Hongjian Zhu, Shumei Song, Rui Wang This email address is being protected from spambots. You need JavaScript enabled to view it.

School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China


 

Received: March 22, 2020
Revised: April 23, 2020
Accepted: May 1, 2020

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.


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Cite this article:

Zhu, H., Song, S. and Wang, R. (2020). Removal of NOx by Adsorption/Decomposition on H3PW12O40·6H2O Supported on Ceria. Aerosol Air Qual. Res. 20: 2273–2279. https://doi.org/10.4209/aaqr.2020.03.0110


HIGHLIGHTS

  • HPW/CeO2 was prepared and used as novel NOx adsorption/decomposition catalyst.
  • The optimal conditions for NOx adsorption by HPW/CeO2 were obtained.
  • The optimized HPW/CeO2 has higher NOx uptake (85.6 mg g−1) than HPW (50.5 mg g−1).
  • The adsorbed NOx can be decomposed to N2 via temperature ramping.
 

ABSTRACT


We synthesized catalysts composed of CeO2 and varying percentages of HPW and assessed their capacity to adsorb and decompose NOx using XRD, FTIR spectroscopy, and BET analysis. The denitrification performance of the catalysts was evaluated by investigating the dynamic NOx adsorption as well as other relevant factors.  The IR spectra of the HPW revealed that the adsorbed NOx entered the bulk phase of the HPW and replaced the crystallization water, combining with its protons to form NOH+. The HPW/CeO2 composites displayed a maximum NOx adsorption rate (85.6 mg-NO2 g-HPW−1) that far exceeded that of HPW alone (50.5 mg-NO2 g-HPW−1). GC-MS confirmed that temperature programming reduced the adsorbed NOx to N2.

 


Keywords: Polyoxometalate; NOx; Adsorption and decomposition; H3PW12O406·H2O/CeO2.


INTRODUCTION


Nitrous oxides NOx (NO + NO2) abatement from stationary power sources and mobile sources has attracted great attention in recent years due to the caused environmental problems (acid rain, photochemical smog, and ozone layer depletion) and harm for human health (Huang et al., 2019; Wang et al., 2019; Shan et al., 2020). To solve this serious environmental problem, many well-developed technologies have been employed for NOx removal. Among these technologies, although selective catalytic reduction of NOx with ammonia (NH3-SCR) has been considered to be one of the most efficient techniques to resolve the NOx issue (Imanaka and Masui, 2012), it has the disadvantages of high cost of NH3 and secondary pollution caused by NH3 leakage. The direct decomposition of NOx is regarded as the most ideal and environmentally friendly technology for the removal of NOx, in which NOx can be direct decomposed into harmless N2 and O2 without reductant (Konsolakis et al., 2015; Sun et al., 2018). In general, the level of NOx adsorption performance determines the alternative strategy for eliminating NOx. Many different types of catalytic decomposition materials have been employed for NOx removal (Sun et al., 2018).

