NOx is a typical pollutant, which is produced through the combustion process of fossil fuels such as coal, petroleum, etc., and is discharged into the atmosphere [1]. Many researchers have conducted various studies to efficiently remove NOx, and a relatively widely applied technique is selective catalytic reduction (SCR) [2]. However, the selective catalytic reduction process requires a high temperature in order to increase the process efficiency while the catalyst is being applied. In the selective catalytic reduction process, NO is difficult to convert, which is a reason why a high temperature is required. Further, a process for oxidizing NO to NO₂ or the like is required to relatively easily process the NO. In this study, the decomposition of H₂O₂ catalysts, which can produce dry oxidants for the oxidation process to convert NO to NO₂, has been studied. Recently, a number of studies have been reported on various types of research results that can produce dry oxidants through H₂O₂ decomposition reaction on heterogeneous catalysts containing transition metals Fe and Zr as main components [3-5]. In addition, the H₂O₂ catalytic cracking reaction proceeds at a relatively low temperature to lower the operating temperature, and when the dry oxidant is applied to the oxidation process, the oxidation efficiency at a low temperature can be obtained [5]. It is believed that this simple facility can complement the disadvantages of the NO treatment process applied by the selective catalytic reduction method through the implementation of the process that can operate at low temperature with high efficiency [6-8]. In this study, H₂O₂ catalytic cracking was applied in order to convert the NO gas, which is difficult to process, into a dry oxidant that can be used simultaneously with the introduction of NO oxidation process. Therefore, the effect of the physico-chemical properties of the catalysts on the degradation of H₂O₂ catalysts was investigated by applying the catalysats having various physical properties in the H₂O₂ catalytic cracking process. The NO conversions characteristics were investigated. The reaction scheme for the production of dry oxidant by the decomposition of H₂O₂ using catalyst is shown in Equation (1), and the NO oxidation reaction formula is shown in (2) to (5) [9-11].

H₂O₂+M(II) →·OH⁺·OH (1)

NO+OH→HNO₂ (2)

NO+OH→NO₂+H (3)

NO+OH→HNO₃ (4)

NO₂+OH→NO₃^-+H⁺ (5)

Catalyst production

Figure 1 shows the process in which γ-Al₂O₃ is used as a carrier and the catalyst carrying Mn-based active material is prepared by the precipitation method. Mn(NO₃)₂ (reagent grade, Aldrich Co., 98% or more) was used as a precursor of the active material. A certain amount of Mn(NO₃)₂ was dissolved in distilled water in a beaker, and γ-Al₂O₃ as a carrier was mixed. Thereafter, precipitation process was carried out by dropping the NH₄OH solution as a precipitant until the pH value of the slurry mixture prepared in the above was within the range of 9 to 10. The precipitated mixture in the slurry state was aged by stirring in a bath at about 80°C for about 4 h, then dried at 110°C for 24 h and fired at about 700°C for 4 h. The obtained Mn-based γ-Al₂O₃ catalyst, which is a solid product, was pulverized and the catalyst performance evaluation was carried out using a particle size ranging from about 100 to 150 μm through a sieving process. Figure 2 shows the preparation process of Mn-based catalyst with K component using Fe₂O₃ as a support. Fe₂O₃ (Powder, 4 (reagent grade, Aldrich Co., 98%) was used. First, the Fe₂O₃ support was mixed with distilled water through a mixing process to form a slurry state, and then mixed with a KMnO₄ aqueous solution as a precursor of the active material. The mixture was then dried at 110 DEG C for about 12 h and then calcined at 700 DEG C for about 4 h to produce a Mn-based Fe₂O₃ support catalyst with a K component added thereto.

Figure 1: Preparation of Mn-Al₂O₃ catalysts.

Figure 2: Preparation of K-Mn/Fe₂O₃ catalysts.

