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Inverse CeO2Fe2O3 catalyst for superior low-temperature CO conversion efficiency
Applied Surface Science 416 (2017) 911–917
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Inverse CeO2 Fe2 O3 catalyst for superior low-temperature CO conversion efficiency Yongming Luo, Ran Chen, Wen Peng, Guangbei Tang, Xiaoya Gao ∗ Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, PR China
a r t i c l e
i n f o
Article history: Received 13 February 2017 Received in revised form 26 April 2017 Accepted 27 April 2017 Available online 28 April 2017 Keywords: CeO2 Fe2 O3 Modification Low-temperature CO oxidation
a b s t r a c t The paper presents a rational design of highly efficient and affordable catalysts for CO oxidation with a low operating temperature. A series of ceria-iron catalysts were inversely built via a co-precipitation method. The catalytic activity of low-temperature CO oxidation was much higher with CeO2 -modified Fe2 O3 (CeO2 Fe2 O3 ) than with Fe2 O3 -modified CeO2 (Fe2 O3 CeO2 ). In particular, the 7.5% CeO2 Fe2 O3 catalyst had the highest activity, reaching 96.17% CO conversion at just 25 ◦ C. Catalyst characterization was carried out to explore the cause of the significantly different CO conversion efficiencies between the Fe2 O3 CeO2 and Fe2 O3 CeO2 catalysts. HRTEM showed a significant inhomogeneous phase in 7.5% CeO2 Fe2 O3 with small CeO2 nanoparticles highly dispersed on the rod-shaped Fe2 O3 surface. Furthermore, the 7.5% CeO2 Fe2 O3 composite catalyst exhibited the highest ratios of Fe2+ /Fe3+ and Ce3+ /Ce4+ as well as the largest pore volume. These properties are believed to benefit the CO conversion in 7.5% CeO2 Fe2 O3 . © 2017 Elsevier B.V. All rights reserved.
1. Introduction Carbon monoxide (CO) is mainly produced from the incomplete combustion of carbon-containing fuels [1]. Because of its toxic effects, CO is considered to be a common ambient air pollutant that most countries are targeting for control. Catalytic oxidation of CO is one of the most promising techniques to remediate CO emissions [2–4]. The design and synthesis of efficient catalysts are the core elements of catalytic technology. Currently, using platinum group metals (PGM) as commercialized catalysts can eliminate most CO, but high temperatures (above 200 ◦ C) are required and result in 80% of CO being emitted during the cold-start stage in car engines [5,6]. Hence, developing catalysts for low-temperature CO oxidation is of particular interest. Various catalysts have been investigated for the lowtemperature oxidation of CO. Catalysts containing noble metals (Pt, Ru, and Au) are highly efficient for CO conversion at low temperatures [7], but their high cost and the limitations of noble metals prohibit their applications on a large scale. Accordingly, much effort has been devoted to developing noble-metal-free catalysts, such as Co3 O4 , Fe2 O3 , CeO2 , and CuO [8–10]. Of these, CeO2 is widely used because of two particular features: the redox pairing of Ce3+ /Ce4+
∗ Corresponding author. E-mail address: [email protected] (X. Gao). http://dx.doi.org/10.1016/j.apsusc.2017.04.225 0169-4332/© 2017 Elsevier B.V. All rights reserved.
and its remarkable oxygen storage capacity (OSC). In the oxidation of CO, CeO2 can act as a medium for an oxygen storage and release, with a complex series of redox reactions occurring via the continuous formation and annihilation of oxygen vacancies and cycling changes in the redox states of Ce3+ /Ce4+ [11]. However, using pure CeO2 is discouraged because the surface redox chemistry of ceria is sensitive to crystal structure defects [12]. This problem can be resolved by substituting some of the Ce cations with other metal ions. The structure and properties of the new compound depend strongly on the sizes and/or charges of the doped metal ions. For example, Pr3+ and Tb3+ doping can reduce the activation energy of oxygen release [13], and can thereby remarkably improve the oxygen exchange capacity of ceria. In contrast, doping with the smaller homovalent Zr4+ enhances the OSC and anti-sintering properties of ceria [14]. The conversion between the two oxidation states of Fe (Fe2+ /Fe3+ ) make Fe an ideal candidate for catalytic applications in CO oxidation. Thus, synergy can be achieved between cerium and iron via the combined cycling of Ce3+ /Ce4+ and Fe2+ /Fe3+ [15–17]. In addition, the amorphous structure of Fe2 O3 provides a favorable specific surface area, which is also important for the catalytic reaction to proceed [18]. Therefore, using Fe-doped CeO2 mixed-oxide catalysts for catalytic oxidation have been widely investigated [19], but high temperatures (>200 ◦ C) are required to obtain high efficiency of CO oxidation [15,20]. To further improve the catalytic activity of Fe2 O3 /CeO2 for low-temperature CO oxidation, Luo and
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co-workers doped nanostructured Pd, and although good catalytic performance was achieved, CO conversion at room temperature was less than 10% [21]. Consequently, rational design of highly efficient and affordable catalysts for CO oxidation with low operating temperatures still remains a challenge. It is generally accepted that optimum catalytic activity for CO oxidation with iron-ceria catalysts is achieved via the formation of Fe-Ce solid solutions obtained with a low Fe molar percentage (below 0.3) [22]. Recently, Hornés reported that enhanced CO catalytic activity was achieved using inverse configurations of ceria and copper [23]. Inspired by Hornés, we endeavored to build inverse iron-ceria catalysts because such a strategy offers an opportunity to form new catalysts with totally different structures and properties that may have superior performance for low-temperature CO oxidation. In this study, Fe2 O3 was modified by CeO2 (CeO2 Fe2 O3 ). As a control, we also fabricated CeO2 catalysts modified by Fe2 O3 (Fe2 O3 CeO2 ). The catalytic activity of low-temperature CO oxidation was much higher with CeO2 Fe2 O3 than with Fe2 O3 CeO2 , and an impressive CO conversion of 96.17% was obtained with the 7.5% CeO2 -Fe2 O3 catalyst at an ambient temperature of 25 ◦ C. The cause for the improvement in CO conversion with 7.5% CeO2 Fe2 O3 was investigated by characterizing the catalysts. Specifically, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), nitrogen sorption, X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2 -TPR), N2 adsorption-desorption, and CO temperature-programmed desorption (CO-TPD) profile analysis were used to characterize the catalysts. Based on the systematic analysis, the dominant factors for controlling the activity of the Ce-Fe catalyst system for low temperature CO oxidation are discussed. 2. Experimental 2.1. Catalyst preparation A series of CeO2 -modified Fe2 O3 composites (denoted as CeO2 Fe2 O3 ) was prepared using a co-precipitation method with CeO2 -to- Fe2 O3 molar ratios of 2.5, 5.0, 7.5, and 10.0. The calculated amounts of Ce(NO3 )3 ·6H2 O and Fe(NO3 )3 ·9H2 O were added to a beaker, followed by the addition of a Na2 CO3 solution. The pH of the solution was adjusted to neutral using HCl (0.25 mol/L) or NaOH (0.25 mol/L). After 2 h, the samples was dried in an air oven at 120 ◦ C for 8 h and then calcined under ambient air at 500 ◦ C for 3 h. The CeO2 Fe2 O3 composites were then obtained via pressing, grinding, and sieving (40–60 mesh). As a control, CeO2 -modified Fe2 O3 composites (denoted as Fe2 O3 CeO2 ) with Fe2 O3 -to-CeO2 molar ratios of 2.5, 5.0, 7.5, and 10.0 were also prepared using a similar procedure as described for CeO2 -Fe2 O3 excluding the adjustments in the relative amounts of Ce(NO3 )3 ·6H2 O and Fe(NO3 )3 ·9H2 O. All reagents were analytical grade and were purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Catalyst characterization The phase compositions and crystalline structures of the samples were determined using a X-ray diffractometer (Rigaku D/max-1200, Japan) at 40 kV and 200 mA with Cu K␣ radiation. The morphologies and particle sizes were examined on a HRTEM (JEOL-100CX, Japan). Specific surface area and pore size distributions of the catalysts were measured using a micromeritics ASAP 2020 (Norcross, GA) based on nitrogen adsorption-desorption. All samples were outgassed at 300 ◦ C for 3 h in a vacuum atmosphere before measurement. XPS was performed using a PHI 5000 Ver-
