Preparation of Ordered Mesoporous Nanocrystalline Ceria and Ceria-zirconia for Soot Oxidation

Ordered mesoporous ceria and ceria-zirconia with high specific surface area were prepared by nanocasting of a mesoporous silica KIT-6 template and used for soot oxidation.The as-synthesized ordered mesoporous ceria and ceria-zirconia were characterized by XRD,TEM,Nitrogen adsorption-desorption,Raman spectroscopy,and XRF.The results indicate that mesoporous ceria and ceria-zirconia possess highly ordered mesoporous structure,and exhibited excellent catalytic performance in soot oxidation.T_（50） of mesoporous ceria and ceria-zirconia are 475 and 470 ℃,respectively.The high catalytic activity of mesoporous materials can be attributed to the mesoporous structure and small crystallite size.Moreover,aged mesoporous materials exhibit high catalytic activity.

Insights into the interface of NiCo_(2)O_(4) spinel/LaCoO_(3) perovskite nano-composite for CO and soot oxidation

In the quest for the development of thermally stable,highly active and low-cost catalysts for use in catalyzed diesel particulate filter,nano-composites are new areas of research.Therefore,we reported the easy synthesis of spinel NiCo_(2)O_(4)/perovskite LaCoO_(3) nano-composite,and its individual oxides NiCo_(2)O_(4)and LaCoO_(3) for comparison.The detailed insights into the physio-chemical characteristics of formed NiCo_(2)O_(4)/LaCoO_(3) nano-composite were done based on various characterization analysis such as X-ray diffraction(XRD),Fourier transform infrared(FT-IR),N_(2) physiosorption,scanning electron microscopy-energy dispersive spectroscopy(SEM-EDX),transmission electron microscopy(TEM),and X-ray photoelectron spectroscopy(XPS).The characterization analysis of NiCo_(2)O_(4)/LaCoO_(3) revealed the successful formation of a chemical interface possessing strong interfacial interaction,resulting in desirable physicochemical characteristics such as small crystallite size,abundant mesoporosity,high specific surface area and activation of surface lattice oxygen.Owing to the desirable characteristics,the activity results over NiCo_(2)O_(4)/LaCoO_(3) nano-composite showed the excellent CO oxidation performance and high soot oxidation activity,recyclability and thermal stability.This work mainly attempts to emphasize the effectiveness of the facile,inexpensive and conventionally used precipitation method for the successful formation of highly efficient nano-composites.

Insight into phase structure-dependent soot oxidation activity of K/MnO_(2) catalyst

In the present study,two nanosized MnO_(2)with β and δ phase structures and potassium loaded MnO_(2)catalysts with varied K loading amounts (denoted as K/MnO_(2)) were prepared.Temperature programmed oxidation and isothermal reactions in loose contact modes were employed to examine the soot oxidation activity of the as-prepared catalysts.Characterization results show that as compared with β-MnO_(2),δ-MnO_(2)has larger surface area and higher content of hydroxyl groups.Upon K loading,abundant hydroxyl groups in δ-MnO_(2)effectively sequestrate K cation to form bound K species and free K species are available only at K loading above 3.0 wt.%.In contrast,the majority of K species present as free state in β-MnO_(2)even at a K loading of 1.0 wt.%due to its very low hydroxyl group content.The O_(2)temperature-programmed desorption (O_(2)-TPD) demonstrates that the catalysts with free K species exhibit strong ability in activating gaseous O_(2),whereas the catalysts only having bound K display minor O_(2)activation capability.As a result,despite of slightly lower activity of β-MnO_(2)than δ-MnO_(2),the K/β-MnO_(2)catalysts exhibit substantially higher activities than K/δ-MnO_(2)catalysts with identical K loadings.The finding in this study clearly demonstrates that for MnO_(2)based catalysts,the enhancement of catalytic activity for soot oxidation is highly K loading amount dependent and the dependency is strongly associated with the phase structure of MnO_(2).

