Darmansyah Darmansyah1,2, Sheng-Jie You2,3, Ya-Fen Wang This email address is being protected from spambots. You need JavaScript enabled to view it.2,4 

1 Department of Civil Engineering, Chung Yuan Christian University, Taoyuan 32023, Taiwan
2 Department of Environmental Engineering, Chung Yuan Christian University, Taoyuan 32023, Taiwan
3 Center for Environmental Risk Management, Chung Yuan Christian University, Taoyuan 32023, Taiwan
4 Sustainable Environmental Education Center, Chung Yuan Christian University, Taoyuan 32023, Taiwan


Received: October 15, 2023
Revised: December 22, 2023
Accepted: January 14, 2024

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.


Download Citation: ||https://doi.org/10.4209/aaqr.230247  


Cite this article:

Darmansyah, D., You, S.J., Wang, Y.F. (2024). Modification of Recycled Industrial Solid Waste Incineration Fly Ash as a Low-cost Catalyst for NOx Removal. Aerosol Air Qual. Res. 24, 230247. https://doi.org/10.4209/aaqr.230247


HIGHLIGHTS

  • Recycled of fly ash as a low-cost catalyst for NOx removal has been investigated.
  • The parameters influencing NOx removal efficiency were thoroughly examined.
  • Over 90% of NOx removal can be achieved using the FA/Zr-10% catalyst.
  • The mechanism of the catalytic oxidation reactions was analyzed and explored.
 

ABSTRACT


Fly ash is solid waste from incinerators that contains complex compounds that have great potential to synthesize valuable materials. This study devises synthesizing a low-cost catalyst from recycled fly ash using a combination of metal oxides (ZrO2, TiO2, and MgO) to remove NOx. This study also investigated the impact of various factors on the synergetic purification of NOx as an air pollution emission. Microscopic characterization showed that increasing the composition of metal oxides can increase the specific surface area of fly ash, crystallinity, binding energy, and the dispersion of metal oxides into the fly ash, favoring the adsorption and the oxidation of NOx onto the surface-modified fly ash catalysts. The NOx removal in raw fly ash (RFA) at 25°C is 10.39%, increasing to 90.3% in the modified fly ash catalyst (FA/Zr-10%) at 250°C. The modified fly ash catalysts can reduce catalyst costs for NOx removal by one-ninth, and the operating temperature is about 10–15% lower than conventional catalysts. In summary, industrial solid waste incineration fly ash holds excellent potential when modified with metal oxides, serving as an economical and highly efficient catalyst for NOx removal.


Keywords: Fly ash, Low-cost catalysts, NOx removal, Selective catalytic reduction, Surface-modified fly ash catalysts


1 INTRODUCTION


Fly ash, a solid waste by-product produced from municipal and industrial solid waste incineration or coal-fired power plants, contains complex compounds and is abundant as industrial waste (Du et al., 2022; Gao and Iliuta, 2022). The complex compounds in fly ash are dominated by SiO2, Al2O3, Fe2O3, CaO, and MgO (Long et al., 2022; Zhan et al., 2022). Furthermore, heavy metals such as mercury, arsenic, and lead at trace levels and toxic gases such as VOCs, CO2, SOx, NOx, furans, and dioxins are also contained in fly ash (Guo et al., 2022; Zhang et al., 2022).

Currently, over 200 million tons of total fly ash, including coal fly ash (CFA), municipal solid waste fly ash (MSWIFA), and industrial solid waste incineration fly ash (ISWIFA), are generated in the world each year, especially in the four major countries of China, India, the USA, and Indonesia (Gollakota et al., 2019; Darmansyah et al., 2023). Some countries have classified fly ash as hazardous material (Zierold and Odoh, 2020; Lan et al., 2022). With the amount dispersed into the ecosystem annually, failing to resolve fly ash issues adequately can harm ecosystems (Zhang et al., 2021a). In the meantime, global efforts to reduce the quantity of fly ash are only between 15% and 30% (Dindi et al., 2019; Lin et al., 2022), and fly ash is typically used as a mixing material in cement and concrete industry, manufacturing, and wastewater industry (Czuma et al., 2020; Tan et al., 2023). Furthermore, fly ash is a porous material, resulting in a substantial specific surface area and excellent adsorption capabilities, which make it an effective adsorbent and catalyst carrier, particularly in NOx elimination (Srivastava et al., 2014; Dindi et al., 2019). The chemical composition of fly ash presents a wide range of opportunities for fly ash utilization, as shown in Table 1. Some researchers have also reported that fly ash, a widely applied catalytic carrier like SiO2, Al2O3, CaO, and TiO2, exhibits constructive physical and chemical properties (Panesar et al., 2019; Long et al., 2022). These characteristics, which include superior thermal and chemical stability, make it a popular choice as a catalytic carrier in NOx removal technology (Panesar et al., 2019; Yang et al., 2019a).

Table 1. Chemical content of industrial solid waste fly ash (untreated) and modified fly ash.

On the other hand, nitrogen oxides (NOx), which include nitrogen monoxide (NO) and nitrogen dioxide (NO2), are two major gaseous pollutants that can cause severe environmental problems such as the greenhouse gas effect, acid rain and photochemical smog, and human health problems, namely acute pulmonary respiratory disorders, miscarriage, nephropathy, colic, and lung cancer (Fang et al., 2019; Zierold and Odoh, 2020). The primary NOx emission sources are stationary oil, coal-fired furnaces, and diesel engines (Chen et al., 2016; Zheng et al., 2021). Despite concerted global efforts to combat emissions, NOx continues to increase, with tens of millions of tons released into the atmosphere annually (Liu et al., 2016). The U.S. Environmental Protection Agency (U.S. EPA) set a limit on the release of NOx based on suggestions provided by the Ozone Assessment Transport Group (OATG). This limit is approximately 0.15 lbs MMBtu1, equivalent to 124 ppm. This data represents a reduction of around 85% from the emission rate recorded in 1990 (U.S. EPA, 2007; Lai and Wachs, 2018). Various strategies for reducing NOx emissions have been researched. Among those approaches, selective catalytic reduction (SCR) of NOx by NH3 has proven the most prevalent and effective (Chen et al., 2022).

