A mechanistic study on the simultaneous elimination of soot and nitric oxide from engine exhaust

The non-catalytic interaction between soot and nitric oxide (NO) resulting in their simultaneous elimination was studied on different types of reactive site present on soot. The reaction mechanism proposed previously was extended by including seven new reaction pathways for which the reaction energetics and kinetics were studied using density functional theory and transition state theory. This has led to the calculation of a new rate for the removal of carbon monoxide (CO) from soot. The new pathways have been added to our polycyclic aromatic hydrocarbon (PAH) growth model and used to simulate the NO–soot interaction to form CO, N₂ and N₂O. The simulation results show satisfactory agreement with experiment for the new CO removal rate. The NO–soot reaction was found to depend strongly on the soot site type and temperature. For a set of temperatures, computed PAH structures were analysed to determine the functional groups responsible for the decrease in the reactivity of soot with NO with increasing reaction time. In isothermal conditions, it was found that as temperature is increased, the number of oxygen atoms remaining on the soot surface decreases, while the number of nitrogen atoms increases for a given reaction time.     

Study on the mechanism of the catalytic conversion of NO x and soot into N2 and CO2 on Fe2O3 in diesel exhaust

The present study deals with the mechanism of the conversion of NO _x and soot into N₂ and CO₂ on Fe₂O₃ catalyst. The results of TPO, TRM, DRIFTS and HRTEM examinations suggest a mechanism, in which NO is reduced by dissociation on active carbon sites leading to the formation of N₂ and surface oxygen groups. The role of the catalyst lies in the activation of the soot by transferring oxygen from Fe₂O₃ to soot surface.     

The Effect of Oxygen Concentration on the Reaction of NOx with Soot Over BaAl2O4

The effect of oxygen concentration on the catalytic reaction of NO_x with soot over BaAl₂O₄ has been studied by Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS). The introduction of O₂ into the NO flow can result in a reactant mixture of NO/NO₂/O₂. Increasing the O₂ concentration from 2 % to 5 % in the NO‐containing flow promotes the formation of NO₂ from the gas phase oxidation of NO. The reactant mixture with high O₂/NO flow, allows for the formation of greater amounts of nitrate species than that with low O₂/NO flow, which further promotes the reaction of soot with NO_x and leads to a high conversion efficiency of NO_x into N₂ and N₂O. In the absence of O₂, N₂O is not observed since the N₂O produced at high temperatures has reacted with soot before it can be detected.     

Temperature‐Programmed Oxidation of Soot in a Hybrid Catalysis‐Plasma System

Non‐thermal plasma (NTP) technology was applied to promote the temperature‐programmed oxidation (TPO) of soot over a perovskites type of La_0.8K_0.2MnO₃ catalyst. The O radicals originating from the decomposition of O₂, as well as NO dissociation if nitrogen oxide were involved, reduce the ignition temperatures of soot. In NO‐O₂‐He, for example, the ignition temperature decreased to 240 °C from 290 °C as the voltage increased from 0 kV to 15 kV. The higher voltage also benefited the adsorption of NO molecules onto the catalyst surface (NO_ad). As a result, the maximum N₂/NO ratio (conversion ratio of NO into N₂) rose from 23 % to 53 %. Some of the NO molecules were dissociated into N and O radicals in plasma, and hence, the N₂/NO ratio was further enhanced due to the combination of N atoms. In any case, the redox process between NO_x and soot proved to be important in soot oxidation.     

XPS investigation of the reaction of carbon with NO, O2, N2 and H2O plasmas

The interaction of graphite with plasmas of pure gases (O₂, N₂ or H₂O), air or mixtures of gases containing NO has been studied by XPS “in situ” analysis. Depending on the type of plasma, different species of nitrogen, oxygen and carbon have been detected on the surface of graphite. The nitrogen containing species have been attributed to pyridinic, pyrrol, quartenary and oxidized groups adsorbed on the surface. The evolution with the treatment time of the relative intensity of the different nitrogen bands for Ar + NO, N₂ + NO, air or N₂ plasmas has served to propose a model accounting for the reactions of graphite with plasmas of NO containing gases. The model explains why carbon materials (in the form of graphite, soot particles, etc.) can be very effective for the removal of the NO present in exhaust combustion gases excited by a plasma. The analysis of the C1s and O1s photoemission peaks reveals the formation of C/O adsorbed species up to a maximum concentration on the surface of around 10% atomic oxygen. A general evolution is the progressive formation of C/O species where the carbon is sp³ hybridized. This tendency is enhanced when graphite is treated with the plasma of water.     

