Performance features of Pt/BaO lean NOx trap with hydrogen as reductant

The performance of a model Pt/BaO/Al₂O₃ monolith catalyst was studied using H₂ as the reductant. The dependence of product selectivities on operating parameters is reported, including the durations of regeneration and storage times, feed composition and temperature, and monolith temperature. The data are explained in terms of a phenomenological model factoring in the transport, kinetic, and spatio‐temporal effects. The Pt/BaO catalyst exhibits high cycle‐averaged NOx conversion above 100°C, generating a mixture of N₂ and byproducts NH₃ and N₂O. The cycle‐averaged NOx conversion exhibits a maximum at about 300°C corresponding to the NOx storage maximum. The N₂ selectivity exhibits a maximum at a somewhat higher temperature, at which point the NH₃ selectivity exhibits a minimum. This trend conveys the intermediate role of NH₃ in reacting with stored NOx. Both N₂ and N₂O are also formed during the storage steps from the oxidation of NH_x species produced during the regeneration. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Cyclic Lean Reduction of NO by CO in Excess H2O on Pt–Rh/Ba/Al2O3: Elucidating Mechanistic Features and Catalyst Performance

This study provides insight into the mechanistic and performance features of the cyclic reduction of NO_x by CO in the presence and absence of excess water on a Pt–Rh/Ba/Al₂O₃ NO_x storage and reduction catalyst. At low temperatures (150–200 °C), CO is ineffective in reducing NO_x due to self-inhibition while at temperatures exceeding 200 °C, CO effectively reduces NO_x to main product N₂ (selectivity >70 %) and byproduct N₂O. The addition of H₂O at these temperatures has a significant promoting effect on NO_x conversion while leading to a slight drop in the CO conversion, indicating a more efficient and selective lean reduction process. The appearance of NH₃ as a product is attributed either to isocyanate (NCO) hydrolysis and/or reduction of NO_x by H₂ formed by the water gas shift chemistry. After the switch from the rich to lean phase, second maxima are observed in the N₂O and CO₂ concentrations versus time, in addition to the maxima observed during the rich phase. These and other product evolution trends provide evidence for the involvement of NCOs as important intermediates, formed during the CO reduction of NO on the precious metal components, followed by their spillover to the storage component. The reversible storage of the NCOs on the Al₂O₃ and BaO and their reactivity appears to be an important pathway during cyclic operation on Pt–Rh/Ba/Al₂O₃ catalyst. In the absence of water the NCOs are not completely reacted away during the rich phase, which leads to their reaction with NO and O₂ upon switching to the subsequent lean phase, as evidenced by the evolution of N₂, N₂O and CO₂. In contrast, negligible product evolution is observed during the lean phase in the presence of water. This is consistent with a rapid hydrolysis of NCOs to NH₃, which results in a deeper regeneration of the catalyst due in part to the reaction of the NH₃ with stored NO_x. The data reveal more efficient utilization of CO for reducing NO_x in the presence of water which further underscores the NCO mechanism. Phenomenological pathways based on the data are proposed that describes the cyclic reduction of NO_x by CO under dry and wet conditions.     

Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration

The so-called 4-way catalytic converter (4WCC) has the ability to simultaneously convert CO, HC, NOx and particulate matter on a single support. It allows diesel vehicles to obey to increasingly stringent emission regulations while at the same time decreasing the space needed by the exhaust aftertreatment system. It is combined with fine engine control strategies so as to ensure conversion of all pollutants. It is hence associated with a large number of catalytic reactions which interact with each other and compete for active sites. The behavior of a commercial 4WCC was characterized on a synthetic gas bench. Gas composition, temperatures and gas hourly space velocity were chosen close to real engine operating conditions. Samples were loaded with soot on an engine bench test. Oxidation reactions were dominant in a lean environment: CO oxidation by NO₂ at low temperature followed by H₂, CO, NO and HC oxidation by O₂. NOx were stored on barium storage sites. In rich conditions H₂, CO and HC were used to reduce NOx. NH₃ production from H₂ was also observed. It could be used to further reduce NOx in lean conditions if stored on a downstream SCR system like in the Honda system [1]. A further conversion of HC was obtained at high temperature due to steam reforming. Interactions and inhibitions were also found. NOx storage appeared to be inhibited by CO oxidation with NO₂ at low temperatures and also by HC, maybe through competition for storage sites with CO₂ produced during HC oxidation. Catalytic reactions were affected by the soot deposit. Continuous oxidation of soot by NO₂ also induced a slower NOx storage rate.     

