LaCoO₃ perovskite in Pt/LaCoO₃/K/Al₂O₃ for the improvement of NO_x storage and reduction performances

Abstract

A series of x wt% Pt/LaCoO₃/K/Al₂O₃ (x = 0, 0.3 and 1.0) and 1.0 wt% Pt/K/Al₂O₃ catalysts were synthesized. The effects of the addition of LaCoO₃ perovskite in x wt% Pt/LaCoO₃/K/Al₂O₃ on NO_x storage and reduction (NSR) performances were studied. 1.0 wt% Pt/LaCoO₃/K/Al₂O₃ exhibits higher NO_x storage capacity and reduction efficiency in NSR than 1.0 wt% Pt/K/Al₂O₃. Interestingly, when the Pt content is lowered from 1.0 to 0.3 wt%, 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ still shows a higher NO_x storage capacity than 1.0 wt% Pt/K/Al₂O₃ and comparable reduction properties. Moreover, 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ possesses higher resistance to SO₂ poisoning and better regenerability than 1.0 wt% Pt/K/Al₂O₃. In situ DRIFTS analysis shows that NO_x storage in 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ is mainly via the nitrate route with small amount of nitrites to nitrates conversion below 200 °C, while NO_x storage at higher temperature mainly proceeds via the nitrate route.

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Introduction

Nitrogen oxides (NO_x) are mainly derived from the combustion of gasoline, diesel and other fossil fuels. They cause serious harm to the environment and human health.^1,2 To meet the more and more stringent regulations of NO_x emissions, several promising techniques, such as NO_x storage and reduction (NSR) and selective catalysis reduction (SCR), have been developed.^3–6 Among them, the NSR technique is regarded as a promising approach for reducing NO_x emissions from lean-burn engines.⁷ NSR catalysts consist of three types of components: noble metals (Pt, Pd and Rh) as active components, alkali or alkaline earth metal oxides (K or Ba) for NO_x storage and supports with high surface area (Al₂O₃) for dispersing the former two parts. During a typical NSR process comprising a lean period and rich period, the adsorbed NO is firstly oxidized into NO₂ by noble metals and then stored on the alkali or alkaline earth metal oxides in the form of nitrites or nitrates under lean conditions. Afterwards, the stored nitrites or nitrates decompose, followed by the reduction into N₂ in the rich period.^8–11 However, sulfur poisoning and thermal deterioration usually result in loss of catalytic activity and deactivation of NSR catalysts in practical application.^9,12 In addition, the scarcity and high price of noble metals have also limited their further development and extensive applications.^10,13 Therefore, it is highly desirable to develop NSR catalysts with high SO₂ tolerance and low dependence on noble metals.^12,14–16

Great efforts have been devoted to the improvement of sulfur-resistant ability and the decrement of platinum content in the NSR catalysts. One of effective strategies is to add the inexpensive transition metal oxides or other composite oxides into Pt-based catalysts.¹⁶ The synergetic effects between the additives and Pt-based catalysts can result in the improvement of NSR performances.¹⁷ The La-based perovskite-type oxides in the form of ABO₃ have been widely employed in heterogeneous catalysis because of their low price, ready synthesis and good redox properties.^18–20 Their performances may be modified through either partial substitution of cations at A and/or B sites or by forming composite perovskites. For instance, Guo et al. found that introducing K and Pd simultaneously into the structure of LaCoO₃ perovskite can enhance NO_x catalytic performance.²¹ However, such perovskite catalysts usually possess low surface area, which engenders low NO_x storage capacity. Alternatively, dispersing perovskites on the supports with relatively large surface area and high thermal stability can improve NSR performances. For example, Meng et al. has reported that CeO₂-supported LaCoO₃ catalyst shows rapid NO_x storage and high NO_x reduction efficiency.²² Wu and his coworkers have reported that LaCoO₃/MgO composite perovskites possess a relatively higher surface area and better catalytic performance in toluene and methane oxidation than LaCoO₃.²³ Therefore, it can be speculated that the addition of inexpensive LaCoO₃ to Pt-based NSR catalysts would be a promising approach for the exploration of low-cost NSR catalysts. Herein, we report a series of x wt% Pt/LaCoO₃/K/Al₂O₃ (x = 0, 0.3 and 1.0) catalysts. The addition of LaCoO₃ improves NSR properties, sulfur resistance and regenerative ability. Moreover, no obvious sacrificing of NSR performances is detected when Pt content is lowered from 1.0 to 0.3 wt%.