Some metal oxide-modified catalysts for the SCR method have recently been used to reduce NOx emissions (Keller et al., 2022; Nannuzzi et al., 2023). The most widely applied commercial catalysts for this process are V2O5/TiO2 and V2O5/MoO3-based catalysts, whose window reaction temperatures are between 300°C–400°C, a narrow temperature window with limited application (Chen et al., 2016; Zhang et al., 2020). Furthermore, the inherent toxicity of vanadium oxides and the formation of N2O at high temperatures were reported by Song et al. (2016). Therefore, developing novel SCR catalysts with high catalytic activity at a wider reactive temperature has become a significant focus in the NOx removal issues (Savva and Efstathiou, 2008). However, using pure metal oxide catalysts on the SCR system to reduce NOx is expensive (Van Allsburg et al., 2022). Based on those problems, it is necessary to find new materials that are low-cost and have high performance to reduce NOx emissions properly. The supported catalysts, such as Al2O3, SiO2, and Ce0.7Zr0.3O2, can increase the adsorption and oxidation of NOx removal (Corbos et al., 2007b; Li et al., 2009). Some researchers have also reported that using metal oxides, namely ZrO2, TiO2, CuO, MnO2, CeOx, and MgO, effectively eliminates NOx emissions (Aissat et al., 2012; Yi et al., 2019; Van Allsburg et al., 2022). Metal oxides facilitate the oxidation of NO to NO2 due to their notable characteristics, such as a substantial surface area, abundant active sites, and effective distribution of active components (Yi et al., 2019). However, as previously mentioned, fly ash offers a distinct surface area and remarkable adsorption capacity (Dindi et al., 2019; Gollakota et al., 2019). Therefore, the combination of fly ash and metal oxide catalysts could be potentially studied further as a promising catalyst for eliminating NOx emissions. Hence, utilizing industrial solid waste incineration fly ash modified by metal oxides (ZrO2, TiO2, and MgO) as catalysts can be suitable for NOx removal.

To fill this gap, in this work, a novel modified fly ash catalyst was systematically prepared from metal oxides and was used to perform SCR treatment with NH3 at 250°C. This was done to enhance the removal efficiency of NOx, offering a low-cost, high-efficiency, environmentally friendliness, and broadly applicable solution. The composition of the modified fly ash catalysts synthesized with various metal oxides was identified through the characterization of X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller (BET). Subsequently, the results of characterizing the modified fly ash were compared to its NOx removal performance to assess the impact of the catalyst material's structure, morphology, crystallinity, binding energy, and activation energy on its effectiveness in the SCR technique. Furthermore, adsorption kinetics, isotherm adsorption, and other possible mechanisms are proposed to enhance the prediction of NOx conversion on the surface of the modified fly ash catalysts.

 
2 MATERIALS AND METHODS


 
2.1 Preparation of Materials

Fly ash, containing primary elements like carbon, calcium, silica, alumina, sodium, and trace elements, underwent analysis using SEM-EDX and an X-ray fluorescence spectroscopy (XRF). The samples were collected from a solid waste incineration plant in Taiwan. Detailed information related to the chemical content of Taiwan industrial solid waste incineration fly ash is shown in Table 1 and Table 2. The metal oxide chemicals, such as zirconium oxide (ZrO2) and titanium oxide (TiO2), were purchased from Sigma-Aldrich, Germany. Magnesium oxide (MgO) was purchased from Alfa Aesar, Thermo-Fischer Scientific, USA.

Table 2. The elemental composition of trace elements in fly ash generated from industrial solid waste incinerator.

 
2.2 Modification of Fly Ash by Metal Oxides

The facile mechanochemical method prepared fly ash with metal oxides, including ZrO2, TiO2, and MgO. Raw fly ash, obtained from a baghouse industrial solid waste incinerator filter in Taiwan, was washed with deionized water to remove impurities and dried at 105°C overnight. In a typical process, 1.95 g of the raw fly ash (RFA) and 0.05 g of metal oxide powder were placed into the ball mill tank. This step was followed by adding a specific amount of steel balls in a 3:1 ratio (10 mm to 20 mm). The ball mill bowl is placed in the ball mill machine (model Retsch-PM100, Germany) and ground for one hour at 300 rpm. A break is taken every five minutes with a change in the grinding direction, resulting in FA/Zr-2.5%wt., FA/Ti-2.5%wt., and FA/Mg-2.5%wt. A simple illustration of the synthesis of a modified fly ash catalyst is shown in Fig. 1. Other various metal oxide compositions (5%wt., 10%wt., and 15%wt.) were used to synthesize different catalysts to observe their effect on the NOx removal performance. Then, all the samples were put into a desiccator to prevent water absorption in the air before the NOx removal performance test process.

Fig. 1. The illustration of the synthesis of a modified fly ash catalyst.Fig. 1. The illustration of the synthesis of a modified fly ash catalyst.

 
2.3 Material Characterization

The phase components of the material samples were determined by X-ray diffraction (XRD) patterns on an X-ray diffractometer (s-7000, Shimadzu, Japan) over a 2 theta in the range of 10°–60° at a step of 2° min1. The electric current is held constant at 40 mA, while the voltage is 40 kV. Scanning electron microscopy (SEM) analysis was conducted using a Zeiss SUPRA 40VP instrument equipped with an INCA X-act Penta-FET Precision spectrometer from Oxford Instruments, UK. An X-ray photoelectron spectroscopy (Thermo-Fisher K-Alpha) was used to determine the O1s, C1s, and Zr multivalent state binding energies on the modified fly ash catalyst, and Brunauer–Emmett–Teller (BET) method and a cylindrical pore model (BJH method) were employed on the adsorption branches to determine specific surface areas and pore size distributions of the samples. The catalyst characterization includes determining the quantity and strength of acid sites using gravimetric methods with pyridine and NH3 as adsorbates. The catalyst's acid site strength is qualitatively determined using infrared spectroscopy (IR).

 
2.4 Catalytic Activity Measurement

Fig. 2 illustrates the schematic of the experimental setup used for NOx removal. The setup comprises a fixed-bed reactor, a NO–NO2–NOx analyzer (Thermo-Scientific model 42i, USA) connected to a gas detector (Sabio, model 1001 zero air source, USA), and an automatic control valve for gas (DELTA, Taiwan). The NOx analyzer continuously monitored the concentration of NOx, NO, and NO2. The experiments were conducted in a fixed-bed reactor with an inner diameter of 8.0 mm and a length of 250 mm, operating at ambient pressure. In each experimental run, a modified fly ash sample weighing 0.8 grams was placed in the middle of the reactor, with weight variations of 2.5%wt., 5%wt., 10%wt., and 15%wt. The gas mixture for NOx ambient consisted of 95%v/v NO and 5%v/v NO2. This mixture was introduced into the reactor at a flow rate of 1.0 L min1, with a concentration of 500 ppb relative to air, and the outlet concentration was continuously monitored using the NO–NO2–NOx analyzer. Finally, the specific analysis of the catalytic performance of the catalyst to NOx removal is described by Yan et al. (2018) in the following equation:

The NOx removal efficiency (η) was calculated as follows:

 

Fig. 2. Experimental setup for NOx removal over the modified fly ash catalyst.Fig. 2. Experimental setup for NOx removal over the modified fly ash catalyst.

The N2 selectivity was evaluated based on the percentage of NOx reduction after SCR process reaction. The N2 selectivity was computed using the formula provided by Zhu et al. (2021).

 

The terms [NOx]in and [NOx]out represent the overall gas concentration of NO and NO2 at the inlet and outlet, respectively. Meanwhile [NH3]in and [NH3]out denote the respective NH3 gas concentrations at the inlet and outlet, and [N2O]out represents the concentration of N2O gas at the outlet.

