In Situ DRIFTS Study of the Low Temperature Selective Catalytic Reduction of NO with NH 3 over MnO x Supported on Multi-Walled Carbon Nanotubes Catalysts

MnOx supported on multi-walled carbon nanotubes (MWCNTs) catalysts were prepared by the pore volume impregnation method and used for low-temperature selective catalytic reduction (SCR) of NO with NH3. Based on the previous study, 10 wt.% loading MnOx/MWCNTs were then selected for investigation of the reaction mechanism by in situ Diffuse Reflectance Infrared Fourier Transform spectroscopy (in-situ DRIFTS). The important intermediates in the SCR of NOx process at 210°C were discussed based on the DRIFTS results. Furthermore, the NH3-SCR reaction pathways over MnOx/MWCNTs catalysts were proposed. The results showed that NH4 species on Brønsted acid sites and coordinate ammonia species on Lewis acid sites existed during the SCR reaction. NH4 species was more active than coordinate ammonia species over the catalysts at 210°C. Most of NOx ad-species would react with NH3 ad-species. However, nitrite species, bidentate and monodentate nitrates contributed to the SCR reaction over the catalysts mostly. Two possible reaction pathways were proposed. One was that NOx ad-species could react with NH4 to form intermediate of NH4N2O4 (a), NH4NO2 (a) or NH4NO3 (a), then to produce N2 and H2O as the final products. The other pathway was that NH3 was initially adsorbed on active site and NH2 was formed, then NH2 reacted with NOx ad-species to produce intermediate NH2NO2 or NH2NO3 which were unstable and would decompose into N2 and H2O.


INTRODUCTION
Selective catalytic reduction (SCR) of NO x with NH 3 is a well-established technology to abate nitrogen oxides from stationary source (Bosch, 1988;Forzatti, 1996).In power plants, the commercial catalysts (V 2 O 5 -WO 3 (MoO 3 )/TiO 2 ), with relatively narrow temperature window of 300-400°C, have been widely used.However, due to the drawbacks of these catalysts, such as high catalytic temperature, narrow temperature window and toxicity of vanadium by SO 2 or dust, there has been a continuing effort to develop a highly low-temperature (≤ 250°C) efficient, stable, environmentalfriendly SCR catalysts for the removal of NO x (Lou et al., 2003., Qi et al., 2004;Jiang et al., 2009;Yang et al., 2012;Liu et al., 2013).
Manganese-based catalysts have shown excellent lowtemperature SCR activity for NO x removal, which have attracted much interest (Wallin et al., 2004;Wu et al., 2008Wu et al., ). 2008)).Furthermore, there are various kinds of labile oxygen in Manganese-based catalysts, which play important roles in completing the SCR reaction cycle (Kijlstra et al., 1997a).It has been reported that Mn-based catalysts, such as MnO x /TiO 2 (Jiang et al., 2009), MnO x /activated carbon fiber (Pasel et al., 1998), and MnO x /activated carbon/ceramic (Tang et al., 2007), have presented good catalytic activity in SCR reaction with excess oxygen at low temperature.In our previous study, we had successfully developed MnO x supported on multi-walled carbon nanotubes (MWCNTs) catalysts which showed high active for low-temperature SCR (Wang et al., 2012).
It is well known that catalyst supports not only could make the active components dispersed uniformly on the supports surface, but also provide higher specific surface area.Furthermore, catalyst supports could improve catalytic efficiency of the active components (Jackson and Hargreaves, 2008;Tian et al., 2010).Carbon materials have wide applications as catalyst supports for low-temperature SCR because of their high specific surface areas and chemical stability.Carbon nanotubes (CNTs), as special orderly carbon materials, have been reported to be excellent as catalyst supports due to their unique electronic properties and structure (Planeix et al., 1994;Santillan-Jimenez et al., 2011;Song et al., 2011).Moreover, CNTs as catalyst supports were also found to be superior and valuable for NO x decomposition (Luo et al., 2000) and reduction (Wang et al., 2004) as catalyst supports.Recently, the MWCNTs have been of interest for application in low-temperature SCR process (Santillan-Jimenez et al., 2011;Li et al., 2013).
The mechanism of SCR reaction in the manganese catalysts has been studied and various possible reaction pathways have been proposed.For some manganese catalysts, such as MnO x /TiO 2 (Pena et al., 2004;Wu et al., 2007;Wu et al., 2010), unsupported MnO x (Qi et al., 2004;Chen, 2007), MnO x /Al 2 O 3 (Kijlstra et al., 1997a, b), and perovskite-type manganese oxides (Zhang et al., 2013b), some aspects of the reaction mechanism were similar, but some were distinguishably different, in particular, at the initial step of adsorption of NH 3 on these catalysts.It was reported that NH 3 adsorbed on the Lewis sites was more active than NH 4 + species in the SCR reaction (Zhu et al., 2013;Fu et al., 2014).On the surface of the manganese catalysts, competition for adsorption between NH 3 and NO exists.If the NO adsorbed on the surface before the NH 3 was introduced, NO would have blocked the SCR reaction.Especially when bidentate nitrate was formed at the surface of the manganese catalysts, it could not react with NH 3 but occupied the activated sites, resulting in deactivation of the catalysts.In general, O 2 in atmosphere can improve the oxidation of NO to higher states (such as bidentate nitrate, bridged nitrate, etc.) on the surface of the catalysts and enhance the adsorption of NO.However, in the case of NH 3 adsorption, the roles of O 2 in atmosphere on the catalysts with various kinds of supports were quite different.In the MnO x /TiO 2 catalysts (Wu et al., 2007;2010), O 2 in atmosphere could enhance the absorption of NH 3 .But in contrast, O 2 appeared to have no effects on the adsorption of NH 3 on the MnO x /Al 2 O 3 catalysts (Kijlstra et al., 1997a, b).Researchers (Reddy and Khan, 2005;Liu et al., 2007) showed that the catalytic properties of supported catalysts remarkably vary with the nature of the supports possibly due to the different reaction pathways over catalysts with various materials of supports for low-temperature SCR.Lots of work based on FT-IR to study the mechanism of SCR reaction has been done for the low-temperature NH 3 -SCR, however, to the best of our knowledge, there has been few studies that focused on the reaction mechanism of manganese supported on carbon materials, especially on MWCNTs.In light of this, mechanism of manganese supported on MWCNTs SCR catalysts should be investigated for further development of efficient low-temperature SCR catalysts.
In this work, the important intermediates and possible reaction mechanism in the SCR of NO x process over MnO x /MWCNTs catalysts have been investigated by using a combination of adsorption, transient response, steady-state response and in situ DRIFTS experiments.We proposed two possible reaction pathways that are significant to understand the behaviour of the formed surface compounds, active intermediates and elucidate the potential mechanism for the catalysts with high low-temperature activity in respect to the SCR reaction cycle.

