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Article

Modified BaMnO3-Based Catalysts for Gasoline Particle Filters (GPF): A Preliminary Study

by
Verónica Torregrosa-Rivero
*,
María-Salvadora Sánchez-Adsuar
and
María-José Illán-Gómez
Carbon Materials and Environment Research Group, Department of Inorganic Chemistry, Faculty of Science, University of Alicante San Vicente del Raspeig, 03690 Alicante, Spain
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(11), 1325; https://doi.org/10.3390/catal12111325
Submission received: 30 September 2022 / Revised: 23 October 2022 / Accepted: 25 October 2022 / Published: 28 October 2022
(This article belongs to the Special Issue Novel and Commercially Applicable Environmental Catalysts and Photocatalysts)
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Versions Notes

Abstract

:
Gasoline engines, mainly gasoline direct injection engines (GDI) require, in addition to three-way catalysts (TWC), a new catalytic system to remove the formed soot. Gasoline Particle Filters (GPF) are, among others, a possible solution. BaMnO3 and copper-doped BaMnO3 perovskites seem to be a feasible alternative to current catalysts for GPF. The physical and chemical properties of these two perovskites determining the catalytic performance have been modified using different synthesis routes: (i) sol-gel, (ii) modified sol-gel and iii) hydrothermal. The deep characterization allows concluding that: (i) all samples present a perovskite-like structure (hexagonal), except BMC3 which shows a polytype one (due to the distortion caused by copper insertion in the lattice), and ii) when a low calcination temperature is used during synthesis, the sintering effect decreases and the textural properties, the reducibility and the oxygen mobility are improved. The study of soot oxidation simulating the hardest GDI scenarios reveals that, as for diesel soot removal, the best catalytic performance involves the presence of oxygen vacancies to adsorb and activate oxygen and a labile Mn (IV)/Mn (III) redox pair to dissociate the adsorbed oxygen. The combination of both properties allows the transport of the dissociated oxygen towards the soot.

1. Introduction

Currently, perovskite oxides (ABO3) are being studied as a promising alternative to PGM-based catalysts for automotive exhaust abatement, due to their high thermal stability, low cost and tunable redox properties mainly achieved by the control of the composition [1,2,3,4,5,6,7]. In several studies focused on developing new strategies to improve the physical, chemical, and catalytic properties of these solids, the following variables are analyzed:
*
The combination of different A and B cations, since almost the 90% of the elements can be stabilized in a mixed oxide with perovskite-like structure [8,9].
*
The partial substitution of A and/or B cations, which allows achieving non-usual oxidation states for the B cation and/or generating oxide ion vacancies [10,11,12,13].
*
The use of perovskite as substrates [1,14,15,16,17] or supporting the perovskite [18,19,20,21,22,23,24].
*
The increase in the surface area by using different synthesis methods: solid-state, sol-gel, reactive grinding and hydrothermal, among others [15,25,26,27,28,29,30,31,32].
It is well-known that manganites (AMnO3) are useful for oxidation applications because of their redox properties due to the electronic configuration of manganese, with oxidation states of (III) and (IV) [11]. Moreover, Mn (III) is affected by the Jahn–Teller effect, a distortion to remove the degeneration due to asymmetrically filled d orbitals achieving lower energy states and modifying the crystal structure [33,34,35], but also generates some structural defects [11]. The coexistence of both oxidation states, especially an enriched Mn (IV) surface, improves the oxygen mobility, enhancing the catalytic activity for soot oxidation [36]. It is also remarkable that manganese is abundant, and then, low cost. On the other hand, copper-doped perovskites, such as LaCo1−xCuxO3 [2], LaFe1−xCuxO3 [3], LaNi0.5Cu0.5O3 [4] or LaMn1−xCuxO3 [29,37], have shown promising results for catalytic oxidation processes. In fact, a remarkable synergistic effect between copper and manganese has been observed for these solids, which boosts the reducibility of both cations and allows the synthesis of bifunctional catalysts [29].
It has been widely proved that soot oxidation can be catalyzed by O2, by NO and by a mixture of both NO-O2 [27,38,39,40]. Nevertheless, the composition of a GDI exhaust is poor in NOx, with the fuel cuts (i.e., when the engine pumps air from the intake to the exhaust [41,42]) being the only source of oxygen, and most of this is used by the three-way catalyst (TWC). Therefore, a good catalyst for this application should be active to adsorb oxygen during the fuel cuts mode and release it in regular operating conditions. T. Boger et al. proved that the passive oxidation of soot could be performed at the high-temperature exhaust of GDI engines (400–700 °C) [41]; thus, the soot oxidation process occurs through different mechanisms as a function of the composition and temperature. Currently, GDI engines present a high fuel economy and reduced CO2 emission (compared to other gasoline engines), which positioned them as the most bought passenger car in 2017 [43]. However, due to the presence of a fuel-rich stage, leading to the incomplete combustion of the remaining fuel into the chamber, a large amount of small soot particles are formed that present a high health risk [42,43].
Perovskite-based catalysts could show a great oxygen storage capacity since O2 can be adsorbed in the oxygen vacancies [38]. Because of that, in this paper, the BaMnO3 solids have been used for soot oxidation under two different atmospheres: in an inert atmosphere (He), simulating the regular operation conditions of a GDI engine; and in a slight oxidant atmosphere (1% O2/He), which reproduces severe fuel cut operation conditions, but also it is the typical O2 concentration at the turbine-GDI engine exit, i.e., upstream of the TWC [44].
In previous studies, BaMnO3 and BaMn1−xCuxO3 mixed oxides with a perovskite-like structure, obtained by different synthesis procedures that provide specific chemical and physical properties, have shown interesting catalytic activity for NO to NO2 oxidation and for NOx-assisted soot oxidation [45,46,47]. In this work, these samples are tested as feasible catalysts for GPF systems. To improve the chemical and physical properties of the BaMnO3 catalyst, two approaches have been employed: (i) partial substitution of manganese by copper, obtaining solids with BaMn1−xCuxO3 composition in which x = 0, 0.1, 0.2, and 0.3 that are prepared by conventional sol-gel synthesis [45]; (ii) synthesis of BaMnO3 samples by modified sol-gel by using carbon black as a pore-forming material and three calcination temperatures (600 °C, 700 °C and 850 °C, the last one being equal to that used in conventional sol-gel synthesis) [46]; and by hydrothermal synthesis followed by a calcination step at 600 °C.

