Selective Lean NO x Reduction over Metal Oxides

The present state of knowledge of NO _x reduction under lean conditions over metal oxides is summarized in this review, with an emphasis on the reaction mechanism. Although a number of nitrogen-containing intermediates have been observed and implied, including organic nitro, nitrite, cyanide, isocyanate, and ammonia, there is yet definitive observation that confirms the relevance of these to the principal reaction pathway. Individual component in a mixed oxide catalyst can participate at different stages of the NO _x reduction reaction. The nitrogen production efficiences for different oxide based catalysts are summarized, and their relationship to the nature of the catalyst is discussed.     

Mechanistic Considerations of the NO x Source and the Reducing Agent for Lean NO x Reduction over H-ZSM-5

This study focuses on the reaction mechanism for lean NO _x reduction by propane. In particular the role of the NO _x -source, i.e. NO or NO₂, for the SCR mechanism has been studied by transient experiments using in-situ FTIR spectroscopy and the results are discussed in relation to isopropylamine and ammonia as reducing agents.     

DME as Reductant for Continuous Lean Reduction of NO x over ZSM-5 Catalysts

Dimethyl ether (DME) is an interesting alternative fuel to diesel, but is not an efficient reductant in lean NO _x conversion over typical diesel HC-SCR catalysts. Comparatively high deNO _x activity was found over H-ZSM-5 in the presence of water, giving reduction of 28% NO and 37% NO₂ respectively with a DME/NO _x -ratio of 4.     

Lean NO x Reduction with Various Bio-Diesels as Reducing Agents

Commercial Cu-ZSM-5- and Ag/Al₂O₃-based lean NO _x catalysts were evaluated in a synthetic exhaust gas bench with the fuels RME, B30, B15, Agrodiesel 15, GTL, NExBTL, and MK1 as reducing agents. The influence of reductant was larger for Ag/Al₂O₃, albeit moderate, whereas the Cu-zeolite showed the highest NO _x conversion at lower temperature for all alternative fuels tested.     

Synthesis of Hexaaluminogallate Catalysts for NO x Reduction

BaCo(Al,Ga)₁₁O₁₉with β-alumina structure was found to be an effective catalyst for NO_xreduction and successfully synthesized by the coprecipitation method using metal nitrates and ammonium carbonate. Anisotropic BaCo(Al,Ga₁₁O₁₉particles crystallized at 1100 °C for 2 h through the coprecipitation method. BaCo(Al,Ga)₁₁O₁₉supported on a cordierite honeycomb had the NO_xremoval activity with methane over 500 °C in the presence of excess oxygen.     

Development of Catalysts for Diesel Particulate NO x Reduction

While diesel vehicles feature high fuel economy with low CO₂ emissions, further suppression of particulate matter (PM) and NO _x in the exhaust stream is demanded worldwide. We have been working to develop a new diesel particulate-NO _x reduction (DPNR) system to decrease both PM and NO _x emissions by combining the NO _x storage-reduction catalyst for direct injection gasoline engines with the most advanced engine control technologies. This paper describes the development of the DPNR system, a post-treatment technology for PM and NO _x , which was achieved through a combination of catalysis and engine control technologies.     

The NO x reduction mechanism by H2 under near isothermal conditions over Pt–Ba/Al2O3 Lean NO x Trap systems

The study of the gas-phase NO reduction by H₂ and of the stability/reactivity of NO _x stored over Pt–Ba/Al₂O₃ Lean NO _x Trap systems allowed to propose the occurrence of a reduction process of the stored nitrates occurring via to a Pt-catalyzed surface reaction which does not involve, as a preliminary step, the thermal decomposition of the adsorbed NO _x species.     

High Efficiency of Noble Metal and Metal Oxide Catalyst Systems for the Selective Reduction of NO with Propene in Lean Exhaust Gas

An investigation was conducted of noble metal and metal oxide catalysts deposited on Al₂O₃. The noble metals Pt, Pd, Rh the metal oxides CuO, SnO₂, CoO, Ag₂O, In₂O₃, catalysts were examined. Also investigated were noble metal Pt, Pd, Rh-doped In₂O₃/Al₂O₃ catalysts prepared by single sol–gel method. Both were studied for their capability to reduce NO by propene under lean conditions. In order to improve the catalytic activity and the temperature window, the intermediate addition propene between a Pt/Al₂O₃ oxidation and metal oxide combined catalyst system was also studied. Pt/Al₂O₃ and In₂O₃/Al₂O₃ combined catalyst showed high NO reduction activity in a wider temperature window, and more than 60% NO conversion was observed in the temperature range of 300–550 °C.     

