Remarkable Enhancement In The N2- Selectivity of NH3-SCR Over The CeNb3Fe0.3/TiO2 Catalyst In The Presence of Chlorobenzene

doi:10.21203/rs.3.rs-607574/v1

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Research Article

Remarkable Enhancement In The N₂- Selectivity of NH₃-SCR Over The CeNb₃Fe_0.3/TiO₂ Catalyst In The Presence of Chlorobenzene

https://doi.org/10.21203/rs.3.rs-607574/v1

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published 29 Oct, 2021

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The simultaneous removal of NO_x and dioxins is the frontier of environmental catalysis, which is still in the initial stage and poses several challenges. In this study, a series of CeNb₃Fe_x/TiO₂ (x = 0, 0.3, 0.6, and 1.0) catalysts were prepared by the sol-gel method and examined for the synergistic removal of NO_x and chlorobenzene. The CeNb₃Fe_0.3/TiO₂ catalyst exhibits an optimum catalytic performance, with an NO_x conversion greater than 95 % at 260-380 °C. It also exhibited an optimal CB oxidation activity, in which CB promoted both the NO_x conversion and N₂ selectivity below 250 °C. Moreover, the more favourable ratios of Ce⁴⁺ to Ce³⁺, and plentiful surface adsorbed oxygen species are the reasons why CeNb₃Fe_0.3/TiO₂ catalyst has better catalytic activity than other catalysts at lower temperature. Simultaneously, owing to the modulation of Fe to the redox properties of Ce and Nb, the large number of oxygen vacancies and acid sites, the CeNb₃Fe_0.3/TiO₂ catalyst is is beneficial to NO_x reduction and CB oxidation. Further, the results of in situ DRIFTS study reveal the NH₃-SCR reactions over CeNb₃Fe_x/TiO₂ catalysts are mainly controlled by the L-H mechanism (< 200 °C) and E-R mechanism (> 200 °C), respectively.

Environmental Chemistry

Health Policy

Toxicology

Ce-based catalysts

NH3-SCR

VOCs removal

Chlorobenzene

Synergy

Redox

With the acceleration in the industrialization in China, air pollution has attracted national attention (Chen et al. 2014, Li et al. 2015). Metallurgical sintering plants, coke ovens, and municipal solid waste incinerators emit nitrogen oxides (NO_x) and dioxins (Fu et al. 2014, Hashimoto et al. 2005, Li et al. 2018). Nitrogen oxides are the main air pollutants, causing some serious environmental problems such as haze, photochemical smog, and acid rain (Wang et al. 2020). In addition to the emission of nitrogen oxides, dioxins are also persistent organic pollutants that are highly toxic to the environment and biology (Liu et al. 2013, Liu &Woo 2006). Therefore, it is imperative to develop high-efficiency technologies that can simultaneously control the release of dioxins and NO_x.

Meanwhile, NH₃ selective catalytic reduction (NH₃-SCR) is a mature and an effective technology to reduce NO_x emissions from stationary sources. V₂O₅–WO₃ and/or V₂O₅–MoO₃ supported on TiO₂ are the most widely used industrial catalysts for SCR (Gan et al. 2018b). However, they still exhibit some major shortcomings, such as a narrow working temperature range (300–400°C), toxic effects of vanadium on the ecological environment and human health, high toxicity of active vanadium substances, and low N₂ selectivity at higher temperatures (Jiang et al. 2019). Cerium is frequently utilized due to its excellent oxygen-storage capacity and high redox capacity via the reduction of Ce⁴⁺ to Ce³⁺ (Lee et al. 2019, Lei et al. 2020). In addition, cerium dioxide can be used as the active ingredient in the SCR reaction; hence, it has been extensively investigated (Chen et al. 2015, Zhang et al. 2020). Sudarsanam has reported that the introduction of Fe into the Ce/TiO₂ catalyst can increase the NH₃-SCR activity by ~ 25% at 200°C (Sudarsanam et al. 2014). Moreover, owing to their excellent reducibility as well as a large amount of surface-active oxygen, iron-based catalysts, Fe-Ti (Ma et al. 2012), and Fe-Ce oxides (Xiong et al. 2013), exhibit satisfactory low-temperature NH₃-SCR activity. Currently, Nb-containing compounds have been used in different reactions, including the catalytic elimination of nitrogen oxides and the catalytic hydrogenation of alkanes (Souza et al. 2017). Previous studies have reported that, binary oxides of cerium and niobium can increase the acidity of the catalyst, which is considered to be a key for in the NH₃-SCR reaction (Ali et al. 2018). Generally, the modification of niobium is apparently an effective strategy in the selective catalytic reduction of NO_x with ammonia or hydrocarbons. Hence, to improve the activity of the catalyst, the combination of Ce and Nb in a catalyst system is strongly recommended (Jawaher et al. 2018). Typically, the catalyst was usually a precious metal or transition metal supported on TiO₂ or Al₂O₃, which exhibit good reducibility as well as large surface oxygen vacancies (Tian et al. 2020, Zhang et al. 2021).

