Mechanisms Of Soot Oxidation Research Articles

High performance of PrMnO3 perovskite catalysts for low-temperature soot oxidation

In the field of heterogeneous catalysis for diesel vehicle exhaust, designing low-cost catalysts with high efficiency and low-temperature catalytic oxidation to reduce the ignition temperature of soot remains a significant challenge. In this study, a series of praseodymium manganese perovskites (PrMnO3) were synthesized using a solution combustion method that regulated the molar ratio of glycine to nitrate (φ). Notably, when φ = 1, the prepared catalyst (PMO-1) exhibited a multistage pore structure and large specific surface area, demonstrating excellent catalytic oxidation performance for soot (T10 = 280 °C, T50 = 377 °C, TOF=16.2 × 10-4 s−1, SCO₂ > 99 %) and NO (maximum NO conversion rate of 80 %). During soot combustion in a NO+O2 reaction atmosphere, the strong NO oxidation capacity of the catalyst provides sufficient NO2 as an oxidant for soot combustion, thereby further enhancing its oxidation performance (T50 = 360 °C). This study concludes that the excellent low-temperature catalytic oxidation performance of the PrMnO3 catalyst is attributed to the large specific surface area provided by its multistage pore structure and the extensive dispersion of active sites, which enhance the contact efficiency between soot particles and active sites. Additionally, based on multiple characterization results such as XPS, TOF calculations, and Soot-TPR, the PMO-1 catalyst exhibits a high content of lattice oxygen and efficient lattice oxygen migration. These properties further supply active oxygen for the NOx-assisted soot oxidation mechanism. This study offers a valuable, simple, and cost-effective strategy for designing perovskite catalysts and efficiently removing PM particles.

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.

Synergistic Effect of Pt and Dual Ni/Co Cations in Hydrotalcite-Derived Pt/Ni1.5Co0.5AlO Catalysts for Promoting Soot Combustion

In this article, the catalysts of hydrotalcite-derived Ni1.5Co0.5AlO nanosheet-supported highly dispersed Pt nanoparticles (Ptn/Ni1.5Co0.5AlO, where n% is the weigh percentage of the Pt element in the catalysts) were elaborately fabricated by the gas-bubble-assisted membrane--reduction method. The specific porous structure formed by the stack of hydrotalcite-derived Ni1.5Co0.5AlO nanosheets can increase the transfer mass efficiency of the reactants (O2, NO, and soot) and the strong Pt-Ni1.5Co0.5AlO interaction can weaken the Ni/Co-O bond for promoting the mobility of lattice oxygen and the formation of surface-oxygen vacancies. The Ptn/Ni1.5Co0.5AlO catalysts exhibited excellent catalytic activity and stability during diesel soot combustion under the loose contact mode between soot particles and catalysts. Among all the catalysts, the Pt2/Ni1.5Co0.5AlO catalyst showed the highest catalytic activities for soot combustion (T50 = 350 °C, TOF = 6.63 × 10-3 s-1). Based on the characterization results, the catalytic mechanism for soot combustion is proposed: the synergistic effect of Pt and dual Ni/Co cations in the Pt/Ni1.5Co0.5AlO catalysts can promote the vital step of catalyzing NO oxidation to NO2 in the NO-assisted soot oxidation mechanism. This insight into the synergistic effect of Pt and dual Ni/Co cations for soot combustion provides new strategies for reducing the amounts of noble metals in high-efficient catalysts.

Effects analysis on soot oxidation performance in the diesel particulate filter based on synergetic passive-active composite regeneration methods

Diesel particulate filters (DPFs) are standard components to control particulate emissions from diesel engine. Different DPF regeneration mechanisms, including passive regeneration and active regeneration, affect the performance of itself and downstream post-processing equipment. In order to study soot oxidation mechanism in DPF, DPF passive-active coupled regeneration model is established, and its kinetic parameters are verified. Active-passive regeneration balance point temperature (APR-BPT), passive regeneration control upper bound temperature (PR-UBT) and active regeneration control lower bound temperature (AR-LBT) are defined to study-three control regimes affected by two soot oxidation mechanisms. Additionally, the effect analysis of key parameters on APR-BPT, PR-UBT and AR-LBT is conducted to reveal the regeneration regime and the regeneration temperature boundary. The research on the soot combustion mechanisms under the synergetic passive-active composite regeneration of the DPF will provide a theoretical basis for DPF regeneration and its control.

