Denox of Nano-Catalyst of Selective Catalytic Reduction Using Active Carbon Loading Mnox-Cu at Low Temperature

catalysts

Article DeNOx of Nano-Catalyst of Selective Catalytic Reduction Using Active Carbon Loading MnOx-Cu at Low Temperature

Tao Zhu 1,2,*, Xing Zhang 1, Wenjing Bian 1, Yiwei Han 1, Tongshen Liu 3,* and Haibing Liu 4

1 Institute of Atmospheric Environmental Management and Pollution Control, China University of Mining & Technology (Beijing), Beijing 100083, China; [email protected] (X.Z.); [email protected] (W.B.); [email protected] (Y.H.) 2 Shaanxi Key Laboratory of Lacustrine Shale Gas Accumulation and Exploitation (Under Planning), Xi’an 710065, China 3 Beijing Municipal Research Institute of Environmental Protection, Beijing 100037, China 4 Solid Waste Management Center of China Ministry of Ecological Environment, Beijing 100029, China; lhblﬀ@126.com * Correspondence: [email protected] (T.Z.); [email protected] (T.L.); Tel.: +86-10-6233-9170 (T.Z.); Fax: +86-10-6233-9170 (T.Z.) Received: 8 November 2019; Accepted: 14 January 2020; Published: 18 January 2020

Abstract: With the improvement of environmental protection standards, selective catalytic reduction (SCR) has become the mainstream technology of flue gas deNOx. Especially, the low-temperature SCR nano-catalyst has attracted more and more attention at home and abroad because of its potential performance and economy in industrial applications. In this paper, low-temperature SCR catalysts were prepared using the activated carbon loading MnOx-Cu. Then, the catalysts were packed into the fiedbed stainless steel micro-reactor to evaluate the selective catalytic reduction of NO performance. The influence of reaction conditions was investigated on the catalytic reaction, including the MnOx-Cu loading amount, calcination and reaction temperature, etc. The experimental results indicate that SCR catalysts show the highest catalytic activity for NO conversion when the calcination temperature is 350 ◦C, MnOx loading amount is 5%, Cu loading amount is 3%, and reaction temperature is 200 ◦C. Under such conditions, the NO conversion arrives at 96.82% and the selectivity to N2 is almost 99%. It is of great significance to investigate the influence of reaction conditions in order to provide references for industrial application.

Keywords: low-temperature SCR; activated carbon; nano-catalyst; reaction condition; NO conversion

1. Introduction

The emission of nitrogen oxides (NOx) has aroused widespread concern in recent years, which is dangerous to human health and brings many negative effects on the environment, such as urban smog, acid rain, and so on. Increasingly stringent limits for flue gas emissions have driven many researchers to look for suitable methods [1,2]. Selective non-catalytic reduction (SNCR) is a more economical NOx control method with a considerable efficiency of 40–70%, which involves the reduction of NOx by a nitrogen agent, usually ammonia (NH3) or urea, and it is relatively simple to implement. Although reasonably inexpensive, SNCR process has a serious limitation. For example, the NOx reduction efficiency drops off drastically at slightly higher or lower temperatures because the temperature window over which nitrogen agents are effective is relatively narrow [3,4]. The selective catalytic reduction (SCR) of NOx with NH3 has been regarded as the most effective method to reduce the emission of NOx. However, the SCR catalysts have to be placed upstream of the electrostatic

Catalysts 2020, 10, 135; doi:10.3390/catal10010135 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 135 2 of 17 precipitator and desulfurization units to avoid costly heating of the flue gas due to its high working temperature window (300–400 ◦C), where the SCR catalysts are susceptible to deactivation by dust accumulation and sulfur poison. As an alternative, low-temperature SCR catalysts could be placed downstream of the electrostatic precipitator and desulfurization units. Therefore, it is crucial to develop low-temperature SCR catalysts, which can work at 200 ◦C or even lower, and adapt to the complex flue gas environment [5–7]. Many scientists whether foreign and domestic had carried out the study of low-temperature SCR catalysts and their main research contents included the active component and catalyst support. However, sintering often occurs in the preparation of single component SCR catalyst, which reduces the dispersion of the active component and the activity of the SCR catalyst. The sintering is avoided by adding transition metal and the performance of SCR catalyst is improved. Therefore, the researches and developments of low-temperature SCR catalyst focused on the transition metal oxide catalyst at home and abroad [8–11]. Copper and copper compounds are thought to have excellent catalytic properties at low temperatures, which can improve the physical properties of the SCR catalyst as a promoter [12,13]. Chmielarz et al. prepared a M-Mg-Al catalyst with a coprecipitation of hydrotalcite (M=Cu2+, Co2+, and Cu2++Co2+), and they compared the catalytic properties of the mixed catalyst. The results showed Cu-Mg-Al > Cu-Co-Mg-Al > Co-Mg-Al and the deNOx efficiency of 80–95% at temperatures of 200–250 ◦C[14]. Manganese (Mn) is thought to have a very outstanding catalytic property at low temperatures, and Mn-based catalyst has been widely studied over the past few decades owing to its low cost. In relation to the supported metal oxide catalysts, the researchers mainly focused on the MnOx catalyst. MnOx is characterized by strong catalytic performance for NOx as the active component of thenSCR catalyst. There are many kinds of MnOx due to the wide distribution of the Mn valence. The Mn with a different valence can be converted to each other, which promotes the selective reduction of NOx by NH3. Therefore, the valence state of Mn in MnOx has a significant effect on the activity of the SCR catalyst. Moreover, composite oxide catalysts are of great interest because one metal element can modify the catalytic properties of another, which results from both electronic and structural influences. For example, binary metal oxide solid solutions might be formed between Mn and Cu [15,16]. Qi et al. used a different precipitation agent to prepare a series of Mn oxide catalyst by coprecipitation, and the catalyst activities were investigated under the condition of NH3-NOx. The results indicated that the catalyst was of high catalytic activity at a low temperature using sodium carbonate as a precipitant preparation of Mn oxide catalysts [17]. Ouzzine et al. studied the Mn-Cu composite catalyst and found that the catalytic temperature range was wide through the activity test. The NOx conversion was more than 60% in the range of 150–400 ◦C. What is more, they found that Cu could absorb sulfur dioxide and form sulfate, which improved sulfur resistance greatly [18]. Yao et al. successfully prepared a series of Mn/CeO2 catalysts via impregnation using deionized water, anhydrous ethanol, acetic acid, and oxalic acid as a solvent and found that the Mn/Ce catalyst exhibited the best water and sulfur tolerance [19]. Therefore, mixing MnOx with suitable metal oxides and loading Mn-based active components on a suitable support could be considered as an efficient strategy [20]. Activated carbon (AC) is a kind of materials with a high specific surface area, unique pore structure, excellent dispersion of active components and chemical stability, which has been widely studied as substrates for supporting low-temperature SCR catalysts. If AC is adopted as the catalyst support, the possibility of sintering deactivation could be reduced [21–25]. Valdés-Sols et al. confirmed that the vanadium loaded on the carbon-ceramic body was of a good low-temperature property, and the ability to resist the poison of catalyst for sulfur dioxide was greater than the Mn catalyst. But water vapor in the flue gas had an influence on the catalyst activation [26]. García-bordejé et al. studied low-temperature SCR characteristics using carbon materials loaded on a honeycomb support, and vanadium loaded on the carbon material. They found that deNOx efficiency was 59.8–72.1% at 150 ◦C and 68–78.6% at 180 ◦C[27]. Shanxi Institute of Coal Chemistry of Chinese Academy of Sciences prepared a SCR catalyst of V2O5 loaded on the carbon honeycomb support. They found the catalyst Catalysts 2020, 10, 135 3 of 17

