A series of vanadium doped Fe2O3 catalysts were synthesized using the homogeneous precipitation method and subjected to laboratory evaluation for selective catalytic reduction of NO x with NH3 (NH3-SCR). The best Fe0.75V0.25Oδ catalyst with a Fe/V mole ratio of 3/1 exhibited superior catalytic performance, achieving 100% NO x conversion at 200 °C over a wide temperature window from 175 to 400 °C, believed to be the best Fe-based low-temperature NH3-SCR catalyst identified to date. The Fe0.75V0.25Oδ catalyst also showed prominent resistance to high gas hourly space velocity (GHSV; 200 000 h–1) and strong durability to SO2 and H2O. Doping of V was shown to remarkably boost the catalytic activity, due to enhancement of the redox ability and surface acidity. XRD, Raman, and morphology results revealed that the incorporation of V had led to the formation of amorphous FeVO4 and Fe2O3. Coupling XPS and UV–vis diffuse reflectance spectra (DRS) results with DFT, it was discovered that the electron inductive effect between Fe and V generated the charge depletion of Fe, resulting in an improvement of the redox ability, facilitating the oxidation of NO to NO2. Meanwhile, the strong interaction between FeVO4 and Fe2O3 species kept V at a higher valence, beneficial for the adsorption and activation of NH3. The synergistic effect of FeVO4 and Fe2O3 thus improved the low-temperature catalytic activity and lowered the apparent activation energy. Combining in situ diffusion Fourier transform infrared spectroscopy (DRIFTS) results with reaction kinetic studies, it was concluded that the SCR reaction mainly followed the Langmuir–Hinshelwood mechanism below 200 °C, since the consumption of adsorbed NH3 species could be divided into the explicit “standard SCR” and “fast SCR” stages, while an Eley–Rideal mechanism proceeded dominantly at and above 200 °C, in which the adsorbed NH3 species were eliminated by gaseous NO directly and linearly. Both the Brønsted and Lewis acid sites played equivalently significant roles in NH3-SCR reaction.

Oxygen vacancy-rich porous Co 3 O 4 nanosheets (OV-Co 3 O 4 ) with diverse surface oxygen vacancy contents were synthesized via facile surface reduction and applied to NO reduction by CO and CO oxidation. The structure−activity relationship between surface oxygen vacancies and catalytic performance was systematically investigated. By combining Raman, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and O 2 -temperature programmed desorption, it was found that the efficient surface reduction leads to the presence of more surface oxygen vacancies and thus distinctly enhance the surface oxygen amount and mobility of OV-Co 3 O 4 . The electron transfer towards Co sites was promoted by surface oxygen vacancies with higher content. Compared with the pristine porous Co 3 O 4 nanosheets, the presence of more surface oxygen vacancies is beneficial for the catalytic performance enhancement for NO reduction by CO and CO oxidation. The OV-Co 3 O 4 obtained in 0.05 mol L −1 NaBH 4 solution (Co 3 O 4 -0.05) exhibited the best catalytic activity, achieving 100% NO conversion at 175 °C in NO reduction by CO and 100% CO conversion at 100 °C in CO oxidation, respectively. Co 3 O 4 -0.05 exhibited outstanding catalytic stability and resistance to high gas hour space velocity in both reactions. Combining in situ DRIFTS results, the enhanced performance of OV-Co 3 O 4 for NO reduction by CO should be attributed to the promoted formation and transformation of dinitrosyl species and −NCO species at lower and higher temperatures. The enhanced performance of OV-Co 3 O 4 for CO oxidation is due to the promotion of oxygen activation ability, surface oxygen mobility, as well as the enhanced CO 2 desorption ability. The results indicate that the direct regulation of surface oxygen vacancies could be an efficient way to evidently enhance the catalytic performance for NO reduction by CO and CO oxidation.

Mn–Fe spinel oxides are considered as promising catalysts for low-temperature selective catalytic reduction of NO x with NH3 (NH3-SCR), but the operation temperature window severely suffers from their excessive redox properties. Here, a novel mesoporous nanostructured Mn0.5Fe2.5O4 spinel catalyst (Mn0.5Fe2.5O4-S) with tailored redox properties was synthesized by a facile self-assembly method and applied for NH3-SCR. The morphological structure and physicochemical properties of the as-prepared catalysts were affirmed through comprehensive characterization methods. Compared with the conventional Mn0.5Fe2.5O4 nanoparticle catalyst (Mn0.5Fe2.5O4-P), the Mn0.5Fe2.5O4-S sample exhibited excellent low-temperature De-NO x performance, a wider operation temperature window, lower apparent activation energy, and higher N2 selectivity. The superior catalytic activity of the Mn0.5Fe2.5O4-S catalyst was mainly attributed to its moderate redox properties derived from the unique mesoporous nanostructure with regular dispersed active sites. In situ DRIFTS results indicated that a large amount of −NH2 species were formed on the Mn0.5Fe2.5O4-S due to the appropriate redox properties. Meanwhile, the optimized redox properties could suppress the unwanted NH3 oxidation and thus broaden the temperature window in the middle temperature region. DFT calculation results proved that the Mn0.5Fe2.5O4-S catalyst with the preferentially exposed (220) crystal plane exhibited a lower energy barrier for the activation of NH3 to −NH2. This should be the key factor for intermediate formation and activity enhancement.

A novel Fe2O3–MnO2/TiO2 catalyst was synthesized using a conventional impregnation method assisted with ethylene glycol and used for NH3–SCR. The catalyst exhibited superior low-temperature activity over a broad temperature window (100–325 °C), low apparent activation energy, and excellent sulfur-poisoning resistance. The characterization results revealed that the catalyst was greatly dispersed with smaller particles, and the partial doping of Fe into the TiO2 lattice thereby led to the formation of the Fe–O–Ti structure, which could strengthen the electronic inductive effect and increase the ratio of surface chemisorption oxygen, resulting in the enhancement of NO oxidation and favoring the low-temperature SCR activity via a “fast SCR” process. The in situ FTIR analysis showed that the NO x adsorption capacity was significantly improved due to the desired dispersion property, further helping both the SCR activity and reaction rate at low temperatures. The present work confirmed that more active sites can be provided on the catalyst surface by modifying the dispersity.

Unique triple-shelled NiMn2O4 hollow spheres are fabricated by a facile solvothermal method. Owing to its particular triple shell structure, the as prepared NiMn2O4 catalyst exhibits superior low-temperature NH3-SCR catalytic performance, achieving above 90% NOx conversion in the temperature range from 100 °C to 225 °C.