NOx Storage Research Articles

One-step fluorine-free synthesis of delaminated, OH-terminated Ti3C2: High photocatalytic NOx storage selectivity enabled by coupling TiO2 and Ti3C2-OH

Difficulties associated with MXene fabrication typified by an environmentally hazardous fluorine-based etching process followed by delamination and fluorine reduction steps create obstacles against the expansion of this outstanding material and its application in air purification. Herein, we demonstrate a one-step hydrothermal process that involves alkali (NaOH)-assisted aluminum etching in the presence of a delaminating agent (hydrazine) for synthesis of delaminated, OH-terminated MXene (Ti3C2-OH). The process does not require the use of fluorine-containing compounds and produces Ti3C2-OH sheets without the need for post-synthesis treatment. Ti3C2-OH shows a strong synergistic effect as a co-catalyst when coupled with TiO2 during photocatalytic oxidation of NOx. Excellent NOx storage selectivity (91 %) and a positive DeNOx index (+ 0.30) were achieved at NO conversion of 54 %. The photocatalytic activity of the hybrid shows high sensitivity to the surface chemistry of the co-catalyst as Ti3C2-OH outperforms delaminated, F-terminated Ti3C2 (fabricated via the traditional process), which shows low NOx storage selectivity (65 %) and a negative DeNOx index (−0.05) at NO conversion of 47 %. We envision that these findings will benefit ongoing efforts aimed at developing effective alternative methods for MXene synthesis and provide a pathway for rational design of efficient NOx abatement photocatalysts.

Advances in Designing Efficient La-Based Perovskites for the NOx Storage and Reduction Process

To overcome the inherent challenge of NOx reduction in the net oxidizing environment of diesel engine exhaust, the NOx storage and reduction (NSR) concept was proposed in 1995, soon developed and commercialized as a promising DeNOx technique over the past two decades. Years of practice suggest that it is a tailor-made technique for light-duty diesel vehicles, with the advantage of being space saving, cost effective, and efficient in NOx abatement; however, the over-reliance of NSR catalysts on high loadings of Pt has always been the bottleneck for its wide application. There remains fervent interest in searching for efficient, economical, and durable alternatives. To date, La-based perovskites are the most explored promising candidate, showing prominent structural and thermal stability and redox property. The perovskite-type oxide structure enables the coupling of redox and storage centers with homogeneous distribution, which maximizes the contact area for NOx spillover and contributes to efficient NOx storage and reduction. Moreover, the wide range of possible cationic substitutions in perovskite generates great flexibility, yielding various formulations with interesting features desirable for the NSR process. Herein, this review provides an overview of the features and performances of La-based perovskite in NO oxidation, NOx storage, and NOx reduction, and in this way comprehensively evaluates its potential to substitute Pt and further improve the DeNOx efficiency of the current NSR catalyst. The fundamental structure–property relationships are summarized and highlighted to instruct rational catalyst design. The critical research needs and essential aspects in catalyst design, including poisoner resistance and catalyst sustainability, are finally addressed to inspire the future development of perovskite material for practical application.

A novel material for passive NOx adsorber: Ce-based BEA zeolite

Passive NOx adsorbers (PNAs) were proposed to address the NOx emissions during the cold start phase. Here we show a novel Ce-based BEA zeolite, as a noble-metal-free passive NOx adsorber. The NOx adsorption capacity of Ce/BEA reaches 36 μmol/g in the feed gas close to realistic exhaust conditions, and the NOx desorption temperature, which is around 290 °C, is ideal for diesel exhaust after-treatment systems. Ce/BEA also behaves notable stability of high temperature CO exposure conditions. Multiple characterizations were performed to explore the NOx adsorption chemistry of Ce/BEA. The Ce(Ⅳ) species in the BEA zeolite serves as the active center for NOx adsorption. The bidentate nitrate species is responsible for the observed NOx storage capacity, and the active oxygen around Ce(Ⅳ) plays a critical role in its formation. Considering the significantly better cost efficiency of Ce compared to Pd, Ce/BEA presents an enormous potential for the PNA applications and provides a novel formulation for the noble-metal choice of PNA materials.

