Trap Catalyst Research Articles

Thermally Induced Structure and Functionality Evolution of a Pt‐based Lean NOx Trap Catalyst

Applying a NOx trap catalyst to effectively remove NOx contaminants from the oxygen‐rich exhaust is still very challenging, due to the lack of use guidance based on deactivation mechanisms. The temperature‐dependent Pt sintering rate constant was described using Pt dispersion determined by pulse CO chemisorption. It was found that 700‐850 ºC was the determining temperature regions for the significant Pt sintering, according to the sintering kinetics and the evolution of active Ptδ+ sites. The weak basicity plays a critical role in trapping NOx at low‐temperature (≤ 300 ºC), and the NOx trap efficiency decreases because of the loss of catalytic activity and medium basicity caused by forming BaAl2O4 phase. The reduction of NOx towards N2 on the Pt crystallites smaller than 18 nm is structure sensitive and decreases with losing the reducibility of catalysts. The security for long‐time use of a NOx trap catalyst should be exposed to the exhaust gas within 750 ºC, and effectively removing NOx contaminants requires it to be worked in the 350‐500 ºC regions. These findings can guide engineers and applied chemists how to efficiently use a NOx trap catalyst in the automotive exhaust purification system.

Lean NOx trap catalyst formulation for selective ammonia synthesis from captured NOx

In the Lean NOx Trap (LNT) technology, ammonia (NH3) occurs as reaction intermediate in conversion of trapped nitrogen oxides (NOx) to nitrogen gas (N2) using hydrogen gas (H2) reductant. The use of an LNT for synthesizing NH3 from NOx is potentially an attractive option for small-scale green NH3 production by converting locally produced NOx pollutant to an important chemical. In this work, it is investigated to what extent the product selectivity towards NH3 on the standard model catalyst Pt/BaO/Al2O3 can be maximized depending on reaction conditions for trapping and reducing NOx with H2 in the temperature range of 75–200 °C. The LNT catalyst preparation was varied by using Pt, Pd and bimetallic combinations, as well as different support materials, namely Al2O3, TiO2, ZrO2 and MgO. NOx adsorption and selectivity of captured NOx reduction to ammonia is discussed in terms of texture and the chemical nature of the support. The NH3 selectivity is influenced by the water vapor content in the gas feed. Storage of NOx in absence of water followed by reduction with hydrogen gas in presence of water vapor was found to be the most favorable operation mode of the LNT for NH3 formation. The NH3 selectivity reached 82% at 150 °C. Explanations for the role of water and in this catalytic chemistry are offered and its consequences for catalyst preparation are discussed. The optimum water content of the reacting gases is predicted to be strongly dependent on catalyst preparation.

Adsorption and desorption properties of toluene on an Ni-encapsulated Beta zeolite catalyst

Hydrocarbon trap (HCT) catalysts are promising automotive catalysts for purifying hydrocarbons in exhaust gas. Zeolites are suitable for use in these catalysts because of their high durability under hydrothermal conditions. Ni-encapsulated Beta zeolite (Ni@Beta) was obtained with Ni phyllosilicate as the Ni precursor. Ni@Beta exhibited higher toluene adsorption and desorption capabilities than those of Ni-impregnated Beta (Ni/Beta), and the Ni@Beta structure was stable after thermal aging at 900 °C. Moreover, Ni@Beta chemically adsorbed toluene after aging owing to the encapsulated Ni particles, which functioned as adsorption sites. In addition, the introduction of Ga into Ni@Beta improved the structural stability, and thus, the specific surface area was maintained at 80% after thermal treatment. Thus, this study provides an effective strategy for synthesizing HCT catalysts.

