Hongjian Zhu1, Meiqing Yu1, Tao Ma1, Rui Wang This email address is being protected from spambots. You need JavaScript enabled to view it.1,2, Korchak Vladimir3Vladimir N. Korchak3Vitaly Edwardovich Matulis4

1 School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China
2 Shenzhen Research Institute of Shandong University, Shenzhen 518057, China
3 Laboratory of Heterogeneous Catalysis, Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow 119991, Russian Federation
4 Laboratory of Chemistry of Condensed Matter of the Scientific-Research Institute for Physical-Chemical Problems, The Belarusian State University, Minsk 220006, Republic of Belarus


Received: September 22, 2021
Revised: November 13, 2021
Accepted: November 16, 2021

 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.


Download Citation: ||https://doi.org/10.4209/aaqr.210251  

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Cite this article:

Zhu, H., Yu, M., Ma, T., Wang, R., Vladimir, K., Korchak, V.N., Matulis, V.E. (2021). Adsorption and Decomposition of NOx on Heteropolyacids: An Evaluation of the Adsorption Performance. Aerosol Air Qual. Res. 21, 210251. https://doi.org/10.4209/aaqr.210251


HIGHLIGHTS

  • W-containing HPAs were superior to Mo-containing HPAs.
  • Various novel NOx adsorption/decomposition catalysts have been screened.
  • The HPW/SnO2 with 50% HPW had the highest adsorption amount of NOx was 85.6 mg g1.
  • The availability of NOx decomposition into N2 was confirmed by GC-MS.
 

ABSTRACT


In order to develop an efficient method by which to eliminate NOx pollution, several new catalyst systems including different heteropolyacids (HPAs) and supported phosphotungstic acids (HPWs) (HPW/SiO2, HPW/SnO2, HPW/USY and HPW/ZSM-5) for adsorption-decomposition of NOx were prepared and studied. The obtained catalysts were characterized using Brunauer-Emmett-Teller (BET) measurement and Fourier transform infrared (FT-IR) analysis. The results showed that W-containing HPAs were superior to Mo-containing HPAs. Among the selected catalysts, HPW/SnO2 with a 50% HPW loading had the highest NOx adsorption rate of 77.3%, for which the amount of saturated NOx adsorption (85.4 mg g–1) was much higher than that of HPW (50.5 mg g–1). The NOx adsorption performance of the catalyst was mainly determined by the interaction between the support and the HPW, which was also affected by the specific surface area of the catalyst. FTIR characterization revealed that the adsorbed NOx mainly existed in the HPW bulk phase in the form of NOH+. A gas chromatograph-mass spectrometer (GC-MS) was used to confirm the effectiveness of NOx decomposition into N2.


Keywords: Polyoxometalate, NOx, Adsorption, Decomposition, Support


1 INTRODUCTION


NOx (NO and NO2) are major air pollutants derived from stationary power sources and mobile sources that not only are extremely toxic to human body, but also can lead to photochemical smog, acid rain, ozone depletion, and the greenhouse effect (Liu et al., 2014; Jiang et al., 2016; Huang et al., 2019). Alternative well-developed technologies have been employed for eliminating NOx emissions in response to environmental protection requirements. Among them, selective catalytic reduction of NOx with NH3 (NH3-SCR) has been considered to be one of the most efficient techniques for NOx removal due to its efficiency, economy, and selectivity (Forzatti et al., 2009; Imanaka and Masui, 2012). However, the leakage of ammonia can cause secondary pollution. The catalytic decomposition of NOx has attracted much attention because it can directly decompose NOx into N2 and O2 without consuming any reducing agent, and the operation process is simple, economical, and without secondary pollutants. In recent years, various catalysts, including precious metals, metal oxides, ion exchange molecular sieves, perovskite-type composite oxides and carbonitrides (Konsolakis and Michalis, 2014; Sun et al., 2014), have been used in direct decomposition of NOx under high temperature conditions (> 500°C).

