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.     

Influence of CeO_2 loading on structure and catalytic activity for NH_3-SCR over TiO_2-supported CeO_2

A series of supported CeO_2/TiO_2 catalysts were prepared to explore the influence of CeO_2 loading on these catalysts for the selective catalytic reduction of NO_3 by NH_3(NH_3-SCR).The catalysts were investigated in detail by means of XRD,Raman,H_2-TPR,NH_3-TPD,XPS,in situ DRIFTS,and NH3-SCR reaction.The activity of the catalyst is closely related to the content of CeO_2.When the loading of CeO_2 is near the dispersion capacity(1.16 mmol Ce~(4+)/100 m~2 TiO_2),the catalytic activity is better.This may be because that the dispersed CeO_2 is the active species and the catalyst has appropriate redox property,along with the larger amounts of surface Ce content and surface adsorbed oxygen species.Finally,a possible reaction mechanism via the Langmuir-Hinshelwood(L-H) mechanism is tentatively proposed to further understand the NH_3-SCR reaction.     

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.     

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.     

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.     

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.     

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 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.     

Preparation and characterization of Ce_(1-x)Pr_xO_2 supports and their catalytic activities

In this work, the addition of praseodymium(Pr) into ceria as a mixed oxide support in a form of Ce_(1-x)Pr_xO_2(x = 0.01,0.025, 0.050, 0.075 and 0.10) was prepared using a co-precipitation method. The structural and textural properties of the synthesized supports were characterized by X-ray diffraction(XRD), N_2 adsorption-desorption, Raman spectroscopy, H_2-temperature programmed reduction(H_2-TPR) and H_2-chemisorption. Upon addition of Pr, XRD patterns and Raman spectra indicated an enlargement of ceria unit cell and the characteristics Raman broad peak at 570 cm~(-1) which was attributed to the existence of oxygen vacancies in the ceria lattice. This indicated that some Ce~(4+) ions in ceria were replaced by larger Pr~(3+) cations. To evidence the incorporation of Pr~(3+) cations into ceria lattice,X-ray absorption near edge structure(XANES) was employed. The results showed that the oxidation states of Ce in mixed oxide supports were slightly lower than 4+ while those of Pr were still the same as a precursor salt. Therefore, the incorporation of Pr~(3+) into ceria lattice would lead to strain and unbalanced charge and result in oxygen vacancies. The reducibility of Ce_(1-x)Pr_xO_2 mixed oxide supports was investigated by H_2-TPR and temperature-resolved X-ray absorption spectroscopy experiment under reduction conditions. XANES spectra of Ce L_3 edges showed a lower surface reduction temperature(Ce~(4+)to Ce~(3+)) of Ce_(0.925)Pr_(0.075)O_2 than that of CeO_2 which agreed with H_2-TPR results. H_2-chemisorption indicated that Pr promoted the dispersion of the metal catalyst on the mixed oxide support and increased the adsorption site for CO. For WGS reaction, 1% Pd/mixed oxide support had higher WGS activity than 1%Pd/ceria. The increase of WGS activity was due to the increase of Pd dispersion on the support and the existence of oxygen vacancies produced from incorporation of Pr into the ceria lattice.     

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.     

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.     

Cerium,manganese and cerium/manganese ceramic monolithic catalysts.Study of VOCs and PM removal

Ceramic supported cerium, manganese and cerium-manganese catalysts were prepared by direct impregnation of aqueous precursor, and characterized by scanning electron microscopy(SEM), X-ray diffraction(XRD), Brunauer-Emmett-Teller method(BET), temperature programmed reduction(H_2-TPR), X-ray photoelectron spectroscopy(XPS) acidity measurements and electrical conductivity. The catalytic activity was evaluated for volatile organic compounds(VOC)(ethanol, methyl ethyl ketone and toluene) oxidation. Additionally, catalysts were tested in particulate matter(PM) combustion. The characterization results indicated that Ce was in the form of Ce~(4+) and Ce~(3+), and Mn existed in the form of Mn~(4+) and Mn~(3+) on the surface of the Mn/AC sample and in the form of Mn~(4+) in the Ce/Mn/AC monolith. VOC oxidation results revealed that the Ce/Mn/AC sample showed an excellent performance compared with ceramic supported CeO_2(Ce/AC) and MnO_x(Mn/AC) samples. The PM combustion was also higher on Ce/Mn/AC monoliths. The enhanced catalytic activity was mainly attributed to the Ce and Mn interaction which enhanced the acidity, conductivity and the reducibility of the oxides.     

Synthesis of neodymium modified CeO2-ZrO2-Al2O3 support materials and their application in Pd-only three-way catalysts

Ce-Zr-Al-Nd2O3 (CZAN) support materials were prepared by co-precipitation and impregnation methods, respectively. They were characterized by X-ray diffraction (XRD), low temperature nitrogen adsorption-desorption, oxygen pulsing technique, H2-temperature pro-grammed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS). The Pd-only three-way catalysts (Pd-TWC) supported on these materials were prepared by incipient wetness method and studied by activity tests. The results demonstrated that the CZAN supports obtained by the two methods showed better structural, textural and redox properties than the CZA without Nd2O3, and the addition of Nd2O3 improved the catalytic activity of TWC. Especially, the CZAN-i support prepared by impregnation method had better thermal stability and redox prop-erty. Meanwhile, the Pd/CZAN-i catalyst exhibited the best catalytic performance. XPS measurements indicated that the Nd-modified sam-ples possessed more Ce3+ and oxygen vacancies on the surface of samples, which led to a better redox property. The excellent redox property of support materials helped to improve the catalytic activity of TWC.     