Soot Oxidation in a Plasma-Catalytic Reactor: A Case Study of Zeolite-Supported Vanadium Catalysts

The plasma-catalytic oxidation of soot was studied over zeolite-supported vanadium catalysts, while four types of zeolites (MCM-41, mordenite, USY and 5A) were used as catalyst supports. The soot oxidation rate followed the order of V/MCM-41 > V/mordenite > V/USY > V/5A, while 100% soot oxidation was achieved at 54th min of reaction over V/MCM-41 and V/mordenite. The CO2 selectivity of the process follows the opposite order of oxidation rate over the V/M catalyst. A wide range of catalyst characterizations including N2 adsorption–desorption, XRD, XPS, H2-TPR and O2-TPD were performed to obtain insights regarding the reaction mechanisms of soot oxidation in plasma-catalytic systems. The redox properties were recognized to be crucial for the soot oxidation process. The effects of discharge power, gas flow rate and reaction temperature on soot oxidation were also investigated. The results showed that higher discharge power, higher gas flow rate and lower reaction temperature were beneficial for soot oxidation rate. However, these factors would impose a negative effect on CO2 selectivity. The proposed “plasma-catalysis” method possessed the unique advantages of quick response, mild operation conditions and system compactness. The method could be potentially applied for the regeneration of diesel particulate filters (DPF) at low temperatures and contribute to the the emission control of diesel engines.

Isotopic study of the influence of oxygen interaction and surface species over different catalysts on the soot removal mechanism

In order to improve the catalytic formulations for soot removal in after-treatment emission control technologies for gasoline and diesel engine vehicle, an isotopic study was approached using transitory labeled oxygen response method over model catalysts that allows the unraveling of soot oxidation mechanism. Ce-based materials promote oxygen exchange associated with the high population of lattice oxygen species (O2-) denoted as OI type. The incorporation of praseodymium produces a Pr3+ enrichment that decrease the energy for oxygen release and increase oxygen mobility through surface and subsurface oxygen centers (OII type) depending on the synthesis procedure. For PtBaK catalyst, OIII species are responsible for oxygen exchange. Gas-solid reaction between soot and gas phase molecular oxygen is responsible for direct uncatalyzed soot oxidation. For ceria containing catalysts, low-temperature soot removal takes place through the intervention of lattice atomic species and superoxide species. For DPNR model catalyst, PtBaK/Al2O3, the soot elimination occurs with the intervention of OIII type centers. In the presence NO, the assisted and cooperative mechanism due to NO2 and the intervention of the adsorbed nitrate species on the trimetallic catalyst enhances soot removal capacity.

Synthesis of YFeO<sub>3-δ</sub> and LaFeO<sub>3-δ</sub> Perovskites with High Catalytic Activity in Carbon Oxidation Reactions

YFeO3-δ (δ = 0.26) and LaFeO3-δ (δ = 0.5) perovskites with a high specific surface and oxygen non-stoichiometry was firstly synthesized by pyrolysis of polymer-salt compositions. It was shown that the catalytic oxidation of carbon in the presence of these complex oxide systems proceeds in the range of 400 - 700 °С, with a maximum temperature at 556 °С for YFeO3-δ; and 380 - 620 °С ,with a maximum temperature at 501 °С for LaFeO3-δ, in one-stage mode for both. By means of thermal analysis and diffractometry, it was shown that there is no contribution to the soot oxidation mechanism by cyclic perovskite surface transformations, due to the reduction of metal oxides by the soot and their subsequent reoxidation. It has been established that the basis of the catalytic reaction mechanism for both perovskites is the presence of oxygen vacancies on the surface of complex oxides.

