The preparation and properties of coprecipitated Cu–Zr–Y and Cu–Zr–La catalysts used for the steam reforming of methanol

Several samples of exhaust diesel soot are investigated by inverse gas chromatography and linear solvation energy relationship (LSER) modelling according to their soluble organic fraction content and their time of exposure in oxidative conditions. The results demonstrate the evolution of the adsorptive properties of the studied materials towards volatile compounds during the oxidation under NO₂.     

复合氧化物的制备及其在碳黑颗粒物氧化中的应用

采用柠檬酸络合燃烧法合成钾铈和钾铈镧复合氧化物催化剂。利用XRD对复合氧化物进行了表征。利用程序升温反应（TPR）方法研究了其对碳黑颗粒物的催化氧化性能,考查了焙烧温度、焙烧时间、前驱体中K、Ce的物质的量比等制备条件对催化剂活性的影响及稳定性。结果表明：钾铈镧复合氧化物对碳黑颗粒物具有较高的催化活性和稳定性。     

The heterogeneous reaction between soot and NO2 at elevated temperature

Two commercial soot samples are analysed by temperature programmed desorption mass spectroscopy (TPD-MS) in order to determine the functional groups at their surface. A systematic approach has been developed, which assures that all functional groups decomposing between 100 °C and 900 °C can be distinguished and assigned. For the understanding of the mechanism of the reaction between soot and NO₂ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and TPD-MS are applied. Both spectroscopic techniques monitor the formation of new species. The reaction with NO₂ results in the formation of an acidic functional group, which decomposes into CO₂ and NO at 140 °C. It is proven that NO₂ reacts directly with the carbon surface. Pre-existing functional groups do not interfere. From these results the reaction sequence has been concluded.     

Study of the influence of the soluble organic fraction of an exhaust diesel soot by a linear solvation energy relationship approach

Two samples of soot are studied in the present work. The results obtained on a virgin diesel soot (VDS) are compared to those obtained on the corresponding material after removal of its soluble organic fraction (SOF). The adsorption properties of both materials towards organic compounds are investigated by gas chromatography (GC) using the linear solvation energy relationship (LSER) equation of Abraham [Chem Soc Rev 1993;22:73-83]. This approach allows each material to be characterized in terms of five quantitative molecular interaction parameters. The results show the large influence of the SOF in the adsorption process of gaseous organic compounds. This study reveals that the extracted diesel soot (EDS) can interact through molecular interactions similar to those obtained on fullerenes or graphite.     

Global kinetic modelling of the reaction of soot with O2 and NOx on Fe2O3 catalyst

This study deals with the catalytic reaction of NO_x and soot on Fe₂O₃ to yield N₂ and CO₂ in excess of oxygen. Based on the three types of kinetic experiments, i.e. temperature programmed oxidation (TPO), transient examinations and gradient-free loop reactor experiments, as well as mechanistic studies presented recently a global kinetic model is established. The model includes catalytic effect of the iron oxide on soot/O₂ reaction, whereas it is assumed that NO_x reduction occurs on the soot without direct participation of Fe₂O₃. Furthermore, the model implies global kinetic expressions for the CO_x formation and NO_x reduction. These equations include the evolution of the surface area of soot and the correlation of reactive carbon sites (C_f) with those specifically involved in NO_x reduction (C*). The kinetic model is sequentially developed by accounting for the catalytic and non-catalytic soot/O₂ as well as soot/NO_x/O₂ conversion. Kinetic parameters are taken from the literature and are also determined from a fit to experimental data. Comparison of measured and calculated data shows accurate reproduction of the experiments and the model. Finally, the kinetic model is validated by some simulations.     

