Fabrication of VO_x/AFCC-PG Catalyst from Waste Support for Hg⁰ Removal in Flue Gas

Abstract

The efficient removal of elemental mercury (Hg⁰) from coal-fired flue gas is a critical challenge in environmental governance. This study proposes utilizing waste fluid catalytic cracking catalyst (WFCC) as the potential support for Hg⁰ catalytic oxidation. After activation (AFCC) via calcination decarbonization, a composite support (AFCC-PG) was fabricated using palygorskite (PG) as a binder. Subsequently, VO_x was loaded onto the support to form the VO_x/AFCC-PG catalyst for Hg⁰ removal. Experimental results demonstrate that the VO_x/AFCC-PG catalyst achieves >95% Hg⁰ removal efficiency under simulated flue gas conditions (150 °C, GHSV = 6000 h⁻¹) while maintaining excellent stability over 60 h. Furthermore, Hg-TPD and XPS analyses indicate that the synergistic lattice oxygen oxidation–adsorption established between VO_x and the AFCC-PG plays a key role in efficient Hg⁰ removal. This study proposes a cost-effective strategy for both the resource utilization of waste catalysts and the control of mercury pollution in coal-fired flue gas.

1. Introduction

Mercury emissions from coal-fired power plant flue gas pose severe hazards to the natural environment and human health [1,2]. Among mercury species, elemental mercury (Hg⁰) has become a focal point in global environmental governance due to its high toxicity, bioaccumulation potential, and long-range transport capability [3,4]. More critically, the low solubility and chemical inertness of Hg⁰ make it difficult for it to be effectively removed by existing air pollution control devices such as wet flue gas desulfurization (WFGD) and electrostatic precipitators (ESPs) [5,6]. Therefore, there is an urgent need to develop efficient and low-cost mercury removal technologies. Catalytic oxidation technology, which converts Hg⁰ into highly water-soluble oxidized mercury (Hg²⁺), has emerged as a promising strategy [7,8,9]. Among various catalysts, supported transition metal oxide catalysts are widely applied for Hg⁰ catalytic oxidation owing to their advantages such as high specific surface area, good dispersion of active components, strong support–metal (SM) interaction, and low cost [10,11,12]. The performance, cost, and application potential of such catalysts are highly dependent on the properties of the support, as well as the sourcing cost of the support [13,14,15].

Notably, a vast amount of solid waste generated from industrial processes, if possessing suitable physicochemical properties, can serve as an ideal low-cost support source for catalysts [16,17,18]. Fluid catalytic cracking, being one of the core processes for heavy oil upgrading, utilizes FCC catalysts that gradually deactivate during operation due to the continuous accumulation of coke and heavy metals (e.g., Ni, V, and Fe) [19]. It is reported that the global refining industry consumes hundreds of thousands of tons of FCC catalysts annually, resulting in enormous quantities of waste FCC catalyst (WFCC) [20,21]. WFCC contains environmentally hazardous metal elements such as Ni, V, and Fe, classifying it as hazardous solid waste whose improper disposal can lead to severe environmental pollution and resource wastage [22,23]. However, from a resource utilization perspective, WFCC possesses unique advantages for application as a support in catalysts [19,24]. Firstly, as a typical zeolitic material, it features a regular pore structure, high specific surface area, and low wear rate. More importantly, WFCC inherently contains catalytically active metal components like vanadium and iron for Hg⁰ oxidation, eliminating the need for complex metal removal processes [25]. Consequently, the secondary utilization of WFCC as a support not only provides potential materials for developing efficient and low-cost Hg⁰ oxidation catalysts but also significantly reduces the disposal cost of hazardous solid waste [19,24].