Experimental section

Catalyst preparation

LaCoO₃ perovskite was synthesized by sol–gel method. Specifically, La(NO₃)₃·6H₂O and Co(NO₃)₂·6H₂O were dissolved in deionized water (100 mL), citric acid (CA) and ethylene diamine tetraacetic acid (EDTA) were added to the above solution with molar ratio of the total metal ions : CA : EDTA = 1 : 1 : 1.5. The aqueous ammonia was added dropwisely to the stirring solution until pH = 4.0–5.0. The mixture was maintained at 80 °C for the evaporation of water and the formation of transparent purple gel, followed by drying at 120 °C for 12 h. The resultant xerogel was calcined at 350 °C for 2 h to remove organic compositions, and then calcined at 800 °C for 4 h to form perovskite-typed LaCoO₃ structure.

The alumina support was synthesized according to literature methods.²⁴ LaCoO₃ (25 wt%) was loaded on Al₂O₃ by mechanical mixing. Subsequently, K (16 wt%) and Pt (0, 0.3 and 1 wt%) were loaded on LaCoO₃/Al₂O₃ by the conventional impregnation method using KNO₃ and H₂PtCl₆·6H₂O as precursors, respectively. The resultant mixture was dried at 120 °C overnight and calcined at 350 °C for 2 h to form x wt% Pt/LaCoO₃/K/Al₂O₃ (x = 0, 0.3 and 1.0).

1.0 wt% Pt/K/Al₂O₃ was prepared using similar procedures except the absence of LaCoO₃ except calcined at 400 °C for 4 h.

Catalyst characterization

Powder X-ray diffraction (XRD) was performed on a Rigaku-DMax2500PC diffractometer using a Cu-K_α radiation (λ = 1.5406 Å) in the range of 2θ from 10 to 80°. N₂ physisorption measurement was carried out on a Micromeritics ASAP 2020 apparatus. The catalysts were degassed in vacuo at 180 °C for at least 6 h before each measurement. H₂ temperature-programmed reduction (H₂-TPR) was performed on AutoChem II 2920 equipped with a TCD detector, in which the catalysts were pretreated under air flow (30 mL min⁻¹) at 400 °C for 0.5 h, and were followed by purging with Ar (30 mL min⁻¹) at the same temperature for 0.5 h. After cooling to room temperature, the temperature was increased to 600 °C at 5 °C min⁻¹ by a temperature-programmed controller in gas flow of 10 vol% H₂/Ar (30 mL min⁻¹). X-ray photoelectron spectroscopy (XPS) analysis was performed on Physical Electronics Quantum 2000, equipped with a monochromatic Al-K_α source (K_α = 1486.6 eV) and a charge neutralizer. NO_x-temperature-programmed desorption (NO_x-TPD) experiments was conducted on an AutoChem 2920 equipped with a TCD detector. A sample of 0.2 g was pretreated in 8 vol% O₂/Ar at 400 °C for 2 h with a flow rate of 100 mL min⁻¹. After cooled to room temperature, the sample was exposed to 500 ppm NO until recovery of the inlet NO_x concentration, followed by flushing with 8 vol% O₂/Ar to remove weakly absorbed NO_x species. NO_x-TPD-MS experiment was carried out from room temperature to 600 °C in a Ar flow at a rate of 3 °C min⁻¹. Mass spectroscopy (Q-MS) was used for the analysis of the gases evolving in NO-TPD quadruple. The signals for NO (m/z = 30), N₂ (m/z = 28), N₂O (m/z = 44), NH₃ (m/z = 17) and NO₂ (m/z = 46) were monitored by a QIC20 bench top gas analysis system connected to an AutoChemII 2920 outlet. In situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) were recorded on a Nicolet Nexus FTIR spectrometer in the range of 650–4000 cm⁻¹ with 32 scans at a resolution of 4 cm⁻¹. In a DRIFTS cell, the test sample was firstly pretreated with Ar at 350 °C for 30 min, and then cooled to 50 °C. After the background spectrum was recorded with a flow of Ar, the sample was exposed to reactant gas atmosphere, and then DRIFTS spectra were recorded at different temperatures.

