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.

Preparation of Cordierite Monolith Catalysts with the Coating of K-Modified Spinel MnCo2O4 Oxide and Their Catalytic Performances for Soot Combustion

Diesel engines are important for heavy-duty vehicles. However, particulate matter (PM) released from diesel exhaust should be eliminated. Nowadays, catalytic diesel particulate filters (CDPF) are recognized as a promising technology. In this work, a series of monolith Mn1−nKnCo2O4 catalysts were prepared by the simple citric acid method. The as-prepared catalysts displayed good catalytic performance for soot combustion and the Mn0.7K0.3Co2O4 catalyst gave the best catalytic performance among all the prepared samples. The T10 and Tm of Mn0.7K0.3Co2O4-HC catalyst for soot combustion are 310 and 439 °C, respectively. The physical and chemical properties of catalysts were characterized by means of SEM, XPS, H2-TPR, Raman and other techniques. The characterization results indicate that K substitution is favorable for the formation of oxygen vacancies, enhancing the mobility of active oxygen species, and improving the redox properties and so on. In-situ Raman results prove that the strength of Co-O bonds in the catalysts became weak during the reaction at high temperatures. In addition, SEM and ultrasonic test results show that the peeling rate of the coat-layer is less than 5%. The as-prepared catalysts can be taken as one kind of candidate catalyst for promising application in soot combustion because of its facile synthesis, low cost and high catalytic activity.

The decisive factor of hollow spherical network morphology of Nd1-xCexCo1-yCuyO3±δ perovskites towards soot oxidation

The perovskites Nd1-xCexCo1-yCuyO3 (x = 0–0.05, y = 0–0.1) have been synthesized using PVP-assisted sol–gel method and applied for soot oxidation reactions. XRD technique reveals the formation of orthorhombic phase with crystal volume of around ~ 214 Å3 and crystal size of ~ 25–40 nm. The interconnected nanoparticles with hollow spherical network morphology of particles are observed for the samples NdCoO3 (NC1) and Nd0.98Ce0.02Co0.95Cu0.05O3 (NC2) with particle sizes of around 300–500 nm. The samples experienced a charge transfer from ligand (O2−) to cobalt cation in UV region (210–260 nm) and also observed broad absorption bands in the visible region (380–600 nm). In addition, the bandgap energy of NC2 showed the lowest value (4.21 eV); as well as surface morphological advantage promoted the transport of surface-chemisorbed oxygen species in the inner and outer surface of catalysts surface due to the reducibility of the catalyst with the soot $$\left( {\frac{{{\text{O}}_{2}^{{{\text{x}} - }} }}{{{\text{O}}_{2}^{{{\text{x}} - }} + {\text{O}}^{2 - } }} = 0.90} \right)$$ O 2 x - O 2 x - + O 2 - = 0.90 . Furthermore, XPS results evidenced the higher content of Co2+ cation upon substitution of Ce/Cu into NC1, which successively formed more amount of Oβ-oxygen species. Remarkably, the perovskite NC2 showed the lowest soot oxidation temperature (T50% = 434 °C) among the investigated perovskites. Besides, the spherically networked morphology of NC2/NC1 samples also decided the soot oxidation process.

Niobium oxide promoted with alkali metal nitrates for soot particulate combustion: elucidating the vital role of active surface nitrate groups

With the target of developing efficient base metal oxide catalysts for soot particulate combustion, Nb₂O₅ catalysts promoted using different alkali metal nitrates have been prepared via an impregnation method. The activity of all the modified catalysts is better than that of the pure Nb₂O₅, and follows the sequence of CsNb1-9 > KNb1-9 > NaNb1-9 > LiNb1-9 > Nb₂O₅. It has been discovered that the original LiNO₃ and NaNO₃ precursors were decomposed into inert Li₂O and Na₂O on LiNb1-9 and NaNb1-9 during the calcination process. However, the KNO₃ and CsNO₃ precursors were intact on KNb1-9 and CsNb1-9 due to the strong stabilization effect of the K⁺ and Cs⁺ cations. As confirmed using different means, surface nitrates are the predominant active centers that contribute to the soot oxidation activity, through the redox cycles between nitrate (NO₃^-) and nitrite (NO₂^-) groups. Due to the existence of a large quantity of active surface NO₃^- groups, KNb1-9 and CsNb1-9 thus exhibit a much better reaction performance than LiNb1-9 and NaNb1-9.

