•Ag/CeO2 catalysts show high activity for naphthalene oxidation.•Cex+ ions serve as main active sites promoted by the impregnated silver.•Ag increases bulk oxygen vacancies (VO-b) in ceria but reduces surface ones (VO-s).•VO-b improves the oxygen availability and VO-s determines the oxygen regeneration.•Both availability and regeneration of active oxygen are crucial in catalytic cycle.A series of Ag/CeO2 catalysts was synthesized by the incipient wetness method. Compared with Ag/Al2O3 and CeO2 catalysts, Ag/CeO2 catalysts are more promising candidates for naphthalene oxidation, in which Cex+ act as the main active sites and are efficiently promoted by Ag species. By characterizing the solid properties and the redox ability, the mechanism of ceria catalysis by the Ag-modified catalysts was investigated. It was found that Ag species can work as an oxygen pump to enhance the oxygen availability in the ceria bulk. They also promote the oxygen regeneration capacity by the oxygen spillover effect. However, such an effect would also decrease the ability for active oxygen regeneration at higher Ag loading, which arises from the lower surface oxygen vacancy concentration. Because of the balance between these two factors, the Ag/CeO2 catalyst with 1 wt.% Ag shows the best performance for isothermal catalytic oxidation of naphthalene.Download high-res image (186KB)Download full-size image

Sulfur poisoning of V2O5/BaSO4–TiO2 (VBT), V2O5/WO3–TiO2 (VWT) and V2O5/BaSO4–WO3–TiO2 (VBWT) catalysts was performed in wet air at 350°C for 3 hr, and activities for the selective catalytic reduction of NOx with NH3 were evaluated for 200–500°C. The VBT catalyst showed higher NOx conversions after sulfur poisoning than the other two catalysts. The introduction of barium sulfate contributed to strong acid sites for the as-received catalyst, and eliminated the redox cycle of active vanadium oxide to some extent, which resulted in a certain loss of activity. Readily decomposable sulfate species formed on VBT-S instead of inactive sulfates on VWT-S. These decomposable sulfates increased the number of strong acid sites significantly. Some sulfate species escaped during catalyst preparation and barium sulfate was reproduced during sulfur poisoning, which protects vanadia from sulfur oxide attachment to a great extent. Consequently, the VBT catalyst exhibited the best resistance to sulfur poisoning.Download high-res image (129KB)Download full-size image

Platinum nanoparticles were synthesized by a classic polyol process in ethylene glycol. The Pt nanoparticles were loaded on H-ZSM5 and γ-Al2O3 supports by wet impregnation. The as-obtained solid was thermally treated in a two-step procedure to remove the organic residues. Due to their larger particle size than the micropore size of H-ZSM5, the impregnated Pt particles were dispersed on the external surface of the zeolite support. With similar Pt particle size distribution and Pt loading, the Pt/H-ZSM5 and Pt/γ-Al2O3 catalysts were evaluated for the oxidation of NO and soot. Despite its lower activity for NO oxidation to NO2, the Pt/H-ZSM5 exhibited a much higher activity for soot oxidation in the presence of NO + O2. This abnormal NOx-assisted soot oxidation behaviour of Pt/H-ZSM5 was explained by the promoting effect of the acid sites on the zeolite.

Platinum was supported on γ-Al2O3 and ultra-stable Y zeolite (USY) by an incipient wetness impregnation method. The catalysts were characterized by nitrogen physisorption, transmission electron microscopy (TEM), CO/C3H8 isothermal oxidations, NH3 temperature-programmed desorption (NH3–TPD) and infrared (IR) spectroscopy of adsorbed probe molecules (CO, C3H8 and C3H8 + O2). Compared with Pt/Al2O3, PtUSY catalyst shows obviously higher activity for the combustion of propane. After estimating the size effect of Pt particles and propane adsorption capacity of USY, the excellent activity of PtUSY is also attributed to the strong interactions between the precious metal and the acidic zeolite. It inhibits the oxidation of Pt in an oxygen-rich atmosphere at high temperatures, which facilitate the initial oxidation step involving the C–H bond activation on metallic Pt as reflected by in situ diffuse reflectance infrared Fourier-transformed (DRIFT) spectra.

