A series of Sb containing CNTs based MnOx/TiO2 catalysts were prepared by the incipient wetness co-impregnation method and tested for the selective catalytic reduction (SCR) of NOx with NH3. The Sb loaded MnOx/TiO2-CNTs catalyst showed better performance than the other catalysts tested and presented the highest activity in the temperature regime of 100–400 oC. Interestingly, Sb loaded catalyst exhibited SO2 and H2O resistance with better thermal stability. Physico-chemical characterizations such as SEM, XRD, BET, XPS, NH3-TPD, and H2-TPR were employed to investigate the influence of Sb loading on NOx removal over MnOx/TiO2-CNTs catalysts. A step-wise addition of SO2 into the gas stream disclosed that MnOx-Sb/TiO2-CNTs catalyst has better SO2 resistance by avoiding the sulphur poisoning than the base catalyst MnOx/TiO2. Moreover, the results of SEM and XRD revealed that the dispersion of MnOx on TiO2/CNT surface was more uniform after Sb loading. We evidenced that the decrease in SCR activity of MnOx based catalyst is due to the formation of MnSO4. Indeed, in Sb loaded catalysts, the SOx species react more preferably with SbOx than that of MnOx. This strong interaction of SO2 with SbOx successfully prevents the poisoning of active sites on MnOx from being sulphated. This significantly improved the SO2 tolerance of Sb-loaded catalysts.
Research Article
Sb loaded MnOx/TiO 2 /CNTs catalysts for Selective catalytic reduction of NOx: insight to the SO 2 and H 2 O tolerance
https://doi.org/10.21203/rs.3.rs-2065222/v1
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NOx removal
Mn/TiO2
Sb
SCR
Carbon nanotubes
SO2 tolerance.
NOx removal is the most important subject in the past few decades due to its irreversible damaging effects to the environment as well as the human health. Therefore, for better performance in industries like coal-fired power stations and diesel engines, ammonia-Selective Catalytic Reduction (NH3-SCR) has been used for deNOx process [1,2]. Till date, V2O5-WO3(MoO3)/TiO2 is considered to be the commercialized SCR catalyst because of its excellent SCR performance in the temperature range of 300 – 400 oC [3]. However, this catalyst has some drawbacks such as conversion of SO2 to SO3, the formation of N2O at high temperatures and the poisonous VOx species production [4,5]. Therefore, transition metal based catalysts (MnOx, CuOx, FeOx, and CeOx, etc.) have to be prepared to replace V-based catalyst [6–8]. Among the recently developed transition metal catalysts, the low-temperature SCR activity of MnOx has attracted much attention because of its various oxidation states [9–11]. The presence of various labile oxygen on MnOx makes it vital to complete the SCR reaction [12]. This makes researchers to concentrate more on investigating the SCR reaction on Mn-based catalysts for SCR process [13].
Meng et al. [14] suggested that the catalytic activity of MnOx catalyst could be improved by doping Sm as a promotor. Similar improved SCR reactivity had also been observed by Guo et al. [15]. However, these studies exhibited a narrow temperature window since they were carried out under low feed flow. Therefore, it is of great interest for the researchers to develop a new MnOx-based catalyst which can deliver high SCR activity in a wide temperature window. Also, the MnOx based catalysts exhibited poor SO2 resistance [13].