Boosting Catalytic Purification of Soot Particles over Double Perovskite-Type La2-xKxNiCoO6 Catalysts with an Ordered Macroporous Structure

The catalytic performances for soot purification over the perovskite-type ABO3 oxides, as one of the most potential non-noble metal catalysts, are closely correlated with the substitution of A-site and B-site ions. Herein, three-dimensional ordered macroporous (3DOM) structural catalysts of double perovskite-type La2-xKxNiCoO6 were prepared by a method of colloidal crystal template. The contact efficiency between the catalyst and soot particles is significantly promoted by the 3DOM structure, and the partial substitution of A-site (La) with low-valence potassium (K) ions in La2-xKxNiCoO6 catalysts boosts the increasing surface density of coordinatively unsaturated active B-sites (Co and Ni) and active oxygen. 3DOM La2-xKxNiCoO6 catalysts exhibited superior performance during the purification of soot particles, and the 3DOM La1.80K0.20NiCoO6 catalyst exhibited the highest activity, that is, the values of T50, SCO2, and turnover frequency are 346 °C, 99.3%, and 0.204 h-1 (at 300 °C), respectively. According to the results of multiple experimental characterizations and density functional theory calculations, the mechanism of the samples during soot removal is proposed: the increase in surface oxygen density induced by the doping of K ions significantly promotes the critical step of the oxidation from NO to NO2 in catalyzing soot purification. It is one new strategy to develop the high-efficient non-noble metal catalysts for soot purification in practical 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.

Potassium promoted macro-mesoporous Co3O4-La0.88Sr0.12CoO3-δ nanotubes with large surface area: A high-performance catalyst for soot removal

The construction of porous perovskite nanotubular materials with a good intrinsic activity, as well as a greater dispersion of the active sites is an effective strategy to obtain a high-performance catalyst used in soot removal. Thence, macro-mesoporous Co₃O₄-La_0.88Sr_0.12CoO_3-δ nanotubes with large specific surface area (154.4 m²·g^-1) from the acid etching of the porous La_0.6Sr_0.4CoO_3-δ nanotubes, are supported by 5% K through bubbling method following calcination for soot combustion. The relationship between the specific surface area and K dispersion and their effect on the activity are studied by a series of isothermal kinetic measurements combined with the characterizations and activity evaluation results. It can be found that the greater the amount is of K⁺ incorporated into perovskite lattice, the better the dispersion of K, as well as the La₂O₂CO₃ formed on the catalyst surface, thus leading to the enhanced performance in the soot catalytic combustion. As a result, the 5% K supported macro-mesoporous Co₃O₄-La_0.88Sr_0.12CoO_3-δ nanotubes after acid etching show good activity and stability, where the T₅₀ is 338 °C (5% O₂ + 500 ppm NO + 6% H₂O) with a good CO₂ selectivity (above 99%), the activate energy is 78.1 kJ·mol^-1, and the turnover frequency is 5.14 × 10^-4 s^-1.

SO2-Tolerant Catalytic Removal of Soot Particles over 3D Ordered Macroporous Al2O3-Supported Binary Pt-Co Oxide Catalysts

The catalytic purification of soot particles is dependent on the SO₂ tolerance and activity of the catalysts in practical application. Herein, we have elaborately fabricated the nanocatalysts of three-dimensionally ordered macroporous (3DOM) Al₂O₃-supported binary Pt-cobalt oxide nanoparticles (NPs) using the method of gas bubbling-assisted membrane precipitation (GBMP), abbreviated as Pt-CoO_x/3DOM-Al₂O₃. Three-dimensionally ordered macroporous Al₂O₃ support can not only improve the contact performance between the soot and active sites but also possess surface acidity to improve the SO₂ tolerance. Supported binary Pt-CoO_x NPs over 3DOM-Al₂O₃ have high-efficient properties for activating NO and O₂. The Pt-CoO_x/3DOM-Al₂O₃ catalyst exhibits super catalytic performance and SO₂ tolerance during the removal of soot particles, whose values of turnover frequency (TOF) and T₅₀ are 0.29 h^-1 and 368 °C, respectively. The catalytic and SO₂-tolerant mechanisms of the Pt-CoO_x/3DOM-Al₂O₃ catalyst for soot purification are systematically studied by in situ diffuse reflectance infrared Fourier transform (DRIFT) spectra. The synergistic effect of binary Pt-CoO_x NPs plays a vital role in the oxidation of NO to NO₂ as a key step during catalytic soot removal, and the surface acidity of 3DOM-Al₂O₃ can not only inhibit the adsorption of SO₂ but also enhance the decomposition of surface hydrosulfate species. This work provides a novel strategy to the development of high-efficient catalysts for SO₂-tolerant catalytic removal of soot particles in both fundamental research and practical applications.

Boosting the Removal of Diesel Soot Particles by the Optimal Exposed Crystal Facet of CeO2 in Au/CeO2 Catalysts

Optimized surface facet of the catalysts is an efficient strategy to boost catalytic purification of diesel soot as important components of atmospheric fine particles. Herein, we have elaborately constructed the nanocatalysts of Au nanoparticles supported on the well-defined CeO₂ (rod, cube, and polyhedron) with predominantly exposed facets of {110}, {100}, and {111}, respectively. The strong interaction between Au and CeO₂ with the optimal crystal facet is crucial to adjust the active site density for activated O₂, and the synergy effect of Au and the CeO₂{110} facet possesses the largest density of active sites compared with other crystal facets of {100} and {111}. The catalytic activity for soot combustion was tuned by exposed crystal facets of CeO₂. The Au/CeO₂-rod catalyst exhibits the highest catalytic activity (T₅₀ = 350 °C, TOF = 0.18 h^-1) and the lowest apparent activation energy (72 kJ mol^-1) during soot combustion. Based on the results of in situ Raman spectra, the formation and stability of oxygen vacancy located at the interface of the Au-O-Ce bond, boosting the key step of NO oxidation to NO₂, are dependent on the exposed crystal facets of CeO₂. It highlights a new strategy for the fabrication of high-efficient CeO₂-based catalysts for the removal of soot particles or other pollution.

