NOx Storage Research Articles

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.

Investigation of NO adsorption and desorption phenomena on a Pd/ZSM-5 passive NOx adsorber

Pd-based zeolite materials have gained significant attention as passive NOx adsorbers (PNAs) for diesel-engine cold-start NOx mitigation due to their ability to store NOx at low temperatures. Pd/ZSM-5 is a promising PNA candidate, however, the NO adsorption mechanism over this material is not well understood. This study combines flow reactor experiments and surface spectroscopy to investigate NO adsorption under a variety of conditions. The state of hydration of the Pd cations played an important role in determining the impact of H2O and CO concentration on the PNA performance. Below 150 °C, the inhibition effect of H2O on NO adsorption was mitigated by CO. Due to dehydration, neither H2O nor CO had an impact on NO uptake at 150 °C, where maximum NO storage capacity of the PNA was also observed. The observed gas composition and temperature effects on NO adsorption and formation of surface intermediates ultimately inform a proposed mechanism.

Low-Temperature NOx Storage Capability of YBaCo4O7+δ Originating from Large Oxygen Nonstoichiometry

Low-Temperature NO<i>_x</i> Storage Capability of YBaCo₄O_7+δ Originating from Large Oxygen Nonstoichiometry

NOx Storage Performance at Low Temperature over Platinum Group Metal-Free SrTiO3-Based Material.

Pt-based catalysts are commonly employed as NOx-trapping catalysts for automobiles, while perovskite oxides have received attention as Pt-free NOx-trapping catalysts. However, the NOx storage performance of perovskite catalysts is significantly inferior at low temperatures and with coexisting gases such as H2O, CO2, and SO2. This study demonstrates that NOx storage reactions proceed over redox site (Mn, Fe, and Co)-doped SrTiO3 perovskites. Among the examined catalysts, Mn-doped SrTiO3 exhibited the highest NOx storage capacity (NSC) and showed a high NSC even at a low temperature of 323 K. Moreover, the high NOx storage performance of Mn-doped SrTiO3 was retained in the presence of poisoning gases (H2O, CO2, and SO2). NO oxidation experiments revealed that the NSC of Co-doped SrTiO3 was dependent on the NO oxidation activity from NO to NO2 via lattice oxygen, which resulted in an inferior NSC at low temperatures. On the other hand, Mn-doped SrTiO3 successfully adsorbed NO molecules onto its surface at 323 K without the NO oxidation process using lattice oxygens. This unique adsorption behavior of Mn-doped SrTiO3 was concluded to be responsible for the high NSC in the presence of poisoning gases.

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.

Analysis of Ion-Exchanged ZSM-5, BEA, and SSZ-13 Zeolite Trapping Materials under Realistic Exhaust Conditions

An industry-defined evaluation protocol was used to evaluate the hydrocarbon trapping (HCT) and passive NOx adsorption (PNA) potential for BEA, ZSM-5, and SSZ-13 zeolites with ion-exchanged Pd or Ag. All materials underwent 700 °C degreening prior to exposure to an industry-derived protocol gas stream, which included NOx, ethylene, toluene, and decane as measured trapping species as well as common exhaust gasses CO, H2O, O2, CO2, and H2. Evaluation showed that BEA and ZSM-5 zeolites were effective at trapping hydrocarbons (HCs), as saturation was not achieved after 30 min of exposure. SSZ-13 also stored HCs but was only able to adsorb 20–25% compared to BEA and ZSM-5. The presence of Ag or Pd did not impact the overall HC uptake, particularly in the first three minutes. Pd/zeolites had significantly lower THC release temperature, and it aided in the conversion of the released HCs; Ag only had a moderate effect in both areas. With respect to NOx adsorption, the level of uptake was much lower than HCs on all samples, and Ag or Pd was necessary with Pd being notably more effective. Additionally, only Pd/ZSM-5 and Pd/SSZ-13 continue to store a portion of the NOx above 200 °C, which is critical for downstream selective catalytic NOx reduction (SCR). Hydrothermal aging (800 °C for 50 h) of a subset of the samples were performed: BEA, Pd/BEA, ZSM-5, Pd/ZSM-5, and Pd/SSZ-13. There was a minimal effect on the HC storage, ~10% reduction in capacity with no effect on release temperature; however, only Pd/SSZ-13 showed significant NOx storage after aging.