2. Results and Discussion

2.1. Characterization

The fresh catalysts were deeply characterized, the results being thoroughly discussed in previous published articles by the authors [45,46,47]. Therefore, in this section, a comparison of the most relevant properties of catalysts has been presented in order to correlate the chemical and physical properties to the catalytic performance for soot oxidation under GDI conditions. Table 1 gathers the main information about the composition synthesis procedure details (as calcination temperature) and the most relevant properties for each sample.

2.1.1. Copper Content (ICP-OES) and Textural Properties

Table 1 shows the nominal and experimental (obtained by ICP-OES analysis) copper content and the BET surface area of the BaMn1−xCuxO3 series and BaMnO3 catalysts obtained through different synthesis routes. The results reveal that the sol-gel procedure allowed the incorporation of the required copper amount for each catalyst, because the actual value was closer to the nominal one. On the other hand, as ceramic solids with poorly developed porosity [48,49,50], the BaMnO3 solids presented a Type IV isotherm (not shown) and low BET specific surface area. However, the synthesis procedure seemed to slightly modify the porosity, and the use of carbon black as pore-forming material allowed a slight increase in the BET surface value for BM-C600 and BM-C700. This is due to the lower calcination temperature used during their synthesis which decreased the sintering effects. On the contrary, the use of the hydrothermal method seemed to not affect the porosity development, because the BET surface area of BM-H was similar to that observed for BM.

2.1.2. Crystal Structure: XRD

The X-ray patterns, presented in Figure 1, show that both synthesis methods allowed the formation of a BaMnO3 perovskite-like structure as the main crystal phase. However, depending on the synthesis method and the copper amount, different minority phases were detected.
For the perovskites obtained through conventional sol-gel synthesis, the crystal phases detected depended on the amount of copper. BM, the sample without copper, presented as the main crystal phase the BaMnO3 hexagonal (PDF number: 026-0168), and for the BaMn1−xCuxO3 series, as the copper content increased, the hexagonal phase decreased in favor of the polytype crystal phase. This phase was formed because the insertion of copper into the perovskite lattice generated a distortion in the original hexagonal structure. For BMC3, the catalyst with the highest amount of copper, the BaMnO3 polytype phase, was the only perovskite structure, and low intense peaks of CuO were observed. The distortion of the structure was also observed in the lattice parameters, calculated for both perovskite phases, and is shown in Table 2.
All the catalysts obtained through the modified sol-gel synthesis showed the BaMnO3 hexagonal structure as the main crystal phase, and the calcination temperature determined the composition of the minority phases. BM-C600 presented some barium carbonate due to the low calcination temperature (600 °C) and, as the calcination temperature increased, the presence of barium carbonate decreased up to BM-C850, for which this crystal phase was not detected at all.
For the samples prepared using hydrothermal synthesis (BM-H), the presence of some barium carbonate was also detected due to the low temperature used during the synthesis process. Moreover, the lattice parameter values for this sample indicated a modification of the BaMnO3 hexagonal structure that could be related to the higher amount of Mn (IV) present in the perovskite structure of BM-H, which is formed to compensate for the positive charge imbalance due to its barium deficiency [51,52].
The average crystal size, calculated by the Scherrer equation and gathered in Table 2, showed the expected trend with the calcination temperature: it decreased with the calcination temperature. Therefore, BM-C600 showed the lowest average crystal size, with the highest values being observed for the samples calcined at 850 °C.

