Sage Journals: Discover world-class research

Abstract

In this study, the CeO₂-Fe₂O₃ mixed oxide catalysts have been prepared by combustion method using gel-created tartaric acid. The ability of effective carbon monoxide (CO) oxidation to carbon dioxide (CO₂) by CeO₂-Fe₂O₃ catalyst under low-temperature conditions was also demonstrated. The calcined CeO₂-Fe₂O₃ material has a porous honeycomb structure and good gaseous absorption-desorption ability. The solid solution of CeO₂-Fe₂O₃ mixed oxides was formed by the substitution of Fe⁺³ ions at some Ce⁴⁺ ion sites within the CeO₂ crystal lattice. The results also showed that the calcination temperature and the molar ratio of Ce³⁺ ions to Fe³⁺ ions (CF) affected the formation of the structural phase and the catalytic efficiency. The catalytic properties of the CeO₂-Fe₂O₃ mixed oxide were good at the CF ratio of 1 : 1, the average crystal size was near 70 nm, and the specific surface area was about 20.22 m².g^-1. The full conversion of CO into CO₂ has been accomplished at a relatively low temperature of 270 °C under insufficient O₂ conditions.

1. Introduction

Every year, the world emits a large amount of CO gas from thermal power plants, metallurgical plants, vehicles, wood burning, and waste burning. According to research in several developmental countries, thousands of people are died each year due to CO gas poisoning [1, 2]. The easy cause of CO gas poisoning is because its colorless, odorless, and nontoxic properties are difficult to recognize. CO gas is the incomplete oxidation product of carbon compounds at low temperatures and O₂ deficiency [3–5]. Under suitable conditions, CO exhibits strong reducing properties for medium metal oxides, so it has been studied and used in the metallurgical industry early [6]. Because of its possible toxic property to humans, the study and treatment of CO gas together with its secondary CO₂ product are very important. In some technical and industrial fields, CO₂ has begun to be captured and applied to reduce the greenhouse effect [7]. There are two popular methods for CO treatment as follows: adsorbing to capture CO gas [8–10] or converting CO to CO₂ by metal oxide catalysts [11].

Nowadays, CeO₂ is one of the important metal oxides in gas conversion catalysis. Cerium (Ce) belongs to the rare earth family and orders the second in the lanthanide series. Ce reserves account for a small proportion of the earth’s crust, so exploitation is difficult and expensive. Ce has the electron configuration [Xe]4f¹5d¹6s², which it can exist in the oxidation states Ce⁺³ ([Xe]4f¹5d6s) and Ce⁺⁴ ([Xe]4f5d6s). Thus, CeO₂ and Ce₂O₃ are the compounds of Ce with oxygen, and CeO₂ is more stable due to the electron configuration of Ce⁺⁴ ([Xe]4f5d6s) being similar to that of the inert xenon gas [12]. The flexible conversion of the two Ce⁴⁺ and Ce³⁺ states through the electron donor mechanisms on the surface of CeO₂ makes it widely used in catalytic reactions such as wastewater treatment [13–16], water-gas phase transition [17], and gas conversion [18–21]. CeO₂ oxides can be prepared by hydrothermal method [21, 22], coprecipitation [23–25], sol-gel [19, 26], impregnation [27], and combustion [18]. The choice of the fabrication method decides the crystal structure, morphology, and physicochemical properties of CeO₂, leading to affect the CO conversion ability of the catalyst [21, 22, 28]. The surface of the fabricated CeO₂ crystals is often imperfect due to lattice defect, vacuous O^2- ion (denoted V_O) sites, and unsaturated bonds leading to the existence of Ce³⁺ and Ce⁴⁺ states [20, 29]. The ratio between the number of Ce³⁺ ions to Ce⁴⁺ ions (Ce³⁺/Ce⁴⁺) is related to the number of V_O vacancies on the CeO₂ crystal surface, so it determines the oxygen storage capability and oxygen lease capability [25]. When the Ce³⁺/Ce⁴⁺ ratio is large, the number of V_O vacancies on the surface is also large [30]. Besides, the number of V_O also depends on the crystal morphology, and it decreases gradually with nanorods > cubes nanoscale > nanopolyhedra [29]. The density functional theory (DFT) calculations show that for low-index faces, the energy to release an O^2- ion from the lattice to create a V_O vacancy of (111) face is the largest in comparison with the (110) and (100) face, so the number of V_O on the faces decreases in the order (110) > (100) > (111) [29]. In addition, CeO₂ is a direct bandgap semiconductor with bandgap energies of 2.56 eV for the bulk sample and 3.23 eV for the nanoscale, corresponding to the absorption transition energy 2p (O^2-) →4f (Ce⁴⁺) [26, 30]. When an O^2- ion on the crystal surface gets an excitation agent to promote the electron transition 2p → 4f, it will separate from the crystal lattice leaving a V_O vacancy in company with the oxidation state transformation Ce⁺⁴(4f^o) → Ce⁺³(4f¹) [11]. V_O vacancies are capable of adsorbing CO or O₂ molecules at these positions, so they play an important role in catalytic reactions. An O₂ molecule can be captured by a V_O vacancy to form an O^2- lattice ion [31]. The combination reactions of CO molecules with the absorbed O₂ molecules on the surface of the oxide catalyst are occurred by the Langmuir-Hinshelwood (L-H) mechanism or with the O^2- ions of the lattice by the Mars-van Krevelen (M-K) mechanism [29]. The CeO₂ oxide catalyst material becomes thermal stable, has a fast catalytic rate, and has a low catalytic temperature when combined with some rare metals such as Pt [32], Pd [33], Au [34], and Ag [35]. However, a current trend uses the low-cost transition metal iron (Fe) element to make Fe₂O₃-CeO₂ mixed oxides. In nature, iron exists in both Fe²⁺ and Fe³⁺ oxidation states. The radius of Fe³⁺ ion (0.64 Å) is smaller than that of Ce⁴⁺ ion (1.01 Å), so the substitution of Fe³⁺ ions in some Ce⁴⁺ ion positions can shrink the CeO₂ crystal lattice, the $2 θ$ diffraction angle positions of the (111) face is shifted towards the larger angle, and the formed possibility of V_O vacancies is also increased [36]. The optical absorption transition properties of Fe₂O₃ oxide include the direct charge transition O^2-(2p) → Fe³⁺(3d) in the ultraviolet region and the indirect charge transition between Fe³⁺(3d) states in the visible region [37]. The conversion ability between Fe³⁺ and Fe²⁺ ions become even more flexible in Fe₂O₃-CeO₂ composition, which is demonstrated through photochemical reactions treating organic pigments [15]. The O^2- ions of the Fe-O-Ce bonds react better with CO than that of the Ce-O-Ce bonds [38], even so than those of Fe-O-Fe bonds. The energy separates an O^2- atom from the surface to create a V_O vacancy of about 3.04 eV [39]. The redox potential of the Fe⁺³/Fe⁺² pair (0.71 eV) is smaller than that of the Ce⁺⁴/Ce⁺³ pair (1.61 eV), which facilitates the electron donor-acceptor processes and the oxidation state transformation of Ce⁺⁴/Ce⁺³ and Fe⁺³/Fe⁺² pairs [20]. Other studies show that the combination of Ce and Fe elements forms CeO₂-Fe₂O₃ mixed oxide, which can reduce the reactive catalytic temperature of 50% ( $T_{50}$ ) and 100% ( $T_{100}$ ) CO conversion in comparison with single oxide catalysts of CeO₂ and Fe₂O₃, depending on the fabrication method [31, 40, 41].