 
2.4.1 The adsorption of NOx

The adsorption process entails binding one or more adsorbates to an adsorbent through physical or chemical interactions (Largitte and Pasquier, 2016). Examining the process of adsorption is essential for accurately determining the rate and dynamics of adsorption. It allows for a meaningful comparison between the predicted adsorption parameters obtained from models and the actual behavior of the adsorbent in the experimental settings. This comparison can be made across various combinations of adsorbents and adsorbates under different experimental conditions (Mishra and Patel, 2009).

The adsorption of gaseous NOx in the surface-modified fly ash at room temperature can be described as:

 

FA/Mi+ is a modified fly ash with metal oxides (Mi+ = Zr4+, Ti4+, or Mg2+).

 
2.4.2 The nitrogen oxide (NO) oxidation mechanism

In this research, the adsorption performance of the modified fly ash catalysts was investigated to observe the reaction rate of NO oxidation to NO2 under different temperature conditions before implementing the selective catalytic reduction (SCR) method. From a thermodynamic standpoint, oxygen allows for the spontaneous oxidation of nitrogen monoxide (NO) to nitrogen dioxide (NO2) (Zhu and Xu, 2022).

 

When assuming a first-order reaction for converting NO to NO2, the normalized initial reaction rate (–r) can be calculated based on the equation developed by Liu et al. (2021) as follows:

 

The normalized initial reaction rate (–r) can be determined by considering the variables mo, SBET, F, P, R, and T. Here, mo represents the mass of the sample in the fixed-bed reactor (g), SBET is the specific surface area of the samples (m2 g1), F denotes the total flow rate of the inlet gas (mol s1), P stands for the standard atmosphere pressure (101,325 Pa), R is the gas constant (8.314 J mol1 K1), T signifies the reaction temperature (K), and A represents the pre-exponential factor. The Arrhenius formula calculates NO oxidation's apparent activation energy (Ea) by modified fly ash:

 

Each NOx adsorption outcome obtained from the NOx analyzer is recorded and repeated for different variations of metal oxide compositions in NOx removal. The results are then contrasted and analyzed to determine the NOx removal efficiency of the various metal oxide compositions.

 
2.4.3 The kinetic reaction mechanism

This study employs the Langmuir-Hinshelwood and Eley-Rideal mechanisms to illustrate the interactions between the adsorbed NOx and NH3 species during the simultaneous reactions of NH3, NO, and NO2 gas species, leading to the formation of nitrogen and water. Additional data was obtained to investigate the kinetic reaction mechanism for reducing NO using NH3 over the fly ash modification catalyst.

 
3 RESULTS AND DISCUSSION



3.1 Structural Morphology of RFA and Modified Fly Ash Catalysts


3.1.1 X-ray diffraction (XRD) analysis

Fig. 3 depicts the x-ray diffraction (XRD) spectra of RFA and modified fly ash material. The XRD analysis was performed to determine the effect of the modification process on the structure of the fly ash material. This modification process involved the incorporation of various metal oxides, such as ZrO2, TiO2, and MgO, into bulk fly ash. First, the XRD of the RFA pattern shows a main crystal peak with a quartz phase at 2 theta of 29.08° and several other weak peaks with a mullite phase at 2 theta of 14.69°, 31.66°, 36.04°, 47.44°, and 52.50°; there are also weak peaks with the hematite phase at 2 theta of 43.16° and 55.85°. The XRD spectra obtained from this study provide similar results to the study conducted by Dana et al. (2004). Subsequently, the XRD result shows that the RFA sample has poor crystallinity or mainly consists of amorphous powder, with a primary peak intensity of only about 200 a.u.

Fig. 3. XRD pattern of RFA, FA/Zr-10%, FA/Ti-10%, and FA/Mg-10%. Note: Q is quartz; H is hematite; and M is mullite.Fig. 3. XRD pattern of RFA, FA/Zr-10%, FA/Ti-10%, and FA/Mg-10%. Note: Q is quartz; H is hematite; and M is mullite.

In comparison, the XRD spectra patterns for fly ash were modified by 10% metal oxides. The figures reveal that all fly ash modified by metal oxide have identical intensity peak positions and lie on the same 2 theta scale as RFA. The result also demonstrated that the fly ash base material continues to dominate the composition of catalyst material (Font et al., 2010; Zhang et al., 2021a). However, metal oxide interaction activity on fly ash was observed, with each metal oxide producing a different effect, as evidenced by an increase in peak intensity more significant than the peaks of RFA. Fig. 3 also shows the XRD spectra for modified fly ash with the addition of 10% ZrO2 (FA/Zr-10%). These XRD spectra results for FA/Zr-10% material depict a main crystalline peak with a quartz phase at 2 thetas at 29.08°, which is dominant with an intensity value of more than 400 a.u. These XRD spectra also have several weak peaks with quartz, hematite, and mullite phases at 14.69°, 31.66°, 36.04°, 43.16°, 47.44°, 52.50°, and 55.85°. While the XRD spectra for FA/Ti-10% show different peaks, the other new dominant peaks in position 2 theta at 27.43°, 31.66°, and 36.05° (mullite phase) and 54.30° (quartz phase) appear quite prominent with an intensity ranging from 100–300 a.u. This phenomenon is due to the crystalline structure of the TiO2 compound being added to the fly ash, which increased its crystallinity (Regonini et al., 2013). The same phenomenon was observed in FA/Mg-10%, with increased in spectral peaks occurring in the same position as FA/Ti-10%, albeit with relatively lower intensity. Compared to RFA, the bonding of fly ash and metal oxides to form a crystal structure shows notable improvements. This enhanced crystalline structure results in a significantly larger surface area, cleaner composition, increased porosity, and greater application versatility. Consequently, the modified fly ash with metal oxides demonstrates great promise in synthesizing adsorbents or catalysts (Lee et al., 2021).

 
3.1.2 Scanning Electron Microscopy (SEM) analysis

Surface morphology was analyzed using SEM for RFA and modified fly ash. The results of the SEM micrograph of all raw fly ash (RFA) and fly ash modification samples can be seen in Fig. 4, revealing the presence of irregularly shaped fly ash particles, mineral aggregates, and agglomerated particles.

Fig. 4. SEM micrographs of (a) RFA, (b) FA/Zr-10%, (c) FA/Ti-10%, and (d) FA/Mg-10%.Fig. 4. SEM micrographs of (a) RFA, (b) FA/Zr-10%, (c) FA/Ti-10%, and (d) FA/Mg-10%.

Fig. 4(a) reveals that the RFA material exhibits porous particles with oval, spherical structures, and irregular and amorphous shapes in its surface morphology. In Fig. 4(b), the surface structure of fly ash modified with 10% ZrO2 (denoted as FA/Zr-10%) is depicted at a magnification of 20,000 times. This evidence reveals the presence of clustered particles with zirconia species adhering to the fly ash surface, indicating that these particles are similar in type but exhibit a higher level of crystallinity. Furthermore, Figs. 4(c) and 4(d) show that amorphous forms still dominate the surface morphology structure. Compared to Fig. 4(a), certain crystalline forms are evident, characterized by multiple hexagonal sections of metal oxides, namely TiO2 and MgO. The surface morphology of this fly ash resembles the findings reported by Srivastava et al. (2014).