Polyoxometalates (POMs) have been extensively studied due to their excellent properties (pseudo-liquid phase, strong Brønsted acidity, and remarkable redox properties) (Ren et al., 2017a, b). As a new type of green environmental protection material, POMs has been used in a variety of catalytic fields (Wang and Yang, 2015; Wei et al., 2018; Liu and Wang, 2019). Especially, the Keggin-type heteropolyacid (HPA) show attractive NOx adsorption behavior (Heylen et al., 2010). Yang and Chen reported that NOx adsorbed on phosphotungstic acid (H3PW12O40·6H2O; HPW) was stimulated in the form of protonated NO (NOH+), which weakened the N-O band. The break of N-O generated in the rapid heating process, thus resulting in the yield of N2 (Chen and Yang, 1995, Yang and Chen, 2004). Moffat et al. (1995) have further claimed that the adsorbed quantities of NO2 followed the order of H3PW12O40 > H4SiW12O40 > H3PMo12O40. However, pure HPA has a small specific surface area, low mechanical strength and is easily soluble in polar molecules. The high heating rate may easily cause the oven temperature to exceed the target temperature, leading to inactivation of HPA with inferior thermal stability, which makes it difficult to reuse in practical applications (Ma et al., 2012; Zhang et al., 2013). In order to increase the adsorption capacity, catalytic decomposition activity and catalyst stability, many heteropolyacid-based adsorbent materials have been developed. Zhang and Wang (2013) designed a new HPW-USY catalyst with a three-dimensional bottle-in-boat structure, which has high thermal stability and good NOx decomposition performance. HPW/CNTs catalysts were prepared and the results show that the yield of N2 by microwave heating is higher than that of rapid heating by resistance furnace (Zhang et al., 2012). In representative lean-gas mixture conditions, the performance of NOx storage and reduction over H3PW12O40·6H2O loaded on Ti-Zr and Ce-Zr were studied, which possess high nitrogen selectivity (Gómez-García et al., 2005a, b). Ceria (CeO2) as a catalyst promoter and catalyst has attracted considerable attention in the three-way catalyst (TWC) system owing to its excellent oxygen storage capacity (OSC), redox properties and ability to serve as a good support (Katta et al., 2010; Zhu et al., 2017). HPW-supported CeO2 composites have been reported as the SCR catalysts (Song et al., 2017; Weng et al., 2016; Geng et al., 2018), and the composites exhibit synergistic interaction due to their improved specific surface area and exposure of more acidic sites. As is well known, there are few reports on HPW-supported CeO2 materials for adsorption and decomposition of NOx. Detailed investigation of the key factors in the experimental process and reaction mechanisms are still need further research.

In this work, to improve the thermal stability and specific surface area of HPW, and to avoid inactivation of HPW due to exceeding the target temperature at high heating rate, a series of heteropolyacid-based adsorbent materials supported by CeO2 were synthesized and used for vigorous removal of NOx. The synthesized HPW/CeO2 were characterized by powder X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy and Brunauer-Emmett-Teller (BET) analysis. The denitrification performance of HPW/CeO2 was determined by the dynamic NOx adsorption study, along with various relevant factors.


SYNTHESIS OF HPW-LOADED MATERIALS



The Transition Metal Doped Ceria-based Solid Solutions

CeO2 support was prepared by precipitation method. NH3·H2O solution were added dropwise to the solution containing Ce(NO3)3·6H2O with continuous stirring until pH = 10. The precipitate was then filtered and washed three times with sufficient deionized water and absolute ethanol. The CeO2 support was obtained after drying at 100°C for 12 h and calcination at 750°C for 4 h.

The HPW-doped CeO2 supports was prepared by impregnation method. A certain mass ratio of the supports was immersed in a quantitative HPW solution for 24 h, and then dried in a 60°C water bath for 1 h to obtain the samples. The samples are denoted as x%HPW/CeO2 (x is the mass ratio of HPW/CeO2).


Catalyst Characterization

FTIR spectroscopy was performed on a Nicolet Avatar 370 and the spectral was recorded in the range of 400–4000 cm−1 with a 4 cm−1 resolution. The XRD patterns were recorded with a Rigaku D/MAX-RA powder X-ray diffractometer, using Cu Kα monochromatized radiation at a scan speed of 5° min−1. BET surface areas of the catalysts were measured by N2 adsorption using a Micromeritics ASAP 2010 apparatus.


Activity Tests


Standard Test Procedure: Adsorption Stage

The test was conducted in a fixed-bed quartz tube reactor of 8 mm internal diameter. The amount of catalyst used is 0.2 g. The sample was first placed in the middle of the reactor between two quartz wool plugs and pretreated in a highly purified N2 stream at 150°C for 1 h. During NOx adsorption, the feed gas containing NO, O2, H2O and balance N2 was passed through the bench blending reactor and partially converted to NO2 (2NO + O2 = 2NO2), obtaining a mixture of NO and NO2. TH-990S NO and NO2 analyzers were used to continually record the concentration of NO and NO2 in the outlet.

