Low temperature performance of selective catalytic reduction of NO with NH3 under a concentrated CO2 atmosphere

: Selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) has been widely investigated to reduce NO x emissions from combustion processes, which cause environmental challenges. However, most of the current work on NO x reduction has focused on using feed gas without CO 2 or containing small amounts of CO 2 . In the future, oxy-fuel combustion will play an important role for power generation, and this process generates high concentrations of CO 2 in ﬂue gas. Therefore, studies on the SCR process under concentrated CO 2 atmosphere conditions are important for future SCR deployment in oxy-fuel combustion processes. In this work, Mn- and Ce-based catalysts using activated carbon as support were used to investigate the effect of CO 2 on NO conversion. A N 2 atmosphere was used for comparison. Different process conditions such as temperature, SO 2 concentration, H 2 O content in the feed gas and space velocity were studied. Under Mn-Ce/AC conditions, the results suggested that Mn metal could reduce the inhibition effect of CO 2 on the NO conversion, while Ce metal increased the inhibition effect of CO 2 . High space velocity also resulted in a reduction of CO 2 inhibition on the NO conversion, although the overall performance of SCR was greatly reduced at high space velocity. Future investigations to design novel Mn-based catalysts are suggested to enhance the SCR performance under concentrated CO 2 atmosphere conditions.


Introduction
NO x emissions are responsible for acid rain and urban smog, and they pose a significant risk to the environment and human health [1].Selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) has been extensively investigated and also been used commercially for NO x reduction.The industrial operation is based on V 2 O 5 -WO 3 (MoO 3 )/TiO 2 catalysts, which are reactive within a high temperature window (300-400 ˝C) [2][3][4].However, many pollutants, e.g., sulphur and dusts, are present in this reactive temperature range, causing the deactivation of selective catalytic reduction (SCR) catalyst [5].Therefore, there is a strong need to develop catalysts for low temperature SCR processes, which can be placed after the electrostatic precipitator and desulfurizer to avoid pollutants such as sulphur and particulates.
Catalysts containing transition metals have been widely researched for low-temperature SCR, due to the effective catalytic ability of transition metals [6][7][8][9][10].Among them, Mn-and Ce-based catalysts have drawn particular attention due to their abundant oxygen vacancies, which promote the redox cycle during SCR reactions [11][12][13][14].In addition, activated carbon (AC) has been used as catalyst support for SCR, since it has high surface for metal loading and the presence of functional groups on the surface of activated carbon can also promote NO x conversion [15][16][17][18][19].
Currently, NH 3 -SCR is mostly investigated under N 2 atmospheres, to simulate the stationary NO x sources of power plants.It is realized that oxy-fuel combustion is attracting increasing interest.Using pure oxygen instead of air for combustion generates a flue gas which consists of mainly CO 2 and H 2 O, where CO 2 can be easily captured [20][21][22][23].Therefore, more understanding of NH 3 -SCR in the presence of an abundance of CO 2 is essential for the future development of SCR technology combined with oxy-fuel combustion processes.However, it is found that there are a few works investigating the influence of CO 2 on SCR [24,25].For example, Kim et al. [24] studied the effect of CO 2 on SCR using a small pore zeolite copper catalyst; and reported that CuSSZ13 catalyst can be deactivated by 10% CO 2 at low temperatures (<300 ˝C).Magnusson et al. [25] studied the influence of 6% CO 2 during SCR, and found no significant influences.To our best knowledge, there is very limited work about the investigation of low-temperature SCR under high concentrations of CO 2 , simulating the flue gas from oxy-fuel combustion processes.
Furthermore, it is known that SCR performance is significantly affected by the process conditions such as temperature, space velocity [26][27][28], concentrations of H 2 O [25,[29][30][31], and the presence of SO 2 [6,11,25,30,32,33].For example, Magnusson et al. [25] investigated SCR using a marine based catalyst; they reported that higher space velocity (18,300 h ´1) resulted in a continuous decrease in catalytic activity, compared with space velocities of 6100 and 12,200 h ´1; in addition, they also reported that at temperatures higher than 300 ˝C, the catalyst showed a stable catalytic reactivity at different SO 2 concentrations, but a significant reduction of SCR activity was observed at a temperature of 250 ˝C for gas streams containing 250 and 750 ppm SO 2 .The decrease of SCR activity with the increase of space velocity has also been reported by other researchers [26].Pan et al. [30] reported that H 2 O has a reversible negative effect on NH 3 -SCR using a MnO x /MWCNTs catalyst, while SO 2 was found to have an irreversible deactivation effect.
In this work, our main objective was to investigate the SCR performance under a concentrated CO 2 atmosphere.Mn-Ce/AC with different process conditions, including temperature, SO 2 and H 2 O and also space velocity were used.In addition, the performance of SCR catalyst under CO 2 atmosphere was compared to an inert N 2 atmosphere at various process conditions.

