A novel ceria hollow nanosphere catalyst for low temperature NOx storage

In this work, a highly active CeO₂ catalyst with hollow nanosphere morphology for low temperature NO_x storage was prepared by a surfactant-assisted solvothermal reaction. The physicochemical properties of ceria samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption–desorption, H₂-temperature programmed reduction (H₂-TPR), X-ray photoelectron spectroscopy (XPS) and in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The as-prepared CeO₂ nanosphere possesses excellent NO oxidation capacity, smaller mesopores, better reducibility and more surface Ce³⁺ content. Compared with CeO₂ with nanorod and nanoparticle morphologies, CeO₂ nanosphere shows better intrinsic low temperature NO_x trapping performance, with a wide operating temperature window (150–300 °C), high NO_x adsorption capacity (NAC, 640–745 μmol/g) and high NO_x storage capacity (NSC, 250–350 μmol/g). Two reaction pathways are speculated for NO_x adsorption on CeO₂ nanosphere, including “nitrate route” and “nitrite route”. The thermally unstable surface nitrites formed on the CeO₂ nanosphere allow ceria to release more NO_x during the desorption process. The present work provides a new ceria morphology for NO_x traps, which may become a potential excellent NO_x storage material.     

Controllable synthesis of argentum decorated CuOx@CeO2 catalyst and its highly efficient performance for soot oxidation

In this paper, CuO_x@Ag/CeO₂ catalysts were synthesized by simple wet-chemical method and equal volume impregnation method. The obtained catalysts were subjected to soot temperature programmed oxidation (soot-TPO) activity tests and were further characterized by various techniques such as X-ray diffraction (XRD), transmission electron microscopy/high-resolution transmission electron microscopy (TEM/HR-TEM), N₂ physisorption, X-ray photoelectron spectroscopy (XPS) and H₂-temperature programmed reduction (H₂-TPR). The results show that CuO_x@Ag/CeO₂ synthesized presents well controlled core-shell structures, with nano-cube like Cu₂O as the core and Ag decorated polycrystalline CeO₂ grafting layers as the shell. Such core-shell structured CuO_x@Ag/CeO₂ can successfully construct a secondary oxygen delivery channel (CuO_x → CeO₂ → Ag) to effectively transfer bulk oxygen of the catalyst to the soot, resulting in its excellent soot oxidation activity compared to CuO_x@CeO₂. The potential benefiting effect by Ag introduction over Cu@Ag/Ce can be concluded as: (i) pumping lattice oxygen and accelerating gaseous O₂ dissociation to generate significantly increased active surface oxygen content; (ii) modulating a moderate surface oxygen vacancies concentration to maintain more highly active O₂^– species.     

Storage-reduction strategy for NOx reduction from gas turbine exhaust with W-Ti-CeOx catalyst in natural gas power plants

Traditional vanadium-based selective catalytic reduction(SCR) deNO_x catalyst can hardly adapt to the gas conditions(much high NO2/NO_x ratio at lower temperature) of the start-up and low loading periods for a gas turbine.Therefore,a W-Ti-CeO_x catalyst with NO_x storage and reduction(NSR)function was developed in this work for gas turbine exhaust NO_x elimination.The experimental results reveal that W-Ti-CeO_x catalyst exhibits high NO...     

Extruded monolith MnOx-CeO2-TiO2 catalyst for NH3-SCR of low temperature flue gas from an industry boiler: Deactivation and recovery

The selective catalytic reduction(SCR) of NO_x with NH₃(NH₃-SCR) technology has been widely applied for reducing NO_x emissions from stationary and mobile sources.In this work,the extruded monolith MnO_x-CeO₂-TiO₂ catalyst was installed in a cement kiln for NH₃-SCR of NO_x,where the flue gas temperature was 110-140℃.It is found that the monolith catalyst is severely deactivated after operating for abou...     