Soot oxidation was conducted over Ag/Al2O3 and Ag/SO42−/Al2O3 catalysts. Spent catalysts were found to exhibit higher activities than fresh ones for soot oxidation in oxygen. Environmental transmission electron microscopy (ETEM) was performed for the catalytic oxidation of soot over fresh and spent catalysts. Aggregation of silver nanoparticles occurred significantly on the alumina support under very low oxygen pressure. On increasing the oxygen pressure, redispersion of large Ag particles was observed on both supports, which explains the superior activities of the spent catalysts. Sulphated alumina was found to anchor the silver species effectively and deter the migration of silver to soot particles. Hence, the aggregation of silver was alleviated over Ag/SO42−/Al2O3, leading to higher activity than the Ag/Al2O3 catalyst.

Cu/SAPO-34 catalysts were prepared by wet impregnation and ion-exchange methods, and both the catalysts were hydrothermally treated at 750 °C in 10 vol% H2O/air for 24 h. Subsequently, the as-received and hydrothermally treated catalysts were exposed to a sulfur poisoning treatment at 350 °C in 100 ppm SO2/10 vol% H2O/air for 24 h and examined for NOx conversion. Sulfur poisoning considerably decreased the NOx conversion efficiency of the catalysts at low temperatures. In contrast, it improved the high-temperature selective catalytic reduction (SCR) activities of the as-received catalysts. The ion-exchange-prepared catalysts displayed higher sulfur tolerance than the impregnation-prepared catalysts at 150–350 °C. The electron paramagnetic resonance, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and H2-temperature-programmed reduction results showed that sulfur poisoning significantly influenced the migration of the copper species and thereby the amount of active isolated Cu2+. More dispersed CuSO4 was produced on the ion-exchange-prepared catalysts than on the impregnation-prepared catalysts after sulfur poisoning. The dispersed CuSO4 showed considerably higher SCR activity than the crystalline CuSO4. Both the dispersed CuSO4 and remaining isolated Cu2+ determined the low-temperature SCR behavior of the sulfur-poisoned catalysts.

Cu/SAPO-34 catalysts were prepared by wet impregnation and ion-exchange methods, and both the catalysts were hydrothermally treated at 750 °C in 10 vol% H2O/air for 24 h. Subsequently, the as-received and hydrothermally treated catalysts were exposed to a sulfur poisoning treatment at 350 °C in 100 ppm SO2/10 vol% H2O/air for 24 h and examined for NOx conversion. Sulfur poisoning considerably decreased the NOx conversion efficiency of the catalysts at low temperatures. In contrast, it improved the high-temperature selective catalytic reduction (SCR) activities of the as-received catalysts. The ion-exchange-prepared catalysts displayed higher sulfur tolerance than the impregnation-prepared catalysts at 150–350 °C. The electron paramagnetic resonance, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and H2-temperature-programmed reduction results showed that sulfur poisoning significantly influenced the migration of the copper species and thereby the amount of active isolated Cu2+. More dispersed CuSO4 was produced on the ion-exchange-prepared catalysts than on the impregnation-prepared catalysts after sulfur poisoning. The dispersed CuSO4 showed considerably higher SCR activity than the crystalline CuSO4. Both the dispersed CuSO4 and remaining isolated Cu2+ determined the low-temperature SCR behavior of the sulfur-poisoned catalysts.

MnOx–CeO2 mixed oxide supported on γ-Al2O3 was sulfated in dry and wet atmospheres to explore the effect of water during sulfur poisoning. The fresh and poisoned catalysts were characterized by Brunauer–Emmett–Teller (BET), thermo-gravimetric analysis (TGA), NH3 temperature-programmed desorption (NH3-TPD), infrared spectroscopy (IR), H2 temperature-programmed reduction (H2-TPR), NO temperature-programmed oxidation (NO-TPO) and soot temperature-programmed oxidation (soot-TPO). The results show that water hinders sulfate deposition on the catalyst. The wet sulfated catalyst with abundant surface hydroxyl groups has higher surface acidity than the dry sulfated one. Although the presence of water does not prevent the deprivation of redox properties, the generated surface acid sites may promote the NO2 utilization efficiency and deep oxidation of soot by O2, resulting in less deactivation of the wet sulfated catalyst.