Emissions of N2O, NH3 and NOx from fuel combustion, industrial processes and the agricultural sectors in China

Based on the methodology and default data recommended by IPCC, N₂O, NH₃ and NOx emissions from fuel combustion, industrial processes, field burning of agricultural residues and fertilization were estimated. Agricultural fertilization is the important contributor of N₂O emission to the atmosphere. Fuel combustion, fertilizer application and animal waste were the important sources of NH₃ and NOx emissions. Estimates of NH₃ and NOx emissions from animal wastes were much lower when Chinese measured nitrogen excrement data were used rather than IPCC default values. The uncertainties in the estimates of N₂O, NH₃and NOx emissions were also analyzed in this paper.     

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.     

NOx removal by non-thermal plasma at low temperatures with amino groups additives

NOx removal from flue gas using direct current (DC) narrow pulsed discharge-induced non-thermal plasma (NTP) was experimentally investigated. Factors such as additives, NOx initial concentrations, residence time, reaction temperatures inside the NTP reactor, and so on were investigated to evaluate their effects on NOx removal efficiencies. The focus was on the effects of additives containing amino groups. The results showed that H₂O addition enhanced NOx removal, NH₃ could further increase the NOx removal efficiencies under the same conditions without an obvious NH₃ slip, and N₂H₄ was the most effective additive by reducing NO _x to N₂. X-Ray diffraction (XRD) analysis of the products collected from the NTP reactor demonstrated that NOx removal inside the NTP reactor was mainly based on NOx oxidation when ammonia or H₂O was used as an additive, while NOx removal was mainly based on NOx reduction with the N₂H₄ additive.     

Enhanced Methane Conversion Under Periodic Operation Over a Pd/Rh Based TWC in the Exhausts from NGVs

The enhancement of methane oxidation performances under periodic operation over a commercial Pd–Rh based three way catalyst (TWC) is investigated at different temperatures. Results confirm that under conditions with periodic oscillating feed around stoichiometry (λ = 1 ± 0.02), higher and more stable CH₄ conversion are obtained than under conditions with constant stoichiometric feed. In particular higher CH₄ conversion is obtained in the rich part of the cycle than in the lean one, the difference being more pronounced at high temperature. A narrow turning point for the TWC activity is finally observed under slightly rich conditions, which is characterised by a marked increase of CH₄ conversion, paralleled by total consumption of O₂ and NO and formation of small amounts of CO, H₂ and NH₃. Results suggest that the oxidation state of palladium plays a key role in the observed enhancement of catalyst performances.     

Reduction of lean NOx by ethanol over Ag/Al2O3 catalysts in the presence of H2O and SO2

The reduction of lean NOx using ethanol in simulated diesel engine exhaust was carried out over Ag/Al₂O₃ catalysts in the presence of H₂O and SO₂. The Ag/Al₂O₃ catalysts are highly active for the reduction of lean NOx by ethanol but the reaction is accompanied by side reactions to form CH₃CHO, CO along with small amounts of hydrocarbons (C₃H₆, C₂H₄, C₂H₂ and CH₄) and nitrogen compounds such as NH₃ and N₂O. The presence of H₂O enhances the NOx reduction while SO₂ suppresses the reduction. The presence of SO₂ along with H₂O suppresses the formation of acetaldehyde and NH₃. By infrared spectroscopy, it was revealed that the reactivity of NCO species formed in the course of the reaction was greatly enhanced in the presence of H₂O. The NCO species readily reacts with NO in the presence of O₂ and H₂O at room temperature, being converted to N₂ and CO₂ (CO). Addition of SO₂ suppresses the formation of NCO species and lowers the reactivity of the NCO species. However, the reduction of NOx is still kept at high conversion levels in the presence of H₂O and SO₂ over the present catalysts. About 80% of NOx in the simulated diesel engine exhaust was removed at 743 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pt dispersion effects during NOx storage and reduction on Pt/BaO/Al2O3 catalysts