Reduction of lean NO2 with diesel soot over metal-exchanged ZSM5, perovskite and γ-alumina catalysts

The catalytic performances of metal-exchanged ZSM5, perovskite and γ-alumina catalysts for the reduction of nitrogen dioxide (NO₂) by diesel soot were investigated. The reaction tests were performed through temperature-programmed reaction (TPR), in which NO₂ and O₂ were passed through a fixed bed of catalyst-soot mixture. On the three types of catalyst, NO₂ was reduced to N₂ by model soot (Printex-U) and most of the soot was converted into CO₂. Pt-, Cu- and Co-exchanged ZSM5 catalysts exhibited reduction activities with conversions of NO₂ into N₂ of about 20%. Among the perovskite catalysts tested, La_0.9K_0.1FeO₃ showed a 32% conversion of NO₂ into N₂. The catalytic activities of the perovskite catalysts were largely influenced by the number and stability of oxygen vacancies. For the γ-alumina catalyst, the peak reduction activity appeared at a relatively high temperature of around 500 °C, but the NO₂ reduction was more effective than the NO reduction, in contrast to the results of the ZSM-5 and perovskite catalysts.     

An update on the mechanism of the graphene–NO reaction

Cognizant of the key experimental facts from studies of carbonaceous solids ranging from soot to graphite, we performed a quantum chemistry study of the interaction of NO monomer or dimer with one or more zigzag sites. Thermodynamic and kinetic results were used to examine two alternative mechanisms proposed in the literature, and to compare them with the graphene–O₂ reaction mechanism. The chemisorption stoichiometry similarities are striking; but the differences, especially regarding the intermediate role of N₂O, have important practical implications. Monomer chemisorption on an isolated site is a dead-end and temporarily inhibiting process, similar to that of formation of a stable C–O surface complex in the graphene–O₂ reaction. When two sites are available, successive monomer adsorption eventually leads to N₂O formation subsequent to parallel reorientation of the first NO molecule. If three contiguous sites are available, N₂ and CO are the principal products. Chemisorption of the dimer provides a straightforward path to N₂ and CO₂ when one site is available and to N₂ and CO when two sites are available. The formation of N₂O is also feasible in this case, both during adsorption and desorption; in the adsorption phase it is very sensitive to the details of the electron pairing processes.     

Simultaneous Catalytic Removal of Nitrogen Oxides and Soot from Diesel Exhaust Gas over Potassium Modified Iron Oxide

Iron oxide modified by potassium, i.e. Fe_1.9K_0.1O₃, exhibits high catalytic performance for the simultaneous conversion of soot and NO_x into CO₂ and N₂. The present study shows that long‐time treatment of the catalyst leads to a drastic decrease in the activity, whereas even the aged catalyst maintains considerable activity. On the other hand, long‐time treatment causes selective N₂ formation, i.e. no more formation of the byproduct N₂O. This alteration of catalytic performance is likely due to agglomeration of the promoter potassium being present at the surface of catalyst. Detailed experiments were carried out with a more realistic diesel model exhaust gas to confirm that Fe_1.9K_0.1O₃ is a suitable catalyst for the simultaneous removal of soot and NO_x between 350 and 480 °C. It was assumed that (CO) intermediates, formed by the catalytic reaction of NO_x and oxygen with the soot surface, are the reactive species in NO_x‐soot conversion.     

Highly efficient oxidation of soot over RuOx/SiO2 in fluidized bed reactor

The potentialities of RuOx/SiO₂ for soot oxidation have been investigated using a temperature programmed reaction technique. Among the noble metal catalysts, RuOx/SiO₂ exhibited the highest activities in fluidized bed reactor. The temperature at 50% soot conversion over RuOx/SiO₂ in the presence of NO and O₂ is about 120 K lower than that only in the presence of O₂. It has been identified that NO was adsorbed on the surface of RuOx in the presence of O₂. The adsorbed NO was highly active for oxidizing soot, which can be attributed to the high mass transferring of the active species in the reaction systems. The mechanism of NO dissociation is also discussed in soot oxidation.     

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.     