Cu/ZSM-5 and CeO2-modified Cu/ZSM-5 catalysts were prepared by a wetness impregnation method. The addition of CeO2 was found to enhance the NOx selective catalytic reduction (SCR) activity of the catalyst at low temperatures, but the high-temperature activity was weakened. The catalysts were characterized by X-ray diffraction (XRD), nitrogen physisorption, inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), H2 temperature-programmed reduction (TPR) and NH3 temperature-programmed desorption (TPD). The results showed that more CuO clusters instead of isolated Cu2+ species were obtained on the modified catalyst. These active CuO clusters, as well as the Cu-Ce synergistic effect, improved the redox property of the catalyst and low-temperatures SCR activity via promoting the oxidation of NO to NO2 and fast SCR reaction. The loss in high-temperatures activity was attributed to the enhanced competitive oxidation of NH3 by O2 and decreased surface acidity of the catalyst.The interactions of ceria with the metal and support lead copper species mainly in the form of surface CuO clusters. It facilitates the fast NH3-SCR at low temperatures via enhanced NO2 production from NO oxidation, but makes against the high-temperature reaction due to the reduced supply of the reductant via catalyzing NH3 oxidation. The inhibition is also attributed to the decreases in the amounts of isolated Cu2+ and acid sites

•NbOx/Ce0.75Zr0.25O2 catalyst shows high NH3-SCR activity in a broad temperature window.•Reaction mechanisms on NbCZ catalyst are verified by DRIFTS and kinetic studies.•“L-H” and “E-R” mechanisms are presented for NbCZ in different temperature ranges.•The NH4NO3 + NO reaction is accelerated over NbCZ catalyst at low temperatures.•The NH2 + NO reaction is also promoted by addition of niobia at high temperatures.A NbOx/Ce0.75Zr0.25O2 (NbCZ) catalyst was synthesized by a citric acid-aided sol-gel method. It shows that above 80% NOx conversion and above 95% N2 selectivity for the selective catalytic reduction of NOx by ammonia over this catalyst are achieved in the temperature range 200–450 °C. Based on the DRIFTS and kinetic studies over NbCZ and Ce0.75Zr0.25O2, the promotion mechanism by niobia loading was elucidated with an overall reaction pathway. Two different reaction routes, “L-H” mechanism via “NH4NO3 + NO” at low temperatures (<200 °C) and “E-R” mechanism via “NH2 + NO” at high temperatures (>350 °C), are presented. The niobia addition increases the surface acidity and promotes the formation of nitrates species at low temperatures. In this way, the reaction between the ads-NH3 and nitrates species is accelerated to form NH4NO3 intermediates, which then decompose to N2 and H2O. The reaction of the ads-NH3 species with gaseous NOx at high temperatures is also promoted due to the enhanced acidity and weakened thermal stability of nitrates after niobia loading.

Pt/H-ZSM5 and Pt/Al2O3 with similar surface Pt particle sizes and chemical states were prepared by incipient wetness impregnation as catalysts for soot oxidation. Pt/H-ZSM5 exhibits obviously higher activities in both O2 and NO + O2 than Pt/Al2O3. On the basis of the results obtained with in situ DRIFT and other structural and surface property characterizations, the high catalytic activity of Pt/H-ZSM5 is currently attributed to two main factors. First, the acidic H-ZSM5 support inhibits NO2 adsorption on the catalyst, leading to a preferential adsorption of NO2 on the surface of soot and providing more chances for NO2–soot reactions. Second, the strong acid sites on the surface of Pt/H-ZSM5 can participate in the catalytic formation and decomposition of surface oxygenated complexes. Consequently, a high catalytic efficiency for soot oxidation is achieved on the Pt/H-ZSM5 catalyst.Keywords: NO2 preferential adsorption; Pt/H-ZSM5; soot oxidation; surface acid sites; surface oxygenated complexes