Herein, WFCC was employed as the source of support, followed by its activation via calcination decarbonization to produce activated FCC support (AFCC). Subsequently, palygorskite (PG) was employed as a binder to fabricate a composite support (AFCC-PG) via a forming process. Building upon this, the VO_x active component was loaded to prepare the VO_x/AFCC-PG catalyst. Then, the Hg⁰ removal performance of this catalyst was systematically investigated in simulated flue gas, and the effects of key reaction conditions (temperature, gas hourly space velocity, and initial Hg⁰ concentration) and critical flue gas components (O₂, SO₂, NO, H₂O, etc.) on mercury removal efficiency were specifically examined. Combined characterization techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and mercury temperature-programmed desorption (Hg-TPD), were employed to elucidate the structure of the catalyst, Hg species, and reaction mechanism. This research provides a theoretical foundation and technical reference for the resource utilization of waste catalysts and mercury pollution control in coal-fired flue gas.

2. Results and Discussion

2.1. Catalyst Characterization

The elemental composition of WFCC was analyzed by XRF (results shown in Table S1). The results indicate that WFCC primarily contains Al and Si elements, consistent with its main components being alumina and silica. Concurrently, metal elements such as vanadium (V) and iron (Fe) are detected, originating from heavy oil impurity deposition during operation of FCC catalysts. Notably, these metals exhibit potential oxidative activity for catalytic Hg⁰ removal. XRD patterns (Figure 1a) reveal distinct characteristic diffraction peaks of Al₂O₃, SiO₂, Y-type zeolite (USY), and ZSM-5 in WFCC, demonstrating the preservation of its fundamental crystal structure [20,24]. To activate the WFCC support, calcination was performed at 500 °C for 1 h under an air atmosphere, with Figure 1b displaying the variation in CO_x concentration released during the process. As the temperature increased to 500 °C, the CO_x concentration began to rise at approximately 140 °C, reaching its peak at around 500 °C, indicating combustion removal of carbon deposits within this temperature range. Carbon elemental analysis (Table S2) further confirms that the carbon content of the activated AFCC dramatically decreases to 0.46% from 26.86% (WFCC), achieving a removal efficiency of 98.29%. Furthermore, comparative XRD patterns (Figure 1a) confirm negligible alterations in the crystal structure of the support after activation, providing essential evidence for its secondary utilization as a catalyst support. Additionally, the VO_x/AFCC-PG catalyst was prepared by first fabricating a composite of AFCC support and PG binder, followed by vanadium oxide deposition. As shown in Figure 1a, the characteristic SiO₂ peak (26.4°) intensity significantly increases after AFCC and PG compositing, consistent with the predominant silica composition of PG. Notably, even at a high loading of 10 wt.%, no characteristic peaks of crystalline vanadium oxides (e.g., V₂O₅) were observed after vanadium species loading. To clarify its phase composition, a vanadium oxide sample without AFCC-PG support was synthesized using identical procedures to those for VO_x/AFCC-PG. XRD analysis (Figure S1) reveals distinct diffraction peaks at 15.6°, 20.5°, and 26.4° corresponding to the (200), (010), and (101) crystal planes of V₂O₅ [26], respectively, confirming the formation of crystalline V₂O₅. Therefore, the absent characteristic peaks of V₂O₅ in the VO_x/AFCC-PG may be attributed to the high dispersion of V₂O₅ on the support, or masking of V₂O₅ signals by the diffraction peaks of the AFCC-PG support.

