De-NOx Performance and Mechanism of Mn-Based Low-Temperature SCR Catalysts Supported on Foamed Metal Nickel

A series of manganese-based catalysts supported on foamed metal nickel (FMN) with various Mn/Ni ratios was prepared for low-temperature selective catalytic reduction (SCR) of NO with NH3 (NH3-SCR). The effects of calcination temperature, amount of added Mn, optimal operating conditions, and H2O on the elimination of nitrogen oxides (de-NOx) performance of catalysts were studied. The catalysts were characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and temperature-programmed desorption experiments of NH3 analyses. The experimental results revealed that the Mn7.5/FMN catalyst calcined at 350 °C exhibited the best NO conversion that was ca. 100% at 120-200 °C. Moreover, it had excellent H2O tolerance. The superior activity of the Mn7.5/FMN catalyst, which was calcined at 350 °C, was attributed to the presence of amorphous manganese oxide, more unsaturated Ni atoms and structural defects, an increase in NH3 adsorbance, and the number of surface acid sites. Based on these studies, we established that the reaction of the NH3-SCR with Mn7.5/FMN catalyst mainly exhibits an Eley-Rideal mechanism.


Introduction
Nitrogen oxides (NO x ) such as NO and NO 2 resulting from fossil fuel combustion are one of the main pollutants in the atmosphere.NO x gases are responsible for acid rain, photochemical smog, and ozone layer depletion.2][3] Currently, several methods are used to eliminate NO x (de-NO x ).][6][7][8] The reactions are as follows: As a core link in this technology, the catalyst performance directly affects the de-NO x efficiency of the whole SCR system. 9,10The V 2 O 5 -WO 3 (MoO 3 )/TiO 2 catalyst is a commonly used commercial SCR catalyst for NO x removal with an active temperature range of 300-400 °C. 11,12However, this catalyst has many disadvantages including the toxicity of vanadium at high temperatures. 13Therefore, significant efforts should be devoted towards the development of non-vanadium catalysts for NH 3 -SCR processes.5][16][17][18][19] Kapteijn et al. 20 studied the singlecomponent MnO x catalyst and established an activity order of MnO 2 > Mn 5 O 8 > Mn 2 O 3 > Mn 3 O 4 > MnO at a temperature range of 385-575 K.Moreover, they determined that Mn 2 O 3 exhibited the highest N 2 selectivity with an NO conversion rate of ca.100% at ca. 450 K. Wang et al. 21studied the low-temperature de-NO x activity of MnO x supported by multi-walled carbon nanotubes.The results revealed an activity order of manganese oxides in different valence states: MnO 2 > Mn 3 O 4 > MnO, proving that the different valence states of Mn exhibit different effects on the de-NO x activity of the catalyst.
Active coke (AC), TiO 2 , and Al 2 O 3 are the most widely used carriers for low-temperature SCR catalysts.Jin and co-workers 22 studied the low-temperature de-NO x activity of Mn-Ce/TiO 2 and Mn-Ce/Al 2 O 3 catalysts.The results indicated that the de-NO x performance of the Mn-Ce/TiO 2 catalyst was better than that of the Mn-Ce/Al 2 O 3 catalyst at temperatures of 80-150 °C.4][25] Moreover, the role of the carrier should not be ignored. 26,27Foamed metal nickel (FMN) with an excellent structure of three-dimensional all-through mesh is used as a sound absorbing "porous metal".It exhibits a number of positive characteristics including good stability, high porosity, good thermal shock resistance, small bulk density and a large surface area, and is mainly used as a positive current collector and active carrier in nickel-based batteries. 28,29However, to date, few studies have concentrated on the use of FMN in the field of low-temperature de-NO x . 30n this paper, FMN is used as a de-NO x catalyst carrier.A series of Mn-based catalysts supported on FMN with different Mn/Ni ratios were studied.The effects of calcination temperature, amount of added Mn, optimum operating conditions, and H 2 O on the de-NO x performance of the catalysts were elucidated and the mechanism was discussed.