Differences Between Al2O3 and Ag/Al2O3 for Lean Reduction of NO x with Dimethyl Ether

Lean reduction of NO _x with DME occurs with high selectivity to N₂ over Al₂O₃ between 300 °C and 550 °C with a maximum of 47% at 380 °C, and with lower selectivity over Ag/Al₂O₃ between 250 °C and 400 °C due to the catalysts’ sensitivity to gas phase radical reactions and activity for NO _x reduction with methanol.     

A Novel Approach to Catalysis for NO x Reduction in Diesel Exhaust Gas

With the introduction of stringent emission standards in the US in 2007 (Tier II Bin8, 5) an exhaust aftertreatment for NO _x reduction is required for compliance with the emission regulations. A new approach consists in the synergetic combination of existing technologies NSC (NO _x Storage Catalyst) and SCR (Selective Catalytic Reduction) with onboard generation of ammonia as the reducing agent for the SCR. It is shown in this work that the performance of the combined system exceeds those of each one considered separately, especially after ageing. The generation of ammonia is correlated to the ammonia selectivity during the regeneration of the NSC. The selectivity is primarily dependent on the temperature, A/F ratio (Air/Fuel) and the rich time. It is shown that the development of a suitable control strategy leads to a high level of NO _x reduction under transient conditions in an FTP driving cycle. Due to the complexity and high development costs of current exhaust aftertreatment systems, modelling and simulation were identified as an important aspect in the development process. A system simulation tool named ExACT (Exhaust Gas Aftertreatment Components Toolbox) developed at Daimler is presented. By using the simulation already at an early stage, specific development work can be carried out prior to the experimental work on an engine test bench.     

Novel MnO x Catalysts for NO Reduction at Low Temperature with Ammonia

Novel MnO_x catalysts for NO reduction at low temperature with NH₃ have been prepared by a simple precipitation method using sodium carbonate. The catalysts thus obtained have exhibited excellent catalytic activity in the temperature range of 348–473 K compared with other MnO_x-based catalysts, which is probably due to its high surface area as well as framework structure and composition. The high catalytic activity is maintained in the presence of 20 vol% water vapor in the feed.     

Synthesis and Catalytic Properties of the Electrochemical NO x Reduction System

Catalytic and electrical properties of an electrochemical NO_x reduction system were investigated. This system had a laminated structure composed of BaCo(Al,Ga)₁₁O₁₉-based catalyst layer on a Pt/YSZ/Pt sheet. The stacked catalyst system can directly reduce more than 65% of NO_x to N₂ under an external bias above 2.5 V at 650 °C. In this system, oxygen existing around the catalyst layer was removed by O²⁻ transportation through the YSZ layer.     

Reduction of NO2 stored in a commercial lean NO x trap at low temperatures

Isothermal storage of NO₂ and subsequent reduction with different reducing agents (H₂, CO or H₂ + CO) in a lean NO _x trap catalyst was tested by Temperature Programmed Desorption (TPD) and Temperature Programmed Reduction (TPR) experiments at temperatures representative of automotive “cold-start” conditions (<200 °C) using a commercial NO _x trap catalyst. Results from the TPR experiments revealed that no reduction of stored NO₂ to N₂ was observed at 100–180 °C, and at 200 °C 10% reduction only of NO₂ to N₂ was measured. A special affinity of H₂ to form NH₃ was observed during the reduction of stored NO₂. The formation of NH₃ increases with increasing amount of stored NO₂ and decreases with increasing storage temperature. Direct relation exists between the amount of adsorbed and/or stored NO₂ and the formation of H₂O and NH₃.     

