Cerium Oxide Promoted Iron-based Oxygen Carrier for Chemical Looping Combustion

Stability Mechanism of Low Temperature C2H4-SCR with Activated-Carbon-Supported MnO x -Based Catalyst

Manganese-based catalysts have shown great potential for use as a hydrocarbon reductant for NO _x reduction (HC-SCR) at low temperatures if their catalytic stability could be further maintained. The effect of CeO₂ as a promoter and catalyst stability agent for activated carbon supported MnO _x was investigated during low temperature deNO _x based on a C₂H₄ reductant. The modern characterization technology could provide a clear understanding of the activity observed during the deNO _x tests. When reaction temperatures were greater than 180 °C and with ceria concentrations more than 5%, the overall NO conversion became stable near 70% during long duration testing. In situ DRIFTS shows that C₂H₄ is adsorbed on the Mn₃Ce₃/NAC catalysts to generate hydrocarbon activated intermediates, R-COOH, and the reaction mechanism followed the E-R mechanism. The stability and the analytical data pointed to the formation of stable oxygen vacancies within Ce³⁺/Ce⁴⁺ redox couplets that prevented the reduction of MnO₂ to crystalline Mn₂O₃ and promoted the chemisorption of oxygen on the surface of MnO _x -CeO _x structures. Based on the data, a synergetic mechanism model of the deNO _x activity is proposed for the MnO _x -CeO _x catalysts.

Perovskites in the Energy Grid and CO2 Conversion: Current Context and Future Directions

CO2 emissions from the consumption of fossil fuels are continuously increasing, thus impacting Earth’s climate. In this context, intensive research efforts are being dedicated to develop materials that can effectively reduce CO2 levels in the atmosphere and convert CO2 into value-added chemicals and fuels, thus contributing to sustainable energy and meeting the increase in energy demand. The development of clean energy by conversion technologies is of high priority to circumvent these challenges. Among the various methods that include photoelectrochemical, high-temperature conversion, electrocatalytic, biocatalytic, and organocatalytic reactions, photocatalytic CO2 reduction has received great attention because of its potential to efficiently reduce the level of CO2 in the atmosphere by converting it into fuels and value-added chemicals. Among the reported CO2 conversion catalysts, perovskite oxides catalyze redox reactions and exhibit high catalytic activity, stability, long charge diffusion lengths, compositional flexibility, and tunable band gap and band edge. This review focuses on recent advances and future prospects in the design and performance of perovskites for CO2 conversion, particularly emphasizing on the structure of the catalysts, defect engineering and interface tuning at the nanoscale, and conversion technologies and rational approaches for enhancing CO2 transformation to value-added chemicals and chemical feedstocks.

Enhanced CH4 and CO Oxidation over Ce1- xFe xO2-δ Hybrid Catalysts by Tuning the Lattice Distortion and the State of Surface Iron Species

CeO₂-Fe₂O₃ mixed oxides are very attractive as catalysts for catalytic oxidation. Herein, we report the structural dependence of the Ce_{1- x}Fe _xO_2-δ catalysts for CH₄ combustion and CO oxidation via changing lattice distortion degrees, surface Fe₂O₃ states, and oxygen vacancy concentrations. The lattice distortion degree and oxygen vacancy concentration of Ce-Fe-O solid solution can be tuned by changing the contents of Fe and the precipitation temperatures in the preparation process. The precipitation at relatively high temperature (70 °C) promotes the lattice distortion, whereas a lower temperature (0 °C) helps the formation of surface oxygen vacancies. The in situ diffuse reflectance infrared/Raman experiments and the physicochemical characterization suggest that both the CO and CH₄ oxidations mainly follow a Mars-van Krevelen mechanism. Both the lattice distortion and the surface iron species play a crucial role in determining the catalytic activity by affecting the redox property of the catalysts. The surface iron species, combined with the oxygen vacancies, improve the catalytic performance by enhancing the adsorption capacity of reactants and reducibility of catalysts. The lattice distortion of CeO₂ contributes to the catalytic activity by tuning the oxygen mobility in the bulk, which promotes the re-oxidation rate of catalysts.

Peroxide route to the synthesis of ultrafine CeO2-Fe2O3 nanocomposite via successive ionic layer deposition

An ultrafine α-CeO2-α-Fe2O3 nanocomposite was prepared from the ultradispersed nanoparticles of cerium (IV) and iron (III) amorphous hydroxides heat-treated at 600 °С and 900 °С in the air. The initial composites were obtained by the successive ionic layer deposition (SILD) method. According to scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and powder X-ray diffraction (PXRD), the cerium/iron ratio in the synthesized nanocomposite is close to 1:2, and the α-CeO2 and α-Fe2O3 nanocrystals are isometrically shaped and have an average size of 4 ± 1 and 7 ± 1 nm (600 °С) and 24 ± 2 and 35 ± 3 nm (900 °С), respectively. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) have shown that nanocrystals are evenly distributed in the composite volume and are spatially conjugated. The formation mechanisms of both initial amorphous composites of cerium (IV) and iron (III) hydroxides and of α-CeO2 and α-Fe2O3 nanocrystals were established. It was shown that synthesis of the initial hydroxide composite using the SILD method proceeds via the formation of amorphous cerium hydroxo-peroxide (CeO(OOH)2). As a result of the study, a schematic mechanism for the formation of a composite based on ultrafine nanocrystals of cerium (IV) and iron (III) oxides has been proposed.

Performance of C2H4 Reductant in Activated-Carbon- Supported MnOx-based SCR Catalyst at Low Temperatures

Hydrocarbons as reductants show promising results for replacing NH3 in SCR technology. Therefore, considerable interest exists for developing low-temperature (<200 °C) and environmentally friendly HC-SCR catalysts. Hence, C2H4 was examined as a reductant using activated-carbon-supported MnOx-based catalyst in low-temperature SCR operation. Its sensitivity to Mn concentration and operating temperature was parametrically studied, the results of which showed that the catalyst activity followed the order of 130 °C > 150 °C > 180 °C with an optimized Mn concentration near 3.0 wt.%. However, rapid deactivation of catalytic activity also occurred when using C2H4 as the reductant. The mechanism of deactivation was explored and is discussed herein in which deactivation is attributed to two factors. The manganese oxide was reduced to Mn3O4 during reaction testing, which contained relatively low activity compared to Mn2O3. Also, increased crystallinity of the reduced manganese and the formation of carbon black occurred during SCR reaction testing, and these constituents on the catalyst’s surface blocked pores and active sites from participating in catalytic activity.