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Article

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

by
Marcos Schöneborn
1,*,
Thomas Harmening
1,
Javier Giménez-Mañogil
2,
Juan Carlos Martínez-Munuera
2 and
Avelina García-García
2,*
1
SASOL Advanced Materials, Anckelmannsplatz 1, 20537 Hamburg, Germany
2
MCMA Group, Department of Inorganic Chemistry and Institute of Materials, University of Alicante, Carretera Sant Vicent del Raspeig s/n, 03690 Sant Vicent del Raspeig, Alacant, Spain
*
Authors to whom correspondence should be addressed.
Materials 2019, 12(13), 2127; https://doi.org/10.3390/ma12132127
Submission received: 29 May 2019 / Revised: 21 June 2019 / Accepted: 1 July 2019 / Published: 2 July 2019
(This article belongs to the Special Issue Rare Earth Oxides and Their Applications)
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Abstract

:
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.

1. Introduction

The implementation of stricter environmental legislations for passenger cars globally, like Euro 6d, US Tier 3 and China 6B calls for improved catalytic systems for emission control [1]. A lot of attention is drawn to the development of improved NOx abatement systems (deNOx) for lean-burn engines. As these engines operate at λ > 1, the abatement of NOx requires dedicated technologies, such as selective catalytic reduction (SCR) and lean NOx trap catalysts (LNT), also known as NOx storage/reduction catalysts (NSR) [2,3].
In SCR systems, an aqueous urea solution is injected into the exhaust gas via an onboard-tank leading to the formation of NH3, which then reacts with NOx over the catalyst via the standard (1) and fast (2) SCR reactions. Currently, the most common SCR catalysts are metal-substituted zeolites, like Fe-ZSM-5 or Cu-CHA [4].
4NO + 4NH3 + O2 → 4N2 + 6H2O
NO + NO2 + 2NH3 → 2N2 + 3H2O
The fast SCR reaction (2) is significantly more effective than reaction (1) at low temperatures in the range of 250–300 °C but it requires the presence of NO2. Therefore, the SCR catalyst is typically positioned downstream of the diesel oxidation catalyst (DOC), which effectively increases the NO2/ NOx ratio [5,6]. Once the NO2 concentration exceeds a certain threshold, the NO2-SCR reaction (3) also takes place.
4NH3 + 3NO2 → 7/2N2 + 6H2O
The NOx abatement mechanism of lean NOx traps is based on alternating lean/rich cycles with long lean phases, in which NOx emissions from the exhaust gases are adsorbed on the catalyst. The stored ad-NOx species are desorbed and reduced to nitrogen on catalytically active noble metals in subsequent short rich periods at elevated temperatures [7]. Most common lean NOx trap catalysts contain a high surface-area material like γ-Al2O3, mixtures of BaO and CeO2 and noble metals, typically Pt or Pd and Rh [8,9].
Barium species, such as BaO or BaCO3 act as the primary NOx storage component. They can adsorb large amounts of NOx in the form of surface nitrites and nitrates during lean operation which involves the progressive oxidation from NO to NO2 and finally NO3−. The mechanism of NOx adsorption on barium species has been investigated intensively [10,11,12,13,14,15]. According to these studies, the NOx adsorption takes place via two pathways in parallel. In the “nitrite” route, NO is catalytically oxidized and directly stored on barium sites in form of nitrite ad-species which can be further oxidized to nitrates.
The “nitrate” route is initiated by the catalytic oxidation of NO to NO2 (4) which then react on barium sites in a disproportionation reaction, resulting in the formation of nitrate and NO (5).
NO + 1/2O2 → NO2
3NO2 + BaO → Ba(NO3)2 + NO
The regeneration of NOx storage sites occurs during rich periods and forms harmless nitrogen via the reduction of NOx species with hydrogen. Cumaranatunge et al. proposed a mechanism that involves the intermediate generation of ammonia [16]. This mechanism has been confirmed in following studies in which various analytical techniques were applied [17,18]. The fundamental chemical reactions can be summarized as:
Ba(NO3)2 + 8H2 → 2NH3 + BaO + 5H2O
3Ba(NO3)2 + 10NH3 → 8N2 + 3BaO + 15H2O
The addition of ceria, which has become a major component in catalyst compositions leads to superior NOx storage efficiencies, especially at temperatures below 300 °C [19,20]. Furthermore, ceria plays a substantial role in the water-gas-shift reaction as it provides hydrogen for the regeneration during rich periods and it has a positive impact on the NOx storage/release chemistry due to its interplay with platinum [21,22,23,24]. The mechanisms of NOx storage on ceria under lean conditions involve the formation of various ad-NOx species, such as linear, bidentate and chelating nitrites and nitrates. It was observed that the low-temperature (<100 °C) adsorption of NO on ceria primarily leads to the generation of surface nitrites whereas at higher temperatures nitrates are preferentially formed [19,25,26]. On the contrary, Ryou et al. did not observe the formation of nitrates on Pd/CeO2 at 120 °C and they proposed that the presence of water suppresses the oxidation of nitrites [27]. The adsorption of NO2 on ceria was studied in detail by Flitschew et al. who combined the well-established diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with in-situ Raman spectroscopy [28,29]. The authors elaborated that NO2 storage on ceria proceeds mainly in two pathways which both lead to the formation nitrate species. The first route involves the adsorption of NO2 on cerium(III) sites which are thereby oxidized and transformed to active Ce(IV)-O species (8) which can then react with additional NO2 to form nitrates (9). In the second pathway, NO2 reacts directly with Ce(IV)-O sites without the contribution of cerium(III) (9).
Ce(III) + NO2→ Ce(IV) ⋯ O2−+ NO
Ce(IV) ⋯ O2− + NO2 → Ce(III) ⋯ O-NO2−
Fine-tuning of the regeneration parameters, like temperature and modes of fuel-injection is a demanding task and crucial for the performance such as the long-time durability of the catalyst. Incomplete regeneration inevitably leads to catalyst degradation over time as relevant surface sites remain occupied with adsorbed NOx species. On the contrary, excess H2 formed during long rich cycles and low temperatures leads to the generation of high levels of ammonia. This can be a serious concern in LNT-only systems [18,30]. Another important aspect to be considered is the additional fuel-consumption required to enrich the exhaust gas, which eventually leads to increased CO2 emissions. Therefore, NOx storage compounds that can effectively be regenerated are urgently needed.
The synergetic combination of passive SCR with LNT catalysts is an efficient solution to attain low NOx tailpipe emissions and to cope with high levels of NH3, which may result from the total reduction of NOx. This approach has become a widespread and well-studied technology for several years [31,32,33,34,35,36,37,38]. The low-temperature performance of LNT complements SCR and the ammonia generated during the rich regeneration phase can replace or supplement urea required for the SCR reactions.
One major deactivation mechanism of LNT catalysts is the solid acid-base reaction between Al2O3 and BaO at elevated temperatures resulting in the formation of BaAl2O4 and thus causing a loss of NOx adsorption sites [39,40]. Improvements in this regard have been reported via the replacement of alumina with the less acidic spinel (MgAl2O4), leading to the formation of BaAl2O4 only at higher temperatures [41,42]. In addition, the use of spinel in LNT catalysts has been described to improve the low temperature NOx storage efficiency and to contribute to enhanced SOx tolerance via the formation of sulphates with low thermal stability [43].
Another reported deactivation mechanism results from reaction of BaO and CeO2 yielding BaCeO3, which was detected in LNT formulations prepared by the impregnation of ceria with barium salts [44].
Despite the technological and chemical improvements achieved over the past 20 years, deactivation at elevated temperatures and optimized NOx storage/release properties are perpetual challenges in the design of new formulations for LNT catalysts.
In this regard, the addition of manganese to LNT compositions aiming at improved performance has been proposed and studied by several authors. MnOx –CeO2 mixed oxides present high oxygen storage abilities and improved redox properties. They are typically obtained by coprecipitation, sol-gel synthesis or similar routes leading to highs level of homogeneity. The interesting properties predestine these mixed oxides for a substitution of pure ceria in various applications and their superior ability to oxidize soot and NO has already been reported [45,46,47]. Le Phuc et al. observed significantly improved performance of MnOx-CeO2 containing Pt/Mn-Ce/Ba/Al LNT catalysts in NOx reduction during rich phases, which they attributed to the improved oxygen mobility of the mixed oxide as compared to pure CeO2 [48].
In another work, Le Phuc studied the contribution of crystalline Mn2O3 as to the NOx storage performance in Mn/Ba/Al compositions which were investigated in the fresh state, that is, without applying thermal ageing prior to testing [49]. It was found that the manganese sesquioxide only led to improved NOx storage performance in a narrow compositional range whereas higher or lower concentrations had a detrimental effect on the storage efficiency.
Zhang et al. detected the formation of BaMnO3 in the system Pd/Mn/Ba/Al. The presence of this phase led to superior NO oxidation abilities and NOx storage performances of the fresh catalysts [50]. Similar but not identical observations were made by Xiao et al, who noticed the occurrence of BaMnO3 only after calcination at 800 °C [51]. This is in accordance with our own investigations, in which we detected this phase in the system Al/Mg/Ce/Mn/Ba after calcination at 850 °C [52].
However, the impact of ageing at elevated temperature as to the NOx storage/release properties of the manganese modified formulations has not been reported yet.
In this work, we present a novel route for the preparation of manganese modified LNT compositions leading to a MnOx layer on ceria/spinel mixtures rather than homogeneous MnOx-CeO2 mixed oxides. Firstly, the impact on this route in altering the NOx storage/release properties of the resulting spinel/ceria/manganese compositions is investigated. Secondly, the stabilizing function of the MnOx layer as a protective barrier between spinel/ceria and barium is rationalized. The materials described herein were thermally aged at 850 °C prior to testing in order to study their thermal stability, which is a common procedure to simulate catalyst ageing under real conditions [53]. NOx storage experiments were carried in a dual-bed with a 1% Pt/Al2O3 catalyst placed upstream from the samples. This arrangement mimics a diesel oxidation catalyst, which is typically present in state-of-the-art lean-burn catalyst systems and generates high concentrations of NO2. It was shown in previous studies that NO2 can be stored much more efficiently than NO on barium and cerium species [19,54].
In order to study the NO oxidation ability provided by the manganese species, additional NOx adsorption experiments were carried out in absence of the Pt/alumina catalyst.
The thermal stability of the ad-NOx species is important to estimate how efficiently the corresponding NOx storage sites can be regenerated. Therefore, the NOx release properties of our new formulations were investigated in temperature programmed desorption (TPD) experiments.