From the above-mentioned characteristics, it shows that the CeO₂-Fe₂O₃ mixed oxide material has many advantageous photochemical properties that need further studying and applying for life. This study was carried out with the purpose of making CeO₂-Fe₂O₃ mixed oxide materials from popular Fe metal, reducing the content of rare earth Ce and the reactive catalytic temperature of complete CO conversion to CO₂ while applying to treat the exhaust gas in simple incinerators.

2. Chemicals and Experiments

2.1. Preparation of Mixed Oxide Catalysts

High-purity chemicals were used as Fe(NO₃)₃·6H₂O (99.98%), Ce(NO₃)₃·6H₂O (99.98%), and tartaric acid (TA) (99.98%). TA was dissolved with twice distilled water at 80 °C to get A solution. The above nitrate salts were also dissolved to give B solution such that the molar ratio of CF mixture to TA solution always was 1 : 3. A regulator matter was utilized to keep the pH of the solution equal to 2. The B solution was slowly added to the A solution, stirred, and heated at 80 °C for 2 h to obtain a pale-yellow homogeneous gel solution. The gel was then dried at 100 °C for 4 h to obtain a porous shape sample. The obtained sample was analyzed by TGA to investigate the phase transition of the sample in accordance with the calcination temperature. The samples calcined in turn at 450, 550, 650, 750, 850, and 950 °C for 2 h to obtain mixed oxides which denote as CF450, CF550, CF650, CF750, CF850, and CF950. To investigate the influence of the CF molar ratio on the structural phase formation of mixed oxides, the weight of the nitrate salts in the B solution was calculated so that the CF molar ratio of the obtained gels is in turn 9 : 1; 3 : 1; 1 : 1; 1 : 3, and 1 : 9. These gels, calcined at 650 °C for 2 h, received the mixed oxides of CF91, CF31, CF11, CF13, and CF19, respectively.

2.2. Analytics

2.2.1. Differential Thermal Analysis and Thermogravimetric Analysis

Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were performed on a Labsys Evo 1600 system. Samples were measured in air, heating rate 10 °C.min^-1.

2.2.2. X-Ray Diffraction Analysis

X-ray diffraction (XRD) measurements of the studied samples were performed on a Siemens D5000 X-meter.

2.2.3. Scanning Electron Microscopy Image

The surface morphology of the materials was recorded by scanning electron microscopy (SEM). Samples were measured on a Hitachi S-4800 instrument operating at 10 kV with a magnification of 80000-100000 times.

2.2.4. Determine the Specific Surface Area

The sample surface area was determined by the Brunauer-Emmett-Teller (BET) method on the Autosorb IQ Station measuring system. BET equation: $P / [V (P_{o} - P)] = 1 / V_{m} C + [(C - 1) / V_{m} C] . P / P_{o}$ _, where $P$ is the equilibrium pressure, $P_{o}$ is the saturated vapor pressure of the adsorbed gas at experimental temperature, $V$ is the volume of the adsorbed gas at pressure $P$ , and $V_{m}$ is the saturated gas volume of monolayer adsorption per 1 gram of adsorbent, and $C$ is the BET constant. The specific surface area BET ( $S_{BET}$ ) of the material is calculated according to the equation: $S_{BET} = (V_{m} / M) . N A_{m} d$ , where $d$ is the density, $M$ is the molar mass of the adsorbent, respectively, $N$ is the Avogadro number ( $N = 6.023 \times 10^{23}$ molecules.mol^-1), and $A_{m}$ is the cross-sectional area of 1 molecule occupied on the surface adsorbent.

2.2.5. Fourier Transform Infrared Spectra

The Fourier transform infrared (FTIR) spectra of the samples were recorded in the range of wavenumbers from 400 cm^-1 to 4000 cm^-1 on the Impact 410 spectrometer.

2.2.6. Determination of CO Conversion

The TPSR temperature surface response program was conducted with a Siemens temperature controller. The CO conversion of the catalytic material performing on the microcurrent device was calculated by the following formula: $H (%) = (C_{f} / C_{o}) .100 %$ , where $H$ is the CO (%) conversion and $C_{o}$ and $C_{f}$ are the CO concentrations before and after catalysis, respectively.

3. Results and Discussions

3.1. DTA and TGA Spectra of CeO₂-Fe₂O₃ Mixed Oxides

The DTA and TGA spectra of the gel sample with the CF/TA ratio of 1 : 3 presented in Figure 1. The DTA curve showed the temperature increased from 100 °C to 250 °C. The sample weight decreased by 38.89%, with an endothermic peak at 217 °C. This reduction may be due to the physical evaporation of the adsorbed water on the gel surface [42, 43]. In the temperature range from 250 °C to 550 °C, the sample weight decreased by 34.83% on the TGA curve corresponding to the exothermic peak of 328 °C on the DTA curve, which was caused by the pyrolysis processes of the nitrate salts released O₂ and NO₂ gas; the combustion process of the TA released CO₂ gas and H₂O according to the following reactions (1), (2), and (3) [43, 44]:

Figure 1

TGA and DTA spectra of the gel sample with the CF/TA ratio of 1 : 3.

2C₄H₆O₆+5O₂ → 8CO₂↑ +6H₂O(1)

4Fe(NO₃)₃ →2Fe₂O₃+12NO₂↑ +3O₂↑(2)

2Ce(NO₃)₃ →2CeO₂+6NO₂↑ + O₂↑(3)

or 4Fe(NO₃)₃ +2Ce(NO₃)₃ +2C₄H₆O₆ + O₂ →2(Fe₂O₃-CeO₂) +18NO₂↑ +8CO₂↑ +6H₂O(4)

When the temperature increased from 550 °C to 900 °C, the sample weight was almost unchanged, showing that the Fe₂O₃-CeO₂ mixed oxide was formed and stable.

3.2. FTIR Spectra of CeO₂-Fe₂O₃ Mixed Oxides

The appearance of vibrations characterized to the bonds of CeO₂-Fe₂O₃ mixed oxide determined via the FTIR spectra. In Figure 2, the FTIR spectra of all samples in the broad absorption range from 3000 cm^-1 to 3500 cm^-1 appeared a peak at about 3378 cm^-1 characterized by the stretching vibration of the O-H group because of the physically adsorbed water; however, this peak intensity decreased gradually as the calcination temperature of samples increased. A peak at about 2337 cm^-1 characterized the stretching vibration of adsorbed CO₂ molecules from the air for CF450, CF550, and CF650 samples, and this peak was completely suppressed for CF750 and CF850 samples. The peaks at 1630, 1380, and 1121 cm^-1 represented the C=O and C-O stretching vibrations of the -COO- group [42, 43]. At the high calcination temperature, the process of CO₂ separation occurred, so the absorption intensity of the band related to -COO- decreased and expanded into a large halo as on the CF850 sample. It is noteworthy that the appearance of the peak at 529 cm^-1 with strong absorption intensity for samples calcined at 850 °C was typical for Fe-O vibrations, the peaks of 643 and 443 cm^-1 were characteristic of Ce-O vibrations [42, 43, 45], and the above peaks appeared clear for the CF650 sample. This also shows that the characteristic phases of the CeO₂-Fe₂O₃ mixed oxide were soon formed, and it was also consistent with the result of the TGA-DTA analysis.

