A highly active VOx-MnOx/CeO2 for selective catalytic reduction of NO: The balance between redox property and surface acidity

Excellent catalysts with low-temperature activity and relatively wide temperature window for selective catalytic reduction of NO with ammonia (NH₃-SCR) are highly demanded in view of the practical treatment of NO. Herein, we have designed a highly active VO_x-MnO_x/CeO₂ material based on the intrinsic requirement of SCR reaction for catalyst, namely redox sites and surface acid sites. The vanadium oxide and manganese oxide are highly dispersed over the ceria mesosphere via simple incipient wetness impregnation. The loading of manganese could introduce acid sites and enhance the redox property remarkably, while the loading of vanadium increases acid sites and weakens redox property. Through tentatively controlling the appropriate loading ratio of the two components, the optimal catalyst achieves a balance between redox property and surface acidity. The work shed light on the development of new SCR catalyst with superior low temperature activity, wide work temperature window and good hydrothermal stability.     

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.     

Novel Mn-Ce bi-oxides loaded on 3D monolithic nickel foam for low-temperature NH3-SCR de-NOx:Preparation optimization and reaction mechanism

This study explored the superior citrate method(CM)to synthesize Mn-Ce bi-oxides on 3 D monolithic Ni-foam(NF)catalysts for the selective catalytic reduction of NO by NH₃(NH₃-SCR).The 17 wt%Mn(7)Ce(3)O_x/NF(CM-17)catalyst shows the NO_xconversion of 98.7%at 175℃and 90%in the presence of 10 vol%H2 O.It is revealed that the combination of surface-active oxygen(formed by high-level oxygen vacancies)and strongly oxidized Mn4+species promots the Fast-SCR reactions,in which Mn4+species play a leading role in NH₃-SCR reaction,and the unsaturated Ni atoms and also Ce3+species promote electron exchange and thus improve the redox performance.The coexistence mechanisms of Fast-SCR reactions and E-R pathways are observed over Mn-CeO_x/NF catalyst,which may be promoted by the Br?nsted sites at low temperature.In addition,the heat resistance,stability,3 D monolithic porous structure and excellent physical properties of foam nickel provide a unique growth substrates for catalysts preparation and reaction sites for NO_xpurification.Therefore,industrial application of Mn-Ce bioxides loaded on 3 D monolithic is proposed to be achieved through reasonable preparation methods.     

Effects of cerium and tungsten addition on acid–base properties of spindle-like α-Fe2O3 in low-temperature SCR of NOx with NH3

Low-temperature selective catalytic reduction (SCR) is important for the elimination of NO_x from stationary sources. In the present study, the loading of Ce and W on α-Fe₂O₃ was achieved through the integration of single-mode microwave and incipient wetness impregnation (IWI) methods. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images reveal that the structure of α-Fe₂O₃ is spindle-like, and the structure remains unchanged after the introduction of Ce and/or W. The results of NH₃-SCR investigation demonstrate that NO_x conversion over Ce–W/α-Fe₂O₃ is more than 85% at 300 °C, which is much higher than that over Ce/α-Fe₂O₃ and α-Fe₂O₃. Our studies illustrate that the addition of Ce can significantly increase the amount of surface oxygen vacancies as well as sites of moderate basicity. On the other hand, the addition of W can obviously decrease the amount of basic sites and increase the number of Brønsted acid sites. The synergistic effect of Ce and W addition on balancing acidity/basicity properties accounts for the high activity of Ce₂₀W₁₀/α-Fe₂O₃ for NO_x removal at low temperatures. The study provides insight into the relationship between acidity/basicity properties and catalytic performance of Ce–W/α-Fe₂O₃ catalysts, which is beneficial to the design of high-performance NH₃-SCR catalyst for NO_x removal at low temperatures.     