Construction of a hollow structure in La0.9K0.1CoO3−δ nanofibers via grain size control by Sr substitution with an enhanced catalytic performance for soot removal

The hollow structure is formed by Sr²⁺ doping in La_0.9K_0.1CoO_3−δ nanofibers for decreasing the grain size, which can improve the contact efficiency of soot–catalyst–gas as well as the intrinsic activity, responsible for the enhancement in activity.

Facile synthesis of three-dimensional ordered macroporous Sr1−xKxTiO3 perovskites with enhanced catalytic activity for soot combustion

The appropriate incorporation of potassium into 3DOM SrTiO₃ perovskites effectively improved the catalytic performance for soot combustion.

Catalysts of self-assembled Pt@CeO2-δ-rich core-shell nanoparticles on 3D ordered macroporous Ce1-xZrxO2 for soot oxidation: nanostructure-dependent catalytic activity

The catalytic performance in heterogeneous catalytic reactions consisting of solid reactants is strongly dependent on the nanostructure of the catalysts. Metal-oxides core-shell (MOCS) nanostructures have potential to enhance the catalytic activity for soot oxidation reactions as a result of optimizing the density of active sites located at the metal-oxide interface. Here, we report a facile strategy for fabricating nanocatalysts with self-assembled Pt@CeO_2-δ-rich core-shell nanoparticles (NPs) supported on three-dimensionally ordered macroporous (3DOM) Ce_1-xZr_xO₂via the in situ colloidal crystal template (CCT) method. The nanostructure-dependent activity of the catalysts for soot oxidation were investigated by means of SEM, TEM, H₂-TPR, XPS, O₂-isothermal chemisorption, soot-TPO and so on. A CeO_2-δ-rich shell on a Pt core is preferentially separated from Ce_1-xZr_xO₂ precursors and could self-assemble to form MOCS nanostructures. 3DOM structures can enhance the contact efficiency between catalysts and solid reactants (soot). Pt@CeO_2-δ-rich core-shell nanostructures can optimize the density of oxygen vacancies (O_v) as active sites located at the interface of Pt-Ce_1-xZr_xO₂. Remarkably, 3DOM Pt@CeO_2-δ-rich/Ce_1-xZr_xO₂ catalysts show super catalytic performance and strongly nanostructure-dependent activity for soot oxidation in the absence of NO and NO₂. For example, the T₅₀ of the 3DOM Pt@CeO_2-δ-rich/Ce_0.8Zr_0.2O₂ catalyst is lowered down to 408 °C, and the reaction rate of the 3DOM Pt@CeO_2-δ-rich/Ce_0.2Zr_0.8O₂ catalyst (0.12 μmol g^-1 s^-1) at 300 °C is 4 times that of the 3DOM Pt/Ce_0.2Zr_0.8O₂ catalyst (0.03 μmol g^-1 s^-1). The structures of 3DOM Ce_1-xZr_xO₂-supported Pt@CeO_2-δ-rich core-shell NPs are decent systems for deep oxidation of solid reactants or macromolecules, and this facile technique for synthesizing catalysts has potential to be applied to other element compositions.

Constructing a three-dimensionally ordered macroporous LaCrOδ composite oxide via cerium substitution for enhanced soot abatement

The addition of Ce into Cr-based perovskite restrained the growth of the crystal size and delayed the transformation from LaCrO₄ to LaCrO₃, and thus, the 3DOM structure was maintained even after calcination at 800 °C.

Design structure for CePr mixed oxide catalysts in soot combustion

A “molten state” appeared during heat treatment of CePr synthesized by a solid-phase grinding method with nitrates as precursors, which rapidly dispersed Ce and Pr. Therefore, CePr catalysts with different structures can be prepared, with catalytic activities independent of grinding time.

The existing states of potassium species in K-doped Co3O4 catalysts and their influence on the activities for NO and soot oxidation

The fresh and washed K-doped Co₃O₄ catalysts were compared with pure Co₃O₄ in order to investigate the existing states of K species and their influence on the activities for NO and soot oxidation.

Boosting soot combustion efficiencies over CuO–CeO2 catalysts with a 3DOM structure

A CuO–CeO₂ catalyst with a well-defined 3DOM structure exhibited superior catalytic activity for soot combustion compared to its 3DOM CeO₂ counterpart.