Catalyst Preparation
Activated carbon (AC, 40-60 mesh, Kecheng Novel Technology Co., Ltd.Beijing, China) was used as catalyst support.It was pre-treated under 67% concentrated HNO 3 for 1.5 h at 80 ˝C, and washed with deionized water to a pH about 7. The washed AC was dried at 110 ˝C for 12 h before using as catalyst support.Catalyst support (treated AC) was added to an aqueous solution (100 mL) containing the desired amount of Mn(NO 3 ) 2 and Ce(NO 3 ) 3 .The solution was stirred for 3 h and left for 2 h without stirring.The precursor was dried at 100 ˝C for 12 h and finally calcined at 500 ˝C for 3 h in N 2 atmosphere.Catalysts with different metal additions were prepared in this work: Mn/AC catalysts contained 3, 7, and 9 wt% Mn, respectively; Ce/AC catalysts had a Ce content of 3, 7 and 9 wt%, respectively.Also 7 wt% (Mn-Ce)/AC catalysts were prepared with the following Mn/Ce ratios: 1:4, 1:2, 1:1 and 2:1.The catalysts prepared in this work have BET surface areas around 300 m 2 ¨g´1 and pore volumes of about 0.3 cm 3 ¨g´1 .

Experimental System
NH 3 -SCR tests were carried out in a fixed bed reaction system shown in Figure 1.Simulated gases (NO, NH 3 and O 2 balanced with N 2 or CO 2 ) with a total flow rate of 1200 mL/min were introduced into the reaction system, where 2 g of catalyst was located.The catalysts were tested at temperatures between 100 ˝C and 300 ˝C.The NO concentration was analysed by a Flue Gas Analyzer (350 XL, Testo, Schwarzwald, Germany) The NO conversion was calculated using the following equation: where [NO] inlet is the introduced NO concentration, vol%; [NO] outlet is the NO concentration in the outlet of the reactor, vol%.

Catalyst Characterization
X-ray diffraction (XRD, Bruker, Karlsruhe, Germany) with monochromatized Cu Kα radiation was used to obtain the crystal structure of the fresh catalyst with scanning speed of 12 min ´1 from 10 ˝to 60 ˝.Scanning electron microscopy (SEM, S-3500N, Hitachi, Tokyo, Japan) and transmission electron microscopy (TEM) (Tecnai-20, Royal Philips, Amsterdam, Netherlands) were used to obtain morphologies of the fresh catalysts.BET surface area and porosity of the fresh catalyst were determined using ASAP2020 equipment (Micromeritics, Norcross, GA, USA); during the analysis, a 0.2 g sample was first degassed under 150 ˝C before being used for adsorption/desorption of N 2 .
Energies 2015, 8 4 microscopy (TEM) (Tecnai-20, Royal Philips, Amsterdam, Netherlands) were used to obtain morphologies of the fresh catalysts.BET surface area and porosity of the fresh catalyst were determined using ASAP2020 equipment (Micromeritics, Norcross, GA, USA); during the analysis, a 0.2 g sample was first degassed under 150 °C before being used for adsorption/desorption of N2.