Figure 2

FTIR spectra of gel samples with the CF/TA ratio of 1 : 3 calcined at different temperatures for 2 h.

3.3. The Role of Calcination Temperature and Metal Ion Molar Ratio in the Structural Phase Formation of CeO₂-Fe₂O₃ Mixed Oxides

3.3.1. The Role of Calcination Temperature

The thermogravimetric analysis shows that the suitable calcination temperature range was from 450 to 900 °C. Figure 3 presents the XRD pattern of the CeO₂-Fe₂O₃ mixed oxides calcined at 450, 550, 650, 750, 850, and 900 °C for 2 h. At 450 °C and 550 °C, the XRD pattern of CF450 and CF550 only appeared the (111), (200), and (220) faces corresponding to the $2 θ$ diffraction angles at 28.96°, 33.11°, and 47.47° (JCPDS No. 34-0394), in which the (111) face had the greatest intensity. These faces characterized the face-centered cubic structure of CeO₂ in mixed oxides [36]. Meanwhile, at these calcination temperatures, there were almost no faces that characterized the structural phase of Fe₂O₃. This may be because the samples had not reached good crystallinity, and the crystal structure was not complete. At 650 °C, the reflection faces characterizing the structural phase of Fe₂O₃ also occurred clearly and intertwined with those of CeO₂ as the represented spectrum of the CF650 sample in Figure 3(b). The (311), (222), (400), (331), (420), (422), and (511) faces corresponded to $2 θ$ diffraction angles of 56.32°, 59.02°, 69.42°, 76.66°, 79.92°, 88.39°, and 95.04° belonged to the CeO₂ crystal structure, but the (012), (110), (024), and (116) faces assigned to $2 θ$ angles of 24.92°, 35.64°, 49.46°, and 54.02°, respectively, representing the hexagonal structure phase of Fe₂O₃ (JCPDS No. 33-0664) [14, 19, 27].

Figure 3

XRD pattern of CeO₂-Fe₂O₃ mixed oxides. Samples with the CF molar ratio of 1 : 1 calcined at different temperatures for 2 h (a) and CF650 sample was redrawn (b).

(b)

The diffraction intensity on all faces became narrower and stronger as the calcination temperature increased. This meant that the samples had better crystallinity, a complete crystal structure, and larger crystal sizes [46]. However, when the samples calcined to 750 °C and 850 °C, a separating tendency of mixed oxide phase into two single oxide phases of CeO₂ and Fe₂O₃ was happened because of these more thermal stable oxides. Based on the XRD pattern and Brass’ formula, it is able to determine the $a$ lattice constant via the (111) face of all the CeO₂-Fe₂O₃ mixed oxides, as seen in Table 1.

Table 1

$2 θ$ diffraction angle and $a$ lattice constant of CeO₂-Fe₂O₃ mixed oxides calcined at different temperatures for 2 h.

Sample	CF450	CF550	CF650	CF750	CF850
Diffraction angle $2 θ$ (degree)	28.96	28.94	28.92	28.89	28.88
Lattice constant $a$ (Å)	5.3385	5.3422	5.3458	5.3512	5.3530

The results showed that there was a slight increase of the $a$ lattice constant from 5.3385 to 5.3530 Å, corresponding to a slight decrease of the $2 θ$ diffraction angle from 28.96 to 28.89 Å. The increase of the $a$ lattice constant together with the left shift of the $2 θ$ angle indicated that the calcination sample at 450°C had formed in a solution of CeO₂-Fe₂O₃ mixed oxide [23]. It is explained that the Fe³⁺(0.64 Å) ion radius is smaller than the Ce⁴⁺ (1.01 Å) ion radius, so when the Ce⁴⁺ ions are replaced by Fe³⁺ ions at some CeO₂ lattice sites, these substitutions cause the shrink of the crystal lattice [36]. In contrast, the partial separation of the CeO₂ phase widened the crystal lattice and shifted the $2 θ$ angle position of the (111) face towards a smaller angle as illustrated by CF450, CF650, and CF850 patterns in the small inset of Figure 3(b). Thus, it can be seen that the change in calcination temperature affected the formation of mixed oxide solution. At 650°C, the crystal was complete with the appearance of both CeO₂ and Fe₂O₃ phases. When the calcination temperature of the sample increased, the mixed oxides had good crystallization and tended to the segregation of single oxide phases.

3.3.2. The Role of Metal Ion Molar Ratio

XRD pattern of CeO₂-Fe₂O₃ mixed oxides with different CF molar ratios calcined at 650 °C for 2 h, as seen in Figure 4. When the molar quantity of Ce³⁺ ions was more than that of Fe³⁺ ions (CF91 and CF31 samples), the XRD pattern manifested the (111), (200), (220), (311), (222), (400), (331), (422), and (511) faces corresponded to the $2 θ$ diffraction angles of 28.82°, 33.11°, 47.47°, 56.32°, 59.07°, 69.41°, 76.64°, 79.91°, 88.39°, and 95.04° and characterized the crystalline structure phase of CeO₂ (for CF91 and CF31 samples in Figure 4(a)) [14, 19, 27].

Figure 4

XRD patterns of CeO₂-Fe₂O₃ mixed oxides. Samples with different CF molar ratios calcined at 650 °C for 2 h (a) and CF11 sample was redrawn (b).

(b)

This may be because with the calcination condition to 650 °C, and the used Fe³⁺ ion content was smaller in comparison with the Ce⁴⁺ ion content, so Fe³⁺ ions were dissolved in the crystal lattice of CeO₂ oxide [23]. When the molar quantity of Ce³⁺ ion and Fe³⁺ ions was equal, the characteristic faces of the Fe₂O₃ phase appeared and interwove with those of CeO₂, and the XRD pattern of CF11 was presented again in Figure 4(b). The presence of the typical (110) face of Fe₂O₃ shows that the mixed oxide has separated into phases [47]. When the molar quantity of Ce³⁺ ions was smaller than those of Fe³⁺ ions (for CF13 and CF19 samples), the diffraction intensity on the (110) face became more narrow and stronger. This phenomenon was known that the molar quantity of Fe³⁺ ions in mixed oxides was high, which led to the split gradually mixed oxides into two individual oxide phases of CeO₂ and Fe₂O₃ [23]. The $a$ lattice constants of CeO₂ were calculated from the (111) face in Table 2. As a result, there was a slight decrease of $a$ lattice constant from 5.3639 Å to 5.3385 Å, while the $2 θ$ diffraction angle of the (111) face shifted towards the greater diffraction angle from 28.82° to 28.98°, respectively. This shift can observe clearly by CF91, CF11, and CF19, as seen in the small inset in Figure 3(b) [20, 23, 24, 36]. The above results showed that the change in CF molar ratio also affected the formation of the crystal structure phase of the mixed oxides. With a CF molar ratio of 1 : 1, there appeared characteristic reflection faces for the structural phases of CeO₂ and Fe₂O₃. When the molar quantity of Fe³⁺ ions were high, the CeO₂-Fe₂O₃ mixed oxide tended to separate into two oxide phases of CeO₂ and Fe₂O₃.