Catalytic effect of platinum on the kinetics of carbon oxidation by NO2 and O2

The effect of a commercial Pt/Al₂O₃ catalyst on the oxidation by NO₂ and O₂ of a model soot (carbon black) in conditions close to automotive exhaust gas aftertreatment is investigated. Isothermal oxidations of a physical mixture of carbon black and catalyst in a fixed bed reactor were performed in the temperature range 300–450 °C. The experimental results indicate that no significant effect of the Pt catalyst on the direct oxidation of carbon by O₂ and NO₂ is observed. However, in presence of NO₂–O₂ mixture, it is found that besides the well established catalytic reoxidation of NO into NO₂, Pt also exerts a catalytic effect on the cooperative carbon–NO₂–O₂ oxidation reaction. An overall mechanism involving the formation of atomic oxygen over Pt sites followed by its transfer to the carbon surface is established. Thus, the presence of Pt catalyst increases the surface concentration of –C(O) complexes which then react with NO₂ leading to an enhanced carbon consumption. The resulting kinetic equation allows to model more precisely the catalytic regeneration of soot traps for automotive applications.     

Co3O4–CeO2 mixed oxide-based catalytic materials for diesel soot oxidation

Co₃O₄–CeO₂ type mixed oxide catalyst compositions have been prepared by using co-precipitation method and, their catalytic activity towards diesel particulate matter (PM)/carbon oxidation has been evaluated under both loose and tight contact conditions. These catalysts show excellent catalytic activity for PM/carbon oxidation, despite their low surface area. The activation energy observed for non-catalyzed and catalyzed reactions are 163 kJ/mol and 140 kJ/mol, respectively, which also confirm the catalytic activity of catalyst for carbon/soot oxidation. The promotional effects of an optimum amount of cobalt oxide incorporation in ceria and presence of a small amount of potassium appears to be responsible for the excellent soot oxidation activity of this mixed oxide type material. The catalytic materials show good thermal stability, while their low cost will also add to their potential for practical applications.     

The effect of H2 treatment on the activity of activated carbon for the oxidation of organic contaminants in water and the H2O2 decomposition

In this work, hydrogen peroxide reactions, i.e. H₂O₂ decomposition and oxidation of organics in aqueous medium, were studied in the presence of activated carbon. It was observed that the carbon pre-treatment with H₂ at 300, 500, 700 and 800 °C resulted in an increase in activity for both reactions. The carbons were characterized by BET nitrogen adsorption, thermogravimetric analyses (TG), temperature programmed reduction (TPR), electron paramagnetic resonance (EPR), iodometric titration and determination of the acid/basic sites. TPR experiments showed that activated carbon reacts with H₂ at temperatures higher than 400 °C. The treatment produces a slight increase in the surface area. EPR analyses indicate the absence of unpaired electrons in the carbon. Iodometric titrations and TG analyses suggested that the treatment with H₂ generates reduction sites in the carbon structure, with concentration of approximately 0.33, 0.53, 0.59, 0.65 and 0.60 mmol/g for carbons treated at 25, 300, 500, 700 and 800 °C, respectively. It was also observed the appearance of basic sites which might be related to the reduction sites. It is proposed that these reducing sites in the carbon can activate H₂O₂ to generate HO^* radicals which can lead to two competitive reactions, i.e. the hydrogen peroxide decomposition or the oxidation of organics in water.     

Heterogeneous reactions of NO2 and NO-O2 on the surface of carbons

The interactions of nitrogen oxides with carbons differing in the chemical structure of surface functional groups were studied using in situ FTIR combined with the measurements of catalytic activity. Microporous carbon samples with similar pore size distribution were prepared from cellulose. The structure and coverage of adsorbates during reactions at temperatures between 295 and 573 K are determined by FTIR. No significant changes in NO_x reaction with carbon surface were found by oxidation of the carbonized film. During the study of the reaction of NO/O₂ mixture with carbons, the infrared absorption bands for the surface species formed are similar to the IR bands observed after the reaction of carbon samples with NO₂. For both reactions, surface species, including C-NO₂, C-ONO, C-NCO and anhydride structures are formed. Catalytic NO_x reduction by carbons has been investigated in the temperature range 295-623 K in the flow reactor equipped with an FTIR gas analyzer. As the surface of carbon is exposed to NO₂ gaseous NO is formed. The reduction of NO₂ to N₂ without the use of an externally supplied reductant can be achieved with microporous carbons. Significant NO₂ conversion to N₂ occurred at 623 K on both oxidized and non-oxidized carbons.     