Polyoxometalates (POMs) have been used in a variety of catalytic fields owing to their strong Brønsted acidity, remarkable redox properties, and pseudo-liquid phase behavior (Wang and Yang, 2015; Ren et al., 2017a, b). In particular, HPAs have shown attractive NOx adsorption performance and can decompose the adsorbed NOx into N2 through rapid heating. Yang and Chen (1994; 1995) reported that NOx was adsorbed on HPW in the form of protonated NO (NOH+), and N-O cleavage was generated in the rapid heating process, thus resulting in N2 yield. However, the small specific surface area and low mechanical strength of HPA and its inferior thermal stability makes it difficult to reuse in practical applications. Therefore, many supported HPA materials have been developed to improve its adsorption performance, catalytic decomposition activity, and stability. In representative lean-gas mixture conditions, the NOx storage and reduction performance of H3PW12O40·6H2O loaded on Ti-Zr and Ce-Zr mixed oxides were studied and shown to have high N2 selectivity (Gómez-García et al., 2005a, b). Platinum (Pt) and HPW doped mesoporous MSU-type silica mixtures were used for NOx adsorption (Hamad et al., 2007). HPW/CNTs catalysts were prepared, and the results showed that the yield of N2 after microwave heating was higher than that obtained with rapid heating using a resistance furnace (Zhang et al., 2012). Zhang et al. (2013) designed a new HPW-USY catalyst with a three-dimensional ship-in-bottle structure. It had high thermal stability and good NOx decomposition performance. In principle, the materials with an isoelectric point of 7 generally do not affect the structure and chemical properties of HPA, which are suitable as HPA supports. A new Keggin-type HPA, germanium-based HPA (H4GeW12O40) was synthesized and utilized as a catalyst for removal of NOx. The results demonstrated a NOx removal rate of 81.5% and N2 selectivity of 68.3% (Wang et al., 2021). H3PW12O40 (HPW)-modified Fe2O3 catalysts were synthesized for the selective catalytic reduction of NOx by NH3 (NH3-SCR). The optimum HPW/Fe2O3-350-0.5 catalyst exhibited nearly a 100% NO conversion at 240–460°C as well as excellent SO2 resistance (Wu et al., 2021). The NOx adsorption performance of the catalyst determines the amount of NOx decomposed. The development of catalysts with high activity and stability have consistently been a research hotspot and have been proven to be quite challenging. The factors affecting the adsorption performance of catalysts also require further exploration.

The objective of this work is to develop a novel, economical, and environmentally friendly NOx conversion catalyst and to further understand the constraints that affect catalytic performance. In this work, various HPAs and supported HPW (HPW/SiO2, HPW/SnO2, HPW/USY and HPW/ZSM-5) were prepared and used for the adsorption and decomposition of NOx. The physical and chemical properties of the catalyst were characterized using BET and FTIR spectra. The relationship between their NOx adsorption performance and their physicochemical properties was analyzed through comparative studies.

 
2 EXPERIMENTAL PROCEDURE


 
2.1 Materials

Commercial heteropolyacids, including H3PW12O40 (HPW), H4SiW12O40 (HSiW), H3PMo12O40 (HPMo), H4SiMo12O40 (HSiMo) and SnO2, were obtained from Sinopharm Chemical Reagent Co., Ltd (China). SiO2 was purchased from Alfa Aesar. USY and ZSM-5 molecular sieves were purchased from the Zhoucun Catalyst Factory and Jiangsu Aoke Petrochemical Science and Technology Company. All reagents were analytical grade and were used directly without further purification.

 
2.2 Synthesis of HPW-Loaded Materials

HPW-loaded materials were prepared using the impregnation method. The mass ratios of HPW/(HPW + support ) were 0, 16.7%, 28.6%, 37.5%, 44.4%, and 50.0%. A specific mass ratio of the support 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 were denoted as HPW/SiO2, HPW/USY, HPW/SiO2, and HPW/ZSM-5.

 
2.3 Catalyst Characterization

FTIR spectra of the samples were recorded using a Nicolet Avatar370, and the spectral domain ranged between 400 and 4000 cm–1 at a 4 cm–1 resolution. The BET surface areas of the catalysts were measured based on the N2 adsorption at –196°C using a Micromeritics ASAP 2010 apparatus.


2.4 Activity Tests


2.4.1 Standard test procedure: Adsorption stage

The experiments were carried out in a fixed-bed quartz tube reactor (inner diameter = 8 mm). 0.3 g of the sample was placed in the middle of the reactor between two quartz wool plugs and pretreated with highly purified N2 at 150°C for 1 h. During NOx adsorption, the feed gas containing 800 ppm NO, 8% O2, 4.2% H2O, and balance He was passed through a bench blending reactor and partially converted to NO2 (2NO + O2 = 2NO2), yielding an NOx mixture. NO and NO2 analyzers (TH-990S) were used to continuously record the concentration at the NO and NO2 outlet.

