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Article

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

by
Sabrina I. Ecker
1,2,*,
Jürgen Dornseiffer
1,
Stefan Baumann
1,
Olivier Guillon
1,3,
Henny J. M. Bouwmeester
1,4 and
Wilhelm A. Meulenberg
1,2
1
Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research, IEK-1: Materials Synthesis and Processing, 52425 Jülich, Germany
2
Inorganic Membranes, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
3
Jülich Aachen Research Alliance: JARA Energy, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
4
Electrochemistry Research Group, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(8), 917; https://doi.org/10.3390/catal11080917
Submission received: 28 June 2021 / Revised: 19 July 2021 / Accepted: 22 July 2021 / Published: 29 July 2021
(This article belongs to the Section Environmental Catalysis)
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Abstract

:
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.

Graphical Abstract

1. Introduction

Lean-burn engines operating with air excess show high emissions of NO and NO2, referred to as NOx [1]. To reduce the emissions of these harmful gases additional after-treatment systems need to be included in the exhaust pathway [2]. Under ideal conditions [3,4] the selective catalytic reduction (SCR) catalyst [5,6] or the lean NOx trap catalyst (LNT), also known as NOx storage and reduction catalyst (NSR) [7], reduce NOx selectively to N2 and water. However, ideal conditions are rarely encountered due to the varying exhaust temperatures (150–500 °C) [8,9] giving rise to different exhaust gas compositions as a result of incomplete combustion processes [9,10] and side reactions [3,11,12,13]. Hence, side product formation of secondary pollutants N2O and NH3 occurs due to the use of catalytic after-treatment systems [14,15]. Whereas ammonia is able to reduce NOx to N2, N2O is less reactive and still being emitted [16,17]. Nitrous oxide is proven to be one of the most dangerous greenhouse gases with a lifetime of 120–150 years and a global warming potential (GWP) which is almost 300 times higher than that of CO2 [18]. Nonetheless, N2O is rarely included in emission regulations. The United States Environmental Protection Agency (U.S. EPA) has set N2O emissions to a limit value of 0.010 g/mile for light-duty vehicles (2012–2016) [18]. In such vehicles, lean NOx traps are the preferred after-treatment catalyst [19] as they are the most space-saving DeNOx systems.
LNTs, originally introduced as Pt-Ba/Al2O3, reduce NOx by alternating lean-rich cycles of the engine. In a long-lasting lean operation phase (in excess of air), dominant NO is oxidised to NO2 by platinum which is stored by sorption onto an (earth-) alkaline metal oxide in the form of nitrites and nitrates. After 60–120 s of storage, the engine operation mode changes to the short rich mode (in excess of fuel), whereby reductives, such as H2, CO and hydrocarbons (HC), are produced. These gases initiate the desorption of the stored nitrites and nitrates, reducing them to N2, N2O and NH3 [7].
As nitrous oxide formation is generally related to the regeneration of LNT catalysts, most studies deal with investigations towards N2O evolution in the rich phase and/or during the switch from rich to lean conditions and vice versa [10,20,21,22]. However, a few studies have reported N2O formation in the lean operation mode due to NOx reduction on Pt-based LNT catalysts in the presence of reductives. Huang et al. [8] observed a decrease of 30% in the NO concentration over Pt/CaO/Al2O3 in excess of oxygen due to reduction by CO and C3H6. Similar behaviour has been observed for the Pt/CeO2-ZrO2 catalyst investigated by Masdrag et al. [23] showing formation of N2O in the temperature range of 200–300 °C. The latter authors studied the impact of H2, CO and C3H6 by single and mixed feed gas experiments and identified a temperature-dependent influence on N2O evolution. Whereas the order of reducibility at 200 °C was H2 > CO > C3H6 (≈0), it was found to change by an increase in temperature to 300 °C to C3H6 > H2 > CO (≈0). The results of Masdrag et al. [23] and Huang et al. [8] are merely consistent with previous results on Platinum Group Metal (PGM)-based catalysts [24,25,26]. In 1991, Hamada et al. [25,27], who first developed PGM/γ-Al2O3 based HC-SCR catalysts successfully, observed a general NOx conversion order of Pt (44%) > Pd (11%) > Rh (4%) at 200 °C. Even though the results identified Pt as the most active catalyst, there is a problem of predominant N2O formation compared to the preferred reaction product N2 and the more reactive NH3, reaching selectivities of 66–83% at 200–250 °C [28,29]. Contrarily, Rh/Al2O3 catalysts with a slightly higher maximum performance temperature (250–300 °C) are found to show a much lower selectivity for N2O (≤30%) [28,29]. However, the NO conversion and N2-selectivity of the precious metal (PGM)-based catalysts are highly support-dependent as the studies of Burch and Millington [30] (Al2O3 vs. SiO2) and Garcia-Cortés et al. [31] (Pt activity order: ZSM-5 >> Al2O3; N2 selectivity: Pt/Al2O3 > Pt/ZSM-5) show.
Burch et al. [26,30,32] noted a correlation between the NOx conversion and the propylene oxidation activity of PGM/alumina catalysts (PGM = Pt, Pd, Rh, Ir) and proposed a multi-step mechanism for the C3H6-SCR reaction [26]. The proposed mechanism involves first a reduction of PGMOx to metallic PGM by propylene (Equation (1)), activating the precious metal sites for NO adsorption. Afterwards, NO is either dissociatively adsorbed as N*- and O*-ad species or as NO*-ad species depending on the temperature (Equation (2)). NO* is the major form at low temperatures and favours the formation of undesired N2O by a recombination of two NO*-ad species (Equation (3)). Additionally, rival adsorption of O2 onto the PGM (Equation (4)) counteracts with the NOx reduction process and limits the NO dissociation as less activated PGM sites become accessible. The competition between NO and O2 adsorption decreases with increasing temperatures leading to a more favoured N2 production from adsorbed N*-ad species (see equation 5) [26,33].
PGMOx → PGM + xO*
NO → NO* → N* + O*
2 NO* + C3H6 + 8 O* → N2O + 3 CO2 + 3 H2O
O2 → 2O*
N* + C3H6 + 9 O* → 0.5 N2 + 3 CO2 + 3 H2O
To reduce N2O formation over Pt-based LNT catalysts at low temperatures there are two options: either conversion of the N2O to N2 or suppressing the C3H6-SCR side reaction. The former option is quite challenging as the O2 concentration is two orders of magnitude higher than that of N2O concentration [34]. The most common catalysts investigated for this specific application are Rh-based due to their high N2 selectivities [29,35], but they commonly suffer from a poor low-temperature activity as the light-off temperatures T50 (temperature of 50% conversion) are around 350 °C [34,36]. Centi et al. [34,36] investigated a zirconia catalyst with 1 wt.% of Rh for its N2O conversion performance in different feed gas mixtures and observed a shift of T50 (~260 °C) to higher temperatures (T50~380–400 °C) in the presence of O2 and H2O due to catalyst deactivation. However, upon removal of O2 and H2O from the feed gas, the good performance (before adding O2 and H2O) at low temperatures could be retrieved [36,37]. The investigations of Beyer et al. [35] and Parres-Esclapez et al. [38] also showed a performance loss in N2O conversion for 0.58 wt.% Rh/Al2O3 (T50 = 341 °C) and 0.5 wt.% Rh/SrAl2O3 (T50 ≈ 300 °C), caused by the presence of O2.
The second approach of SCR reaction inhibition can be achieved with a reduction of available propylene by oxidation. Again, supported noble metal catalysts possess a remarkable ability for the oxidation of hydrocarbons and CO at low temperatures. Palladium is the most common commercially used precious metal [39], for example, in the compositions Pd/CeO2 [40,41], Pd/Al2O3 [42] and Pd/CeO2-Al2O3 [42]. The survey of Shen et al. [41] displays a strong relation between the C3H6 and CO light-off temperatures and the loaded Pd content. It was shown that the light-off temperature T50(C3H6) shifts from around 225 °C to 140 °C upon increasing the Pd loading from 1 to 7 wt.% [41]. The same shift was observed for the CO oxidation T50 values, which are commonly at lower temperatures than for propylene oxidation (e.g., T50(CO) at ~200 °C for 1 wt.% Pd). Even though Faria et al. [43] proposed CeO2 to enhance the oxidation ability of Pd/Al2O3 due to a better precious metal dispersion, Guimarães et al. [42] observed a T50 shift to higher temperatures with the incorporation of ceria (T50(Pd/Al2O3): ~375 °C and T50(Pd/CeO2/Al2O3): ~490 °C). However, it must be pointed out that both catalysts were pre-treated differently.
Infiltration composites with 20 wt.% loadings of B-site substituted lanthanum strontium ferrates (La0.5Sr0.5Fe1-xMxO3-δ M = Nb, Ti, Zr) on an alumina support showed promising NOx storage capacities (NSC) for the substituting elements Nb, Ti and Zr under laboratory testing conditions [44]. Depending on the perovskite composition a maximal NSC of around 120 to 164 µmol/g was measured in the temperature range from 250 °C to 350 °C. Hereby, an increasing maximum NSC and a temperature-dependent shift to lower temperatures are found in the order of Nb < Ti < Zr [44].
The main focus of this study is to investigate nitrous oxide formation on 2.5 wt.% Pt/20 wt.% La0.5Sr0.5Fe1-xMxO3/Al2O3 (M = Nb, Ti, Zr)-based lean NOx trap catalysts in the lean operation mode. After determining the underlying mechanism of formation in the presence of CO and C3H6 reductives [23], different approaches to reduce or avoid nitrous oxide formation have been examined. The first approach considers an additional precious metal coating of 0.125 wt.% Rh obtained via incipient wetness impregnation onto the LNT catalyst in order to increase the number of active NOx oxidation and reduction sites on the catalyst surface as well as the N2 selectivity. The second approach comprises the usage of an oxidation catalyst (2 wt.% Pd/20 wt.% CeO2/Al2O3) deposited on top of the LNT material in a two-layer configuration within the reactor to decrease the reductive concentrations before passing through the LNT catalyst.

