Modified red mud catalyst for the selective catalytic reduction of nitrogen oxides: Impact mechanism of cerium precursors on surface physicochemical properties

Synergistic optimization of bio-oil quality and heavy metal solidification during microwave co-pyrolysis of cow dung and red mud

To decrease the environmental risks caused by heavy metals (HMs) in red mud (RM) and improve the quality of pyrolysis oil from biomass, high-temperature pretreated RM and cow dung (CD) were microwave co-pyrolyzed. Then, the optimization potential of energy consumption was evaluated and the interaction mechanism between RM and CD was explored. The results showed that the increase in transition metal oxides and specific surface area improved the microwave-absorption and catalytic capacity of the pretreated RM. By optimizing the parameters, a pretreatment temperature of 650 °C resulted in a 21.65% reduction in acid content of bio-oil, higher HMs immobilization rates (>91%) and a 7.44% reduction in energy consumption. The synergistic optimization of bio-oil quality, HMs immobilization and energy consumption was achieved. After microwave co-pyrolysis with cow dung, the larger specific surface area (92.90 m² g^-1) and higher carbon crystallinity (I_D/I_G = 1.02) of pyrolysis residues enhanced the physical adsorption to HMs. The complexation of HMs with -OH could further enhance the solidification of HMs. This work will provide support to efficient resource utilization of solid waste, and demonstrate the great potential of microwave co-pyrolysis in HMs immobilization.

Advances in the fabrication, modification, and performance of biochar, red mud, calcium oxide, and bentonite catalysts in waste-to-fuel conversion

Various catalysts are being used in fuel production from biomass and polymeric waste for the obtention of an alternative energy source with both environmental friendliness and economic viability. Biochar, red mud bentonite, and calcium oxide have been shown to play a pertinent role as catalysts in waste-to-fuel conversion processes, such as transesterification and pyrolysis. In this line of thought, this paper has provided a compendium of the fabrication and modification technologies of bentonite, red mud calcium oxide, and biochar, together with their various performances in their application in the waste-to-fuel processes. Additionally, an overview of the structural and chemical attributes of these components is discussed regarding their efficiency. Ultimately, research trends and future points of focus are evaluated, and it is observed that techno-economic optimization of catalyst synthetic routes and investigation of new catalytic formulations, such as biochar and red mud-based nanocatalysts, are potential prospects. This report also offers future research directions that are anticipated to contribute to the development of sustainable green fuel generation systems.

Red mud-based catalysts for the catalytic removal of typical air pollutants: A review

Red mud, as a solid waste produced during the alumina production, can cause severe eco-environmental pollution and health risks to human. Therefore, the resourcing of this type of solid waste is an effective way for the sustainable development. This paper reviews the recent progress on red mud-based catalysts for the removal of typical air pollutants, such as the catalytic reduction of nitrogen oxides (NO_x) by NH₃ (NH₃-SCR) and the catalytic oxidation of CO and volatile organic compounds (VOCs). The factors influencing the catalytic performance and the structure-activity relationship have been discussed. Future prospects and directions for the development of such catalysts are also proposed. This review would benefit for the high value-added utilizations of red mud in mitigating atmospheric pollutions.

Insight into catalytic activation of bisulfite for lomefloxacin degradation by simple composite of calcinated red mud

Antibiotic was detected in many environments, and it had posed a serious threat to human health. The advanced oxidation process has been considered an effective way to treat antibiotics. In this work, using industrial waste red mud (RM) as raw material, a series of modified RM (MRM-T; T donates the calcination temperature) was obtained via a facile calcination method and applied to activate sodium bisulfite (NaHSO₃) for the lomefloxacin (LOM) degradation. Among all MRM-T, MRM-700 exhibited superior catalytic activity, and approximately 89% of LOM (10 mg/L) was degraded at 30 min through the activation of NaHSO₃ ([NaHSO₃] = 0.5 g/L) by MRM-700 ([MRM-700] = 0.9 g/L). Moreover, the kinetic constant of LOM removal in the MRM-700/NaHSO₃ system (0.082 min^-1) was 16.4 times higher than that of the RM-raw/NaHSO₃ system (0.005 min^-1). The as-synthesized product of MRM-700 was characterized by N₂ adsorption-desorption isotherms, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectra. The result indicated that the catalyst possessed excellent pore structure, high specific area, and abundant Fe³⁺ sites, and the lattice of Fe₂O₃ was doped after calcination, both of which were favorable for the activation of NaHSO₃. The quenching experiment proved that •SO₄^- and •OH^- active species were produced in MRM-700/NaHSO₃ system, and •SO₄^- played a dominant role in LOM removal. In addition, the potential LOM degradation pathway was analyzed via UPLC-MS technology and density functional theory (DFT) calculation, and the toxicity of the treated LOM solution was tested by the culture of mung bean sprouts. This study not only provided a feasible strategy for the valuable use of RM to activate NaHSO₃ but also offered a cost-effective catalyst for the efficient removal of pollutants in wastewater.

