The Role of CeO2 as a Gateway for Oxygen Storage over CeO2

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The Role of CeO as a Gateway for Oxygen Storage over CeO-Grafted FeO Composite Materials 2

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Masato Machida, Takahiro Kawada, Hiroaki Fujii, and Satoshi Hinokuma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09876 • Publication Date (Web): 15 Oct 2015 Downloaded from http://pubs.acs.org on October 21, 2015

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The Journal of Physical Chemistry

The Role of CeO2 as a Gateway for Oxygen Storage over CeO2-Grafted Fe2O3 Composite Materials

Masato Machida,* Takahiro Kawada, Hiroaki Fujii, Satoshi Hinokuma Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto, 860-8555 Japan

Corresponding author: Prof. Masato Machida Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto, 860-8555 Japan TEL/FAX:+81-96-342-3651, E-mail: [email protected]

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ABSTRACT The surface grafting of CeO2 onto Fe2O3 with a 1:5 molar ratio produced a thermally stable composite material with greater and faster oxygen storage/release than its separate constituents. In the composite, CeO2 and Fe2O3 were intimately contacted by interfacial Ce–O–Fe bonding, and no solid solutions or mixed Ce and Fe oxides were formed after heating at 900 °C. The oxygen storage capacity and initial rate of oxygen release/storage were both increased in the composite structure by virtue of the Fe2O3 and CeO2, respectively. The reduction–oxidation cycles in which Fe2O3 is reduced via Fe3O4 to Fe metal by CO or H2 and then reoxidized by O2 were stabilized by surface-grafting Fe2O3 with CeO2. In situ Raman spectra demonstrated that the surface-grafted CeO2 acts as an oxygen gateway, activating the dissociation of O2 into oxide ions or the recombination of oxide ions into O2 and transferring oxide ions to/from Fe2O3. Meanwhile, Fe2O3 acts as an oxygen reservoir that expands the O2 storage capacity. The composite material was tested in a simulated exhaust gas stream with lean/rich perturbations (which occur in automotive three-way catalysts). The synergistic effect of the surface grafting effectively buffered the system against air-to-fuel ratio fluctuations.

Keywords: Oxygen storage, Cerium oxide, Iron oxide, Surface grafting, Three-way catalyst


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INTRODUCTION Oxygen storage materials reversibly store and release large amounts of oxygen depending on the partial oxygen pressure in the gas phase. This function is crucial in current automotive three-way catalyst converters,1-13 where the oxygen partial pressure in the auto exhaust must be regulated near the ideal air-to-fuel ratio (A/F = 14.6). If the A/F value is suboptimal, the conversion of noxious pollutants (NOx, CO, and hydrocarbons) over the precious metal catalysts (Pt, Rh, and/or Pd) is incomplete. Although cerium-based oxides such as CeO2–ZrO2 have been widely used for this purpose, the redox cycles between Ce4+ and Ce3+ limit their oxygen storage capacity (OSC) to below 0.25 mol-O2·(mol-Ce)−1. To improve the OSC, further studies have been extended to other cerium-based solid solutions containing various transition metals (Mn, Fe, Rh, Pd, etc.),14-19 in which both Ce and transition metals contribute to redox cycles. However, most of these solid solutions are thermodynamically metastable phases, which should be decomposed at high temperatures (≥ 800 °C). Many attempts were also directed at alternative redox mechanisms, such as lanthanide oxysulfates (La2O2SO4 and Pr2O2SO4),20-25 delafossite-type oxides (CuMO2: M = Al, Fe, Mn, and Ga),26 perovskite-related oxides (Dy1–xYxMnO3+δ,27 BaYMn2O5+δ,28 Ca2AlMnO5+δ,29 YBaCo4O7+δ,30 MgFeAlOx,31 BaLnFe2O5+δ32 and Sr3Fe2O733), alkaline earths-doped SnO2,34 and non-ceria fluorite-type oxides (PrOx–ZrO235). However, cerium-based oxides offer high rates of oxygen release and storage, and stability against thermal and chemical deterioration, which have yet to be matched by any candidate materials. In a series of previous studies,20-25 we determined the OSC of lanthanide oxysulfates (Ln2O2SO4) as 2 mol-O2·(mol-S)−1. Such a large capacity is attributed to the wider redox range of sulfur (S6+ ↔ S2−) compared to Ce. However, these oxysulfates



