Low Temperature 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.

PGM-free metal oxide nanoarray forests for water-promoted low-temperature soot oxidation

In this study, PGM free metal oxide based nanorod array (nanoarray) forests have been conformally integrated onto the millimeter-sized channel walls and microscale pores of SiC diesel particulate filter (DPF) substrates using scalable microwave-assisted hydrothermal and dip-coating methods. High-efficiency and robust low temperature soot oxidation is successfully achieved using such a new type of cDPFs as a result of i) promoted tight soot-catalyst contact through spontaneous soot deposits in-between nanorods, ii) highly dispersed loading of LaSrCoO3 perovskite oxidation catalysts onto ZnO nanorod arrays, effectively mitigating active site blockage by soot deposition. The light-off temperature of soot oxidation was lowered to 250 °C in oxygen, a 250 °C decrease compared to that of the commercial Pt based catalyst, due to the abundant oxygen vacancies enabled by the LaSrCoO3/ZnO (LSCO/ZnO) nanorod array catalysts. The performance of the catalyst was further enhanced to initiate the soot oxidation at ∼150 °C with co-fed gas of NO, due to the LSCO catalyzed NO2 formation and its favorable oxidation with soot at low temperature. Furthermore, the nanoarray supported perovskite catalyst shows excellent stability and activity after 120 h of hydrothermal aging at 650 °C. Soot oxidation Tmax is lowered from 542 °C to 510 °C under 6 % water steam condition due to the formation and promotion effect of oxidative -OH species. In situ DRIFTS study reveals that such -OH species form at various catalyst surfaces and interfaces, pointing to a potentially generic water promotion effect for solid/solid interfacial catalytic oxidation under humidity condition. Finally, the mechanistic soot oxidation pathways under O2, NO2, and H2O feeds are proposed for understanding the low-temperature soot oxidation behaviors over the nanoarray supported perovskite catalysts.

Nickel and cobalt transfigured natural clay: a green catalyst for low-temperature catalytic soot oxidation

Modified 'natural clay' with Ni and Co nanoparticles explored as efficient catalyst for low-temperature soot oxidation activity studies.

Why Soot is not Alike Soot: A Molecular/Nanostructural Approach to Low Temperature Soot Oxidation

Due to worldwide increasingly sharpened emission regulations, the development of Gasoline Direct Injection and Diesel Direct Injection engines not only aims at the reduction of the emission of nitrogen oxides but also at the reduction of particulate emissions. Regarding present regulations, both tasks can be achieved solely with the help of exhaust after treatment systems. For the reduction of the emission of particulates, Gasoline (GPF) and diesel Particulate Filters (DPF) offer a solution and their implementation is intensely promoted. Under optimal conditions particulates retained on particulate filters are continuously oxidized with the exhaust residual oxygen so that the particulate filter (PF) is regenerated possibly without any additional intervention into the engine operating parameters. The regeneration behavior of PF depends on the reaction rates of soot particles with oxidative reactants at exhaust gas temperatures. The reaction rates of soot particles from internal combustion engines (ICE) often are discussed in terms of order/disorder on the particle nanoscale, the concentration and kind of functional groups on the particle surfaces, and the content of (mostly polycyclic aromatic) hydrocarbons in the soot. In this work the reactivity of different kinds of soot (soot from flames, soot from ICE, carbon black) under oxidation conditions representative for PF regeneration is investigated. Soot reactivity is determined in dynamic Temperature Programmed Oxidation (TPO) experiments and the soot primary particle morphology and nanostructure is investigated by High-Resolution Transmission Electron Microscopy (HRTEM). An image analysis method based on known methods from the literature and improving some infirmities is used to evaluate morphology and nanostructural characteristics. From this, primary particle size distributions, length and separation distance distributions as well as tortuosities of fringes within the primary particle structures are obtained. Further, UV–visible spectroscopy and Raman scattering and other diagnostic techniques are used to study the properties connected to the reactivity of soot and to corroborate the experimental findings. It is found that nanostructural characteristics predominantly affect reactivity. Oxidation rates are derived from TPO and interpreted on a molecular basis from quantum chemistry calculations revealing a replication/activation oxidation mechanism.