This study provides insight into the effect of Pt dispersion on the overall rate and product distribution during NOx storage and reduction. The storage and reduction performance of Pt/BaO/A₂O₃ monoliths with varied Pt dispersion (3%, 8%, and 50%) and fixed Pt (2.48 wt.%) and BaO (13.0 wt.%) loadings is reported. At low temperature (<200 °C), the differences in storage and reduction activity were the largest between the three catalysts. The amount of NOx stored increased with increased dispersion, as did the amount of stored NOx that was reduced. These trends are attributed to larger Pt surface area and Pt–BaO interfacial perimeter, the latter of which enhances the spillover of surface species between the precious metal and storage components. At high temperature (370 °C), the stored NOx was almost completely regenerated for the three catalysts. However, the regeneration of the 3% dispersion catalyst was much slower, suggesting a rate limitation involving the reverse spillover of stored NOx to Pt and/or of adsorbed hydrogen from Pt to BaO. The results indicate that the catalyst dispersion and operating conditions may be tuned to achieve the desired ammonia selectivity. For the aerobic regeneration feed, the most (net) NH₃ was generated by the 50% dispersion catalyst at the lowest temperature (125 °C), by the 3% dispersion catalyst at the highest temperature (340 °C), and by the 8% dispersion catalyst at the intermediate temperatures (170–290 °C). Similar trends were observed for the net production of NH₃ with an anaerobic regeneration feed. A phenomenological picture is proposed that describes the effects of Pt dispersion consistent with the established spatio-temporal behavior of the lean NOx trap.     

Characterization of highly active silver catalyst for NOx reduction in lean-burning engine exhaust

A new Ag/Al₂O₃ catalyst for removing NOx in lean exhaust gas was developed. Oxidized Ag/Al₂O₃ catalyst is highly active for reduction of NOx with ethanol and propene, whereas reduced Ag/Al₂O₃ catalyst is less active for these reactions. Selectivity to N₂ is also high on the oxidized Ag/Al₂O₃ compared to that on the reduced Ag/Al₂O₃. XRD and SEM studies of these two types of Ag catalysts suggest that oxidation induces an interaction between Ag and the support, where the particles are grown in large size. In contrast, the metallic Ag particles are finely dispersed by the reduction process. Although dispersion of Ag particles is decreased by the oxidation process, the catalytic activity is increased. This suggests that the Ag-alumina sites created in the high temperature oxidizing environment are active in catalytic reduction of NOx. This revised version was published online in July 2006 with corrections to the Cover Date.     

An Investigation of Catalysts for the On Board Synthesis of NH3. A Possible Route to Low Temperature NOx Reduction for Lean-Burn Engines

Platinum group metal catalysts have been investigated for the formation of NH₃ from NO + H₂ at low temperatures in the absence and presence of CO. Although CO inhibits the formation of NH₃, substantial amounts are still observed with a Pt catalyst. By combining Pt with a support (ceria–zirconia) that has low temperature NO_x storage characteristics it has been shown in transient experiments that NH₃ can be formed and stored in situ under rich conditions, and may then be used to reduce NO_x under lean burn conditions.     

