Active sites assembly effect on CeO2-WO3-TiO2 catalysts for selective catalytic reduction of NO with NH3

The influence of H2O or/and O2 introduction during the low-temperature gas-phase sulfation of organic COS + CS2 on the conversion and deposition of sulfur-containing species in the sulfated CeO2-OS catalyst for NH3-SCR

Herein, the typical components of blast furnace gas, including H2O and O2, were introduced to improve the NH3-SCR activity of the sulfated CeO2-OS catalyst during the gas-phase sulfation of organic COS + CS2 at 50 °C. The characterization results demonstrate that the introduction of O2 or H2O during gas-phase sulfation enhances the conversion of organic COS + CS2 on a cubic fluorite CeO2 surface and reduces the formation of sulfur and sulfates in the catalyst, but decreases the BET surface area and pore volume of the sulfated CeO2-OS catalyst. However, the introduction of O2 or H2O during the gas-phase sulfation increases the molar ratios of Ce3+/(Ce3+ + Ce4+) and Oβ/(Oα + Oβ + Oγ) on the sulfated CeO2-OS catalyst surface, thus promoting the formation of surface oxygen vacancies and chemisorbed oxygen, and these properties of the catalyst are further enhanced by the co-existence of O2 and H2O. Furthermore, the reduction of sulfates formed under the action of O2 or H2O decreases the weak acid sites of the sulfated CeO2-OS catalyst, but the few and highly dispersive sulfates present stronger reducibility, and the proportion of medium-strong acid sites of the catalyst increases. These factors help to improve the NH3-SCR activity of the sulfated CeO2-OS catalyst. Thus, there exists a synergistic effect of H2O and O2 introduction during gas-phase sulfation on the physical-chemical properties and catalytic performance of the sulfated CeO2-OS catalyst by organic COS + CS2 at 50 °C.

Effect of CO2 on the Desulfurization of Sintering Flue Gas with Hydrated Lime

The effect of carbon dioxide (CO₂) on the desulfurization of sintering flue gas with hydrate (Ca(OH)₂) as an absorbent was investigated, and the formation of calcium carbonate (CaCO₃) and its effect on the desulfurization was discussed. The competitive relationship between carbon dioxide (CO₂) and sulfur dioxide (SO₂) with the deacidification agent in sintering flue gas is discussed thermodynamically, showing that sulfates are more likely to be generated under high oxygen potential conditions and that SO₂ reacts more preferentially than CO₂ under a thermodynamic standard state. The amount of produced CaCO₃ increases under the condition that the CO₂ concentration is absolutely dominant to SO₂ in the sintering flue gas atmosphere. The effect of temperature, humidity and CO₂ concentration on the desulfurization of Ca(OH)₂ are discussed experimentally. The increasing temperature is not conducive to desulfurization, and the humidity can promote desulfurization, while excessive humidity could inhibit desulfurization. The suitable relative humidity is 20%. In situ generated calcium carbonate has a certain desulfurization effect, but the desulfurization effect is not as good as Ca(OH)₂. However, a large proportion of CaCO₃ was produced in the desulfurization ash under the condition that CO₂ concentration was absolutely dominant to SO₂ in the sintering flue gas atmosphere.

Ce1−xFexVO4 with Improved Activity for Catalytic Reduction of NO with NH3

A series of Ce1−xFexVO4 (x = 0, 0.25, 0.50, 0.75, 1) catalysts prepared by modified hydrothermal synthesis were used for selective catalytic reduction (SCR) of NOx with NH3. Among them, Ce0.5Fe0.5VO4 showed the highest catalytic activity. The catalysts were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction using H2 (H2-TPR), and temperature-programmed desorption of NH3 (NH3-TPD). The results indicated the formation of Ce-Fe-V-O solid solutions. The average oxidation states (AOS) of Ce, Fe, V, and O atoms changed obviously with the incorporation of Fe3+ into CeVO4, and the acidity of Ce0.5Fe0.5VO4 differs from that of CeVO4 and FeVO4. The presence of more acid sites and a sharp increase in active oxygen species in Ce0.5Fe0.5VO4 effectively improved the selective catalytic reduction (SCR) activity.

Abatement of Nitrogen Oxides via Selective Catalytic Reduction over Ce1-W1 Atom-Pair Sites

Environmentally benign CeO₂-WO₃/TiO₂ catalysts are promising alternatives to commercial toxic V₂O₅-WO₃/TiO₂ for controlling NO_x emission via selective catalytic reduction (SCR), but the insufficient catalytic activity of CeO₂-WO₃/TiO₂ catalysts is one of the obstacles in their applications because of a lack of an in-depth understanding of the CeO₂-WO₃ interactions. Herein, we design a Ce₁-W₁/TiO₂ model catalyst by anchoring Ce₁-W₁ atom pairs on anatase TiO₂(001) to investigate the synergy between Ce and W in SCR. A series of characterizations combined with density functional theory calculations and in situ diffuse-reflectance infrared Fourier-transform experiments reveal that there exists a strong electronic interaction within Ce₁-W₁ atom pairs, leading to a much better SCR performance of Ce₁-W₁/TiO₂ compared with that of Ce₁/TiO₂ and W₁/TiO₂. The Ce₁-W₁ synergy not only shifts down the lowest unoccupied states of Ce₁ near the Fermi level, thus enhancing the abilities in adsorbing and oxidizing NH₃ but also makes the frontier orbital electrons of W₁ delocalized, thus accelerating the activation of O₂. The deep insight of the Ce-W synergy may assist in the design and development of efficient catalysts with an SCR activity as high as or even higher than V₂O₅-WO₃/TiO₂.

Starch bio-template synthesis of W-doped CeO2 catalyst for selective catalytic reduction of NOx with NH3: influence of ignition temperature

A novel tungsten-doped CeO₂ catalyst was fabricated via the sweet potato starch bio-template spread self-combustion (SSC) method to secure a high NH₃-SCR activity. The study focuses on the influence of ignition temperature on the physical structure and redox properties of the synthesized catalyst and the catalytic performance of NO_x reduction with NH₃. These were quantitatively examined by conducting TG-DSC measurements of the starch gel, XRD analysis for the crystallites, SEM and TEM assessments for the morphology of the catalyst, XPS and H₂-TPR measurements for the distribution of cerium and tungsten, and NH₃-TPD assessments for the acidity of the catalyst. It is found that the ignition temperature shows an important role in the interaction of cerium and tungsten species, and the optimal ignition temperature is 500 °C. The increase of ignition temperature from 150 °C is beneficial to the interactions of species in the catalyst, depresses the formation of WO₃, and refines the cubic CeO₂ crystallite. The sample ignited at 500 °C shows the biggest BET surface area, the highest surface concentration of Ce species and molar ratio of Ce³⁺/(Ce³⁺+Ce⁴⁺), and the most abundant surface Brønsted acid sites, which are the possible reasons for the superiority of the NH₃-SCR activity. With a high GHSV of 200,000 mL (g h)^-1 and the optimal ignition temperature, Ce₄W₂O_z-500 can achieve a steadily high NO_x reduction of 80% or more at a lowered reduction temperature in the range of 250~500 °C.