Ceria-based catalysts for soot oxidation: a review

Developments in ceria-based soot oxidation catalysts, especially during the last decade, are reviewed. Based on the com- parisons of the activity, durability and cost-efficiency of different soot oxidation catalysts, four kinds of applicable ceria-based cata- lysts have been screened out, which are： （1） CexZrl-xO2 catalyst with high cerium content （x〉0.76）, （2） rare-earth metals （especially Pr） modified ceria, （3） transition metals （especially Mn and Cu） modified ceria, and （4） Ag/CeO2. Moreover, a general review of recent developments on the morphology-controlled ceria-based catalysts, as well as that on the soot oxidation mechanisms over different ceria-based catalysts, is also presented.

Catalytic oxidation of soot particulates over MnO_x-CeO_2 oxides prepared by complexation-combustion method

MnOx-CeO2 oxides prepared by complexation-combustion method were used for soot oxidation. The highest conversion rate of soot was obtained on a MnOx-CeO2 oxide prepared under mild acid condition of pH = 4, where the oxidation temperature corresponding to maximum activity was decreased more than 150 ℃ compared with that of un-catalytic soot oxidation. The structure and property of the catalysts were investigated by X-ray powder diffraction （XRD） and temperature programmed reduction （TPR）. The results indicated that there were at least two kinds of Mn species present in MnOx-CeO2 catalysts, i.e. Mn ions within CeO2 lattice and high dispersion MnOx on the surface of CeO2. The presence of Mn ions in the CeO2 lattice improved the oxygen vacancy due to the charge difference, and the CeO2 considerably decreased the reduction temperature of MnOx. The capability to activate oxygen through the oxygen exchange between O2 in gas phase and lattice oxygen species in MnOx-CeO2 oxide contributed to the high catalytic activity for the reaction.

Preparation methods and thermal stability of Ba-Mn-Ce oxide catalyst for NOx-assisted soot oxidation

Manganese oxide-loaded and -doped ceria as well as the corresponding barium-modified oxide catalysts were prepared for soot oxidation in the presence of NOx, and were characterized by using X-ray diffraction, Brunauer-Emmett-Teller and NO temperature- programmed oxidation measurements. The activity of catalyst depended strongly on the NO 2 production capacity, and the importance of surface nitrates was weakened without heat transfer limitations. The formation of perovskite-type oxides after the high-temperature calcination caused the loss of NOx storage capacity for the Ba-modified catalysts, but did not seem to affect the NO oxidation activity obviously. The addition of barium did not prevent the phase separation of MnOx-CeO 2 solid solutions, whereas it inhibited the sintering of oxide crystallites effectively. This, as well as the relatively high surface area, resulted in a small increase in soot oxidation temperature for the thermally aged Ba/Mn-Ce catalyst.

Rare earth metal doped CeO_2-based catalytic materials for diesel soot oxidation at lower temperatures

In this work, the influence of trivalent rare-earth dopants（Sm and La） on the structure-activity properties of CeO2 was thoroughly studied for diesel soot oxidation. For this, an optimized 40% of Sm and La was incorporated into the CeO2 using a facile coprecipitation method from ultra-high dilute aqueous solutions. A systematic physicochemical characterization was carried out using X-ray diffraction（XRD）, transmission electron microscopy（TEM）, Brumauer-Emmett-teller method（BET） surface area, X-ray photoelectron spectroscopy（XPS）, Raman, and H2-temperature programmed reduction（TPR） techniques. The soot oxidation efficiency of the catalysts was investigated using a thermogravimetric method. The XRD results suggested the formation of nanocrystalline single phase CeO2-Sm2O3 and CeO2-La2O3 solid solutions. The Sm- and La-doped CeO2 materials exhibited smaller crystallite size and higher BET surface area compared with the pure CeO2. Owing to the difference in the oxidation states of the dopants（Sm3＋ and La3＋） and the Ce4＋, a number of oxygen vacancies were generated in CeO2-Sm2O3 and CeO2-La2O3 samples. The H2-TPR studies evidenced the improved reducible nature of the CeO2-Sm2O3 and CeO2-La2O3 samples compared with the CeO2. It was found that the addition of Sm and La to the CeO2 outstandingly enhanced its catalytic efficiency for the oxidation of diesel soot. The observed 50% soot conversion temperatures for the CeO2-Sm2O3, CeO2-La2O3 and CeO2 were ~790, 843 and 864 K（loose contact）, respectively, and similar activity order was also found under the tight contact condition. The high soot oxidation efficacy of the CeO2-Sm2O3 sample was attributed to numerous catalytically favourable properties, like smaller crystallite size, larger surface area, abundant oxygen vacancies, and superior reducible nature.