Additionally, the particles are stacked together. It can be attributed to the hygroscopic nature of the CaO, SiO2, and Al2O3 compounds in fly ash, which readily absorb moisture from the surrounding air during the sample transportation process for analysis (Panesar et al., 2019). Consequently, aggregation occurs in the RFA and modified fly ash samples, leading to an accumulation effect in the test results.

 
3.1.3 Brunauer–Emmett–Teller (BET) analysis

The BET analysis in this experiment involved two samples: RFA and FA/Zr-10% modified fly ash. The FA/Zr-10% sample exhibited the previously mentioned characteristics, including the highest crystallinity, good binding energy, and good morphology structure compared to other fly ash modifications. Fig. 5(a) displayed variations in the specific surface area and porosity between RFA and the modified fly ash catalyst samples. RFA and the optimized sample, FA/Zr-10%, exhibited a type II N2 adsorption isotherm with type H4 hysteresis loops, per the IUPAC definition (Liu et al., 2022), indicating that both sample particles had mesoporous structures corresponding to monolayer formation. Moreover, ball milling increased the specific surface area of the fly ash particles (Liu et al., 2021).

The N2 adsorption‐desorption isotherm in Fig. 5(a) reinforces the distinction in gas adsorption capacity, showing that fly ash modified with FA/Zr-10% particles have five times greater adsorption capability than RFA particles. Notably, both RFA and the modified fly ash displayed a turning point of 0.95 for nitrogen-desorbed volume, and the N2 adsorbed volume showed a steep increase at higher relative pressures, primarily attributed to the presence of large mesopores (> 10 nm) in the samples. Fig. 5(b) demonstrated that both RFA and the modified fly ash catalyst displayed distinct peaks in pore widths, ranging from approximately 8–12 nm, which could be attributed to the arrangement of the original fly ash structure. The pore distribution analysis indicated several large mesopores in both RFA and modified fly ash samples.

Fig. 5. The BET results of RFA and modified fly ash (FA/Zr-10%) include: (a) N2 adsorption-desorption isotherms, and (b) pore size distribution.Fig. 5. The BET results of RFA and modified fly ash (FA/Zr-10%) include: (a) N2 adsorption-desorption isotherms, and (b) pore size distribution.

Table 3 provides an overview of the findings from the BET analysis. The data highlights that the untreated fly ash, referred to as RFA, exhibits lower BET-specific surface area and pore volume than the modified fly ash. This difference could be attributed to the significant interaction between the metal oxides (ZrO2, TiO2, and MgO) and the fly ash.

Table 3. Summary of BET analysis results of RFA and modified fly ash with metal oxides.

The FA/Zr-10% catalyst with the largest surface area, pore volume, and minor pore diameter was conducive to high ZrO2 dispersion on the carrier, which improved the circulation and transformation of active oxygen species on the catalyst. The specific surface area data can be used to determine the activation energy of the catalyst in the oxidation reaction of NO to NO2. The BET analysis for other various metal oxides is provided in Table S1.

 
3.1.4 X-Ray Photoelectron Spectroscopy (XPS) analysis

X-ray photoelectron spectroscopy (XPS) was utilized to examine the chemical states of elements in the subsurface area (Mudgal et al., 2021). In this experiment, focused analysis was conducted on various compositions of ZrO2 metal oxide in fly ash. In Figs. S1 and S2, the survey O1s and C1s spectra of four different samples, namely FA/Zr-2.5%, FA/Zr-5.0%, FA/Zr-10%, and FA/Zr-15%, indicate the presence of atoms O, C, and metal oxides from the reference material. As illustrated in Fig. S1, the XPS spectra for O1s did not show significant changes in forming new peaks and binding energy scale position due to metal oxides in the modified fly ash. All samples are still dominated by bulk fly ash. However, each sample has differences in the relative intensity of the lattice oxygen surface (Oα) and the chemisorbed oxygen surface (Oβ). Fig. S1(a) displays the main peak at 531.92 eV, corresponding to the lattice oxygen surface (Oα) in all the samples. This lattice oxygen surface is like the one found in SiO2 and Al2O3, as determined by XRD analysis, and these compounds are known to be the primary constituents of fly ash particles. This figure also obtained that the presence of zirconia (ZrO2) in the modified fly ash catalyst for FA/Zr-2.5%, FA/Zr-5.0%, FA/Zr-10%, and FA/Zr-15% will increase the relative intensity of lattice oxygen surface (Oα) by 10319,18 a.u., 10642.14 a.u., 10915.55 a.u., and 11206.26 a.u., respectively. However, the chemisorbed oxygen surface (Oβ) of FA/Zr-15% experienced a decrease to 6587.15 a.u. from 7147.25 a.u. on FA/Zr-10%. This proves that adding metal oxide to fly ash can increase the intensity of the modified fly ash catalysts and improve the oxidation capability of SiO2 and Al2O3 in the bulk fly ash and optimum on adding 10% of metal oxide (Font et al., 2010; Mu et al., 2023). The FA/Zr-15% catalyst exhibited higher intensity in the lattice oxygen surface peak (Oα) and lower intensity in the chemisorbed oxygen surface peak (Oβ) compared to the other catalyst samples. This higher intensity in the lattice oxygen surface peak (Oα) correlates with the highest NO conversion efficiency observed in the SCR reaction using FA/Zr-10% as the catalyst.

The C1s XPS spectra of modified fly ash are shown in Fig. S2. The C1s XPS spectra also give similar trends that indicate modified fly ash has good binding energy differences between FA/Zr-2.5%, FA/Zr-5.0%, FA/Zr-10%, and FA/Zr-15% about 2.50, 2.85, 3.55, and 3.65 eV, respectively.

Fig. 6 presents XPS spectra of Zr-3d (3d 3/2 and 3d 5/2) for various ZrO2 variations on modified fly ash catalysts. The addition of ZrO2 metal oxides to fly ash catalysts is indicated by FA/Zr-2.5%, FA/Zr-5.0%, FA/Zr-10%, and FA/Zr-15%, resulting in sequential increases in 3d 3/2 by 183.65 eV, 183.69 eV, 183.73 eV, and 183.75 eV, respectively. Meanwhile, the valence of Zr4+ 3d 5/2 experiences consecutive decreases of 180.80 eV, 180.78 eV, 180.76 eV, and 180.75 eV. The range of binding energy of Zr multivalence is similar to the research conducted by Cheng et al. (2023). The binding energy value is produced because of the difference in valences that exist between Zr-3d 3/2 and Zr-3d 5/2. Increasing the amount of binding energy that is created results in an increase in the catalytic capacity to convert NO to NO2. In the SCR mechanism, this can potentially improve the effectiveness of the removal of NOx (Yang et al., 2019b).

Fig. 6. XPS spectra of Zr 2p for FA/Zr-2.5%, FA/Zr-5.0%, FA/Zr-10%, and FA/Zr-15%.Fig. 6. XPS spectra of Zr 2p for FA/Zr-2.5%, FA/Zr-5.0%, FA/Zr-10%, and FA/Zr-15%.

