NOx Storage Research Articles

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.

Macroporous carrier-free Sr-Ti catalyst for NOx storage and reduction

A novel concept is presented, consisting of a macroporous carrier-free catalyst. As a proof of concept, a novel Cu-containing Sr-Ti NSR catalyst with a macroporous network has been synthesized, and its maximum NOx storage capacity (1500μmolNOx/gcatalyst) significantly surpasses that of conventional Pt/Ba/Al2O3 formulations (∼600–800μmolNOx/gcatalyst). This high NOx storage capacity is achieved because the active phases (mainly SrCO3 and Cu-containing Sr-Ti perovskites, as deduced by XRD) are not diluted in an inert carrier, and this can be done because the macroporous structure obtained using a polymethylmethacrylate colloidal crystal template allows the access of gases to the particles bulk. In situ DRIFTS experiments showed that NOx were chemisorbed on the novel Cu-containing Sr-Ti macroporous catalyst as a mixture of nitrite and nitro species, and this suggests that several NOx chemisorption pathways are simultaneously taking place probably involving chemisorption of both NO and NO2. Additionally, this novel catalyst is totally selective towards N2 formation as NOx reduction product, without traces of N2O nor NH3. CO2 and H2O compete with NOx for being chemisorbed on the catalyst, and this hinders the utilization of this catalyst in real diesel exhausts. However, we believe that this new concept of macroporous carrier-free catalyst could be extended to other heterogeneous catalyzed reactions or materials to avoid the diluting effect of the catalyst support.

Pt/Ba/Co1Mg2Al1Ox with dual adsorption sites: A novel NOx storage and reduction catalyst

A novel NOx storage and reduction catalyst, namely 1Pt/20Ba/Co1Mg2Al1Ox with dual NOx adsorption sites was developed. The layered double hydroxide derived mixed metal oxide Co1Mg2Al1Ox not only serves as catalyst support but also provides additional NOx adsorption catalytic sites. The influence of Pt and BaO loadings, adsorption and calcination temperatures, H2O resistance and lean-rich cycling conditions on the performance of the stated catalyst composition were systematically investigated. We demonstrate that this new 1Pt/20Ba/Co1Mg2Al1Ox catalyst possesses much higher NOx storage capacity (1.38mmol/g) than the conventional Pt/Ba/γ-Al2O3 (0.56mmol/g). It also exhibits good H2O resistance, stability in lean-rich cycling conditions and a high NOx removal efficiency (ca. 82.2%), thus suggesting a great potential for practical lean-burn deNOx applications.

The function of Pt in plasma-assisted NOx storage and reduction

The storage and reduction of NOx was examined over a series of mechanically mixed LaMn0.9Fe0.1O3 (LMF)+Ba/Al2O3 (BA), Pt-LMF+BA and LMF+BA+Pt/Al2O3 (PA) catalysts in order to investigate the function of Pt in plasma-assisted NSR reaction. Efforts were also made to optimize the location of Pt to give the best H2 reduction ability.

Rapidly pulsed reductants for diesel NOx reduction with lean NOx traps: Effects of pulsing parameters on performance

