Diesel Soot Combustion over Mn2O3 Catalysts with Different Morphologies: Elucidating the Role of Active Oxygen Species in Soot Combustion

Catalytic diesel soot combustion was examined using a series of Mn₂O₃ catalysts with different morphologies, including plate, prism, hollow spheres and powders. The plate‐shaped Mn₂O₃ (Mn₂O₃‐plate) exhibited superior carbon soot combustion activity compared to the prism‐shaped, hollow‐structured and powdery Mn₂O₃ under both tight and loose contact modes at soot combustion temperatures (T₅₀) of 327 °C and 457 °C, respectively. Comprehensive characterization studies using scanning electron microscopy, scanning transmission electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy, temperature‐programmed reduction and oxygen release measurements, revealed that the improved activity of Mn₂O₃‐plate was mainly attributed to the high oxygen release rate of surface‐adsorbed active oxygen species, which originated from oxygen vacancy sites introduced during the catalyst preparation, rather than specific surface‐exposed planes. The study provides new insights for the design and synthesis of efficient oxidation catalysts for carbon soot combustion as well as for other oxidation reactions of harmful hydrocarbon compounds.     

NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript>/rGO hybrid nanostructures for efficient electrocatalytic oxygen evolution

We report the synthesis of NiCo₂O₄/reduced graphene oxide (NiCo₂O₄/rGO) hybrid hierarchical structures with unique nanonet and microsphere morphologies by organic polar solvent-assisted solvothermal method. The electrocatalytic oxygen evolution reaction (OER) activity of these materials is studied by cyclic voltammetry, linear sweep voltammetry and chronoamperometry methods in O₂-saturated 0.1 M KOH solution. The NiCo₂O₄/rGO hybrid nanocomposite materials are found to be highly active electrocatalysts for OER at lower overpotentials. The nanonet and microsphere-like NiCo₂O₄/rGO catalysts require overpotentials of 0.450 and 0.530 V at a current density of 10 mA cm^?2, and their corresponding Tafel slopes are 53 and 62 mV dec^?1, which are much lower than values reported for non-precious electrocatalysts. Further, both NiCo₂O₄/rGO catalysts show good catalytic stability with current retention more than 92 % over long period of 15,000 s determined by chronoampirometry and at the end of 1000th cycle determined by linear sweep voltammetry. The enhanced OER activity of nanostructured NiCo₂O₄/rGO hybrid catalysts is attributed to synergistic interaction between rGO and NiCo₂O₄, which seems to be essential for maintaining the large contact area at the electrode-electrolyte interface, better mass, and charge transport and to minimize the aggregation of NiCo₂O₄ nanoparticles.     

超微粒La-Co氧化物催化剂对柴油机尾气碳黑催化燃烧性能的研究

用有机酸络合法制备了Co3O4，NiCo2O4和LaCo2O43种催化剂。通过程序升温氧化反应(TPO)技术对这3种催化剂进行模拟柴油碳黑催化燃烧反应的活性评价。研究发现以Co3O4为活性成分的催化剂能显著降低碳黑燃烧的温度。以Ni和La部分取代Co3O4后形成的复合氧化物NiCo2O4和混合氧化物LaCo2O4能改进Co3O4的氧化活性。但是NiCo2O4不能改进碳黑在松散接触时的燃烧活性；而混合氧化物LaCo2O4由于形成了超微粒含缺陷的LaCoO3钙钛矿型结构，它具有良好的低温氧化活性和表面原子移动性，因而能显著改进碳黑在松散接触时的燃烧活性。     

Environmental Transmission Electron Microscopy Study of Diesel Carbon Soot Combustion under Simulated Catalytic‐Reaction Conditions

Environmental transmission electron microscopy (ETEM) is used to monitor the catalytic combustion of diesel carbon soot upon exposure to molecular oxygen at elevated temperatures by using a gas‐injection specimen heating holder. The reaction conditions simulated in the ETEM experiments reconstruct real conditions effectively. This study demonstrated for the first time that soot combustion occurs at the soot–catalyst interface for both Ag/CeO₂ and Cu/BaO/La₂O₃ catalysts.     

