Low-temperature H₂-plasma-assisted NO_x storage and reduction over a combined Pt/Ba/Al and LaMnFe catalyst

Abstract

A model catalyst of the Pt/Ba/Al₂O₃ (PBA) type exhibited poor activity in NO_x storage–reduction (NSR) at low temperatures (≤300 °C) even under a plasma-enhanced process. In order to achieve optimal NO_x removal efficiency during plasma-assisted NO_x storage–reduction in the presence of H₂O and CO₂, a PBA+LMF (LaMn_0.9Fe_0.1O₃) catalyst was prepared by mechanical mixing. Compared to the PBA and LMF references, the combined PBA+LMF catalyst showed an obvious synergistic effect with respect to NO_x storage capacity. With the assistance of H₂ plasma in rich phase, higher NO_x conversions could be obtained over the PBA+LMF sample over a wide temperature range (200–350 °C). The origin of the synergy was clarified on the basis of structure–activity correlations. The combination of non-thermal plasma and heterogeneous catalysis was proven to be very effective in improving the low temperature activity of lean NO_x trap (LNT) catalysts.

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1. Introduction

NO_x storage and reduction (NSR) (also known as lean NO_x trap (LNT)) technology is practically used for lean burn engines' NO_x removal. The commercial Pt/Ba/Al₂O₃ (PBA) catalyst exhibits good catalytic performance at temperatures higher than 300 °C due to the strong surface basicity of BaO. However, typical engine-out temperatures in the FTP-75 (the U.S. Federal Test Procedure used to certify vehicle emissions and fuel economy) are about 180–350 °C, therefore how to improve the low temperature activity of LNT catalysts is especially important for light duty diesel applications.

Though there are some efforts on this issue being published,^1–4 such as doping transition metal oxides including CeO₂, MnO_x, CoO_x, etc., to improve the low temperature activity of the Pt/Ba/Al₂O₃ catalyst, the activity below 250 °C is still quite lower than practical demands. On the other hand, when the low temperature activity becomes a bottleneck for thermal catalysis, non-thermal plasma might provide another possibility. In a previous study, we found that by combining the high NO_x storage capacity of perovskite catalysts with non-thermal plasma-assisted activation of H₂ in rich phase, high NO_x conversions were achieved over the LaMn_0.9Fe_0.1O₃ catalyst over a broad temperature range in the absence of H₂O and CO₂.⁵ However, the ubiquity of CO₂ and H₂O in vehicle exhaust emphasizes the importance of understanding their roles—beyond mere dilution—with respect to the function of NO_x trap catalysts.⁶

Therefore, in the present study, the performance of representative LNT catalysts in the presence of H₂O and CO₂ was studied in a typical H₂-plasma-assisted NSR process. It was found that the activity of the LaMn_0.9Fe_0.1O₃ (LMF) catalyst was greatly depressed when H₂O and CO₂ were in co-existence. The traditional Pt/Ba/Al₂O₃ catalyst also exhibited poor low-temperature activity even in a H₂-plasma-assisted NSR process. Meanwhile, it is exciting to find that when LMF was physically mixed with PBA (weight ratio of 1 : 1), strong synergistic effects were exhibited in both NO_x storage capacity and NO_x storage and reduction performance, and higher NO_x conversions could be obtained over the PBA+LMF catalyst over a wide temperature range (200–350 °C). The present study not only exhibited a highly active NSR catalyst, but also gave a general understanding of the formula of a NSR catalyst especially active for a low-temperature H₂-plasma-assisted NSR process.

2. Experimental

2.1. Catalyst preparation

Pt/Ba/Al₂O₃ (PBA) was prepared by incipient wetness impregnation: the γ-Al₂O₃ support (CNPC, surface area of 202 m² g⁻¹) was impregnated with aqueous Ba(O₂CCH₃)₂ (Sinopharm Chemical Reagent Co., Ltd, AR), followed by drying at 120 °C and calcination at 500 °C in air. The resulting solid was impregnated with aqueous chloroplatinic acid (H₂PtCl₆·6H₂O, Sinopharm Chemical Reagent Co., Ltd, AR), dried and further calcined at 500 °C for 5 h. The target Pt and Ba loadings were 1 wt% and 30 wt%, respectively. The LaMn_0.9Fe_0.1O₃ catalyst (LMF) was prepared by the combustion synthesis method according to a literature procedure.⁵ Pt-LMF was similarly prepared by incipient wetness impregnation: LMF was impregnated with aqueous chloroplatinic acid, dried and further calcined at 500 °C for 5 h; the target Pt loading was 1 wt%. The PBA+LMF catalyst was prepared by mechanically mixing equal masses of PBA and LMF by means of grinding, followed by calcining the mixture at 500 °C in air. All samples were pressed, crushed, and sieved to 20–40 mesh to obtain suitable particles for the experiments.

2.2. Catalyst characterization

X-ray powder diffraction (XRD) analysis was conducted using an XRD-6000 (Shimadzu) instrument with Cu Kα radiation (λ = 0.1542 nm), operating at 40 kV and 30 mA; phase identification was achieved through comparison of XRD patterns to those of the Joint Committee on Powder Diffraction Standards (JCPDS). Two types of scans were used to record the XRD measurements, hereby designated as normal scan (2° min⁻¹, step size 2θ = 0.04°) and slow scan (0.24° min⁻¹, step size 2θ = 0.01°). The crystallite sizes of BaCO₃ in the different samples were determined using the Scherrer equation based on the width of the peak (slow scan) at 2θ = 24.08° corresponding to BaCO₃ (111). X-ray photoelectron spectroscopic (XPS) measurements were conducted using an Al Kα source operated at 10 kV and 15 mA. Binding energies were corrected for charging effects by referencing them to adventitious carbon (284.6 eV). High-angle annular dark field STEM (HAADF-STEM) investigations were conducted in combination with spot energy-dispersive X-ray spectroscopy analysis (EDXS).