In the NH3-SCR mechanism reaction, the C1s spectra, valence variation of ZrO2, and lattice oxygen (Oα) play a crucial role in catalyzing the decomposition of NOx into nitrogen and water (Wang et al., 2023).

 
3.1.5 Surface acidity and basicity of modified fly ash catalysts analysis

In this study, to determine the analysis of surface acidity and basicity analysis of a modified fly ash catalyst, reference was made to the concept developed by Karge (2008). The characterization used to assess the acidic strength of a catalyst involved gravimetric techniques utilizing ammonia and pyridine. Determining the number of acid sites using ammonia as the adsorbate base provides the total number of catalyst acid sites if the small size of NH3 molecules allows them to penetrate the catalyst pores. The number of pyridine acid sites as the adsorbate base represents the number of surface acid sites, assuming that the relatively large size of pyridine molecules allows them to bind only to the catalyst's surface. The results of the quantitative determination of acid sites for each catalyst are presented in Table 4.

Table 4. Summary of the modified fly ash catalyst surface acidity with the gravimetric of Ammonia and Pyridine.

As seen in Table 4, adding various metal oxides can significantly increase the acidity of the modified fly ash catalyst when compared to RFA, whether using ammonia or pyridine as the adsorbate base. The Surface acidity total with NH3 testing findings show that the RFA, FA/Mg-10%, FA/Ti-10%, and FA/Zr-10% catalysts have experienced an increase in total surface acidity, as follows: The values are 4.625, 7.077, 8.307, and 9.664 mmol g1, respectively. By comparison, the pyridine values for the same catalysts are 0.164, 0.256, 0.285, and 0.329 mmol g1. The increase in acidity with ammonia is relatively larger compared to using pyridine. This is presumed to be because ammonia molecules can more effectively reach the metal oxide-modified catalyst's active sites than pyridine molecules (Barzetti et al., 1996). The measurements of acid content are consistent when compared to the specific surface area and average pore size of the modified fly ash catalyst in Table S1.

To characterize the types of acid sites, present on the surface of both RFA and the modified fly ash catalysts, the adsorption of pyridine adsorbate to explain Brønsted and Lewis acid sites is considered. Estimating the type of acid sites that interact with adsorbates in particular adsorption bands is possible. This calculation considers the nature of the contact as well as the intensity of the interaction. The pyridine-FTIR spectra presented in Fig. 7 for RFA, FA/Mg-10%, FA/Ti-10%, and FA/Zr-10% were examined to explore the acid site characteristics in modified fly ash catalysts with varying crystallinities. The peaks observed at 1605 cm1, 1574 cm1, and 1444 cm1 indicate the presence of Lewis acids, while the peak at 1547 cm1 corresponds to Brønsted acids, and another peak at 1477 cm1 suggests the coexistence of Lewis and Brønsted acids (Si et al., 2022; Che et al., 2023). Notably, the enhanced acidity of FA/Zr-10% surpasses that of other modified fly ash catalysts (such as FA/Mg-10% and FA/Ti-10%). This observation aligns with the pyridine-FTIR spectrum of FA/Zr-10%, indicating more acid sites on the catalyst's surface, consistent with BET and NH3 analyses. 

Fig. 7. Pyridine spectra of RFA, FA/Mg-10%, FA/Ti-10%, and FA/Zr-10%Fig. 7. Pyridine spectra of RFA, FA/Mg-10%, FA/Ti-10%, and FA/Zr-10%.

 
3.2 Effects of Metal Oxides Loading over Fly Ash for NOx Removal

The performance of metal oxide-modified fly ash for NOx removal at room temperature (25°C) was investigated. The complete NOx removal illustration can be seen in Fig. S3. The calculation of NOx removal efficiency in this study was carried out based on the experimental data obtained and implemented in Eq. (1). The findings indicated that NO2 did not form because of NO oxidation. This phenomenon was attributed to the absence of sufficient room temperature heat. Metal oxides must act as catalysts to increase their activation energy and interact with fly ash to convert NO to NO2. As a result, the adsorption process was more influential in reducing NOx than the oxidation process. The investigation results also show that the NOx removal performance between RFA and modified fly ash with various metal oxides such as ZrO2, TiO2, and MgO gives different values.

The conversion percentage of NOx removal in RFA only reached 10.39%, occurring after the initial value (first 15 minutes) of the NOx removal test. The low performance of NOx adsorption on RFA was due to the physical properties of fly ash, which are not reactive to NOx without active agents (Cui et al., 2020). Therefore, it is necessary to increase the amount of metal oxides as an active substance to increase NOx adsorption into the pores of the fly ash.

In Fig. 8(a), the results show the addition of various metal oxides to fly ash NOx reduction. Specifically, the NOx reduction percentage for FA/Zr-2.5%, FA/Zr-5%, FA/Zr-10%, and FA/Zr-15% were 13.02%, 21.90%, 35.04%, and 35.71%, respectively. Notably, when the metal oxide loading did not exceed 10%, further increasing the metal oxide content did not result in a proportional increase in the initial NOx reduction value. For instance, a one-and-a-half-fold increase in ZrO2 loading from 10% to 15% resulted in just under a 1% increase in NOx removal at room temperature. This phenomenon may be attributed to potential active site saturation, causing a disproportionate rise in the initial value (Liu et al., 2021). Similarly, increasing the composition of other metal oxides, such as TiO2 and MgO, by up to 10% by weight also enhanced NOx removal efficiency.

Fig. 8. (a) effects of RFA and modified fly ash on NOx removal efficiency at room temperature by different compositions of metal oxides, (b) effects of RFA and modified fly ash on NOx removal efficiency at room temperature by different types of metal oxides, and (c) NO oxidation activity to NO2 at 25°C on the RFA and the modification of fly ash catalysts.Fig. 8. (a) effects of RFA and modified fly ash on NOx removal efficiency at room temperature by different compositions of metal oxides, (b) effects of RFA and modified fly ash on NOx removal efficiency at room temperature by different types of metal oxides, and (c) NO oxidation activity to NO2 at 25°C on the RFA and the modification of fly ash catalysts.
 

Furthermore, as shown in Fig. 8(b), the fly ash-modified NOx removal tests at room temperature for FA/Zr-10%, FA/Ti-10%, FA/Mg-10%, and RFA were 35.04%, 29.06 %, 24.05%, and 10.39%, respectively. This significant increase in NOx removal suggests that modifying the metal oxide deposition on fly ash can enhance the adsorption properties of this material (Corbos et al., 2007a).

Fig. 8(c) depicts the NO oxidation activity to NO2 at 25°C for 90 minutes with a gas hour space velocity of 75,000 GHSV. The NOx gas utilized in this study contained 95%v/v NO and 5%v/v NO2. At room temperature, RFA exhibited minimal conversion of NO to NO2, accounting for only 5.17%v/v, or a mere 0.17% of the initial composition. This result indicates that RFA lacks efficient oxidizing ability under these conditions. On the other hand, modified fly ash with ZrO2, TiO2, and MgO at room temperature increased the NO2 composition by 16.3%, 11.3%, and 10.1%, respectively. This suggests that metal oxides possess the capability to convert NO to NO2.