Controllable synthesis of argentum decorated CuO_(x)@CeO_(2) catalyst and its highly efficient performance for soot oxidation

In this paper,CuO_(x)@Ag/CeO_(2) 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),N_(2) physisorption,X-ray photoelectron spectroscopy(XPS)and H_(2)-temperature programmed reduction(H_(2)-TPR).The results show that CuO_(x)@Ag/CeO_(2) synthesized presents well controlled core-shell structures,with nano-cube like Cu_(2)O as the core and Ag decorated polycrystalline CeO_(2) grafting layers as the shell.Such core-shell structured CuO_(x)@Ag/CeO_(2) can successfully construct a secondary oxygen delivery channel(CuO_(x)→CeO_(2)→Ag)to effectively transfer bulk oxygen of the catalyst to the soot,resulting in its excellent soot oxidation activity compared to CuO_(x)@CeO_(2).The potential benefiting effect by Ag introduction over Cu@Ag/Ce can be concluded as:(ⅰ)pumping lattice oxygen and accelerating gaseous O_(2) dissociation to generate significantly increased active surface oxygen content;(ⅱ)modulating a moderate surface oxygen vacancies concentration to maintain more highly active O_(2) species.

Size effect of Pt nanoparticles in acid-assisted soot oxidation in the presence of NO

Pt/Al2O3 catalysts with mean Pt particle size ranged from 2.7 to 7.1 nm were synthesized by chemical reduction method,and the sulfated counterparts were prepared by impregnation of sulfuric acid.The turnover frequency of platinum for soot oxidation under loose contact conditions in a feed flow containing NO and O2 are positively correlated with the size of platinum.The sulfated Pt/Al2O3 exhibits higher catalytic activity for soot oxidation in the presence of NO despite their reduced ability for NO2 production.Such a contradiction is more significant for those catalysts with smaller platinum particles.Herein,the catalysts were characterized by X-ray diffraction(XRD),Brunauer-Emmett-Teller(BET),transmission electron microscopy(TEM),inductive coupled plasma(ICP)emission spectrometry,CO chemisorption,thermogravimetric analysis(TGA),NH3 temperature-programmed desorption(NH 3-TPD),NO temperature-programmed oxidation(TPO)and NO x temperatureprogrammed desorption(TPD).Possible effect of Pt particle size for the catalytic oxidation of soot in the presence of NO was presented based primarily on the promoted NO2 transfer efficiency onto the soot pushed by the acidic catalysts.

In-situ generation of platinum nanoparticles on LaCoO_(3) matrix for soot oxidation

The LaCo_(0.94)Pt_(0.06)O_(3) catalyst is reduced under 5% H_(2)/Ar at different temperatures to get Pt/LaCoO_(3) with high catalytic activity for soot oxidation.Transmission electron microscopy(TEM),scanning electron microscopy(SEM),X-ray diffraction(XRD),Brunauer-Emmett-Teller method(BET),X-ray photoelectron spectroscopy(XPS),H_(2)-temperature programmed reduction(H_(2)-TPR),O_(2)-temperature programmed desorption(O_(2)-TPD) and thermogravimetric analysis(TGA) were used to study the physicochemical properties of the catalyst.SEM and TEM results indicate that Pt nanoparticles(<10 nm) are grown homogeneously on the surface of the LaCoO_(3) matrix after in-situ reduction.XRD shows that the reduced catalyst has a high symmetrical structure.TGA results indicate that all reduced catalysts exhibit an excellent activity,especially the catalyst reduced at 350℃(T_(10)=338℃,T_(50)=393℃,T_(90)=427℃).And perovskite is the primary active component.According to XPS study,the high symmetrical structure benefits the mobility of oxygen vacancy,and Pt nanoparticles induce the oxygen vacancy to move to its adjacent situation,resulting in more adsorbed oxygen on the surface of the reduced catalyst and increasing the activity.The possible reaction principle is also proposed.

