Novel Nano-Fe2O3-Co3O4 Modified Dolomite and Its Use as Highly Efficient Catalyst in the Ozonation of Ammonium Solution

Abstract

Catalytic ozonation is a new method used for removal of NH₄OH solution. Therefore, high catalytic performance (activity and selectivity) should be achieved. In this work, we report the synthesis and catalytic performance of Fe₂O₃-Co₃O₄ modified dolomite in the catalytic ozonation of NH₄OH solution. Dolomite was successfully activated and modified with Fe₂O₃ and Co₃O₄. Firstly, dolomite was activated by heating at 800°C for 3 h and followed by KOH treatment. Activated dolomite was modified with Fe₂O₃ by the atomic implantation method using FeCl₃ as Fe source. Fe₂O₃ modified dolomite was further modified with Co₃O₄ by precipitation method. The obtained catalysts were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), N₂ adsorption–desorption (BET), and temperature-programmed reduction (H₂-TPR). From SEM image, it was revealed that nano-Fe₂O₃ and Co₃O₄ particles with the size of 80–120 nm. Catalytic performance of activated dolomite, Fe₂O₃ modified dolomite, and Fe₂O₃-Co₃O₄ modified dolomite in catalytic ozonation of NH₄⁺ solution was investigated and evaluated. Among 3 tested catalysts, Fe₂O₃-Co₃O₄ modified dolomite has the highest NH₄⁺ conversion (96%) and N₂ selectivity (77.82%). Selectivity toward N₂ over the catalyst was explained on the basis of bond strength M-O in oxides through the standard enthalpy $\Delta {H^{°}}_{\text{f}}$ of oxide. Catalyst with lower $\Delta {H^{°}}_{\text{f}}$ value has higher N₂ selectivity and the order is the following: Co₃O₄ ($\Delta {H^{°}}_{\text{f}}$ of 60 kcal (mole O)) > Fe₂O₃ ($\Delta {H^{°}}_{\text{f}}$ of 70 kcal (mole O)) > MgO ($\Delta {H^{°}}_{\text{f}}$ of 170 kcal (mole O)). Moreover, high reduction ability of Fe₂O₃-Co₃O₄ modified dolomite could improve the N₂ selectivity by the reduction of NO₃^- to N₂ gas.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Agricultural and industrial effluents containing ammonia (NH₄⁺-N) discharged from production operation, dairy processor, and fertilizer plants could reduce water quality and cause a danger to human health [1]. Methods for removal of NH₄OH solution such as biological nitrification processes, ion exchange, membrane separation, chlorination, and catalytic degradation have often been used [2–6]. However, ion exchange and membrane separation methods require expensive operating costs and can cause secondary pollution, while biological methods require a long treatment time and strict operation control [3–5]. An alternative approach is the catalytic ozonation. Liu et al. investigated the employing of MgO as an effective catalyst for the catalytic ozonation of NH₄⁺ solution. This MgO catalyst exhibited high NH₄OH conversion (95%) but the N₂ selectivity was very low; nitrites and nitrates were the main products [6]. Ichikawa et al. have examined a series of metal oxides such as MgO, Co₃O₄, Fe₂O₃, and CuO for catalytic ozonation of NH₄⁺ ion to nitrogen gas (N₂O, N₂) and found that Co₃O₄ exhibited the highest N₂ selectivity as compared to that of MgO, CuO, and Fe₂O₃ [4]. Liu et al. demonstrated that the SrO-Al₂O₃ composite was an effective catalyst for ultrasound-assisted ozonation of NH₄⁺ solution. NH₄⁺ conversion of 83.2% and gaseous nitrogen selectivity of 51.8% were achieved [7]. Chen et al. 2018 reported the synthesis of MgO-Co₃O₄ (molar ratio 8 : 2) and claimed that an ammonia nitrogen removal and gaseous nitrogen selectivity reached to the value of 85.2% and 44.8%, respectively [8].

