. Significantly however, the XANES and XRD do exhibit continuing oxidation of the reduced ferrites after this first stage. Thus, it would seem that the reduced ferrites are still reducing carbon dioxide but to a much reduced extent with the reaction rate being slowed considerably so that the activity is not detected by the mass spectrometer. This can be understood if the rate of reaction in the final stage is controlled by diffusion of carbon dioxide to the active surface in which case the surface is being continuously covered by oxidized reaction products during the reaction. The final stage is time-consuming (several hours) by which time the measurements were stopped when no further changes in the XANES were observed. Diffusion controlled reaction rates in the later stages of the carbon dioxide reaction have been reported.8-10 Whereas enhanced activity toward decomposing carbon dioxide is desirable, reoxidation to the original ferrite is important because this maintains reducibility and decomposition activity. The reversibility of the individual ferrites is discussed in the following. Zinc Ferrite. After reoxidation in carbon dioxide for 4 h, the diffractograms (Figure 4c,d) show ferrite peaks, confirming that the spinel structure is regenerated. However, the diffractograms also contain zinc oxide peaks, albeit with low peak intensities relative to those of the reduced material. The presence of small fractions of zinc oxide means that reoxidized zinc ferrite has a higher Fe/Zn ratio than the original. This cannot be explained by oxidation of magnetite because the diffractogram precludes magnetite, showing only zinc ferrite. The XANES at the iron edge (Figure 5) is in accordance with iron(II) being oxidized to the trivalent state in zinc ferrite, and the linear combination (Figure 7) shows that the major part (95%) of iron after reduction is in zinc ferrite. The XANES modeling was not improved by including magnetite as a standard, hence excluding that material as an additional reaction product. The conversion of zinc oxide to zinc ferrite accompanying the oxidation of iron(II) by carbon dioxide is confirmed by the XANES at the zinc edge. Both of the zinc XANES analyses (linear combination and energy position of the inflection point in the first-derivate spectrum) shows that in the reoxidized material, zinc is predominately present as zinc ferrite with a minor fraction of zinc oxide (∼10-20%). Magnetite. After 3 h reoxidation in carbon dioxide, the diffractograms (Figure 2c,d) show only magnetite peaks. The broadening of the peaks is less than those for the original magnetite, and the Scherrer equation yields an average crystallite size of 80 nm (Table 1). The XANES at the iron edge also shows that magnetite is formed during the reaction, and the XANES after reoxidation is virtually identical with assynthesized magnetite. The linear combination modeling (Figure 7) also shows that 90% of the sample is magnetite after only 10 min in carbon dioxide, and that longer reaction times are necessary for complete reoxidation. Nickel Ferrite. Diffractograms c and d of Figure 3 are of reoxidized nickel ferrite after 3 h in carbon dioxide. The diffractograms contain ferrite peaks that confirm the presence of the spinel. However, the asymmetrical peaks, especially at high 2θ, suggest that there is a mixture of ferrite phases in the reoxidized material. This is confirmed by XANES at the iron edge (Figure 5), which shows that iron is not reoxidized exclusively to the trivalent state in nickel ferrite but to an average lower valence state. From the linear combination of the iron XANES (Figure 7), it is evident that magnetite is formed as the major product when nickel-iron alloy is oxidized in

