《1. Introduction》

1. Introduction

Compared with conventional stoichiometric engines, lean-burn engines such as lean-burn gasoline or diesel engines provide higher fuel efficiency and produce lower carbon dioxide (CO2) emissions. However, the nitrogen oxides (NOx, referring to nitric oxide (NO) and nitrogen dioxide (NO2)) emitted from lean-burn engines render traditional three-way catalysts (TWCs) ineffective due to the ample oxygen (O2) presence in the exhaust gas. Therefore, two promising de-NOx technologies for the abatement of NOx emissions—namely, selective catalytic reduction and NOx storage and reduction (NSR), which were initially proposed by Toyota researchers in the 1990s—have been employed to meet regulations [1,2]. NSR catalysts mainly consist of alkali or alkaline earth metals (e.g., BaO and CeO2) for NOx storage, and noble metals such as rhodium (Rh) and platinum (Pt) that are well dispersed on a support such as alumina (Al2O3), which possesses a high specific surface area. Many research papers are also reporting that other additives such as cerium oxide (CeO2) effectively promote the stability and durability of catalysts. NSR technology periodically involves a regular long lean (oxidation) period and short rich (reduction) excursion. The NO existing in the exhaust gas is oxidized to NO2 by the noble metals and is stored in the storage component in the form of nitrites or nitrates during the lean phase. In rich atmospheres, reductants such as hydrogen (H2), carbon monoxide (CO), propene (C3H6), and so forth are introduced into the exhaust gas, leading to the decomposition of the nitrites or nitrates into NOx, which is subsequently reduced to nitrogen (N2) on the noble metal sites [3].

With the aim of obtaining high NOx conversions, numerous studies have been conducted on the optimization of catalytic formulations and operation conditions [4–11]. Traditional NSR catalysts are in the form of Pt–Ba/γ-Al2O3; however, the utilization of CeO2 in recent studies has revealed the excellent abilities of this catalyst. CeO2 favors NOx storage efficiency (NSE) as a stored NOx species, facilitates steam reforming reactions and water–gas reactions, and it stabilizes on a high dispersion of noble metals [12]. CeO2-containing catalytic formulations have been proposed in which CeO2 is used as a support because of its known capability in NOx storage related to nitrate and nitrite species [13–15]. The action of CeO2 as a promoter has also been thoroughly investigated. Le Phuc et al. [16] prepared a series of Pt/Ba/Ce/Al2O3 catalysts with different cerium/barium (Ce/Ba) molar ratios and found that the presence of Ce increased the NOx conversion rate to some extent. Recent papers have focused on NSR conditions, including the effects of reductant type, reductant amount, and different lean and rich durations [17,18]. In terms of reductant types, Masdrag and coworkers [19] investigated the effect of H2, CO, C3H6, and H2 + CO + C3H6 on the NOx conversion rate over Pt/10%BaO/Al2O3. The results showed that H2 was a superior reductant in the testing temperature range of 200–400 °C, obtaining a maximum conversion rate of 78% at 400 °C; this finding was similar to observations reported by Abdulhamid and coworkers [20]. The influence of the lean and rich durations on NSR was also studied. AL-Harbi and Epling [18] examined different regeneration protocols in the temperature window from 200 to 500 °C over a model NSR catalyst. The results revealed an obvious improvement in catalyst performance in the wake of increasing regeneration times at 200, 300, and 400 °C. In addition, Ansari et al. [21–24] synthesized CoTiO3/CoFe2O4 nanostructures via a sol–gel auto-combustion technique and found evenly distributed spherical nanoparticles in X-ray diffraction (XRD) patterns and energy dispersive spectroscopy (EDS) patterns when the molar ratio of cobalt/titanium (Co/Ti) was 1:1. Mahdiani et al. [23,24] prepared PbFe12O19 nanostructures using sol–gel auto-combustion, and found the valine for the nanostructures to be 5123 Oe.