Influence of the soluble organic fraction on the thermal behaviour, texture and surface chemistry of diesel exhaust soot

Diesel soot samples collected on a SiC filter while using or not an oxidation catalyst, were solvent extracted or heat treated under inert atmosphere. All studied soot were characterised by XPS, DRIFTS, TGA-MS, Py-GC-MS and gas sorption. In the first part, the obtained results are discussed in order to show the influence of an oxidation catalyst on some properties of the raw soot materials, such as their soluble organic fraction (SOF) content and composition, their surface chemistry and textural characteristics. Then, it is seen that the thermal decomposition of SOF adsorbed on the particulate matter leads under inert atmosphere to the formation of a microporous carbonaceous layer at 600 °C. On the other hand, textural analyses have revealed that the structure of the raw soot material is non-porous in nature, as subsequent extractions in DCM and toluene do not seem to create any porosity within the samples. Finally, we have described and discussed the composition of the particulate matter as a function of its respective components, namely SOF, volatile organic fraction (VOF), carbonaceous matrix and ashes.     

Oxidation of carbon by NOx, with particular reference to NO2 and N2O

The reactions reviewed here concern those between elemental carbon and NO₂, N₂O and NO, sometimes in the presence of oxygen. The section on NO includes only updates to recent reviews. Soots, activated carbons and carbon blacks are more reactive than graphite. The magnitudes of the reaction rates are found to be: NO₂ > N₂O ≈ NO ≈ O₂. The presence of a soluble organic fraction (SOF) in soot is found to influence some reactions, and all three reactions suffer from inhibition by surface products. The mechanisms proposed for the surface adsorbates are summarised. All authors found that two types of active site were present; one forming weak bonds (physisorption), and the other undergoing chemisorption to form groupings such as -C-ONO, -C-ONO₂ or -C-NO₂. The latter decompose to give oxides of carbon, and are sometimes called redox reactions. The adsorbates appear to be the same for all NO_x species. Some elemental nitrogen adsorption takes place, and can involve incorporation into the C skeleton. The attack of NO on carbon proceeds via NO₂, so that catalysts that facilitate this oxidation are effective. Gaseous SO₂ and H₂O assist in the process by forming acids which are good oxidants. The change in activation energy with temperature found experimentally for NO and N₂O may be due to the form of nitrogen on the edge carbon atoms.     

The catalytic activity of CuO–CeO2 mixed oxides for diesel soot oxidation with a NO/O2 mixture

Copper doped ceria and ceria–zirconia mixed oxides were synthesized using the citric acid sol–gel method. The temperature-programmed oxidation (TPO) results showed that the Cu modification improved the low-temperature activity and the selectivity to CO₂ of ceria for soot oxidation in the presence of NO and excess oxygen even after ageing at 800 °C for 20 h in flow air. Meanwhile, not only the segregation of Cu and sintering of CuO, but also the separation of Ce- and Zr-rich phases worsened the activity of the Cu–Ce–Zr catalyst after the high-temperature calcination.     