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release and store oxygen only at temperatures above 600 °C, which is much higher than the lowest possible temperature for CeO2–ZrO2 (≤300 °C). To effectively reduce the temperature range, we trialed a structural modification by substituting Ln with Ce; a typical example is (La1−xCex)2O2SO4.25 Although Ce was partly deposited as CeO2 on the oxysulfate, this improved the oxygen release/storage by 4–8 times probably because the easy shift between Ce3+ and Ce4+ states on the surface facilitates the redox of sulfur. Thus, Ce redox reactions should provide an oxygen gateway for accelerating the oxygen release/storage of other redox materials that are either contacted with the Ce material. The notion of Ce as an oxygen gateway is supported in a recent study on an inverse oxide/metal catalyst configuration for CO oxidation36-37 in which CeO2 nanoparticles supported on metal substrate such as Cu gain catalytic activity by interaction between the reactants and the defect sites of CeO2, the metal sites of the support, or the metal–CeO2 interface. Loading CeO2 nanoparticles onto the surface of Fe2O3 was also reported to significantly improve the reducibility of Fe2O3.38-40 Although this effect seems to be attributed to the chemical interaction between CeO2 and Fe2O3, the detailed mechanism has not been clarified. Owing to the high concentration and mobility of oxygen vacancies, CeO2 is considered as an efficient catalyst for both O2 dissociation and the recombination of oxides ions for O2 evolution. The catalytic activity of supported CeO2 can be further enhanced by decreasing the particle size, thereby increasing the concentration of coordinatively unsaturated Ce sites.41 The use of CeO2 as an oxygen gateway requires another contacting component that plays a role of an oxygen reservoir. This concept will open up new oxygen storage materials consisting of these heterogeneous junctions, which are different from the conventional ceria-based solid solution materials.


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Extending this idea, we propose that CeO2 and transition metal oxides can provide an oxygen gateway and an oxygen reservoir, respectively, when combined into the same system. Therefore, we prepared a composite material of hematite (α-Fe2O3) and surface-grafted CeO2 nanoparticles and investigated the synergistic effect of the bonded constituents on oxygen storage performance. To ensure intimate contact of both oxide components, CeO2 was grafted onto the surfaces of Fe2O3 particles by a nanometric colloidal sol technique.42-43 The local structure was characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray absorption fine structure (XAFS), and in situ Raman spectroscopy. The oxygen release/storage properties were evaluated by various unsteady state techniques. The synergistic effects of the CeO2-grafted Fe2O3 on the oxygen storage capacity and the rate of oxygen release/storage were confirmed, and a detailed mechanism was derived from the structural characterization.

EXPERIMENTAL SECTION Sample preparation. Surface grafting of CeO2 onto Fe2O3 was conducted using α-Fe2O3 (hematite) powders (Wako Chemicals, 99.9%) and a nanometric colloidal sol of CeO2 (Nissan Chemical Ind., CE-20B) dispersed in a basic (pH = 9.6) solution. The CeO2 sol was coated onto Fe2O3 by wet-incipient impregnation, drying in vacuo at ambient temperature, and subsequent calcination at 600 °C for 4 h. The composition of the product is expressed as nCeO2–(1–n)Fe2O3 (where n = 0, 0.16, 0.67, 0.86, and 1.0). Unless otherwise stated, the experimental sample was 0.16CeO2–0.84Fe2O3 (i.e., n = 0.16), denoted simply as CeO2–Fe2O3; the other samples were used as references. Supported Pd catalysts



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(Pd/γ-Al2O3, 2 wt% Pd loading) were prepared by wet-incipient impregnation of Pd(NO3)2 (Tanaka Precious Metals) to γ-Al2O3 (ALO-8, supplied by the Catalysis Society of Japan) followed by drying and calcination at 600 °C for 3 h in air. Powders of CeO2–Fe2O3 (n = 0.16) and Pd/Al2O3 (50 wt% each) were ground for 1 h in an automatic agate mortar (Retsch, RM200) and then thermally aged in flowing 10% H2O in air at 900 °C for 25 h before use. Similarly, the references were prepared by mixing dried CeO2 sol or Fe2O3 with the Pd/Al2O3 catalyst, followed by thermal aging under the same conditions.

