Electrospinning synthesis and characterization of nanofibers of Co, Ce and mixed Co-Ce oxides. Their application to oxidation reactions of diesel soot and CO

Co,Ce nanoparticles supported on stacked wire mesh cartridges. Activity and stability in diesel soot combustion

In this work, Co3O4 nanoparticles were successfully synthesized by precipitating a precursor salt solution in the form of microdroplets generated by a nebulizer, as an efficient, fast and low-cost approach. After drying and calcination, synthesized particles were deposited on stacked wire mesh monoliths by immersing the structures in a suspension containing synthesized Co3O4 particles and commercial ceria nanoparticles as a binder. These structured catalysts were evaluated for the combustion of diesel soot which constitutes a crucial step in the regeneration of catalytic particulate filters (CDPFs). Thermal and mechanical stability of Co,Ce washcoated monoliths were investigated. For this, successive catalytic evaluations of the structured system, with intermediate treatments at 900 °C (accelerated aging), were carried out indicating a very good activity and stability of the catalysts developed. Adherence tests showed good adhesion of the catalytic layer to the metallic substrate. Fresh and aged catalysts were fully characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Laser Raman Spectroscopy (LRS) and Temperature-Programmed Reduction (TPR). It was found that the catalytic coating resulted composed of nanometric CeO2 and Co3O4 along with chromium, iron and manganese oxides coming from the migration of the metallic substrate, in the catalytic cartridge calcined at 600 °C. Despite after calcination at 900 °C spinels of Co, Fe, Cr and Mn were observed, these oxides did not significantly affected the catalytic activity. Although this aging treatment at 900 °C was severe and is not expected under real conditions, it highlights the potential application of the catalytic metallic cartridges here developed.

Insights into non-crystalline structure of solid solution Ce-Mn co-oxide nanofibers for efficient low-temperature toluene oxidation

The controllable preparation of efficient non-crystalline solid solution catalysts is a great challenge in the catalytic oxidation of volatile organic compounds. In this work, series non-crystalline solid solution structured Ce-Mn co-oxide nanofibers were creatively prepared by adjusting Ce/Mn molar ratios using electrospinning. 0.20CeMnOx (the ratio of Ce to Mn was 0.2) displayed an outstanding low-temperature toluene oxidation activity (T90 = 233 °C). The formation of the amorphous solid solution and the unique nanofiber structure both contributed to a large specific surface area (S = 173 m2 g-1) and high adsorbed oxygen content (Oads/O = 41.3%), which enhanced the number of active oxygen vacancies. The synergies between non-crystalline structure and active oxygen species markedly improved oxygen migration rate as well as redox ability of the catalysts. Additionally, in situ diffuse reflectance infrared Fourier transform spectra showed that the absorbed toluene could be completely oxidized to CO2 and H2O with benzyl alcohol, benzaldehyde, benzoic acid, and maleic anhydride as intermediates. In summary, this study provided an alternative route for the synthesis of non-crystalline metal co-oxide nanofibers.

Electrospun Ce-Mn oxide as an efficient catalyst for soot combustion: Ce-Mn synergy, soot-catalyst contact, and catalytic oxidation mechanism

Increasing the contact efficiency and improving the intrinsic activity are two effective strategies to obtain efficient catalysts for soot combustion. Herein, the electrospinning method is used to synthesize fiber-like Ce-Mn oxide with a strong synergistic effect. The slow combustion of PVP in precursors and highly soluble manganese acetate in spinning solution facilitates the formation of fibrous Ce-Mn oxides. The fluid simulation clearly indicates that the slender and uniform fibers provide more interwoven macropores to capture soot particles than the cubes and spheres do. Accordingly, electrospun Ce-Mn oxide exhibits better catalytic activity than reference catalysts, including Ce-Mn oxides by co-precipitation and sol-gel methods. The characterizations suggest that Mn³⁺ substitution into fluorite-type CeO₂ enhances the reducibility through the acceleration of Mn-Ce electron transfer, improves the lattice oxygen mobility by weakening the Ce-O bonds, and induces oxygen vacancies for the activation of O₂. The theoretical calculation reveals that the release of lattice oxygen becomes easy because of a low formation energy of oxygen vacancy, while the high reduction potential is beneficial for the activation of O₂ on Ce³⁺-Ov (oxygen vacancies). Due to above Ce-Mn synergy, the CeMnO_x-ES shows more active oxygen species and higher oxygen storage capacity than CeO₂-ES and MnO_x-ES. The theoretical calculation and experimental results suggest that the adsorbed O₂ is more active than lattice oxygen and the catalytic oxidation mainly follows the Langmuir-Hinshelwood mechanism. This study indicates that electrospinning is a novel method to obtain efficient Ce-Mn oxide.