Enhanced low-temperature NOx storage and reduction performance using Cu-loaded lean NOx trap catalysts

Lean NOx trap (LNT) used in diesel-powered vehicles exhibit poor NOx storage and reduction (NSR) performance during low-temperature operation, particularly under cold-start conditions. To overcome this limitation, Cu was added to a conventional LNT catalyst (Pt-Ba/γ-Al2O3, PBA) to improve the NOx storage and regeneration performance of the Cu-loaded Ba adsorbent (Cu-Ba/γ-Al2O3) during low-temperature operation. However, a Pt-Cu complex was formed when Pt and Cu were used together as a one-body catalyst (Pt-Cu-Ba/γ-Al2O3, PCBA), leading to decreased catalytic performance. Physical mixing was investigated as a method to prevent the catalytic performance degradation caused by the Pt-Cu complex. The physical mixture of Pt/γ-Al2O3 and Cu-Ba/γ-Al2O3 (PA + CBA) could prevent the formation of Pt-Cu complex and yielded the highest NSR performance at 150 ℃ among the synthesized LNT catalysts. PA + CBA maintained a significantly improved NSR performance in lean-rich cycling tests using CO or H2 as a reducing agent.

Effects of substituting iron for aluminum on the low-temperature catalytic activity and sulfur resistance of hydrotalcite-derived LNT catalysts.

The storage and reduction of NOx on a series of Fe-modified hydrotalcite-based lean NOx trap catalysts were assessed, together with the product selectivity. The crystal structures and micromorphologies of these materials were characterized using X-ray diffraction and scanning electron microscopy, while in situ diffuse reflectance Fourier transform infrared spectroscopy was used to evaluate the evolution of transition state species. The introduction of Fe was found to improve the synergistic interaction between the Mg and Fe in the hydrotalcite structure, allowing these catalysts to work efficiently at low temperatures. In addition, both Pt/BaO/MgAlO and Pt/BaO/MgFeO catalysts exhibited better NOx adsorption and reduction performance compared with Pt/BaO/Al2O3. The superior performance of the former two materials was attributed to the enhanced adsorption of NOx in the form of nitrates and nitrites by Fe and Mg and to the ready decomposition of these nitrates at low temperatures. A Pt/BaO/MgFeO catalyst showed excellent low temperature activity and high selectivity for N2 together with superior sulfur resistance compared with Pt/BaO/Al2O3.

Development of the Lean NOx Trap Catalyst with High NOx Conversion and Improved Sulfur-purge Performance

Development of the Lean NOx Trap Catalyst with High NOx Conversion and Improved Sulfur-purge Performance

Выбор метода изготовления пористых порошковых изделий фильтрующего назначения

The article considers three methods of increasing the distribution uniformity of properties over the filtration area of powder filter materials at different stages of their manufacturing. It is shown that granulation of metal powders with a pore former increases the uniformity of permeability distribution over the filtration area by a factor of 2.3–3.5. Dry radial isostatic pressing ensures high distribution uniformity of properties, while the coefficient of variation of local permeability does not exceed 0.17. Radial compression allows increasing the uniformity of permeability distribution over the filtration area by 15 ... 22 % compared to that for the original powder filter materials. The selection of the method used in practice is determined by the shape, size and properties of the manufactured products and initial powders. The considered methods can be used in petrochemical engineering for the manufacture of porous powder products for filtering purposes, used to trap catalyst particles, filters for fine and coarse fuel and oil purification.

Low-dimensional models for real time simulations of monolith reactors with dual washcoat layers

We present a novel and accurate procedure with local property dependent internal and external mass transfer coefficient matrices for real time simulations of monolith reactors with dual washcoat layers. Our approach extends the prior work based on the Thiele matrix method for accurately describing the washcoat diffusional effects in single layered systems. It combines the vector form of the multi-scale averaged model with calculation of the local property dependent internal, external and cross-Sherwood number matrices for multi-component systems. We illustrate the usefulness of the reduced order model using the example of a dual-layered model system with first washcoat layer of selective catalytic reduction (SCR) catalyst and second washcoat layer of lean NOx trap (LNT) catalyst. We determine the mesh size independent solutions of the full 1+1D detailed model and compare the same with that obtained with the reduced order model. We show that the proposed method has the same accuracy as the mesh size independent detailed solution while speeding up calculations by about three orders of magnitude for the example studied, and possibly by several more orders of magnitude for real systems with complex and detailed micro-kinetic models to describe the storage and reactions in the catalytic layers.