The objective of the present study was to optimize NSR capabilities in order to gain a higher NOx conversion based on simulated gas tests, and to investigate the roles of temperature variances and the scale time of H2 on the evolution mechanism of NSR during a complete NSR process and a cycle-average NSR process over the Pt–Ba–Ce/γ-Al2O3 (denoted herein as PBCA) catalyst, respectively. The catalyst was prepared by incipient wetness impregnation and was then characterized in terms of Pt dispersion, XRD, morphology and size of the catalyst, transmission electron microscope (TEM), surface chemical composition oxidation state, and X-ray photoelectron spectroscopy (XPS) in order to study the physicochemical properties of the catalyst.

《2. Experimental procedure》

2. Experimental procedure

《2.1. Catalyst preparation》

2.1. Catalyst preparation

A series of PBCA catalysts were prepared by incipient wetness impregnation. First, γ-Al2O3 (Umicore) was impregnated in Ce(NO3)2·6H2O solution (AR, Sinopharm Chemical Reagent Co., Ltd., China) to give a Ce loading of 15 wt%. Ba(O2CCH3)2 (AR, Sinopharm Chemical Reagent Co., Ltd., China) as a precursor was subsequently incorporated into the sample using the same method, with a nominal Ba loading of 10 wt%. The sample was dried at 120 °C for 24 h and calcined at 550 °C in the air for 5 h. H2PtCl6·6H2O (AR, Sinopharm Chemical Reagent Co., Ltd., China) was dissolved in deionized water and subjected to ultrasonic concussion for 30 min in order to obtain a homogeneous dispersion solution; it was then added to the abovementioned sample to gain Pt loadings of 0.285%, 0.577%, and 0.855%, respectively. After each impregnation, the sample was dried and calcined under the same conditions described above. Finally, the ball-milled sample was sieved using 40–60 mesh before the NSR experiment.

《2.2. Catalyst characterization》

2.2. Catalyst characterization

XRD of the as-prepared sample was measured on a Bruker D8 Advance X-ray diffractometer with a nickel (Ni)-filtered copper (Cu) Kα (γ= 0.154068 nm) radiation source at 40 kV and 40 mA. Powder XRD patterns were recorded at 0.02° intervals in the range of 20°–80° with a scanning velocity of 7°·min-1 . Jade software (Jade Software Co., Ltd., New Zealand) was used for data treatment; identification of the crystalline phrase was performed using Joint Committee on Powder Diffraction Standards (JCPDS) cards. TEM was performed using a FEI Tecnai 12 electron microscope (Thermo Fisher Scientific Inc., USA) operating with a 120 kV accelerating voltage. Before being imaged, the powder sample was finely ground and dispersed ultrasonically in anhydrous ethanol at room temperature; a drop of the suspension was then dropped onto a lacey carbon-coated Cu grid of 200 mesh. XPS analysis was carried out on an ESCALABTM 250Xi spectrometer from Thermo Fisher Scientific fitted with a micro-focused, monochromatic aluminum (Al) Kα source ( = 1486.6 eV), with a base pressure of about 5 × 10-14 MPa; the binding energies were referenced to the C 1s line at 284.6 eV from adventitious carbon.

《2.3. Catalyst activities evaluation》

2.3. Catalyst activities evaluation

The lab apparatus for the NSR experiments is shown in Fig. 1. The apparatus was composed of a reactor, gas paths, a gas paths control part, and a gas analyzer. The catalyst evaluation was carried out with 0.3 mL (0.40 g) of catalyst using a fixed-bed quartz micro-reactor with an inside diameter of 10 mm. The catalyst was placed inside a tubular electric resistance furnace, the temperature of which was controlled by a thermocouple dipped into the catalyst bed. The catalyst was plugged in and sandwiched between two silica wool layers to prevent the sample from moving away. The gas paths and the gas paths control part included gas cylinders, reducing valves, valve controllers, and mass-flow controllers. All gases were introduced by mass-flow controllers with a total flow rate of 280 mL·min-1 , giving a space velocity of 5.6 × 104 h-1 . The outlet NOx (NO and NO2) concentration was detected by a NOx analyzer (Thermo ScientificTM Model 42i-HL, Thermo Fisher Scientific Inc., USA). In our study, prior to completing the NSR experiments, the catalyst sample temperature was increased (20 °C·min-1 ) to 450 °C in N2 and was then exposed to a regeneration gas mixture consisting of 1% H2 and a balance of N2 for 30 min. The catalyst was then cooled to the required measurement temperature in N2, and the reaction temperature was varied from 250 to 400 °C. When cycling the NSR experiments performed at 350 °C, a fixed 240 s lean phase and varied rich phase (30, 45, and 60 s) were used. The NOx conversion was based on the average of two cycles when a steady cycle-to-cycle performance was achieved. The gas composition and parameters used for completing and cycling the NSR experiments are summarized in Table 1.