Catalyst Preparation
The MWCNTs were purchased from Shenzhen Nanotech Port Co. Ltd.Prior to supporting manganese, the MWCNTs were purified with 3 mol/L HNO 3 solution at 100°C for 4 h and pretreated in oxygen dielectric barrier discharge plasma at the discharge power of 15 W for 40 min.The detailed process was described in previous study (Wang et al., 2011).
The catalysts were prepared by pore volume impregnation method.According to previous study (Wang et al., 2012), when the manganese loading was 10 wt.%, the catalyst showed the highest SCR activity.Therefore, in this study, a manganese acetate solution was used as the Mn precursor and was controlled to 10 wt.% loading.After the impregnation, the catalysts were dried at 110°C for 12 h followed by the calcination at 400°C in air for 2 h.The detailed information of catalyst preparation was described in previous study (Wang et al., 2012).The catalysts were denoted as MnO x /MWCNTs.

DRIFTS Experiments
In situ DRIFTS experiments were conducted on a Nicolet 6700 FTIR spectrometers (2 cm -1 resolution with 100 accumulated scans) equipped with a MCT detector cooled by liquid nitrogen and a diffuse reflection accessory with a high temperature reaction cell.The catalysts were first mixed with KBr in a ratio of 1/100 by weight, and it was then loaded into a DRIFTS cell.Prior to each experiment, the sample was pretreated at 350°C in Ar atmosphere for 1 h to remove any adsorbed species, then cooled down to the reaction temperature of 210°C.The background spectrum was recorded in flowing Ar and was automatically subtracted from the sample spectrum during the experiment.Then the Ar flow was switched to a stream containing one or more reactants, such as NH 3 , NO, and O 2 .
In situ DRIFTS experiments included transient response and steady-state response experiments.It should be noted that new catalysts sample pretreated under the same condition were used in each in situ DRIFTS experiment.The feed composition in each experiment was described at the bottom of each figure.