saprobe to get information of the surface element compositions and the chemical states of all of the catalysts. H2 -TPR was used to evaluate the redox behaviors of the as-prepared catalysts. An Autochem 2910 with a thermal conductivity detector (TCD) was used. Prior to analysis, 0.1 g of the catalyst was pretreated with Ar/H2 (90%/10%, V/V) at 100 ◦ C for 30 min. The sample was then heated to 900 ◦ C (10 ◦ C/min), and then a TCD was used. The CO-TPD profiles of the catalyst were carried out on an apparatus equipped with a TCD detector. 0.2 g of the catalyst was put on the reactor bed, dried at 500 ◦ C for 1 h with a He flow, and then cooled to 50 ◦ C. Next, a gaseous mixture of CO/He (5%/95.0%, V/V, 30 mL/min−1 ) was added to the reactor for 2 h at a temperature of 50 ◦ C. Last, the desorption step was conducted with a He flow for 30 min, increasing the temperature from 50 ◦ C to 800 ◦ C at 10 ◦ C/min. 2.3. Catalytic activity test The catalytic activity of CO oxidation was performed in a fixedbed micro-reactor (i.d. = 6 mm) under atmospheric pressure. 0.2 g of the catalyst was added to the reactor, which was filled with a mixture gas of CO/O2 /He (2.0%/1.0%/97.0%, V/V/V, 30 mL/min−1 , corresponding to a space velocity of 12590 h−1 ). The effluents were analyzed online using a gas chromatograph (GC-9790) equipped with a TCD detector. The efficiency of CO conversion was calculated according to the following equation: COConversion (%) =
C[CO]in − C[CO]out ∗ 100% C[CO]in
where C[CO]in and C[CO]out represent the initial concentration of CO and the concentration of CO after catalytic oxidation irradiation, respectively. 3. Results and discussion 3.1. Catalytic activity The catalytic effectiveness with CeO2 Fe2 O3 and Fe2 O3 CeO2 is shown in Fig. 1, which presents the efficiency of CO conversion in the temperature range of 25–100 ◦ C. As can be seen, all of the CeO2 Fe2 O3 composites displayed better CO catalytic activity than pure Fe2 O3 or CeO2 . It is interesting to observe, however, that much higher catalytic activity for low-temperature CO oxidation was achieved with CeO2 Fe2 O3 than with Fe2 O3 CeO2 . The 7.5% CeO2 Fe2 O3 catalyst exhibited the most pronounced improvement of CO conversion with an impressive CO conversion efficiency of 96.17% at 25 ◦ C. When the temperature was increased to 100 ◦ C, the efficiency of CO conversion gradually increased to 98.66%. At the same temperature, there was negligible catalytic activity with either pure Fe2 O3 or pure CeO2 . The Fe2 O3 CeO2 catalyst also showed unsatisfactory catalytic activity with a maximum CO conversion efficiency of 23.57% at 25 ◦ C. 3.2. Characterization of the catalysts 3.2.1. Structure and morphology Fig. 2 shows the XRD patterns of the samples. Two characteristic peaks of CeO2 in 2 = 28.6◦ and 47.6◦ were observed for the CeO2 Fe2 O3 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0 (Fig. 2A), and this suggests the formation of crystalline CeO2 in the CeO2 -Fe2 O3 composite catalysts. Meanwhile, the XRD diffraction peaks from Fe2 O3 decreased with a small amount of CeO2 doping (2.5 and 5.0). However, this phenomenon was different with high amounts of CeO2 doping (modified molar ratios of 7.5 and 10). In particular, the 7.5% CeO2 -Fe2 O3 catalyst had the strongest diffraction peak intensity. CeO2 doping also shifted the diffraction angles
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Fig. 1. CO conversion (A) CeO2 and 10.0.
Fe2 O3 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0; (B) Fe2 O3
Fig. 2. XRD patterns of the as-synthesized samples. (A) CeO2 Fe2 O3 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0; (B) Fe2 O3
of 33.2◦ , 35.7◦ , 40.9◦ , and 49.5◦ in Fe2 O3 . This indicated that the crystalline structure of Fe2 O3 changed with the presence of ceria. The Fe2 O3 CeO2 composite catalysts were prepared by changing the molar ratios of iron-to-ceria to make CeO2 a major component. No detectable Fe2 O3 crystals were found in the Fe2 O3 CeO2 composite catalysts (Fig. 2B). However, the intensity and position of the XRD diffraction peak corresponding to cubic CeO2 continuously changed with an increasing amount of Fe, indicating the formation of Fe-Ce solid solution without an appreciable phase separation [24]. However, the Fe-Ce solid solution was thermodynamically mesostable. Fe segregated in the CeO2 lattice and formed a thermodynamically more stable hematite at high Fe concentration [22]. This was not evident in our Fe2 O3 CeO2 samples because of the relatively low amount of Fe2 O3 doping. However, hematite crystals were distinct in the CeO2 Fe2 O3 composite catalysts (in which Fe2 O3 was a major component) (Fig. 2A). Fig. 3 shows the morphologies of the catalyst samples. Completely different morphologies were observed in the catalyst samples depending on whether Fe2 O3 or CeO2 were dominant. A significant inhomogeneous phase was observed in 7.5% CeO2 Fe2 O3 (Fig. 3A), where small CeO2 nanoparticles were highly dispersed on the rod-shaped Fe2 O3 surface. This is different from the previously reported microstructure of the CeO2 Fe2 O3 mixed oxide composite [25,26]. In contrast, the 7.5% Fe2 O3 CeO2 composite catalyst consisted of nanoparticles with spherical or elliptical shapes with a uniform size of 5 nm (Fig. 3B).
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CeO2 composite catalysts with molar ratios of 2.5, 5.0, 7.5,
CeO2 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0.
Table 1 XPS and texture data of as-prepared samples. Samples
Fe2+ /Fe3+
Ce3+ /Ce4+
Surface Area (m2 /g)
Pore Volume (cm3 /g)
Fe2 O3 2.5% 5% 7.5% 10%
0.517 0.480 0.495 0.563 0.364
– 0.125 0.129 0.152 0.118
17.73 33.79 38.51 40.77 32.02
0.123 0.173 0.185 0.193 0.179
CeO2 2.5% 5% 7.5%3 10%
– 0.296 0.258 0.294 0.374
0.089 0.149 0.138 0.128 0.114
87.03 106.3 102.2 69.95 36.77
0.189 0.129 0.160 0.104 0.190
3.2.2. Compositions XPS analysis was performed to examine the oxidation states and relative proportions of the elements on the surface of the catalysts. The core-level spectra of Ce 3d, Fe 2p, and O 1s are shown in Fig. 4. The obtained ratios of Fe2+ /Fe3+ and Ce3+ /Ce4+ are summarized in Table 1. In the Ce 3d spectra (Fig. 4A for CeO2 -Fe2 O3 and Fig. 4B for Fe2 O3 CeO2 ), the peaks denoted as v, v// , v/// , u, u// , u/// and v/ , u/ , were considered as the characteristic peaks of the 3d10 4f0 state of Ce4+ and the 3d10 4f1 initial electronic state of the Ce3+ ions, respectively. These peaks indicate the coexistence of Ce3+ and Ce4+ in both of the CeO2 -Fe2 O3 and Fe2 O3 CeO2 samples. In contrast to
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Fig. 3. TEM images of the as-synthesized samples (A) CeO2 of 7.5.
