NOx Storage Catalysts Research Articles

Concepts for Hydrogen Internal Combustion Engines and Their Implications on the Exhaust Gas Aftertreatment System

Hydrogen as carbon-free fuel is a very promising candidate for climate-neutral internal combustion engine operation. In comparison to other renewable fuels, hydrogen does obviously not produce CO2 emissions. In this work, two concepts of hydrogen internal combustion engines (H2-ICEs) are investigated experimentally. One approach is the modification of a state-of-the-art gasoline passenger car engine using hydrogen direct injection. It targets gasoline-like specific power output by mixture enrichment down to stoichiometric operation. Another approach is to use a heavy-duty diesel engine equipped with spark ignition and hydrogen port fuel injection. Here, a diesel-like indicated efficiency is targeted through constant lean-burn operation. The measurement results show that both approaches are applicable. For the gasoline engine-based concept, stoichiometric operation requires a three-way catalyst or a three-way NOX storage catalyst as the primary exhaust gas aftertreatment system. For the diesel engine-based concept, state-of-the-art selective catalytic reduction (SCR) catalysts can be used to reduce the NOx emissions, provided the engine calibration ensures sufficient exhaust gas temperature levels. In conclusion, while H2-ICEs present new challenges for the development of the exhaust gas aftertreatment systems, they are capable to realize zero-impact tailpipe emission operation.

Ordered Nanostructure Catalysts Efficient for NOx Storage/Reduction (NSR) Processes

NOx emissions in the atmosphere can cause various environmental problems, which should be strictly controlled and regulated. Furthermore, because of the limited amount of crude oil resources in the world and severe global warming, the development of fuel-efficient vehicles has long been desired. Accordingly, efficient NOx storage and reduction catalysts have been developed over the decades, called NSR (NOx storage/reduction) catalysts. In the present article, recent advances in NSR catalysts which possess ordered nanostructures will be summarized, including our noble Pt/KNO3/K-titanate nanobelt (KTN), Pt-KNO3/CeO2 and Pt-KNO3/ZrO2 catalysts, as well as nanoporous Ni-phosphate (VSB-5) and Co-substituted VSB-5 catalysts.

Spatially and Temporally Resolved Measurements of NO Adsorption/Desorption over NOx-Storage Catalyst.

The two‐dimensional (2D) temporal evolution of the NO‐concentration over a NOx‐storage catalyst is investigated in situ with planar laser‐induced fluorescence (PLIF) in an optically accessible parallel wall channel reactor. Signal accumulated phase‐correlated 2D‐recordings of repetitive adsorption/desorption cycles are obtained by synchronizing the switching of the NO gas flow (on/off) with the laser and detection system, thereby significantly increasing the signal‐to‐noise ratio. The gas compositions at the reactor outlet are additionally monitored by ex‐situ analytics. The impacts of varying feed concentration, temperature and flow velocities are investigated in an unsteady state. Transient kinetics and the mass transfer limitations can be interpreted in terms of the NO concentration gradient changes. The technique presented here is a very useful tool to investigate the interaction between surface kinetics and the surrounding gas flow, especially for transient catalytic processes.

Characterization of the sensitive material for a resistive NOx gas dosimeter by DRIFT spectroscopy

Resistive NOx gas dosimeters measure continuously the integrated NOx concentration, i.e., the NOx dose. The impedance of potassium-manganese based NOx storage catalyst films decrease at 350 °C linearly with the NOx dose. The slope of the signal is proportional to the actual NOx concentration. The dosimeter principle is based on the sorption of NOx on the sensitive material. During NOx absence, the sensor signal remains constant, i.e. no desorption occurs. When a certain sorption state is reached, the sorbed species has to be desorbed thermally. To study the sorption processes on the NOx storage material, Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) was used. Different material compositions were investigated by DRIFTS on powdered catalysts and operando DRIFTS on NOx dosimeters themselves. On Al2O3 (catalyst support) no NOx sorbs. Manganese addition (Mn-Al2O3) leads to few detectable nitrites on Aluminum-sites during NOx exposure. On Potassium-loaded alumina (K-Al2O3), nitrites and nitrates form. The comparison of NO2 and NO proves that NO2 is sorbed directly as nitrate species and NO cannot be strongly sorbed as nitrate on K-Al2O3. The complete material composition K/Mn-Al2O3 shows strong nitrate bands during NO and NO2 exposure. An interplay of potassium and manganese when forming nitrates was found. Operando DRIFTS confirms the direct relation between formed nitrates and complex impedance. Furthermore, it is proven that during thermal regeneration, sorbed nitrites and nitrates desorb completely and the sensor signal returns to its initial value. On the whole, the sorption behavior of the powders and the dosimeter behavior can be explained.