Catalytic activity measurement. Catalytic activity tests were carried out in a stainless steel reactor (inner diameter = 8 mm) with a fixed bed. A 0.5 g of catalyst was pretreated under Ar (30 mL min⁻¹) at 400 °C for 4 h to remove surface-adsorbed species, and then cooled to test temperature. The feed gas was introduced using mass-flow controller at a total flow rate of 600 mL min⁻¹. NSR experiments were performed by a series of periodic operations from lean conditions (120 s; 8 vol% O₂, 500 ppm NO and Ar balance) to rich conditions (120 s; 500 ppm NO, 3.5 vol% H₂ and Ar balance). Outlet NO_x concentration was monitored by an on-line chemiluminescence NO–NO₂–NO_x analyzer (Model 42i-HL, Thermo Scientific). N₂ selectivity was analyzed by GC7820 A. The outlet N₂O and NH₃ were also analyzed using FTIR spectrometer (Nicolet Nexus 6700) with a heated, multiple-path gas cell. The corresponding N₂ selectivity was calculated by the following formula:

N₂ selectivity (%) = (NO_{x inlet} − NO_{x outlet} − NH_{3 outlet} − 2N₂O_outlet)/(NO_{x inlet} − NO_{x outlet}) × 100%.

NO_x storage capacity (NSC) was calculated according to the following formula after reaching a steady state^3c

NSC = (NO_{x inlet} × V × t)/(N₀ × m) × storage ratio × 10⁻³

here, NO_x is the concentration in ppm unit, V is the flow rate of feed gas, i.e. 600 mL min⁻¹, N₀ is a constant, i.e. 22.4 L mol⁻¹, m is the weight of the catalyst, i.e. 0.5 g, and storage ratio is the percentage of the amount of the stored NO_x to that of the inlet NO_x.

Sulfur poisoning. After NSR measurements, the catalysts were sulfated by exposing to a feed gas containing 8 vol% O₂, 500 ppm NO, 100 ppm SO₂ and Ar balance for 45 min.

Regeneration from sulfur poisoning. The sulfated catalysts were reduced using 3.5 vol% H₂ at 550 °C for 10, 30, 60 and 90 min, respectively, and then used for NSR measurements.

Results and discussion

Structural and textural properties

XRD patterns of LaCoO₃ and x wt% Pt/LaCoO₃/K/Al₂O₃ are shown in Fig. 1. Strong reflections peaks at 32.8 and 33.3° are assigned to LaCoO₃ with rhombohedral distortion (JCPDS 84-0848), corresponding to (1 1 0) and (1 0 4) planes, respectively,²⁵ suggesting the formation of a perovskite-typed structure. For x wt% Pt/LaCoO₃/K/Al₂O₃, such two characteristic peaks have no obvious variation, the additional weak peaks at 29.5, 41.3, 44.2 and 66.90 are related to KNO₃ species. No other phases, such as La₂O₃, CoO_x and PtO_x are detected, which suggests that these species are absent or their contents are too low to be detected by XRD measurements.²⁶

Fig. 1 XRD patterns of LaCoO₃ and x wt% Pt/LaCoO₃/K/Al₂O₃.

BET specific surface area, pore volume and pore size of LaCoO₃ and x wt% Pt/LaCoO₃/K/Al₂O₃ are listed in Table 1. The specific surface area of LaCoO₃ is only 9 m² g⁻¹, and is increased to 125 m² g⁻¹ in LaCoO₃/K/Al₂O₃. The presence of 0.3 wt% Pt has no obvious effect on the specific surface area. However, the specific surface area decrease from 126 to 68 m² g⁻¹ as Pt contents increase from 0.3 to 1.0 wt% in x wt% Pt/LaCoO₃/K/Al₂O₃, which probably attributed that the continuous increment of Pt content leads to some pores of Al₂O₃ are plugged, thus BET surface area significantly decreases in 1.0 wt% Pt/LaCoO₃/K/Al₂O₃.²⁷