Insight into CO induced degradation mode of Pd/SSZ-13 in NOx adsorption and release: Experiment and modeling

Passive NOx adsorption (PNA) on Pd zeolites is an important technique to remove NOx during the cold start of the engine. However, the stability of Pd zeolites under high concentrations of CO is still challenging in multiple cold starts of an engine. Herein, we illustrate the CO-induced degradation mechanism of Pd zeolite by combining experiments and kinetic models. Pd/SSZ-13 has been used in multicycle processes containing NOx adsorption at low temperature and temperature programmed desorption, which represents the PNA degradation in multiple cold start periods. A kinetic model was developed to describe the NOx storage and degradation behavior of Pd/SSZ-13. Both experimental and modelling observations suggested that two Pd sintering modes are occurring under high CO concentration (4000 ppm), namely Ostwald ripening and particle migration. Apart from the degradation behavior, this model is also adequate for describing multi-cycle NOx storage and release behavior under low CO concentration.

Effect of H2, H2O, and CO2 on the deNOx characteristics of a combined passive NOx adsorber and NOx reduction catalyst

AbstractThe effect of H2, H2O, and CO2 on the NOx removal characteristics of a non‐PGM material (Ag/MgO/γ‐Al2O3) is investigated, which is shown to be active for both NOx adsorption and reduction. H2O facilitates NOx adsorption at early times but inhibits it at longer times. The inhibiting effect of CO2 is observed at low temperatures and is proposed to be due to bicarbonates. However, the inhibiting effect of both H2O and CO2 decreases at higher temperatures. The formation of NO2 at low temperatures is attributed to NO oxidation, and the contribution of stored NOx to NO2 formation increases with temperature. H2O inhibits NH3 oxidation at high temperatures as well as NOx reduction by NH3‐SCR at low temperatures. However, the NOx reduction activity is not affected by CO2. H2 significantly promotes the NH3‐SCR reactions, which is attributed to the facilitation of NOx spillover from the reduced Ag sites to the support.

Role of Ba in an Al2O3‐Supported Pd‐based Catalyst under Practical Three‐Way Catalysis Conditions

AbstractBarium (Ba) has been widely and successfully added to three‐way catalysts to effectively control gasoline vehicle emissions. While the introduction of Ba is well‐known to have a very significant impact on three‐way catalysis (TWC) performance, the role of Ba under practical TWC conditions remains insufficiently understood. In this study, we investigated the role of Ba in a Pd‐based catalyst in a standard TWC process using a gasoline vehicle and in situ/operando infrared (IR) spectroscopy. The addition of Ba was found to reduce all of the emissions considered (NOx, CO, and non‐methane hydrocarbons) in the vehicle exhaust gas; it also suppressed supported‐metal aggregation, which is a requirement of a high‐performance catalyst. In addition, IR studies using model powder catalysts revealed that the efficient formation of intermediate surface nitrate species and their subsequent reactions with reductant gases, such as CO, H2, and C3H6 (as a hydrocarbon), play key roles in reducing emissions. This process is akin to the NOx storage and reduction (NSR) mechanism commonly used to control diesel emissions.

Synergetic promotion of 4Mn10Ce/γ-Al2O3 catalyst to Pt/xBa/Ce0.5Zr0.5O2 catalysts for NOx removal from diesel exhausts

In this investigation, Pt/xBa/Ce0.5Zr0.5O2 catalysts were prepared by deposition and incipient wetness impregnation method and the physio-chemical properties of samples were characterized with XRD and BET. The experiments were carried out to evaluate the effect of 4Mn10Ce/γ-Al2O3 on the removal of NOx over the Pt/xBa/Ce0.5Zr0.5O2 catalysts and NO-TPD was employed for determining the NOx adsorption. The results showed that the larger diffraction peak width of Ce-Zr oxides and specific area were found over Pt/15Ba/Ce0.5Zr0.5O2. Based on the NOx storage and storage-reduction experiments, it was found that the decline of NOx storage efficiency was attributed to the decomposition of nitrites and nitrates under high temperatures beyond 350℃, and the inhibition of reactions of NOx with Ba by the NO oxidation. NO2 was stored more easily and faster than NO, which contributed to NOx storage after 4Mn10Ce/γ-Al2O3 catalyst added. The stability of stored species was undermined at higher temperature due to H2 consumption by 4Mn10Ce/γ-Al2O3 catalyst leading to shortage of reductant and more reaction heat. The optimal concentration of O2 at around 7.5% can provide more OⅡ for the oxidation process of NO and improve the performance of NOx reduction.