2.3. NSC and NOC experiments

NO_x storage capacity (NSC) and NO oxidation capacity (NOC) experiments were conducted in a microreactor using a chemiluminescence NO_x analyzer (Ecotech ML9841AS) as the detector. The catalyst (500 mg) was firstly pretreated at 500 °C in 1% H₂/Ar for 1 h, and then exposed to gases including 500 ppm NO, 8% O₂, with or without 2% H₂O and 2% CO₂ present, and balance N₂. The storage time was 50 min for the NSC experiments. NOC was measured when the steady state had been achieved (typically requiring 2–3 h). The NSC and NOC were calculated according to the following formulae:where t_s is the storage time (s) and m_cat is the mass of the catalyst (g).

2.4. In situ DRIFTS

In situ DRIFTS measurements were performed on a Tensor 27 (Bruker) FTIR spectrometer. The catalyst was firstly pretreated in 1% H₂/Ar at 500 °C for 30 min and then cooled to the desired temperature in Ar. A gas mixture of 500 ppm NO, 8% O₂ with or without 2% H₂O and 2% CO₂ present, and Ar as the balance gas was then introduced, and a series of NO_x adsorption spectra were obtained over time. All spectra were measured with a resolution of 4 cm⁻¹ and accumulating 128 scans. A background spectrum was subtracted from each spectrum.

CO adsorption measurements were conducted to investigate the chemical state of Pt on the surface of the PBA catalyst. After reduction in H₂ at 500 °C, the sample was pre-treated in flowing helium (99.999%, 50 ml min⁻¹) at 500 °C and cooled to 20 °C, then exposed to flowing 3% CO/He (50 ml min⁻¹) for 15 min (to saturate the sample) and finally flushed with flowing He. DRIFT spectra were then recorded. All spectra reported here were collected at a resolution of 4 cm⁻¹ for 60 scans.

2.5. NO_x-TPD and H₂-TPSR

Each sample was first pretreated at 500 °C for 60 min in 1% H₂/Ar and then cooled to the desired temperature in Ar. NO_x adsorption was carried out in 500 ppm NO, 8% O₂ and balance Ar, with or without 2% H₂O and 2% CO₂ present at 200 °C for 50 min. After purging the catalyst with Ar at 200 °C for 30 min, the temperature was raised to 700 °C at a rate of 10 °C min⁻¹ in flowing Ar. H₂-TPSR was performed after NO_x adsorption at 300 °C for 50 min, and then temperature programmed reduction in 2% H₂/Ar at a rate of 10 °C min⁻¹. NO_x-TPD and H₂-TPSR were conducted in a microreactor using a mass spectrometer (Pfeiffer Vacuum GSD 301) as the detector.

2.6. Catalyst evaluation

Catalytic activities were measured in a dielectric-barrier discharge (DBD) reactor as described in the literature,⁷ loaded with 500 mg of catalyst particles (20–40 mesh). The voltage was applied with a high-voltage power source (CTP-2000K, Nanjing Suman Electronic Co., Ltd) working at about 40 kHz. Effluent gases were analyzed on-line using a mass spectrometer (Pfeiffer Vacuum GSD 301). The catalyst was firstly pretreated in 2% H₂/Ar at 500 °C for 1 h, and then cooled to the desired temperature for lean–rich cycling measurements until the catalyst reached a cycle-averaged steady state. Briefly, NO_x is first stored on the catalyst during the fuel lean phase (plasma off) and then the stored NO_x is reduced to N₂ and H₂O by the plasma during the fuel rich phase (plasma on). The NO_x concentration was averaged over 5 cycles to give a mean conversion according to the following formula:

The cycling parameters and gas composition used for the cycling experiments are summarized in Table 1.

Table 1 Gas composition used for NO_x storage/reduction cycling experiments

Parameter	Lean	Rich
Duration (s)	600	120
Temperature (°C)	150–350	150–350
Power input (W)	—	1.8
Space velocity (ml gcat^−1 h^−1)	14 400	14 400
NO (ppm)	500	—
O2 (%)	8	—
H2 (%)	—	2
Ar (%)	Balance	Balance

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3. Results

3.1. Catalyst characterization

The X-ray diffraction patterns of PBA, LMF and PBA+LMF catalysts are shown in Fig. 1. For the PBA catalyst, the main phases detected were BaCO₃ (JCPDS no. 45-1471) and γ-Al₂O₃ (JCPDS no. 47-1770). The characteristic reflections of Pt or PtO_x were not observed due to the low concentration of Pt in the PBA catalyst.² In the case of LMF, the positions and relative intensities of the XRD peaks of the as-prepared LaMn_0.9Fe_0.1O₃ sample are consistent with those of the standard LaMnO_3.11 compound (JCPDS no. 50-0297).⁵ For the PBA+LMF sample, the sample showed diffraction patterns for BaCO₃ and LaMnO_3.11 crystallites. The estimated BaCO₃ particle size (according to the Scherrer equation) was ca. 15 nm and ca. 11 nm for PBA and PBA+LMF, respectively. The decrease in BaCO₃ particle size for the PBA+LMF sample is suggestive of a chemical interaction between LMF and PBA, i.e., possible formation of a new phase such as BaLaMn₂O_5.5,⁸ which has the same diffraction pattern as LaMnO_3.11.⁵

Fig. 1 XRD patterns of PBA, LMF and PBA+LMF.