Development and identification of a control oriented model of NOx Storage Catalyst for automotive application

The NOχ Storage Catalyst is currently envisaged to be implemented in light-duty passenger cars for nitrogen oxides reduction, in order to comply with strict emission legislation targets. Since robustness and durability of the engine and emission control system is the first priority in automotive application, to satisfy the need of robust on-board real time monitoring, diagnosis and control, computing efficient methods are needed. In this framework, a control oriented model that describes the dynamics of the main physical-chemical processes within the NSC catalyst, while still maintaining affordable computational burden, has been developed and validated. Model calibration has been performed, for light-duty application, along the NEDC test cycle, by using a statistical-based sub-optimal procedure, based on a parametric analysis which allows identifying the more suitable section of NeDC cycle for model identification, without the need to perform cost- and time- expensive experiments on the engine test bench. The procedure also accounts for missing information and sensors inaccuracies. The great potential of this methodology is the possibility to adopt not optimal designed tests for model parameters identification. The proposed methodology is proven to be effective for real time control strategy, directly embedded in ECU, and provide a sub-optimal but effective strategy for complex models calibration.

Enhancement of photocatalytic NOx abatement on titania via additional metal oxide NOx-storage domains: Interplay between surface acidity, specific surface area, and humidity

In this work, we propose a simple and effective preparation procedure to obtain ternary mixed oxides composed of titania (TiO2, P25), alumina (γ-Al2O3) and calcium oxide (CaO) functioning as efficient photocatalytic NOx oxidation and storage (PHONOS) catalysts that are capable of facile NOx abatement under ambient conditions in the absence of elevated temperatures and pressures with UVA irradiation. In this architecture, titania was the photocatalytic active component and CaO and/or γ-Al2O3 provided NOx storage domains revealing dissimilar specific surface areas (SSA) and surface acidities. We show that photocatalyst formulation can be readily fine-tuned to achieve superior photocatalytic performance surpassing conventional P25 benchmark in short (1 h) and long term (12 h), as well as humidity-dependent photocatalytic tests. We demonstrate the delicate interplay between the surface acidity, SSA and humidity and provide detailed mechanistic insights regarding the origin of photocatalytic activity, selectivity and deactivation pathways.

NOx 吸蔵触媒を装着したディーゼル乗用車における実路走行時 NOx 排出予測モデルに関する研究

NOx 吸蔵触媒を装着したディーゼル乗用車における実路走行時 NOx 排出予測モデルに関する研究

Modelling NOX Storage and Thermal Desorption: Is It Possible to Model the Effect of Changing the Catalyst Composition?

In this work, a series of model NOX storage catalysts with different Pt and Ba loadings (in the range 0–2 wt% and 0–20 wt%, respectively) have been studied for NOX storage and desorption. The samples have been characterised by in-situ DRIFTS to identify the nature of the stored species and by EXAFS to gather information about the Pt particle size. A correlation between the average Pt particle size and the Pt/Ba ratio has been found. A simple kinetic model for NOX storage and thermal desorption has been developed, which can reasonably capture the experimental results over a wide range of conditions. The fitting parameters have been optimised independently for each of the samples and have been found to be dependent on the catalyst’s structural properties (average Pt particle size, total Pt perimeter and Ba loading). The so found correlations have been introduced in the model, which can now be used to simulate the behaviour of catalysts with different Pt and Ba loadings as well as to optimise the catalyst composition for different applications.