《Fig. 1》

Fig. 1. Scheme of the lab apparatus for NSR experiments. MCF: mass flow meter.

《Table 1》

Table 1 Details of flow conditions used in the complete NSR experiments and cycling NSR experiments.

The activity of the catalyst sample during the lean and rich periods was evaluated by the following formula. The NOx storage capacity (NSC) was defined as follows:

where fNO;in is the NO molar flow (mol·min-1 ) at the inlet, fNOx;out is the NOx molar flow at the outlet, tL is the duration of the lean phase, and mcat is the mass of the catalyst (g).

The cycle-average NOx reduction conversion was based on two consecutive steady-state cycles, and the mean conversion was calculated according to the following formula:

where t is the total cycle length.

《3. Results and discussion》

3. Results and discussion

《3.1. Characterization of the catalyst》

3.1. Characterization of the catalyst

Fig. 2 shows the X-ray diffractograms of the PBCA catalysts. XRD spectra were taken on a freshly calcined sample. The XRD analysis showed that the main phase in PBCA was γ-Al2O3 (JCPDS No. 48-0366), which exhibited a typical cubic fluorite structure. The presence of BaCO3 (JCPDS No. 05-0378) in the International Center for Diffraction Data (ICDD) database was observed, and was consistent with the decomposition of Ba(O2CCH3)2 into crystalline BaCO3 during catalyst calcination at 500 °C [25]. The peak of the BaCO3 tended to decrease along with the increase of Pt content, and disappeared entirely when the Pt content was 0.855 wt%. Neither Pt nor PtOx were found in the XRD profile, which indicates that Pt exists as an amorphous or poorly crystalline phase monolayer, or else greatly disperses on the surface of the storage component and γ-Al2O3 and thus cannot easily be observed through XRD. The peaks belong to BaO when 2θ is equal to 28.04°, 28.68°, and 66.80°, and CeO2 (JCPDS No. 43-1002) peaks emerge at the degrees of 28.68° and 47.02°.

《Fig. 2》

Fig. 2. XRD profiles of the PBCA samples. 2θ: scattering angle.

Fig. 3 shows the TEM pattern, which was used to examine the morphology and size of the 0.577Pt–10Ce–15Ba/γ-Al2O3 catalyst. The TEM image reveals that there are many nanoparticles with sizes varying in the range of 1–5 nm on the catalyst (as indicated by the red circles), which can be assigned to Pt or PtOx particles; this indicates that the Pt species is well dispersed, perhaps because of the strong interaction between Pt and CeO2 [26]. The larger black particle in the image (indicated by the white circle) can be regarded as the aggregation of CeO2 and BaCO3. The light grey zone on which the PtOx, CeO2, and BaCO3 are deposited can be regarded as γ-Al2O3.

《Fig. 3》

Fig. 3. TEM image of 0.577Pt–10Ce–15Ba/γ-Al2O3 catalyst.

