Promotion Effect and Mechanism of the Addition of Mo on the Enhanced Low Temperature SCR of NOx by NH3 over MnOx/γ-Al2O3 Catalysts

Abstract A series of Mn/γ-Al2O3 and MnxMoy/γ-Al2O3 catalysts were prepared by using Incipient Wetness Impregnation (IWI) method. The catalytic performance tests showed that the Mn3Mo1.25/γ-Al2O3 demonstrated a higher SCR performance (NO conversion of around 96%) at a broad low temperature range (150–300 °C). The characterization showed that the addition of Mo to the Mn/γ-Al2O3 catalysts promoted the dispersion of MnOx on the surface of γ-Al2O3. The adsorption of NO could form two different species, i.e., nitrites and nitrates, on the surface of the catalyst. The presence of nitrites is beneficial to low temperature SCR. It is found that the existence of Mo in the catalyst favours the formation of Mn3+, which plays a critical role in the adsorption of NH3 and therefore improves NH3 adsorption capacity of the MnOx/γ-Al2O3 catalysts. The low temperature SCR of the Mn3Mo1.25/γ-Al2O3 catalyst was found to mainly follow L-H mechanism, but E-R mechanism also plays a role to some extent. Moreover, it is found that the addition of Mo not only mitigates the deactivation of the catalysts, but also broadens the effective temperature range of the SCR catalysts.


Introduction 26
The emission of nitrogen oxides (NOx) from combustion processes is associated with a series of 27 severe environmental problems, such as acid rain and ozone depletion, has become an issue of 28 great concern for decades [1][2][3]. To address this problem, the selective catalytic reduction (SCR) 29 of NOx by NH 3 has been applied to treat flue gas from stationary and mobile sources [4,5]. At 30 coal-fired power stations, V2O5-WO3/TiO2 is the most commonly used SCR catalyst. However, 31 the high operating temperature window of 300 -400°C is associated with a variety of problems [6, 32 7], such as the possible oxidation of SO2 and the high energy consumption [8,9]. Therefore, there 33 is a need for the development of low temperature SCR catalysts, which has attracted a wide 34 attention in recent years [10,11]. 35 The manganese-based catalyst is a good alternative to vanadium-based SCR catalysts, which has 36 demonstrated high catalytic activity and selectivity at low temperature [9,[12][13][14][15][16][17][18][19][20]. Mn-based oxides 37 catalysts, such as MnOx-CeO2/meso-TiO2[21], MnO2-(Co3O4)/TiO2 [22] and nano-flaky MnOx 38 supported on carbon nanotubes [16], have outstanding SCR activity at low temperatures. The 39 addition of transition and/or rare earth metals, such as Fe, Ce and Sb etc, has been found to have 40 positive effects on the performance of these Mn-based catalysts [20,[23][24][25][26]. However, their 41 operating temperature window was narrow. The development of novel SCR catalysts, which are 42 highly efficient at different temperature levels for different applications is highly desirable but 43 remains very challenging [14]. 44 Previous studies have shown that Mo can promote the distribution of active constituents on the 45 support and subsequently enhances the activity of the catalyst [27]. Moreover, most researchers 46 believed that low temperature SCR reaction follows Eley-Rideal (E-R) mechanism [28,29].

Effects of Mo addition on Mn dispersion 133
Structural and morphological properties of the catalysts were investigated by BET and XRD 134 analyses. As shown in Table. 1, the specific surface area of the Mn3Mo1.25/γ-Al2O3 catalyst is larger 135 than that of the Mn3/γ-Al2O3 catalyst, which provides more active sites for low temperature NH3-136 SCR reaction. In comparison with pure γ-Al2O3, the surface area of the γ-Al2O3 loaded with Mo 137    which was due to its low content. Pijun Gong's et al. [37] claimed that β-MnO2 has the worst SCR 151 activity in among different MnO2 species, while α-Mn2O3 was found to demonstrate high SCR 152 activity and selectivity by many researchers [38,39]. Except for γ-Al2O3, the intensity of diffraction 153 peaks of Mn compounds was weak. However, in Fig 2(b), when Mn loading varied from 0 to 6 154 wt%, the diffraction peaks of α-Mn2O3, MnO, β-MnO2, especially that of α-Mn2O3, started to 155 appear when Mn loading raised to 3 wt%, and the peaks intensity became higher with the increase 156 in Mn loadings, which means bulk MnOx species began to form and accumulated on the surface 157 of the catalyst. However, bulk MnOx species occupied great amount of surface space but 158 performed poor low temperature SCR activity [40]. While doping Mo first on the support, the 159 intensity of diffraction peaks intensity of Mn species decreased, as shown in Fig 2 (a) 20 40  Mn3Mo1.25/γ-Al2O3, e. Mn4/γ-Al2O3, f. Mn4Mo1.25/γ-Al2O3, g. Mn6/γ-Al2O3, h. Mn6Mo1.25/γ-167 Al2O3 168