Chemical poisoning by zinc and phosphorous of Pt/Ba/Al2O3 NOx storage catalysts

The effects of phosphorous and zinc on the performance of Pt/Ba/Al2O3 catalysts were investigated through wet impregnation of ammonium phosphate and zinc acetate aqueous solutions. Six different sample combinations were studied; 1 wt-% P, 1 wt-% Zn, 1 wt-% P with 1 wt-% Zn, 2 wt-% P, 2 wt-% Zn, 2 wt-% P with 2 wt-% Zn. NOx storage and reduction (NSR) activity and NO2 temperature programmed desorption (TPD) profiles were measured before and after impregnation of P and Zn. Samples containing P performed significantly worse than samples only containing Zn in both NSR activity and TPD measurements. The increased NOx slip during lean phase in activity measurements for P-poisoned samples is mainly related to an increased slip of NO2. This was found for both NO and NO2 in the gas feed during lean phase and suggests that it is mainly the storage component that is poisoned and not the noble metal. Furthermore, the combination (1 wt-% P and 1 wt-% Zn) proved to result in slightly worse performance than only 1 wt-% P, however this was not the case for samples containing 2 wt-%, where the addition of zinc reduced the negative effect of phosphorous. Measurements from NO2-TPD experiments showed that NOx release at low temperature was not affected by the addition of P, while desorption in the temperature range 425–475 °C was significantly reduced. It can therefore be concluded that the poisoning mainly is related to barium NOx storage sites and not to alumina sites. Moreover, X-ray diffraction measurements indicate that some of the barium species are affected by phosphorous. Images from scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping were in line with the results seen in both the activity tests and NO2-TPD experiments. Phosphorous was concentrated at the same position as barium in the observed images, whereas zinc was more evenly distributed over the surface. For the sample with both 2 wt-% P and 2 wt-% Zn, X-ray photoelectron spectroscopy measurements indicate that Zn and P have a low interaction and this suggests that most of the zinc and phosphorous are separated. However, STEM-EDX showed agglomerates of some zinc and phosphorous, which could be zinc phosphates. This is a plausible explanation of the decreased deactivation observed after introducing 2 wt-% Zn compared to the sample only containing 2 wt-% P.

Radio Frequency-Based Determination of the Oxygen and the NOx Storage Level of NOx Storage Catalysts

In recent years, research on radio frequency-based (also known as “microwave-based”) catalyst state diagnosis for diesel engines has focused mainly on SCR systems. To increase the efficiency of exhaust aftertreatment systems, SCR catalysts can be combined with NOx storage catalysts (NSC). For optimum performance, the degree of loading of the storage components has to be monitored. Up to now, in commercial application this has been done indirectly by using mathematical models. In this paper, the opportunities of a simple radio frequency (RF) based method for a direct determination are highlighted. Therefore, a commercial NSC was investigated between 210 and 390 °C. A linear dependence between the RF signals and the stored amount of NOx was found. Furthermore, the NOx and oxygen gas concentrations do not directly affect the RF parameters, but they have an indirect influence, since the stored NOx amount is NOx concentration dependent. By evaluating multiple RF signals, which are based on different material effects, a separate determination of the oxygen and NOx storage level could be realized.

Steady state and lean-rich cycling study of a three-way NOX storage catalyst: Modeling

A nonisothermal monolithic reactor model with a global kinetic formulation is developed to predict the steady-state and cyclic oxidation and reduction with CO and H2 of a model three-way NOx storage catalyst (TWNSC). Data were used from a parallel experimental study conducted by Malamis et al. [1] along with kinetics measurements made in the current study. The reactor model comprises a low-dimensional treatment of the transport processes and multi-site NOx and O2 storage combined with catalytic kinetics of the reaction steps. Kinetic sub-models of NO and CO single and co-oxidation are developed to capture the CO and NO light-off trends including inhibition effects. The sub-models are combined to simulate the catalyst performance under both steady-state and cyclic conditions. The TWNSC model predicts the steady-state product distribution spanning a range of rich conditions, along with cyclic operation features, including cycle-averaged NOx conversion and NH3 selectivity, and the transient species concentrations and temperature profiles over a wide range of total cycle time (TCT, 10 s–200 s) at 50% duty cycle rich. The model captures the key feature of the TWNSC using CO and H2 as reductants. The results point to the importance of sizing the TWNSC for either NO reduction to N2 or NH3 formation, depending on the application. The TWNSC reactor model provides insight about reaction and transport interactions, spatio-temporal features, etc., along with guidance in the catalyst design and operation strategy optimization.

Steady state and lean-rich cycling study of a three-way NOX storage catalyst: Experiments