Comparison of CO2 and N2 Atmosphere with Different Catalysts
NH3-SCR was carried out at temperatures between 100 °C and 300 °C with Mn/AC, Ce/AC and Mn/Ce/AC catalysts, respectively.As shown in Figure 2, activated carbon (AC) without metal loading showed very low NO reduction activity, with about 35% NO conversion.In addition, with little catalytic impact from catalyst (only using AC), CO2 showed inhibition of NO conversion in the SCR test.

Results and Discussion
3.1.Comparison of CO 2 and N 2 Atmosphere with Different Catalysts NH 3 -SCR was carried out at temperatures between 100 ˝C and 300 ˝C with Mn/AC, Ce/AC and Mn-Ce/AC catalysts, respectively.As shown in Figure 2, activated carbon (AC) without metal loading showed very low NO reduction activity, with about 35% NO conversion.In addition, with little catalytic impact from catalyst (only using AC), CO 2 showed inhibition of NO conversion in the SCR test.

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Energies 2015, 8, 12331-12341 NH3-SCR was carried out at temperatures between 100 °C and 300 °C with Mn/AC, Ce/AC and Mn/Ce/AC catalysts, respectively.As shown in Figure 2, activated carbon (AC) without metal loading showed very low NO reduction activity, with about 35% NO conversion.In addition, with little catalytic impact from catalyst (only using AC), CO2 showed inhibition of NO conversion in the SCR test.With the increase of reaction temperature from 100 ˝C to 300 ˝C, NO conversion was increased significantly in the presence of Mn/AC catalyst, for both N 2 and CO 2 atmospheres.The enhanced NO conversion at a relatively higher temperature using Mn-based catalyst has been reported [34][35][36][37].Similar results were also found while using other catalysts (Figures 3 and 4).
From Figure 2, the conversion of NO was between 58% and 70% in the presence of 3% Mn/AC catalyst under N 2 atmosphere; the NO conversion was much lower in the CO 2 atmosphere (33%-58%).Using 7% Mn/AC catalyst, the N 2 atmosphere also gave higher NO conversion (65%-75%) compared with the CO 2 atmosphere (49%-70%).However, it seems that the difference of NO conversion between N 2 and CO 2 atmosphere was reduced at higher reaction temperatures, when the catalyst was changed from 3% Mn/AC (10% difference) to 7% Mn/AC (5% difference).
The advantage of N 2 atmosphere over CO 2 in terms of NO conversion was reduced when the Mn loading in the Mn/AC catalyst was increased to 9%.For example, the same level of NO conversion was observed from Figure 2 for both CO 2 and N 2 atmospheres at 200 ˝C using the 9% Mn/AC catalyst.When the reaction temperature was increased to 300 ˝C in the presence of the 9% Mn/AC catalyst, the NO conversion in the CO 2 atmosphere (69%) became higher than under the N 2 atmosphere (64%).
The NO conversion in relation to the reaction atmosphere has also been studied using Ce/AC catalysts.As shown in Figure 3, the difference of NO conversion between the CO 2 and N 2 atmosphere was also affected by the reaction temperature.When the reaction temperature was lower than 200 ˝C, the NO conversion was higher in the atmosphere of N 2 compared with the CO 2 one.When the reaction temperature was increased to 300 ˝C, in most of the cases, the N 2 atmosphere gave higher NO conversion compared with the CO 2 atmosphere; except when using the 3%Ce/AC catalyst, where the NO conversion was higher in the atmosphere of CO 2 (80%) compared with the N 2 one (75%).