Table 2

$2 θ$ diffraction angle and $a$ lattice constant of CeO₂-Fe₂O₃ mixed oxides with different CF molar ratios calcined at 650 °C for 2 h.

Sample	CF91	CF31	CF11	CF13	CF19
Diffraction angle 2θ (degree)	28.82	28.87	28.92	28.94	28.98
Lattice constant $a$ (Å)	5.3639	5.3548	5.3458	5.3421	5.3385

3.4. Morphology of Fe₂O₃-CeO₂ Mixed Oxides

SEM images of the CeO₂-Fe₂O₃ mixed oxides were shown in Figure 5 with the CF molar ratio of 1 : 1 which were calcined at temperatures of 450 °C, 650 °C, and 850 °C for 2 h. It is observed that the surface of the CF450 sample was porous. This is the characteristic of materials that are prepared by the combustion method using organic compounds to create gels (Figure 5(a)) [48]. For the CF650 sample, the porous property became clear to many honeycomb-like cavities. It can be explained that the molecular structure of TA consists of strongly polar functional groups HOOC(HO)CC(OH)COOH that played the role of stretching and uniformly dispersing Ce³⁺ and Fe³⁺ ions in solution [49]. At 650 °C, the decomposition of the organic component and the pyrolysis of nitrate salts led to a decrease in volume and mass, creating a system of space-connected microcapillary tubes (Figure 5(b)). This created a porous property of mixed oxides as well as the obtained result in other reports [50]. The average crystal size of the oxide was determined to be about 70 nm. For the CF850 sample, the crystals tended to break the spatial porous block into discrete nanosized particles (Figure 5(c)) [42].

Figure 5

SEM images of CeO₂-Fe₂O₃ mixed oxides with the CF ratio of 1 : 1 calcined at 450 °C (a), 650 °C (b), and 850 °C (c) for 2 h.

(b)

(c)

From the dependence of $P / [V (P_{o} - P)]$ on $P / P_{o}$ in the BET equation, the adsorption-desorption isotherm representing the relation between the adsorbed volume and the relative pressure was shown in Figure 6. The linear shape of the isotherm in the low $P / P_{o}$ value range indicated that monolayer adsorption occurred on the surface of the porous catalyst, and it is also suitable for the linear region in the range of $P / P_{o}$ from 0.05 to 0.35 of this material [51]. The specific surface area of the CeO₂-Fe₂O₃ mixed oxide, CF650 sample, was determined at about 20.22 m².g^-1.

Figure 6

Nitrogen adsorption-desorption isotherms of CeO₂-Fe₂O₃ mixed oxide with the CF molar ratio of 1 : 1 calcined at 650 °C for 2 h.

3.5. Process of CO Conversion to CO₂

The process of CO oxidation by CeO₂-Fe₂O₃ mixed oxide catalysts and the curves of CO conversion according to temperature is described via Figure 7. Under the catalytic temperature condition below 300 °C and deficient O₂ gas, the activities of CO oxidation took place [6]. The CF91, CF31, CF13, and CF19 samples with the different molar quantities of Ce³⁺ and Fe³⁺ ions shown that the CO conversion curves were similar variation, and the ability of CO conversion was lower than that of the CF11 sample (Figure 7(a)). For the CF11 sample (also named as CF11(CF650) or CF650(CF11)), the CO conversion curve was left-skewed, meaning that at the same catalytic temperature, the CO conversion of the CF11 sample was higher and finished earlier than other samples. This can be because the molar quantity of Ce³⁺ and Fe³⁺ ions was the same, the quantity of O^2- ions in the -Ce-O-Fe- bonds was more dominant than that in two -Ce-O-Ce and -Fe-O-Fe- bonds of other samples, so the catalytic reactions of CF11 sample also happened better than the others [38]. The CO conversion for all samples can be explained by the following reactions:

Figure 7

CO conversion of CeO₂-Fe₂O₃ samples according to temperature. Samples with different CF molar ratios calcined at 650 °C for 2 h (a) and samples calcined at different temperatures for 2 h (b).

(b)

2CO + O₂ →2CO₂↑(7)

2CeO₂ + CO → Ce₂O₃ + CO₂↑(8)

3Fe₂O₃ + CO →2Fe₃O₄ (or FeO.Fe₂O₃) + CO₂↑(9)

2FeO+2CeO₂ (or 2FeO.CeO₂) → Ce₂O₃ + Fe₂O₃ (or Ce₂O₃.Fe₂O₃)(10)

2Ce₂O₃ + O₂ →4CeO₂(11)

When the catalytic temperature increased from 75 °C to 175 °C, the amount of CO gas was converted about 12%; this may be because a part of CO gas was oxidized by O₂ in (7) [11]. In addition, these reactions can also occur between adsorbed CO and O₂ gases on the surface of the CeO₂-Fe₂O₃ catalyst to emit CO₂ by the J-H mechanism. When the temperature increased from 175 °C to 260 °C, the CO conversion was rapid for all samples but reached the fastest 50% at 211 °C for CF11. The intensification of CO oxidation can be because CO combines with an O^2- at the lattice sites in -Ce-O-Fe-, -Ce-O-Ce- and -Fe-O-Fe- bonds to release CO₂ by the M-K mechanism in (8) and (9) [27, 29]. The Fe₃O₄ product was a mixture of two FeO·Fe₂O₃ oxides, meaning that only a part of Fe³⁺ ions were reduced to Fe²⁺ ions. This low-temperature reaction was also carried out early by another researcher [6].

When the temperature was over 260 °C, the CO conversion process slowed down because it needed the recovery of the CeO₂ oxidizing agent from Ce₂O₃ according to (10) and (11) and then reached 100% at 275 °C faster for the CF11 sample. The recovery here was due to the redox potential of Fe⁺³/Fe⁺² (0.711 V) < Ce⁺⁴/Ce⁺³ (1.61 V) [20], so a tendency happened the redox process Ce⁺⁴ + Fe⁺² → Ce⁺³ + Fe⁺³ [23, 52], or this was also the process of transferring an O^2- ion from Ce⁴⁺ ion to a neighboring Fe²⁺ ion, creating the charge balance [38]. The (11) reaction turned Ce³⁺ into initial Ce⁴⁺, which was an important agent that helped Fe ion form a closed-loop bridge of oxidation states by the (9) and (10) reactions as follows: Fe₂O₃ → Fe₃O₄ → FeO → Fe₂O₃. The participation of Fe³⁺ ions in the mixed oxide made the deformation of lattice structure and increased the reaction centers of gases storage and release. Therefore, the conversion of CO to CO₂ became more efficient under low temperatures and O₂ deficiency [4, 5]. It can be seen that thanks to the redox process between Fe⁺² and Ce⁺⁴, the catalyst system was restored to its original properties and was not degraded.

With the same catalytic mechanisms above, among all CeO₂-Fe₂O₃ mixed oxide samples calcined at different temperatures for 2 h, the CO conversion of the CF650 sample was better than all of those (Figure 7(b)). The CF650 curve was also skewed to the left and rapidly reached 100% at 270 °C. It is possible that for the samples CF450 and CF550, the crystallization of the oxides had not been complete, and there were still carbonate components of organic combustion products as analyzed in the TGA-DGT and FTIR spectra, which interfered with the process of CO oxidation. For the CF750 and CF850 samples, the crystal grain size increased, and the surface area decreased, leading to a decrease in the contact and oxidation capacity. Moreover, at high calcination temperature, there was a tendency of phase separation into two single oxides as mentioned in the XRD pattern, so their oxidation ability became less than that of mixed oxide catalysts [23]. Thus, the calcination temperature changed the crystallinity and affected the CO catalytic ability of the CeO₂-Fe₂O₃ mixed oxides. From the above arguments, it is able to describe the mechanisms of CO oxidation, capture O₂, and release O^2- ions, creating V_O vacancies on the surface of CeO₂ crystal, as shown in Figure 8.

