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Feb 5, 2013 - Carbonate-Based Lean-Burn NOx Trap Catalysts Pt–K2CO3/ZrO2 with Large NOx Storage ... sequential impregnation, which show extremely go...
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Carbonate-Based Lean-Burn NOx Trap Catalysts Pt−K2CO3/ZrO2 with Large NOx Storage Capacity and High Reduction Efficiency Nana Hou, Yuxia Zhang, and Ming Meng* Tianjin Key Laboratory of Applied Catalysis Science and Engineering, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, P. R. China

ABSTRACT: A series of carbonate-based lean-burn NOx trap (LNT) catalysts Pt−K2CO3/ZrO2 with different K2CO3 loading were prepared by sequential impregnation, which show extremely good performance for lean NOx storage and reduction. The catalyst containing 15 wt % K2CO3 exhibits a large NOx storage capacity of 2.16 mmol/g and a very high NOx reduction percentage of 99%. Multiple techniques including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), temperature-programmed decomposition (TPD), Fourier-transform infrared spectroscopy (FT-IR), and in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) were employed for catalyst characterization. The results of XRD, FT-IR, and HR-TEM conformably show that, at room temperature, the K species exist as amorphous K2CO3; while at NOx storage temperature (350 °C), three kinds of K species including −OK groups, K2O, and K2CO3 are simultaneously present in the catalysts as revealed by in situ DRIFTS, TPD, and FT-IR results. Surface carbonates are identified as the most active species for NOx storage, showing the best NOx storage performance. Higher K2CO3 loading than 15 wt % leads to the formation of more bulk or bulk-like K2CO3 species, which are unfavorable to NOx storage. As K2CO3 loading is 10 wt % or less, the NOx is mainly stored as nitrates species such as monodentate nitrates, ionic nitrates, and bridging bidentate nitrates, while at higher K2CO3 loading, the NOx is only stored as bidentate nitrite species. The presence of excess amount of K2CO3 can decrease the ability of the catalysts for NO adsorption and oxidation, making the NOx oxidized only to nitrite species. stability of K and Li sulfates than Ba sulfate.8−14 Thermodynamic evaluation and reaction data demonstrate that the basicity of the component is directly related to the NOx trapping performance; at 350 °C, the performance of different basic components was found to decrease in the following order: K > Ba > Sr ≥ Na > Ca > Li.1,15 So, in this work, K was selected as NOx storage component. It is known that NOx storage performance of NSR catalysts is not only related to the kinds of basic components but also to their existing states. In our previous work, it was found that the presence of K2CO3 is favorable to NOx storage.8,16 The amount and surface states of K2CO3 is mainly determined by the kinds of starting Kcontaining salts such as K2CO3,8,9,16 CH3COOK,17−20 and KNO3.11,14,21 In many cases11,14 potassium oxides or surface −OK groups are considered as the main NOx storage

1. INTRODUCTION The much higher fuel utilizing efficiency and lower CO2 emission of lean-burn engines make them more and more attractive to vehicle manufacturers as compared with the traditional engines operating at the chemical stoichiometric point.1,2 However, under lean condition, nitrogen oxides (NOx) are extremely hard to reduce over the traditional three-way catalysts due to the presence of excess oxygen. So, exploration of new kinds of catalysts or techniques is necessary. Lean NOx storage and reduction (NSR) also called lean NOx trap (LNT) technique proposed by Toyota in mid-19903−5 is now still regarded as one of the most promising techniques for lean NOx removal. At present, the most commonly studied catalyst is the Pt/Ba/Al2O3 system. However, the Ba-containing catalysts are highly sensitive to sulfur species, showing very low sulfur-resisting performance.5−7 By replacement of Ba with other basic components, such as K or Li, the sulfur-resistance and regeneration performance of the NSR catalysts could be greatly improved due to the much lower thermodynamic © 2013 American Chemical Society

Received: November 29, 2012 Revised: January 17, 2013 Published: February 5, 2013 4089