XPS analysis was carried out to elucidate the surface chemical composition and oxidation state on the surface of the catalyst. The survey scan analysis was performed in a binding energy range between 0 and 1200 eV, and is presented in Fig. 4(a). The survey’s spectrum shows that Ce, Ba, O, C, Pt, and Al are present on the 0.577Pt–10Ce–15Ba/γ-Al2O3 catalyst surface, and there are no obvious impurities present. The XPS spectrum of the chemical state of Ce is shown in Fig. 4(b). The complex Ce 3d level spectrum has spin–orbit doublets that are 3d5/2 and 3d3/2. The peaks V and U correspond to Ce 3d5/2 and Ce 3d3/2, respectively. In addition, the bands V (882.6 eV), V” (889.2 eV),  (898.4 eV), U (901.1 eV), U”(906.9 eV), and (916.9 eV) refer to the presence of Ce4+, and V0 (885.6 eV) and U0 (903.6 eV) are related to Ce3+. These peaks clearly indicate that the Ce4+ oxidation state is predominant, while the peaks for Ce3+ are very weak. According to the spectrum areas, the ratio of Ce3+/Ce4+ is 0.06. The binding energy of the Ba 3d5/2 core-level is 780.1 eV, suggesting that Ba is carbonated; this is consistent with the observation of the C 1s component near 280.9 eV, which is the fingerprint of carbonate structures. The narrow spectrum of O 1s, which is fitted into two peaks, is depicted in Fig. 4(c). The peak located at 529.1 eV (hereafter denoted as Oα) corresponds to the surface lattice oxygen species (O2–) [27], of which a small contribution could be due to PtOx species [28]. Moreover, the second peak at 531.2 eV (hereafter denoted as Oβ) corresponds to the chemisorbed oxygen [29]. It has been reported that surface chemisorbed oxygen is the most active oxygen, playing an important role in the NO-to-NO2 oxidation reaction [30,31]. For Pt, only the Pt 4d5/2 spectrum is recorded due to the peak of Pt 4f, which is overlapped by the Al 2p peak (74.1 eV) [32,33]. As can be seen in Fig. 4(d), a major peak at a lower binding energy (314.5 eV) is associated with metallic Pt (Pt0+), and a minor one at higher position (316.8 eV) signifies the presence of oxidized Pt (PtO or PtO2).

《Fig. 4》

Fig. 4. XPS patterns of 0.577Pt–10Ba–15Ce/γ-Al2O3 catalyst: (a) Survey; (b) Ce 3d; (c) O 1s; (d) Pt 4d.

《3.2. Complete NOx storage and reduction》

3.2. Complete NOx storage and reduction

In order to obtain a good understanding of the NSR process, a complete NSR experiment was conducted. The reaction was not switched to the rich phase until the spilled NOx concentration was equal to the inlet NO concentration, and the next cycle was initiated only when the outlet NOx concentration in the rich phase decreased to zero. The experiment was carried out at 250, 300, 350, and 400 °C using a continuously flowing mixture of 500 ppm NO, 10% O2 and N2 (in balance) in the lean phase, and 1% H2 and N2 (in balance) in the rich phase. The outlet NOx (NO + NO2) concentration value was obtained via a NOx analyzer. Fig. 5 shows the evolution of NOx (NO + NO2) at the reactor during the complete NSR period at different temperatures. The results of the complete NSR experiments are summarized in Table 2. As shown in Fig. 5(a), when the experiment began (t = 0 min)—that is, when the gas mixtures were switched to lean compositions—the NOx-containing mixture showed a delay of about 3 min, and the concentration then increased to the asymptotic values corresponding to the inlet concentration of 500 ppm NO. Of the NOx, NO occurred initially, while NO2 was also observed due to the oxidation of NO by O2 on Pt or oxidized Pt sites via Reaction (3). As previously reported in the literature [34,35], NOx molecules were absorbed at different Ba absorption sites; the most active component among these was BaO, although this mainly existed as BaCO3 under real working conditions [36]. The overall reactions are given below, as Reactions (3) to (7):