Effect of experimental conditions on the parameters used for evaluating the performance of the catalyst Mo/Al2O3 in diesel soot combustion

The literature reported different studies of soot combustion reaction under very distinct experimental conditions, which can include different values of catalyst:soot weight ratios, gas flow and heating rates. Therefore, avoiding screening of innumerable catalysts or empirical experiments, this work aims to present a general methodology based on a statistical experimental design of experiments with soot combustion, evaluating different reaction conditions and parameters that can be used for any other similar study. In this way, the effect of experimental conditions on the parameters used for evaluating the performance of Mo/Al₂O₃, a promising system previously studied, and Pt/Al₂O₃, a notorious catalytic system, were studied by a complete factorial experimental design. The results have shown that the experimental conditions strongly interfere with the parameters used for evaluating the catalytic performance and then it may generate incorrect conclusions.The effects of interaction between different conditions on the activity and mainly on the selectivity of CO₂ permitted to explain the performance of catalysts on soot combustion and to distinguish different pathways of catalytic and non-catalytic reactions under specific reaction conditions.The most appropriate conditions for studying soot combustion seem to be high cat:soot ratios, low heating rates and high gas flow rates, which, according to this work, must be equal to: 95:1, 2 K min⁻¹ and 115 mL min⁻¹, respectively.     

An exploratory study of diesel soot oxidation with NO2 and O2 on supported metal oxide catalysts

A number of supported metal oxide catalysts were screened for their catalytic performance for the oxidation of carbon black (CB; a model diesel soot) using NO₂ as the main oxidant. It was found that contact between the carbon and catalyst was a key factor in determining the rate of oxidation by NO₂. Oxides with low melting points, such as Re₂O₇, MoO₃ and V₂O₅ showed higher activities than did Fe₃O₄ and Co₃O₄. The activities of MoO₃ and V₂O₅ on various supporting materials were also examined. MoO₃/SiO₂ was the most active catalyst among the supported MoO₃ examined, whereas, V₂O₅/MCM-41 showed the highest activity among the supported V₂O₅. Different performances of the supported MoO₃ catalysts were explained by the interaction of MoO₃ with the supports: a strong MoO₃/support interaction may result in a poor mobility of MoO₃ and a poor activity for oxidation of carbon by NO₂. The high activity of V₂O₅/MCM-41 was associated with its catalysis of the oxidation of SO₂ by NO₂ to form SO₃, which substantially promotes oxidation of carbon by NO₂. Addition of transition metal oxides or sulfates to supported MoO₃ and V₂O₅ was also investigated. Combining MoO₃ or V₂O₅ with CuO on SiO₂, adding VOSO₄ to MoO₃/SiO₂ or MoO₃/Al₂O₃ and adding TiOSO₄ or CuSO₄ to V₂O₅/Al₂O₃ improved the catalytic performance.     

A novel laboratory bench for practical evaluation of catalysts useful for simultaneous conversion of NOx and soot in diesel exhaust

This study deals with the development of a laboratory bench for the practical evaluation of catalysts that are useful for the direct conversion of NO_x and soot in the exhaust of diesel engines. The employed model exhaust is generated by using a diffusion burner with additionally dosing some gaseous components to the burner gas to obtain a realistic feed composition. The produced soot is extensively characterized by employing thermogravimetry, transmission electron microscopy, N₂ physisorption and temperature programmed techniques. The results of the different characterization methods show that the present soot is suitable for the intended catalytic investigations. The simultaneous conversion of NO_x and soot is examined like in practice, i.e. the soot is separated from the tail gas by a diesel particulate filter (DPF) that is coated with the catalyst. The deposited soot is then catalytically converted by NO_x and O₂ to form N₂ and CO₂. The conversions of NO_x and soot are measured by exclusively applying gas analysers, whereby a special experimental procedure is developed to determine the soot removal. Hence, additional soot related analytics are not required. To show the suitability of the constructed bench a Pt/Fe₂O₃/β-zeolite sample is taken as test catalyst that is reported to be very active in NO_x/soot reaction. The measurements performed with and without catalyst clearly show the effect of the used sample in simultaneous NO_x/soot conversion. We therefore consider the constructed laboratory bench to be a useful tool for testing and ranking catalytic materials.     