Key Properties and Parameters of Pd/CeO2 Passive NOx Adsorbers

In this paper, a series of Pd/CeO2 catalysts prepared by different synthesis routes and showing different morphological and textural properties have been investigated for passive NOx adsorption (PNA) applications. The results obtained by NOx adsorption/desorption tests demonstrated that NOx storage capacity and NOx storage efficiency of Pd/CeO2 materials depend strictly on their surface area, whereas the morphology of the support and the Pd deposition method do not seem to play a key role. In contrast, the Pd deposition method does impact the dynamics of NOx desorption by affecting the amount of NOx desorbed at different temperatures. This seems to be connected to Pd–Ce interactions at the nanoscale that favor NOx desorption at higher temperatures suitable for PNA application. These findings are relevant in designing and optimizing the properties of Pd/CeO2 materials for their function as passive NOx adsorbers.

Mesoporous LaCoO3 perovskite oxide with high catalytic performance for NOx storage and reduction

A mesoporous LaCoO3 perovskite oxide (LaCoO3-Meso) with three-dimensionally ordered helical interwoven structure was synthesized by a nano-casting method using KIT-6 as the hard template. The obtained LaCoO3-Meso with high surface area was tested for its catalytic performance in the NOx storage and reduction (NSR) reaction and compared with a sample synthesized by the conventional sol-gel method. The LaCoO3-Meso showed a significant advantage for NOx storage, with a NOx storage capacity 2 times higher than the regular sample. LaCoO3-Meso also exhibited improved NSR catalytic performance in the 150–450 °C temperature range, especially within 350–400 °C, where the NOx conversion was raised for 40%. The results of X-ray photoelectron spectroscopy and X-ray absorption fine structure measurements suggested the presence of a high concentration of oxygen defects on the LaCoO3-Meso surface. Further results provided by temperature programmed reduction and temperature programmed desorption indicated that the oxygen defects not only increase the amount of trapped NOx, but also improve the low-temperature redox performance of the catalyst. The lower stability of NOx species adsorbed on oxygen defects promotes the NOx release step in the NSR reaction and benefits the regeneration of storage sites.

Experimental and numerical investigation of NO oxidation on Pt/Al2O3- and NOxstorage on Pt/BaO/Al2O3-catalysts

Effects of temperature and inlet conditions on NO oxidation and NOxstorage, as well as reduced NOxstorage capacity over time – reflected by changes of measured NO concentration, which are reproduced by CFD using detailed reaction mechanisms.

The upgrading role of Al at T1/T2 sites in stabilizing Pd ions over Pd-beta passive NOx adsorbers under a reducing atmosphere

The preserved Pd ions as γ and β sites related with Al at T1–T2 sites corresponded to the stabilized NOx storage capability and rendered a suitable desorption temperature.

Analysis of the NOx storage behaviour during cold start of modern SCR flow-through substrates and SCR on-filter substrates