Measures to Reduce the N2O Formation at Perovskite-Based Lean NOx Trap Catalysts under Lean Conditions

The net oxidising atmosphere of lean burn engines requires a special after-treatment catalyst for NOx removal from the exhaust gas. Lean NOx traps (LNT) are such kind of catalysts. To increase the efficiency of LNTs at low temperatures platinised perovskite-based infiltration composites La0.5Sr0.5Fe1-xMxO3-δ/Al2O3 with M = Nb, Ti, Zr have been developed. In general, platinum based LNT catalysts show an undesired, hazardous formation of N2O in the lean operation mode due to a competing C3H6-selective catalytic reduction (SCR) at the platinum sites. To reduce N2O emissions an additional Rh-coating, obtained by incipient wetness impregnation, besides the Pt coating and a two-layered oxidation catalyst (2 wt.% Pd/20 wt.% CeO2/alumina)-LNT constitution, has been investigated. Though the combined Rh-Pt coating shows a slightly increased NOx storage capacity (NSC) at temperatures above 300 °C, it does not decrease N2O formation. The layered oxidation catalyst-LNT system shows a decrease in N2O formation of up to 60% at 200 °C, increasing the maximum NSC up to 176 µmol/g. Furthermore, the NSC temperature range is broadened compared to that of the pure LNT catalyst, now covering a range of 250–300 °C.

Thermal ageing of a commercial LNT catalyst: Effects on the structure and functionalities

Hydrothermal degreening and ageing procedures were applied to a tri-metal (Pt-Pd-Rh) fully formulated lean NOx Trap catalyst to evaluate the effects of thermal stress on the performances and structural properties. X-ray absorption fine structure (XAFS) analysis revealed that the average size of the platinum particles was the same after degreening and ageing treatments. The formation of a new phase of alloyed Pt-Pd was observed to increase with the thermal load. The size of the ceria particles also increased after the ageing treatment. NOx storage capacity experiments revealed a substantial decrease of the concentration of active NOx storage sites which correlated with both ageing and degreening protocols. The performances of the treated catalyst were evaluated through spatially resolved (SpaciMS) lean-rich cycles. During the lean phase, the impact of the decrease in storage sites was significant on the aged sample, where an enlargement of the area required to achieve full storage was observed. On the other hand, the regeneration functionalities did not appear to be particularly affected by ageing. Rather, the aged sample showed a decrease of oxygen storage capacity (OSC), which promoted a lower reductant consumption and therefore a quicker and more efficient reduction process. On the other hand, the different distributions of adsorbed species by the end of the lean phase produced greater spread presence of NH3 and NOx slip along the channels of the aged sample during the reduction.

Composition/Performance Evaluation of Lean NOx Trap Catalysts for Coupling with SCR Technology

AbstractThe washcoat composition and the catalytic properties of two commercially available lean NOx traps (LNTs) were investigated. Both catalysts contained nominally the same NOx storage and catalytic materials but differed strongly in their amount and activity as well as in the composition of their layered washcoat architecture. In lean‐rich cycle experiments under realistic engine‐out gas compositions using a laboratory gas test bench, the LNTs showed comparable NOx storage behavior. At temperatures below 250 °C, the lean phase durations last up to 300 s until 50 % of the NOx storage capacity is reached. The simultaneously calculated NOx storage efficiencies drop rapidly below 35 %, resulting in a high NOx slip. Strong variations were observed in N2O and NH3 selectivity and in CO slip during regeneration of both LNTs caused by the different oxygen storage capacity (OSC), water gas shift (WGS) activity and rhodium distribution in the catalytic layers. Based on the obtained results, proposals were made to optimize the storage and regeneration performance, leading to highly efficient LNT catalysts for coupling with a downstream SCR catalyst.