Study of the reaction of NOx and soot on Fe2O3 catalyst in excess of O2

This study addresses the catalytic reaction of NO_x and soot into N₂ and CO₂ under O₂-rich conditions. To elucidate the mechanism of the soot/NO_x/O₂ reaction and particularly the role of the catalyst -Fe₂O₃ is used as model sample. Furthermore, a series of examinations is also made with pure soot for reference purposes. Temperature programmed oxidation and transient experiments in which the soot/O₂ and soot/NO reaction are temporally separated show that the NO reduction occurs on the soot surface without direct participation of the Fe₂O₃ catalyst. The first reaction step is the formation of CC(O) groups that is mainly associated with the attack of oxygen on the soot surface. The decomposition of these complexes leads to active carbon sites on which NO is adsorbed. Furthermore, the oxidation of soot by oxygen provides a specific configuration of active carbon sites with suitable atomic orbital orientation that enables the chemisorption and dissociation of NO as well as the recombination of two adjacent N atoms to evolve N₂. Moreover, carbothermal reaction, high resolution transmission electron microscopy and isotopic studies result in a mechanistic model that describes the role of the Fe₂O₃ catalyst. This model includes the dissociative adsorption of O₂ on the iron oxide, surface migration of the oxygen to the contact points of soot and catalyst and then final transfer of O to the soot. Moreover, our experimental data suggest that the contact between both solids is maintained up to high conversion levels thus resulting in continuous oxygen transfer from catalyst to soot. As no coordinative interaction of soot and Fe₂O₃ catalyst is evidenced by diffuse reflectance infrared Fourier transform spectroscopy a van der Waals type interaction is supposed.     

Reactivity of NO/NO2–NH3 SCR system for diesel exhaust aftertreatment: Identification of the reaction network as a function of temperature and NO2 feed content

We present a systematic study of the NH₃-SCR reactivity over a commercial V₂O₅–WO₃/TiO₂ catalyst in a wide range of temperatures and NO/NO₂ feed ratios, which cover (and exceed) those of interest for industrial applications to the aftertreatment of exhaust gases from diesel vehicles. The experiments confirm that the best deNO_x efficiency is achieved with a 1/1 NO/NO₂ feed ratio. The main reactions prevailing at the different operating conditions have been identified, and an overall reaction scheme is herein proposed.

Particular attention has been paid to the role of ammonium nitrate, which forms rapidly at low temperatures and with excess NO₂, determining a lower N₂ selectivity of the deNO_x process. Data are presented which show that the chemistry of the NO/NO₂–NH₃ reacting system can be fully interpreted according to a mechanism which involves: (i) dimerization/disproportion of NO₂ and reaction with NH₃ and water to give ammonium nitrite and ammonium nitrate; (ii) reduction of ammonium nitrate by NO to ammonium nitrite; (iii) decomposition of ammonium nitrite to nitrogen. Such a scheme explains the peculiar deNO_x reactivity at low temperature in the presence of NO₂, the optimal stoichiometry (NO/NO₂ = 1/1), and the observed selectivities to all the major N-containing products (N₂, NH₄NO₃, HNO₃, N₂O). It also provides the basis for the development of a mechanistic kinetic model of the NO/NO₂–NH₃ SCR reacting system.     

Application of NOX storage/release materials based on alkali-earth oxides supported on Al2O3 for high-temperature diesel soot oxidation

Alkali-earth oxides and nitrates supported on alumina were studied as model systems for NO_X storage/release. Their impact on the high-temperature soot oxidation has been investigated. The stability of surface nitrates and temperature of NO_X release increase parallel to the basicity of the cation. The presence of soot decreases the temperature of NO_X release. The storage capacity depends on the several factors, such as basicity, dispersion of the cation, and pre-treating conditions. Adsorption of NO with O₂ at 200 °C leads to the formation of surface nitrates that mainly exist as ionic nitrates. Stored nitrates contribute to the soot oxidation and assist to lower the temperature of soot oxidation up to almost 100 °C. In the presence of only NO_X storage material the efficiency of NO_X utilization is, however, quite low, around 30%. Therefore, the presence of an oxidation catalyst is essential to increase the efficiency of NO_X utilization for soot oxidation up to 140% and selectivity towards CO₂. A combination of oxidation catalyst with NO_X storage materials enables to lower the temperature of soot oxidation more than 100 °C for the Sr- and Ca-based systems.     