Characterization. The powder XRD was measured on a Rigaku Multiflex diffractometer with monochromated CuKα radiation (40 kV, 20 mA). The chemical composition was determined by energy-dispersive X-ray fluorescence analysis (Horiba, MESA-500W). The Brunauer‒Emmett‒Teller (BET) surface area was calculated from N2 desorption isotherms measured at 77 K (Bel Japan, Belsorp). X-ray photoelectron spectroscopy (XPS) was conducted using a Thermo K-alpha spectrometer with AlKα radiation (12 kV). The binding energy calculation was checked using the line position of C1s as an internal reference. Microstructure images were taken under a FEI TECNAI F20 transition electron microscope (TEM) operating at 200 keV. The XAFS of the Ce K-edge was recorded on the NW-10A instrument of the Photon Factory–Advanced Ring for Pulse X-rays (PF–AR), High Energy Accelerator Research Organization (KEK) at Tsukuba (Proposal #2006G344), with a ring energy of 6.5 GeV and a stored current of around 35–60 mA. A Si(311) double-crystal monochromator was used, and the spectra were recorded at room temperature in


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transmission mode. The incident and transmitted X-rays were monitored by ionization chambers filled with N2 and Kr gasses, with lengths of 17 and 31 cm, respectively. Each sample was volume-adjusted by boron nitride powder to ensure an appropriate absorbance at the edge energy for the XAFS measurement and pressed into a disk (diameter 10 mm). The disk was packed into a polyethylene package under an Ar atmosphere. The XAFS data were processed by a REX 2000 program (Rigaku). The extended X-ray absorption fine structure (EXAFS) oscillation was extracted by fitting a cubic spline function through the post edge region. The k3-weighted EXAFS oscillation in the 3.0−16.0 Å−1 region was Fourier transformed. Phase shifts and backscattering amplitudes of Ce−O and Ce−O−Ce were obtained from the EXAFS data of CeO2. The XAFS spectrum of the Fe K-edge was recorded on the BL-7C instrument of the Photon Factory (PF)-KEK, with a ring energy of 2.5 GeV and a stored current of approximately 300–450 mA. A Si(111) double-crystal monochrometor was used. The incident and transmitted X-rays were monitored in an ionization chamber filled with N2 and Ar/N2 (Ar/N2 = 15/85) gasses. in situ XRD was performed using a Rigaku RINT-Ultima diffractometer (30 kV, 40 mA) equipped with a high-speed two dimensional detector to observe the phase change during redox in a flow of 1.4% H2/He or 0.7% O2/He at 500 °C. In situ Raman scattering study was performed on a Horiba Jobin Yvon LabRAM HR Evolution spectrometer with a 457 nm-laser excitation source. The spectrometer was installed with an infrared image heating stage (Yonekura, MS-TPS) connected to a gas-flow system, to facilitate high-temperature measurements under controlled gas environments. The powder catalysts were heated at 500 °C under a 5% O2/N2 flow to remove any adsorbed gases. This procedure was followed by flushing with N2 for 30 s and subsequent



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reduction in 5% H2/N2 for 10 min. The end of the first oxidation step was followed by N2 flushing and subsequent admission of 5% O2/N2 for 10 min. During two sets of these reduction/oxidation cycles, the spectra were recorded every 30 s.