Conventionally, adsorption on activated carbon has been employed to control aromatic chloride emissions during the exothermic process (Kulkarni et al. 2008). However, this technology could only achieve phase transfer between pollutants, and not eliminate them (Addink &Olie 1995). Moreover, the conventional technology of adsorption of activated carbon and capture of dioxins does not easily satisfy emission regulation (Baldrian et al. 2006). In recent years, the catalytic oxidation of metal oxides has attracted widespread attention due to its cost-effectiveness and high efficiency (Goemans et al. 2003). To the best of our knowledge, few studies have reported the synergistic elimination of NO_x and dioxins (Gan et al. 2018c, Wang et al. 2021). V₂O₅-TiO₂-based catalysts has been widely used for the reduction of NO_x, and the catalysts also been successfully used for the oxidation of dioxins (Gan et al. 2018b). However, vanadium species are still limited due to their high toxicity and low N₂ selectivity at high temperatures. Gan has reported that a MnO_x-CeO₂ composite oxide demonstrates promise as a catalyst for the oxidation of CB as CeO₂ is prone to produce highly active chemisorbed oxygen, and CB improves the N₂ selectivity of the NH₃-SCR reaction due to the inhibition of NSCR (Gan et al. 2018a). Therefore, it is crucial to develop a new multi-component catalyst with high activity, low toxicity, and cost-effectiveness to synergistically eliminate dioxins and NO_x.

In this study, a series of CeNb₃Fe_x/TiO₂ (x = 0, 0.3, 0.6, and 1.0) catalysts were prepared by the sol-gel method and investigated for the synergistic removal of NO_x and chlorobenzene. The effect of doping Fe on the synergistic removal of NO_x and CB on the CeNb₃/TiO₂ catalyst was investigated.

The information on a detailed description of the characterization techniques used in the passage is given in supplementary materials.

Catalyst preparation

The CeNb₃Fe_x/TiO₂ (x = 0, 0.3, 0.6 and 1.0) catalyst was prepared by the sol-gel method, and the molar ratio of Ce, Nb, Fe, and Ti was 1:3:x:1. The reagents used for the catalyst preparation included cerium nitrate (Ce(NO₃)₃∙6H₂O), niobium chloride(NbCl₅), and ferric nitrate (Fe(NO₃)₃∙9H₂O). TiO₂ was used as the support. Ethanol and acetic acid were used as the solvents. All analytical-grade reagents were used.

First, cerium nitrate, niobium chloride, and ferric nitrate were added in the designed ratio to prepare precursor solution A. Next, ethanol, acetic acid, and tetrabutyl titanate were added to prepare precursor solution B. Subsequently, solution B was slowly added into solution A to form gel. The resulting a gel was dried at 110°C, calcined at 450°C, and ground for further use.

Activity Catalyst

The catalytic performance for NO and CB removal was examined in a fixed-bed quartz tube flow reactor in the steady-flow mode. Under atmospheric pressure, 0.3 g of 40–60 mesh catalyst was placed between glass wool plugs in a fixed-bed quartz tube reactor (inner diameter = 9 mm). The feed gas composition was 500 mg/L of NO, 500 mg/L of NH₃, 50 mg/L of CB, and 5 vol.% of O₂, with N₂ as the balance gas. Under ambient conditions, the gas hourly space velocity (GHSV) was 250,000 mL/(g·h). The experiment was performed at 60–500°C. At each temperature, when the reaction reached a steady state (at least 40 min), the corresponding data were collected.

The removal efficiencies of NO, chlorobenzene, and N₂ selectivity were defined by Eqs. (1)–(3), respectively:

(1)

(2)

(3)

where NO_x contains NO and NO₂. Cⁱⁿ and C^out are the related concentrations of the inlet and outlet gas phase, e.g., NO_x, NH₃, N₂O, and CB.