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350 °C. So, it is still necessary to perform further in situ research work to obtain new and deep insights into the kinds and states of K species, and their performance during NOx storage. In the present work, a series of potassium carbonate-based NSR catalysts Pt−K2CO3/ZrO2 with different K2CO3 loading were prepared by impregnation. Several techniques, such as Xray diffraction (XRD), high-resolution transition electronic microscopy (HR-TEM), Fourier-transform infrared spectroscopy (FT-IR), temperature-programmed decomposition (TPD), and in situ diffuse reflectance infrared Fouriertransform spectroscopy (in situ DRIFTS) were employed to characterize the catalysts, especially the kinds and states of K species. Direct spectroscopic evidence for the existence and involvement of K2CO3 species in NOx storage was obtained. The NOx storage mechanism over these K2CO3-based NSR catalysts was revealed by in situ DRIFTS. The correlation between the existing states of K species and their NOx storage performance was well established.

components after the NSR catalysts are prereduced by H2 at 350−500 °C. However, the presence of potassium carbonates and their involvement in NOx storage could not be excluded because potassium carbonates often decompose at very high temperature and cannot be reduced by H2, far different from potassium nitrates. Up to now, little attention has been paid to potassium carbonates used as a NOx storage component, and little is known about the relationship between the existing states and NOx storage performance of potassium carbonates. Therefore, in the present work, K2CO3 was selected as the starting salt of NOx storage components. It is known that the surface properties of supports can directly affect the interaction between K2CO3 and supports, so the kinds of support are also important to the stability, amount, and existing states of K2CO3 species. It is revealed that the decomposition of K2CO3 could be initiated by the hydroxyl groups on support surface, forming −OK groups, which is relatively less active to NOx storage as compared with potassium oxides and carbonates.22 ZrO2 as a weak basic support23,24 possesses very small amounts of hydroxyl groups on its surface as compared with traditional supports Al2O3, SiO2, etc.; the employment of ZrO2 as the support can avoid the formation of large amounts of −OK groups and enhance the NOx storage performance of NSR catalysts. So, ZrO2 was chosen as the support of the studied NSR catalysts in this work. The existing states of storage components such as potassium or barium species in NSR catalysts are determined not only by the support property but also by their loadings. In the early research, a lot of work has been performed on the states of barium species and their performance for NOx storage. Scholz et al. proposed a multiple-storage-sites mechanism and considered that fast NOx storage mainly occurred on surface barium species and that slow NOx storage took place on semibulk or bulk barium species where diffusion played a major role.25 Other researchers divided the barium species into three kinds, namely, the amorphous BaO, amorphous barium carbonates (LT-BaCO3), and crystalline barium carbonates (HT-BaCO3); among them, the LT-BaCO3 exhibited the highest reactivity for NOx storage.12,26−29 For K-containing NSR catalysts, it is reported that there are also several kinds of K species identified and that NOx could be preferentially adsorbed by K2O, then by KOH (with release of water), and finally by K2CO3 (with CO2 release).17 Additionally, surface −OK groups are thought as another kind of K species in Kbased NSR catalysts, which can also act as NOx storage sites though they are not as active as K2O and K2CO3 species for NOx storage.5,8,16,22 Since K species often possess high dispersion, it is hard to characterize these K-related species including surface −OK groups, potassium oxides, and carbonates. Up to now, our knowledge about these K species is very limited, and it is still in controversy that what kind of K species are the main NOx storage phases and how these species influence the NOx storage behavior of the NSR catalysts. Our earlier studies8,16 suggest that high K loading and high calcination temperature of support facilitate the formation of K2CO3, but no direct evidence was obtained to demonstrate the existence and involvement of K2CO3 in NOx storage; meanwhile, it should be noted that the previous characterization work on the kinds and distribution states of K species was performed at room temperature, which cannot reflect the real situation of K species in the course of NOx storage because the NSR catalysts were always prereduced at 350−500 °C in H2-containing atmosphere before being used in NOx storage at