Effect of gas composition on the NOx adsorption and reduction activity of a dual-function Ag/MgO/[formula omitted]-Al2O3 catalyst

An Ag/MgO/γ-Al2O3 catalyst is developed, which can perform the dual role of a passive NOx adsorber (PNA) at low temperatures and a NOx reduction catalyst at higher temperatures. The effect of H2O and CO2, as well as various reductants such as H2, NH3, and C3H6 on its deNOx performance is investigated. The NOx storage is comparable to that obtained by Pd-based PNAs, and occurs through a spillover mechanism. The presence of H2 remarkably increases the low-temperature NOx adsorption and binding strength, and is attributed to the significant formation of nitrates as opposed to the predominant formation of nitrites in the absence of H2. Moreover, the positive effect of H2 is not related to the H2-SCR activity of the catalyst or an enhancement in the NO oxidation activity. C3H6 inhibits NOx storage, but the presence of H2 promotes it considerably. A significantly high NH3-SCR activity with negligible N2O formation is measured at 150 °C, while C3H6-SCR activity is exhibited at higher temperatures. However, the presence of NH3 does not affect the binding of NOx on the various adsorption sites, which is primarily governed by the presence of H2.

Investigation of crystal size effect on the NOx storage performance of Pd/SSZ-13 passive NOx adsorbers

The crystal size effect on the NOx adsorption and desorption over Pd/SSZ-13 passive NOx adsorbers (PNA) was investigated. SSZ-13 zeolites with various crystal size (0.4, 0.8, and 2.3 μm) presented highly textural uniformity. 0.4 μm Pd/SSZ-13 showed the highest NOx storage capacity and robust hydrothermal stability. According to the combined XRD, HAADF-STEM, H2-TPR, and CO-TPR results, ionic Pdn+ ions were dispersed better on Pd/SSZ-13 with a smaller crystal size. The nature of Pdn+ ion distribution was investigated by bulky and external surface acidic characterization. It was found that more external surface acid sites were formed on as-prepared smaller-sized SSZ-13 zeolites, which contributed to easier Pdn+ ion diffusion. The kinetic study showed that the NOx adsorption on Pd/SSZ-13 obeyed the pseudo-second-order model and the NOx adsorption activation energy for Pd/SSZ-13 was 25∼38 kJ/mol, independent of the crystal size. This work might guide the design of state-of-the-art zeolite supported PNAs.

A novel ceria hollow nanosphere catalyst for low temperature NOx storage

In this work, a highly active CeO2 catalyst with hollow nanosphere morphology for low temperature NOx storage was prepared by a surfactant-assisted solvothermal reaction. The physicochemical properties of ceria samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption–desorption, H2-temperature programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS) and in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The as-prepared CeO2 nanosphere possesses excellent NO oxidation capacity, smaller mesopores, better reducibility and more surface Ce3+ content. Compared with CeO2 with nanorod and nanoparticle morphologies, CeO2 nanosphere shows better intrinsic low temperature NOx trapping performance, with a wide operating temperature window (150–300 °C), high NOx adsorption capacity (NAC, 640–745 μmol/g) and high NOx storage capacity (NSC, 250–350 μmol/g). Two reaction pathways are speculated for NOx adsorption on CeO2 nanosphere, including “nitrate route” and “nitrite route”. The thermally unstable surface nitrites formed on the CeO2 nanosphere allow ceria to release more NOx during the desorption process. The present work provides a new ceria morphology for NOx traps, which may become a potential excellent NOx storage material.