Figure 8

The mechanisms of CO oxidation and the oxidation state conversion of metal ions between Ce⁺⁴/Ce⁺³ and Fe⁺³/Fe⁺² pairs on the CeO₂-Fe₂O₃ catalyst surface.

For the J-H mechanism, a CO molecule reacted with adsorbed an O₂ molecule at V_O vacancy to emit a CO₂ molecule (7):

For the M-K mechanism, a CO molecule combined with an O^2- ion of the crystal lattice to release a CO₂ molecule and a V_O vacancy (8) and (9).

The transformation of Ce⁺⁴ to Ce⁺³ and Fe⁺² to Fe⁺³ was in (10).

The transformation of Ce⁺⁴ from Ce⁺³ after an O₂ molecule was captured by a V_O vacancy (11):

The CO conversion temperatures at $T_{50}$ and $T_{100}$ of all CeO₂-Fe₂O₃ samples are shown in Table 3. It can show that the change of the CF molar ratio and calcination temperature of samples affected the CO conversion of CeO₂-Fe₂O₃ mixed oxide catalysts. The $T_{50}$ and $T_{100}$ temperatures of all samples are quite low, so it is considered an advantage of this fabrication method. Of all the samples, the $T_{50}$ and $T_{100}$ temperatures of the CF11 (CF650) samples are 200 °C and 270 °C, respectively, which are lower than those of the other samples (Table 3(a) and 3(b)).

Table 3

$T_{50}$ and $T_{100}$ temperatures of CeO₂-Fe₂O₃ samples. Samples with different CF molar ratios calcined at 650 °C for 2 h (a) and samples calcined at different temperatures for 2 h (b).

	a			b
Sample	$T_{50}$ (°C)	$T_{100}$ (°C)	Sample	$T_{50}$ (°C)	$T_{100}$ (°C)
CF91	211	275	CF450	211	275
CF31	211	275	CF550	211	280
CF11	200	270	CF650(CF11)	200	270
CF13	211	275	CF750	205	275
CF19	211	295	CF850	221	275

Some publications have shown that the $T_{50}$ and $T_{100}$ of oxide catalysts are different depending on the fabrication method, as shown in Table 4. In some cases, if the single CeO₂ or Fe₂O₃ oxide is used as a catalyst, the $T_{50}$ and $T_{100}$ also decrease quite low [22, 53], but the rest is very high [23, 41, 46, 52, 54–56]. For CeO₂-Fe₂O₃ catalyst materials, several studies have achieved immediate maximum CO conversion (M) at room temperature but are not capable of 100% CO conversion [25]. Some results are quite similar [19, 27, 40], but the others are very high [23, 24, 31, 41].

Table 4

$T_{50}$ and $T_{100}$ temperatures of CeO₂, Fe₂O₃, and CeO₂-Fe₂O₃ oxides cited in some other references.

Oxide	Fabricated method	$T_{50}$	$T_{M /} T_{100}$ ( $T_{M} < T_{100}$ )	Reference
CeO₂	Polyol method	250°C	350°C	[57]
CeO₂	Combustion method	x	~240°C^a	[58]
CeO₂	Hydrothermal method	216	~230°C	[22]
CeO₂	Combustion method	330°C	~410°C	[52]
CeO₂	Combustion method	~210°C	~260°C^b	[48]
CeO₂	Co-precipitation method	500 K	700 K	[23]
CeO₂	Microwave and combustion method	362°C	~475°C	[41]
CeO₂	Hydrothermal method	x	~610 K	[55]
CeO₂	Surfactant-templated method	~230°C	~300°C	[46]
CeO₂	Thermal decomposition method	300°C	x	[40]
CeO₂	Solvothermal reaction method	~320°C	~400°C	[56]
Fe₂O₃	Co-precipitation method	620 K	750 K	[23]
Fe₂O₃	Co-precipitation method	290°C	450°C	[54]
Fe₂O₃	Surfactant-assisted method	~220°C	270°C	[53]
CeO₂-Fe₂O₃	Co-precipitation method	500 K	600 K	[23]
CeO₂-Fe₂O₃	Cyclic molecular designed dispersion method	~465 K	548 K	[27]
CeO₂-Fe₂O₃	Co-precipitation method	x	25°C^c	[25]
CeO₂-Fe₂O₃	Hydrothermal method	166°C	~280°C	[19]
CeO₂-Fe₂O₃	Co-precipitation method	480 K	~575 K	[24]
CeFe10	Thermal decomposition method	203°C	~280°C	[40]
Ce_xFe_1-xO_2-δ	Co-precipitation method	250°C	~450°C	[31]
Ce_0.98Fe_0.03O₂	Microwave and combustion method	298°C	~375°C	[41]

^a,b,cThe maximum CO conversion catalytic temperatures ( $T_{M}$ ) of 30%, 70%, and 96.17%, respectively. x is the reaction catalyst temperature that is unknown or out of measurement scale.

The correlation between $T_{50}$ and $T_{100}$ temperatures of the CeO₂-Fe₂O₃ mixed oxides is shown in Figure 9. It is seen that the $T_{50}$ and $T_{100}$ are quite low for all samples and the conversion ability is the best corresponding to the CF11 sample calcined at 650°C for 2 h.

Figure 9

$T_{50}$ and $T_{100}$ temperatures of CeO₂-Fe₂O₃ samples. Samples with different CF molar ratios calcined at 650 °C for 2 h (a) and samples calcined at different temperatures for 2 h (b).

(b)

Figure 10 shows five oxidation cycles at $T_{100}$ that were used to study the structural and mixed catalytic stability of sample CF11(650). After 5 oxidation cycles, the CO conversion efficiency was lowered to 97.18% (Figure 10(a)). This reduction in CO oxidation performance might be attributed to phase separation in the mixed oxide, as evidenced by the presence of the Fe2O3 phase at peak (110) (Figure 10(b)).

Figure 10

CO conversion of sample CF650(CF11) at $T_{100}$ after 5 recycles (a) and corresponding XRD patterns (A) 1^st, (B) 2^nd, (C) 3^rd, (D) 4^th, and (E) 5^th (b).

(b)

The studied results showed that Fe³⁺ ions joined in CeO₂ crystal lattice, which has caused the deformation of the lattice structure, increasing the quantity of V_O vacancies. The V_O vacancies acted as the reaction centers, thereby promoting easier oxidation processes. The CeO₂-Fe₂O₃ mixed oxide catalysts are fabricated by the combustion method using gel-created TA matter for CO conversion is effective.