2.1.3. Surface Composition

In Figure 2, the spectra corresponding to the Mn 2p3/2, O 1s and Cu 3p3/2 transitions of catalysts are shown. Briefly, Figure 2a,b reveal the coexistence of two oxidation states of manganese: Mn (III), due to the signal over 645 eV assigned to a satellite peak of Mn (III), and the signal at c.a. 643 eV [53,54] corresponding to Mn (IV). The Mn (IV)/Mn (III) ratio, shown in Figure 3a, points out Mn (III) as the main oxidation state on the surface for all catalysts, since all of them are lower than 1. For the BaMn1−xCuxO3 series, the copper inserted into the perovskite lattice (which promotes the polytype phase) led to an increase in the amount of Mn (III); therefore, the electroneutrality must be achieved by the generation of oxygen vacancies. On the other hand, for the BM-CX samples, the amount of Mn (IV) increased with the calcination temperature, and also, some oxygen vacancies should be present on the surface to compensate for the deficit of positive charge due to the presence of Mn (III). For BM-H, a Mn (IV)/Mn (III) ratio higher than for BM was detected, as the positive charge imbalance due to the barium deficiency has to be compensated [51,52]; so, the ratio was close to 1, meaning that the amount of both oxidation states on the surface was almost the same.
Figure 2c,d compare the XPS spectra corresponding to the O1s transition, showing three different contributions assigned to lattice oxygen (OLattice, ca. 529 eV), oxygen from the surface groups (ca. 531 eV), and oxygen from moisture (ca. 533 eV) [55,56,57]. In these spectra, it is clearly observable that copper introduction promoted the generation of surface oxygen species, such as adsorbed surface oxygen and hydroxyl/carbonate species (corresponding to moisture), as the area corresponding to these species increased from BM to the BMC3 catalyst. Therefore, it seems that to compensate for the deficit of positive charge due to copper and to achieve the electroneutrality, a lower amount of oxygen exits on the surface.
In Figure 3a, the values of the OLattice/(Ba + Mn + Cu) ratio for all the catalysts are gathered. It reveals the presence of oxygen vacancies because the values are lower than the nominal one (1.5) [58] for the BM-CX series and for the copper-containing catalysts. The generation of oxygen vacancies is a mechanism to compensate for the positive charge defect due to the presence of Mn (III), Mn (III) and Cu (II) for copper-containing samples, and barium deficit for BM-H. Therefore, as was expected, as the copper amount increased, the OLattice/(Ba + Mn + Cu) ratio decreased. It was observed that the modified sol-gel and hydrothermal synthesis allowed an increase in the oxygen vacancies on the surface since the ratio was lower than for BM. The calcination temperature for the BM-CX samples did not play a significant role since no significant differences were observed.
Figure 2e shows the Cu2p3/2 transition spectra of the samples, in which the position of the main band over 933 eV and the shake-up satellite peaks (at c.a. 940 eV and 944 eV) indicate the presence of Cu (II) [2,17,31,59]. In general, all the catalysts show a similar Cu2p3/2 spectrum, in which two different contributions, corresponding to two degrees of copper interaction with the perovskite, are suggested: (i) the signal over 935 eV could be assigned to copper with a strong electronic interaction with the perovskite, and (ii) the signal at c.a. 933.5 eV could correspond to copper with a weak electron interaction with the perovskite [59,60]. The Cu/(Ba + Mn + Cu) ratio provides information about the distribution of copper over the perovskite surface. In Figure 3b, the ratios are compared to the nominal value and it seems that copper was inserted into the perovskite lattice only for BMC3, as concluded from the XRD data. Therefore, copper was homogeneously distributed on the perovskite for BMC1 and BMC2, since these two copper-containing samples showed surface ratios close to the nominal one (cross on Figure 3b).

2.1.4. Reducibility: H2-TPR

The hydrogen consumption profiles, shown in Figure 4, revealed the presence of two manganese species with different oxidation states in all the BaMnO3 samples, as has been also observed on the surface using XPS. The profiles can be divided into three regions, in which different reduction processes occur.
The hydrogen consumption between 200 °C and 600 °C was due to the reduction of manganese species. The asymmetry of the peak reveals that the reduction of manganese was carried out in two steps, pointing out the presence of Mn (IV), which was reduced at a lower temperature. The reduction of Mn (IV) to Mn (III) was detected as a shoulder of the main peak corresponding to Mn (III) reduction to Mn (II), including the initial Mn (III) present and that formed by Mn (IV) reduction [35,61].
For the BaMn1−xCuxO3 series (Figure 4a), it can be concluded that copper incorporation promoted the reduction of manganese since the reduction temperature of Mn (IV)/Mn (III) was shifted to lower temperatures as the copper content increased. For BMC3, the reduction of Cu (II) and Mn (IV)/Mn (III) was overlapped; therefore, a synergetic effect between copper and manganese should be taking place.
In Figure 4b, the profiles of the samples obtained using the modified sol-gel and the hydrothermal method reveal that the catalysts calcined at lower temperatures (BM-C600, BM-C700 and BM-H) show an improved reducibility since the reduction occurred at a lower temperature than in the BM catalyst. This fact has also been observed by S. Irusta et al. for lanthanum manganites [62] and by Y. Gao et al. for barium manganite [20,63], obtained through conventional sol-gel synthesis and calcined at different temperatures. The H2-TPR data point out that BM-C600, BM-C700 and BM-H catalysts should present more available surface for the interaction with the gas phase, since surface species are more reducible than bulk species.
It should be noted that, only for the BM-CX series, a hydrogen consumption peak at c.a. 550 °C was observed, which could be due to the presence of the remaining carbon black used as hard template, as has been detected through TGA [46].
At the highest temperature range, the hydrogen consumption around 750 °C was assigned to the decomposition of surface species, while, up to 750 °C, the reduction of Mn (III) bulk to Mn (II) took place. This process was mainly observed in the BM profile as it presented the largest crystal size and, therefore, the lowest amount of boundary manganese [62].