Catalysts 2020, 10, x FOR PEER REVIEW 3 of 17 not only had a deNOx effect, but also could resist SO2 poisoning. But the engineering practicability of the low-temperatureAlthough the catalyst SCR catalyst varied, stillit is needs the key further problem research on how [28, 29to ].effectively reduce NOx emission from flue gas at a low temperature and with low energy. At the same time, the SCR catalysts in Although the catalyst varied, it is the key problem on how to effectively reduce NOx emission industrialfrom flue application gas at a low are temperature facing many and problems, with low such energy. as high At cost thes, same poisoning time, and the SCRtransformations. catalysts in Theindustrial steady application increase of areNO facingx emissions many necessitates problems, such the improvement as high costs, of poisoning abatement and technologies transformations. [30]. In this context, the research on low-temperature SCR catalyst was launched. According to the data, The steady increase of NOx emissions necessitates the improvement of abatement technologies [30]. In 95%this context,of NOx thein flue research gas is on in low-temperature the form of NO. SCR The catalyst by-products was launched. NO2 and According N2O were to formed the data, in 95%the reaction, but the amount generated was very small. Therefore, NO was used to simulate the catalytic of NOx in flue gas is in the form of NO. The by-products NO2 and N2O were formed in the reaction, reductbut theion amount of NO generatedx in this research was very and small. the NO Therefore, conversion NO was was used taken to as simulate an index the of catalytic the SCR reduction catalyst [31,32]. The influence of reaction conditions on NO conversion was investigated, including the MnOx- of NOx in this research and the NO conversion was taken as an index of the SCR catalyst [31,32]. The Cu loading amount, calcination and reaction temperatures, etc. The optimal preparation and reaction influence of reaction conditions on NO conversion was investigated, including the MnOx-Cu loading conditionsamount, calcination of low-temperature and reaction SCR temperatures, catalyst were etc. determined The optimal in preparation a series of andexperiments reaction conditions so that it couldof low-temperature provide basic SCRresearch catalyst data were for industrial determined application. in a series of experiments so that it could provide basic research data for industrial application. 2. Results and Discussion 2. Results and Discussion 2.1. The Characterization of Support 2.1. The Characterization of Support The AC was purified by 1%, 5% and 10% nitric acid, then filtered, washed and dried, respectively. ObservedThe AC by wasthe X purified-ray diffraction by 1%, 5% (XRD and) pattern 10% nitric as shown acid, then in F filtered,igure 1,washed there are and two dried, dispersed respectively. peaks inObserved the scope by thefrom X-ray 20 to di ff30°raction, and (XRD)there are pattern three as dispers showne ind Figurepeaks1 in, there the arescope two from dispersed 30 to peaks40°. The in skeletonthe scope structure from 20 to of 30 AC◦, and is similar there are to three amorphous dispersed carbon peaks and in the it scope is mainly from composed 30 to 40◦. The of graphite skeleton microcrystalsstructure of AC in is which similar 2– to4 layers amorphous single carbon layer graphite and it is mainlysheets composedare stacked. of The graphite dispersed microcrystals peaks are in whichgraphite 2–4 mic layersrocrystals single layer with graphite different sheets structures, are stacked. which The indicates dispersed that peaks the are AC graphite sample microcrystals has a low withgraphitization different structures,structure and which silica indicates is an inorganic that the ACcomponent sample hasof AC. a low There graphitization is almost no structure amorphous and peaksilica in is the an inorganicXRD diffraction component pattern of AC.of the There AC, iswhich almost indicates no amorphous that AC is peak of good in the crystallinity XRD diffraction after processingpattern of the. AC, which indicates that AC is of good crystallinity after processing.

Figure 1. XRD pattern of AC AC..

The Brunauer Brunauer-Emmett-Teller-Emmett-Teller ( (BET)BET) surface areas, pore pore volumes and and pore pore sizes of AC are summarized in in Table 11.. FromFrom TableTable1 1,, the the BET BET surface surface area area of of AC AC increases increases by by about about 9.10% 9.10% after after the the nitric acid purification purification process, and total pore volume isis not significant.significant. It is worth noting that the purifiedpurified micropore volume of AC increases and the average pore diameter decreases. Obviously, Obviously, the nitric acid process enhances the microporous natur naturee of AC, which is conducive to the dispersion of MnO and Cu on the AC surface. Furthermore, it is beneficial for SCR catalyst in terms of mass transfer, MnOx and Cu on the AC surface. Furthermore, it is beneficial for SCR catalyst in terms of mass transfer,adsorption adsorption and activation and of activation reactants of during reactants the reaction during [33 the]. reactionThe AC average [33]. The pore AC diameter average falls pore in diameterthe range falls of 8–11 in the nm, range favoring of 8– gas11 nm, molecule favoring diff gasusion molecule in the pores diffusion and thusin the ruling pores out and the thus diff rulingusion outlimitation the diffusion in the adsorptionlimitation in and the desorption adsorption process. and desorption process.

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Catalysts 2020, 10, 135 Table 1. Physical characteristics of AC. 4 of 17

BET Surface Total Pore Micropore Average Pore Sample Area Volume Volume Table 1. Physical characteristics of AC. Diameter (nm) (m2/g) (cm3/g) (cm3/g) Before BET Surface Area Total Pore Volume Micropore Volume Average Pore Sample 638 ± 1.96 0.29 ± 0.01 0.26 ± 0.02 10.27 ± 0.03 processing (m2/g) (cm3/g) (cm3/g) Diameter (nm) After Before processing 638 6961.96 ± 2.28 0.290.30 ± 0.020.01 0.31 ±0.26 0.03 0.028.56 ± 0.02 10.27 0.03 processing ± ± ± ± After processing 696 2.28 0.30 0.02 0.31 0.03 8.56 0.02 ± ± ± ±