The NOx adsorption amount is calculated by integrating the curve below the baseline (1000 ppm) and expressed in mg-NO2 g-HPW−1. The calculation formula is shown below:

where m: mass of adsorbent (g), Q: total gas flow (m3 min−1), C: NOx removal concentration (mg m−3), t: adsorption time (min).


Decomposition Stage

After NOx adsorption saturation, the reactor containing catalyst was purge with He flow (5 mL min−1) for 1 h. Then reactor placed in an oven was rapid heated from 30°C to 450°C at a heating rate of 150°C min−1. The decomposition product was detected by GC-MS analyzer.

NO conversion was then calculated by: N2 yield = 2N2 formation/NOx adsorbed, where NOx adsorbed represents the NOx adsorbed amount of HPW and N2 formation represents the generated N2 amount during NOx decomposition.

Fig. 1. XRD spectra of HPW/CeO2 with different loading capacities: (a) 0%, (b) 40%, (c) 50%, (d) 70%, and (e) HPW.Fig. 1. XRD spectra of HPW/CeO2 with different loading capacities: (a) 0%, (b) 40%, (c) 50%, (d) 70%, and (e) HPW.


RESULTS AND DISCUSSION



Characterizations

Figs. 1 and 2 are XRD and FTIR spectra of HPW and HPW/CeO2 with different loadings, respectively. It could be seen that XRD patterns of CeO2 correspond to that of a mixture of cubic fluorite structure (PDF: 43-1022). The crystalline-phase HPW diffraction peak appears when the HPW loading is 40%. With the increase of the HPW loading, the diffraction peaks attributed to the HPW crystal phase and its peaks intensity gradually increases. It can be seen from Fig. 2, pure CeO2 has only a strong peak assigned to symmetrical stretching mode of Ce-O at about 450 cm−1. For HPW, several distinct characteristic peaks can be observed. According to the previous studies (Yang et al., 2005; Cheng and Wang, 2013), the peaks at 1083 and 988 cm−1 can be ascribed to the stretching vibration of P-Oa and W-Od, and the stretching of W-Oc-W between edge-sharing octahedra and vibration of W-Ob-W between vertex-sharing W3O13 octahedra appeared approximately at about 802 and 891 cm1, respectively, which is an accepted way to identify Keggin structure of HPA. In addition, the band at 1640 cm−1 are characteristic peak of lattice water in HPA. When the HPW loading is 70%, the characteristic peaks of the Keggin structure appear at 700–1100 cm−1, indicating that the basic Keggin structure does not change after the HPW loaded on CeO2. The results of BET analysis showed that the specific surface area of HPW is only 3.2 m2 g−1. When HPW was supported on CeO2, its specific surface area is as high as 13.8 m2 g−1 (70%HPW/CeO2), which also indicates that HPW was effectively loaded on the CeO2 support. 

Fig. 2. FTIR spectra of (a) CeO2, (b) 70%HPW/CeO2, and (c) HPW.Fig. 2. FTIR spectra of (a) CeO2, (b) 70%HPW/CeO2, and (c) HPW.


Effect of HPW Loading on NO
x Adsorption

The effect of HPW loading on NOx adsorption was investigated over CeO2 as support. As can be seen from Fig. 3, the adsorption rate of NOx of the mixture was higher than pure oxide. After the loading of HPW is increased to 50%, the adsorption rate of the system rapidly increases to 74.6%, and the best removal rate, i.e., 75.3%, is obtained over 70%HPW/CeO2. Combined with the XRD and IR characterization results of the HPW/CeO2 system, it can be seen that when the HPW loading is 70%, the crystal phase of HPW on the support surface is almost identical to the pure HPW. Although HPW is the active component of the system, the adsorption rate of NOx of the mixture increased by nearly 10% compared to HPW (66.3%). This indicates that CeO2 is an excellent carrier, and there is a synergistic effect between CeO2 and HPW. Considering this fact, 70%HPW/CeO2 is used as the representative material to further explore the effect of different experimental factors on NOx adsorption performance in the following section. 