2. Results

2.1. N2O Formation on Pt/La0.5Sr0.5Fe1-xMxO3-δ/Al2O3 (M = Nb, Ti, Zr; x = 0.25 or 0.5) during Lean Operation Conditions

The temperature-dependent NOx storage capacity investigations were carried out up to the NOx saturation of the catalysts in order to determine not only the maximum storage capacity of the materials but also any possible side reactions, e.g., N2O or NH3 formation. Figure 1 shows the detected temperature-dependent N2O formation curves for 2.5 wt.% Pt/20 wt.% La0.5Sr0.5Fe0.5Ti0.5O3/Al2O3 (LSFT_Pt), 2.5 wt.% Pt/20 wt.% La0.5Sr0.5Fe0.75Nb0.25O3/Al2O3 (LSFN_Pt) and 2.5 wt.% Pt/20 wt.% La0.5Sr0.5Fe0.5Zr0.5O3/Al2O3 (LSFZ_Pt) along with that for the reference catalyst 2.5 wt.% Pt/20 wt.% BaO/Al2O3 (BaO_Pt). The general curve trend is that a profound increase in the amount of N2O formed at low temperature (up to ~19 to 25 ppm at 250 °C), is followed by a profound decline at temperatures above 350 °C to an almost zero level. These results demonstrate similar behaviour for the perovskite-type oxide-based infiltration composites as is known for other (earth-) alkaline-based LNT catalysts [9,11,16,45]. Nevertheless, a small difference is found between the performance of LSFN_Pt and that of the other catalysts. The Nb-containing catalyst displays a much lower activity as significantly lower N2O formation values are found in the temperature range of 250–300 °C when compared to values found for the other catalysts.
To examine the impact of reductive gases CO and C3H6 on the N2O formation in more detail, additional temperature-dependent NOx storage measurements were carried out under different synthetic exhaust gas compositions (Table 1). These experiments were conducted only for the LSFZ_Pt. Corresponding results are given in Figure 2.
Whereas the exhaust gas mixture without CO (labelled w/o_CO) shows a curve progression analogous to that of the lean gas mixture (i.e., with CO and C3H6), the feed gas without propylene (labelled w/o_C3H6) shows a completely temperature-independent behaviour with a constant, almost negligible N2O formation of around 1–2 ppm. The results clearly reveal a N2O evolution dependency on the presence of propylene only, and so do verify a propylene-SCR mechanism. Furthermore, when comparing the progression curves of the lean exhaust gas and that without CO, a higher N2O evolution at 200 °C and 250 °C is observed for the latter. These results suggest that CO counteracts the undesired N2O formation mechanism at low temperatures.