Heterogeneous mechanism of NOx-assisted soot oxidation in the passive regeneration of a bench-scale diesel particulate filter catalyzed with nanostructured equimolar ceria-praseodymia

An experimental study of passive regeneration of bench-scale diesel particulate filters (DPF) catalyzed by morphologically varied ceria and ceria-praseodymia (equimolar mixture) washcoats was carried out. The microstructured washcoat was prepared in the DPF via in situ solution combustion synthesis, whereas the nanostructured counterpart was deposited in the DPF via wetness impregnation with its suspension. Soot model was loaded via impregnation until the weight ratio of soot to catalytic washcoat was 1–20. The experiments revealed that after 4 h of isothermal regeneration at 400 °C, in the presence of 10% O2 and 1000 ppm NO, the reaction over nanostructured ceria-praseodymia washcoat resulted in the highest soot conversion (82%). Moreover, this catalyst achieved the highest NO2 exploitation efficiency in soot conversion, estimated as the ratio of the converted soot C atoms per mole of consumed NO2 (formed by NO oxidation), resulting in a C-to-NO2 ratio of 3.43. The catalyst reducibility analysis via Soot- and NO-TPR showed that nanostructured ceria-praseodymia had the highest catalytic activity toward soot oxidation in the absence of bulk-phase oxygen (i.e. via Mars-van Krevelen mechanism), as well as the ability to adsorb NOx at lower temperatures. Additional experiments with NO2-assisted soot oxidation were conducted to characterize the participation of NO2 in the catalytic reaction. They revealed that, in the absence of bulk-phase oxygen, the oxidation by NO2 over nanostructured ceria-praseodymia was more intense at low temperatures than that without catalyst. It was concluded that the participation of NO2 during the passive DPF regeneration was not only in the bulk phase, where NO2 directly oxidizes soot, but also on the catalyst surface, where adsorbed NO2 species and possibly reactive oxygen species, generated through an interaction between NO2 and the surface, attack soot at soot–catalyst interface, i.e. via a heterogeneous mechanism. As a matter of fact, the heterogeneous pathway of NO2-assisted soot oxidation was not observed to the same extent for the microstructured ceria-praseodymia catalyst, leading to the observation of different structure-sensitive mechanisms of soot oxidation by NO2. Finally, in the upscaled reaction system, nanostructured ceria-praseodymia remained the most performing catalyst despite the shorter residence time and the weaker soot–catalyst contact.

Promoting effect of H2O over macroporous Ce-Zr catalysts in soot oxidation

Gasoline particulate filters (GPFs) contain small amounts of O2 and excess H2O; however, the effect of H2O on soot oxidation has not been thoroughly explored to date. Thus, it is necessary to understand the effect of H2O on soot oxidation to improve the catalytic performance in GPFs. This study investigates the role of H2O in soot oxidation on macroporous Ce-Zr mixed oxide catalysts (M-CeZr). The results revealed an improvement in the catalytic activity of soot oxidation in the presence of H2O over that afforded under oxygen-only conditions. Since the mechanism of soot oxidation under dry conditions involves the conversion of gaseous oxygen to active oxygen (Ox−) species on the oxygen vacancies of the catalyst, Ox− and oxygen vacancy are critical factors that affect the catalytic performance in the absence of H2O. Notably, when H2O was introduced into the reaction, it was predominantly used as an oxidant rather than gaseous oxygen. Further, the dissociation of H2O into active oxygen over the catalyst surface was not related to the number of oxygen vacancies. Therefore, even when the catalyst comprised few oxygen vacancies, its activity improved under wet conditions. In addition, although the catalysts were damaged by high temperatures, the catalytic performance was maintained in the presence of H2O, unless the morphology of the catalysts collapsed.

Thermal Transformation of Birnessite (OL) Towards Highly Active Cryptomelane (OMS-2) Catalyst for Soot Oxidation

A layered potassium–manganese oxide, birnessite (OL: KMn4O8) was subjected to long-term post-calcination thermal treatment aimed at stimulating its activity in catalytic soot oxidation. The 12 h thermal treatment at several temperatures (425, 500, 550 and 650 °C) in atmospheric conditions resulted in the swift transformation of the layered material into the tunnel-shaped octahedral molecular sieve cryptomelane (OMS-2: KMn8O16). The obtained materials were thoroughly characterized using powder X-ray diffraction, Raman spectroscopy, N2-BET specific surface area analysis, and transmission electron microscopy techniques. With increasing temperature of thermal treatment, the concentration of OMS-2 increased, which was confirmed by Rietveld analysis, and dominated the material properties, notably the lowering of work function of the catalysts (Δϕ = 0.2 and 0.4 eV in air and vacuum, respectively). The obtained results reveal the high catalytic activity of the OMS-2 formed from the thermal transformation of the OL material compared to both the parent OL material and the uncatalyzed soot oxidation reaction. The achieved catalytic activity showed direct correlation with the temperature of pre-treatment, with the most active catalyst being calcined at 550 °C/12 h, and lowering the temperature of 50% soot conversion (T50%) by spectacular 150 °C in loose contact. The catalytic activities were found to correlate well with the work function (low work function–high catalytic activity), confirming the electron transfer from the catalyst surface to oxygen molecule as the important step in the soot oxidation mechanism over mixed potassium–manganese oxides. The beneficial effect of thermal pre-treatment was found to last over multiple runs, maintaining a T50% lower by 100 °C compared to the untreated parent material. The obtained results indicate the importance of the temperature treatment for the catalytic performance of potassium promoted transition metal oxides as catalytically active phases for efficient soot removal in the conditions present in combustion engine exhaust gases.Graphical