Fig. 2(a)–(c) show the XPS spectra of O 1s for the PBA, LMF and PBA+LMF samples. By deconvolution, the distribution of oxygen species was estimated. Two kinds of oxygen species were present, the peak at BE = 530.8 eV being ascribed to surface lattice oxygen (O_latt²⁻) species, while that at BE = 532.2 eV being assigned to adsorbed oxygen species (O_ads).^2,3,5 The ratios of O_ads/O_latt²⁻ were 0.817 and 0.609 for LMF and PBA+LMF, respectively, significantly higher than that of PBA (0.196). This means that the LMF and PBA+LMF samples are enriched with surface oxygen compared to PBA. The Mn 2p XPS spectra are shown in Fig. 2(d) and (e). By means of curve fitting, there are two kinds of Mn species present; the Mn 2p_3/2 binding energy at 641.4 eV is assigned to Mn³⁺, while that at 642.9 eV is ascribed to Mn⁴⁺.^3,9 The ratio of Mn⁴⁺/Mn³⁺ is 0.940 in LMF, higher than that in PBA+LMF (0.630). These results indicate that LMF is enriched with surface Mn⁴⁺ species as compared to PBA+LMF. This is consistent with the O_ads/O_latt²⁻ data which show that LMF has more surface oxygen than PBA+LMF. On the other side, the change of Mn⁴⁺/Mn³⁺ ratio upon mixing LMF with PBA is also suggestive of a chemical interaction between LMF and PBA.

Fig. 2 O 1s XPS spectra of PBA (a), LMF (b), PBA+LMF (c) and Mn 2p XPS spectra of LMF (d), PBA+LMF (e).

DRIFTS measurements were performed after CO chemisorption on the reduced PBA, LMF and PBA+LMF samples in order to probe the state of the Pt sites, and the results are shown in Fig. 3. For comparison purposes, analogous DRIFTS measurements were also recorded for Pt-LMF. For PBA, there is a strong peak observed at 2063 cm⁻¹, with a shoulder at 2042 cm⁻¹ also detected. Given that the alkaline BaO/Al₂O₃ support is electron-rich, electron donation to the 5d orbital of Pt occurs when Pt is supported.^10,11 As a result, increased back-donation of electron density from the Pt 5d orbital to CO (2π*) occurs when CO is chemisorbed, leading to a red shift of the CO vibration.¹¹ On this basis, the band at 2042 cm⁻¹ can be assigned to CO linearly adsorbed on Pt sites close to Ba species, while the other band at 2063 cm⁻¹ is assigned to CO chemisorbed on Pt in contact with Al₂O₃. The other peaks at 1830,¹¹ 1845,¹² and 1868 cm⁻¹ (ref. 13) can be assigned to bridged-CO (B-CO) species.¹³ In addition, a small band at 2122 cm⁻¹ was observed (Fig. 3); in general, carbonyls stabilized on cationic Pt sites give rise to a band above 2100 cm⁻¹,^14,15 and thus the band at 2122 cm⁻¹ can be attributed to linearly coordinated CO species on Pt²⁺.

Fig. 3 DRIFT spectra of CO chemisorbed on the different catalysts.

In the case of the Pt-LMF sample, two obvious bands at 2079 and 2105 cm⁻¹ were observed. According to the foregoing assignments, the band at 2079 cm⁻¹ can be ascribed to CO linearly adsorbed on Pt sites interacting with LMF. Since the oxide support is electron-deficient, charge transfer should occur from Pt to the oxide, resulting in the observed blue shift in the CO vibration. The band at 2105 cm⁻¹ can be assigned to CO adsorption on partially oxidized platinum Pt^δ+.¹³ For the PBA+LMF sample, an obvious band at 2079 cm⁻¹ was observed which was attributed to CO adsorption on Pt sites close to LMF, as indicated above. Notably, the band at 2042 cm⁻¹, which is related to CO adsorbed on Pt sites close to Ba species (PBA), was not observed. In addition, similar to PBA, both linear-CO on Pt²⁺ (2122 cm⁻¹) and bridged-CO species (1830, 1845 and 1868 cm⁻¹) were observed in the DRIFT spectrum. The above results give clear evidence of CO adsorption on Pt sites close to LMF over the PBA+LMF sample.

The morphology change was confirmed by high-angle annular dark field STEM (HAADF-STEM) combined with spot energy-dispersive X-ray spectroscopy analysis (EDXS). The investigations were performed by comparison of PBA with PBA+LMF samples. As shown in Fig. 4(a), the bright white rod-like species present in the PBA sample should be ascribed to crystalline BaCO₃ as indicated by the corresponding intense barium peaks (as indicated for the red square labeled region 2). Conversely, the dark region marked as region 1 (again indicated by a red square) corresponds to an Al-rich area. In the enlarged images shown in Fig. 4(b) and (c), highly dispersed, individual bright spots with an average size of ca. 1 nm are observed, which correspond to Pt nanoparticles. The corresponding X-ray spectrum shows both Pt peaks and Al peaks, while the Ba peaks are noticeably weak, indicating that Pt is preferentially dispersed on γ-Al₂O₃ instead of BaCO₃.

Fig. 4 STEM images of PBA (a–c) and PBA+LMF (d–f) samples. The EDX spectra obtained from the outlined areas are shown.

In the case of PBA+LMF (Fig. 4(d)), rod-like BaCO₃ structures can't be clearly observed. In region 1 (indicated by a red square), strong Ba and La peaks coexist, suggesting the possible formation of a new phase such as BaLaMn₂O_5.5. It is also noteworthy that there is no Pt observed in this region. However, as characterized by the dark region marked in the red square 2, strong peaks due to La, Mn and Fe are observed, as well as Pt, indicating the dispersion of Pt on LMF. The bright white region, marked by the red square 3, displays strong peaks for Pt, La and Mn, indicating the re-dispersion of Pt on LMF. In the enlarged images shown in Fig. 4(e) and (f), individual bright spots due to Pt particles are observed, the observed Pt particle size distribution of 2–4 nm being a little larger than that observed for PBA.

3.2. NO oxidation capacity

Shown in Fig. 5(a) are the conversions of NO to NO₂ as a function of reaction temperature in the absence of H₂O and CO₂. All samples displayed low NO conversion at 150 °C (<20%). As the temperature increased from 150 to 300 °C, the LMF and PBA+LMF samples showed enhanced NO conversions. However, the conversion over the PBA catalyst remained quite low (23.2% at 300 °C). Upon further increase of the temperature to 350 °C, the conversion of NO to NO₂ continued to increase for PBA and PBA+LMF; however, the conversion decreased from 78.4% to 66.2% for LMF under the same conditions, which is due to the thermodynamic limitation associated with the NO/NO₂ equilibrium. Based on the XPS results presented above, it is apparent that surface adsorbed oxygen plays an important role in NO oxidation,⁹i.e., the higher surface oxygen concentration of the LMF catalyst, as compared to PBA, is consistent with its superior NO oxidation capacity.