2.2. The Influence of MnOx Loading Amount on Catalytic Activity 2.2. The Influence of MnOx Loading Amount on Catalytic Activity The influence of MnOx loading amount on SCR catalyst activity was investigated when the Cu The influence of MnO loading amount on SCR catalyst activity was investigated when the Cu loading amount (% wt) wasx 1%, 3%, 5%, 7%, 10%, respectively. The calcination temperature was loading amount (% wt) was 1%, 3%, 5%, 7%, 10%, respectively. The calcination temperature was 350 ◦C. 350 °C. The experiments compared the different MnOx loading amount of SCR catalyst under The experiments compared the different MnO loading amount of SCR catalyst under different reaction different reaction temperatures for NO conversion.x Figure 2 presents the NO conversion changes temperatures for NO conversion. Figure2 presents the NO conversion changes with MnO x loading with MnOx loading amounts and reaction temperatures when Cu loading amounts were 1%, 3%, 5%, amounts and reaction temperatures when Cu loading amounts were 1%, 3%, 5%, 7%, 10%, respectively. 7%, 10%, respectively. When MnOx loading amounts change in the scope of 1–5%, NO conversion increases with When MnOx loading amounts change in the scope of 1–5%, NO conversion increases with the the increase of MnOx loading amounts. There are more active sites if the catalyst has more active increase of MnOx loading amounts. There are more active sites if the catalyst has more active components, which is helpful to enhance catalytic activity. However, NO conversion decreases when components, which is helpful to enhance catalytic activity. However, NO conversion decreases when MnOx loading amounts change in the scope of 5–10%. All the experimental results indicate that SCR MnOx loading amounts change in the scope of 5–10%. All the experimental results indicate that SCR catalysts show optimal performance when the MnOx loading amount reaches 5%, and NO conversion catalysts show optimal performance when the MnOx loading amount reaches 5%, and NO conversion arrives at 95.31%, 96.82%, 95.20%, 92.65%, 88.00% at a reaction temperature of 200 C. As can be seen arrives at 95.31%, 96.82%, 95.20%, 92.65%, 88.00% at a reaction temperature of 200 °C◦ . As can be seen from Figure2, the SCR catalyst shows strong activity at 200 C, which indicates that the SCR catalyst from Figure 2, the SCR catalyst shows strong activity at 200 °C,◦ which indicates that the SCR catalyst is suitable for deNOx of low temperature flue gas. Whatever the Cu loading amount change, NO is suitable for deNOx of low temperature flue gas. Whatever the Cu loading amount change, NO conversion reaches the maximum with a MnOx loading amount of 5%, and the N2 selectivity remains conversion reaches the maximum with a MnOx loading amount of 5%, and the N2 selectivity remains stable at nearly 99%. stable at nearly 99%.

(a) 1% Cu

(b) 3% Cu

Figure 2. Cont.

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(d) 7% Cu

(e) 10% Cu

FigureFigure 2. 2. RelationshipRelationship between between MnO MnOxx loadloadinging amount amount and and NO NO conversion conversion with with a a Cu Cu load loadinging amount amount ofof ( (aa)) 1% 1%,, ( (bb)) 3%, 3%, ( (cc)) 5%, 5%, ( (d)) 7%, 7%, ( (e)) 10%, respectively respectively..

2.3.2.3. The The I Influencenfluence of of Cu Cu L Loadingoading A Amountmount on on C Catalyticatalytic A Activityctivity The influence of Cu loading amount on SCR catalyst activity was explored when MnO loading The influence of Cu loading amount on SCR catalyst activity was explored when MnOx loading amountamountss (% wt)wt) werewere 1%,1%, 3%, 3%, 5%, 5%, 7%, 7%, 10%, 10% respectively., respectively. The The calcination calcination temperature temperature was wa 350s 350◦C. The°C. Theexperiments experiments compared compared the di thefferent different Cu loading Cu load amounting amount of the SCR of the catalyst SCR catalyst under di underfferent different reaction reactiontemperatures temperature for NOs conversion.for NO conversion. Figure3 Figureillustrates 3 illustrat the influencees the influence of the Cu of loadingthe Cu loading amount amount on NO conversion with different MnO loading amounts. NO conversion arrives at 93.58%, 93.79%, 96.82%, on NO conversion with differentx MnOx loading amounts. NO conversion arrives at 93.58%, 93.79%, 96.82%91.82%,, 87.93%91.82%, when 87.93% the w Cuhen loading the Cu amount loading is amount 3%, and i thes 3%, reaction and the temperature reaction temperature is 200 ◦C, respectively. is 200 °C , respectively.When the Cu loading amount changes in the scope of 1–3%, NO conversion always increases withWhen the increase the Cu of load theing Cu loadingamount amount.changes However,in the scope NO of conversion 1–3%, NO decreases conversion when always the Cu increase loadings withamount the changesincrease inof the scopeCu load ofing 3–10%. amount. Whatever However, the MnONO conversionx loading amount decreases changes, when the NO Cu conversion loading reaches the maximum with a Cu loading amount of 3%, and the N selectivity remains stable at nearly amount changes in the scope of 3–10%. Whatever the MnOx loading2 amount changes, NO conversion 99%. On the whole, NO conversion decreases gradually when the reaction temperature exceeds 200 C. reaches the maximum with a Cu loading amount of 3%, and the N2 selectivity remains stable at nearly◦ 99%This. resultOn the reveals whole, that NO the conversion MnOx-Cu/ AC decreases catalyst gradually has a good when activity the at reaction a low temperature. temperature exceeds 200 °C. This result reveals that the MnOx-Cu/AC catalyst has a good activity at a low temperature.

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(a) 1% MnOx

(b) 3% MnOx

(d) 7% MnOx

(e) 10% MnOx

FigureFigure 3. RelationshipRelationship between between the the Cu Cu load loadinging amount and NO conversion with the MnOx loadloadinging amountamount of of ( (aa)) 1% 1%,, ( (bb)) 3%, 3%, ( (cc)) 5%, ( (d)) 7%, ( e)) 10% 10%,, respectively respectively..

2.4. The Influence of Calcination Temperature on Catalytic Activity

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In four different calcination temperatures (250 °C , 300 °C , 350 °C and 400 °C ), the SCR catalysts (Cu loading amount of 3%, and MnOx loading amount of 5%) were prepared. Figure 4 shows that NO conversion is highest up to 96.49% when the calcination temperature is 350 °C, and the N2 Catalystsselectivity2020, 10, 135remains stable at nearly 99%. 7 of 17 The SCR catalyst could not give full play to the function because MnOx and Cu might not integrate adequately due to the calcination temperature of 300 °C . When the calcination temperature 2.4. Thereaches Inﬂuence 400 °C of, th Calcinatione temperature Temperature may lead onto the Catalytic inside Activitypore channels of SCR catalyst to be collapsed or closed due to sintering, which reduces the number of active sites on the surface of the SCR catalyst. CatalystsIn four 2020 di, 10ﬀ,erent x FOR calcinationPEER REVIEW temperatures (250 ◦C, 300 ◦C, 350 ◦C and 400 ◦C), the SCR catalysts7 of 17 The high calcination temperature may reduce the dispersion of active sites [34]. Huang et al. found (Cu loading amount of 3%, and MnOx loading amount of 5%) were prepared. Figure4 shows that NO that the suitable calcination temperature could improve the attrition strength of the SCR catalyst, conversionIn four is highestdifferent up calcination to 96.49% temperatures when the calcination (250 °C , 300 temperature °C , 350 °C isand 350 400C, °C and), the the SCR N catalystsselectivity while the continuous increase of the calcination temperature could change the◦ mechanical property2 remains(Cu load stableing amount at nearly of 99%. 3%, and MnOx loading amount of 5%) were prepared. Figure 4 shows that and catalytic performance of the SCR catalyst in a perverse way [35]. NO conversion is highest up to 96.49% when the calcination temperature is 350 °C, and the N2 selectivity remains stable at nearly 99%. The SCR catalyst could not give full play to the function because MnOx and Cu might not integrate adequately due to the calcination temperature of 300 °C . When the calcination temperature reaches 400 °C , the temperature may lead to the inside pore channels of SCR catalyst to be collapsed or closed due to sintering, which reduces the number of active sites on the surface of the SCR catalyst. The high calcination temperature may reduce the dispersion of active sites [34]. Huang et al. found that the suitable calcination temperature could improve the attrition strength of the SCR catalyst, while the continuous increase of the calcination temperature could change the mechanical property and catalytic performance of the SCR catalyst in a perverse way [35]. (a) (b)