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The support ZrO2 was prepared by precipitation using an aqueous solution of zirconium oxychloride and aqueous ammonia. The precipitate was kept at room temperature for 24 h, then filtered and washed with deionized water until no chloride ions were detected by adding AgNO3 solution to the filtrate. After drying at 120 °C for 24 h, the precipitate powder was calcined at 500 °C for 5 h in air to obtain the final ZrO2 support. The Pt−K2CO3/ZrO2 catalysts were prepared by sequential impregnation. At first, the ZrO2 support was impregnated with an aqueous solution of H2PtCl6·6H2O, then dried at 120 °C overnight, and calcined in air at 500 °C for 1 h to obtain the precursor. The loading of Pt is 1 wt %. Next, the precursor was impregnated with an aqueous solution of K2CO3 (5, 10, 15, and 20 wt %). After drying and calcination under the same condition as above, a series of Pt−K2CO3/ZrO2 catalysts were successfully prepared, which were denoted as Pt−XK2CO3/ ZrO2, where X represents the weight loading of K2CO3. Before being used in NOx storage, the powder sample was first pressed, crushed, and sieved to 40−60 mesh, then reduced at 500 °C for 1 h in the atmosphere of 20 vol % H2 in N2. After NOx storage experiments, the catalysts were reduced at 350 °C for 30 min in a mixture gas of 20 vol % H2 in N2. The obtained samples were denoted as Pt−YK2O/ZrO2. 2.2. Catalyst Characterization. XRD patterns were recorded on an X’pert Pro diffractometer (PANAlytical Company) with a rotating anode using Co Kα as radiation source (λ = 0.1790 nm) at 40 kV and 40 mA. The data of 2θ from 10° to 90° were collected with the step size of 0.02°. HRTEM images were obtained using a Philips TecnaiG2F20 system operating at 200 kV. FT-IR experiments were registered with a Nexus FT-IR spectrometer (Thermal Nicolet Co.) in KBr pellets. The spectra were recorded from 400 to 4000 cm−1 at a resolution of 4 cm−1. TPD for the samples impregnated with K2CO3 was conducted on a TPDRO 1100 apparatus supplied by Thermo-Finnigan Company. Each time, 20 mg of the sample were heated from room temperature to 900 °C at a rate of 10 °C min−1 in the atmosphere of pure helium. Before detection by thermal conductivity detector (TCD), the effluent gas was 4090

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purified by a trap containing Mg(ClO4)2 in order to remove H2O. In situ DRIFT spectra were recorded on the same Nexus FT−IR spectrometer in the range of 650−4000 cm−1 after accumulating 32 scans at a resolution of 4 cm−1. In order to study the decomposition of K2CO3 at different temperatures, the powder samples were first pretreated in N2 at 150 °C for 1 h, and subsequently, the background spectrum was recorded; then the temperature was increased from 150 to 500 °C at an interval of 50 °C. The DRIFT spectra were collected after exposure to N2 atmosphere for 10 min at each temperature plateau. The NOx sorption/storage over different samples at 350 °C was also characterized by in situ DRIFTS. The powder sample was first reduced in 5% H2/N2 at 500 °C for 1 h; then the temperature was dropped to 350 °C to record the background spectrum, and subsequently, the sorption gas of 400 ppm NO + 5% O2 + N2 was introduced to the sample cell. The DRIFT spectra were collected at different exposure time up to 60 min to follow the evolution of different surface species. After NOx storage, some K exists as nitrite/nitrate species, which influence the identification of carbonate species; to eliminate the contribution of nitrite/nitrate species to the spectra, the sample was reduced in 5% H2/N2 for 15 min at the same temperature, and the spectra were recorded again. The flow rate of the mixture gas was always 50 mL/min. 2.3. NOx Storage Capacity (NSC) and Reduction Performance Measurement. NSC measurements were carried out in a quartz-tubular continuous flow reactor (i.d. = 8 mm) containing 0.5 g of each catalyst (40−60 mesh). After the temperature reached 350 °C, a mixture gas consisting of 400 ppm NO and 5% O2 and the balance N2 were introduced at a rate of 400 mL/min, corresponding to a space velocity of ∼20 000 h−1. The concentrations of NO, NO2, and total NOx at the reactor outlet were monitored online by a Chemiluminescence NO−NO2−NOx Analyzer (Model 42i-HL, Thermo Scientific). The NOx reduction performance of the samples was evaluated in lean/rich cycles. The experiments were conducted in the same reactor as above, using a lean period of 5 min and a rich period of 1 min. In lean period, the mixture gas of 400 ppm NO + 5 vol % O2 + balance N2 was introduced to the sample with a flow rate of 150 mL/min, while in rich period, the gas of 1000 ppm C3H6 was introduced with a flow rate of 150 mL/ min. In each case, enough cycles were performed so that the data presented can reflect the average level.