Recent study reported that the Sb loading could improve the SCR performance of MnOx based catalysts [2]. One of the major issues in SCR process is SO2 poisoning. SO2 is being produced when fossil fuels or their by-products such as petrol and diesel were used in combustion reaction. When SO2, present in the exhaust flue gas, reacts with MnOx present in the catalyst it produces MnSO4 [16]. The produced SO4-2 significantly reduced the number of active sites, thus the NOx reduction efficiency decreased. SO2 itself competes with NO for the adsorption sites which exalts the SO2 poisoning effect on the SCR reaction. In addition, reaction between SO2 and NH3 results in formation of (NH4)2SO4 and (NH4)2SO3. This in turn depletes the specific surface area of the catalyst by blocking the active sites which further could inhibit the NOx reduction [17]. These salts, ((NH4)2SO4 and (NH4)2SO3, need higher temperature for decomposition than the SCR operating temperature [18]. Moreover, the poisoning effect is magnified by the presence of water vapour in the flue gas [19]. Water vapour in the flue gas reduces the number of active sites on the catalyst surface. Competitive adsorption between H2O and NH3/NOx leads to decrease in number of available active sites for SCR reaction, therefore, NOx removal efficiency decreases [20]. Dissociative adsorption and decomposition of water vapour produces hydroxyls groups on the surface (-OH) which causes irreversible catalyst deactivation at temperatures below 473 K [19].
In the present study, we report the role of Sb loading on SCR activity of MnOx/TiO2/CNT catalysts. Mn-Sb/TiO2-CNT catalysts with different Sb/Mn molar ratios were synthetized and used in NH3-SCR reaction. Various characterization techniques were performed to explore the effect of catalysts physico-chemical properties on SCR performance. Moreover, the SO2 and H2O tolerance of the catalyst with different Sb have also been investigated.
2.1 Catalyst preparation
Catalysts were prepared by incipient wetness co-impregnation method [21]. The purified multiwalled carbon nanotubes (CNTs) dispersed in ethanol and the TiO2 in deionized water were mixed and sonicated for 15 min. To that, a solution of manganese acetate (Mn(CH3CO2) 2·(H2O)n) and antimony acetate (Sb(CH3CO2)3) dissolved in oxalic acid, were added under continuous ultrasonic treatment for 30 min. The solution was stirred at 100 ◦C for 8 h and then filtered. The solid was washed with distilled water and ethanol then dried at 120 ◦C for 3 h, and then calcined at 500 ◦C for 5 h. The as-prepared catalysts were denoted as Mn-Sbx/TiO2-CNTs, where x represents the wt.% of Sb loading on the catalyst. The amount of MnOx and CNTs is maintained to be 12 wt.% and 10 wt.% respectively, in all the catalysts. For comparison, the MnOx/TiO2 catalyst, without CNTs and Sb, was also prepared by the same method and employed for DeNOx process.
2.2 Catalyst characterisation
Brunauer–Emmett–Teller (BET) method employed to measure the total surface area, pore diameter and pore volume of the catalysts using N2 as an adsorbate at -196 °C. Scanning Electron Microscope (SEM) analysis was performed using FEI, Quanta 200 equipped with an energy-dispersive X-ray spectroscopy (EDS) analyser to obtain the surface topology and composition of the catalysts. The and structural parameters such as grain size, Crystallinity and particle size were measured using X-Ray Diffraction (XRD) spectrophotometer (Cu-Kα radiation at 45 kV and 40 mA in the scanning angle (2θ) range 5◦ < 2θ < 85◦). X-ray Photoelectron Spectroscopy (XPS) was used (PHI-Versa Probe III with Al-Kα anode and a dual X-ray source) to analyse the chemical state and elemental composition of the catalyst. Temperature-programmed desorption with ammonia (NH3-TPD) is carried out using Chemisorb 2750 (Micrometrics, USA) to obtain the acidity and ammonia storage capacity of the catalyst. Test samples were treated under He atmosphere at 200 °C for 60 min and cooled down to 100 °C before the experiment begins. After cooling, it was exposed to 10% NH3 gas at 30 mL/min. Once the sample was saturated, excess ammonia was removed by purging with N2 (30 mL/min). Tests were done by heating the catalysts between 100 °C and 750 °C in N2 atmosphere at 10 °C/min. Similarly, Temperature-programmed reduction (H2-TPR) experiments were carried out under 5%H2-N2 atmosphere to understand the reductive capacity of the catalyst (Chemisorb 2750, Micrometrics, USA).