FigureFigure 4. The 4. The inﬂuence influence ofof calcination calcination temperature temperature on catalytic on catalytic activity. activity. (a) NO conversion (a) NO; (b conversion;) N2

(b)N2 selectivity. selectivity.

The2.5. The SCR Influence catalyst of couldReaction not Temperature give full play on C toatalytic the function Activity because MnOx and Cu might not integrate adequatelyFrom due the to above the calcination experiments, temperature NO conversion of 300 reaches◦C. Whena maximum the calcination at a calcination temperature temperature reaches 400 ◦C,of 350the temperature°C , MnOx load maying leadamount to the of 5%, inside and pore Cu loadchannelsing amount of SCR of catalyst 3%. Then to bethe collapsed MnOx-Cu/AC or closed due tocatalyst sintering,s were which prepare reducesd under the number that condition of active. The sites influence on the surface of reaction of the temperature SCR catalyst. on The NO high calcinationconversion temperature was investigated may reduce at 100 °C, the 150 dispersion °C , 200 °C of, 250 active °C , 300 sites °C ,[ 35034]. °C, Huang respectively et al.. found that the suitable calcinationFigure 5 present temperatures that the could NO conversionimprove the of attrition MnOx-Cu strength catalyst of increases the SCR firstly catalyst, and while then the (a) (b) continuousdecreases increase with the of increas the calcinatione in reaction temperature temperature could, and change the N2 theselectivity mechanical remains property stable at and nearly catalytic 99%. Especially, the NO conversion reaches 96.82% when the reaction temperature is 200 °C. performanceFigure of 4. the The SCR influence catalyst of calcination in a perverse temperature way [35 on]. catalytic activity. (a) NO conversion; (b) N2 According to Ren’s research, the reason why NO conversion increases rapidly with the increase in selectivity. 2.5. Thetemperature Influence is of that Reaction the consumption Temperature of on NH Catalytic3 is mainly Activity for the NO reduction reaction. Therefore, NO 2.5.conversion The Influence increases of Reaction rapidly Temperature in the temperature on Catalytic range Activity of 100 to 200 °C. After that, NO reaction will Frombe restricted the above for the experiments, occurrence NOof NH conversion3 oxidation reaches reaction a if maximum the reaction at temper a calcinationature continue temperatures to of 350 ◦C,rise,From MnO that xtheisloading because above amounttheexperiments, NH3 ofoxidation 5%, NO and reactionconversion Cu loading will reaches easily amount occur a maximum of at 3%. a high Then atreaction a the calcination MnO temperaturex-Cu temperature/AC [36]. catalysts wereof 350prepared °C , MnO underx load thating condition. amount of The 5%, influenceand Cu load of reactioning amount temperature of 3%. Then on NOthe conversion MnOx-Cu/AC was catalysts were prepared under that condition. The influence of reaction temperature on NO investigated at 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C, 300 ◦C, 350 ◦C, respectively. conversion was investigated at 100 °C, 150 °C , 200 °C , 250 °C , 300 °C , 350 °C, respectively. Figure5 presents that the NO conversion of MnO x-Cu catalyst increases firstly and then decreases Figure 5 presents that the NO conversion of MnOx-Cu catalyst increases firstly and then with the increase in reaction temperature, and the N2 selectivity remains stable at nearly 99%. decreases with the increase in reaction temperature, and the N2 selectivity remains stable at nearly Especially, the NO conversion reaches 96.82% when the reaction temperature is 200 ◦C. According 99%. Especially, the NO conversion reaches 96.82% when the reaction temperature is 200 °C. to Ren’s research, the reason why NO conversion increases rapidly with the increase in temperature According to Ren’s research, the reason why NO conversion increases rapidly with the increase in is that the consumption of NH3 is mainly for the NO reduction reaction. Therefore, NO conversion temperature is that the consumption of NH3 is mainly for the NO reduction reaction. Therefore, NO increases rapidly in the temperature range of 100 to 200 ◦C. After that, NO reaction will be restricted for conversion increases rapidly in the temperature range of 100 to 200 °C. After that, NO reaction will the occurrence of NH3 oxidation reaction if the reaction temperature continues to rise, that is because be restricted for the occurrence of NH3 oxidation reaction if the reaction temperature continues to the NH oxidation reactionFigure will 5. The easily influence occur of reac at ation high temperature reaction on temperature catalytic activity [36. ]. rise, that3 is because the NH3 oxidation reaction will easily occur at a high reaction temperature [36]. 2.6. The Catalytic Stability Test of SCR Catalyst

FigureFigure 5. 5.The The inﬂuenceinfluence ofof reactionreaction temperature on on catalytic catalytic activity activity..

2.6. The Catalytic Stability Test of SCR Catalyst

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Catalysts 2020, 10, x FOR PEER REVIEW 8 of 17 2.6.Catalysts The 20 Catalytic20, 10, x FOR Stability PEER REVIEW Test of SCR Catalyst 8 of 17 Long-time stability is a crucial indicator to evaluate the catalytic performance of SCR catalysts in practicalLong-timeLong-time industrial stabilitystability application. isis aa crucialcrucial indicator Asindicator shown to to inevaluate evaluate Figure the 6,the catalytic a catalytic 25 h stability performance performance test was of of SCR carried SCR catalysts catalysts out toin practical industrial application. As shown in Figure6, a 25 h stability test was carried out to investigate investigatein practical catalytic industrial stability application. at 200 °C. As T shownhe calcination in Figure temperature 6, a 25 h of stability the MnO testx- Cu/AC was carried catalyst out wa tos catalytic stability at 200 ◦C. The calcination temperature of the MnOx-Cu/AC catalyst was 350 ◦C. 350investigate °C. Throughout catalytic stabilitythe 25 h atcontinuous 200 °C. The procedure, calcination the temperature deNOx activity of the of MnO MnOx-Cu/ACx-Cu/AC catalyst catalyst wa iss Throughout the 25 h continuous procedure, the deNOx activity of MnOx-Cu/AC catalyst is rather rather350 °C. stable Throughout with a slightthe 25 fluctuation h continuous around procedure, ±1% over the time,deNO thex activity NO conversion of MnOx -remainsCu/AC catalyst at nearly is stable with a slight ﬂuctuation around 1% over time, the NO conversion remains at nearly 96%, and 96%,rather and stable the selectiwith avity slight of Nfluctuation2 is almost ± around99%. There ±1% is over almost time, no tdeactivationhe NO conversion occurr ingremains during at 25nearly h at the selectivity of N2 is almost 99%. There is almost no deactivation occurring during 25 h at 200 ◦C, 20096%, °C, and which the selecti furthervity demonstrates of N2 is almost that 99%. the MnOTherex- isCu/AC almost catalyst no deactivation behaves with occurr excellenting during stability. 25 h at which further demonstrates that the MnOx-Cu/AC catalyst behaves with excellent stability. 200 °C, which further demonstrates that the MnOx-Cu/AC catalyst behaves with excellent stability.