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Energies 2015, 8, 12331-12341 affected by the reaction temperature.When the reaction temperature was lower than 200 °C, the NO conversion was higher in the atmosphere of N2 compared with the CO2 one.When the reaction temperature was increased to 300 °C, in most of the cases, the N2 atmosphere gave higher NO conversion compared with the CO2 atmosphere; except when using the 3%Ce/AC catalyst, where the NO conversion was higher in the atmosphere of CO2 (80%) compared with the N2 one (75%).The SCR-NH3 process was enhanced under the CO2 atmosphere compared with the N2 atmosphere, when the Mn loading in the catalyst was increased (Figures 2 and 3).This is consistent with the results shown in Figure 4, where the SCR experiments were carried out using Mn-Ce/AC catalysts with The SCR-NH 3 process was enhanced under the CO 2 atmosphere compared with the N 2 atmosphere, when the Mn loading in the catalyst was increased (Figures 2 and 3).This is consistent with the results shown in Figure 4, where the SCR experiments were carried out using Mn-Ce/AC catalysts with different Mn:Ce ratios.From Figure 4, catalyst with low Mn loading e.g., 7% Mn-Ce/AC (Mn:Ce = 1:4) showed a lower NO conversion in the CO 2 atmosphere (57%) compared with the N 2 atmosphere (71%) at the temperature of 100 ˝C.
Energies 2015, 8 6 different Mn:Ce ratios.From Figure 4, catalyst with low Mn loading e.g., 7% Mn-Ce/AC (Mn:Ce = 1:4) showed a lower NO conversion in the CO2 atmosphere (57%) compared with the N2 atmosphere (71%) at the temperature of 100 °C.With the increase of Mn:Ce ratio, the NO conversion was increased for the CO2 atmosphere, and fluctuated (increased first and then decreased) for the N2 atmosphere.For example, the NO conversion was around 73% for the CO2 and only 64% for the N2 in the presence of the 7% Mn-Ce/AC (Mn:Ce = 2:1) catalyst at 100 °C.In addition, the overall performance of SCR was better in the CO2 atmosphere compared with the N2 atmosphere, when the catalyst has higher Mn:Ce ratio.
In summary, in this work we found that the depression of SCR activity under CO2 atmosphere can be reduced by adding Mn metals in the catalyst system, while the presence of Ce in the catalyst enhanced the CO2-induced SCR depression.With the increase of Mn:Ce ratio, the NO conversion was increased for the CO 2 atmosphere, and fluctuated (increased first and then decreased) for the N 2 atmosphere.For example, the NO conversion was around 73% for the CO 2 and only 64% for the N 2 in the presence of the 7% Mn-Ce/AC (Mn:Ce = 2:1) catalyst at 100 ˝C.In addition, the overall performance of SCR was better in the CO 2 atmosphere compared with the N 2 atmosphere, when the catalyst has higher Mn:Ce ratio.