Figure 1c,d display the N₂ adsorption–desorption isotherms and corresponding pore size distributions of the AFCC, PG, AFCC-PG, and VO_x/AFCC-PG catalysts. All samples exhibit typical Type IV isotherms with H3-type hysteresis loops, indicating mesoporous structures. However, AFCC shows a lower capillary condensation pressure (Figure 1c), which indicates a smaller average pore size. As shown in Figure 1d, the pore size distributions of all four samples are predominantly concentrated around 3.8 nm. Notably, compared with the AFCC, a distinct broad peak appears at 20 nm for the PG material, indicating the presence of larger mesopores. Furthermore, for the composite sample (AFCC-PG, and VO_x/AFCC-PG), this broad peak shifts significantly toward smaller pore sizes due to the combined pore structure characteristics inherited from AFCC and PG materials. Detailed parameters including specific surface area, pore volume, and average pore size are listed in Table S3. The specific surface areas of AFCC, PG, AFCC-PG, and VO_x/AFCC-PG are 85.6 m²/g, 135 m²/g, 84.2 m²/g, and 48.8 m²/g, respectively. Compared to AFCC, PG exhibits a higher specific surface area, while AFCC-PG shows a slightly lower specific surface area than AFCC. After further loading of VO_x, the specific surface area of the VO_x/AFCC-PG catalyst significantly decreases to 48.8 m²/g, attributed to the deposition of VO_x active components on the support surface and within the pores, occupying partial pore space. To investigate the microscopic morphology of the samples, SEM analysis was performed (Figure 2). AFCC primarily forms a blocky structure composed of densely packed spherical or quasi-spherical particles (Figure 2a–c), consistent with its low specific surface area and pore size. After compositing with PG (Figure 2d–f), rod-like PG structures are observed attached to the blocky AFCC particles for the AFCC-PG support. The VO_x/AFCC-PG catalyst loaded with vanadium active components (Figure 2g–i) retains morphological features similar to the AFCC-PG support.

2.2. Catalytic Activity

2.2.1. Effect of Reaction Conditions on Catalytic Activity

Figure 3a compares the Hg⁰ removal performance of AFCC, PG, AFCC-PG, and VO_x/AFCC-PG catalysts under simulated flue gas (N₂, 5% O₂, 3% H₂O, 1500 ppm SO₂, 200 μg·m⁻³ Hg⁰) at 150 °C with a gas hourly space velocity (GHSV) of 6000 h⁻¹. The results show that the Hg⁰ removal efficiency of the heat-treated AFCC catalyst is only ~22%. PG exhibits higher mercury removal activity (~40%) due to its superior adsorption capability [27]. After compositing AFCC with PG (AFCC-PG), the activity is slightly higher than that of AFCC but still lower than PG, likely attributable to the reduced specific surface area, which weakens adsorption capacity. Notably, the performance increases markedly after VO_x loading, maintaining >95% Hg⁰ removal efficiency for 300 min. This indicates that VO_x loading effectively combines its catalytic oxidation capability with the adsorption effect of support, synergistically promoting efficient Hg⁰ removal.

Figure 3b examines the effect of reaction temperature (120–210 °C) on the performance of the VO_x/AFCC-PG catalyst. The Hg⁰ removal efficiency initially increases and then decreases with rising temperature. At lower temperatures (<150 °C), adsorption of Hg⁰ on the catalyst surface dominates, but the oxidation activity of VO_x is limited. At higher temperatures (>150 °C), oxidation activity enhances while adsorption weakens. At 150 °C, optimal synergy between adsorption and oxidation processes is achieved, resulting in peak mercury removal efficiency [28]. GHSV is a critical parameter for industrial applications [29]. As shown in Figure 3c, the Hg⁰ removal efficiency of the VO_x/AFCC-PG catalyst decreases with increasing GHSV. When GHSV rises to 12,000 h⁻¹, efficiency drops to ~80%, attributed to shortened flue gas residence time restricting sufficient contact with active sites. Actual flue gas exhibits fluctuations in Hg⁰ concentration [30]. Figure 3d evaluates the impact of inlet Hg⁰ concentration (140–226 μg·m⁻³). Within this range, the VO_x/AFCC-PG catalyst demonstrates excellent Hg⁰ removal capability, indicating strong resistance to Hg⁰ concentration fluctuations.