Synthesis and catalytic properties of Ce0.6Zr0.4O2 solid solutions in the oxidation of soluble organic fraction from diesel engines

Ce_0.6Zr_0.4O₂ solid solutions were synthesized by co-precipitation, sol–gel like method, solution combustion and surfactant-assistant approaches, respectively. The catalytic properties of bulk and γ-Al₂O₃ supported Ce_0.6Zr_0.4O₂ solid solutions were studied for the oxidation of soluble organic fractions (SOF) from diesel engines by TG-DTA method. The physicochemical properties were characterized by XRD, BET surface area and pore distribution, SEM, TEM, and particle size distribution techniques. XRD and TEM results show that a Ce_0.6Zr_0.4O₂ solid solution was formed for samples as-prepared and heat-treated at 900 °C for 2 h in air. The co-precipitation derived Ce_0.6Zr_0.4O₂ has as high BET surface area as 153.71 m²/g by controlling preparation conditions. Notable is that the surface area and particle size for fresh Ce_0.6Zr_0.4O₂ ignited at 350 °C decreased little after a thermal treatment in air at 900 °C for 2 h. Furthermore, its bulk density is lowest. The commercial engine oil (SJ5W/40) for FAW-VOLKSWAGEN, which was used by Bora 1.9 TDI diesel cars in China market was substituted for SOF. The catalytic activity was evaluated by normalized peak areas and extrapolated onset temperatures of DTA curves. A computer program was developed by direct non-linear regression model for simulation of TG/DTG curves to determine the thermal processes and kinetic parameters. It is found that lube evaporation/decomposition and thermal decomposition (pyrolysis) were observed under a nitrogen atmosphere. Lube evaporation fractions were inhibited by Ce_0.6Zr_0.4O₂ and γ-Al₂O₃. While under an air atmosphere, namely, in the process of lube oxidation (combustion), evaporation/decomposition, low-temperature oxidation and high-temperature oxidation were distinguished. Ce_0.6Zr_0.4O₂ solid solutions are active catalysts for lube oxidation, in which the sample prepared by solution combustion has the highest activity, mainly due to the maintenance of the surface area and particle size upon sintering and its lowest bulk density. However, γ-Al₂O₃ is more like a support. There exists synergism between Ce_0.6Zr_0.4O₂ and γ-Al₂O₃: γ-Al₂O₃ adsorbs lube retaining it within its pore structure, whereas, Ce_0.6Zr_0.4O₂ solid solutions initiate oxidation reactions when light-off temperatures reach. The application of CeO₂-ZrO₂ solid solution prepared by solution combustion at lower temperature would be promising in diesel oxidation catalysts.     

The influence of NOx on the oxidation of metal activated diesel soot

The influence of NO on the oxidation of metal (cerium, copper, and iron)-activated soot was studied. Without NO in the gas phase, the activation energy of soot is ≈170 kJ/mol, independent of the type of metal applied in the soot. The rate-limiting step in the oxidation with oxygen is probably the decomposition of surface oxygen complexes. In presence of NO, the oxidation rate of soot mixed with a supported platinum catalyst is increased significantly, especially for cerium-activated soot. The activation energy of the oxidation reaction is decreased by the presence of NO in the gas phase. The increase in reaction rate as a result of NO and a platinum catalyst is explained by a cycle of two catalytic reactions, where platinum oxidises NO to NO₂, which subsequently oxidises soot using cerium as a catalyst, forming NO which can participate in the reaction more than once. This oxidation mechanism can be put into practice by combining a platinum-activated particulate trap with a combination of platinum and cerium fuel additives. This combination might be a breakthrough in the search for an applicable catalytic soot removal system.