Fe2 O3 composite catalysts with the molar ratio of 7.5; (B) Fe2 O3
pure CeO2 , the binding energies of the peaks were slightly less in Fe2 O3 CeO2 , indicating an improved reducibility. Fig. 4C and D shows the Fe 2p core-level spectra of CeO2 -Fe2 O3 and Fe2 O3 CeO2 , respectively. The binding energies at 711.2 and 724.2 eV with two satellite peaks at 718.7 and 732.4 eV correspond to Fe 2p3/2 and Fe 2p1/2 , respectively, and confirm the presence of Fe in all of the samples [27]. The O 1s spectra of CeO2 -Fe2 O3 and Fe2 O3 CeO2 are shown in Fig. 4E and F, respectively. The observed peak at 529.2 eV was assigned to the lattice oxygen in CeO2 . The existence of surface oxygen on the catalyst was evidenced by the appearance of another peak at 531.2 eV. The O 1s spectra shifted to a high binding energy in Fe2 O3 CeO2 compared to that of pure CeO2 . The ratios of Fe2+ /Fe3+ and Ce3+ /Ce4+ for all of the samples were calculated from the XPS analysis (Table 1). There was a continuous increase in Fe2+ /Fe3+ and Ce3+ /Ce4+ ratios with an increase in the amount of CeO2 from 0 to 7.5%. Meanwhile, the Fe2+ /Fe3+ and Ce3+ /Ce4+ ratios decreased when the amount of CeO2 was further increased to 10%. It is believed that high ratios of Fe2+ /Fe3+ and Ce3+ /Ce4+ are beneficial for the CO conversion, and this was clearly seen with the 7.5% CeO2 Fe2 O3 sample that showed the most pronounced promotion of CO conversion.
3.2.3. Redox behaviors Fig. 5A shows the H2 -TPR profiles of the CeO2 -Fe2 O3 samples. Pure Fe2 O3 exhibited a sharp peak at 450 ◦ C and a broad peak around 600 ◦ C, indicating a reduction route for Fe2 O3 of Fe2 O3 → Fe3 O4 → Fe0 and indicating that two valence states of Fe2+ /Fe3+ were present [11]. Pure CeO2 had two reduction peaks at 500 ◦ C and 820 ◦ C corresponding to the reduction of surface ceria and the reduction of bulk ceria, respectively [28]. For CeO2 Fe2 O3 (Fig. 5A), the peaks of 450 ◦ C and 600 ◦ C shifted to remarkably lower temperatures, suggesting an increase in reduction behavior. This was different from the report that the first low-temperature peak shifted to higher temperature in the Fe2 O3 CeO2 mixed oxides [17]. In the case of Fe2 O3 CeO2 (Fig. 5B), more complex TPR profiles, which involved more reduction peaks, were obtained. It was reported that different reduction processes for the Fe-Ce samples occurred with the coexistence of the free Fe2 O3 cluster and the solid solution, and that this resulted in modified TPR profiles [17]. The formation of Fe-Ce solid solutions shifts the reduction peaks to lower temperatures because of the generation of oxygen vacancies [29]. The consumption of adsorbed oxygen caused the peak at 280 ◦ C. The reduction of Fe3+ generated the peak around 350–400 ◦ C, and this continuously decreased correspond-
CeO2 composite catalysts with the molar ratio
ing to the amount of Fe2 O3 . The peaks around 600 ◦ C and 800 ◦ C were attributed to the reduction of Fe2+ and the reduction of bulk Ce4+ , respectively [30].
3.2.4. Surface area and pore structure analysis The surface area and porous structure of the as-prepared catalysts were examined using N2 adsorption-desorption isotherms. As shown in Fig. 6, the isotherms of the CeO2 Fe2 O3 catalyst were a IV isotherm with a H3 hysteresis loop, and this is characteristic of mesoporous solids. The values of the surface area and pore volume are summarized in Table 1. As shown, the BET surface area of Fe2 O3 significantly improved after the CeO2 was incorporated. The incorporation of Ce has been reported to increase the distance between adjacent Fe particles [31]. Nevertheless, a volcano-type dependence on the CeO2 content was observed in CeO2 Fe2 O3 , and the largest increases in surface area (40.77 m2 /g) and pore volume (0.193 cm3 /g) were seen with 7.5% CeO2 Fe2 O3 ; and this contributed to the superior CO conversion efficiency. In the case of the Fe2 O3 CeO2 catalyst, the lower content of Fe2 O3 increased the surface area of CeO2 , while the surface area decreased when CeO2 was modified with a high content Fe2 O3 . On the contrary, the Fe2 O3 CeO2 catalyst showed a general decrease in pore volume compared to that of pure CeO2 .
3.3. Correlation between catalyst characterization and activity The activity and characterization results were compared to provide an understanding of the cause of the enhanced CO conversion in 7.5% CeO2 Fe2 O3 . The XRD patterns (Fig. 1B) show that a Ce-based solid solution formed in the Fe2 O3 CeO2 catalysts. The presence of a Ce-based solid solution is usually related to the improved reducibility, which is evident in the TPR data (Fig. 5). Furthermore, a Ce-based solid solution can promote oxygen storage and mobility capacity [32,33]. Hence, the formation of a Ce-based solid solution always contributes mainly to catalytic performance with Ce Fe catalysts. Given the relatively low Ce content in the CeO2 Fe2 O3 sample, there should be more CeO2 -based solid solution in the Fe2 O3 CeO2 sample, but since there is not, the excellent performance of the CeO2 Fe2 O3 catalysts cannot be primarily due to the presence of a CeO2 -based solid solution. The surface area of the catalyst is considered to be crucial for catalytic activity. However, this was not evident in our current results. The Fe2 O3 CeO2 catalyst generally showed a higher surface area than the CeO2 Fe2 O3 catalyst but had an inferior CO conversion efficiency. Pore volume could play an important role in the CO con-
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Fig. 4. XPS analysis of Ce 3d, Fe 2p, and O 1s in the as-synthesized samples. (A) (C) (E) for CeO2 Fe2 O3 composite catalysts (B) (D) (F) for Fe2 O3 CeO2 composite catalysts.
version. Large pore structure is beneficial for transporting the CO molecule, thereby improving the catalytic activity of CeO2 Fe2 O3 . CO-TPD analysis was chosen to further investigate the behavior of CO oxidation over the catalysts, and these results are shown in Fig. 7. The desorption peaks for 7.5% CeO2 Fe2 O3 are wide and complicated, indicating CO was adsorbed strongly on multifold active sites. In general, the stronger the adsorption of CO on a catalyst, the higher the desorption temperature is [34]. Clearly, the desorption temperature of 7.5% CeO2 Fe2 O3 was much higher than that of 7.5% Fe2 O3 CeO2 , illustrating a stronger adsorption of CO on the 7.5% CeO2 Fe2 O3 catalyst. Furthermore, compared to 7.5% Fe2 O3 CeO2 , the peak area for desorption with 7.5% CeO2 -Fe2 O3
was much larger, suggesting an increased number of active sites for CO adsorption. HRTEM showed a significant inhomogeneous phase in 7.5% CeO2 Fe2 O3 with small CeO2 nanoparticles highly dispersion on the rod-shaped Fe2 O3 surface. The interaction between CeO2 and Fe2 O3, especially in the interface between the two oxides, facilitated the formation of Fe2+ [17]. It was reported that Fe atoms on the surface of Fe2 O3 are very close to some of the Ce-O species in the interface between CeO2 and Fe2 O3 , and this may generate a Fe O Ce species [17,29]. It should be noted that the ratio of Fe2+ /Fe3+ in 7.5% CeO2 Fe2 O3 was the highest of all of the catalyst samples. Meanwhile, 7.5% CeO2 Fe2 O3 had the highest ratio of
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Fig. 5. H2 -TPR profiles of the as-synthesized samples (A) CeO2 with molar ratios of 2.5, 5.0, 7.5, and 10.0.
