New insights into the role of vanadia species as active sites for selective catalytic reduction of NO with ammonia over VO_x/CeO_2 catalysts

A series of VO_x/CeO_2 catalysts we re synthesized via vanadia supported on ceria with different BET surface areas.The catalysts were employed to investigate the active sites for the selective catalytic reduction of NO with NH_3(NH3-SCR).The kinetic results show that VO_x/CeO_2 catalysts exhibit nearly constant apparent activation energies(E_a),indicating the same SCR reaction mechanism.The V-O-Ce bridging modes and oligomeric VO_x were identified and quantified by Raman,FT-IR and H_2-TPR.The amounts of the V-O-Ce bridging modes calculated by H_2-TPR are correlated to the NH_3-SCR intrinsic reaction rates.The turnover frequencies(TOFs) show a constant value at the same temperature,which were calculated based on the number of V-O-Ce bridging modes of VO_x/CeO_2 catalysts.Therefore,it can be concluded that the V-O-Ce bridging modes are the active sites of VO_x/CeO_2 catalysts for the NH3-SCR reaction.     

Effect of praseodymium additive on CeO_2(ZrO_2)/TiO_2 for selective catalytic reduction of NO by NH_3

A series of praseodymium added CeO_2(ZrO_2)/TiO_2 catalysts separately prepared by methods of sol-gel and impregnation were tested for selective catalytic reduction of NO, and characterized by X-ray diffraction(XRD), N_2-brumauer-emmett-teller(N_2-BET), NH_3-temperature programmed desorption(NH_3-TPD), H_2-temperature programmed reduction(H_2-TPR), PL spectra, Raman spectra, electron paramagnetic resonance(EPR) and X-ray photoelectron spectroscopy(XPS), respectively. Influence of preparation method on catalytic performance was studied. Results showed that the influence of Pr addition on catalytic performance of the CeO_2(ZrO_2)/TiO_2 catalysts was different between the sol-gel method and the impregnation method. The Pr addition tended to interact with TiO_2 and formed the structure of Ti-O-Pr in the sol-gel method while it was more likely to interact with CeO_2 forming the structure of Ce-O-Pr in the impregnation method. The total acid amount and redox properties of the catalysts prepared by sol-gel method decreased with the addition of Pr element, which resulted in decrease of catalytic activity. In contrast, the Pr-added catalyst prepared by impregnation method was found to possess easier reducibility, more total acid amount and higher proportion of Ce~(3+) species, which was favourable for higher catalytic activity.     

Synergistic catalytic removals of NO,CO and HC over CeO2 modified Mn-Mo-W-Ox/TiO2-SiO2 catalyst

A series of Mn-Mo-W-O_x/TiO_2-SiO_2 catalysts was modified with CeO_2 using an extrusion molding method. The catalytic activities of the obtained catalysts were tested for the synergistic catalytic removals of CO, NO and C_3H_8. The ratio of catalyst composition on catalytic activities for NH_3-SCR was optimized, which reveals that the molar ratio of Ti/Si was 9:1 and the catalyst containing 1.5 wt% CeO_2 and 12 wt% Mn-Mo-W-O_x exhibits the best catalytic performances. These samples were characterized by XRD, N_2-BET, Py-IR, NH_3-TPD, SEM/element mapping, H_2-TPR and XPS, respectively. Results show that the optimal catalyst exhibits more than 99% NO conversion, 86% CO conversion and 100% C_3H_8 conversion under GHSV of 5000 h~(-1). In addition, the GHSV has little influence on removal of NO when it is less than 15,000 h~(-1). Furthermore, the addition of CeO_2 will enhance the surface acidity, increase Mn~(4+)concentration and inhibit the grain growth, which are favorable for the excellent catalytic performance.Anyway,the 1.5 wt% CeO_2-12 wt% Mn-Mo-W-O_x/TiO_2-SiO_2 possesses outstanding redox properties,abundant acid sites and high Mn~(4+) concentration, which provide a guarantee for synergistic catalytic removal of CO, NO and HC.     