4. Conclusion

CeO₂-Fe₂O₃ mixed oxides have been prepared successfully by the combustion method using gel-created tartaric acid. The solid solution of CeO₂-Fe₂O₃ mixed oxides formed a molar ratio of Ce⁺³ ions to Fe³⁺ ions of 1 : 1 at 650 °C for 2 h with a uniform average crystal size of 70 nm and a surface area of 20.22 m².g^-1. In particular, the transformation of metal-ion states in Fe³⁺/Fe²⁺ and Ce⁴⁺/Ce³⁺ pairs through the redox processes have formed a closed loop of Fe-ion oxidation states: Fe₂O₃ → Fe₃O₄ → FeO → Fe₂O_3, and maintains the catalytic properties of the CeO₂-Fe₂O₃ mixed oxides. The participation of Fe-metal ions in CeO₂-Fe₂O₃ mixed oxide solution enhanced the density of V_O vacancies and promoted the catalytic reactions of CO conversion. The choice of Ce³⁺ to Fe³⁺ molar ratio of 1 : 1 has halved the needed Ce content. The complete conversion of CO into CO₂ has taken place at a low temperature of 270 °C under deficient O₂ conditions. The studied results can open a prospect of using CeO₂-Fe₂O₃ mixed oxide catalysts for simple CO emission incinerators.

Footnotes

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research is funded by the Vietnam Academy of Science and Technology (reference number TĐVLTT.01/21-23).

References

Mattiuzzi

Lippi

Worldwide epidemiology of carbon monoxide poisoning

Human & Experimental Toxicology 2020 39

5945169

4 387 392

10.1177/0960327119891214

31789062

Kinoshita

Türkan

Vucinic

Naqvi

Bedair

Rezaee

Tsatsakis

Carbon monoxide poisoning

Toxicology Reports 2020 7

5945169

169 173

10.1016/j.toxrep.2020.01.005

32015960

Eilenberg

S. R.

Bilsback

K. R.

Johnson

Kodros

J. K.

Lipsky

E. M.

Naluwagga

Fedak

K. M.

Benka-Coker

Reynolds

Peel

Clark

Field measurements of solid-fuel cookstove emissions from uncontrolled cooking in China, Honduras, Uganda, and India

Atmospheric Environment 2018 190

5945169

116 125

10.1016/j.atmosenv.2018.06.041

2-s2.0-85049926171

Zhang

Meng

Chen

Carbon monoxide formation and emissions during waste incineration in a grate–circulating fluidized bed incinerator

Waste Management & Research 2011 29

5945169

3 294 308

10.1177/0734242X10368581

2-s2.0-79952391767

Xiu

Stevanovic

Rahman

M. M.

Pourkhesalian

A. M.

Morawska

Thai

P. K.

Emissions of particulate matter, carbon monoxide and nitrogen oxides from the residential burning of waste paper briquettes and other fuels

Environmental Research 2018 167

5945169

536 543

10.1016/j.envres.2018.08.008

2-s2.0-85051818247

30142630

Richard

M. A.

Soled

S. L.

Fiato

R. A.

DeRites

B. A.

The behavior of iron oxides in reducing atmospheres

Materials Research Bulletin 1983 18

5945169

7 829 833

10.1016/0025-5408(83)90060-0

2-s2.0-0040263989

Vansant

Koziel

P. W.

Aresta

Karimi

Kawi

Technical and industrial applications of CO2

An Economy Based on Carbon Dioxide and Water 2019

Cham

Springer

73 103

10.1007/978-3-030-15868-2_3

Gao

Wang

Duan

Dong

Chen

Adsorption separation of CO from syngas with CuCl@AC adsorbent by a VPSA process

RSC Advances 2018 8

5945169

69 39362 39370

10.1039/C8RA08578A

2-s2.0-85057738055

Hogendoorn

J. A.

van Swaaij

W. P. M.

Versteeg

G. F.

The absorption of carbon monoxide in COSORB solutions: absorption rate and capacity

The Chemical Engineering Journal 1995 59

5945169

3 243 252

10.1016/0923-0467(94)02959-8

2-s2.0-0029407968

10.

Xue

Hao

Cheng

Xue

Hao

CO adsorption performance of CuCl/activated carbon by simultaneous reduction–dispersion of mixed Cu(II) salts

Materials 2019 12

5945169

10, article 1605

10.3390/ma12101605

2-s2.0-85066832549

31100801

11.

Della Mea

G. B.

Matte

L. P.

Thill

A. S.

Lobato

F. O.

Benvenutti

E. V.

Arenas

L. T.

Jürgensen

Hergenröder

Poletto

Bernardi

Tuning the oxygen vacancy population of cerium oxide (CeO2−x, 0 < x <0.5) nanoparticles

Applied Surface Science 2017 422

5945169

1102 1112

10.1016/j.apsusc.2017.06.101

2-s2.0-85020942908

12.

Scirè

Palmisano

Cerium Oxide (CeO₂): Synthesis, Properties and Applications 2020 firt ed

Amsterdam, Netherlands

Elsevier B.V

10.1016/C2017-0-02724-6

13.

Maria Magdalane

Kaviyarasu

Matinise

Mayedwa

Mongwaketsi

Douglas

Mola

G. T.

AbdullahAl-Dhabi

Arasu

M. V.

Henini

Kennedy

Maaza

Jeyaraj

Evaluation on La₂O₃ garlanded ceria heterostructured binary metal oxide nanoplates for UV/ visible light induced removal of organic dye from urban wastewater

South African Journal of Chemical Engineering 2018 26

5945169

49 60

10.1016/j.sajce.2018.09.003

2-s2.0-85054074249

14.

Wang

Zhang

Cai

Wang

Djerdj

Surfactant-assisted synthesis of CeO2 nanoparticles and their application in wastewater treatment

RSC Advances 2012 2

5945169

32 12413 12423

10.1039/C2RA21590J

2-s2.0-84871102673

15.

Sabari Arul

Mangalaraj

Ramachandran

Nirmala Grace

In Han

Fabrication of CeO2/Fe2O3 composite nanospindles for enhanced visible light driven photocatalyst and supercapacitor electrode

Journal of Materials Chemistry A 2015 3

5945169

29 15248 15258

10.1039/C5TA02630J

2-s2.0-84951925268

16.

Tambat

Umale

Sontakke

Photocatalytic degradation of milling yellow dye using sol–gel synthesized CeO2

Materials Research Bulletin. 2016 76

5945169

466 472

10.1016/j.materresbull.2016.01.010

2-s2.0-84955462771

17.

Kovacevic

Mojet

B. L.

van Ommen

J. G.

Lefferts

Effects of morphology of cerium oxide catalysts for reverse water gas shift reaction

Catalysis Letters 2016 146

5945169

4 770 777

10.1007/s10562-016-1697-6

2-s2.0-84969350952

18.

Zedan

A. F.

AlJaber

A. S.

Combustion synthesis of non-precious CuO-CeO₂ nanocrystalline catalysts with enhanced catalytic activity for methane oxidation

Materials 2019 12

5945169

6 878

10.3390/ma12060878

2-s2.0-85063598779

30875991

19.

Lykaki

Stefa

Carabineiro

S. A.

Pandis

P. K.

Stathopoulos

V. N.

Konsolakis

Facet-dependent reactivity of Fe2O3/CeO2 nanocomposites: effect of ceria morphology on CO oxidation

Catalysts 2019 9

5945169

4, article 371

10.3390/catal9040371

2-s2.0-85065718733

20.

