Effect of Hydroxylation and Carboxylation on the Catalytic Activity of Fe₂O₃/Graphene for Oxidative Desulfurization and Denitration

Abstract

Iron-based particles loaded on porous carbon materials have attracted extensive attention as catalysts for denitration and desulfurization reactions. However, the carbon support of a high-temperature denitration catalyst is inevitably oxidized in the presence of H₂O and O₂. The mechanism of denitration catalyst oxidation and its influence on the catalytic reaction remain to be further explored. Fe₂O₃-loaded graphene models with carbon vacancy (G_def), hydroxyl (HyG), and carboxyl (CyG) were constructed to investigate the effects of hydroxylation and carboxylation on the catalytic activity of Fe₂O₃/graphene for oxidative desulfurization and denitration by using density functional theory (DFT) calculations. According to the analysis of structural properties and adsorption energy, the adsorption process of Fe₂O₃ on HyG and CyG was observed to have proceeded more favorably than that on G_def. The density-of-states (DOS) results also affirmed that HyG and CyG promote the electron delocalization of Fe₂O₃ around the Fermi level, enhancing the chemical activity of Fe₂O₃. Moreover, adsorption energy analysis indicates that hydroxylation and carboxylation enhanced the adsorption of SO₂ and H₂O₂ on Fe₂O₃/graphene while also maintaining preferable adsorption stability of NO. Furthermore, mechanistic research explains that adsorbed H₂O₂ on HyG and CyG directly oxidizes NO and SO₂ into HNO₂ and H₂SO₄ following a one-step reaction. The results provide a fundamental understanding of the oxidized catalyst on catalytic denitration and desulfurization reactions.

1. Introduction

Environmental pollution caused by energy consumption is becoming more and more serious. When fossil fuels are burned, sulfur and nitrogen elements are mainly oxidized to SO₂ and NO [1]. Air pollution, acid rain, and the greenhouse effect, caused by harmful substances such as flue gas, sulfur oxides (mainly SO₂), and nitrogen oxides (mainly NO), are serious threats to human survival [2]. Therefore, it is urgent to strengthen the control of SO₂ and NO in flue gas. Many countries have issued strict standards to control the emission of SO₂ and NO below 50 ppm [3,4], which has prompted extensive efforts to develop effective technologies to reduce or remove these toxic gases [5]. The technologies that have been developed include wet flue gas desulfurization, dry sorbent injection, spray dryer absorption for desulfurization [6], low nitrogen oxide combustion technology, selective catalytic reduction (SCR), and selective noncatalytic reduction (SNCR) for denitration [7]. SCR technology was widely used in the flue gas treatment of coal-fired power plants, but its large scale and high cost limit its utilization in developing countries [8].

In order to make up for the deficiencies of current industrial technology, other methods are being studied, such as integrated oxidative desulfurization and denitration (ODSN) [9]. ODSN is expected to be a promising method for catalytic oxidization of SO₂ and NO in mild conditions, using a suitable catalyst for catalytic oxidation. Cu [9,10,11], Mn [12,13], Co [14], and Ce [15] are commonly used as catalysts by researchers. For example, Jie Ding et al. [16] used a Ce-Ti catalyst to catalyze the oxidation of NO_x and SO₂ by ozone, with the assistance of a glassy amino scrubber to remove sulfur and nitrogen. This process has no secondary pollution, but does have reduced waste production and a lower operating cost. Li et al. [17] used TiO₂/Cu₂O supported on activated carbon (AC) fiber as a photocatalyst to improve the adsorption ability of AC fiber for NO and SO₂. In addition, hematite (α-Fe₂O₃) is expected to be an ideal catalyst due to its low cost, good activity, and environmental performance. Alumina (Fe-Al) and H₂O₂ on the surface of hematite can achieve flue gas desulfurization and denitration simultaneously. Therefore, hematite-based catalysts have attracted more and more attention [18].

