Porous Zeolitic Imidazolate Framework Loaded Mn as an Efficient Catalyst for the Selective Catalytic Reduction of NO x with NH 3

Zinc 2-methylimidazolate (Zn(Me-Im) 2 , ZIF-8) was synthesized through solvothermal method, and the Mn@ZIF-8 was synthesized by loading the Mn species onto the prepared ZIF-8 through impregnation. The effect of the addition amount of Mn species on the activity in selective catalytic reduction of NO x with NH 3 (NH 3 -SCR), the stability of catalysts at ideal temperatures and the resistance to H 2 O and SO 2 were investigated. Moreover, the characterizations of the catalysts including XRD, SEM, EDS, XPS, BET, TGA and H 2 -TPR were carried out. The results indicated that SCR activity of the catalysts was related to the addition amount of Mn species, the Mn@ZIF-8(0.8) showed the best NO conversion of over 90% ranging from 225 ° C – 400 ° C with good stability in a long-time test. Besides, the Mn@ZIF-8 showed a high resistance to SO 2 . The ZIF-8 has a great potential to be an excellent support, and Mn@ZIF-8 is promising catalyst in SCR reaction.


INTRODUCTION
Nitrogen oxides (NOx), mainly NO and NO2, is a kind of common gaseous pollutant, and the removal of NOx has long been a difficult problem in air pollution control.The sources of NOx can be divided into two categories, one is the mobile sources and the other is the fixed sources.The fixed sources mainly refer to the flue gas emitted from the combustion of fossil fuels such as coal and oil in the process of industrial production, and the mobile sources are generally thought to be the exhaust gas emitted from automobile engine.With the rapid development of modern industrialization, a large amount of fossil fuels have been exploited and consumed, making the NOx emitted into the atmosphere far exceed the carrying capacity and self-purification capacity of the atmospheric environment.NOx brought severe harm to environment, such as the photochemical smog, acid rain and ozone holes.All these environmental pollution phenomena are related to the emission of NOx, and have a serious threat to our human beings.Consequently, it is essential to control the NOx in the atmosphere (Brandenberger et al., 2008;Busca et al., 2008;Forzatti, 2001;Twigg, 2007).
NH3-SCR has been proven to be the most effective technique for abatement of NOx (Chen et al., 2016).And the catalyst with high denitrification efficiency is the key of the SCR technique.So far, the SCR catalysts can be divided into the following categories according to their active components: noble metal catalysts (Li et al., 2010;Qi et al., 2004), metal oxides catalysts (Qi et al., 2003a;Wei et al., 2018;Shen et al., 2016) and molecular sieve catalysts (Brandenberger et al., 2008;Delahay et al., 2005;Gao et al., 2017).Though the commercial catalysts V2O5-WO3(MoO3)/TiO2 have been widely used, the defects like narrow temperature window and toxicity of vanadium pentoxide still exist (Yi et al., 2016).Hence, the novel catalysts with wide temperature window and environment-friendly properties is desirable.
