High Soot Oxidation Activity Research Articles

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.

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.

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.

CuO/Zn-CeO2 Nanocomposite as an Efficient Catalyst for Enhanced Diesel Soot Oxidation

Development of non-noble metal catalysts with improved structural, surface, and redox properties for catalytic soot oxidation has gained an enormous interest. Among practical alternatives, ceria-supported transition metal oxides have been proved to be the satisfactory catalysts for soot oxidation due to their outstanding redox properties and oxygen transfer capability promoted by the strong interaction between metal oxide and ceria interfaces. Following the above considerations, in the present work, CuO/Zn-CeO2 nanocomposite was prepared by a wet impregnation method and investigated for catalytic soot oxidation. To probe the significance of nanocomposites, pure counterparts, namely, CuO/CeO2, CuO/ZnO, and Zn-CeO2, were also synthesized. Various characterization techniques, namely, TEM-HRTEM, N2O chemisorption, XRD, ICP-OES, BET, Raman, XPS, and H2-TPR, were employed to investigate the structural, surface, and redox properties. Pure CuO/ZnO and CuO/CeO2 catalysts showed a soot oxidation activity with a T50 of 613 and 526 °C, respectively, under the practical conditions of NO concentration of 500 ppm and 20% O2. Interestingly, the CuO/Zn-CeO2 nanocomposite exhibited a remarkable higher soot oxidation activity with a T50 of 460 °C. The enhancement in the soot oxidation activity has been attributed to a strong interaction between the highly dispersed CuO and Zn-CeO2 support, which resulted in the desired textural properties and abundant surface defects (Ce3+ species as well as oxygen vacancies). In addition, the long-term stability test verifies an excellent reusability of the CuO/Zn-CeO2 nanocomposite towards soot oxidation without appreciable loss in the activity. The present study demonstrates the significance of ceria-based nanocomposite catalysts for environmental pollutant abatement.

A robust core-shell silver soot oxidation catalyst driven by Co3O4: Effect of tandem oxygen delivery and Co3O4-CeO2 synergy

In this work, tandem oxygen delivery systems were built by coating cube-like Co3O4 with Ag/CeO2, which exhibited high soot oxidation activity that overwhelmed the Fe2O3-based catalysts. The results of TPR and activity tests indicated that a large amount of Co3O4 lattice oxygen could be transferred into Ox− with the assistance of Ag/CeO2. These active oxygen species accelerated the oxidation of soot and NO at low temperature. Furthermore, the Co3O4-CeO2 synergy facilitated Co3+ → Co2+ transformation during reactions, resulting in boosted NO conversion and thereby NO2-assisted soot combustion at mild and high temperatures. Given the Ag/Co3O4@CeO2-type catalysts were low-cost, hydrothermally stable and could be fabricated via co-precipitation methods easily, they were highly promising for application in both gasoline and diesel particulate filters.

Catalytic Performance of Ag2O and Ag Doped CeO2 Prepared by Atomic Layer Deposition for Diesel Soot Oxidation

The catalytic behaviour of Ag2O and Ag doped CeO2 thin films, deposited by atomic layer deposition (ALD), was investigated for diesel soot oxidation. The silver oxide was deposited from pulses of the organometallic precursor (hfac)Ag(PMe3) and ozone at 200 °C with growth rate of 0.28 Å/cycle. Thickness, crystallinity, elemental composition, and morphology of the Ag2O and Ag doped CeO2 films deposited on Si (100) were characterized by ellipsometry, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and field emission scanning electron microscopy (FESEM), respectively. The catalytic effect on diesel soot combustion of pure Ag2O, CeO2, and Ag doped CeO2 films grown on stainless steel foil supports was measured with oxidation tests. Nominally CeO2:Ag 10:1 doped CeO2 films were most effective and oxidized 100% of soot at 390 °C, while the Ag2O films were 100% effective at 410 °C. The doped films also showed much higher stability; their performance remained stable after five tests with only a 10% initial reduction in efficiency whereas the performance of the Ag2O films reduced by 50% after the first test. It was concluded that the presence of Ag+ sites on the catalyst is responsible for the high soot oxidation activity.

Catalytic performance of supported Ag nano-particles prepared by liquid phase chemical reduction for soot oxidation

The effect of size and morphology of Ag catalysts supported on ZrO2 on the soot oxidation activity was investigated. Spherically shaped Ag nano-particles with the size of ca. 16nm, prepared by liquid-phase chemical reduction, supported on ZrO2 (Ag/ZrO2(NP)) exhibited outstanding performance for soot oxidation with gaseous O2 in tight and loose contact modes, compared with hemispherically shaped Ag particles prepared by impregnation (Ag/ZrO2(IMP)). The exceptional performance of Ag/ZrO2(NP) for soot oxidation in loose contact mode was explained by relatively good catalyst-soot contact state and effective migration of active oxygen species from the catalyst to soot. In addition to morphological effect, Ag nano-particles supported on ZrO2 were found to be stabilized in the surface valence state of Ag+, and then oxygen species participating in soot oxidation can be effectively activated on Ag+ sites. We concluded that the presence of Ag+ sites on the surface of Ag nano-particles is responsible for high soot oxidation activity.

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.

Iron Doped Lanthanum Stannate Pyrochlores (La<sub>2</sub>Sn<sub>2-X</sub>Fe<sub>X</sub>O<sub>7</sub>) for the Simultaneous Catalytic Removal NOx and Soot

A series of La2Sn2-xFexO7 (x=0, 0.2, 0.6, 1.0, 1.4, 1.8, 2.0) catalysts were prepared by a constant pH co-precipitation method, and their catalytic activity was investigated for simultaneous removal of NOx and soot. The incorporation of Fe strongly influences the crystal phase composition, surface area and redox properties of the catalysts. All the catalysts displayed soot oxidation activity with nearly 100% selectivity towards CO2. The doped solids exhibit higher activities than the undoped one, which may be related to the enhancement of reducibility derived from structure defects induced by doping. LSF0.6 may be a good catalyst with high soot oxidation activity (Ti = 334 °C) and a high maximum productivity of N2 (PN2= 12.4%).

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.

Low Cost, Ceria Promoted Perovskite Type Catalysts for Diesel Soot Oxidation

Perovskite type catalysts with SrCoO3 and Sr0.8Ce0.2CoO3 compositions have been prepared by co-precipitation and other methods and, their catalytic activity towards diesel particulate matter (PM)/carbon oxidation has been evaluated under the loose contact con- dition. These catalysts show excellent catalytic activity for PM/carbon oxidation, despite their low surface area and under the loose contact condition. The synergistic effects of Ce incorporation in perovskite and presence of a small amount of potassium appears to be responsible for the high soot oxidation activity of these perovskite type materials. The Ce incorporation seems to be contributing by enhancing the redox property of the catalyst, while it appears unlikely that potassium is contributing by improving the catalyst-soot contact through its volatiza- tion. The catalysts show excellent thermal stability and stable activity under repeated cycles of use.