NO Temperature-programmed Oxidation Research Articles

Low temperature oxidation of diesel soot particles over one-dimensional 2 × 3 tunnel-structured Na2Mn5O10 catalysts

Tunnel-structured manganese oxides have drawn widespread research attention due to their excellent oxidation properties for the soot oxidation reaction. In this study, a series of romanechite Na2Mn5O10 catalysts with a 2 × 3 tunnel structure were synthesized via a facile hydrothermal method. The as-synthesized catalysts were investigated using X-ray diffraction, scanning and transmission electron microscopy, H2 and soot temperature-programmed reduction, O2 temperature-programmed desorption, NO temperature-programmed oxidation, X-ray photoelectron spectroscopy, and other measurements. Among all the catalysts, the Na1MnyOδ-120 catalyst exhibited the best catalytic performance for soot combustion, with T10, T50, and T90 values of 281 ℃, 317 ℃, and 343 ℃, respectively, which were considerably reduced to 271 ℃, 310 ℃, and 335 ℃ in the existence of 10% H2O. The results of multiple characterization methods indicated that the Na2Mn5O10 catalysts exhibit good reducibility, strong oxygen adsorption and activation capacity, and NO to NO2 oxidation ability. Moreover, the active sites (Mn and oxygen vacancies) and reaction mechanism (Langmuir–Hinshelwood) were also revealed by in-situ DRIFT and density functional theory calculations results. This study details a new strategy for the design and synthesis of efficient soot oxidation catalysts with practical application prospects.

Sintering behavior of platinum and modification effect of ceria on silica-alumina supports for diesel soot oxidation

In this work, silica-alumina mixed oxides with different SiO2 contents (5% and 30%) were adopted as acidic supports for platinum catalysts for soot oxidation. The obtained catalysts were thermally aged in air at 550, 650 and 750 °C for 2 h, respectively. The catalysts were characterized by transmission electron microscopy (TEM), CO chemisorption, inductively coupled plasma (ICP), N2 adsorption/desorption, NH3 temperature-programmed desorption (TPD), H2 temperature-programmed reduction (TPR), infrared (IR) spectroscopy of CO and NH3 adsorption, and NO temperature-programmed oxidation (TPO). The acidic support Pt catalysts had higher catalytic activities for NO oxidation and soot oxidation. After aging at 750 ℃, severer sintering of Pt occurred on the acidic supports, and some Pt particles larger than 50 nm formed, resulting in serious deactivation. It was ascribed to metallic state of Pt on the acidic supports with weakened metal-support interactions (SMI). Thus, CeO2 was added to the silica-alumina support by ball milling to strengthen the SMI via forming Pt-O-Ce bonds. In this way, Pt nanoparticles were anchored to the modified support to inhibit sintering at high temperatures, and the obtained Pt/CeO2/SiO2-Al2O3 catalyst exhibited both high soot oxidation activity and superior thermal stability.

Alkali/alkaline-earth metal-modified MnOx supported on three-dimensionally ordered macroporous–mesoporous TixSi1-xO2 catalysts: Preparation and catalytic performance for soot combustion

The performance of catalysts used in after-treatment systems is the key factor for the removal of diesel soot, which is an important component of atmospheric fine particle emissions. Herein, three-dimensionally ordered macroporous–mesoporous TixSi1-xO2 (3DOM-m TixSi1-xO2) and its supported MnOx catalysts doped with different alkali/alkaline-earth metals (AMnOx/3DOM-m Ti0.7Si0.3O2 (A: Li, Na, K, Ru, Cs, Mg, Ca, Sr, Ba)) were prepared by mesoporous template (P123)-assisted colloidal crystal template (CCT) and incipient wetness impregnation methods, respectively. Physicochemical characterizations of the catalysts were performed using scanning electron microscopy, X-ray diffraction, N2 adsorption–desorption, H2 temperature-programmed reduction, O2 temperature-programmed desorption, NO temperature-programmed oxidation, and Raman spectroscopy techniques; then, we evaluated their catalytic performances for the removal of diesel soot particles. The results show that the 3DOM-m Ti0.7Si0.3O2 supports exhibited a well-defined 3DOM-m nanostructure, and AMnOx nanoparticles with 10–50 nm were evenly dispersed on the inner walls of the uniform macropores. In addition, the as-prepared catalysts exhibited good catalytic performance for soot combustion. Among the prepared catalysts, CsMnOx/3DOM-m Ti0.7Si0.3O2 had the highest catalytic activity for soot combustion, with T10, T50, and T90 (the temperatures corresponding to soot conversion rates of 10%, 50%, and 90%) values of 285, 355, and 393°C, respectively. The high catalytic activity of the CsMnOx/3DOM-m Ti0.7Si0.3O2 catalysts was attributed to their excellent low-temperature reducibility and homogeneous macroporous–mesoporous structure, as well as to the synergistic effects between Cs and Mn species and between CsMnOx and the Ti0.7Si0.3O2 support.