2. Materials and Methods

2.1. Sample Preparation

MnOx modified carriers with loadings of 9 wt% (calculated as MnO2) were prepared by incipient wetness impregnation of homogeneous compositions MgAl2O4/CeO2 (PURALOX MG20 Ce20, commercially available from SASOL) using an aqueous solution of manganese acetate tetrahydrate. The dried samples were then calcined in air at 600 °C.
Both, the Mn-free and the Mn-modified materials were used as starting materials for further wet-impregnation with a barium acetate solution to obtain a loading of approximately 15 wt% BaO. All samples were aged in air at 850 °C for 4 h prior to analyses. Henceforth, the samples are referred to as MgCe (PURALOX MG20 Ce20), MgCe-Mn (MgCe modified with MnOx), MgCe-Ba (MgCe modified with BaO) and MgCe-Mn-Ba (MgCe subsequently modified with MnOx and BaO). The chemical compositions of the samples are summarized in Table 1.

2.2. Sample Characterization

Surface area (Brunauer-Emmett-Teller method, BET) and porosity measurements were performed by nitrogen adsorption at −196 °C using a Micromeritics Tri-Star 3000 system. The samples were outgassed overnight at 300 °C under vacuum prior to the measurements. X-ray powder diffraction was conducted on a Phillips X’Pert diffractometer using Cu-Kα radiation (λ = 1.540598 Å). Powder diffractograms were recorded between 5° and 90° (2Ɵ), with a step-width of 0.02°. The sample compositions were determined after digestion in an MLS 1200 microwave apparatus by Inductively Coupled Plasma Atomic Emissions Spectrometer (ICP-OES) using a Spectroflame instrument (SPECTRO). X-ray photoelectron spectra (XPS) were obtained using a K-alpha spectrophotometer (Thermo-Scientific), with a high-resolution monochromator. It comprises a source of electrons and ions for automated load compensation. The X-ray radiation source is equipped with an Al anode (1486.6 eV). The pressure of the analysis chamber was constantly set at 5 × 10−9 mbar. The detector was kept in constant energy mode with a pass energy of 200 eV for the survey spectrum and 50 eV for the sweep in each individual region. The binding energy was adjusted using the C-1s transition, appearing at 284.6 eV. Binding energy values measured are accurate to ±0.2 eV. The values of binding energy and kinetic energy were adjusted with the Peak-Fit software of the spectrophotometer. The Mg-1s, Al-2p, Ce-3d, Mn-2p, Ba-3d regions (along with C-1s and O-1s regions) were employed to analyse the surface composition of the carriers in the present study.

2.3. NOx Storage/Release Tests

NOx adsorption experiments were conducted in a quartz tubular reactor connected to specific NDIR-UV gas analysers for NO, NO2, CO, CO2 and O2, with the measurement data recorded every 10 seconds. The NOx adsorption (500ppm NO/5%O2/balance N2) was performed at 300 °C in a dual-bed configuration with a 1% Pt/Al2O3 commercial catalyst (supplied by Sigma-Aldrich) placed upstream of the sample using a global flow gas of 500 mL/min. The catalyst effectively oxidizes NO to NO2, which is required for effective NOx storage. Subsequent temperature programmed desorption (TPD) experiments were carried out under N2 from 300 °C to 700 °C (5 °C/min) in order to study the thermal stability of the various stored NOx species. Besides NOx storage ability, this is another important aspect in order to assess the suitability of the different materials for the development of new LNT catalysts.
In addition, NOx adsorption experiments in single-bed configuration were performed without the Pt/alumina catalyst in order to study the NO oxidation ability of the individual samples and to gain further insight into the role of the individual manganese species in the NOx storage processes.