The use of Rapidly Pulsed Reductants (RPR) provides a method for injecting hydrocarbons in rapid pulses ahead of a lean NOx trap (LNT) with the goal of achieving greater than 90% NOx conversion at temperatures as high as 600°C. This approach was recently discussed under the name Di-Air by Toyota. It may be useful for diesel vehicles with smaller engines that cannot conveniently accommodate urea SCR technology. A further goal of RPR is to reduce the fuel penalty associated with the fuel injections used to purge the LNT.The focus of this study was on understanding this approach in the exhaust temperature range of 450°C to 600°C. The impact of pulsing parameters, i.e., pulse frequency, pulse amplitude, and duty cycle (i.e., ratio of pulse-on/total-cycle-time) on RPR performance was investigated using ethylene injection into simulated exhaust with a research LNT catalyst over a wide range of conditions. A novel injection system was designed that allowed for the investigation of previously unexplored areas of high frequency (up to f=100Hz) and produced pulse durations as short as a millisecond. NOx storage capability was found to be essential for RPR operation. An optimal pulsing frequency was always found for a given set of flow conditions that produced a maximum NOx conversion while holding the fuel penalty (i.e., hydrocarbon dose) constant. The optimal pulsing frequency, usually on the order of 1Hz, was dependent on a number of parameters including the flow velocity, concentration of injected hydrocarbons, and operating temperature. The effects of pulse duty cycle and amplitude, which together determine the amount of injected reductant or the fuel penalty, were investigated separately. Regardless of pulsing frequency and duty cycle, pulses of injected hydrocarbon large enough in magnitude to generate a rich mixture were necessary during the pulse in order to achieve high NOx conversion. Lean or even stoichiometric pulses did not achieve significant NOx conversion. However, even with the rich pulses, the cycle-averaged air/fuel ratio remained net lean in this work.In the main temperature range of this study (450–600°C), where the NOx storage capacity of the LNT was very limited, RPR operation could be divided into two frequency regimes, one below and one above the optimum frequency. At injection frequencies below the optimum frequency, it was observed that the NOx conversion improved as the frequency increased. This was primarily because the duration of the lean period was decreasing, increasing the average NOx storage efficiency during the lean period while still maintaining sufficient durations of the rich period to adequately purge the LNT. At injection frequencies above the optimum frequency, reductant mixing and its parasitic consumption by the gas phase O2 resulted in inadequate purging of the LNT, causing the NOx conversion to drop from the maximum value. These competing factors led to the maximum NOx conversion observed during RPR operation for a LNT at temperatures as high as 600°C for a specified fuel penalty.

Barium Oxide (BaO) Infiltrated Lanthanum Strontium Manganese Oxide (LSM)-Gadolinium Doped Ceria (GDC) Solid Oxide Electrochemical Reduction Cells (SOERC) for Reduced Diesel NOx Emissions

Diesel engines are widely used in automobiles for their high fuel efficiency, low carbon monoxide emissions, and low hydrocarbon emissions [1]. Unfortunately, due to their lean-burn operating environment, diesel engines emit higher amounts of NOx than gasoline engines. Although NOx Storage and Reduction (NSR) and Selective Catalytic Reduction (SCR) technologies have been commercialized and offer high deNOx (i.e. NOx decomposition) efficiency, they suffer from a loss of fuel efficiency during catalyst regeneration and a need for large-volume, on-board ammonia storage and replenishment, respectively [2]. Further, expensive precious metal catalysts are needed to enable deNOx chemical reactions in these conventional technologies. In contrast, Solid Oxide Electrochemical Reduction Cells (SOERCs) utilizing an external bias to electrochemically drive NOx decomposition promise precious-metal-free operation without the need for catalyst regeneration or large-volume, on-board storage. Here, the deNOx performance of barium oxide (BaO) infiltrated lanthanum strontium manganese oxide (LSM)-gadolinium doped ceria (GDC) SOERCs were measured as a function of operating temperature, AC amplitude, and AC frequency. Under all test conditions, SOERC performance was reproducible across different samples with the application, removal, and subsequent reapplication of electrical bias. The BaO-LSM-GDC SOERCs tested here displayed better low-temperature NOx conversion efficiencies than those previously reported in the literature; achieving 22% at 350oC compared to the 3% reported in the literature [3, 4]. In agreement with this literature 1) the application of an AC electric bias was found to produce higher BaO-LSM-GDC NOx conversion efficiencies than a DC bias of the same magnitude, and 2) BaO-LSM-GDC SOERCs exhibited current and thermodynamic efficiencies of a few percent, and a few tenths of a percent, respectively. Unfortunately, as shown previously [3, 4], these conversion efficiencies decreased by several percent after only a few hours of operation at 450oC due to microstructural coarsening of the BaO infiltrate.

Oxidizing, trapping and releasing NOx over model manganese oxides in alternative lean-burn/fuel-rich atmospheres at low temperatures

Herein, we report the performance of the catalytic oxidizing, trapping and releasing NO over the MnOx (Mn2O3, Mn3O4 and MnO2) model Lean NOx Traps (LNT) catalysts at low temperatures (≤200°C). Among these typical MnOx samples, Mn2O3 has the high NO oxidizability, and can efficiently trap and release NOx in the alternative lean-burn/fuel-rich atmospheres. Our characterization results show that enrichment of the Mn3+ cations and active surface oxygen species probably plays the important role for NO oxidation. Although MnO2 has the biggest NOx storage capacity in the lean-burn atmosphere, the strong adsorption strength between MnO2 and NOx inhibits the release of the stored NOx species in the fuel-rich atmosphere. The in situ DRIFTS and NO-TPD studies indicate that the trapped NOx on Mn2O3 can transform from nitro compounds to nitrites, and then to nitrates along with the increased adsorption temperature.

