Effect of iron minerals during coaling on the transformation of NO in the presence of NH3: Take pyrite as an example

Tungsten modified natural limonite catalyst for efficient low-temperature selective catalytic reduction of NO removal with NH3: preparation and characterization

With natural limonite as the precursor and an ammonium tungstate hydrate as modification, the W/limonite composite catalysts were synthesized by the impregnation method. Their structures and properties were systematically characterized and analyzed; the denitrification activity and resistance to water and sulfur on catalysts were investigated. The results indicated that the W/limonite composite with W/Fe mass ratio of 9% and calcination temperature of 300 °C had highly catalytic activity, enhanced resistance to sulfur and water. The NO conversion efficiency was maintained over 85% with NO initial concentration of 500 ppm, the gas hourly space velocity (GHSV) of 36,000 h^-1, and reaction temperature of 100 °C, while it was greater than 98% with addition of 200 ppm SO₂ and 3 vol. % H₂O at the reaction temperature of 250 °C. The superior performance was mainly ascribed to the formation of W-OH species and W = O species with wide dispersion on the surface of goethite or in Fe₂O₃ lattice defects, to generate more acidic hydroxyl groups and more oxygen defects and strong acidity Brønsted for the SCR reaction.

Effect of MnOx/α-Fe2O3 Prepared from Goethite on Selective Catalytic Reduction of NO with NH3

A low-cost goethite and manganese acetate were used to prepare a MnOx/α-Fe2O3 composite catalyst by simple impregnation method for novel high-efficiency selective catalytic reduction (SCR) of NO with NH3, and its denitration performance of composite catalyst under different conditions was investigated in a thermal fixed-bed catalytic reaction system. The results showed that MnOx/α-Fe2O3 with Mn/Fe molar ratio of 0.1 and calcination temperature of 400 °C had the best low-temperature catalytic activity and wider reaction temperature window compared with α-Fe2O3. It achieved over 90% NO conversion efficiency with a space velocity of 72,000 h−1 at 200~350 °C and possessed a good resistance of H2O and SO2. Characterization by XRD, BET, H2-TPR, and NH3-TPD revealed that the main reason for the high catalytic activity of MnOx(0.1)/α-Fe2O3(400) was that the addition of Mn changed phase types of catalyst and valence composition of Fe, resulting in a larger specific surface area, more acidic sites, and higher redox performance.

Mesoporous poorly crystalline α-Fe2O3 with abundant oxygen vacancies and acid sites for ozone decomposition

In this work, mesoporous poorly crystalline hematite (α-Fe₂O₃) was prepared using mesoporous silica (KIT-6) functionalized with 3-[(2-aminoethyl)amino]propyltrimethoxysilane as a hard template (SMPC-α-Fe₂O₃). The disordered atomic arrangement structure of SMPC-α-Fe₂O₃ promoted the formation of oxygen vacancies, which was confirmed using X-ray photoelectron spectroscopy (XPS), O₂-temperature-programmed desorption (TPD), H₂-temperature-programmed reduction (TPR), and in situ diffuse reflectance infrared Fourier transform (DRIFT) analyses. Density functional theory calculations (DFT) also proved that reducing the crystallinity of α-Fe₂O₃ decreased the formation energy of oxygen vacancies. TPD and in situ DRIFT analyses of NH₃ adsorption suggested that the surface acidity of SMPC-α-Fe₂O₃ was considerably higher than those of mesoporous poorly crystalline α-Fe₂O₃ (MPC-α-Fe₂O₃) and highly crystalline α-Fe₂O₃ (HC-α-Fe₂O₃). The oxygen vacancies and acid sites formed on α-Fe₂O₃ surface are beneficial for ozone (O₃) decomposition. Compared with MPC-α-Fe₂O₃ and HC-α-Fe₂O₃, SMPC-α-Fe₂O₃ exhibited a higher removal efficiency for 200-ppm O₃ at a space velocity of 720 L g^-1 h^-1 at 25 ± 2 °C under dry conditions. Additionally, in situ DRIFT and XPS results suggested that the accumulation of peroxide (O₂^2-) and the conversion of O₂^2- to lattice oxygen over the oxygen vacancies caused catalyst deactivation. However, O₂^2- could be desorbed completely by continuous N₂ purging at approximately 350 °C. This study provides significant insights for developing highly active α-Fe₂O₃ catalysts for O₃ decomposition.