Optimized Pt-MnOx interface in Pt-MnOx/3DOM-Al2O3 catalysts for enhancing catalytic soot combustion

The catalysts of three-dimensionally ordered macroporous (3DOM) Al₂O₃-supported core-shell structured Pt@MnO_x nanoparticles (3DOM-Pt@MnO_x/Al₂O₃) were successfully prepared by the gas bubbling-assisted membrane reduction-precipitation (GBMR/P) method. Pt@MnO_x core-shell nanoparticles (NPs) are highly dispersed on the inner surface of 3DOM-Al₂O₃ support. Pt@MnO_x/3DOM-Al₂O₃ catalysts, which combine both advantages of high-efficiency soot-catalyst contact by 3DOM-Al₂O₃ structure and the abundant active sites by the optimized Pt-MnO_x interface, exhibit high catalytic activities for soot combustion, and the catalytic activities are strongly dependent on the thickness of MnO_x shell. Among the catalysts, 3DOM-Pt@MnO_x/Al₂O₃-1 catalyst with optimized Pt-MnO_x interface shows the highest catalytic activity for soot combustion, i.e., its values of T₅₀ and S_CO2^m are 351 °C and 98.6%, respectively. The highest density of Pt-MnO_x active sites for adsorption-activation of gaseous O₂ is responsible for enhancing catalytic activity for soot combustion. Pt@MnO_x/3DOM-Al₂O₃ catalysts are promising to practical applications for the emission reduction of soot particles.     

Low charge overpotentials and extended full cycling capability in lithium-oxygen batteries by controlling the nature of discharge products

Hollow NiCo₂O₄ microspheres with a highly hierarchical porous structure were synthesized and conducted as catalysts for lithium-oxygen batteries. The influence of NiCo₂O₄ on the discharge products was investigated. The NiCo₂O₄ showed the capability to promote the formation of lithium deficient Li_2 − xO₂ and exerted a significant influence on the electrochemical performance of lithium-oxygen batteries with a low charge overpotential and extended full cycling over 50 cycles.     

Catalytic removal of soot particles over MnCo<Subscript>2</Subscript>O<Subscript>4</Subscript> catalysts prepared by the auto-combustion method

Soot removal for exhaust gas from diesel engine has been addressed due to the more stringent legislation and environmental concerns. MnCo₂O₄ catalysts were systematically prepared using glucose as a fuel via the auto-combustion method and applied for soot removal. The as-prepared samples were characterized by X-ray diffraction (XRD), O₂-temperature-programmed oxidation (TPO) reaction and H₂-temperature-programmed reduction reaction (H₂-TPR). The catalytic activities for soot combustion were evaluated by micro activity test (MAT) with a tight contact mode between soot and catalysts. Compared with catalysts prepared by the solid state method without glucose, auto-combustion method in the presence of glucose can decrease the synthetic temperature, avoiding high temperature treatment and sintering. The catalysts prepared with glucose could catalyze soot oxidation effectively and the derived values of T₁₀, T₅₀, and T₉₀ were 326, 408, and 468 °C in a tight contact mode, respectively, showing a significant drop of T₁₀, T₅₀, and T₉₀ by 156, 177, and 178 °C for non-catalytic reaction.     

Promotional effect of cobalt addition on catalytic performance of Ce<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>O<Subscript>2</Subscript> mixed oxide for diesel soot combustion

A series of Co-modified Ce_0.5Zr_0.5O₂ catalysts with different concentrations of Co (mass %: 0, 2, 4, 6, 8, 10) was investigated for diesel soot combustion. Ce_0.5Zr_0.5O₂ was prepared using the coprecipitation method and Co was loaded onto the oxide using the incipient wetness impregnation method. The activities of the catalysts were evaluated by thermogravimetric (TG) analysis and temperature-programmed oxidation (TPO) experiments. The results showed the soot combustion activities of the catalysts to be effectively improved by the addition of Co, 6 % Co/Ce_0.5Zr_0.5O₂ and that the 8 % Co/Ce_0.5Zr_0.5O₂ catalysts exhibited the best catalytic performance in terms of lower soot ignition temperature (T_i at 349°C) and maximal soot oxidation rate temperature (T_m at 358°C). The reasons for the improved activity were investigated by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), H₂ temperature-programmed reduction (H₂-TPR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). These results revealed that the presence of Co could lower the reduction temperature due to the synergistic effect between Co and Ce, thereby improving the activity of the catalysts in soot combustion. The 6 % Co catalyst exhibited the best catalytic performance, which could be attributed to the greater amounts of Co³⁺ and surface oxygen species on the catalyst.     