2.1.5. O2 Desorption: O2-TPD

The evolved oxygen profiles, shown in Figure 5a,b, reveal that the BaMnO3 catalysts mainly evolve high-temperature oxygen, called β-O2, which comes from the BaMnO3 perovskite lattice, being related to the reduction of Mn (IV) to Mn (III) [64,65,66] and boosted by the presence of lattice oxygen vacancies. Thus, the O2-TPD data inform about the oxygen mobility through the lattice [66,67]. The total amount of emitted β-O2, calculated using the area under the peaks between 700 °C and 950 °C, is included in Figure 5c.
The oxygen emission profiles revealed a clear effect of the copper amount, of the calcination temperature, and of the synthesis method. It was observed that the β-O2 emission was higher as the copper amount increased, suggesting that the amount of copper boosted the oxygen mobility through the ratio of the perovskite lattice, and hence, the manganese reducibility was also improved (as was concluded from the H2-TPR experiments). The hydrothermal synthesis allowed the formation of a BaMnO3 perovskite-like solid (BM-H) with enhanced oxygen mobility, probably due to the higher Mn (IV)/Mn (III) ratio on the surface by XPS (see Figure 3a), and by H2-TPR for the bulk, and the presence of a high amount of surface oxygen vacancies, promoted by its barium deficiency. However, for the BM-CX series, only BM-C700 and BM-C850 showed a significantly higher emission of β-O2 than that observed for BM. Note that the BM-C700 sample also evolved oxygen at a low and medium temperature, called α (corresponding to the desorption of oxygen adsorbed on the surface) and α’-O2 (corresponding to the desorption of oxygen adsorbed on the vacancies), respectively.
In summary, the incorporation of copper and the use of modified sol-gel and hydrothermal synthesis methods yield perovskites, in which Mn (III) and Mn (IV) oxidation states coexist (Mn (III) being the main oxidation state in most of them) and a higher amount of oxygen vacancies on the surface than BM. BMC3 (with the highest amount of copper), BM-C700 and BM-H present an improved reducibility and, hence, a higher oxygen mobility than BM perovskite, which plays an important role in catalyzing oxidation reactions.

2.2. Activity Tests

In previous studies, we found that the combination of a high amount of oxygen defects, a balanced Mn (IV)/Mn (III) ratio, and the presence of surface copper provide high activity for NO and NO2-assisted soot oxidation processes. In this work, the synthetized samples have been tested as catalysts for soot oxidation under simulated GDI exhaust conditions.

2.2.1. Preliminary Phase: TG-MS Experimental System

The preliminary study for soot oxidation in GDI conditions has been performed in a TG-MS system, which allows the use of low amounts of soot and low gas flow to avoid external mass transfer limitations [68]. In order to simulate the hardest scenario, the soot oxidation tests were carried out under inert atmosphere (0% O2/He), where the available oxygen only came from the perovskite, and under a slightly oxidant atmosphere (1% O2/He) which was achieved during fuel cuts [40].
The CO and CO2 emission profiles during both reaction atmospheres (0% and 1% O2) are presented in Figure 6. These profiles are expressed as μmol of CO or CO2 per gram of catalyst and initial soot; therefore, although the bare soot was also tested (see Figure A1 in Appendix A), the corresponding profiles were not included. However, the soot conversion and CO2 selectivity are included in Figure 7, which gathers the selectivity to CO2, based on the 44 m/z and 28 m/z MS signals registered during the soot oxidation experiments, and the percentage of removed soot, which was calculated from the weight loss during the TGA experiment in synthetic air carried out after the catalytic test. As some samples presented barium carbonates (due to the low calcination temperature used during synthesis), an additional experiment was carried out in the absence of soot (see Figure A2 in Appendix A) and the CO and CO2 emissions were used to correct the values for each catalyst.
Figure 6a,b shows the CO (dotted lines) and CO2 (solid line) emissions in 1% O2/He and He atmosphere, respectively, for the BaMn1−xCuxO3 series. The results in 1% O2/He reveal that all the catalysts were active to catalyze the oxidation of soot to CO2 because the reaction started at ca. 450 °C, the maximum being located at ca. 700 °C. However, as the amount of copper increased, the catalytic behavior worsened. This could be related to an increase in the amount of Mn (III) (supported by XPS data in Figure 3a) which seems to be less active than Mn (IV) [69]. Similar results have been observed for BaFe1−xCuxO3 catalysts [70,71]. Moreover, as is expected for manganese-based catalysts, independently of the copper amount, all the samples presented a high CO2 selectivity, being higher than 90%.
On the contrary, in the absence of oxygen, the catalysts were active at temperatures higher than 700 °C because the unique oxygen for soot oxidation came from the perovskite lattice (β-O2), which seemed to be able to catalyze the soot oxidation, as was also reported for BaFe1−xCuxO3 catalysts [71]. In the conditions used, almost all the soot was removed in the absence of the catalyst, but the CO2 selectivity was quite low, whereas in presence of the BaMn1−xCuxO3 catalysts, the selectivity to CO2 increased as the copper amount increased. The CO and CO2 profiles (Figure 6b,d) followed the thermodynamically expected product distribution according to the Ellingham diagram, showing a strong correlation between the CO2 formation and the temperature since, below 800 °C, soot is oxidized only to CO2, but above 800 °C, CO is the main product of soot oxidation. Therefore, depending on the presence or absence of oxygen, the copper improved or worsened the catalytic performance.
The effect of the synthesis method is shown by the CO and CO2 profiles presented in Figure 6c,d. As for the copper-containing catalysts, in the absence of oxygen, the catalysts were active at temperatures higher than 700 °C, because it is the temperature at which the β-O2 is released (as observed in the O2-TPD profiles in Figure 5). Note that the BaMnO3 catalysts obtained through the hydrothermal and modified sol-gel methods presented a similar catalytic performance than BM; so, the synthesis method seems to not significantly affect the catalytic activity. A similar conclusion has also been reported for La0.9M0.1MnO3 [40] and La0.6Sr0.4MnO3 [44].
The results show that only in the presence of catalysts was the oxidation carried out, but the synthesis method was not relevant since all the catalysts were able to remove almost 90% of the soot and they showed similar CO2 selectivity (around 50%) at the end of the experiment.
On the contrary, the CO and CO2 profiles under a 1% O2/He atmosphere (Figure 6a,c) reveal that the catalytic activity was largely improved since the soot oxidation started at ca. 400 °C with the maximum being located at ca. 700 °C, while in inert atmosphere, the soot oxidation started at ca. 600 °C and the maximum was achieved at ca. 850 °C. Indeed, a similar catalytic performance was reported for other manganese-based perovskites, such as La0.6Sr0.4MnO3 [72], La1−x−ySrxAgyMnO3 [69], SrMnO3 and SrMn0.98(Co,Cu)0.02O3 [73]. This catalytic performance was attributed to the high capacity of manganese to activate oxygen, which was adsorbed in oxygen vacancies and then dissociated by the reduction of Mn (IV) to Mn (III) between 400 and 500 °C. Additionally, above 500 °C, the soot oxidation could also be performed by β-O2 coming from the perovskite lattice. As the catalyzed soot oxidation is carried out at lower temperatures in the presence of 1% O2/He, the soot oxidation to CO2 is thermodynamically favored, according to the Ellingham diagram, so, as shown in Figure 7, higher CO2 selectivity values were achieved.
In the absence of oxygen (He), as for the BaMn1−xCuxO3 series, the effect of the synthesis method or calcination temperature was not observed, and the only modification over the reaction was an increase in the selectivity to CO2.
To summarize, the catalysts were slightly active to oxidize soot under the highest demanding GDI conditions, i.e., in the absence of oxygen, but they presented a noticeably high CO2 selectivity. The catalyst with the highest amount of copper (BMC3) showed the lowest percentage of soot removed but presented the highest CO2 selectivity as it evolved the highest amount of β-O2 of the series. For the BaMnO3 catalysts, the highest activity was observed for BM-H, also because of the high amount of available oxygen (β-O2), but no effect of the calcination temperature for the BM-CX samples was observed.