Silver sulfate catalyst for soot oxidation with high resistance to sulfur poisoning

This study investigates the soot oxidation activity and mechanism of an Al2O3-supported Ag2SO4 catalyst with novel active species. The Ag2SO4/Al2O3 catalyst exhibits higher soot oxidation activity under loose contact conditions compared to Ag/Al2O3 and Ag/CeO2 catalysts. Moreover, it is hardly affected by sulfur poisoning and maintains its high activity after sulfur poisoning treatment. High-temperature X-ray diffraction measurements suggest that a part of Ag2SO4 on the Al2O3 support melts and creates contact between the Ag2SO4 and soot at temperatures over 400 °C, resulting in higher soot oxidation activity even under loose contact conditions. The soot oxidation mechanism over the Ag2SO4/Al2O3 catalyst is probed by oxygen isotope exchange analysis and operando X-ray absorption fine structure measurements, which reveal that the oxygen chemically bound in Ag2SO4 is utilized for soot oxidation. In particular, the above oxidation features a reversible redox cycle comprising partial reduction of Ag2SO4 to metallic Ag that is further re-oxidized to Ag2SO4 by the gas-phase oxygen.

Quantum chemical study of the mechanism of oxidation of C15H9 by atomic oxygen

An important environmental problem related to the use of fossil fuels is the formation of soot during combustion. Mechanisms of soot oxidation, which alleviates its emission into the environment, are not fully understood. The reaction of O with a radical C15H9 may play an important role in combustion. In this article, the C15H9 molecule was chosen as a model of soot surface. The paper discusses various pathways resulting from the C15H9 + O reaction. Relative energies, frequencies and optimal geometries of the reactants, products, intermediates and transition states of the C15H9 + O reaction have been calculated using the quantum-chemical Gaussian and Molpro program packages. The reaction pathways leading to carbon monoxide CO elimination have been found and dscussed.

Mechanism of Soot Oxidation over CeO2–ZrO2 under O2 Flow

Abstract Soot oxidation over CeO2–ZrO2 (CZ) has been studied herein. The soot-CZ mixtures were observed under different contact conditions using transmission electron microscopy (TEM) and evaluated using thermogravimetric-differential thermal analysis (TG-DTA). These results indicate that the soot ignition temperature depends on the soot/CZ contact degree, and the soot oxidation rate depends on the soot/CZ contact area. The TEM observation of soot-CZ mixture quenched at T50 (50% soot conversion) indicates that soot oxidation occurs only at the soot/CZ interface. Furthermore, the soot oxidation under 18O2 flow and under He flow suggests that CZ lattice oxygen is a more active oxygen species than the adsorbed oxygen on the CZ. The CZ lattice oxygen mainly oxidizes soot; however, the adsorbed oxygen on the CZ surface does not oxidize soot at lower temperatures. Thus, the adsorbed oxygen oxidizes the reaction intermediates such as adsorbed CO on the CZ surface, which shows that CZ lattice is more active than the adsorbed oxygen.

Detailed, sterically-resolved modeling of soot oxidation: Role of O atoms, interplay with particle nanostructure, and emergence of inner particle burning