In the present report, the catalysts with strong adsorption capacity for SO₂ and NO and high selectivity for ODSN are still to be discovered. Carbon sorbent materials such as AC, graphite oxide (GO), single-walled carbon nanotubes (SWNTs), and graphene (G) are considered promising adsorbent materials for the selective removal of SO₂ and NO_x due to their high surface area and uniform pore size distribution. AC can effectively adsorb SO₂ and NO_x in flue gas at an aerobic low temperature, and its adsorption rate can reach over 99% [19]. Ammar et al. prepared metal oxide particles loaded on GO (PMO-Fe₃O₄ /rGO) as a heterogeneous catalyst/adsorbent, and hydrogen peroxide as an oxidant, achieving a denitration efficiency of 85.6% [20]. In the extractive catalytic oxidative desulfurization system, a sandwich-type polyoxometalate (K₁₀[Co₄(H₂O)₂(PW₉O₃₄)₂]₁₀-(Co₄-POM) was covalently immobilized on an amino-modified magnetic GO. When the catalyst was introduced into the reaction system, the oxidative desulfurization efficiency was 45.8%, and when acetonitrile was added as an extraction solvent, the sulfur was completely removed (100%) [21]. MoO₃ nanoparticles, supported on a carbon nanotubes catalyst, show high activity for oxidative desulfurization; the optimum sulfur removal efficiency was 96% [22]. Graphene has shown great potential in ODSN due to its unique single-atom-layer structure, high specific surface area, mechanical strength, fascinating thermal properties, and many surface-active sites [23,24,25]. Giulia Costamagna tailored the carbon surface with anchored iron nanoparticles and tested them for catalytic oxidative desulphurization of high sulfur content oil, proving the reliability of those materials as promising catalysts for upgrading sulfur-rich drop-in fuels [26]. Liao et al. used graphene to accelerate iron transport, significantly enhancing biological denitration and reducing intermediate accumulation [27]. Iron-based particles loaded on porous carbon material have attracted great attention due their low cost, non-toxicity, environmental protection, and excellent adsorption capacity to SO₂ and NO. However, the oxidation of carbon materials, such as hydroxylation and carboxylation, will inevitably occur in the oxidizing atmosphere [28], thus changing the adsorption and catalytic reaction capacity and reaction mechanisms of catalysts [29]. For example, high concentrations of oxygen and carbon dioxide would lead to the oxidation of AC [1], enhancing the NO and SO₂ adsorption capacity of AC, but reducing the efficiency of desulfurization and denitration [30]. A careful literature search revealed that the oxidation of carbon materials changes the catalytic activity. However, the mechanism of ODSN catalyst oxidation and its influence on the catalytic reaction has not yet been sufficiently studied. Accordingly, the present work is devoted to thoroughly investigating the adsorption and catalytic oxidation of SO₂ and NO on Fe₂O₃-loaded graphene models with carbon vacancy (G_def), hydroxyl (HyG), and carboxyl (CyG), by means of the DFT method. For the studied SO₂ and NO on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG, the analyses of geometric structures, the adsorption energies, and the electronic properties were performed. Furthermore, the mechanism of the influence of H₂O₂ sprayed by the traditional hydrogen peroxide oxidative denitration technology on the catalytic desulfurization and denitration of Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG was studied. The research results reveal the variation characteristics of the catalyst performance, and have guiding significance for the development of new catalysts for ODSN.