2.2. Impact of Rh on the NOx Storage Capacity and N2O Evolution of the Infiltration Composites

Rhodium catalysts are most effective in reducing N2O emissions due to their excellent N2O decomposition properties at temperatures above 300 °C [34,35,36]. However, Hamada [29] observed a slight C3H6-SCR activity for Rh/Al2O3 in a net oxidising atmosphere already at 200 and 250 °C. Due to this activity, the addition of rhodium to the platinised infiltration composites (2.5 wt.% Pt/20 wt.% La0.5Sr0.5Fe1-xMxO3-δ/Al2O3) seems to be a possibility to reduce N2O emission at 250 °C, corresponding to the temperature of maximal N2O formation, by a more selective catalytic reduction reaction (see Figure 1).
Since Abduhlhamid et al. [20] reported a low catalytic activity of Rh in NO oxidation, a combined Pt-Rh coating on the infiltration composites was used to maintain a good NSC, while decreasing the N2O evolution. According to the commonly commercially used Pt to Rh ratio, the Rh amount was set to 0.125 wt.%, leading to a Pt-Rh ratio of 1:0.05. Figure 3 compares obtained data for the NSC and N2O formation of as-prepared LSFT_Pt and LSFT_PtRh. Starting with a comparison of the NSC (Figure 3A) of the bare Pt-coated and that of mixed Pt-Rh coated samples, a general shift of the adsorption curve by 50 °C towards higher temperatures occurs due to the addition of 0.125 wt.% Rh. As a result, the Pt-Rh combination shows a higher NSC than bare Pt in the range of 300–450 °C. Additionally, the maximum storage temperature increases from 142 µmol/g for LSFT_Pt (250 °C) to 151 µmol/g for LSFT_PtRh (300 °C).The expectation that N2O formation for LSFT_PtRh is decreased with slightly increasing the NSC is refuted by the results shown in Figure 3B. Here it can be seen that with additional Rh coating, a higher N2O formation over the entire temperature range occurs for LSFT_PtRh. While the average amounts of N2O formed in the moderate to high-temperature range continue to converge, ending at values of 1–3 ppm, the difference in the low-temperature range is substantial. While the difference is about 15 ppm at 200 °C, it is reduced to 2 ppm at temperatures above 250 °C.

2.3. Performance Study of a Mixed Composition of 80 vol.% LNT (Bottom) and 20 vol.% Oxidation Catalyst (Top)

The second approach to avoid N2O evolution deals with the oxidation of propylene before it reaches the LNT catalyst. Here, the choice for the oxidation catalyst was set to 2 wt.% Pd/20 wt.% CeO2/Al2O3 (Ce_Pd) as this composition has advantageous properties for both oxidation of propylene and CO. Next to the outstanding and well-established oxidation properties of Pd, CeO2 is expected to promote the oxidation ability of the catalyst due to the easy change between Ce3+ and Ce4+ oxidation states [46,47]. Furthermore, former studies on combined Pd and CeO2-based oxidation catalysts showed a more homogeneously dispersed palladium coating and thus sinter stable Pd particles [43] and a performance shift to lower temperatures depending on the coated Pd amount [41].
In this experiment first, the activity of the Ce_Pd catalyst composition for CO and C3H6 oxidation has been investigated in separate (w/o_CO and w/o_C3H6) and co-feed (full lean gas mixture; see Table 1) experiments and the resulting light-off curves are shown in Figure 4. The CO oxidation occurs in the temperature range of 70–185 °C for the separate (w/o_C3H6) feed mixture and the propylene oxidation is observed between 185 and 280 °C for the feed gas mixture without CO. With these feed conditions the T50 values are estimated to be 157 and 252 °C for CO and propylene, respectively. Due to the co-feed of 500 ppm CO and 200 ppm C3H6, the light-off curves of both reducing gases have converged and show similar start and end temperatures for the oxidation curves. In principle, the light-off curve of CO was shifted on average by 40 °C to higher temperatures (T50(CO) = 191 °C), while the C3H6 light-off curve was shifted by around 20 °C to lower temperatures (T50(C3H6) = 232 °C) than in the separate feed experiment. With that, the results of the full lean gas feed experiment indicate a kind of promoting role of CO for C3H6 oxidation and an improved performance in the low-temperature range of 200–300 °C.
In the following, the reactor filling changed to a layered two component system. Therefore, 2 mL (20 vol.%) Ce_Pd was fixed on top of 8 mL NOx storage active infiltration composite (see Figure 5A), keeping the reactor filling constant at a total filling volume of 10 mL, to effectively change the gas mixture by the conversion of propylene and CO to CO2 and water before entering the LNT catalyst (80 vol.%).
Figure 5B reveals the temperature-dependent NSC for the single LSFZ_Pt sample and the layered Ce_Pd-LSFZ_Pt system. The comparison shows that the layered catalyst system keeps the general Gaussian-type curve progression with a maximum at 250 °C as already observed for the pure LSFZ_Pt sample. The maximal performance for bare LNT and the layered catalyst system are similar, as the average values are 171 µmol/g and 176 µmol/g. However, by the addition of the oxidation catalyst the stored NOx amount is increased at temperatures above 250 °C until 400 °C, resulting in a performance enhancement of 15–22%. Due to the increase an almost equal performance at 300 °C (169 µmol/g) than for 250 °C occurs thereby broadening the high-performance window of the LNT catalyst. In conjunction with the increased NOx storage capacity, a decrease in the formed N2O amount can be observed in Figure 5C due to the addition of the oxidation catalyst. The decrease occurs in the range of 200–350 °C and accounts for between 60% (200 °C) and 13% at the maximum evolution temperature.