Lean NOx trap catalysts with high low-temperature activity and hydrothermal stability

A combination of the Pt-BaO/CeO2 and Cu/CeO2 (PBC + CC) catalyst offers high NOx storage and reduction (NSR) performance at low temperatures; however, the PBC + CC catalyst is prone to deactivation upon hydrothermal aging. In this study, we used a reduced γ-Al2O3 support with unsaturated penta-coordinated Al3+ sites, which can anchor catalytic materials (Pt, Cu, BaO, and CeO2). The physical mixture of Pt-BaO-CeO2 and Cu-CeO2 deposited on the reduced Al2O3 (PBCrA + CCrA) was prepared and tested for the NOx removal. The PBCrA + CCrA catalyst showed little degradation after hydrothermal aging at 750 °C. XRD, HR-TEM, CO chemisorption, N2O chemisorption, and N2 physisorption results showed that catalytically active phases were barely changed even after the hydrothermal aging, particularly preventing ceria sintering. CO-TPR and XPS results demonstrated that the PBCrA + CCrA catalyst has a superior reducibility of lattice oxygen on CeO2, leading to excellent low-temperature activity.

Improvement of Fuel Economy and Greenhouse Gases Reduction in Gasoline Powered Vehicles Through the TWC-NOx Trap Catalyst

The three way catalyst NOx trap catalyst (TWC-NT) is consisted of a conventional three way catalyst (TWC) compositions and NOx storage material (NSM) to mitigate NOx breakthrough occurred during the fuel cut event to improve fuel economy of stoichiometric (A/F = 14.6) gasoline powered vehicles. Typical fuel cut operation generates very lean exhaust mixture that oxidizes the catalyst surface in TWC system, and NOx is slipped on the oxygen rich catalyst surface when the engine is switched back to the stoichiometric air to fuel ratio. The fully batch formulated TWC and TWC-NT catalyst were prepared and aged on engine dynamometer equivalent to 150,000 miles at underfloor location. The aged catalysts was evaluated at LEV-3 SULEV30 passenger car to check their effects on NOx efficiency, fuel saving and N2O suppression ability. The results revealed TWC-NT showed higher NOx removal performance by 71 % with multiple the fuel cuts and O2 purge period through the thermally enhanced NOx storage and reduction function, and 2 % higher fuel economy on FTP-75 cycle than the conventional TWC. Also the catalyst had good N2O suppression ability by the removal of NOx as the main reactant of N2O formation in the exhaust gas reactions over catalyst.

Optimizing the lean hydrocarbon NOx trap: Sequential and dual-layer configurations

Vehicular emission control catalysts are ineffective in eliminating CO, hydrocarbons, and NOx during engine cold-start when exhaust temperatures are below 200 °C. In this study the performance of coupled low temperature NOx, n-C12H26 (C12), and C3H6 trapping, release and conversion for a series of model Lean Hydrocarbon NOx Trap (LHCNT) catalysts are examined. Pd and Pt supported on small-pore (SSZ-13) and large-pore (BEA) zeolites are selected based on the performance during transient NO and C12 uptake, release and conversion experiments. These catalysts are combined into sequential (Pt + Pd/BEA → Pd/SSZ-13; Pd/SSZ-13 → Pt + Pd/BEA) and dual-layer (Pt + Pd/BEA top, Pd/SSZ-13 bottom) configurations in an attempt to improve the trapping and conversion performance. While all three configurations trap between 75 and 100 μmolNOx/g-cat, the Pd/SSZ-13 → Pt + Pd/BEA sequential configuration is most effective in simultaneously trapping C12 and NO in the presence of H2O, resulting in excellent NO and C12 storage below 100 °C with release and/or conversion at or above 200 °C. For each configuration, C12 oxidation lights-off below 300 °C and NO oxidation achieves ∼35 % conversion in the absence of C12. Neither the presence of C12 nor the order of the sequential configuration has a significant impact on NO uptake. C12 significantly delays NO and NO2 desorption to temperatures exceeding 300 °C. The more compact dual-layer catalyst is most effective in forming NO2 as the release temperature lines up with the maximum NO conversion temperature but traps less C12 than the sequential configurations. The addition of C3H6 in the feed on the dual-layer catalyst leads to further delay in the NOx desorption as well as increased NO and C12 conversion at high temperatures. The overall findings provide guidance in the optimizing LHCNT configuration for realistic feeds.

