Analysis of Mn valence in the Mn based catalyst for NH 3-SCR process

A series of MnWOx/TiO2 catalysts were prepared by liquid phase deposition. The MnWOx/TiO2 catalyst was characterized by N2 physical adsorption, X-ray diffraction, H2 programmed temperature reduction, transmission electron microscopy and X-ray electron spectroscopy, and their NH3-SCR performance were tested. The effect of Mn valence on the NH3-SCR performance of MnWOx/TiO2 catalyst was analyzed and discussed. The results show that the active components are uniformly dispersed on the surface, and the average valence of manganese are different. The average valence state of manganese in Mn3WOx/TiO2 catalyst is the highest and the activity at low temperature is the best. On the contrary, the average valence of manganese in MnWOx/TiO2 catalyst is the lowest while the N2 selectivity is the best, which means that high valence of manganese is beneficial to its low temperature activity while low valence is favorable for its N2 selectivity. Original article, Published date: 2018-06-06 DOI: 10.23977/metf.2018.21001 ISSN 2515-1282 https://www.clausiuspress.com/journal/METF.html


Introduction
Nitrogen oxides (NO x ) are one type of the major pollutants that affect air quality and can cause many environmental problems such as acid rain, smog, and photochemical pollution.NO x can also bring harm to the human body.High concentrations of NO x can influence the human respiratory system.In the past decades, nitrogen removal has been a hot topic in the field of environmental catalysis, and it still receives considerable attention today [1][2][3] .NO x is mainly derived from the emission of exhaust gas from stationary sources or mobile sources such as power plants and automobiles.Many different denitration methods have been developed to deal with different situations.Representative denitrification technologies include selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), adsorption, electron beam, pulse corona cryogenic plasma, and direct catalytic decomposition.While using NH 3 as the reducing agent, NO x can be efficiently converted to environmentally harmless N 2 through selective catalytic reduction reaction(NH 3 -SCR), which has thereby become one of the major technologies for NO x removal [4][5][6] .
For the NH 3 -SCR technology, the catalyst is the basic unit part.Till now, the representative NH 3 -SCR catalyst is vanadium-based catalyst V 2 O 5 -WO 3 (MoO 3 )/TiO 2 , which has been widely used in the industry [7][8] .One of the disadvantages of the V 2 O 5 -WO 3 (MoO 3 )/TiO 2 catalyst is that the operating temperature window is mainly in the range of 300-400 o C, which means that it cannot effectively remove the nitrogen oxides in the low temperature environment and raise the energy consumption.Therefore, Researchers are working hard to develop new catalysts that have high NH 3 -SCR activity at lower temperatures.Among these potential catalyst systems, manganese oxide exhibits the best deNO x activity in the low temperature range due to its variable valence and good redox performance [9] .However, due to the variable valence of manganese, the active components of manganese-based catalysts are very complex and has become one of the main factors affecting the catalytic activity of manganese-based catalysts [10][11][12][13][14][15].In this paper, a series of MnWO x /TiO 2 catalysts with different average valence states were prepared using manganese nitrate and tungstate as raw materials.The influence of valence state of manganese on the NH 3 -SCR was discussed.

Preparation of MnWO x /TiO 2 Catalyst
The MnWO x /TiO 2 catalyst was prepared by liquid deposition.According to the target Mn: W molar ratios, suitable amount of manganese nitrate solution and ammonium tungstate were measured.Equal mass of oxalic acid was used to help dissolving ammonium tungstate.The total amount of metal salt precursor is 0.01 mol.The metal precursor was dissolved by adding an appropriate amount of deionized water, and then the TiO 2 support was added to the solution with stirring.0.5 mol/L ammonia was used as a precipitant, and it was added dropwisely to the solution at a rate of 3 ml/min.The pH was adjusted to 10.The obtained product was filtered, washed, and calcined in a muffle furnace at 300 o C for 2 h.The target catalyst was obtained and was labelled as Mn 3 WO x /TiO 2 , Mn 2 WO x /TiO 2 and MnWO x /TiO 2 according to the molar ratio of manganese to tungsten.

