Low temperature selective catalytic reduction using molding catalysts Mn-Ce/FA and Mn-Ce/FA-30%TiO2

: Mn-Ce/FA (M) and Mn-Ce/FA-TiO 2 (M), using ﬂy ash (FA) with and without TiO 2 as the carriers, were prepared by an integral molding technique. With the increase of TiO 2 content, the rates of NO conversion and denitration of Mn-Ce/FA-TiO 2 (M) increased, and the NO 2 and N 2 O formation rates decreased. It is found that TiO 2 could effectively inhibit the excessive oxidation of NO and the generation of N 2 O. The effects of space velocity, oxygen concentration and ammonia nitrogen ratio on three types of nitrogen oxides (NO, NO 2 , N 2 O) and denitration rates of the Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO 2 (M) were further investigated. In addition, it is demonstrated that Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO 2 (M) were more suitable to be used in the environment of low sulfur and less water.


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Catalysts with higher TiO 2 content demonstrated lower formation rates of N 2 O and NO 2 .

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With the Mn-Ce/FA-30%TiO 2 (M), NO was mainly converted to N 2 and N 2 O. • SO 2 preferentially reacted with NH 3 to form ammonium sulfate to reduce the production of N 2 O.

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The hydroxyl groups in H 2 O inhibited the production of N 2 O.