Figure 1. NOx storage curves for the samples Pt−XK2CO3/ZrO2.

Table 1. NOx Storage Capacity (NSC) of the Fresh Pt− XK2CO3/ZrO2 and the Samples after Being Used in One Lean-Rich Cycle As Listed in Parentheses, the NSC Decreasing Percentage (%) Based on the Fresh Samples, and the NOx Reduction Percentage (%) after 8 Lean-Rich Cycles sample Pt−5%K2CO3/ ZrO2 Pt−10% K2CO3/ZrO2 Pt−15% K2CO3/ZrO2 Pt−20% K2CO3/ZrO2

3. RESULTS AND DISCUSSION 3.1. NOx Storage and Reduction. Figure 1 presents the isothermal NOx storage curves at 350 °C over the fresh catalysts with different K2CO3 loading. It can be seen that, at the beginning, most NOx is quickly adsorbed or stored by the catalysts with its concentration decreasing to the bottom within several minutes. After some time, the NOx concentration increases slowly, approaching a steady-state value. The NOx storage capacity (NSC) for the samples calculated from their isothermal NOx storage curves is listed in Table 1. It is found that the sample Pt/ZrO2 without K exhibits very low NSC (0.1 mmol/g), while those containing K2CO3 show large NSC. With the increase of K2CO3 loading the NSC increases remarkably first, then decreases gradually, exhibiting a volcano-type tendency. The maximal NOx storage capacity (2.16 mmol/g) is achieved on the sample Pt−15%K2CO3/ZrO2. Figure 2

NSC (mmol/g)

NSC decreasing percentage (%)

NOx reduction percentage (%)

0.57 (0.45)

21

90

1.43 (1.12)

22

96

2.16 (1.49)

31

99

2.09 (1.80)

14

92

Figure 2. NOx storage curves for the samples Pt−YK2O/ZrO2.

displays the typical NOx storage curves of Pt−YK2O/ZrO2 samples, which were reduced after used in one lean-rich NOx storage and reduction cycle. Their NOx storage capacity (NSC) is also listed in Table 1. From these data, it can be seen that the NOx storage capacity of Pt−YK2O/ZrO2 increases monotonically with the increase of K2CO3 loading. At the same K2CO3 loading, the catalyst Pt−YK2O/ZrO2 always exhibits much lower NSC than the corresponding catalyst Pt−XK2CO3/ZrO2. The largest decreasing percentage of NSC is also observed on the sample with 15% K2CO3. The different performance for NOx storage of these two series of catalysts suggests that the states of K species including kinds and distribution in them may be very different. 4091

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Figure 3. NOx storage and reduction over Pt−XK2CO3/ZrO2 catalysts during lean/rich cycles: (a) Pt−5%K2CO3/ZrO2, (b) Pt−10%K2CO3/ZrO2, (c) Pt−15%K2CO3/ZrO2, and (d) Pt−20%K2CO3/ZrO2.