 
3.3 Effects of Temperature on NOx Removal Performance

Based on the results of reducing NOx by modification of fly ash and metal oxides, and considering the economic feasibility and simplifying laboratory experiments, the explanation of the effect of temperature on the removal of NOx was taken from the three best samples of each metal oxide, namely FA/Zr-10%, FA/Ti-10%, and FA/Mg-10% and RFA as a standard. In Fig. 9(a), each NOx removal test with various temperature variations was carried out for 90 minutes for each experiment. In addition, the experiment was repeated three times for the performance test in NOx removal to assure the reliability and validity of the experimental data. However, the temperature increase was observed to have an insignificant effect on NOx removal in RFA, with levels remaining below 10%. Because of its inert properties, RFA has no increased adsorption or oxidation activity in reducing NOx removal (Lei et al., 2021).

Fig. 9. Effect of temperature on fly ash modification: (a) NOx removal for various metal oxides, (b) NO2 generation for various metal oxides, (c) NO vs. NO2 selectivity for FA/Zr-10%.Fig. 9. Effect of temperature on fly ash modification: (a) NOx removal for various metal oxides, (b) NO2 generation for various metal oxides, (c) NO vs. NO2 selectivity for FA/Zr-10%.

In comparison, fly ash modified by metal oxides (ZrO2, TiO2, and MgO) demonstrated increased adsorption and oxidation activity, which was beneficial for NOx removal. Fig. 9(a) also shows the NOx removal efficiency values for each metal oxide (FA/Zr-10%, FA/Ti-10%, and FA/Mg-10%) were 35.6%, 29.06%, and 23.96% at room temperature (25°C), increasing to 38.15%, 32.15%, and 26.75% at 50°C respectively. In addition, the optimum increase in NOx removal efficiency is achieved at 250°C, reaching 63.55%, 49.12%, and 42.5%, respectively. Beyond 250°C, the NOx removal efficiency gradually decreases until it stabilizes between 300°C and 400°C. Fig. 9(b) shows the NO2 generation in ppb (part per billion) for the RFA and the modification of fly ash catalysts at 50°C–400°C. The effect of temperature can increase the production of NO2 gas species on every single catalyst, namely RFA, FA/Zr-10%, FA/Ti-10%, and FA/Mg-10% at 10.20 ppb, 50.25 ppb, 56.23 ppb, and 68.11 ppb at 50°C, respectively, to 30.20 ppb, 245.10 ppb, 292.70 ppb, and 323.50 ppb at 250°C, respectively. In contrast, the NO2 generation can be decreased gradually after a temperature of 250°C–400°C. This result indicates that the catalytic activity of certain metal oxides becomes saturated at temperatures exceeding 250°C, reducing their ability to oxidize NO to NO2. The phenomenon of decreased catalytic activity at high temperatures has been previously observed and described by Han et al. (2019) and Wang et al. (2017).

Fig. 9(c) also clearly shows the percentage composition of NO and NO2 for the most optimum catalyst modification of fly ash, namely FA/Zr-10%, where at 250°C, the ratio of NO and NO2 is 20% and 80%, respectively.

 
3.4 Mechanism and Kinetic Study


3.4.1 The oxidation of the NO-NO2 mechanism

The results indicate that the NOx adsorption capability of FA/Zr-10% depends on two factors. Firstly, numerous selective adsorption sites are formed by the Zr4+ ion and ZrO2 on the sample, enabling interaction with the adsorbate NOx. However, increasing the amount of ZrO2 does not lead to an increase in the number of effective adsorption sites; instead, excessive ZrO2 loading obstructs the channels of the fly ash, resulting in reduced adsorption capacity.

The second factor is associated with physical adsorption, which is influenced by the structure of the modified fly ash, its surface area, micropore volume, and the experimental conditions. The combined effect of these two factors determines the NOx adsorption on the surface of the modified fly ash adsorbent. A surface reaction occurs when gas NOx is adsorbed on FA/Zr-10%, as shown in Rxns. (R4) and (R5):

 

The surface reaction takes place as described by Rxn. (R6) when NO and O2 adsorb on the FA/Zr-10% sample:

 

These four simultaneous reactions ((R4), (R5), (R6), and (R7)) can be approximated using first-order reactions, which allows to derive Eqs. (3–4). Specifically, it indicates that NO is significantly less strongly adsorbed on modified fly ash than NO2. Therefore, examining the impact of O2 concentration on the adsorption capacity of FA/Zr-10% samples for NOx removal is crucial.

Fig. 10 presents the Arrhenius plots for NO oxidation for the three samples based on Eq. (3) and Eq. (4). The slope of the plot allows for the calculation of the reaction activation energy and reaction rate of oxidation (–r) (Liu et al., 2021). The activation energy for the modified fly ash FA/Zr-10% of 75.2 kJ mol–1 is higher compared to that of FA/Ti-10% (30.9 kJ mol–1) and FA/Mg-10% (14.9 kJ mol–1). This difference suggests that the surface-adsorbed oxygen species on FA/Zr-10% are more easily activated, making it a more effective catalyst for NOx removal. This is because NO2 is more reactive than NO (Li et al., 2017), and FA/Zr-10% can be further modified using other methods, such as SCR.

Fig. 10. Arrhenius plots the NO oxidation rate on FA/Zr-10%, FA/Ti-10%, and FA/Mg-10%.Fig. 10. Arrhenius plots the NO oxidation rate on FA/Zr-10%, FA/Ti-10%, and FA/Mg-10%.

Table S2 revealed the comparison of oxidation reaction rates for various metal oxides. The oxidation reaction rate of FA/Mg-10%, FA/Ti-10%, and FA/Zr-10% were 8.0 × 10–5 kJ mol–1, 1.10 × 10–4 kJ mol–1, and 3.37 × 10–4 kJ mol–1, respectively. The oxidation reaction rate of the FA/Zr-10% catalyst is three times faster than the FA/Ti-10% catalyst and four times faster than the FA/Mg-10% catalyst, indicating the modified fly ash with 10%wt. ZrO2 (denoted as FA/Zr-10%) has good potential as a catalyst for NOx removal. These findings suggest a strong correlation between the characterization analysis data and the oxidizing capability of the modified fly ash catalyst. The combination of FA/Zr-10%, which exhibits high crystallinity, substantial binding energy, and a large specific surface area, demonstrates the highest oxidizing ability among all the modified fly ash catalysts.

 
3.4.2 Adsorption kinetics model analysis

Different kinetics models, such as the Weber–Morris kinetics model, Elovich kinetics model, first-order pseudo-kinetics model, and pseudo-second kinetics model, were used to study how modified fly ash binds to NOx in the flue gas. Once the NOx removal concentration data is obtained from the experiment, the subsequent task involves processing the data by creating plots using various equations of the adoption kinetic model. A suitable adsorption kinetics model was developed based on these investigations. The setup of this model is depicted in Fig. 11(a).