Nordhei et al. carbon dioxide. Shin et al.7,19 have reported a similar result when nickel-iron alloy is oxidized by carbon dioxide at 750 °C. The quality of fit in the modeling of the reoxidized iron XANES is significantly improved when nickel ferrite is included in addition to magnetite. This suggests that nickel(II) and iron(II) compete and that nickel ferrite forms at the expense of magnetite. The assymmetry of the ferrite peaks in the diffractograms (Figure 3c,d) can be explained from the above because the d-spacings of nickel ferrite and magnetite are similar,22 and hence, the peaks of the two materials are not completely separated. The linear combination at the nickel edge (Figure 8) is also consistent with the formation of nickel ferrite in addition to magnetite. The XANES (Figure 6) shows that nickel is oxidized in carbon dioxide alongside iron and, therefore, also contributes to the reaction. Metallic nickel has been found not to decompose carbon dioxide at 750 °C.7,19 But, in this study, the XANES at the nickel edge clearly shows that metallic nickel is actually oxidized. Linear combination analysis suggests that the major part of the oxidized material is nickel(II) oxide, formed by oxidizing nickel from a nickel-iron alloy. XRD and XANES (Table 2) show that the reoxidation of the nickel-iron alloy by carbon dioxide is not complete after 3 h. The diffractograms (Figure 3c,d) exhibit low-intensity nickel-iron alloy peaks, and the linear combination analyses (Figures 7 and 8) show that after reoxidation approximately 20% and 40% of the total iron and nickel, respectively, are metallic. However, the fraction of metallic iron is significantly reduced after 10 min in carbon dioxide and is in accordance with the quick XANES scan, which suggests that metallic iron is an electron donor in the reaction. The incomplete reoxidation of the metal to oxide by carbon dioxide would be consistent with further reaction being inhibited by a surface layer oxide. The metal particles are relatively large (average crystallite size ∼70 nm, Table 1), which leads to longer reaction times. Cobalt Ferrite. Whereas the metal fraction is inactive during the initial stage of the reaction, the XANES (Figures 5 and 6) shows that metallic cobalt and iron are oxidized in the later stage of reaction. Linear combination analyses of the iron and cobalt XANES (Figures 7 and 8c) show that after 1 h in carbon dioxide, the cobalt-iron alloy is completely oxidized in carbon dioxide. Accompanying the decreased metal fraction on oxidation is a corresponding increase in the cobalt ferrite content. This is paralleled by gradual decreases in the fractions of iron(II) oxide and cobalt(II) oxide. Taken together, this information shows that carbon dioxide oxidizes the metallic iron and cobalt fractions directly to cobalt ferrite. After 2.5 h in carbon dioxide, the cobalt XANES analysis shows that 20% of cobalt is in cobalt oxide. However, the iron XANES, which were collected over longer reaction times, show that all material is reoxidized to cobalt ferrite. XRD exhibits only ferrite reflections also consistent with complete reoxidation to cobalt ferrite (Figure 1c,d). Khedr et. al.9 have reported that magnetite is an additional product formed when cobalt-iron alloy is oxidized by carbon dioxide at similar temperatures. This is not supported in the present study. The d-spacings of cobalt ferrite and magnetite are similar, but there is no asymmetry in the spinel peaks that would indicate magnetite. Also, in the linear combination analysis, the XANES could not be fitted using magnetite as a standard, which supports the view that magnetite is not formed as is the case for the oxidation of nickel-iron alloy. Table 2 lists the reoxidized products obtained from XANES and XRD. Although nickel ferrite and zinc ferrite consumes more carbon dioxide during the initial stage of reaction than cobalt ferrite and magnetite, the latter are the only materials

Decomposition of CO2 over Fe, Co, Ni, and Zn Ferrites that are completely reoxidized to the original states. Significantly, nickel ferrite shows excellent reducibility and initial stage carbon dioxide consumption relative to the other ferrites, although the reoxidation is not complete. The reoxidation products are a mix of several oxides, and hence, cyclic reducibility and decomposition efficiency are progressively degraded for nickel ferrite. Oxidation from iron(II) oxide seems to promote reoxidation to the original ferrite spinel. For the reduced nickel ferrite, the fast solid-state reaction of carbon dioxide with the nickel-iron alloy stabilizes iron as magnetite thereby inhibiting reoxidation back to nickel ferrite. Although the cobalt-iron alloy is reoxidized in carbon dioxide without forming magnetite, this reaction occurred in the slow diffusion controlled stage, which may favor oxidation to cobalt ferrite rather than magnetite. However, this may also be a property of the metals involved. This study shows that reducibility, decomposition efficiency, and reoxidation ability are all important to the reduction of carbon dioxide on reduced ferrites at 500 °C. Relative to magnetite, exchange of iron(II) as an A metal by cobalt(II), nickel(II), or zinc(II) gives advantages to the properties of the ferrites toward the decomposition of carbon dioxide. IV. Conclusion (1) The combination of XANES and XRD shows that the reducibility of a series of nanophase ferrites in hydrogen strongly depends on the type of A metal (A ) Fe(II), Co(II), Ni(II), or Zn(II)). The exceptional reducibility exhibited by nickel ferrite is rationalized in terms of the reduction of nickel(II) to the metallic state at low temperatures, thereby destabilizing the spinel and promoting further reduction. Iron(III) in the ferrites is gradually reduced to iron(II) oxide, with the latter being further reduced to the metallic state. At the same time, the A metals are expelled from the spinel as A(II) oxide or the metal. (2) The reduction of carbon dioxide to carbon monoxide on the reduced ferrites is accompanied by oxidation of iron. XANES shows that iron(II) oxide when present is active in the reaction. Reduced nickel and zinc ferrite show the largest consumptions of carbon dioxide. This may partly be attributed to their higher surface-to-volume ratios. (3) In carbon dioxide, the reduced ferrites are directly reoxidized to ferrites. However, only cobalt ferrite and magnetite are completely reoxidized to the original states. Although reduced zinc ferrite is oxidized to