Given the information in Fig. 5 and Table 2, it can be seen that the time required for complete NOx storage was 41 min, the NSC was 119.8 , and the corresponding NSE was 18.7% at 250 °C. When the gas compositions were switched to the rich condition, it is worth noting that NO was quickly reduced to zero by H2, while NO2 showed a huge pulse to 650 ppm before quickly decreasing to zero (as clearly shown in the zoomed-in zone in Fig. 5(a)). The NOx desorption peak area was mainly composed of NO2, which indicates that NO2 was more easily stored than NO. When H2 was introduced, the surface adsorbing NOx was quickly reduced and the exothermic reaction with NOx caused the catalyst temperature to rise, accelerating the decomposition of the nitrate or nitrite. NOx-removal efficiency is not only related to NSC in the lean phase, but is also concerned with the reduction of trapped NOx during the rich phase; therefore, the released NOx in the rich phase led to a decrease in the NOx conversion (13.5%) compared with the NSE. When the temperature increased from 250 to 400 °C, the NOx evolution curves were similar to those at 250 °C. However, the NSC was greatly enhanced in the range of 250– 350 °C, despite a slight decrease at 400 °C, and attained the maximum value of 315.3 1 at 350 °C, which is ascribed to the increase in NO2 yield. Furthermore, the amount of NO2 increased with temperature, and the NO2/NOx ratio increased from 0.09 to 0.59 in the temperature range of 250–400 °C. The NSC increase in the temperature range of 250–350 °C can be mainly attributed to the increased NO2/NOx ratio, with NO2 being more favorably trapped on BaO sites as nitrates or nitrites than NO. At 400 °C, the NSC reached a value close to that observed at 350 °C, but the NO2/NOx was much higher (Table 2). These results indicate that higher temperatures accelerate the thermal decomposition of nitrates or nitrites in the NOx storage process, and that the former predominates at higher temperatures, leading to decreased NSC. The catalyst reached its maximum NSC at 350 °C, much higher than that of 250 and 300 °C. However, the NOx conversion for the whole testing temperature varied in a narrow range from 13.5% to 21.3%, as shown in Table 2.

《Fig. 5》

Fig. 5. Evolution of NOx (NO + NO2) at the reactor exit during the complete NSR period at different temperatures: (a) 250 °C; (b) 300 °C; (c) 350 °C; (d) 400 °C.

《Table 2》

Table 2 Complete NSR measured at different temperatures.

Fig. 6 depicts the NSE as a function of the storage time at different temperatures. It is clear that the NSE decreases with storage time for all measured temperatures during the initial 5 min, the NSE for all temperatures is greater than 90%, and the maximum is 94.7% at 350 °C, indicating that almost all of the introduced NO during the initial lean phase is trapped by the catalyst. As the storage time reaches 40 min, the NSE drops to 22.5%, 28.5%, 42.9%, and 43.7% for 250, 300, 350, and 400 °C, respectively, which is due to the fact that the NOx trapping sites are gradually reduced and eventually saturated. In actual application, a higher NSE is required to reduce more NOx in order to meet the strict emission regulations. Using our proposed strategy to gain a better NOx conversion, we optimized the experiment conditions by shortening the storage time to 4 min accompanied by a 30, 45, and 60 s rich-phase duration.

《Fig. 6》

Fig. 6. NSE as a function of storage time at different temperatures.

《3.3. Cycling NOx storage and reduction》

3.3. Cycling NOx storage and reduction

As described above, the maximum NSC was obtained at 350 °C. To further study the real-world applications, a series of lean–rich cycling experiments was carried out at 350 °C by varying the rich-phase durations (30, 45, and 60 s) while maintaining the lean phase at 240 s. Fig. 7 depicts the evolution of NOx in the outlet feed of the catalyst’s initial 6–8 lean–rich cycles. The catalytic system reached a steady cycle-to-cycle performance after about five cycles. A summary of the results is provided in Table 3. As can be seen from the enlarged view of the steady cycles, NOx was effectively trapped with very low emissions during the lean condition, and the NOx emissions were stabilized at 116, 102, and 81 ppm for 240-lean/30-rich, 240-lean/45-rich, and 240-lean/60-rich conditions, respectively. The NOx outlet concentrations were much lower than the NO feed concentration (500 ppm), indicating that NOx can be effectively stored over the catalyst; the amounts of trapped NOx were 112.6, 117.2, and 118.4   for the 30, 45, and 60 s regeneration times, corresponding to an NSE of 90.0%, 93.4%, and 94.7%, respectively. Furthermore, for each experiment, NO2 was observed during the lean phase of the cycling, revealing that NSE was not limited by the kinetics of NO oxidation. When the feed gases were switched to a rich condition, a sharp NOx spike occurred, which can be determined indicating that the decomposition rate of the nitrates or nitrites (Reactions (8)–(10) [37,38]) was higher than the NOx reduction rate (Reactions (11) and (12)), and that the NOx conversion rate was limited by the reduction step.