Catalyst preparation
Mn x /FMN catalysts (x represents the mass ratio of Mn loading and FMN, x = 2.5, 5.0, 7.5, 10.0, 12.5, and 15.0) were prepared by the impregnation method using FMN (commercial foamed metal nickel, 98% purity) as support and manganese nitrate (50% Mn(NO 3 ) 2 solution) as precursor, 31 both purchased from Sinopharm (China).A certain amount of FMN was added to the manganese nitrate solution according to the ratio of Mn and FMN.The mixture was magnetically stirred for 1 h and then dried in an oven at 105 °C for 12 h.The dried catalyst samples were subsequently calcined at 250, 350, 450, and 550 °C in a muffle furnace for 5 h.The prepared catalysts were sealed and stored until further use in subsequent experiments.

Catalytic experiments
Catalytic measurements were performed in a fixedbed reactor using 0.3 g catalyst for each experiment. 32he simulated mixed flue gas comprised 500 ppm NO, 500 ppm NH 3 , 5 vol% O 2 and was balanced with N 2 .The total gas flow rate of the mixed flue gas was 100 mL min -1 and the gas hourly space velocity (GHSV) was 13000 h -1 .A thermocouple (K-type) was used to measure the reactive temperature of the catalyst in the fixed-bed reactor.The flue gas concentrations at the inlet and outlet of the system were measured with a flue gas analyzer (ECOM J2KN, Germany).Prior to the initiation of the experiment, the simulated mixed flue gas was fed into the reactor for 0.5 h to ensure stability.Catalytic activity was evaluated from the amount of NO conversion, calculated by equation 5: (5)   where η is the NO conversion and [NO] in and [NO] out indicate the NO inlet and outlet concentrations at steady state, respectively.The NO 2 concentration was negligible.

Scanning electron microscope
A scanning electron microscope (SEM) (Hitachi S-4800, Japan) was used to investigate the microstructural features of the catalyst surface.
X-ray diffraction X-ray diffraction (XRD) measurements were performed on an X-ray diffractometer with CuKα radiation (Bruker D8 Advance, Germany).The diffraction patterns were recorded in the 2θ range of 10-80° at a scan speed of 10° min -1 and a resolution of 0.02°.

X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS; Thermo Scientific K-Alpha spectrometer, USA) was used to study the valence state, elemental content, and energy level structure.Test conditions included an Al Kα excitation source, 30 mA target current, 15 kV target voltage, a vacuum chamber pressure less than 10 -7 Pa, and a 0.1 eV scan step size.The binding energies of the samples (O, Ni, and Mn) were calibrated according to contaminant carbon (C 1s = 284.6 eV).

Temperature-programmed desorption analysis of ammonia
Temperature-programmed desorption analyses of NH 3 (NH 3 -TPD) were performed on Micromeritics 2920 (USA) auto-adsorption apparatus and the desorbed amount of NH 3 was also monitored by thermal conductivity detection (TCD).The 100 mg catalyst sample was placed in a reaction tube and heated to 300 °C.Next, it was pretreated under 30 mL min -1 He atmosphere for 0.5 h, cooled down, and subsequently maintained at a temperature of 80 °C.At the beginning of the adsorption process, the system was purged with a mixed gas comprising NH 3 (10 mL min -1 ) and N 2 (30 mL min -1 ).After adsorption was saturated, the system was purged with He at a flow rate of 30 mL min -1 until the TCD detector signal was stable.Finally, the temperature was raised from 100 to 700 °C under He atmosphere (30 mL min -1 ) and the working curve was recorded.