Diesel soot combustion. KNO3 and KOH catalysts supported on zirconia

KNO₃/ZrO₂ and KOH/ZrO₂ catalysts were studied and found active in the catalytic soot combustion. Two equipments were used to carry out the combustion experiments: a thermogravimetric reactor with an O₂/He feed and a fixed bed microreactor with NO/O₂/He feed.     

Controllable Synthesis of Fe-Doped NiCo2O4 Nanobelts as Superior Catalysts for Oxygen Evolution Reaction

As one of the promising clean and renewable technologies, water splitting has been a hot topic, especially the half-reaction of oxygen evolution reaction (OER) due to its sluggish and complex kinetics. Hence, Fe-doped NiCo₂O₄ nanobelts were designed and prepared as catalysts toward OER. By increasing the Fe amount, the catalytic performances of the as-synthesized products went up and then decreased. Profiting from the synergistic effect between Fe atom and NiCo₂O₄, all the Fe-NiCo₂O₄ catalysts exhibited superior catalytic activities to the corresponding NiCo₂O₄. In addition, the characteristic nanobelt architecture facilitates the conduction of electrons and the exposure of active sites. With the optimal Fe content, the 9.1 % Fe-NiCo₂O₄ yielded the smallest overpotential and Tafel slope among the catalysts, distinctly lower than that of RuO₂.     

三维有序大孔SnO2基固溶体用于有效燃烧碳烟颗粒

柴油车尾气排放的碳烟颗粒对人类的生存环境和身体健康带来了严重危害.催化燃烧是消除碳烟颗粒污染的有效途径.碳烟颗粒催化燃烧是固-固-气相反应,因此催化剂本身具有活泼的氧中心且其能与碳烟颗粒有效接触是提高反应效率的关键因素.为改善碳烟颗粒与催化剂的接触,设计制备三维有序大孔(3DOM)催化剂,使碳烟颗粒可以进入催化剂孔道内部,增加其与催化剂的有效接触,是提高反应活性的有效途径.此外,在催化剂晶格中掺杂其它金属离子形成固溶体结构,可提高其氧化还原性能,也可有效提高其碳烟燃烧活性.SnO2富含活泼的表面缺位氧和可还原的晶格氧,且其熔点高达1630 oC,具有良好的热稳定性,被广泛用于制备气体传感、电化学和催化等材料.在过去的6年中,本课题组在SnO2催化化学领域做了大量系统的工作,将SnO2基催化材料用于多种环保和能源反应.发现通过其它阳离子Fe3+,Cr3+,Ta5+,Ce4+和Nb5+等的掺杂,替换晶格中部分Sn4+形成金红石型SnO2固溶体结构,可显著提高催化剂氧物种的流动性、活性和本身的热稳定性.本文采用胶体晶体模板法制备出了Ce4+,Mn3+和Cu2+离子掺杂的SnO2三维有序大孔固溶体催化剂用于松散接触条件下的碳烟催化燃烧.采用SEM,TEM,XRD,STEM-mapping,O2-TPD和XPS等手段对催化剂进行表征,研究其碳烟催化燃烧性能.SEM和TEM结果表明已成功合成三维有序大孔结构样品.XRD,Raman和STEM-mapping结果表明,Ce4+,Mn3+和Cu2+离子均进入四方金红石型SnO2晶格形成固溶体结构.另外,Raman,H2-TPR,XPS和O2-TPD等结果发现上述离子掺杂三维大孔SnO2后,催化剂表面形成了更活泼、丰富的氧物种,有利于碳烟颗粒燃烧.其中3DOM-Cu1Sn9催化剂具有最丰富的活泼氧中心,因此表现出最高的活性.     