Catalytic efficiency

Figure 1 shows the results of NO conversion, CB conversion, and N₂ selectivity on CeNb₃Fe_x/TiO₂ catalyst. After the NO_x conversion rate of the catalyst first increased, it decreased later due to the limitation by the thermodynamics and kinetics of the NH₃-SCR reaction (Fig. 1(a)) (Yang et al. 2011). The denitrification efficiency of the catalysts between 220 and 420°C exhibited excellent results. Notably, after the addition of Fe, the NO removal efficiency was significantly improved at all of the reaction temperature ranges. Especially for CeNb₃Fe_0.3/TiO₂, these catalysts exhibited highest catalytic activity, and the NO_x conversion rate exceeded 90 % at 250–400°C. However, at an Fe/(Ce + Nb + Fe) molar ratio of 1, the activity at 350–500°C decreased significantly that excess Fe can inhibit the SCR over the CeNb₃/TiO₂ catalyst. Figure 1(c) show the results of N₂ selectivity. Clearly, the N₂ selectivity over the CeNb₃Fe_x/TiO₂ (x = 0.3, 0.6, and 1) catalyst was greater than that over CeNb₃/TiO₂, indicative of its good SCR performance. With the increase in temperature, the conversion rate of CB over different catalysts increased (Fig. 1(b)). CeNb₃Fe_0.3/TiO₂ exhibited the highest CB conversion rate in the entire active window. However, with the increase in the Fe content, the CB conversion rate gradually decrease further confirming that excess Fe can inhibit the CB catalytic oxidation of CeNb₃Fe_x/TiO₂ catalysts.

Figure 2 shows the effect of CB on the NO_x conversion and N₂ selectivity over the CeNb₃Fe_0.3/TiO₂ catalyst. The presence of CB considerably affected the activity of the catalyst. In Fig. 2(a), by the introduction of CB into the flue gas at a temperature of greater than 250°C, the NO_x conversion rate of the catalyst started to decrease, with only a maximum decrease of 4.3% at 340°C. Notably, CB oxidation promoted N₂ selectivity over the CeNb₃Fe_0.3/TiO₂ catalyst at a low temperature (Fig. 2(b)). The results revealed that CB exerts an inhibitory effect on the SCR activity over the CeNb₃Fe_0.3/TiO₂ catalyst at temperatures greater than 250°C, but with the increase in the reaction temperature, the catalytic removal efficiency of CB can be improved.

XRD and Raman

Figure 3(a) shows the XRD patterns of the CeNb₃Fe_x/TiO₂ catalyst. Characteristic diffraction peaks of anatase TiO₂ (PDF#21-1272) and Nb₂O₅ (PDF#26–0885) can be detected in the CeNb₃/TiO₂ catalyst. The absence of characteristic diffraction peaks of CeO_x species may be related to highly dispersed or amorphous phase. Compared with that of CeNb₃/TiO₂, the increased content of Fe in CeNb₃Fe_x/TiO₂ composites lead to the conversion of anatase to rutile TiO₂ (PDF#21-1276), coupled with the CeO₂ peak appears. The Fe and Nb species were not detected in the CeNb₃Fe_x/TiO₂, indicating that Fe and Nb oxide species are highly dispersed or exist in an amorphous form. demonstrating the strong interaction between Ce, Nb, and Fe oxide caused by the introduction of Fe.

Figure 3(b) shoes the Raman spectra of the CeNb₃Fe_x/TiO₂ catalyst. The Raman spectrum of the Fe-containing catalysts revealed a characteristic peak near 462 cm^− 1, corresponding to the F_2g vibration mode of the octahedron around the CeO₂ cubic fluorite structure (Fan et al. 2020). In addition, the CeNb₃Fe_0.3/TiO₂ catalyst exhibited similar peaks at 509 (A_1g) and 632 cm^− 1 (E_g), while the CeNb₃/TiO₂ catalyst only a peak at 632 cm^− 1 (E_g) (Liu et al. 2012, Tian et al. 2012). It exhibited a typical rutile TiO₂ phase, which was consistent with the XRD results. Notably, the wavenumber of TiO₂ with Fe-containing catalyst shifts to a higher direction compared with that of CeNb₃/TiO₂, possibly related to the rearrangement of the electron cloud. Moreover, a weak band corresponding to oxygen vacancies at 245 cm^− 1 was observed (Yao et al. 2014). The peak intensity of CeNb₃Fe_0.3/TiO₂ catalyst at 245 cm^− 1 is significantly higher than other catalysts. This indicates that the addition of a small amount of Fe promotes the increase of oxygen vacancies, which can weaken the N-O bond and accelerate the decomposition of NO, thereby improving the NH₃-SCR reaction and promoting the catalytic performance of the decomposition of chlorobenzene at higher temperatures.