Dolomite is a mineral clay composed of calcium magnesium carbonate CaMg(CO₃)₂ [9]. However, raw dolomite cannot be used as adsorbent and catalyst due to its low surface and high impurities. Therefore, the activation or modification of dolomite is necessary to expand its application. Chaudhary and Prasad reported that thermally activated dolomite could enhance its surface area, pore size distribution, and pore volume, consequently increasing fluoride removal from aqueous solution [10]. Modified dolomite can be used as an efficient adsorbent for removal of arsenic and heavy metals and dyes from aqueous solution [11]. Thermally activated dolomite can be used as an efficient catalyst for biodiesel production and decomposition of pentachlorophenol [12, 13]. To our best knowledge, Fe₂O₃-Co₃O₄ modified dolomite used as an efficient catalyst in catalytic ozonation of NH₄⁺ solution has not yet been reported in the literature.

In this work, we report the activation and Fe₂O₃ and Fe₂O₃-Co₃O₄ modification of dolomite. Activated and Fe₂O₃ and Fe₂O₃-Co₃O₄ modified dolomites were tested in the catalytic ozonation of NH₄OH solution, and catalytic ozonation performances of activated and modified dolomites were evaluated and rationalized.

2. Materials and Methods

2.1. Materials

The natural dolomite was provided by Ninh Binh mineral company Vietnam. Iron(III) chloride hexahydrate (FeCl₃·6H₂O, 98%), cobalt(II) chloride hexahydrate (CoCl₂·6H₂O, 98%), ammonium chloride (NH₄Cl, 99%), cetyltrimethylammonium bromide (CTAB, ≥98%), and natri hydroxit (NaOH, 97%) were provided by Sigma Corporation.

2.2. Materials

2.2.1. Activation of Raw Dolomite

Firstly, 10 g of dolomite was activated by heating at 800°C for 3 h in a furnace with a heating rate of 10°C/min. After calcination, CaMg(CO₃)₂ was converted into CaCO₃, Ca(OH)₂, and MgCO₃ as the following reactions: $$\begin{array}{c}\text{(1)}& \text{CaMg}{\left({\text{CO}}_3\right)}_2\to {\text{CaCO}}_3+{\text{MgCO}}_3{\text{CaCO}}_3\to \text{CaO}+{\text{CO}}_2\\ {\text{MgCO}}_3\to \text{MgO}+{\text{CO}}_2\end{array}$$

Activated dolomite was treated with a solution of 10 M KOH for 24 h at 80°C. The solid product was separated from the mixture by centrifuge and then was washed several times with distilled water until a pH of 7 and dried overnight at 80°C in a furnace.

2.2.2. Modification of Activated Dolomite with Fe₂O₃

The incorporation of Fe ions into activated dolomite framework (denoted as Fe₂O₃/dolomite) was carried out by using an “atomic implantation” method. Atomic implantation has occurred in a tubular quartz reactor with two compartments, separated by a membrane of quartz fibers. 0.483 g of FeCl₃·6H₂O and 0.9 g of activated dolomite are introduced into each compartment. Tubular quartz reactor is placed in a tubular furnace and heated up at 500°C with a heating rate of 20°C/min. At this temperature, FeCl₃ is evaporated and with N₂ flow (60 mL/min) went through the membrane to the compartment containing the activated dolomite (Scheme 1). At this stage, FeCl₃ is dissociated into Fe³⁺, Cl^-, and Fe³⁺ ions incorporated into an activated dolomite matrix. After one hour of heating at 500°C, the reactor is cooled down to the room temperature. The scheme of modifying dolomite by the atomic implantation method is illustrated as below.

[figure omitted; refer to PDF]

2.2.3. Modification of Fe₂O₃/Dolomite with Co₃O₄ (Denoted as Fe₂O₃-Co₃O₄/Dolomite) Was Performed by Precipitation Method

0.8 g of Fe₂O₃/dolomite was put into a glass reactor that contained 50 mL of H₂O (solution 1). After that, 0.8068 g of CoCl₂·6H₂O was added into 50 mL of H₂O and ultrasonic treatment for 60 minutes (solution 2). 2.4708 g of CTAB was put into 20 mL of H₂O, and then, the mixture was stirred to form a clear solution (solution 3). Solution 2 was dropwise added to solution 1, and then, solution 3 was dropwise added to the above mixture solution $1+2$ and adjusted pH of 10 by adding 2 M NaOH solution. The mixture is further stirred and aged at 80°C for 24 h. The solid product is separated by centrifuge filtration and washed with distilled water until pH of 7. The product is dried at 90°C and calcined at 450°C for 3 h and then cooled down to room temperature.

