Zhongpeng Wang

Catalytic combustion of soot particulates over rare-earth substituted Ln2Sn2O7 pyrochlores (Ln=La, Nd and Sm).

Catalytic combustion is one of the most promising methods for diesel soot removal. Ln2Sn2O7 pyrochlores substituted with different rare-earth (RE) elements (Ln=La, Nd and Sm) were prepared through co-precipitation method for catalytic combustion of soot particulates. The structural, textural and redox properties, together with the oxygen vacancy of the catalysts were investigated systematically. Their catalytic activities were evaluated by both temperature-programmed oxidation and isothermal reaction techniques. With the increasing in RE ionic radius (r), the SnO bond strength in Ln2Sn2O7 pyrochlores evaluated from the stretching IR band was decreased, resulting in the improved reducibility and enhanced oxygen vacancies of catalysts. The increase of oxygen vacancy concentration was further confirmed by photoluminescence (PL) investigations wherein upon excitation with UV radiation, the pyrochlores nanoparticles exhibited strong and sharp transition at 408nm attributed to oxygen vacancies. Catalytic combustion and isothermal reactions revealed that the ignition activity (ignition temperature, T5) and the intrinsic activity (turnover frequency, TOF) were shown to depend correlatedly on redox properties and oxygen vacancy concentrations, both of which were influenced by the substitution of different RE elements. Among the pyrochlore oxides, the as-synthesized La2Sn2O7 sample displayed relatively the highest ignition activity and the largest intrinsic activity with TOF of 2.33×10(-3)s(-1).

NOx storage and soot combustion over well-dispersed mesoporous mixed oxides via hydrotalcite-like precursors

A series of mixed oxides with highly dispersed redox components were prepared via hydrotalcite-like precursors in which Mg was partly substituted with copper and cobalt, which were employed for NOx storage and soot combustion. The physico-chemical properties of the catalysts were characterized by XRD, TGA, IR, N2 adsorption, H2-TPR and in situ FTIR techniques. The results show the transition metal cations have isomorphously replaced Mg2+ in the layered structures forming a single hydrotalcite type phase. After calcination, the transition metal oxides exist in a highly dispersed form in the Mg(Al)O matrix and there is a cooperative effect between the copper and cobalt on the redox properties of the catalyst. The as-prepared oxide catalysts exhibit large surface areas, basic characters and improved redox properties, resulting in high performances in NOx storage and soot combustion. Both the NOx storage and desorption are catalytically accelerated due to the highly dispersed transition metal oxides. The presence of NOx positively affects the activity of all the oxides catalysts for soot combustion, which may be related to the production of NO2 during NO oxidation. NO2-assisted mechanism and active oxygen mechanism may occur simultaneously in soot/NO/O2 reaction.

Synthesis and characterization of Co–Al–Fe nonstoichiometric spinel-type catalysts for catalytic CO oxidation

A series of CoAlFe nonstoichiometric spinel-type oxides were synthesized from hydrotalcite precursors prepared through a co-precipitation method, and their catalytic activities for CO oxidation were investigated. The solids were characterized by XRD, BET, SEM, TG-DTG, H2-TPR and in situ FTIR. The calcined hydrotalcite-like precursors were composed of spinel-like CoAlFe mixed oxide with crystallite sizes in the range 8–10.5 nm. The nanosized spinel oxide catalysts showed higher surface area as calcination led to dehydroxylation and carbonate decomposition of anions in interlayer spaces. FTIR results showed two vibrational frequency bands (ν1 and ν2) for tetrahedral and octahedral sites, confirming the formation of the Co3O4 spinel. Iron ions were introduced into the spinel system leading to improved redox properties, as confirmed by TPR. Furthermore, the CoAlFe ternary oxide nanoparticles exhibited superior catalytic performance in CO oxidation in contrast with CoAl and CoFe binary oxides, which can be ascribed to the improved reducibility. According to the in situ FT-IR analysis, CO adsorbed on the catalyst surface reacted with surface lattice oxygen to form CO2. In addition, CO2 could adsorb on the surface and form intermediate carbonate species.

Catalytic oxidation of soot on mesoporous ceria-based mixed oxides with cetyltrimethyl ammonium bromide (CTAB)-assisted synthesis

Mesoporous ceria and transition metal-doped ceria (M0.1Ce0.9O2 (M=Mn, Fe, Co, Cu)) catalysts were synthesized via CTAB-assisted method. The physicochemical properties of the prepared catalysts were characterized by XRD, DLS analysis, SEM, BET, Raman, H2-TPR and in situ DRIFT techniques. The catalytic activity tests for soot oxidation were performed under tight contact of soot/catalyst mixtures in the presence of O2 and NO+O2, respectively. The obtained results show that mesoporous ceria-based solid solutions can be formed with large surface areas and small crystallite size. Transition metals doping enhances the oxygen vacancies and improves redox properties of the solids, resulting in the increased NO oxidation capacity and NOx adsorption capacity. The soot oxidation activity in the presence of O2 is enhanced by doping transition metal, which may be related to their high surface area, increased oxygen vacancies and improved redox properties. The soot combustion is accelerated by the NO2-assisted mechanism under NO+O2 atmosphere, facilitating an intimate contact between the soot and the catalyst.