2.2.2. Effect of Flue Gas Components on Catalytic Activity

To investigate the effect of O₂ on the VO_x/AFCC-PG catalyst, Hg⁰ removal experiments were conducted at O₂ concentrations of 0%, 5%, 10%, and 20% (Figure 4a). The results show that in pure N₂ atmosphere, the catalyst activity slightly decreases as the reaction proceeds. Under 5% O₂ concentration, removal efficiency increases to 95%, but further elevating the O₂ concentration to 10% and 20% yields limited improvement. This is primarily attributed to O₂ replenishing the consumed lattice oxygen of VO_x during catalytic oxidation, restoring its oxidative activity, while simultaneously generating chemisorbed oxygen on the catalyst surface for direct Hg⁰ oxidation [30]. At higher O₂ concentrations, chemisorbed oxygen and the oxygen required for lattice oxygen regeneration approach saturation, resulting in diminished enhancement of Hg⁰ removal. Additionally, the transient O₂ response experiment (Figure 4b) further confirms its critical role. Upon O₂ cessation, Hg⁰ removal efficiency rapidly declines due to rapid consumption of lattice/surface-adsorbed oxygen, while reintroducing O₂ restores efficiency to 78% swiftly with gradual stabilization, but it fails to recover to >90%. This indicates that severe oxygen deficiency induces local structural changes in vanadium oxides, and structural recovery requires overcoming energy barriers upon re-exposure to O₂, preventing full reconstruction of partial active sites [31,32].

The effect of SO₂ on Hg⁰ removal varies with catalysts, manifesting as inhibition, promotion, or no significant impact [33,34,35]. Figure 4c shows that upon adding 500 ppm SO₂ to N₂, the mercury removal efficiency of VO_x/AFCC-PG sharply decreases from 88% to 52%. When the SO₂ concentration increases to 1000 ppm and 1500 ppm, efficiency further drops to ~37% and ~33%, respectively. This indicates a pronounced concentration-dependent inhibition by SO₂, attributed to competitive adsorption of SO₂ at VO_x active sites hindering Hg⁰ removal. After introducing 5% O₂ (Figure 4c), SO₂ inhibition is significantly mitigated with efficiency increasing to 92%, 91%, and 86% in N₂ containing 500, 1000, and 1500 ppm SO₂, respectively. This likely stems from O₂ weakening competitive adsorption of SO₂ against Hg⁰, and O₂ can also react with SO₂ to form SO₃, which subsequently combines with Hg⁰ to generate HgSO₄, thereby promoting mercury capture [34]. The transient SO₂ response experiment (Figure 4d) reveals that after introducing 1500 ppm SO₂, Hg⁰ removal efficiency first rapidly decreases to 67.3%, then slowly recovers to ~85%. Upon ceasing SO₂ supply, efficiency only recovers to 87%, failing to return to the initial level. This confirms irreversible poisoning of VO_x/AFCC-PG by SO₂, due to SO₂ occupying active sites and forming sulfate species covering active centers, degrading catalytic performance [34].

To investigate the effect of NO on Hg⁰ removal over the VO_x/AFCC-PG catalyst, experiments were conducted at varying NO concentrations (Figure 4e). The results show that introducing NO into N₂ atmosphere slightly enhances Hg⁰ removal efficiency, indicating a promotional effect of NO [36]. This likely occurs because VO_x oxidizes NO to NO₂, and NO₂ synergistically enhances Hg⁰ oxidation by VO_x/AFCC-PG. When adding 5% O₂ to NO-containing N₂ flue gas, the NO oxidation rate and promotional effect of NO₂ are further strengthened, leading to additional efficiency improvement. H₂O in flue gas also affects catalyst performance. As shown in Figure 4f, adding 3% H₂O to N₂ rapidly reduces Hg⁰ removal efficiency from 88% to 75% for VO_x/AFCC-PG. Further increasing the H₂O content causes additional efficiency decline, primarily due to competitive adsorption of H₂O at active sites against Hg⁰, degrading catalytic performance [37].

