Soot Oxidation Activity Research Articles

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.

Enhanced Soot Oxidation Activity of a CuO-Doped CeO2 Catalyst via Acid Etching

Copper oxides tend to agglomerate on the surface of CeO2, with a high amount of Cu. In this study, a CeO2 catalyst with a high CuO doping amount was treated with nitric acid to improve its catalytic performance for soot oxidation. The effect of acid etching on the structural properties of the CuO-doped CeO2 catalyst were elucidated. The characterization results indicated that aggregated CuO particles formed over CuCe. The acid etching resulted in a remarkable increase in the surface area of CuCe. Additionally, acid etching promoted the formation of surface-adsorbed oxygen species and oxygen vacancy, and reduced the content of CuOx species with weak interaction with CeO2. The soot temperature-programmed oxidation results show the acid etching of CuCe catalyst could reduce the T50 from 443 to 383 °C. The isothermal reaction results also suggest that acid etching of CuCe leads to an increase in reaction rate from 16.2 to 46.0 μmol min−1 g−1.

Ni-BaMnO3 Perovskite Catalysts for NOx-Assisted Soot Oxidation: Analyzing the Effect of the Nickel Addition Method

In this study, we analyzed the role of a series of BaMn1−xNixO3 (x = 0, 0.2, and 0.4) mixed oxide catalysts, synthesized using the sol–gel method, in NOx-assisted diesel soot oxidation. ICP-OES, XRD, XPS, and H2-TPR techniques were used for characterization and Temperature-Programmed Reaction experiments (NOx-TPR and Soot-NOx-TPR), and isothermal reactions at 450 °C (for the most active sample) were carried out to determine the catalytic activity. All samples catalyzed NO and soot oxidation at temperatures below 400 °C, presenting nickel-containing catalysts with the highest soot conversion and selectivity to CO2. However, the nickel content did not significantly modify the catalytic performance, and in order to improve it, two catalysts (5 wt % in Ni) were synthesized via the hydrothermal method (BMN2H) and the impregnation of nickel on a BaMnO3 perovskite as support (M5). The two samples presented higher activity for NO and soot oxidation than BMN2E (obtained via the sol–gel method) as they presented more nickel on the surface (as determined via XPS). BMN2H was more active than M5 as it presented (i) more surface oxygen vacancies, which are active sites for oxidation reactions; (ii) improved redox properties; and (iii) a lower average crystal size for nickel (as NiO). As a consequence of these properties, BMN2H featured a high soot oxidation rate at 450 °C, which hindered the accumulation of soot during the reaction and, thus, the deactivation of the catalyst.

NOx-assisted soot oxidation with alkaline-earth metals (Ca, Sr, Ba) loaded on Ag/2MnOx-1CeO2

Previously, we demonstrated that Ag/2MnOx-1CeO2 is the optimal catalyst for NOx-assisted soot oxidation and that the amount of NOx adsorption is major factor for NOx-assisted soot oxidation activity. Thus, in this study, alkaline-earth metals were introduced into Ag/2MnOx-1CeO2 to promote NOx-assisted soot oxidation activity by increasing NOx adsorption. As a results, Ag(Er)/2MnOx-1CeO2 catalysts exhibited enhanced NOx adsorption, which improved their NOx-assisted soot oxidation activity compared with that of Ag/2MnOx-1CeO2. Among them, AgSr/2MnOx-1CeO2 and AgBa/2MnOx-1CeO2 exhibited high NOx adsorption capacity. In particular, AgSr/2MnOx-1CeO2 greatly improved NOx-assisted soot oxidation activity by desorbing NOx at an appropriate temperature (approximately 350 °C). On the other hand, AgBa/2MnOx-1CeO2 adsorbed NOx even at high temperature, so its NOx-assisted soot oxidation activity was lower than that of AgSr/2MnOx-1CeO2. Therefore, the T20 of AgSr/2MnOx-1CeO2 decreased to 80 °C compared with Ag/2MnOx-1CeO2. Consequently, the developed AgSr/2MnOx-1CeO2 catalyst is an optimal catalyst for NOx-assisted soot oxidation activity compared to existing catalysts. Finally, this study demonstrates that the increase in NOx adsorption enhances the NOx-assisted soot oxidation activity, but the NOx desorption temperature is a more important factors for improving the NOx-assisted soot oxidation activity.