Oxidation of Finely Dispersed Carbon on Coated Oxide Catalysts

The catalytic activity of soot filters for internal combustion gas exhausts made from synthetic cordierite with a catalytic covering of transition metal oxides (Co₃O₄, MnO₂, CuO, Cr₂O₃) or binary oxides (CuCr₂O₄, CuCo₃O₅, Co₃MnO₆, CuMnO₃) has been studied. The most active catalyst for the oxidation of CO and hexane and for the combustion of soot forming compounds in the exhaust gases is copper chromite. A secondary carrier based on -Al₂O₃ increased the soot capacity of the filters and increased the selectivity the process of combustion of soot to CO₂.     

In-situ UV-Raman study on soot combustion over TiO2 or ZrO2-supported vanadium oxide catalysts

UV-Raman spectroscopy was used to study the molecular structures of TiO₂ or ZrO₂-supported vanadium oxide catalysts. The real time reaction status of soot combustion over these catalysts was detected by in-situ UV-Raman spectroscopy. The results indicate that TiO₂ undergoes a crystalline phase transformation from anatase to rutile phase with the increasing of reaction temperature. However, no obvious phase transformation process is observed for ZrO₂ support. The structures of supported vanadium oxides also depend on the V loading. The vanadium oxide species supported on TiO₂ or ZrO₂ attain monolayer saturation when V loading is equal to 4 (4 is the number of V atoms per 100 support metal ions). Interestingly, this loading ratio (V₄/TiO₂ and V₄/ZrO₂) gave the best catalytic activities for soot combustion reaction on both supports (TiO₂ and ZrO₂). The formation of surface oxygen complexes (SOC) is verified by in-situ UV Raman spectroscopy and the SOC mainly exist as carboxyl groups during soot combustion. The presence of NO in the reaction gas stream can promote the production of SOC.     

Hierarchical NiCo2O4 nanorods as an efficient cathode catalyst for rechargeable non-aqueous Li–O2 batteries

NiCo₂O₄ nanorods were synthesized by a hydrothermal method followed by low temperature calcination. FESEM and TEM analyses confirmed that the as-prepared materials consist of a hierarchical nanorod structure. When applied as cathode catalysts in rechargeable Li–O₂ batteries, NiCo₂O₄ nanorods exhibited a superior catalytic activity, including low charge over-potential, high discharge capacity and high-rate capability.     

Ceria–Zirconia Particles Wrapped in a 2D Carbon Envelope: Improved Low‐Temperature Oxygen Transfer and Oxidation Activity

Engineering the interface between different components of heterogeneous catalysts at nanometer level can radically alter their performances. This is particularly true for ceria‐based catalysts where the interactions are critical for obtaining materials with enhanced properties. Here we show that mechanical contact achieved by high‐energy milling of CeO₂–ZrO₂ powders and carbon soot results in the formation of a core of oxide particles wrapped in a thin carbon envelope. This 2D nanoscale carbon arrangement greatly increases the number and quality of contact points between the oxide and carbon. Consequently, the temperatures of activation and transfer of the oxygen in ceria are shifted to exceptionally low temperatures and the soot combustion rate is boosted. The study confirms the importance of the redox behavior of ceria‐zirconia particles in the mechanism of soot oxidation and shows that the organization of contact points at the nanoscale can significantly modify the reactivity resulting in unexpected properties and functionalities.     

Catalytic combustion of diesel soot over perovskite-type catalyst: Potassium titanates

Potassium titanates with a high crystallinity were successfully prepared by the sol-gel method and characterized by XRD, SEM, and BET surface area measurements. K₆Ti₄O₁₁, K₂Ti₂O₅, K₂Ti₄O₉ were found to have better soot oxidation performance compared with Pt/TiO₂ and CeO₂ based catalysts. K₂Ti₂O₅ may be an excellent candidate for soot oxidation due to its high oxidation activity, water-stability, resistance to sulfur poisoning and economical advantages. Certain amount of NO _x can contribute to the catalytic combustion of diesel over potassium titanates, implying that K₂TiO₅ may be a kind of catalyst for simultaneous removal of NO _x and soot.     

