Catalytic Soot Oxidation Research Articles

Ce(III) formate-derived hierarchical cerium oxide particles with icosahedral symmetry-based architectures: Effect of third level of structural hierarchy in soot and propane oxidation

The article investigates the role of architecture of complex hierarchically structured ceria and gadolinium doped ceria particles in soot combustion (solid-solid reaction) and propane oxidation (gas-solid reaction). Synthesis of hierarchical ceria mesostructures showing icosahedral symmetry-based architecture has been presented for the first time. Carefully choosing synthetic conditions allows to obtain particles with intended morphology of dodecahedron, rhombic hexecontahedron, great stellated dodecahedron, fractal-like branched great stellated dodecahedron and great dodecahemicosacron polyhedra. Twinning-based multiple branching of Ce(III) formate crystals by {101} twin orientation relation was proposed as a driving mechanism for the formation of macroparticles architecture that has been confirmed by the construction of macroparticle models using the identified twinning law. In the catalytic studies, the third level of the material organizational structure referring to shape of hierarchical macroparticles was chosen as a tested variable, while other architecture-related properties were controlled to be alike. In result, the shape of hierarchical particles plays a vital role in the catalytic soot oxidation, and architecture modulation allows the catalytic properties of the material to be enhanced through increasing the geometrical fit between ceria and soot particles. The best sample lowers temperature of half oxidation by 143 °C as compared to non-hierarchically organized ceria nanocrystals. The shape of hierarchical particles does not play leading role in gas-solid propane oxidation, but introduction of Gd dopant diversifies catalytic activity. The architecture of the particles may serve as a modifier of the catalytic activity through grain boundary engineering of gadolinium doped ceria.

Boosting the removal of diesel soot particles by regulating the Pr−O strength over transition metal doped Pr6O11 catalysts

The content of active lattice oxygen and oxygen vacancies is crucial for the catalytic oxidation of soot. Herein, we adjust the Pr-O bond strength in Pr6O11 by doping several common transition metals (Mn, Fe, Co, Ni) to promote the formation of oxygen vacancies and the activation of lattice oxygen. This strategy does not compromise its crystal structure, allowing for improved catalytic performance while maintaining stability. The Mn-doped Pr6O11 catalyst shows the best soot catalytic oxidation performance. Its T50 (the temperature of soot conversion reaching 50 %) value is 396 °C under loose contact. Further characterizations and density functional theory (DFT) calculations demonstrate that PMO possesses a large specific surface area. Additionally, the weakening the strength of the Pr-O bond leaded to an increase in oxygen vacancies, which in turn enhanced the redox ability of catalyst. This work will provide a reference for the development of Pr-based catalysts for soot combustion.

Ash effects on diesel soot–catalytic oxidation and its chemical reaction kinetics: the comparison of powder sample and CDPF waffle sample

Ash effects on diesel soot–catalytic oxidation and its chemical reaction kinetics: the comparison of powder sample and CDPF waffle sample

High-temperature calcination dramatically promotes the activity of Cs/Co/Ce-Sn catalyst for soot oxidation

Catalytic oxidation of soot is of great importance for emission control on diesel vehicles. In this work, a highly active Cs/Co/Ce-Sn catalyst was investigated for soot oxidation, and it was unexpectedly found that high-temperature calcination greatly improved the activity of the catalyst. When the calcination temperature was increased from 500 °C to 750 °C, T50 decreased from 456.9 °C to 389.8 °C in a NO/O2/H2O/N2 atmosphere. Characterization results revealed that high-temperature calcination can promote the ability to transfer negative charge density from Cs to other metal cations in Cs/Co/Ce-Sn, which will facilitate the production of more oxygen defects and the generation of more surface-active oxygen species. Surface-active oxygen species are beneficial to the oxidation of NO to NO2, leading to the high yield of NO2 exploitation. Therefore, the Cs/Co/Ce-Sn catalyst calcined at 750 °C demonstrated higher activity than that calcined at 500 °C. This work provides a pathway to prepare high efficiency catalysts for the removal of soot and significant insight into the effects of calcination on soot oxidation catalysts.