2.3. Catalytic Mechanism and Application Potential

XPS analysis of V, O, and Hg elements was performed to investigate valence state and surface species changes on the VO_x/AFCC-PG catalyst before and after Hg⁰ removal. Figure 5 displays V 2p, O 1s, and Hg 4f XPS spectra of the fresh and used catalysts, with detailed parameters listed in Table S4. The V 2p spectra of fresh and used VO_x/AFCC-PG catalysts (Figure 5a,d) reveal a dominant V⁵⁺ peak at 517.4 eV and a minor V⁴⁺ peak at 516.2 eV, confirming that +5 valence V₂O₅ species dominate the catalyst surface [26]. The decreased V⁵⁺/V⁴⁺ ratio of used VO_x/AFCC-PG catalyst indicates a partial reduction of V⁵⁺ to V⁴⁺ during Hg⁰ catalytic oxidation. O 1s spectra of fresh and used catalysts are shown in Figure 5b,e. The O 1s spectrum of fresh catalyst (Figure 5b) can be deconvoluted into three peaks: lattice oxygen (O_lat) at 530.5 eV, adsorbed oxygen (O_ads) at 532.0 eV, and hydroxyl oxygen (O_OH) at 533.2 eV. O_ads exhibits the highest concentration (71.3 at%), while O_lat accounts for only 15.53 at%, demonstrating abundant adsorbed oxygen on the surface. After reaction (Figure 5e), the O_lat proportion decreases from 15.53% to 7.77%, whereas O_ads increases from 71.3% to 81.63%, evidencing consumption of VO_x lattice oxygen for Hg⁰ oxidation [38,39]. The Hg 4f peak overlaps with Si 2p in binding energy (Figure 5c,f). Given the trace Hg adsorption relative to high Si content in the catalyst, the single peak at 102.8 eV is primarily attributed to Si 2p. To verify the presence of mercury species, we performed SEM-EDS elemental mapping on the post-reaction catalyst. As shown in Figure S2, the distinct Hg elemental mapping confirms the existence of mercury species.

To investigate the mercury species adsorbed on the sample surface, Hg-TPD experiments were performed on the used catalysts (Figure 6a). For the PG support, the low-temperature desorption peak (190 °C) detected after reaction is attributed to adsorbed Hg⁰ species, while the minor high-temperature desorption peak (300 °C) corresponds to the desorption of surface HgO. As for the AFCC, the desorption peak primarily originates from surface HgO desorption, indicating that inherent elements (V, Fe, Ni, etc.) in AFCC can participate in the Hg⁰ catalytic oxidation to HgO. Notably, the used VO_x/AFCC-PG catalyst exhibits significantly enhanced Hg-TPD peak intensity, consistent with its high catalytic oxidation activity. Similarly, peaks observed at approximately 156 °C and 313 °C are assigned to the Hg⁰ and HgO species, respectively, while the newly emerged peak around 443 °C corresponds to HgSO₄. Further comparison of peak areas shows that HgO is the dominant species after Hg⁰ removal over the VO_x/AFCC-PG catalyst, with HgSO₄ being a minor species.

Based on the above experimental and characterization results, this study investigates the Hg⁰ removal process over the VO_x/AFCC-PG catalyst (Figure 6b). Initially, gaseous Hg⁰(g) from flue gas adsorbs onto active sites of the catalyst to form adsorbed Hg⁰(ad), which then combines with lattice oxygen from V₂O₅ and is oxidized to HgO (Mars–Maessen mechanism) [38,39], while V₂O₅ is reduced to V₂O₄. During this process, O₂ can react with V₂O₄ to replenish the consumed lattice oxygen, forming V₂O₅ and restoring its oxidative activity. Concurrently, abundant chemisorbed oxygen species (O_ads, Figure 5b) on the catalyst surface can react with gaseous Hg⁰(g) via the Eley–Rideal mechanism or with adsorbed Hg⁰(ad) via the Langmuir–Hinshelwood mechanism to generate HgO [40,41]. Furthermore, SO₂ adsorbs on active sites and is oxidized to SO₃, which subsequently reacts with adsorbed Hg⁰(ad) to form HgSO₄ [34,42]. A portion of SO₃ may also react with H₂O to produce H₂SO₄, which further reacts with Hg⁰(ad) to generate HgSO₄, accounting for the presence of a small amount HgSO₄ after reaction (Figure 6a, Hg-TPD). Moreover, while VO_x catalyzes Hg⁰ oxidation, PG and AFCC concurrently adsorb gaseous Hg⁰ and stabilize HgO/HgSO₄ products within their mesopores. This adsorption–oxidation process synergistically prolongs the reactant residence time and enhances both catalytic oxidation and adsorption.