Enhancing low-temperature NH3-SCR performance of Fe–Mn/CeO2 catalyst by Al2O3 modification

In the work, supported catalysts of FeO_x and MnO_x co-supported on aluminum-modified CeO₂ was synthesized for low-temperature NH₃-selective catalytic reduction (NH₃-SCR) of NO. Impressively, the SCR activity of the obtained catalyst is markedly influenced by the adding amount of Al and the appropriate Ce/Al molar ratio is 1/2. The activity tests demonstrate that Fe–Mn/Ce1Al2 catalyst shows over 90% NO conversion at 75–250 °C and exhibits better SO₂ resistance compared to Fe–Mn/CeO₂. Fe–Mn/Ce1Al2 shows the expected physicochemical characters of the ideal catalyst including the larger surface and increased active reaction active sites by controlling the amount of Al doping. Also, the better catalytic activity is well correlated with the present advantaged surface adsorption oxygen species, Mn⁴⁺ species, Ce³⁺ species and the enhanced reducibility of Fe–Mn/Ce1Al2, which is superior to the Fe–Mn/CeO₂ catalyst. More importantly, we further demonstrate that the amount and strength of surface acid sites are improved by Al-doping and more active intermediates (monodentate nitrate) is generated during NH₃-SCR reaction. This work provides certain insight into the rational creation of simple and practical denitration catalyst environmental purification.     

Promotion effect of niobium on ceria catalyst for selective catalytic reduction of NO with NH3

The CeO₂, Ce–Nb–O_x and Nb₂O₅ catalysts were synthesized by citric acid method and the promotion effect of Nb on ceria for selective catalytic reduction (SCR) of NO with NH₃ was investigated. The catalytic activity measurements indicate that the mixed oxide Ce–Nb–O_x presents a higher SCR activity than the single oxide CeO₂ or Nb₂O₅ catalyst. In addition, the Ce–Nb–O_x catalyst shows high resistance towards H₂O and SO₂ at 280 °C. The Raman, X-ray photoelectron spectra and temperature programmed reduction with H₂ results indicate that the incorporation of Nb provides abundant oxygen vacancies for capturing more surface adsorbed oxygen, which provides a superior redox capability and accelerates the renewal of active sites. Furthermore, the Fourier transform infrared spectra and temperature programmed desorption of NH₃ results suggest that niobium pentoxide shows high surface acidity, which is partly retained in the Ce–Nb–O_x catalyst possessing a high content of Lewis and Brønsted acid sites. Therefore, the incorporation of Nb improves both the redox and acidic capacities of Ce–Nb–O_x catalyst for the SCR reaction. Here, the redox behavior is primarily taken on Ce and the acidity is well improved by Nb, so the synergistic effect should exist between Ce and Nb. In terms of the reaction mechanism, in situ DRIFT experiments suggest that both NH₃ on Lewis acid sites and NH₄⁺ on Brønsted acid sites can react with NO species, and adsorbed NO and NO₂ species can both be reduced by NH₃. In the SCR process, O₂ primarily acts as the accelerant to improve the redox and acid cycles and plays an important role. This work proves that the combination of redox and acidic properties of different constituents can be feasible for catalyst design to obtain a superior SCR performance.     

Novel promoting effects of cerium on the activities of NO_x reduction by NH_3 over TiO_2-SiO_2-WO_3 monolith catalysts

A series of catalysts were prepared by doping different loadings of CeO2 over TiO2-SiO2-WO3 and used for the selective catalytic reduction of NOx by NH3. The experimental results showed that the selective catalytic reduction（SCR） performance and SO2-resistant ability of TiO2-SiO2-WO3 were greatly enhanced by the introduction of cerium. The catalyst containing 10% CeO2 showed the highest NO conversion in a wide temperature range and good N2 selectivity with broad operation temperature window at the gas hourly space velocity（GHSV） of 30000 h–1, which was a very promising catalyst for NOx abatement from diesel engine exhaust. The catalysts were characterized by X-ray diffraction（XRD）, scanning electron microscopy with energy dispersive X-ray spectroscopy（SEM-EDS）, N2 adsorption-desorption（BET） and X-ray photoelectron spectroscopy（XPS）. The characterization results showed that the bigger pore radius, higher surface atomic concentration and dispersion of Ce and the abundant adsorbed oxygen on the surface of catalyst contributed to the best NH3-SCR performance of CeO2/TiO2-SiO2-WO3 catalyst containing 10% CeO2.     