Modification of SmMn2O5 Catalyst with Silver for Soot Oxidation: Ag Loading and Metal–Support Interactions

A series of Ag-modified manganese-mullite (SmMn2O5) catalysts with different Ag contents (1, 3, and 6 wt.%) were prepared via a citric acid sol–gel method for catalytic soot oxidation. The catalysts were characterized by powder X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Raman spectroscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy analysis (HRTEM), X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR). The soot oxidation activity of the mullite was significantly promoted by the addition of silver and affected by the loading amount of the metal. Herein, the influences of silver loading on the metal size distribution and its interactions with the mullite were studied. Based on these characterizations, a possible soot oxidation reaction mechanism was proposed for silver-modified SmMn2O5.

Cerium Doping Effect in 3DOM Perovskite-Type La2−xCexCoNiO6 Catalysts for Boosting Soot Oxidation

Herein, we present an in-depth investigation into the enhancement of catalytic soot oxidation through cerium-doped three-dimensional ordered macroporous (3DOM) La-Co-Ni-based perovskites synthesized with the colloidal crystal template (CCT) method. The 3DOM structure significantly contributes to the accessibility and interaction efficiency between soot and catalyst. Based on the results of powder X-ray diffraction (XRD), N2 adsorption-desorption measurements, scanning electron microscopy (SEM), temperature-programmed oxidation of NO (NO-TPO), temperature-programmed reduction of H2 (H2-TPR), in situ infrared Fourier transform spectroscopy (In-situ DRIFTS), and temperature-programmed oxidation (TPO) reactions, the role of cerium doping in modifying the structural and catalytic properties of 3DOM perovskite-type La2−xCexCoNiO6 catalysts was investigated systematically. The optimized cerium doping ratio in La2−xCexCoNiO6 catalysts can improve the microenvironment for efficient soot-catalyst contact, enhancing the catalytic activity of soot oxidation. Among the catalysts, the 3DOM La0.8Ce1.2CoNiO6 catalyst shows the highest catalytic activity for soot oxidation, whose T10, T50, and T90 values are 306 °C, 356 °C, and 402 °C, respectively. The mechanism of the cerium doping effect for boosting soot oxidation is proposed: The doping of Ce ions can increase the surface oxygen species, which is the main active species for promoting the key step of NO oxidation to NO2 in catalyzing soot oxidation. This research provides a new strategy to develop high-efficient non-noble metal catalysts for soot oxidation in pollution control and sustainable environmental practices.

Study on the differences between non-catalytic and catalytic oxidation of soot based on catalyst CeO2

Exploring the differences between soot non-catalytic and catalytic oxidation process is beneficial to enhance the catalyst performance and soot removal efficiency of catalytic diesel particulate filters. In this paper, the soot samples are pretreated under high temperature conditions at both non-catalytic and catalytic oxidation atmosphere to obtain target pretreated soot samples with different mass loss. Five typical oxidation stages are analyzed with corresponding soot mass loss rate of 15 %, 25 %, 50 %, 70 %, 85 %. The soot physicochemical properties including oxidation activity, porous structure, particle layer, nanostructure, graphitization and functional groups are explored; further, the correlations among these physicochemical properties are analyzed, and three levels of catalytic effect mechanism are proposed based on CeO2 catalyst. This paper helps in understanding the differences of the mechanisms between non-catalytic and catalytic oxidation of soot.

Enhanced Catalytic Soot Oxidation over Co-Based Metal Oxides: Effects of Transition Metal Doping.

A series of Co-M (M = Fe, Cr, and Mn) catalysts were synthesized by the sol-gel method for soot oxidation in a loose contact mode. The Co-Fe catalyst exhibited the best catalytic activity among the tested samples, with the characteristic temperatures (T10, T50, and T90) of 470 °C, 557 °C, and 602 °C, respectively, which were 57 °C, 51 °C, and 51 °C lower than those of the CoOx catalyst. Catalyst characterizations of N2 adsorption-desorption, X-ray diffraction (XRD), X-ray photo-electron spectrometry (XPS), and the temperature programmed desorption of O2 (O2-TPD) were performed to gain insights into the relationships between the activity of catalytic soot oxidation and the catalyst properties. The content of Co2+ (68.6%) increased due to the interactions between Co and Fe, while the redox properties and the relative concentration of surface oxygen adsorption (51.7%) were all improved, which could significantly boost the activity of catalytic soot oxidation. The effects of NO and contact mode on soot oxidation were investigated over the Co-Fe catalyst. The addition of 1000 ppm of NO led to significant reductions in T10, T50, and T90 by 92 °C, 106 °C, and 104 °C, respectively, compared to the case without the NO addition. In the tight contact mode, the soot oxidation was accelerated over the Co-Fe catalyst, resulting in 46 °C, 50 °C, and 50 °C reductions in T10, T50, and T90 compared to the loose contact mode. The comparison between real soot and model Printex-U showed that the T50 value of real soot (455 °C) was 102 °C lower than the model Printex-U soot.

