A highly active and stable SO2 reduction catalyst, 3%La-15%Fe/γ-Al2O3, was successfully synthesized using γ-Al2O3 derived from coal gangue. The structural properties of the synthesized catalyst were analyzed using XPS, SEM, TEM, EDS, BET, XRD, and H2-TPR. Characterization studies revealed a high BET specific surface area of 288.55 m2/g for the coal gangue-derived mesoporous γ-Al2O3. Furthermore, lanthanum doping inhibited iron crystallite growth, enhancing dispersion on the support and contributing to superior catalytic performance. Under optimized conditions (380 °C, 6000 h-1 GHSV, and a CO/SO2 ratio of 2), the catalyst achieved 98.92 ± 1.3% SO2 conversion and 99.43 ± 0.7% selectivity to elemental sulfur. This high performance remained stable for over 50 h, demonstrating the catalyst's potential for industrial application. The utilization of coal gangue-derived γ-Al2O3 offers both economic and environmental benefits by providing a cost-effective support material and addressing coal gangue disposal challenges. This work presents a promising strategy for sustainable SO2 abatement and resource utilization.
The rapid expansion of various industries, particularly steel and power generation, coupled with the rising reliance on fossil fuels for example oil and coal, has caused a sharp rise in annual sulfur dioxide (SO) emissions. For instance, SO emissions from coal combustion alone can reach up to 20% by weight. SO is a pungent, colorless gas and a major air pollutant. Mammalian exposure to SO can cause various health problems, including DNA damage, respiratory illnesses, oxidative organ damage, and genetic mutations. Furthermore, SO aids in the formation of photochemical smog and acid rain, causing damage to soil and vegetation, thereby impacting ecosystems and raising significant public health concerns. Economic damage resulting from deterioration of infrastructure and buildings is also a considerable consequence. So, the WHO and the EPA have established emission standards to mitigate the detrimental effects of SO pollution. The EPA modified its National Ambient Air Quality Standards (NAAQS) for the primary one-hour SO standard from 140 ppb to 75 ppb in 2012. Similarly, European Union policymakers, through Regulation (EU) 2016/2284, adhere to this global trend, mandating a 70% reduction in SO emissions by 2029.
Wet and dry scrubbing, traditional gas desulfurization techniques, utilize aqueous solutions of alkali and alkaline earth metals, and dry alkaline earth metal salts for example limestone slurry and calcium hydroxide. While these techniques can achieve removal efficiencies exceeding 90%, their high energy consumption, substantial equipment costs, extensive water usage, and the generation of low-value byproducts pose significant economic challenges. Furthermore, the production of large volumes of wastewater and the accumulation of substantial amounts of unrecoverable desulfurization gypsum and ash, which can cause secondary pollution, create significant environmental concerns.
Therefore, the development of green and cost-effective SO removal methods is crucial. Catalytic SO removal has gained significant attention due to its lower cost, reduced environmental impact, high efficiency, and potential for byproduct recovery. A particularly attractive feature is the production of valuable byproducts and the absence of waste generation. This method can employ either catalytic oxidation or catalytic reduction:
1. In catalytic oxidation, SO is converted to SO on a catalyst in the presence of O, which can subsequently react with HO to form HSO (Eqs. (1) and (2)).
2. In catalytic reduction, SO is converted to elemental sulfur utilizing reducing agents like carbon monoxide, hydrogen, or methane (Eq. (3)).
While sulfuric acid is cheap and has possessing minimal added value, its transportation costs are high, and overproduction leads to market saturation. Conversely, the demand for elemental sulfur is increasing across various industries. Elemental sulfur is attractive due to its ease of storage, convenient transportation, and high monetary value, making it a desirable product from SO conversion. Therefore, converting SO to elemental sulfur presents an efficient approach for resource recovery and preventing secondary pollution.
It is noteworthy that in industrial processes, SO is typically present in the tail gas alongside various other gaseous compounds. Prior to conducting catalytic SO removal processes, it is necessary to address the issues caused by these species. Typically, incineration units in refineries and similar industries convert different sulfur-containing compounds such as HS and CS into sulfur dioxide, thereby simplifying and enhancing the efficiency of the SO removal process. Additionally, to improve the removal efficiency and mitigate the effects of residual interfering species, the use of supplementary methods such as filtration and advanced separation systems is recommended.