Fe2 O3 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0; (B) Fe2 O3
Fig. 6. Nitrogen adsorption-desorption isotherms of the as-synthesized samples (A) CeO2 Fe2 O3 CeO2 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0.
Fig. 7. Temperature programmed desorption (TPD) analysis of the as-synthesized samples. CeO2 -Fe2 O3 composite catalysts with molar ratios of 7.5 (Red line) and Fe2 O3 CeO2 composite catalysts with molar ratios of 7.5 (black line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
CeO2 composite catalysts
Fe2 O3 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0; (B)
Ce3+ /Ce4+ . With the coexistence of Fe2+ and Ce3+ , a Fe-O Ce species could achieve the coupling of the Ce4+ Ce3+ and Fe3+ Fe2+ cycles in the CeO2 Fe2 O3 interface. It has been reported that Fe2+ /Fe3+ ion pairs are catalytic centers for CO oxidation [10]. When Ce3+ Ce4+ couples coexist, bivalent catalytic centers are present in the CeO2 -Fe2 O3 catalyst. In particular, the redox potential differences of Fe3+ /Fe2+ (0.77 V) and Ce4+ /Ce3+ (1.61 V) form mixed Ce4+ Fe2+ and/or Ce3+ Fe3+ ion pairs as a result of mutual charge interaction [15]. Therefore, the presence of both Fe2+ /Fe3+ and Ce3+ /Ce4+ facilitated electron hopping between the Ce4+ Fe2+ and/or Ce3+ Fe3+ systems. Such electrons can be captured by O2 molecules on the surface of the catalyst and thus enhance the catalytic activity of the 7.5% CeO2 Fe2 O3 catalyst towards CO oxidation [10]. Furthermore, the existence of Fe2+ and Ce3+ has been reported to correlate with the formation of oxygen vacancies [35], and this is also favorable for the catalytic behavior of 7.5% CeO2 Fe2 O3 in CO conversion. Thus, the inverse 7.5% CeO2 Fe2 O3 catalyst obtained superior low-temperature CO conversion efficiency. We compared our results with those reported in the literature for ceria nanoparticles. For example, Konsolakis and co-coworkers evaluated the impact of synthesis parameters on the solid state properties and the CO oxidation performance of ceria nanoparticles and found that complete CO conversion was obtained at a high temperature (>400 ◦ C)
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for all ceria nanoparticles [36]. However, pure CeO2 has always shown zero CO oxidation activity at room temperature. The lowtemperature CO conversion efficiency of CeO2 has been achieved by loading noble metals of Au [37], but even then, the low temperature (<100 ◦ C) conversion was still less than 80%. Similarly, transition metal copper has been shown to greatly improve the CO oxidation activity of Ce-related oxides. For example, Luo et al. [38] demonstrated the superior low-temperature CO conversion efficiency of nanosized CuO CeO2 catalysts, but they still did not obtain a high CO conversion efficiency at room temperature. Here, the 7.5% CeO2 Fe2 O3 catalyst showed a 96.17% CO conversion at a relatively low temperature of just 25 ◦ C. Furthermore, the synthesis method of the catalyst was green, and the advantages of the obtained 7.5% CeO2 Fe2 O3 catalyst are its low cost (without noble metals) and its easy availability, making it a promising candidate for the removal of cold-start emissions. 4. Conclusions Inverse CeO2 -Fe2 O3 catalysts were synthesized via a coprecipitation method. All of the mixed oxides showed superior catalytic activities, and in particular, the 7.5% CeO2 Fe2 O3 catalyst showed the highest activity, reaching 96.17% CO conversion at 25 ◦ C. The large increase of pore volume in 7.5% CeO2 Fe2 O3 contributed to the superior efficiency of CO oxidation. Furthermore, elevated ratios of Fe2+ /Fe3+ and Ce3+ /Ce4+ were determining factors that resulted in the outstanding catalytic activity of 7.5% CeO2 Fe2 O3 . These results indicate that the 7.5% CeO2 Fe2 O3 catalyst is promising for the removal of cold-start emission. Acknowledgement We gratefully acknowledge the financial supports of the National Natural Science Foundation of China (Grant No. 21307046, 21367015 and U1402233). References [1] EPA, https://www3.epa.gov/airquality/carbonmonoxide/. [2] Y.H. Zheng, Y. Cheng, Y.S. Wang, F. Bao, L.H. Zhou, X.F. Wei, Y.Y. Zhang, Q. Zheng, Quasicubic-Fe2 O3 nanoparticles with excellent catalytic performance, J. Phys. Chem. B 110 (2006) 3093–3097. [3] Y.N. Tang, W.G. Chen, Z.G. Shen, S.S. Chang, M.Y. Zhao, X.Q. Dai, Nitrogen coordinated silicon-doped graphene as a potential alternative metal-free catalyst for CO oxidation, Carbon 111 (2017) 448–458. [4] E. Moretti, A.I. Molina, G. Sponchia, A. Talon, R. Frattini, E. Rodriguez-Castellon, L. Storaro, Low-temperature carbon monoxide oxidation over zirconia-supported CuO–CeO2 catalysts: effect of zirconia support properties, Appl. Surf. Sci. 403 (2017) 612–622. [5] E. Kolobova, A. Pestryakov, G. Mamontov, Yu. Kotolevich, N. Bogdanchikova, M. Farias, A. Vosmerikov, L. Vosmerikova, V. Cortes Corberan, Low-temperature CO oxidation on Ag/ZSM-5 catalysts: influence of Si/Al ratio and redox pretreatments on formation of silver active sites, Fuel 188 (2017) 121–131. [6] R. Westerholm, A. Christensen, Regulated and unregulated exhaust emissions from two three-way catalyst equipped gasoline fuelled vehicles, Atmos. Environ. 30 (1996) 3529–3536. [7] R.H. Liu, N.S. Gao, F. Zhen, Y.Y. Zhang, L. Mei, X.W. Zeng, Doping effect of Al2 O3 and CeO2 on Fe2 O3 support for gold catalyst in CO oxidation at low-temperature, Chem. Eng. J. 225 (2013) 245–253. [8] J. Luo, M. Meng, X. Li, Y. Zha, T. Hu, Y. Xie, J. Zhang, Mesoporous Co3 O4 -CeO2 and Pd/Co3 O4 -CeO2 catalysts: synthesis, characterization and mechanistic study of their catalytic properties for low-temperature CO oxidation, J. Catal. 254 (2008) 310–324. [9] X. Zheng, X. Zhang, X. Wang, S. Wang, S. Wu, Low-temperature CO oxidation over a ternary oxide catalyst with high resistance to hydrocarbon inhibition, Appl. Catal. A 295 (2005) 142–149. [10] A.El-A.A. Said, M.M.M.A. El-Wahab, M.N. Goda, Synthesis and characterization of pure and (Ce, Zr, Ag) doped mesoporous CuO-Fe2 O3 as highly efficient and stable nanocatalysts for CO oxidation at low temperature, Appl. Surf. Sci. 390 (2016) 649–665. [11] F.J. Perez-Alonso, I. Melián-Cabrera, M. López Granados, F. Kapteijn, J.L.G. Fierro, Synergy of Fex Ce1-x O2 mixed oxides for N2 O decomposition, J. Catal. 239 (2006) 340–346.