Design of highly efficient NOx storage-reduction catalysts from layered double hydroxides for NOx emission control from naphtha cracker flue gases

Although there are many reports on the removal of NOx from vehicle and power plant emissions, little effort has been devoted to the removal of NOx from cracker flue gases. Here we report a systematic investigation on the design of highly efficient NOx storage-reduction (NSR) catalysts using highly dispersed aqueous miscible organic-layered double hydroxides (AMO-LDHs) derived mixed oxides (LDOs) for NOx emission control from naphtha cracker flue gases. For a series of binary M2+Al3+ LDOs, the influence of six divalent cations (Ca, Zn, Cu, Ni, Co, and Mg) on the NOx adsorption capacity, NO oxidation property, and the thermal stability of adsorbed NOx was systematically investigated. The optimal NOx storage temperature range for each binary LDO was also determined. Based on the fundamental findings of binary LDOs, a ternary LDH CoMgAl-CO3 derived LDO CoMgAlOx with a significantly improved NOx storage capacity was designed. By tuning the Co/Mg ratio, the NOx storage capacity was greatly improved from 0.14mmol/g for Mg3Al1Ox to 0.50mmol/g for Co1Mg2Al1Ox at 300°C. By further doping Pt and K2CO3, a new NSR catalyst composed of 1wt% Pt/15wt% K2CO3/Co1Mg2Al1Ox was obtained, which exhibited a very high NOx storage capacity of 1.2mmol/g. Finally, the NSR cycling performance, and the CO2, SO2 and H2O resistance of this catalyst were also investigated. Thanks for its superior NOx storage capacity, NSR cycling performance, and CO2, SO2 and H2O resistance, this new NSR catalyst showed great potential for the NOx control from cracker flue gases.

NOx conversion and selectivity in multi-layer and sequential DOC-LNT automotive exhaust catalysts: Influence of internal transport

In order to pass strict emission standards, automobiles with internal combustion engines need to be equipped with complex exhaust gas aftertreatment systems. For example, Diesel engines are usually fitted with an oxidation catalyst (DOC), deNOx catalyst (either SCR or lean NOx trap – LNT) and a particulate filter. Integrating them into a single multi-functional device saves space, cost and weight while improving heat management during cold starts as well. This work investigates a compact DOC+LNT converter combining both catalyst types in a single device – porous layer of a DOC catalyst is added on top of a LNT washcoat. The performance of this dual-layer converter is compared with traditional single-layer DOC and LNT coated on two separate monoliths in series.Laboratory reactor experiments showed that the performance of the dual-layer sample was affected by transport limitations – mainly internal diffusion through the catalytic layers. An increased NOx slip was observed during NOx adsorption in the dual-layer sample. The lower amount of stored NOx was however more than compensated during NOx reduction at lower temperatures near the light-off, where the compact dual-layer monolith exhibited higher efficiency of NOx reduction and lower selectivity to N2O during rich regeneration periods. This was attributed to the fact that the NOx released from the bottom LNT layer has to pass through the top DOC layer where it can further react with the reducing agents from the rich feed. At higher temperatures, the two single-layer monoliths outperformed the compact dual-layer monolith.

The influence of xMnyCe/γ-Al2O3 on NOx catalysts on the properties of NOx storage and reduction over Pt-Ce-Ba/γ-Al2O3 catalysts