Evaluation of catalytic activity
Effects of calcination temperature and Mn loading on catalytic activity Figure 1a illustrates the NO conversion percentage values of catalysts with different Mn loadings at 120-240 °C.For FMN, the conversions were basically zero at 120-240 °C, indicating its low catalytic activity.NO conversion first increased and then decreased with increasing Mn loading, which reached a maximum at a Mn loading of 7.5% and was ca.100% at 120-200 °C.A small amount of Mn loading provides less active site and so the activity of the Mn x /FMN catalyst is low.On the other hand, a high amount of Mn loading may cause agglomeration of the active component that in turn deteriorates the catalytic activity.The Mn loading of 7.5% is suitable, so we further studied the Mn 7.5 /FMN catalyst.
The NO conversion activity of the Mn 7.5 /FMN catalyst uncalcined and calcined at 250-550 °C is presented in Figure 1b, indicating that the calcination temperature had great influence on the NO x conversion.The NO conversion activity of the Mn 7.5 /FMN catalyst firstly increased and then decreased with the rise of calcination temperature.The Mn 7.5 /FMN catalyst exhibited the highest activity at a calcination temperature of 350 °C.The NO conversions were ca.100% at 120-220 °C.Many factors may be responsible for the high NO conversions, such as surface area and surface acidity of catalyst. 33,34We will conduct comprehensive studies in our future work.
Effects of the NH 3 /NO ratio and O 2 concentration on catalytic activity NH 3 plays an important role in the NH 3 -SCR reaction and is usually adsorbed onto the catalyst surface where it reacts with NO.Excess NH 3 molecules may result in secondary pollution in the NH 3 -SCR reaction and thus, it is necessary to understand the effect of the NH 3 /NO ratio on catalytic activity.Figure 2a demonstrates the effect of different NH 3 /NO ratios on the NO conversion activities of the Mn 7.5 /FMN catalyst calcined at 350 °C.At an NH 3 /NO ratio < 0.8, NO conversion increased linearly with an increase in NH 3 /NO ratio.According to reaction 1, it can be known that the amount of NH 3 was not enough, so the NO conversion increased linearly with an increase in NH 3 /NO ratio.However, when the NH 3 /NO ratio reached 1.0, the NH 3 reacted on NO completely, and thus the conversion activity became stable even increasing the amount of NH 3 .
In NH 3 -SCR, both O 2 and NO have oxidative capacity and thus, the oxidation reaction at the catalyst surface is a very critical step. 35Hence, it is essential to study the effect of O 2 concentration on the catalytic activity.Figure 2b demonstrates that although the NO conversion of the Mn 7.5 /FMN catalyst is only ca.20% at an O 2 concentration of 0 ppm, it increases rapidly with an increase in O 2 concentration until it becomes stable at a concentration of ca.1%.This indicates that the O 2 molecules participate in the reaction and the adsorbed and lattice oxygen molecules on the catalyst surface are formed rapidly, thereby increasing NO conversion.When the adsorbed and lattice oxygen molecules on the catalyst surface reach the saturation point (ca.1%, Figure 2b), NO conversion remains stable, even if the O 2 concentration increases.This further explains why the oxidation capacity of O 2 on the catalyst surface is much stronger than that of NO and why the importance role of O 2 in the NH 3 -SCR reaction cannot be ignored.

Effect of H 2 O on catalytic activity
H 2 O has an inhibition effect on the NH 3 -SCR of NO. [36][37][38] To investigate whether the Mn 7.5 /FMN catalyst is affected by H 2 O, an H 2 O resistance test was carried out at 180 °C. Figure 3 displays the effect of H 2 O on the NO conversion activity of the Mn 7.5 /FMN catalyst.Notably, the Mn 7.5 /FMN catalytic activity was not affected by H 2 O, even at 10 vol%.In the presence of H 2 O, the H 2 O and NH 3 molecules compete for the active sites on the catalyst. 39However, the Mn 7.5 /FMN catalyst exhibits superior H 2 O resistance, indicating that the Mn 7.5 /FMN catalytic activity can be ascribed to the strong adsorption of the NH 3 molecules present on the acid sites of the catalyst.

Adsorption performance of the catalyst
To understand the adsorption characteristics of NH 3 on the surface of Mn 7.5 /FMN catalyst, the transient response characteristics of NH 3 were studied at 180 °C. Figure 4a presents the changes of NO conversion activity under different conditions.In the presence of NH 3 , NO conversion reaches ca.100% during the first stage, indicating that the more active NH 3 species existing on the Mn 7.5 /FMN catalyst surface can participate in the NO reduction reaction.When the NH 3 flow is switched off, NO conversion rapidly decreases until it stabilizes after half an hour.NO conversion rate is only 10%, suggesting that NO reacts with O 2 to produce nitrates and nitrites that are deposited on the catalyst surface, thus, the catalyst maintains lower NO conversion activity. 40When NH 3 is reintroduced into the reaction system, NO conversion rapidly increases and reaches a value close to the original after ca. 10 min.It then increases slowly because the adsorbed NH 3 must first react with the nitrate that deposits onto the catalyst surface to regain the active site. 41Finally, NO conversion returns to the original value and remains stable.The NH 3 desorption time is therefore longer than its adsorption time, indicating strong adsorption onto the catalytic surface.The reaction cannot occur when NH 3 and NO are in the gas phase. 42f the reaction of NO with NH 3 occurs, the latter, which should be in the adsorption state, would react with NO. Figure 4a illustrates that the NH 3 desorption rate is lower than its adsorption rate, indicating that NH 3 can be adsorbed  onto the Lewis acid or Brønsted acid sites present on the catalytic surface. 43,44he transient response characteristics of NO were also studied at 180 °C. Figure 4b presents the changes of NO concentration and conversion under different conditions.When the NO flow was cut off, the NO concentration dropped from 500 to 0 ppm in ca.20 min and the NO conversion also decreased.When NO was added back to the reaction system, the NO concentration rapidly (in ca.20 min) returned to the initial level (500 ppm), indicating that the NO adsorption capacity on the Mn 7.5 /FMN catalyst surface is relatively weak.However, this does not imply that NO cannot be adsorbed onto the catalyst surface.Figure 4b illustrates that the catalyst exhibits high NO conversion activity, indicating that it is capable of adsorbing NO.