Performance characterization of a selective catalytic reduction on filter aftertreatment system in a heavy-duty diesel engine tested on a laboratory dynamometer

Increasingly stringent emission regulations call for improvement in the current emissions control technology in heavy-duty diesel engines. In the existing aftertreatment system utilized in diesel engines, there is already a challenge with space and volume, and an attempt to add more components would lead to the consumption of more space and impracticable. The Selective Catalytic Reduction on Filter (SCRF)-an integration of Selective Catalytic Reduction (SCR) and Diesel Particulate Filter (DPF) in a single component, offers space saving and favorable thermal inertia opportunities. However, literatures are conflicting on the effect of the SCR and DPF activities on the operations of the SCRF, as both require NO2 for fast SCR reaction, and passive soot oxidation respectively. This paper presents a detailed experimental characterization of a Silicon-Carbide (Si–C) coated with Copper-Zeolite (Cu-Ze) catalyst SCRF after-treatment component in a heavy-duty diesel engine, evaluated on a laboratory dynamometer. The effect of particulate matter loading on the SCRF activities in steady state, and transient operations was investigated, with emphasis on the simultaneous utilization of NO2 between the passive soot oxidation activities and SCR reactions-which is a determinant of the NOx conversion efficiency. The SCRF was loaded from 0 g/L up to 5 g/L while passive regeneration and NOx conversion activities were investigated over steady and transient cycles. The investigation has shown that both NOx conversion efficiency and passive soot oxidation increase with particulate matter loading in the SCRF system of a heavy-duty diesel engine.

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.

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.

Tailoring the Composition of BaxBO3 (B = Fe, Mn) Mixed Oxides as CO or Soot Oxidation Catalysts in Simulated GDI Engine Exhaust Conditions.

Mixed oxides with perovskite-type structure (ABO3) are promising catalysts for atmospheric pollution control due to their interesting and tunable physicochemical properties. In this work, two series of BaxMnO3 and BaxFeO3 (x = 1 and 0.7) catalysts were synthesized using the sol-gel method adapted to aqueous medium. The samples were characterized by μ-XRF, XRD, FT-IR, XPS, H2-TPR, and O2-TPD. The catalytic activity for CO and GDI soot oxidation was determined by temperature-programmed reaction experiments (CO-TPR and soot-TPR, respectively). The results reveal that a decrease in the Ba content improved the catalytic performance of both catalysts, as B0.7M-E is more active than BM-E for CO oxidation, and B0.7F-E presents higher activity than BF for soot conversion in simulated GDI engine exhaust conditions. Manganese-based perovskites (BM-E and B0.7M-E) achieve better catalytic performance than iron-based perovskite (BF) for CO oxidation reaction due to the higher generation of actives sites.

Insight into the effect of catalytic reactions on correlations of soot oxidation activity and microspatial structures

A catalyst is usually coated on Diesel particulate filter (DPF) for assisted regeneration. In this paper, the oxidation activity and pore structure evolutions of soot under the effect of CeO2 are explored. CeO2 effectively increases the oxidation activity of soot and reduces the initial activation energy; in the meantime, the addition of CeO2 changes the soot oxidation mode. Pure soot particles tend to produce the porous structure in the oxidation process. Mesopores promote the diffusion of oxygen, and macropores contribute to reduce the agglomeration of soot particles. Additionally, CeO2 provides the active oxygen for soot oxidation and promotes the multi-point oxidation at the beginning of soot oxidation. With the oxidation proceeding, catalysis causes the collapsion of soot microspatial structures, in the meantime, the macropores caused by the catalytic oxidation are filled by CeO2. It results in the tight contact between soot and catalyst, further promoting the formation of the available active oxygen for soot oxidation. This paper is meaningful to analyze the oxidation mechanism of soot under catalysis, which lays a foundation for improving the regeneration efficiency of DPF and reducing the particle emission.