A comparison of various reducing agents reveals that methane requires a temperature approximately 200 °C higher than other reducing agents for catalytic reduction, and produces a broader range of byproducts. Hydrogen (H), despite its high yield and selectivity, is expensive and its storage and transportation are difficult and hazardous, hindering its industrial applicability. Carbon monoxide (CO), however, offers both high yield and selectivity, similar to H. Furthermore, CO is readily available and cost-effective as it is a byproduct in various industrial processes, including blast furnace gas, yellow phosphorus production tail gas, converters in iron and steel industries, and synthetic ammonia feedstock gas. Consequently, CO presents promising prospects for industrial applications.
Various metals, including Co, Ni, Fe, Zr, Ca, Ru, Mo, Ce, and Ba, have been employed as catalysts for SO reduction. These metal catalysts can be used in their promoted or bimetallic forms. Recently, bimetallic catalysts and their synergistic effects have attracted considerable interest due to their tunable oxidation states. One metal element can alter the catalytic properties of another through electronic and structural modifications, leading to improved water resistance, enhanced adsorption/desorption, increased redox ability, and the creation of more acidic sites, ultimately boosting catalytic activity and selectivity. Research indicates that rare earth elements are particularly active components, enhancing the performance of other metal catalysts in converting SO to S. This enhancement stems from their abundant of oxygen vacancies and facile oxygen mobility, facilitating the reaction. However, the efficiency of this process is highly dependent on the catalyst type and composition, the reducing agent used, operating conditions, and other factors.
Supports play a crucial role in enhancing catalyst performance. A suitable support provides a high surface area, preventing the formation of crystalline aggregates or sintering of the active components, while simultaneously stabilizing the dispersion and chemical stability of the active phase. The support can also enhance mass transfer by providing space for catalytic reactions and can act as a ligand or promoter. Since most SO catalytic reduction mechanisms follow Langmuir-Hinshelwood or Eley-Riedel kinetics, which require chemisorption of SO, γ-alumina (γ-AlO), with its high chemical stability and abundant Lewis's acid sites, appears to be an ideal support for adsorbing electron-rich SO.
Currently, the synthesis of mesoporous γ-AlO is commonly carried out using expensive aluminum precursors such as Al(NO)·9HO, aluminum isopropoxide, and other similar compounds. This significantly increases the final production cost and poses limitations for the large-scale development of industrial applications. In this context, the utilization of industrial wastes such as coal gangue (CG) -- a byproduct of coal mining generated in enormous quantities with an annual global production of approximately one billion tons -- has attracted considerable attention as a sustainable and low-cost source of aluminum. This approach not only drastically reduces the cost of raw materials and the overall price of γ-AlO, but also offers significant environmental benefits, as it helps to decrease the volume of waste and the associated disposal costs, thus mitigating environmental pollution. Moreover, this process enables the simultaneous production of high-purity silica as a valuable byproduct, thereby considerably increasing the overall economic value of the process. Therefore, exploiting free and sustainable aluminum resources not only represents an effective step toward reducing alumina production costs, but is also considered a promising and environmentally sustainable solution.
This study presents a novel and sustainable approach to SO catalytic reduction by synthesizing a high-purity, mesoporous γ-AlO support directly from coal gangue, an abundant industrial waste. Unlike conventional catalysts that rely on costly commercial alumina, this valorization of coal gangue not only significantly reduces material costs but also addresses environmental challenges associated with waste disposal. The mesoporous structure derived from coal gangue provides a high surface area and enhanced mass transfer, which are critical for catalytic performance. Furthermore, the catalyst system employs iron (Fe) as the active metal, promoted by lanthanum (La), a rare earth element known for its ability to create abundant oxygen vacancies and facilitate oxygen mobility. This promotion enhances SO adsorption and redox reactions, leading to improved catalytic activity, selectivity, and stability compared to previously reported Fe-based catalysts on commercial supports. Comprehensive characterization and systematic optimization of reaction parameters demonstrate the synergistic effect between the coal gangue-derived support and lanthanum promoter, resulting in superior CO-assisted SO reduction efficiency. This integrated strategy not only advances catalyst design by combining waste valorization with enhanced catalytic function but also offers a cost-effective and environmentally friendly solution for industrial SO emission control.
In this study, high-purity, high-surface-area mesoporous γ-AlO was extracted from CG and used as a catalyst support. Iron and lanthanum oxides were then dispersed onto the γ-alumina as active components using an impregnation method. These compounds were subsequently operated for the catalytic reduction of SO using CO as the reducing agent. The synthesized catalysts were characterized utilizing XRD, BET, XPS, SEM, H-TPR, EDS and TEM techniques. Furthermore, to verify the optimal conditions, the effects of numerous factors such as reaction temperature, molar ratio of SO to CO, iron and lanthanum percentages, gas hourly space velocity (GHSV), and catalyst stability were investigated.