The three-way NOx storage catalyst (TWNSC) combines components from a conventional three-way catalyst (TWC) and NOX storage and reduction (NSR) catalyst to improve NOX emissions while exploiting the higher fuel economy of lean-burn gasoline vehicles. The performance of a commercial monolithic TWNSC was studied to understand the NOX trapping and reduction performance over a range of conditions with emphasis on identifying conditions leading to optimal performance in terms of a standalone TWNSC or one coupled with a downstream selective catalytic reduction (SCR) device. Using H2, CO, and C3H6 in various combinations, the impact of cycle timing (cycle time, rich duty fraction), reductant and O2 feed concentrations, and feed temperature on NOX conversion and product (N2, NH3, N2O) selectivities was determined. Steady state experiments were conducted to assess catalyst activity and selectivity and to help interpret the phenomena observed during lean-rich switching. Cycling experiments reveal maxima in the NOX conversion and ammonia-to-NOx ratio (ANR) at distinct, intermediate cycle times. The existence of operating conditions giving these maxima depends on the reductant type, feed temperature, and O2 feed concentration. For example, a large disparity in the lean/rich ratio (stoichiometric number) of the lean and rich feeds tends to lead to a NOx conversion maximum. Where possible, the data trends are interpreted in terms of known performance features of the TWC and NSR catalysts. The study findings provide guidance for optimizing the TWNSC formulation and operation strategy.

Evaluating the NOx Storage Catalysts (NSC) Aging: A Preliminary Analytical Study with Electronic Microscopy

This paper describes an expeditious and reliable method for determining the thermal effects in a static condition of commercial NOx storage catalysts (NSCs) using scanning electron microscopy with an energy dispersive X-ray analytical system (SEM/EDS). It is worth remarking that possible changes in the morphology and in the elemental composition of the catalyst may be considered as the most important causes of the lower conversion of NOx. The information attained in this work indicates that Pt nanoparticle sintering is strongly increased by the oxygen exposure, and this can be considered a very useful preliminary investigation for the studies already present in the literature on the efficiency of NSCs.

Numerical Simulations of the flow field and chemical reactions of the Storage/Oxidation process within a NSC Pt - BaO Catalyst

A NOx Storage Catalyst (NSC) has been studied by means of reactive CFD simulations. In the scenario of automotive pollutant emission reduction, due to the stringent regulation, the detailed description of the chemical and physical phenomena within catalysts represents a key point in order to improve their conversion efficiency. The active part of the catalyst has been simulated as a porous medium. In this zone, surface reactions take place, which are modelled by means of an Arrhenius chemical kinetic approach, involving the Pt and BaO sites on the active surface of the matrix. Actually, two chemical mechanisms are considered, the simplest involves only BaO site, the other one includes both BaO and Ptsites. Both models are validated against data available in the literature and then applied to a real automotive catalyst geometry. Thus, a detailed description of the spatial distribution of the species is provided for both models. Lean condition is simulated in order to check the catalyst behaviour according to experimental results.

Effect of Calcination Heating Rate on Microstructure and Performances of NSR Catalyst

The Be/Ce/γ-Al2O3 compound catalysts were prepared by sol-gel process using γ-Al2O3, Ba (AC)2 and Ce (NO3)3·6H2O as raw materials, and the effect of calcination heating rate on microstructure and performance of the NOx storage and reduction (NSR) catalyst was investigated. The crystal structure, microstructure, absorption and reduction performances of the NSR catalyst were characterized and analyzed by X-Ray diffractometer (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), Brunauer Emmett Teller (BET) and H2 temperature program reduction (H2-TPR). The experimental results show that the Ba and Ce elements mainly exist in the forms of BaCO3 and CeO2, respectively, and partial CeO2 is amorphous. The calcination heating rate can play a key role in the grain size and distribution of BaCO3 and CeO2, so that it can affect the surface area, pore volume and pore diameter of the NSR catalyst. Moreover, the reduction temperature of the NSR catalyst decrease first and then increase with the increase of calcination heating rate, and the reduction temperature of as-received NSR catalyst is the lowest as the reduction temperature is 6 °C/min, that is its reduction performance is the optimal.

Isocyanic acid and ammonia in vehicle emissions

Vehicles are considered to be an important source of ammonia (NH3) and isocyanic acid (HNCO). HNCO and NH3 have been shown to be toxic compounds. Moreover, NH3 is also a precursor in the formation of atmospheric secondary aerosols. For that reason, real-time vehicular emissions from a series of Euro 5 and Euro 6 light-duty vehicles, including spark ignition (gasoline and flex-fuel), compression ignition (diesel) and a plug-in electric hybrid, were investigated at 23 and −7°C over the new World harmonized Light-duty vehicle Test Cycle (WLTC) in the Vehicle Emission Laboratory at the European Commission Joint Research Centre Ispra, Italy. The median HNCO emissions obtained for the studied fleet over the WLTC were 1.4mgkm−1 at 23°C and 6mgkm−1 at −7°C. The fleet median NH3 emission factors were 10mgkm−1 and 21mgkm−1 at 23 and −7°C, respectively. The obtained results show that even though three-way catalyst (TWC), selective catalytic reduction (SCR), and NOx storage catalyst (NSC) are effective systems to reduce NOx vehicular emissions, they also lead to considerable emissions of the byproducts NH3 and/or HNCO. It is also shown that diesel light-duty vehicles equipped with SCR can present NH3 emission factors as high as gasoline light-duty vehicles at both, 23 and −7°C over the WLTC. Therefore, with the introduction in the market of this DeNOx technology, vehicular NH3 emissions will increase further.