XPS, XRF and TEM-EDX 169
The XPS of Mn 2p (a), Mo 3d (b) are shown in Fig. 3. Different MnOx species have specific and 170 unique spectrums. In Fig. 3(a), Mn3p3/2 peaks consist of three MnOx species, Mn 4+ (641.5-171 641.7eV), Mn 3+ (541.5-541.7eV) and satellite [41]. The area ratio, respectively, represent the 172 relative amount of species on the surface. A significant decrease in area ratio of Mn 4+ /Mn 3+ from 173 1.26 to 1.08 was observed as a result of Mo addition, which is consistent with the results of XRD 174 analysis. It can be seen from Table 2  loaded on the support. In Fig. 4

H2-TPR 197
The H2-TPR results of the γ-Al2O3, Mo1.25/γ-Al2O3, Mn3/γ-Al2O3, Mn3Mo1.25/γ-Al2O3 are shown 198 in Fig. 5 The amount and strength of surface acid sites of Mn3/γ-Al2O3 catalyst before and after Mo 218 addition was investigated using NH3-TPD, which is shown in Fig. 6. There are three distinct peaks, 219 which could be divided into weak, medium and strong acid sites, respectively. The temperature 220 range of τ1, τ2 and τ3 is 150-250°C, 250-400°C and 400-500°C respectively. The Mn3Mo1.25/γ-221 Al2O3 had higher intensity at all peaks, which suggests more medium and strong acid sites existed. Moreover, catalytic activity of the Mnx/γ-Al2O3 with different Mo loadings was investigated as 227 shown in Fig. 7. The strong and medium acid sites of catalyst increased significantly with the 228 increase in Mo loadings. It is also found that a higher Mo loading led to a higher catalytic 229 temperature of the catalysts in Fig. 1(c). Therefore, the low Mo content (1.25 wt%) favoured the 230 low temperature SCR.  Fig. 9 shows the NO-TPD profiles of the Mn3/γ-Al2O3 catalyst before and after Mo addition. Two 248 desorption peaks can be observed in Fig. 9, which contained a broad peak in the low temperature 249 region (LT-peak) and a strong peak at higher temperature region (HT-peak). The nitroso species 250 formed from the adsorbed NO at LT-peak will react with ammonia [45]. In contrast, the nitro 251 compounds formed from the NO adsorbed at HT-peak only decomposed at high temperature and 252 reacted with -NH2 [47]. Therefore, the area ratio between LT-peak and HT-peak could be utilized 253 to evaluate the activity of SCR catalyst and investigate the mechanism of catalytic process. In Fig.  254 9, after the addition of Mo, there is a shift in LT-peak and HT-peak toward lower temperature 255 region and the height of peak decreased, which indicates that the Mn3Mo1.25/γ-Al2O3 had a lower 256 SCR activity temperature. In addition, the area ratio between LT-peak and HT-peak increased from 257 4.6 to 5.13, which means that more nitroso species formed on the Mn3Mo1.25/γ-Al2O3 so that higher 258 low temperature SCR activity. 259 19 II: The adsorption of NH3 (500ppm) + O2 (3%) at 25°C for 1h, switched to the adsorption 275 of NO (500ppm) + O2 (3%) at 25°C for 1h, and then purged with N2 until outlet 276 concentration of NO became below 5ppm, followed by performing TPD process at 10°C 277 /min; 278 III: The adsorption of NO (500ppm) +O2 (3%) at 25°C for 1h, switched to the adsorption 279 of NH3 (500ppm) + O2 (3%) at 25°C for 1h, and then purged with N2 until outlet 280 concentration of NO became below 5ppm, followed by performing TPD Process at 10°C 281 /min; 282 IV: The adsorption of NO (500ppm) +O2 (3%) at 25°C for 1h, and then purged with N2 283 until outlet concentration of NO became below 5ppm, followed by performing TPD 284