3. Results

3.1. Structural Properties

The phase composition of the samples was analysed by powder X-ray diffraction. It can be seen in Figure 1 that all samples contain MgAl2O4 and CeO2 in crystalline form.
The addition of MnOx to the MgAl2O4/CeO2 composition does not lead to the occurrence of additional reflections in the powder pattern, thus pointing to a homogeneous dispersion of the MnOx species among the surface without the formation of additional crystalline domains detectable by X-ray diffraction (XRD).
The Ba-containing samples with and without manganese exhibit remarkable differences. In MgCe-Ba, a substantial formation of crystalline BaAl2O4 is observed. This phase is also present in MgCe-Mn-Ba but the intensity of the relevant reflections is decreased in the X-ray pattern of this material. Most importantly, the formation of BaMnO3 is detected in MgCe-Mn-Ba. This phase adopting the perovskite structure is more accurately described as BaMnO3-δ, reflecting the possible formation of vacancies in the oxygen sublattice in conjunction with a partial reduction of manganese (IV). The presence of this phase indicates a close proximity of barium and manganese achieved via the preparation procedure and is in accordance with previous studies, in which the occurrence of BaMnO3 in the systems Ba/MnOx-CeO2 and Mn/Ba/Al2O3 was also reported [48,50,51,55,56]. No indication of crystalline binary manganese oxides is found in the XRD pattern of MgCe-Mn-Ba. Other authors, however, reported on the occurrence of isolated and crystalline species Mn2O3 or MnO2 in similar systems [49,50]. Remarkable structural and compositional differences of manganese modified supports as a function of the use of either Mn-acetate or Mn-nitrate for preparation were observed and studied by Kapteijn et al., underlining the crucial role of the preparation route as to the resulting phase composition and arrangement [57]. No indication of the presence of BaCeO3 is found in either sample, proving that the formation of this phase is thermodynamically unfavoured under the conditions of preparation and ageing.
Data on the physical properties of all samples is summarized in Table 2.
Incorporation of about 15% of BaO on the solid precursors by means of wet impregnation followed by aging at 850 °C leads to a significant decrease in BET surface areas and pore volumes of the resulting samples. Nevertheless, the large average pore radius present in all samples is a good indication of their suitability as functional supports for LNT catalysts.
The analysis of the XPS results (Table 3) becomes complex because the samples suffer from significant levels of carbon contamination due to the presence of carbonates. Carbonation also inevitably occurs under real exhaust conditions. Therefore, purging of the samples via heat treatment in inert gas prior to the NOx storage tests was not applied. Considering the sequential preparation of the samples, both the atomic surface analysis and the metal ratios broadly reflect the decrease in Al, Mg and Ce content after addition of manganese and barium. In addition, the decrease in manganese content after subsequent impregnation with barium is reflected by the XPS results.

3.2. NOx Storage

3.2.1. Dual-bed Experiments

The results of the NOx adsorption tests measured in dual-bed configuration are displayed in Table 4.
The total NOx retention capacity expressed as mmol of NOx stored/m2 of carrier follows the order—MgCe-Mn < MgCe < MgCe-Ba << MgCe-Mn-Ba. It is worth mentioning that the surface areas of the samples with barium are significantly lower, resulting in a different order of NOx retention capacity if expressed as mmol of NOx stored/g of carrier—MgCe-Mn < MgCe-Ba < MgCe << MgCe-Mn-Ba. This is of importance as the reduction in surface area upon Ba-addition and ageing clearly contribute to the deactivation of the support in terms of NOx storage ability. In order to study the role of the different species present in the samples, the surface area related expression is applied in this work.
In the case of the Ba-free samples, the presence of MnOx slightly reduces the NOx storage capacity, thus illustrating that the MnOx as such does not contribute to the NOx storage process at all or only to a negligible extend. Apparently, relevant NOx storage sites of ceria and spinel are partially occupied and thus inactivated by Mn-species. This can also be deduced from the XPS analyses revealing a lower surface concentration of cerium and also a higher surface concentration of carbon species in the MnOx modified samples. MgCe-Ba (less cerium and magnesium but barium on the surface) shows a marked but unexpected small increase in NOx retention capacity compared to MgCe, which indicates that only a small amount of unreacted (and X-ray invisible) BaO or BaCO3 is still present after ageing. MgCe-Mn-Ba on the contrary, exhibits superior NOx storage ability, pointing to a significant stabilizing and hence beneficial effect of MnOx on the Ba-assisted NOx storage process. The introduction of MnOx as protective layer between BaO and spinel/ceria leads to the formation of BaMnO3 and reduces the amount of undesired BaAl2O4 generated after ageing, as observed in the XRD pattern. Thus, the occurrence of the perovskite phase most likely accounts for the high retained NOx storage ability after ageing.

3.2.2. Single-bed Experiments

Single-bed experiments without the Pt/alumina catalyst were conducted to study the effect of Mn-addition on NO oxidation ability. The absence of the noble metal catalyst is required as its outstanding oxidation capacity does not allow for a proper investigation of the NO oxidation contribution of the carriers. Table 5 compiles the NO and NO2 levels quantified for the samples MgCe and MgCe-Mn during NOx adsorption in single-bed experiments. The adsorption curves obtained for MgCe and MgCe-Mn observed in single-bed (SB) and dual-bed (DB) experiments are shown in Figure 2).
MgCe-Mn presents a significantly higher NO2 production than MgCe evident from the NO2/NO ratios obtained in single-bed experiments, revealing its superior NO oxidation activity promoted by MnOx addition. The ability of binary manganese oxides, especially Mn2O3, to oxidize NO has also been observed and described in detail by Guo et al. [58]. Although the analysis of the oxidation state of manganese is not part of this study, the presence of highly dispersed Mn2O3 species in MgCe-Mn therefore appears to be a valid assumption.
Although the difference in NO2/NO ratios between MgCe-Ba and MgCe-Mn-Ba is in the same range as in the Ba-free samples, a remarkable higher NOx storage ability is found in MgCe-Ba-Mn in the single-bed experiments. The amount of stored NOx is in the same range as observed in dual-bed experiments and is around 4.5 times higher than that observed for MgCe-Ba. The amounts of stored NOx observed in single-bed and dual-bed experiments are summarized in Figure 3. The NO and NO2 adsorption progressions shown in Figure 2c,d prove that the high NOx storage efficiency of MgCe-Mn-Ba results from the effective NO2 production in conjunction with the availability of storage sites. NO as such is only adsorbed to a negligible extend. The value of Ba-Mn interactions for fast NOx storage has previously been reported by Zhang et al. [50]. They have found that the NO oxidation ability of Mn-sites in close proximity to NOx storage sites of barium leads to very high NOx storage efficiencies. It can be speculated that the perovskite BaMnO3, which accommodates manganese in the rather high oxidation state +IV, combines both, high redox potential and efficient NOx storage sites in one solid. The high activity of BaMnO3 for NO and soot oxidation has also been pointed out by Gao et al. [55].