2. Results and Discussion

2.1. Structure and Properties of Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG

Firstly, we compare and analyze the structural properties of Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG, as shown in Figure 1a. It can be inferred that after Fe₂O₃ is loaded on G_def, the Fe and O atoms are hybridized with the C atoms of G_def, the charge density is transferred from the Fe₂O₃ cluster to G_def, and the total Mulliken charge on the Fe₂O₃ cluster is 0.370. From Figure 1b, it can be concluded that for Fe₂O₃/HyG, the O atoms of Fe₂O₃ cannot bond with the C atoms of HyG, while Fe hybridizes with both the C atoms of HyG and the O atoms of hydroxyl (OH), resulting in the redistribution of charge density on atoms of Fe₂O₃/HyG. The total Mulliken charge of the Fe₂O₃ cluster loaded on HyG is −0.03. Compared with Fe₂O₃/G_def, the Fe₂O₃ cluster on HyG shows a weak negative charge. Figure 1c shows the stable configuration of Fe₂O₃/CyG, with only the Fe atom of the Fe₂O₃ cluster hybridizes with the C atom of CyG, and without direct interaction between the Fe₂O₃ cluster and the carboxyl (COOH) group. There was no hybridization between the metal oxides and carboxyl groups, which can be regarded as the common characteristics of Fe₂O₃ and CyG.

The stability of Fe₂O₃ on G_def, HyG, and CyG is evaluated by calculating the adsorption energy ($E_{ads}$) with Equation (13). Accordingly, the $E_{ads}$ for Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG are −1.81 eV, −7.71 eV, and −9.61 eV, respectively. The results show that the presence of hydroxyl and carboxyl groups has a positive effect on the stability of the Fe₂O₃ cluster on the support.

To detail an understanding of the effect of the hydroxyl group and the carboxyl group on the electronic properties of the catalyst systems, we further analyzed the DOS for the Fe₂O₃ cluster supported on G_def, HyG, and CyG. As can be revealed in Figure 2, compared with the DOS of the pure Fe₂O₃ cluster, the DOS change of Fe₂O₃ loaded on G_def, HyG, and CyG indicated that there was a chemical interaction between Fe₂O₃ and G_def, HyG, and CyG. Specifically, HyG and CyG promote the electron delocalization of Fe₂O₃ around the fermi level (E_f = 0.0 eV), which can improve the chemical activity of Fe₂O_3.

2.2. NO and SO₂ Adsorption on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG

The adsorption of NO and SO₂ on Fe₂O₃/G_def Fe₂O₃/HyG and Fe₂O₃/CyG is compared and discussed herein. Figure 3a–c depict the optimized models, which are the stable adsorption configurations of NO on the surface of Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG, respectively. The N atom interacts with the Fe atom chemically, and the bond lengths are 1.711 Å, 1.652 Å, and 1.675 Å, respectively. The electronic interaction between NO and the catalyst leads to charge rearrangement, and the bond lengths of N-O become 1.180 Å, 1.187 Å, and 1.179 Å, respectively, which are larger than the bond lengths of free NO molecule (1.151 Å). For the configurations of SO₂ adsorption on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG, as shown in Figure 3d–f, the S atom was adsorbed by the Fe atom to form an S-Fe bond, with bond lengths of 2.222 Å, 2.211 Å, and 2.641 Å, respectively. After adsorption, the bond length and bond angle of SO₂ changed slightly, while the charge density of SO₂ increased by 0.167, 0.197, and 0.143 on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG, respectively.

The $E_{ads}$ for NO [and SO₂] on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG are calculated by the Equation (13). As shown in

Table 1. $E_{ads}$ for NO [and SO₂] adsorption on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG.

	Adsorption Energy (eV)
NO-Fe2O3/Gdef	−2.20
NO-Fe2O3/HyG	−1.22
NO-Fe2O3/CyG	−1.06
SO2-Fe2O3/Gdef	−0.16
SO2-Fe2O3/HyG	−0.33
SO2-Fe2O3/CyG	−0.41

, $E_{ads}$ are all negative, which indicates that the adsorption of NO and SO₂ on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG are exothermic processes. Hydroxylation and carboxylation decreased the $E_{ads}$ of NO on surfaces and increased the $E_{ads}$ of SO₂ on surfaces. However, the $E_{ads}$ of SO₂ on surfaces was much less than that of NO on these catalysts. The interaction between NO and the surfaces are stronger than those of SO₂.