3. Discussion

3.1. Catalyst Characterisation

The infiltration composites Pt/La0.5Sr0.5Fe0.5Ti0.5O3-δ/Al2O3 (LSFT_Pt), Pt/La0.5Sr0.5Fe0.75Nb0.25O3-δ/Al2O3 (LSFN_Pt) and Pt/La0.5Sr0.5Fe0.5Zr0.5O3-δ/Al2O3 (LSFZ_Pt) were already introduced as potential LNT materials in a previous work and extensively characterised by BET, X-ray diffraction (XRD), selected area electron diffraction (SAED) and transmission electron microscopy (TEM) [44]. The analysis of the specific surface areas (SSA) of the infiltration composites showed that with the use of the incipient wetness impregnation method the loss of the SSA, starting from γ-Al2O3, could be kept low despite the multi-step synthesis route. As a result, an SSA between 99 and 106 m2/g was measured for the three nanocomposite compositions at a perovskite loading of 20 wt.% (equivalent prepared Pt/BaO/Al2O3 displayed an SSA of 80 m2/g). The varying specific surface areas depending on the substituting B-site cations in the perovskites could be explained by the different crystallisation behaviour due to the material composition. SAED measurements performed on the freshly prepared samples showed that the degree of crystallisation of the perovskites decreased in the order Ti > Nb > Zr, thus contradicting the decreasing order of the SSA. Furthermore, the successful in situ crystallisation of the expected perovskite compositions could be confirmed by additional ACOM-TEM images besides the SAED measurements. The results identified the infiltration composites as an ultrafine, uniform mixture of platinum, perovskite and alumina particles.
As the titanium-containing infiltration composite has shown the best crystallisation behaviour and NSC of all investigated infiltration composite mixtures, this material composition was chosen to investigate the impact of the additional 0.125 wt.% Rh coating. The resulting Pt/Rh/La0.5Sr0.5Fe0.5Ti0.5O3-δ/Al2O3 (LSFT_PtRh) specimen shows the same well-matched perovskite composition due to the synthesis procedure using a perovskite precursor solution and has an SSA of 104 m2/g. Thus, the impact of the Rh-coating is marginal in the case of the morphological material properties.

3.2. N2O Formation Investigations

Figure 1 shows the N2O formation on perovskite-based lean NOx trap (LNT) catalysts, and Pt/BaO/Al2O3 as a commonly known reference catalyst, under lean operation conditions as a function of temperature. Even though all examined materials show a similar Gaussian-type N2O formation curve, a slight difference between LSFN_Pt and the other specimen can be observed. A comparable behaviour pattern was also observed in the NSC experiments [44]. Assuming that the weaker NSC and N2O evolutions depend on the perovskite composition, this behaviour could be due to a slowed ad-species spill-over from the precious metal to the storage material. Hodjati et al. [48] postulated a reversible opening mechanism for the storage of NOx on ABO3 perovskites (A = Ba, Ca, Sr; B = Sn, Ti, Zr) according to Equations (6) and (7).
BaSnO3 + 3 NO2 → Ba(NO3)2 + SnO2 + NO
Ba(NO3)2 + SnO2 → BaSnO3 + 2 NO2 + 0.5 O2
In this case, the incorporation of the niobium cations may have led to a more stable perovskite lattice compared to Ti4+ (0.74 Å) [49] and Zr4+ (0.86 Å) [49] due to the equal ionic radii of Fe3+ (0.78 Å) [49] and Nb5+ (0.78Å) [49]. The high redox stability of niobium oxides at temperatures below 900–1000 °C discussed by Gervasini [50] could also contribute to a lower crystal lattice opening probability. Under this assumption, the Nb-containing perovskite would be less reactive with respect to NOx uptake and the ad-species would have to dwell longer on the platinum, thus inhibiting the overall activity of the precious metal with respect to NOx storage and the N2O forming side reaction.
By a following survey concerning different feed gas compositions (Figure 2), the underlying N2O formation mechanism for our infiltration composites was clearly identified as a C3H6-SCR. Interestingly, a kind of inhibiting effect is observed, when CO and C3H6 are dosed together. The resulting lower N2O evolution at 200 and 250 °C might be explained by an increased availability of active (reduced) Pt sites due to an additional PGMOx to PGM0 transformation by CO leading to more likely formed N*- and O*- ad-species and so a lower N2O formation (see Equations (3) and (5)).
To the best of our knowledge, only Masdrag et al. [23] have so far investigated the influence of different reducing agents on N2O formation on an LNT catalyst (Pt/CeO2-ZrO2) in lean operation. In their study, they defined a temperature-dependent reductive influence with CO > C3H6 ≈ 0 at 200 °C and C3H6 > CO ≈ 0 at 300 °C in lean operation mode. [23].
Hence, the catalyst investigations in our study and in the study of Masdrag et al. were conducted entirely in a net oxidising atmosphere (see Table 1), the Pt/La0.5Sr0.5FexM1−xO3−δ/Al2O3 (M = Nb, Ti, Zr) infiltration composites show a pronounced N2O formation by the C3H6-SCR reaction at 200 °C (see Figure 1), while Pt/CeO2-ZrO2 was completely inactive (reference [23], Figure 7A,B). It seems that the Pt particles (2.5 wt.%) in the composites can be reduced more effectively by C3H6 at low temperatures and O2 excess than in the LNT catalyst studied by Masdrag et al. (2.12 wt.% Pt, CeO2 content unknown) [23]. This difference can most likely be explained by the influence of the support materials and/or the Pt particle sizes (Pt particle size Masdrag et al.: 6.2 nm, this study: 2–4 nm).
Since the focus of C3H6-SCR catalyst studies is mostly on NOx conversion and N2 selectivity, no literature is known at this time that explicitly addresses the undesired N2O formation as a function of different support materials and/or PGM particle size.