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

 

where m represents the mass of the catalyst (g); Q is the total gas flow (m3 min–1); C is the NOx removal concentration (mg m–3), and t is the adsorption time (min).

 
2.4.2 Decomposition stage

After NOx adsorption saturation, the reactor containing the catalyst was purged with He flow (5 mL min–1) for 1 h. Then, the reactor was placed in a tube furnace and rapidly heated from 30°C to 450°C at a heating rate of 150°C min–1. The decomposition product was detected with a GC-MS analyzer.

NO conversion was then calculated using the following formula:

 

where NOx (adsorbed) represents the amount of HPW the NOx adsorbed, and N2 formation represents the amount of N2 generated during NO conversion.

 
3 RESULTS AND DISCUSSION


 
3.1 FTIR Analysis

FTIR spectra are the most commonly used research and measurement method applied to polyanions. The FTIR spectra of the supports before and after loading with HPW are shown in Fig. 1(a–d), and the FTIR spectrum of the HPW used in this work is shown in Fig. 1(e). Four characteristic peaks of the Keggin structure can be observed at 1080 cm–1, 983 cm–1, 892 cm–1, and 801 cm–1, corresponding to P-Oa, W = Od, W-Ob-W, W-Oc-W, respectively, which is consistent with the literature (Yang and Chen, 1994). As shown in Fig. 1(a), for HPW/SiO2, the P-Oa absorption peak of HPW at 1080 cm–1 became broader due to being masked by a wide SiO2 absorption peak at 1086 cm–1. The other three absorption peak positions of HPW shifted slightly, and their peak intensity decreased significantly. These findings implied that the HPW Keggin structure still existed after loading HPW on SiO2. The change in the characteristic peaks indicated that an interaction between HPW and SiO2 occurred. The negative charge of the heteropolyanion increased after the HPW was loaded on SiO2, where the increased negative charge could fill its antibonding molecular orbital, leading to a reduction in the chemical bond strength and a decrease in the vibration frequency.

Fig. 1. FTIR spectra of HPW and various supports with a 50% HPW loading.Fig. 1. FTIR spectra of HPW and various supports with a 50% HPW loading.

In Fig. 1(b), a broad peak can be observed at 400–750 cm–1 (650 cm–1), which was mainly attributed to the stretching vibration of the Sn-O band (Abello et al., 1998; Singh et al., 2014). When comparing the IR spectra of SnO2 and HPW/SnO2, it can be seen that the four characteristic peaks of the Keggin structure on HPW/SnO2 were stable, indicating that HPW was definitely loaded on the SnO2, and there was no strong interaction between the SnO2 and the HPW, thereby preserving the heteropolyanion phase.

As shown in Fig. 1(c), the USY framework structure can be identified by the peaks at 1700–400 cm–1 (Zhang et al., 2001; Munir and Usman, 2018). The absorption band at 1636 cm–1 was assigned to the bending vibration of the -OH group. The broad band from 1000 to 1250 cm–1 represented the asymmetric stretching vibration of Si-O-Si. The absorption bands at 815 cm–1 and 455 cm–1 were attributed to the bending vibration of Si-O-Si. In addition, a band at 590 cm–1 was observed, indicating the presence of a zeolite framework with tetrahedral units of SiO4 and AlO4. The characteristic peak of the of USY continued to exist on HPW/USY, indicating that the cage-type micropore structure of USY was preserved, and the strong interaction between USY and HPW caused the characteristic peak of the zeolite to shift. The broader absorption peak of USY at 1054 cm–1 covered the P-Oa band and the HPW W = Od band. The W-Ob-W band and the W-Oc-W band shifted slightly, and their peak intensity was greatly reduced or even disappeared. This indicated that the Keggin structure in HPW/USY basically existed, but the strong interaction between HPW and USY may have deformed the PO4 tetrahedral structure in the heteropolyanion and weakened W-O bond outside the heteropolyanion structure.