Introduction
In 2016, the total energy consumption including coal, oil, gas, hydro, nuclear, wind, etc. in China reached 4.36 billion tce (ton of standard coal equivalent) and coal consumption accounted for 62.0% [1].Meanwhile, coal will be the dominant energy source in China for a long time.NOx emission will reach 29.34 million tons and the emission from thermal power plants will account for 17.51 million tons by 2020 in China [2].NOx can pollute the atmosphere and cause photochemical smog, ozone hole and acid rain.In addition, it significantly affects the human central nervous system endangering human health.Thus reducing the nitrogen oxide pollution is an urgent task.At present, control methods of NOx emission can be divided into three categories: free nitrogen combustion, low NOx combustion and flue gas purification technology [3].Among them, the flue gas NH 3 -SCR (Selective Catalytic Reduction) denitration has become the mainstream technology for industrial applications because of its high removal efficiency of NOx.
Energies 2017, 10, 2084 2 of 14 The core of SCR technology is the development of a catalyst.Currently, commercial SCR catalysts are mainly vanadium based with an active temperature range of 350-450 • C [4].In this case, the SCR device must be installed before air preheater and after fuel economizer, so this installation is usually accompanied by a high concentration of ash and SO 2 which can cause catalyst poisoning [5].However, the exhaust temperature zone of most boiler systems in China is mainly 120-250 • C. If a vanadium catalyst is to be used, flue gas preheating device must be added, which increased the overall energy consumption and cost [6].Therefore, low temperature SCR has attracted extensive attention.The excellent activity of manganese oxides and great oxygen carrying capacity of cerium oxides are very promising among the existing low temperature SCR catalysts.For example, Liao et al. [7] compared Mn-Ce/TiO 2 catalysts prepared by coprecipitation, impregnation and deposition, respectively.The results showed that the denitration rate of the Mn-Ce/TiO 2 could reach 90% at 100-160 • C. Chen et al. [8] prepared a modified Mn-Ce/TiO 2 denitration catalyst by impregnation.With the increase of Ce content, oxygen storage capacity of the catalyst was enhanced to promote the removal rate of NO.Meanwhile, the surface active acid sites and active oxygen quantity of the catalyst were increased with the doping of Co, which effectively improved the efficiency of De-NOx.
TiO 2 has a good chemical stability [9].At present, research on low temperature SCR monolithic catalysts is mainly related to TiO 2 based catalysts.Sun et al. [10] studied the influence of additives on the denitration rate of Ce-Mn/TiO 2 .It was found that a proper amount of glycerin could play a role of lubrication and the presence of glass fiber could increase the mechanical strength of the catalyst.Meanwhile, 2.5% active carbon could enhance the activity of the catalyst at low temperature and the removal rate of NO was increased by 16% at 80 • C compared to the catalyst without active carbon.Although the TiO 2 catalyst with different active components exhibit good denitration activity, it is difficult to be applied to actual industrial production due to the high cost.Thus, the research and development of a low cost and highly active low temperature molding catalyst have great significance in practical application.
Our previous work studied Mn-and Ce-based catalysts using activated carbon as support for low temperature SCR [11].Fly ash (FA), which is much cheaper than activated carbon, is a solid waste collected and discharged by thermal power plant [12], which is generally alkaline and mainly consists of SiO 2 , Al 2 O 3 and Fe 2 O 3 .With the rapid development of thermal power, the amount of fly ash from coal-fired power plants has increased largely.Since 2012, China's annual emission amount of fly ash is about 0.3 billion tons, but the recovery utilization rate is less than 30%.FA is commonly used for road construction, brick making, cement and concrete mixing, etc. [13].In recent years, FA has been used in the field of environmental protection because of its good adsorption properties, especially in the treatment of wastewater.Because FA is usually a powder of irregular size and shape with large specific surface area and many pores, which can be used as the support of the catalyst.For example, Zhang et al. [14] prepared a TiO 2 -FA catalyst for the degradation of methyl orange wastewater.Xuan et al. [15] prepared a catalyst supported by FA for denitration.Transition metals such as Fe, Cu, V, Ni were used as active components.The results showed that the catalytic activity of FA as carrier was inferior to that of active carbon.When Cu was used as active component at 280 • C, the efficiency of denitration was the best and the conversion rate of NO could reach 92%.Shi et al. [16] used FA composite attapulgite (PG) as support for Fe and Mn.At a temperature range of 150-300 • C, the denitration rate of the Mn-Fe/PG catalyst could reach more than 80%, performing superior low temperature SCR activity.
Although extensive research of granular catalysts for low temperature SCR has been done and the denitration rate can be up to 90%, the granular catalyst cannot be directly installed for industrial applications because of its large bed resistance, poor fixation, poor mechanical, etc.Therefore, the development of the catalyst molding process is a necessary step to practical application using low temperature SCR technique.
At present, molding catalyst can be broadly divided into coating and integral types.The coating type adopts ceramic as carrier of active components loaded by impregnation method, but the catalyst Energies 2017, 10, 2084 3 of 14 framework is easy to be washed by flue gas, leading to weak abrasive resistance, short life-span and poor denitration rate.Integral molding catalyst has attracted wide attention because of its honeycomb, homogeneous pores, strong mechanical properties, uniform distribution of active components and high denitration rate.However, the commercial available monolithic honeycomb catalysts are mostly used at medium temperature (350-450 • C), and cannot be directly applied to the existing boiler exhaust system where low temperature SCR is required.A new molding catalyst for low temperature SCR commercial application is required.
FA is a good catalyst support for low temperature SCR due to its low cost and its suitability.However, it has poor moldability.Thus a mixture of FA and TiO 2 was used in this work to improve the extrusion.MnOx and CeOx were used as active components.Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO 2 (M) were prepared by the integral squeezing molding method.The effects of TiO 2 content, space velocity, oxygen concentration and ammonia nitrogen ratio on the performance of NO conversion and denitration were studied.In addition, the NO 2 and N 2 O formation rates were investigated in relation to the resistance of the catalysts against water and sulfur.

Catalyst Preparation
FA and FA-TiO 2 were used as catalyst supports.The active precursors (Mn(NO 3 ) 2 , Ce(NO 3 )•6H 2 O) were dissolved in deionized water.The mass fraction of active components (Mn, Ce) was 8.5%, and the mole ratio of Mn:Ce was kept 1:1.The required amounts of carriers of FA and FA-30%wt TiO 2 were dry-mixed for 20 min with methylcellulose, respectively.After mixing evenly, the active component precursor and proper water were added, the mixture was stirred for 30 min, and then kneaded mud for 2.5 h and stayed for 24 h.The embryo of catalyst was obtained by squeezed molding with a mold.The embryo was dried at 80 • C for 4 h and 110 • C for 3 h, and finally calcined at 500 • C for 2.5 h in N 2 atmosphere.When the catalyst was cooled to room temperature in N 2 atmosphere, the required molding catalyst was obtained, namely Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO 2 (M), respectively.The two cylindrical catalysts have the diameter of 23 mm and the length of 40 mm.And seven channels were evenly distributed in each molding catalyst.