Oxygen storage performance. The temperature of oxygen release from CeO2–Fe2O3 (n = 0.16) mixed with the Pd/Al2O3 catalyst was determined by CO-based temperature programmed reduction (CO-TPR). Following pretreatment in flowing 20% O2/He at 500 °C, the sample was heated at 10 °C min−1 in flowing 1% CO/He. The effluent CO and CO2 were analyzed by a mass spectrometer (Pfeiffer, Omnistar) as the temperature was ramped up to 900 °C. The OSC and initial rate of oxygen release were determined by anaerobic CO oxidation under cycle-feed stream conditions at constant reaction temperatures (300–500 °C) in a dual-supply flow system. Two gas feeds, 1% CO/He and 0.5% O2/He, were alternately switched at programmed time intervals (15 min). The rate of gas feed (F) to the sample (W = 0.050 g) was controlled at W/F = 1.0 × 10–3 g min cm–3. The concentrations of each gas component (CO, CO2, and O2) were recorded before and after the catalyst bed using a quadrupole mass spectrometer (Pfeiffer, Omnistar). The reduction and reoxidation cycles were studied by a microbalance (Rigaku, 8120) connected to the dual-gas supplying system. The sample mixed with the Pd/Al2O3 catalyst (approximately 10 mg) was first heated in a N2 stream up to 500 °C, and the weight was stabilized within 30 min. The gas feed to the sample was then switched between 1.4% H2/He (140 min) and 0.7% O2/He (40 min) while recording the sample weight. The buffering effect on A/F was evaluated in a flow reactor, in which the sample


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(W = 0.050 g) was subjected to a simulated exhaust gas mixture containing NO (0.050%), CO (0.51%), C3H6 (0.039%), O2 (0.17 or 0.83%), H2O (7%), and He (balance) at a flow rate F = 100 cm3 min–1 (W/F = 5.0 × 10−4 g min cm−3). In this measurement, the O2 gas feed was switched between 0.83 and 0.17% O2 (A/F = 15.0 and 14.2, respectively) at intervals of 10 or 120 s, whereas concentrations of other gases (NO, CO, C3H6, and H2O) and the rate of the total gas feed were maintained constant. The A/F value was calculated from the concentrations of each gas in the gas feed, as reported by Tanaka et al.44 The A/F value of the gas stream at the outlet of the catalyst bed was monitored by a solid electrochemical sensor (Horiba MEXA-730). Thermodynamic calculations for equilibrium O2 pressures were done on a commercially available software (HSC Chemistry®, Outokumpu Research Oy).

RESULTS AND DISCUSSION Microstructure of CeO2-grafted Fe2O3. The diffraction peaks in the powder XRD patterns of thermally aged CeO2–Fe2O3 were assignable only to CeO2 with a cubic fluorite structure and to α-Fe2O3 with a rhombohedral hematite structure (Supporting Information). These diffraction peaks exhibited no shift and thus negligible formation of CeO2–Fe2O3 solid-solution. Figure 1 shows TEM images of the colloidal CeO2 sol (a) and CeO2–Fe2O3 before and after thermal aging (b–d). Most of the colloidal CeO2 sol particles were 6–8 nm in size (Figure 1a), consistent with the size calculated from the BET surface area (8 nm; 130 m2g−1). In colloidal suspension at neutral pH, these basic colloidal CeO2 particles have a positive surface charge,43 whereas Fe2O3 particles (with an isoelectric point of pH ~6) have a negative surface charge.45-46 Therefore, the colloidal CeO2 nanoparticles are



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attracted to the surfaces of the Fe2O3 particles (average size approximately 200 nm) by electrostatic forces. This attraction may partly explain the efficient adsorption of CeO2 nanoparticles onto Fe2O3 (b). During subsequent thermal aging at 900 °C, the adsorbed CeO2 caused granule growth by coalescence and strong bonding to the Fe2O3 surface. Finally, the Fe2O3 particles (with a size of several hundred nanometers) became surface-grafted with hemispherical CeO2 particles (with a size of several ten nanometers); however, unbound particles of Fe2O3 and CeO2 could not be observed (c). The surface atomic ratio determined by XPS was (Ce/Fe)s = 0.38. An enlarged image (d) reveals intimate interfacial contact at the boundary between CeO2 and Fe2O3; however, the boundary structure suggests no evidence of solid-state reactions forming solid solutions and/or mixed oxide phases.47-48 This result is consistent with the reported phase diagrams containing Fe2O3 and CeO2, which reveal no thermostable binary phases in air.48-49