Fig. 1(d) shows the IR spectrum of HPW/ZSM-5. The peaks appearing at 1223 cm–1, 1090 cm–1, 794 cm–1, 544 cm–1, and 448 cm–1 can be ascribed to the typical characteristic peaks of a ZSM-5 molecular sieve framework (On et al., 1995; Shirazi et al., 2010). In the case of HPW/ZSM-5, the characteristic peak of the ZSM-5 framework structure and the HPW Keggin structure can be found with a weak deviation. Therefore, HPW/ZSM-5 not only maintained the porous zeolite structure of ZSM-5, but also the HPW Keggin structure on ZSM-5 was not destroyed, indicating that the interaction between HPW and ZSM-5 was relatively small, which was similar to HPW/SnO2. The combination of the characteristics of HPW and the support was more conducive to the improvement in the deNOx performance.

 
3.2 BET

Table 1 lists the specific surface area of the four catalyst systems with the preferred HPW loading. The specific surface area of HPW was only 3.2 m2 g–1. For the supports, an obvious difference in specific surface area was observed. The specific surface areas of SiO2, SnO2, USY, and ZSM-5 were 334.2 m2 g–1, 10.4 m2 g–1, 559.8 m2 g–1, and 305.4 m2 g–1, respectively. In general, for adsorption reactions, a larger specific surface area is beneficial in terms of providing more adsorption sites, thereby improving the adsorption performance of the catalyst. Actually, it can be found that for all supports loaded with HPW, as the HPW loading increased, the specific surface area of the sample gradually decreased to varying degrees. This indicated that the HPW was effectively loaded on the supports, and the interaction between the highly dispersed HPW and the supports had an inhibiting effect in specific surface areas.

Table 1. NOx adsorption rate and the specific surface area of the various samples.

 
3.3 Adsorption Performance of Heteropoly Acids for NOx

The NOx adsorption performance of four heteropoly acids (HPW, HSiW, HPMo, and HSiMo) was investigated. As shown in the Fig. 2, during the experimental period, relatively stable adsorption rates were observed on the four heteropoly acids. The NOx adsorption rates after a 40 min reaction time with HPW, HSiW, HSiMo, and HPMo were 66.5%, 62.9%, 30.7%, and 24.4%, respectively. Among the four heteropolyacids, HPW showed the best NOx adsorption performance. From the experimental results, the NOx adsorption rates of the W-containing heteropolyacids (HPW and HSiW) were much higher than that of Mo-containing heteropolyacids (HSiMo and HPMo), indicating that the NOx adsorption performance of heteropolyacid could be closely related to the composition of its anions, especially the type of coordination atoms in the anions. This result was consistent with the conclusion of Belanger and Moffat (1995). Indeed, the acid strength of the W-based heteropolyacids was higher than that of the Mo-based heteropolyacids (Jozefowicz et al., 1993). It can be inferred that the NOx adsorption rates of the four heteropolyacids were correlated with their acid strengths.

Fig. 2. NOx adsorption performance of HPW, HSiW, HSiMo and HPMo.Fig. 2. NOx adsorption performance of HPW, HSiW, HSiMo and HPMo.

 
3.4 Effect of HPW Loading on NOx Adsorption for Various Supports

NOx adsorption experiments were carried out on various supports with different HPW loading amounts. The NOx adsorption rate was plotted as a function of the adsorption time reached by each catalyst in the NO-TPO experiment (Fig. 3). These results were derived as shown in Fig. 4 in order to better compare the effect of different HPW loading amounts on the NOx adsorption performance of the catalyst. As shown in Fig. 4(a), the loading amounts of HPW were 0, 16.7%, 28.6%, 37.5%, 44.4%, and 50.0%, for which the corresponding NOx adsorption rates of HPW/SiO2 were 35.2%, 39.6%, 62.3%, 56.6%, 49.3%, and 50.9%, respectively. It was found that the NOx adsorption performance of pure SiO2 was very low. After loading with HPW, the NOx adsorption efficiency of the catalytic system increased immediately. When the HPW loading exceeded 30%, the NOx adsorption efficiency of HPW/SiO2 decreased steadily with increases in the loading, but it was still higher than the performance of the pure SiO2. The best deNOx performance was obtained when the HPW loading was 28.6% for the HPW/SiO2 system. The NOx adsorption rate for HPW/SnO2 is shown in Fig. 4(b). Pure SnO2 exhibited an extremely low NOx adsorption rate (13.7%), where after a small amount of HPW was loaded, the NOx adsorption performance increased sharply. The deNOx performance of HPW/SnO2 basically increased with increases in the amount of loaded HPW, but the performance decreased slightly at 37.5% and then rebounded again. It should be noted that the NOx adsorption rate of HPW/SnO2 with a 50% HPW loading amount reached 77.3%. Fig. 4(c) shows the NOx adsorption rate of HPW/USY. HPW/USY exhibited a similar trend to that of the HPW/SiO2 system; that is, the NOx adsorption rate increased first and then decreased with increases in the HPW loading amounts. For HPW/USY, the optimal HPW load was 16.7%. Fig. 4(d) shows the NOx adsorption rates of HPW/ZSM-5 corresponding to HPW loadings of 0, 28.6%, 37.5%, 44.4% and 50% were 27.7%, 57.6%, 62.6%, 65.2% and 70.5%, respectively. The results showed that as the HPW loading amount increased, the NOx adsorption performance of HPW/ZSM-5 increased steadily. This may have been due to the fact that the ZSM-5 is a molecular sieve material with a large specific surface area and a rich pore structure. The heteropoly acid loading can cause the specific surface area and pores to decrease, but it does not lead to the destruction of the structure. Thus, a synergistic effect on the NOx adsorption performance was observed.