Catalyst Characterization
The surface area and pore volume of the catalysts were analyzed by the method of Brunauer-Emmett-Teller (BET) (ASAP2020HD88).X-ray diffraction (XRD) (D8 FOCUS) with monochromatized Cu-Ka radiation and ceramic X tube (voltage 40 KV, current 40 mA) was used for phase analysis with scanning speed of 0.02 • per 0.1 s from 10 • to 60 • .The catalyst for characterization experiments was ground into 500 mesh powder, uniformly filled on the sample vessel and pressed with glass slides.The surface morphology of the catalyst was analyzed by scanning electron microscopy (SEM) with Hitachi S-3500N.

Experimental System
An NH 3 -SCR test was carried out in a fixed bed reaction system.The simulated gases (NH 3 , NO and O 2 balanced with N 2 ) were supplied by cylinders.The catalytic reaction conditions were 800 ppm NH 3 , 800 ppm NO, 6% O 2 , and gas total flow was 1200 mL/min (Figure 1).The gas was controlled by mass flow meters and entered a fixed bed reactor through a gas mixer.The NO content of the inlet and outlet was measured by a Fuel Gas Analyzer (J2KNpro (e-Com)), and recorded through a computer.

BET Analysis
The Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO2 (M) were characterized by BET.As shown in Table 1, the surface area and pore volume of the Mn-Ce/FA-30%TiO2 (M) were about twice as much as those of the Mn-Ce/FA (M).This indicates that the surface area and pore volume of the catalysts can greatly increase by the addition of TiO2.In general, a larger surface area can improve gas adsorption capacity of catalysts, which increases the contact area and reaction rate of reactants.

Activity Evaluation Method
The NO conversion rate was calculated using Equation ( 1): The NO 2 formation rate was calculated using Equation ( 2): The N 2 O formation rate was calculated using Equation (3): The denitration rate was calculated using Equation (4): where NO inlet , NO outlet , NO 2outlet , and N 2 O outlet represent inlet NO concentration (ppm), outlet NO concentration (ppm), outlet NO 2 concentration (ppm), and outlet N 2 O concentration (ppm), respectively.

BET Analysis
The Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO 2 (M) were characterized by BET.As shown in Table 1, the surface area and pore volume of the Mn-Ce/FA-30%TiO 2 (M) were about twice as much as those of the Mn-Ce/FA (M).This indicates that the surface area and pore volume of the catalysts can greatly increase by the addition of TiO 2 .In general, a larger surface area can improve gas adsorption capacity of catalysts, which increases the contact area and reaction rate of reactants.

SEM Analysis
The molding Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO 2 (M) were characterized by SEM.The results were shown in Figure 2. FA is an excellent carrier of catalysts due to the irregular micro-spherical structure, developed porous porosity, smooth surface, substantial pore channel and large pore size.Figure 2a is SEM picture of the Mn-Ce/FA (M).The large pore channels and holes of molding catalyst with binder decreased significantly, and the bonding state of a spongy surface appeared, but still retained a portion of pore space.Figure 2b is SEM picture of the Mn-Ce/FA-30%TiO2 (M).The surface of the catalyst formed TiO2 clusters.The active component directly loaded on the surface of FA and combined with TiO2 [17].Most of the holes and spherical structure of the Mn-Ce/FA-30%TiO2 (M) disappeared, and pore channels of the surface were difficult to be observed.The addition of binder results in the aggregation of the active component to block the pore channels of the carrier.

XRD Analysis
From the XRD spectra, the Mn-Ce/FA (M) had obvious SiO2 diffraction peaks at 27.8°, and no diffraction peaks of Al2O3 or other materials were observed clearly.Two peaks simultaneously appeared on the Mn-Ce/FA-30%TiO2 (M), namely anatase TiO2 and SiO2 diffraction peaks, indicating that adding TiO2 did not change the crystal structure of the original carrier.No characteristic peaks of MnOx and CeOx were found on the catalysts.There might be no crystalline metal oxides, or MnOx and CeOx highly disperses on the supports [17], leading to low crystallinity and amorphous noncrystalline structure.Liu et al. [18] reported manganese oxide phase state could affect denitration activity and increase the dispersion, promoting the generation of amorphous manganese oxide and significantly improving denitration rate.As shown in Figure 3, the relative intensity of SiO2 diffraction peak of the catalyst doped with TiO2 was lower than that of the catalyst based on FA alone.The unique electronic cloud structure of anatase TiO2 and the highly dispersed active component are beneficial for catalytic activity.After the impregnation of metals, the active component can interact with anatase TiO2 and SiO2, resulting in the reduction of carrier crystallinity.FA is an excellent carrier of catalysts due to the irregular micro-spherical structure, developed porous porosity, smooth surface, substantial pore channel and large pore size.Figure 2a is SEM picture of the Mn-Ce/FA (M).The large pore channels and holes of molding catalyst with binder decreased significantly, and the bonding state of a spongy surface appeared, but still retained a portion of pore space.Figure 2b is SEM picture of the Mn-Ce/FA-30%TiO 2 (M).The surface of the catalyst formed TiO 2 clusters.The active component directly loaded on the surface of FA and combined with TiO 2 [17].Most of the holes and spherical structure of the Mn-Ce/FA-30%TiO 2 (M) disappeared, and pore channels of the surface were difficult to be observed.The addition of binder results in the aggregation of the active component to block the pore channels of the carrier.