Ce(SO4)2/α-Fe2O3 selective catalytic reduction of NOx with NH3: preparation, characterization, and performance

To achieve a low-cost, high-activity denitrification catalyst with excellent water and sulfur resistance, goethite and Ce(SO₄)₂·4H₂O were used to prepare Ce(SO₄)₂/α-Fe₂O₃ composite catalyst by the impregnation way and investigated the effect of Ce(SO₄)₂ on the properties of goethite. Ce(SO₄)₂/α-Fe₂O₃ with various preparation conditions for denitration was systematically discussed, and its structure and properties were characterized by XRD, BET, TEM, XPS, H₂-TPR, and NH₃-TPD methods. The results showed that Ce(SO₄)₂/α-Fe₂O₃ over the Ce/Fe molar ratio of 0.02 and calcination temperature of 350 ℃ had excellent catalytic activity, resistance to sulfur, and water properties and stability. When NO_x initial concentration was 500 ppm, gas hourly space velocity was 36,000 h^-1 and its reaction temperature was 300 ℃; the NO_x conversion efficiency was maintained at over 95% along with 300 ppm SO₂ and nearly 100% couple with 10% H₂O. Its superior performance was mainly attributed to the enhancement of the surface adsorbed oxygen and acidity of α-Fe₂O₃ by cerium sulfate. The multiple advantages of Ce_0.02/α-Fe₂O₃(350) made it feasible for practical engineering application.

Selective Leaching of Rare Earth Elements from Ion-Adsorption Rare Earth Tailings: A Synergy between CeO2 Reduction and Fe/Mn Stabilization

The increasing demand for rare earth elements (REEs) motivates the development of novel strategies for cost-effective REE recovery from secondary sources, especially rare earth tailings. The biggest challenges in recovering REEs from ion-adsorption rare earth tailings are incomplete extraction of cerium (Ce) and the coleaching of iron (Fe) and manganese (Mn). Here, a synergistic process between reduction and stabilization was proposed by innovatively using elemental sulfur (S) as reductant for converting insoluble CeO2 into soluble Ce2(SO4)3 and transforming Fe and Mn oxides into inert FeFe2O4 and MnFe2O4 spinel minerals. After the calcination at 400 °C, 97.0% of Ce can be dissolved using a diluted sulfuric acid, along with only 3.67% of Fe and 23.3% of Mn leached out. Thermodynamic analysis reveals that CeO2 was indirectly reduced by the intermediates MnSO4 and FeS in the system. Density functional theory calculations indicated that Fe(II) and Mn(II) shared similar outer electron arrangements and coordination environments, favoring Mn(II) over Ce(III) as a replacement for Fe(II) in the FeO6 octahedral structure of FeFe2O4. Further investigation on the leaching process suggested that 0.5 mol L-1 H2SO4 is sufficient for the recovery of REEs (97.0%). This research provides a promising strategy to selectively recover REEs from mining tailings or secondary sources via controlling the mineral phase transformation.

Fabrication of Nanohybrid Spinel@CuO Catalysts for Propane Oxidation: Modified Spinel and Enhanced Activity by Temperature-Dependent Acid Sites

Modulating the catalyst electronic structure is a promising direction to enhance the catalytic oxidation performance. Herein, we report an innovative synthesis of the nanohybrid spinel@CuO catalyst with a broad biphasic interface for propane oxidation. The reaction rate of spinel@CuO catalyst was significantly increased compared to the physically mixed spinel+CuO catalyst. Lattice distortions and severe blurring of lattice fringes adjacent to the interface (between the spinel and CuO) comes with the spinel@CuO system, which enhanced interfacial interaction to form defect structures. The cobalt cations were selectively doped into the spinel lattice and occupied both the A and the B sites, while the CuO was not affected. At lower temperatures (∼200 °C), the enrichment of Brønsted acid sites increased the adsorption energy of propane. At higher temperatures (∼350 °C), the A and B sites cobalt weakened the Cu-O bond to make the oxygen vacancies form more readily, thereby enriching the Lewis acid sites. The substitution doping also resulted in lattice distortion in the spinel species, promoting the formation of a defect structure. The broad interface and temperature-dependent acid sites were conducive to propane oxidation.

Key intermediates from simultaneous removal of NOx and chlorobenzene over a V2O5–WO3/TiO2 catalyst: a combined experimental and DFT study

Chlorobenzene suppresses the active cis-N2O22− formation and the dissociated Cl− combines with vanadium to form vanadium chloride which enhanced surface Brønsted acidity.