Promoting effect of microwave irradiation on CeO_2-TiO_2 catalyst for selective catalytic reduction of NO by NH_3

In this work we prepared several CeO₂-TiO₂ catalysts for the NH₃-SCR reactionusing co-precipitation with assistance of microwave irradiation. The catalytic NH₃-SCR activities over CeO₂-TiO₂ catalysts at low temperatures are largely enhanced by the treatment of microwave irradiation, the operation temperature window is also broadened. For better understanding the promotion mechanism, the catalyst prepared by conventional co-precipitation with and without microwave irradiation treatment was characterized with H₂-TPR, NH₃-TPD, XPS, XRD and BET. Microwave irradiation treatment accelerates the crystallite rate of CeO₂-TiO₂ catalysts, and greatly enlarges their surface area by adjusting their microstructures. The resistance to SO₂ and H₂O is also improved via regulating the hierarchical pore structure by the microwave irradiation. Microwave irradiation treatment can also improve the redox property and increase the acid sites over the catalyst surfaces. The result of in situ DRIFTS suggests that the microwave irradiation treatment generates more Brønsted acid sites on CeO₂-TiO₂-2 h catalyst, helpful in SCR reactions. XPS results show that after microwave irradiation on the CeO₂-TiO₂ catalysts, the surface demonstrates an elevated concentration of chemisorbed oxygen, consequently leading to better oxidation of NO to NO₂. Additionally, the molar ratio of Ce³⁺/Ce⁴⁺ has been elevated after being treated by microwave irradiation, a vital factor in enhancing the NH₃-SCR activities.     

Selective catalytic reduction of NOx with NH3 over core-shell Ce@W catalyst

An environmentally benign WO₃ wrapped cubic CeO₂ core–shell catalyst (Ce@W) was developed for the selective catalytic reduction of NO_x with NH₃. Compared with CeW particles prepared via the conventional co-precipitation method, this core–shell catalyst not only displays higher tolerance to SO₂ and H₂O, but also exhibits a wider activity temperature window of 250–450 °C, in which NO_x conversion and N₂ selectivity reaches 100%. The improved performance of Ce@W catalysts can be contributed to the strong interactions between CeO₂ (100) and WO₃, which generates more Ce³⁺ and surface chemisorbed oxygen. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) reveal that the more thermally stable Brønsted acid sites on Ce@W lead to its excellent high-temperature activity.     

Ce–Ti catalysts modified with copper and vanadium to effectively remove slip NH3 and NOx from coal-fired plants

In this work, V/Ce–Ti catalysts were modified with different kinds of transition metals (Cu, Fe, Co, Mn) by sol–gel and impregnation methods. The NH₃ oxidation performance of them was tested to select the most active catalyst in NH₃-selective catalytic oxidation (NH₃–SCO). The effect of NO, SO₂ and H₂O was also investigated. The experimental results indicate that 1% Cu–V/Ce–Ti catalyst exhibits the most significant ability to remove slip ammonia discharged from coal-fired plants and its NH₃ conversion efficiency reaches 90% at 300 °C. In addition, 97% NO_x can be removed when NO is introduced in the gas. Cu–V/Ce–Ti catalyst also obtains good resistance to H₂O and SO₂. Based on the characterization experiment, the introduced Cu and V are highly dispersed on Ce–Ti catalyst and they can increase the redox properties and the number of acidic sites. Besides, the redox cycles among Cu, V and Ce species on Cu–V/Ce–Ti catalyst surface are conducive to generating more active oxygen and promoting the oxidation capacity of the catalyst.     

In-situ DRIFT assessment on strengthening effect of cerium over FeO_x/TiO_2 catalyst for selective catalytic reduction of NO_x with NH_3

Fe-based catalysts have a great potential to be used for selective catalytic reduction(SCR) of NO_x with NH3 reaction due to their low cost,nontoxicity and excellent catalytic activity.The aim of this paper is to investigate Ce doping effect on activity of NH_3-SCR over the FeO_x/TiO_2 catalyst.In-situ diffuse reflectance infrared fourier transform(DRIFT) technology was utilized to verity the adsorbed species on the surface of FeO_x/TiO_2 and FeO_x-CeO_2/TiO_2 catalysts.With respect to the obtained results,among the four catalysts studied,the FeO_x-CeO_2/TiO_2 with the FeO_x/CeO_2 ratio of 3/8 shows the best NO conversion more than 98%in the temperature range of 230—350℃,The active centers for NH_3 adsorption and activation are assigned to Lewis acid sites over the FeO_x-CeO_2/TiO_2 and monodentate nitrates can act as the key intermediate in the NH3-SCR.Moreover,both of Langmuir-Hinshelwood and Eley-Rideal mechanisms are observed over the FeO_x-CeO_2/TiO_2 catalysts in the SCR.     

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.     

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.     