Morphology‐Tuned Synthesis of NiCo2O4‐Coated 3D Graphene Architectures Used as Binder‐Free Electrodes for Lithium‐Ion Batteries

Nanostructured NiCo₂O₄ is directly grown on the surface of three‐dimensional graphene‐coated nickel foam (3D‐GNF) by a facile electrodeposition technique and subsequent annealing. The resulting NiCo₂O₄ possesses a distinct flower or sheet morphology, tuned by potential or current variation electrodeposition, which are used as binder‐free lithium‐ion battery anodes for the first time. Both samples exhibit high lithium storage capacity, profiting from the unique binder‐free electrode structures. The flower‐type NiCo₂O₄ demonstrates high reversible discharge capacity (1459 mAh g^?1 at 200 mA g^?1) and excellent cyclability with around 71 % retention of the reversible capacity after 60 cycles, which are superior to the sheet‐type NiCo₂O₄. Such superb performance can be attributed to high volume utilization efficiency with unique morphological character, a well‐preserved connection between the active materials and the current collector, a short lithium‐ion diffusion path, and fast electrolyte transfer in the binder‐free NiCo₂O₄‐coated 3D graphene structure. The simple preparation process and easily controllable morphology make the binder‐free NiCo₂O₄/3D‐GNF hybrid a potential material for commercial applications.     

DTA,TG studies of catalytic oxidation of carbon particles over M2 III O3 (MIII=Al,Cr, Fe,Ni)

Carbonizate as a model soot has been submitted to oxidation using Al₂O₃, Cr₂O₃, Ni₂O₃ and Fe₂O₃ as catalysts in the temperature range from RT up to 1000°C. The results obtained indicate that Fe₂O₃ is the most active catalyst in soot oxidation. However, all the catalysts examined are active in transformation of carbonizate components. It has been shown that DTA and TG methods can be used as fast methods testing the carbonizate oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

In-situ UV-Raman study on soot combustion over TiO2 or ZrO2-supported vanadium oxide catalysts

UV-Raman spectroscopy was used to study the molecular structures of TiO₂ or ZrO₂-supported vanadium oxide catalysts. The real time reaction status of soot combustion over these catalysts was detected by in-situ UV-Raman spectroscopy. The results indicate that TiO₂ undergoes a crystalline phase transformation from anatase to rutile phase with the increasing of reaction temperature. However, no obvious phase transformation process is observed for ZrO₂ support. The structures of supported vanadium oxides also depend on the V loading. The vanadium oxide species supported on TiO₂ or ZrO₂ attain monolayer saturation when V loading is equal to 4 (4 is the number of V atoms per 100 support metal ions). Interestingly, this loading ratio (V₄/TiO₂ and V₄/ZrO₂) gave the best catalytic activities for soot combustion reaction on both supports (TiO₂ and ZrO₂). The formation of surface oxygen complexes (SOC) is verified by in-situ UV Raman spectroscopy and the SOC mainly exist as carboxyl groups during soot combustion. The presence of NO in the reaction gas stream can promote the production of SOC. Supported by the National Natural Science Foundation of China (Grant Nos. 20473053, 20773163 and 20525621), the Beijing Natural Science Foundation (Grant No. 2062020), and the 863 Program of China (Grant No. 2006AA06Z346)     

Role of nitrogen dioxide in the oxidation of diesel soot on promoted mixed catalysts with fluorite and perovskite structures

The oxidation of soot on catalysts with the perovskite and fluorite structures (including platinum-promoted catalysts) in the presence and in the absence of NO₂ was studied using in situ IR spectroscopy and temperature-programmed techniques (TPR, TPD, and TPO). It was found that, as a rule, the temperature of the onset of soot oxidation considerably decreased upon the addition of NO₂ to a flow of O₂/N₂, whereas the amount of oxygen consumed in soot oxidation considerably increased. To explain these facts, we hypothesized that the initiation of soot combustion in the presence of NO₂ was related to the activation of the NO₂ molecule through the formation (at a low temperature) and decomposition (at a high temperature) of nitrate structures on the catalyst. Superequilibrium amounts of NO₂ resulted from the decomposition of nitrate complexes immediately on the catalyst for soot combustion. Based on a comparison between catalyst activities and data obtained by TPR and the TPD of oxygen, a conclusion was drawn that the presence of labile oxygen in the catalyst is a necessary but insufficient condition for the efficient occurrence of a soot oxidation reaction in the presence of NO₂. The introduction of platinum as a constituent of the catalyst increased the amount of labile oxygen and, as a consequence, increased the amount of highly reactive nitrate complexes. As a result, this caused a decrease in the temperature of the onset of soot combustion.