Table 1 Textural properties of x wt% Pt/LaCoO₃/K/Al₂O₃

Sample	BET surface area (m^2 g^−1)	Average pore radius (nm)	Pore volume (cm^3 g^−1)
LaCoO3	9	20.13	0.10
LaCoO3/K/Al2O3	125	2.45	0.20
0.3 wt% Pt/LaCoO3/K/Al2O3	126	2.06	0.16
1.0 wt% Pt/LaCoO3/K/Al2O3	68	2.41	0.12

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NO_x storage and reduction performance

In order to evaluate the effect of LaCoO₃ addition on catalytic performances, NSR measurements of 1.0 wt% Pt/K/Al₂O₃ and x wt% Pt/LaCoO₃/K/Al₂O₃ were carried out. The outlet NO_x concentrations as a function of time during lean/rich cycles after the catalytic system reaching the steady state are shown in Fig. S1,† their NO_x breakthrough profiles show similar variation trends. Under lean conditions, the outlet NO_x concentration drops to a minimum value in 2 min, because NO_x is mainly stored on K species in the form of nitrates and/or nitrites.²⁸ Obviously, the outlet NO_x concentration in 1.0 wt% Pt/LaCoO₃/K/Al₂O₃ is much lower than that of 1.0 wt% Pt/K/Al₂O₃ at test temperatures, suggesting that the former possesses higher NO_x storage capability (NSC). When Pt content decreases from 1.0 to 0.3 wt%, the outlet NO_x concentration in 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ is also lower than that of 1.0 wt% Pt/K/Al₂O₃. Notably, the outlet NO_x concentration in LaCoO₃/K/Al₂O₃ is higher than that of 1.0 wt% Pt/K/Al₂O₃ at 200–300 °C, but it is lower than the latter at 300–400 °C. The corresponding NSC was calculated by integrating concentration curves of NO_x. As shown in Table 2, NSC in 1.0 wt% Pt/LaCoO₃/K/Al₂O₃ and 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ is much higher than that in 1.0 wt% Pt/K/Al₂O₃, suggesting the addition of LaCoO₃ facilitates NO_x storage. Interestingly, LaCoO₃/K/Al₂O₃ also shows higher NSC at 350–400 °C than 1.0 wt% Pt/K/Al₂O₃ in spite of the absence of Pt, mainly because Pt particles readily undergo severe deactivation caused by thermal aging at high temperature.²⁵ For example, NSC at 350 °C in LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ reach 21.4 and 16.6 μmol g⁻¹, respectively.

Table 2 NO_x storage capacity (NSC), surface La/Co atomic ratio, O_ads/(O_ads + O_latt) ratio and S 2p content of x wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃

Sample	NSC (μmol g^−1)					Surface La/Co atomic ratio (%)			Oads/(Oads + Olatt) (%)
	Temperature (°C)
	200	250	300	350	400	Fresh	Sulfated	Regenerated
LaCoO3/K/Al2O3	2.4	2.8	9.2	21.4	24.4	1.46	—	—	53.2
0.3 wt% Pt/LaCoO3/K/Al2O3	29.4	31.8	35.2	35.0	32.0	1.90	1.08	1.53	60.7
1.0 wt% Pt/LaCoO3/K/Al2O3	34.2	41.2	43.6	44.6	44.8	1.47	—	—	64.4
1.0 wt% Pt/K/Al2O3	11.4	13.2	15.4	16.6	18.0		—	—	34.2

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Under rich conditions (Fig. S1†), the outlet NO_x concentrations initially increase because the stored NO_x is released and can't be completely reduced in a short time. After reaching the maximum, the outlet NO_x concentrations decrease rapidly to a low level, since the stored nitrates/nitrites are decomposed and the released NO_x are eventually reduced into N₂ on platinum and LaCoO₃. With the increment of temperature, the outlet NO_x concentration decreases and reaches the lowest values at 350–400 °C, suggesting that the best NO_x reduction ability is achieved at this temperature range.²⁵ The corresponding NO_x conversions of x wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are shown in Fig. 2. An apparent increment in NO_x conversion is observed with elevating temperature at 200–350 °C. NO_x conversion follows the order of 1.0 wt% Pt/LaCoO₃/K/Al₂O₃ > 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ > LaCoO₃/K/Al₂O₃. It should be mentioned that NO_x conversion in 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ is higher than that of 1.0 wt% Pt/K/Al₂O₃ except for 300 and 350 °C. The corresponding N₂ selectivities are listed in Table S1.† The N₂ selectivities at 300 °C in 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are 90 and 88%, respectively, which reveals that the addition of LaCoO₃ in x wt% Pt/LaCoO₃/K/Al₂O₃ has no apparent influence on N₂ selectivity.