2.3. Characterization

The X-ray diffraction (XRD) measurements were performed on a D8 Advance diffractometer (Bruker, Germany) using CuK_α as radiation source, $\lambda =0.15406$ nm, and a range of $2\theta ={10}^{°}–{80}^{°}$. The morphology of samples was examined by scanning electron microscopy on JEOL JSM 6500F. The FT-IR measurement was performed on a Jasco 4700 spectrometer. The surface area of samples was determined by BET measurements on Tristar-3000 instruments using nitrogen adsorbate. EDS of samples were measured on a JEOL JED-2300 spectrometer. Temperature-programmed reduction of hydrogen (H₂-TPR) was performed on an Autochem II 29020 instrument (micromeritics) with a thermal conductor detector (TCD).

2.4. Evaluation of Catalytic Activity and Selectivity

Catalytic ozonation of NH₄⁺ solution is carried out in a glass reactor equipped with heating and stirring systems. The reaction conditions are the following: reaction temperature of 50°C, NH₄⁺ solution of 50 mg/L, catalyst dosage of 0.6 g/L, reaction time of 2 h, ozone flow of 30 mg/min, and NH₄Cl used as NH₄⁺ source. NH₄⁺ concentration was analyzed by the Nessler method. 1 mL of Nessler reagent is added to 25 mL of the mixture of reaction product. NO₃^- and NO₂^- concentration was determined by using the spectrometric methods, which complies to ISO (ISO 7890-3:2006 and ISO 6777-1984). The solution was transferred to the cuvette of the spectrophotometer (UV-Vis Lambda 25, Germany) and measured under UV light at a wavelength of 425 nm [14–16].

The conversion and selectivity are calculated as follows: $$\begin{array}{c}\text{(2)}& \text{Conversion}\text{of}\text{N}{{\text{H}}_4}^{+}=\frac{\text{reacted}\text{N}{{\text{H}}_4}^{+}\left(\text{mole}\right)}{\text{initial}\text{N}{{\text{H}}_4}^{+}\left(\text{mole}\right)}.100\%\text{Selectivity}\text{of}\text{N}{{\text{O}}_3}^{-}=\frac{\text{N}{{\text{O}}_3}^{-}\left(\text{mole}\right)}{\text{reacted}\text{N}{{\text{H}}_4}^{+}\left(\text{mole}\right)}.100\%\\ \text{Selectivity}\text{of}{\text{N}}_2=\frac{{\text{N}}_2\left(\text{mole}\right)}{\text{reacted}\text{N}{{\text{H}}_4}^{+}\left(\text{mole}\right)}.100\%\end{array}$$

3. Results and Discussion

3.1. X-Ray Diffraction

In X-ray diffraction pattern (XRD) of raw dolomite (Figure 1(a)) appeared the peaks at 30.9°, 41.1°, 44.9°, and 51.3° which are characteristic for CaMg(CO₃)₂. In addition, the appearance of peaks at 2θ of 23°, 29.6°, 36°, 39.6°, 43.2°, 47.6°, 48.5°, 57.3°, and 60.8° was assigned to CaCO₃ phase [17]. In the XRD pattern of activated dolomite (Figure 1(b)), specific peaks of CaMg(CO₃)₂ phase disappear. The new peaks at 2θ of 42.8° and 61.9° appear which is attributed to the reflecting plane (101) and (103) which are typical for the structure of MgO [18]. In the XRD pattern of Fe₂O₃/dolomite (Figure 1(c)) appeared typical peaks of Fe₂O₃ phase at 2θ of 33.1°, 35°, 49.5°, and 54° which is attributed to the reflecting plane (104), (110), (024), and (116), respectively [19]. In the XRD pattern of Fe₂O₃-Co₃O₄/dolomite (Figure 1(d)), beside the peaks of Fe₂O₃ phase, appear the peaks at 2θ of 37.9°, 44.8°, 55.6°, 59.3°, and 65.2^o which are assigned to the reflecting plane (311), (400), (422), (511), and (440) of the Co₃O₄ phase, respectively [20].