2.3 Catalytic activity measurements
The SCR activity experiment was performed in a fixed bed quartz reactor (i.d. = 10 mm). For each experiment a fresh 0.5 g of catalyst was used. The flue gas consists of 500 ppm NO, 500 ppm NH3, 5 vol% O2, 200 ppm SO2 (only for SO2 resistance test), 4 vol% H2O, and N2 as balance gas was sent to the reactor inlet. The total feed flow was controlled by mass flow controllers and the total flow rate was set at 500 mL/min (GHSV: 60,000 h−1). All experiments were performed in the temperature range of 100 to 400 ◦C, and the temperature was raised 5 ◦C/min rate at atmospheric pressure. The concentrations of NO, NO2, N2O, and NH3 were measured using the gas analyser (VE FT-8000, Vasthi Instruments, India). The NO conversion efficiency and N2 selectivity were calculated using the following equations 1 and 2:
3.1 Characterization studies
3.1.1 BET analyses
From Table 1, it can be seen that addition of CNT and loading of Sb into the Mn/TiO2 catalyst increases the total surface area. Among the catalyst studied MnOx-Sb2/TiO2-CNTs exhibited the highest BET surface area (127.7 m2/g). High surface area facilitates the reactants adsorption and enhances the NH3-SCR reaction efficiency.
Table 1 Physico-chemical properties of the synthesised catalysts.
| Catalysts | BET Surface area (m2/g) |
Pore volume (cm3/g) |
Average pore diameter (nm) |
| MnOx/TiO2 | 56.5 | 0.089 | 3.82 |
| MnOx-Sb/TiO2 | 66.1 | 0.112 | 3.85 |
| MnOx/TiO2-CNTs | 124.6 | 0.223 | 3.86 |
| MnOx-Sb1/TiO2-CNTs | 125.3 | 0.247 | 3.87 |
| Mn-Sb2/TiO2-CNTs | 127.7 | 0.275 | 3.92 |
| Mn-Sb3/TiO2-CNTs | 125.6 | 0.250 | 3.86 |
3.1.2 XRD analyses
The XRD patterns of MnOx/TiO2, MnOx-Sb1/TiO2, MnOx-Sb2/TiO2 and Mn-Sb3/TiO2 are reported in Fig.1. Based on the XRD patterns, the dominant phase of TiO2 is rutile. However, there were few peaks attributed to TiO2 anatase phase. The peak located between 25 - 30° 2Q could be attributed to TiO2 of the MnOx/TiO2/CNT catalyst. From these peaks, it can be seen that the structure of TiO2 remains unchanged even after doping CNTs and SbOx at various concentrations. Remarkably, no diffraction peaks correspond to MnOx and SbOx were observed. This finding emphasis the fact that these metal oxides are well dispersed in the TiO2 surface, and the particle size would be very small [22]. Following the addition of Sb, the dominant phase of TiO2 remains as rutile which confirms that the added SbOx has no effect on the structure of TiO2.
3.1.3 XPS analyses
The surface elements concentration and their chemical states play a crucial role in SCR reaction, which could be obtained by XPS analysis. The XPS spectra of Mn 2p, O 1s and Ti 2p for all catalysts are presented in Fig. 2.
In Fig.2(a), it can be seen that the Mn(2p3/2) spike of all the catalysts was split into three peaks located at 639.5–641.0 eV, 641.5–642.5 eV and 642.8–644.5 eV, corresponding to Mn2+, Mn3+ and Mn4+, respectively [23,24]. The Mn4+/Mn ratio of MnOx/TiO2 catalyst is 25.3%, whereas for MnOx-Sb2/TiO2-CNTs catalyst is 42.6%, which is about 1.7 times higher than that of MnOx/TiO2 catalyst. This enhanced Mn4+/Mn ratio indicates that higher oxidation of Mn increased upon loading SbOx into the catalyst.