We further researched the adsorption of NO and SO₂ on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG. Figure 4 displays the DOS of N_2p, Fe_3d, and SO₂ on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG. The comparison before and after adsorption revealed that N_2p-DOS and Fe_3d-DOS change significantly around the Fermi level. N_2p-DOS and Fe_3d-DOS split to form bonding and antibonding orbitals, and the orbital energies overlap well, further confirming that NO is chemisorbed on Fe₂O₃/G_def and Fe₂O₃/HyG. A Mulliken charge analysis revealed that the total charges of the adsorbed NO on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG were −0.050, 0.032, and 0.023, respectively. Hydroxylation and carboxylation slightly modulated the charge population between NO and the catalyst; that is, the oxidation of graphene will affect the adsorption of NO. For the adsorption of SO₂ on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG, the SO₂-DOS shifts to the left, indicating that part of the charge is transferred to SO₂ after adsorption. However, the DOS only shows a left shift, without any splitting after SO₂ adsorption on Fe₂O₃/HyG and Fe₂O₃/CyG, which implies that only physical adsorption occurs for SO₂-Fe₂O₃/HyG and SO₂-Fe₂O₃/CyG.

2.3. Catalytic Oxidation of SO₂ and NO on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG

In this section, the catalytic oxidation reaction mechanism of desulfurization and denitration was studied. The reaction mechanism of NO oxidation via hydroxyl radical ·OH in the gas phase is as follows:

$$·\mathrm{OH}+\mathrm{NO} \to {\mathrm{HNO}}_2\hspace{1em}{\Delta }_{r}H=-2.15 \mathrm{eV}$$

(1)

$${\mathrm{HNO}}_2+·\mathrm{OH} \to {\mathrm{H}}_2{\mathrm{NO}}_3\hspace{1em}{\Delta }_{r}H=-0.31 \mathrm{eV}$$

(2)

The corresponding rate Equation:

$$\mathrm{r}=\frac{k\mathrm{T}}{h}\mathrm{Exp}\left(\frac{-0.21 \mathrm{eV}\times 6.02\times {10}^{23}\times 1.6\times {10}^{-19}}{R}\right)$$

(3)

The reaction mechanism of SO₂ oxidation via hydroxyl radical ·OH in the gas phase is as follows:

$$·\mathrm{OH}+{\mathrm{SO}}_2 \to {\mathrm{HSO}}_3\hspace{1em}{\Delta }_{r}H=-1.54 \mathrm{eV}$$

(4)

$${\mathrm{HSO}}_3+·\mathrm{OH} \to {\mathrm{H}}_2{\mathrm{SO}}_4\hspace{1em}{\Delta }_{r}H=-3.77 \mathrm{eV}$$

(5)

The thermal effect of SO₂ oxidation via ·OH was more obvious than that of NO oxidation. NO and SO₂ oxidation via ·OH is thermodynamic feasible, and the process is exothermic.

The traditional hydrogen peroxide oxidation denitration technology is to spray H₂O₂ at the flue gas at above 400 °C. H₂O₂ decomposes into the free hydroxyl radical ·OH, which oxidizes NO into NO₂, and then the alkaline solution is used to absorb the generated NO₂. However, H₂O₂ also generates radical ·OOH, which cannot oxidize NO, but combines with ·OH to generate H₂O and O₂. Moreover, the decomposition reaction of H₂O₂ into ·OH in the gas phase is thermodynamically unfeasible. Therefore, we discussed the adsorption of H₂O₂ on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG.