For the preparation of SO₄^2-/ZrO₂ catalyst, Zr (OC₃H₇)₄ (reagent grade, Aldrich Co., 9 8% or more) was precipitated with ammonia water and dried at 110°C for 12 h. The dried precipitate was mixed with 1 N H₂SO₄ Aging for 12 h, drying at 120°C for 12 h, and calcining at 500°C.

Analysis of catalyst characteristics

The effect of various physical and chemical properties of various catalysts on H₂O₂ decomposition was investigated by various techniques as follows. XRD analysis was performed to investigate the crystal structure and properties of the prepared catalysts. The quantitative and qualitative analysis of the elements constituting the catalysts was performed using EDX (Energy Dispersive X-ray Spectrometer for FE-SEM, FISONS Co., LEO SUPRA 55, GENESIS 2000 (Carl Zeiss, EDAX), and resolution: 138 eV/5B-92U. X-ray photoelectron spectroscopy (X-ray photoelectron spectroscopy, K-Alpha model) was used to analyse the qualitative and quantitative analysis of the sample surface and the chemical bonding state of the constituent elements. The temperature programmed desorption (TPD) experiment using NH₃ gas was conducted as follows to investigate the acidity characteristics of the heterogeneous catalysts prepared further. NH₃ gas having a concentration of about 10 volume % is filled in the catalyst bed for 6 hours. After the adsorption process is proceeded, N₂ gas is flowed and the temperature is increased from room temperature to 800°C at a rate of 5°C/min to be desorbed. The acid characterization was analysed by quantitative analysis of NH₃ with a thermal conductivity detector (TCD). In order to investigate the decomposition characteristics of H₂O₂ catalysts, 55 g of H₂O₂ and 0.1 g of the prepared catalyst were reacted on the balance. The weight change of H₂O₂ over time was recorded in 2 s.

Decomposition of H₂O₂ and NO conversion experiment by type of catalyst

The experimental apparatus for evaluating the catalytic performance is composed of the H₂O₂ catalytic cracking process in which a dry oxidant is produced and the NO oxidizing process capable of oxidizing NO by injecting the dry oxidant prepared there from. The H₂O₂ catalytic cracking reaction, which can evaluate the catalytic performance, was carried out by passing H₂O₂ through an appropriate amount of flow rate together with the elevated temperature after a fixed amount of the catalyst was filled in the central portion of the tubular reactor of SUS material having an outer diameter of 1/2 inch. The reaction temperature was controlled through thermal transfer to the inside of the reactor using an automatic temperature controller after locating the heat conduction band (K-type) installed in the electric furnace at the same point as the catalyst filling layer. As the reactant, H₂O₂ (Duksan Co., 28-30%) to be decomposed was heated to be injected into the reactor in a vaporized state in the process of injecting the desired amount by using a metering pump. N₂ was then transferred by using it as carrier gas and injected into the tubular catalytic reactor after mixing in a chambered mixer. The gas used in the experiment was adjusted to the experimental parameters using a calibrated MFC (mass flow controller, Linetech Co.). At the rear end of the reactor, a condenser and an adsorbent loading layer were used to remove moisture at the end of the heat line. The H₂O₂ decomposition efficiency (%) was calculated by substituting into equation 6. The system for the NO oxidation process, which is connected to the downstream of the H₂O₂ catalytic cracking process, was constructed so that the dry oxidant produced by the H₂O₂ decomposition and NO as the main reaction target can be simultaneously supplied at the desired concentration. In the NO oxidation process, a tubular reactor (material: SUS; outer diameter: 1/2 inch, length: 50 cm) filled with a fixed layer of bead type alumina having a diameter of 2.5 mm was used to increase the contact efficiency between the injected dry oxidant and NO gas ) Was used. Reactivity experiments were carried out by simultaneously injecting dry oxidant and NO gas produced by the decomposition of H₂O₂ catalyst into the tubular reactor. At this time, NO was used for the gas composition of 1,000 ppm and N₂ was used as the dilution gas. The concentration values for the basic feed gas are shown in Table 1. The gas flow rate was controlled through a mass flow controller (Linetech Co.) with a calibrate d MFC. The reaction conditions are shown in Table 2. Reactants an d products were monitored by injecting every 5 min on-line to a gas chromatograph (DS science, Korea) equipped with a PDD (Pulsed Discharge Detector). In order to analyze NO and NO₂ with G.C, a SUS separator filled with Hayesep Q was used. The detailed operating conditions for G.C are shown in Table 3. The NO conversion rate (% NO) to obtain the reactivity results was calculated by substituting the concentration of gases such as NO and NO₂ before and after the reaction into the equation (7) as follows.