In situ fabrication of Au–CoFe2O4: an efficient catalyst for soot oxidation

Diesel soot particulate is one of the main sources of air pollution, which should be eliminated effectively to minimize the carcinogenic and lethal effect of air. Herein, an effort has been made to synthesize Au–CoFe2O4 nanocomposite by a facile route and use as an effective catalyst for soot oxidation. The physicochemical properties of Au–CoFe2O4 were carried out through SEM, EDX mapping, HRTEM, XRD, XPS, hydrogen temperature-programmed reduction (H2-TPR), and NO temperature-programmed oxidation (NO-TPO). Au–CoFe2O4 catalyst showed a remarkable catalytic property for soot oxidation in both NO + O2 and O2 owing to the formation of oxygenated species that enhanced the oxidizing properties of the catalyst. The CoFe2O4 composite showed a remarkable and stable catalytic property for soot oxidation, and low T50 (50% soot conversion) 411 °C value has been achieved in the presence of O2. While as we introduced gold (Au) to form Au–CoFe2O4 then its catalytic performance for soot oxidation was found to be tremendously increased to T50 341 °C owing to the formation of surface-oxygenated species that enhanced the oxidizing properties of the catalyst surface. The durability performance of the catalyst including reusability, this catalyst showed even better soot oxidation activity after sulfur and hydrothermal aging. Owing to the tremendous soot oxidation activity, this catalyst can be used as an environmentally friendly and efficient soot oxidizer for the removal of soot.

SmMn2O5 catalysts modified with silver for soot oxidation: Dispersion of silver and distortion of mullite

The mullite SmMn2O5 (SMO) was modified with silver for soot oxidation by two methods, i.e. sol-gel (Ag/SMO-sg) and impregnation (Ag/SMO-im). The catalysts were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectrometry (ICP-AES), H2-O2 titration, H2 temperature-programmed reduction (H2-TPR), CO temperature-programmed reduction (CO-TPR), soot temperature-programmed reduction (soot-TPR), O2 temperature-programmed desorption (O2-TPD), cycled H2 temperature-programmed reduction (cycled H2-TPR), NO temperature-programmed oxidation (NO-TPO) and soot temperature-programmed oxidation (soot-TPO). Metallic silver nanoparticles were observed in the mullite matrix for both the catalysts. For Ag/SMO-sg, the possible incorporation of silver into the mullite crystal cell and strong electronic interaction between the highly dispersed metal and the support increase the Mn4+/Mn3+ ratio and create more structural defects in the mullite, resulting in boosted NO oxidation and thereby NO2-assisted soot combustion. The introduction of Ag can promote the generation/regeneration of active oxygen, which accelerates the ignition of soot in O2 at low temperatures. Therefore, the sol-gel-synthesized Ag/SmMn2O5 is highly promising for application in diesel particulate filters.

Size effect of Pt nanoparticles in acid-assisted soot oxidation in the presence of NO

Pt/Al2O3 catalysts with mean Pt particle size ranged from 2.7 to 7.1 nm were synthesized by chemical reduction method, and the sulfated counterparts were prepared by impregnation of sulfuric acid. The turnover frequency of platinum for soot oxidation under loose contact conditions in a feed flow containing NO and O2 are positively correlated with the size of platinum. The sulfated Pt/Al2O3 exhibits higher catalytic activity for soot oxidation in the presence of NO despite their reduced ability for NO2 production. Such a contradiction is more significant for those catalysts with smaller platinum particles. Herein, the catalysts were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), inductive coupled plasma (ICP) emission spectrometry, CO chemisorption, thermogravimetric analysis (TGA), NH3 temperature-programmed desorption (NH3-TPD), NO temperature-programmed oxidation (TPO) and NOx temperature-programmed desorption (TPD). Possible effect of Pt particle size for the catalytic oxidation of soot in the presence of NO was presented based primarily on the promoted NO2 transfer efficiency onto the soot pushed by the acidic catalysts.