Lean NOx Reduction with H2 and CO in Dual-Layer LNT–SCR Monolithic Catalysts: Impact of Ceria Loading

A series of monolithic catalysts consisting of a layer of selective catalytic reduction (SCR) catalyst deposited on top of lean NO_x trap (LNT) catalyst were synthesized for lean reduction of NO_x (NO&NO₂) with H₂ and CO. The LNT catalyst exhibited a rather low NO_x conversion below 250 °C due to CO inhibition. The top SCR layer comprising Cu/ZSM5 significantly increased the NO_x conversion at low temperature by its reaction with NH₃ formed during the regeneration phase. The addition of CeO₂ to the LNT layer promoted the water gas shift reaction (CO + H₂O ? H₂ + CO₂). The WGS reaction mitigated the CO inhibition and the generated H₂ enhanced the low-temperature catalyst regeneration. The ceria addition decreased the performance at high temperatures due to increased oxidation of NH₃. The ceria loading was optimized by applying a non-uniform axial profile. A dual-layer catalyst with an increasing ceria loading axial profile improved the performance over a wide (low and high) temperature range.     

Simultaneous removal of particulates and NO by the catalytic bag filter containing MnOx catalysts

A catalytic bag filter which can remove particulates and NOx simultaneously was prepared and tested in a laboratory and a pilot plant. Manganese oxides (MnOx), active for the selective NO reduction with NH₃ at low temperatures, was utilized as the catalyst. This MnOx-coated bag filter showed 92.6% NOx conversion at 423 K with a space velocity of 400,000 h⁻¹.     

Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/Reduction Catalysts

Over the last several years, nitrogen oxide(s) (NOx) storage/reduction (NSR) catalysts, also referred to as NOx adsorbers or lean NOx traps, have been developed as an aftertreatment technology to reduce NOx emissions from lean-burn power sources. NSR operation is cyclic: during the lean part of the cycle, NOx are trapped on the catalyst; intermittent rich excursions are used to reduce the NOx to N₂ and restore the original catalyst surface; and lean operation then resumes. This review will describe the work carried out in characterizing, developing, and understanding this catalyst technology for application in mobile exhaust-gas aftertreatment. The discussion will first encompass the reaction process fundamentals, which include five general steps involved in NOx reduction to N₂ on NSR catalysts; NO oxidation, NO₂ and NO sorption leading to nitrite and nitrate species, reductant evolution, NOx release, and finally NOx reduction to N₂. Major unresolved issues and questions are listed at the end of each of the reaction process fundamental sections. Degradation mechanisms and their effects on NSR catalyst performance are also described in relation to these generalized reactions. Since at this stage it does not appear possible to arrive at a complete and consistent mechanistic model describing the broad, existing experimental phenomenology for these processes, this review is primarily focused on summarizing and evaluating literature data and reconciling the many differences presented.     

Evaluation of amines for the selective catalytic reduction (SCR) of NOx from diesel engine exhaust

With a view to developing onboard generation of selective reductants for NO_x removal from diesel engine exhaust we compared the performance of a primary, secondary and tertiary amine to NH₃ using a typical mini core NH₃-SCR catalyst. Primary amines with short hydrocarbon chains, e.g. CH₃NH₂ (maximum NO_x conversion, 50%) approached the NO_x conversion obtained using NH₃ (maximum NO_x conversion, 70%). Increasing the amine to NO_x ratio greater than 1 results in NO_x conversions closer to those of NH₃ (maximum NO_x conversion increased to 60%). Secondary and tertiary amines had smaller NO_x conversions as a function of temperature and the drop in NO and NO_x conversion decreased with increasing amine hydrogen substitution. Also, the maximum NO_x conversion for each reductant tends to move to a lower temperature as the degree of substitution increases.Unlike NH₃, the amines can react in the gas phase at temperatures within the range of diesel engine exhaust. Due to this gas phase reactivity the NO_x conversions measured using the mini core SCR catalyst also contain a gas phase conversion component. Gas phase conversions were investigated by replacing the mini core SCR catalyst with an equivalent length of quartz beads. Subtraction of the two results highlighted the differences between the mini core catalytic and gas phase conversions measured in this manner over the temperature range investigated. These differential NO_x conversions for the three amines had maxima at about 375 °C.     

MnOx-WO3/TiO2NH3选择性还原NOx的催化性能与动力学

研究了Mn-W/TiO2用于NH3选择性催化还原NOx体系的催化反应性能,在很宽的温度范围和各种气体条件下,该催化剂显示了较高的催化活性.在GHSV 18900 h-1、100～350℃条件下,NOx转化率高达80.3%～99.6%,Nz选择性达98.7%～100%;当反应气体中有0.01%SO2(分压比,下同)和6%...     