AbstractSelective catalytic reduction (SCR) systems are the state-of-the-art technology to reduce nitrogen oxide emissions (NOx) of modern diesel engines. The system behaviour is well understood in the common temperature working area. However, the system properties below light-off temperature are less well known and offer a wide scope for further investigations. Vehicle measurements show that under specific conditions during cold start, NOx can be partially stored and converted on on-filter and flow-through SCR catalysts. The purpose of this work was in a first step to analyse the main influence parameters on the NOx storage behaviour. Therefore, synthetic gas test bench measurements have been carried out, varying the gas concentrations, temperature, and gas hourly space velocity (GHSV). These investigations showed that the NOx storage effect strongly depends on the NH3 level stored in the catalyst, GHSV, the adsorbed water (H2O) on the catalyst, and the temperature of the catalyst. Further influence parameters such as the gas composition with focus on carbon monoxide (CO), short-chain hydrocarbons and long-chain hydrocarbons have been analysed on a synthetic gas test bench. Depending on operating conditions, a significant amount of NOx can be stored on a dry catalyst during the cold start phase. The water vapor from the combustion condenses on the cold exhaust pipe during the first seconds, or up to a few minutes after a cold start. As the water vapor reaches the surface of the catalyst, it condenses and adsorbs onto it, leading to a sudden temperature rise. This exothermal reaction causes the stored NOx to be desorbed, and furthermore it is partially reduced by the NH3 stored in the catalyst.

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.

Palladium/Ferrierite versus Palladium/SSZ-13 Passive NOx Adsorbers: Adsorbate-Controlled Location of Atomically Dispersed Palladium(II) in Ferrierite Determines High Activity and Stability.

Pd-loaded FER and SSZ-13 zeolites as low-temperature passive NOx adsorbers (PNA) are compared under practical conditions. Vehicle cold start exposes the material to CO under a range of concentrations, necessitating a systematic exploration of the effect of CO on the performance of isolated Pd ions in PNA. The NO release temperature of both adsorbers decreases gradually with an increase in CO concentration from a few hundred to a few thousand ppm. This beneficial effect results from local nano-"hot spot" formation during CO oxidation. Dissimilar to Pd/SSZ-13, increasing the CO concentration above ≈1000 ppm improves the NOx storage significantly for Pd/FER, which was attributed to the presence of Pd ions in FER sites that are shielded from NOx. CO mobilizes this Pd atom to the NOx accessible position where it becomes active for PNA. This behavior explains the very high resistance of Pd/FER to hydrothermal aging: Pd/FER materials survive hydrothermal aging at 800 °C in 10 % H2 O vapor for 16 hours with no deterioration in NOx uptake/release behavior. Thus, by allocating Pd ions to the specific microporous pockets in FER, we have produced (hydro)thermally stable and active PNA materials.

Palladium/Ferrierite versus Palladium/SSZ‐13 Passive NOx Adsorbers: Adsorbate‐Controlled Location of Atomically Dispersed Palladium(II) in Ferrierite Determines High Activity and Stability**

AbstractPd‐loaded FER and SSZ‐13 zeolites as low‐temperature passive NOx adsorbers (PNA) are compared under practical conditions. Vehicle cold start exposes the material to CO under a range of concentrations, necessitating a systematic exploration of the effect of CO on the performance of isolated Pd ions in PNA. The NO release temperature of both adsorbers decreases gradually with an increase in CO concentration from a few hundred to a few thousand ppm. This beneficial effect results from local nano‐“hot spot” formation during CO oxidation. Dissimilar to Pd/SSZ‐13, increasing the CO concentration above ≈1000 ppm improves the NOx storage significantly for Pd/FER, which was attributed to the presence of Pd ions in FER sites that are shielded from NOx. CO mobilizes this Pd atom to the NOx accessible position where it becomes active for PNA. This behavior explains the very high resistance of Pd/FER to hydrothermal aging: Pd/FER materials survive hydrothermal aging at 800 °C in 10 % H2O vapor for 16 hours with no deterioration in NOx uptake/release behavior. Thus, by allocating Pd ions to the specific microporous pockets in FER, we have produced (hydro)thermally stable and active PNA materials.

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.