Fig. 5 NO conversion to NO₂ measured under lean conditions (500 ppm NO, 8% O₂, with or without 2% CO₂ and 2% H₂O, Ar balance, WHSV = 14 400 ml g_cat⁻¹ h⁻¹): without H₂O and CO₂ (a) and with H₂O and CO₂ (b).

Fig. 5(b) shows the conversion of NO to NO₂ as a function of reaction temperature in the presence of H₂O and CO₂. Compared to the experiments performed using 500 ppm NO/8% O₂/Ar as feed gas (Fig. 5(a)), the addition of H₂O and CO₂ decreased the NOC for all the samples. Notably, LMF is still the most active catalyst for NO oxidation, followed by the PBA+LMF and PBA catalysts.

3.3. NO_x storage capacity

Fig. 6 shows the NO_x storage capacities (NSCs) of the catalysts measured at different temperatures in a continuously flowing feed in both the absence and presence of H₂O and CO₂. In the absence of H₂O and CO₂ (Fig. 6(a)), PBA exhibited its highest NSC value at 350 °C. The LMF sample showed higher NSC compared to the PBA sample even at 150 °C, and the NSC showed little dependence on temperature. However, the NSC decreased when the temperature was raised to 350 °C, which can be explained by the decreased thermal stability of the stored nitrites at higher temperatures.^5,16,17 Notably, much higher NSC values were obtained over the PBA+LMF sample, and compared with PBA, the enhancement of NSC on PBA+LMF was particularly evident at higher temperatures.

Fig. 6 NO_x storage capacity measured under lean conditions in the absence (a) and presence (b) of H₂O and CO₂ at different temperatures; 50 min storage time, WHSV = 14 400 ml g_cat⁻¹ h⁻¹.

Compared to the experiments performed using only 500 ppm NO/8% O₂/Ar as the feed gas (Fig. 6(a)), the addition of H₂O and CO₂ significantly decreased the NSC for the PBA and LMF samples (Fig. 6(b)), especially for the LMF sample. While the latter showed relatively high NSC in the absence of H₂O and CO₂ over a broad temperature range (150–300 °C, as shown in Fig. 6(a)), its NSC significantly decreased when H₂O and CO₂ were introduced into the feed gas. Notably, in the case of PBA+LMF, the presence of H₂O and CO₂ caused only a slight reduction in the NSC over the full temperature range studied, as shown in Fig. 6. Although both the LMF and PBA+LMF catalysts displayed relatively high NO oxidation activity compared to PBA (as noted previously), even in the presence of H₂O and CO₂ (Fig. 5), LMF showed the lowest NSC over the full temperature range. On this basis, it can be concluded that the poor NO_x storage capacity of LMF in the presence of H₂O and CO₂ cannot be ascribed to its lower NO oxidation activity. Rather, other explanations must be considered, e.g., relatively weaker NO_x adsorption (relative to the adsorption of H₂O/CO₂) on the LMF sample or the possibility that NO_x storage may be accomplished via different mechanisms for the different catalysts.^18–20 In the case of PBA-type catalysts, much work has been done to elucidate the mechanism of NO_x storage.^21–25 Specifically, two parallel pathways for NO_x storage on Pt/Ba/Al have been proposed by Nova et al.:^18,20,26 a “nitrite route”, in which O₂ is activated by Pt sites and transferred to neighboring Ba sites, favoring the stepwise oxidation of NO to nitrite ad-species; nitrites are then progressively oxidized into nitrate species, which are the predominant species at catalyst saturation. In parallel with the formation of nitrite ad-species, oxidation of NO to NO₂ can also occur on Pt sites; the NO₂ thus formed can be stored on BaO in the form of nitrates, one NO molecule being released for every three molecules of NO₂ consumed via reaction with BaO. It has been proposed that the “nitrite” pathway dominates over the disproportion route in the case of NO/O₂ adsorption over PBA. In addition, from the comparison of the NSC data in the presence of H₂O and CO₂ for PBA, LMF and PBA+LMF, it is clear that the combination of PBA and LMF greatly enhanced the NSC at all temperatures, indicating a strong synergistic effect on NSC resulting from the physical mixing of PBA and LMF.

3.4. Adsorbed NO_x species

In situ DRIFT spectra showing the evolution of surface NO_x species in the different feed gases are shown in Fig. 7. After exposing the PBA catalyst to a NO/O₂/Ar mixture for 50 min at 200 °C, two obvious bands at 1230 and 1325 cm⁻¹ and a small band at 1548 cm⁻¹ were observed (Fig. 7(a)). According to the literature, the band at 1230 cm⁻¹ can be attributed to nitrite species on Ba,^27,28 and the band at 1325 cm⁻¹ can be assigned to nitrates on Ba.⁴ The small band centered at 1548 cm⁻¹ can also be assigned to nitrates, although it is difficult to assign the location of these species (i.e., to BaO or γ-Al₂O₃) due to the overlap of bands in this region.²⁹ Upon exposure of the LMF sample to the NO/O₂/Ar mixture at 200 °C, a strong band at 1260 cm⁻¹ and a weak band at 1552 cm⁻¹ were observed. These bands can be assigned to nitrite species on La or Mn sites.^16,30–32 The main surface species on PBA+LMF corresponded to nitrates (1315 and 1548 cm⁻¹) at 200 °C. Notably, the nitrite band (1230 cm⁻¹) associated with the PBA component of the PBA+LMF sample was very weak (Fig. 7(a)), which is likely a consequence of the stronger oxidation capacity of PBA+LMF compared to PBA. In addition, a shoulder peak could be observed in the spectrum of PBA+LMF at 1245 cm⁻¹ which is assigned to monodentate nitrates associated with Al.²⁹

Fig. 7 In situ DRIFT spectra of PBA (a), LMF (b) and PBA+LMF (c) catalysts after 50 min of NO adsorption at 200 °C.

