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

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).

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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