Structured NSR-SCR hybrid catalytic technology: Influence of operational parameters on deNOx activity

An intermediate approximation using structured reactors between the study of deNOx catalysts at laboratory scale and in motor bench has been addressed. The hybrid NSR (NOx Storage and Reduction) and SCR (Selective Catalytic Reduction) technology has been analyzed for Pt-Ba-K/Al2O3 and Cu-SAPO-34 catalysts structured in monolithic form. In order to analyze the influence of the way of driving on deNOx efficiency, operational parameters such as NO inlet concentration, space velocity in terms of total flow rate, temperature, and the concentration of reductant agent, using H2 and C3H6, have been modified. The coupled NSR-SCR configuration is more efficient than the single NSR alternative, since the former used the ammonia produced in NSR monolith to reduce the non-adsorbed NOx through the SCR process over the system placed downstream. However, NH3 production depended on NO concentration, temperature, and reductant agent content in the process, limiting the promoting effect at high temperatures and NO concentrations, and low hydrogen concentration. N2 selectivity increased when temperature increased, and NO concentration and hydrogen content decreased. On the other hand, NOx to N2 reduction efficiency of the combined NSR-SCR system was not affected by variations in space velocity, with a maximum range studied up to 90,000 h−1. Based on these results, the location of the catalytic system should be adjusted in order to work in the 250–350 °C temperature range, where conversion and N2 selectivity values were around 70% and 95%, respectively. On the other hand, the hydrogen concentration of 2% and 3% can be suitable for the efficient operation of the system, since as reductant agent increased the ammonia formation and the surface regeneration also increased. The substitution of H2 by propylene as reducing agent caused a decrease in the amount of ammonia produced in the NSR configuration, which limits the improvement of deNOx activity of the double NSR-SCR configuration, and overproduces CO2, CO and N2O through parallel reactions as reverse water gas shift or C3H6 oxidation, which could poison the system and require emission control through other technologies.

Experimental and density functional theory studies on Cu/Ba-coimpregnated γ-Al2O3 for low-temperature NOx storage and adsorbent regeneration

Although the lean-NOx trap (LNT) has been used to reduce NOx emissions from diesel-powered vehicles, LNT application is limited because its performance degrades at low temperatures (e.g., during cold starting). Therefore, to enhance low-temperature NOx storage and adsorbent regeneration, γ-Al2O3 was coimpregnated with both Cu and Ba (Cu–Ba/γ-Al2O3). The experimentally measured NOx storage capacities (NSCs) and NOx storage efficiencies (NSEs) were compared. Density-functional-theory (DFT) calculations were performed to reveal the NOx storage mechanism. The Cu/Ba-coimpregnated γ-Al2O3 improved both NSC and NSE of NO storage and enhanced NSE of NO2 storage at initial stage. In addition, it desorbed NOx at lower temperatures than the conventional Ba-impregnated γ-Al2O3 (Ba/γ-Al2O3). The in-situ diffuse reflectance infrared Fourier-transform spectroscopy analysis and DFT calculations for NO storage showed that NO adsorption was superior on the Cu-compound surfaces and that stable hyponitrite was stored on the Ba-compound surfaces. In NO2 storage, Cu/Ba coimpregnation offered high preferential NO2 coverage on the CuO surface and produced the most stable ionic nitrate on the Ba-compound surfaces. The experimental and theoretical results confirmed that the Cu/Ba-coimpregnated adsorbent exhibited both superior NOx storage and adsorbent regeneration compared to the conventional Ba-containing LNT adsorbent.

NOx Storage on BaTi0.8Cu0.2O3 Perovskite Catalysts: Addressing a Feasible Mechanism.

The NOx storage mechanism on BaTi0.8Cu0.2O3 catalyst were studied using different techniques. The results obtained by XRD, ATR, TGA and XPS under NOx storage–regeneration conditions revealed that BaO generated on the catalyst by decomposition of Ba2TiO4 plays a key role in the NOx storage process. In situ DRIFTS experiments under NO/O2 and NO/N2 show that nitrites and nitrates are formed on the perovskite during the NOx storage process. Thus, it seems that, as for model NSR catalysts, the NOx storage on BaTi0.8Cu0.2O3 catalyst takes place by both “nitrite” and “nitrate” routes, with the main pathway being highly dependent on the temperature and the time on stream: (i) at T < 350 °C, NO adsorption leads to nitrites formation on the catalyst and (ii) at T > 350 °C, the catalyst activity for NO oxidation promotes NO2 generation and the nitrate formation.

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.