Formation of nano-rod structures in manganese-rich ceria–manganese mixed oxides and their soot oxidation activity

EDTA-citrate synthesized manganese-rich ceria–manganese oxide catalysts (Ce1−xMnxO2−δ; where x = 1 to 0.6, termed CM100, CM90, CM80, CM70, CM60, CM50 and CM40 respectively) were obtained after calcination at 600 °C/5 h, characterized and tested for soot oxidation activity. Mn3O4 and Mn2O3 phases were observed in the XRD spectra; with the increase in the ceria concentration, cubic fluorite phase formation was observed from CM80 to CM40. BET and BJH analyses revealed that the pore diameter and volume had a meaningful impact on the soot oxidation activity, as seen for CM80 which had a BET surface area of 31 m2/g, pore diameter of 21.119 nm, and a pore volume of 0.1637 cc/g. The formation of the Mn3O4 peak in all the synthesized catalysts and ceria’s cubic fluorite F2g peak (452 cm −1) were observed from CM80 to CM40 from Raman spectral analysis. FESEM analysis of all the CMX samples revealed an intriguing aspect, the appearance of nano-rods. These nano-rods were observed in the HRTEM imaging as well. The high-resolution imaging and SAED analysis proved that the nano-rod structures were primarily due to the α-MnO 2 phase. The mixture of agglomerates and nano-rods profoundly affected the soot oxidation activity of the synthesized samples. CM70 had a minimum T50 of 389 °C among the CMX catalysts. The activation energy was reckoned with the Ozawa and KAS methods. CM50, with 131 kJ/mol for Ozawa and 127 kJ/mol for the KAS method, had the lowest values among the chosen catalysts. Synergistic interactions between ceria and the manganese oxides, considerable specific surface area, pore diameter, pore volume, substantially higher exposed (220) and (200) planes and the formation of the nano-rods helped to lower the activation energy for soot oxidation activity and consequently a reduction in the soot oxidation temperature of CM50 to CM70. The concentration of the nano-rods rendered a considerable role in reducing the T50 temperatures of the CMX catalysts.

An investigation on copper-loaded ceria-praseodymium catalysts for soot oxidation activity and its kinetics

An investigation on copper-loaded ceria-praseodymium catalysts for soot oxidation activity and its kinetics

Insights into the interface of NiCo2O4 spinel /LaCoO3 perovskite nano-composite for CO and soot oxidation

In the quest for the development of thermally stable, highly active and low-cost catalysts for use in catalyzed diesel particulate filter, nano-composites are new areas of research. Therefore, we reported the easy synthesis of spinel NiCo2O4/perovskite LaCoO3 nano-composite, and its individual oxides NiCo2O4 and LaCoO3 for comparison. The detailed insights into the physio-chemical characteristics of formed NiCo2O4/ LaCoO3 nano-composite were done based on various characterization analysis such as X-ray diffraction (XRD), Fourier transform infrared (FT-IR), N2 physiosorption, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDX), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The characterization analysis of NiCo2O4/LaCoO3 revealed the successful formation of a chemical interface possessing strong interfacial interaction, resulting in desirable physicochemical characteristics such as small crystallite size, abundant mesoporosity, high specific surface area and activation of surface lattice oxygen. Owing to the desirable characteristics, the activity results over NiCo2O4/LaCoO3 nano-composite showed the excellent CO oxidation performance and high soot oxidation activity, recyclability and thermal stability. This work mainly attempts to emphasize the effectiveness of the facile, inexpensive and conventionally used precipitation method for the successful formation of highly efficient nano-composites.