The successively alternative lean/rich cycling experiments for NOx storage and reduction were also performed at 350 °C over the Pt−XK2CO3/ZrO2 catalyst, the results of which are shown in Figure 3. By comparison, it is found that the NOx storage behavior of the Pt−XK2CO3/ZrO2 catalysts with different K2CO3 loading is different during the lean stage. As displayed in Figure 3a, over Pt−5%K2CO3/ZrO2, the NOx concentration gradually increases with the time going since the storage sites are gradually saturated;14 at the end of the lean period (5 min) for the last two cycles, the NOx concentration reaches about 30 ppm (dash line). With the loading of K2CO3 increasing, the corresponding NOx concentration obviously decreases, as shown in Figure 3b−d, especially for the sample with 15% K2CO3, over which nearly no NOx is detected during the whole lean period, further confirming that this sample possesses the best NOx storage performance. When the atmosphere is switched from lean to rich flow, the NOx is released suddenly with its concentration increasing to a considerable value at this moment, but it decreases rapidly and sharply due to its reduction by C3H6. After 8 lean-rich cycles, the NOx reduction percentages over these catalysts are calculated and listed in Table 1. It can be seen that the sample Pt−15%K2CO3/ZrO2 shows the best NOx reduction performance, giving a NOx reduction percentage as high as 99%. 3.2. Catalyst Characterization. The XRD patterns of all the samples Pt−XK2CO3/ZrO2 with different K2CO3 loading are shown in Figure 4. All the diffraction peaks in the patterns are attributed to ZrO2 phase; no diffraction peaks corresponding to Pt- or K-related species appear. Many works reported a similar situation that the Pt- and K-containing species are hardly recognized by XRD analysis.15−18 So, it is deduced that

Figure 4. XRD patterns of the samples Pt−XK2CO3/ZrO2.

the K2CO3 phase probably exists in amorphous state and that the Pt concentration may be below the XRD detection limit.15 The morphological and structural features of the samples Pt−XK2CO3/ZrO2 are investigated by HR-TEM, as shown in Figure 5. The (1,0,0) and (0,1,1) planes of ZrO2 with the lattice spacing of 0.51 and 0.34 nm are clearly observed, but no crystal planes of Pt and K2CO3 phases are found; so, it is inferred that Pt and K2CO3 species should be present in the amorphous or highly dispersed state on the support surface, which is in good agreement with the XRD results above. Ghiotti et al.18 revealed the presence of a highly dispersed crystalline thin layer of monoclinic K2CO3 and cubic K2O phases; and the K2O rapidly converted into K2CO3 as re-exposed to the laboratory atmosphere. By careful observation on the HR-TEM images 4092

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ZrO2, three characteristic IR bands of K2CO3 at 1327, 1582, and 1652 cm−1 are identified. They are easily assigned to bridged carbonates (CO2 asymmetric stretch), carboxylate ions (CO2 asymmetric stretch), and bridged bidentate carbonates (CO asymmetric stretch), respectively.18,19,22 With the increase of K2CO3 loading, these bands are enhanced; meanwhile, two additional bands appear at 1363 and 1457 cm−1, which can be attributed to chelating bidentate carbonates (CO2 asymmetric stretch) and bicarbonates (CO2 asymmetric stretch).22 HR-TEM and FT-IR results conformably indicate that the K species in the catalysts mainly exist as carbonates. 3.3. Temperature-Programmed Decomposition of the Samples. Although K2CO3 has been identified as the main existing K species in the catalysts Pt−XK2CO3/ZrO2 at room temperature, the real K species during NOx storage are still unclear since before NOx storage the fresh samples were prereduced at 500 °C for 1 h in a gas flow of 20 vol % H2 in N2. To better know the real existing state of K-species during NOx storage, the temperature-programmed decomposition of K2CO3 in the catalysts was performed. As seen from Figure 7, several CO2 desorption peaks are observed at different

Figure 5. HR-TEM images of the samples Pt−XK2CO3/ZrO2: (a) Pt−5%K2CO3/ZrO2, (b) Pt−10%K2CO3/ZrO2, (c) Pt−15%K2CO3/ ZrO2, and (d) Pt−20%K2CO3/ZrO2. (e) EDS analysis of Pt−15% K2O/ZrO2.

in Figure 5, we do find a thin layer of amorphous material covering the surface of support; the thickness of this layer obviously increases with K2CO3 loading increasing. To know more about the amorphous material, the energy dispersive spectroscopy (EDS) analysis was performed on Pt−15% K2CO3/ZrO2. The results show that the amorphous material contains the elements of K, C, O, and Pt. Since all K oxides can readily react with CO2 in air to form K2CO3, the presence of K oxides could be excluded. The simultaneous existence of K, C, and O elements suggests that the main K species in the amorphous material should be K2CO3. To further confirm this point, FT-IR characterization was carried out on the samples, the results of which are shown in Figure 6. For Pt−5%K2CO3/