Optimization of the Washcoat Slurry for Hydrotalcite-Based LNT Catalyst

This work aimed to optimize the washcoat slurry for hydrotalcite-based lean NOx trap (LNT) catalyst. The effects of the slurry properties including pH, solid content, binder and additive on the hydrotalcite-based slurry viscosity were investigated. The particle size distribution of the optimal hydrotalcite-based slurry was measured. A cordierite material was used to coat the optimal slurry, and the washcoat was characterized by X-ray diffraction, scanning electron microscopy and N2 adsorption. The optimal slurry containing Pt and Ba was coated on the cordierite for the preparation of hydrotalcite-based LNT catalyst, and the performances of this catalyst were evaluated by NOx storage test, temperature programmed desorption and NOx reduction. For comparison, the performance of the commercial LNT catalyst with Pt/BaO/Al2O3 was analyzed. After coating, the hydrotalcite-based washcoat was closely contacted with the support, being the main phase MgO and presenting a specific surface area of 86.3 m2/g. The hydrotalcite-based LNT catalyst had better NOx storage and desorption ability, selectivity to N2 and LNT efficiency than the Pt/BaO/Al2O3 catalyst.

Deactivation reactions on a commercial lean nox-trap - Effect of hydrocarbon nature, concentration and operation temperature

Engine-out emissions during rich phase operation of a lean NOX trap (LNT) catalyst contain a mixture of different hydrocarbons (HCs) depending on engine operation and fuel type. The impact of temperature, as well as type and concentration of HC on reactions and deactivation potential on a commercial LNT catalyst was analyzed by lean-rich cycle measurements and subsequent temperature programmed oxidation. Deactivation during the rich phase is omitted initially by O species adsorbed or stored by CeO2, but formation of amorphous carbon and significant deactivation of noble metal sites occur in the presence of C2H2. Carbon formation and deactivation of Pt sites correlate with the reforming reaction of C2H2, thus emphasizing the significance of this reaction in the rapid build-up of carbonaceous deposits. Coking and deactivation decreases with decreasing C2H2 concentration and reaction temperature that determine the extent of reforming reaction. DFT calculations of adsorption energies on sites of Pt (111) or Pt10/CeO2 verify that abundance of HCs on the Pt surface, relative to other feed components, determines which reactions occur and thus the coking degree. This aligns with the finding that HCs with lower adsorption energy, such as C2H4 or C3H6, are less reactive and significant catalyst deactivation is absent during their presence.

Improved NOx Storage/Release Properties of Ceria-Based Lean NOx Trap Compositions with MnOx Modification

Ceria/spinel-based lean NOx trap compositions with and without barium were modified with MnOx via incipient wetness impregnation. The effect of the MnOx layer on the aged materials (850 °C) as to the NOx storage and release properties was investigated via NOx adsorption (500 ppm NO/5% O2/balance N2) carried out at 300 °C in a dual-bed with a 1% Pt/Al2O3 catalyst placed upstream of the samples to generate sufficient amounts of NO2 required for efficient NOx storage. Subsequent temperature programmed desorption (TPD) experiments were carried out under N2 from 300 °C to 700 °C. The addition of MnOx to the barium free composition led to a slightly reduced NOx storage capacity but all of the ad-NOx species were released from this material at significantly lower temperatures (ΔT ≈ 100 °C). The formation of a MnOx layer between ceria/spinel and barium had a remarkable effect on ageing stability as the formation of BaAl2O4 was suppressed in favour of BaMnO3. The presence of this phase resulted in an increased NOx storage capacity and lower desorption temperatures. Furthermore, NOx adsorption experiments carried out in absence of the Pt-catalyst also revealed an unexpected high NOx storage ability at low NO2/NO ratios, which could make this composition suitable for various lean NOx trap catalysts (LNT) related applications.