Fig. 3. NOx adsorption performance of various supports with different HPW loading amounts. (a) HPW/SiO2, (b) HPW/SnO2, (c) HPW/USY, and (d) HPW/ZSM-5.Fig. 3. NOx adsorption performance of various supports with different HPW loading amounts. (a) HPW/SiO2, (b) HPW/SnO2, (c) HPW/USY, and (d) HPW/ZSM-5.

Fig. 4. Effect of HPW loading on NOx adsorption for various supports (a) HPW/SiO2, (b) HPW/SnO2, (c) HPW/USY, and (d) HPW/ZSM-5. Reaction conditions: 800 ppm NO, 8% O2, 4.2% H2O, T = 150°C, SV = 10580 h–1.Fig. 4. Effect of HPW loading on NOx adsorption for various supports (a) HPW/SiO2, (b) HPW/SnO2, (c) HPW/USY, and (d) HPW/ZSM-5. Reaction conditions: 800 ppm NO, 8% O2, 4.2% H2O, T = 150°C, SV = 10580 h–1.

Comparing the four sets of data, it was found that the adsorption curve showed two different changes, which was a meaningful for determining the nature of the NOx adsorption. When unloaded supports were used for NOx adsorption, their adsorption rate sequences were USY > SiO2 > ZSM-5 > SnO2, which was consistent with their specific surface area sequence, indicating that a specific surface area plays a certain role in NOx adsorption. When SiO2 and USY were used as the HPW supports for NOx adsorption, the basic trend in the adsorption performance indicated similar volcanic changes. It can be seen that the highest NOx adsorption rates of HPW/SiO2 and HPW/USY were 62.4% and 65.6%, respectively, both still slightly lower than that of pure HPW (66.5%), which meant that HPW was the main active component leading to NOx adsorption. According to the BET data, the increased HPW loading on SiO2 and USY caused a significant decrease in their specific surface area. For HPW/SiO2 and HPW/USY with the 50% HPW loading, the specific surface area decreased by 59.3% and 56.1%, respectively, compared to the pure supports. The high loading amount made it possible for a significant amount of HPW to enter the support pores, which blocked its porous passages or collapsed the microporous structure, leading to a reduction in the specific surface area. After the highly dispersed HPW was wrapped in the pores of the support, it may have instead reduced the probability of contact between the HPW body phase and reactive molecules, thus limiting the HPW-specific pseudo liquid phase behavior.

This may be one reason for the initial increase in the deNOx performance of HPW/SiO2 and HPW/USY and the subsequent decrease with HPW loading. In addition, the strong interaction between HPW and the surface properties (hydroxyl groups and acid sites) of the support was an important factor affecting the deNOx performance. After loading, the HPW acid amount and acid type remain unchanged, but the acid strength decreased, and the degree to which the acid strength was decreased was also related to the strength of the surface interaction, where a stronger surface interaction led to the lower acid strength. Rocchiccioli-Deltcheff et al. (2010) believed that the effect of HPW and hydroxyl groups on the surface of SiO2 would cause the acidity and redox of the heteropoly acid to decrease at the same time, thereby leading to reduced catalytic performance. Mccormick et al. (1998) also reported that HPW interacts strongly with the hydroxyl groups on the surface of the SiO2 support, resulting in a completely different secondary structure from HPW.