Unveiling the Surface Chemical Reactions during Multi-Phase Catalytic Oxidation of Soot on Nanoengineering/Interfacing/Doping-Prepared Mn-CeO2 Catalysts Using TG-MS and Operando DRIFTS-MS.

The aerosol pyrolysis method from nitrate precursors was used to prepare the Mn-CeO2 catalyst containing Mn2O3, CeO2, and Mn-doped CeO2 nanoparticles for catalyzing carbonous soot oxidation. The prepared Mn-CeO2 catalysts have high specific surface areas, Ce3+ ratio, and oxygen vacancy defects; these are a benefit for soot oxidation. The T50 for soot oxidation on the 0.57Mn-CeO2 catalyst is as low as 355 °C, which is 329 °C lower than that for soot oxidation without a catalyst. The catalysts were characterized using XRD, SEM-EDS, HRTEM, XPS, Raman spectroscopy, H2-TPR-MS, O2-TPD-MS, soot-TPR-MS, and operando DRIFTS-MS. The functions of Mn2O3, CeO2, and Mn-doped CeO2 in the 0.57Mn-CeO2 catalyst are unveiled. Mn-doped CeO2 plays a key role and CeO2 participates in soot oxidation, while Mn2O3 is used to enhance higher ratios of Ce3+, via the reaction of Mn3+ + Ce4+ = Mn4+ + Ce3+. The mechanism of soot oxidation on Mn-CeO2 was proposed.

Effect of ash participation on the catalytic activity of as-prepared catalysts to promote diesel soot combustion

This paper studies the effects of ash species and ash aging deposition on the physical and chemical properties of KC catalysts. By thermogravimetric analysis (TGA), the pure KC catalyst has good stability and catalytic activity at 600 °C. In the loosely contacting soot particles (PU)/ash/KC mixtures, the negative effect of ash on KC catalytic soot oxidation is relatively weak. The addition of ash makes the active site areas between PU and KC catalyst lose contact without changing the physicochemical properties of the catalyst. By contrast, the ash aging deposition on KC at 400 °C has an obviously negative effect on the soot combustion catalyzed by KC, because ash aging deposition leads to enormous changes in the chemical properties and catalytic activity of KC. This finding is consistent with characterization analyses such as XRD, SEM, XPS and Raman. Ash aging deposition destroys the KC structure and reduces the activity of KC to catalyze soot oxidation, and CaSO4 has the greatest effect on the oxidation activity. Consequently, ash aging deposition causes structural property damage of as-prepared catalysts, leading to the failure of CDPF catalytic layer. This study provides a new idea for the selection and optimization strategy of catalysts in CDPF.

Multi-objective optimization study of hydrothermal aging on NOx-assisted soot catalytic combustion and secondary pollution emission in regeneration of CeO2 catalytic diesel particulate filter