Directions in vehicle efficiency and emissions

This paper provides a general review of light-duty (LD) and heavy-duty (HD) regulations, engine technology, and key emission control strategies. The US is placing a stronger emphasis on laboratory emissions, and the LD regulations are about an order of magnitude tighter than Euro 6, but Europe is focusing on real-world reductions. The California HD low-NOx regulation is advancing and may be proposed in 2017/18 for implementation in 2023+. The second phase of US HD greenhouse gas regulations propose another 25-30% tightening beyond Phase 1, beginning in 2021. LD and HD engine technology continues showing marked improvements in engine efficiency. LD gasoline concepts are closing the gap with diesel. HD engines are demonstrating more than 50% BTE using methods that can reasonably be commercialized. LD and HD diesel NOx technology trends are also summarized. NOx storage catalysts and SCR combinations are the lead approach to meeting the LD regulations. Numerous advanced NOx technologies are being evaluated and some promise for meeting the California HD low NOx targets. Oxidation catalysts are improved for both diesel and methane oxidation applications. Gasoline particulate filters (GPF) are the lead approach to reducing particles from gasoline direct injection (GDI) engines. They reduce PAH emissions, and catalyzed versions can be designed for low back pressure. Regeneration largely occurs during hot decelerations.

Review on Radio Frequency Based Monitoring of SCR and Three Way Catalysts

Knowledge of the actual catalyst state plays a key role in automotive exhaust gas aftertreatment. The oxygen loading degree of three-way catalysts (TWC), the amount of stored ammonia in selective reduction catalysts (SCR), or the NOx loading degree in NOx storage catalysts (NSC) are important parameters. Today, they are determined indirectly and/or model-based, applying models that are typically calibrated by gas sensors installed up- and/or downstream of the catalysts. A novel approach to determine directly the catalyst state by microwaves (radio frequencies, rf) emerged recently. For this method, the catalyst housing serves as an electrical cavity resonator. As “sensor”, one or two simple antennas are mounted in the canning. The electrical properties of the honeycomb incl. coating change with gas loading, affecting either the resonance frequencies or the power transmission. Such contactless-obtained information is strongly correlated with the catalyst state as will be discussed here for TWC and SCR systems. This contribution reviews the progress in the past 3 years that exceeds by far the status of initial studies.

Sulfur Poisoning of a NOx Storage Catalyst - A Comprehensive Modelling Approach

Sulfur Poisoning of a NO_x Storage Catalyst - A Comprehensive Modelling Approach

New operation strategy for driving the selectivity of NOx reduction to N2, NH3 or N2O during lean/rich cycling of a lean NOx trap catalyst

Periodical regeneration of NOx storage catalyst (also known as lean NOx trap) by short rich pulses of CO, H2 and hydrocarbons is necessary for the reduction of nitrogen oxides adsorbed on the catalyst surface. Ideally, the stored NOx is converted into N2, but N2O and NH3 by-products can be formed as well, particularly at low-intermediate temperatures. The N2 and N2O products are formed concurrently in two peaks. The primary peaks appear immediately after the rich-phase inception, and tail off with the breakthrough of the reductant front accompanied by NH3 product. The secondary N2 and N2O peaks then appear at the rich-to-lean transition as a result of reactions between surface-deposited reductants/intermediates (CO, HC, NH3, NCO) and residual stored NOx under increasingly lean conditions.Based on these mechanistic insights, we propose and demonstrate a novel strategy for driving the selectivity of the secondary peaks towards desired products. It is based on a transition phase of neutral or slightly lean (nearly stoichiometric) character inserted between the rich and the fully lean phase. This strategy allows more complete regeneration of the catalyst with higher N2 yield and without the undesired formation of a secondary N2O peak. Furthermore, NH3 can be formed during this slightly lean transition phase without any CO or hydrocarbons breakthrough. Such ammonia formation is desirable in the exhaust gas aftertreatment systems combining LNT with passive SCR technology.