Developments in ceria-based soot oxidation catalysts, especially during the last decade, are reviewed. Based on the comparisons of the activity, durability and cost-efficiency of different soot oxidation catalysts, four kinds of applicable ceria-based catalysts have been screened out, which are: (1) CexZr1–xO2 catalyst with high cerium content (x<0.76), (2) rare-earth metals (especially Pr) modified ceria, (3) transition metals (especially Mn and Cu) modified ceria, and (4) Ag/CeO2. Moreover, a general review of recent developments on the morphology-controlled ceria-based catalysts, as well as that on the soot oxidation mechanisms over different ceria-based catalysts, is also presented.Comparsion between different modified ceria and Pt/Al2O3 as soot oxidation catalysts

The promotion effect of ceria modification on the low-temperature activity of V2O5-WO3/TiO2 catalyst was evaluated for the selective catalytic reduction of NO with NH3 (NH3-SCR). The catalytic activity of 1 wt% V2O5-WO3/TiO2 was significantly enhanced by the addition of 8 wt% ceria, which exhibited a NOx conversion above 80% in a broad temperature range 190–450 °C. This performance was comparable with 3 wt%V2O5-WO3/TiO2, indicating that the addition of ceria contributed to reducing the usage of toxic vanadia in developing low-temperature SCR catalysts. Moreover, V1CeWTi exhibited approximately 10% decrease in NOx conversion in the presence of 60 ppm SO2. The characterization results indicated that active components of V, W and Ce were well dispersed on TiO2 support. The synergetic interaction between Ce and V species by forming V–O–Ce bridges enhanced the reducibility of VCeWTi catalyst and thus improved the low-temperature activity. The sulfur poisoning mechanism was also presented on a basis of the designed TPDC (temperature-programmed decomposition) and TPSR (temperature-programmed surface reaction) experiments. The deposition of (NH4)2SO4 on V1CeWTi catalyst was much smaller compared with that on V1Ti. On the other hand, the oxidation of SO2 to SO3 was significantly promoted on the CeO2-modified catalyst, accompanied by the formation of cerium sulfates. Therefore, the deactivation of this catalyst was mainly attributed to the vanishing of the V–Ce interaction and the sulfation of active ceria.The enhanced low-temperature activity of V2O5-WO3/TiO2 catalyst by CeO2 modification for NH3-SCR reaction and the SO2 deactivation mechanism over V1CeWTi catalyst. Figure optionsDownload full-size imageDownload as PowerPoint slide

Developments in diesel engines and gasoline direct injection (GDI) engines have spawned the requirement of soot oxidation catalysts that work well in both NO + O2 and O2. In this study, we found that supporting Pt on zeolites such as H-ZSM5 and USY may receive better soot oxidizers than commercial Pt/Al2O3 catalyst. Durability tests indicate that, even after the hydrothermal aging at 800 °C, the aged Pt/H-ZSM5 is still a better soot oxidizer than Pt/Al2O3 in NO + O2, and the aged Pt/USY exhibits the best activity in O2. Further explorations reveal that the NO2 preferential adsorption on soot is dependent on acid sites both inside and outside micropores of zeolites, while the decomposition of surface oxygenated complexes (SOCs) can be promoted by strong acid sites on the external surface. These two factors contribute mainly to the high activity of the Pt/zeolite catalysts in NO + O2 and O2, respectively. Considering a relatively high external surface area is always essential to inhibiting the severe sintering of Pt, it is important to choose a zeolite support with a relatively large external surface area and a certain amount of acid sites after the hydrothermal aging.

The BaO-modified Pd/Al2O3 exhibits much better catalytic activity than Pd/Al2O3 for C3H8 oxidation both before and after the hydrothermal aging treatment. Further studies ascribe its good activity to the influence of BaO species on the physicochemical characteristics of the catalyst and the reaction routes. Firstly, the BaO species inhibits the phase transformation of alumina, resulting in higher surface area of the catalysts and hereby a better dispersion of Pd. Secondly, the basic nature and electron-withdrawing effect of barium oxide maintain palladium at high oxidation state, which leads to a higher PdO content on surface of the BaO-modified catalyst. Finally, the formation/decomposition of carbonate/bicarbonate species can be promoted by the addition of BaO, which provide extra reaction routes and are important for the deep oxidation of C3H8.