Local structure analysis by XAFS. To elucidate why the interface between CeO2 and Fe2O3 is closely contacted (Figure 1d), we studied the local structure in an XAFS experiment of model compounds with compositions nCeO2–(1−n)Fe2O3 (n = 0, 0.16, 0.66, 0.86, and 1.0) prepared by the same procedure. Figure 2 shows the Fourier transforms of the k3-weighted Ce K-edge EXAFS for nCeO2–(1−n)Fe2O3, without correcting the data for phase shifts. The two peaks attributed to the atomic distances in Ce–O and Ce–O–Ce were identical at all compositions (n). The atomic distances (r) and coordination numbers (CN) around the Ce atom, obtained from the curve-fitting analysis of EXAFS, were the same in nCeO2–(1−n)Fe2O3 and in pure CeO2 (n = 1.0) (Table 1). This indicates that the


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surface-grafted CeO2 particles in Figure 1d were mostly composed of pure Ce oxide. Figure 3 shows the Fourier transforms of the k3-weighted Fe K-edge EXAFS for nCeO2–(1−n)Fe2O3 (without corrections for phase shift); the results of the curve fitting analysis are shown in Table 2. The sole peak at 1.95 Å and the overlapped peaks in the range 3–4 Å for Fe2O3 (n = 0) are attributed to the Fe–O (CN = 6.0) and Fe–O–Fe (CN = 3.0 and 6.0) atomic distances, respectively. In CeO2–Fe2O3 with a smaller Fe content (n = 0.86), the CN of Fe–O increased to 7.7, larger than in Fe2O3 (CN = 6.0) and close to that of Ce–O in CeO2 (CN = 8.0). In addition, the CN (=7.8) of the third shell at 3.26 Å cannot be explained by the Fe2O3 structure with an Fe–O–Fe coordination number of CN = 3 (r = 2.97 Å), 3 (3.36 Å), and 6 (3.71 Å). These results imply similar local environments of the Fe species and CeO2, although this unusual feature was not obvious in samples with higher Fe contents (n < 0.86). Given that some of the Fe3+ ions in CeO2–Fe2O3 inhabit the coordination environment of CeO2, strong interfacial bonding can establish between CeO2 and Fe2O3. This finding does not indicate mutual diffusion layers, because the Ce K-edge EXAFS exhibited no Ce species in the Fe2O3 structure. Only Fe ions near the interface boundary can dissolve into the CeO2 structure. According to the Fe K-edge XANES spectra, the Fe species in these samples were in the Fe3+ state, regardless of the sample composition (n) (Supporting Information). As stated above, no binary compound and no solid solubility were reported in the Fe2O3–CeO2 equilibrium. Hegde and his coworker synthesized a Ce0.67Fe0.33O2−δ solid solution by sonochemical method, which is stable up to 600 °C only but decomposes when heated at above 600 °C.14 Li et al. reported hydrothermal synthesis of the CeO2-based solid solutions, Ce0.85Fe0.15O2.39 Tabakova et al. prepared CeO2–Fe2O3 as a support for Au catalysts and suggested the coexistence of hematite and cubic CeO2-like



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solid solution which contained Fe3+ at the Ce4+ site and interstitial sites.50 These studies imply the presence of metastable CeO2-like solid solutions, while Fe2O3-like ones are unlikely formed as the present result suggested.

Oxygen release/storage performance. To evaluate their oxygen release/storage performances, the samples were physically mixed with the Pd/Al2O3 catalyst (50 wt%). The essential role of Pd/Al2O3 is to activate reducing gases (CO, H2, and/or C3H6) and O2, to supply them to the interface with CeO2–Fe2O3, Fe2O3, or CeO2 and thus to accelerate the redox reactions thereon. First, the oxygen release profiles were analyzed by CO–TPR. Figure 4 shows the temperature dependence of the CO2 formation rate, which corresponds to the rate of oxygen release of CeO2–Fe2O3 (n = 0.16), Fe2O3, and CeO2. A half-amount of Pd/Al2O3 was used as a reference sample. All samples showed a small peak at approximately 150 °C, characterizing the reduction of PdO to Pd metal. Thus, Pd in the mixture leads to redox cycles (PdO ⇄ Pd + 0.5O2), whereas its OSC should be less than 5 × 10−2 mmol-O2 (g-mixture)−1 (approximately 0.15 wt% of total weight) because of the small loading of Pd. Its contribution to the observed OSC is thus very small for CeO2–Fe2O3 and Fe2O3, when the reaction temperature is higher than 400 °C. CeO2 exhibited very weak and broad peaks around 400 and 850 °C. The CO2 evolution of Fe2O3 was much enhanced from 400 to 900 °C, reflecting its higher reducibility than CeO2. CeO2–Fe2O3 exhibited similar behavior, although some differences are notable. First, the greatest rate of CO2 formation (at approximately 500 °C) was 1.5-fold higher than that of Fe2O3. Second, a new peak appeared at approximately 340 °C, indicating oxygen release at lower temperatures than in Fe2O3.