Ce_(1–x)La_xO_y solid solution prepared from mixed rare earth chloride for soot oxidation

Ce_（1–x）La_xO_y solid solution was simply prepared using mixed rare earth chloride（RECl_3·x H_2O, RE=Ce, La99%, containing unseparated Ce and La from rare earth metallurgical industry） as precursor by ultrasonic-assisted co-precipitation method with different ultrasonic frequencies（CLf, f=200, 400, 600, 800, 1000 Hz）. A compared Ce_（1–x）La_xO_y solid solution（CL＊） was also prepared by the same mothod with 10% less precipitant. X-ray diffraction results confirmed the formation of Ce1–xLaxOy solid solution, and the crystal structures of these catalysts were not very sensitive to ultrasonic frequency and precipitant amount. However, both of the factors had obvious effect on morphology and surface area of CL, and precipitant amount seem to play a more crucial role than ultrasonic frequency for Ce_（1–x）La_xO_y solid solution preparation. When soot and catalyst were tight contacted, the peak temperature（Tpeak） of soot oxidation and oxygen reducing temperature for CLf catalysts decreased linearly with increasing surface area. Under loose contact condition, the Tpeak had obvious negative correlation with H_2 consumption. It was inferred that good reducibility of the Ce_（1–x）La_xO_y solid solution favored the soot oxidation reaction. The Ce_（1–x）La_xO_y solid solution prepared from unseparated rare earth chloride showed a good soot oxidaiton activity. Controlling the preparation conditions to prepare a CL catalyst would high surface area will enhance its reducibility and activity.

Catalytic combustion of soot particulates over La_(2-x)K_xNiMnO_6 catalysts

Nanosized La2-xKxNiMnO6 catalysts with ABO3 type perovskite-like structure were prepared by auto-combustion method using citric acid as a ligand to control particle size and morphology.The structures and properties of these perovskite-like oxides were investigated by X-ray powder diffraction（XRD）and temperature-programmed reduction（TPR）.The catalytic activities for soot combustion were evaluated by temperature- programmed oxidation（TPO）with pure O2 and O2/NOx as oxidant,respectively.In the La2-xKxNiMnO6 catalysts,the partial substitution of K at A-site leads to an increase of the concentrations of high valence cation and oxygen vacancy,which enhance the catalytic activity for soot combustion.The optimal substitution amount of K was equal to x=0.4 among these samples.Tp（peak temperature）in O2-TPO profile was 420-C and Tp in O2/NOx-TPO profile was 370-C over La1.6K0.4NiMnO6 catalyst for soot particulates combustion under loose contact conditions between catalyst and soot.

Metal-Support Interactions on Ag/Co_(3)O_(4)Nanowire Monolithic Catalysts Promoting Catalytic Soot Combustion

Tuning metal-support interactions(MSIs)is an important strategy in heterogeneous catalysis to realize the desirable metal dispersion and redox ability of metal catalysts.Herein,we use pre-reduced Co_(3)O_(4)nanowires(Co-NWs)in situ grown on monolithic Ni foam substrates to support Ag catalysts(Ag/Co-NW-R)for soot combustion.The macroporous structure of Ni foam with crossed Co_(3)O_(4)nanowires remarkably increases the soot-catalyst contact effi ciency.Our characterization results demonstrate that Ag species exist as Ag 0 because of the equation Ag^(+)+Co^(2+)=Ag^(0)+Co^(3+),and the pre-reduction treatment enhances interactions between Ag and Co_(3)O_(4).The number of active oxygen species on the Ag-loaded catalysts is approximately twice that on the supports,demonstrating the signifi cant role of Ag sites in generating active oxygen species.Additionally,the strengthened MSI on Ag/Co-NW-R further improves this number by increasing metal dispersion and the intrinsic activity determined by the turnover frequency of these oxygen species for soot oxidation compared with the catalyst without pre-reduction of Co-NW(Ag/Co-NW).In addition to high activity,Ag/Co-NW-R exhibits high catalytic stability and water resistance.The strategy used in this work might be applicable in related catalytic systems.