Fig. 11. The modified fly ash (FA/Zr-10%) data on (a) fitting of the adsorption kinetics models, (b) fitting of the adsorption isotherm models, and (c) the NOx removal efficiency vs N2 selectivity at various temperatures of the modified fly ash catalyst.Fig. 11. The modified fly ash (FA/Zr-10%) data on (a) fitting of the adsorption kinetics models, (b) fitting of the adsorption isotherm models, and (c) the NOx removal efficiency vs N2 selectivity at various temperatures of the modified fly ash catalyst.

The adsorption capacity of modified fly ash for NOx progressively increases with extended adsorption periods, eventually reaching equilibrium after 60 hours of reaction time. The saturated adsorption capacities of NOx were found to be 0.763 mg g–1. From the results presented in Table S3, the coefficient of determination for the Weber–Morris Model is lower than 0.9 or only 0.729, while the coefficients of determination for the Elovich, the first-order pseudo-kinetic, and the pseudo-second-order pseudo-kinetic models are 0.942, 0.994, and 0.974, respectively. This evidence suggests that these three models describe the adsorption process satisfactorily and indicate the simultaneous occurrence of intraparticle diffusion, chemisorption, and physisorption during adsorption (Largitte and Pasquier, 2016).

Nevertheless, significant differences exist between the real saturated and theoretical adsorption capacity estimated through the pseudo-second-order kinetics and the Elovich model. The coefficient of determination (R2) for these models is inferior to that of the first-order pseudo-kinetic model, suggesting that chemical adsorption and internal diffusion do not predominantly govern the process (Deihimi et al., 2018; Liu et al., 2022).

On the other hand, the first-order pseudo-kinetic model exhibits a theoretical adsorption capacity that closely corresponds to the experimental saturation adsorption capacity, with an R2 value surpassing 0.994. Consequently, the pseudo-first-order kinetic model provides a more precise representation of the adsorption characteristics of modified fly ash on NOx, indicating that adsorption mass transfer and external diffusion are crucial factors and physical adsorption primarily governs the process (Largitte and Pasquier, 2016). Moreover, the increased specific surface area of modified fly ash boosts the physical adsorption of NOx.

 
3.4.3 Adsorption isotherm model analysis

For a more comprehensive examination of NOx adsorption using modified fly ash, the Langmuir adsorption isotherm model and the Freundlich adsorption isotherm model were utilized to analyze the data and create an adsorption isotherm model (Kalam et al., 2021; Liu et al., 2022). The Langmuir model accurately describes monolayer adsorption, in which a single solute molecule is adsorbed on each adsorption site of the adsorbent surface. On the other hand, the Freundlich model is ideal for describing non-ideal adsorption behavior on heterogeneous adsorbent surfaces (Liu et al., 2022).

Fig. 11(b) illustrates the application of the adsorption isotherm model to the NOx removal process onto the surface of the modified fly ash. The results reveal that the correlation coefficient (R2) for NOx adsorption using the Langmuir equation is 0.998. This high R2 value indicates that the Langmuir adsorption isotherm model accurately captures the adsorption process of modified fly ash on NOx. The detail of adsorption isotherm parameter information is shown in Table S4.

However, the Freundlich adsorption isotherm model produced a correlation coefficient (R2) marginally lower at 0.878. This result suggests that the NOx adsorption characteristics of modified fly ash are not entirely consistent with the Freundlich adsorption isotherm model and that the adsorption behavior does not exhibit layered adsorption (Largitte and Pasquier, 2016; Liu et al., 2022). The NOx combined with the fitting outcomes of the kinetic model, it becomes evident that the NOx adsorption process by modified fly ash involves a combination of chemical and physical adsorption.

The primary purpose of the Langmuir adsorption isotherm model is to estimate the maximum adsorption capacity of modified fly ash under optimal conditions. The theoretical maximum adsorption capacity of NOx is determined to be 0.993 mg g1.

 
3.4.4 The selective catalytic reduction (SCR) reaction mechanism

SCR using NH3 to remove NOx over modified fly ash has been investigated. In this study, the modified fly ash catalyst material used was FA/Zr-10%, which involved 500 ppm of NH3 as an excess reactant in this NH3-SCR process with a gas hour space velocity of 75,000 GHSV and a temperature range of 50°C–400°C for the SCR experiment.

In order to comprehend the NH3-SCR reaction process of NO on the FA/Zr-10% catalyst, the initial focus was investigating the adsorption properties of the reactants, potential intermediates, and products. The calculations considered all possible adsorption sites (ZrO2, SiO2, and Al2O3) and orientations (parallel and perpendicular). Through the analysis of adsorption energy, structure, and equilibrium distance, the active sites for the adsorption of different substances were determined. The dehydrogenation reaction of NH3 on the FA/Zr-10% catalyst was also examined based on the dissociation barrier. The primary reaction pathway for N2 formation was established by comparing the energy barriers of various reactions.

The experimental results of NOx removal using NH3-SCR can be observed in Fig. 11(c). This figure provides evidence of the effectiveness of the NH3-SCR method in removing NOx. The efficiency of NOx removal exhibits a positive correlation with temperature, wherein the removal efficiency gradually increases from 50°C–250°C, reaching a peak efficiency of 90.3%. However, beyond 250°C and up to 400°C, the removal efficiency experiences a gradual decline. This decline can be attributed to the saturation of active sites on the FA/Zr-10% modified fly ash, resulting in a slower NH3-SCR reaction (Zhang et al., 2024). Additionally, this decrease may also be caused by the production of water (Zengel et al., 2021), which becomes entrapped within the catalyst pores and may be due to the inhibition of surface oxygen species (Lisi and Cimino, 2020), thereby potentially disrupting the performance of the catalyst's active sites in reducing NOx to N2 at temperatures above 250°C up to 400°. Furthermore, at high temperatures, the catalyst's ability to oxidize NO increases, resulting in a decline in catalyst selectivity. This condition was consistent with the previous study findings regarding other categories of catalysts (Chung et al., 2020). Fig. 11(c) also illustrates N2 selectivity in the modified catalyst FA/ZrO2 10%. At a temperature of 25°C, N2 selectivity shows excellent performance, exceeding 80% and increasing with the operating temperature up to 150°C, where N2 selectivity approaches 83%. However, as the temperature reaches 200°C, N2 selectivity declines to 80.57%, dropping further to around 76.61% at 250°C, and continuing to decrease to 60.3% at 400°C. This decrease may be attributed to the increased production of H2O, which affects the reduction of the catalyst's active surface at higher temperatures, thereby reducing the selectivity for N2. Additionally, the decline in N2 selectivity is also due to the heightened oxidation capability of NO to NO2, strongly influencing the catalyst's selectivity (Che et al., 2023).

Several reaction steps coincide in the NH3-SCR process, including the adsorption of NH3 and NO onto the surface of the catalyst, followed by an oxidation reaction of NO, then dehydrogenation of NH3 to N2O and the primary SCR reaction converting NOx to N2 and H2O involving NH3 and O2 (Chen et al., 2020). A simpler mathematical reaction mechanism approach is needed to solve the problem of the kinetics of the NH3-SCR reaction that occurs, namely using the Langmuir-Hinshelwood mechanism and the Eley-Rideal mechanism, as described in the following equation.