Table 1: Basic composition in the gas bomb.

Table 2: Basic reaction conditions for the catalytic H₂O₂ decomposition.

Table 3: Basic operation conditions of G.C.

(6)

(7)

Characteristic analysis by catalyst

Table 4 shows the EDX analysis results for the constituents and their contents of K-Mn/Fe₂O₃, K-Mn/Al₂O₃, Mn/Al₂O₃ and ZrO₂ catalysts prepared for H₂O₂ decomposition. The K-Mn/Fe₂O₃ catalysts with K and Mn contents of about 30 and 70 weight %. In support Fe₂O₃ had Mn contents of 5.3 and 10.7 at.%, Fe contents of 17.3 and 10.3 at.%, Respectively, K component is present at about 3.6 and 8.1 at.%. The Mn/ Al₂O₃ catalysts supported on Al₂O₃ support at 10, 30 and 70 weight %, Respectively, increased in the order of 3.5, 7.2 and 13.5% as the loading of Mn component increased. ZrO₂ and SO₄^2-/ZrO₂ catalysts according to surface treatment by sulphuric acid show that the Zr component corresponding to about 25~26 at.% is formed irrespective of surface treatment and the S component was not detected because the content was very small even though it was surface-treated. The amount of acid sites corresponding to the total NH₃ adsorption amount can be determined as an integrated value for a linear graph corresponding to the amount of desorbed NH₃ that varies with temperature. The total amount of acid sites for the NH₃ adsorption is Fe₂O₃22O₃ and Lt; 30 wt.% Mn/Al₂O₃ & lt; K-Mn/Fe₂O₃. Figure 3 shows the intensity and distribution of acid sites along with the total amount of acid sites. As shown in Figure 4 and Table 5, the NH₃ desorption temperature corresponding to the acid strength corresponding to each catalyst is as follows. First, the ZrO₂ catalyst having the lowest acid amount has an acid point corresponding to a relatively low level of about 293°C, The amount of acid sites is also relatively low at 0.138 mmol/g. The 10 wt.% Mn/Al₂O₃ catalyst having a somewhat larger amount of acid sites than the ZrO₂ catalyst also has a total amount of acid sites of 0.889 mmol/g. However, the acid sites are at a desorption temperature of about 294°C as shown in Figure 5. On the other hand, the 30 wt.% Mn/Al₂O₃ catalyst with a few acid sites has relatively high acid sites ranging from about 270°C to 370°C and the acid sites are also 0.392 and 1.424 mmol/g. The K-Mn/Fe₂O₃ catalysts having relatively many acid sites have various acidic sites corresponding to about 217, 297, 355, 399, 452, 689, and 762°C and 1.337 mmol/g, respectively. The K-Mn/Fe₂O₃ catalyst rapidly undergoes electron transfer to determine the reaction rate during the H₂O₂ catalytic cracking reaction. This is due to the fact that the distribution of the Lewis acid sites in the catalyst phase, H₂O₂. It is because it exists in abundance. K is converted to ionic form after reduction and is placed on the carrier or Mn to change the electronic properties of Mn. When K is occupied by the active site of Mn, this active site is decreased and when the NH₃^-TPD is observed, the acid concentration of the catalyst is weakened, so that the distribution of weak acid sites is increased [12]. It is also