H₂O₂ is physically adsorbed on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG, forming a stable adsorption configuration with two hydrogen bonds, as shown in Figure 5. It can be seen that H₂O₂ adsorbed on the catalyst surface site without decomposition. After H₂O₂ is adsorbed on the surface of the Fe₂O₃/G_def, the O-O bond length was 1.438 Å, which was close to that (1.439 Å) of the pure H₂O₂. The two H atoms of H₂O₂ and the two O atoms of Fe₂O₃ form hydrogen bonds with lengths of 2.325 Å and 2.466 Å, respectively. While H₂O₂ is adsorbed on the surface of Fe₂O₃/HyG, the O-O bond length is 1.467 Å, which is longer than that of pure H₂O₂. One H of H₂O₂ adsorbed on Fe₂O₃/HyG forms a hydrogen bond (1.708 Å) with one O atom of Fe₂O₃, while another O atom of H₂O₂ forms another hydrogen bond (1.724 Å) with an H atom of hydroxyl. For the stable configuration of H₂O₂-Fe₂O₃/CyG, the O-O has a bond length of H₂O₂ 1.437 Å. One H atom of H₂O₂ adsorbed on Fe₂O₃/ CyG forms a hydrogen bond (1.708 Å) with one O atom of Fe₂O₃, and another O atom of H₂O₂ forms a hydrogen bond (1.641 Å) with an H atom of the carboxyl.

Similar to Equation (13), the $E_{ads}$ of H₂O₂ on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG can be calculated as −0.35 eV, −1.27 eV, and −2.29 eV, respectively. Hydroxylation and carboxylation significantly enhance the interaction between H₂O₂ and the catalysts.

In order to explain the adsorption characteristics of H₂O₂ more accurately, we analyzed the electron characteristics of H₂O₂ adsorbed on the catalyst surface. Figure 6 shows the DOS of H₂O₂ adsorption on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG. Before adsorption, the outer orbital π* of H₂O₂ was half-filled. After adsorption, H₂O₂ accepted electron transferring by Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG. The outer orbitals were fully filled, and the DOS of H₂O₂ shifted to the left. The DOS of H₂O₂ adsorbed on Fe₂O₃/G_def and H₂O₂-Fe₂O₃/HyG shifted to the left by approximately 1 eV, while the DOS of H₂O₂ adsorbed on Fe₂O₃/CyG shifted to the left by approximately 2 eV. Hydroxylation and carboxylation can significantly change the DOS of H₂O₂. The carboxylation enhances the adsorption stability of H₂O₂ on the catalyst surface, which corresponds to the adsorption energy calculated above.

Although the physical adsorption of H₂O₂ on Fe₂O₃/HyG and Fe₂O₃/CyG cannot lead to the generation of ·OH directly; it adjusts the extent of the reaction balance and promotes the oxidation of NO and SO₂. It was found that Fe₂O₃/G_def catalyzes the oxidation of NO by H₂O₂ to form the adsorbed HNO₂, and releases an ·OH group to the gas phase for further oxidation as:

NO + H₂O₂-Fe₂O₃/G_def → HNO₂-Fe₂O₃/G_def + ·OH

(6)

·OH + NO → HNO₂

(7)

HNO₂ + ·OH → H₂NO₃

(8)

As shown in Figure 7a, differing from the reaction on Fe₂O₃/G_def, H₂O₂ oxidizes NO into free nitrous acid molecules on Fe₂O₃/HyG and Fe₂O₃/CyG. Hydroxylation and carboxylation of Fe₂O₃/HyG and Fe₂O₃/CyG can catalyze H₂O₂ to oxidize NO to HNO₂ in one step, as:

NO + H₂O₂-Fe₂O₃/HyG → H₂NO₃ + Fe₂O₃/HyG

(9)

NO + H₂O₂-Fe₂O₃/CyG → H₂NO₃ + Fe₂O₃/CyG

(10)

Similarly, as shown in Figure 7b, the surface adsorption of H₂O₂ on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG cannot directly decompose into ·OH, but adjusts the extent of the reaction balance to promote the oxidation reaction of SO₂. Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG can directly adsorb H₂O₂ and oxidize SO₂ into sulfuric acid molecules on the surface. Therefore, the mechanism of Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG catalyzing H₂O₂ to oxidize SO₂ is as follows:

$${\mathrm{H}}_2{\mathrm{O}}_2 \stackrel{\mathrm{Cat}.}{\to }{\mathrm{H}}_2{\mathrm{O}}_2\text{-}\mathrm{Cat}.$$

(11)

SO₂ + H₂O₂-Cat.→ H₂SO₄ + Cat.