Sulfur Poisoning Effects on Modern Lean NOx Trap Catalysts Components

In the present work, a series of different materials was investigated in order to enhance the understanding of the role of modern lean NOx trap (LNT) components on the sulfur poisoning and regeneration characteristics. Nine different types of model catalysts were prepared, which mainly consisted of three compounds: (i) Al2O3, (ii) Mg/Al2O3, and (iii) Mg/Ce/Al2O3 mixed with Pt, Pd, and Pt-Pd. A micro flow reactor and a diffuse reflectance infrared Fourier transform spectrometer (DRIFTS) were employed in order to investigate the evolution and stability of the species formed during SO2 poisoning. The results showed that the addition of palladium and magnesium into the LNT formulation can be beneficial for the catalyst desulfation due mainly to the ability to release the sulfur trapped at relatively low temperatures. This was especially evident for Pd/Mg/Al2O3 model catalyst, which demonstrated an efficient LNT desulfation with low H2 consumption. In contrast, the addition of ceria was found to increase the formation of bulk sulfate species during SO2 poisoning, which requires higher temperatures for the sulfur removal. The noble metal nature was also observed to play an important role on the SOx storage and release properties. Monometallic Pd-based catalysts exhibited the formation of surface palladium sulfate species during SO2 exposure, whereas Pt-Pd bimetallic formulations presented higher stability of the sulfur species formed compared to the corresponding Pt- and Pd-monometallic samples.

BaTi0.8B0.2O3 (B = Mn, Fe, Co, Cu) LNT Catalysts: Effect of Partial Ti Substitution on NOx Storage Capacity

The effect of partial Ti substitution by Mn, Fe, Co, or Cu on the NOx storage capacity (NSC) of a BaTi0.8B0.2O3 lean NOx trap (LNT) catalyst has been analyzed. The BaTi0.8B0.2O3 catalysts were prepared using the Pechini’s sol–gel method for aqueous media. The characterization of the catalysts (BET, ICP-OES, XRD and XPS) reveals that: i) the partial substitution of Ti by Mn, Co, or Fe changes the perovskite structure from tetragonal to cubic, whilst Cu distorts the raw tetragonal structure and promotes the segregation of Ba2TiO4 (which is an active phase for NOx storage) as a minority phase and ii) the amount of oxygen vacancies increases after partial Ti substitution, with the BaTi0.8Cu0.2O3 catalyst featuring the largest amount. The BaTi0.8Cu0.2O3 catalyst shows the highest NSC at 400 °C, based on NOx storage cyclic tests, which is within the range of highly active noble metal-based catalysts.

Synergistic Effect of Cu/CeO2 and Pt-BaO/CeO2 Catalysts for a Low-Temperature Lean NO x Trap.

A lean NO x trap (LNT) catalyst has been widely used for removing NO x exhaust from lean-burn engines. However, the operation range of LNT has been limited because of the poor activity of LNT catalysts at low temperatures (≤300 °C), especially in urban driving conditions. To increase NO x removal efficiency during lean-rich cycle operation, a Cu/CeO2 (CC) catalyst was added to a Pt-BaO/CeO2 (PBC) catalyst. In comparison to PBC- or CC-only catalysts, the physical mixture of PBC and CC catalysts (PBC + CC) exhibited a significant synergy for both NO x storage and reduction efficiencies. In particular, low-temperature activity below 200 °C was greatly enhanced. A Pt-BaO-Cu/CeO2 (PBCC) catalyst, which was synthesized by depositing Pt and Cu together on a ceria support, showed poorer NO x removal efficiency. The origin of the synergistic effect over PBC + CC was investigated. Under lean conditions, the CC showed much better activity for NO oxidation, allowing for faster NO x storage on PBC. Under rich conditions, H2 was generated in situ on the CC by a water-gas shift reaction then accelerated the reduction of NO x, which had been stored on PBC, with a higher selectivity to N2. This simple modification in the catalyst can provide an important clue to enhance low-temperature activity of the commercial LNT system.