Adsorption Experiments
The purpose of the NH 3 or NO + O 2 adsorption experiments was to learn the main adsorption species on the surface of the catalysts, which could be conducive to the following analysis of the intermediates and reaction pathways.

NH 3 Adsorption on MnO x /MWCNTs
As shown in Fig. 1, several weak bands at 930, 965, 1549, 1559, 1684 cm -1 and a broad band in the range of 3350-3100 cm -1 were detected when NH 3 was injected at 210°C for various times.After NH 3 was fed into the DRIFTS cell for about 10 min, the bands at 1549 cm -1 , 1559 cm -1 due to presence of amide species (-NH 2 ) were detected (Qi and Yang, 2004;Sun et al., 2009), which might be oxidized by lattice oxygen on the surface of manganese oxide (Kijlstra et al., 1997a).With the continuous injection of ammonia, the new band at 1684 cm -1 which could be attributed to the ammonium ions bond to Brønsted acid sites was detected (Jin et al., 1986;Pan et al., 2013).It was noted that several adsorbed species were detected after injection of NH 3 for about 25 min.The weak bands corresponding to N-H stretching vibration modes of NH 3 , especially for the coordinated ammonia bonded to Lewis acid sites (3345, 3325, 3249 and 3152 cm -1 ) (Galvez et al., 2008;Sun et al., 2009;Jin et al., 2010;Liu et al., 2012) were observed at a broad band in the range of 3350-3100 cm -1 (Kijlstra et al., 1997a;Wu et al., 2007;Liu and He, 2010;Zhou et al., 2011).And the bands at 965 and 930 cm -1 attributed to gaseous state or weakly adsorbed NH 3 (Jiang et al., 2010;Jin et al., 2010;Liu et al., 2012) were also observed.However, the intensity of adsorption of ammonia was very weak, which was in agreement with previous studies by Fan et al. (2011) and Chang et al. (2001).On the whole, Fig. 1 reveals that the main ammonia adsorption species were ammonium ions bonded to Brønsted acid sites, coordinated ammonia bonded to Lewis acid sites and weakly adsorbed NH 3 on the surface of MnO x /MWCNTs catalysts.

NO and O 2 Co-Adsorption on MnO x /MWCNTs
The DRIFTS results of feeding NO and O 2 mixture over MnO x /MWCNTs catalysts for various times are shown in Fig. 2. With the supply of NO and O 2 mixture, the bands at 1741,1697,1648,1627,1559,1508, and 1457 cm -1 were clearly observed.These bands could be assigned to N 2 O 4 (a) (1741 and 1697 cm -1 ) (Zhou et al., 2011), bridging nitrate species (1648 and 1627 cm -1 ) (Kijlstra et al., 1997a;Zhao et al., 2009), bidentate nitrate (1559 cm -1 ) (Liu and He, 2010;Wu et al., 2010;Zhou et al., 2011;Zhang et al., 2013a), monodentate nitrate (1539 cm -1 ) (Zhang et al., 2013a), and nitrite species (1508 and 1457 cm -1 ) (Kijlstra et al., 1997a;Jin et al., 2010).When NO and O 2 were absorbed onto the surface of MnO x /MWCNTs catalysts for about 15 min, the intensity of bands at 2236 and 2221 cm -1 were detected and became larger afterwards.These bands could be attributed to N 2 O in NH 3 -SCR (Liu et al., 2012).After NO-s adsorbed on the surface of MnO x /MWCNTs, the NO x ad-species formed N 2 O may due to oxygen vacant sites in MnO x (Kijlstra et al., 1997a) which were able to dissociate adsorbed NO-s.This could provide a route for the formation of the N 2 O gaseous species (NO-s + N-s → N 2 O + 2s).The gaseous NO bands on the surface of the catalysts at around 1907 cm -1 and 1843 cm -1 (Zhang et al., 2013b) were hardly detected, indicating NO adsorbed on the catalysts quickly then translated to NO x ad-species.
Compared to the process of ammonia adsorption (Fig. 1), the rate of the NO x adsorption was much faster and more prominent.This result is in agreement with the study of Long and Yang (2001) reported that carbon nanotubes as superior sorbents for nitrogen oxides.