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To more quantitatively evaluate the oxygen release/storage performance, anaerobic CO oxidation was performed under cycled-feed stream conditions (Supporting Information). Figure 5 compares the obtained OSC and initial rate of oxygen release (rR) for CeO2, Fe2O3 and CeO2–Fe2O3 at different temperatures. Here, it should be noted that the OSC value is a partial capacity measured under the present cycle condition and is much smaller than a theoretical capacity as shown in Figure 6. Although pure CeO2 exhibited the smallest OSC at all temperatures, its rR values were comparable with that of CeO2–Fe2O3 at 300 °C, but less temperature-dependent. By contrast, the OSC and rR of pure Fe2O3 alone were strongly temperature-dependent; the rR was comparable to that of CeO2–Fe2O3 at 500 °C, but much smaller at 300 °C. The combined CeO2 and Fe2O3 yielded the largest OSC and rR at all temperatures. From the temperature dependence of rR, the apparent activation energies were calculated as 2.8 kJ mol−1 (CeO2–Fe2O3), 1.8 kJ mol−1 (CeO2), and 15.7 kJ mol−1 (Fe2O3). The smaller values for CeO2–Fe2O3 and CeO2 imply similar fast oxygen release processes in these samples. Next, we investigated the oxygen release/storage cycle by measuring the weight change of the sample. The reducing gas was H2, which is more reactive than CO. Figure 6 plots the typical weight changes under oscillating feed-stream conditions (0.7% O2 or 1.4% H2; He balance). Because the sample loses and gains weight in a stream of H2/He and O2/He respectively, the observed oscillation amplitude corresponds to the OSC. The weight changes in the CeO2 sample were rapid but small, indicating an OSC below 20% of the theoretical OSC, 0.25 mol-O2·(mol-Ce)−1 (1.45 mmol-O2·g−1), which was estimated from the stoichiometric reaction between CeO2 and Ce2O3. By contrast, the weight change of Fe2O3 was much greater during the first cycle, and decreased as the



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cycles continued. The first weight loss suggests that Fe2O3 reduced to a mixture of FeO (Fe2+) and metallic Fe (Fe0). After the 6th cycle, the weight loss had decreased to nearly 1/6th of the original loss, because of irreversible reoxidation. This behavior can be rationalized by the substantial agglomeration of the Fe metal particles during the redox cycles, forming large dense grains exceeding several tens of µm across (Supporting Information). Therefore, the oxygen release/storage property of Fe2O3 easily deteriorates under reduction to the metallic state. By contrast, the weight of CeO2–Fe2O3 reversibly changed by approximately 25 wt%, indicating that more than 90% of the total OSC occurred between Fe2O3 (Fe3+) and Fe metal (Fe0). The oxygen release and storage rates were estimated from the slopes of the weight changes presented in Figure 6. From the initial slope of the weight change, the initial oxygen release rate of CeO2–Fe2O3 during the first few cycles was calculated as 0.575 mmol-O2 g−1 min−1, which was almost unchanged after 10 cycles (0.526 mmol-O2 g−1 min−1). The oxygen release rate of Fe2O3 was much less stable, decreasing from 0.898 mmol-O2 g−1 min−1 during the first few cycles to 0.009 mmol-O2 g−1 min−1 during the 10th cycle. Unlike the Fe2O3 sample, the CeO2–Fe2O3 particles remained small sizes (≤ 1 µm across) even after segregation of the Fe metal (Supporting Information). Such retention of the highly dispersed microstructure is necessary for the cyclic stability of oxygen release/storage. The reduction and reoxidation behavior of CeO2‒Fe2O3 were further studied by in situ XRD experiment (Figure 7). With a progress of the reduction in a stream of H2/He at 500 °C, the phase was transformed from α-Fe2O3 via Fe3O4 to Fe metal, whereas CeO2 showed no obvious change of diffraction peaks (a). The reoxidation of Fe metal via Fe3O4 to α-Fe2O3 was completed more rapidly compared to the reduction (b).