3.3. NOx Release

3.3.1. TPD after NOx Adsorption under Dual-bed Configuration

The temperature programmed desorption (TPD) of NOx, NO and NO2 after NO/O2 retention under dual-bed configuration was analysed in the temperature range of 300–700 °C. Figure 4 shows the TPD profiles of the different samples. The shape of the TPD profile of MgCe with two maxima—the first one centred at about 360 °C and the second centred at about 450 °C—indicates that there are two main ad-NOx species present on this sample. The individual contribution of NO and NO2 with the quantification given in Table 6 reveals that a significant fraction of the stored NOx is released from this sample in the form of NO2. The NO2 desorption takes places in a wide temperature window between 300 and 510 °C. The release of NO takes place in two steps, the first centred at about 350 °C and the second centred at about 490 °C.
The presence of barium leads to distinct changes in the TPD profiles in MgCe-Ba and MgCe-Mn-Ba. Significantly more NO is released from MgCe-Ba than from MgCe with the concentration of NO2 being almost identical. Similar to MgCe, the NO profile of MgCe-Ba shows two peaks. The first one is centred at about 350 °C and shows a comparative shape and peak area than the corresponding peak in MgCe. The second peak is centred at about 530 °C, with a much larger peak area. The NO release is taking place between 440 °C and 610 °C, so that the maximum desorption temperature of NO is increased by about 120 °C compared to MgCe. The concentrations and release temperatures of NO2 are very similar for MgCe and MgCe-Ba although the main contribution of NO2 in MgCe-Ba originates from the desorption processes taking place between 400 °C and 520 °C.
The observations made in the TPD profiles of MgCe and MgCe-Ba lead to the conclusion that the active barium species form comparatively strongly bound ad-NOx species, which mainly form NO upon decomposition. This is in line with studies from Lietti et al. [11,12], who reported that NO is the main decomposition product of NOx species stored on barium, with TPD profiles very similar to the high temperature peak shown in Figure 4e. This furthermore supports the assumption (see Section 3.2.1) that there are still active barium species left on MgCe-Ba although only BaAl2O4 can be detected in the X-ray pattern.
The presence of MnOx forming a layer on spinel/ceria displays unexpected substantial effects on the NOx release chemistry of the samples with and without BaO. The NOx desorption is dominated by NO in the Mn-containing samples with only minor contribution of NO2 in marked contrast to the Mn-free samples. In the case of the Mn-Ba combination, an additional contribution of NO species released in the range of 420–520 °C is observed, thus pointing to the generation of additional storage sites. This contribution can be linked to the presence of BaMnO3 since this desorption peak is absent in MgCe-Mn. Secondly, the NOx release process is much more efficient in the MnOx modified samples, in particular in MgCe-Mn. As it can be seen in Figure 4a–c, the NOx desorption is taking place in the narrow range 300–450 °C. The high temperature desorption of NO and NO2 is completely absent in the MnOx modified sample. A similar observation is made in the TPD profile of MgCe-Mn-Ba, in which the decreased fraction of NOx desorbed at higher temperatures upon manganese modification is also apparent.

3.3.2. TPD after NOx Adsorption under Single-bed Configuration

Effects induced by MnOx addition are even more pronounced in the TPD profiles of MgCe and MgCe-Mn after NOx storage under single-bed configuration (Figure 5 and Table 7).
The majority of stored NOx species is released from MgCe in the range of about 320–490 °C in the form of NO2. The asymmetry of the NO2 peak indicates the presence of differently bound ad-NOx species with the main population being desorbed at the peak maximum of about 440 °C.
TPD profiles of MgCe-Mn show a significant increase in NO desorption whereas the concentration of released NO2 is close to zero. The high-temperature desorption peak of NO detected in MgCe is not present in MgCe-Mn but a new one is observed ranging from 300–460 °C. This can be understood in conjunction with the dual-bed experiments, see Section 3.3.1, Figure 4b. Upon the addition of manganese, the low-temperature NO peak gains intensity whereas the high-temperature peak disappears. The absence of the high-temperature NO peak and the low concentration of detectable NO2 in MgCe-Mn are in accordance with the dual-bed experiments and support the theory of inhibited ceria/spinel sites by manganese species.
The NOx-TPD profile of MgCe-Mn-Ba is very similar to the profile obtained after adsorption in dual-bed experiments whereas the low-temperature peak associated with NO release is absent in the profile of MgCe-Ba, thus illustrating the similarity of ad-NOx species formed on BaMnO3 with and without the platinum oxidation catalyst. The presence and intensity of the low-temperature NO peaks of MgCe-Mn and especially MgCe-Mn-Ba demonstrate that the corresponding ad-NOx species are only formed if considerable amounts of NO2 are available in the feed gas or in other words: if an effective NO → NO2 oxidant is present.

4. Discussion

The effect of MnOx addition to LNT compositions based on MgAl2O4/CeO2 on the NOx storage/release properties was investigated at an adsorption temperature of 300 °C. The results clearly show that both, compositions with and without barium, can be improved significantly by the formation of a MnOx layer on the spinel/ceria mixture. The advantages of adding manganese are different in both type of materials indicating that at least two directions in the development of novel LNT catalysts may be pursued.
1. Barium-free compositions. The formation of a manganese oxide layer on MgAl2O4/CeO2 primarily alters the chemical nature of the generated ad-NOx species, as they exhibit lower thermostabilities than those generated without Mn-addition. This approach is clearly different from the formation of MnOx-CeO2 mixed oxides, which is described in several studies [45,46,47].
The presence of the MnOx layer significantly narrows the temperature range, in which the full amount of stored NOx is desorbed and lowers the maximum desorption temperature by about 100 °C. This finding makes the manganese modified composition an appealing candidate for the development of passive NOx adsorbers (PNA). In contrast to conventional LNT catalysts, passive NOx adsorbers are regenerated at elevated temperatures without additional rich pulses. This approach has already been described in the early 2000s [59,60] but has gained particular attention just recently due to the stricter legal requirements for NOx emissions and the implementation of the more realistic and challenging test procedure WLTP (Worldwide Harmonized Light-Duty Vehicles Test Procedure) for type approval. PNAs in combination with passive SCR catalysts can effectively improve the cold start NOx conversion at temperatures lower than 200 °C without causing additional fuel penalties [61]. The performance of PNAs is determined by (a) the ability to store NOx at low temperatures of about 150 °C and (b) effective NOx release in a narrow and low temperature range, so that the conversion over the SCR catalyst such as the recovery of NOx storages sites can take place efficiently. Ceria- and manganese-based PNAs have already been described in literature but our newly developed composition MgAl2O4/CeO2 with MnOx appears to be an interesting and sophisticated alternative due to its narrow NOx release window from 300–450 °C [62,63]. Although the observed NOx release behaviour is very promising, the assessment of the low-temperature NOx storage efficiency has not been investigated yet and is part of ongoing studies.
2. Compositions with barium. In the case of Mn-free sample, BaAl2O4 is formed upon ageing at 850 °C, evidently leading to a decrease in NOx storage sites. The introduction of a protective layer of MnOx between spinel/ceria and barium beneficially suppresses the formation of BaAl2O4 in favour of BaMnO3. This perovskite phase accommodating manganese (IV) was proven to be very active for NOx storage even at low NO2/NO ratios. This beneficial effect was not observed in former investigations of Ba/Mn/alumina compositions prepared by a sequential impregnation of γ-Al2O3 with barium and manganese salts followed by calcination at only 500 °C, not leading to the formation of BaMnO3 [49]. This illustrates that the temperature treatment along with the resulting phase composition has a crucial impact on the resulting NOx storage properties. The NOx retention ability and the NOx release mechanisms of the manganese modified material do not change if the sample is positioned downstream of an external Pt-catalyst. This is of enormous relevance as state of the art barium-based LNT catalysts require high levels of NO2 for effective NOx conversion [54]. The observation suggests that LNT catalysts based on this formulation can tolerate a reduced noble metal (Pt/Pd) content without losing performance. Furthermore, these materials offer the opportunity for the development of new deNOx concepts, in which the LNT function is not mandatorily placed downstream of the DOC function or in which the DOC requires a certain level of NOx retention ability itself.
A detailed understanding of the chemistry involved in the generation of the various ad-NOx species presenting different thermostabilities could be gained by operando DRIFTS measurements in combination with Raman spectroscopy and other sophisticated techniques. These investigations might provide a complementary overview on the diverse processes taking place on the surface of the materials during NOx storage and release. The individual contributions of the different compounds present in the compositions could be elucidated and rationalized this manner. These studies are in progress and will be presented in following publications.