Effect of low-sulfur fuels upon NH3 and N2O emission during operation of commercial three-way catalytic converters

The use of low-sulfur fuel is known to improve the performance of the three-way catalytic converter (TWC). However, in this work we report how low-sulfur operation of commercial TWC also favors formation of N₂O and NH₃ as by products. We found that low-sulfur rich operation above 300 °C increases the production of NH₃, inhibiting the formation of N₂O characteristic of high-sulfur operation. During lean operation, the production of N₂O near the stoichiometric point is not significantly affected by the sulfur level. The large production of N₂O observed during light-off is not affected by SO₂ when the operation is lean, but under rich conditions N₂O is produced up to 575 °C. The increased production of NH₃ and N₂O in TWC as a result of the introduction of low-sulfur gasoline is an area that requires further analysis because of its implication upon public health in large urban settings.     

Modeling and simulation of a smart catalytic converter combining NOx storage, ammonia production and SCR

Dynamic simulation of the smart catalytic converter, proposed by Daimler AG, is presented. The smart catalytic converter combines NOx storage, on-board ammonia production and selective catalytic reduction (SCR) and functions in a dual-mode operation, alternating between lean burn and rich burn. It relies on intrinsic dynamic operation and synchronization of all units and its development demands a reliable dynamic simulator. A platform capable of simulating the dynamic behavior of multiple-unit aftertreatment system was developed based on COMSOL package. Predictive kinetic models were developed for NOx storage unit that includes ammonia formation function and for NH₃-SCR unit. Using these kinetic models, two-unit smart catalytic converter was simulated on the developed simulator. The results of the simulator were validated using two-unit experimental data. The simulator was also employed to control and optimize the performance of smart catalytic converter. It was shown that the simulator is vital for optimization of lean and rich periods in order to ensure stable lean–rich cycles.     

A novel integrated rotary reactor for NOx reduction by CO and air preheating: NOx removal performance and mechanism

A novel integrated rotary reactor for NOx reduction by CO and air preheating (iNA reactor) was proposed. NOx removal performance was investigated in a fixed-bed reactor, which was used to simulate the working conditions change in the iNA reactor. Lab-synthesized Cu/FeCeO_x were used as the catalyst. Two different modes were tested with the iNA reactor: short cycles and long cycles. Excellent NOx removal efficiencies of over 95% and 90% for short cycles and long cycles, respectively, were observed in the iNA reactor. Moreover, compared with the constant-temperature rotary reactor, better H₂O and SO₂ resistances were also found in the iNA reactor. The reaction mechanism was proposed based on in situ diffuse reflectance infrared Fourier transform study. In the iNA process, NOx was stored as nitrates in the adsorption zone, and then decomposed rapidly by both high temperatures and CO, leading to the deep catalyst regeneration. Therefore, temperature swinging and the feed of CO were key to having high iNA reactor performance for NOx removal.     

Application of computational fluid dynamics analysis for improving performance of commercial scale selective catalytic reduction

The performance of commercial scale selective catalytic reduction (SCR) system is strongly dependant upon the degree of mixing between NH₃ and NOx or NH₃ concentration distribution at the catalyst layer according to the reaction kinetics of SCR catalysts. Insufficient mixing of the reduction agent and NOx mass flow necessitates an uneconomically large catalyst volume and high NH₃ slip to meet the required NOx emission values. The effective methodology which can increase the performance of commercial scale SCR through improving NH₃ concentration distribution at the catalyst layer using computational fluid dynamics (CFD) analysis was suggested and applied to the real operations. The operation results have shown that the performance of commercial SCR was improved from 54.4% to 74.8% as NH₃ concentration deviation at the catalyst layer was reduced from 23.6% to 8.6%. It is established that the increase of NH₃ concentration uniformity at the catalyst layer contributes to improvement of performance of commercial scale SCR.