believed that it is advantageous for the H₂O₂ decomposition reaction because it is supposed to be due to the high electron and oxygen mobility due to the high oxidation state on the MnO₄ structure involved. In the case of using a catalyst based Al₂O₃ carrier-based MnO₂ catalyst, as the MnO₂ content is increased, the catalyst is converted into a relatively high oxidation state corresponding to Mn₂O₃, thereby improving the reactivity, but there is a limit to improve the catalyst performance And it is considered that there is a limit in the catalyst design to enrich the amount and intensity of the Lewis acid point mentioned above. As a result of the performance test of various catalysts containing Mn component, it is considered that the reactivity is low, and it is difficult to expect high activity due to low Mn content, which is the basic active substance on the decomposition of H₂O₂ catalyst. Especially, in case of SO₄^2-/ZrO₂ catalyst, Although a strong acid point is expected, it is considered that it is not suitable for the decomposition of H₂O₂ catalyst due to low performance because it corresponds to the kind of Brӧnsted acid sites at the same time as the absence of Mn component as a basic active material. Figure 5 shows the XRD analysis results for Fe₂O₃, Al₂O₃, K-Mn/Fe₂O₃, and Mn/Al₂O₃ catalysts. Fe₂O₃ catalyst was found to have crystallinity of rhombohedrol structure. On the K-Mn/Fe₂O₃ catalyst prepared by using it as a support, Fe₂O₃ of rhombohedrol structure exists and at the same time, K1.33Mn8O16 compound form and α-MnO₂ compound form. Al₂O₃ is present as a γ-Al₂O₃ compound. On the Mn/Al₂O₃ as a support, γ-Al₂O₃ and K1.33Mn8O16 type compounds are present at the same time, and α-MnO₂ compound is not present. Figure 6 shows the results of XRD analysis of Mn/Al₂O₃ catalyst prepared by supporting Al₂O₃ and Mn as support. Al₂O₃ is present as a compound of γ-Al₂O₃ type. In the case of 10 wt.% Mn/Al₂O₃ catalyst containing 10 wt.% Mn as a support, the morphology of MnO₂ compound is added by XRD analysis As shown in Figure 7. In the case of the 30 wt.% Mn/Al₂O₃ catalyst, γ-Al₂O₃-type compounds and Mn₂O₃-type compounds were present. MnO₂ and α-Mn₂O₃-type compounds could not be confirmed by XRD analysis. As a result, it can be seen that, when the Mn component is supported on the Al₂O₃ support, the Mn component is converted into another oxidation state compound as the Mn content increases. This is because the oxidation, and the oxidation state of the Mn component, which is an active material, is considered to be a major influential factor. Figure 7 shows the XRD analysis of the SO₄^2-/ZrO₂ catalyst prepared by treating the surface with ZrO₂ catalyst and sulphuric acid. As a result, it can be understood that ZrO₂ catalyst is a ZrO₂ compound having monoclinic and tetragonal structures coexisting. In case of SO₄^2-/ZrO₂ catalyst surface treated with sulphuric acid, it is difficult to bring about crystal change of sulphate functional group and related ZrO₂ compound itself Changes in structure and related characteristics cannot be confirmed.