Cu-Mn-Ce mixed oxides catalysts for soot oxidation and their mechanistic chemistry

A series of flower-like CuxMnyCez mixed oxides catalysts for soot removal were prepared via a facile co-precipitation approach followed by calcination. The structural and surface properties of these catalysts were investigated by powder X-ray diffraction, N2 physisorption, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction, oxygen temperature-programmed desorption and NO temperature-programmed oxidation. Superior soot oxidation performance was obtained over these ternary mixed oxides catalysts and the corresponding mechanism was explored. In particular, the Cu1Mn1Ce1 catalyst showed a remarkable improvement in soot oxidation activity in both NO + O2 and O2 atmospheres. The reusability and durability of these catalysts were assessed, and these materials showed high soot oxidation activity after the hydrothermal stability test.

Effect of Biodiesel impurities (K, Na, P) on non-catalytic and catalytic activities of Diesel soot in model DPF regeneration conditions

The impact of Biodiesel impurities (Na, K and P) on the non-catalytic and catalytic reactivity of Diesel soot was evaluated under model DPF (Diesel Particulate Filter) regeneration conditions. Temperature-programmed oxidation (TPO) measurements confirmed that Na and K depositing into soot or on the surface of the catalyst enhanced the oxidative reactivity of soot under both O2 and NOx + O2 and Na-doped samples showed better results. However, the presence of P inhibited the non-catalytic and catalytic reactivity. These findings can be mainly attributed to the changes in nanostructure and surface chemical properties of the doped samples, characterized by Raman, high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR) and NO temperature-programmed oxidation (NO-TPO). The result of this characterization evidenced that the presence of Na and K increased structural defects of soot and reduction ability of the catalyst. Moreover, Na-/K-doped catalysts presented higher oxidizing ability of NO into NO2, whereas the opposite trend was observed for the P-containing catalysts. In addition, higher structural disorder of Na-doped soot and higher alkali metal content on the surface of Na-doped catalyst might lead to enhanced reactivity in comparison to K-doped soot and catalyst.

Promoting Effects of Barium Substitution on the Catalytic Performances of FeCeO2−δ for Soot Oxidation

In this work, the promoting effects of the introduction of barium to FeCeO2−δ on soot oxidation were thoroughly studied. A series of FeCeO2−δ, BaFeCeO2−δ, and BaCeO2−δ was prepared by a self-propagating high-temperature synthesis method. The physiochemical properties of catalysts were investigated using Brunauer–Emmett–Teller, X-ray diffraction, transmission electron microscopy, NO temperature-programmed oxidation (NO-TPD), and NOx-TPD analysis. Ba(10%)FeCe displays the best catalytic performance and possesses the lowest soot ignition temperature of 282 °C in the presence of NO. However, FeCeO2−δ was the most active one, showing the lowest soot ignition temperature of 342 °C in the absence of NO. NO-TPD demonstrates that barium loading in FeCeO2−δ dramatically improves the NOx adsorption and desorption capacity. In addition, in situ diffuse reflectance infrared Fourier transform spectroscopy indicates that NO can be adsorbed on catalysts’ surfaces to form nitrite and nitrate species and that the correspon...

Study on hydrothermal deactivation of Pt/MnO x -CeO2 for NO x -assisted soot oxidation: redox property, surface nitrates, and oxygen vacancies.

The study mainly focuses on surface properties to investigate the deactivation factors of Pt/MnO x -CeO2 by H2 temperature-programmed reduction, CO chemical adsorption, NO x -temperature-programmed desorption (TPD), O2-TPD, NO temperature-programmed oxidation, SEM, TEM, in situ diffuse reflectance infrared Fourier transform spectra, Raman, and thermogravimetric methods. The results show that there are three main factors to lead to hydrothermal deactivation of the catalyst: redox property, oxygen vacancy, and surface nitrates. The loss of oxygen vacancies decreases the generation and desorption of active oxygen and that of surface nitrates weakens the production of NO2 and surface peroxides (-O2-). These factors greatly result in the damage of the C-NO2-O2 cooperative reaction.