SEM analysis
The morphology and structure of the FMN, Mn 7.5 /FMN and Mn 15 /FMN catalysts were studied by SEM.The surface of FMN is very smooth and it provides an interconnected porous framework that serves as a support for the active MnO x species distributed on the catalyst surface (Figure 5).The surfaces of Mn 7.5 /FMN and Mn 15 /FMN catalysts are much rougher than that of FMN, indicating that the MnO x species was loaded on the FMN surface.These rough surfaces can easily adsorb a large amount of gas and, thus, the activity of the catalyst increases.Notably, the surface of the Mn 7.5 /FMN catalyst is rougher than that of Mn 15 /FMN catalyst, indicating that the excess amount of MnO x loaded can lead to the agglomerate of active substance.

XRD analysis
The overall crystal structure and phase purity of the Mn x /FMN catalysts were verified by XRD.The XRD patterns of the different catalysts calcined at 350 °C are illustrated in Figure 6a.The diffraction peaks at 44.7, 52.0 and 76.5° are ascribed to the Ni from the FMN substrate in the FMN, Mn 2.5 /FMN, Mn 7.5 /FMN, Mn 10 /FMN, and Mn 15 /FMN catalysts (Figure 6a).Only the Ni diffraction peaks were detected and the absence of manganese oxide diffraction peaks was attributed to the amorphous character of manganese oxides.As the calcination temperature increased further, the intensity of the Ni diffraction peaks decreased, while that of the NiO and α-Mn 2 O 3 peaks increased.This suggests that NiO and α-Mn 2 O 3 formations increase with an increase in temperature.The activity of the Mn 7.5 /FMN catalyst calcined at 250 and 350 °C was higher than that at 450 and 550 °C (Figure 1b), indicating that for this catalyst, the afforded NiO and α-Mn 2 O 3 exhibit an inhibitory effect on the NO conversion activity of the Mn 7.5 /FMN catalyst.