When H₂O and CO₂ were added to the feed gas (shown in Fig. 7(a)–(c)), the intensities of all of the above bands were significantly reduced, while new bands appeared at 1336 and 1592 cm⁻¹ for PBA and PBA+LMF, and at 1278 and 1566 cm⁻¹ for LMF. These new bands are assigned to carbonates on Ba,²⁸ La or Mn.³³ Besides the foregoing bands, a new band was observed at 1635 cm⁻¹ for all samples that can be ascribed to adsorbed water δ(H–O–H).^34,35 Notably, compared with LMF, the intensity of the δ(H–O–H) band was much higher for the PBA and PBA+LMF samples, especially for PBA.

3.5. Thermal stability of the stored NO_x

The thermal stability of stored NO_x was investigated by means of NO_x-TPD experiments, and the results are shown in Fig. 8. In the absence of H₂O and CO₂ (Fig. 8(a)), two obvious NO_x desorption peaks (m/z = 30) were observed at 307 and 514 °C for the PBA sample. Correlated with the DRIFT results, the former can be attributed to weakly adsorbed nitrite species, while the latter desorption event, which was accompanied by O₂ (m/z = 32) generation, can be assigned to the decomposition of nitrate at Ba sites.⁵ In the case of LMF, a broad NO_x desorption peak was observed from 250 to 500 °C with a maximum near 307 °C, indicating the poor thermal stability of the adsorbed NO_x. Moreover, no oxygen release was observed. This is consistent with the DRIFTS results that the adsorbed NO_x species are nitrite species on La or Mn sites. For the PBA+LMF sample, a very broad NO_x desorption peak was observed, commencing at ca. 370 °C and ending at ca. 600 °C, with an additional small peak at 301 °C. The latter desorption event is associated with NO_x release from γ-Al₂O₃, as indicated in the case of the PBA sample. Based on literature reports, it is reasonable to ascribe the broad peak to the decomposition of several types of adsorbed NO_x species. Deconvolution of the TPD profile reveals the presence of a strong desorption peak at 467 °C (Fig. 8(a)), indicating that a large fraction of the stored NO_x desorbed at a temperature lower than that observed for PBA (514 °C).

Fig. 8 NO_x-TPD profiles after NO_x adsorption in 500 ppm NO/8% O₂/Ar (a) and 500 ppm NO/8% O₂/2% CO₂/2% H₂O/Ar (b) for 50 min at 200 °C.

Compared to the case when H₂O and CO₂ were absent (Fig. 8(a)), the intensity of the NO_x desorption peaks significantly decreased after H₂O and CO₂ co-existed (Fig. 8(b)). This is mainly due to the inhibition of NO_x adsorption, as demonstrated by the decreased NSC measured in the presence of H₂O and CO₂ (Fig. 6(b)). In addition, the high-temperature NO_x desorption peak of PBA (Fig. 8(b)) shifted to an even higher temperature in the presence of H₂O and CO₂ (from 514 °C to 553 °C). According to Theis et al.,³⁶ the presence of H₂O enhances the spillover of NO₂ from Pt sites to the NO_x storage sites (Ba) during lean operation, possibly via the formation of nitric acid (HNO₃) which then reacts with NO_x storage sites to form the stored NO_x species. Consequently, it can be speculated that the higher NO_x mobility in the presence of H₂O results in NO_x storage at sites further away from Pt, which might explain the higher desorption temperature.³ For the LMF sample, the intensity of the NO_x desorption peak at ca. 300 °C decreased compared with the case in the absence of H₂O and CO₂, implying the strong competitive adsorption between NO_x and H₂O/CO₂ on the same sites, being a consequence of the significantly reduced NSC obtained in the presence of H₂O and CO₂ (as shown in Fig. 6). Over PBA+LMF, there was a broad and large NO_x desorption peak, accompanied by O₂ formation, centered at 486 °C. Compared with NO_x desorption from PBA, NO_x desorbed in a larger amount but at lower temperature from PBA+LMF, which is consistent with the enhanced NSC, especially at lower temperatures, observed for PBA+LMF (shown in Fig. 6).

3.6. Reduction behavior of the stored NO_x

Reduction of stored NO_x was investigated by means of H₂-TPSR experiments. After NO_x adsorption in 500 ppm NO/8% O₂/Ar, two obvious H₂ consumption peaks (m/z = 2) accompanied by N₂ (m/z = 28) formation were observed at 92 and 207 °C for the PBA sample as shown in Fig. 9(a). This result suggests that surface nitrates/nitrites on Al and Ba sites reacted with H₂ at 92 °C and 207 °C,⁵ respectively, with the simultaneous formation of N₂. In the case of the LMF sample, the N₂ formation peak was observed at 368 °C, which did not line up with the H₂ consumption peak (410 °C). This discrepancy can be attributed to the reduction of surface or lattice oxygen on the catalyst, which evidently occurs at slightly higher temperature than NO_x reduction.⁵ In addition, unlike PBA, significant NO_x desorption was observed in the H₂-TPSR process from 250 to 400 °C. Evidently, due to the higher NO_x reduction temperature and the poor thermal stability of the NO_x stored on LMF, a fraction of the NO_x was released before H₂ reduction could occur. According to the H₂-TPSR results for PBA+LMF shown in Fig. 9(a), two H₂ consumption maxima (m/z = 2) were observed at 100 and 180 °C. The peak at 100 °C can be attributed to the reduction of NO_x weakly adsorbed on the γ-Al₂O₃ support as previously described, while the peak at 180 °C is assigned to the reduction of nitrate or nitrite on Ba sites. Notably, the reduction of adsorbed NO_x on Ba occurred at a lower temperature on PBA+LMF than that on the PBA sample (the H₂ consumption maxima were observed at 180 and 207 °C, respectively). Comparing the reduction behavior of the LMF, PBA and PBA+LMF catalysts, it is clear that reduction of stored NO_x requires much higher temperatures over LMF than over the PBA and PBA+LMF catalysts. This explains the poor performance of LMF during lean–rich cycling experiments, in which NO_x was released before H₂ reduction.