A newly-developed detailed mechanism of soot oxidation was tested against experimental observations. The computations were performed at an atomistic level and with a detailed consideration of soot particle surface sites. Several additional reactions were investigated theoretically and one of them, oxidation of embedded five-member rings by O atoms, was included in the model. The primary focus of the study was on the high-temperature shock-tube experiments of Roth et al. (1991). The reaction model was able to reproduce the experimental results, but required coupling to particle nanostructure: partial oxidation of PAH molecules and the decrease in PAH initial sizes along the oxidation path. The principle reaction mechanism was identified to be the formation of oxyradicals, their decomposition, formation of hard-to-oxidize embedded five-member rings, and oxidation of the latter predominantly by O atoms. The analysis identified O as the most effective oxidizer of the embedded five-member rings, which thus controls the rate of the overall oxidation. The model thus predicts fast oxidation during a brief initial period followed by a slow-oxidation one. The model of partial oxidation of an aromatic molecule and switching to the next intact molecule suggests pore formation and subsequent inner particle burning. We also investigated the ability of the present model to reproduce recent measurements of soot oxidation rates performed by Camacho et al. (2015) at about 1000 K. The initial reaction model failed to predict these results, and no adjustment could reconcile the differences. The only way to bring the model to experimental values was by assuming a catalytic decomposition of water on the reactor wall supplying additional radicals, H and OH, to the reacting gas mixture. Additional chemistry, oxidation through complex formation at neighboring surface sites, was required to fully reproduce the experimental observations. These additional reactions were found to play no role in the high-temperature simulations, nor were they sufficient to reproduce the low-temperature experiment on their own, without the assumed catalytic decomposition of water.

Ceria–Zirconia Particles Wrapped in a 2D Carbon Envelope: Improved Low‐Temperature Oxygen Transfer and Oxidation Activity

AbstractEngineering the interface between different components of heterogeneous catalysts at nanometer level can radically alter their performances. This is particularly true for ceria‐based catalysts where the interactions are critical for obtaining materials with enhanced properties. Here we show that mechanical contact achieved by high‐energy milling of CeO2–ZrO2 powders and carbon soot results in the formation of a core of oxide particles wrapped in a thin carbon envelope. This 2D nanoscale carbon arrangement greatly increases the number and quality of contact points between the oxide and carbon. Consequently, the temperatures of activation and transfer of the oxygen in ceria are shifted to exceptionally low temperatures and the soot combustion rate is boosted. The study confirms the importance of the redox behavior of ceria‐zirconia particles in the mechanism of soot oxidation and shows that the organization of contact points at the nanoscale can significantly modify the reactivity resulting in unexpected properties and functionalities.

Ceria-Zirconia Particles Wrapped in a 2D Carbon Envelope: Improved Low-Temperature Oxygen Transfer and Oxidation Activity.

Engineering the interface between different components of heterogeneous catalysts at nanometer level can radically alter their performances. This is particularly true for ceria-based catalysts where the interactions are critical for obtaining materials with enhanced properties. Here we show that mechanical contact achieved by high-energy milling of CeO2–ZrO2 powders and carbon soot results in the formation of a core of oxide particles wrapped in a thin carbon envelope. This 2D nanoscale carbon arrangement greatly increases the number and quality of contact points between the oxide and carbon. Consequently, the temperatures of activation and transfer of the oxygen in ceria are shifted to exceptionally low temperatures and the soot combustion rate is boosted. The study confirms the importance of the redox behavior of ceria-zirconia particles in the mechanism of soot oxidation and shows that the organization of contact points at the nanoscale can significantly modify the reactivity resulting in unexpected properties and functionalities.

Soot Oxidation Kinetics of Different Ceria Nanoparticle Catalysts

Catalysts for direct soot oxidation in catalyzed diesel particulate filters (CDPFs) consist typically of various mixed oxide compositions (frequently with CeO2 as the dominant component) that assist soot oxidation by enhancing the supply of oxygen from the catalyst to the soot. Apart from the composition, the material morphological characteristics may also contribute to the catalytic activity and this is the motivation for the present study. Different CeO2 nanoparticle catalysts have been obtained employing aerosol-based synthesis (ABS) and sol–gel methods. The obtained catalyst particles have been characterized with respect to their physical and morphological properties as well as with respect to their catalytic soot oxidation activity. The results have been analyzed with the aid of a multi-population kinetics model where soot is found to consist of three fractions reacting with different activation energies, namely 120, 180, and 240 kJ/mol. The occurrence of these three fractions is attributed to the formation of distinct families of surface oxygen complexes (SOCs) on the carbon surface which are subsequently gasified and hence cause soot oxidation, in agreement with accepted mechanisms of soot oxidation in the literature. The CeO2 nanoparticles oxidize catalytically all three fractions of soot, but with different “enhancement factors,” while the activation energies during catalytic oxidation remain the same. A comparison of the catalytic pre-exponentials to those of plain soot shows, in most cases, enhancements, which for some catalysts, can be up to ∼4.5, 6.5, and 2 times larger than those of plain soot, reflecting the relative capacity of the catalyst to generate more of the respective SOCs. The developed approach provides a more detailed but tractable way to describe soot oxidation (plain and catalytic), which can be readily incorporated into simulations of actual emission control systems to increase their reliability.