XPS analysis
XPS measurements were used to determine the atomic ratios and the valences of the surface components.
Figure 7 displays the O 1s and Ni 2p 3/2 XPS spectra of the FMN and Mn 7.5 /FMN catalysts calcined at 350 °C.The O 1s spectrum was fitted with two characteristic peaks at ca. 529.23 and 530.90 eV ascribed to lattice oxygen (O α ) and surface-adsorbed oxygen (O β ), respectively.Notably, O α exhibits a higher activity in the redox reaction than O β , thereby promoting NO oxidation and simultaneously accelerating the "fast SCR" reaction. 45,46Therefore, a study of O α / (O α + O β ) helps to understand the oxidation performance of the catalyst.Similarly, Ni 2p 3/2 spectra were fitted with three characteristic peaks at ca. 853.55, 856.45, and 860.67 eV, ascribed to the main peak and two satellite peaks (represented as sat I and sat II), respectively.Sat I corresponds to Ni 3+ species, Ni 2+ -OH species, and Ni 2+ vacancies, while sat II was attributed to ligand-metal charge transfer. 47,48The results afforded from the XPS analysis (Table 1) provide a better insight into the effects of each component on de-NO x .A comparison of the results afforded by the Mn 7.5 /FMN and FMN catalysts reveals an improvement in the sat I and sat II ratios after Mn was added.This indicates the presence of more unsaturated Ni atoms and structural defects on the surface of the Mn 7.5 /FMN catalyst as well as interactions between the Mn and Ni atoms.In addition, the presence of O α species is beneficial to NH 3 -SCR. 49,50hese factors contribute towards the increase in activity observed for the Mn 7.5 /FMN catalyst.
Figure 8 presents the O 1s, Ni 2p 3/2 and Mn 2p 3/2 spectra for the Mn 7.5 /FMN catalysts calcined at 350 and 550 °C.The Mn 2p 3/2 spectra were fitted with three characteristic peaks at ca. 640.0, 642.0, and 644.2 eV, ascribed to Mn 2+ , Mn 3+ , and Mn 4+ , respectively. 51Mn 4+ has been reported to exhibit a strong redox ability that can promote a "fast SCR" reaction. 52Moreover, out of all its valence states the Mn 3+ intermediate valence state exhibits better conversion capacity and thus, it can promote the catalytic activity of the catalyst.We therefore concluded that Mn 4+ and Mn 3+ play very important roles in the low-temperature NH 3 -SCR reaction.
The results from Mn 7.5 /FMN XPS analysis are listed in Table 2.The Mn 4+ and Mn 3+ contents on the catalyst surface exhibit little change with increasing calcination temperature, however, there are significant changes in sat I and sat II.Notably, the sat I (main peak) and sat II (main peak) ratios for the Mn 7.5 /FMN catalyst calcined at 350 °C are larger than those of the Mn 7.5 /FMN catalyst calcined at 550 °C, illustrating that there are more unsaturated Ni atoms and structural defects on the surface of the former catalyst.In addition, the ratio of O α on the surface of the Mn 7.5 /FMN catalyst calcined at 550 °C is larger than that of the Mn 7.5 /FMN catalyst calcined at 350 °C.As discussed above (sub-section "Effects of calcination temperature and Mn loading on catalytic activity"), the Mn 7.5 /FMN catalyst calcined at 350 °C exhibits higher activity.This indicates that the O α ratio does not have a crucial effect on the activity of the Mn 7.5 /FMN catalyst.For the Mn 7.5 /FMN catalysts at different calcination temperatures, sat I and sat II play more important roles in NO conversion.Moreover, a combination of these results with the XRD results demonstrates that when the calcination temperature is raised to 550 °C, the Mn crystallinity of the Mn 7.5 /FMN catalyst improves, while active Mn decreases.Thus, the presence of active Mn, more unsaturated Ni atoms, and structural defects on the surface of the Mn 7.5 /FMN catalyst all contribute towards better NH 3 -SCR performance.

NH 3 -TPD analysis
Based on the mechanism of the NH 3 -SCR reaction at low temperature, the surface acidity of the catalyst plays a very important role in the whole process. 52,53Thus, the acid sites on the catalyst surface were investigated via NH 3 -TPD analysis.Figure 9 illustrates the NH 3 -TPD profiles of the Mn 7.5 /FMN catalyst calcined at 350 and 550 °C.The Mn 7.5 /FMN catalyst calcined at 350 °C exhibits four distinct desorption peaks located at 127.2, 277.1, 421.1, and 645.6 °C.On the other hand, three peaks centered at 127.2, 421.1, and 645.6 °C were observed in the TPD profile of the Mn 7.5 /FMN catalyst calcined at 550 °C.Desorption peaks at temperatures of 80-150, 150-300, and > 300 °C  have been assigned to weak, medium, and strong acid sites, respectively. 54,55Comparison of the TPD profile of the Mn 7.5 /FMN catalyst calcined at 350 and 550 °C suggests that the peak areas located at the weak and medium acid sites are larger for the catalyst calcined at 350 °C, while the peak areas of its strong acid sites are smaller.This indicates an increase in NH 3 adsorbance and in the number of acid sites on the surface of the Mn 7.5 /FMN catalyst calcined at 350 °C at low temperature.Generally, more acid sites are conducive to the adsorption of reactive gases and thus, the activity of the Mn 7.5 /FMN catalyst calcined at 350 °C is higher than that of the Mn 7.5 /FMN catalyst calcined at 550 °C at low temperature.This demonstrates that the surface acidity plays an important role in the low-temperature NH 3 -SCR reaction of the Mn 7.5 /FMN catalyst and that NH 3 adsorption onto the weak acid sites of the Mn 7.5 /FMN catalyst calcined at 350 °C is more favorable.