Table 4: EDX analysis of the various catalysts for the catalytic H₂O₂ decomposition.

Table 5: Acidic properties of the various catalysts by NH₃-TPD.

Figure 3: Schematic diagram of the experimental setup for catalytic H₂O₂ decomposition and NO oxidation.

Figure 4: Acidic properties of the various catalysts by NH₃-TPD.

Figure 5: XRD analysis of the various catalysts.

Figure 6: XRD analysis of the various catalysts (A: Al₂O₃, B: 10 wt.% Mn/ Al₂O₃, C: 30 wt.% Mn/Al₂O₃).

Figure 7: XRD analysis of the various catalysts (A: ZrO₂, B: SO₄^2-/ZrO₂).

Figure 8 shows the results of XPS analysis of the binding energy of elemental components to confirm the binding state of constituent elements of K-Mn/Fe₂O₃ catalyst supporting K and Mn in support Fe₂O₃. As shown in Figure 8a, the binding energy of the Mn component was obtained in various peaks, but at binding energies corresponding to about 642.2 and 654.1 eV. The type of compound containing Mn component corresponding to this binding energy position is KMnO₄. Figure 8b also shows the binding energy of the K component, which is 292.6 eV, indicating the binding structure of K and KMnO₄. The bond energy for the Fe component shown in Figure 8c was two peaks corresponding to about 710.05 and 723.5 eV, and this binding energy was found to be the binding energy of the Fe component contained in the Fe₂O₃ compound. Therefore, it is considered that the property of K-Mn/Fe₂O₃ catalyst is composed of Fe₂O₃ which is a support, mostly containing KMnO₄ component. Figure 9 shows the results of XPS analysis of the binding energy of constituent elements for the 10 wt.% Mn/Al₂O₃ catalyst prepared by loading 10 wt.% Of Mn on the support γ-Al₂O₃. As shown in Figure 9a, the binding energy of the Mn component is about 641.8 and 653.8 eV, indicating that the main Mn compound with such binding energy is MnO₂. The bond energy for the Al component contained in Al₂O₃, another component, was obtained at 73.9 eV as a single peak and is shown in Figure 9b. It can be seen that such a peak position is maintained as a support of the compound type introduced before production as the Al₂O₃ compound. Figure 10 shows the results of XPS analysis of the binding energy of the constituent elements for the 70 wt.% Mn/Al₂O₃ catalyst loaded with Mn on support Al₂O₃. Figure 10a shows a relatively large number of binding energy peaks as compared to the above-mentioned 10 wt% Mn-supported catalyst. As Mn content increases, various Mn compounds are formed in addition to MnO₂ Able to know. However, the peaks corresponding to 641.7 and 652.8 eV were observed at the positions of the most important binding energies, respectively, and this binding energy tendency can be obtained as a result of the MnO₂ compound. In Figure 10b, the binding energy position for the Al component is 73.6 eV, which means that it is present as a typical Al₂O₃ compound. Based on these results, it is considered that the Mn component is present in the MnO₂ phase and the catalyst supported on Al₂O₃ is supported. As shown in Figure 11, the position of the peak corresponding to the bonding energy of the Zr component is 186.5 eV regardless of whether the surface treatment is performed or not. As can be seen in Figure 11, the peak position of the binding energy for the S component present in the SO₄^2-/ZrO₂ catalyst surface treated with sulfuric acid is about 168.8 eV. This means that SO₄^2-functional groups are formed on the surface of ZrO₂ due to surface treatment with sulfuric acid as binding energy corresponding to formation of SO₄^2-functional group. These SO₄^2-functionalized catalysts are known to enhance the catalytic effect in reactions involving related reaction mechanisms by further enhancing the strength and quantity of acid sites.

Figure 8: XPS analysis of Fe₂O₃ & K-Mn/ Fe₂O₃ catalysts.

Figure 9: XPS analysis of 10 wt.% Mn/Al₂O₃ catalysts.

Figure 10: XPS analysis of SO₄^2-/ZrO₂ catalysts (A: Before sulphuric acid treatment, B: after sulphuric acid treatment)

Figure 11: Effect of the various catalysts on the catalytic H₂O₂ decomposition.

