Synergic effect of Pd/γ-alumina and Cu/ZSM-5 on the performance of NO x storage reduction catalyst

NO _x reduction with a combination of catalysts, Pd catalyst, NO _x storage reduction (NSR) catalyst and Cu/ZSM-5 in turn, was investigated to elucidate for the high NO _x reduction activity of this catalyst combination under oxidative atmosphere with periodic deep rich operation. The catalytic activity was evaluated using the simulated exhaust gases with periodically fluctuation between oxidative and reductive atmospheres, and it was found that the NO _x reduction activity with this catalyst combination was apparently higher than that of the solely accumulation of these individual activities, which was caused by the additional synergic effect by this combination. The Pd catalyst upstream of the NSR catalyst improved NO _x storage ability by NO₂ formation under oxidative atmosphere. The stored NO _x was reduced to NH₃ on the NSR catalyst, and the generated NH₃ was adsorbed on Cu/ZSM-5 downstream of the NSR catalyst under the reductive atmosphere, and subsequently reacted with NO _x on the Cu/ZSM-5 under the oxidative atmosphere.     

Deactivation of barium oxide-based NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> storage and reduction catalyst by hydrothermal treatment

The deactivation of a barium oxide-based NO _x storage and reduction (NSR) catalyst with hydrothermal treatment was studied by treating it with 10 vol% water vapor diluted in nitrogen at 850°C. XRD, XPS, SEM, IR of CO adsorption, and the N₂ adsorption was used to investigate the physical and chemical changes of the NSR catalyst caused by the hydrothermal treatment. The 12 h hydrothermal treatment decreased its NO₂ storage capacity by 20%. However, the hydrothermal treatment significantly decreased its ability to reduce the stored NO₂. The formation of an inactive phase consisting of platinum and aluminum is believed to be the cause of the severe deactivation of the NSR catalyst.     

NOx Abatement by Catalytic Traps: On the Mechanism of NOx Trapping Under Automotive Conditions

NO _x adsorption was measured with a barium based NO_x storage catalyst at an engine bench equipped with a lean burn gasoline direct injection engine (GDI). In order to study the influence of gas phase NO₂ on the NO_x storage efficiency two different pre-catalysts were used: One with excellent NO oxidation activity to produce a high NO₂ concentration and another pre-catalyst without NO oxidation activity and therefore high NO concentration at the NO _x storage catalyst inlet. Both pre-catalyst had excellent HC and CO conversion efficiency and therefore the CO and HC concentration at the NO _x storage catalyst inlet was practically zero. No lean NO _x reduction was observed. Under that conditions, experiments with NO _x storage catalysts of different length show that a high NO₂ inlet concentration did not enhance the NO _x storage efficiency. Moreover, we observed reduction of NO₂ to NO over the NO_x storage catalyst. However, in presence of a high NO inlet concentration NO₂ formation was observed which may proceed parallel to NO _x storage.     

FT-IR study of NO X storage mechanism over Pt/BaO/Al2O3 catalysts. Effect of the Pt–BaO interaction

NO _x storage mechanism over a model NSR catalyst has been analysed by means of in-situ FTIR. The results indicated that a two-step mechanism involving nitrite formation, without requirement of NO evolution to NO₂, followed by oxidation to nitrate species, being both steps assisted by O₂, would describe the overall process at 350 °C. This mechanism could be also extended to a wider temperature range. The interaction between Pt and Ba sites was crucial in this mechanism, since spillover process of oxidising agents appeared to play a key role. NO₂ direct interaction with BaO surface may also occur, but this process was only dominant on Ba sites away from Pt interaction.     

A New Simulation Model for NOx Storage Catalyst Dynamics

The NO _x storage concept has been studied experimentally and by two differently detailed numerical simulation models. The first detailed model simulates the concentration fronts of the solid components in the barium particles. It shows the slow, diffusion-hindered formation of dense nitrate layers around barium nanoparticles during NO _x storage and their rapid break-up during regeneration. Based upon this knowledge a new simplified model was developed which is able to describe well the storage and regeneration and to explain the main chemical and physical processes in the NO _x storage catalyst.     