2.2.2. Second Phase: GC Experimental System

The study for soot oxidation in GDI conditions has been also analyzed using a GC, which allows the use of higher amounts of soot and a higher flow, these conditions being closer to the real application. Considering the results obtained in the preliminary study: (i) BMC2 was not included as BMC2 and BMC1 show the same catalytic behavior; and (ii) BM-C700 was used as the representative sample of the BM-CX series since no significant effect of the calcination temperature was observed.
The soot conversion, CO, and CO2 emission profiles during the soot oxidation tests in He and 1% O2/He atmospheres are shown in Figure 8 and Figure 9, respectively. The temperature required to achieve 10% soot conversion (T10%) and the CO2 selectivity values are gathered in Figure 10 in order to compare the activity data in both reaction atmospheres. Note that, in Figure 8a, 50% soot conversion was not achieved in the absence of oxygen; then, the T10% was compared instead of T50%. Figure 8a shows the soot conversion profiles under inert atmosphere, revealing that all catalysts were active since the soot oxidation was not carried out in the absence of the catalyst (blank). However, no effect of the amount of copper or the synthesis method was observed because all the BaMnO3 catalysts removed over the 40% of the initial soot at the end of the test, showing a low CO2 selectivity, which is the opposite trend observed in TG-MS conditions (Figure 7). This fact is a consequence of the different soot: catalyst ratio used (1:4 for GC and 1:8 for TG-MS) because, under inert atmosphere, oxygen is only supplied by the perovskite (β-O2) [40,42,44,69,73]. Thus, once the perovskite is not able to supply oxygen, the passive oxidation of soot is performed, yielding CO as the main product, as is observed in Figure 9a.
On the contrary, in the presence of 1% O2/He, the catalyzed and uncatalyzed (blank) soot oxidation achieved 100% soot conversion (see Figure 10), but, in the presence of catalysts, the CO2 selectivity significantly increased (reaching almost 100%), as has also been observed in TG-MS conditions. Therefore, BaMnO3 catalysts are able to adsorb/activate oxygen from the reaction atmosphere and then transport it to the soot due to their high oxygen mobility, as was proposed for perovskite-based catalysts [44] and for other metal-oxide-based catalysts [74]. Finally, an effect of the synthesis method (BM, BM-C700 and BM-H) or the copper amount (BMC1 and BMC3) was not observed since all the BaMnO3 catalysts showed a similar T10%, which was close to that of the blank, and all the BaMnO3-based catalysts showed a high CO2 selectivity.
In summary, all the BaMnO3 samples tested seemed to be active for the soot oxidation in the regular operation conditions for GDI engines [41], i.e., in the absence of oxygen, due to their capacity to supply oxygen coming from the perovskite lattice. However, a revealing effect of the synthesis method was not observed.

3. Materials and Methods

Materials: chemicals used as precursors: Citric acid (C6H8O7 > 99.0%, Sigma Aldrich, St. Louis, MO, USA); ammonia (NH3 30%, Panreac, Darmstadt, Germany); barium acetate (Ba(C2H4O2)2 > 99.0%, Sigma Aldrich, St. Louis, MO, USA); magnesium (II) nitrate tetrahydrated (Mn(NO3)2·4H2O > 99.0%, Sigma Aldrich, St. Louis, MO, USA); copper (II) nitrate trihydrated (Cu(NO3)2·3H2O > 99.0%, Panreac, Darmstadt, Germany); carbon black (Vulcan XC-72R, CABOT, Boston, MS, USA); and model soot (PRINTEX-U, DEGUSA, Madrid, Spain).