The present study aimed to explore the effect of hydrothermal aging on the heterogeneous catalytic combustion process of soot in CeO2 catalytic diesel particular filter and the formation of secondary pollutants (N2O and CO).The present study was based on a fixed-bed experimental bench for NOx-assisted soot catalytic oxidation in CeO2-CDPF with different soot loading and catalyst coating amounts. A non-linear regression surrogate model and a multi-objective optimization decision approach were used to analyze the trade-off relationship between the design variables and the optimization objectives. The results showed that lower levels of soot loading (2.5 g/L), CeO2 catalyst coating amount (2.5 g/L) and higher NOx concentration (1000 ppm) in fresh catalysts were favorable for improving the performance of soot-assisted catalytic combustion and minimizing the formation of N2O and CO. However, it was observed that higher levels of catalyst coating amount (10 g/L) resulted in better soot catalytic combustion performance with lower N2O and CO emissions. Global sensitivity analysis showed a positive correlation between N2O and CO peak concentrations and peak temperatures with soot loading, but a negative correlation with catalyst loading. Over time, hydrothermal aging of CDPF caused the inhibitory effect of CeO2 catalyst on N2O to shift from negative to positive correlation, while also decreasing the synergistic performance of NOx in soot catalytic combustion.

The Catalyst of Ruthenium Nanoparticles Decorated Silicalite-1 Zeolite for Boosting Catalytic Soot Oxidation

Herein, the Ruthenium nanoparticles (NPs) with the size of 12 nm were decorated on the hexagonal prism silicalite-1 (Ru/S-1) by the gas bubbling-assisted membrane reduction method (GBMR). The adsorption/activation properties are improved for reactant molecules due to the formation of an interfacial structure that enhances the interaction between the Ru NPs and S-1. The Ru/S-1 catalyst displays the highest catalytic activity (T50 = 356 °C) and CO2 selectivity (SCO2m = 99.9%). Moreover, no obvious deactivation was observed over the Ru/S-1 catalyst even after five cycles, and the values of T50 and SCO2m after cycling five times are similar to the fresh catalyst. The Ru/S-1 catalyst with excellent catalytic performance can be compared with a series of noble metal catalysts for soot oxidation. The catalytic mechanism of the Ru/S-1 catalyst was revealed by in situ characterization for soot oxidation. The interfacial effect between Ru NPs and S-1 plays an important role in the conversion of NO to NO2 during soot oxidation. Preparation of Ru/S-1 catalyst provides a hopeful way to obtain considerably low-cost and highly stable auto-exhaust treatment catalysts.

Transition metal modified manganese-based catalysts for soot oxidation promoted by noncompetitive adsorption of oxygen: Experiments and DFT calculations

A series of manganese-based oxide catalysts were synthesized by the hydrothermal method. By adjusting the doping amounts of W and Cu in the system to study the catalytic performance of transition metal-doped manganese oxides on soot combustion. They were deeply studied through various characterizations and theoretical calculations. The results show that W and Cu doping modifications can significantly improve the activity of manganese-based oxides in soot oxidation. W-Cu0.1/MnOx prepared by co-modification of W and Cu at 395 °C can convert 90% of soot into CO2. The multiple characterization results show that by adjusting the doping amount of copper and tungsten, the nano-flower-like morphology can be obtained and the crystal plane can be reconstructed, which is helpful to increase the specific surface area, Mn3+ content, and oxygen vacancy content. The theoretical calculation results show that the oxygen vacancy on the surface of the catalyst shows the ability to activate O2 and H2O, and they show non-competitive adsorption. In addition, W-Cu0.1/MnOx has a good application prospect in the catalytic oxidation of soot because of its excellent catalytic stability. This work provides a reference for the design and preparation of non-noble metal-based catalysts for the elimination of soot particles from diesel engine exhaust.

Preparation and characterization of Na4Mn9O18 whiskers on channel surface of cordierite honeycomb ceramics for soot catalytic combustion

Na4Mn9O18 whiskers were prepared on the wall of cordierite honeycomb ceramics using the molten salt synthesis, and Na4Mn9O18 whiskers were grew on channel surface of cordierite honeycomb ceramics to form hierarchical microstructure for catalytic combustion of soot. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were used to characterize the structural morphology and phase compositions. Na4Mn9O18 whiskers with aspect ratio of 24.4 measured by statistics. It was found that the loading rate of the monolithic catalyst reaches 3.588% while the binding rate after sonication almost no change. Whiskers are tightly combined with the cordierite ceramic substrate by the binding strength experiments and EDS analysis. The oxygen temperature-programmed desorption (O2-TPD) measurements were performed to investigate the O2 adsorption property. The catalytic performance of the samples were examined by semi-quantitative thermal gravimetric analysis (TG-DTG) and temperature-programmed oxidation (TPO). The catalyst performance study demonstrates that Na4Mn9O18 whiskers decrease the soot combustion temperature apparently. Experiments show the characteristic temperature could be reduced by increasing the catalyst loading. The Na4Mn9O18 decreased the ignition temperature of soot particles oxidation, and the cycle stability experiment confirmed that the sample (0h-m-k) has good structural stability and stable catalytic performance. The coarseness of the interface increased because Na4Mn9O18 whiskers grew on the cordierite substrate, and the trapping ability improved. Na4Mn9O18 is an important type of catalyst which is rarely studied in catalytic oxidation of soot. This catalyst displays good activity and stability in the soot catalytic oxidation. This unique microstructure has potential application in the DPF field.