For HPW/USY, there was a strong interaction and a weak interaction between the HPW and the silicon hydroxyl group (Si-OH) on the surface of USY, respectively, where the strong interaction consumed H+ to produce water, and the weak interaction caused a H+ transfer (Liu et al., 2003). Indeed, there were various hydroxyl groups on the surface of the USY channel, including surface Si-OH, skeleton surface Al-OH, and non-framework Al-OH, so the source of acidity of HPW/USY was more complicated. HPW may also interact weakly with the abundant Si-OH in the hydroxyl socket of the USY secondary pore, resulting in a decrease in the acid strength. In addition, the acid-base interaction between the Al species in the USY and HPW may have decreased the acid strength. The decrease in the total acid content on HPW/USY with a high HPW loading may have been due to the decreased surface area of the catalyst, which led to a reduction in the available surface acid centers, thereby reducing the adsorption performance of the catalyst. ZSM-5 and USY are both zeolite molecular sieves with a Si-Al framework structure. However, they have different frameworks, pore structures, and surface properties. It is clear that the specific surface area of HPW/ZSM-5 from pure ZSM-5 to the 50% loading was only decreased by 44.1%, which indicated that the negative effect of increasing the HPW loading amount and the clogging of the carrier pores was less than that in the case of USY. Furthermore, the amount of non-framework Al remaining in the pores of ZSM-5 may have been much less than that in USY (Shen et al., 2007), which reduced the probability of non-framework Al damaging the acidic center of the HPW with a Keggin structure. Thus, the NOx adsorption rate and the trend of the changes in the deNOx performance with the HPW loading amount were obviously different. It is worth noting that SnO2 was completely different from the other conventional supports in terms of interacting with HPW to participate in NOx adsorption. Conventional porous supports generally rely on their own excellent pore structure and high specific surface area to efficiently disperse HPW active centers to facilitate the entry of NOx molecules into the HPW body. However, the specific surface area of SnO2 is very small and clearly does not have the properties characteristic of porous materials. In addition, SnO2 is a semiconductor material with a specific concentration of oxygen vacancies. It has thus been speculated that SnO2 can be used as a high-efficiency catalytic promoter in the place of ordinary supports. Zhang et al. (2002) studied the desorption and decomposition of NO by HPW/TiO2 and speculated the trapping effect of oxygen vacancies on the surface of TiO2. O2 is essential in the process of HPW adsorption of NOx. The doping of SnO2 can increase the O2 adsorption capacity of the catalyst surface in the NOx adsorption process. Therefore, this combination can demonstrate significantly increased NOx adsorption capacity through synergistic effects that occur between SnO2 and HPW, where HPW/SnO2 with the 50% loading had the best deNOx performance.

 
3.5 Adsorption Capacity

Adsorption capacity is an important indicator to evaluate the performance of an adsorbent. The NOx saturation adsorption curves for HPW and HPW/SnO2 with a 50% HPW loading amount at 150°C were investigated, as shown in Fig. 5. The calculated saturation adsorption capacity of HPW was 50.5 mg g–1. The NOx saturation adsorption capacity of HPW/SnO2 was equivalent to 85.4 mg g–1, which was higher than that of Ti-Zr (28 mg g–1) and TiO2 (46 mg g–1) as supports reported in the previous literature (Hodjati et al., 2001; Gómez-García et al., 2005b). It was thus found that the amount of HPW used was reduced, and the stability of the catalyst increased after the HPW was supported on SnO2. Furthermore, this combination still maintained excellent NOx adsorption performance.

Fig. 5. The saturation adsorption capacity of HPW and HPW/SnO2 (50% loading).Fig. 5. The saturation adsorption capacity of HPW and HPW/SnO2 (50% loading).

 
3.6 Adsorption Mechanism

In this experiment, the HPW and HPW/SnO2 samples after adsorption saturation were characterized using FTIR, as shown in Fig. 6. It can be observed that four characteristic peaks of the Keggin structure still existed in the range of 700–1100 cm–1, indicating that the two samples maintained a stable Keggin structure after being saturated with NOx. In addition, a new characteristic peak appeared at 2266 cm–1 for the two samples. Yang and Chen (1994, 1995) carried out IR characterization of pure HPW saturated with NOx and found an absorption peak at 2270 cm–1. According to TGA and the calculation of the mass conservation principle of nitrogen, the absorption peak at 2270 cm–1 was regarded as ionic protonated NO (NOH+). Thus, a classic NO adsorption mechanism was proposed where the adsorbed NO was retained in the bulk HPW phase in the form of NOH+ by replacing the crystal water in HPW. Herring et al. (1998, 2010) improved the NO adsorption mechanism and confirmed that the protons existed in the secondary structure of heteropoly acid in the form of H3O+ or H5O2+. Particularly, protonated water was a prerequisite for NO to enter the HPA secondary structure and exist as NOH+.