3.3. Attempts to Reduce N2O Evolution

The first attempt to reduce the N2O formation on LNT catalysts under lean conditions was the usage of an additional Rh coating on the infiltration composites. By the addition of 0.125 wt.% Rh the NOx storage capacity above 300 °C is slightly improved (Figure 3A). Though an auxiliary effect of the Rh coating at higher temperatures seems to be plausible, as the studies of Kubiak et al. [51], Andonova et al. [52] and Castoldi et al. [53] showed appreciable NO oxidation abilities and NOx storage capacities for Rh-based BaO/Al2O3 catalysts above 350 °C. Figure 3B compares the N2O evolution on LSFT_Pt and LSFT_PtRh. The observed N2O formation increase caused by the Rh addition could be related to an interplay between the generally high light-off temperatures for NO oxidation and N2O decomposition of Rh-based catalysts on the one hand, and the slight NOx adsorption capacity at low temperatures proceeding via the nitrite route on the other hand. The latter usually proceeds by a subsequent transfer of activated oxygen from the PGM to the storage material, allowing the adsorption of NO in the form of nitrite adsorbents. Consequently, Kubiak et al. [51] observed a low NOx adsorption of 0.106 mmol/g for Rh/BaO/Al2O3 at 150 C in 1000 ppm NO + 3 vol.% O2 balanced with helium. Due to the feed gas composition used here, which contains reducing agents during the lean phase (propylene and CO), it can be assumed that the nitrite pathway is inhibited by the partial reduction of the noble metal sites. Thus, the available amount of activated oxygen would decrease and consequently lead to the recombination of the weakly bound NO-ad species at the PGM and storage material sites and the formation of N2O. However, the dramatic increase of about 500% of evolving N2O at 200 °C cannot only be related to additional N2O formation over 0.125 wt.% Rh, hence indicating a strong impeding interaction between the Rh and Pt concerning NO oxidation at low temperatures in the presence of reductives.
The second attempt deals with the use of a layered oxidation and LNT (ratio 1:4) catalyst as illustrated in Figure 5A. The chosen oxidation catalyst 2 wt.% Pd/20 wt.% CeO2/Al2O3 (Ce_Pd) showed a promising oxidation ability for propylene and CO in separate and co-feed experiments (Figure 4), thus seeming able to suppress N2O production via SCR reaction. In general, Ce_Pd showed a similar performance compared to the results of Shen et al. [41] and Burch and Millington [30], who observed a complete activation of their investigated Pd/CeO2-ZrO2 and Pd/Al2O3 catalysts for propylene oxidation in a temperature range of around 125 °C (T50 = 203 °C) and 100 °C (T50 = 250 °C). The oxidation activity for both- CO and C3H6- raises from 0 to 100% by a temperature increase of around 100 °C in all feed gas experiments. Interestingly, due to the co-feed of CO and propylene, a convergence of the light-off curves (LO) was observed, shifting the CO-LO to higher and the C3H6-LO to lower temperatures. Similar experiments on Pt/Al2O3 [54,55,56] or Pt/CeO2 [57] oxidation catalysts have shown that under co-feed conditions both LO curves commonly shift to higher temperatures as the two reducing gases compete for the active sites and thus inhibit the oxidation of the other gas. Only Lang et al. [58] observed a similar behaviour for 1 wt.% Pd/CeO2-ZrO2 as for the herein investigated 2 wt.% Pd/20 wt.% CeO2/Al2O3 with a more pronounced approximation of the light-off curves for CO and propylene (T50(CO) shift from 164 to 229 °C and T50(C3H6) shift from 226 to 232 °C) and an improved C3H6 conversion at temperatures above ~240 °C (T85(C3H6) ≈ 261 °C shifts to ~250 °C) due to the CO and propylene co-feed. Lang et al. [58] explained this behaviour by a possible alternating oxidation mechanism and/or lower activation barriers due to the available ceria compared to alumina-based catalysts (CO reaction order Pd/Al2O3: 1st order and Pd/CeO2-ZrO2: 0th order). Related to the mentioned literature, we presume a combination of both theories to be the reason for the observed curve shifts due to the CO and C3H6 co-feed. The observed shift of the CO-LO to higher temperatures seems to be caused by the general inhibition of the CO oxidation by competing propylene molecules and/or oxidation intermediates for the active catalyst sites. Instead, the promotion of the C3H6 oxidation by co-dosed CO might be explained by either an alternating oxidation mechanism dependent on the Pt supporting oxides or by more available Pt0 sites due to the prior or simultaneously elapsing CO oxidation consuming the oxygen species bound on the precious metal (PtOx) [56]. Hazlett et al. [56] already observed that the addition of propylene to the feed gas resulted in a strong increase in multiple precious metal-bound surface species (triple bound adsorbates, carbonyl and dicarbonyl species) on Pt-and Pd-based alumina catalysts competing with the single precious metal-bound CO. In this case, C3H6 needs, on average, three times the amount of available active catalyst sites (Pt0) compared to CO for the oxidation reaction, which is commonly described as a Langmuir-Hinshelwood mechanism using surface co-adsorbed reactants [54]. However, the results in Figure 4 reveal that the catalyst offers the necessary oxidation activity in the desired temperature range of 200–500 °C to be used as an oxidation catalyst top layer for our approach to reduce C3H6 concentrations in the feed gas of the LNT catalyst. The resulting NOx storage and N2O evolution curves for the layered system are compared to the bare LNT material (LSFZ_Pt) in Figure 5B,C. Due to the addition of Ce_Pd an increase of 15–22% of the NOx storage capacity (NSC) in the temperature range of 250–400 °C is obtained. In parallel to the NSC enhancement, a N2O formation decrease can be observed at 200 to 350 °C. The results thus show a N2O evolution diminishment of 13–60% in the general N2O formation window. Consequently, an oxidation catalyst layered on top of the LNT seems to be a suitable means to suppress N2O formation during the lean storage phase of the LNT due to a competing propylene-SCR. Nevertheless, nitrous oxide emissions were not reduced to a zero level which might be caused by the chosen configuration with only a small oxidation catalyst layer on top of the LNT materials. To enhance the effect on NSC and N2O evolution, the optimal thickness of the oxidation catalyst has to be found.