Figure 6. The catalytic stability of the SCR catalyst with the loading amount of 5% MnOx and 3% Cu.

Figure 6.6. The catalytic stability of the SCR catalyst with the loadingloading amountamount ofof 5%5% MnOMnOxx and 3% Cu. 2.7. The Catalytic Activity Analysis of the SCR Catalyst 2.7. The C Catalyticatalytic A Activityctivity AnalysisAnalysis of the SCR CatalystCatalyst In order to explore the relationship between the structure and performance in detail, microstructureIn order order to exploretoand explore micromorphology the relationship the relationship betweenanalysis between thewere structure performed. the and structur performance Thee XRDand patternperformance in detail, of microstructureSCR incatalysts detail, withandmicrostructure micromorphologya loading amount and micromorphology of analysis 3%, 5% were MnO performed. xanalysis and 3%, were5% The Cu performed. XRDis shown pattern in The F ofigure XRD SCR 7, pattern catalysts which presentsof with SCR a c loadingthatatalysts the obviousamountwith a loading of peaks 3%, 5%amountare MnO between ofx and 3%, 20°3%,5% MnO and5% Cu 50°.x and is shown Its 3%, main 5% in Cu Figurephase is shown 7 is, which graphite in F presentsigure microcrystal 7, which that the presents obvious with different that peaks the structures.areobvious between peaks The 20◦ MnOareand between 50x is◦. not Its main found 20° andphase in the 50°. is XRD graphite Its mainpattern, microcrystal phase which is graphiteis with consistent di ﬀ microcrystalerent with structures. Jiang’s with report The different MnO andx indicatesisstructures. not found that The in the theMnO active XRDx is component pattern,not found which inis presentthe is consistentXRD in pattern, an amorphous with which Jiang’s isstate. reportconsistent Moreover, and indicateswith it Jiang’s is very that reportlikely the active thatand n+ Mncomponentindicatesn+ ions that areis presentthe incorporated active in ancomponent amorphous into the is state.present copper Moreover, in oxide an amorphous latticeit is very to likely state. form that Moreover, a solidMn soions itlution is are very incorporated during likely that the calcinationsintoMnn+ the ions copper are [37]. incorporated oxide The latticeamorphous to into form MnO the a solid copperx may solution be oxide one during lattice of the tothekey form calcinations factors a solid for the [37 so ].lution excellent The amorphous during catalytic the activityMnOcalcinationsx may in deNO be [37]. onex Theofperformance the amorphous key factors according MnOfor thex may excellentto Tang be ’ ones catalytic research of the activity [38]. key factorsCuO in deNO shows forx performance the a peak excellent at 37.2°, according catalytic and Wangtoactivity Tang’s et in researchal. deNO foundx [performance 38 that]. CuO Cu wasshows according present a peak nearly atto 37.2Tang ◦ e,xclusively’ ands research Wang in et[38]. al.the foundCuO form shows that of mononuclear Cu a waspeak present at 37.2°, ions nearly andand oligomericexclusivelyWang et al. species in found the form [39]. that of Cu mononuclear was present ions nearly and oligomeric exclusively species in the [39 form]. of mononuclear ions and oligomeric species [39].

Figure 7. TheThe XRD XRD pattern pattern of of SCR SCR catalysts catalysts with with the the loading loading amount amount of 3%, of 3%, 5% 5%MnO MnOx andx and 3%, 3%,5% Cu, 5%

respectively.Cu,Figure respectively. 7. The XRD pattern of SCR catalysts with the loading amount of 3%, 5% MnOx and 3%, 5% Cu, respectively. XPS analysis analysis was was carried carried out out to determine to determine the atomic the atomic concentration concentration and chemical and chemical states of states diﬀerent of differentspeciesXPS presented species analysis presented on was catalysts carried on catalysts surface. out to determinesurface. Figure 8F igureshows the atomic8 shows the oxidation concentration the oxidation state ofstate and Mn chemicalof species Mn species and states theand of thebindingdifferent binding energy species energy peaks presented peaks of Mn of on 2p Mn catalysts for 2p MnO for surface.x MnO/Cu-ACx/Cu F catalyst.igure-AC 8catalyst. shows The Mn theThe 2P oxidation 3Mn/2 peak 2P3/2 ofstate peak 5% of MnOof Mn 5%x species-3% MnO Cux /-andAC3% Cu/ACcatalystthe binding catalyst can beenergy separated can peaks be separated into of Mn three 2p into peaks, for threeMnO i.e., xpeaks,/Cu 642.3-AC eV, i.e. catalyst. 643.4, 642.3 eV The eV, and 643.4Mn 644.7 2P eV eV,3/2 peak and which 644.7of correspond5% eV,MnO whichx-3% to 2+ 3+ 4+ correspondMnCu/ACspecies, catalyst to MnMn can2+ species, species be separated andMn3+ Mn species intospecies, three and Mnpeaks, respectively.4+ species, i.e., 642.3 Therespectively. active eV, 643.4 components The eV active and are 644.7components mainly eV, which in are the mainlyformcorrespond of in MnO, the to form Mn 2+2 O ofspecies,3 MnO,and MnO Mn3+22O .species3 The and Mn MnO and valence 2Mn. The4+ species, bindingMn valence respectively. energy binding distribution energyThe active distribution of SCRcomponents catalysts of SCR are as 3+ catalystsmainly in as the shown form in of T MnO,able 2, Mn and2O the3 and valence MnO 2distribution. The Mn valence of Mn binding oxides are energy dominated distribution by Mn of SCRand Mncatalysts4+. The as MnO shownx catalytic in Table activity 2, and has the been valence ascribed distribution to the potential of Mn oxides of Mn are to formdominated several by oxides Mn3+ andand 3+ 4+ toMn provide4+. The MnOoxygenx catalytic selectively activity from has its crystallinebeen ascribed lattice to the [40]. potential The proportion of Mn to of form Mn several and Mn oxides reaches and to provide oxygen selectively from its crystalline lattice [40]. The proportion of Mn3+ and Mn4+ reaches