Figure 7. TPD profiles of the samples Pt−XK2CO3/ZrO2.

temperatures, suggesting multiple existing states of K2CO3 in the samples. In general, the decomposition of pure K2CO3 takes place at very high temperature (∼900 °C).30 The much lower temperature for K2CO3 decomposition should be resulted from the interaction between K2CO3 and the support. It has been reported that, when K2CO3 was loaded on γ-Al2O3, its decomposition started at much lower temperature of about 200 °C.22 This decomposition was initiated by the interaction between K2CO3 and the hydroxyl groups on γ-Al2O3 surface, generating −OK groups as described in the following reaction: K 2CO3 + 2( −OH) → CO2 + H 2O + 2( −OK)

(I)

On the surface of ZrO2, a small amount of hydroxyl groups are present as confirmed by pyridine-IR (not shown), which can react with K2CO3 and enhance its decomposition at lower temperature. According to this analysis, in Figure 7, the peaks in low-temperature region (below 400 °C) can be attributed to the decomposition of the K2CO3 species, which directly interact with the surface hydroxyl groups, and the decomposition process should take place via reaction I above. Those peaks between 400 and 750 °C may come from the decomposition of surface K2CO3 species, which have high

Figure 6. FT-IR spectra of the samples Pt−XK2CO3/ZrO2. 4093

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Figure 8. In situ DRIFT spectra recorded at different temperatures during the decomposition of the samples Pt−XK2CO3/ZrO2 in pure N2: (a) Pt− 5%K2CO3/ZrO2, (b) Pt−10%K2CO3/ZrO2, (c) Pt−15%K2CO3/ZrO2, and (d) Pt−20%K2CO3/ZrO2.

temperature. The appearance of negative peaks indirectly reflects the disappearance of some K2CO3 species. Combined with the FT-IR results in Figure 6 (bands of K2CO3 center at 1327, 1363, 1457, 1582, and 1652 cm−1), it is inferred that some K2CO3 species can decompose below 500 °C, following the reactions I and II. Such deduction is consistent with the TPD results above; however, TPD results also indicate that some K2CO3 species decompose at higher temperature than 500 °C; so, it is definite that there is still some K2CO3 species existing in the samples after being heated to 500 °C. During NOx storage capacity measurement, the samples were also pretreated at 500 °C; therefore, it is believed that there are at least three kinds of K species, namely, surface −OK groups, K2O, and K2CO3 existing in the pretreated samples, which should be the active species for NOx sorption and storage. 3.4. In Situ DRIFTS Characterization on NOx Sorption and Storage. Figure 9 shows the DRIFT spectra of NO and O2 coadsorption on the samples Pt-XK2CO3/ZrO2. The spectra of the sample with 5% K2CO3 show three bands around 1456, 1373, and 1282 cm−1, which are attributed to monodentate nitrates, ionic nitrates, and bridging bidentate nitrates,18,21 respectively. For Pt−10%K2CO3/ZrO2, a trace band at 1243 cm−1 is detected, which is assigned to bidentate nitrite.18 When K2CO3 loading reaches 15% or higher, the bands of nitrates species disappear, and only one dominant band of bidentate nitrite around 1243 cm−1 is observed, which suggests that high loading of K2CO3 is favorable to NOx storage as nitrite species. As indicated by TPD results (Figure 7) and in situ DRIFT spectra recorded in N2 atmosphere (Figure 8), there are several kinds of K species in the samples during NOx storage, including −OK groups, K2O, and K2CO3. When the K2CO3 loading is

dispersion or exist in amorphous state. The peaks in the hightemperature region (above 750 °C) should correspond to the decomposition of bulk or bulk-like K2CO3 species. The decomposition of both the surface K2CO3 and bulk-like or bulk K2CO3 species can generate K2O species via the following reaction: K 2CO3 → CO2 + K 2O

(II)