Synthesis of Co,Ce Oxide Nanoparticles Using an Aerosol Method and Their Deposition on Different Structured Substrates for Catalytic Removal of Diesel Particulate Matter

The synthesis of Co and Ce oxide nanoparticles using precipitation of precursor salt solutions in the form of microdroplets generated with a nebulizer proved to be an efficient, fast and inexpensive method. Different morphologies of single oxides particles were obtained. Ceria nanoparticles were almost cube-shaped of 8 nm average size, forming 1.3–1.5 μm aggregates, whereas cobalt oxide appeared as rounded-edged particles of 37 nm average size, mainly forming nanorods 50–500 nm. Co3O4 and CeO2 nanoparticles were used to generate structured catalysts from both metallic (stainless steel wire mesh monoliths) and ceramic (cordierite honeycombs) substrates. Ceria Nyacol was used as a binder to favor the anchoring of catalytic particles thus enhancing the adhesion of the coating. The resulting structured catalysts were tested for the combustion of diesel soot with the aim of being used in the regeneration of particulate filters (DPFs). The performance of these structured catalysts was similar to or even better than that exhibited by the catalysts prepared using commercial nanoparticles. Among the catalysts tested, the structured systems using ceramic substrates were more efficient, showing lower values of the maximum combustion rate temperatures (TM = 410 °C).

Diesel particulate filter regeneration mechanism of modern automobile engines and methods of reducing PM emissions: a review

Diesel particulate filter (DPF) is considered as an effective method to control particulate matter (PM) emissions from diesel engines, which is included in the mandatory installation list by more and more national/regional laws and regulations, such as CHINA VI, Euro VI, and EPA Tier3. Due to the limited capacity of DPF to contain PM, the manufacturer introduced a method of treating deposited PM by oxidation, which is called regeneration. This paper comprehensively summarizes the most advanced regeneration technology, including filter structure, new catalyst formula, accurate soot prediction, safe and reliable regeneration strategy, uncontrolled regeneration and its control methods. In addition, due to the change of working conditions in the regeneration process, the additional emissions during regeneration are discussed in this paper. The DPF is not only the aftertreatment device but also can be combined with diesel oxidation catalyst (DOC), selective catalytic reduction (SCR) and exhaust recirculation (EGR). In addition, the impact of DPF modification on the original system of some old models has been reasonably discussed in order to achieve emission targets.

Research Progress on the Applications of Electrospun Nanofibers in Catalysis

During the last two decades, electrospinning has become a very popular technique for the fabrication of nanofibers due to its low cost and simple handling. Nanofiber materials have found utilization in many areas such as medicine, sensors, batteries, etc. In catalysis, these materials also present important advantages, since they present a low resistance to internal diffusion and a high surface area to volume ratio. These advantages are mainly due to the diameter–length proportion. A bibliographic analysis on the applications of electrospun nanofibers in catalysis shows that there are two important groups of catalysts that are being investigated, based on TiO2 and in carbon materials. The main applications found are in photo- and in electro-catalysis. The present study contributes by reviewing these catalytic applications of electrospun nanofibers and demonstrating that they are promising materials as catalysts, underlining some works to prove the advantages and possibilities that these materials have as catalysts. On one hand, the possibilities of synthesis are almost infinite, since with coaxial electrospinning quite complex nanofibers with different layers can be prepared. On the other hand, the diameter and other properties can be controlled by monitoring the applied voltage and other parameters during the synthesis, being quite reproducible procedures. The main advantages of these materials can be grouped in two: one related to their morphology, as has been commented, relative to their low resistance and internal diffusion, that is, their fluidynamic behavior in the reactor; the second group involves advantages related to the fact that the active phases can be nanoscaled and dispersed, improving the activity and selectivity in comparison with conventional catalytic materials with the same chemical composition.