Catalysts 2020, 10, 135 9 of 17 shown in Table2, and the valence distribution of Mn oxides are dominated by Mn 3+ and Mn4+. The MnOCatalystsx catalytic 2020, 10, x activityFOR PEER has REVIEW been ascribed to the potential of Mn to form several oxides and to provide9 of 17 oxygen selectively from its crystalline lattice [40]. The proportion of Mn3+ and Mn4+ reaches maximum whenmaximum the loadingwhen the amount loading of amount Cu is 3%, of whichCu is 3%, indicates which that indicates the reducibility that the reducibility of the SCR of catalyst the SCR is 3+ 3+ relativelycatalyst is strong.relatively Xin strong. et al. suggested Xin et al. thatsuggested Mn could that Mn activate could NH activate3 into NH NH2 intermediate3 into NH2 intermediate that would 4+ readilythat would react readily with NO react to producewith NO N to2 [produce41]. Yao N et2 al.[41]. conﬁrmed Yao et al. that confirmed an ample that amount an ample of Mn amountions of is 4+ oneMn of ions the is key one factors of the enhancingkey factors NHenhancing3 oxidation NH3 in oxidation the NH3 in-SCR the processNH3-SCR due process to the due strong to the oxidizing strong stateoxidizing of Mn state (IV) of [42 Mn]. According (IV) [42]. toAccording Lee’s research, to Lee the’s research, existence the of variousexistence Mn of valences various increasedMn valences the electronincreased transfer the electron during transfer the NO conversion during the processes, NO conversion which could processes, promote which the denitration could promote reaction the fordenitration NH3-SCR reaction [43]. Fang for NH et al.3-SCR found [43]. that Fang there et al. was found a strong that interactionthere was a between strong interaction manganese between oxides andmangane copperse oxides oxides, and and copper the valence oxides, distribution and the valence of Mn distribution was aﬀected of by Mn the was loading affected amount by the of loading Cu [44]. Furthermore,amount of Cu a[44]. synergistic Furthermore, interaction a synergistic between interaction Mn and Cu between might exist Mn accordingand Cu might to the exist redox according couple: Mnto the3+ +redoxCu2 +couple:Mn 4Mn+ +3+Cu + Cu+. 2+ → Mn4+ + Cu+. →

(a) Full spectrum (b) Peak spectrum

Figure 8. The XPS pattern of SCR catalysts with the loading amount of 3%, 5% MnO x andand 3%, 3%, 5% 5% Cu, Cu, respectively. (a)) Full spectrum;spectrum; ((b)) PeakPeak spectrum.spectrum.

Table 2. The Mn valence binding energy distribution of SCR catalysts. Table 2. The Mn valence binding energy distribution of SCR catalysts. 2+ 3+ 4+ 3+ 4+ x+ SCR CatalystsSCR Catalysts MnMn(eV)2+ (eV) Mn Mn3+ (eV)(eV) Mn Mn4+ (eV)(eV) N(Mn N(Mn3+ + Mn4++)/n(MnMn )/x+n(Mn) ) 5% MnO5%x -3%MnO Cux-3% Cu 642.3642.3 643.4 643.4 644.7 644.7 0.86 0.86 5% MnO -5% Cu 642.3 643.4 644.8 0.85 5%x MnOx-5% Cu 642.3 643.4 644.8 0.85 3% MnOx-5% Cu 642.1 643.2 644.8 0.82 3% MnOx-5% Cu 642.1 643.2 644.8 0.82 3% MnOx-3% Cu 640.2 641.8 643.5 0.78 3% MnOx-3% Cu 640.2 641.8 643.5 0.78

The distribution of acidic sites were characterized by NH -TPD with the loading amount of 3%, The distribution of acidic sites were characterized by NH3-TPD with the loading amount of 3%, 5% MnO and 3%, 5% Cu, respectively. The calcination temperature was 350 C. As is shown in 5% MnOxx and 3%, 5% Cu, respectively. The calcination temperature was 350 °C. As◦ is shown in Figure Figure9a, the9 weakera, the weaker desorption desorption peak appears peak appears at 68 °C, at 68 which◦C, whichcorresponds corresponds to the toweak the weakacidic acidic sites of sites SCR of SCRcatalysts. catalysts. In addition, In addition, the thestronger stronger desorption desorption peak peak appears appears at at 142 142 °C,◦C, which which corresponds corresponds to to the strong acidic sites. The peak area and intensity of the catalyst with a loading amount of 5% MnO -3% strong acidic sites. The peak area and intensity of the catalyst with a loading amount of 5% MnOx-3% Cu are are higher higher than than others, others, which which indicates indicates that that there there are are more more acidic acidic sites sites in the in catalyst the catalyst surface. surface. The The optimum combination ratio makes the active component of MnO dispersed uniformly, and it is optimum combination ratio makes the active component of MnOx dispersedx uniformly, and it is conducive to to improvingimproving the the activity activity of the of the SCR SCR catalyst catalyst [45]. [45]. Many Many research research showed showed that surface that surface acidity playedacidity a played critical a role critical in SCR role reaction in SCR [46 reaction,47]. Therefore, [46,47]. the Therefore, increased the acidic increased sites might acidic represent sites might one of the most important reasons for the higher catalytic activity of the MnO -Cu/AC catalyst. Yao et al. represent one of the most important reasons for the higher catalytic activityx of the MnOx-Cu/AC foundcatalyst. that Yao the et synergistic al. found that effects the between synergistic Mn effects and Cu between ions could Mn further and Cu enhance ions could the low-temperaturefurther enhance performancesthe low-temperature via modified performances active sites via[ 48 modified]. Moreover, active Shu sites et al.[48]. found Moreover, that surface Shu et acidity al. found played that a criticalsurface roleacidity in the played SCR reactiona critical [ 49role]. Consequently,in the SCR reaction the increased [49]. Consequently acidic sites, might the increased represent acidic one of sites the most important reasons for the higher catalytic activity of the MnO -Cu/AC catalyst. might represent one of the most important reasons for the higherx catalytic activity of the MnOx- Cu/ACThe catalyst. redox property is an important index to evaluate the performance of catalysts, and H2-TPR technology provides an effective way in measuring the reducibility of the SCR catalyst. The sample The redox property is an important index to evaluate the performance of catalysts, and H2-TPR amounttechnology of SCRprovides catalyst an effective was 0.10 way g and in themeasuring concentration the reducibility of H2 was of 3%the inSCR the catalyst. process The of analysis. sample amount of SCR catalyst was 0.10 g and the concentration of H2 was 3% in the process of analysis. Figure 9b presents the TPR profiles of the MnOx-Cu/AC catalyst, and the H2 consumption amounts in the temperature range of 100–220 °C are summarized in Table 3. As can be seen from Figure 9b, one reduction peak is observed for MnOx-Cu/AC catalyst, which can be attributed to the reduction