Fig. 3. Effect of HPW loading on NOx adsorption efficiency. Reaction conditions: 800 ppm NO, 8% O2, 4.2% H2O, T = 150°C, space velocity (SV) = 5298 h−1.Fig. 3. Effect of HPW loading on NOx adsorption efficiency. Reaction conditions: 800 ppm NO, 8% O2, 4.2% H2O, T = 150°C, space velocity (SV) = 5298 h−1.


Effect of Oxygen Content on NOx Adsorption

The oxygen content is an important factor for affecting the NOx adsorption performance. NOx adsorption experiments were carried out at different oxygen content over 70%HPW/CeO2 and the results were shown in Fig. 4. It can be seen that under oxygen-free conditions, the adsorption rate of the system is low at only about 20%. As the oxygen content increases, the adsorption rate of the system gradually increases. When the oxygen content reaches 8%, the maximum adsorption rate is obtained, and the adsorption rate remains stable even with further increase of oxygen content. Therefore, the suitable oxygen content for NOx adsorption by HPW/CeO2 system is 8%. 

Fig. 4. The effect of oxygen content on the NOx adsorption over 70%HPW/CeO2. Reaction conditions: 800 ppm NO, 8% O2, 4.2% H2O, SV = 5298 h−1.Fig. 4. The effect of oxygen content on the NOx adsorption over 70%HPW/CeO2. Reaction conditions: 800 ppm NO, 8% O2, 4.2% H2O, SV = 5298 h−1.


Effect of Water Vapor Content on NO
x Adsorption

Fig. 5 shows the adsorption performance of 70%HPW/CeO2 at different water vapor content. It can be clearly seen that the adsorption rate shows a volcanic curve with the increasing water vapor content. When the water vapor content is less than 4.2%, the adsorption rate increases with the water vapor content. However, when the water vapor content is higher than 4.2%, the adsorption rate is gradually decreasing. The suitable water vapor content for NOx adsorption in HPW/CeO2 system is about 4.2%. 

Fig. 5. The effect of water vapor content on the NOx adsorption performance. Reaction conditions: 800 ppm NO, 8% O2, T = 150°C, SV = 5298 h−1.Fig. 5. The effect of water vapor content on the NOx adsorption performance. Reaction conditions: 800 ppm NO, 8% O2, T = 150°C, SV = 5298 h−1.


Effect of Reaction Temperature on NO
x Adsorption

Temperature is a key factor for chemical reactions, especially for adsorption experiments. Fig. 6 shows the effect of temperature on the NOx adsorption performance of HPW/CeO2 system. In the temperature range of 125–250°C, NOx adsorption rate is maintained above 60%. When the reaction temperature is 170°C, the adsorption rate can reach to 76.3%. After the temperature rises to 300°C, the adsorption rate drops sharply to less than 50%. The experimental results show that HPW has better thermal stability when supported on CeO2, and the suitable temperature for HPW/CeO2 is 170°C. 

Fig. 6. NOx adsorption curves of 70%HPW/CeO2 at different temperatures. Reaction conditions: 800 ppm NO, 8% O2, 4.2% H2O, SV = 5298 h−1.Fig. 6. NOx adsorption curves of 70%HPW/CeO2 at different temperatures. Reaction conditions: 800 ppm NO, 8% O2, 4.2% H2O, SV = 5298 h−1.


The Effect of Space Velocity on NO
x Adsorption

The space velocity (SV) of reaction gas could be also a factor for its removal in the process due to the residence time of the gas in the reactor can be actually affected by the change of space velocity. Therefore, the effect of space velocity of NOx on NOx adsorption performance of HPW/CeO2 at 150°C was studied (Fig. 7). It should be noted that the adsorption rate of the system decreases as the space velocity increases, indicating that free diffusion of gas has little effect on the adsorption surface reaction. The experimental results are mainly due to the reduction of the residence time of gas in the reactor by the changing space velocity. Considering the space velocity is in the range of 2649–5298 h−1, the adsorption rate of NOx is relatively stable, and its decrease sharply due to the increasing space velocity. Therefore, the suitable space velocity for the adsorption of NOx by the HPW/CeO2 system is 2649–5298 h−1

Fig. 7. NOx adsorption performance under different space velocity. Reaction conditions: 800 ppm 8% O2, 4.2% H2O, T = 150°C.Fig. 7. NOx adsorption performance under different space velocity. Reaction conditions: 800 ppm 8% O2, 4.2% H2O, T = 150°C.