Effect of Y on improving the thermal stability of MnOx-CeO2 catalysts for diesel soot oxidation

A series of MnOx-CeO2-Y2O3 catalysts with different Y loadings (0, 1, 3, 6, and 10 wt%) were prepared by a co-precipitation method and investigated for NOx-assisted soot oxidation. The thermal stabilities of these catalysts were evaluated by treating them at 800 °C for 12 h under dry air flow. The catalysts were characterized by X-ray diffraction, N2 adsorption-desorption, Raman spectroscopy, H2 temperature-programmed reduction, oxygen storage capacity, NO temperature-programmed oxidation, X-ray photoelectron spectroscopy, and soot temperature-programmed oxidation. The addition of Y led to decreased BET surface areas and poor low-temperature reduction abilities and oxygen storage capacities, which affected NO and soot oxidation activities. However, after aging, the doping of Y had effectively enhanced the stability of the catalytic activities for NO and soot oxidations, where the addition of 6%–10% Y achieved the optimum result because the maximal soot oxidation rate temperature was increased by only 34–35 °C. Additionally, the catalytic activity and deactivation of MnOx-CeO2-containing catalysts were closely related to the presence of Mn4+ and oxygen species on the surface.

Formation of BaMnO3 in Ba/MnOx–CeO2 catalyst upon the hydrothermal ageing and its effects on oxide sintering and soot oxidation activity

MnOx–CeO2 mixed oxides were synthesized using a sol–gel method and the barium-loaded catalyst was prepared by a wet impregnation. Both catalysts were hydrothermally aged at 800°C for 10h. The fresh and aged catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), high-resolution transmission electron microscopy (HRTEM), H2 temperature-programmed reduction (H2-TPR), NO temperature-programmed oxidation (NO-TPO) and soot temperature-programmed oxidation (soot-TPO). Compared with MnOx–CeO2, the Ba-modified mixed oxides show a much higher hydrothermal stability. Barium exists in the form of orthorhombic carbonate and cubic nitrate on the fresh sample. BaMnO3 nanoparticles are identified to form at the interface of CeO2 crystallites upon the hydrothermal ageing. The formation of the perovskites prohibits the sintering of ceria significantly, and consequently maintains the redox property of the aged mixed oxides to a great degree. Additionally, the nanoscaled perovskites themselves also contribute to the catalytic activity of the aged oxides catalyst.

MnOx–CeO2 supported on a three-dimensional and networked SBA-15 monolith for NOx-assisted soot combustion

This paper reports the preparation and characterization of MnOx–CeO2/SBA-15 monolith (MnCe/SM) catalysts for NOx-assisted soot combustion. The SM with a three-dimensional (3D) network structure was synthesized by a sol–gel method, in which the shearing force and the acidity of the solution were finely tuned to direct the formation and assembly of the primary particles. The MnCe/SM catalysts were further prepared by a facile isovolumetric impregnation method. The samples were characterized by nitrogen adsorption, scanning electron microscopy, energy dispersed spectroscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, O2 temperature-programmed desorption, NO temperature-programmed desorption, and NO temperature-programmed oxidation. It is found that the MnOx–CeO2 nanoparticles were dispersed in the channels or/and on the outer surface of the SM support depending on the loading, and had a strong synergistic effect. The MnCe/SM catalysts with an appropriate MnOx–CeO2 loading showed much higher catalytic performance for soot combustion than that of the unsupported MnOx–CeO2 mixture. This is attributed to the combination of the MnOx–CeO2 active component with the 3D network structure of SM. The later not only provides large surface area and high accessibility for the soot particulates to the active sites, but also acts as a particulate filter.

Synergistic effect between MnO and CeO2 in the physical mixture: Electronic interaction and NO oxidation activity

MnO and CeO2 powders were mechanically mixed by a spatula and by milling to obtain loose-contact and tight-contact mixed oxides, respectively. The monoxides and their physical mixtures were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), Raman, O2 temperature-programmed desorption (O2-TPD), H2 temperature-programmed reduction (H2-TPR) and NO temperature-programmed oxidation (NO-TPO). The MnOx-CeO2 solid solutions did not form without any calcination process. The oxidation state of manganese tended to increase while the ionic valence of cerium decreased in the mixed oxides, accompanied with the formation of oxygen vacancies. This long-ranged electronic interaction occured more significantly in the tight-contact mixture of MnO and CeO2. The formation of more Mn4+ and oxygen vacancies promoted the catalytic oxidation of NO in an oxygen-rich atmosphere.