Fig. 2(b) presents the XPS spectra of O 1s for all the investigated catalysts. The O 1 s spectra could be divided into two different characteristic peaks of oxygen species. The peak located at 531.3 eV was assigned to the surface chemisorbed oxygen (Oα) and the other peak centered at low 529.9 eV was attributed to the lattice oxygen (Oβ) [25,26]. It is evident from Fig. 2(b) that the relative concentration ratio of Oα/(Oα + Oβ) on the surface of Mn-Sb2/TiO2-CNTs catalyst is higher than that on the surfaces of Mn/TiO2 catalyst, which depicted that the lattice oxygen concentration was decreased with the addition of Sb species. Generally, the chemisorbed oxygen species (Oα) are more active than the lattice oxygen species (Oβ) because of its higher mobility [27]. Moreover, it was reported that the presence of NO2 promotes the SCR reaction on the catalysts [28]. The oxidation of NO to NO2 was promoted by the higher relative concentration ratio of Oα/(Oα + Oβ) on the catalyst’s surface which is beneficial for the NH3-SCR reaction and successive assistance of the “fast SCR” reaction. Therefore, NOx conversion through the “fast SCR” route could be effectively accelerated [29].
Fig. 2(C) presents the XPS spectra of Ti 2p for different catalysts. Two major peaks observed and assigned to Ti 2p1/2 and Ti 2p3/2 centered around 464 eV and 459 eV, respectively. The binding energy of all Sb loaded catalysts shifted to lower value as compared to the Mn-TiO2 catalyst. These findings indicate that some of the Ti4+ are reduced to Ti3+ which could be attributed to the strong interaction among Mn, Sb and TiO2 species [30].
3.1.4 NH3-TPD analyses
Surface acidity of the catalysts play a vital role in the NH3-SCR reaction [31]. Therefore, NH3-TPD analysis has been performed to study the surface acidity of the prepared catalysts. The NH3-TPD profiles for Mn/TiO2, Mn-Sb1/TiO2, Mn-Sb2/TiO2 and Mn-Sb3/TiO2 are presented in Fig 3. For all the catalysts, a broad peak appeared between ~200 and ~400 °C. The acidity strength of the Sb loaded catalysts followed the following order: Mn-Sb3/TiO2 > Mn-Sb2/TiO2 >Mn-Sb1/TiO2 > Mn/TiO2. This acid strength is in accordance with the order of their catalytic activities as well. The Sb loading on Mn/TiO2-CNTs catalyst widens the NH3 desorption peak, which covers from 100 to 400 ℃, representing the coexistence of Brønsted and Lewis acid sites. Moreover, with increase in the amount of Sb loading, high surface area is available which further improves the interaction between the acid and redox sites [32]. Indeed, this synergistic effect between acidic and redox sites further influences the SCR activity of the catalyst.
3.1.5 H2-TPR analyses
The redox properties of the catalysts are an important factor related to their respective performances in the NH3-SCR reaction. Therefore, H2-temperature-programmed reduction (H2-TPR) analysis was performed and the results are presented in Fig.4. All three Sb loaded catalysts displayed three reduction peaks between 300 and 600 °C. The low temperature peaks (300-450 °C) are attributed to the reduction of Mn4+ to Mn3+ and Ti4+ to Ti3+ owing to the interaction between MnO2 and TiO2 [33]. The high temperature peaks (450- 600 °C) are associated to the reduction of Mn3+ to Mn2+ [34]. It was noted that the reduction peak centre varied with the amount of Sb in the catalysts. For Mn-Sb3/TiO2-CNTs, Mn-Sb2/TiO2-CNTs, Mn-Sb1/TiO2-CNTs and Mn/TiO2 catalysts, the reduction peaks at low temperature were cantered at 335 °C, 330 °C, 332 °C and 330 °C respectively. The low peak temperature of Mn-Sb2/TiO2 implies the higher oxidative property and oxygen mobility of the catalyst. Moreover, as shown in Fig.4, the total areas of the reduction peak were maximum for Mn-Sb2/TiO2-CNTs. Since the peak areas correspond to H2 consumption, it can be seen that the redox properties of the catalysts are enhanced at the incorporation of Sb species which promotes the NH3-SCR reaction.