Effect of Initial NO Concentration

Fig. 8 shows the effect of initial NO concentration on the NOx adsorption performance of the system. With the increase of the NO concentration of intake gas, the adsorption rate of the system gradually decreases. It should be noted that a linear relationship is exhibited between the NOx adsorption rate and the initial NO concentration. When the initial concentration of NO is 387 ppm, the adsorption rate of the system is up to 79.3%. 

Fig. 8. The effect of initial NO concentration on the NOx adsorption performance. Reaction conditions: 8% O2, 4.2% H2O, T = 150°C, SV = 5298 h−1.Fig. 8. The effect of initial NO concentration on the NOx adsorption performance. Reaction conditions: 8% O2, 4.2% H2O, T = 150°C, SV = 5298 h−1.


Adsorption Mechanism

Under optimized experimental conditions (800 ppm NO, 8% O2, 4.2% H2O, T = 150°C, SV = 5298 h−1), the saturated adsorption capacity was calculated. The saturation adsorption capacity of the system is significantly increased after the HPW was supported on CeO2, and the saturation adsorption time became shorter. Based on experimental data, the saturated adsorption of NOx by HPW catalyst is 50.5 mg-NO2 g-HPW−1. In addition, as shown in Table 1, the saturated adsorption of NOx by 70%HPW/CeO2 catalyst is up to 85.6 mg-NO2 g-HPW−1, which is higher than that of Ti-Zr (28 mg-NO2 g-HPW−1) and TiO2 (46 mg-NO2 g-HPW−1) as support reported in previous literature (Hodjati et al., 2001b; Gómez-García et al., 2005a). Combined with the results of BET analysis, when HPW is loaded on CeO2 support (70%HPW/CeO2), its specific surface area is as high as 13.8 m2 g−1, which is much higher than the specific surface area of HPW (3.2 m2 g−1). That indicates that HPW could be highly dispersed on the CeO2, leading to more available adsorption sites were used for NOx adsorption. 

Table 1. NOx adsorption capacity of different catalyst systems.

The IR spectra of HPW before and after adsorption saturation are shown in Fig. 9. In the range of 700–1100 cm−1, four characteristic peaks of Keggin structure for 70%HPW/CeO2 after adsorption saturation still exist, indicating that the Keggin structure of HPW has not changed. In addition, a new characteristic peak appears at 2266 cm−1 and the characteristic peak of H3O (in secondary structure of HPW) at 1617 cm−1 become weaker. 

Fig. 9. FTIR spectra of 70%HPW/CeO2 before and after adsorption saturation.Fig. 9. FTIR spectra of 70%HPW/CeO2 before and after adsorption saturation.

According to previous reports, there are two different viewpoints on the characteristic peak at 2266 cm−1. Bélanger and Moffat (1995) reported that the characteristic peak at 2266 cm−1 is caused by HNO2+ vibration. The interaction of NO2 and HPW is not only appeared on the surface. NO2 can also enter the secondary structure of HPW and combine with protons in the secondary structure to form HNO2+. Chen and Yang (1995) and Yang and Chen (2004) carried out NOx adsorption tests over HPW, and considered that the characteristic peak at 2266 cm−1 was caused by NOH+ vibration. In the presence of oxygen, NO can pass through the surface of HPW into its bulk to replace H5O2+ in the secondary structure, and finally NOH+ is formed. Thermogravimetric analysis (TGA) and calculation of mass conservation principle of nitrogen proved that the bond with HPW is NO instead of NO2. Herring et al. (1998) used KPW as an adsorbent to perform NO2 adsorption experiments, and the DRIFTS results showed that the ion of NO2 is only adsorbed on the surface of KPW (KPW does not contain crystal water). Besides the other characteristic peaks of NO+, NO2+ and NOxy are detected on the surface of KPW after adsorption saturation. This result supported the view of Yang and Chen (2004), and the mechanism of NOH+ formation was proposed.