5. Patents

WO2016/142058A1 (SASOL Germany GmbH).

Author Contributions

Conceptualization, M.S. and T.H.; methodology, M.S., T.H. and A.G.-G.; investigation, T.H., J.G.-M. and J.C.M.-M.; writing—original draft preparation, M.S.; writing—review and editing, A.G.-G. and J.C.M.-M.; visualization, M.S.; supervision, M.S. and A.G.-G.

Funding

This research received no external funding.

Acknowledgments

The authors thank Dirk Worch and his team (SASOL Germany GmbH) for ICP, XRD and N2-physisorption measurements. A.G.-G. gratefully acknowledges the general financial support of Generalitat Valenciana (PROMETEO/2018/076), MINECO (CTQ2015-64801-R) and the UE (FEDER funding). J.C.M.-M. also acknowledges Spanish Ministry of Science, Innovation and Universities for the financial support through a FPU grant (FPU17/00603). Valuable comments provided by Karen Beckhusen, Muxin Han, Jonathan Paiz and Patrick Bussian are kindly acknowledged. The authors thank the reviewers for useful suggestions.

Conflicts of Interest

The authors declare no conflict of interest

References

  1. Walker, J. Johnson Matthey Sector Call—Clean Air. Available online: https://seekingalpha.com/article/4187530-johnson-matthey-jmply-sector-call-clean-air-slideshow (accessed on 2 May 2019).
  2. Twigg, M.V. Catalytic control of emissions from cars. Catal. Today 2011, 163, 33–41. [Google Scholar] [CrossRef]
  3. Heck, R.M.; Farrauto, R.J.; Gulati, S.T. Catalytic Air Pollution Control, 3rd ed.; Wiley: Hoboken, NJ, USA, 2009; pp. 158–163. [Google Scholar]
  4. Metkar, P.S.; Harold, M.P.; Balakotaiah, V. Experimental and kinetic modeling study of NH3-SCR of NOx on Fe-ZSM-5, Cu-chabazite and combined Fe- and Cu-zeolite monolithic catalysts. Chem. Eng. Sci. 2013, 87, 51–66. [Google Scholar] [CrossRef]
  5. Koebel, M.; Madia, G.; Elsener, M. Selective catalytic reduction of NO and NO2 at low temperatures. Catal. Today 2002, 73, 239–247. [Google Scholar] [CrossRef]
  6. Stadlbauer, S.; Waschl, H.; Schilling, A.; del Re, L. DOC Temperature Control for Low Temperature Operating Ranges with Post and Main Injection Actuation; SAE International: Warrendale, PA, USA, 2013; Volume 1. [Google Scholar] [CrossRef]
  7. Epling, W.S.; Campbell, L.E.; Yezerets, A.; Currier, N.W.; Parks, J.E. Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/Reduction Catalysts. Catal. Rev. 2004, 46, 163–245. [Google Scholar] [CrossRef]
  8. Chan, D.; Gremminger, A.; Deutschmann, O. Effect of Hydrothermal Aging on Physical and Chemical Properties of a Commercial NOx-Storage Catalyst. Top. Catal. 2013, 56, 293–297. [Google Scholar] [CrossRef]
  9. Kubiak, L.; Castoldi, L.; Lietti, L.; Andonova, S.; Olsson, L. Mechanistic Investigation of the Reduction of NOx over Pt- and Rh-Based LNT Catalysts. Catalysts 2016, 6, 46. [Google Scholar] [CrossRef]
  10. Fridell, E.; Skoglundh, M.; Westerberg, B.; Johansson, S.; Smedler, G. NOx storage in barium-containing catalysts. J. Catal. 1999, 183, 196–209. [Google Scholar] [CrossRef]
  11. Lietti, L.; Forzatti, P.; Nova, I.; Tronconi, E. NOx Storage Reduction over Pt@Ba/γ-Al2O3 Catalyst. J. Catal. 2001, 204, 175–191. [Google Scholar] [CrossRef]
  12. Nova, I.; Castoldi, L.; Lietti, L.; Tronconi, E.; Forzatti, P.; Prinetto, F.; Ghiotti, G. NOx adsorption study over Pt–Ba/alumina catalysts: FT-IR and pulse experiments. J. Catal. 2004, 222, 377–388. [Google Scholar] [CrossRef]
  13. Kwak, J.H.; Mei, D.; Yi, C.-W.W.; Kim, D.H.; Peden, C.H.F.F.; Allard, L.F.; Szanyi, J. Understanding the nature of surface nitrates in BaO/γ-Al2O3 NOx storage materials: A combined experimental and theoretical study. J. Catal. 2009, 261, 17–22. [Google Scholar] [CrossRef]
  14. Kumar, A.; Harold, M.P.; Balakotaiah, V. Isotopic studies of NOX storage and reduction on Pt/BaO/Al2O3 catalyst using temporal analysis of products. J. Catal. 2010, 270, 214–223. [Google Scholar] [CrossRef]
  15. Forzatti, P.; Castoldi, L.; Nova, I.; Lietti, L.; Tronconi, E. NOx removal catalysis under lean conditions. Catal. Today 2006, 117, 316–320. [Google Scholar] [CrossRef]
  16. Cumaranatunge, L.; Mulla, S.S.; Yezerets, A.; Currier, N.W.; Delgass, W.N.; Ribeiro, F.H. Ammonia is a hydrogen carrier in the regeneration of Pt/BaO/Al2O3 NOx traps with H2. J. Catal. 2007, 246, 29–34. [Google Scholar] [CrossRef]
  17. Bhatia, D.; Clayton, R.D.; Harold, M.P.; Balakotaiah, V. A global kinetic model for NOx storage and reduction on Pt/BaO/Al2O3 monolithic catalysts. Catal. Today 2009, 147, 250–256. [Google Scholar] [CrossRef]
  18. Pereda-Ayo, B.; González-Velasco, J.R.; Burch, R.; Hardacre, C.; Chansai, S. Regeneration mechanism of a Lean NOx Trap (LNT) catalyst in the presence of NO investigated using isotope labelling techniques. J. Catal. 2012, 285, 177–186. [Google Scholar] [CrossRef]
  19. Luo, J.-Y.; Epling, W.S.; Qi, G.; Li, W. Low Temperature Ceria-Based Lean NOx Traps. Catal. Lett. 2012, 142, 946–958. [Google Scholar] [CrossRef]
  20. Shi, C.; Ji, Y.; Graham, U.M.; Jacobs, G.; Crocker, M.; Zhang, Z.; Wang, Y.; Toops, T.J. NOx storage and reduction properties of model ceria-based lean NOx trap catalysts. Appl. Catal. B Environ. 2012, 119–120, 183–196. [Google Scholar] [CrossRef]
