Study on oxidation activity of Ce-Mn-K composite oxides on diesel soot

Investigation of the Dual-Regulatory Mechanism of Zr in MnRE1-xZrxO2+δ Oxide Catalysts for NO Oxidation

Utilizing Diesel Oxidation Catalysts (DOC) to partially oxidize NO to NO2 is a crucial step in controlling NOx emissions from diesel engines. However, enhancing both catalytic activity and hydrothermal stability remains a significant challenge. Benefiting from abundant asymmetric oxygen vacancies and increased Mn4+ content, MnRE0.5Zr0.5 exhibits superior NO oxidation performance (T63 = 337 °C) and hydrothermal aging resistance (T52 = 340 °C) compared to the undoped sample (T53 = 365 °C). XPS, Raman, TPR, and XAS are employed to verify the elevation of oxygen vacancy concentration and Mn valence state modulation due to Zr introduction. Furthermore, compared to MnRE (1.36 eV), the formation energy of oxygen vacancies in MnRE0.5Zr0.5 is significantly reduced (0.17 eV). This work elucidates the dual regulatory role of Zr in the Mn-RE-Zr ternary system, providing theoretical support and guidance for the design of catalysts for atmospheric pollutant purification and industrial catalysis.

Enhancement of Ni-NiO-CeO2 Interaction on Ni-CeO2/Al2O3-MgO Catalyst by Ammonia Vapor Diffusion Impregnation for CO2 Reforming of CH4

Ni-based catalysts have been widely used for the CO2 reforming of methane (CRM) process, but deactivation is their main problem. This study created an alternative electronic Ni-NiO-CeO2 interaction on the surface of 5 wt% Ni-5 wt% CeO2/Al2O3-MgO (5Ni5Ce(xh)/MA) catalysts to enhance catalytic potential simultaneously with coke resistance for the CRM process. The Ni-NiO-CeO2 network was developed on Al2O3-MgO through layered double hydroxide synthesis via our ammonia vapor diffusion impregnation method. The physical properties of the fresh catalysts were analyzed employing FESEM, N2 physisorption, and XRD. The chemical properties on the catalyst surface were analyzed employing H2-TPR, XPS, H2-TPD, CO2-TPD, and O2-TPD. The CRM performances of reduced catalysts were evaluated at 600 °C under ambient pressure. Carbon deposits on spent catalysts were determined quantitatively and qualitatively by TPO, FESEM, and XRD. Compared to 5 wt% Ni-5 wt% CeO2/Al2O3-MgO prepared by the traditional impregnation method, the electronic interaction of the Ni-NiO-CeO2 network with the Al2O3-MgO support was constructed along the time of ammonia diffusion treatment. The electronic interaction in the Ni-NiO-CeO2 nanostructure of the treated catalyst develops surface hydroxyl sites with an efficient pathway of OH* and O* transfer that improves catalytic activities and coke oxidation.

Combined Steam and CO2 Reforming of Methane over the Hierarchical Ni-ZrO2 Nanosheets/Al2O3 Catalysts at Ultralow Temperature and under Low Steam

This research developed hierarchical 10 wt % Ni-1 wt % ZrO2/Al2O3 catalysts for combined steam and CO2 reforming of methane (CSCRM) reaction to produce syngas for gas-to-liquid (GTL) application under the ultralow temperature and low steam condition. The hierarchical nanosheet catalysts were prepared via a novel impregnation technique assisted by ammonia vapor diffusion with various times (1, 6, and 12 h) to develop the different magnitude of hierarchical nanosheets on the surface. Then, CSCRM at 600 °C was performed on the catalysts for 6 h. The results evidenced the improvement of H2 selectivity, reaching an appropriate H2/CO ratio (1.9-2.0) in FT subunits in the GTL process when nanosheets existed on the surface due to the increase in H2O adsorption-dissociation sites. The good dispersion of hierarchical nanosheets accompanied by the ZrO2 promoter successfully enhanced the CH4 conversion and the coke prevention through the spread nanosheets because of the increase in the number of active sites and the surface interaction. The interaction of hierarchical nanosheets created the H2O activation-dissociation sites that allowed CO2 to be selective on the oxygen vacancy sites, producing more OH* and OH* on the catalyst surface to resist the carbon deposition during CSCRM operation.