Novel Y2O3 Doped MnO x Binary Metal Oxides for NO x Storage at Low Temperature in Lean Burn Condition

MnO _x -Y₂O₃ binary metal oxide catalysts are synthesized by a constant-pH co-precipitation method, their ability of NO _x storage capacity and absorbing process were investigated. The pure MnO _x and Y₂O₃ calcined at 500 °C for 4 h in static air are both of body-centre structure, while the binary metal oxides containing Mn and Y are mainly of amorphous phase. The adulteration of Y₂O₃ can remarkably improve the specific surface areas of the catalysts, which probably result of the enhancement on NO storage capacity and catalytic oxidation ability of NO at 100 °C. The XPS results indicate that both Mn and Y have 3+ chemical states in the binary oxides. FT-IR spectra could be beneficial to explain the NO storage process on the binary metal oxide: NO can be adsorbed on the MnO _x and Y₂O₃ sites as nitrates and nitrites, respectively, and then the nitrites on Y₂O₃ site are shifted to Mn₂O₃ site and then is oxidized to nitrates. As a result, the NO storage capacity is enhanced due to the adulteration of Y₂O₃, finally the NO _x are adsorbed on the Mn₂O₃ site as nitrate species.     

Methane as Alternative in the Selective Reduction of NO over Supported Palladium Catalysts in Lean Conditions: Role of Redox Properties of Support Materials

The reduction of CH₄ by NO has been investigated in the presence of oxygen on palladium supported on alumina, ceria–zirconia mixed oxides and perovskite materials, mainly LaCoO₃. The activation procedure, under oxygen or hydrogen, drastically influences the catalytic performances of both catalysts. The stabilisation of a metallic or oxidic Pd phase leads to poor activity in the conversion of NO in the absence of oxygen. On the other hand, oxygen enhances the activity, particularly on the reduced Pd/LaCoO₃, in the CH₄ + NO reaction. Such results have been explained by different interactions between palladium and the support.     

Effect of Ceria on the Sulfation and Desulfation Characteristics of a Model Lean NO x Trap Catalyst

The effect of ceria addition on the sulfation and desulfation characteristics of a model Ba-based lean NO _x trap (LNT) catalyst was studied. According to DRIFTS and NO _x storage capacity measurements, ceria is able to store sulfur during catalyst exposure to SO₂, thereby helping to limit sulfation of the main (Ba) NO _x storage phase and maintain NO _x storage capacity. Temperature programmed desulfation experiments revealed that desulfation of a model ceria-containing catalyst occurred in two stages, corresponding to sulfur elimination from the ceria phase at ~450 °C, followed by sulfur loss from the Ba phase at ~650 °C. Significantly, the ceria-containing catalyst displayed relatively lower sulfur evolution from the Ba phase than its non-ceria analog, confirming that the presence of ceria lessened the degree of sulfur accumulation on the Ba phase.     

Effect of Ceria on the Storage and Regeneration Behavior of a Model Lean NO x Trap Catalyst

In this study the effect of ceria addition on the performance of a model Ba-based lean NO _x trap (LNT) catalyst was examined. The presence of ceria improved NO _x storage capacity in the temperature range 200–400 °C under both continuous lean and lean-rich cycling conditions. Temperature-programmed experiments showed that NO _x stored in the ceria-containing catalyst was thermally less stable and more reactive to reduction with both H₂ and CO as reductants, albeit at the expense of additional reductant consumed by reduction of the ceria. These findings demonstrate that the incorporation of ceria in LNTs not only improves NO _x storage efficiency but also positively impacts LNT regeneration behavior.     

The Effects of CO2 and H2O on the NO x Destruction Performance of a Model NO x Storage/Reduction Catalyst

The effects of CO₂ and H₂O on the NO _x storage and reduction characteristics of a Pt/Ba/Al₂O₃ catalyst were investigated. The presence of CO₂ and H₂O, individually or together, affect the performance and therefore the chemistry that occurs at the catalyst surface. The effects of CO₂ were observed in both the trapping and reduction phases of the experiments, whereas the effect of H₂O seems limited to the trapping phase. The data also indicate that multiple types of sorption sites (or mechanisms for sorption) exist on the catalyst. One mechanism is characterized by a rapid and complete uptake of NO _x . A second mechanism is characterized by a slower rate of NO _x uptake, but this mechanism is active for a longer time period. As the temperature is increased, the effect of H₂O decreases compared to that of CO₂. At the highest temperatures examined, the elimination of H₂O when CO₂ is present did not affect the performance.