[figure omitted; refer to PDF]

3.1.1. Fourier-Transform Infrared Spectroscopy

As observed in Figure 2(a), in the FTIR spectrum of dolomite (raw ores) appeared the bands at 720; 873; 1429; 1797; 2516; and 2868 cm^-1 which are assigned to the occurrence of fluctuations in dolomite structure [21, 22]. In the FTIR spectrum of activated dolomite (Figure 2(b)) appeared the band at 3448 cm^-1 which was assigned to the vibration of OH groups [23]. In addition, the peak at 428 cm^-1 is attributed to the stretching of Mg-O bonds [24]. In the FTIR spectrum of Fe₂O₃/dolomite (Figure 2(c)) appeared the bands at 548 and 638 cm^-1 which are characteristic for Fe-O bonding [25, 26]. In the FTIR spectrum of Fe₂O₃-Co₃O₄/dolomite (Figure 2(d)) additionally appeared the band at 666 cm^-1, which assigned to the stretching of Co-O bond [27, 28].

[figure omitted; refer to PDF]

3.1.2. Energy-Dispersive X-Ray Spectroscopy Analysis

From Figure 3 and Table 1, it was observed that the chemical composition of activated dolomite was slightly changed in O and Ca content between raw dolomite and activated dolomite. Thus, oxygen content decreases and calcium content increases. This is due to the decomposition of CaCO₃ to CaO and CO₂. Comparing the chemical composition of activated, Fe₂O₃/dolomite and Fe₂O₃-Co₃O₄/dolomite, the redistribution of elements in these samples was observed. Oxygen and calcium content were reduced and replaced by Fe and Co content.

[figures omitted; refer to PDF]

Table 1

Element composition (wt%) of dolomite (raw ores), activated dolomite, Fe₂O₃/dolomite, and Fe₂O₃-Co₃O₄/dolomite samples.

Sample	Dolomite (raw ores) (% weight)	Activated dolomite (% weight)	Fe2O3/dolomite (% weight)	Fe2O3-Co3O4/dolomite
				% weight	% atomic percentage
O	61.50	57.98	51.15	44.19	58.47
Mg	11.55	11.98	9.91	10.24	9.03
Ca	16.89	19.96	16.89	4.59	2.43
C	10.06	10.09	12.67	10.74	18.95
Cl	—	—	0.35	0.37	0.22
Fe	—	—	9.59	10.06	3.63
Co	—	—	—	20.27	7.27
Total	100	100	100	100	100

3.1.3. Scanning Electron Microscopy (SEM) Analysis

SEM images of raw dolomite, activated dolomite, Fe₂O₃/dolomite, and Fe₂O₃-Co₃O₄/dolomite samples were illustrated in Figure 4. SEM image of raw dolomite (Figure 4(a)) showed the rough surface with high disorder and particles size of about 2 μm. Activated dolomite (Figure 4(b)) shows a smaller particle size of ca. 0.2-0.4 μm with uniform distribution. The strong reduction in size of dolomite can be explained by the fact that the thermal cracking of dolomite fragments and decomposition of carbonates to magnesium and calcium oxide and CO₂. SEM image of Fe₂O₃/dolomite (Figure 4(c)) presented crystals of sheet shape, with particle size of ca. 0. .3 μm. SEM image of Fe₂O₃-Co₃O₄/dolomite (Figure 4(d)) showed the nano-Co₃O₄ particles size of 80-120 nm, with aggregated form of 0.2-0.3 μm size.

[figures omitted; refer to PDF]

3.1.4. N₂ Adsorption–Desorption Isotherm (BET) Analysis

N₂ adsorption–desorption isotherms of raw dolomite, activated dolomite, Fe₂O₃/dolomite, and Fe₂O₃-Co₃O₄/dolomite samples are presented in Figure 5. N₂ adsorption–desorption isotherms of all dolomite samples look like a type IV according to the IUPAC classification [29]. However, the hysteresis loop on activated, Fe₂O₃/dolomite, and Fe₂O₃-Co₃O₄/dolomite samples was differentiated. This hysteresis loop is due to the capillary condensation of nitrogen which is often observed on mesoporous materials. Specific surface charge ($S_{\text{BET}}$), pore diameter ($D_{\text{BJH}}$), and pore volume ($V_{\text{pore}}$) are listed in Table 2.