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.

Effect of oxygen vacancy and highly dispersed MnOx on soot combustion in cerium manganese catalyst

Cerium manganese bimetallic catalysts have become the focus of current research because of their excellent catalytic performance for soot combustion. Two series of cerium manganese catalysts (Na-free catalysts and Na-containing catalysts) were prepared by coprecipitation method and characterized using XRD, N₂ adsorption-desorption, SEM, Raman, XPS, H₂-TPR, O₂-TPD, Soot-TPR-MS and in situ IR. The effects of abundant oxygen vacancies and surface highly dispersed MnO_x on soot catalytic combustion of cerium manganese catalysts prepared by different precipitants were analyzed. The activity test results show that the active oxygen species released by a large number of oxygen vacancies in the cerium manganese catalyst are more favorable to the soot catalytic combustion than MnO_x which is highly dispersed on the surface of the catalyst and has good redox performance at low temperature. Because the catalytic effect of MnO_x on the surface of Na-free catalysts is more dependent on the contact condition between the catalyst and the soot, this phenomenon can be observed more easily under the loose contact condition than under the tight contact condition. The activity cycle test results show that these two series of catalysts show good stability and repeated use will hardly cause any deactivation of the catalysts.

Revealing the promoting effect of multiple Mn valences on the catalytic activity of CeO2 nanorods toward soot oxidation

Mn-modified CeO₂ nanomaterials have attracted extensive attention as efficient and promising catalysts for soot combustion due to their low cost and high catalytic activity. However, a detailed mechanism of how Mn promotes soot oxidation over CeO₂ is still not clearly elucidated, which is crucial to further optimize the catalyst for achieving its practical applications. We here report a Mn-doped CeO₂ catalyst with tunable surface Mn chemical valence states to study the Mn-promoting mechanism for improving CeO₂ catalyst activity in soot oxidation. Experimental results show that Mn-doped CeO₂ nanorods with surface Mn chemical valence states being optimized (Mn_0.19Ce_0.81O₂) can lower the eliminating temperature of soot to 410 °C (T₉₀) when in a loose contact and exhibit a strong resistance towards water molecules. The catalytic performances of Mn_0.19Ce_0.81O₂ nanorods are comparable with those of other reported oxide catalysts both in the mimetic realistic and ideal reaction environments. Detailed characterization and theoretical calculation results demonstrate that balanced multiple Mn valences can dramatically enhance the catalysts' redox properties and their ability to activate O₂ molecules, as well as improve the dynamic contact efficiency during the oxidation, which synergistically result in superior catalytic performances. This work might provide insight for the future design and preparation of catalysts to efficiently eliminate soot particles.

Effects of La incorporation in catalytic activity of Ag/La-CeO2 catalysts for soot oxidation

Owing to strengthened regulations toward vehicle emissions, the use of diesel particulate filter technology to reduce particulate matter emissions has attracted significant attention. To achieve low temperature oxidation of particulate matter, numerous studies on Ag/CeO₂ catalysts for soot oxidation have been reported. Herein, Ag/La-CeO₂ catalysts with different La contents are synthesized and compared to analyze the effect of La. Hydrogen temperature programmed reduction analysis confirms that the reducibility increases with an increase in the La content in La-CeO₂. X-ray photoelectron spectroscopy and Raman analysis confirm an increase of oxygen vacancies with La doping. Accordingly, the soot oxidation performances estimated by temperature programmed oxidation experiments increase with La doping. However, the catalytic activity of Ag/La-CeO₂ exhibits a volcano trend. When an appropriate amount of La is incorporated in Ag/CeO₂, peroxide generation and reducibility improve, thereby enhancing the soot oxidation performance. Conversely, the catalytic activities gradually decrease with excess La-doping. Scanning transmission electron microscopy analysis and density functional theory calculations confirm that excess amounts of La induce the sintering of Ag particles, which lead to the degradation of peroxide generation and reducibility of the catalysts. Consequently, an optimal amount of La incorporation on Ag/La-CeO₂ results in the best soot oxidation performance.