Determination of Mechanism for Soot Oxidation with NO on Potassium Supported Mg‐Al Hydrotalcite Mixed Oxides

Soot oxidation with NO (in the absence of gas phase O₂) on potassium‐supported Mg‐Al hydrotalcite mixed oxides (K/MgAlO) was studied using a temperature‐programmed reaction and in situ FTIR techniques. Nitrite and the ketene group were identified as the reaction intermediates and thus a nitrite‐ketene mechanism was proposed in which surface active oxygen on K sites of K/MgAlO is transferred to soot by NO through nitrites. In the absence of gas phase O₂, soot oxidation with NO at lower temperatures (below 450 °C) is limited by the amount of active oxygen on the K sites. This kind of active oxygen is not reusable but can be replenished in the presence of gas phase O₂.     

The reduction of NO with H2 over Ru/MgO

Ruthenium supported on magnesia was found to be a highly active and selective catalyst for the reduction of NO to N₂ with H₂. The adsorption of NO on Ru/MgO was studied at room temperature by applying frontal chromatography with a mixture of 2610 ppm NO in He. Subsequently, temperature‐programmed desorption (TPD) and temperature‐programmed surface reaction (TPSR) experiments in H₂ were performed. The adsorption of NO was observed to occur partly dissociatively as indicated by the formation of molecular nitrogen. The TPD spectrum exhibited a minor NO peak at 340 K indicating additional molecular adsorption of NO during the exposure to NO at room temperature, and two N₂ peaks at 480 K and 625 K, respectively. The latter data are in good agreement with previous results with Ru(0001) single‐crystal samples, where the interaction with NH₃ was found to lead to two N₂ thermal desorption states with a maximum coverage of atomic nitrogen of about 0.38. Heating up the catalyst after saturation with NO at room temperature in a H₂ atmosphere revealed the self‐accelerated formation of NH₃ after partial desorption of N₂, whereby sites for reaction with H₂ become available. As a consequence, the observed high selectivity towards N₂ under steady‐state reduction conditions is ascribed to the presence of a saturated N+O coadsorbate layer resulting in an enhanced rate of N₂ desorption from this layer and a very low steady‐state coverage of atomic hydrogen. The formation of H₂O by reduction of adsorbed atomic oxygen is the slow step of the overall reaction which determines the minimum temperature required for full conversion of NO. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dinitrogen formation over low-exchanged Cu-ZSM-5 in the selective reduction of NO by propane

The role of gaseous NO and C₃H₈ has been studied over low-exchanged Cu-ZSM-5 zeolite employing TPD, FTIR and pulse technique with the alternate introduction of NO or C₃H₈ onto the catalyst surface. The rate of the N₂ formation is directly proportional to the content of gaseous NO and the surface coverage with 2-nitrosopropane. There was no formation of N₂ during interaction of gaseous C₃H₈ with NO adsorbates. However, 2-nitrosopropane and its isomer acetone oxime were also formed in this reaction. This revised version was published online in November 2006 with corrections to the Cover Date.     

The catalytic reduction of nitric oxide over supported ruthenium catalysts

The activity of supported ruthenium catalysts for reducing NO to N₂ in an exhaust-like feedstream has been examined in laboratory experiments. The rate and temperature of NO removal is largely dependent on the NO inlet concentration and independent of the concentration of reducing agents in the system. The selectivity for nitrogen formation, however, is dependent on the concentration of the reducing agents CO and H₂ as well as the concentration of NO. No evidence was found for an ammonia intermediate in the conversion of nitric oxide to elemental nitrogen over ruthenium. The high selectivity of ruthenium for the NO to N₂ conversion is explained and compared with the behavior of platinum and palladium catalysts.     

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.     

Catalytic Reduction of Nitric Oxide by Hydrogen Sulfide Over γ-alumina

The ability of H₂S to reduce NO in a fixed bed reactor using a γ-alumina catalyst was studied with the objective of generating new methods for conversion of NO to N₂. Compared to the homogenous reaction of NO with H₂S, the catalyzed reaction showed improved conversions of NO to N₂. Using a gas space velocity of 1000 h⁻¹ and a feed of 1% NO and 1% H₂S in argon, it was found that the conversion of NO to N₂ was complete at 800 °C. This result compared to a 38% conversion of NO to N₂ for the homogeneous gas phase reaction at 800 °C. At temperatures below 800 °C, a short fall in the nitrogen balance was discovered when the γ-alumina was employed as a catalyst. This discrepancy was explained by conversion of NO to NH₃ and subsequent reaction of the NH₃ with any SO₂ in the system to form ammonium sulfur oxy-anion salts. This suggestion is supported by the finding that when larger amounts of H₂S were used relative to NO, more NH₃ was formed together in tandem with lower N₂ mass balances. Several reaction pathways have been proposed for the catalytic reduction of NO by H₂S.     