Manganese oxide (MnOx), a kind of transition metal oxide, has attracted great attentions due to its various labile oxygen species and high activity in NH3-SCR reaction (Wu et al., 2007).Min et al. (2007) prepared different types of MnOx catalysts through a precipitation method, and got a high NOx conversion for the SCR of NOx with NH3.To enhance the performance of MnOx catalyst, a series of doping species and supporter was applied.Peng et al. (2016) used Eu to modify MnOx catalyst to promote SCR activity, and the results showed that MnEuOx exhibited a high NOx conversion in a wide temperature range of 150-400°C.CeO2 was a another metal species used to improve the SCR activity of MnOx.Qi et al. (2003bQi et al. ( , 2004) ) prepared mixed manganese and cerium oxides, and the MnOx-CeO2 was highly active in SCR reaction obtaining almost 95% NO conversion at 150°C.Singoredjo et al. (2010) prepared the alumina supported MnOx exhibiting a high NOx conversion during 120-300°C.TiO2 is another kind of support commonly used to facilitate the SCR activity of MnOx, Park et al. (2013) loaded Mn on synthesized TiO2 through impregnation method, exhibiting a high NO conversion of almost 100% at 150°C.Besides, a series of metal species such as Fe, Ce, V, Sm, Zr, were used to modify the MnOx/TiO2 to avoid poor resistance to SO2 of MnOx.Yet, the result could not meet the requirements of industrialization, further investigation is still needed (Yang et al., 2016;Niu et al., 2016;Zhang et al., 2018;Sun et al., 2018).
Metal organic frameworks (MOFs), the porous crystalline materials, have drawn extensive research interests due to their higher specific surface area, higher porosity, and more stable porous structure compared to the metal oxides (Lee et al., 2009).It is advantageous for them to adsorb the reactants and provide great gas storage, separation and catalysis, besides, the high specific surface, well ordered porous structure, and regular crystal structure have proven the MOFs to be ideal support (Zhang et al., 2016;Wang et al., 2016;Zhang et al., 2017).Zhang et al. (2017) loaded Mn and Ce onto MOFs through in situ and impregnation methods, and studied their catalytic activities in SCR reaction, the results showed that MnCe@MOF had a high NOx conversion in a wide temperature range.Wang et al. (2016) prepared the CeO2/MIL-100(Fe) catalysts by encapsulating ceria nanoparticles into MIL-100(Fe) through impregnation, the CeO2/MIL-100(Fe) catalyst showed a high NH3-SCR activity and great resistance to SO2 at 196-300°C.Zeolitic imidazolate framework (ZIFs), a sort of MOFs, has higher specific surface area and better thermal stability than ordinary MOFs (Huang et al., 2010;Park et al., 2006).Among them, ZIF-8 is the most widely studied ZIFs with a specific surface area of 1400 m 2 g -1 and great thermal stability (Park et al., 2006;Venna et al., 2010), and has been widely used in gas adsorption, hydrogen storage and catalysis.However, few researchers have applied ZIF-8 in NH3-SCR reaction (Banerjee et al., 2008;Küsgens et al., 2009;Nguyen et al., 2012).
In this work, ZIF-8 was synthesized through a solvothermal method, and MnOx was firstly loaded on ZIF-8 by impregnation.The activity of the catalysts in NH3-SCR reaction, the stability of the catalysts, the effect of the addition amount of Mn species on catalytic activity as well as the resistance to H2O and SO2 were investigated.Besides, the characterization of the catalysts including XRD, SEM, EDS Mapping, XPS, BET, and H2-TPR, TGA was carried out to research the catalysts for further information.