Surface exsolution and local atomic structure of amorphous CeTiOx

We made precipitated nano-ceria (～5 nm) on the surface of the catalyst by heat treatment of Ce-supersaturated amorphous CeTiO_x to improve the oxygen storage properties of CeO₂. The catalysts were prepared by sol-gel methods and TiO₂ nanoparticles were preferentially generated as a core material to form selective Ce-supersaturated structure on the catalyst surface. Reaction temperature and amount of doping element are optimized to induce selective crystallization of CeO₂. CeCe (2nd shell) bond around 0.38 nm of Ce L₃-edge extended X-ray absorption fine structure is reduced and nanostructure of precipitated ceria on the surface is observed by HREM. The catalyst is present as amorphous with precipitated nano-CeO₂ on the surface. The de-NO_x efficiency of the catalyst, which has precipitated CeO₂, improves by ～50% owing to the simultaneous reactions of the nano CeO₂ and the amorphous CeTiO_x.     

Promotion effect of H2 pretreatment on CeO2 catalyst for NH3-SCR reaction

In this study, the promotion effect of H₂ pretreatment on the SCR performance of CeO₂ catalyst was investigated based on the characterization results of XRD, H₂-TPR, Raman and in situ DRIFT techniques. Lower crystallinity, higher reducibility and surface acidity can be found on CeO₂-H catalyst. The results of DRIFT study reveal that the pretreatment of CeO₂ catalyst with H₂ can facilitate the adsorption of NH₃ and NO_x species, while the adsorbed NO_x is basically inactive in the NH₃-SCR reaction. Moreover, the reaction mechanism of the NH₃-SCR reaction over CeO₂ catalyst is not changed by H₂ pretreatment, which is mainly under the control of Eley-Rideal (E-R) mechanism. The enhanced SCR performance of CeO₂-H catalyst is mainly due to the promoted NH₃ adsorption and the subsequent facilitation of SCR reaction through E-R pathway.     

Hydrothermal and sulfur aging of CeTi/CeWTi catalysts for selective catalytic reduction of NO_x with NH_3

The CeO_2-TiO_2(CeTi)and CeO_2/WO_3-TiO_2(CeWTi)catalysts were prepared by sol-gel method.The asprepared catalysts were hydrothermally treated at 760 ℃ for 48 h in air containing 10 vol% H_2O to obtain the hydrothermal aged catalysts.The sulfur aged catalysts were treated at 400 ℃ with 100 ppm SO_2,10%water vapor,air balance for 48 h and catalysts.The powder X-ray diffraction(XRD)and Raman results indicate that the crystallization of hydrothermal aged catalysts is more serious than sulfur aged catalysts.In addition,tungsten species can stabilize the CeTi catalyst from grain growth.According to the results of in situ diffuse reflectance infrared Fourier transform spectra(DRIFTS),temperatureprogrammed desorption of ammonia(NH_3-TPD),H_2 temperature-programmed reduction(H_2-TPR)and ammonia oxidation,the aging process leads to loss of surface area,redox properties,surface acidities and surface ceria concentration,especially for the hyd rothermal aging.The NH_3-NO/NO_2 SCR perfo rmances of sulfur aged catalysts are better than that of hydrothermal aged catalysts.Compared with CeTi catalyst,the addition of tungsten inhibits the crystallization of catalyst.So that more acid sites and active sites are retained.This is also the reason why tungsten addition improves the NH_3-NO/NO_2 SCR performance of CeTi catalyst.     