3.1.6 In situ DRIFT analyses
3.1.6.1 NH3 adsorption
Fig. 5. presents the NH3 adsorption DRIFT spectra over Mn/TiO2 and Mn-Sb2/TiO2-CNTs catalysts at different temperatures. It is clear from Fig. 5(a) that only two bands were observed at 1310 and 1610 cm−1 for Mn/TiO2 catalyst which could be attributed to the NH3 species adsorbed on Lewis acid sites [35,36]. On Mn-Sb2/TiO2-CNTs catalyst five bands were observed, out of which 3 bands centred at 1600, 1290 and 1170 cm−1 could be ascribed to the NH3 species associated to Lewis acid sites and two bands centred at 1490, 1400 cm−1 could be attributed to the NH4+ species adsorbed on Brønsted acid sites [37–39]. It clearly depicted that the presence of Sb on the catalyst leads to formation of Brønsted acid sites. Moreover, the band intensities are much stronger on Mn-Sb2/TiO2-CNTs catalyst. These finding emphasis the fact that high amount of adsorbed NH3 species are available on Mn-Sb2/TiO2-CNTs catalyst. The intensity decreases at high temperature owing to the desorption of adsorbed NH3 species, and all the adsorbed NH3 on Mn/TiO2 catalyst have been desorbed at 300 to 350 ℃. Therefore, it can be suggested that the desorption of NH3 from the catalyst surface sorption could decrease the NOx conversion efficiency.
3.2 Activity studies
3.2.1 NOx conversion study
The SCR activity of the prepared catalysts as a function of temperature is reported in Fig. 6. For a comparison study the activity of Mn/TiO2 was also investigated. The SCR activity of Mn/TiO2 catalyst is very low in the entire investigated temperature range, and maximum of 80% NOx conversion is obtained at 300 ◦C. NO conversion is also increased when the CNTs were introduced as an additional support to the Mn/TiO2 catalyst. This result suggested that the interaction between TiO2 and the CNTs is important for the SCR reaction. Indeed, the NOx conversion is further enhanced by adding Sb to the catalyst. Notably, for Mn-Sb/TiO2 catalyst, the SCR is higher than the Mn/TiO2 and Mn/TiO2-CNTs catalysts. The activity was further improved when the Sb and CNTs were incorporated to the Mn/TiO2 catalyst. The Mn-Sb1/TiO2-CNTs catalyst exhibited the best SCR activity of more than 90% NOx conversion in a wide temperature range between 200 and 300 ◦C. Although Mn-Sb/TiO2 catalyst showed similar catalytic activity as Mn-Sb/TiO2-CNTs but its operational temperature window is wider than that of Mn-Sb/TiO2 catalyst.
3.2.2 Influence of Sb loading on NOx conversion efficiency
The effect of Sb loading on NOx conversion as a function of temperature is reported in Fig.7(a). Globally, all catalysts have shown increase in NOx conversion efficiency up to 250 °C, thereafter it decreases with increasing the temperature. Mn/TiO2-CNT has exhibited the lowest NOx conversion efficiency of 84% at 250 °C. Interestingly, when 1 wt.% amount of Sb was doped, the NOx conversion increased by 10% (94%). This finding shows that even a small amount of Sb doping could contribute to the catalyst performance enhancement. When the Sb content in the catalyst was increased to 2 wt%, the catalyst shows excellent SCR activity reaching a maximum conversion efficiency upwards of 98.6% at 250 °C. The N2 selectivity for the Mn/TiO2, Mn/TiO2-CNTs, Mn-Sb1/TiO2, Mn-Sb2/TiO2 and Mn-Sb3/TiO2 catalysts for temperatures between 100 °C and 400 °C is shown in Fig.7(b). The general trend shows that N2 selectivity decreases with increase of reaction temperature which is associated with the formation of Nitrous oxide (N2O) and NO2 from NH3 oxidation [40]. It can be observed that the addition of Sb improves the N2 selectivity over the entire temperature range. Among all the catalysts, Mn-Sb2/TiO2 exhibited the highest N2 selectivity for all the investigated temperature range. Thus, it can be proposed that Mn-Sb2/TiO2 is the best catalyst for the DeNOx process.