Effect of pore-size distribution on Ru/ZSM-5 catalyst for enhanced N_2 activation to ammonia via dissociative mechanism

In the present study,a series of Ru/ZSM-5 catalysts with different pore-size distributions were prepared and investigated for NH_3 synthesis.Our studies indicate that Ru/ZSM-5-Mic with micropore structure exhibits superior NH_3 synthesis rate,which is much higher than those of Ru/ZSM-5-Mac(with macroporous structure) and Ru/ZSM-5-Mes(with mesoporous structure) catalysts.A series of TPD experiments demonstrate that pore-size distributions play an important role in N_2 adsorption and activation over Ru/ZSM-5.Moreover,the addition of La significantly promotes the NH_3 synthesis performance over Ru/ZSM-5-Mic.Additionally,in situ DRIFTS studies indicate that the main intermediate species over Ru/ZSM-5-Mic are-NH_2,and most of the surface hydrogen species desorb following the H_2O-formation pathway.     

Catalytic performance of a Pt-Rh/CeO_2-ZrO_2-La_2O_3-Nd_2O_3 three-way compress nature gas catalyst prepared by a modified double-solvent method

A Pt-Rh three-way catalyst(M-DS) supported on CeO_2-ZrO_2-La_2O_3-Nd_2O_3 and its analogous supported catalyst(DS) were developed via a modified double-solvent method and conventional double-solvent method, respectively. The as-prepared catalysts were characterized by N_2 adsorption-desorption, X-ray diffraction(XRD), CO-chemisorption, X-ray photoelectron spectroscopy(XPS) and hydrogen temperature-programmed reduction(H_2-TPR). The preformed Pt nanoparticles generated using ethanol as a reducing agent on M-DS presented enhanced Pt dispersion regardless of aging treatment as confirmed by XRD and CO-chemisorption measurements. The textural properties and reduction ability of M-DS were maintained to a large extent after aging treatment. This result was consistent with those of the N_2 adsorption-desorption and H_2-TPR, respectively. Meanwhile, the XPS analysis demonstrated that higher Pt~0 species and larger Ce~(3+) concentration could be obtained for M-DS. In the conversion of a simulated compressed natural gas(CNG) vehicle exhaust, both fresh and aged M-DS showed a significant enhancement in the activity and N_2-selectivity. Particularly, the complete conversion temperature(T_(90)) of CH_4 over the aged M-DS catalyst was 65 oC lower than that over the aged catalyst by conventional double-solvent method.     

Nd3+-TiO2催化剂的制备及对合成甲醇的影响

用等体积浸渍法和溶胶凝胶法制备了两种Nd3+-TiO2催化剂,并用X射线衍射、透射电镜和紫外可见吸收光谱等技术表征了催化剂的形貌、结构及吸光性能,对比研究了两种Nd3+-TiO2催化剂对二氧化碳和水合成甲醇反应体系的催化活性,探索了最佳的Nd掺杂量、体系反应温度、二氧化碳压力、光照时间。试验表明,尽管两种合成方法所得的催化剂最佳的Nd掺杂量相差不大,但用溶胶凝胶法制备的Nd3+-TiO2催化剂平均粒径更小,分布均匀,对目的反应的催化效果更好。     

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%.     

Effects of sol-gel method and lanthanum addition on catalytic performances of nickel-based catalysts for methane reforming with carbon dioxide

The nickel-based catalysts were prepared by the sol-gel method and used for the CH4 reforming with CO2. The effects of the sol-gel method on the specific surface area, catalytic activity, desorption, and reduction performances of catalysts were investigated with BET, TPR, and TPD. Compared with the catalyst prepared by the impregnation method, the results indicated that the catalysts prepared by the sol-gel method had larger specific surface area, showing higher catalytic activities and exhibiting perfect desorption and reduction performances. In addition, the modification effects of adding La were studied, and it was found that the 0.75NLBT catalyst constituted of 5wt.%Ni-0.75wt.%La was optimal.     

Rare earths (Ce,Y, Pr) modified Pd/La_2O_3-ZrO_2-Al_2O_3 catalysts used in lean-burn natural gas fueled vehicles

14%REO_x-2.5%La_2 O_3-33.5%ZrO_2-50%Al_2 O_3(RE = Ce, Y, Pr) composites were prepared by a coprecipitation method. The Pd catalysts were obtained by an aqueous solution of Pd(NO_3)_2 loaded on the rare earths modified composites with an initial wet impregnated method. The experiment results show that catalytic activity of the rare earths modified Pd/La_2 O_3-ZrO_2-Al_2 O_3 catalysts is better than bare sample for methane oxidation. The structural characterization results reveal that the rare earths modified Pd catalysts increase amounts of surface active oxygen species by X-ray photoelectron spectroscopy(XPS) analysis and improve the dispersion of active component from H2-temperature programmed reduction(H2-TPR) measurement compared with bare sample. Especially,Pd/14%Y_2 O_3-2.5%La_2 O_3-33.5%ZrO_2-50%Al_2 O_3 sample exhibits highly active stability, it is related to the Pd particles highly dispersion,which was observed by transmission electron microscope(TEM) images.     

制备条件对大颗粒氧化铈流动性的影响

对草酸沉淀法和碳酸氢铵沉淀法制备大颗粒氧化铈的流动性进行了研究。在草酸沉淀法中,研究了搅拌速度、沉淀温度、陈化时间、沉淀剂的加入速度和灼烧温度等条件对C eO2流动性能的影响,这些制备条件对C eO2粉体的流动性影响不是很大,制得的大颗粒氧化铈的流动性指数介于70~80之间,流动性良好;碳酸盐沉淀法制备大颗粒C eO2,晶种和添加剂可以改善C eO2粉体的流动性,加入适量的晶种和添加剂,也可以制得流动性指数都介于70~80之间,流动性良好的大颗粒C eO2。