NO x removal test
Catalyst performance was tested with simulated flue gas.The composition of the reaction gas was as follows: the volume fraction of NO and NH 3 was 0.05%, the volume fraction of O 2 was 5%, and N 2 was used as the equilibrium gas.The total flow rate of the mixture was 500 ml/min and the calculated space velocity was 60,000 ml/g -1 h -1 .
The MnWO x /TiO 2 catalyst was tableted, broken, and sieved to give 20-60 mesh solid particles.0.5g catalyst was placed in a tube furnace, and the NO x concentration before and after NH3-SCR reaction was measured by a TH200 nitrogen oxide analyzer and a Thermo 1500 chromatography.The NO x conversion and N 2 selectivity are calculated as follows:

BET
Table 1 lists the texture properties of the MnWO x /TiO 2 catalysts.The specific surface area, pore volume and pore size of TiO 2 are 197.9m 2 /g, 0.305 cm 3 /g, and 6.2 nm, respectively.the loading of the active component MnWO x reduces of the specific surface area and pore volume of the catalyst, but the specific surface area and pore volume of MnWO x /TiO 2 catalysts are close to each other, indicates that there texture properties are similar.

XRD
Fig. 1 shows the XRD pattern of MnWO x /TiO2 catalyst.Only anatase TiO 2 characteristic diffraction peaks at 2θ = 25.3°,37.7°, 48.1°, 55.1°, 62.7°, 70.4°, 75.1° are observed in the patterns.No characteristic peaks of the active species MnWOx are found, indicating that the active species are evenly distributed on the support and there maybe no crystalline phase generated.Interestingly, lattice fringes of MnWO 4 can be observed in Fig. 2c & 2d, where the 0.483 nm interplanar spacing is attributed to the (100) face and the 0.245-nm interplanar spacing is attributed to the (200) face of MnWO 4 , indicating that manganese and tungsten may interact with each other.The formation MnWO 4 phase [16] also proved that the active component has been successfully loaded on the support surface.Fig. 2 The TEM images of the Mn a WO x /TiO 2 catalysts.a, b)Mn 3 WO x /TiO 2 ; c) Mn 2 WO x /TiO 2 ; d) MnWO x /TiO 2 .

XPS
XPS is one of the most powerful characterizations for determining the valence states of the surface elements.Fig. 3 shows the Mn2p3/2 spectra of the MnWO x /TiO 2 catalyst.The Mn2p3/2 of the MnWO x /TiO 2 catalyst can be divided into three sub-peaks by peak deconvolution: 642.0-642.6eVcan be attributed to Mn 4+ , 640.6-641.4eV .combining Fig. 3 and Table 2, the valence of Mn species in the Mn 3 WO x /TiO 2 catalyst is mainly Mn 4+ , while in the Mn 2 WO x /TiO 2 catalyst and the MnWO x /TiO 2 catalyst, the Mn species are mainly at Mn 3+ and Mn 2+ , respectively..In the Mn 3 WO x /TiO 2 catalyst, the reduction peak is mainly MnO 2 →Mn 2 O 3 , and the valence state of Mn in MnO 2 is +4.The reduction peak of Mn in Mn 2 WO x /TiO 2 and MnWO x /TiO 2 catalysts shifted to high temperature, indicating the decrease of Mn valence in the catalyst.The results are consistent with the results of XPS characterization.