XRD Analysis
From the XRD spectra, the Mn-Ce/FA (M) had obvious SiO 2 diffraction peaks at 27.8 • , and no diffraction peaks of Al 2 O 3 or other materials were observed clearly.Two peaks simultaneously appeared on the Mn-Ce/FA-30%TiO 2 (M), namely anatase TiO 2 and SiO 2 diffraction peaks, indicating that adding TiO 2 did not change the crystal structure of the original carrier.No characteristic peaks of MnOx and CeOx were found on the catalysts.There might be no crystalline metal oxides, or MnOx and CeOx highly disperses on the supports [17], leading to low crystallinity and amorphous non-crystalline structure.Liu et al. [18] reported manganese oxide phase state could affect denitration activity and increase the dispersion, promoting the generation of amorphous manganese oxide and significantly improving denitration rate.As shown in Figure 3, the relative intensity of SiO 2 diffraction peak of the catalyst doped with TiO 2 was lower than that of the catalyst based on FA alone.The unique electronic cloud structure of anatase TiO 2 and the highly dispersed active component are beneficial for catalytic activity.After the impregnation of metals, the active component can interact with anatase TiO 2 and SiO 2 , resulting in the reduction of carrier crystallinity.
activity and increase the dispersion, promoting the generation of amorphous manganese oxide and significantly improving denitration rate.As shown in Figure 3, the relative intensity of SiO2 diffraction peak of the catalyst doped with TiO2 was lower than that of the catalyst based on FA alone.The unique electronic cloud structure of anatase TiO2 and the highly dispersed active component are beneficial for catalytic activity.After the impregnation of metals, the active component can interact with anatase TiO2 and SiO2, resulting in the reduction of carrier crystallinity.

Effects of TiO 2 Content on Catalyst Activity
The temperature of experiment ranged from room temperature to 250 • C. The other conditions were: NO inlet 800 ppm, ammonia nitrogen ratio 1:1, space velocity 3000 h −1 , and N 2 as equilibrium gas.In Figure 4a,c Meanwhile, with the increase of TiO 2 content at the same reaction temperature, the amount of NO 2 decreased (Figure 4b).It is indicated that the existence of TiO 2 reduced the excessive oxidation of NH 3 and O 2 .Figure 4d shows the denitration rates using different catalysts.At 250 • C, the denitration rates of catalysts with different TiO 2 contents (10%, 30%, 50%, 80% and 100%) were 71.4%, 79.0%, 84.5%, 88.4% and 90.1%, respectively.It is demonstrated that when the TiO 2 content was more than 30%, the denitration rates of catalysts were similar.Considering the economy and efficiency, the Mn-Ce/FA-30%TiO 2 (M) is suggested to be optimal for low temperature SCR in this work.The surface of TiO 2 has a large number of Lewis acid sites, which can greatly promote the adsorption capacity of NH 3 at low temperature to improve the NO conversion rate.Chmielarz et al. [20] prepared the catalysts Mn-Ce/TiO 2 and NH 3 -TPD was carried out.It was found that the surface of TiO 2 had massive Lewis acid sites, and the number of acid sites directly determined the activity of catalysts.At low temperature, the obvious desorption peak of NH 3 appeared.This might be due to desorption of NH 3 which adsorbed on a large number of Lewis acid sites of TiO 2 .
formation of N2O follows the Langmuir-Hinshelwood mechanism, while it follows the Eley-Rideal mechanism at 250 °C [19].The results show that the doping of TiO2 has obvious inhibitory effect on the formation of N2O.It is probable that the unique electronic cloud structure and a large amount of hydroxyl of nanophase TiO2 avoid excessive NH3 dehydrogenation.It results in the reduction of NH groups on the surface of the catalyst, effectively inhibiting the production of N2O and improving the selectivity of N2.Meanwhile, with the increase of TiO2 content at the same reaction temperature, the amount of NO2 decreased (Figure 4b).It is indicated that the existence of TiO2 reduced the excessive oxidation of NH3 and O2. Figure 4d shows the denitration rates using different catalysts.At 250 °C , the denitration rates of catalysts with different TiO2 contents (10%, 30%, 50%, 80% and 100%) were 71.4%, 79.0%, 84.5%, 88.4% and 90.1%, respectively.It is demonstrated that when the TiO2 content was more than 30%, the denitration rates of catalysts were similar.Considering the economy and efficiency, the Mn-Ce/FA-30%TiO2 (M) is suggested to be optimal for low temperature SCR in this work.The surface of TiO2 has a large number of Lewis acid sites, which can greatly promote the adsorption capacity of NH3 at low temperature to improve the NO conversion rate.Chmielarz et al. [20] prepared the