Preparation of MnOx–CeO2–Al2O3 mixed oxides for NOx-assisted soot oxidation: Activity, structure and thermal stability

Three kinds of MnOx–CeO2–Al2O3 mixed oxides were prepared by introducing Al2O3 to MnOx–CeO2 at different stages of a sol–gel method. MnCeAl and MnCe/Al were received by adding aluminum nitrate and alumina powders to the Mn–Ce precursor solution, respectively, while MnCe+Al was obtained by mixing the sol–gel-synthesized MnOx–CeO2 and Al2O3 powders mechanically. The catalysts were calcined in air at 800°C for 20h to obtain the thermally aged samples. The catalysts were characterized by soot and NO temperature-programmed oxidation (soot-TPO and NO-TPO), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), extended X-ray absorption fine structure (EXAFS), energy dispersive spectroscopy (EDS) and H2 temperature-programmed reduction (TPR). The introduction of Al2O3 degrades the soot oxidation activity of the fresh catalysts compared with MnOx–CeO2 mixed oxides, but it helps to maintain relatively high catalyst surface area and high MnOx dispersion after the rigorous aging treatment. Among the catalysts prepared, MnCe/Al exhibits the highest thermal stability with a small increase of Tm by 17°C for soot oxidation after aging. The active surface area, which refers to the exposure area of MnOx on the catalyst, is suggested to determine the NO oxidation and soot oxidation activities of the aged catalysts.

Combined promoting effects of platinum and MnOx–CeO2 supported on alumina on NOx-assisted soot oxidation: Thermal stability and sulfur resistance

MnOx–CeO2 mixed oxides were introduced into Pt/Al2O3 catalyst to reduce the amount of platinum and improve the catalytic activity for soot oxidation in presence of NOx. The catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM), CO chemisorption, thermogravimetric (TG) analysis, Fourier transform infrared spectroscopy (FTIR), H2 temperature-programmed reduction (H2-TPR), NO temperature-programmed oxidation (NO-TPO) and soot temperature-programmed oxidation (soot-TPO). Due to the superior redox property, the introduction of MnOx–CeO2 mixed oxides effectively enhanced the catalytic activities of Pt/Al2O3 catalyst for NO oxidation and hereby for soot oxidation in NO+O2. The Pt/MnOx–CeO2/Al2O3 catalyst also exhibited high activities after the thermal ageing and sulfur poisoning, which are related to the intrinsic characteristics of MnOx–CeO2/Al2O3 and Pt as well as their synergistic effects. The deactivation mechanisms of different catalysts (Pt/Al2O3,MnOx–CeO2/Al2O3 and Pt/MnOx–CeO2/Al2O3) were discussed.

NOx-Assisted Soot Oxidation on Pt–Mg/Al2O3 Catalysts: Magnesium Precursor, Pt Particle Size, and Pt–Mg Interaction

Pt–Mg/Al2O3 soot oxidation catalysts were prepared by impregnating either magnesium acetate or magnesium nitrate on alumina-supported platinum catalyst. The influence of Mg addition on the structure and catalytic behaviors of Pt/Al2O3 catalysts were investigated by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, transmission electron microscopy (TEM), H2 chemisorption, thermogravimetric (TG) analysis, Fourier transform infrared spectroscopy (FTIR), NOx temperature-programmed desorption (NOx-TPD), NO temperature-programmed oxidation (NO-TPO), and soot temperature-programmed oxidation (soot-TPO). In spite of the coverage of surface Pt by magnesium species and the weakened oxidation resistance of Pt, the Pt–Mg/Al2O3 catalyst derived from magnesium acetate exhibits a higher soot oxidation activity than that prepared with magnesium nitrate, which is mainly determined by the larger Pt particle size on this catalyst. Additionally, the synergistic effect between Pt and Mg enhances the NO oxidation activity and NOx storage capacity of Pt–Mg/Al2O3 catalyst. More NO2 is produced in the temperature range of soot oxidation on this catalyst than on the Mg-free Pt/Al2O3 catalyst with a similar Pt particle size, which efficiently promotes the ignition of soot.