Fig. 2 NO_x conversion of 1.0 wt% Pt/K/Al₂O₃ and x wt% Pt/LaCoO₃/K/Al₂O₃.

SO₂ poisoning and regeneration performances

The superior NSR properties of 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ over 1.0 wt% Pt/K/Al₂O₃ prompt us to further examine their SO₂ tolerance and regeneration ability. The NSC of fresh, sulfated and regenerated catalysts are listed in Table 2. The sulfated catalysts show a dramatic decrement of NSC in comparison with fresh catalysts. Specifically, NSC at 350 °C for fresh 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are 35.0 and 16.6 μmol g⁻¹, respectively, which are lowered to 23.2 and 5.6 μmol g⁻¹ in the sulfated counterparts. The significant decrement is related to the formation of sulfites or sulfates on the catalysts. It has been reported that sulfated catalysts can be recovered to some extent by the reduction by H₂. The effects of regeneration time on NSC of sulfated catalysts were investigated. After 90 min, NSC of regenerated 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are 33.7 and 8.0 μmol g⁻¹, respectively. NSC of regenerated 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ reach 96.3 and 48.2% of fresh counterparts after 90 min, respectively. These results suggest that both 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are quickly deactivated in the presence of SO₂, but 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ possesses better regeneration ability than 1.0 wt% Pt/K/Al₂O₃ in the lean periods. NO_x conversions at 350 °C for fresh, sulfated and regenerated catalysts in the rich period are illustrated in Fig. S2.† NO_x conversions of fresh 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are 78 and 65%, respectively, which are lowered to 25 and 24% in sulfated catalysts, respectively. NO_x conversion was also recovered to some extent after sulfated catalysts are regenerated by H₂ at 550 °C. NO_x conversion gradually increases with the increment of regeneration time. For instances, after sulfated 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are regenerated for 60 min, their NO_x conversions are increased to 66 and 59%, respectively. All these results suggest that 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ possesses higher resistance to SO₂ poisoning and better regenerability than 1.0 wt% Pt/K/Al₂O₃ in the rich period.

H₂-TPR analysis

In order to investigate the influence of LaCoO₃ addition in x wt% Pt/LaCoO₃/K/Al₂O₃ on redox properties, H₂ temperature-programmed reduction (H₂-TPR) experiments were performed and the corresponding profiles are shown in Fig. 3. The reduction peaks at 181 °C with a shoulder peak at 233 °C are observed in 1.0 wt% Pt/K/Al₂O₃, which are ascribed to stepwise reduction of PtO₂ to PtO or Pt.²¹ The reduction peaks shift to lower temperature in 1.0 wt% Pt/LaCoO₃/K/Al₂O₃, revealing that the addition of LaCoO₃ may promote the reduction of PtO_x through hydrogen spill-over from LaCoO₃ to Pt due to the activation and dissociation of hydrogen species on Pt.²⁹ Notably, a strong reduction peak at 471 °C is detected in LaCoO₃/K/Al₂O₃, which may be assigned to the reduction of Co³⁺ to Co²⁺, forming oxygen-deficient La₂CoO₄.³⁰ It should be mentioned that the reduction peak of LaCoO₃ shifts to lower temperature in the presence of Pt, which may be associated with synergetic effect between LaCoO₃ and Pt. The consumption of H₂ in 1.0 wt% Pt/LaCoO₃/K/Al₂O₃ (713 μmol g⁻¹) is much higher than that of 1.0 wt% Pt/K/Al₂O₃ (175 μmol g⁻¹) and LaCoO₃/K/Al₂O₃ (237 μmol g⁻¹), resulting in the improvement of NSR activities, SO₂ tolerance and regeneration ability.

Fig. 3 H₂-TPR profiles of 1.0 wt% Pt/K/Al₂O₃ and x wt% Pt/LaCoO₃/K/Al₂O₃.