The catalytic oxidation of ammonia: influence of water and sulfur on selectivity to nitrogen over promoted copper oxide/alumina catalysts

The CuO/Al₂O₃ system is active for ammonia oxidation to nitrogen and water. The principal by-products are nitrous oxide and nitric oxide. Nitrous oxide levels increase with the addition of various metal oxides to the basic copper oxide/alumina system. Addition of sulfur dioxide to the reaction stream sharply reduces the level of ammonia conversion, but has a beneficial effect on selectivity to nitrogen. Added water vapour has a lesser effect on activity but is equally beneficial in terms of selectivity to nitrogen. The CuO/Al₂O₃ is also active for the selective catalytic reduction of nitric oxide by ammonia, but this reaction is not effected by sulfur dioxide addition. A mechanism for ammonia oxidation to nitrogen is proposed wherein part of the ammonia fed to the catalyst is converted into nitric oxide. A pool of monoatomic surface nitrogen species of varying oxidation states is established. N₂ or N₂O are formed depending upon the average oxidation state of this pool. An abundance of labile lattice oxygen species on the catalyst surface leads to overoxidation and to N₂O formation. On the other hand, reduced lability of surface lattice oxygen species favours a lower average oxidation state for the monoatomic surface nitrogen pool and leads to N₂ formation.     

NO Adsorption on ZnAl2O4/Al2O3 Powder: A NEXAFS Study at the Nitrogen K Edge

In order to understand the mechanism of the selective catalysis of nitrogen oxide reduction by hydrocarbons on a ZnAl₂O₄/Al₂O₃ catalyst, the NO adsorption step has been studied as a function of the surface state of the catalyst by using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the nitrogen K edge. The role of oxygen, whose presence is essential for the reaction to occur, is examined. In absence of a preliminary surface oxidation, nitric oxide was found not to be adsorbed on the ZnAl₂O₄/Al₂O₃ surface. After this preliminary treatment, we observed that the nitrogen atom of the NO molecule was linked to a surface oxygen with an adsorption mode parallel or slightly tilted with respect to the catalyst surface. Through these experiments we clearly demonstrate the advantages of soft X-ray experiments in catalysis research even in the case of practical application to real materials.     

A highly efficient and porous catalyst for simultaneous removal of NOx and diesel soot

A highly efficient and porous catalyst (La_0.8K_0.2Cu_0.05Mn_0.95O₃) for simultaneous removal of nitrogen oxides (NOx) and diesel soot was synthesized and the sample was characterized by XRD, BET, SEM–EDS, XRF, and XPS. The results indicate that this catalyst is perovskite and it possesses the very good characteristics for multiphase catalytic reactions. The catalytic properties were appraised under simulated diesel engine exhaust by temperature programmed reaction (TPR). TPR results indicate that this catalyst is a promising candidate for simultaneous removal of NOx and diesel soot. The maximum NO conversion into N₂ and the ignition temperature of soot are 54.8% and 260 °C, respectively. Comparing with those in previous reports, the comprehensive performance of this catalyst is greatly improved by partial substitution of La with K and Mn with Cu at the same time.     

Ab Initio Calculation of BN Generation from Boron and Nitrogen Oxides

Boron Nitride (BN) is one of the products produced in the burning of boron‐containing propellant. A possible reaction mechanism for the reactions of boron and nitrogen oxides (NO, NO₂, N₂O) has been studied using the G2MP2 method. The BN product can be formed in the reactions of B(⁴P) with NO, NO₂ and N₂O. Among these three reactions, B(⁴P)+NO₂ and B(⁴P)+N₂O are 181.42 kJ/mol and 160.92 kJ/mol more‐exothermic than the B(⁴P)+NO reaction. The barrier heights from intermediates to transition states are 64.85 kJ/mol and 111.75 kJ/mol for B(⁴P)+NO₂ and B(⁴P)+N₂O, respectively. However, in the reaction B(⁴P)+NO , the transition from intermediate to product (IM3→BN+O) is very endothermic by 420.70 kJ/mol. So B(⁴P)+ N₂O→BN+NO and B(⁴P)+NO₂→BN+O₂ are more likely reactions to generate BN than B(⁴P)+NO→BN+O.