Use of potassium conductors in the electrochemical promotion of environmental catalysis

This study summarizes the effect of the electrochemical promotion of catalysis (EPOC) by using potassium conductors in environmental catalytic reactions applied to the removal of several automotive pollutants, such us CO, C₃H₆ and nitrogen oxides (N₂O and NO_x). It has been shown the extraordinarily potential of using Pt/K–βAl₂O₃ in a wide variety of environmental reactions (oxidation, reduction), activating the catalyst at lower reaction temperatures and decreasing the inhibitory effect of poisons such as water in the reaction atmosphere. In addition, a new application of potassium conductors-based electrochemical catalysts has been developed for the NO_x storage/reduction process (NSR). The idea of coupling catalysis and solid-state electrochemistry on this process would allow to improve and simplify the current NSR technology. Finally, the results have been obtained under reaction conditions compatible with the treatment of automotive exhaust emissions. This demonstrates the potential for the practical use of the phenomenon of electrochemical promotion by using potassium conductors on this kind of process.     

Transient Surface Processes of Storage and Conversion of NOx Species on Pt–Me/Al2O3 Catalysts (Me = Ba,Ce, Cu)

The transient reactivity and surface phenomena of storage and conversion of NO _x species on Pt(1%)–Me/Al₂O₃ catalysts, where Me = Ba, Ce and Cu, were studied by the RWF (rectangular wavefront) method. The Me component has a relevant influence on the processes of surface storage and transformation. The reduction of NO _x by propene in the presence of oxygen is promoted by adding Cu to a Pt/Al₂O₃ catalyst, while cerium promotes transient conversion of NO in the absence of propene, but inhibits the reduction of NO _x in the presence of propene. Copper is suggested to be a promising element to add together with Ba for new NO _x storage-reduction catalysts due to its capacity to act both as a storage element and as promoter for NO _x reduction.     

The Effect of Carbon Monoxide and Hydrocarbons on NOx Storage at Low Temperature

One possible way to reduce NO _x in lean exhausts is by using NO _x trap catalysts. This paper addresses storage of NO _x on such catalysts at temperatures below the catalyst light-off. Experiments carried out on commercial samples in synthetic exhausts revealed a large capacity for storage of NO _x when NO₂ was added at temperatures below 150°C. In contrast, when NO was added instead, no storage took place. CO was found to decrease the storage by reacting with NO₂ and forming NO and CO₂. Propene inhibited the reaction between NO₂ and CO and therefore gave rise to larger NO _x storage when CO was present. The paper concludes with a discussion of a possible mechanism for the storage of NO _x at low temperatures.     

Effects of Lean/Rich Timing and Nature of Reductant on the Performance of a NOx Trap Catalyst

A NO _x trap catalyst was studied in a laboratory reactor under simulated diesel passenger car conditions. The effects of lean/rich duration and the nature of reductant are investigated. At 300°C, the average NO _x conversion decreases with increasing lean duration; conversely the NO _x conversion increases with increasing rich duration. The NO _x conversion at this temperature was found to be a direct function of reaction stoichiometry. That is, the quantity of trapped NO _x under lean conditions must be balanced by the quantity of reductant during the rich trap regeneration step. At extreme temperatures, other factors, reaction kinetics (at lower temperatures) and NO _x storage capacity (at higher temperatures), dominate the NO _x conversion process. Overall, carbon monoxide was found to be the most effective reductant. Hydrocarbon, e.g., C₃H₆, is effective at higher temperatures (T>350°C), while H₂ is more efficient than other reductants at low temperatures (T<200°C). The individual steps of the NO _x conversion process are discussed.     