Transient Reaction Experiments The Reaction between NO + O 2 Ad-Species and NH 3 on MnO x /MWCNTs
The MnO x /MWCNTs catalysts were first purged with NO and O 2 mixture for 30 min, then NO and O 2 were shut down when NH 3 was introduced at 210°C.As shown in Fig. 3, after NH 3 was introduced, the intensity of the bands due to bridging nitrate species (1648 cm -1 ), bidentate nitrate (1559 cm -1 ), and nitrite species (1508 and 1457 cm -1 ) on the catalysts surface were consumed rapidly before 10 min.And the bands at 1627 cm -1 attributed to the gaseous or weakly adsorbed NO 2 became weaker afterwards.However, with the continuous injection of ammonia, new bridging nitrate species were formed on the surface of the catalysts, and these active nitrate species kept the dynamic equilibrium.Fig. 3 indicates that the gaseous or weakly adsorbed NO 2 were transformed to bridging nitrate species and nitrite species on MnO x /MWCNTs.Additionally, H 2 O with the surface O-H stretching bands at 3800-3500 cm -1 was observed, which is a final product of the SCR reaction.Furthermore, it also suggested that these nitrate species and nitrite species were active on catalysts surface.In contrast, the bands could be assigned to NH 3 ad-species began to grow, including coordinated ammonia bonded to Lewis acid sites (3256 and 1389 cm -1 ) and the gaseous or weakly adsorbed NH 3 (966 and 930 cm -1 ).Obviously, the bands due to the coordinated ammonia bonded to Lewis acid sites in this experiment were more remarkable than those in ammonia adsorption experiment (Fig. 1).The results in Fig. 3 indicated that competition for adsorption between NH 3 and NO, which would block the SCR reaction, did not occur or might be very weak in this SCR reaction.However, this does not coincide with the works of Kijlstra et al. (1997b) and Chen (2007).The reason might be different in the number of adsorption sites between the catalysts with various kinds of supports.

The Reaction between NH 3 Ad-Species and NO + O 2 on MnO x /MWCNTs
The catalysts were first purged with NH 3 for 30 min, then the NO and O 2 mixture was injected when NH 3 was shut down simultaneously.The spectra were recorded as a function of time.The variations of adsorption and desorption were recorded and shown in Fig. 4.After injecting NO and O 2 , the bands could be assigned to the surface O-H stretching (3800-3500 cm -1 ), N 2 O 4 (a) (1735 cm -1 ), bridging nitrate species (1684 and 1652 cm -1 ), bidentate nitrate (1559 cm -1 ), monodentate nitrate (1539 cm -1 ) and nitrite species (1457 cm -1 ) were observed.Band intensity increased gradually with time.And it was noted that new bands at 1684 cm -1 assigned to the ammonium ions bonded to Brønsted acid sites (Tuo et al., 1986;Pan et al., 2013) was clearly observed and intensified with addition of NO and O 2 .DRFITS spectra of ammonia adsorption in Fig. 1 and Fig. 4 have demonstrated that the majority of coordinated ammonia consumed rapidly by NO x ad-species, so that the bands due to ammonia adsorption were barely observed.Thus, the water was detected which could be confirm by the surface O-H stretching (3800-3500 cm -1 ).However, a small portion of adsorbed NH 3 species reacted with water and generated NH 4 + species on Brønsted acid sites.Reversible reaction can be described by the following equation: Because the intensity of the band due to NH 4 + species (1684 cm -1 ) grow larger gradually, and the reaction is reversible and slow, it turned out that NH 4 + species was still increase in about 30 min.