Investigations of soot combustion on yttria-stabilized zirconia by environmental transmission electron microscopy (ETEM)

Environmental transmission electron microscopy (ETEM) equipped with an aberration corrector was used to get further insights into the mechanism of soot oxidation on yttria-stabilized zirconia (YSZ), an O2− ionic conductor. In situ observations, with a resolution of around 1nm, of a “tight” YSZ/real soot mixture were performed at 550°C in the presence of a few mbar of oxygen (2 and 3mbar). Preliminary experiments were conducted to validate that moderate electron beam illumination conditions do not control the soot combustion. Video sequences recorded at 550°C clearly confirmed that bulk YSZ oxygen atoms are the active species for soot oxidation at the soot/YSZ interface as no oxidation takes place without contact. The number of soot/YSZ contact points and their evolution with time are the key parameters for soot oxidation. The contact length between the soot and the YSZ grains could be estimated to be around 20–40nm only.

Oxidation characteristics of gasoline direct-injection (GDI) engine soot: Catalytic effects of ash and modified kinetic correlation

In this paper, experimental analyses are conducted into the GDI soot oxidation characteristics as dependent on engine operating conditions. Soot is sampled at various engine operating conditions of a commercial 2.4L GDI engine with a naturally aspirated, homogeneous, and stoichiometric operation strategy. The oxidation reactivity, ash composition, and carbon nanostructure of the GDI soot samples are analyzed using thermogravimetric analysis (TGA), scanning electron microscope–energy-dispersive spectroscopy (SEM–EDS), high-resolution transmission electron microscopy (HR-TEM), and Raman spectroscopy. Based on the analyses, a global GDI soot oxidation mechanism is proposed which includes the effects of soluble organic fractions (SOF)/weakly bonded carbon (WBC), and three types of ash on GDI soot oxidation. The results show that GDI soot contains an order of magnitude higher ash fraction than does conventional diesel soot, and oxidation reactivity is significantly enhanced by the catalytic effects of ash, as a function of ash content in soot. A modified empirical kinetic correlation for GDI soot oxidation is suggested on the basis of the results, and the modified kinetic correlation predicts the GDI soot oxidation rate accurately for various engine operation points at wide ranges of soot conversion and temperature without modifying kinetic parameters. The kinetic parameters are determined from isothermal and non-isothermal thermogravimetric analysis (TGA) soot oxidation tests; the methods are elucidated in detail.

Influence of the surface potassium species in Fe–K/Al2O3 catalysts on the soot oxidation activity in the presence of NOx

Cordierite monolith-supported and powder Fe–K/Al2O3 catalysts were prepared and thoroughly characterized by bulk (XRD, Raman spectroscopy, XRF, ICP–OES, TPR) and surface (XPS, IR, N2-BET, NOx-TPD, K-TPD) sensitive methods. The catalytic activity was tested in TPO (temperature programmed oxidation) of model soot. The influence of the calcination temperature, i.e. 450 and 650°C, on the catalysts physicochemical properties and reactivity, type of surface states of K promoter and their role in the soot oxidation mechanism in the presence of NOx was evaluated. In the catalysts calcined at 450°C potassium was found to be mostly as free KNO3 whereas calcination at 650°C successfully transformed KNO3 into K2O, which during the soot oxidation yielded K2CO3 species. Such carbonates underwent decomposition in the presence of the oxygen surface groups of the support, leading to the formation of active basic O2− groups. NOx species were found to adsorb strongly on these O2− sites. The increased stability of thus formed NOx-species resulted in lower soot oxidation activity. In the same time, the catalysts calcined at 650°C evidenced higher ability towards the reduction of NOx, occurring simultaneously with the soot oxidation process. Additionally, the higher calcination temperature led to significantly higher stability of K promoter species as evidenced by potassium thermodesorption experiments. An overall schematic model of the catalyst morphology and relative distribution of the active components (K, Fe) over the Al2O3 support is proposed.