N₂-physisorption

To further investigate the structural characteristics of the CeNb₃Fe_x/TiO₂ catalyst, N₂ adsorption-desorption isotherm results were recorded (Fig. S1). All samples exhibited a type IV isotherm, which is related to the capillary condensation that occurs in the mesopore. Table 1 summarizes the specific structure information. The specific surface area of these samples decreased in the order of CeNb₃/TiO₂ > CeNb₃Fe_0.3/TiO₂ > CeNb₃Fe_0.6/TiO₂ > CeNb₃Fe/TiO₂. The large specific surface area and even porous structures can provide more active sites and easily facilitate the adsorption of reactants, which was beneficial to the excellent catalytic performance. Although CeNb₃/TiO₂ had largest specific surface area among all the samples, its catalytic activity was relatively poor due to its single active component.

SEM and TEM

The structural morphologies of different samples were visualized by SEM and TEM. The results show that the catalyst has an irregular surface and porous multi-metal oxides (Fig. S2). With the addition of Fe in the CeNb₃/TiO₂ catalyst, the catalyst exhibited a small number of non-porous block structures on the surface with fewer pore structures, reducing the reaction units on the catalysts surface. For the TEM results of CeNb₃Fe_x/TiO₂ (Fig. 4a-d), with the increase of the amount of incorporated Fe, the iron species were loaded on the catalyst surface, thereby reducing the pore size and increasing the catalyst particle size. This result was consistent with BET-N₂ detection and catalyst activity detection results. In Fig. 4(e), the TEM image of CeNb₃Fe_0.3/TiO₂ clearly showed that the lattice fringes with an interplanar distance of 3.260, 3.218 and 1.695 nm can be assigned to the (112), (110) and (211) plane of CeO₂ and TiO₂, respectively. For CeNb₃Fe_0.3/TiO₂, the lattice fringes with an interplanar distance of 1.953, 3.124 and 3.622 nm can be assigned to the (420), (400) and (101) plane of Nb₂O₅ and TiO₂, respectively. Therefore, the highly dispersed Fe and Nb species in CeNb₃Fe_x/TiO₂ catalysts should be considered the major reason for the difference in catalytic performance.

XPS

The surface states and atomic concentrations of the catalysts were investigated by X-ray photoelectron spectroscopy (XPS), and the corresponding results are shown in Fig. 5 and Table 1. To distinguish and understand the surface species, by fitting Gaussian peaks after Shirley background subtraction, the XPS spectra of Ce 3d, Nb 3d, Fe 2p, and O1s were deconvolved into several peaks. As displayed in Fig. 5(a), the XPS of Ce 3d exhibited eight components, corresponding to four pairs of spin-orbit doublets(Perret et al. 2014, Romeo et al. 1993). The peaks of Ce 3d_5/2 at 882.4 (v), 889.1 (v''), and 898.5 (v''') eV corresponded to Ce⁴⁺ species, and the peak at 885.7 (v') and 903.7 (u') eV indicates that the initial electronic state of 3d₁₀4f₁ corresponded to Ce³⁺ species (Gomez et al. 2013). Similarly, the Ce3d_3/2 peaks at Ce 901.0 (u), 907.3 (u''), and 916.8 (u''') eV corresponded to Ce⁴⁺ species (Perret et al. 2014, Yao et al. 2017), Clearly, CeNb₃Fe_x/TiO₂ comprised a mixture of Ce³⁺ and Ce⁴⁺ oxidation states on the catalyst surface. Meanwhile, by calculating the Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio of different samples, the Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio of CeNb₃Fe_0.3/TiO₂ (0.44) was found to be greater than those of CeNb₃Fe_0.6/TiO₂ (0.32) and CeNb₃Fe/TiO₂ (0.28), indicating the presence of more oxygen vacancies on the CeNb₃Fe_0.3/TiO₂ surface. Those unsaturated chemical bonds caused by Ce³⁺ could help for denitrification and CB decomposition activity (Ma et al. 2020a). Furthermore, the presence of oxygen vacancy is beneficial to the dissociation of the NO amd CB (Li et al. 2011), thereby promoting the NO_x reduction and CB oxidation.