SCR Steady-State Response Experiments on MnO x /MWCNTs Catalysts
The formation of surface species on MnO x /MWCNTs catalysts under an atmosphere of NH 3 , NO and O 2 mixture is also investigated by DRIFTS.The results were presented in Fig. 5.After all the reaction gases were supplied, the bands due to N 2 O (2241 and 2203 cm -1 ), nitrosyl NO− (1911 and 1847 cm -1 ) (Zhou et al., 2011), bridging nitrate (1627 cm -1 ) (Zhao et al., 2009), bidentate nitrate (1559 cm -1 ), monodentate nitrate (1539 cm -1 ), coordinate ammonia species on Lewis acid site (1597 and 1263 cm -1 ) (Zhao et al., 2009) and the gaseous or weakly adsorbed NH 3 (966 and 928 cm -1 ) were detected.The bands due to bridging nitrate, coordinate ammonia species on Lewis acid sites, and the gaseous or weakly adsorbed NH 3 increased obviously with time.However, the bands that could be assigned to bidentate and monodentate nitrates increased only slightly.Considering the sharp peaks of bidentate and monoentate nitrates species in the NO and O 2 co-adsorption experiment, it indicated that these species were favoured in SCR reaction for the catalysts at low-temperature.The bands due to N 2 O 4 (1697-1754 cm -1 ) and nitrosyls (1508 cm -1 ) were barely observed in this experiment.Considering that the N 2 O 4 and nitrosyls species could react with NH 3 ad-species in the NH 3 transient reaction experiment (Fig. 3), it could be suggested that the N 2 O 4 and nitrosyls species were also important intermediate during the formation of NO x adspecies.Moreover, the band at 1684 cm -1 attributed to the ammonium ions bonded to Brønsted acid sites was hardly detected in SCR steady-state response experiments, and considering NH 4 + species on Brønsted acid sites observed in the NH 3 adsorption experiment (Fig. 1), it indicated that the NH 4 + participated in the reaction and NH 4 + species was important intermediate in the SCR reaction cycle at 210°C.

Shut-off Experiments NO +O 2 Shut-Off after SCR Reaction on MnO x /MWCNTs
Fig. 6 shows the variation of the DRIFTS spectra when the supply of NO and O 2 stopped after SCR reaction with MnO x /MWCNTs catalysts at 210°C for various times.It was shown that intensity of the bands due to N-H stretching vibration modes of NH 3 , the coordinate ammonia species on Lewis acid sites (1597 and 1263 cm -1 ), and the gaseous or weakly adsorbed NH 3 became stronger with time.However, because the intensity of the bands (928 and 966 cm -1 ) due to the gaseous or weakly adsorbed NH 3 grew with time very clearly, suggesting that these species might not react with NO x ad-species directly.In addition, it is worth noting that the very weak band (1684 cm -1 ) attributed to Brønsted acid sites could be detected after 25 min.However, due to stopping the supply of NO and O 2 and introducing NH 3 into the DRIFTS cell continuously, it is also observed in Fig. 6 that the band due to bridging nitrate (1627 cm -1 ) decreased remarkably with time, and the bands due to nitrosyl NO− (1911 and 1847 cm -1 ), bidentate nitrate (1559 cm -1 ),

NH 3 Shut-Off after SCR Reaction on MnO x /MWCNTs
The catalysts were first purged with mixture of NO, O 2 and NH 3 for 30min at 210°C.Fig. 7 shows the evolution of species on the MnO x /MWCNTs surface, when NH 3 was switched off from the NH 3 , NO and O 2 mixture.The intensity of the bands could be assigned to NO x ad-species, including nitrosyls (1911 and 1847 cm -1 ), bridging nitrate (1627 cm -1 ), bidentate nitrate (1559 cm -1 ), and monodentate nitrate (1539 cm -1 ).These bands did not vary apparently with the time when ammonia in the feed gases was turned off.In the case of ammonia ad-species, the bands of gaseous or weakly adsorbed NH 3 (966 and 928 cm -1 ) decreased remarkably once without ammonia.By contrast, the intensity of the bands that could be assigned to coordinate ammonia species on Lewis acid sites (1597 and 1263 cm -1 ) did not vary firstly, and started to decrease after the gaseous or weakly adsorbed NH 3 disappeared, suggesting the translation of Considering the increase of gaseous or weakly adsorbed NH 3 in section 3.4.1, it indicated that gaseous or weakly adsorbed NH 3 might not or slightly react with the NO x adspecies directly.In addition, the band due to NH 4 + species on Brønsted acid sites was disappeared after the gaseous or weakly adsorbed NH 3 vanished, it suggesting that NH 4 + species on the surface of catalysts were consumed rapidly by NO x ad-species and they were very active in SCR reaction.After adsorbing on the surface of MnO x /MWCNTs catalysts, the gaseous or weakly adsorbed NH 3 would translate to NH 4 + species on Brønsted acid sites and coordinate ammonia species on Lewis acid site then participate into the SCR reaction.Moreover, the intensity of decreased obviously compared with that of coordinate ammonia species, which indicated that NH 4 + species as one of intermediates in the SCR reaction was more active than coordinate ammonia species on the surface of MnO x /MWCNTs catalysts at 210°C.