Fig. 9 H₂-TPSR profiles obtained under 2% H₂/Ar after NO_x adsorption in 500 ppm NO/8% O₂/Ar (a) and 500 ppm NO/8% O₂/2% CO₂/2% H₂O/Ar (b) gas stream for 50 min; NO_x adsorption was performed at 300 °C.

Reduction of stored NO_x (H₂-TPSR) was also investigated subsequent to the adsorption of NO in feed gas containing H₂O and CO₂, the results being shown in Fig. 9(b). For PBA, H₂ consumption (m/z = 2) accompanied by N₂ (m/z = 28) formation was observed at 221 °C. Compared with the experiments conducted in the absence of H₂O and CO₂ during NO adsorption (Fig. 9(a)), the low-temperature H₂ consumption peak was not observed. As indicated above, this low-temperature peak can be ascribed to NO_x stored on the Al₂O₃ support, which was inhibited in the presence of H₂O.³⁷ In the case of LMF, the majority of stored NO_x was desorbed at ca. 300 °C, such that only a very weak N₂ formation peak was observed. In this case, H₂ consumption was mainly associated with the reduction of surface or lattice oxygen.⁵ In the case of PBA+LMF, compared to the experiments performed in the absence H₂O and CO₂ (Fig. 9(a)), the H₂ consumption peak at lower temperature was not observed. In addition, the reduction of stored NO_x was slightly inhibited (i.e., occurred at a higher temperature) compared with the results obtained in the absence of H₂O and CO₂. H₂O is known to adsorb on Pt at low temperature,³⁸ so it is reasonable to expect that this leads to the blockage of sites on the Pt surface for hydrogen adsorption, this being the rate determining step for noble metal-catalyzed hydrogen oxidation.³⁹ Notably, NO_x release from PBA and PBA+LMF was not observed during the H₂-TPSR process. However, almost all of the stored NO_x desorbed from LMF during H₂-TPSR due to an imbalance in the rates of NO_x release and NO_x reduction. Given the absence of Pt, LMF shows poor activity for NO_x reduction; hence, most of the NO_x is released without being reduced.

3.7. NO_x storage and reduction by catalysis only

NO_x storage and reduction were investigated under cycling conditions using H₂ as the reductant, the NO_x conversion profiles being shown in Fig. 10. The cycle-averaged lean phase NO_x storage efficiency and rich phase NO_x release values are collected in Table 2. In the absence of H₂O and CO₂ (Fig. 10(a)), the NO_x removal efficiency increased with increasing temperature and the highest conversion for the PBA catalyst was obtained at 350 °C, which is consistent with results reported in the literature.² Compared with PBA, the LMF sample displayed much lower NO_x removal efficiency at all temperatures, the poor catalytic performance of LMF during lean–rich cycling being attributed to the low activity for reduction of the stored NO_x, as revealed by the much higher NRE values at low temperatures (≤250 °C) and the poor NO_x storage capacities at higher temperatures due to the lack of strong basicity components such as Ba, K, etc. (shown in Table 2). In the case of PBA+LMF, considerably higher cycle-averaged NO_x conversions were obtained at 150 °C to 350 °C as a result of its excellent NO_x storage (higher NSE) and NO_x reduction properties (lower NRE) over a broad temperature range (as shown in Fig. 6 and 9).

Fig. 10 NO_x conversion during lean/rich cycling in the absence (a) and presence (b) of H₂O and CO₂ at different temperatures; WHSV = 14 400 ml g_cat⁻¹ h⁻¹.

Table 2 Comparison of NO_x storage and release during NO_x storage/reduction cycling (catalysis only)^a

Catalyst	Temperature (°C)	NSC (10 min) (μmol g^−1)	NOx release (μmol g^−1)	NSE (%)	NRE^b (%)
a “w/o” means “without”, “w” means “with”. b (NOx released in rich purge/NOx stored in lean phase) × 100%.
PBA (w/o H2O/CO2)	150	24.9	19.5	46.6	36.4
	200	34.4	8.04	64.1	15.0
	250	43.6	4.82	81.3	9.01
	300	47.3	1.77	88.3	3.31
	350	48.6	0.91	90.7	1.72
LMF (w/o H2O/CO2)	150	28.52	25.19	94.6	47.1
	200	29.91	22.94	95.3	42.8
	250	24.66	11.26	95	21.1
	300	21.23	0.70	90.7	1.32
	350	14.74	0.38	90.9	0.71
PBA+LMF (w/o H2O/CO2)	150	42.02	4.45	78.4	8.37
	200	45.99	1.66	85.8	3.15
	250	49.63	1.29	92.6	2.48
	300	53.22	1.34	99.3	2.56
	350	52.26	0.96	97.5	1.82
PBA (w/ H2O/CO2)	150	6.11	1.72	11.4	3.21
	200	19.19	3.64	35.8	6.83
	250	41.59	8.20	77.6	15.3
	300	44.65	2.30	83.3	4.34
	350	46.90	2.95	87.5	5.52
LMF (w/ H2O/CO2)	150	3.00	1.29	5.61	2.43
	200	8.15	2.25	15.2	4.21
	250	14.79	2.47	27.6	4.61
	300	17.96	2.79	33.5	5.23
	350	14.74	0.38	27.5	0.75
PBA+LMF (w/ H2O/CO2)	150	11.04	3.00	20.6	5.67
	200	24.23	4.93	45.2	9.26
	250	41.59	1.39	77.6	2.61
	300	50.12	3.59	93.5	6.74
	350	47.49	1.55	88.6	2.93

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When H₂O and CO₂ were added to the feed gas (Fig. 10(b)), each of the samples exhibited decreased NO_x removal efficiency in comparison with the results reported in Fig. 10(a). Notably, the presence of H₂O and CO₂ in the feed decreased the NO_x conversion of PBA+LMF greatly, especially at low temperatures. Analysis of the NSE and NRE data shown in Table 2 shows that the loss in activity should be due to its decrease in NO_x storage capacity, as well as the enhanced NRE in the presence of H₂O and CO₂, especially at low temperatures (≤250 °C). However, as shown in Fig. 6, the presence of H₂O and CO₂ in the feed did not exhibit such a big negative effect on NO_x storage capacities during steady measurement, which suggests that the catalyst mustn't be fully regenerated during the lean–rich cycles leading to the decreased NSC. Accordingly, the activity for reduction of the stored NO_x must be improved.