Synthesis of ZIF-8 particles
ZIF-8 was synthesized by solvothermal method according to the following procedure (Gee et al., 2013;Tran et al., 2011): A solid mixture of Zn(NO3)2•6H2O (0.717 g, 2.41 mmol) and 2methylimidazole (H-MeIM) (0.18 g, 2.19 mmol) was dissolved in 50 mL DMF, the solution was transferred into a teflon-line stainless steel autoclave, and heated to 140°C at a rate of 5°C min -1 in a temperature-programmed oven and maintained for 24 h.After cooling to ambient temperature, the crystal was collected and washed with DMF for three times.Finally, these samples were dried at 60°C overnight to remove the residual solvents.

Characterizations of the Catalysts
The XRD patterns were obtained on a Bruker D8 diffractometer using Cu Kα radiation between 2θ = 5° and 45°.The micro-morphology of Mn@ZIF-8 was investigated by scanning electron microscope (SEM) (SU 8010), and energy dispersive spectroscopy (EDS) mapping images were obtained on the same device.The X-ray photoelectron spectroscopy (XPS) analysis was carried on a Thermo Fisher Scientific ESCALAB 250 spectrometer, the correction of binding energy shift was referenced to C 1s line at 284.6 eV, and the spectra of the Mn 2p and O 1s were recorded.The specific surface area analysis was conducted on Micromeritics Tri StarII 3020 Surface Arae and Porosity Analyzer with a N2 adsorption, and the results were calculated with the multi-point Brunauer-Emmett-Teller (BET) approach.The thermogravimetric analysis (TGA) was carried on a TGA SDT Q600 thermogravimetric analyzer with a heating rate of 10°C min -1 from 25°C to 600°C under nitrogen atmosphere.The hydrogen temperature programmed reduction (H2-TPR) analysis was performed in which 100 mg of the samples were preheated to 450°C with a ramp rate of 10°C min -1 under N2 atmosphere.After cooling to the ambient temperature, the samples were heated up to 800°C at a ramp rate of 8°C min -1 with a flow of H2, and the consumption of H2 was continuously recorded using the TCD detector.

Activity Tests
The tests of NH3-SCR activity were carried out in a fix-bed quartz reactor (8 mm in diameter) heated by a tube furnace.The simulated flue gas contained 500 ppm NO, 500 ppm NH3, 5% O2, 3% H2O (when used), 200 ppm SO2 (when used) and balance N2, which were regulated by mass flow controllers respectively.In each run of SCR test, 0.2 g of Mn@ZIF-8 was used and the total flow rate was controlled at 100 mL min -1 corresponding to a gas hourly space velocity (GHSV) of approximately 240000 h -1 .The catalyst was heated to the temperature ranging from 100°C to 450°C at a rate of 10°C min -1 , and held at each temperature spots for 50 min.Concentrations of NO and NO2 were simultaneously monitored by the NO and NO2 analyzers (TH-9905), and N2O in the outlet gas was measured by an Antaris™ IGS Gas Analyzer from Thermo Fisher Scientific Inc.The NOx conversion rate and the N2 selectivity were calculated as follows: where the subscripts "in" and "out" refers to the inlet concentration and outlet concentration at steady state, respectively.

XRD
To determine the crystal structure of the catalysts, the XRD analysis of the synthesized ZIF-8 and Mn@ZIF-8 was carried out.It could be easily seen from Fig. 1 that the characteristic peak of synthesized ZIF-8 was in accordance with the simulated ZIF-8, which approved the ZIF-8 was successfully synthesized.As for the XRD pattern of Mn@ZIF-8, no apparent difference was found after the impregnation, indicating that the crystalline structure of ZIF-8 retained well.Yet, the characteristic peaks of Mn species were not observed which may be interpreted by that the Mn species were in amorphous phase (Li et al., 2017).

SEM and EDS Mapping
SEM was carried to investigate the structure of Mn@ZIF-8.It can be seen from Fig. 2 that the synthesized catalysts still retained its dodecahedral shape, which matched well with the results of XRD analysis.Moreover, EDS Mapping was performed to demonstrate the distribution of Mn, Zn species, and it could be seen that the elements of Mn and Zn are uniformly distributed on the whole surface of the catalysts, which was beneficial for adsorption and activation of the reactants, and was conducive to the NH3-SCR catalytic activity.

XPS spectra
To further investigate the chemical state of Mn elements in the catalysts, XPS spectra were recorded and displayed in Fig. 3(a), from which it could be seen obviously that two prominent peaks at 642.89, 654.73 eV accompanied by two satellite peaks at 641.53, 653.46 eV were gained after fitting by Gaussian components.The peaks at 641.53 eV, 653.46 eV could be attributed to Mn 2+ species, while the peaks at 642.89 eV and 654.73eV could be assigned to Mn 3+ (Becerra et al., 2011;Guo et al., 2016).According to previous reports, the co-existence of Mn 2+ and Mn 3+ on the catalysts could facilitate the formation of oxygen vacancies, which played an important role in low temperature oxidation property (Liu et al., 2012).Besides, the high valence state of Mn 3+ species would improve the reducibility of the sample (Cheng et al., 2017).
The O 1s XPS spectra of the Mn@ZIF-8 were shown in Fig. 3(b), two asymmetric peaks could be observed, indicating there were two distinct types of oxygen species.The lower binding energy peak at 530.27 eV was ascribed to the lattice oxygen species O 2-in the catalysts, and the sub-bands at 531.71 ev was attributed to surface chemisorbed oxygen species, such as O 2-or O - (Atribak et al., 2011).