DISCUSSION
It was believed that different adsorbed ammonia species present different activity profiles.Zhu et al. (2013) suggested that ammonia adsorbed on Lewis acid sites in Cu-SSZ-13 was more active at low temperature than NH 3 adsorbed on Brønsted acid sites.In this work, with reaction temperature of 210°C, the main ammonia adsorption species on MnO x /MWCNTs catalysts were ammonium ions bonded to Brønsted acid sites, coordinated ammonia bonded to Lewis acid sites and weakly adsorbed NH 3 during the NH 3 -SCR reaction.From section 3.4.2, it was speculated that gaseous or weakly adsorbed NH 3 may not or slightly react with the NO x ad-species directly.They would translate to NH 4 + species on Brønsted acid sites and coordinate ammonia species on Lewis acid site then participate into the SCR reaction.Moreover, NH 4 + species on Brønsted acid sites was important intermediates and they significantly contributed to the SCR reaction on MnO x /MWCNTs catalysts.
When NO and O 2 mixture was injected to the preadsorbed ammonia catalysts, adsorbed ammonia species were consumed rapidly, and H 2 O, N 2 O 4 (a), bridging nitrate, bidentate nitrate, monodentate nitrate, and nitrite were produced.Fig. 1 and Fig. 2 have indicated that the adsorption of NO and O 2 on the MnO x /MWCNTs catalysts was more quickly and remarkable compared to the adsorption of ammonia.The NO x ad-species, including bridging nitrate species, bidentate nitrate, and nitrite species, N 2 O, N 2 O 4 (a), began to grow once NO and O 2 mixture was injected to the MnO x /MWCNTs catalysts.Most of the NO x ad-species were consumed in the SCR reaction with ammonia.It was different from the results obtained by Kijlstra et al. (1997a, b).In their report, the formation of bidentate nitrates would block the catalytic sites on the surface of the MnO x /Al 2 O 3 catalysts below 500 K.It further suggested that the activity of bidentate nitrates was improved with MnO x /MWCNTs as catalysts in this study.It was worth noting that N 2 O 4 (a) and nitrite species were not detected in the SCR steadystate response process (section 3.3).However, these species were observed in other experiments in this study.The disappearance of N 2 O 4 (a) and nitrite species might be attributed to the lower generation rates of these species compared to their consumption.Nevertheless, evidence of small amount of N 2 O 4 (a) species in the adsorption experiments in section 3.1.2suggested that the N 2 O 4 (a) species did not react with other species and their reaction activity might be damped.For the nitrite species, the result is in good agreement with a previous literature (Liu et al., 2012) in which nitrite (NO 2 -) species were easy to bond with adsorbed NH 3 species to form NH 4 NO 2 , these species acted as active intermediates in SCR process and were easy to decompose to N 2 and H 2 O. From these results, it is believed that nitrite species are quite active in SCR reaction over MnO x /MWCNTs catalysts.In order to improve the activity of the MnO x /MWCNTs catalysts, and it is important to increase the ability of the forming nitrite species during adsorption of NO and O 2 over the catalysts.In conclusion, nitrite species, bidentate and monodentate nitrates are the principal NO x species which participated in the SCR reaction over MnO x /MWCNTs catalysts.
The mechanism of NH 3 -SCR of NO x over various Mnbased catalysts has been discussed in many literatures (Kijlstra et al., 1997b;Wu et al., 2007;Liu et al., 2012;Zhang et al., 2013b) in which NH 4 + species was reported to react with the active nitrites to produce the unstable ammonium nitrite for a further N 2 generation.Kijlstra et al. (1997b) reported the mechanism for NH 3 -SCR of NO over MnO x /Al 2 O 3 catalysts, proposing both E-R (Eley-Rideal) and L-H (Langmuir-Hinshelwood) mechanisms for this reaction.However, in the case of Mn-based catalysts with carbon materials as catalysts supports, there are rare studies on SCR reaction mechanism with DRIFTS.Sun et al. (2009) reported that the NH 3 -SCR reaction followed L-H mechanism for V 2 O 5 /AC catalysts with low reactivity presented for the reaction between the adsorbed NH 3 and gas-phased NO.V 2 O 5 promoted the formation of -NH 2 , which is the main intermediate of the SCR reaction in their work.However, in our work, NH 3 participated in the SCR reaction as in the form of NH 4 + on Brønsted acid sites and coordinate ammonia species on Lewis acid sites.Moreover, NH 4 + species as one of intermediates in the SCR reaction was more active than other NH 3 ad-species.It competes with NO ad-species, including bridging nitrate species, bidentate nitrates, monodentate nitrates, nitrite species, N 2 O and N 2 O 4 (a), but mainly bidentate nitrates, monodentate nitrates and nitrite species.Hence, there are two possible reaction pathways for low-temperature SCR over the MnO x /MWCNTs catalysts at 210°C.One reaction took place between coordinated NH 3 and NO x ad-species.Coordinate ammonia species NH 2 NO 2 or NH 2 NO 3 were further converted to N 2 and H 2 O. Therefore, the SCR reaction of NO by NH 3 over the MnO x /MWCNTs catalysts could take place as follows: The ammonia ad-species were probably formed according to the reactions: The mechanism of the SCR reaction had been studied extensively elsewhere and different hypotheses had been proposed (Li et al., 2011;Zhou et al., 2011;Fu et al., 2014;Yu et al., 2014).Fig. 8 showed that the reaction pathways existed in the NH 3 -SCR process over the catalysts.However, in this study there are two reaction pathway existing in the NH 3 -SCR process over the MnO x /MWCNTs catalysts at 210°C.