4. Materials and Methods

4.1. Lean NOx Trap Catalyst Preparation

All investigated LNT storage materials were synthesised by a four-step synthesis pathway using incipient wet impregnation (IWI) processes ending up with a milling and granule formation step to give the materials the necessary geometry for the NOx adsorption tests. A detailed description of each single preparation step can be found elsewhere [44].
In brief, firstly a citric acid stabilised aqueous perovskite precursor solution for each of the three investigated perovskite compositions La0.5Sr0.5Fe0.75Nb0.25O3, La0.5Sr0.5Fe0.5Ti0.5O3 and La0.5Sr0.5Fe0.5Zr0.5O3 was prepared. Therefore, the starting materials La(NO3)3·6H2O (Alfa Aesar, Karlsruhe, Germany, 99.9%), Sr(NO3) (Alfa Aesar, Karlsruhe, Germany, ≥99.0%), Fe(NO3)3·9H2O (Alfa Aesar, Karlsruhe, Germany, ≥98.0%) were dissolved in distilled water and stabilised with citric acid (VWR chemicals, Pensylvannia, USA, metal:citric acid 1:3). The varying fourth ingredient was added as a highly concentrated (around 5 wt.% transition metal load) complex solution, gained from titanium(IV) iso-propoxides (Alfa Aesar, Karlsruhe, Germany), zirconium(IV) n-propoxide in n-propanole (70 wt.%, Alfa Aesar, Karlsruhe, Germany) or ammonium niobate(V) oxalate hydrates (Sigma Aldrich, Taufkirchen, Germany) complexed with citric acid, to yield a perovskite precursor solution of either 1:1:1:1 or 1:1:1.5:0.5 stoichiometry. Secondly, a commercial alumina powder (Puralox TH 100/150/L4, Sasol, Hamburg, Germany) was infiltrated via IWI with the precursor solutions resulting in perovskite-alumina composites after subsequent thermal treatment. Exemplarily, 100 g of the two-component composite with a 20 wt.% Ti containing perovskite loading were prepared by the infiltration of 120 mL precursor solution into 80 g alumina. After calcination of the overnight-dried perovskite-alumina composites (100 °C/h, 700 °C, 5 h), additional 2.5 wt.% platinum was coated on the material via IWI using an aqueous solution of H2Pt(OH)6 (abcr GmbH, Karlsruhe, Germany) and (CH3)4NOH·5H2O (Alfa Aesar, Karlsruhe, Germany) in a proportion of 1:2. The coated infiltration composite powders were dried at 120 °C and finally calcined at 550 °C for 1 h (175 °C/h). After milling, granules in fractions of 1.5 to 2.5 mm were formed. The resulting catalysts were abbreviated with LSFT, LSFN or LSFZ related to the perovskite-type oxide constructing elements followed by the platinum element symbol. Exemplarily, 2.5 wt.% platinum on 20 wt.% La0.5Sr0.5Fe0.5Ti0.5O3−δ/Al2O3 is abbreviated as LSFT_Pt.
The platinised titanium perovskite-type oxide-based catalyst was further impregnated with 0.125 wt.% rhodium by another IWI coating step. Therefore 50 g of the LSFT_Pt composite was infiltrated with a solution of 0.174 g Rh(NO3)3 hydrate (36 wt.% Rh content, Merck, Darmstadt, Germany) dissolved in 60 mL distilled water. The wet powder was dried at 140 °C and calcined at 550 °C for 1 h using a heating rate of 175 °C/h. Afterwards, an analogous milling and granulation procedure as for the Pt only containing infiltration composites was carried out to examine their NSC and N2O evolution at lean conditions. The composite 0.125 wt.% Rh-2.5 wt.% Pt on 20 wt.% La0.5Sr0.5Fe0.5Ti0.5O3−δ/Al2O3, was abbreviated as before but with the additional element symbol of rhodium resulting in LSFT_PtRh.
Additionally, a comparable 2.5 wt.% Pt/20 wt.% BaO/Al2O3 sample, condensed as BaO_Pt, was synthesised by IWI. A 100 g synthesis batch was prepared by impregnating 80 g alumina (Puralox TH 100/150/L4, Sasol, Hamburg, Germany) with an aqueous barium solution. For the latter 33.32 g Ba(CH3COO)2 were dissolved in 120 mL distilled water. After impregnation and homogenisation, the wet powder was dried at 120 °C overnight and calcined at 700 °C for 3 h. Similar to all former specimen, the composite powder was additionally coated with 2.5 wt.% Pt, grounded and formed to granules.

4.2. Oxidation Catalyst Preparation

50 g of the used 2 wt.% Pd/20 wt.% CeO2/Al2O3 oxidation catalyst was prepared via incipient wetness impregnation. First, 28 g of an aqueous (NH3)4Pd(NO3)2 solution (10 wt.% Pd content, Alfa Aesar, Karlsruhe, Germany) were further diluted with 32 mL distilled water. The created solution was infiltrated on 49 g of commercial 20 wt.% ceria containing alumina powder (Puralox SCFa-160/Ce20, Sasol, Hamburg, Germany). After drying at 140 °C overnight, a final calcination was performed at 550 °C for 1 h using a heating rate of 175 °C/h. The catalyst powder was milled down and granulated in the way as described before. The catalyst is abbreviated as Ce_Pd.