Preparation methods and thermal stability of Ba-Mn-Ce oxide catalyst for NO x-assisted soot oxidation

Manganese oxide-loaded and -doped ceria as well as the corresponding barium-modified oxide catalysts were prepared for soot oxidation in the presence of NO x, and were characterized by using X-ray diffraction, Brunauer-Emmett-Teller and NO temperature-programmed oxidation measurements. The activity of catalyst depended strongly on the NO 2 production capacity, and the importance of surface nitrates was weakened without heat transfer limitations. The formation of perovskite-type oxides after the high-temperature calcination caused the loss of NO x storage capacity for the Ba-modified catalysts, but did not seem to aifect the NO oxidation activity obviously. The addition of barium did not prevent the phase separation of MnO x-CeO 2 solid solutions, whereas it inhibited the sintering of oxide crystallites effectively. This, as well as the relatively high surface area, resulted in a small increase in soot oxidation temperature for the thermally aged Ba/Mn-Ce catalyst.

Effects of tungsten oxide on soot oxidation activity and sulfur poisoning resistance of Pt/Al2O3 catalyst

Tungsten oxide was impregnated into the alumina-supported platinum catalyst in order to improve the soot oxidation activity and the resistance to sulfur dioxide. The catalysts were characterized by X-ray diffraction (XRD), Raman, UV-vis spectroscopy, Brunauer–Emmett–Teller (BET), Fourier transform infrared (FT-IR) spectroscopy, temperature-programmed desulfation, NOx temperature-programmed desorption (NOx-TPD), NO temperature-programmed oxidation (NO-TPO) and soot temperature-programmed oxidation (soot-TPO). The deposition of WOx was found to reduce the availability of Pt active sites on the fresh catalyst. This, as well as the acidic property of tungsten oxide, decreases the NO oxidation and the NOx adsorption abilities. However, a higher soot oxidation activity was achieved on the WOx-modified catalyst in the presence of NO and O2, which is associated with the presence of more platinum in the metallic state by interacting with the electronegative tungsten oxide. The acidity of tungsten oxide and the improved oxidation-resistance of platinum may be critical to the NO ↔ NO2 recycling efficiency and decomposition of surface oxygen complexes. After the sulfur poisoning treatment, fewer sulfates are formed on the WOx-modified catalyst and decompose at lower temperatures. The IR spectra of CO adsorption indicate that less platinum active sites on this catalyst are affected by sulfates and a higher level of Pt dispersion is obtained, which is responsible for the high NO oxidation and soot oxidation activities.

Effects of adsorbed and gaseous NOx species on catalytic oxidation of diesel soot with MnOx–CeO2 mixed oxides

MnOx–CeO2, MnOx and CeO2 catalysts were synthesized by the sol–gel method. The structural, redox and adsorption properties of the individual and mixed oxides were investigated by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), CO temperature-programmed reduction (TPR), NO temperature-programmed oxidation (TPO), NOx temperature-programmed desorption (TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The TPO tests were performed with the soot–catalyst mixture under loose contact conditions to evaluate the catalytic activity of the oxide catalysts for soot oxidation. The MnOx–CeO2 mixed oxide catalyst presents the lowest soot oxidation temperature among the catalysts investigated in the presence of NO and O2. The synergetic effect between manganese oxide and ceria restrains the growth of oxide crystallites, increases the specific surface area and improves the low-temperature redox property. Especially, the activity of MnOx–CeO2 mixed oxides for NO oxidation and its capacity for NO2 storage in the form of surface nitrates are greatly enhanced. Not only the NO2 released from decomposition of surface nitrates but also that formed by catalytic oxidation of gaseous or adsorbed NO on the catalyst is confirmed important for soot oxidation. It is found by the in situ DRIFTS tests with the soot–catalyst mixture that the generation of surface oxygen complexes (SOCs), such as carboxylic anhydrides, lactones, quinine, ceto-enol groups, ethers and phenols, occurs at about 100°C lower temperature with exposure to NO than in the absence of NO, which is an important step for soot oxidation.