Preparation of M/Ce_(1-x)Ti_(x)O_(2)(M=Pt,Rh,Ru)from sol-gel method and their catalytic oxidation activity for diesel Soot

A series of Ce_(1-x)Ti_(x)O_(2)mixed oxide catalysts were synthesized by sol-gel method and then loading of noble metal(M=Pt,Rh,Ru)was used for soot oxidation.Ti-doped Ce_(1-x)Ti_(x)O_(2)catalysts(x is the molar ratio of Ti/(Ti+Ce)and ranges from 0.1 to 0.5)exhibit much better oxidation performance than CeO_(2)catalyst,and the Ce_(0.9)Ti_(0.1)O_(2)catalyst calcined at 500℃has the best catalysis activity.Each noble metal(1 wt%)was supported on Ce_(0.9)Ti_(0.1)O_(2)(M/C9 T1)and the properties of the catalysts were characterized by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),Raman,Brunauer-Emmett-Teller(BET)method,and H_(2)-temperature programmed reduction(H_(2)-TPR)results.Results show that the introduction of Ti into CeO_(2)forming Ti-O-Ce structure enhances the catalytic activity and increases the number of oxygen vacancies at the catalyst surface.The noble metal is highly dispersed over Ce_(0.9)Ti_(0.1)O_(2),and M/C9 T1 catalysts present enhanced activity in comparison to Ce_(0.9)Ti_(0.1)O_(2).It is found that noble metals can greatly increase the activity of the catalyst and the corresponding oxidation rate of soot can enhance the electron transfer capacity and oxygen adsorption capacity of the catalyst.A small amount of Ti doping in CeO_(2)can significantly improve the activity of the catalyst,while a large amount of Ti reduces the performance of the catalyst because a large amount of Ti is enriched on the surface of the catalyst,which hinders the contact and reaction between the catalyst and the soot.

Catalytic combustion of soot over ceria-zinc mixed oxides catalysts supported onto cordierite

Modified substrates as outer heterogeneous catalysts was employed to reduce the soot generated from incomplete combustion of diesel or diesel/biodiesel blends, a process that harms the environment and public health. The unique storage properties of ceria （CeO2） makes it one of the most efficient catalysts available to date. Here, we proposed that ceria-based catalysts can lower the temperature at which soot combustion occurs; more specifically, from 610℃ to values included in the diesel exhausts operation range （300-450℃）. The sol-gel method was used to synthesize mixed oxide-based catalysts （CeO2：ZnO）; the resulting catalysts were deposited onto cordierite substrates. In addition, the morphological and structural properties of the material were evaluated by XRD, BET, TPR-H2, and SEM. Thermogravimetric （TG/DTA） analysis revealed that the presence of the catalyst decreased the soot combustion temperature by 200℃ on average, indicating that the oxygen species arise at low temperatures in this situation, promoting highly reactive oxidation reactions. Comparative analysis of soot emission by diffuse reflectance spectroscopy （DRS） showed that catalyst-impregnated cordierite samples efficiently oxidized soot in a diesel/biodiesel stationary motor： soot emission decreased by more than 70%.

Recent advances in soot combustion catalysts with designed micro-structures

With the enhancement of the people consciousness of environment protection, soot particulates(PM)elimination has drawn wide attention in recent years. Efficient after-treatment with well-designed catalysts is one of the best ways to eliminate soot particulates that come from diesel engines. Catalysts coated on the DPF(diesel particulate filter) are considered as the main factor to lower soot ignition temperature.Improvement of the structures of the catalysts is significantly important in order to achieve good catalytic performance and high stability. Based on the structures, soot combustion catalysts can be mainly divided into three types: particle-based catalysts, 3 DOM catalysts and nanoarray catalysts. This review mainly summarized recent advances in soot combustion catalysts with different designed micro-structures, each category is explained with critical assessment and several typical examples, aiming to guide the synthesis of advanced soot combustion catalysts.