Furthermore, to evaluate the application potential of the catalyst, we have tested its long-term stability and regeneration performance. As shown in Figure 6c, the VO_x/AFCC-PG catalyst exhibits excellent stability, maintaining 98% removal performance during a 60 h test in simulated flue gas at 150 °C. SEM images also demonstrate that the catalyst morphology remains essentially unchanged after reaction (Figure S3), confirming the structural stability of the catalyst. Meanwhile, the catalyst after the first regeneration (Figure 6d) achieves 86% mercury removal efficiency and retains 70% activity even after four regeneration cycles, suggesting its potential for practical applications. Furthermore, comparative analysis with reported catalysts (Table S5) demonstrates that the VO_x/AFCC-PG catalyst achieves comparable activity in Hg⁰ removal and stability in the presence of SO₂. Significantly, utilizing hazardous-waste-derived materials as catalytic support offers distinct advantages in reducing direct production costs, minimizing secondary environmental pollution, and promoting circular resource utilization.

3. Materials and Methods

3.1. Synthesis of Catalysts

Activation of WFCC support: First, 5 g of WFCC catalyst was heated to 500 °C at a heating rate of 10 °C/min in an air atmosphere and maintained for 1 h to remove the surface carbon deposits, thus obtaining the activated FCC support, denoted as AFCC.

Preparation of AFCC-PG support: To mold the powdered AFCC support, palygorskite (PG) was utilized as a binder to construct the AFCC-PG support. Specifically, AFCC, PG, and deionized water were mixed in a mass ratio of 6:4:1. Then, the mixture was stirred into a homogeneous paste, spread evenly on a glass plate, and air-dried. Finally, it was then dried in an oven at 110 °C for 10 h to obtain the AFCC-PG support.

Preparation of VO_x/AFCC-PG catalyst: The VO_x/AFCC-PG catalyst was prepared via the impregnation method. Specifically, an aqueous solution was prepared by dissolving predetermined amounts of NH₄VO₃ (Aladdin, Shanghai, China) and H₂C₂O₄·2H₂O (Aladdin, Shanghai, China). Then, the AFCC-PG support was immersed in this solution (targeting a 10 wt.% VO_x loading). The mixture was stirred into a paste, spread evenly, and air-dried for 24 h. Next, the sample was subsequently dried in an oven at 50 °C for 5 h, followed by 110 °C for 10 h. Finally, it was calcined at 350 °C for 4 h under an air atmosphere, yielding the catalyst designated as VO_x/AFCC-PG.

Preparation of VO_x: To clarify the VO_x phase composition on the VO_x/AFCC-PG, a vanadium oxide sample without AFCC-PG support was synthesized using identical procedures to those for VO_x/AFCC-PG, yielding the material designated as VO_x.

Regeneration of VO_x/AFCC-PG catalyst: The used VO_x/AFCC-PG catalyst after reaction was regenerated through thermal treatment. Specifically, the catalyst was placed in a tube furnace, heated to 300 °C, and maintained for 2 h under an air atmosphere. The regenerated catalyst is denoted as R_x-VO_x/AFCC-PG, where x represents the number of regeneration times.

3.2. Activity Measurement

Mercury removal experiments were performed in an atmospheric-pressure fixed-bed reactor system composed of four main components (Figure S4): a simulated flue gas delivery system, a temperature control unit, an online mercury monitoring system, and an exhaust gas treatment unit. The simulated flue gas contained 5 vol% O₂, 1500 ppm SO₂, 3 vol% H₂O, and 200 μg/m³ Hg⁰, with N₂ as a balance gas, maintaining a total flow rate of 100 mL/min. A 0.5 g catalyst sample (20–40 mesh) was loaded into an 8 mm internal diameter quartz reactor positioned in a tubular resistance furnace. Hg⁰ concentration monitoring was conducted using a Lumex RA-915M mercury analyzer (Mission, BC, Canada). The exhaust gases passed through the Hg⁰ analyzer before being treated in the activated carbon adsorption unit—a final-stage safety measure prior to venting. The calculation formula for the efficiency of Hg⁰ removal by the catalyst is as follows (Hg⁰ in and Hg⁰ out is the inlet/outlet Hg⁰ concentration, respectively):