Highly dispersed CeO2–x nanoparticles with rich oxygen vacancies enhance photocatalytic performance of g-C3N4 toward methyl orange degradation under visible light irradiation

CeO₂/g-C₃N₄ photocatalysts have attracted tremendous attention in the photocatalytic degradation of organic pollutants. The design and construction of highly active CeO₂/g-C₃N₄ photocatalysts without harsh conditions are still challenging. Herein, highly dispersed CeO_2–x nanoparticles with rich oxygen vacancies were successfully precipitated on the surface of g-C₃N₄ under mild conditions. The fabricated CeO_2–x/g-C₃N₄ exhibits remarkable activity and stability for photocatalytic degradation of MO pollutant. The optimal rate constant of MO degradation over CeO_2–x/g-C₃N₄ is about 0.031 min^?1, which is three times higher than that of g-C₃N₄. A negligible activity decrease is observed after three cycling runs. The enhanced catalytic performance can be ascribed to the excellent dispersion of CeO_2–x with rich oxygen vacancies that benefit O₂ adsorption and visible light absorption. In addition, the proper band alignment between CeO_2–x and g-C₃N₄ is conducive to the highly efficient separation of photogenerated electron–hole pairs.     

Effect of manganese and/or ceria loading on V_2O_5-MoO_3/TiO_2 NH_3 selective catalytic reduction catalyst

The effect of manganese and/or ceria loading of V_2 O_5-Mo_O_3/TiO_2 catalysts was investigated for selective catalytic reduction(SCR) of NO_x by NH_3.The manganese and/or ceria loaded V_2 O_5-MoO_3/TiO_2 catalysts we re prepared by the wetness impregnation method.The physicochemical characteristics of the catalysts were thoroughly characterized.The catalytic performance of 1.5 wt% V_2 O_5-3 wt% MoO_3/TiO_2(V1.5 Mo3/Ti) is greatly enhanced by addition of 2.5 wt% MnO_x and 3.0 wt% CeO_2(V1.5 Mo3 Mn2.5 Ce3/Ti) below450℃.Compared with the V1.5 Mo3/Ti catalyst with NO_x conversion of 75% at 275 ℃,V1.5 Mo3 Mn2.5 Ce3/Ti exhibits higher NO_x conversion of 84% with good resistance to SO_2 and H_2 O at a gas hourly space velocity value of 150000 h~(-1).The active manganese,cerium,molybdenum,and vanadium oxide species are highly dispersed on the catalyst surface and some synergistic effects exist among these species.Addition of MnO_x significantly enhances the redox ability of the cerium,vanadium,and molybdenum species.Addition of Ce increases the acidity of the catalyst.More active oxygen species,including surface chemisorbed oxygen,form with addition of Mn and/or Ce.Because of the synergistic effects,appropriate proportions of manganese in different valence states exist in the catalysts.In summary,the good redox ability and the strong acidity contribute to the high NH3-SCR activity and N2 selectivity of the V1.5 Mo3 Mn2.5 Ce3/Ti catalyst in a wide temperature range.And the V1.5 Mo3 Mn2.5 Ce3/Ti catalyst shows good resistance to H_2 O and SO2 in long-time catalytic testing,which can be ascribed to the highly sulfated species adsorbed on the catalyst.     

Regulating local coordination environment of rhodium single atoms in Rh/CeO2 catalysts for N2O decomposition

Rh single atom catalysts(SACs) have been insensitively investigated recently due to the maximum utilization efficiency of Rh,one of the most expensive precious metals.Although great efforts have been made in the development and application of Rh SACs,there are few reports on the precise control of the local coordination environment of Rh single sites on CeO₂ and their catalytic performance for N₂O decomposition.Herein,Rh/CeO₂ catalysts with different Rh-O coordin...     