The Langmuir-Hinshelwood mechanism can be explained as follows:

 

Meanwhile, NO reduction in fly ash modification through the Eley-Rideal mechanism can be explained as follows:

 

The combination of the Langmuir-Hinshelwood mechanism and the Eley-Rideal mechanism is provided in detail in Supplementary Information. It has been simplified by Xiong et al. (2015) into the following equation:

 

The parameters described are as follows: kN2(L-H) represents the reaction rate constant for N2 formation over the modified fly ash catalyst via the Langmuir-Hinshelwood mechanism, kN2(E-R) is the reaction rate constant for N2 formation through the Eley-Rideal mechanism, and kN2O stands for the reaction rate of N2O formation.

After the experiments, the obtained data regarding the decrease in NOx concentration are plotted using Rxn. (R13). This plot enables the determination of several parameters, including the main reaction rate (–rNOx), the Langmuir-Hinshelwood reaction kinetics rate constant (kN2(L-H)), the Eley-Rideal reaction kinetics rate constant (kN2(E-R)), and N2O formation kinetics rate constant (kNO2). The solver application on Microsoft Excel can determine these parameters, providing the parameter data.

A summary of the NH3-SCR reaction mechanism parameter data can be seen in Table S5. This table exhibits the results of the SCR mechanism conducted between 50°C–400°C. As the temperature increases, there is a gradual enhancement in the rate of the NOx reduction reaction. The highest rate of NOx reduction is achieved at 250°C, reaching 6.287 × 10–2 min–1, and no further increase in the reaction rate is observed beyond this temperature. This behavior is likely due to the saturation of the catalyst's active sites at temperatures exceeding 250°C (Lai and Wachs, 2018).

Moreover, the SCR process involves a reaction that leads to the formation of N2O species, and this reaction rate also rises with increasing temperature during the SCR process. This is evident from the peak reaction rate of 250°C, measuring 1.653 × 10–2 mol g–1 min–1. This emphasizes the significant impact of reaction temperature on the SCR mechanism and establishes it as a crucial factor. Furthermore, it is essential to note that the optimum temperature is at 250°C, where the kinetic rate constants for the Langmuir-Hinshelwood reaction, the Eley-Rideal reaction, and the formation of N2O are reported as 1.496 × 10–2 mol g–1 min–1, 2.09 mol g–1 min–1, and 1.658 × 102 mol g–1 min–1, respectively. The data on the reaction rate constants indicate that the most significant one is attributed to the Eley-Rideal mechanism, with a value of 2.09 mol g–1 min–1. This evidence implies that the Eley-Rideal mechanism dominates the reaction route of the SCR reaction of the NOx removal process over the FA/Zr-10% catalyst for any temperature. The same research findings regarding the dominance of the reaction route of the Eley-Rideal mechanism in the SCR method have also been disclosed by Zhang et al. (2021b).

A summary comparison has been made for the current modified fly ash catalysts with other SCR catalysts reported in the literature (Table 5). This comparative data demonstrates that fly ash modified with ZrO2 of 10% can compete with other catalysts in terms of cost-effectiveness and has the potential for further performance enhancement in NOx removal.

Table 5. The summary comparison of SCR catalyst performance for NOx removal.


4 CONCLUSION 


The novel fly ash modification catalysts were successfully synthesized using ZrO2, TiO2, and MgO as NOx removal precursors, demonstrating excellent SCR performance. Among the catalysts examined, the best combination was FA/ZrO2-10%, which exhibited superior NOx removal efficiency. This catalyst also demonstrated a substantial activation energy of 75.19 kJ mol–1 in the oxidation reaction of NO to NO2. Furthermore, the kinetic adsorption model employed in this study followed a pseudo-first-order model, highlighting the importance of adsorption mass transfer and external diffusion. The adsorption process mainly relied on physical adsorption, specifically monolayer adsorption.

In the NH3-SCR experiment using the Langmuir-Hinshelwood mechanism and the Eley-Rideal mechanism approach, a notable NOx removal efficiency of 90.3% was obtained at 250°C, with a reaction rate of 6.287 × 10–2 min–1. The results indicate a strong correlation between the characterization analysis data and the oxidizing capacity of the modified fly ash catalyst. Another advantage of this novel catalyst is its utilization of solid waste incineration by modifying recycled fly ash with 10% ZrO2. This evidence reduces the catalyst cost to one-ninth of conventional catalysts and enables operation at lower temperatures (250°C). This discovery underscores the potential and feasibility of combining modified fly ash waste with metal oxides for future low-cost NOx removal applications.

 
NOMENCLATURES


Co      Initial concentration of adsorbate (mg L–1)
Ce  Concentration of adsorbate at time t (mg L–1)
qe  Experimental amount of adsorbate sorbed at instant t (mg g–1)
q Amount of adsorbate sorbed at time t (mg g–1)
qm  Maximum amount of adsorbate sorbed (mg g–1)
k   Reaction rate constant (mol g–1 min–1)
kN2(E-R)  The Eley-Rideal reaction kinetics rate constant (mol g–1 min–1)
kN2(L-H)  The Langmuir-Hinshelwood reaction kinetics rate constant (mol g–1 min–1)
kNO2  The N2O formation kinetics rate constant (mol g–1 min–1)
K  Langmuir rate constant
K Freundlich rate constant
α Initial sorption rate in the Elovich model (mg g–1 s–1)
β  Constant related to the extent of surface coverage and activation energy for Chemisorption in the Elovich model (g mg–1)
η the efficiency of NOx removal
Oα  Lattice oxygen surface peak
Oβ Chemisorbed oxygen surface peak
ΔE Binding energy (eV)
Ea Activation energy (J mol–1)
mo Weight of adsorbent (g)
SBET  Specific surface area (m2 g–1)
Total flow rate (mol s–1)
Standard atmosphere (101,325 Pa)
R Gas constant (8.314 J mol–1 K–1)
T Reaction temperature (K)
CFA Coal fly ash
ISWIFA  Industrial solid waste incineration fly ash
MSWIFA Municipal solid waste incineration fly ash
RFA  Raw fly ash (untreated)
SCR Selective catalytic reduction

   

 
ACKNOWLEDGMENT


This work was supported by financial aid from Chung Yuan Christian University, Taiwan (Project No. 109609432). The authors thank the Circular and Society Laboratory group, Chung Yuan Christian University, Taiwan, for their cooperation and support.

 
ADDITIONAL INFORMATION AND DECLARATIONS


Declaration of Competing Interest

The authors declare that they have no competing financial interests or personal relationships that could have influenced the work reported in this paper.

 
Credit Author Contribution Statement

Darmansyah Darmansyah: Data curation, Formal analysis, Investigation, Writing – original draft preparation. Sheng-Jie You: Writing – review and editing, Supervision. Ya-Fen Wang: Conceptualization, Funding acquisition, Writing – review and editing, Supervision.


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