Modification of Cu/ZSM-5 catalyst with CeO_2 for selective catalytic reduction of NO_x with ammonia

Cu/ZSM-5 and CeO_2-modified Cu/ZSM-5 catalysts were prepared by a wetness impregnation method. The addition of CeO_2 was found to enhance the NO_x selective catalytic reduction(SCR) activity of the catalyst at low temperatures, but the high-temperature activity was weakened. The catalysts were characterized by X-ray diffraction(XRD), nitrogen physisorption, inductively coupled plasma optical emission spectrometry(ICP-OES), X-ray photoelectron spectroscopy(XPS), electron paramagnetic resonance(EPR), H_2 temperature-programmed reduction(TPR) and NH_3 temperature-programmed desorption(TPD). The results showed that more CuO clusters instead of isolated Cu~(2+) species were obtained on the modified catalyst. These active CuO clusters, as well as the Cu-Ce synergistic effect, improved the redox property of the catalyst and low-temperatures SCR activity via promoting the oxidation of NO to NO_2 and fast SCR reaction. The loss in high-temperatures activity was attributed to the enhanced competitive oxidation of NH_3 by O_2 and decreased surface acidity of the catalyst.     

New insights into MnCe(Ba)O_x/TiO_2 composite oxide catalyst:Barium additive accelerated ammonia conversion

MnCeO_x/TiO_2 has been widely used in selective catalytic reduction(SCR) of NO_x at low temperature.However,it is often poisoned in the presence of water vapor and sulfur dioxide.In this work,the promotion mechanism of Ba modification was investigated.Results show that the doped BaO reacts with CeO_2 and forms BaCeO_3.This unique perovskite structure of BaCeO_3 significantly enhances NO oxidation and NH_3 activation of MnCeO_x/TiO_2 catalyst so that the NO conversion and the resistances to SO_2 improve.It is found that Ba species obviously promotes the NO adsorption ability and improve the redox properties of MnCeO_x/TiO_2 catalyst.While the acid properties of the catalyst are inhibited by Ba modification and among which Lewis acid sites are dominant for both MnCeO_x/TiO_2 and MnCe(Ba)O_x/TiO_2 catalysts.Furthermore,in situ DRIFT experiments reveal that the NO reduction upon MnCeO_x/TiO_2 and MnCe(Ba)O_x/TiO_2 catalysts follows both E-R and L-H mechanisms,in which L-H is preferred.Ba species enhances the formation of active nitrate species,which accelerates the NO reduction through L-H mechanism.It is interesting that although Ba species weakens the NH_3 adsorption,it induces the ammonia conversion to coordination ammonia,which in turn accelerates the catalytic reaction.     

High activity of CeO_2-TiO_2 composites for deep oxidation of 1,2-dichloroethane

CeO_2-TiO_2 catalysts prepared by different methods were investigated for deep oxidation of 1,2-dichloroethane(DCE),as a typical representative of the chlorinated volatile organic compounds(CVOCs).Characterization analysis reveals that CeO_2-TiO_2 catalysts prepared by sol-gel and coprecipitation methods exhibit higher specific area,CeO_2 and TiO_2 particles are highly dispersed into each other and the reducibility and mobility of active oxygen species are obviously promoted due to the strong interaction between the two catalysts CeO_2 and TiO_2,resulting in higher catalytic activity for DCE oxidation to and less chlorinated byproduct.The high calcination temperature would lead to the formation of a new monoclinic phase Ce_(0.3)Ti_(0.7)O_2 and sintering,which is the main reason for the catalytic activity for DCE oxidation markedly decreases.     

Effects of support property on the catalytic performance of CeO2-ZrO2-CrOx for 1,2-dichloroethane oxidation

HZSM-5, Al₂O₃, TiO₂ and SiO₂ supported CeO₂-ZrO₂-CrO_x catalysts were prepared by deposition-precipitation method and tested for deep catalytic oxidation of 1,2-dichloroethane（DCE）, as one of the common chlorinated organic pollutants. All the catalysts were characterized by means of N₂ adsorption-desorption, X-ray photoelectron spectroscopy（XPS）, ammonia-temperatureprogrammed desorption（NH₃-TPD） and hydrogen temperature-programmed reduction（H2-TPR）. The characterization results revealed that there was strongly synergistic effect between the oxidizability of CZCr species and the acidity of supports, which obviously promoted the catalytic activity for DCE degradation. 20% CZCr/HZSM-5 showed the highest activity and good durability during the long-term continuous test. The catalytic activity decreased in the order: 20%CZCr/HZSM-5>CZCr>20%CZCr/TiO₂>20%CZCr/Al₂O₃>20%CZCr/SiO₂.     