One-pot synthesis of Cu-Ce co-doped SAPO-5/34 hybrid crystal structure catalysts for NH_3-SCR reaction with SO_2 resistance

SAPO-34,SAPO-5/34 based catalysts doped with Cu,Ce as active components were synthesized via a one-pot hydrothermal method by using different amounts of additive(a-cellulose),and their catalytic activities were measured for selective catalytic reduction(SCR) of NO with NH3.The synthesized Cu-Ce co-doped products switch from cubic SAPO-34,to flower-like aggregated SAPO-5/34,hybrid crystal SAPO-5/34,and finally to spherical aggregated SAPO-34 with the increase of α-cellulose amount.The Cu-Ce co-doped SAPO-5/34 hybrid crystal structure catalysts with 0.75 mol ratios of C/P(Cu-Ce/SP-0.75)exhibit excellent NH3-SCR activity with higher than 90% NO_x conversion in the temperature range of 180-450℃,at WHSV of 20000 mL/(g·h).Furthermore,the catalyst displays outstanding sulfur resistance and NO_X conversion maintains above 90% at 200-450℃ after adding 100 ppm of SO_2.The characteristic results suggest that the high deNO_X performance of Cu-Ce/SP-0.75 is due to the enhanced accessibility,abundant activity species,excellent redox property and high adsorptive and activated capacity for NH3.     

Role of cerium as a promoter and process optimization studies for dehydration of glycerol to acetol over copper chromite catalyst

Cerium-promoted silica supported copper chromite catalyst was synthesized from acid hydrolysis of sodium silicate by sol-gel method.The catalyst was characterized by Brunauer-Emmett-Teller(BET)method,field-emission scanning electron microscopy(FESEM),X-ray diffraction(XRD),H₂-temperature programmed reduction(H₂-TPR),NH3-temperature programmed desorption(NH3-TPD)and pyridine adsorbed Fourier transform infrared spectroscopy(Py-FTIR).Among cerium doped catalysts,5 wt%of Ce promoted copper chromite supported by 40 wt%of silica(SiCuCr40-Ce5)shows the largest BET surface area.XRD analysis of the reduced form of the catalyst shows both CeO₂/Ce₂O₃ redox system and CuO/Cu₂O/Cu redox system.Py-FTIR shows the maximum number of Lewis acid sites for SiCuCr40-Ce5 than others.The highest acetol selectivity with analytical reagent(AR)grade glycerol conversion is observed for SiCuCr40-Ce5 at 200℃for 3 h in a batch reactor at atmospheric pressure.Cerium promotion lowers the reaction te mperature with enhanced glycerol conve rsion and increased acetol selectivity.Though the above catalyst shows higher conversion for laboratory reagent(LR)grade glycerol but it reduces acetol selectivity.The addition of glucose into the LR grade glycerol further reduces glycerol conversion and decreases the acetol selectivity to zero.This may be due to the presence of iron as impurity in LR grade glycerol.XRD analysis of spent catalyst shows the absence of redox catalytic system and the pore volume reduces identified by BET analysis.Raman analysis of the spent catalyst shows graphite-like carbon deposition in the spent catalvst.     

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.     

In situ DRIFTS study of NOx adsorption behavior on Ba/CeO2 catalysts

A series of Ba/CeO2 catalysts with different Ba loading amounts were prepared by incipient wetness impregnation. Their NOx adsorption behaviors under NO and NO＋O2 conditions were investigated by in situ DRIFTS. It was found that NOx was ad-sorbed and stored in the form of nitrites and nitrates on both Ba and Ce sites on the surface of the catalysts. The less thermally stable BaCO3 was suggested to be the main active phase for NOx trapping. Ceria served primarily as an oxygen supplier in the absence of O2, and the reaction from nitrites to nitrates on Ba sites was the key step in this case. In the presence of O2, however, gaseous O2 became the main oxygen source. The NOx adsorption capacity of the catalyst was dominated by the Ba content. Moreover, the stability of ni-trites and nitrates formed on Ce sites was found to be lower than those formed on Ba sites which existed in the form of the ionic bar-ium nitrate species.     