Diesel soot oxidation over potassium-promoted urchin-structured NiO-NiCo2O4 catalyst

Catalytic oxidation is an effective strategy to eliminate soot emitted from diesel engines. However, the insufficient contact sites and low intrinsic activity of catalysts severely limit the development of the technique. Herein, we designed and prepared a potassium-promoted NiO-NiCo2O4 composite endowed with an urchin-like structure (K/NiO-NiCo2O4 US), which displayed high catalytic activity for soot oxidation with temperatures of soot conversion at 10% (T10) and 50% (T50) as low as 307 °C and 347 °C, respectively. The enhanced performance was unveiled based on a series of characterization results. On one hand, the urchin-like structure increased the contact efficiency between the soot and the catalyst. On the other hand, the loading of potassium generated more Co3+ species and active oxygen species, improving the oxidation ability.

Effect of doping copper on K-OMS-2 over soot oxidation activity

Effect of doping copper on K-OMS-2 over soot oxidation activity

BaFe1−xNixO3 Catalysts for NOx-Assisted Diesel Soot Oxidation

AbstractIn this work, it is analyzed the effect of the partial substitution of Fe by Ni in a BaFeO3 perovskite to be used as the catalyst for NOx-assisted diesel soot oxidation. A series of BaFe1−xNixO3 (x = 0, 0.2, 0.4 and 0.8) catalysts have been synthesized by using the sol–gel method. The catalysts have been characterized by ICP-OES, XRD, XPS, O2-TPD, H2-TPR- and TEM. The catalytic activity for NO to NO2 oxidation and NOx-assisted diesel soot oxidation have been determined by Temperature Programmed Reaction experiments (NOx -TPR and Soot-NOx-TPR, respectively) and by isothermal reaction at 450 °C. Ni seems not to be inserted in the BaFeO3 perovskite and, instead of that, BaNiO3 perovskite and NiO are detected on the surface of the perovskite BaFeO3. XPS data reveal the coexistence of Fe(III) and Fe(IV) on the catalyst’s surface (being Fe(III) the main oxidation state) and the presence of oxygen vacancies. All catalysts are active for NO oxidation to NO2, showing BaFeO3 and BaFe0.6Ni0.4O3 the best catalytic performance. BaFe0.6Ni0.4O3 shows the highest proportion of nickel on surface and it combines the highest activity and stability for NOx-assisted diesel soot oxidation. Also, this catalyst presents the highest initial soot oxidation rate which minimizes the accumulation of unreacted soot during reaction.

Soot oxidation over Pt-loaded CeO2-ZrO2 catalysts under gasoline exhaust conditions: Soot-catalyst contact efficiency and Pt chemical state

Decreasing the ignition temperature of soot combustion is a key of the improved catalyst performances, but a much lower concentration of oxygen from gasoline vehicle exhaust makes the research of the catalyst extremely challenging. Improving the adsorption and activation of oxygen molecules on the catalyst surface and the effective migration rate of active oxygen species are the key factors to enhance soot oxidation. In this work, Pt/CeO2-ZrO2-re catalyst was synthesized by synergistically adjusting CeO2-ZrO2 pore structure and Pt chemical state, as a result, the conversion temperature (T50, 50% soot was oxidized) was reduced by 113 ℃ in 1% O2/N2. The pore structure of CeO2-ZrO2 was optimized by copolymerization microsphere template assisted method, and better soot-contact efficiency and intrinsic properties were obtained, resulting in a higher availability of reactive oxygen species. In addition, adjusting Pt chemical state by using the liquid phase reduction method is significant for the adsorption and activation of oxygen molecules and catalytic cycle of soot oxidation. Structure-activity relationship studies showed that soot oxidation performance is highly related to the initial oxidation state of Pt and pore structure in 1% O2/N2. Better soot-contact efficiency, improved intrinsic properties and the existence of metallic Pt are responsible for the higher soot oxidation activity of Pt/CeO2-ZrO2-re catalyst.