Figure 5(b) shows the deconvoluted XPS spectra of Fe 2p for different catalysts. Bands characteristic of Fe2p_1/2 and Fe2p_3/2 were observed at 724 eV and 711 eV, respectively (Liu et al. 2017). Gauss fitting revealed that the peaks observed at 709.1-709.3 eV are assigned to typical Fe²⁺ cations, while peaks at 712.2-713.3 eV correspond to characteristics Fe³⁺ (Fan et al. 2020). From the above results, iron in our sample was present as Fe²⁺ and Fe³⁺. The Fe³⁺/(Fe³⁺ + Fe²⁺) of the CeNb₃Fe_0.6/TiO₂ (0.58) catalyst was slightly greater than those of CeNb₃Fe_0.3/TiO₂ (0.40) and CeNb₃Fe/TiO₂ (0.57), indicating the introduction of an appropriate amount of Fe can promote the formation of Fe³⁺ content on the surface. According to previously reported study, Fe³⁺ species can promote the SCR reaction (Liu et al. 2009). Therefore, this may be one of the reasons for the good SCR activity of the CeNb₃Fe_0.3/TiO₂ catalyst.

The Nb 3d and Ti 2p XPS spectra were deconvoluted into two separate peaks (Figs. 5(c) and S3). The spin-orbit splitting peaks at 206.9-207.2 eV and 209.7-210.1 eV corresponded to Nb3d_3/2 and Nb3d_5/2, respectively (Fig. 5(c)) (Ali et al. 2018, Qu et al. 2013). The Nb⁵⁺/(Nb⁵⁺ + Nb⁴⁺) of the CeNb₃Fe_x/TiO₂ catalysts was greater than that of the CeNb₃/TiO₂ catalyst. Compared with the CeNb₃/TiO₂ sample, the binding energy of the CeNb₃Fe_x/TiO₂ shifted to a high value, indicating that the mixing of Fe and Nb oxides together changes the chemical environment around the Nb species. As displayed in Fig. S3, after the introduction of Fe, the binding energy increased to higher values, with an increase to 458.6 and 464.2 eV, and Ti still existed as Ti⁴⁺ (Du et al. 2020), indicating that there is a strong interaction between Ce, Nb, Fe, and Ti species. Combining the Ce 3d, Fe 2p, and Nb 3d XPS results (Table 1), Fe-doping may cause redox cycling (Ce³⁺ + Nb⁴⁺ = Ce³⁺ + Nb⁵⁺ and Fe³⁺ + Ce³⁺ = Fe²⁺ + Ce⁴⁺) to promote the generation of oxygen vacancies on the catalyst surface, thereby improving the catalytic activity of NO_x reduction and CB oxidation to a certain extent.

Figure 5(d) shows the O 1s spectrum of the sample, which can be divided into two types of oxygen: lattice oxygen (529.8 eV, called O_α) and chemically adsorbed oxygen (532.1 eV, called O_β) (Shan et al. 2012, Zhang et al. 2020). The related studies indicated that O₂²⁻ or O⁻ forms O_β with a higher mobility and a better oxygen carrying capacity than those of O_α, indicating that a catalyst with O_β than O_α can clearly improve its catalytic activity (Jawaher et al. 2018). Therefore, the high O_β ratio is beneficial for the oxidation of NO by CB and oxidation to NO₂ in the SCR reaction, promoting “fast SCR”: NO + NO₂ + 2NH₃ = 2N₂ + 3H₂O (Ma et al. 2020b, Yan et al. 2020). The change trend of O_β/(O_α + O_β) decreased in the order of CeNb₃Fe_0.3/TiO₂ (0.42) > CeNb₃Fe_0.6/TiO₂ (0.39) > CeNb₃Fe/TiO₂ (0.38) > CeNb₃/TiO₂ (0.36) (Table 1). Therefore, the high active oxygen level of CeNb₃Fe_0.3/TiO₂ can promote the oxidation of chlorinated aromatic compounds and the NH₃-SCR reaction.

Table 1 The physical, chemical and surface properties of the sample.