$$\mathrm{Efficiency} =\mathrm{ }{\displaystyle \frac{\mathrm{H}{\mathrm{g}}_{\mathrm{in}}^0-\mathrm{H}{\mathrm{g}}_{\mathrm{out}}^0}{\mathrm{H}{\mathrm{g}}_{\mathrm{in}}^0}} \times 100\%$$

3.3. Materials Characterization

The crystal structure of the catalyst was characterized by X-ray diffraction (XRD-7000S, Shimadzu, Kyoto, Japan) using Cu Kα radiation, with a scanning rate of 7°/min over the 2θ range of 2–50°. The elemental composition of the samples was analyzed using an X-ray fluorescence spectrometer (XRF, PANalytical Axios, Malvern Panalytical, The Netherlands). The release of CO_x during the activation process was monitored by placing the sample in a tube furnace under an air atmosphere. The temperature was raised from room temperature to 500 °C at a constant heating rate and maintained for 1 h. The effluent gas was continuously analyzed for CO_x concentration using a flue gas analyzer (Testo 350, Testo SE & Co KGaA, Lennestadt-Obermehre, Germany). The carbon content of the WFCC before and after activation were analyzed by XRF (PANalytical Axios, Malvern Panalytical, The Netherlands). Textural properties including specific surface area, pore volume, and pore size distribution were determined using an ASAP 2460 analyzer (Micromeritics Instrument Corporatio, Norcros, GA, USA). The catalyst microstructure and elemental mapping were observed using scanning electron microscopy (SEM, ZEISS Sigma 300, Oberkochen, Germany). X-ray photoelectron spectroscopy (XPS) analyses were carried out on a Thermo Scientific ESCALAB 250 spectrometer (Waltham, MA, USA) with a monochromatic Al Kα X-ray source. The mercury adsorption form of the catalyst was investigated through temperature-programmed desorption of mercury (Hg-TPD). Specifically, following Hg⁰ removal from simulated flue gas, the used catalyst was loaded into a quartz tube reactor within a tube furnace. After purging with N₂ for 30 min, the temperature was increased from room temperature to 800 °C at a heating rate of 10 °C/min under nitrogen flow. The mercury released from the catalyst was continuously monitored using a mercury detector, following reduction through KBH₄ solution at the reactor outlet.

4. Conclusions

The VO_x/AFCC-PG catalyst achieves efficient gaseous Hg⁰ removal from coal-fired flue gas through resource utilization of waste FCC catalyst, combined with VO_x loading and PG binder molding. This catalyst exhibits over 95% mercury removal efficiency at 150 °C with robust resistance to Hg⁰ concentration fluctuations. Herein, O₂ plays a critical role in catalytic oxidation by replenishing lattice oxygen and generating surface-active oxygen species to promote the reaction cycle. SO₂ causes reversible inhibition via competitive adsorption and irreversible deactivation through sulfate deposition, while coexisting O₂ mitigates its poisoning effects. Furthermore, Hg-TPD and XPS analyses indicate that the synergistic lattice oxygen oxidation–adsorption established between VO_x and AFCC-PG plays a key role in efficient Hg⁰ removal. The catalyst maintains 98% stability during 60 h testing in simulated flue gas (200 μg/m³ Hg⁰, 5% O₂, 1500 ppm SO₂, 3% H₂O), retaining 86% of initial activity after regeneration, confirming its practical application potential. This study provides an economically efficient and environmentally friendly strategy for solid waste resource utilization and flue gas mercury removal technology. Ongoing and future efforts will center on deepening the exploration of catalyst scale-up synthesis and validating performance in simulated or pilot-scale setups that closely mimic real flue gas conditions to evaluate the engineering feasibility.