3.2.3 Effect of Sb loading on SO2 and H2O poisoning
The influence of SO2 and H2O on the NOx conversion efficiency for all the prepared catalysts have been studied for various temperature and reported in Fig.8. In the previous section we evidenced that Mn-Sb2/TiO2-CNTs catalyst showed the best NOx removal efficiency. Therefore, the SO2 and H2O poisoning on deNOx process have been investigated for Mn-Sb2/TiO2-CNTs catalyst and compared with base Mn/TiO2 catalyst. 200 ppm of SO2 and 5% H2O were introduced into the feed gas at the reactor inlet.
As shown in Fig.8(a) the NOx conversion efficiency decreased significantly when H2O and SO2 were introduced in the feed flow. Below 250 °C the decrease in NOx conversion efficiency is about 20% for Mn/TiO2 catalyst. However, when temperature is increased the decrease in NOx removal efficiency minimized. This occurs as the deposition rate of sulphate species exceeds its consumption rate at lower temperatures [41]. Thus, sulphates deposit on the catalyst surface and deactivates the catalytic sites. Indeed, Mn-Sb2/TiO2-CNTs catalyst exhibited better SO2 and H2O resistance in all investigated temperature. About 10% decrease in NOx removal efficiency has been observed. Incorporation of CNTs into Mn/TiO2 catalyst has significantly increased the specific surface area, therefore, the number of active sites for NOx conversion have also been increased [42].
SO2 and H2O poisoning on catalytic activity of Mn/TiO2 and Mn-Sb2/TiO2-CNTs catalysts have been monitored for 30 h and reported in Fig.8(b). Before introducing SO2 and H2O into the feed stream, NOx conversion efficiency of Mn/TiO2 and Mn-Sb2/TiO2-CNTs catalysts were 78% and 98.6%, respectively. Notably, this decreased drastically and attained stable values of about 45% and 86%, respectively when SO2 (200 ppm) and H2O (5%) were added to the feed gas. However, the catalytic activity retained for more than 15 h. Remarkably, when the supply of SO2 and H2O were turned off, the NOx conversion efficiency of Mn-Sb2/TiO2-CNTs catalyst recovered to 96%. Nevertheless, for Mn/TiO2 the catalytic activity recovered only to 54% which is 20% lower than that of initial catalytic activity. These findings suggest that the Sb loading not only enhanced NOx removal efficiency but also improved the self-regeneration of SO2 and H2O poisoning. Mn-Sb2/TiO2-CNTs exhibited far superior SCR performance compared to Mn/TiO2 catalyst in the presence of SO2 and H2O.
In the present investigation, the effect of Sb and CNTs loading on Mn/TiO2 catalyst on NOx conversion and N2 selectivity have been investigated. The SCR activity and SO2 resistance of Mn/TiO2 catalyst has been effectively enhanced with the Sb loading. Moreover, the introduction of CNTs as an additional catalytic support increased the total surface area of the catalyst which increased the amount of NH3 adsorption on the catalyst surface. The more adsorbed NH3 and NOx species could be found on the surface of Mn-Sb2/TiO2-CNTs catalyst which accelerated the NH3-SCR reaction over Mn-Sb2/TiO2-CNTs catalyst through the L-H mechanism. Mn-Sb2/TiO2-CNTs catalyst exhibited more than 90% NOx conversion even with the presence of 200 ppm SO2 in the feed gas in the temperature between 200 to 300 ◦C. Catalyst characterization demonstrated that the NO oxidation was remarkably increased when Sb was introduced into Mn/TiO2-CNTs catalyst. Moreover, the addition of Sb brought more Mn4+ ions and adsorbed oxygen species on the catalyst surface, which has remarkably enhanced the NO oxidation to N2.
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No competing interests reported.