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.

A Robust Mesoporous Al2 O3 -Based Nanocomposite Catalyst for Abundant NOx Storage with Rational Design of Pt and Ba Species.

The nanostructural design of heterogeneous catalysts has often been demanded for assessing synergetic effects, which should be developed further by using high-surface-area porous metal oxide supports. However, such opportunities have been undermined by the poor stability of ordered mesoporous structures. Herein, rational design is demonstrated to obtain nanocomposite catalysts showing improved NOx storage properties owing to the presence of Ba species over a well-designed mesoporous alumina (Al2 O3 ) support. It is found that Ba species are impregnated successfully only after the stabilization of the mesoporous structure by full crystallization of Al2 O3 frameworks to the γ-phase, with the formation of Pt nanoparticles coinciding with complete removal of organic components. All the insights during this synthetic procedure are essential for designing high-performance catalysts to purify and recover NOx molecules, and are applied for designing a variety of cutting-edge mesoporous nanocomposite catalysts.

Soot combustion over CeO2 catalyst: the influence of biodiesel impurities (Na, K, Ca, P) on surface chemical properties.

This work assessed the impact of biodiesel impurities on CeO2 catalyst for soot combustion via soot-TPO experiments. The results showed that Na- and K-doped catalysts were assisted for soot combustion, while Ca- and P-doped catalysts had a negative effect. N2 adsorption-desorption and XRD results indicated that doping biodiesel impurities led to smaller surface area by blocking small pores. Surface chemical properties are suggested as major reasons for promoting soot combustion by means of XPS, H2-TPR, and O2-TPD. Na- and K-doped catalysts showed stronger redox ability and surface lattice oxygen mobility, poorly for Ca- and P-doped catalysts. Interestingly, a large number of surface oxygen species were observed on P-doped catalyst and it enhanced the ignition of bio soot. In the presence of NO, surface chemical properties including NOx storage/release capacity and NO oxidation ability were characterized by NO-adsorption DRIFTS, NO-TPO and NOx-desorption DRIFTS, alkali-doped catalyst with excellent NOx storage capacity that can release active oxygen species and gaseous NO2 accelerate heterogeneous soot combustion, and the poor NO conversion ability to NO2 that weakens the promotion effect of soot combustion. Particularly in the existence of P, the promotion effect of soot elimination in NO + O2 was further weakened by the reason of poor NOx storage capacity and NO oxidation ability.

Novel low-temperature lean NOx storage materials based on La0.5Sr0.5Fe1-xMxO3-δ/Al2O3 infiltration composites (M = Ti, Zr, Nb)

Low-temperature NOx storage materials for lean NOx traps based on nanocomposites containing perovskite-type oxides of La0.5Sr0.5Fe1-xMxO3-δ (M = Ti, Nb or Zr; x = 0.25 or 0.5) are prepared in a highly porous alumina with different loadings of 15−25 wt. %. The resulting storage materials are characterised by X-ray diffraction, BET and transmission electron microscopy with coupled ASTAR system for crystallite phase mapping. For all nanocomposites high surface areas around 100 m²/g are found. With perovskite loadings of 20 wt. % the platinised composites reveal NOx storage capacities between 120–164 μmol/g below 300 °C with a performance ranking of Zr > Ti > Nb. After hydrothermal ageing at 750 °C only for the Nb and Zr containing composites a decrease of the NSC up to 20 % can be observed. The NSC of the LSFT system can be improved by decreasing the perovskite loading to 15 wt. %.

Understanding of Soot Removal Mechanism over DeNOx-Catalysts as Passive Converters

Although diesel particulate filters (DPFs) are able to efficiently remove soot and coupled NOx storage and reduction and selective catalytic reaction (NSR–SCR) technologies can considerably decreas...