  21. Hilaire, S.; Wang, X.; Luo, T.; Gorte, R.J.; Wagner, J. A comparative study of water-gas-shift reaction over ceria-supported metallic catalysts. Appl. Catal. A Gen. 2004, 258, 271–276. [Google Scholar] [CrossRef]
  22. Panagiotopoulou, P.; Kondarides, D.I. Effect of the nature of the support on the catalytic performance of noble metal catalysts for the water–gas shift reaction. Catal. Today 2006, 112, 49–52. [Google Scholar] [CrossRef]
  23. Infantes-Molina, A.; Righini, L.; Castoldi, L.; Loricera, C.V.; Fierro, J.L.G.; Sin, A.; Lietti, L. Characterization and reactivity of Ce-promoted PtBa lean NOx trap catalysts. Catal. Today 2012, 197, 178–189. [Google Scholar] [CrossRef]
  24. Say, Z.; Vovk, E.I.; Bukhtiyarov, V.I.; Ozensoy, E. Influence of ceria on the NOx reduction performance of NOx storage reduction catalysts. Appl. Catal. B Environ. 2013, 142–143, 89–100. [Google Scholar] [CrossRef]
  25. Philipp, S.; Drochner, A.; Kunert, J.; Vogel, H.; Theis, J.; Lox, E.S. Investigation of NO Adsorption and NO/O2 Co-adsorption on NOx -Storage-Components by DRIFT-Spectroscopy. Top. Catal. 2004, 30, 235–238. [Google Scholar] [CrossRef]
  26. Ji, Y.; Toops, T.J.; Graham, U.M.; Jacobs, G.; Crocker, M. A kinetic and DRIFTS study of supported Pt catalysts for NO oxidation. Catal. Lett. 2006, 110, 29–37. [Google Scholar] [CrossRef]
  27. Ryou, Y.; Lee, J.; Lee, H.; Kim, C.H.; Kim, D.H. Low temperature NO adsorption over hydrothermally aged Pd/CeO2 for cold start application. Catal. Today 2018, 307, 93–101. [Google Scholar] [CrossRef]
  28. Filtschew, A.; Stranz, D.; Hess, C. Mechanism of NO2 storage in ceria studied using combined in situ Raman/FT-IR spectroscopy. Phys. Chem. Chem. Phys. 2013, 15, 9066–9069. [Google Scholar] [CrossRef]
  29. Filtschew, A.; Hess, C. Unravelling the mechanism of NO and NO2 storage in ceria: The role of defects and Ce-O surface sites. Appl. Catal. B Environ. 2018, 237, 1066–1081. [Google Scholar] [CrossRef]
  30. AL-Harbi, M.; Epling, W.S. Effects of different regeneration timing protocols on the performance of a model NOx storage/reduction catalyst. Catal. Today 2010, 151, 347–353. [Google Scholar] [CrossRef]
  31. Corbos, E.C.; Haneda, M.; Courtois, X.; Marecot, P.; Duprez, D.; Hamada, H. Cooperative effect of Pt–Rh/Ba/Al and CuZSM-5 catalysts for NOx reduction during periodic lean-rich atmosphere. Catal. Commun. 2008, 10, 137–141. [Google Scholar] [CrossRef]
  32. Corbos, E.C.; Haneda, M.; Courtois, X.; Marecot, P.; Duprez, D.; Hamada, H. NOx abatement for lean-burn engines under lean–rich atmosphere over mixed NSR-SCR catalysts: Influences of the addition of a SCR catalyst and of the operational conditions. Appl. Catal. A Gen. 2009, 365, 187–193. [Google Scholar] [CrossRef]
  33. Weibel, M.; Waldbüer, N.; Wunsch, R.; Chatterjee, D.; Bandl-Konrad, B.; Krutzsch, B. A novel approach to catalysis for NOx reduction in diesel exhaust gas. Top. Catal. 2009, 52, 1702–1708. [Google Scholar] [CrossRef]
  34. Pereda-Ayo, B.; Duraiswami, D.; González-Velasco, J.R. Control of NOx storage and reduction in NSR bed for designing combined NSR–SCR systems. Catal. Today 2011, 172, 66–72. [Google Scholar] [CrossRef]
  35. Castoldi, L.; Bonzi, R.; Lietti, L.; Forzatti, P.; Morandi, S.; Ghiotti, G.; Dzwigaj, S. Catalytic behaviour of hybrid LNT/SCR systems: Reactivity and in situ FTIR study. J. Catal. 2011, 282, 128–144. [Google Scholar] [CrossRef]
  36. Can, F.; Courtois, X.; Royer, S.; Blanchard, G.; Rousseau, S.; Duprez, D. An overview of the production and use of ammonia in NSR+SCR coupled system for NOx reduction from lean exhaust gas. Catal. Today 2012, 197, 144–154. [Google Scholar] [CrossRef]
  37. Wang, J.; Ji, Y.; Jacobs, G.; Jones, S.; Kim, D.J.; Crocker, M. Effect of aging on NOx reduction in coupled LNT–SCR systems. Appl. Catal. B Environ. 2014, 148–149, 51–61. [Google Scholar] [CrossRef]
  38. Wittka, T.; Holderbaum, B.; Dittmann, P.; Pischinger, S. Experimental Investigation of Combined LNT + SCR Diesel Exhaust Aftertreatment. Emiss. Control Sci. Technol. 2015, 1, 167–182. [Google Scholar] [CrossRef] [Green Version]
  39. Eberhardt, M.; Riedel, R.; Göbel, U.; Theis, J.; Lox, E.S. Fundamental investigations of thermal aging phenomena of model NOx storage systems. Top. Catal. 2004, 30, 135–142. [Google Scholar] [CrossRef]
  40. Kim, D.H.; Chin, Y.-H.; Kwak, J.H.; Szanyi, J.; Peden, C.H.F. Changes in Ba Phases in BaO/Al2O3 upon Thermal Aging and H2O Treatment. Catal. Lett. 2005, 105, 259–268. [Google Scholar] [CrossRef]
  41. Kwak, J.H.; Kim, D.H.; Szanyi, J.; Cho, S.J.; Peden, C.H.F. Enhanced High Temperature Performance of MgAl2O4-Supported Pt–BaO Lean NOx Trap Catalysts. Top. Catal. 2012, 55, 70–77. [Google Scholar] [CrossRef]
  42. Jeong, S.; Youn, S.; Kim, D.H. Effect of Mg/Al ratios on the NOx storage activity over Pt-BaO/Mg–Al mixed oxides. Catal. Today 2014, 231, 155–163. [Google Scholar] [CrossRef]
  43. Roy, S.; van Vegten, N.; Baiker, A. Single-step flame-made Pt/MgAl2O4—A NOx storage-reduction catalyst with unprecedented dynamic behavior and high thermal stability. J. Catal. 2010, 271, 125–131. [Google Scholar] [CrossRef]