It should be noted that, with the increase of K2CO3 loading, the amount of −OK groups arising from the reaction between K2CO3 and surface hydroxyl groups are increased, suggesting the more complete covering of the support surface by K2CO3 species; in addition, the amounts of the bulk or bulk-like K2CO3 species, which decompose above 750 °C, are also increased although their decomposition peaks have not ended until 900 °C. The amounts of the K2CO3 species, which decompose at medium temperature, show relatively small change as K2CO3 loading increases. To obtain deep insights into the decomposition behavior of different K2CO3 species in the catalysts with different K2CO3 loading, the in situ DRIFT spectra of Pt−XK2CO3/ZrO2 samples were recorded during a heating process in N2, the results of which are shown in Figure 8. In each measurement, the samples were first pretreated in pure N2 at 150 °C for 1 h to remove surface adsorbed water; then the background spectrum was collected. For Pt−5%K2CO3/ZrO2, several negative peaks of K2CO3 species (1327, 1580, and 1652 cm−1) are detected as the temperature increases to 200 °C, and their intensity is enhanced with temperature increasing. For other samples with higher loading of K2CO3, only the main negative peaks appear at ∼1652 cm−1, which also get stronger with the elevation of 4094

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Figure 9. In situ DRIFT spectra of Pt−XK2CO3/ZrO2 recorded at 350 °C after exposure to the atmosphere of 400 ppm NO + 5% O2 + N2 for different time: (a) Pt−5%K2CO3/ZrO2, (b) Pt−10%K2CO3/ZrO2, (c) Pt−15%K2CO3/ZrO2, and (d) Pt−20%K2CO3/ZrO2. (e) DRIFT spectra of the samples after being reduced by H2.

not higher than 10%, most of these species may be available to NOx storage, forming different nitrite and nitrate species. However, with the increase of K2CO3 loading (≥15%), most of the surface −OK and K2O species may be covered by the excess K2CO3, thus only the outer surface K2CO3 species could react with NOx species, forming nitrite species. In addition, too large amounts of K2CO3 may also cover some Pt sites, decreasing the oxidation ability of the catalysts and making the NOx mainly stored as nitrite species. To confirm the involvement of K2CO3 species in NOx storage, extra DRIFTS experiments were performed on the after-NOx-storage samples to get direct spectroscopic evidence. It is known that, if K2CO3 species participate in NOx storage, there will appear negative bands of K2CO3 due to its consumption. However, the increased bands of nitrite/nitrate

species could compensate the negative bands of K2CO3 because they appear in similar regions; as a result, negative bands of K2CO3 may be weakened or hardly observed. In order to fully reveal the negative bands of K2CO3, the after-NOx-storage samples containing nitrite/nitrate species were in situ reduced with H2, after this treatment, the DRIFT spectra were recorded, as shown in Figure 9e. Two obvious negative peaks around 1320 and 1600 cm−1 attributed to K2CO3 (see Figure 6) are clearly observed. The intensity of these bands remarkably increases with the increase of K2CO3 loading, suggesting more and more carbonate species involved in NOx storage. The sample Pt−15%K2CO3/ZrO2 with the strongest inverse bands should possess the largest amount of available K2CO3 for NOx storage. In Figure 6, it has been found that, when K2CO3 loading is more than 5%, some other bands of K2CO3 at 1363 4095

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Figure 10. Models of the distribution states of K species in the samples Pt−XK2CO3/ZrO2 before and after NOx storage.