Sample	BET (m²/g)	Pore volume (cm³/g)	Average pore diameter (nm)	Ce³⁺/Ce^a	Fe³⁺/Fe^b	Nb⁵⁺/Nb^c	O_α/O^d
CeNb₃Fe_0.3/TiO₂	136	0.13	3.12	0.44	0.40	0.42	0.42
CeNb₃Fe_0.6/TiO₂	124	0.11	3.39	0.32	0.58	0.40	0.39
CeNb₃Fe/TiO₂	75	0.08	4.12	0.28	0.57	0.39	0.38
CeNb₃/TiO₂	158	0.10	2.59	0.30	/	0.37	0.36

^a Ce³⁺/Ce presents Ce³⁺/(Ce³⁺+Ce⁴⁺).

^b Fe³⁺/Fe presents Fe³⁺/(Fe³⁺+Fe²⁺).

^c Nb⁵⁺/Nb presents Nb⁵⁺/(Nb⁵⁺+Nb⁴⁺).

^d O_α/O presents O_α/(O_α+O_β).

H₂-TPR analysis

H₂-TPR was employed to investigate the effect of Fe doping on the reducibility of the CeNb₃/TiO₂ catalyst (Fig. 6). The CeNb₃/TiO₂ exhibits four TPR signals, which can be assigned to the reduction of the high dispersed surface CeO₂ (460°C) (Jawaher et al. 2018), Nb₂O₅ to NbO₂ (535°C) (Lian et al. 2019), the reduction of the Ce-O-Ti species (60°C) (Zhao et al. 2017) and the reduction of the bulk CeO₂ (801°C), respectively (Fan et al. 2020). After addition of Fe, the peak shape of CeNb₃Fe_x/TiO₂ changed considerably. The reduction of surface CeO₂ and Nb₂O₅ temperature of the CeNb₃Fe_0.3/TiO₂ catalyst is lower than that of the CeNb₃Fe_x/TiO₂ catalyst, indicating that the redox ability of CeNb₃Fe_0.3/TiO₂ increases compared with CeNb₃Fe_x/TiO₂. In addition, the new reduction peak at 468°C for the CeNb₃Fe/TiO₂ catalyst, corresponding to the reduction of Fe₂O₃-Fe₃O₄ (Xu et al. 2015). Moreover, the introduction of Fe to CeNb₃Fe_x/TiO₂ (x > 0.3) could lead to the peak located at about 801°C shift to higher temperatures (826°C) compared with the CeNb₃/TiO₂ catalyst. The incorporation of Fe led to a decrease in the number of NbO_x and bulk CeO₂ species and reduced their reducibility. The results show that the introduction of excessive Fe is not conducive to improving the oxidation-reduction capacity of the CeNb₃Fe_x/TiO₂ catalysts. Therefore, the CeNb₃Fe_0.3/TiO₂ catalyst has a lower reduction temperature, it exhibits the best reduction performance, which is obviously the main factor affecting the catalytic activity.

NH₃-TPD analysis

As the essential processes in the NH₃-SCR reaction, NH₃ adsorption and activation play key roles in the generation of reactive intermediates. NH₃ is used as a probe molecule to detect surface acid sites, acid strength, and acid content. The NH₃-TPD curves shown in Fig. 7 exhibited a wide range of desorption temperatures, indicative of the presence of different acid sites with different strengths. Current studies have reviewed the desorption signals of physically adsorbed ammonia and NH⁴⁺ adsorbed on the weak acid sites at 100–200°C, as well as the desorption signals of NH₃ on the catalyst surface at the acid site at 200–350°C and the strong acid site at 350–600°C (Kim et al. 2018, Wang et al. 2019). Based on the desorption peak area, the percentages of acid sites with different strength distributions and the desorption temperature of desorption peak were calculated for the CeNb₃Fe_x/TiO₂ catalysts (Table 2). Compared to CeNb₃/TiO₂, the introduction of Fe increased the ammonia desorption amount at lower and medium temperature (A1 and A2) which corresponds to weak acid sites. The results indicate that the CeNb₃Fe_x/TiO₂ catalyst comprise a higher number of Bronsted acids after loading with Fe. The adsorbed ammonia coordinated to the Bronsted acid site exhibited a lower thermal stability compared to the Lewis acid site (Wang et al. 2016). Nevertheless, after the addition of Fe, the total amount desorbed ammonia (A_total) decreased slightly, and the ammonia desorption peak corresponding to the strong acid site at high temperature (T₃) shifted to a low temperature range. This phenomenon indicated that the addition of Fe leads to the partial destruction or occupation of a part of the acid sites on the CeNb₃/TiO₂ catalyst, as well as to the transformation of some strong acid sites to intermediate acid sites (Zi et al. 2019). This may result from that the introduction of Fe can improve the redox ability of the catalyst slightly, and low-medium acid can effectively promote the adsorption process (Liu et al. 2016, Shu et al. 2020). Compared to strong acid sties, medium-low strong acid sites could prevent excess oxidation performance, thereby inhibiting N₂O formation and promoting NO conversion (Niu et al. 2016). In conclusion, after the addition of Fe, the NO and CB removal efficiencies increased (Sec 3.1 for evaluation results), which was related to the increase in the weak acid and middle acid. Therefore, CeNb₃Fe_0.3/TiO₂ catalyst has the largest amount of low and medium temperature acid sites, which explains why the catalyst exhibits the best reactivity.