Based on the above analysis, we believe that the characteristic peak at 2266 cm−1 is caused by the NOH+ vibration entering the HPW secondary structure for 70%HPW/CeO2. There is an antisymmetric stretching vibration peak near 2300 cm−1 for NO+, which is only 34 cm−1 different from the vibration peak at 2266 cm−1 detected by this experiment. The stretching vibration peak of NO2+ mainly exist in 2350–2375 cm−1. The minimum difference between stretching vibration peak of NO2+ and the peak at 2266 cm−1 detected by the experiment is 84 cm−1. On the other hand, the characteristic peaks of bending vibration of NO2+ and the characteristic peaks of N2O3 on the surface of 70%HPW/CeO2 were not detected. Furthermore, the weakening characteristic peak of H3O at 1617 cm−1 is mainly caused by the partial crystal water in the HPW secondary structure being replaced by NOH+. This experimental result is consistent with the conclusion of Herring et al. (1998). The heteropolyacid used in this work is H3PW12O40·6H2O. At most, six crystal water molecules can be replaced in one HPW molecule and there is competitive adsorption between H2O and NOx. Therefore, a conclusion can be draw for HPW/CeO2 system, namely, NOx is adsorbed on the surface of HPW in the form of NOH+, and NOH+ enters its bulk phase by replacing part of the crystal water in the secondary structure of HPW.


Catalytic Decomposition of Adsorbed NO
x

GC-MS was used to detect the decomposition products, and N2 was found in the products. The results are shown in Fig. 10. The decomposition characteristics of adsorbed NOx on HPW and HPW/CeO2 catalytic systems are similar. The detected N2 reaches a peak at about 10 min, and the decomposition is basically completed at about 20 min. The results of this experiment show that for NO in an oxygen-enriched atmosphere, HPW/CeO2 catalytic systems can achieve a better decomposition and conversion effect due to the interaction between two components. Therefore, a new heteropoly compound catalysis system has been design for NOx elimination by adsorption/decomposition and the catalytic conversion of NOx to N2 can be achieved without reducing agents. The used catalyst can be placed in humid air (5% water vapor), and HPW can replenish the lattice water replaced by NOx by adsorbing water molecules, thereby regenerating the catalyst. Detailed decomposition mechanism and process of the conversion of the adsorbed intermediate product (NOH+) into N2 by the temperature programming process need to be further investigated. 

Fig. 10. Catalytic decomposition of adsorbed NOx over HPW and 70%HPW/CeO2.Fig. 10. Catalytic decomposition of adsorbed NOx over HPW and 70%HPW/CeO2.


CONCLUSIONS


In this study, we synthesized catalysts composed of CeO2 and varying percentages of HPW and assessed their capacity to adsorb and decompose NOx. The adsorption performance of the most efficient catalyst, 70%HPW/CeO2, was approximately linearly proportional to the initial NO concentration. The optimal conditions for adsorption were an environment containing 8% O2 and 4.2% H2O, a reaction temperature of 170°C, and a space velocity of 2649–5298 h−1. The IR spectra of the HPW revealed similar adsorption mechanisms for HPW/CeO2 and HPW catalysts: The adsorbed NOx entered the bulk phase of the HPW and replaced the crystallization water, combining with its protons to form NOH+. However, the maximum adsorption rate of the HPW/CeO2 catalysts (85.6 mg-NO2 g-HPW−1) greatly exceeded that of pure HPW (50.5 mg-NO2 g-HPW−1). GC-MS confirmed that temperature programming reduced the adsorbed NOx to N2.


ACKNOWLEDGEMENTS


This work was supported by the National Natural Science Foundation of China (Nos. 20576064 and 20776080) and the Key Research and Development Program of Shandong Province, China (2017GSF217006; 2008GG10006005).


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