Defective zeolite TS-1 confined Pt nanoclusters with superior performance for CO and soot catalytic oxidation

Catalytic oxidation is an important reaction in the treatment of diesel exhaust. Herein, a facile self-assembly strategy has been developed to synthesize defective zeolite confined-catalyst Pt@TS-1 with ultra-low Pt loading, which shows super catalytic activity in the CO conversion (T90 = 175 °C) and soot oxidation (T90 = 490 °C). Owing to the confinement effect of zeolite, the obtained catalyst Pt@TS-1 exhibits a high percentage of Pt2+, smaller Pt nanoparticles and sufficient Ti3+-VÖ defects, which can greatly accelerate the adsorption and conversion of CO, and the deep oxidation of soot. In-situ DRIFTS results demonstrate that rich and smaller Pt nanoparticles confined in zeolite may be responsible for excellent CO conversion. Significantly, the catalyst Pt@TS-1 also shows excellent catalytic stability and good resistance to water and sulfur, showing the potential for industrial applications.

Simulating Catalytic Reaction and Soot Oxidation in Coated Particulate Filters: a Simplified Modelling Framework Including Diffusion Effects

Simulating Catalytic Reaction and Soot Oxidation in Coated Particulate Filters: a Simplified Modelling Framework Including Diffusion Effects

Multiple doping of cryptomelane catalysts by Co, Cu, Ag and Ca for efficient soot oxidation and its effect on NO2 formation and SO2 resistance

Two series of cryptomelane doped with Cu2+ and Ag+ ions were synthesized by coprecipitation method, and additionally impregnated with Ca2+, Co2+ and Ag+ ions to obtain bi- and tri-metallic catalysts. The samples were thoroughly characterized by XRD, RS, TEM/EDX, XRF and XPS. The catalysts were tested for soot oxidation in the loose contact mode in various reaction conditions (O2, O2 + NO, O2 + SO2, and O2 + NO + SO2). The role of doping was examined and discussed in terms of its influence on catalytic soot oxidation performance, NO2 formation in the course of the combustion, and resistance to SO2. It was found that NO2 production in the course of soot oxidation is strongly enhanced upon doping, and clearly depends on the dopants nature. However, it does not directly correlate with the catalysts activity, which in the O2/NO mixture is rather weakly sensitive to the type of doping. This observation was interpreted in terms of a critical amount of NO2, which actually can be utilized effectively for soot combustion. The excess of NO2 gains in importance in the presence of SO2, accounting for the beneficial role of doping in improving the resistance to sulphur poisoning of the cryptomelane catalyst.

Pivotal role of MnOx physicochemical structure in soot oxidation activity

Arising an interest in climate and health problems, catalytic soot oxidation and its mechanism need to be understood, in order to effectively reduce PM (particulate matter) emissions from industry and transportation. Here, a synergistic effect of the Mn physicochemical structure on the soot oxidation activity of Ti-based oxide catalysts is investigated. To unravel the structural change and role of different Mn species, a comparison of different Mn loading and pretreatment conditions was done. While a fine loading with different Mn arrangements is observed by XRD analysis, a comparative XPS study of these Mn catalysts reveals the predominant presence of Mn3+ oxidation state and optimal combination of surface adsorbed and lattice oxygen species (ratio of 1.07) imparts a promotional role in soot oxidation activity. Furthermore, the TPR, O2-TPD, and TEM analysis represent that the increased Mn amount with different configurations led to a decreased reducibility, difficulty in lattice oxygen accessibility, and changed the morphology of MnOx species from cubic to hexapod and/or truncated octahedra associated with the presence of α-Mn2O3-C, β-MnO2 and/or α-Mn2O3-TO, respectively. The presence of smaller segregated cubic particles, ease of reducibility, and predominant presence of Mn3+ species led to superior performance even at lower 10 %Mn supported TiO2 catalyst and was revealed with an activation energy of 112.9 kJ mol−1. This proves the consequence of a proper design of an optimum Mn structure on oxide supports for an efficient soot oxidation reaction.