Song

Wang

Sun

Study of oxidative dehydrogenation of ethylbenzene with CO2 on supported CeO2-Fe2O3 binary oxides

Arabian Journal of Chemistry 2020 13

5945169

10 7357 7369

10.1016/j.arabjc.2020.08.013

21.

Stefa

Lykaki

Fragkoulis

Binas

Pandis

P. K.

Stathopoulos

V. N.

Konsolakis

Effect of the preparation method on the physicochemical properties and the CO oxidation performance of nanostructured CeO₂/TiO₂ oxides

PRO 2020 8

5945169

7 847

10.3390/pr8070847

22.

Feng

Han

Wang

Xue

Highly reducible nanostructured CeO2 for CO oxidation

Catalysts 2018 8

5945169

11 535

10.3390/catal8110535

2-s2.0-85057248382

23.

Bao

Chen

Fang

Jiang

Huang

Structure-activity relation of Fe2O3–CeO2 composite catalysts in CO oxidation

Catalysis Letters 2008 125

5945169

1-2 160 167

10.1007/s10562-008-9540-3

2-s2.0-51849106036

24.

Sudarsanam

Mallesham

Naga Durgasri

Reddy

B. M.

Physicochemical characterization and catalytic CO oxidation performance of nanocrystalline Ce-Fe mixed oxides

RSC Advances 2014 4

5945169

22 11322 11330

10.1039/C3RA45778H

2-s2.0-84894476785

25.

Luo

Chen

Peng

Tang

Gao

Inverse CeO₂![single bond](https://sdfestaticassets-us- east-1.sciencedirectassets.com/shared- assets/55/entities/sbnd.gif)Fe₂O₃ catalyst for superior low-temperature CO conversion efficiency

Applied Surface Science 2017 416

5945169

C 911 917

10.1016/j.apsusc.2017.04.225

2-s2.0-85018433120

26.

Ansari

A. A.

Optical and structural properties of sol–gel derived nanostructured CeO2 film

Journal of Semiconductors 2010 31

5945169

5, article 053001

10.1088/1674-4926/31/5/053001

2-s2.0-77952905208

27.

Tavakoli

Nasiri

Mortazavi

Khodadadi

A. A.

Cyclic molecular designed dispersion (CMDD) of Fe₂O₃ on CeO₂ promoted by Au for preferential CO oxidation in hydrogen

International Journal of Hydrogen Energy 2020 45

5945169

58 33598 33611

10.1016/j.ijhydene.2020.09.123

28.

Shang

Zhang

Han

Effects of preparation methods on the activity of CuO/CeO2 catalysts for CO oxidation

Frontiers of Chemical Science and Engineering 2017 11

5945169

4 603 612

10.1007/s11705-017-1661-z

2-s2.0-85028759016

29.

Dey

Dhal

G. C.

Cerium catalysts applications in carbon monoxide oxidations

Materials Science for Energy Technologies 2020 3

5945169

6 24

10.1016/j.mset.2019.09.003

30.

Vazirov

R. A.

Sokovnin

S. Y.

Ilves

V. G.

Bazhukova

I. N.

Pizurova

Kuznetsov

M. V.

Physicochemical characterization and antioxidant properties of cerium oxide nanoparticles

Journal of Physics: Conference Series. 2018 1115

5945169

3, article 032094

10.1088/1742-6596/1115/3/032094

2-s2.0-85058234634

31.

Zhu

Wei

Dong

Cheng

Wang

Enhanced CH4 and CO oxidation over Ce_1-xFe_xO₂ hybrid catalysts by tuning the lattice distortion and the state of surface iron species

ACS Applied Materials & Interfaces 2019 11

5945169

21 19227 19241

10.1021/acsami.9b05409

2-s2.0-85066908206

32.

Boronin

A. I.

Slavinskaya

E. M.

Figueroba

Stadnichenko

A. I.

Kardash

T. Y.

Stonkus

O. A.

Fedorova

E. A.

Muravev

V. V.

Svetlichnyi

V. A.

Bruix

Neyman

K. M.

CO oxidation activity of Pt/CeO₂ catalysts below 0 °C: platinum loading effects

Applied Catalysis B: Environmental 2021 286, article 119931

5945169

10.1016/j.apcatb.2021.119931

33.

Stonkus

O. A.

Kardash

T. Y.

Slavinskaya

E. M.

Zaikovskii

V. I.

Boronin

A. I.

Thermally induced structural evolution of palladium–ceria catalysts. Implication for CO oxidation

ChemCatChem 2019 11

5945169

15 3505 3521

10.1002/cctc.201900752

2-s2.0-85068511759

34.

Saoud

K. M.

El-Shall

M. S.

Physical and chemical synthesis of Au/CeO2 nanoparticle catalysts for room temperature CO oxidation: a comparative study

Catalysts 2020 10

5945169

11 1351

10.3390/catal10111351

35.

Kibis

L. S.

Svintsitskiy

D. A.

Kardash

T. Y.

Slavinskaya

E. M.

Gotovtseva

E. Y.

Svetlichnyi

V. A.

Boronin

A. I.

Interface interactions and CO oxidation activity of Ag/CeO₂ catalysts: a new approach using model catalytic systems

Applied Catalysis A: General 2019 570

5945169

51 61

10.1016/j.apcata.2018.11.005

2-s2.0-85056468678

36.

Galvita

V. V.

Poelman

Bliznuk

Detavernier

Marin

G. B.

CeO2-modified Fe2O3 for CO2 utilization via chemical looping

Industrial and Engineering Chemistry Research 2013 52

5945169

25 8416 8426

10.1021/ie4003574

2-s2.0-84879517805

37.

Taffa

D. H.

Hamm

Dunkel

Sinev

Bahnemann

Wark

Electrochemical deposition of Fe2O3 in the presence of organic additives: a route to enhanced photoactivity

RSC Advances 2015 5

5945169

125 103512 103522

10.1039/C5RA21290A

2-s2.0-84949939840

38.

Zhang

Dong

Wei

Zhang

Determination of active site densities and mechanisms for soot combustion with O₂ on Fe-doped CeO₂ mixed oxides

Journal of Catalysis 2010 276

5945169

1 16 23

10.1016/j.jcat.2010.08.017

2-s2.0-78049351959

39.

Hoh

S. W.

Thomas

Jones

Willock

D. J.

A density functional study of oxygen vacancy formation on α-Fe2O3(0001) surface and the effect of supported au nanoparticles

Research on Chemical Intermediates 2015 41

5945169

12 9587 9601

10.1007/s11164-015-1984-7

2-s2.0-84953349338

40.

Laguna

O. H.

Centeno

M. A.

Arzamendi

Gandía

L. M.

Romero-Sarria

Odriozola

J. A.

Iron-modified ceria and Au/ceria catalysts for total and preferential oxidation of CO (TOX and PROX)

Catalysis Today 2010 157

5945169

1-4 155 159

10.1016/j.cattod.2010.04.011

2-s2.0-78049314041

41.

Sahoo

T. R.

Armandi

Arletti

Piumetti

Bensaid

Manzoli

Panda

S. R.

Bonelli

Pure and Fe-doped CeO2 nanoparticles obtained by microwave assisted combustion synthesis: physico-chemical properties ruling their catalytic activity towards CO oxidation and soot combustion

Applied Catalysis B: Environmental 2017 211

5945169

31 45

10.1016/j.apcatb.2017.04.032

2-s2.0-85017576172

42.