Decomposition of H₂O₂ and NO conversion according to the type of catalyst

Mn/Al₂O₃>10 wt.% Mn/Al₂O₃ catalyst was used as a catalyst. The results showed that the decomposition efficiency of H₂O₂ by the catalyst was K-Mn/Fe₂O₃ and Mn/Al₂O₃>Fe₂O₃>SO₄^2-/ZrO₂. The decomposition rate of H₂O₂ varies depending on the type of active material and the amount of dry oxidant generated at that time varies [13-15]. Figure 12 shows the conversion efficiency as H₂O₂ decomposition efficiency depending on the type of catalyst performed for the production of dry oxidant. These results are strongly influenced by the distribution of the acid sites of the catalysts, which are most related to the characteristics of the H₂O₂ decomposition reaction, among the physicochemical properties of the various catalysts produced. H₂O₂ decomposition reaction characteristics are dependent on the oxidation-reduction reaction due to electron mobility, and the reaction characteristic is related to the Lewis acid point of the solid catalyst. In addition, it is considered that the size of reactivity depends on the distribution and amount of weak acid sites in the characteristics of H₂O₂ as a reaction target in the acid sites. Figure 11 shows the amount of decomposition of H₂O₂ by decomposition of H₂O₂ catalyst by type of catalyst for the production of dry oxidant. The catalysts used were 10, 30, and 70 wt.% Mn/Al₂O₃ catalysts and SO₄^2-/ZrO₂ catalysts, which were different in content from K-Mn/Fe₂O₃ catalysts. As a result, it was found that the decomposition efficiency of H₂O₂ gradually increased with time regardless of the kind of catalyst, and it was found that the time required for stabilizing the decomposition efficiency of H₂O₂ catalyst, that is the order of magnitude of reactivity was as follows: K-Mn/Fe₂O₃>70 wt.% Mn/Al₂O₃>30 wt.% Mn/Al₂O₃>10 wt.% Mn/Al₂O₃>SO₄^2-/ZrO₂. The same tendency as the test result was obtained. To investigate the characteristics of the NO oxidation reaction proceeding in the downstream oxidation process by using the dry oxidant prepared according to the decomposition reaction characteristics of the performed H₂O₂ catalysts. Figure 12 shows the NO conversion efficiency (value in the graph, in%) by injecting the dry oxidant obtained after the H₂O₂ decomposition reaction carried out according to the above catalyst type into the oxidation process together with the exhaust gas. The reaction temperature was controlled at 150°C and the amount of H₂O₂ injected and the amount of catalyst loaded were 0.3 g/min and 0.5 g respectively. The oxidation process also controlled the reaction temperature to 150°C, and the flow rate of injected simulated flue gas and NO concentration were 300 ml/min and 1,000 ppm respectively. The conversion efficiency of NO in the oxidation process varied depending on the type of catalyst, and the NO conversion rate varied depending on the performance of the catalyst obtained in the H₂O₂ decomposition process. As a result, the NO conversion efficiency of the catalysts used in the H₂O₂ decomposition process was K-Mn/Fe₂O₃ and Mn/Al₂O₃ catalysts, and the conversion of NO obtained in the oxidation process reached about 100%. The NO conversion rates of the catalysts used in the H₂O₂ decomposition process were K-Mn/Fe₂O₃≈Mn/Al₂O₃>>30 wt.% Mn/Al₂O₃>10 wt.% Mn/Al₂O₃>Fe₂O₃>SO₄^2-/ZrO₂ And so on.

Figure 12: Effect of the various catalysts on NO oxidation.

In this study, the conversion of dry oxidant obtained from H₂O₂ catalytic cracking was investigated by injecting the dry oxidant obtained from it into the subsequent NO oxidation process. First, various catalysts were prepared and applied to H₂O₂ decomposition reaction. As a result, it was found that the acid sites possessed by the catalyst had the greatest effect on the decomposition efficiency of H₂O₂, and the acid sites present on the catalyst surface were uniformly distributed from weak acid sites to strong acid sites. The H₂O₂ decomposition efficiency was higher. Also, when the dry oxidant obtained from this process was injected into the NO oxidation process, the NO conversion efficiency was increased as the decomposition efficiency of H₂O₂ was increased. Therefore, Mn-based Fe₂O₃ catalyst with K component as the catalyst having a uniform and rich acid point ranging from the weakest point to the strongest point among the various catalysts produced showed the highest decomposition efficiency of H₂O₂, The NO conversion rate obtained under optimized operating conditions on the NO oxidation process using dry oxidant reached about 100%.

This study was supported by the Ministry of Environment’s environmental industry advancement technology development project.