3.1. Synthesis of Catalysts

3.1.1. Conventional Sol-Gel Method for the Synthesis of BaMn1−xCuxO3 Series [45]

For the conventional sol-gel synthesis of BaMnO3(BM) and BaMn1−xCuxO3 (BMC3), barium, manganese and copper (for BMC3) precursors were added to a solution of citric acid (1M) with a pH of 8.5 (basified with ammonia solution); then, the solution was heated up to 65 °C for 5 h to obtain a gel, which was dried at 90 °C for 48 h, and finally was calcined at 150 °C (1 °C/min) for 1 h and at 850 °C (5 °C/min) for 6 h.

3.1.2. Modified Sol-Gel Method for The Synthesis of BM-CX Series [46]

The modified sol-gel method followed the above-mentioned steps but, after adding the precursors, carbon black was incorporated into the solution, with a mass ratio 1:1 regarding the mass of BaMnO3, and vigorously stirred for 1 h. Then, the mixture was dried at 90 °C for 48 h, calcined at 150 °C (1 °C/min) for 1 h, and finally, at 600 °C, 700 °C or 850 °C (5 °C/min) for 6 h, to obtain BM-C600, BM-C700, and BM-C850.

3.1.3. Hydrothermal Method for The Synthesis of BM-H

For the hydrothermal synthesis of BM-H, the barium precursor was solved in deionized water at 90 °C for 1 h. Then, manganese precursors were added. After 15 min under magnetic stirring, the mixture was transferred to a stainless-steel autoclave and heated up to 90 °C for 72 h. The obtained solid was filtered, washed with deionized water, dried overnight at 90 °C, and calcined at 600 °C (5 °C/min) for 6 h.

3.2. Characterization Techniques

ICP-OES (Perkin-Elmer device model Optimal 4300 DV, Waltham, MA, USA) was used to measure the amount of copper on the copper-containing samples. The samples were mineralized by using a diluted aqua regia solution (HNO3:HCl, 1:3) and stirring at room temperature for 1 h.
N2 adsorption at −196 °C was achieved in an Autosorb-6B instrument from Quantachrome (Anton Paar Austria GmbH, Graz, Austria) to determine the textural properties. The samples were degassed at 250 °C for 4 h before the N2 adsorption experiments.
The X-ray patterns were recorded between 20 and 80° 2 θ angles with a step rate of 0.4°/min and using Cu Kα (0.15418 nm) radiation in a Bruker D8-Advance device (Billerica, MA, USA) in order to study the crystalline structure, and the average crystal size was determined by using the Scherrer equation, assuming k = 0.9.
The chemical surface properties were obtained through X-ray photoelectron spectroscopy (XPS), using a K-Alpha Photoelectron Spectrometer from Thermo-Scientific (Waltham, MA, USA) with an Al Kα (1486.7 eV) radiation source. To obtain the XPS spectra, the pressure of the analysis chamber was maintained at 5 × 10−10 mbar. The binding energy (BE) and kinetic energy (KE) scales were adjusted by setting the C 1 s transition at 284.6 eV, and the BE and KE values were then determined with the peak-fit software of the spectrometer. The XPS ratios OLattice/(Ba + Mn), Cu/(Ba + Mn + Cu) and Mn (IV)/Mn (III) were calculated using the area under the suggested deconvolutions of O 1 s, Mn 3p3/2, Cu 2p3/2 and Ba 3d5/2 bands.
The reducibility of the catalysts was studied by temperature programmed reduction with H2 (H2-TPR) in a Pulse Chemisorb 2705 (from Micromeritics, Norcross, GA, USA) with a thermal conductivity detector (TCD) and using 30 mg of sample, heated at 10 °C/min from 25 °C to 1000 °C in 5%H2/Ar atmosphere (40 mL/min). The quantification of the H2 consumption was carried out using a CuO reference sample.
O2-TPD experiments were performed in a TG-MS (Q-600-TA and Thermostar from Balzers Instruments (Pfeiffer Vacuum GmbH, Asslar, Germany), respectively), with 16 mg of the sample heated at 5 °C/min from room temperature to 900 °C under a 100 mL/min of helium atmosphere. The 18, 28, 32 and 44 m/z signals were followed for H2O, CO, O2 and CO2, respectively, evolved during these experiments. The 40 m/z signal was also followed to ensure the tightness of the experimental system. The amount of evolved oxygen was estimated using a CuO reference sample.

3.3. Activity Test

The study of the soot oxidation in GDI conditions, in the absence (He) and very restrictive amounts of oxygen (1% O2/He), was developed in two phases to obtain complementary information. In both phases, the catalyst and the model soot were mixed with a spatula to ensure loose contact.
The preliminary phase was performed using the most favorable reaction conditions, i.e., low model soot:catalyst ratio (1:8) and a medium gas flow (100 mL/min). The experiments were carried out in a TG (Q-600-TA) coupled with a mass spectrometer (Omnistar-Vacuum Pfeiffer), to follow the m/z signals 18 (H2O), 28 (CO), 32 (O2), and 44 (CO2). To prove the tightness of the system, the m/z 40 (Ar) signal was also followed. Previously, the catalyst–soot mixture was dried at 150 °C for 1 h to remove the moisture, and then the temperature was raised from 150 °C to 900 °C (10 °C/min). The calibration was performed with calcium oxalate for 18 (H2O), 28 (CO), and 44 (CO2) m/z signals, and copper oxide for a 32 (O2) m/z signal.
The second phase was developed under conditions closer to the real application, and similar to those employed in the NO to NO2 oxidation and diesel soot oxidation studies, that is, a higher ratio of model soot: catalyst (1:4) and a higher gas flow (500 mL/min). The reaction was carried out in a U-shaped quartz reactor, loaded with a diluted mixture of 80 mg of catalyst and 20 mg of model soot diluted in 300 mg SiC. The exit of the reactor was connected to a gas chromatograph (HP6890 series), equipped with two columns (Porapack-Q and MolSieve-13X) and a TCD. Before the experiment, the mixture was dried at 150 °C for 1 h to remove the moisture; then, the temperature was raised from 150 °C to 900 °C (5 °C/min).
The soot conversion and CO2 selectivity were determined using Equations (1) and (2).
Soot   conversion   ( % ) = ∑ 0 t CO 2 + CO ∑ 0 final ( CO 2 + CO ) · 100
CO 2   Selectivity   ( % ) = CO 2 ∑ 0 final ( CO 2 + CO ) · 100