[figures omitted; refer to PDF]

Table 2

Textual characteristics of dolomite (raw ores), activated dolomite, Fe₂O₃/dolomite, and Fe₂O₃-Co₃O₄/dolomite samples.

Sample	$S_{\text{BET}}$ (m2/g)	$V_{\text{pore}}$ (cm3/g)	$D_{\text{BJH}}$ (nm)
Dolomite (raw ores)	2.0	0.008	18.35
Activated dolomite	22	0.159	32.31
Fe2O3/dolomite	39	0.206	21.87
Fe2O3-Co3O4/dolomite	60	0.262	16.65

As seen in Table 2, $S_{\text{BET}}$, $V_{\text{pore}}$, and $D_{\text{BJH}}$ of activated dolomite were much higher than those of raw dolomite. This indicated the efficiency of activation, making dolomite structure more porous and consequently increasing the surface area, pore volume, and pore diameter. Modification of activated dolomite by Fe₂O₃ and Co₃O₄ led to increase the surface are and pore volume (from 22 m²/g to 60 m²/g). This clearly indicated the formation of new mesoporous systems by the agglomeration of nano-Fe₂O₃ and Co₃O₄ particles. The decrease in pore diameter observed on Fe₂O₃/dolomite and Fe₂O₃-Co₃O₄/dolomite can be explained by the fact that nano-Fe₂O₃ and Co₃O₄ particles are located within the pores of activated dolomite, causing the narrowing pore diameter.

3.1.5. H₂-TPR Analysis

In the H₂-TPR profile of activated dolomite material (Figure 6(a)) appeared a peak at ca. 706°C which was assigned to the reduction of MgO to Mg [5]. In the H₂-TPR profile of Fe₂O₃/dolomite (Figure 6(b)) appeared peaks at 318°C, 408°C, 550°C, and 689°C which are characteristic for the reduction Fe₂O₃ to Fe₃O₄, reduction of Fe₃O₄ to FeO, and the reduction of MgO to Mg, respectively [5, 30]. H₂-TPR profile of Fe₂O₃-Co₃O₄ modified dolomite (Figure 6(c)) shows peaks at 328°C, 420°C, 548°C, and 688°C which is attributed to the reduction of Co³⁺ to Co²⁺, Co²⁺ to Co^o, Fe₃O₄ to FeO, and MgO to Mg [30–32]. From the obtained results, it can be concluded that the Fe₂O₃ and Fe₂O₃-Co₃O₄ modified dolomite exhibits much higher reduction ability as compared to that of activated dolomite. Thus, much lower reduction temperature of modified dolomites (320-420°C) than that of activated dolomite (680-730°C) was noted. The Fe₂O₃-Co₃O₄/dolomite exhibited the highest peak intensity at 318-340°C, indicating a larger amount of H₂ reduction as compared to that of Fe₂O₃ modified dolomite. This lowest reduction temperature of Fe₂O₃-Co₃O₄ modified dolomite is due to the fact that M-O strength of this catalyst is weak. The high reduction ability of this catalyst played a decisive role in the improvement of N₂ selectivity, by enhancing the reduction of NO₃^- to N₂.

[figure omitted; refer to PDF]