and 1457 cm−1 are detectable, but in Figure 9e, no negative bands at 1363 and 1457 cm−1 are observed, which implies that these carbonate species may be still remaining in the catalysts even after pretreatment at 500 °C. This analysis is also consistent with the TPD results (Figure 7). The K2CO3 species with the IR bands around 1320 and 1600 cm−1 should correspond to surface carbonate species, which are active and available to NOx storage; while the other K2CO3 species with the IR bands at 1363 and 1457 cm−1 may be the bulk or bulklike carbonate species. On the basis of the maximal conversion of K2CO3 to nitrite/nitrate on the sample Pt−15%K2CO3/ ZrO2, it is inferred that the amount of accessible surface carbonate species on this sample has reached the maximum; further increase of K2CO3 loading can only increase the amount of bulk or bulk-like carbonate species. Therefore, the optimal loading of K2CO3 in Pt−XK2CO3/ZrO2 should be 15%. This conclusion is strongly supported by the NSC measurement results (Table 1). In section 3.1, it is elucidated that the samples Pt−YK2O/ ZrO2 always show lower NSC than Pt−XK2CO3/ZrO2 and that the NSC of Pt−15%K2CO3/ZrO2 decreases to the largest extent (Table 1). This suggests that potassium carbonate species possess higher NOx storage ability than K2O species. The transformation of more carbonate species to K2O species results in more remarkable decrease in NSC. In Pt−15% K2CO3/ZrO2, the largest amount of carbonate species has been converted to nitrite/nitrate species during NOx storage, as revealed by the in situ DRIFT spectra in Figure 9e; so, it is natural that its NSC decreases to the largest extent. 3.5. Distribution State of K-Containing Species. By combining all the results of NSC, XRD, HRTEM, FT-IR, TPD, and in situ DRIFTS, models describing the existing states of K species in the samples with different K2CO3 loading before and after NOx storage are proposed, as shown in Figure 10. On Pt− 5%K2CO3/ZrO2, as shown in Figure 10a, most K species

including −OK groups, K2O, and K2CO3 are believed to be available to NOx storage since the low loading of K2CO3 in this sample makes the K species have relatively high dispersion or small crystallite size. The presence of NOx can enhance the decomposition of K2CO3 at much lower temperature by transforming carbonates to nitrite/nitrate species. With the increase of K2CO3 loading to 10%, the crystallites of K2CO3 species will naturally increase to some extent; meanwhile, some −OK groups and K2O species may be partially covered by K2CO3 species, as seen in Figure 10b. However, the NSC still increases due to the increased amount of carbonate species, whose outer surface is accessible to NOx storage. When the K2CO3 loading is increased to 15%, the amount of accessible surface carbonate species reaches the maximum; as a result, the largest NSC is achieved on Pt−15%K2CO3/ZrO2, as displayed in Figure 10c. Further increase of K2CO3 loading to 20% will increase the crystallite size of K species, enhancing the formation of bulk or bulk-like K2CO3 species, as described in Figure 10d. Since only the thin outer surface of bulk or bulklike K2CO3 could be involved in NOx storage, Pt−20%K2CO3/ ZrO2 even shows lower NSC than Pt−15%K2CO3/ZrO2.

4. CONCLUSIONS The carbonates-based NSR catalysts Pt−XK 2 CO 3 /ZrO 2 possess extremely good performance for lean NOx storage and reduction, whose NOx storage capacities strongly depend on the kinds and distribution of K species. The largest NSC of 2.16 mmol/g and highest NOx reduction percentage of 99% are achieved over the sample Pt−15%K2CO3/ZrO2. The K2CO3 loading determines the states of K species in the catalysts. Three kinds of K species including −OK groups, K2O, and K2CO3 are identified in the catalysts. Surface carbonate species exhibit higher NOx storage capability than −OK groups and K2O. Too high of a loading of K2CO3 leads to the formation of more bulk or bulk-like K2CO3 species, which are unfavorable to 4096

dx.doi.org/10.1021/jp3117598 | J. Phys. Chem. C 2013, 117, 4089−4097

The Journal of Physical Chemistry C

Article

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NOx storage. In situ DRIFTS results reveal that, as K2CO3 loading is not higher than 10%, the NOx is mainly stored as nitrates species such as monodentate nitrates, ionic nitrates, and bridging bidentate nitrates, while at higher K2CO3 loading, the NOx is only stored as bidentate nitrite species. The presence of excess amount of K2CO3 may cover some of the Pt sites, making them unavailable to NO adsorption and oxidation; as a result, the NOx can only be oxidized and stored as nitrite species.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-(0)22-2789-2275. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 21076146 and 21276184), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120032110014), and the Program of New Century Excellent Talents in University of China (No. NCET-07-0599). We are grateful to the support from the Program for Introducing Talents of Discipline to Universities of China (No. B06006) and the Engineering Education Funding of Tianjin University.

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dx.doi.org/10.1021/jp3117598 | J. Phys. Chem. C 2013, 117, 4089−4097