NO-TPD 247
Process at 10°C /min. 285 In Fig. 11, Curve III coincided with Curve IV resulted from the adsorption of NO on the fresh 286 catalyst. However, Curve I shifted as compared with Curves III and IV when NH3 and NO were 287 simultaneously introduced, which indicates that NH3 adsorbs on certain sites competitively with 288 NO. Curve II shifted up significantly in the first 25 min, and then shifted down to the level of curve 289 III and IV. It can therefore be concluded that some of the adsorption sites are occupied randomly 290 by NH3 owing to NH3 preferentially adsorbed on the catalyst. According to the calculation, the 291 adsorption capacity of Curve I was larger than that of Curve II, which is attributed to gas phase 292 NH3 being competitively adsorbed with NO on the catalyst surface. Therefore, the adsorption sites 293 of the catalysts could be classified into four types: Type 1, adsorbs NH3 preferentially; Type 2, 294 adsorbs NO preferentially; Type 3, adsorbs NH3 competitively, and Type 4, random adsorption 295 sites, on which both NH3 and NO can be adsorbed depending on their molecular movement. 296 As shown in Fig. 12, TPD Curves i and ii did not show high temperature desorption peak of NO 297 and NO2. TPD Curves iii and iv are similar, but low temperature desorption peaks of curve iii for 298 NO are weak and high temperature desorption peaks also shifted. Compared Curve i with iii, TPD 299 results did not show high temperature desorption peaks, which indicated that bidentate nitrate and 300 bridge nitrate were not easy to form on the surface treated by NH3 [5]. It is speculated that O2 will 301 20 accelerate the formation of these stable NO complexes, which only react with NH3 at high 302 temperature and are responsible for the deactivation of SCR catalysts. It is generally believed that SCR reaction starts with the adsorption of NH3. But the mechanisms 308 of low temperature SCR for catalysts with different active components and support are different. 309 Marban et al. [48] suggested that there are two different SCR mechanisms associated with different 310 NH3 species. In Fig. 12, NO-TPD peaks disappeared in Curves i and ii, which is attributed to the 311 reaction between NO and ad-NH3 on the catalyst surface. As shown in Fig. 13 21 order to compare, a similar test without Process III was taken into consideration, which is also 316 shown in Fig. 13 (dashed line). The amount of NOx being adsorbed at low temperature around 317 250˚C decreased, even disappeared, and high temperature species were also reduced to some 318 extent, which directly proves that low temperature SCR proceeds between the adsorbed NH3 319 species and the adsorbed NO species via Langmuir-Hinshelwood (L-H) mechanism. 320 321 In this study, the addition of Mo was found to improve NH3 adsorption capacity of the catalyst. 325 With the increase in Mo loadings, the amount of surface acid sites increased, which was vital to 326 SCR reaction. In Fig. 14, when in the presence of gas phase O2, the adsorption peaks of NO and 327 NO2 decreased. Therefore, it can be concluded that the addition of Mo could reduce NO adsorption 328 on the catalysts surface but did not result in a lower low temperature SCR activity. Instead, the 329 low temperature SCR efficiency of the Mn3Mo1.25/γ-Al2O3 was much higher than that of the Mn3/γ-330 Al2O3 catalyst. Additionally, in Fig.12 Secondly, the addition of Mo enhances the formation of Mn2O3 on the catalyst, which accelerates 342 the formation of intermediates, whereafter the -NH3 is transformed into -NH2 via H-abstraction. 343 Thirdly, the addition of Mo on the catalysts mitigates the deactivation of the catalysts. In Fig. 13, 344 NO adsorbed species at HT-Peak region were very difficult to react with NH3 at 150˚C. The reason 345 is that these NO formed some complexes (bridged and bidentate nitrates) that are thermally stable. 346 However, the addition of Mo could inhibit the transformation of nitrites into nitrates thus slow 347 down the self-deactivation of the catalysts. 348 Therefore, the low temperature SCR reaction is composed of 4 steps as shown in Fig. 15

Conclusions 357
In this study, the Mn3Mo1.25/γ-Al2O3 catalyst achieved a high NO conversion of around 96% at 358 150 -300°C. It is found that the addition of Mo to Mn-based SCR catalyst could not only inhibit 359 the growth of MnOx bulks, favour the formation of Mn 3+ state and promote the NH3 adsorption 360 capacity of the catalyst, but also act as a moderator to adjust the effective operating temperature 361 window of the SCR reaction, which could be achieved by adjusting Mo loading. Moreover, the 362 addition of Mo was found to mitigate the deactivation of the catalysts. The study on SCR 363 mechanism showed that the low temperature SCR starts from the adsorption of NH3 on Mn 3+ sites. 364 The low temperature SCR followed mainly E-R mechanism, but L-H mechanism also plays a role 365 to some extent. 366