4. Conclusions

The conventional sol-gel, modified sol-gel, and hydrothermal synthesis methods were used to prepare BaMnO3 perovskite-based solids. The characterization and activity results draw off the following conclusions:
  • All synthesis methods allow the formation of BaMnO3 solids with a perovskite-like structure, and only BMC3 presents the polytype structure as the majority phase due to the partial substitution of manganese by copper into the perovskite lattice.
  • Mn (III) and Mn (IV) oxidation states coexist in all catalysts, with Mn (III) being the main oxidation state. The presence of copper causes an increase in the Mn (III) and in the amount of oxygen surface vacancies.
  • Both strategies, the insertion of copper and using two different synthesis methods (hydrothermal and modified sol-gel synthesis), promote the formation of oxygen vacancies, the manganese reducibility and, hence, an improvement in the oxygen mobility.
  • In the most favorable reaction conditions (preliminary study), a relationship was observed between the ability to catalyze the soot oxidation and the amount of β-O2 evolved, showing that BM-H and BMC3 samples show the best performances under the hardest GDI scenarios. In the absence of oxygen in the reaction atmosphere, the oxygen supplied by the perovskite (i.e., the oxygen evolved during the O2-TPD experiments) allows a high selectivity to CO2, since the passive oxidation of soot is carried out and BaMnO3 catalyzes the CO to CO2 oxidation. In the presence of a low amount of oxygen (1% O2 in He), the catalysts present a high activity to catalyze the CO to CO2 oxidation.
  • In the reactions carried out in more realistic conditions (second phase), all BaMnO3-based catalysts, independently of the copper amount or the method used for synthesis, were active for soot oxidation in the absence of oxygen (He). In the presence of low amounts of oxygen (1% O2/He), the catalysts present a high CO2 selectivity.
  • All BaMnO3 samples tested seem to be active for the soot oxidation in the regular operation conditions for GDI engines [41], i.e., in the absence of oxygen, due to their capacity to supply oxygen coming from the perovskite lattice. However, a revealing effect of the synthesis method was not observed.

Author Contributions

Conceptualization, V.T.-R. and M.-J.I.-G.; methodology, V.T.-R.; validation, V.T.-R. and M.-J.I.-G.; formal analysis, V.T.-R., M.-S.S.-A. and M.-J.I.-G.; investigation, V.T.-R.; resources, V.T.-R., M.-S.S.-A. and M.-J.I.-G.; data curation, V.T.-R.; writing—original draft preparation, V.T.-R.; writing—review and editing, M.-J.I.-G.; visualization, M.-S.S.-A.; supervision M.-J.I.-G.; project administration M.-J.I.-G.; funding acquisition M.-J.I.-G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Generalitat Valenciana (CIPROM/2021/70), Spanish Government (PID2019-105542RB-I00) and EU (FEDER Founding).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Catalysts 12 01325 g0a1 550
Figure A1. Evolved CO2 and CO profiles for uncatalyzed soot oxidation: (a) in 1% O2/He and (b) in He.
Figure A1. Evolved CO2 and CO profiles for uncatalyzed soot oxidation: (a) in 1% O2/He and (b) in He.
Catalysts 12 01325 g0a1
Catalysts 12 01325 g0a2 550
Figure A2. Evolved CO2 and CO profiles for catalysts: (a,c) in (1% O2/He) and (b,d) in He.
Figure A2. Evolved CO2 and CO profiles for catalysts: (a,c) in (1% O2/He) and (b,d) in He.
Catalysts 12 01325 g0a2