MnOx–CeO2–Al2O3 mixed oxide catalyst, which is efficient for soot oxidation in NO + O2 with T50 = 437 °C, shows a poor resistance to sulfur dioxide with T50 raised to 534 °C. The sulfated catalyst was treated in a reducing atmosphere (5% H2/He) at different temperatures (500, 600, and 700 °C). The catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, infrared (IR) spectroscopy, thermogravimetric analysis (TGA), temperature-programmed decomposition (TPD) of sulfates, and temperature-programmed reduction (TPR) of hydrogen. The catalytic activities for soot and NO oxidation were evaluated using the temperature-programmed oxidation (TPO) process. The sulfates can be only partially removed by the reduction treatment at low temperatures, whereas the sintering of metal oxides turns to be a more important factor responsible for the deactivation of catalysts reduced at high temperatures. The optimized treatment temperature is 600 °C, at which the reduced catalyst has a T50 value of 466 °C for soot oxidation.

V2O5-WO3/TiO2 catalyst was poisoned by impregnation with NH4Cl, KOH and KCl solution, respectively. The catalysts were characterized by X-ray diffraction (XRD), inductively coupled plasma (ICP), N2 physisorption, Raman, UV-vis, NH3 adsorption, temperature-programmed reduction of hydrogen (H2-TPR), temperature-programmed oxidation of ammonia (NH3-TPO) and selective catalytic reduction of NOx with ammonia (NH3-SCR). The deactivation effects of poisoning agents follow the sequence of KCl>KOH≫NH4Cl. The addition of ammonia chloride enlarges the pore size of the titania support, and promotes the formation of highly dispersed V = O vanadyl which improves the oxidation of ammonia and the high-temperature SCR activity. K+ ions are suggested to interact with vanadium and tungsten species chemically, resulting in a poor redox property of catalyst. More importantly, potassium can reduce the Brønsted acidity of catalysts and decrease the stability of Brønsted acid sites significantly. The more severe deactivation of the KCl-treated catalyst can be mainly ascribed to the higher amount of potassium resided on catalyst.

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

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.

Two thermal stable phases, Ce0.75Zr0.25O2 and Ce0.16Zr0.84O2, with different surface area were prepared by coprecipitation. The oxygen storage capacity (OSC) measurements were carried out at 500 °C under both transient (CO–O2 cycle at 0.05, 0.1 and 0.25 Hz) and stationary reaction conditions (CO pulse). In the oxygen storage/release process, the rate-determine step is surface reactions when the specific surface area is lower than 60 m2/g. When the surface area increases further, the influence of surface area is less important. The increased surface area favors diesel soot catalytic combustion via providing more redox sites to activate adsorbed oxygen. Nevertheless, this effect is less important when the specific surface area is larger than 40 m2/g, especially under loose contact condition.

The BaO-modified Pd/Al2O3 exhibits much better catalytic activity than Pd/Al2O3 for C3H8 oxidation both before and after the hydrothermal aging treatment. Further studies ascribe its good activity to the influence of BaO species on the physicochemical characteristics of the catalyst and the reaction routes. Firstly, the BaO species inhibits the phase transformation of alumina, resulting in higher surface area of the catalysts and hereby a better dispersion of Pd. Secondly, the basic nature and electron-withdrawing effect of barium oxide maintain palladium at high oxidation state, which leads to a higher PdO content on surface of the BaO-modified catalyst. Finally, the formation/decomposition of carbonate/bicarbonate species can be promoted by the addition of BaO, which provide extra reaction routes and are important for the deep oxidation of C3H8.