3.8. Combination of non-thermal plasma and heterogeneous catalysis for NO_x storage and reduction

Based on the promise of NTP-assisted catalysis, we opted to employ NTP in the rich phase to aid catalyst regeneration. Fig. 11(a) depicts the cycle-averaged NO_x conversions as a function of reaction temperature, measured in the absence of H₂O and CO₂, from which it is evident that the NO_x removal efficiency of PBA, LMF and PBA+LMF over the temperature range 150–350 °C was greatly enhanced compared with the performance obtained in the absence of the NTP in the rich phase (Fig. 10(a)). Most notable was the improvement in the cycle-averaged NO_x conversion obtained over the LMF sample. According to our previous study, the reduction of stored NO_x is the rate limiting step over LMF;⁵ clearly, by employing NTP in the rich phase to assist reduction of the stored NO_x, the NO_x conversion was greatly improved, especially at low temperature. Although the reduction activity was enhanced by NTP, the NO_x removal efficiency of PBA below 300 °C was lower than those obtained over the LMF and PBA+LMF samples. This can be attributed to the poor NO oxidation capacity of PBA at low temperatures, resulting in its relatively low NO_x storage capacity. In contrast, over the mixed PBA+LMF sample, higher NO_x conversions (>95%) could be obtained over the whole temperature range from 150 to 350 °C.

Fig. 11 NO_x conversion during lean/rich cycling at different temperatures (lean phase: 500 ppm NO, 8% O₂, balance Ar, without (a) or with (b) 2% H₂O and 2% CO₂, duration: 10 min; rich phase: 1% H₂, balance Ar, with or without 2% H₂O and 2% CO₂, discharge power: 1.8 W, duration: 2 min, WHSV = 14 400 ml g_cat⁻¹ h⁻¹).

Plasma-assisted NO_x storage and reduction experiments were also conducted over the PBA, LMF and PBA+LMF samples in the presence of H₂O and CO₂ (Fig. 11(b)). LMF exhibited decreased NO_x removal efficiency at all temperatures, a consequence of the fact that the NO_x storage capacity of LMF was significantly inhibited by the addition of H₂O and CO₂ as shown in Fig. 6. Notably, the presence of H₂O and CO₂ in the feed did not decrease the NO_x conversion over PBA as much as that over LMF, although again the decrease in NO_x conversion can be ascribed to the decreased NSC in the presence of H₂O and CO₂. Notably, addition of H₂O and CO₂ to the feed resulted in only a minor decrease in NO_x removal efficiency for PBA+LMF during NTP-assisted NSR (Fig. 11(b)). NO_x conversions were close to 99% at all temperatures from 200 to 350 °C, while a cycle-averaged conversion of 84% was obtained at 150 °C.

The cycle-averaged lean phase NO_x storage efficiency and rich phase NO_x release values are collected in Table 3. A rather low NO_x release percentage was observed for PBA above 300 °C. The comparatively higher NO_x release at 200 and 250 °C for PBA can be ascribed to an imbalance between the rates of nitrate decomposition and NO_x reduction, i.e., the rate of NO_x reduction was too low for the released NO_x to be reduced. Higher temperatures of 300–350 °C therefore represent an optimum at which a balance is achieved between the rates of nitrate decomposition and NO_x reduction. In the case of LMF, significant NO_x release was observed above 200 °C, especially in the presence of H₂O and CO₂, due to the poor thermal stability of surface NO_x species as previously verified in the NO_x-TPD measurements (Fig. 8). In contrast, almost no NO_x release was observed from PBA+LMF in the absence/presence of H₂O and CO₂. This result can be attributed to the high thermal stability of the NO_x species formed (as shown in Fig. 8) and the excellent NO_x reduction activity (as shown in Fig. 9), thereby maintaining a balance between the rates of nitrate decomposition and NO_x reduction for PBA+LMF over the full operating temperature range, even in the presence of H₂O and CO₂.

Table 3 Comparison of NO_x storage and release during NO_x storage/reduction cycling (plasma-assisted catalysis)^a

Catalyst	Temperature (°C)	NSC (10 min) (μmol g^−1)	NOx release (μmol g^−1)	NSE (%)	NRE^b (%)
a “w/o” means “without”, “w” means “with”. b (NOx released in rich purge/NOx stored in lean phase) × 100%.
PBA (w/o H2O/CO2)	150	31.2	0.08	58.2	0.13
	200	38.4	1.96	71.7	5.1
	250	44.4	1.73	82.9	3.9
	300	51.2	0.2	95.6	0.21
	350	51.5	0.02	96.1	0.02
LMF (w/o H2O/CO2)	150	50.7	0.82	94.6	1.61
	200	51.1	1.83	95.3	3.58
	250	50.9	2.39	95	4.69
	300	48.6	4.56	90.7	9.38
	350	48.7	9.81	90.9	20.1
PBA+LMF (w/o H2O/CO2)	150	51.1	0.26	95.4	0.51
	200	50.8	0.37	94.8	0.73
	250	51.6	0.35	96.3	0.68
	300	51.9	0.24	96.9	0.46
	350	51.3	0.18	95.8	0.35
PBA (w/ H2O/CO2)	150	27.5	0.98	51.3	3.56
	200	34.9	2.15	65.2	6.16
	250	42	3.35	76.5	8.17
	300	49.8	0.2	91.2	0.41
	350	50.1	0.88	93.1	1.76
LMF (w/ H2O/CO2)	150	28.5	2.64	53.1	9.27
	200	34.3	1.56	64	4.55
	250	42.1	3.81	78.6	9.06
	300	48.6	7.54	90.8	15.5
	350	43.7	16.3	81.6	37.3
PBA+LMF (w/ H2O/CO2)	150	44.8	0.15	84	0.035
	200	51.7	0.05	96.5	0.009
	250	52	0.16	97.1	0.032
	300	52.2	0.23	97.4	0.043
	350	51.7	0.05	96.5	0.096