Excellent sulfur resistance of Pt/BaO/CeO2 lean NOx trap catalysts

In this work, we investigated the NO_x storage behavior of Pt/BaO/CeO₂ catalysts, especially in the presence of SO₂. High surface area CeO₂ (110 m²/g) with a rod like morphology was synthesized and used as a support. The Pt/BaO/CeO₂ sample demonstrated slightly higher NO_x uptake in the entire temperature range studied compared with Pt/BaO/γ-Al₂O₃. More importantly, this ceria-based catalyst showed higher sulfur tolerance than the alumina-based one. The time of complete NO_x uptake was maintained even after exposing the sample to 3 g/L of SO₂. The same sulfur exposure, on the other hand, eliminated the complete NO_x uptake time on the alumina-based NO_x storage catalysts. TEM images show no evidence of either Pt sintering or BaS phase formation during reductive de-sulfation up to 600 °C on the ceria-based catalyst, while the same process over the alumina-based catalyst resulted in both a significant increase in the average Pt cluster size and the agglomeration of a newly formed BaS phase into large crystallites. XPS results revealed the presence of about five times more residual sulfur after reductive de-sulfation at 600 °C on the alumina-based catalysts in comparison with the ceria-based ones. All of these results strongly support that, besides their superior intrinsic NO_x uptake properties, ceria-based catalysts have (a) much higher sulfur tolerance and (b) excellent resistance against Pt sintering when they are compared to the widely used alumina-based catalysts.     

Thermal ageing phenomena and strategies towards reactivation of NO x - storage catalysts

The thermal ageing and reactivation of Ba/CeO₂ and Ba/Al₂O₃ based NO _x -storage/ reduction (NSR) catalysts was studied on model catalysts and catalyst systems at the engine. The mixed oxides BaAl₂O₄ and BaCeO₃, which lower the storage activity, are formed during ageing above 850 °C and 900 °C, respectively. Interestingly, the decomposition of BaCeO₃ in an atmosphere containing H₂O/NO₂ leads again to NO _x -storage active species, as evidenced by comparison of fresh, aged and reactivated Pt-Ba/CeO₂ based model catalysts. This can be technically exploited, particularly for the Ba/CeO₂ catalysts, as reactivation studies on thermally aged Ba/CeO₂ and Ba/Al₂O₃ based NSR catalysts on an engine bench showed. An on-board reactivation procedure is presented, that improved the performance of a thermally aged catalyst significantly.     

Evaluation of a NOx Reduction System on an Engine Rig Under Stationary Operation

The NO _x storage and reduction approach was applied on a full-scale engine rig under stationary operation. NO _x reduction experiments were performed and a catalyst model developed and tested. The exhaust system was equipped with a bypass system. A NO _x reduction of 25–53% was achieved. At low temperature, higher values were reached when the exhaust gas bypass was longer than the injection period.     

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.     

Dynamics and selectivity of NOx reduction in NOx storage catalytic monolith

Several nitrogen compounds can be produced during the regeneration phase in periodically operated NO_x storage and reduction catalyst (NSRC) for conversion of automobile exhaust gases. Besides the main product N₂, also NO, N₂O, and NH₃ can be formed, depending on the regeneration phase length, temperature, and gas composition. This contribution focuses on experimental evaluation of the NO_x reduction dynamics and selectivity towards the main products (NO, N₂ and NH₃) within the short rich phase, and consequent development of the corresponding global reaction-kinetic model. An industrial NSRC monolith sample of PtRh/Ba/CeO₂/ -Al₂O₃ type is employed in nearly isothermal laboratory micro-reactor. The oxygen and NO_x storage/reduction experiments are performed in the temperature range 100–500 °C in the presence of CO₂ and H₂O, using H₂, CO and C₃H₆ as the reducing agents.The spatially distributed NSRC model developed earlier is extended by the following reactions: NH₃ is formed by the reaction of H₂ with NO_x and it can further react with oxygen and NO_x deposited on the catalyst surface, producing N₂. Considering this scheme with ammonia as an active intermediate of the NO_x reduction, a good agreement with experiments is obtained in terms of the NO_x reduction dynamics and selectivity. A reduction front travelling in the flow direction along the reactor is predicted, with the NH₃ maximum on the moving boundary. When the front reaches the reactor outlet, the NH₃ peak is observed in the exhaust gas. It is assumed that the ammonia formation during the NO_x reduction by CO and HCs at higher temperatures proceed via the water gas shift and steam reforming reactions producing hydrogen. It is further demonstrated that oxygen storage effects influence the dynamics of the stored NO_x reduction. The temperature dependences of the outlet ammonia peak delay and the selectivity towards NH₃ are correlated with the effective oxygen and NO_x storage capacity.     