3.1.6. Catalytic Ozonation of NH₄⁺ Solution

NH₄⁺ conversion to NO₃^- and N₂ overactivated and modified dolomites is presented in Figure 7. As seen in Figure 7, all samples exhibited high NH₄⁺ conversion (86-96%) and among these catalysts, Fe₂O₃-Co₃O₄/dolomite has the highest activity (conversion of 96%). The N₂ selectivity is decreased in the order activated dolomite (45%) < Fe₂O₃/dolomite (65.13%) < Fe₂O₃-Co₃O₄/dolomite (77.68%) and NO₃^- selectivity is increased in the order: activated dolomite (54.32%) > Fe₂O₃/dolomite (34.87%) > Fe₂O₃-Co₃O₄/dolomite (22.32%). Among the tested samples, Fe₂O₃-Co₃O₄/dolomite exhibited the highest N₂ selectivity of 77.68% and lowest NO₃^- selectivity of 22.32%. The result obtained in this work is better than that reported in the literature. Thus, Chen et al. [8] have shown the N₂ selectivity of 44.8% over MgO-Co₃O₄ catalyst in the catalytic ozonation of ammonium solution. Liu et al. [7] have shown the conversion of 83.2% and gaseous nitrogen selectivity of 51.8% on SrO-Al₂O₃ catalyst in the catalytic ozonation of ammonium solution. The mechanism of catalytic ozonation of NH₄⁺ solution is involved with the following process. The ozonation reaction was conducted at pH of 9, and ammonium exists as NH₃ molecule, (Equation (3)). At pH of 9, the ozonation catalyzes the formation of HO₂^· and OH radical, according to Equations (4) and (5) [8]. $$\begin{array}{}\text{(3)}& \text{N}{{\text{H}}_4}^{+}+\text{O}{\text{H}}^{‐}\to \text{N}{\text{H}}_3+{\text{H}}_2\text{O}\text{(4)}& {\text{O}}_3+{\text{OH}}^{‐}\to \text{H}{\text{O}}_2·+·{{\text{O}}_2}^{‐}\text{(5)}& {\text{O}}_3+\text{H}{\text{O}}_2·\to ·\text{OH}+2{\text{O}}_2\end{array}$$

[figure omitted; refer to PDF]

The decomposition of NH₃ in water through ozonation with a metal oxide catalyst is performed by hydroxyl radicals (Equation (6)). In addition, the hydroxyl radical oxidizes NH₃ in the solution to intermediate product nitrite, which rapidly oxidizes to nitrate (Equation (7)) and (Equation (8)) [33]. NH₃ in solution can be oxidized to nitrate nitrogen by hydroxyl radical (Equation (9)). $$\begin{array}{}\text{(6)}& 2\text{N}{\text{H}}_3+·\text{OH}\to {\text{N}}_2+{\text{H}}_2\text{O}+5{\text{H}}^{+}\text{(7)}& \text{N}{\text{H}}_3+6^{·}\text{OH}\to \text{N}{{\text{O}}_2}^{‐}+4{\text{H}}_2\text{O}+3{\text{H}}^{+}\text{(8)}& \text{N}{{\text{O}}_2}^{‐}+2^{·}\text{OH}\to \text{N}{{\text{O}}_3}^{‐}+{\text{H}}_2\text{O}\text{(9)}& \text{N}{\text{H}}_3+8^{·}\text{OH}\to \text{N}{{\text{O}}_3}^{‐}+5{\text{H}}_2\text{O}+{\text{H}}^{+}\end{array}$$

In summary, the reactions occurring during ammonium decomposition by catalytic ozonation are as follows: $$\begin{array}{}\text{(10)}& \text{N}{{\text{H}}_4}^{+}+3/2{\text{O}}_3\to 1/2{\text{N}}_2+3/2{\text{H}}_2\text{O}+3/2{\text{O}}_2+{\text{H}}^{+}\text{(11)}& \text{N}{{\text{H}}_4}^{+}+3{\text{O}}_3\to \text{N}{{\text{O}}_2}^{‐}+{\text{H}}_2\text{O}+3{\text{O}}_2+2{\text{H}}^{+}\text{(12)}& \text{N}{{\text{H}}_4}^{+}+4{\text{O}}_3\to \text{N}{{\text{O}}_3}^{‐}+{\text{H}}_2\text{O}+4{\text{O}}_2+2{\text{H}}^{+}\end{array}$$