Effect of Space Velocity
The Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO 2 (M) catalysts were studied for the influences of space velocities (3000 h −1 , 5000 h −1 , 7000 h −1 , 9000 h −1 , 11,000 h −1 ), while the reaction conditions were at 250 • C, NO inlet concentration 800 ppm, ammonia nitrogen ratio 1:1, oxygen concentration of six percent and N 2 as balanced gas.As shown in Figure 5a,c,d, the rates of NO conversion, N 2 O formation and denitration of the two catalysts were decreased with the increase of space velocity.At 3000 h −1 , the denitration rates using the Mn-Ce/FA (M) and the Mn-Ce/FA-30%TiO 2 (M) were 65.3% and 78.7%, NO conversion rates were 79.2% and 87.5%, and N 2 O formation rates were 13.1% and 8.1%, respectively.With the increase of space velocity to 7000 h −1 , the rates of NO conversion, N 2 O formation and denitration of the Mn-Ce/FA (M) and the Mn-Ce/FA-30%TiO 2 (M) decreased slowly.But among the three types of nitrogen oxides, NO 2 formation changed slightly with the variation of space velocity (Figure 5b).
The space velocity can affect mass transfer of reactants and products during SCR reactions.With the increase of space velocity, the flow rate of gases increases resulting in an enhanced gas diffusion.However, high flow rate of gases can reduce the contacts with catalytic sites.It was reported that excessive space velocity would decrease contact time between reaction gas and catalysts, which decreased the activity of the catalyst [21].
The space velocity can affect mass transfer of reactants and products during SCR reactions.With the increase of space velocity, the flow rate of gases increases resulting in an enhanced gas diffusion.However, high flow rate of gases can reduce the contacts with catalytic sites.It was reported that excessive space velocity would decrease contact time between reaction gas and catalysts, which decreased the activity of the catalyst [21].