(12)

where Cat. is the catalysts (Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG).

3. Model and Methods

The DFT [31] calculations were carried out using Dmol³ module in the Materials Studio software package, with Generalized Gradient Approximation (GGA) and Perdew-Burke-Ernzerhof (PBE) functional [32] for exchange and correlation potentials. Double numerical basis sets, plus polarization function (DNP), were used for atomic basis functions. The DFT semi-core pseudopots method was used for core treatment. The plane wave cutoff energy was set to 600 eV. The self-consistent accuracy was set to 2 × 10⁻⁵ eV/atom. A maximum force tolerance of 0.002 Ha/Å was applied. Following previous work [33,34], the 6 × 6 graphene flake, composed of 72 carbon atoms and a vacuum layer with a thickness of 15 Å, was established. One C atom of the graphene sheet was removed in order to obtain the C-defect graphene (G_def) by geometric optimization. The hydroxyl and carboxyl groups were grafted onto the defect sites of G_def to obtain the configurations of HyG and CyG, respectively. Three stable catalyst models of Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG were obtained by loading the Fe₂O₃ cluster on the modified graphene surfaces.

The adsorption energy ($E_{ads}$) was calculated as:

$$E_{ads}=E_{\mathrm{adsorbate},\mathrm{surface}}-E_{\mathrm{surface}}-E_{\mathrm{adsorbate}}$$

(13)

where $E_{\mathrm{adsorbate},\mathrm{surface}}$, $E_{\mathrm{surface}}$, and $E_{\mathrm{adsorbate}}$ represent the total energy of the surface slabs with adsorbates, the bare slabs, and adsorbates, respectively. A negative $E_{ads}$ value indicates that the adsorption process is exothermic, whereas a positive value indicates an endothermic process. The adsorption stability of Fe₂O₃ on G_def, HyG, and CyG, as well as the NO and SO₂ on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG, can be evaluated by calculating the $E_{ads}$.

4. Conclusions

In the current study, the effects of hydroxylation and carboxylation on the catalytic activity of Fe₂O₃/graphene for ODSN are investigated using DFT calculations. Adsorption energies, charge transfer analyses, and DOS calculations were performed for NO, SO₂, H₂O₂, and Fe₂O₃/graphene configurations. The hydroxyl group and carboxyl group permitted more stable adsorption of the Fe₂O₃ clusters on graphene and promoted the electron delocalization of Fe₂O₃ around the Fermi level, evidencing the enhanced chemical activity of Fe₂O₃. Hydroxylation and carboxylation improved the adsorption of SO₂ and H₂O₂ on Fe₂O₃/graphene, and maintained the preferable adsorption stability of NO. The DOS of N_2p, Fe_3d, and SO₂ on Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG verified that NO was chemisorbed on Fe₂O₃/G_def and Fe₂O₃/HyG, while only physical adsorption occurred for the SO₂-Fe₂O₃/HyG and SO₂-Fe₂O₃/CyG. Fe₂O₃/G_def catalyzed the oxidation of NO by H₂O₂ to form the adsorbed HNO₂ and to release a ·OH group to the gas phase. H₂O₂ oxidizes NO into free nitrous acid molecules on the surface of Fe₂O₃/HyG, and Fe₂O₃/CyG via a one-step reaction mechanism. Similarly, Fe₂O₃/G_def, Fe₂O₃/HyG, and Fe₂O₃/CyG can directly adsorb H₂O₂ and oxidize SO₂ into sulfuric acid molecules on the surface. The results provide a fundamental understanding of catalyst oxidative denaturation on catalytic denitration and desulfurization reactions.