  44. Casapu, M.; Grunwaldt, J.-D.; Maciejewski, M.; Wittrock, M.; Göbel, U.; Baiker, A. Formation and stability of barium aluminate and cerate in NOx storage-reduction catalysts. Appl. Catal. B Environ. 2006, 63, 232–242. [Google Scholar] [CrossRef]
  45. Wu, X.; Lin, F.; Xu, H.; Weng, D. Effects of adsorbed and gaseous NOx species on catalytic oxidation of diesel soot with MnOx-CeO2 mixed oxides. Appl. Catal. B Environ. 2010, 96, 101–109. [Google Scholar] [CrossRef]
  46. Shen, Q.; Zhang, L.; Sun, N.; Wang, H.; Zhong, L.; He, C.; Wei, W.; Sun, Y. Hollow MnOx-CeO2 mixed oxides as highly efficient catalysts in NO oxidation. Chem. Eng. J. 2017, 322, 46–55. [Google Scholar] [CrossRef]
  47. Qi, G.; Li, W. NO oxidation to NO2 over manganese-cerium mixed oxides. Catal. Today 2015, 258, 205–213. [Google Scholar] [CrossRef]
  48. Le Phuc, N.; Courtois, X.; Can, F.; Royer, S.; Marecot, P.; Duprez, D. NOx removal efficiency and ammonia selectivity during the NOx storage-reduction process over Pt/BaO(Fe, Mn, Ce)/Al2O3 model catalysts. Part II: Influence of Ce and Mn–Ce addition. Appl. Catal. B Environ. 2011, 102, 353–361. [Google Scholar] [CrossRef]
  49. Le-Phuc, N. Removal of NOx in the presence of oxygen over Mn/BaO/Al2O3 catalysts. Mater. Sci. Nanaotechnol. 2017, 1, 37–40. [Google Scholar]
  50. Zhang, Z.; Chen, B.; Wang, X.; Xu, L.; Au, C.; Shi, C.; Crocker, M. NOx storage and reduction properties of model manganese-based lean NOx trap catalysts. Appl. Catal. B Environ. 2015, 165, 232–244. [Google Scholar] [CrossRef] [Green Version]
  51. Xiao, J.; Li, X.; Deng, S.; Wang, F.; Wang, L. NOx storage-reduction over combined catalyst Mn/Ba/Al2O3–Pt/Ba/Al2O3. Catal. Commun. 2008, 9, 563–567. [Google Scholar] [CrossRef]
  52. Schöneborn, M.; Harmening, T.; Niemeyer, D.; Rolfs, S.; Fabian, J. NOx Trap Catalyst Support Material with Improved Stability against BaAl2O4 Formation. Patent No. WO2016142058A1, 15 September 2016. [Google Scholar]
  53. Adouane, D.; Dutilleul, H.; Guibert, P.; Darcy, P.; Costa, P. Da Effect of thermal ageing on structure and reactivity of commercial Lean NOx trap : From laboratory to engine scale. In Proceedings of the 7th International Conference on Environmental Catalysis, Lyon, France, 2–6 September 2012. [Google Scholar]
  54. Al-Harbi, M.; Epling, W.S. Investigating the effect of NO versus NO2 on the performance of a model NOx storage/reduction catalyst. Catal. Lett. 2009, 130, 121–129. [Google Scholar] [CrossRef]
  55. Gao, Y.; Wu, X.; Liu, S.; Weng, D.; Zhang, H.; Ran, R. Formation of BaMnO3 in Ba/MnOx-CeO2 catalyst upon the hydrothermal ageing and its effects on oxide sintering and soot oxidation activity. Catal. Today 2015, 253, 83–88. [Google Scholar] [CrossRef]
  56. Xiao, J.H.; Li, X.H.; Deng, S.; Xu, J.C.; Wang, L.F. The NOx oxidation-storage and tolerance of SO2 poison of Mn/Ba/Al2O3 catalyst. Acta Phys.-Chim. Sin. 2006, 22, 815–819. [Google Scholar] [CrossRef]
  57. Kapteijn, F.; Vanlangeveld, A.D.; Moulijn, J.A.; Andreini, A.; Vuurman, M.A.; Turek, A.M.; Jehng, J.M.; Wachs, I.E. Alumina-Supported Manganese Oxide Catalysts: I. Characterization: Effect of Precursor and Loading. J. Catal. 1994, 150, 94–104. [Google Scholar] [CrossRef]
  58. Guo, L.H.; Guo, L.; Zhao, D.Y.; Gao, Z.N.; Tian, Y.; Ding, T.; Zhang, J.; Zheng, L.R.; Li, X.G. Oxidizing, trapping and releasing NOx over model manganese oxides in alternative lean-burn/fuel-rich atmospheres at low temperatures. Catal. Today 2017, 297, 27–35. [Google Scholar] [CrossRef]
  59. Feeley, J.S.; Farrauto, R.J.; Deeba, M. Method and Apparatus for NOx Abatement in Lean Gaseous Streams. U.S. Patent No. 6471924B1, 29 October 2002. [Google Scholar]
  60. Jarvis, M.; Adams, K.M. Method for Converting Exhaust Gases from a Diesel Engine using Nitrogen Oxide Absorbent. U.S. Patent No. 6182443B1, 6 February 2001. [Google Scholar]
  61. Gu, Y.; Epling, W.S. Passive NOx adsorber: An overview of catalyst performance and reaction chemistry. Appl. Catal. A Gen. 2019, 570, 1–14. [Google Scholar] [CrossRef]
  62. Jones, S.; Ji, Y.; Bueno-Lopez, A.; Song, Y.; Crocker, M. CeO2-M2O3 Passive NOx Adsorbers for Cold Start Applications. Emiss. Control Sci. Technol. 2017, 3, 59–72. [Google Scholar] [CrossRef]
  63. Ji, Y.; Xu, D.; Crocker, M.; Theis, J.R.; Lambert, C.; Bueno-Lopez, A.; Harris, D.; Scapens, D. Mn-based mixed oxides for low temperature NOx adsorber applications. Appl. Catal. A Gen. 2018, 567, 90–101. [Google Scholar] [CrossRef]
Materials 12 02127 g001 550
Figure 1. Powder X-ray diffraction (XRD) patterns of (a) MgCe and MgCe-Mn (b) MgCe-Ba and MgCe-Mn-Ba with simulated reflection positions of MgAl2O4, CeO2, BaAl2O4 and BaMnO3. For better clarity, the reflections of MgAl2O4 and CeO2 are not marked in (b).
Figure 1. Powder X-ray diffraction (XRD) patterns of (a) MgCe and MgCe-Mn (b) MgCe-Ba and MgCe-Mn-Ba with simulated reflection positions of MgAl2O4, CeO2, BaAl2O4 and BaMnO3. For better clarity, the reflections of MgAl2O4 and CeO2 are not marked in (b).
Materials 12 02127 g001
Materials 12 02127 g002 550