Ce0.5Zr0.5O2, Ce0.5Zr0.2Mn0.3O2 and Ce0.5Mn0.5O2 were prepared by citric acid sol–gel method. The effect of manganese on the structural and redox properties of ceria-based mixed oxides was investigated by means of powder X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller analyses, temperature-programmed reduction and catalytic activity evaluation in the presence of excess O2. The results showed that some Mn cations could enter into the ceria lattice to form solid solutions. Mn3O4 appeared due to the instability of the mixed oxides with increment of the Mn doping ratio while another oxide Mn2O3 is detected in the physical mixture of ceria and manganese oxide. These Mn-doped mixed oxides, especially Ce0.5Mn0.5O2, presented better catalytic activities than Ce0.5Zr0.5O2 and even Pt-loaded catalyst for total oxidation of C3H8 and oxidative sorption of NO in the presence of excess oxygen. The oxidation ability of Mn and the strong interaction between Mn and Ce were suggested to promote the oxygen storage/transport capacity of the mixed oxides as well as reactive adsorption of nitric oxide and hydrocarbons.

A series of Ba–Cu–Ce catalysts were prepared by loading different amounts of Ba(Ac)2 on the sol–gel-synthesized CuOx–CeO2 mixed oxides. The activities of the catalysts for soot oxidation were evaluated in 1000 ppm NO/10% O2/N2 under loose contact conditions. The catalysts were characterized by XRD, BET, H2-TPR, in situ-DRIFTS and NO-TPO measurements. When Ba loading is between 6 and 10 mol.% of Cu + Ce, the catalysts exhibit an onset temperature (Ti) as low as 376 °C. NO is oxidized at Cux+ and Cex+ sites and then stored on the adjacent Ba species in form of barium nitrate. Ba(NO3)3 begins to decompose, releasing abundant NO2 at around 370 °C under the driving of soot as the reductant. The nitrate-derived NO2 and the NO-derived NO2 initiate the soot combustion over the Ba-modified catalysts with a significantly lowered Ti. The ternary catalysts lose their NOx storage capacity after hydrothermally aged at 800 °C for 10 h due to the formation of BaCeO3. However, Ba restrains the sintering of (Cu,Ce)Ox, resulting in relatively more favorable redox properties and thus higher activity for NO oxidation. The barium-modified catalysts show lower onset temperatures (470–480 °C) of soot oxidation than pure CuCe (509 °C) after aging.Graphical abstractDownload high-res image (166KB)Download full-size imageHighlights► The soot oxidation activity of CuOx–CeO2 catalyst in NOx is enhanced by loading Ba. ► The optimum loading amount of barium is 6–10 at.% of (Cu + Ce). ► The nitrate-derived NO2 assists the soot catalytic combustion. ► The NOx storage capacity deteriorates due to formation of BaCeO3 after aging. ► The loading of Ba restrains the sintering of CuO and CeO2 during hydrothermal aging.

The Cu- and Mn-doped ceria were prepared using the citric acid sol–gel method. The structural and redox properties of the mixed oxides were investigated by means of XRD, BET, O2-TPD, CO-TPR and CO reduction under isothermal conditions. TPO tests were performed to evaluate the catalytic activity for soot oxidation. The results showed that Mnx+ cations entered into the ceria lattice to form solid solutions, which increased the amount of oxygen vacancies and promoted surface oxygen chemisorption. CuxO clusters were more likely to be dispersed on the surface of ceria particles. The interaction between copper and cerium greatly enhanced the rapid release of lattice oxygen of the oxides in the reducing atmosphere. These two mixed oxides showed improved catalytic activities and selectivities to CO2 for soot oxidation compared with pure ceria. The order of activities under different contact conditions was believed to be related to active oxygen species released by the catalysts.

Platinum nanoparticles were synthesized by a classic polyol process in ethylene glycol. The Pt nanoparticles were loaded on H-ZSM5 and γ-Al2O3 supports by wet impregnation. The as-obtained solid was thermally treated in a two-step procedure to remove the organic residues. Due to their larger particle size than the micropore size of H-ZSM5, the impregnated Pt particles were dispersed on the external surface of the zeolite support. With similar Pt particle size distribution and Pt loading, the Pt/H-ZSM5 and Pt/γ-Al2O3 catalysts were evaluated for the oxidation of NO and soot. Despite its lower activity for NO oxidation to NO2, the Pt/H-ZSM5 exhibited a much higher activity for soot oxidation in the presence of NO + O2. This abnormal NOx-assisted soot oxidation behaviour of Pt/H-ZSM5 was explained by the promoting effect of the acid sites on the zeolite.

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.