The aim of the study was to investigate the influence of Diesel Oxidation Catalysts (DOC) xMnyCe/γ-Al2O3catalysts on the properties of NOx storage and reduction (NSR) under the treatment of Pt-Ce-Ba/γ-Al2O3 catalysts. The physicochemical properties of DOC and NSR were determined with XRD, TEM and XPS. The results showed that Pt-CeO2 interaction had a great impact on the dispersion of the NSR catalyst. Furthermore, the performance of NO oxidation by DOC and NOx desorption by DOC/NSR combined system were evaluated based on the simulation experiments. Moreover, optimal operation of the single NSR catalyst and the DOC/NSR combined system were attained at 400°C, while the durations of the lean and rich periods were tL=240s and tR=30s/45s/60s, respectively. The single NSR catalyst showed that the highest NOx conversion and maximum NOx adsorption conversion were 58.0% and 74.8%. The DOC/NSR combined system allowed Pt10Ce15Ba/γ-Al2O3 catalyst storing the NO2 oxidized on the 6Mn10Ce/γ-Al2O3 catalyst, favoring the NOx storage process during the lean phase. And it also allowed the stored NOx species of NSR catalysts desorbing due to H2 oxidation by the 6Mn10Ce/γ-Al2O3 catalyst, favoring the NOx desorption process during the rich phase.

Fast cycling in a non-isothermal monolithic lean NOx trap using H2 as reductant: Experiments and modeling

A combined experimental and modeling study of fast cycling NOx storage and reduction with H2 for emissions control of lean burn gasoline and diesel vehicles is conducted to provide mechanistic insight and detailed understanding to Toyota’s Di-Air technology. Experiments reveal NOx conversion enhancement over a model Pt/Rh/BaO/CeO2/Al2O3 lean NOx trap (LNT) catalyst for cycle times less than 10s (fast cycling) compared to the longer cycle times typical of conventional NSR. Increased cycle frequency results in the increase (decrease) of the cycle-averaged NOx conversion (NH3 selectivity) over a wide temperature range. The more frequent storage and regeneration of fast cycling increases the utilization of stored NOx capacity by decreasing NOx breakthrough during the lean phase. The decreased NH3 selectivity is in part a result of a combination of increased efficiency of the NH3 oxidation by oxygen stored on the ceria component and of the adsorption of NH3 on the support. A global kinetic model incorporated into a non-isothermal 1-D monolith reactor model is effective in capturing most of the experimental trends. Quantitative agreement requires treatment of the extent of mixing of the lean and rich feeds upstream of the reactor, as well as of mixing of the product stream in the downstream FTIR cell. The predicted axial temperature profile shows a front-monolith temperature rise as high as twice that measured in the middle-monolith under slower cycling. Under faster cycling a predicted 100°C front-monolith temperature rise contrasts to a measured near-constant middle-monolith temperature. Non-isothermal effects are especially important when catalyst temperatures exceed 450°C, which lowers the NOx storage capacity, resulting in an increased amount of slipped NOx during early lean phase.

Rapidly Pulsed Reductants for Diesel NOx Reduction With Lean NOx Traps: Comparison of Alkanes and Alkenes as the Reducing Agent

Lean NOx traps (LNTs) are often used to reduce NOx on smaller diesel passenger cars where urea-based selective catalytic reduction (SCR) systems may be difficult to package. However, the performance of LNTs at temperatures above 400 °C needs to be improved. Rapidly pulsed reductants (RPR) is a process in which hydrocarbons are injected in rapid pulses ahead of the LNT in order to improve its performance at higher temperatures and space velocities. This approach was developed by Toyota and was originally called Di-Air (diesel NOx aftertreatment by adsorbed intermediate reductants) (Bisaiji et al., 2011, “Development of Di-Air—A New Diesel deNOx System by Adsorbed Intermediate Reductants,” SAE Int. J. Fuels Lubr., 5(1), pp. 380–388). Four important parameters were identified to maximize NOx conversion while minimizing fuel penalty associated with hydrocarbon injections in RPR operation: (1) flow field and reductant mixing uniformity, (2) pulsing parameters including the pulse frequency, duty cycle, and magnitude, (3) reductant type, and (4) catalyst composition, including the type and loading of precious metal and NOx storage material, and the amount of oxygen storage capacity (OSC). In this study, RPR performance was assessed between 150 °C and 650 °C with several reductants including dodecane, propane, ethylene, propylene, H2, and CO. Under RPR conditions, H2, CO, C12H26, and C2H4 provided approximately 80% NOx conversion at 500 °C; however, at 600 °C the conversions were significantly lower. The NOx conversion with C3H8 was low across the entire temperature range. In contrast, C3H6 provided greater than 90% NOx conversion over a broad range of 280–630 °C. This suggested that the high-temperature NOx conversion with RPR improves as the reactivity of the hydrocarbon increases.