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Energies 2015, 8, 12331-12341 In summary, in this work we found that the depression of SCR activity under CO 2 atmosphere can be reduced by adding Mn metals in the catalyst system, while the presence of Ce in the catalyst enhanced the CO 2 -induced SCR depression.
Mn-based catalysts have been widely researched for SCR, as various types of labile oxygen are present in the catalyst [38][39][40].In addition, abundant oxygen vacancies present in amorphous Mn-based catalysts were reported to greatly improve the SCR activity [40].The enhancement of SCR activity using Mn-based catalyst is also due to the promoted adsorption of NH 3 and NO oxidation to NO 2 [41], which play important roles in SCR [42].In this work, it seems that amorphous Mn-species were present in the Mn-based catalyst; as shown in Figure 5, XRD analysis of the selected catalysts is presented.Diffraction peaks related to Mn or Ce metals could barely be observed, as little differences could be found from XRD analysis between AC and Mn-Ce/AC.The weak diffraction in the XRD analysis might be due to that the particle size of metal were very small.As shown in Figure 6 (TEM analysis), particles with size about 10 nm were observed in the fresh 7% Mn-Ce/AC (Mn:Ce = 1:4).
Kim et al. [24] reported that CO 2 -induced deactivation in SCR was due to the adsorption of NH 3 by CO 2 , and supression of the formation of nitrates, which are a key reaction intermediate for NO x reduction.Yang et al. [43] also reported that CO 2 may compete for the adsorbed NH 3 , which should be desirable for NO conversion.Therefore, in order to reduce the negative effect of CO2 in SCR, the adsorption of NH3 by CO2 should be depressed.In this work, we suggest that the formed Mn-based species might reduce the adsorption of NH3 by CO2.The addition of Mn in the catalysts changed some Lewis (weak) acid sites to Bronsted (strong) acid sites, which have been reported to be less affected by CO2 [43].It has also been reported that weak acid sites in the catalyst were changed into strong acid sites with the increase of Therefore, in order to reduce the negative effect of CO2 in SCR, the adsorption of NH3 by CO2 should be depressed.In this work, we suggest that the formed Mn-based species might reduce the adsorption of NH3 by CO2.The addition of Mn in the catalysts changed some Lewis (weak) acid sites to Bronsted (strong) acid sites, which have been reported to be less affected by CO [43].It has also Therefore, in order to reduce the negative effect of CO 2 in SCR, the adsorption of NH 3 by CO 2 should be depressed.In this work, we suggest that the formed Mn-based species might reduce the adsorption of NH 3 by CO 2 .The addition of Mn in the catalysts changed some Lewis (weak) acid sites to Bronsted (strong) acid sites, which have been reported to be less affected by CO 2 [43].It has also been reported that weak acid sites in the catalyst were changed into strong acid sites with the increase of Mn loading [8].In addition, addition of Ce into MnTi catalyst resulted in an increase of Lewis acid sites, which enhances the inhibiting of CO 2 on SCR [43]; thus the literature supports the results obtained in this work, where catalysts with high Ce loading showed lower NO conversion under the CO 2 atmosphere compared with the N 2 atmosphere (Figure 4).
From the above results, we propose that the performance of SCR in CO 2 atmosphere is affected by the type of catalyst and also the reaction temperature.It is suggested that CO 2 has a more negative effect on SCR at low temperatures.In the following sections, we will discuss the influence of process conditions on SCR in the CO 2 and N 2 atmosphere.

Selective Catalytic Reduction of NO x with NH 3 (NH 3 -SCR) Test under CO 2 and N 2 Atmosphere with Different Process Conditions
The SCR performance under concentrated CO 2 was compared with a N 2 atmosphere under different process conditions.Figure 7 shows the effect of space velocity on the SCR performance using the 7% Mn/AC and the 7% Ce/AC catalysts.The higher space velocity resulted in a lower NO conversion might be due to the reduction of resisdence time of the reactants [25,26].Although Ce-based catalyst has higher SCR activity compared with the Mn-based catalyst, we focus on discussions about the influence of the reaction atmosphere.