Catalysts 2020, 10, 135 10 of 17

Figure9b presents the TPR profiles of the MnO x-Cu/AC catalyst, and the H2 consumption amounts in Catalyststhe temperature 2020, 10, x FOR range PEER of REVIEW 100–220 ◦C are summarized in Table3. As can be seen from Figure9b,10 one of 17 reduction peak is observed for MnOx-Cu/AC catalyst, which can be attributed to the reduction of MnOx, ofand MnO thex peak, and the area peak of 5% area of of the 5% MnO of thex-3% MnO Cux-/AC3% Cu/AC catalyst catalyst is higher is higher than others. than others. The reduction The reduction peak peakappears appears around around 200 C, 200 and °C, the and reduction the reduction of MnO of MnOMn2 →O Mn2MnOO3 → mayMnO occur may atoccur this processat this process [50,51]. ◦ 2 → 2 3 → [50,51The TPR]. The profiles TPR profiles of the MnO of thex-Cu MnO/ACx-Cu/AC catalyst catalyst are able are to able deconvoluted to deconvoluted into three into Gaussianthree Gaussian peaks peaks(dotted (dotte curves).d curves). The lowest The lowest temperature temperature peak (150–177peak (150◦C)–177 is attributed°C) is attributed to the reductionto the reduction of highly of highlydispersed dispersed MnOx surface MnOx surface species, species, the higher the temperaturehigher temperature peak (194–208 peak (194◦C)– corresponds208 °C) corresponds to the bulk-like to the bulkMnO-likex, and MnO thex, highest and the temperature highest temperature peak (200–212 peak ◦ (200C) appearing–212 °C) appearing in the MnO inx -Cuthe/ ACMnO catalystx-Cu/AC is catalystrelated tois therelated MnO tox strongly the MnO interactedx strongly with interacted CuO [52 with,53]. CuO It could [52,53 be]. concluded It could be that concluded H2 consumption that H2 consumptionamounts increase amounts with increasethe increase with of the the increase Mn and ofCu the loading Mn and amount, Cu loading and the amount, H2 consumption and the H of2 consumption5% of the MnO ofx -3%5% of Cu the/AC MnO catalystx-3% isCu/AC obviously catalys highert is obviously than that ofhigher others, than which that suggestsof others, that which the suggestsreducibility that is the relatively reducibility strong is relatively and verifies strong that and 5% verifies MnOx -3%that Cu5% MnO is thex- optimal3% Cu is ratio. the optimal Dong ratio. et al. Dongfound et that al. Cufound and that Mn Cu elements and Mn might elements form might the copper form galaxitethe copper of CuMngalaxite2O 4of, andCuMn the2O interaction4, and the interactionbetween MnO betweenx and MnO Cu improvedx and Cu improved the activity the of activity the catalyst of the ecatalystffectively effectively and was and beneficial was beneficial for NO forconversion NO conversion [54]. [54].

(a) NH3-TPD (b) H2-TPR

Figure 9. TheThe acid acid and and redox redox properties properties of the SCR catalysts with the loading amount of 3%, 5% MnOxx and 3%, 5% Cu, respectively. (a) NH3--TPD;TPD; ( (b))H H2--TPR.TPR.

Table 3. The H2 consumption of the SCR catalysts. Table 3. The H2 consumption of the SCR catalysts.

SCR Catalysts Peak Area H2 Consumption (µmol/g) SCR Catalysts Peak Area H2 Consumption (μmol/g) 5% MnO -3% Cu 7197 828 5%x MnOx-3% Cu 7197 828 5% MnOx-5% Cu 6883 791 5% MnOx-5% Cu 6883 791 3% MnOx-5% Cu 6790 780 3% MnOx-5% Cu 6790 780 3% MnOx-3% Cu 6612 759 3% MnOx-3% Cu 6612 759

2.8. The Reaction Mechanism of NH3-SCR 2.8. The Reaction Mechanism of NH3-SCR According to the above research, the optimal component ratio of the SCR catalyst is loaded 5% According to the above research, the optimal component ratio of the SCR catalyst is loaded 5% MnOx and 3% Cu on AC. The large specific surface area of AC is beneficial to the uniform loading of MnOx and 3% Cu on AC. The large specific surface area of AC is beneficial to the uniform loading of active component Mn. NH3 and NO simulate the catalytic reduction in this work. As for the reaction active component Mn. NH3 and NO simulate the catalytic reduction in this work. As for the reaction mechanism of SCR, more and more scholars think that the SCR catalytic reaction should comply with mechanism of SCR, more and more scholars think that the SCR catalytic reaction should comply with the Eley-Rideal (E-R) mechanism. Another view with the Langmuir-Hinshelwood (L-H) mechanism the Eley-Rideal (E-R) mechanism. Another view with the Langmuir-Hinshelwood (L-H) mechanism is now accepted for the NH3-SCR process reaction mechanism. For the E-R mechanism, the redox is now accepted for the NH3-SCR process reaction mechanism. For the E-R mechanism, the redox reaction takes place between active NH3 and gaseous NO. The adsorbed NH3 is activated at the weak reaction takes place between active NH3 and gaseous NO. The adsorbed NH3 is activated at the weak acid site on the surface of the catalyst, with the adsorption and activation of NH3 as the critical step. acid site on the surface of the catalyst, with the adsorption and activation of NH3 as the critical step. For the L-H mechanism, the NO of the gas phase first interacts with O2 and subsequently the NOx is For the L-H mechanism, the NO of the gas phase first interacts with O2 and subsequently the NOx is adsorbed onto the surface of the catalyst where it is oxidized to nitrates and nitrites in the role of lattice adsorbed onto the surface of the catalyst where it is oxidized to nitrates and nitrites in the role of oxygen [55–59]. The NO removal mechanism of the MnOx-Cu/AC catalyst can be seen in Figure 10. In lattice oxygen [55–59]. The NO removal mechanism of the MnOx-Cu/AC catalyst can be seen in Figure both E-R and L-H catalytic mechanisms, acidic sites are beneficial to the adsorption and activation of 10. In both E-R and L-H catalytic mechanisms, acidic sites are beneficial to the adsorption and3+ NH3 and NOx in a low-temperature NH3-SCR process [60–62]. Kijlstra et al. suggested that the Mn activation of NH3 and NOx in a low-temperature NH3-SCR process [60–62]. Kijlstra et al. suggested that the Mn3+ location on the surface of MnOx/Al2O3 was the Lewis acid center [63]. On the MnOx-Cu catalysts, a mechanism similar with that on the MnOx/Al2O3 were pointed out by researchers. Most researchers believe that NH3 is adsorbed to the Lewis acid center and intermediates like NH2* or adsorbed NH3 formed, then they react with NO through the E-R mechanism getting N2 and H2O

Catalysts 2020, 10, 135 11 of 17

location on the surface of MnOx/Al2O3 was the Lewis acid center [63]. On the MnOx-Cu catalysts, a mechanism similar with that on the MnOx/Al2O3 were pointed out by researchers. Most researchers believeCatalysts that 20 NH20, 103 ,is x FOR adsorbed PEER REVIEW to the Lewis acid center and intermediates like NH2* or adsorbed11 of 17 NH3 formed, then they react with NO through the E-R mechanism getting N2 and H2O[64,65]. Yang et al. found[64,65 that]. chemisorption Yang et al. found mechanism that chemisorption was responsible mechanism for the was adsorption responsible of reactants for the adsorption through density of reactants through density functional theory calculations and NH2* produced from NH3 functional theory calculations and NH * produced from NH dehydrogenation was identiﬁed as a dehydrogenation was identified as a 2key reactive intermediate3 in the selective catalytic reduction key reactive intermediate in the selective catalytic reduction activity of NO with NH over CuMn O activity of NO with NH3 over CuMn2O4 spinel [66]. For the catalytic process, the optimal3 reaction 2 4 spinelpathway [66]. For of the N2 catalyticformation process, in NH3- theSCR optimal of NO is reaction a two-step pathway process: of N(1)2 formation NH2 formation in NH from3-SCR NH of3 NO is a two-stepdehydrogenation process: (NH (1) NH3* + 2* →formation NH2* + H*), from and NH (2)3 thedehydrogenation reaction between (NH NH23 *and+ * NO NH(NH22** + NO*H*), → and (2) → the reactionN2* + H2O*). between NH and NO (NH * + NO* N * + H O*). 2 2 → 2 2

FigureFigure 10. 10.NO NO removal removal mechanism mechanism of of the the MnO MnOx-Cu/ACx-Cu/AC catalyst. catalyst.