Nanostructured Potassium-Manganese Oxides Decorated with Pd Nanoparticles as Efficient Catalysts for Low-Temperature Soot Oxidation

Two nanostructured potassium–manganese oxides, with a layered (OL) and tunneled structure (OMS-2), were synthesized and their surface decorated with 1% Pd. All prepared samples were characterized by means of X-ray diffraction, Raman spectroscopy, N2-BET specific surface area analysis, TPR, SEM/TEM. Catalytic activity in soot combustion in different reaction conditions was investigated (tight contact, loose contact, loose contact with NO addition). The obtained results revealed, that manganese oxides are highly catalytically active in soot combustion, shifting the reaction temperature window by 280 °C for OMS-2 and 300 °C for OL in comparison to the non-catalytic process. Furthermore, Pd promotion is beneficial in all cases, lowering the window of soot combustion compared to the unpromoted oxides, with the most significant effect for loose contact conditions. The difference in activity between tight and loose contacts can be successfully bridged in the presence of NO due to its transformation into NO2. The particular activity of 1% Pd/OMS-2 and 1% Pd/OL pave the road for their further development towards catalytic system for efficient soot removal in the conditions present in diesel exhaust gases.Graphical

Catalytic combustion of soot over Cu, Mn substitution CeZrO2-δ nanocomposites catalysts prepared by self-propagating high-temperature synthesis method

This study concentrates on improving the low temperature performance of CDPF catalysts. The self-propagating high temperature combustion synthesis method was applied to synthesize a series of CeZrO2-δ, CuCeZrO2-δ, and MnCuCeZrO2-δ catalysts, which were coated on DPF substrate. The performance of the catalysts was fully investigated by the soot temperature programmed combustion, which showed that these catalysts with nanometer size have a high catalytic activity of soot combustion at low temperatures. Compared with Ce0.05Zr0.05O2-δ, Cu0.9Ce0.05Zr0.05O2-δ and Mn0.09Cu0.81Ce0.05Zr0.05O2-δ showed excellent low temperature soot oxidation activities and much higher CO2 selectivity. BET, XRD, TEM, and XPS were employed to characterize the structural and physical-chemical properties of catalysts. The XRD results indicated that catalysts with 90% Cu possess evenly distributed crystalline grains and small particle size, which is responsible for its excellent oxidation activity by providing more active sites as well as forming good connection between the catalyst and soot. The XPS results demonstrated that the Cux+ has a significant association with the formation of NO2, thus promoting the soot oxidation, and that the synergetic effect between Cux+ and Mnx+ contributes most to the improvement of the catalytic activity. The partial substitution with Mn could enhance the chemisorbed oxygen, which further improved NO oxidation and CO2 selectivity. Furthermore, the soot combustion reaction mechanism was examined with in situ DRIFTS. The results exhibited that the active sites of Cu0.9Ce0.05Zr0.05O2-δ and Mn0.09Cu0.81Ce0.05Zr0.05O2-δ could generate more reactive oxygen for NO oxidation and that the uniformly dispersed Cux+ and Mnx+ in Ce lattice could adsorb NO2 to form nitrite and nitrates, which behaved as a carrier of NO2 and transported it around the surface of soot, thus facilitating the soot combustion, especially at low temperatures.

Study of Ag promoted Fe2O3@CeO2 as superior soot oxidation catalysts: The role of Fe2O3 crystal plane and tandem oxygen delivery

In this work, model α-Fe2O3 catalysts with different morphologies were synthesized to investigate crystal plane effects on soot oxidation. The results revealed that the electron-rich state of Fe2O3 {113} planes conferred them more surface Ox− and thus better catalytic performance than the {014} and {012} planes. Surface grafting of a polycrystalline CeO2 layer onto Fe2O3 gave rise to Fe2O3@CeO2 catalysts with drastically increased oxygen utilization. More importantly, by loading Ag nano-particles on the surface of Fe2O3@CeO2, a tandem oxygen delivery route was opened, resulting in strong Ox- generation/regeneration ability and superior low temperature soot oxidation activity. These Ag/Fe2O3@CeO2 catalysts overwhelmed the nano-cubic Ag/CeO2 in both catalyst cost and activity, making them very promising for application in catalyzed gasoline particulate filters (CGPFs).

Soot combustion over Ce1-xFexO2-δ and CeO2/Fe2O3 catalysts: Roles of solid solution and interfacial interactions in the mixed oxides

Two series of CeO2–Fe2O3 catalysts (CeO2–based and Fe2O3–supported oxides) with varying composition were synthesized by a hydrothermal method and characterized using various techniques. The comparison on the activity and thermal stability of different catalysts for low-temperature soot oxidation was also performed. The presence of both Ce–Fe–O solid solution and CeO2–Fe2O3 interaction were observed over the two types of catalysts. The oxygen vacancy in the solid solution is the crucial active site to facilitating the soot combustion over the CeO2–based samples. Small CeO2 nanoparticles are well dispersed on the Fe2O3–supported catalysts, which results in the formation of Fe–O–Ce species due to the strong CeO2–Fe2O3 interaction. The Fe–O–Ce species could achieve the coupling of the Ce4+–Ce3+ and Fe3+–Fe2+ couples in the CeO2–Fe2O3 interface, which is also identified as an active species for catalytic soot oxidation. The concentration of oxygen vacancy is closely related to the content of iron in ceria lattice, while the formation of Fe–O–Ce species strongly relies on the particle size of CeO2. It is also found that the oxygen vacancy is more active than the Fe–O–Ce species for soot oxidation, but it is very easy to decompose at high temperature, resulting in obvious deactivation of catalysts. By contrast, the Fe–O–Ce species is very stable under high-temperature treatments. For the fresh samples, the CeO2–based and Fe2O3–supported catalysts showed comparable catalytic activity. After long term aging at 800°C, the loss on activity over the CeO2–based catalyst (Ce–Fe–O solid solution) is much higher than that over the Fe2O3–supported sample. The Fe2O3–supported catalysts are more suitable for practical application than the Ce–Fe–O solid solution.