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Inverse CeO2 Fe2 O3 catalyst for superior low-temperature CO conversion efficiency Yongming Luo, Ran Chen, Wen Peng, Guangbei Tang, Xiaoya Gao ∗ Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, PR China
a r t i c l e
i n f o
Article history: Received 13 February 2017 Received in revised form 26 April 2017 Accepted 27 April 2017 Available online 28 April 2017 Keywords: CeO2 Fe2 O3 Modification Low-temperature CO oxidation
a b s t r a c t The paper presents a rational design of highly efficient and affordable catalysts for CO oxidation with a low operating temperature. A series of ceria-iron catalysts were inversely built via a co-precipitation method. The catalytic activity of low-temperature CO oxidation was much higher with CeO2 -modified Fe2 O3 (CeO2 Fe2 O3 ) than with Fe2 O3 -modified CeO2 (Fe2 O3 CeO2 ). In particular, the 7.5% CeO2 Fe2 O3 catalyst had the highest activity, reaching 96.17% CO conversion at just 25 ◦ C. Catalyst characterization was carried out to explore the cause of the significantly different CO conversion efficiencies between the Fe2 O3 CeO2 and Fe2 O3 CeO2 catalysts. HRTEM showed a significant inhomogeneous phase in 7.5% CeO2 Fe2 O3 with small CeO2 nanoparticles highly dispersed on the rod-shaped Fe2 O3 surface. Furthermore, the 7.5% CeO2 Fe2 O3 composite catalyst exhibited the highest ratios of Fe2+ /Fe3+ and Ce3+ /Ce4+ as well as the largest pore volume. These properties are believed to benefit the CO conversion in 7.5% CeO2 Fe2 O3 . © 2017 Elsevier B.V. All rights reserved.
1. Introduction Carbon monoxide (CO) is mainly produced from the incomplete combustion of carbon-containing fuels [1]. Because of its toxic effects, CO is considered to be a common ambient air pollutant that most countries are targeting for control. Catalytic oxidation of CO is one of the most promising techniques to remediate CO emissions [2–4]. The design and synthesis of efficient catalysts are the core elements of catalytic technology. Currently, using platinum group metals (PGM) as commercialized catalysts can eliminate most CO, but high temperatures (above 200 ◦ C) are required and result in 80% of CO being emitted during the cold-start stage in car engines [5,6]. Hence, developing catalysts for low-temperature CO oxidation is of particular interest. Various catalysts have been investigated for the lowtemperature oxidation of CO. Catalysts containing noble metals (Pt, Ru, and Au) are highly efficient for CO conversion at low temperatures [7], but their high cost and the limitations of noble metals prohibit their applications on a large scale. Accordingly, much effort has been devoted to developing noble-metal-free catalysts, such as Co3 O4 , Fe2 O3 , CeO2 , and CuO [8–10]. Of these, CeO2 is widely used because of two particular features: the redox pairing of Ce3+ /Ce4+
∗ Corresponding author. E-mail address: [email protected] (X. Gao). http://dx.doi.org/10.1016/j.apsusc.2017.04.225 0169-4332/© 2017 Elsevier B.V. All rights reserved.
and its remarkable oxygen storage capacity (OSC). In the oxidation of CO, CeO2 can act as a medium for an oxygen storage and release, with a complex series of redox reactions occurring via the continuous formation and annihilation of oxygen vacancies and cycling changes in the redox states of Ce3+ /Ce4+ [11]. However, using pure CeO2 is discouraged because the surface redox chemistry of ceria is sensitive to crystal structure defects [12]. This problem can be resolved by substituting some of the Ce cations with other metal ions. The structure and properties of the new compound depend strongly on the sizes and/or charges of the doped metal ions. For example, Pr3+ and Tb3+ doping can reduce the activation energy of oxygen release [13], and can thereby remarkably improve the oxygen exchange capacity of ceria. In contrast, doping with the smaller homovalent Zr4+ enhances the OSC and anti-sintering properties of ceria [14]. The conversion between the two oxidation states of Fe (Fe2+ /Fe3+ ) make Fe an ideal candidate for catalytic applications in CO oxidation. Thus, synergy can be achieved between cerium and iron via the combined cycling of Ce3+ /Ce4+ and Fe2+ /Fe3+ [15–17]. In addition, the amorphous structure of Fe2 O3 provides a favorable specific surface area, which is also important for the catalytic reaction to proceed [18]. Therefore, using Fe-doped CeO2 mixed-oxide catalysts for catalytic oxidation have been widely investigated [19], but high temperatures (>200 ◦ C) are required to obtain high efficiency of CO oxidation [15,20]. To further improve the catalytic activity of Fe2 O3 /CeO2 for low-temperature CO oxidation, Luo and
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co-workers doped nanostructured Pd, and although good catalytic performance was achieved, CO conversion at room temperature was less than 10% [21]. Consequently, rational design of highly efficient and affordable catalysts for CO oxidation with low operating temperatures still remains a challenge. It is generally accepted that optimum catalytic activity for CO oxidation with iron-ceria catalysts is achieved via the formation of Fe-Ce solid solutions obtained with a low Fe molar percentage (below 0.3) [22]. Recently, Hornés reported that enhanced CO catalytic activity was achieved using inverse configurations of ceria and copper [23]. Inspired by Hornés, we endeavored to build inverse iron-ceria catalysts because such a strategy offers an opportunity to form new catalysts with totally different structures and properties that may have superior performance for low-temperature CO oxidation. In this study, Fe2 O3 was modified by CeO2 (CeO2 Fe2 O3 ). As a control, we also fabricated CeO2 catalysts modified by Fe2 O3 (Fe2 O3 CeO2 ). The catalytic activity of low-temperature CO oxidation was much higher with CeO2 Fe2 O3 than with Fe2 O3 CeO2 , and an impressive CO conversion of 96.17% was obtained with the 7.5% CeO2 -Fe2 O3 catalyst at an ambient temperature of 25 ◦ C. The cause for the improvement in CO conversion with 7.5% CeO2 Fe2 O3 was investigated by characterizing the catalysts. Specifically, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), nitrogen sorption, X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2 -TPR), N2 adsorption-desorption, and CO temperature-programmed desorption (CO-TPD) profile analysis were used to characterize the catalysts. Based on the systematic analysis, the dominant factors for controlling the activity of the Ce-Fe catalyst system for low temperature CO oxidation are discussed. 2. Experimental 2.1. Catalyst preparation A series of CeO2 -modified Fe2 O3 composites (denoted as CeO2 Fe2 O3 ) was prepared using a co-precipitation method with CeO2 -to- Fe2 O3 molar ratios of 2.5, 5.0, 7.5, and 10.0. The calculated amounts of Ce(NO3 )3 ·6H2 O and Fe(NO3 )3 ·9H2 O were added to a beaker, followed by the addition of a Na2 CO3 solution. The pH of the solution was adjusted to neutral using HCl (0.25 mol/L) or NaOH (0.25 mol/L). After 2 h, the samples was dried in an air oven at 120 ◦ C for 8 h and then calcined under ambient air at 500 ◦ C for 3 h. The CeO2 Fe2 O3 composites were then obtained via pressing, grinding, and sieving (40–60 mesh). As a control, CeO2 -modified Fe2 O3 composites (denoted as Fe2 O3 CeO2 ) with Fe2 O3 -to-CeO2 molar ratios of 2.5, 5.0, 7.5, and 10.0 were also prepared using a similar procedure as described for CeO2 -Fe2 O3 excluding the adjustments in the relative amounts of Ce(NO3 )3 ·6H2 O and Fe(NO3 )3 ·9H2 O. All reagents were analytical grade and were purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Catalyst characterization The phase compositions and crystalline structures of the samples were determined using a X-ray diffractometer (Rigaku D/max-1200, Japan) at 40 kV and 200 mA with Cu K␣ radiation. The morphologies and particle sizes were examined on a HRTEM (JEOL-100CX, Japan). Specific surface area and pore size distributions of the catalysts were measured using a micromeritics ASAP 2020 (Norcross, GA) based on nitrogen adsorption-desorption. All samples were outgassed at 300 ◦ C for 3 h in a vacuum atmosphere before measurement. XPS was performed using a PHI 5000 Ver-