Effect of Oxygen Concentration
Oxygen concentration plays an important role on the activity of catalyst.At 250 • C, NO inlet concentration 800 ppm, ammonia nitrogen ratio 1:1, space velocity 3000 h −1 , and N 2 as balanced gas, the effects of oxygen concentrations (1%, 3%, 6%, 9%, and 12%) on NOx were investigated using the Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO 2 (M) catalysts.Under the condition of one percent oxygen concentration, the NO conversion rates of the Mn-Ce/FA (M) and the Mn-Ce/FA-30%TiO 2 (M) were 61.9% and 73.2%, and the NO 2 formation rates were 1.1% and 0.63%, respectively.With the increase of oxygen concentration to six percent, the NO conversion and NO 2 formation rates of the two catalysts gradually increased.This shows that the presence of oxygen greatly improved the catalytic reaction.However, with the increase of oxygen content, the dehydrogenation of NH 3 was also promoted to produce more N 2 O.Although the amount of N 2 O increased, the rates of NO conversion and denitration of the Mn-Ce/FA (M) and the Mn-Ce/FA-30%TiO 2 (M) were not affected greatly (Figure 6a,d).At 6% O 2 , the denitration rates of the Mn-Ce/FA (M) and the Mn-Ce/FA-30%TiO 2 (M) were 65.3% and 78.6%, respectively.When the oxygen concentration increased from 6% to 12%, slight changes are observed for the rates of NO conversion, N 2 O formation, and denitration.It was reported that oxygen of adsorbed state was firstly adsorbed on the surface of catalyst, and then diffused inside the catalyst to participate in catalytic reaction.On the one hand, the existence of oxygen is more conducive to oxidize NO to NO 2 , and NO 2 is easier to react with ammonia to generate nitrogen and water.On the other hand, O 2 promotes dehydrogenation of NH 3 to NH 2 on the catalyst, which can enhance the SCR reaction by L-H mechanism.When oxygen is at saturated adsorption state on the catalyst, the surface of catalyst is not vacant for excess oxygen.As reported by other researchers, there Energies 2017, 10, 2084 9 of 14 was a negative influence for further NO removal, and the excessive oxygen would diffuse in the reactor to oxidize NO to NO 2 [22].
changes are observed for the rates of NO conversion, N2O formation, and denitration.It was reported that oxygen of adsorbed state was firstly adsorbed on the surface of catalyst, and then diffused inside the catalyst to participate in catalytic reaction.On the one hand, the existence of oxygen is more conducive to oxidize NO to NO2, and NO2 is easier to react with ammonia to generate nitrogen and water.On the other hand, O2 promotes dehydrogenation of NH3 to NH2 on the catalyst, which can enhance the SCR reaction by L-H mechanism.When oxygen is at saturated adsorption state on the catalyst, the surface of catalyst is not vacant for excess oxygen.As reported by other researchers, there was a negative influence for further NO removal, and the excessive oxygen would diffuse in the reactor to oxidize NO to NO2 [22].Oxygen Concentration (%)

Effect of Ammonia to Nitrogen Ratio
In engineering applications, the ammonia nitrogen ratio is generally controlled between 1.0 and 1.2.In this experiment, the ratios of ammonia to nitrogen were 0.5, 0.8, 1, 1.2, and 1.5.As shown in Figure 7a,d, with the increase of ammonia nitrogen ratio, the rates of NO conversion and denitration of the Mn-Ce/FA (M) and the Mn-Ce/FA-30%TiO 2 (M) increased.When the ammonia nitrogen ratio was higher than 1, NO conversion rate and denitration rate increased slowly and remained at a stable level.Figure 7b shows that the amount of NO 2 remained constant.It is indicated that the change of ammonia nitrogen ratio did not affect the NO 2 formation rate.But the N 2 O formation rates of the two catalysts increased with the increase of ammonia nitrogen ratio (Figure 7c).When the amount of ammonia increased, NH 3 was automatically dehydrogenated to generate a large amount of NH.Then NH was oxidized to form NHNO by oxygen, and NHNO combined with NO to generate N 2 O [23].However, when the ammonia ratio was higher than 1, the increase of N 2 O was not obvious.Because the NO adsorption sites of catalyst reach saturation, the ammonia of dehydrogenation cannot react with NO to produce N 2 O. Ammonia is firstly adsorbed on the acid site of catalyst according to the L-H mechanism.When ammonia is overloaded, acid adsorption on the surface of catalyst reaches saturation.Excessive ammonia would cause ammonia slip.García-Bordejé et al. [24] studied NH 3 transient experiment on carbon coated cordierite catalyst.It was found that NH 3 with high adhesion adsorbed on the surface of the catalyst, and excessive ammonia would replace the vacancy of active substances, leading to the decrease of denitration rate.
with NO to produce N2O.Ammonia is firstly adsorbed on the acid site of catalyst according to the L-H mechanism.When ammonia is overloaded, acid adsorption on the surface of catalyst reaches saturation.Excessive ammonia would cause ammonia slip.García-Bordejé et al. [24] studied NH3 transient experiment on carbon coated cordierite catalyst.It was found that NH3 with high adhesion adsorbed on the surface of the catalyst, and excessive ammonia would replace the vacancy of active substances, leading to the decrease of denitration rate.