The effects of exhaust gas recirculation on NOx storage pathway

A LNT (lean NOx trap) model coupled with EGR (exhaust gas recirculation) was developed based on the Langmuir–Hinshelwood mechanism to investigate the EGR effects on NOx adsorption pathway of LNT catalysts with temperature changed in range 150℃~550℃. Both the nitrate and nitrite adsorption paths were considered for the NOx storage process in the model as well as the spillover of stored NOx between Ba and Pt sites. The data and validation for modelling were from literatures of predecessors and our previous lean-burn gasoline engine experiment*. The model quantified the contributions of both nitrate route and nitrite route to the NOx storage with change of EGR rate (0%~30%) under raw emission atmosphere from tested gasoline engine. The model captured key feature of different trends of nitrate route and nitrite route with increasing temperature (150℃~550℃) under EGR rate varying from 0% to 25%. The LNT model provided insight of reaction mechanism for interpreting the behaviour of NOx storage with change of GER rate and temperature, which contributed to improve the NOx storage capacity when mapping EGR rate for lean-burn engine and catalyst operation strategy optimization.

Integration of an Oxidation Catalyst with Pd/Zeolite-Based Passive NOx Adsorbers: Impacts on Degradation Resistance and Desorption Characteristics

Fully ion-exchanged Pd/SSZ-13 model passive NOx adsorbers (PNA) exhibit significant NOx storage capability, achieving NOx-to-Pd ratios of 1, and NOx desorption at higher temperatures, where downstream NOx reduction catalysts are active, under simulated exhaust conditions. However, CO has been found to induce PNA degradation, which limits the potential for practical application. In this study, in an attempt to understand the consequences of limiting CO exposure, we integrated a model Pt/Al2O3 diesel oxidation catalyst (DOC) with a Pd/SSZ-13 PNA and performed NOx adsorption and temperature-programmed desorption (TPD) cycles with the PNA, the DOC, and the DOC+PNA integrated system. Despite the high initial NOx-to-Pd ratio, Pd/SSZ-13 experienced significant degradation over 15 adsorption and desorption cycles when including CO, with the NOx-to-Pd ratio dropping from 0.98 to 0.75. CO oxidation over the DOC+PNA integrated system lights off at significantly lower temperature compared to the PNA, limiting the PNA CO exposure at temperatures above 200 °C. As a result, the durability of the DOC+PNA integrated system is enhanced, and only a 0.02 decrease in NOx-to-Pd ratio was observed over the 15 test cycles. A further benefit with integration of the PNA with the DOC was a lower temperature NOx release, within a more practical temperature window. This is due to the enhanced oxidation activity of the DOC+PNA integrated system and consequently an early onset of NO2 formation, which was found to trigger the low temperature NOx release. Low temperature NOx adsorption and TPD experiments with controlled exposure of CO and NO2 reveal two types of NOx storage mechanisms, one of which is destabilized by the presence of NO2, leading to the evolution of a lower temperature NOx release. Overall, integrating the PNA with a highly active low temperature CO oxidation catalyst was beneficial by lowering the NOx release temperature window and leading to significantly less NOx capacity loss.

Copper‐Lanthanum Catalysts for NOx and Soot Removal

AbstractThe individual and simultaneous removal of NOx by Storage and Reduction with H2 (H2‐NSR) and combustion of soot have been studied for La and CuLa catalysts. The NOx storage capacity of pure lanthanum oxide is improved either by copper addition or by using a colloidal crystal hard template for the synthesis of a macroporous structure. Although the NOx storage capacity of the CuLa macroporous material is around ten times lower regarding conventional PtBa catalysts, the kinetics of the H2‐NSR cycles are fast enough to allow high NOx removal (until 90 % removal). Lanthanum oxide catalyst prepared with uncontrolled structure has no activity for soot combustion, but the activity increases for a macroporous counterpart prepared using the colloidal crystal template because the macroporous structure improves the soot‐catalyst contact and allows the nitrogen‐oxygen surface groups yielded upon NOx chemisorption to oxidize soot. The soot combustion activity of lanthanum catalysts is also enhanced upon copper loading, because copper improves the NO2‐assisted soot combustion mechanism and favors the chemisorption of NOx, enhancing soot oxidation by nitrogen‐oxygen surface species. Finally, it is demonstrated that the simultaneous removal of NOx by H2‐NSR and soot combustion can be carried out with a CuLa catalyst at 450 °C. The simultaneous removal does not affect the combustion of soot and only has a small effect in NOx removal.