Synergetic catalytic removal of chlorobenzene and NOx from waste incineration exhaust over MnNb0.4Ce0.2Ox catalysts: Performance and mechanism study

Nb doped MnCe_0.2O_x complex oxides catalysts prepared via a homogeneous precipitation method were investigated for synergistic catalytic removal of NO_x and chlorobenzene (CB) at low temperatures. The MnNb_0.4Ce_0.2O_x catalyst with a molar ratio of Nb/Mn = 0.4 exhibits excellent activity and the NO_x and CB removal efficiency reaches 94.5% and 96% at 220 °C, respectively. Furthermore, the NO_x and CB removal efficiency of MnNb_0.4Ce_0.2O_x still remains above 80% after injecting 300 ppm SO₂ and 7 vol% H₂O for 36 h. In addition, the presence of CB and NO_x + NH₃ can improve the NO_x and CB removal efficiency of MnNb_0.4Ce_0.2O_x, respectively. The analysis results from N₂-BET, Py-IR, H₂-TPR and NH₃-TPD reveal that the introduction of Nb increases the average pore size, pore volume and surface area, promoted the growth of Lewis acid amount obviously, and enhances redox ability of MnCe_0.2O_x at 100–250 °C. Moreover, the molecular migration process of NO_x, NH₃, CB and SO₂ in NH₃-SCR and CB oxidation reaction over MnNb_0.4Ce_0.2O_x catalysts were systematically studied. In situ DRIFTS, FT-IR and XPS also confirm that the adsorption of sulfate species and SO₂ on the surface of MnNb_0.4Ce_0.2O_x is inhibited effectively by the introduction of Nb in the presence of SO₂ and H₂O. Moreover, Nb additives also enhance the structural stability of MnNb_0.4Ce_0.2O_x, due to the interactions among Mn, Nb and Ce. The NH₃-TPD, H₂-TPR and in situ DRIFTS results also confirm that the MnNb_0.4Ce_0.2O_x still retains abundant acid sites and high redox ability in the presence of SO₂ and H₂O. In summary, MnNb_0.4Ce_0.2O_x catalysts represent a promising and effective candidate for controlling NO_x and CB at low temperatures.     

Fabrication of wide temperature lanthanum and cerium doped Cu/TNU-9 catalyst with excellent NH3-SCR performance and outstanding SO2+H2O tolerance

The effects of La on the catalytic performance,SO₂ and H2O resistance of Cu-Ce/TNU-9 catalyst were studied in the selective catalytic reduction of NO_x via ammonia(NH₃-SCR).The results show that the La doped Ce-Cu/TNU-9(CCL/T9) catalyst exhibits better SCR performance than Ce-Cu/TNU-9(CC/T9) and Cu/TNU-9(C/T9) in the wide temperature window(200-450 ℃) due to La benefiting from enhancing Cu⁺+Ce⁴⁺?Cu²⁺-+Ce³⁺ to facilitate ...     

Effect of different doping elements on performance of Ce-Mn/TiO2 catalyst for low temperature denitration

Sm and Ho were doped in Ce-Mn/TiO₂ catalyst respectively to enhance its denitration performance at low temperature.X-ray diffraction(XRD),N₂ adsorption-desorption,X-ray photoelectron spectroscopy(XPS),NH₃-temperature programmed desorption(NH₃-TPD),H₂-temperature programmed reduction(H₂-TPR) and in situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS) techniques were used to analyze the structure and performance ...     

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...     

Catalysis of Ce_xZr_(1–x)O_2 for di-iso-propyl-ether hydration

The catalytic potential of CexZr1–xO2 in isopropyl ether(DIPE) hydration was explored. While the acidic H-zeolite catalyst was favorable for propylene formation through carbenium ion mechanism, Ce-ZSM-5 and CexZr1–xO2 catalysts improved product selectivity of isopropyl alcohol(IPA) through redox mechanism. The catalytic property of CexZr1–xO2 depended on the preparation method and variable, type of cerium precursor and Ce/Zr ratio. By means of characterizations with X-ray diffraction, N2 adsorption isotherms and NH3 temperature programmed desorption, tetragonal phase of CexZr1–xO2 was proposed as the active phase in which CeO2 and ZrO2 catalyzed synergistically the DIPE hydration with IPA product selectivity. The CexZr1–xO2 prepared from cerium sulfate precursor with co-precipitation hydrothermal method exhibited maximum catalytic activity and IPA product selectivity. The precursor effect was attributed to the stabilization of SO42– species on the tetragonal phase of CexZr1–xO2 and super solid acidity.     

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.