Promising ceria–samaria-based nano-oxides for low temperature soot oxidation: a combined study of structure–activity properties

Nanostructured Ce–Sm mixed oxides are promising catalysts for diesel soot oxidation through facile active oxygen participation.

Lattice Oxygen Activity in Pr- and La-Doped CeO2for Low-Temperature Soot Oxidation

Effects of Pr- and La-doped CeO2 on desorption of active lattice oxygen to soot oxidation were studied. Thermogravimetric analysis under N2 flow indicated that, despite the decrease in reducible Ce content, the amount of oxygen deficiency increases with increasing La content (x) to 15 mol % in Ce0.80–xPr0.20LaxO2. In inert atmosphere, the introduction of oxygen vacancies by doping La increased the available lattice oxygen for desorption. In the oxygen isotopic exchange, under 18O2 flow, desorption of oxygen molecules containing lattice oxygen atom (16O) was favored because of Pr and La doping in CeO2. These dopants increase the exchange rate constant between gas-phase and lattice oxygen atoms. The apparent activation energy, derived from an Arrhenius plot, for CeO2, Ce0.80Pr0.20O2, and Ce0.65 Pr0.20La0.15O2 were 311, 230, and 135 kJ/mol, respectively. The values indicated that Pr and La doping causes facile oxygen exchange between the gas phase and the solid. The comparison of the rate constants for 16O18O formation under 18O2 flow and 18O2/16O2 mixture flow showed that lattice oxygen predominantly contributes to the exchange reaction. Surface exchange coefficient (k) and diffusion coefficient (D) were obtained by secondary ion mass spectrometry (SIMS) depth analysis of the oxides after diffusion annealing in 18O2. The values of both k and D increase by doping Pr and La, and the surface exchange coefficient value, k, especially increases significantly. Hence, Pr and La doping enhances the oxygen exchange with lattice oxygen, and so high soot oxidation activity at low temperature could be related to the increased contribution of lattice oxygen.

Morphology of active species of Ag/ZrO2 for low-temperature soot oxidation by oxygen

The catalytic oxidation of carbon black, a model soot, over Ag/ZrO2 was studied. Transmission electron microscopy observations revealed that Ag nanoparticles with sizes of 2–10 nm on the catalysts presented 3–10 wt% Ag loading. Temperature-programmed reaction and transient activity measurements of carbon black oxidation revealed that the oxidation rate also increased with an increasing number of Ag nanoparticles on the catalysts up to 5 wt% Ag loading. The catalytically active Ag species thus was concluded to be nanoparticles with sizes of 2–10 nm.

Doped nanosized ceria solid solutions for low temperature soot oxidation: Zirconium versus lanthanum promoters

The relative efficacies of zirconium and lanthanum promoters in ceria solid solutions were investigated systematically for low temperature soot oxidation. Thus nanosized CeO2–ZrO2 (CZ, 1:1) and CeO2–La2O3 (CL, 8:2) solid solutions, and a reference CeO2 (C) were synthesized and investigated by means of various characterization techniques namely, XRD, TEM, BET surface area, XPS, UV–vis DRS, Raman and TPR. The catalytic efficiency for oxygen storage/release capacity (OSC) and soot oxidation were evaluated by a thermogravimetric (TG) method. The XRD results suggested formation of solid solutions and TEM studies confirmed nanosized nature of the particles. Raman and XPS measurements disclosed the presence of oxygen vacancies and lattice defects in both CZ and CL samples and these were more prominent in the case of CL sample. XPS studies revealed existence of both Ce3+ and Ce4+ (confirming redox nature), and the concentration of former is relatively higher in CL sample. The CL solid solution exhibited high intense and more resolved O2−→Ce3+ charge transfer transition band compared to CZ in the UV–vis DRS study. TPR measurements indicated a facile reduction of ceria after La3+ incorporation. Activity studies revealed that La3+ doped ceria accelerates the oxidation rate of soot compared to pure ceria and Zr4+ doped ceria. An attempt has been made to explain the enhanced activity of CL sample in terms of active oxygen formation provoked by defective structure of ceria due to the presence of La3+.