H2-TPR
To investigate the redox behavior of Mn@ZIF-8, the H2-TPR analysis was performed, and the results were presented in Fig. 4, from which it could be seen that there are two main reduction peaks.The first peak located at around 250°C could be assigned to the reduction of MnO2 to Mn2O3 and the following peak around 520°C could be attributed to the further reduction of Mn2O3 to MnO (Zhao et al., 2016;Du et al., 2018).Normally, the operational temperature should be lower than 450°C.This prominent peak could be caused by the reduction of Mn 3+ → Mn 2+ and the decomposition of ZIFs based on the TGA curves.Then, it could be speculated that the doping of Mn species prompted the catalysts to have the stronger redox behavior and oxygen storage capacity of the catalyst, which played an important role in the SCR activities (Li et al., 2016).

BET analysis
The BET surface area of a catalyst support plays a significant role in the NH3-SCR reaction.Besides, the high BET surface area of the support can improve the catalytic activity considerably

Catalysts
BET surface area (m 2 g -1 ) ZIF-8 1380.73 0.4Mn@ZIF-8 1623.260.6Mn@ZIF-8 1758.19 0.8 Mn@ZIF-8 1911.34 1 Mn@ZIF-8 1689.17 by means of enhancing the dispersion of active components to avoid aggregation, as well as the transfer of charge (Han et al., 2015;Wang et al., 2013).The BET surface areas of all the samples were listed in Table 1.It can be seen that the high surface area of ZIF-8 would be beneficial to the high NOx conversion and the surface area of the samples was increased with the increase of the addition of Mn species, however, when the addition amount of Mn species reached 1 mL (in Mn(NO3)2 solution volume), the surface area began to decline.The decrease in specific surface area may be due to the excessive Mn loading caused formation of large solids and pores clogging of the ZIFs materials.The variation trend of the surface area was in accordance with the SCR activities, which indicated that the BET surface area was an important factor that influenced the SCR activities of the samples.
3.1.6Thermogravimetric analysis of Mn@ZIF-8 As was well known, the MOFs usually have relatively poor thermal stability, yet, the thermal stability of the catalysts has a great impact on the SCR reaction.To investigate the stability of Mn@ZIF-8, the thermal gravimetric analysis was conducted, and the result shown in Fig. 5 exhibited a slightly weight loss of 5% from room temperature to 450°C, corresponding to the depletion of guest molecules, mainly H2O, indicating the structure of Mn@ZIF-8 maintained well under 450°C, and Mn@ZIF-8 has a high thermal stability.When the temperature was higher than 450°C, a sharp weight loss of the sample appeared, which could be ascribed to the oxidation of organic ligands and the collapse of the structure of Mn@ZIF-8, which may account for the decrease in activity of the catalysts.

The Effect of Loading Amount of Mn Species
The NOx conversion of all the catalysts in the range of 200-450°C was shown in Fig. 6, from which it could be seen that more Mn species leaded to higher catalytic activity of the samples till the Mn@MOF(0.8)which presented the highest deNOx efficiency compared with the rest  Mn loadings.The NOx conversion of Mn@ZIF-8(0.8) was over 90% in the range of 225-400°C, and reached 100% at 350°C.However, when the loading amount of Mn species exceeded 1 mL (in Mn(NO3)2 solution volume), the catalytic activity began to decline, which may be caused by the severe decrease of BET surface area.On the other hand, the NOx conversion of all the samples increased with the rising of temperature within 200-350°C, and when the temperature went up to 400°C, the NOx conversion began to decrease, which may be inferred from the TGA analysis that this decreased activity may be caused by the collapse of ZIF-8 framework.
N2O is also an important pollutant in the air.Using traditional Mn-based catalysts, a large amount of N2O was produced by side reaction and therefore limited its application.In view of this, the formation of N2O in the SCR reaction was tested, and the N2 selectivity of catalysts with different loadings was studied.As shown in Fig. 6(b), the N2O was formed in all catalysts.With the increase of temperature, ammonia was oxidized to N2O gradually, and the nitrogen selectivity of the catalyst decreased continuously.Besides, the nitrogen selectivity of the catalyst decreased when the loading amount of Mn species increased.Yet the nitrogen selectivity was still over 70% with higher catalyst activity in the temperature window of 250-350°C.