Core-shell MnCeO catalysts for NO oxidation and mild temperature diesel soot combustion

Environmental contamination such as soot particles and NO x have aroused extensive attraction recently. However, the main challenge lies in the oxidation of soot at mild temperature with the assistance of NO x . Here, a series of core–shell MnCeO x catalysts were successfully synthesized by hydrothermal method and employed for low-temperature catalytic oxidation of soot in the presence of NO x . X-ray diffraction (XRD), inductively coupled plasma - optical emission spectrometry (ICP-OES), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), N 2 adsorption/desorption, H 2 -temperature programmed reduction (H 2 -TPR), O 2 -temperature programmed desorption (O 2 -TPD), X-ray photoelectron spectroscopy (XPS) and Raman were conducted for identifying the structure of prepared catalysts and investigating redox properties of the core–shell mixed oxide, which shows excellent performance for NO oxidation and the T 50 about 350 °C for soot oxidation. Besides, soot particle is oxided at 304 °C in presence of NO, while T 50 augments up to 340 °C in the absence of NO. In the NO x -assisted elimination cases of soot, NO 2 generated from NO in emission gas is generally reckoned as a facilitator which significantly reduces ignition temperature of soot particles. Possible catalytic mechanism over core–shell MnCeO x catalysts is also proposed in this paper. Core-shell MnCeO x catalysts with different Mn contents were successfully synthesized through hydrothermal method and they exhibit superior catalytic ability for soot oxidation. This work achieves NO x -assisted soot combustion at mild temperature and can be used as alternative option in catalytic processes of soot particles. • A series of MnCeO x catalysts were successfully synthesized by hydrothermal method and the core-shell structure is confirmed. • MnCeO x catalysts showed superior catalytic capability for low-temperature catalytic oxidation of soot in the presence of NO x and the lowest catalytic temperature was achieved at 304 °C. • Possible catalytic mechanism over core-shell MnCeO x catalysts was also proposed.

Surface adsorbed and lattice oxygen activated by the CeO2/Co3O4 interface for enhancive catalytic soot combustion: Experimental and theoretical investigations

Metal oxide-oxide interface on supported catalyst has been rarely studied due to the complex interfacial structure and synthetic challenge. Herein, different Ag-supported CeO2/Co3O4 samples with various covered-state of CeO2 were prepared for catalytic soot oxidation. In comparison, catalytic activity was significantly improved by grafting CeO2 on Co3O4, in which the best performing Ag/CoCe-2 exhibited remarkable catalytic performance towards soot oxidation with a T50 of 290.5 ℃ under 10 % O2/N2. Catalyst characterization investigated by Scanning Electron Microscope (SEM), quasi in-situ X-ray Photoelectron Spectroscopy (XPS), in-situ Raman, etc. revealed that this outstanding promotion in catalytic activity can be principally ascribed to the formation of the CeO2/Co3O4 interface. An appropriate CeO2 dosage maximized the contact and interaction between Co3O4 and CeO2, resulting in the largest CeO2/Co3O4 interface featured with abundant generated superoxide species and activated surface lattice oxygen. Density functional theory (DFT) calculations were also carried out for the oxygen vacancy formation energy, Gibbs free energy, etc. In presence of the CeO2/Co3O4 interface, a charge density redistribution around the adsorbed reactants at oxygen vacancies could be formed, owing to the efficient charge transfer enhanced by the electron-appealing effect. The change in electronic structure favored reducing the oxygen vacancy formation energy and boosting the lattice oxygen activation induced by the hybridized Co-O-Ce bonds, finally lowering the adsorption and activation barriers for reactive species and accelerating the reaction kinetics.