Figure 2. Profiles of NO and NO2 of during NOx adsorption at 300 °C under single-bed (SB) and dual-bed (DB) conditions. (a) NO and (b) NO2 profiles of MgCe and MgCe-Mn. (c) NO and (d) NO2 profiles of MgCe-Ba and MgCe-Mn-Ba. Profiles of Pt/alumina are included for comparison.
Figure 2. Profiles of NO and NO2 of during NOx adsorption at 300 °C under single-bed (SB) and dual-bed (DB) conditions. (a) NO and (b) NO2 profiles of MgCe and MgCe-Mn. (c) NO and (d) NO2 profiles of MgCe-Ba and MgCe-Mn-Ba. Profiles of Pt/alumina are included for comparison.
Materials 12 02127 g002
Materials 12 02127 g003 550
Figure 3. Stored amounts of NOx observed in single-bed and dual-bed experiments.
Figure 3. Stored amounts of NOx observed in single-bed and dual-bed experiments.
Materials 12 02127 g003
Materials 12 02127 g004 550
Figure 4. Temperature programmed desorption (TPD) profiles (dual-bed configuration) of MgCe and MgCe-Mn (a) NOx, (b) NO (c) NO2 and MgCe-Ba and MgCe-Mn-Ba (d) NOx, (e) NO (f) NO2.
Figure 4. Temperature programmed desorption (TPD) profiles (dual-bed configuration) of MgCe and MgCe-Mn (a) NOx, (b) NO (c) NO2 and MgCe-Ba and MgCe-Mn-Ba (d) NOx, (e) NO (f) NO2.
Materials 12 02127 g004
Materials 12 02127 g005 550
Figure 5. TPD profiles (single-bed configuration) of MgCe and MgCe-Mn (a) NOx, (b) NO (c) NO2 and MgCe-Ba and MgCe-Mn-Ba (d) NOx, (e) NO (f) NO2.
Figure 5. TPD profiles (single-bed configuration) of MgCe and MgCe-Mn (a) NOx, (b) NO (c) NO2 and MgCe-Ba and MgCe-Mn-Ba (d) NOx, (e) NO (f) NO2.
Materials 12 02127 g005
Table
Table 1. Chemical composition (wt%) of the samples determined by Inductively Coupled Plasma (ICP) analysis.
Table 1. Chemical composition (wt%) of the samples determined by Inductively Coupled Plasma (ICP) analysis.
SampleAl2O3MgOCeO2MnO2BaO
MgCe63.017.020.000
MgCe-Mn58.314.917.79.10
MgCe-Ba52.814.717.0015.5
MgCe-Mn-Ba48.413.315.57.315.5
Table
Table 2. Physical properties of the samples studied in this work.
Table 2. Physical properties of the samples studied in this work.
SampleSBET (m2/g)rP (nm)VP (cm3/g)
MgCe96130.64
MgCe-Mn104100.53
MgCe-Ba68110.37
MgCe-Mn-Ba59110.31
Table
Table 3. Surface atomic contents and ratios estimated by X-ray photoelectron spectroscopy (XPS).
Table 3. Surface atomic contents and ratios estimated by X-ray photoelectron spectroscopy (XPS).
ElementSample
MgCeMgCe-MnMgCe-BaMgCe-Mn-Ba
C33.4643.750.458.1
O40.037.735.530.1
N0.40.30.70.5
Al17.311.110.58.5
Ce2.92.50.70.5
Mg2.32.31.11.0
Mn0.02.50.00.7
Ba0.00.01.20.8
Al/(Al + Ce + Mg + Mn + Ba) 10.770.600.780.74
Ce/(Al + Ce + Mg + Mn + Ba) 10.130.140.050.04
Mg/(Al + Ce + Mg + Mn + Ba) 10.100.130.080.09
Mn/(Al + Ce + Mg + Mn + Ba) 1-0.13-0.06
Ba/(Al + Ce + Mg + Mn + Ba) 1--0.090.07
1 The ratios were calculated based on the content of the elements.
Table
Table 4. Amounts of NOx stored on the samples in dual-bed experiments.
Table 4. Amounts of NOx stored on the samples in dual-bed experiments.
Samples
without Ba		NOx Stored (10−3 mmol/m2carrier)Samples
with Ba		NOx Stored (10−3 mmol/m2carrier)
MgCe3.23MgCe-Ba4.41
MgCe-Mn2.60MgCe-Mn-Ba6.27
Table
Table 5. NO2/NO ratios and amounts of NOx stored on the different samples obtained under single-bed configuration.
Table 5. NO2/NO ratios and amounts of NOx stored on the different samples obtained under single-bed configuration.
Sample NO2/NO RatioNOx Stored (10−3 mmol/m2carrier)
MgCe0.071.27
MgCe-Mn0.431.03
MgCe-Ba0.081.40
MgCe-Mn-Ba0.486.39
Table
Table 6. Released amounts of NOx, NO and NO2 after adsorption under dual-bed configuration.
Table 6. Released amounts of NOx, NO and NO2 after adsorption under dual-bed configuration.
Sample NOx Released (10−3 mmol/m2carrier)NO Released (10−3 mmol/m2carrier)NO2 Released (10−3 mmol/m2carrier)
MgCe2.470.791.68
MgCe-Mn0.970.780.19
MgCe-Ba4.693.151.54
MgCe-Mn-Ba4.764.390.37
Table
Table 7. Released amounts of NOx, NO and NO2 after adsorption under single-bed configuration.
Table 7. Released amounts of NOx, NO and NO2 after adsorption under single-bed configuration.
Sample NOx Released (10−3 mmol/m2carrier)NO Released (10−3 mmol/m2carrier)NO2 Released (10−3 mmol/m2carrier)
MgCe0.590.180.41
MgCe-Mn0.360.320.04
MgCe-Ba1.341.340
MgCe-Mn-Ba5.054.610.44

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Schöneborn, M.; Harmening, T.; Giménez-Mañogil, J.; Martínez-Munuera, J.C.; García-García, A. Improved NOx Storage/Release Properties of Ceria-Based Lean NOx Trap Compositions with MnOx Modification. Materials 2019, 12, 2127. https://doi.org/10.3390/ma12132127

AMA Style

Schöneborn M, Harmening T, Giménez-Mañogil J, Martínez-Munuera JC, García-García A. Improved NOx Storage/Release Properties of Ceria-Based Lean NOx Trap Compositions with MnOx Modification. Materials. 2019; 12(13):2127. https://doi.org/10.3390/ma12132127

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Schöneborn, Marcos, Thomas Harmening, Javier Giménez-Mañogil, Juan Carlos Martínez-Munuera, and Avelina García-García. 2019. "Improved NOx Storage/Release Properties of Ceria-Based Lean NOx Trap Compositions with MnOx Modification" Materials 12, no. 13: 2127. https://doi.org/10.3390/ma12132127

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