saprobe to get information of the surface element compositions and the chemical states of all of the catalysts. H2 -TPR was used to evaluate the redox behaviors of the as-prepared catalysts. An Autochem 2910 with a thermal conductivity detector (TCD) was used. Prior to analysis, 0.1 g of the catalyst was pretreated with Ar/H2 (90%/10%, V/V) at 100 ◦ C for 30 min. The sample was then heated to 900 ◦ C (10 ◦ C/min), and then a TCD was used. The CO-TPD profiles of the catalyst were carried out on an apparatus equipped with a TCD detector. 0.2 g of the catalyst was put on the reactor bed, dried at 500 ◦ C for 1 h with a He flow, and then cooled to 50 ◦ C. Next, a gaseous mixture of CO/He (5%/95.0%, V/V, 30 mL/min−1 ) was added to the reactor for 2 h at a temperature of 50 ◦ C. Last, the desorption step was conducted with a He flow for 30 min, increasing the temperature from 50 ◦ C to 800 ◦ C at 10 ◦ C/min. 2.3. Catalytic activity test The catalytic activity of CO oxidation was performed in a fixedbed micro-reactor (i.d. = 6 mm) under atmospheric pressure. 0.2 g of the catalyst was added to the reactor, which was filled with a mixture gas of CO/O2 /He (2.0%/1.0%/97.0%, V/V/V, 30 mL/min−1 , corresponding to a space velocity of 12590 h−1 ). The effluents were analyzed online using a gas chromatograph (GC-9790) equipped with a TCD detector. The efficiency of CO conversion was calculated according to the following equation: COConversion (%) =
C[CO]in − C[CO]out ∗ 100% C[CO]in
where C[CO]in and C[CO]out represent the initial concentration of CO and the concentration of CO after catalytic oxidation irradiation, respectively. 3. Results and discussion 3.1. Catalytic activity The catalytic effectiveness with CeO2 Fe2 O3 and Fe2 O3 CeO2 is shown in Fig. 1, which presents the efficiency of CO conversion in the temperature range of 25–100 ◦ C. As can be seen, all of the CeO2 Fe2 O3 composites displayed better CO catalytic activity than pure Fe2 O3 or CeO2 . It is interesting to observe, however, that much higher catalytic activity for low-temperature CO oxidation was achieved with CeO2 Fe2 O3 than with Fe2 O3 CeO2 . The 7.5% CeO2 Fe2 O3 catalyst exhibited the most pronounced improvement of CO conversion with an impressive CO conversion efficiency of 96.17% at 25 ◦ C. When the temperature was increased to 100 ◦ C, the efficiency of CO conversion gradually increased to 98.66%. At the same temperature, there was negligible catalytic activity with either pure Fe2 O3 or pure CeO2 . The Fe2 O3 CeO2 catalyst also showed unsatisfactory catalytic activity with a maximum CO conversion efficiency of 23.57% at 25 ◦ C. 3.2. Characterization of the catalysts 3.2.1. Structure and morphology Fig. 2 shows the XRD patterns of the samples. Two characteristic peaks of CeO2 in 2 = 28.6◦ and 47.6◦ were observed for the CeO2 Fe2 O3 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0 (Fig. 2A), and this suggests the formation of crystalline CeO2 in the CeO2 -Fe2 O3 composite catalysts. Meanwhile, the XRD diffraction peaks from Fe2 O3 decreased with a small amount of CeO2 doping (2.5 and 5.0). However, this phenomenon was different with high amounts of CeO2 doping (modified molar ratios of 7.5 and 10). In particular, the 7.5% CeO2 -Fe2 O3 catalyst had the strongest diffraction peak intensity. CeO2 doping also shifted the diffraction angles
Y. Luo et al. / Applied Surface Science 416 (2017) 911–917
Fig. 1. CO conversion (A) CeO2 and 10.0.
Fe2 O3 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0; (B) Fe2 O3
Fig. 2. XRD patterns of the as-synthesized samples. (A) CeO2 Fe2 O3 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0; (B) Fe2 O3
of 33.2◦ , 35.7◦ , 40.9◦ , and 49.5◦ in Fe2 O3 . This indicated that the crystalline structure of Fe2 O3 changed with the presence of ceria. The Fe2 O3 CeO2 composite catalysts were prepared by changing the molar ratios of iron-to-ceria to make CeO2 a major component. No detectable Fe2 O3 crystals were found in the Fe2 O3 CeO2 composite catalysts (Fig. 2B). However, the intensity and position of the XRD diffraction peak corresponding to cubic CeO2 continuously changed with an increasing amount of Fe, indicating the formation of Fe-Ce solid solution without an appreciable phase separation [24]. However, the Fe-Ce solid solution was thermodynamically mesostable. Fe segregated in the CeO2 lattice and formed a thermodynamically more stable hematite at high Fe concentration [22]. This was not evident in our Fe2 O3 CeO2 samples because of the relatively low amount of Fe2 O3 doping. However, hematite crystals were distinct in the CeO2 Fe2 O3 composite catalysts (in which Fe2 O3 was a major component) (Fig. 2A). Fig. 3 shows the morphologies of the catalyst samples. Completely different morphologies were observed in the catalyst samples depending on whether Fe2 O3 or CeO2 were dominant. A significant inhomogeneous phase was observed in 7.5% CeO2 Fe2 O3 (Fig. 3A), where small CeO2 nanoparticles were highly dispersed on the rod-shaped Fe2 O3 surface. This is different from the previously reported microstructure of the CeO2 Fe2 O3 mixed oxide composite [25,26]. In contrast, the 7.5% Fe2 O3 CeO2 composite catalyst consisted of nanoparticles with spherical or elliptical shapes with a uniform size of 5 nm (Fig. 3B).
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CeO2 composite catalysts with molar ratios of 2.5, 5.0, 7.5,
CeO2 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0.
Table 1 XPS and texture data of as-prepared samples. Samples
Fe2+ /Fe3+
Ce3+ /Ce4+
Surface Area (m2 /g)
Pore Volume (cm3 /g)
Fe2 O3 2.5% 5% 7.5% 10%
0.517 0.480 0.495 0.563 0.364
– 0.125 0.129 0.152 0.118
17.73 33.79 38.51 40.77 32.02
0.123 0.173 0.185 0.193 0.179
CeO2 2.5% 5% 7.5%3 10%
– 0.296 0.258 0.294 0.374
0.089 0.149 0.138 0.128 0.114
87.03 106.3 102.2 69.95 36.77
0.189 0.129 0.160 0.104 0.190
3.2.2. Compositions XPS analysis was performed to examine the oxidation states and relative proportions of the elements on the surface of the catalysts. The core-level spectra of Ce 3d, Fe 2p, and O 1s are shown in Fig. 4. The obtained ratios of Fe2+ /Fe3+ and Ce3+ /Ce4+ are summarized in Table 1. In the Ce 3d spectra (Fig. 4A for CeO2 -Fe2 O3 and Fig. 4B for Fe2 O3 CeO2 ), the peaks denoted as v, v// , v/// , u, u// , u/// and v/ , u/ , were considered as the characteristic peaks of the 3d10 4f0 state of Ce4+ and the 3d10 4f1 initial electronic state of the Ce3+ ions, respectively. These peaks indicate the coexistence of Ce3+ and Ce4+ in both of the CeO2 -Fe2 O3 and Fe2 O3 CeO2 samples. In contrast to
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Fig. 3. TEM images of the as-synthesized samples (A) CeO2 of 7.5.