Preparation of K Modified Three-Dimensionally Ordered Macroporous MnCeOx/Ti0.7Si0.3O2 Catalysts and Their Catalytic Performance for Soot Combustion

Soot particles in diesel engine exhaust is one of the main reasons for hazy weather and elimination of them is urgent for environmental protection. At present, it is still a challenge to develop new catalysts with high efficiency and low cost. In this paper, a kind of K modified three-dimensionally ordered macroporous (3DOM) MnCeOx/Ti0.7Si0.3O2 catalysts are designed and synthesized by a sample method. Due to the macroporous structure and synergistic effect of K, Mn, and Ce, the KnMnCeOx/Ti0.7Si0.3O2 (KnMnCeOx/M-TSO) catalysts exhibit good catalytic performance for soot combustion. The catalytic activity of K0.5MnCeOx/M-TSO was the best, and the T10, T50, and T90 are 287, 336, and 367 °C, respectively. After the prepared catalyst was doped with K, the physicochemical properties and catalytic performance changed significantly. In addition, the K0.5MnCeOx/M-TSO catalyst also somewhat exhibits sulfur tolerance owing to it containing Ti. Because of its simple synthesis, high activity, and low cost, the prepared KnMnCeOx/M-TSO catalysts are regarded as a promising candidate for application.

Surface acid etching for efficient anchoring of potassium on 3DOM La0.8Sr0.2MnO3 catalyst: An integration strategy for boosting soot and NOx simultaneous elimination

The emission of soot and NOx is one of the most severe environmental issues, and the key factor is the development of catalysts in after-treatment systems. In this study, an innovative non-noble metal catalyst, named HKLSM, was fabricated by etching 3DOM La_0.8Sr_0.2MnO₃ with citric acid and synchronously anchoring potassium salt, for soot and NOx simultaneous removal. The citric acid could not only slightly erode the 3DOM skeleton, thereby beneficial to the dispersion of potassium, but also react with high-valence state Mn to generate abundant coordination unsaturated Mn³⁺ sites, which could produce more active oxygen species. Moreover, HKLSM showed a higher NOx adsorption capability than the samples that were not subjected to acid etching. This adsorbed NOx could be stored as NO₃^- species, which could facilitate soot combustion. Among all the as-prepared catalysts, HKLSM demonstrated a competitive soot combustion activity with a T₅₀ value of 368 °C, a TOF value of 3.24 × 10^-4 s^-1, a reaction rate of 1.87 × 10^-7 molg^-1s^-1, a total NOx to N₂ yield of 42.0% and favorable reusability and water-resistance. This integration strategy can rationalize an alternative protocol to soot and NOx simultaneous elimination or even other catalysis systems.

Study of spatial and temporal aging characteristic of catalyzed diesel particulate filter catalytic performance used for diesel vehicle

Catalyzed diesel particulate filters (CDPFs) have been widespread used as a technically and economically feasible mean for meeting increasingly stringent emissions limits. An important issue affecting the performance of a CDPF is its aging with using time. In this paper, the effects of noble metal loadings, regions and using mileage on the aging performance of a CDPF were investigated by methods of X-ray diffraction (XRD), X-ray photoelectron spectroscopy and catalytic activity evaluation. Results showed that aging of the CDPF shifted the XRD characteristic diffraction peaks towards larger angles and increased the crystallinity, showing a slowing downward trend with the increase of the noble metal loadings. In addition, the increase of the noble metal loading would slow down the decline of Pt and Pt⁴⁺ concentration caused by aging. The characteristic temperatures of CO, C₃H₈ conversion and NO₂ production increased after aging, and the more the noble metal loadings, the higher the range of the increase. But noticeably, excessive amounts of noble metals would not present the corresponding anti-aging properties. Specifically, the degree of aging in the inlet region was the deepest, the following is the outlet region, and the middle region was the smallest, which were also reflected in the increase range of crystallinity, characteristic temperatures of CO, C₃H₈ conversion and NO₂ production, as well as the decrease range of Pt and Pt⁴⁺ concentrations. The increase of aging mileage reduced the size of the aggregates of the soot and ash in CDPFs, however, improved the degree of tightness between particles. Meanwhile Carbon (C) concentration in the soot and ash increased with the aging mileage.