4.3. NOx Adsorption and N2O Evolution Measurements as a Function of Temperature

Temperature-dependent NOx adsorption measurements were performed in a laboratory exhaust gas test bench under constant lean conditions representing the exhaust gas composition on a lean burn engine at a lambda situation of λ = 1.5. In general, this was realised by the lean gas mixture (see Table 1) and a gas hourly space velocity set to 80,000 h−1. In order to examine the dependence of the N2O formation on the dosed reductives, additional NOx storage measurements with altered gas compositions, either without CO (w/o_CO) or without propylene (w/o_C3H6) were dosed (see Table 1). A more detailed description of the used test bench could be found elsewhere [44]. In brief, all gas concentrations were controlled by dynamic gas flow controller (MFC) of type 5850 from Brooks Instrument, LLC (Hatfield, PA, USA). The inert carrier gas stream of N2, O2 and CO2 was heated up to 200 °C before the water vapour was added, dosed by a gear type pump from GATHER Industrie (Wülfrath, Germany) and vaporised by an evaporator from LINSEIS Messgeräte (Selb, Germany). The gas mixture was passed through a furnace (DLERH, Horst, Lorsch, Germany) and heated to the respective adsorption temperature (200–450 °C in 50 °C steps) in advance. The further gas flow towards the reactor with the sample was carried out in heated pipes and shortly before entering the reactor the reactive gases NO, CO and propylene were mixed into the carrier gas stream. To guarantee a constant temperature during the measurement, the titanium reactor was wrapped with a heat jacket (Horst, Lorsch, Germany) and the temperature was controlled by thermocouples (NiCrNi) at two points in the gas stream. The first thermocouple was placed before the reactor entrance and the second directly within the sample. The sample holder was filled with 10 mL of granules of a sole infiltration composite (see Figure 6) or a top-layered 20 vol.% oxidation catalyst on 80 vol.% infiltration composite setting (see Figure 5A). In both cases, the catalysts were covered up- and downstream with 100 mg quartz wool and a titanium sieve. The outlet gas composition was analysed using a Fourier-transformed infrared spectrometer (FT-IR) of the type MultiGas 2030 from MKS instruments to monitor the formation of C- and N-containing components (e.g., NO, NO2, N2O, propylene, CO) during the measurements.
The NOx storage capacity was measured between 200 and 450 °C in 50 °C steps by saturating the sample with NOx. To control the gas composition before and after this adsorption phase, the gas mixture was switched via solenoid valves through a bypass, which directed the gas around the reactor. Afterwards, the stored NOx was desorbed by heating up the sample to 550 °C within the same gas mixture but without NO dosing. Finally, the specific molar storage capacity was calculated from the NOx desorption peak.

4.4. Propylene and CO Oxidation Measurements on 2 wt.% Pd/20 wt.% CeO2/Al2O3 as a Function of Temperature

The measurements of the propylene and CO oxidising ability of the Ce20_Pd catalyst were carried out on the same laboratory gas test bench as the NOx storage measurements (see Figure 6) at a gas hourly space velocity of 80,000 h−1 with three different feed gas compositions shown in Table 1. For the measurements, 2 mL of the catalyst was filled into the reactor and the gas mixtures were constantly passed through the sample. For propylene the temperature range of 165–295 °C and for CO the range between 60 and 215 °C was investigated. The heating rate in all cases was 2 °C/min. The cycles were repeated three times each. The resulting conversion curves are calculated with the following equations:
CO conversion [%] = (COinlet − COoutlet)/COinlet × 100
C3H6 conversion [%] = (C3H6inlet − C3H6outlet)/C3H6inlet × 100

5. Conclusions

This study focuses on the undesirable N2O formation on newly developed Pt/La0.5Sr0.5Fe1−xMxO3/Al2O3 (M = Nb, Ti, Zr) lean NOx trap (LNT) catalysts under laboratory gas test bench conditions. In addition to the investigations on the formation mechanism of the undesired by-product, which caused a decreased NOx storage capacity (NSC) for the LNT materials, attempts were made to reduce these emissions. To this end, two approaches were pursued, firstly an additive Rh coating on the LNT catalyst and secondly an oxidation catalyst-LNT layered system.
The mechanism of N2O evolution was identified as a low-temperature C3H6-SCR. Nitrous oxide formation occurred due to the competing partial reduction of PtOx to Pt0 by C3H6 during the general NOx oxidation and storage procedure on LNT catalysts in net oxidising atmosphere. The investigations showed that even low concentrations of propylene (200 ppm), as generally present in lean exhaust gases, lead to high N2O formation of 20 ppm on average at low temperatures.
The additional Rh coating on the Pt/La0.5Sr0.5Fe0.5Ti0.5O3/Al2O3 nano composite resulted in a dramatic increase in N2O formation in the range 200–250 °C. The comparison with the platinised nano composite sample leads to the assumption that this result can only be related to an additional N2O formation at the Rh particles. It seems that the Rh nitrites route for NOx storage is prevented by Pt0 and Rh0 particles. Thus the necessary oxygen spill-over is omitted causing a recombination of NO* ad-species. Although the additional Rh coating is unsuitable for the reduction of N2O formation, it should be mentioned that a slightly better NSC for the Pt and Rh coated sample was observed in the range of 300–450 °C.
In contrast, the catalyst installation with an oxidation catalyst connected upstream of the LNT showed promising results. With a replacement of 20 vol.% of the LNT by the oxidation catalyst (2 wt.% Pd/20 wt.% CeO2/Al2O3), 13–60% less N2O emissions were measured in the low temperature range. The results thus show that the oxidation of the propylene is the most effective way to reduce N2O formation. Nevertheless, nitrous oxide formation could not yet be completely suppressed, which may be due to the catalyst configuration with a low oxidation catalyst content. To further improve the effectiveness of the oxidation catalyst, the palladium content and the layer thickness have to be optimised.