According to the mentioned reactions, in order to obtain high N₂ selectivity, reactions (11) and (12) should be suppressed. Ichikawa et al. studied the relationship between N₂ selectivity and standard enthalpy change of formation per mole of oxygen atom ($\Delta {H^{\text{o}}}_{\text{f}}$) of the metal oxides. Ichikawa et al. found that the N₂ selectivity was high for the catalyst with a low value of $\Delta {H^{\text{o}}}_{\text{f}}$ and N₂ selectivity was low for the catalyst with high value of $\Delta {H^{\text{o}}}_{\text{f}}$. For example, Co₃O₄ with $\Delta {H^{\text{o}}}_{\text{f}}$ of 60 kcal (mole of O)^-1 has the highest N₂ selectivity of 90% and MgO with $\Delta {{\text{H}}^{\text{o}}}_{\text{f}}$ of 140 kcal (mole O)^-1 has the lowest N₂ selectivity of 20 kcal (mole O)^-1 [4]. They claimed that $\Delta {H^{\text{o}}}_{\text{f}}$ indicates the bond strength between the metal cation (M) and lattice oxygen (M-O) in the metal oxides. Thus, the metal oxides with low $\Delta {H^{\text{o}}}_{\text{f}}$ value have weak M-O bond strength. This means the weak bond strength between M on the surface and active oxygen (O$\ast$) formed by the reaction of O₃ with the surface. O$\ast$ would be unstable, easy to participate in the NH₄⁺ oxidation. Since NH₄Cl was used as a NH₄⁺ source, Cl^- in solution could participate in the NH₄⁺ oxidation. Cl^- anions could react with O₃ to form ClO^- which oxidized NH₄⁺ ion to nitrogen [4, 7]. To investigate the stability of the catalysts, we carry out three cycles of catalytic ozonation of NH₄⁺ solution over Fe₂O₃-Co₃O₄/dolomite, and the result is presented in Figure 8.

[figure omitted; refer to PDF]

As observed in Figure 8, NH₄⁺ conversion over Fe₂O₃-Co₃O₄/dolomite after 3 cycles was slightly decreased, less than 5% as compared to that of the fresh catalyst. N₂ selectivity over Fe₂O₃-Co₃O₄/dolomite after 3 cycles of reaction decreased only ca. 7-8% as compared to that of the fresh catalyst, while NO₃^- selectivity slightly increased (ca. 5%). From the obtained result, it could confirm that the Fe₂O₃-Co₃O_4/dolomite has high activity stability and it can be reused.

To check the change of structure and morphology of the Fe₂O₃-Co₃O₄/dolomite after 3 cycles of reaction, XRD pattern and SEM image of the spent catalyst were performed.

From XRD patterns (Figures 9(a) and 9(b)) and SEM images (Figures 9(c) and 9(d)), it can be seen that the structure and morphology of the spent catalyst were maintained; no change of metal oxide phase as well as agglomerating of metal oxide particles were noted. Based on the above results, the high catalytic activity and stability of the Fe₂O₃-Co₃O₄/dolomite catalyst could be proved.

[figure omitted; refer to PDF]

4. Conclusions

From the obtained results, some conclusions could be drawn.

It was successful to modify dolomite with Fe₂O₃ by using the “atomic implantation” method. By this method, nano-Fe₂O₃ particles with small size and high dispersion on dolomite surface were achieved.

It was also successful to modify Fe₂O₃/dolomite with Co₃O₄ by the precipitation method. As seen in SEM images, nano-Fe₂O₃, Co₃O₄ particles of 80-130 nm with high dispersion on dolomite surface were obtained.

Catalytic performance of activated dolomite, Fe₂O₃/dolomite, and Fe₂O₃-Co₃O₄/dolomite in catalytic ozonation of NH₄⁺ solution was investigated and evaluated. Among three catalysts, Fe₂O₃-Co₃O₄/dolomite has the highest NH₄⁺ conversion (96%) and N₂ selectivity (77.82%). Selectivity toward N₂ over the catalyst can be rationalized by bond strength M-O through the standard enthalpy $\Delta {H^{°}}_{\text{f}}$. The catalyst with low $\Delta {H^{°}}_{\text{f}}$ value has higher N₂ selectivity. Moreover, the H₂ reduction ability also plays an important role in the improvement of N₂ selectivity by the reduction of NO₃^- to N₂ gas.

Acknowledgments

The authors thank Vietnam Academy of Science and Technology (VAST.07.01/18−19) and the Institute of Chemistry (VHH. 2019.01.02) for the financial support.

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