Fig. 6. FTIR of HPW and HPW/SnO2 (50% loading) after adsorption saturation.Fig. 6. FTIR of HPW and HPW/SnO2 (50% loading) after adsorption saturation.

According to the above analyses, the vibration peak at 2266 cm–1 detected in this experiment could be ascribed to NO+. Furthermore, the characteristic bending vibration peaks of NO2+ (570 cm–1) were not detected. The HPA used in this work was H3PW12O406H2O. At most, 6 crystal water molecules could be replaced in one HPW molecule, and there was competitive adsorption between H2O and NOx. Consequently, it could be inferred that for the HPW/SnO2 system, the characteristic peak at 2266 cm–1 should also be created by the NOH+ vibration entering the HPW secondary structure. The interaction between SnO2 and HPW was correspondingly small and did not cause changes in the HPW structure. HPW with a pseudo liquid phase acted as the main component for NOx adsorption, and SnO2 acted as a catalyst assistant. In the presence of O2 and H2O, NOx could be adsorbed on the surface of the HPW or further into its bulk phase to replace part of the crystal water in the secondary HPW structure, where, finally, adsorbed NOx existed in the catalyst in the form of NOH+.

 
3.7 Catalytic Decomposition of Adsorbed NOx

Catalytic decomposition products of adsorbed NOx on HPW and HPW/SnO2 was detected using GC-MS. N2 was found in the products, for which the obtained results are shown in Fig. 7. For HPW, N2 formation was detected 2 min after the start of the increase in the temperature, and a N2 peak was detected around 8 min later. The decomposition basically ended after 16 min. For HPW/SnO2, the detected N2 reached a peak in about 14 min, and the decomposition was basically completed in about 24 min. The experimental results showed that the decomposition time of HPW/SnO2 with the 50% loading was longer than that for HPW, so it can be reasonably concluded that the addition of the SnO2 support was not only beneficial to the NOx adsorption performance, but the combination also contributed to improving the efficiency of NO conversion, thereby leading to more N2 conversion. The used catalyst could be regenerated by placing it in humid air (5% water vapor) below 100°C, where HPW can replenish the lattice water replaced by NOx by adsorbing water molecules. Therefore, the new NOx adsorption-decomposition system screened in this work can achieve NOx to N2 without the use of reducing agents.

Fig. 7. Catalytic decomposition of adsorbed NOx on HPW and HPW/SnO2 (50% loading).Fig. 7. Catalytic decomposition of adsorbed NOx on HPW and HPW/SnO2 (50% loading).

 
4 CONCLUSIONS


In summary, several new catalyst systems, different HPAs, and supported HPWs (HPW/SiO2, HPW/SnO2, HPW/USY and HPW/ZSM-5) were prepared and used for the adsorption and decomposition of NOx. The results showed that W-containing HPAs were superior to Mo-containing HPAs. Among the selected catalysts, HPW/SnO2 with a 50% HPW loading had the highest NOx adsorption rate of 77.3%, and its saturated NOx adsorption amount (85.4 mg g–1) was much higher than that of HPW (50.5 mg g–1). With HPW as the main active component, the interaction between HPW and the support affected the HPW properties, in turn affecting its NOx adsorption performance. When the interaction was stronger, the adsorption rate of the catalyst tended to become lower. The adsorbed NOx mainly existed in the HPW bulk phase in the form of NOH+. The GC-MS detection confirmed that the adsorbed NOx can be decomposed to N2 using a temperature programming process. This study provides a new experimental basis for which to find a new multi-functional NOx catalytic decomposition system.

 
ACKNOWLEDGEMENTS


This work was supported by the National Natural Science Foundation of China [No. 20776080, 20911120088] and the Scientific Innovation program of Shenzhen city, China, under basic research program (JCYJ20170818102915033).


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