Fe2 O3 composite catalysts with the molar ratio of 7.5; (B) Fe2 O3
pure CeO2 , the binding energies of the peaks were slightly less in Fe2 O3 CeO2 , indicating an improved reducibility. Fig. 4C and D shows the Fe 2p core-level spectra of CeO2 -Fe2 O3 and Fe2 O3 CeO2 , respectively. The binding energies at 711.2 and 724.2 eV with two satellite peaks at 718.7 and 732.4 eV correspond to Fe 2p3/2 and Fe 2p1/2 , respectively, and confirm the presence of Fe in all of the samples [27]. The O 1s spectra of CeO2 -Fe2 O3 and Fe2 O3 CeO2 are shown in Fig. 4E and F, respectively. The observed peak at 529.2 eV was assigned to the lattice oxygen in CeO2 . The existence of surface oxygen on the catalyst was evidenced by the appearance of another peak at 531.2 eV. The O 1s spectra shifted to a high binding energy in Fe2 O3 CeO2 compared to that of pure CeO2 . The ratios of Fe2+ /Fe3+ and Ce3+ /Ce4+ for all of the samples were calculated from the XPS analysis (Table 1). There was a continuous increase in Fe2+ /Fe3+ and Ce3+ /Ce4+ ratios with an increase in the amount of CeO2 from 0 to 7.5%. Meanwhile, the Fe2+ /Fe3+ and Ce3+ /Ce4+ ratios decreased when the amount of CeO2 was further increased to 10%. It is believed that high ratios of Fe2+ /Fe3+ and Ce3+ /Ce4+ are beneficial for the CO conversion, and this was clearly seen with the 7.5% CeO2 Fe2 O3 sample that showed the most pronounced promotion of CO conversion.
3.2.3. Redox behaviors Fig. 5A shows the H2 -TPR profiles of the CeO2 -Fe2 O3 samples. Pure Fe2 O3 exhibited a sharp peak at 450 ◦ C and a broad peak around 600 ◦ C, indicating a reduction route for Fe2 O3 of Fe2 O3 → Fe3 O4 → Fe0 and indicating that two valence states of Fe2+ /Fe3+ were present [11]. Pure CeO2 had two reduction peaks at 500 ◦ C and 820 ◦ C corresponding to the reduction of surface ceria and the reduction of bulk ceria, respectively [28]. For CeO2 Fe2 O3 (Fig. 5A), the peaks of 450 ◦ C and 600 ◦ C shifted to remarkably lower temperatures, suggesting an increase in reduction behavior. This was different from the report that the first low-temperature peak shifted to higher temperature in the Fe2 O3 CeO2 mixed oxides [17]. In the case of Fe2 O3 CeO2 (Fig. 5B), more complex TPR profiles, which involved more reduction peaks, were obtained. It was reported that different reduction processes for the Fe-Ce samples occurred with the coexistence of the free Fe2 O3 cluster and the solid solution, and that this resulted in modified TPR profiles [17]. The formation of Fe-Ce solid solutions shifts the reduction peaks to lower temperatures because of the generation of oxygen vacancies [29]. The consumption of adsorbed oxygen caused the peak at 280 ◦ C. The reduction of Fe3+ generated the peak around 350–400 ◦ C, and this continuously decreased correspond-
CeO2 composite catalysts with the molar ratio
ing to the amount of Fe2 O3 . The peaks around 600 ◦ C and 800 ◦ C were attributed to the reduction of Fe2+ and the reduction of bulk Ce4+ , respectively [30].
3.2.4. Surface area and pore structure analysis The surface area and porous structure of the as-prepared catalysts were examined using N2 adsorption-desorption isotherms. As shown in Fig. 6, the isotherms of the CeO2 Fe2 O3 catalyst were a IV isotherm with a H3 hysteresis loop, and this is characteristic of mesoporous solids. The values of the surface area and pore volume are summarized in Table 1. As shown, the BET surface area of Fe2 O3 significantly improved after the CeO2 was incorporated. The incorporation of Ce has been reported to increase the distance between adjacent Fe particles [31]. Nevertheless, a volcano-type dependence on the CeO2 content was observed in CeO2 Fe2 O3 , and the largest increases in surface area (40.77 m2 /g) and pore volume (0.193 cm3 /g) were seen with 7.5% CeO2 Fe2 O3 ; and this contributed to the superior CO conversion efficiency. In the case of the Fe2 O3 CeO2 catalyst, the lower content of Fe2 O3 increased the surface area of CeO2 , while the surface area decreased when CeO2 was modified with a high content Fe2 O3 . On the contrary, the Fe2 O3 CeO2 catalyst showed a general decrease in pore volume compared to that of pure CeO2 .
3.3. Correlation between catalyst characterization and activity The activity and characterization results were compared to provide an understanding of the cause of the enhanced CO conversion in 7.5% CeO2 Fe2 O3 . The XRD patterns (Fig. 1B) show that a Ce-based solid solution formed in the Fe2 O3 CeO2 catalysts. The presence of a Ce-based solid solution is usually related to the improved reducibility, which is evident in the TPR data (Fig. 5). Furthermore, a Ce-based solid solution can promote oxygen storage and mobility capacity [32,33]. Hence, the formation of a Ce-based solid solution always contributes mainly to catalytic performance with Ce Fe catalysts. Given the relatively low Ce content in the CeO2 Fe2 O3 sample, there should be more CeO2 -based solid solution in the Fe2 O3 CeO2 sample, but since there is not, the excellent performance of the CeO2 Fe2 O3 catalysts cannot be primarily due to the presence of a CeO2 -based solid solution. The surface area of the catalyst is considered to be crucial for catalytic activity. However, this was not evident in our current results. The Fe2 O3 CeO2 catalyst generally showed a higher surface area than the CeO2 Fe2 O3 catalyst but had an inferior CO conversion efficiency. Pore volume could play an important role in the CO con-
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Fig. 4. XPS analysis of Ce 3d, Fe 2p, and O 1s in the as-synthesized samples. (A) (C) (E) for CeO2 Fe2 O3 composite catalysts (B) (D) (F) for Fe2 O3 CeO2 composite catalysts.
version. Large pore structure is beneficial for transporting the CO molecule, thereby improving the catalytic activity of CeO2 Fe2 O3 . CO-TPD analysis was chosen to further investigate the behavior of CO oxidation over the catalysts, and these results are shown in Fig. 7. The desorption peaks for 7.5% CeO2 Fe2 O3 are wide and complicated, indicating CO was adsorbed strongly on multifold active sites. In general, the stronger the adsorption of CO on a catalyst, the higher the desorption temperature is [34]. Clearly, the desorption temperature of 7.5% CeO2 Fe2 O3 was much higher than that of 7.5% Fe2 O3 CeO2 , illustrating a stronger adsorption of CO on the 7.5% CeO2 Fe2 O3 catalyst. Furthermore, compared to 7.5% Fe2 O3 CeO2 , the peak area for desorption with 7.5% CeO2 -Fe2 O3
was much larger, suggesting an increased number of active sites for CO adsorption. HRTEM showed a significant inhomogeneous phase in 7.5% CeO2 Fe2 O3 with small CeO2 nanoparticles highly dispersion on the rod-shaped Fe2 O3 surface. The interaction between CeO2 and Fe2 O3, especially in the interface between the two oxides, facilitated the formation of Fe2+ [17]. It was reported that Fe atoms on the surface of Fe2 O3 are very close to some of the Ce-O species in the interface between CeO2 and Fe2 O3 , and this may generate a Fe O Ce species [17,29]. It should be noted that the ratio of Fe2+ /Fe3+ in 7.5% CeO2 Fe2 O3 was the highest of all of the catalyst samples. Meanwhile, 7.5% CeO2 Fe2 O3 had the highest ratio of
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Fig. 5. H2 -TPR profiles of the as-synthesized samples (A) CeO2 with molar ratios of 2.5, 5.0, 7.5, and 10.0.