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The reduction/reoxidation was therefore reversible and any other phases, such as γ-Fe2O3, FeO and CeFeO3, were not detected. Because γ-Fe2O3 consists of the cubic spinel structure similar to Fe3O4, in situ XRD is difficult to differentiate between Fe3O4 and γ-Fe2O3. However, the absence of γ-Fe2O3 can be confirmed by comparing these phase changes with the weight change of CeO2‒Fe2O3 versus the time-on-stream (c). The XRD peaks appeared after the reduction with 10 min was assigned to Fe3O4 (a), which was almost consistent with the stoichiometric weight loss (2.8 wt%) from α-Fe2O3 to Fe3O4 (c). The slope of the observed weight loss (c) indicates that the phase transformation of α-Fe2O3 → Fe3O4 is faster than that of Fe3O4 → Fe. In addition, the weight gain due to reoxidation occurred much faster than the weight loss in a reduction step. This is also in accordance with the result of in situ XRD (b). Oxygen storage materials serve an important buffering function against A/F fluctuations between fuel-rich and fuel-lean exhausts in automotive three-way catalysis.1-13 Therefore, we evaluated the A/F buffering effects of CeO2 and CeO2–Fe2O3 at 500 °C in simulated gas streams under A/F perturbation conditions (Figure 8). When the two simulated gas feeds with A/F = 15.0 and 14.2 were periodically switched at intervals of 10 and 120 s (dotted lines), the evolutions of A/F were recorded after the catalyst bed (blue- and red-solid lines). The A/Fs versus the time on stream are plotted in Figure 8. During the 10 s interval, the amplitude of the oscillation was reduced by either of CeO2 or CeO2–Fe2O3, suggesting that the A/F perturbation was buffered by oxygen release/storage; moreover, the extent of buffering was almost identical in these two samples. At the longer interval (120 s), however, the buffering effect was not obvious in the CeO2 sample, because the recorded A/Fs nearly coincided with the periodic A/Fs. By contrast, CeO2–Fe2O3 still exhibited some buffering effects, as



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highlighted by yellow shaded regions, in concordant with its larger OSC than that of CeO2. Therefore, the CeO2-grafted Fe2O3 composite can potentially achieve fast and large-capacity oxygen release/storage under automotive exhaust conditions.

Possible mechanism of oxygen release/storage. Notably, both OSC and rR were higher in the CeO2-grafted Fe2O3 than in the separate oxide components. To elucidate the possible mechanism of this synergistic effect, we discuss the phase relationships of the Ce and Fe oxides in terms of their oxygen partial pressures and temperatures. The oxygen equilibrium reactions are as follows: 2/x CeO2 → 2/x CeO2−x + O2 6Fe2O3 → 4Fe3O4 + O2 2Fe3O4 → 6FeO + O2 2FeO → 2Fe + O2 The equilibrium partial oxygen pressures (pO2) of each reaction were calculated from the standard Gibbs energy, ∆G° = −RT log pO2, as a function of temperature. Figure 9 compares the as-calculated phase equilibria in the Ce and Fe oxide systems. At a reaction temperature of 500 °C, Fe2O3 begins reducing at approximately pO2 = 10−19 atm, close to the midpoint between A/F = 14.2 and 15.0, and reaches the metallic state (Fe) below pO2 = 10−30 atm, close to A/F = 14.2. On the other hand, the CeO2 to Ce2O3 reduction requires a much lower pO2 (

The Role of CeO2 as a Gateway for Oxygen Storage over CeO2

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