Hydrothermal Synthesis of Lanthanide Stannates Pyrochlore Nanocrystals for Catalytic Combustion of Soot Particulates

Nanocrystalline La2Sn2O7 and La2Sn1.8Co0.2O7 with a phase-pure pyrochlore structure were synthesized by a hydrothermal method, and their catalytic activity was investigated for soot combustion. The as-synthesized catalysts presented relatively larger surface area, and pore volume, which was benefit to the gas molecule diffusion in the reaction. A uniform spherical structure with particle size of 200–500 nm was found in SEM. The samples via hydrothermal route are more active for catalytic soot combustion, ascribing to the spherical morphology, high surface area and improved oxygen mobility. After Co, the reducibility was improved and surface oxygen vacancy was produced, resulting in the enhanced activity and selectivity to CO2 formation.

Catalytic oxidation of CO over mesoporous copper-doped ceria catalysts via a facile CTAB-assisted synthesis

Nanosized copper-doped ceria CuCe catalysts with a large surface area and well-developed mesoporosity were synthesized by a surfactant-assisted co-precipitation method. The prepared catalysts with different Cu doping concentrations were characterized by XRD, DLS analysis, TEM, BET, Raman, H2-TPR and in situ DRIFTS techniques. The influence of Cu content on their catalytic performance for CO oxidation was also studied. The XRD results indicate that at a lower content, the Cu partially incorporates into the CeO2 lattice to form a CuCe solid solution, whereas a higher Cu doping causes the formation of bulk CuO. Copper doping favors an increase in the surface area of the CuCe catalysts and the formation of oxygen vacancies, thereby improving the redox properties. The CuCe samples exhibit higher catalytic performance compared to bare CeO2 and CuO catalysts. This is ascribed to the synergistic interaction between copper oxide and ceria. In particular, the Cu0.1Ce catalyst shows the highest catalytic performance (T50 = 59 °C), as well as excellent stability. The in situ DRIFTS results show that CO adsorbed on surface Cu+ (Cu+–CO species) can easily react with the active oxygen, while stronger adsorption of carbonate-like species causes catalyst deactivation during the reaction.

A Rational Design of the Sintering-Resistant Au-CeO2 Nanoparticles Catalysts for CO Oxidation: The Influence of H2 Pretreatments

The redox pretreatment of samples is one of the crucial ways of altering the catalytic properties of the supported noble metal materials in many heterogeneous reactions. Here, H2-reducing pretreatment is reported to enhance the thermal stability of Au-CeO2 catalysts prepared by the deposition–precipitation method and calcination at 600 °C for CO oxidation. In order to understand the improved activity and thermal stability, a series of techniques were used to characterize the physico-chemical changes of the catalyst samples. H2 pretreatment may lead to: (i) a strong metal–support interaction (SMSI) between Au nanoparticles (NPs) and CeO2, evidenced by the particular coverage of Au NPs by CeO2, electronic interactions and CO adsorption changes. (ii) the production of surface bicarbonates which can accelerate CO oxidation. As a result, the H2 pretreatment makes the Au NPs more resistant to sintering at high temperature and enhances the CO oxidation activity. Furthermore, this reduction pretreatment strategy may provide a potential approach to enhance the thermal-stability of other supported noble metal catalysts.

Catalytic Oxidation of Soot on a Novel Active Ca-Co Dually-Doped Lanthanum Tin Pyrochlore Oxide

A novel active Ca-Co dually-doping pyrochlore oxide La2−xCaxSn2−yCoyO7 catalyst was synthesized by the sol-gel method for catalytic oxidation of soot particulates. The microstructure, atomic valence, reduction, and adsorption performance were investigated by X-ray powder diffraction (XRD), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), H2-TPR (temperature-programmed reduction), and in situ diffuse reflection infrared Fourier transformed (DRIFTS) techniques. Temperature programmed oxidation (TPO) tests were performed with the mixture of soot-catalyst under tight contact conditions to evaluate the catalytic activity for soot combustion. Synergetic effect between Ca and Co improved the structure and redox properties of the solids, increased the surface oxygen vacancies, and provided a suitable electropositivity for oxide, directly resulting in the decreased ignition temperature for catalyzed soot oxidation as low as 317 °C. The presence of NO in O2 further promoted soot oxidation over the catalysts with the ignition temperature decreased to about 300 °C. The DRIFTS results reveal that decomposition of less stable surface nitrites may account for NO2 formation in the ignition period of soot combustion, which thus participate in the auxiliary combustion process.