Research on Sulfur and Water Resistances of Catalysts
3.4.1.Sulfur Resistance sulfur resistance using the Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO2 (M) was studied under the conditions of 250 °C , NO 800 ppm, NH3 800 ppm, six percent O2 and 3000 h −1 .SO2 with the  The sulfur resistance using the Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO 2 (M) was studied under the conditions of 250 • C, NO 800 ppm, NH 3 800 ppm, six percent O 2 and 3000 h −1 .SO 2 with the concentration of 100 ppm and 200 ppm was added into the reaction at 90 min lasting for a period of time, then stopped at 270 min, respectively.When the addition of 100 ppm SO 2 was stopped, the rates of NO conversion and denitration of the Mn-Ce/FA (M) recovered to 74.4% and 58.3%, and the two rates of the Mn-Ce/FA-30%TiO 2 (M) recovered to 83.5% and 75.1%, respectively.Compared with the SCR without SO 2 , the two rates decreased by only about 5% while decreased by about 10% at 200 ppm SO 2 (Figure 8a,d).It is indicated that the Mn-Ce/FA (M) and the Mn-Ce/FA-30%TiO 2 (M) were relatively suitable for low sulfur environment.Figure 8b showed that SO 2 had almost no significant effect on NO 2 formation.However, the N 2 O formation rate became slightly lower as SO 2 was added and then higher than the initial value after SO 2 was stopped (Figure 8c).It is suggested that the added SO 2 first reacted with ammonia to form ammonium sulfite, which effectively inhibited the dehydrogenation of NH 3 .When the dehydrogenation of NH 3 was inhibited, the NH group from NH 3 also decreased.This resulted in the decrease of generation of N 2 O, which was from the reaction of NO and NH.However, when the addition of SO 2 was stopped, NH 3 was dehydrogenated to produce more N 2 O. SO 2 may aggravate the deposition of ammonium sulfate on the surface of catalyst at low temperature.You et al. [25] studied Mn and Ce catalysts for SCR.Their FTIR results proved that the deposition of ammonium sulfate hindered the adsorption of NO and cut off the L-H path in SCR reaction to accelerate the poisoning of catalyst.Wei et al. [26] found that SO 2 reacted with MnOx to form sulfate.The generated sulfate not only consumed active components of catalysts, but also deposited on the surface of the catalyst, which inhibited adsorption and desorption of the reaction gas and decreased denitration rate.
NO and NH.However, when the addition of SO2 was stopped, NH3 was dehydrogenated to produce more N2O.SO2 may aggravate the deposition of ammonium sulfate on the surface of catalyst at low temperature.You et al. [25] studied Mn and Ce catalysts for SCR.Their FTIR results proved that the deposition of ammonium sulfate hindered the adsorption of NO and cut off the L-H path in SCR reaction to accelerate the poisoning of catalyst.Wei et al. [26] found that SO2 reacted with MnOx to form sulfate.The generated sulfate not only consumed active components of catalysts, but also deposited on the surface of the catalyst, which inhibited adsorption and desorption of the reaction gas and decreased denitration rate.

Water Resistance
Figure 9 showed the water stability of the Mn-Ce/FA (M) and the Mn-Ce/FA-30%TiO2 (M) during low temperature SCR reactions.A concentration of three or six percent H2O was added into the reaction at 90 min lasting for a period of time, then stopped at 270 min.When the addition of three

Water Resistance
Figure 9 showed the water stability of the Mn-Ce/FA (M) and the Mn-Ce/FA-30%TiO 2 (M) during low temperature SCR reactions.A concentration of three or six percent H 2 O was added into the reaction at 90 min lasting for a period of time, then stopped at 270 min.When the addition of three percent water was terminated, the rates of NO conversion and denitration of the Mn-Ce/FA and the Mn-Ce/FA-30%TiO 2 decreased by about five percent, compared to the SCR process without water addition.The reduction of SCR efficiency was about 10% in the presence of six percent H 2 O.It is indicated that the Mn-Ce/FA and Mn-Ce/FA-30%TiO 2 were more suitable to be used in less water environment.
Figure 9b shows that H 2 O had almost no significant effect on the amount of NO 2 production.H 2 O competes with reactive gas NH 3 for the active sites on the surface of catalyst.The adsorption of water molecules produces hydroxyl groups, resulting in a reduction of active sites of catalyst.Although the addition of water decreased denitration rate, it also inhibited the production of N 2 O.It was reported that water could reduce active sites of catalysts, but would be adsorbed on the surface of catalysts to produce massive hydroxyl in free state [27].Huang [28] reported that the presence of steam in the flue gas inhibited SCR reaction of NH 3 and NO which adsorbed on the Lewis acid sites, and the effect of inhibition increased with the increase of H 2 O concentration.
water molecules produces hydroxyl groups, resulting in a reduction of active sites of catalyst.Although the addition of water decreased denitration rate, it also inhibited the production of N2O.It was reported that water could reduce active sites of catalysts, but would be adsorbed on the surface of catalysts to produce massive hydroxyl in free state [27].Huang [28] reported that the presence of steam in the flue gas inhibited SCR reaction of NH3 and NO which adsorbed on the Lewis acid sites, and the effect of inhibition increased with the increase of H2O concentration.