Selective catalytic reduction of NOx with NH3 (NH3-SCR) Test under CO2 and N2 Atmosphere with Different Process Conditions
The SCR performance under concentrated CO2 was compared with a N2 atmosphere under different process conditions.Figure 7 shows the effect of space velocity on the SCR performance using the 7% Mn/AC and the 7% Ce/AC catalysts.The higher space velocity resulted in a lower NO conversion might be due to the reduction of resisdence time of the reactants [25,26].Although Ce-based catalyst has higher SCR activity compared with the Mn-based catalyst, we focus on discussions about the influence of the reaction atmosphere.From Figure 7, it seems that for the Mn/AC catalyst, higher NO conversion was observed under the CO2 atmosphere compared with the N2 atmosphere, under high space velocity (13,632 h −1 ).The better SCR performance using CO2 over N2 was not observed in the presence of Ce/AC catalysts; this is consistent with the previous discussions that Mn reduces the negative effect of CO2 while Ce does not.In addition, since the depression of SCR under CO2 is mainly ascribed to the adsorption of NH3 by CO2, thus a higher space velocity could reduce/avoid the negative effects of CO2.However, the overall NO efficiency could be greatly reduced at higher space velocity.
The influence of CO2 atmosphere on SCR was also investigated under different SO2 concentrations.SO2 is known to have a negative effect on NO conversion due to catalyst poisoning [6,11,30].As shown in Figure 8, for all of the tested three catalysts (Mn/AC, Ce/AC and Mn-Ce/AC), the CO2 From Figure 7, it seems that for the Mn/AC catalyst, higher NO conversion was observed under the CO 2 atmosphere compared with the N 2 atmosphere, under high space velocity (13,632 h ´1).The better SCR performance using CO 2 over N 2 was not observed in the presence of Ce/AC catalysts; this is consistent with the previous discussions that Mn reduces the negative effect of CO 2 while Ce does not.In addition, since the depression of SCR under CO 2 is mainly ascribed to the adsorption of NH 3 by CO 2 , thus a higher space velocity could reduce/avoid the negative effects of CO 2 .However, the overall NO efficiency could be greatly reduced at higher space velocity.
The influence of CO 2 atmosphere on SCR was also investigated under different SO 2 concentrations.SO 2 is known to have a negative effect on NO conversion due to catalyst poisoning [6,11,30].As shown in Figure 8, for all of the tested three catalysts (Mn/AC, Ce/AC and 12337 Energies 2015, 8, 12331-12341 Mn-Ce/AC), the CO 2 atmosphere showed an inhibition of the NO conversion.In particular, the difference of NO conversion between CO 2 and N 2 atmosphere was increased with the increase of SO 2 concentration in the feed gas stream.For example, the NO conversion was similar for both CO 2 and N 2 atmosphere with the SO 2 concentration of 0 ppm; however, the N 2 atmosphere gave about 10% of NO conversion higher than the CO 2 atmosphere when the SO 2 concentration was increased to 400 ppm.Therefore, it is suggested that the presence of SO 2 enhances the inhibition of NO conversion in the CO 2 atmosphere.
The influences of a CO 2 atmosphere on catalytic SCR at different H 2 O concentrations are shown in Figure 9.The increase of H 2 O content from 0% to 8% resulted in a reduction of NO conversion for all the catalysts; this is consistent with the literature on the effect of H 2 O addition [30,44].It seems that the influence of H 2 O concentration on NO conversion is small, when the CO 2 atmosphere is compared with the N 2 atmosphere, although a little higher NO conversion was observed for the N 2 atmosphere with 8% of H 2 O.It is suggested that the presence of large amounts of H 2 O, which would compete with NO and NH 3 adhering to the AC surface, has a more negative effect on the SCR performance in the CO 2 atmosphere, compared with the N 2 atmosphere.
Energies 2015, 8 9 It seems that the influence of H2O concentration on NO conversion is small, when the CO2 atmosphere is compared with the N2 atmosphere, although a little higher NO conversion was observed for the N2 atmosphere with 8% of H2O.It is suggested that the presence of large amounts of H2O, which would compete with NO and NH3 adhering to the AC surface, has a more negative effect on the SCR performance in the CO2 atmosphere, compared with the N2 atmosphere.Energies 2015, 8 9

Conclusions
It seems that the influence of H2O concentration on NO conversion is small, when the CO2 atmosphere is compared with the N2 atmosphere, although a little higher NO conversion was observed for the N2 atmosphere with 8% of H2O.It is suggested that the presence of large amounts of H2O, which would compete with NO and NH3 adhering to the AC surface, has a more negative effect on the SCR performance in the CO2 atmosphere, compared with the N2 atmosphere.12338

Conclusions
In this work, the SCR process in the presence of Mn-and Ce-based catalysts under a concentrated CO 2 atmosphere was investigated in order to obtain information for developing SCR technologies combined with oxy-fuel combustion power plants.It is found that CO 2 can depress the conversion of NO, in particular at low reaction temperatures and with high SO 2 concentration in the feed gas.Under the Mn-Ce/AC catalysis conditions, the results showed that with the increase of Mn loading, the inhibitory effect of CO 2 on NO conversion was reduced, while adding Ce metal in the catalyst system enhanced the depression effect of CO 2 on SCR.

Figure 1 .
Figure 1.Schematic diagram of the selective catalytic reduction of NOx with NH3 (NH3-SCR) reaction system.

Figure 1 .
Figure 1.Schematic diagram of the selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) reaction system.