Chawla

S. K.

Prabhjyot

Mudsainiyan

R. K.

Meena

S. S.

Yusuf

S. M.

Effect of fuel on the synthesis, structural, and magnetic properties of M-type hexagonal ferrites by sol-gel auto-combustion method

Journal of Superconductivity and Novel Magnetism 2015 28

5945169

5 1589 1599

10.1007/s10948-014-2893-5

2-s2.0-84940006885

43.

Ali

A. A.

Ahmed

I. S.

Elfky

E. M.

Auto-combustion synthesis and characterization of iron oxide nanoparticles (α-Fe2O3) for removal of lead ions from aqueous solution

Journal of Inorganic and Organometallic Polymers and Materials 2021 31

5945169

1 384 396

10.1007/s10904-020-01695-3

44.

Strydom

C. A.

van Vuuren

C. P. J.

The thermal decomposition of cerium(III) nitrate

Journal of thermal analysis 1987 32

5945169

1 157 160

10.1007/BF01914558

2-s2.0-0023174286

45.

Lassoued

Dkhil

Gadri

Ammar

Control of the shape and size of iron oxide (α-Fe₂O₃) nanoparticles synthesized through the chemical precipitation method

Results in Physics 2017 7

5945169

7 3007 3015

10.1016/j.rinp.2017.07.066

2-s2.0-85028872208

46.

Guo

M.-N.

Guo

C.-X.

Jin

L.-Y.

Wang

Y.-J.

J.-Q.

Luo

M.-F.

Nano-sized CeO₂ with extra-high surface area and its activity for CO oxidation

Materials Letters 2010 64

5945169

14 1638 1640

10.1016/j.matlet.2010.04.018

2-s2.0-77953134696

47.

Moog

Feral-Martin

Duttine

Wattiaux

Prestipino

Figueroa

Majimel

Demourgues

Local organization of Fe3+ into nano-CeO2 with controlled morphologies and its impact on reducibility properties

Journal of Materials Chemistry A 2014 2

5945169

47 20402 20414

10.1039/C4TA02631D

2-s2.0-84910002350

48.

Nhiem

D. N.

Dai

L. M.

Van

N. D.

Lim

D. T.

Catalytic oxidation of carbon monoxide over nanostructured CeO2-Al2O3 prepared by combustion method using polyvinyl alcohol

Ceramics International 2013 39

5945169

3 3381 3385

10.1016/j.ceramint.2012.08.069

2-s2.0-84872981407

49.

Seibert

Grégr

Kejzlar

The preparation of iron oxide nanoparticles by a self-combustion method

Manufacturing Technology 2019 19

5945169

4 680 684

10.21062/ujep/355.2019/a/1213-2489/MT/19/4/680

50.

Pullar

R. C.

Novais

R. M.

Caetano

A. P.

Barreiros

M. A.

Abanades

Oliveira

F. A. C.

A review of solar thermochemical CO2 splitting using ceria-based ceramics with designed morphologies and microstructures

Frontiers in Chemistry 2019 7, article 601

5945169

10.3389/fchem.2019.00601

2-s2.0-85072839490

31552219

51.

Brackmann

Toniolo

F. S.

Schmal

Synthesis and characterization of Fe-doped CeO2 for application in the NO selective catalytic reduction by CO

Topics in Catalysis 2016 59

5945169

19-20 1772 1786

10.1007/s11244-016-0698-4

2-s2.0-84998679070

52.

Gupta

Kumar

Waghmare

U. V.

Hegde

M. S.

Origin of activation of lattice oxygen and synergistic interaction in bimetal-ionic Ce0.89Fe0.1Pd0.01O2−δCatalyst

Maternité 2009 21

5945169

20 4880 4891

10.1021/cm901906e

2-s2.0-70350259955

53.

Cao

J.-L.

Wang

X.-L.

Wang

S.-R.

S.-H.

Yuan

Z.-Y.

Mesoporous CuO–Fe2O3 composite catalysts for low-temperature carbon monoxide oxidation

Applied Catalysis B: Environmental 2008 79

5945169

1 26 34

10.1016/j.apcatb.2007.10.005

2-s2.0-37549053705

54.

Biabani-Ravandi

Rezaei

Fattah

Catalytic performance of Ag/Fe2O3 for the low temperature oxidation of carbon monoxide

Chemical Engineering Journal 2013 219

5945169

124 130

10.1016/j.cej.2012.12.094

2-s2.0-84873171857

55.

Tana

M. Z.

Shen

Morphology-dependent redox and catalytic properties of CeO2 nanostructures: nanowires, nanorods and nanoparticles

Catalysis Today 2009 148

5945169

1-2 179 183

10.1016/j.cattod.2009.02.016

2-s2.0-74249111891

56.

Zhang

Hou

Yang

Wang

Liu

Chen

Yang

A facile synthesis for cauliflower like CeO2 catalysts from Ce-BTC precursor and their catalytic performance for CO oxidation

Applied Surface Science 2017 423

5945169

771 779

10.1016/j.apsusc.2017.06.235

2-s2.0-85021660253

57.

J. C.

Kwong

Mak

A. C.

Lai

Morphology-controllable synthesis of mesoporous CeO2 nano- and microstructures

Chemistry of Materials 2005 17

5945169

17 4514 4522

10.1021/cm0507967

2-s2.0-24944456806

58.

Kerkar

R. D.

Salker

A. V.

Significant effect of multi-doped cerium oxide for carbon monoxide oxidation studies

Materials Chemistry and Physics 2020 253

5945169

123326

10.1016/j.matchemphys.2020.123326

Synthesis of CeO 2 -Fe 2 O 3 Mixed Oxides for Low-Temperature Carbon Monoxide Oxidation

Abstract

1. Introduction

2. Chemicals and Experiments

2.1. Preparation of Mixed Oxide Catalysts

2.2. Analytics

2.2.1. Differential Thermal Analysis and Thermogravimetric Analysis

2.2.2. X-Ray Diffraction Analysis

2.2.3. Scanning Electron Microscopy Image

2.2.4. Determine the Specific Surface Area

2.2.5. Fourier Transform Infrared Spectra

2.2.6. Determination of CO Conversion

3. Results and Discussions

3.1. DTA and TGA Spectra of CeO2-Fe2O3 Mixed Oxides

3.2. FTIR Spectra of CeO2-Fe2O3 Mixed Oxides

3.3. The Role of Calcination Temperature and Metal Ion Molar Ratio in the Structural Phase Formation of CeO2-Fe2O3 Mixed Oxides

3.3.1. The Role of Calcination Temperature

3.3.2. The Role of Metal Ion Molar Ratio

3.4. Morphology of Fe2O3-CeO2 Mixed Oxides

3.5. Process of CO Conversion to CO2

4. Conclusion

Footnotes

Data Availability

Conflicts of Interest

Acknowledgments

References

3.1. DTA and TGA Spectra of CeO₂-Fe₂O₃ Mixed Oxides

3.2. FTIR Spectra of CeO₂-Fe₂O₃ Mixed Oxides

3.3. The Role of Calcination Temperature and Metal Ion Molar Ratio in the Structural Phase Formation of CeO₂-Fe₂O₃ Mixed Oxides

3.4. Morphology of Fe₂O₃-CeO₂ Mixed Oxides

3.5. Process of CO Conversion to CO₂