《Fig. 7》

Fig. 7. NOx concentration profiles during lean–rich cycling at 350 °C with a fixed 240 s lean phase and varied rich phase: (a) 30 s, (b) 45 s, and (c) 60 s.

《Table 3 》

Table 3 Cycling NSR measured at different rich phase time.

In addition, the amount of desorbed NOx was significantly augmented, especially as unreduced NO2 during the rich period steadily increased as the regeneration time extended. This finding was attributed to the amount of released NOx that was a function of the NOx trapped during the previous lean phase. As illustrated in Table 3, the amount of trapped NOx increased when the regeneration time was prolonged. The NOx rapidly decreased to a low level after that, but the degree of this reduction differed; for a 30 s rich-phase duration, the lowest NOx was 50 ppm, but when the rich period was progressively lengthened to 45 s and then 60 s, the NOx was reduced to a minimum of 30 and 17 ppm, respectively. This finding can be ascribed to the acquired H2 amount increasing when the regeneration time was extended, which would lead to the NOx stored in the catalyst previously being desorbed and restored more thoroughly, and which would provide more oxygen vacancies. Hence, the initial NOx adsorption concentration in the next cycle decreased and superior catalytic performance was exhibited. However, the fuel consumption would be increased as the regeneration time extended under the practical conditions. It could be concluded that when the duration of the lean and rich phases was 240 and 60 s, respectively, the NSE and NOx conversion rate reached the highest level. This finding reveals that prolonging the regeneration time appropriately is favorable due to the increased decomposition of nitrates or nitrites and the refreshing of trapping sites for the next cycle.

《4. Conclusions》

4. Conclusions

In this study, a PBCA catalyst prepared by incipient wetness impregnation was characterized by XRD, TEM, and XPS technologies in order to study its physical and chemical properties. Both complete and cycling NSR experiments were adopted to research the effect of H2 on the evolution mechanism of NSR over the NSR catalyst. The results showed that each component of the catalyst was well-crystallized. The TEM image showed that the activity component—and especially PtOx, which had a particle size in the 1–5 nm range—was equally dispersed on the γ-Al2O3. In addition, Ce existing with a Ce3+/Ce4+ ratio of 0.06 facilitated the migration of active oxygen over the catalyst. In the complete NSR experiment, the NSC increase that occurred in the temperature range of 250– 350 °C was mainly attributed to the increased NO2/NOx ratio due to NO2 being more favorably trapped on BaO sites as nitrates or nitrites than NO. When the gas compositions were switched to the rich condition, NO was quickly reduced to zero by H2, while NO2 showed a huge pulse before decreasing to zero. In cycling NSR experiments at 350 °C, the results demonstrated that the NOx conversion rate increased substantially in comparison with the complete NSR experiment. Furthermore, the NSE and conversion rate increased gradually when the regeneration time was extended (30, 45, and 60 s) due to the increased decomposition of nitrates or nitrites and the refreshment of trapping sites for the next cycle. The maximum NOx conversion of the catalyst reached 83.5% when the duration of the lean and rich phases was 240 and 60 s, respectively, indicating that prolonging the regeneration time appropriately had a favorable result.

《Acknowledgements》

Acknowledgements

The authors wish to acknowledge financial support of this research by the National Natural Science Foundation of China (51676090), the Natural Science Foundation of Jiangsu Province (BK20150513), and the Six Talent Peaks Project in Jiangsu Province. The authors acknowledge the contribution of Professor Guanjun Qiao for technical support.

《Compliance with ethics guidelines》

Compliance with ethics guidelines

Pan Wang, Jing Yi, Chuan Sun, Peng Luo, and Lili Lei declare that they have no conflict of interest or financial conflicts to disclose.