Preparation of Monoclinic Pyrrhotite by Thermal Decomposition of Jarosite Residues and Its Heavy Metal Removal Performance

Jarosite residues produced by zinc hydrometallurgical processing are hazardous solid wastes. In this study, monoclinic pyrrhotite (M-Po) was prepared by the pyrolysis of jarosite residues in H2S atmosphere. The influence of gas speed, reaction temperature, and time was considered. The mineral phase, microstructure, and elemental valence of the solids before and after pyrolysis were analyzed using X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy, respectively. The performances of the prepared M-Po on the removal of Zn and Pb from aqueous solution were evaluated. The results show M-Po to be the sole product at the reaction temperatures of 550 to 575 °C. Most of the M-Po particles are at the nanometer scale and display xenomorphic morphology. The phase evolution process during pyrolysis is suggested as jarosite → hematite/magnetite → pyrite → pyrite+M-Po → M-Po+hexagonal pyrrhotite (H-Po) → H-Po. The formation rate, crystallinity, and surface microtexture of M-Po are controlled by reaction temperature and time. Incomplete sulfidation may produce coarse particles with core–shell (where the core is oxide and the shell is sulfide) and triple-layer (where the core is sulfate, the interlayer is oxide, and the shell is sulfide) structures. M-Po produced at 575 °C exhibits an excellent heavy metal removal ability, which has adsorption capacities of 25 mg/g for Zn and 100 mg/g for Pb at 25 °C and pH ranges from 5 to 6. This study indicates that high-temperature sulfidation is a novel and efficient method for the treatment and utilization of jarosite residues.

Investigation on the transformation behaviours of Fe-bearing minerals of coal in O2/CO2 combustion atmosphere containing H2O

The transformation behaviors of Fe-bearing minerals in coals of Xinjiang (XJC) and Shenhua (SHC) were investigated in an O₂/CO₂ atmosphere containing H₂O in a drop-tube-furnace (DTF). The solid products were characterized using XRD, Mössbauer spectroscopy, particle size analyzer and SEM-EDX techniques. The results show that the change in the combustion atmosphere does not significantly alter the main phases of Fe-bearing minerals in the coal ashes, but does affect their relative contents. The ratio of Fe²⁺-glass to Fe³⁺-glass in the ashes produced from the O₂/CO₂ combustion atmosphere was significantly increased. During the XJC combustion and under different combustion conditions examined, the content of Fe-glass phases remained almost unaltered. However, in SHC samples, combustion under O₂/CO₂ atmosphere resulted in a higher amount of iron melting into Fe-glass phases and less amount of iron oxide formation. This could be attributed mainly to the presence of Fe-bearing minerals mostly included in nature in SHC samples, which more easily interacted with clays or other silicates inside coal-formed Fe-glass phases. Increasing the O₂ level of the O₂/CO₂ atmosphere during SHC combustion could promote the formation of iron oxides. In O₂/CO₂ atmosphere, with the same oxygen level, the replacement of 10% of CO₂ with H₂O promoted the formation of iron oxides, regardless of the occurrence form (included or excluded) of iron minerals in coal. Furthermore, the addition of steam resulted in an increase in the size of the particles in ash, resulting probably in a decrease in the deposition and slagging propensity of coal ash.

The ratio of Fe²⁺-glass to Fe³⁺-glass in ashes from O₂/CO₂ atmosphere is significantly increased. The iron oxides (hematite or magnetite) formation of included iron minerals may be delayed in O₂/CO₂. H₂O promotes iron oxides formation.