Stability Test of Mn@ZIF-8
Besides, the stability tests of the catalysts at 350°C was carried out, and as illustrated in Fig. 7, the NOx conversion of Mn@ZIF-8 maintained high level after 12 h, presenting a good stability under the identical temperature, which may be ascribed to the porous structures of ZIF-8 and the strong interaction between Mn species and ZIF-8 (Zhang et al., 2014).

Effect of SO2 and H2O on Mn@ZIF-8
As the existence of H2O and SO2 is unavoidable in the exhaust, even after desulfurization, the residual H2O and SO2 still exert a notable impact on SCR performance.Therefore, the effects of SO2 and H2O on the NOx conversion of Mn@ZIF-8 were investigated.The results shown in Fig. 8 revealed that the NOx conversion decreased slightly after introducing 200 ppm of SO2, and then reached a relatively steady state.After cutting off the input of SO2, the conversion of NOx was gradually restored its original level and remained stable.Hence, it could be concluded that the Mn@ZIF-8 had a great resistance to SO2, and the process of SO2 inhibition is reversible.Besides, the negative effect of SO2 on SCR reaction could be attributed to the following aspects: on one hand, the SO2 would compete with the reactants for the active sites, on the other hand, the formed sulfate depositing (ammonium sulfate and ammonium bisulfate species) would block the active sites (Lu et al., 2015).Yu et al. (2010) proposed that the porous structure was in favor of the great SO2 resistance.The high sulfur resistance of Mn based catalysts was related to porous silica support (Huang et al., 2008).Thus, it could be inferred that the great SO2 resistance of Mn@ZIF-8 may be ascribed to the porous structure of ZIF-8.
The impact of H2O and SO2 was also investigated.The NOx conversion decreased to 65% under the coexistence of H2O and SO2 and could not recover to the original value after eliminating the SO2+H2O feeding as shown in Fig. 9, which demonstrated that there was a synergistic inhibition effect between H2O and SO2, and massive sulfate species and deposition were formed and blocked the active sites.

CONCLUSIONS
The XRD patterns and SEM image demonstrated that the Mn@ZIF-8 was synthesized successfully, and the Mn species were highly dispersed on ZIF-8.The Mn 2p XPS spectra proved that the Mn 2+ and Mn 3+ coexisted on the catalysts and played an important role in redox process.
The prepared Mn@ZIF-8 showed an over 90% NO conversion at the temperatures ranging from 225-400°C, the excellent catalytic activity may attributed to the huge specific surface area of ZIF-8,   as the variation trend of the surface area was in accordance with the SCR activities.Besides, the activity and stability for Mn@ZIF-8 was proved to be ideal after 12 h.Furthermore, the Mn@ZIF-8 exhibited strong tolerance against SO2, which may be ascribed to the porous structure of ZIF-8.It is noteworthy the catalytic activity of Mn@ZIF-8 was inhibited considerably in the presence of H2O and SO2.As a whole, this study demonstrated that the ZIF-8 is a kind of promising support and the use of Mn@ZIF-8 as a novel catalyst for SCR reaction is feasible.

CONFLICTS OF INTEREST
There are no conflicts to declare.

Table 1 .
BET surface area of the catalysts.