CONCLUSIONS
In this paper, low-temperature NH 3 -SCR process over the MnO x /MWCNTs catalysts was investigated by in situ DRIFTS.The main conclusions were drawn as follows: Ammonia adsorption species were ammonium ions, coordinated ammonia and weakly adsorbed NH 3 on the surface of MnO x /MWCNTs catalysts at 210°C.The gaseous and weakly adsorbed NH 3 will translate to NH 4 + species and coordinate ammonia then participate into the SCR reaction.NH 4 + species as one of intermediates in the SCR reaction was more active than coordinate ammonia species on the surface of the catalysts at 210°C.In terms of NO x ad-species, most of them would react with NH 3 ad-species, including N 2 O 4 (a), bridging nitrate, bidentate nitrate, monodentate nitrate, and nitrite.And nitrite species, bidentate and monodentate nitrates contributed to the SCR reaction over the catalysts mostly.

Fig. 2 .
Fig. 2. DRIFTS spectra of NO and O 2 co-adsorption on MnO x /MWCNTs at 210°C for various times.

Fig. 3 .
Fig. 3.In situ DRIFTS spectra of MnO x /MWCNTs in a flow of 1000 ppm NO + 5% O 2 after adding 1000 ppm NH 3 at 210°C for various times.

Fig. 4 .
Fig. 4. In situ DRIFTS spectra of MnO x /MWCNTs in a flow of 1000 ppm NH 3 after adding 1000 ppm NO + 5% O 2 at 210°C for various times.

Fig. 6 .
Fig. 6.In situ DRIFTS spectra of MnO x /MWCNTs in a flow of 1000 ppm NH 3 , 1000 ppm NO, 5% O 2 then removed NO and O 2 at 210°C for various times.

Fig. 7 .
Fig. 7.In situ DRIFTS spectra of MnO x /MWCNTs in a flow of 1000 ppm NH 3 , 1000 ppm NO, 5% O 2 then removed NH 3 at 210°C for various times.
formation of various NO x ad-species probably takes place as follows: NO (g) + e → NO-(a) happened between NH 4 + and NO x adspecies.NH 4 + could react with NO x ad-species to form NH 4 NO 2 , NH 4 NO 2 O 4 or NH 4 NO 3 , and these species were unstable and were further converted to N 2 and H 2 O. NH 3 (g) + H + → NH 4

Fig. 8 .
Fig. 8.The schematically plot of two reaction pathways over the catalysts at 210°C.