NO-TPD-MS

The adsorption behavior for NO is known to substantially affect NSR performances of catalysts.³² NO_x temperature-programmed desorption (NO_x-TPD) experiments were carried out in order to determine the effect of LaCoO₃ addition on the adsorption ability of NO. As shown in Fig. 4, NO desorption peaks at 99–109 °C, 378–393 °C and 467–518 °C are ascribed to desorption of NO_x, surface adsorbed nitrite/nitrate and bulk nitrate species, respectively.³³ Compared to 1.0 wt% Pt/K/Al₂O₃, the larger desorption area and lower desorption temperature between 300 and 550 °C are observed in 1.0 wt% Pt/LaCoO₃/K/Al₂O₃, revealing the addition of LaCoO₃ can facilitate desorption of adsorbed and bulk nitrate species. Besides, the desorption temperature of the surface adsorbed nitrite/nitrate species in 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ shifts to low temperature in comparison with that in 1.0 wt% Pt/K/Al₂O₃. To evaluate desorption species from NO-TPD, an online mass tracking was performed. The temperature-dependent mass profiles in x wt% Pt/LaCoO₃/K/Al₂O₃ suggest that NO and N₂ are the major products, the trace amount of N₂O, NO₂ and NH₃ are also generated during NO desorption (Fig. S3†). Notably, the amounts of N₂O and NH₃ in x wt% Pt/LaCoO₃/K/Al₂O₃ is higher than that in LaCoO₃/K/Al₂O₃.

Fig. 4 NO-TPD profiles of x wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃.

XPS analysis

X-ray photoelectron spectroscopy (XPS) is an effective technique to gain information of surface element compositions, metal oxidation states and the adsorbed species of a catalyst. As shown in Table 2, the surface La/Co atomic ratio in all of the LaCoO₃-containing catalysts is more than 1, suggesting the surface of these catalysts is enriched by La. The oxygen vacancies are known to play an important role in catalytic reactions.³⁴ since they may accelerate the adsorption and dissociation of oxygen molecules, resulting in the formation of highly active electrophilic oxygen molecules.²⁶ XPS spectra of O 1s for x wt% Pt/LaCoO₃/K/Al₂O₃ are depicted in Fig. 5A, and the related quantitative O_ads/(O_ads + O_latt) ratio are summarized in Table 2. Based on the deconvolution of XPS peaks for O 1s, there are two peaks of binding energies, corresponding to two types of oxygen species. The low binding energy peak at 530.4 eV is related to lattice oxygen, while high one at 532.0–532.2 eV may be assigned to surface adsorbed oxygen species, which are derived from the adsorption of gaseous O₂ on oxygen vacancies.³⁵ The O_ads/(O_latt + O_ads) ratio in the catalysts follows the trend: 1.0 wt% Pt/K/Al₂O₃ < LaCoO₃/K/Al₂O₃ < 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ < 1.0 wt% Pt/LaCoO₃/K/Al₂O₃, revealing that 1.0 wt% Pt/LaCoO₃/K/Al₂O₃ possesses the highest amount of adsorbed surface oxygen species. These results suggest that the addition of LaCoO₃ into 1.0 wt% Pt/K/Al₂O₃ can promote the formation of adsorbed surface oxygen species, which is favorable for the activation and oxidation of NO to NO₂.

Fig. 5 XPS spectra of La 3d, Co 2p, O 1s, S 2p in x wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃.

S 2p XPS spectra of sulfated 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are shown in Fig. 5B and C. The peak at 168.4 eV is assigned to sulfate species owing to sulfur uptake after SO₂ exposure.³⁶ Sulfur contents in sulfated 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are 2.55 and 2.60%, respectively, while sulfur contents in regenerated counterparts decrease to 1.42 and 2.25% after sulfated samples were regenerated for 90 min, respectively (Table 2). The sulfur elimination ratios of 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are 44 and 13.46%, respectively. These results further show superior regeneration ability of 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ in comparison with 1.0 wt% Pt/K/Al₂O₃.