J. Phys. Chem. C, Vol. 113, No. 45, 2009 19577 zinc ferrite, not all zinc is reallocated to the spinel. For nickel ferrite, magnetite is formed as the main product mixed with nickel ferrite, nickel(II) oxide, and unreacted nickel-iron alloy. Acknowledgment. Financial support from the Norwegian Research Council and Faculty of Natural Science and Technology, Norwegian University of Science and Technology (NTNU) is much appreciated. References and Notes (1) Nordhei, C.; Mathisen, K.; Bezverkhyy, I.; Nicholson, D. J. Phys. Chem. C 2008, 112, 6531. (2) Zhang, C. L.; Shuang, L.; Wang, L. J.; Wu, T. H.; Peng, S. Y. Mater. Chem. Phys. 2000, 62, 44. (3) Kodama, T.; Sano, T.; Yoshida, T.; Tsuji, M.; Tamaura, Y. Carbon 1995, 33, 1443. (4) Ma, L. J.; Chen, L. S.; Chen, S. Y. J. Phys. Chem. Solids 2007, 68, 1330. (5) Kodama, T.; Tabata, M.; Tominga, K.; Yoshida, T.; Tamaura, Y. J. Mater. Sci. 1993, 28, 547. (6) Zhang, C. L.; Shuang, L.; Wu, T. H.; Peng, S. Y. Mater. Chem. Phys. 1999, 58, 139. (7) Shin, H. C.; Choi, S. C.; Jung, K. D.; Han, S. H. Chem. Mater. 2001, 13, 1238. (8) Khedr, M. H.; Farghali, A. A. Appl. Catal., B 2005, 61, 219. (9) Khedr, M. H.; Omar, A. A.; Abdel-Moaty, S. A. Mater. Sci. Eng., A 2006, 432, 26. (10) Farghali, A. A.; Khedr, M. H.; Khalek, A. A. A. J. Mater. Process Tech. 2007, 181, 81. (11) Ferna´ndez-Garcı´a, M. Cat. ReV. 2002, 44, 59. (12) Nordhei, C.; Ramstad, A. L.; Nicholson, D. G. Phys. Chem. Chem. Phys. 2008, 12, 1053. (13) Ravel, B.; Newville, M. J. Synchrotron Rad., 2005, 12, 537–541. (14) PDF Card Reference; PDF No. 47-1405; International Centre of Diffraction Data: Newtown Square, PA. (15) PDF Card Reference; PDF No. 4-0850; International Centre of Diffraction Data: Newtown Square, PA. (16) PDF Card Reference; PDF No. 49-1568; International Centre of Diffraction Data: Newtown Square, PA. (17) PDF Card Reference; PDF No. 6-0696; International Centre of Diffraction Data: Newtown Square, PA. (18) Ma, L. J.; Chen, L. S.; Chen, S. Y. Mater. Chem. Phys. 2007, 105, 122. (19) Shin, H. C.; Oh, J. H.; Lee, J. C.; Han, S. H.; Choi, S. C. Phys. Status Solidi A 2002, 189, 741. (20) Paris, E.; Tyson, T A. Phys. Chem. Miner. 1994, 21, 299–308. (21) Ohzono, H.; Kouno, M.; Miyake, H.; Ohyama, H. Anal. Sci. 2001, 17, i135. (22) PDF Card Reference; PDF No. 85-1436 and No. 01-074-2081; International Centre of Diffraction Data: Newtown Square, PA.

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Decomposition of Carbon Dioxide at 500 Â°C over Reduced Iron

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