Kinetics of Soot Oxidation by NO2

Modern technologies use NO(2) to promote low-temperature soot oxidation for diesel particulate filter regeneration. In this study, the online aerosol technique of high-temperature oxidation tandem differential mobility analysis is used to study kinetics of soot oxidation by NO(2). Soot particles are exposed to varying temperature and NO(2) mixing ratio inside the furnace resulting from thermal decomposition of NO(2) to NO. This causes soot oxidation rates to vary throughout the furnace. Variations in temperatures and NO(2) mixing ratio are thoroughly accounted for the first time. Soot oxidation rates are calculated as a function of frequency factor A(soot), activation energy E(soot), and concentration of NO(2) within the furnace at temperatures ranging from 500 to 950 degrees C. Results suggest A(soot) and E(soot) values for soot oxidation of 2.4 x 10(-14) (nm K(-0.5) s(-1) cm(3) molecule(-1)) and 47.1 kJ mol(-1), respectively, when reaction order to NO(2) is assumed as unity. The activation energy for soot oxidation with NO(2) is significantly lower than oxidation with air. However, parts per million levels of NO(2) cause soot oxidation at low temperatures suggesting NO(2) is a stronger oxidant than O(2).

Copper promoted ceria–zirconia based bimetallic catalysts for low temperature soot oxidation

Abstract In this investigation, ceria–zirconia composite oxide based monometallic (CoO/CeO2–ZrO2 and NiO/CeO2–ZrO2) and bimetallic (CuO–CoO/CeO2–ZrO2 and CuO–NiO/CeO2–ZrO2) catalysts were evaluated for soot oxidation. These catalysts were prepared by co-precipitation and impregnation methods, and were characterized by X-ray diffraction, Raman spectroscopy, temperature programmed reduction and BET surface area techniques. Characterization results reveal formation of spinel phases in various samples, and Ce1−xCuxO2 solid solution in case of CuO–CoO/CeO2–ZrO2 sample. Among various catalysts investigated, the CuO–CoO/CeO2–ZrO2 combination exhibited a high activity and nearly 100% selectivity towards CO2.

A combinatorial approach for the discovery of low temperature soot oxidation catalysts

High-throughput syntheses using a commercial pipetting robot with the aid of the software “Plattenbau” as well as a highly parallel screening technique were used in the search of low temperature soot oxidation catalysts. It has been shown that emissivity-corrected infrared thermography which monitors the heat changes resulting from the heat of reaction on catalyst surfaces is an efficient and fast screening technique of soot oxidation catalysts. It was found that alkali metal mixed oxides have the potential to decrease the soot oxidation temperature to diesel exhaust realistic temperatures. From the first generation, the oxides K3Ce97 and Cs3Co97 were found to be the most active and stable samples. Further doping and composition spread of the CsCo catalyst resulted in even more active samples. HT-screening experiment hits were successfully confirmed by thermogravimetric analysis.

Redox properties of a TiO2 supported Cu-V-K-Cl catalyst in low temperature soot oxidation

Abstract Strong activity of a Cu-V-K-Cl/TiO 2 catalyst for soot oxidation was found by comparing results of constant temperature carbon oxidation tests in the presence and in the absence of catalyst. In addition, temperature programmed reductions (TPR) with different reductants (H 2 or soot) and re-oxidations (TPO) with air were carried out. Mass-spectrometric (MS) analyses of the gaseous products of the mentioned TPR and TPO processes were also performed on line. The aim was to investigate the behaviour of the catalyst under reducing and oxidising environments in order to draw indications on the role of the redox properties in the catalytic reaction mechanism. Results showed that the catalyst can be reduced and re-oxidised under proper environments. In particular, the catalyst delivers oxygen in the whole range of temperatures from 480 to 1000 K when carbon or H 2 are employed as reducing substances. The relevant differences between the two cases are the main products of the reductions (H 2 O with H 2 and CO 2 with soot) and the high HCl desorption in the case of reduction by H 2 . In addition, the temperature range within which the catalyst re-oxidises after such a reduction is practically coincident with those where soot catalytic oxidation occurs.

Experimental methods for activity measurements in environmental catalysis

The very specific operating conditions of catalytic processes for the abatement of pollutants from mobile or fixed sources, require an unusual experimental set-up to determine the catalyst performance. Selected examples of environmental catalysis applications such as deNO x with NH 3 or hydrocarbons, deSO x with either powder or monolith catalysts, low temperature soot oxidation, are discussed with respect to the experimental methods to be employed for catalytic activity measurements.