K 2p XPS spectra of fresh and sulfated 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are given in Fig. 5D and E, the binding energy peaks of K 2p in fresh 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ and 1.0 wt% Pt/K/Al₂O₃ are at 293.02 and 292.74 eV, respectively. After sulfation, these peaks shift to 292.06 and 292.60 eV, respectively, owing to the formation of surface K₂SO₄ and/or K₂SO₃. After the sulfated catalysts were regenerated for 90 min, the peaks shift to 292.94 and 292.61 eV, respectively, indicating that most of K₂SO₄ and/or K₂SO₃ are reduced by hydrogen for 0.3 wt% Pt/LaCoO₃/K/Al₂O₃, while only partial K₂SO₄ and/or K₂SO₃ are reduced for 1.0 wt% Pt/K/Al₂O₃. For La 3d spectra of fresh and sulfated 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ (Fig. 5F), no obvious peak shift is observed, but their intensity is different, suggesting the presence of SO₂ has no obvious effect on La because cobalt inhibits neighboring La forming sulfite/sulfate species. For Co 2p spectra (Fig. 5H), the binding energy peaks in fresh and sulfated catalysts are 780.89 and 781.07 eV, respectively, and the surface La/Co atomic ratio decreases from 1.90 to 1.08% after sulfation and then increases to 1.53% after regeneration for 90 min (Table 2). These results suggest that sulfate species are formed on cobalt in the presence of SO₂, and partial cobalt sulfate is reduced in the regeneration process. For O 1s spectra (Fig. 5I), the broad peak at 531.31 eV in fresh 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ is attributed to oxygen in metal oxides, while the peak shifts to 531.38 eV after sulfation, suggesting that oxides are partially sulfated in the presence of SO₂.²¹ However, XPS spectra of Pt 4f and Al 2p can't be distinguished from XPS due to the overlap of their binding energy.

In situ DRIFTS

To understand NO_x storage process, in situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) of 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ at different temperatures were carried out. As shown in Fig. 6, when 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ was exposed to 1250 ppm NO and 7.5 vol% O₂ balanced by Ar at 100 °C, the peak at 1268 cm⁻¹ suggests the formation of free nitrite ions.¹⁷ The peaks at 1318 and 1422 cm⁻¹ are assigned to bidentate and monodentate nitrates, respectively. The peaks at 1043 and 1104 cm⁻¹ correspond to free ionic nitrates.³¹ With the increment of temperature, the peak at 1268 cm⁻¹ gradually weakens, while the nitrate peak at 1318 cm⁻¹ becomes more intensified gradually, and the peak reaches the maximum at 350 °C, suggesting that the increasing temperature favors the activation of oxygen species and subsequent oxidation of nitrites to nitrates. However, all the peaks intensities decrease when the temperature is increased to 400 °C, which may be related to decomposition of nitrite or nitrate species. The negative peaks around 1765 cm⁻¹ probably result from the transformation of carbonates to nitrates/nitrites as proved by the released gaseous CO₂ with its characteristic bands at 2363 cm⁻¹.²² Therefore, NO_x storage in 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ mainly proceeds via nitrate route with small amount of nitrites to nitrates conversion below 200 °C, while storage process mainly is nitrate route at high temperature. It was reported that the nitrites can be converted to nitrates up to at least 250 °C over Pt/K/Al₂O₃,^37,38 the lower oxidation temperature of nitrites to nitrates in 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ than that of Pt/K/Al₂O₃ may be related to the existence of a strong interaction between Pt and LaCoO₃, which is beneficial to promote NO_x storage capacity.

Fig. 6 In situ DRIFTS spectra of 0.3 wt% Pt/LaCoO₃/K/Al₂O₃.

Conclusions

A series of x wt% Pt/LaCoO₃/K/Al₂O₃ (x = 0, 0.3 and 1.0) catalysts were prepared for NSR. The addition of LaCoO₃ into 1.0 wt% Pt/K/Al₂O₃ results in promotion of NSR activity, SO₂ tolerance and regenerability. The superior NSR performances in LaCoO₃-additive Pt/K/Al₂O₃ are associated with stronger interaction between LaCoO₃ and Pt, lower NO desorption temperature and more surface adsorption oxygen species. More importantly, when the Pt content decreases from 1.0 wt% in Pt/K/Al₂O₃ to 0.3 wt% in Pt/LaCoO₃/K/Al₂O₃, higher NSC and comparable NO_x reduction properties are demonstrated, which provides a new route for development of effective and cheap Pt-substituted NSR catalysts.

Acknowledgements

The authors acknowledge the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2013CB933200), National Natural Science Foundation of China (21471151) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18273a

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