Effect of Ce doping into V_2O_5-WO_3/TiO_2 catalysts on the selective catalytic reduction of NO_x by NH_3

In this work, the effectiveness of V_2O_5-WO_3/TiO_2 catalysts modified with different CeO_2 contents by impregnation and co-precipitation methods on the selective catalytic reduction of NOxby NH3 have been studied comparatively by various experimental techniques. The results showed that the NO conversion of V_2O_5-WO_3/CeO_2-TiO_2 catalysts modified by co-precipitation method obviously increased with the Ce doping contents in the studied range below 20%(All Ce contents are in mass fractions), but the NO conversion of V_2O_5-WO_3/CeO_2/TiO_2 catalysts modified by impregnation methods was lower than V_2O_5-WO_3/CeO_2-TiO_2 catalysts especially beyond 2.5% Ce doping contents. The V_2O_5-WO_3/CeO_2-TiO_2 catalysts showed better SCR activity, wider reaction window, and higher sulfur and water resistance. The characterization results elucidated that the modified catalysts by co-precipitation method exhibited higher specific surface area, much better dispersity of Ce component, more Ce~(3+)species and more Br?nsted acid sites than that by impregnation. The vacancies caused by more Ce~(3+)species were favorable for more NO oxidation to NO_2, and the interaction between Ce species and WOxspecies generated more Br?nsted acid sites. It could be supposed that dispersed Ce Oxspecies and WOxspecies offered more second active centers respectively to adsorb oxygen and activate ammonia as co-catalysis to the primary active center of V ions, thus facilitated the better SCR activity of modified V_2O_5-WO_3/CeO_2-TiO_2 catalysts by coprecipitation methods. The co-precipitation methods with Ce component were more suitable for production of modified commercial V_2O_5-WO_3/TiO_2 catalysts.     

Catalytic oxidation of formaldehyde over CeO_2-Co_3O_4 catalysts

A series of CeO_2-Co_3O_4 mixed oxide catalysts with different Co/(Co+Ce) atomic ratios were synthesized by citric acid sol-gel method and used for catalytic oxidation of formaldehyde(HCHO). Many techniques such as Brumauer-Emmett-Teller(BET), X-ray diffraction(XRD), scanning electron microscopy(FE-SEM), temperature programmed reduction(H_2-TPR), temperature-programmed desorption(O_2-TPD) and X-ray photoelectron spectroscopy(XPS) were used to characterize catalysts. The results of catalytic performance tests showed that the catalyst CeO_2-Co_3O_4 with Co/(Co+Ce) ratio of 0.95 exhibited the best performance, and the temperature of complete oxidation of HCHO was 80 oC. The analytical results indicated that the addition of CeO_2 enhanced the specific surface area of Co_3O_4 and the fine dispersion of both of them. Moreover, the strong interaction between CeO_2 and Co_3O_4 resulted in the unique redox properties, which enhanced the available surface active oxygen and led to high valence state of cobalt oxide species. All those effects played crucial roles in the excellent performance of CeO_2-Co_3O_4 for the HCHO oxidation.     

CeO_2/CuO catalysts using different template agent for preferential CO oxidation in H_2-rich stream

The CeO_2/CuO catalysts using different template agent(F68 L64, F127 and P123) were synthesized by the simple template and impregnation method. They were characterized by FESEM, XRD, N2 physisorption and H2-TPR techniques. It is found that the CeO_2/CuO catalysts are double pore distribution, and CeO_2 can enter into the gap of CuO supports and form the contact interface of copper and cerium. Among the asprepared catalysts, the CeO_2/CuO-F127 catalyst displays better activity at lower temperature and the CeO_2/CuO-P123 catalyst presents higher activity at higher temperature. The CeO_2/CuO-P123 catalyst has the smallest crystallite sizes of CuO and CeO_2 as well as the largest size of cubes, which may improve the interaction of copper and cerium and enhance the performance of CO oxidation.     

From nanoparticles to single atoms for Pt/CeO_2:Synthetic strategies,characterizations and applications

Pt/CeO_2 catalysts with unitary Pt species,nanoparticles,clusters or single atoms,often exhibit excellent activity and unique selectivity in many catalytic reactions benefiting from their small size,abundant unsaturated active sites,and unique electro nic structure.In recent years,a tre mendous number of related articles have provided great inspiration to future research and development of Pt/CeO_2 catalysts.In this review,the state-of-the-art evolution of Pt nanoparticles to Pt single atoms on CeO_2 is reviewed with the emphasis on synthetic strategies,advanced characterization techniques(allowing one to clarify the single atoms from clusters),the catalytic applications and mechanisms from the viewpoint of theoretical calculation.Finally,the critical outlooks and the challenges faced in developing the single-atom Pt/CeO_2 catalysts are highlighted.     