Conclusions
A cost-effective molding fly ash-based catalyst was investigated in this work for low temperature SCR process.The main conclusions are: (1) With the increase of TiO2 content, the NO conversion and denitration rates of catalysts increased, and the amount of NO2 and N2O decreased.(2) The rates of NO conversion, denitration and N2O formation of the Mn-Ce/FA (M) and the Mn-Ce/FA-30%TiO2 (M) increased with the increase of oxygen concentration and ammonia nitrogen ratio, and the decrease of space velocity.The NO2 formation rate was not affected by the change of space velocity and ammonia nitrogen ratio, but increased with the increase of oxygen concentration.

Conclusions
A cost-effective molding fly ash-based catalyst was investigated in this work for low temperature SCR process.The main conclusions are: (1) With the increase of TiO 2 content, the NO conversion and denitration rates of catalysts increased, and the amount of NO 2 and N 2 O decreased.(2) The rates of NO conversion, denitration and N 2 O formation of the Mn-Ce/FA (M) and the Mn-Ce/FA-30%TiO 2 (M) increased with the increase of oxygen concentration and ammonia nitrogen ratio, and the decrease of space velocity.The NO 2 formation rate was not affected by the change of space velocity and ammonia nitrogen ratio, but increased with the increase of oxygen concentration.(3) In the experiments of water and sulfur resistances, the rates of NO conversion and denitration of Mn-Ce/FA (M) and Mn-Ce/FA-30%TiO 2 (M) under 100 ppm SO 2 and three percent H 2 O conditions decreased by about five percent, respectively, compared with water and sulfur free environment.It was found that H 2 O and SO 2 had almost no effect on the production of NO 2 .The presence of SO 2 could inhibit the excessive dehydrogenation of NH 3 and decreased production of N 2 O.The hydroxyl groups in H 2 O could also inhibit the formation of N 2 O.

Figure 1 .
Figure 1.Schematic diagram of the reaction system.

Figure 1 .
Figure 1.Schematic diagram of the reaction system.

Figure 3 .
Figure 3. XRD analyses of the prepared catalysts.Figure 3. XRD analyses of the prepared catalysts.

Figure 3 .
Figure 3. XRD analyses of the prepared catalysts.Figure 3. XRD analyses of the prepared catalysts.
, with the increase of TiO 2 content at different temperature, the NO conversion rate increased and N 2 O formation rate decreased.At 250 • C, the NO conversion rate and N 2 O formation rate of the Mn-Ce/100%TiO 2 (M) were 95% and 4.8%, respectively.Compared with the Mn-Ce/FA (M), NO conversion was increased by 16.8% and N 2 O formation was decreased by 8.3%.N 2 O originated from the direct oxidation of NH 3 and the reactions of NH 3 and NO.At 100-150 • C, the formation of N 2 O follows the Langmuir-Hinshelwood mechanism, while it follows the Eley-Rideal mechanism at 250 • C[19].The results show that the doping of TiO 2 has obvious inhibitory effect on the formation of N 2 O.It is probable that the unique electronic cloud structure and a large amount of hydroxyl of nanophase TiO 2 avoid excessive NH 3 dehydrogenation.It results in the reduction of NH groups on the surface of the catalyst, effectively inhibiting the production of N 2 O and improving the selectivity of N 2 .

Figure 7 .
Figure 7. Effects of ratio of ammonia to nitrogen on catalytic activity.(a) NO conversion; (b) NO 2 formation rate; (c) N 2 O formation rate; (d) Denitration rate.

Table 1 .
BET analyses of the prepared catalysts.

Table 1 .
BET analyses of the prepared catalysts.