A Discussion on the Unique Features of Electrochemical Promotion of Catalysis (EPOC): Are We in the Right Path Towards Commercial Implementation?

The phenomenon of “Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA)” or “Electrochemical Promotion of Catalysis (EPOC)” has been extensively studied for the last decades. Its main strength, with respect to conventionally promoted catalytic systems, is its capability to modify in-situ the activity and/or selectivity of a catalyst by controlling the supply and removal of promoters upon electrical polarization. Previous reviews have summarized the main achievements in this field from both the scientific and technological points of view. However, to this date no commercial application of the EPOC phenomenon has been developed, although numerous advances have been made on the application of EPOC on catalyst nanostructures (closer to those employed in conventional catalytic systems), and on the development of scaled-up reactors suitable for EPOC application. The main bottleneck for EPOC commercialization is likely the choice of the right chemical process. Therefore, from our point of view, future efforts should focus on coupling the latest EPOC advances with the chemical processes where the EPOC phenomenon offers a competitive advantage, either from an environmental, a practical or an economic point of view. In this article, we discuss some of the most promising cases published to date and suggest future improvement strategies. The considered processes are: (i) ethylene epoxidation with environmentally friendly promoters, (ii) NOx storage and reduction under constant reaction atmosphere, (iii) CH4 steam reforming with in-situ catalyst regeneration, (iv) H2 production, storage and release under fixed temperature and pressure, and (v) EPOC-enhanced electrolysers.

Core‐crown Quantum Nanoplatelets with Favorable Type‐II Heterojunctions Boost Charge Separation and Photocatalytic NO Oxidation on TiO2

AbstractFunctionalization of TiO2 (P25) with oleic acid‐capped CdSe(core)/CdSeTe(crown) quantum‐well nanoplatelets (NPL) yielded remarkable activity and selectivity toward nitrate formation in photocatalytic NOx oxidation and storage (PHONOS) under both ultraviolet (UV‐A) and visible (VIS) light irradiation. In the NPL/P25 photocatalytic system, photocatalytic active sites responsible for the NO(g) photo‐oxidation and NO2 formation reside mostly on titania, while the main function of the NPL is associated with the photocatalytic conversion of the generated NO2 into the adsorbed NO3− species, significantly boosting selectivity toward NOx storage. Photocatalytic improvement in NOx oxidation and storage upon NPL functionalization of titania can also be associated with enhanced electron‐hole separation due to a favorable Type‐II heterojunction formation and photo‐induced electron transfer from the CdSeTe crown to the CdSe core of the quantum well system, where the trapped electrons in the CdSe core can later be transferred to titania. Re‐usability of NPL/P25 system was also demonstrated upon prolonged use of the photocatalyst, where NPL/P25 catalyst surpassed P25 benchmark in all tests.

Cobalt oxide with flake-like morphology as efficient passive NOx adsorber

A series of Co-based oxides were synthesized and investigated as passive NOx adsorbers (PNAs). The morphology variation of Co3O4 oxide from nanoparticle to flake shape enhanced the NOx storage efficiency (NSE) and improved the NOx desorption capability. The enhanced NSE of the flake-like sample is related to its abundant surface oxygen species and large surface area. DRIFTS and CO2-TPD experimental results indicated that Co3O4-flake and Co7Ni3Ox-flake samples with moderate basicity could store NOx as nitrite species, which facilitated NOx desorption at lower temperatures.