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4. Discussion

Hybrid plasma–catalyst systems have been proven to be very efficient in VOC oxidation, automobile catalysis, water purification, and reforming/hydrogen generation reactions.^40–44 In general, two ways have been proposed for combining NTP with catalysts. In the first method, the NTP is placed upstream of the catalyst bed in order to activate the gas prior to contact with the surface of the catalyst; this is essentially a “two-stage” plasma-catalysis system. In the second way, the discharge occurs over the surface of the catalyst; this is a so-called “single-stage” plasma-catalysis system, with the goal of achieving higher efficiency compared with the two-stage configuration.⁴⁵ However, no matter which kind of system is being used, the problem associated with the plasma-catalysis approach is the relatively high energy cost required to remove very low concentrations of pollutants. Therefore, an improved plasma process has been proposed by several groups.^46–49 Our group has reported a “cyclic storage discharge” approach for VOC removal, where the low concentration of VOC is first enriched on an adsorbent, after which the NTP is briefly switched on for the desorption (decomposition) and/or oxidation of the adsorbed VOC.⁴⁸ Since the plasma is only applied for a very short time compared with the operating time for VOC enrichment, the energy consumption is greatly reduced.

Following on from these studies, we propose a non-thermal plasma-assisted NSR process, especially for low-temperature activity improvement. In the lean phase, a catalyst with high NSC is required to store NO_x over a wide temperature range (150–350 °C). In the rich phase, H₂-plasma is introduced to assist the catalyst regeneration. It is supposed that the DBD plasma, possessing an average electron energy of 1–5 eV, enhances the dissociation of H₂ into H (the binding energy of H₂ being ca. 4.4 eV), which should be the key step for H₂ reduction in the rich phase. Actually, when the reaction rate is high enough at higher temperatures (>350 °C), there is no necessity for the assistance of NTP. In other words, NTP could be switched on only at low temperatures, at which H₂-reduction was limited. As indicated by the results presented in this paper, NO_x conversions of greater than 90% can be obtained over the mixed PBA+LMF sample over a wide temperature range (200–350 °C) in a H₂-plasma-assisted NSR process (Fig. 11), even in the presence of H₂O and CO₂.

For optimal results when applying a H₂-plasma in the rich phase, it is important that the NSR catalyst possesses high NO_x storage capacity, especially in the low temperature region. Indeed, the LMF catalyst, which exhibited the highest NSC at low temperatures, gave generally higher NO_x conversions than PBA in H₂-plasma-assisted NSR.⁵ However, the co-existence of H₂O and CO₂ in the feed led to a considerable loss of NSC, thereby limiting its usefulness. By mechanically mixing PBA with LMF, a synergetic effect was obtained with respect to the NSC, which was manifested in the NO_x conversion obtained during NSR cycling, especially during H₂-plasma-enhanced NSR. To understand this synergetic effect, structure characterization was carried out, providing evidence of a re-construction that occurred upon calcination of the mixture. Specifically, a decrease in the BaCO₃ crystal size was observed, suggesting that the BaCO₃ phase may have undergone re-dispersion due to an interaction with LMF (possibly forming a BaLaMnO_5.5 phase). Moreover, the blue band shift observed for CO adsorbed on Pt is indicative of charge transfer from Pt to LMF, suggesting a close interaction between the two phases. This morphology change was confirmed by HAADF-STEM combined with EDXS analysis. As is well known, the proximity of NO oxidation and NO_x storage sites is crucial for NSR catalysts. As indicated in Fig. 5, LMF exhibited the highest NO oxidation ability (NO → NO₂), in both the absence and presence of H₂O and CO₂. When it was mixed with PBA, which provided storage sites for NO₂, the PBA+LMF mixture showed a synergetic effect on NSC, due to the proximity of the sites for NO oxidation and NO_x storage. However, the PBA+LMF catalyst still exhibited poor activities at low temperatures in typical catalysis-only NSR cycling because of NO_x release in the rich phase as indicated in Table 2. Consequently, given the high lean phase NSC and H₂ plasma-assisted NO_x reduction in the rich phase, high NO_x conversions (>90%) could be obtained over the PBA+LMF sample over a wide temperature range (200–350 °C).

5. Conclusions

According to a comparative study of traditional Pt/Ba/Al₂O₃ (PBA), perovskite-type LMF and physically mixed PBA+LMF catalysts, the three catalysts exhibit strikingly different NO_x storage capacities in the absence and presence of H₂O and CO₂. Notably, the combination of PBA and LMF prepared by mechanical mixing exhibited improved NSC compared to the other catalysts, even in the presence of H₂O and CO₂. This is attributed to the reconstruction of the mixture upon calcination, which provided sites for NO oxidation and storage which were in close proximity. Hence, by combining the high NO_x storage capacity of PBA+LMF with non-thermal plasma-assisted activation of the reducing agent (H₂), high NO_x conversions were achieved over a broad temperature range.

Acknowledgements

This work was supported by the National Natural Foundation of China (no. 21373037 and 21577013), the Fundamental Research Funds for the Central Universities (no. DUT12LK23, DUT13YQ107, DUT15TD49 and DUT16ZD224) and the China Postdoctoral Science Foundation of MOE (2014M560201). Z. S. Zhang is thankful to the Shandong Provincial Natural Science Foundation (no. ZR2016BB27). MC thanks the National Science Foundation and the U.S. Department of Energy (DOE) for financial support under award no. CBET-1258742. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE.

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