Influence of cobalt on performance of Cu-CeO_2 catalysts for preferential oxidation of CO

Copper and cobalt oxides supported on CeO_2 were investigated for preferential oxidation of carbon monoxide(CO-PROX) in the presence of excess hydrogen and CO_2.(Cuo)_(1-x)(Co_3 O_4)_(x/3)-(CeO_2)_(2.5)(x=0,0.25,0.50,0.75,0.85 and 1) catalysts were prepared by coprecipitation method.These mixed oxide catalysts were characterized by several physicochemical techniques,such as BET surface area(S_(BET)),X-ray diffraction(XRD),high resolution transmission electron microscopy(HRTEM),temperature programmed reduction(TPR) and X-ray photoelectron spectroscopy(XPS).XRD studies show the peaks related to CuO and Co_3 O_4 phases in copper and cobalt containing CeO_2 catalysts.The average particle size of the CeO_2 crystallites is in the range of 8-10 nm as evaluated from HRTEM studies.XPS studies demonstrate that Cu,Co and Ce in(cuO)_(1-x)(Co_3O_4)_(x/3)-(CeO_2)_(2.5) catalysts are presented in+2 and +1,+3 and +2 and +4 and +3 oxidation states,respectively.The catalyst with x=0.75 shows better activity and selectivity towards CO-PROX.Though the catalyst with only copper(CuO-CeO_2,x=0) shows good activity but reverse water gas shift(RWGS) reaction is noticed at high temperature.On the other hand,RWGS reaction is suppressed on the cobalt containing CuO-ceO_2 catalyst.Cobalt on CeO_2 with x=1 shows hardly any activity for PRoX reaction at low temperatures.No methanation activity is observed on CuO-CeO_2 or Co_3O_4-CeO_2 catalysts.In contrast,combination of copper and cobalt on CeO_2 shows methanation of CO where enhanced activity is observed with increasing in cobalt content.     

CeO2with diverse morphologies-supported IrOx nanocatalysts for efficient oxygen evolution reaction—Commemorating the 100th anniversary of the birth of Academician Guangxian Xu

IrOx-based catalysts are considered the most promising candidates for oxygen evolution reaction(OER)due to their high efficiency.However,improving their intrinsic catalytic activity is essential for practical application.In this work,CeO₂with three different morphologies(rod,cube,octahedron)and supported IrOx nanoparticles were fabricated,and they display morphology-dependent OER activity.The IrOx/CeO₂-rod shows the highest activity;the catalysts have a catalytic activity sequence of rod>cube>octahedron.A plausible mechanism was proposed:the CeO₂support with different morphologies modulates the electronic structure of IrOx by the synergistic interaction promoted by oxygen vacancies between the active component and the support,thereby altering the catalytic activity of the IrOx/CeO₂catalyst.     

Effect of cerium oxide prepared under different hydrothermal time on electrocatalytic performance of Pt-based anode catalysts

In this paper,CeO_2 with a pore size of 2-4 nm was synthesized by hydrothermal method.The CeO_2 modified graphene-supported Pt catalyst was prepared by the microwave-assisted ethylene glycol reduction chloroplatinic acid method,and the effect of the addition of CeO_2 prepared by different hydrothermal reaction time on the catalytic performance of Pt-based catalysts was investigated.The microstructures of CeO_2 and catalysts were characterized by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),specific surface area and pore size analyzer(BET),scanning electron microscopy(SEM) and electron spectroscopy(EDAX),transmission electron microscopy(TEM),and the catalysts electrochemical performance was tested by electrochemical workstation.The results show that the catalytic performance of the four catalysts with CeO_2 is better than that of the catalyst without CeO_2.Adding CeO_2 with a specific surface area of 120.15 m~2/g prepared by hydrothermal reaction time of 39 h to Pt/C synthesis catalyst,its electrocatalytic performance,stability and resistance to poisoning are the best.The electrochemical active surface area is 102.83 m~2/g,the peak current density of ethanol oxidation is 757.17 A/g and steady-state current density of 1100 s is 108.17 A/g which shows the lowest activation energy for ethanol oxidation reaction.When the cyclic voltammogram is scanned for 500 cycles,the oxidation peak current density retention rate is 87.74%.     

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.     