NOx storage/reduction catalysts based in cobalt/copper hydrotalcites

The use of materials based on hydrotalcites as NO_x storage/reduction (NSR) catalysts has been investigated, examining their activity at low temperature and their resistance to poisons such as H₂O and SO₂. The results obtained show that catalysts derived from Mg/Al hydrotalcites containing copper or cobalt is active at low temperatures, specially the samples containing 10 or 15% of Co. The addition of 1 wt% of transition metals with redox properties such as Pt, Pd, V and Ru to the hydrotalcite increases its activity because the combination of the redox properties of these metals and the acid-base properties of the hydrotalcite. The best results were obtained with the catalyst derived from a hydrotalcite with a molar ratio Co/Mg/Al = 15/60/25 and containing 1 wt% V. This material shows a higher activity, at low temperatures and in the presence of H₂O and SO₂, than a Pt–Ba/Al₂O₃ reference catalyst.     

NOx storage behavior and sulfur-resisting performance of the third-generation NSR catalysts Pt/K/TiO2–ZrO2

The NO_x storage and reduction (NSR) catalysts Pt/K/TiO₂–ZrO₂ were prepared by an impregnation method. The techniques of XRD, NH₃-TPD, CO₂-TPD, H₂-TPR and in situDRIFTS were employed to investigate their NO_x storage behavior and sulfur-resisting performance. It is revealed that the storage capacity and sulfur-resisting ability of these catalysts depend strongly on the calcination temperature of the support. The catalyst with theist support calcined at 500 °C, exhibits the largest specific surface area but the lowest storage capacity. With increasing calcination temperature, the NO_x storage capacity of the catalyst improves greatly, but the sulfur-resisting ability of the catalyst decreases. In situ DRIFTS results show that free nitrate species and bulk sulfates are the main storage and sulfation species, respectively, for all the catalysts studied. The CO₂-TPD results indicate that the decomposition performance of K₂CO₃ is largely determined by the surface property of the TiO₂–ZrO₂ support. The interaction between the surface hydroxyl of the support and K₂CO₃ promotes the decomposition of K₂CO₃ to form –OK groups bound to the support, leading to low NO_x storage capacity but high sulfur-resisting ability, while the interaction between the highly dispersed K₂CO₃ species and Lewis acid sites gives rise to high NO_x storage capacity but decreased sulfur-resisting ability. The optimal calcination temperature of TiO₂–ZrO₂ support is 650 °C.     

The role of NO2 in the reduction of NO by hydrocarbon over Cu-ZrO2 and Cu-ZSM-5 catalysts

The reduction of NO _x with propene or propane in the presence of 1 or 4% O₂ was studied at low conversions over a 7.4 wt% Cu-ZrO₂ and a 3.2 wt% Cu-ZSM-5 catalyst. The rates of N₂ production were compared in experiments using only NO or a mixture of NO and NO₂ in the feed. They were also compared with the rates of NO₂ reduction to NO under the same conditions, and of NO oxidation to NO₂ in the absence of hydrocarbon. It was found that the reduction of NO₂ to NO was very fast, consistent with literature data. The data were best explained by a reaction scheme in which the hydrocarbon was activated primarily by reaction with adsorbed NO₂ to form an adsorbed oxidized N-containing hydrocarbon intermediate, the reaction of which with NO was the principal route to produce N₂ under lean NO _x conditions.On leave from State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China.     

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.