Fe2 O3 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0; (B) Fe2 O3
Fig. 6. Nitrogen adsorption-desorption isotherms of the as-synthesized samples (A) CeO2 Fe2 O3 CeO2 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0.
Fig. 7. Temperature programmed desorption (TPD) analysis of the as-synthesized samples. CeO2 -Fe2 O3 composite catalysts with molar ratios of 7.5 (Red line) and Fe2 O3 CeO2 composite catalysts with molar ratios of 7.5 (black line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
CeO2 composite catalysts
Fe2 O3 composite catalysts with molar ratios of 2.5, 5.0, 7.5, and 10.0; (B)
Ce3+ /Ce4+ . With the coexistence of Fe2+ and Ce3+ , a Fe-O Ce species could achieve the coupling of the Ce4+ Ce3+ and Fe3+ Fe2+ cycles in the CeO2 Fe2 O3 interface. It has been reported that Fe2+ /Fe3+ ion pairs are catalytic centers for CO oxidation [10]. When Ce3+ Ce4+ couples coexist, bivalent catalytic centers are present in the CeO2 -Fe2 O3 catalyst. In particular, the redox potential differences of Fe3+ /Fe2+ (0.77 V) and Ce4+ /Ce3+ (1.61 V) form mixed Ce4+ Fe2+ and/or Ce3+ Fe3+ ion pairs as a result of mutual charge interaction [15]. Therefore, the presence of both Fe2+ /Fe3+ and Ce3+ /Ce4+ facilitated electron hopping between the Ce4+ Fe2+ and/or Ce3+ Fe3+ systems. Such electrons can be captured by O2 molecules on the surface of the catalyst and thus enhance the catalytic activity of the 7.5% CeO2 Fe2 O3 catalyst towards CO oxidation [10]. Furthermore, the existence of Fe2+ and Ce3+ has been reported to correlate with the formation of oxygen vacancies [35], and this is also favorable for the catalytic behavior of 7.5% CeO2 Fe2 O3 in CO conversion. Thus, the inverse 7.5% CeO2 Fe2 O3 catalyst obtained superior low-temperature CO conversion efficiency. We compared our results with those reported in the literature for ceria nanoparticles. For example, Konsolakis and co-coworkers evaluated the impact of synthesis parameters on the solid state properties and the CO oxidation performance of ceria nanoparticles and found that complete CO conversion was obtained at a high temperature (>400 ◦ C)
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for all ceria nanoparticles [36]. However, pure CeO2 has always shown zero CO oxidation activity at room temperature. The lowtemperature CO conversion efficiency of CeO2 has been achieved by loading noble metals of Au [37], but even then, the low temperature (<100 ◦ C) conversion was still less than 80%. Similarly, transition metal copper has been shown to greatly improve the CO oxidation activity of Ce-related oxides. For example, Luo et al. [38] demonstrated the superior low-temperature CO conversion efficiency of nanosized CuO CeO2 catalysts, but they still did not obtain a high CO conversion efficiency at room temperature. Here, the 7.5% CeO2 Fe2 O3 catalyst showed a 96.17% CO conversion at a relatively low temperature of just 25 ◦ C. Furthermore, the synthesis method of the catalyst was green, and the advantages of the obtained 7.5% CeO2 Fe2 O3 catalyst are its low cost (without noble metals) and its easy availability, making it a promising candidate for the removal of cold-start emissions. 4. Conclusions Inverse CeO2 -Fe2 O3 catalysts were synthesized via a coprecipitation method. All of the mixed oxides showed superior catalytic activities, and in particular, the 7.5% CeO2 Fe2 O3 catalyst showed the highest activity, reaching 96.17% CO conversion at 25 ◦ C. The large increase of pore volume in 7.5% CeO2 Fe2 O3 contributed to the superior efficiency of CO oxidation. Furthermore, elevated ratios of Fe2+ /Fe3+ and Ce3+ /Ce4+ were determining factors that resulted in the outstanding catalytic activity of 7.5% CeO2 Fe2 O3 . These results indicate that the 7.5% CeO2 Fe2 O3 catalyst is promising for the removal of cold-start emission. Acknowledgement We gratefully acknowledge the financial supports of the National Natural Science Foundation of China (Grant No. 21307046, 21367015 and U1402233). References [1] EPA, https://www3.epa.gov/airquality/carbonmonoxide/. [2] Y.H. Zheng, Y. Cheng, Y.S. Wang, F. Bao, L.H. Zhou, X.F. Wei, Y.Y. Zhang, Q. Zheng, Quasicubic-Fe2 O3 nanoparticles with excellent catalytic performance, J. Phys. Chem. B 110 (2006) 3093–3097. [3] Y.N. Tang, W.G. Chen, Z.G. Shen, S.S. Chang, M.Y. Zhao, X.Q. Dai, Nitrogen coordinated silicon-doped graphene as a potential alternative metal-free catalyst for CO oxidation, Carbon 111 (2017) 448–458. [4] E. Moretti, A.I. Molina, G. Sponchia, A. Talon, R. Frattini, E. Rodriguez-Castellon, L. Storaro, Low-temperature carbon monoxide oxidation over zirconia-supported CuO–CeO2 catalysts: effect of zirconia support properties, Appl. Surf. Sci. 403 (2017) 612–622. [5] E. Kolobova, A. Pestryakov, G. Mamontov, Yu. Kotolevich, N. Bogdanchikova, M. Farias, A. Vosmerikov, L. Vosmerikova, V. Cortes Corberan, Low-temperature CO oxidation on Ag/ZSM-5 catalysts: influence of Si/Al ratio and redox pretreatments on formation of silver active sites, Fuel 188 (2017) 121–131. [6] R. Westerholm, A. Christensen, Regulated and unregulated exhaust emissions from two three-way catalyst equipped gasoline fuelled vehicles, Atmos. Environ. 30 (1996) 3529–3536. [7] R.H. Liu, N.S. Gao, F. Zhen, Y.Y. Zhang, L. Mei, X.W. Zeng, Doping effect of Al2 O3 and CeO2 on Fe2 O3 support for gold catalyst in CO oxidation at low-temperature, Chem. Eng. J. 225 (2013) 245–253. [8] J. Luo, M. Meng, X. Li, Y. Zha, T. Hu, Y. Xie, J. Zhang, Mesoporous Co3 O4 -CeO2 and Pd/Co3 O4 -CeO2 catalysts: synthesis, characterization and mechanistic study of their catalytic properties for low-temperature CO oxidation, J. Catal. 254 (2008) 310–324. [9] X. Zheng, X. Zhang, X. Wang, S. Wang, S. Wu, Low-temperature CO oxidation over a ternary oxide catalyst with high resistance to hydrocarbon inhibition, Appl. Catal. A 295 (2005) 142–149. [10] A.El-A.A. Said, M.M.M.A. El-Wahab, M.N. Goda, Synthesis and characterization of pure and (Ce, Zr, Ag) doped mesoporous CuO-Fe2 O3 as highly efficient and stable nanocatalysts for CO oxidation at low temperature, Appl. Surf. Sci. 390 (2016) 649–665. [11] F.J. Perez-Alonso, I. Melián-Cabrera, M. López Granados, F. Kapteijn, J.L.G. Fierro, Synergy of Fex Ce1-x O2 mixed oxides for N2 O decomposition, J. Catal. 239 (2006) 340–346.
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