NO removal mechanism
The catalytic mechanism of the Mn 7.5 /FMN catalyst, based on the data afforded in the above mentioned studies (sections "Adsorption performance of the catalyst" and "NH 3 -TPD analysis"), is presented in Figure 10.Data from the NH 3 and NO transient responses and NH 3 -TPD analysis confirm that the Mn 7.5 /FMN catalyst can adsorb NH 3 as well as adsorb NO, leading to the conclusion that the NH 3 -SCR reaction of the Mn 7.5 /FMN catalyst corresponds to the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. 31,56For the E-R mechanism, the redox reaction takes place between active NH 3 and gaseous NO.The adsorbed NH 3 is activated at the weak acid site on the surface of the catalyst, with the adsorption and activation of NH 3 as the critical step.On the other hand, for the L-H mechanism, the NO of the gas phase first interacts with O 2 and subsequently the NO x is adsorbed onto the surface of the catalyst where it is oxidized to nitrates and nitrites in the role of lattice oxygen.Finally, the adsorbed activated NH 3 reacts with the nitrates and nitrites to generate nitrogen and water.However, as discussed above (section "Adsorption performance of the catalyst"), the Mn 7.5 /FMN catalyst adsorption capacity of NH 3 is stronger than that of NO, indicating the E-R mechanism as the main mechanism of the NH 3 -SCR reaction of the Mn 7.5 /FMN catalyst.

Conclusions
A series of Mn x /FMN catalysts were prepared by the impregnation method to study the effects of calcination temperature, amount of added Mn, optimal operating conditions, and H 2 O on low-temperature NO conversion.The Mn 7.5 /FMN catalyst calcined at 350 °C exhibited excellent NH 3 -SCR activity of NO and very superior H 2 O resistance.The Mn 2 O 3 and NiO species were found   to exhibit a better crystallinity when the calcination temperature exceeded 350 °C.However, these species brought about a decrease in catalytic activity.The unsaturated Ni atoms, structural defects, and number of surface acid sites on the surface of the Mn 7.5 /FMN catalyst calcined at 350 °C were greater than those of the Mn 7.5 /FMN catalyst calcined at 550 °C.This is responsible for the excellent catalytic activity of the former catalyst.The superior H 2 O resistance of the Mn 7.5 /FMN catalyst calcined at 350 °C at low temperature can be attributed to the strong NH 3 adsorption on the acid sites.The reaction of Mn 7.5 /FMN catalysts corresponds to both E-R and L-H mechanisms, however, the E-R mechanism plays a role in the NH 3 -SCR reaction of the Mn 7.5 /FMN catalyst.

Figure 1 .
Figure 1.NO conversion activities of (a) Mn x /FMN catalysts with different Mn loadings calcined at 350 °C and (b) Mn 7.5 / FMN catalyst uncalcined and calcined at different temperatures.Reaction conditions: 500 ppm NO, 500 ppm NH 3 , 5 vol% O 2 , balanced with N 2 .

Figure 3 .
Figure 3.Effect of H 2 O on the NO conversion of the Mn 7.5 /FMN catalyst.Reaction conditions: 500 ppm NO, 500 ppm NH 3 , 5 vol% O 2 , 10 vol% H 2 O balanced with N 2 .

Figure
Figure 6b presents the X-ray diffraction patterns of the Mn 7.5 /FMN catalyst calcined at different temperatures.Only the Ni diffraction peaks were detected at calcination temperatures of 250 and 350 °C, indicating the presence of amorphous manganese oxides on the surface of the Mn 7.5 /FMN catalyst.When the calcination temperature reached 450 °C, NiO diffraction peaks were detected at 37.3, 43.3, 55.5, and 62.8° (JCPDS card No. 44-1159), while an α-Mn 2 O 3 diffraction peak appeared at 33.1°.This indicates that part of the Ni was oxidized and the amorphous manganese oxides partially transformed into α-Mn 2 O 3.As the calcination temperature increased further, the intensity of the Ni diffraction peaks decreased, while that of the NiO and α-Mn 2 O 3 peaks increased.This suggests that NiO and α-Mn 2 O 3 formations increase with an increase in temperature.The activity of the Mn 7.5 /FMN catalyst calcined at 250 and 350 °C was higher than that at 450 and 550 °C (Figure1b), indicating that for this catalyst, the afforded NiO and α-Mn 2 O 3 exhibit an inhibitory effect on the NO conversion activity of the Mn 7.5 /FMN catalyst.

Figure 10 .
Figure 10.NO removal mechanism of the Mn 7.5 /FMN catalyst.