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Catalysts 12 01325 g001 550
Figure 1. X-ray patterns of (a) BaMn1−xCuxO3 series and (b) BaMnO3 catalysts.
Figure 1. X-ray patterns of (a) BaMn1−xCuxO3 series and (b) BaMnO3 catalysts.
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Catalysts 12 01325 g002 550
Figure 2. XPS transition spectra of Mn2p3/2 (a,b), O1s (c,d) and Cu 2p3/2 (e) for BaMn1−xCuxO3 and BaMnO3 catalyst series.
Figure 2. XPS transition spectra of Mn2p3/2 (a,b), O1s (c,d) and Cu 2p3/2 (e) for BaMn1−xCuxO3 and BaMnO3 catalyst series.
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Catalysts 12 01325 g003 550
Figure 3. XPS ratios (a) Mn (IV)/Mn (III) and OLattice/(Ba + Mn + Cu) and (b) Cu/(Ba + Mn + Cu) for the BaMn1−xCuxO3 and BaMnO3 catalyst series.
Figure 3. XPS ratios (a) Mn (IV)/Mn (III) and OLattice/(Ba + Mn + Cu) and (b) Cu/(Ba + Mn + Cu) for the BaMn1−xCuxO3 and BaMnO3 catalyst series.
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Catalysts 12 01325 g004 550
Figure 4. Hydrogen consumption profiles in TPR conditions of (a) BaMn1−xCuxO3 series and (b) BaMnO3 catalysts.
Figure 4. Hydrogen consumption profiles in TPR conditions of (a) BaMn1−xCuxO3 series and (b) BaMnO3 catalysts.
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Catalysts 12 01325 g005 550
Figure 5. O2-TPD profiles for (a) BaMn1−xCuxO3 catalysts serie, (b) BaMnO3 catalysts serie and (c) amount of β-O2 emitted during O2-TPD experiments.
Figure 5. O2-TPD profiles for (a) BaMn1−xCuxO3 catalysts serie, (b) BaMnO3 catalysts serie and (c) amount of β-O2 emitted during O2-TPD experiments.
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Catalysts 12 01325 g006 550
Figure 6. Evolved CO2 (solid line) and CO (dotted line) profiles during soot oxidation: (a,c) in (1% O2/He) and (b,d) in He.
Figure 6. Evolved CO2 (solid line) and CO (dotted line) profiles during soot oxidation: (a,c) in (1% O2/He) and (b,d) in He.
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Figure 7. Percentage of removed soot and CO2 selectivity values in He and 1% O2/He for BaMn1−xCuxO3 and BaMnO3 catalyst series.
Figure 7. Percentage of removed soot and CO2 selectivity values in He and 1% O2/He for BaMn1−xCuxO3 and BaMnO3 catalyst series.
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Catalysts 12 01325 g008 550
Figure 8. Soot conversion profiles in TPR conditions in (a) He and (b) 1% O2/He for BaMnO3 catalyst series.
Figure 8. Soot conversion profiles in TPR conditions in (a) He and (b) 1% O2/He for BaMnO3 catalyst series.
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Catalysts 12 01325 g009 550
Figure 9. CO (x) and CO2 (·) emission profiles in TPR conditions: (a) in He and (b) in 1% O2/He for BaMNO3 catalysts.
Figure 9. CO (x) and CO2 (·) emission profiles in TPR conditions: (a) in He and (b) in 1% O2/He for BaMNO3 catalysts.
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Figure 10. Temperature to achieve 10% soot conversion (T10%) and CO2 selectivity values in He and 1% O2/He for BaMnO3 catalysts.
Figure 10. Temperature to achieve 10% soot conversion (T10%) and CO2 selectivity values in He and 1% O2/He for BaMnO3 catalysts.
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Table
Table 1. Chemical composition, copper content by ICP-OES and BET surface area of the catalysts.
Table 1. Chemical composition, copper content by ICP-OES and BET surface area of the catalysts.
CatalystCompositionSynthesis
Procedure		Tcal (°C)Nominal Cu
(wt%)		Cu-ICP-OES
(wt%)		SBET
(m2·g−1)
BMBaMnO3Conventional
Sol-gel		850--5
BMC1BaMn0.9Cu0.1O32.62.44
BMC2BaMn0.8Cu0.2O35.24.84
BMC3BaMn0.7Cu0.3O37.87.64
BM-C600BaMnO3Modified sol-gel600--20
BM-C700BaMnO3700--21
BM-C850BaMnO3850--7
BM-HBa0.9MnO3Hydrothermal600--10
Table
Table 2. Crystal phase, average crystal size and lattice parameters of the catalysts.
Table 2. Crystal phase, average crystal size and lattice parameters of the catalysts.
CatalystIdentified Crystal
Phases (XRD)		Average Crystal Size (nm) aLattice Parameters b
a (nm)c (nm)
BMBaMnO3 hexagonal
Ba3Mn2O8		405.7034.831
BMC1BaMnO3 hexagonal
BaMnO3 polytype		39 (38)5.698 (5.774)4.808 (4.364)
BMC2BaMnO3 polytype
BaMnO3 hexagonal, CuO		43 (36)5.695 (5.784)4.811 (4.365)
BMC3BaMnO3 polytype,
CuO		(33)(5.792)(4.368)
BM-C600BaMnO3 hexagonal
Ba3Mn2O8
BaCO3		275.6934.803
BM-C700BaMnO3 hexagonal
Ba3Mn2O8
BaCO3		315.6934.798
BM-C850BaMnO3 hexagonal
Ba3Mn2O8
BaCO3		335.6934.805
a by Scherrer equation using the main diffraction peak of BaMnO3 hexagonal (≈31.4°); in brackets are the crystallographic data calculated for the BaMnO3 polytype phase (main diffraction peak ≈30.9°) b from the main diffraction peaks of the BaMnO3 hexagonal ≈31.4° (110) and ≈25.8° (101); in brackets are the crystallographic data calculated for the BaMnO3 polytype phase (main diffraction peaks ≈30.9° (110) and ≈27.0° (101).
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Torregrosa-Rivero, V.; Sánchez-Adsuar, M.-S.; Illán-Gómez, M.-J. Modified BaMnO3-Based Catalysts for Gasoline Particle Filters (GPF): A Preliminary Study. Catalysts 2022, 12, 1325. https://doi.org/10.3390/catal12111325

AMA Style

Torregrosa-Rivero V, Sánchez-Adsuar M-S, Illán-Gómez M-J. Modified BaMnO3-Based Catalysts for Gasoline Particle Filters (GPF): A Preliminary Study. Catalysts. 2022; 12(11):1325. https://doi.org/10.3390/catal12111325

Chicago/Turabian Style

Torregrosa-Rivero, Verónica, María-Salvadora Sánchez-Adsuar, and María-José Illán-Gómez. 2022. "Modified BaMnO3-Based Catalysts for Gasoline Particle Filters (GPF): A Preliminary Study" Catalysts 12, no. 11: 1325. https://doi.org/10.3390/catal12111325

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