Table 2 NH₃-TPD amounts of adsorbed NH₃ in the prepared catalysts.

^a The amount of the CeNb₃Fe_0.3/TiO₂ acidity was assigned as 1.0, and compared with the other samples.

In situ DRIFT analysis

NO + O₂ adsorption

Figure 8 shows the in situ DRIFTS results for the NO + O₂ adsorption on these CeNb₃Fe_x/TiO₂ catalysts at 250°C. Several vibration bands can be observed in the range of 1132–1660 cm^− 1 including bidentate nitrates (1132 and 1660 cm^− 1), monodentate nitrate (1370 and 1556 cm^− 1) (Chen et al. 2010a, Larrubia et al. 2001) and bidentate nitrate(1563 cm^− 1) (Zawadzki &Wiśniewski 2003). Compared to the CeNb₃/TiO₂, the shift in the band observed at 1565 cm^− 1 to 1563 cm^− 1 for the CeNb₃Fe_x/TiO₂ catalyst was related to the deformation of nitrate species. The adsorption peak of nitrate on the CeNb₃/TiO₂ catalyst was less than the CeNb₃Fe_x/TiO₂ catalysts. The adsorbed nitrate species can rapidly react with the adjacent adsorbed NH⁴⁺ or NH₃ to generate an increased amount of reactive intermediates, which can further react with gaseous NO to generate N₂ and H₂O; this in turn promoted a rapid SCR reaction. Therefore, doping with Fe can promote the NH₃-SCR reaction activity over the CeNb₃/TiO₂ catalyst.

Reactions between NO + O₂ and pre-adsorbed NH₃

The effect of the adsorption of NH₃ species over CeNb₃Fe_x/TiO₂ in the NH₃-SCR reaction was investigated, and the in situ DRIFTS for the reaction of NO + O₂ and pre-adsorbed NH₃ at 250°C was recorded (Fig. 9). Figure 9(a) shows the different NH₃ species with L acid sites and B acid sites on the CeNb₃/TiO₂ surface after exposure to NH₃ for 60 min. NH₃ coordination peaks (1604, 1422 and 1214 cm^− 1), NH⁴⁺ coordination peaks (1665 cm^− 1), and several N-H peaks (3345, 3251, and 3176 cm^− 1) were observed for the CeNb₃/TiO₂ catalyst (Zawadzki &Wiśniewski 2003). After the introduction of NO + O₂, the bands of adsorbed NH₃ substances disappeared within 3 min. During the reaction, some bands of surface nitrate substances were observed, namely bidentate and bridging nitrates (1554, 1113 cm^− 1), monodentate nitrates (1370 cm^− 1), and absorbed NO₂ species (1625 cm^− 1) (Brandenberger et al. 2009, Wang et al. 2013). The results revealed that the NH⁴⁺ on the B acid sites and the coordinated NH₃ on the L acid sites are the main active substances in the NH₃-SCR reaction over CeNb₃/TiO₂. For CeNb₃Fe_x/TiO₂ catalysts, the same conclusion is also applicative as shown in Fig. 9(b-d), respectively. After the NO + O₂ was introduced for 3 min, the band of NH₃ species didn’t completely disappear, which indicate that CeNb₃Fe_x/TiO₂ adsorbs more NH₃ species than CeNb₃/TiO₂, which is consistent with the result of NH₃-TPD. This may be one reason for much higher NO conversion over the CeNb₃Fe_0.3/TiO₂ catalysts.