Author Contributions

The conceptualization was done by J.D. and S.I.E. All investigations were carried out by S.I.E. The necessary resources were provided by O.G. Supervision was carried out by J.D.; S.B.; H.J.M.B. and W.A.M. The original draft was prepared by S.I.E. and J.D.; S.B.; O.G.; H.J.M.B. and W.A.M. contributed to the writing-review and editing process. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the German Federal Ministry of Education and Research, Grant-No.: 13XP5042B.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the complexity of the analysis which needs guidance for reproduction.

Acknowledgments

Reprints from Ecker, S.I.; Dornseiffer, J.; Werner, J.; Schlenz, H.; Sohn, Y.J.; Sauerwein, F.S.; Baumann, S.; Bouwmeester, H.J.M.B.; Guillon, O.; Weirich, T.E.; Meulenberg, W.A. Novel low-temperature lean NOx storage materials based on La0.5Sr0.5Fe1-xMxO3-δ/Al2O3 infiltration composites (M = Ti, Zr, Nb). Appl. Catal. B Environ. 2021, 286, 119919–119929 with permission from Elsevier.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Temperature dependence of N2O formation during the adsorption phase at lean operation conditions for different Pt-coated perovskite-based LNT catalysts. Data for the reference catalyst 2.5 wt.% Pt/20 wt.% BaO/Al2O3 (BaO_Pt) are given for comparison.
Figure 1. Temperature dependence of N2O formation during the adsorption phase at lean operation conditions for different Pt-coated perovskite-based LNT catalysts. Data for the reference catalyst 2.5 wt.% Pt/20 wt.% BaO/Al2O3 (BaO_Pt) are given for comparison.
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Figure 2. Temperature dependence of N2O formation over LSFZ_Pt in different synthetic exhaust gas compositions. Feed gas compositions are listed in Table 1.
Figure 2. Temperature dependence of N2O formation over LSFZ_Pt in different synthetic exhaust gas compositions. Feed gas compositions are listed in Table 1.
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Figure 3. Temperature dependence of the (A) NOx storage capacity and (B) N2O formation of LSFT_PtRh. Data for LSFT_Pt from [44] also shown for comparison.
Figure 3. Temperature dependence of the (A) NOx storage capacity and (B) N2O formation of LSFT_PtRh. Data for LSFT_Pt from [44] also shown for comparison.
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Figure 4. Separate CO and propylene (filled symbols) and full lean feed (CO + C3H6; open symbols) oxidation measurement data as a function of temperature using 2 wt.% Pd/20 wt.% CeO2/Al2O3 (Ce_Pd) as oxidation catalyst. The compositions of the feed gas mixtures are given in Table 1.
Figure 4. Separate CO and propylene (filled symbols) and full lean feed (CO + C3H6; open symbols) oxidation measurement data as a function of temperature using 2 wt.% Pd/20 wt.% CeO2/Al2O3 (Ce_Pd) as oxidation catalyst. The compositions of the feed gas mixtures are given in Table 1.
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Figure 5. (A) Scheme of the filling constitution for the top-layered oxidation catalyst-LNT combination. Comparison of the temperature dependent NOx storage capacity (B) and N2O formation (C) for LSFZ_Pt (▲, from reference [44]) and a catalyst combination of 80 vol.% LSFZ_Pt covered with 20 vol.% of Ce_Pd (▼).
Figure 5. (A) Scheme of the filling constitution for the top-layered oxidation catalyst-LNT combination. Comparison of the temperature dependent NOx storage capacity (B) and N2O formation (C) for LSFZ_Pt (▲, from reference [44]) and a catalyst combination of 80 vol.% LSFZ_Pt covered with 20 vol.% of Ce_Pd (▼).
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Figure 6. Technical drawing of the test bench and the reactor filling during only LNT catalyst examination experiments.
Figure 6. Technical drawing of the test bench and the reactor filling during only LNT catalyst examination experiments.
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Table
Table 1. Exhaust gas compositions used in C3H6-SCR experiments.
Table 1. Exhaust gas compositions used in C3H6-SCR experiments.
Componentssynthetic Exhaust Gas Mixtures (Balanced with N2)
leanw/o_COw/o_C3H6
CO29 vol.%9 vol.%9 vol.%
H2O9 vol.%9 vol.%9 vol.%
O26.5 vol.%6.5 vol.%6.5 vol.%
CO500 ppm-500 ppm
C3H6200 ppm200 ppm-
NO500 ppm500 ppm500 ppm
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Ecker, S.I.; Dornseiffer, J.; Baumann, S.; Guillon, O.; Bouwmeester, H.J.M.; Meulenberg, W.A. Measures to Reduce the N2O Formation at Perovskite-Based Lean NOx Trap Catalysts under Lean Conditions. Catalysts 2021, 11, 917. https://doi.org/10.3390/catal11080917

AMA Style

Ecker SI, Dornseiffer J, Baumann S, Guillon O, Bouwmeester HJM, Meulenberg WA. Measures to Reduce the N2O Formation at Perovskite-Based Lean NOx Trap Catalysts under Lean Conditions. Catalysts. 2021; 11(8):917. https://doi.org/10.3390/catal11080917

Chicago/Turabian Style

Ecker, Sabrina I., Jürgen Dornseiffer, Stefan Baumann, Olivier Guillon, Henny J. M. Bouwmeester, and Wilhelm A. Meulenberg. 2021. "Measures to Reduce the N2O Formation at Perovskite-Based Lean NOx Trap Catalysts under Lean Conditions" Catalysts 11, no. 8: 917. https://doi.org/10.3390/catal11080917

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