NH 3 -SCR Performance
Fig. 5 shows the NH 3 -SCR performance of the MnWO x /TiO 2 catalysts.Taking 90% NO x conversion as criteria,in Fig. 5a, the Mn 3 WO x /TiO 2 catalyst activity temperature window is between 100 o C -250 o C; while Mn 2 WO x /TiO 2 and MnWO x /TiO 2 catalysts have an activity temperature window of 120 o C -280 o C and 130 o C -340 o C, respectively.It can be clearly observed that when the valence state of Mn species is higher, its low temperature activity NH 3 -SCR is better; when the valence state of Mn species is decreased, the activity window moves to high temperature attitude.
Fig. 5b shows the N 2 selectivities variation of the MnWO x /TiO 2 catalysts, which exhibits a different trend with the activity profiles.The Mn 3 WO x /TiO 2 catalyst possesses the worst N 2 selectivity in the high temperature range.while with the decrease of Mn valence, the N 2 selectivity of the MnWO x /TiO 2 catalyst increases gradually.Although it can not exclude the effect of tungsten, it is known that high valence Mn has a strong redox ability, which is easy to promote the nonselective oxidation of NH 3 in the high-temperature range and cause the formation of N 2 O. Therefore, high valence may be not favourable for the N 2 selectivity.The decrease of the Mn valence reduces the redox properties and decreases the NH 3 -SCR activity of the MnWO x /TiO 2 catalysts in the low temperature region, but it may also help to avoid the occurrence of nonselective oxdidation, thereby improving the N 2 selectivity.

SO 2 resistance
Fig. 6 shows the SO 2 resistance of the MnWO x /TiO 2 catalyst.When SO 2 is introduced, the NO x conversion of Mn 3 WO x /TiO 2 and Mn 2 WO x /TiO 2 catalysts immediately decreases and begins to stabilize at a certain degree, while the decrement degree on Mn 2 WO x /TiO 2 catalyst is lower than that of Mn 3 WO x /TiO 2 .For MnWO x /TiO 2 catalyst, the introduction of SO 2 does not immediately lead to a decrease in the conversion, i.e., its SO 2 resistance is improved.Similarly, this cannot exclude the effect of tungsten, but it also means that the reduction in the valence of manganese contributes to the improvement of its sulfur resistance.One reason may be that high valence Mn has strong oxidative ability to SO 2 , which will quickly oxidize SO 2 to SO 3 , generating sulfate and blocking the pores of the catalyst, thus decreasing the NO x removal ability of the catalyst.The oxidizing ability of low valence Mn to SO 2 is decreased, which also slows down the formation of sulfate on the catalyst.

Conclusions
In this paper, a series of MnWO x /TiO 2 catalysts with different Mn/W molar ratios were prepared by liquid deposition method.The MnWO x /TiO 2 catalysts were characterized and their NH 3 -SCR performances were investigated.Through the characterization, it was found that the as-prepared MnWO x /TiO 2 catalyst had good dispersion of active components, and the valence of Mn decreased with the decrease of Mn/W molar ratio.Among MnWO x /TiO 2 catalysts with different Mn valences, Mn 3 WO x /TiO 2 catalyst exhibits the highest low-temperature NH 3 -SCR activity, while MnWO x /TiO 2 catalyst exhibits the best N 2 selectivity, which indicates that the high valence is favourable for the NH 3 -SCR activity and low valence is for N 2 selectivity.

Fig. 2
Fig. 2 displays TEM images of MnWO x /TiO 2 catalysts, which disclose the microstructure of the MnWO x /TiO 2 catalysts.It can be seen that each catalyst are made up of homogeneous nanoparticles.Fig. 2b points out the (101) and (102) face lattice fringes of anatase TiO 2 .Interestingly, lattice fringes of MnWO 4 can be observed in Fig.2c & 2d, where the 0.483 nm interplanar spacing is attributed to the (100) face and the 0.245-nm interplanar spacing is attributed to the (200) face of MnWO 4 , indicating that manganese and tungsten may interact with each other.The formation MnWO 4 phase[16] also proved that the active component has been successfully loaded on the support surface.

2 OFF-SO 2 OFF-SO 2 Fig. 6
Fig. 6 The activity of MnWO x /TiO 2 at 260 o C in the presence of 100ppm SO 2 .

Table 1
The texture properties of the MnWO x /TiO 2 catalysts.

Table 2
Quantitative results of Mn2p XPS spectra of MnWO x /TiO 2 .TPR profiles of the MnWO x /TiO 2 catalysts.The MnWO x /TiO 2 catalysts show 4 reduction peaks at 4 different temperatures of 220 o C, 300 o C, 400 o C and 460 o C. The latter three reduction peaks can be assigned as MnO 2 →Mn 2 O 3 , Mn 2 O 3 →Mn 3 O 4 and Mn 3 O 4 →MnO