Mesoporous CeO_2-C hybrid spheres as efficient support for platinum nanoparticles towards methanol electrocatalytic oxidation

The development of direct methanol fuel cells(DMFCs) is partially limited by the poor kinetics of methanol oxidation reaction(MOR) at the anode side.It was reported that the interaction between Pt and CeO_2 enhances the electrocatalytic performance of Pt catalyst for MOR.In this work,a hybrid material(CeO_2-C) composed of CeO_2 and carbon was successfully prepared by a simple hydrothermal method followed by calcination in inert atmosphere.The hierarchically porous nanostructure and especially good electronic conductivity of CeO_2-C make it an excellent support for Pt particles for application in electrocatalytic process.TEM investigation reveals that triple-phase interface of Pt,carbon and CeO_2 forms in Pt/CeO_2-C catalyst.Performance of the as-prepared catalyst for MOR was studied in alkaline medium.The Pt/CeO_2-C catalyst shows superior catalytic performance for MOR compared with Pt/CeO_2 and the physical mixture of Pt/CeO_2 and acetylene black(Pt/CeO_2+C).The significantly improved performance can be attributed to the synergetic effect between Pt particles and CeO_2-C support,and the better conductivity of CeO_2-C.This study provides a possible method to expand the application potential of CeO_2 materials in MOR,and may also be used in other electrocatalytic process.     

聚合物稳定的Au-Pd/TiO2-Al2O3催化剂的还原方法及其催化性能的研究

以PVP为稳定剂，乙醇和K1BH4为还原剂制备了Au-Pd／TiO2-Al2O3催化剂，考察了还原剂对Au-Pd负载型双金属催化剂加氢脱硫性能的影响，并运用XRD，TPD，TPR等技术对催化剂进行表征。结果表明，以乙醇还原的Au-Pd／TiO2-AlO3催化剂的加氢脱硫活性较好。乙醇还原的Au-Pd／TiO2．Al2O3催化剂中Au-Pd之间及活性组分与载体之间的相互作用较强，形成Au-Pd，合金的晶粒较小，活性组分的分散度和活性表面积较大，反应活化能较低，这些均有利于催化剂活性的提高。     

Au-Pd/ZnO-CeO_2催化剂甲醇部分氧化制氢性能的研究

采用聚合物保护乙醇还原法制备了Au-Pd/ZnO-CeO_2催化剂,考察了CeO_2对Au-Pd/ZnO催化剂甲醇部分氧化制氢反应性能的影响,并运用BET、XRD、TPR、H_2-TPD和CH_3OH-TPD等手段对催化剂进行了表征.结果表明,CeO_2对Au-Pd/ZnO催化剂具有较好的改性效果,CeO_2的引入能提高Au-Pd/ZnO催化剂的活性和氢气的选择性,归结于CeO_2的加入增加了催化剂的比表面积、分散度和对反应物甲醇的吸附,同时减少了对生成物H_2脱附,这些均有利于甲醇部分氧化制氢反应.     

Effects of MO_x(M=Mn,Cu,Sb,La)on V-Mo-Ce/Ti selective catalytic reduction catalysts

A series of MO_x-V_2O_5-MoO_3-CeO_2/TiO_2(M=Mn,Cu,Sb,and La) catalysts were prepared via an impregnation method.The physico-chemical properties of the catalysts were characterized and their NH_3-SCR of NO performance was compared.The Mn-loaded catalyst(Mn5V1Mo3Ce7/Ti) exhibits a large number of acid sites of varying strength,and together with good reducibility of the catalyst,contributes to the optimal SCR performance.The sulphate species formed in the presence of SO_2 significantly enhance the H_2O and SO_2 tolerance of Mn5V1Mo3Ce7/Ti.The Cu-loaded catalyst(Cu5V1Mo3Ce7/Ti)demonstrates potential in flue gas applications in the absence of SO_2 at low temperatures because of the excellent redox ability observed and the high degree of weak acid sites.The Sb and La loaded catalysts(Sb5V1Mo3Ce7/Ti and La5V1Mo3Ce7/Ti),especially La5V1Mo3 Ce7/Ti,exhibit the largest number of acid sites and the lowest reducibility,and therefore,may be suitable for use in high temperature denitrification applications.     

Rare earth containing catalysts for selective catalytic reduction of NO_x with ammonia:A Review

Increasingly stringent regulations in many countries require effective reduction and control of NOx emissions. To meet these limits, various methods have been exploited, among which the selective catalytic reduction of NOx using ammonia as the reductant(NH3-SCR) is the most favored technology. High catalytic activity, N2 selectivity and resistance to deactivation by sulfur, alkaline metals and hydrothermal conditions are the optimal properties of a successful SCR catalyst. Rare earth oxides, particularly CeO2, have been increasingly used to improve the catalytic activity and resistance to deactivation of deNOx catalysts, both modifying traditional vanadium catalysts, and also developing novel catalysts, especially for low temperature applications. This review summarized the open literature concerning recent research and development progresses in the application of rare earths for NH3-SCR of NOx. Additionally, the roles of rare earths in enhancing the performance of NH3-SCR catalyst were reviewed.