Core-shell MnCeOx catalysts for NO oxidation and mild temperature diesel soot combustion

Environmental contamination such as soot particles and NO_x has aroused extensive attraction recently.However,the main challenge lies in the oxidation of soot at mild temperature with the assistance of NO_x.Here,a series of core-shell MnCeO_x catalysts were successfully synthesized by hydrothermal method and employed for low-temperature catalytic oxidation of soot in the presence of NO_x.X-ray diffraction(XRD),inductively coupled plasma-optical emission sp...     

Highly efficient K-doped Mn-Ce catalysts with strong K-Mn-Ce interaction for toluene oxidation

In this study,K_x-Mn-Ce catalysts prepared by sol-gel method were investigated for toluene oxidation.Compared with Mn-Ce,the catalytic performance of K_x-Mn-Ce was further improved.X-ray diffraction(XRD),high resolution transmission electron microscopy(HRTEM) and Raman analyses demonstrate that K ions enter the lattice of CeO₂ and disperse uniformly.The results of X-ray photoelectron spectroscopy(XPS),H₂-temperature programmed reduction(H₂-TPR...     

CeO2-x modified Ru/γ-Al2O3 catalysts for ammonia decomposition reaction

Developing high-performance ammonia decomposition catalysts for preparing CO_x-free hydrogen shows great practical significance.Herein,CeO₂ is used as a promoter to modulate the metal-support interaction to enhance the catalytic performance of Ru/Al₂O₃ catalysts.A series of 1Ru/xCe-10AI(x=0.5,1,or 3)catalysts was prepared by a facile colloidal deposition method.We find that the optimized 1 Ru/1Ce-10Al catalyst exhibits excellent activity for the decompo...     

Effect of hydration on the surface basicity and catalytic activity of Mg-rare earth mixed oxides for aldol condensation

Magnesium and rare earth mixed oxides (Mg₃REO_x (RE = La, Y, Ce)) were prepared and characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, infrared spectra and microcalorimetry of CO₂. The results reveal that the Mg₃CeO_x catalyst is present in the form of Mg-Ce-O solid solution, while the Mg₃LaO_x and Mg₃YO_x catalysts are probably rare earth oxides dispersed on MgO surface. As a result, among the calcined Mg₃REO_x catalysts, the Mg₃CeO_x catalyst presents the highest rate constant for acetone aldolization, which is well correlated to its more homogeneous distribution of basic sites. In contrary, the Mg₃YO_x catalyst exhibit the lowest catalytic activity for acetone aldolization. Upon hydration pre-treatment, the basic properties on the surface of the Mg₃REO_x catalysts were changed markedly. The Mg₃YO_x catalyst after hydration treatment shows the highest amount of basic sites on catalyst surface, and then exhibits the highest activity among the hydrated Mg₃REO_x catalysts. These results make it possible to fine-tune basic sites for acetone aldolization.     

Promotional mechanism of activity of CeEuMnOx ternary oxide for low temperature SCR of NOx

Due to strong synergistic effect of the elements,a series of XEuMnO_x ternary oxides(X=Ce,Ni,Co,Sb,Sn,Mo) were synthesized by one-pot co-precipitation method,and composite components were identified and optimized to maintain high activity and superior SO₂ and H₂O endurance in selective catalytic reduction of NO_x with NH₃(NH₃-SCR).NO_x conversion of CeEuMnO_x ternary oxide catalysts attains more than 90% at 100-2...     

Low-temperature catalytic combustion of trichloroethylene over MnOx-CeO2 mixed oxide catalysts

The cerium-based catalysts were investigated for the catalytic co mbustion of trichlo roethylene(TCE) and exhibit a surprising catalytic activity.MnO_x was doped into CeO₂ by a citric acid(CA) sol-gel method,and the effect of Mn content on the physicochemical properties and catalytic activities of MnO_x-CeO₂ mixed oxides was investigated systemically.The introduction of MnO_x into CeO₂ can evidently improve the catalytic activity and...