Micro-calorimetric studies of NO2 adsorption on Pt/BaO-supported on γ-Al2O3 NOx storage and reduction (NSR) catalysts—Impact of CO2

The adsorption of NO2 on Pt/BaO/γAl2O3 catalyst has been investigated by micro calorimetry at atmospheric pressure, NOx storage tests and temperature-programmed desorption (TPD). The heat of adsorption of NO2 (ΔHads(NO2)) was determined over a wide range of NOx coverages, as the catalyst was exposed to 500/900ppm NO2 in the absence/presence of 5% CO2 in the range of 423–773K. The temperature dependent changes of ΔHads(NO2) verified the presence of energetically different NOx storage sites with different binding strength. The ΔHads(NO2) was found to follow a linear correlation versus temperature, ranging for example from −134.5 to −178.8kJ/mol for NOx storage over Pt/BaO/γAl2O3 at 423–673K. Thus, at high temperature mostly strongly bound nitrates were formed, while at lower temperature more loosely bound species were also present. Interestingly, the heat of adsorption was higher when using higher NO2 concentration, indicating more bulk barium nitrate formation. This is consistent with the TPD data where a clear high temperature peak was visible after adsorption using 900ppm NO2 at 423 and 473K, which was not the case for 500ppm NO2. Moreover, the micro-calorimetric data also provided evidence in support of the detrimental effect of CO2 on the NOx uptake process. The heat released during the NOx storage in 500ppm NO2+5% CO2 was determined to be significantly reduced ca. −97.8kJmol−1 at 423K, but ca. −134.5kJmol−1 without CO2. Furthermore, our results show that it is critical to measure heat of adsorption for surface compounds since they are significantly different compared to thermodynamic data for bulk materials.

Performance Studies and Correlation between Vehicle- and Rapid- Aged Commercial Lean NOx Trap Catalysts

Even though substantial improvements have been made for the lean NOx trap (LNT) catalyst in recent years, the durability still remains problematic because of the sulfur poisoning and sintering of the precious metals at high operating temperatures. Hence, commercial LNT catalysts were aged and tested in order to investigate their performance and activity degradation compared to the fresh catalyst, and establish a proper correlation between the aging methods used. The target of this study is to provide useful information for regeneration strategies and optimize the catalyst management for better performance and durability. With this goal in mind, two different aging procedures were implemented in this investigation. A catalyst was vehicle-aged in the vehicle chassis dynamometer for 100000 km, thus exposed to real conditions. Whereas, an accelerated aging method was used by subjecting a fresh LNT catalyst at 800 °C for 24 hours in an oven under controlled conditions. Engine dynamometer studies were performed with a Volvo mid-sized diesel engine with the purpose of testing the NOx storage and reduction performance, as well as the THC and CO conversion activity of the catalysts under controlled conditions. The aged catalysts activity was shown to be significantly degraded, mainly at low working temperatures compared to the fresh LNT, and one reason for this could be limited NO oxidation. In addition, the oven-aged sample was found to be well correlated to the vehicle-aged catalyst. On top of that, several vehicle emission cycles were carried out in the vehicle chassis dynamometer with a 2.0 l Volvo XC90 diesel vehicle in order to study the catalysts performance under real driving conditions and monitor the gradual deterioration of the vehicle-aged catalyst during the vehicle aging testing.

The Effects of CO, C2H4, and H2O on the NOx Storage Performance of Low Temperature NOx Adsorbers for Diesel Applications

The Effects of CO, C₂H₄, and H₂O on the NO_x Storage Performance of Low Temperature NO_x Adsorbers for Diesel Applications

Impact of Soot on NOx Adsorption over Cu-Modified Hydrotalcite-Derived Lean NOx Trap Catalyst.