Reaction of NH₃ + NO + O₂

Figure 10 shows the in situ DRIFTS for the co-adsorption of NO + O₂ + NH₃ on all sample at different temperatures. During the whole SCR reaction process, several bands coordinated with NH₃ were observed on the L acid sites (3389 − 3043 cm^− 1) in the N-H region (Xiong et al. 2013). The bands at 1666–1718 cm^− 1 corresponded to the σ_s and σ_as of NH⁴⁺ at the B acid site, respectively (Chen et al. 2010b, Jiang et al. 2018). However, the intensity of the nitrate species over the CeNb₃Fe_x/TiO₂ catalyst decreased with the increase in temperature, and a band corresponding to the bridged nitrate species was not observed at 400°C. Notably, the L acid site for the energy band of the CeNb₃Fe_0.3/TiO₂ catalyst at 3300 cm^− 1 disappeared at 250°C (Fig. 10(b)), indicating that the CeNb₃Fe_0.3/TiO₂ catalyst comprises an increased number of L acids than those in the other three catalysts, which is consistent with the results of NH₃-TPD. For CeNb₃Fe_0.3/TiO₂, two new peaks appeared at 1225 cm^− 1 compared with CeNb₃/TiO₂ at below 200°C (Lei et al. 2020). The new peaks were ascribed to bidentate nitrates (1543 cm^− 1) (Liu et al. 2020) and surface nitrosyls of Fe³⁺-NO type (1715 cm^− 1) (Chen et al. 2018). The NH₃-SCR reaction over CeNb₃Fe_x/TiO₂ catalyst follows the Langmuir-Hinshelwood (L-H) mechanism at below 200°C and the E-R mechanism at above 200°C.

In this study, a series of CeNb₃Fe_x/TiO₂ catalysts were prepared by the sol-gel method, which exhibited good catalytic activity with the simultaneous removal of NO_x and chlorobenzene. An NO removal rate of greater than 95 % at 260–380°C was observed over CeNb₃Fe_0.3/TiO₂. Notably, in the presence of CB, the CeNb₃Fe_0.3/TiO₂ catalyst exhibited > 95% N₂ selectivity over the entire temperature range (140–500°C). The CeNb₃Fe_0.3/TiO₂ presented better NO_x reduction and CB oxidation which might be related to its more favourable ratios of Ce³⁺/Ce, Nb⁵⁺/Nb and abundant surface adsorbed oxygen species, which afforded optimal reduction ability, suitable NO_x and CB adsorption capacity. Owing to the modulation of Fe to the redox properties of Ce and Nb, the large number of oxygen vacancies and acid sites, the CeNb₃Fe_0.3/TiO₂ catalyst is is beneficial to NO_x reduction and CB oxidation. Finally, the results of in situ DRIFTS study reveal the NH₃-SCR reactions over CeNb₃Fe_x/TiO₂ catalysts are mainly controlled by the L-H mechanism (< 200°C) and E-R mechanism (> 200°C), respectively.

Compliance with ethical standards

Ethical approval and consent to participate Not applicable.

Consent for publication Not applicable.

Authors contributions Pengchao Zang: Conceptualization, data curation, methodology, writing—original draft. Jun Liu: Data curation, visualization. Xiaoqing Liu: Visualization and editing. Guojie Zhang: Review and editing. Jianjun Chen: Writing—review and editing and software. Junhua Li: Review and editing. Yongfa Zhang: Supervision and editing. All authors read and approved the manuscript.

Funding information This work was supported by the Postdoctoral Science Program of China (No. 2019M660061) and the Applied Basic study Program of Shanxi Province (No. 201801D221349).

Competing interests The authors declare that they have no conflict of interest.

Data and materials availability All data generated or analyzed during this study are included in this published article.

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Remarkable Enhancement In The N₂- Selectivity of NH₃-SCR Over The CeNb₃Fe_0.3/TiO₂ Catalyst In The Presence of Chlorobenzene

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NH₃-TPD analysis

In situ DRIFT analysis

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Remarkable Enhancement In The N2- Selectivity of NH3-SCR Over The CeNb3Fe0.3/TiO2 Catalyst In The Presence of Chlorobenzene

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Abstract

Figures

Introducton

Experiment

Catalyst preparation

Activity Catalyst

Results

Catalytic efficiency

XRD and Raman

N2-physisorption

SEM and TEM

XPS

H2-TPR analysis

NH3-TPD analysis

In situ DRIFT analysis

Conclusion

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References

Supplementary Files

Status:

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Remarkable Enhancement In The N₂- Selectivity of NH₃-SCR Over The CeNb₃Fe_0.3/TiO₂ Catalyst In The Presence of Chlorobenzene

N₂-physisorption

H₂-TPR analysis

NH₃-TPD analysis