Fe‐Ce Composite Oxide Catalysts Supported on 3D Nickel Foam for Catalytic Oxidation of Soot

AbstractTo reduce the combustion temperature of diesel soot, the Fe−Ce composite oxide catalysts with varying composition supported by three‐dimensional nickel foams were prepared by hydrothermal reaction and impregnation method. The internal structure and performance of the monolithic catalysts were characterized by X‐ray diffraction, Raman, scanning electron microscope, X‐ray photoelectron spectroscopy and catalyst activity evaluation techniques. The results revealed that when the molar ratio of Ce and Fe was 0.5, the CeO2/Fe2O3@NF catalyst showed the best catalytic activity of soot oxidation, good stability and water resistance. Besides, the addition of NO and H2O was helpful to improve the catalytic performance for soot oxidation, when 10 % H2O and 800 ppm NO were mixed into 20 %O2/N2 atmosphere, the T10, T50 and T90 values could decrease to 299 °C, 344 °C and 389 °C, respectively.

Single-crystalline α-MnO2 catalysts with tailored exposed crystal facets for boosting catalytic soot oxidation: The crystal facet-dependent activity

The distribution and coordination states of surface oxygen and metal ions in pure-phase metal oxides are dominated by the exposed crystal facets, which directly affects its physicochemical properties. Herein, we have elaborately constructed three catalysts of single-crystalline α-MnO2 nanowires with predominantly exposed {310}, {110} and {100} facets, respectively. The α-MnO2 catalyst exhibits a strong facet-dependent catalytic activity for the three-phase reaction of catalytic soot oxidation. Compared with the low-index {110} and {100} facets, the α-MnO2 catalyst with exposed high-index {310} facet exhibits the highest catalytic activities (T50 = 333 °C, TOF = 1.127 h−1), the lowest apparent activation energy (68.4 KJ mol−1), and excellent catalytic stability during soot oxidation. Combining the results of characterizations and density functional theory (DFT) calculations, the intrinsic mechanism of crystal facet-dependent catalytic activity for soot oxidation is revealed: The selectively exposed α-MnO2{310} facets can induce the formation of coordinative unsaturated atoms and surface defect sites, enhancing the adsorption-activation ability for O2 and NO. In-depth understanding of the face-dependent catalytic activity of α-MnO2 catalyst is valuable for the acquisition of high-efficient non-precious metal catalysts during soot oxidation.

Manganese doped Ceria (Ce1−xMnxO2−δ (x = 0–0.3)) catalysts synthesized by EDTA–Citrate method for soot oxidation activity

Manganese doped Ceria (Ce1−xMnxO2−δ (x = 0–0.3)) catalysts synthesized by EDTA–Citrate method for soot oxidation activity

Enhanced catalytic oxidation of soot over 3DOM LaMnO3 by adding Ag and CeO2: Improving the generation and delivery of active oxygen species

In this work, Ag-Ce modified 3DOM LaMnO3 catalysts were first synthesized by redox etching precipitation and used for NOx-assisted soot oxidation. Compared with wetness impregnation method, Ag introduced by redox etching precipitation method showed smaller size and stronger interaction with 3DOM LaMnO3, thus presented better catalytic performance. Moreover, Ag content was selected as variable (Ag mass ratio = 1–5 wt%) to explore the role of Ag in catalytic soot oxidation. The characterization results had demonstrated that Ag could activate oxygen and convert it into highly active superoxide species (O2–). Besides, Ag could enhance the oxygen mobility and NO2 consumption rate, leading to better activity for NOx-assisted soot oxidation. However, excessive Ag (>4 wt%) would cause too low oxygen vacancy amount, causing worse catalytic performance. Further modification with CeO2 mainly increased oxygen vacancy and transferred O2– produced on Ag. Besides, NO2 utilization rate were further enhanced. Hence, 4Ag-Ce/LM with optimal Ag amount exhibited the best catalytic activity for NOx-assisted soot oxidation with T50 value of 367 ℃. Moreover, it also showed good reusability and water resistance. This work synthesized a promising catalyst for soot oxidation and was expected to offer a new insight for catalyst design in other fields.