The impact of soot on NOx adsorption was studied over a Cu-modified hydrotalcite-derived lean NOx trap catalyst in a NO + O2 atmosphere. Powder X-ray diffraction, scanning electron microscopy, Raman scattering spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the surface properties of the pure catalyst and the soot/catalyst mixture. The adsorbed NOx species on the samples were evaluated by in situ diffuse reflectance Fourier transform spectroscopy. The soot coverage decreases the available adsorption sites on the surface of the catalyst, and a portion of active oxygen species are consumed by the soot oxidation during He pretreatment process. The NOx adsorption on two catalyst samples simultaneously undergoes two routes: the "nitrite route" and the "nitrate route". The "nitrite route" is more dominant than the "nitrate route". During NOx adsorption, the soot oxidation weakens the NO oxidation to NO2, and the released CO2 competes with NOx on the adsorption sites. Moreover, the temperature-programmed desorption tests indicate that the presence of soot reduces the NOx storage capacity of the catalyst and shifts the NO desorption peak to the lower temperature range by 50 °C.

Preparation and characterization of Pt-Ba-Al2O3 coatings obtained by spray pyrolysis

Pt-Ba-Al2O3 coatings were obtained by spray pyrolysis deposition on stainless steel foil. The XRD results showed that the coatings consisted from Ba carbonate and Pt (fcc) phases. SEM micrographs demonstrated that the coatings are smooth and compact. The XPS data confirmed that Ba and Pt are in close contact. Pt crystallite size is only slightly influenced by the order of Pt and Ba deposition and the dispersion of Pt is in the narrow range from 28 to 33% for all samples. Catalyst component deposition order affects NOx storage capacity. Simultaneous deposition of Ba and Pt on already formed Al2O3 substrate produces the most efficient catalyst. Pt deposited on top of the Ba layer results in slightly reduced NOx storage capacity, while deposition of Pt on the separated layer with Ba layer on top significantly reduces the ability for NOx storage.

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.

Effect of support material Al2O3vs ZrO2-TiO2 on the Ba availability for NSR catalyst: An in situ and operando IR study

The evolution of Ba sites in two distinct NOx storage and reduction (NSR) catalysts that are based on alumina or zirconia-titania mixed oxide (ZrO2-TiO2 or ZT) during NOx adsorption/desorption was investigated by in situ and operando IR spectroscopy. Based on various evidences from the in situ study, medium sized Ba sites on alumina supported fresh catalysts are proposed to experience sintering under NOx adsorption to form bigger particles, while for ZT, initially possessing smaller sized Ba particles, the formation of a thin layer or very fine particles of Ba would proceed under the same condition. This evolution can also be affirmed by observations from the operando IR study showing that NOx adsorption on ZT supported catalyst is initially faster than on alumina supported catalyst (time on stream lower than 300s), but after long adsorption time (about 50min) the two catalysts show similar storage capacity. This new mechanistic insight suggests that NOx ad/desorption rate, which is critical for optimizing NSR performance, needs to be controlled by support materials whose interaction with the Ba particles not only determines their initial size (fresh catalyst) but also their resistance towards sintering during the NOx adsorption

Synthesis of Pt/K2CO3/MgAlOx\u2013reduced graphene oxide hybrids as promising NOx storage\u2013reduction catalysts with superior catalytic performance

Pt/K2CO3/MgAlOx–reduced graphene oxide (Pt/K/MgAlOx–rGO) hybrids were synthesized, characterized and tested as a promising NOx storage and reduction (NSR) catalyst. Mg–Al layered double hydroxides (LDHs) were grown on rGO via in situ hydrothermal crystallization. The structure and morphology of samples were thoroughly characterized using various techniques. Isothermal NOx adsorption tests indicated that MgAlOx–rGO hybrid exhibited better NOx trapping performance than MgAlOx, from 0.44 to 0.61 mmol · g−1, which can be attributed to the enhanced particle dispersion and stabilization. In addition, a series of MgAlOx–rGO loaded with 2 wt% Pt and different loadings (5, 10, 15, and 20 wt%) of K2CO3 (denoted as Pt/K/MgAlOx–rGO) were obtained by sequential impregnation. The influence of 5% H2O on the NOx storage capacity of MgAlOx–rGO loaded with 2 wt% Pt and 10% K2CO3 (2Pt/10 K/MgAlOx–rGO) catalyst was also evaluated. In all, the 2Pt/10 K/MgAlOx–rGO catalyst not only exhibited high thermal stability and NOx storage capacity of 1.12 mmol · g−1, but also possessed excellent H2O resistance and lean–rich cycling performance, with an overall 78.4% of NOx removal. This work provided a new scheme for the preparation of highly dispersed MgAlOx–rGO hybrid based NSR catalysts.