3. Materials3. Materials and and Methods Methods

3.1. Catalyst3.1. Catalyst Preparation Preparation AC wasAC wa useds us ased theas the support support to to prepare prepare thethe low-temperaturelow-temperature SCR SCR catalysts catalysts in this in thisexperiment experiment,, whichwhich was was purchased purchased from from Sinopharm Sinopharm Chemical Chemical Reagent Ltd Ltd Co Co.. Twenty Twenty mesh mesh beforehand beforehand and and smallersmaller AC particlesAC particles were were selected selected in in order order toto ensureensure the the uniform uniform particle particle size size of AC of. AC. The Theoriginal original AC containedAC contain manyed many impurities, impurities, such such as as amorphous amorphous carbon carbon and and Fe, Fe, Ni Ni and and Co, Co, etc. etc. Therefore, Therefore, it wa its was necessarynecessary to purify to purify AC AC to avoidto avoid the the influence influence ofof these impurities impurities on on catalyst catalyst preparation preparation.. Firstly, Firstly, AC AC particles were added to different concentrations of nitric acid in turn and stirred for 2 h. Next, these particles were added to different concentrations of nitric acid in turn and stirred for 2 h. Next, these activated carbon particles experienced dilution, filtering, deionized water flushing to pH = 6–7 in activated carbon particles experienced dilution, filtering, deionized water flushing to pH = 6–7 in turn. Then, 110 °C drying in electrothermal blowing for 12 h. The AC was supported with MnOx and turn.Cu Then, using 110 impregnation◦C drying inmethods, electrothermal where Mn blowing and Cu fornitrates 12 h. were The dissolved AC was in supported deionized with water, MnO andx and Cu usinga certain impregnation amount of the methods, AC was where added. Mn The and mixture Cu nitrates of metal were nitrates dissolved and carbon in deionized was heated water, and and a certaincontinuously amount stirred of the for AC 5 h was after added. impregnated The mixturewith a rotary of metal evaporation nitrates instrument and carbon for 2 was h, and heated then and continuouslythese particles stirred were for dried 5 h after respectively impregnated in 110 °C with for 5 a h rotary and inevaporation 50 °C for 12 h. instrument After drying, for the 2 sample h, and then thesewas particles calcined were in a dried certain respectively temperature in under 110 ◦ CN2 for atmosphere 5 h and in for 50 2◦ h.C Finally, for 12 h. the After sample drying, was cooled the sample down to room temperature with protection of N2, and the MnOx-Cu/AC catalyst was packed into a was calcined in a certain temperature under N2 atmosphere for 2 h. Finally, the sample was cooled fiedbed stainless steel micro-reactor for the performance evaluation of SCR for NO conversion. down to room temperature with protection of N2, and the MnOx-Cu/AC catalyst was packed into a fiedbed stainless steel micro-reactor for the performance evaluation of SCR for NO conversion. 3.2. Catalyst Characterization

Catalysts 2020, 10, 135 12 of 17

3.2. Catalyst Characterization X-ray diffraction (XRD) measurement was performed by an XRD-7000 S system (Shimadzu 1 Corporation, Japan). The 2θ scans covered the range from 10◦ to 80◦ at a 5◦ min− scan rate and a step of 0.02◦. Cu Kα radiation was employed, and the X-ray tube was operated at 40 kV and 40 mA. Catalysts 2020, 10, x FOR PEER REVIEW 12 of 17 Specific surface area and pore size were calculated by the Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-HalendaX-ray diffraction (XRD) (BJH) measurement method, respectively. was performed The by surface an XRD- atomic7000 S system states (Shimadzu of the catalysts were analyzedCorporation, by Japan). X-ray The photoelectron 2θ scans covered spectroscopy the range from (XPS, 10° AXISto 80° at Ultra a 5° min DLD)−1 scan with rate Al and K αa stepradiation (hv = 1486.6of 0.02°. eV) Cu asΚα the radiation excitation was employed, source at and150 theW. XC1-ray s-binding tube was energy operated of at 284.6 40 kV eV and was 40 taken mA. as a Specific surface area and pore size were calculated by the Brunauer-Emmett-Teller (BET) method and reference. The hydrogen temperature programmed reduction (H2-TPR) experiments of the prepared the Barrett-Joyner-Halenda (BJH) method, respectively. The surface atomic states of the catalysts catalysts were performed on a Tianjin XQ TP5080 auto-adsorption apparatus. Prior to the H2-TPR were analyzed by X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD) with Al Κα radiation (hv experiment, 0.10 g of the catalyst was pretreated with Ar at a total flow rate of 50 mL/min at 200 C = 1486.6 eV) as the excitation source at 150 W. C1 s-binding energy of 284.6 eV was taken as a reference. ◦ for 1 h, and subsequently cooled to room temperature under an Ar atmosphere. Finally, the reactor The hydrogen temperature programmed reduction (H2-TPR) experiments of the prepared catalysts temperaturewere performed was raised on a to Tianjin 700 ◦ XQC at TP5080 a constant auto-adsorption heating rate apparatus. of 10 ◦C Prior/min to under the H2 a-TPR flow experiment, of H2 (10%) /Ar (50 mL0.10/min). g of Thethe catalyst H2 consumption was pretreated was with monitored Ar at a total by flow a thermal rate of conductivity50 mL/min at 200 detector °C for (TCD)1 h, and during the experiment.subsequently The cooled curve to dataroom oftemperature temperature under programmed an Ar atmosphere. desorption Finally, of the ammonia reactor temperature (NH3-TPD) were recordedwas byraised a thermal to 700 °C conductivity at a constant detectorheating rate (TCD) of 10 in°C/min a Tianjin under XQ a flow TP5080 of H2 auto-adsorption (10%)/Ar (50 mL/min). apparatus. First, theThe H quartz2 consumption tube is was added monitored to the b catalystsy a thermal and conductivity a helium detector gas of 30(TCD) mL during/min wasthe experiment. introduced and The curve data of temperature programmed desorption of ammonia (NH3-TPD) were recorded by a the temperature of the furnace was raised from room temperature to 200 C with a 10 C/min heating thermal conductivity detector (TCD) in a Tianjin XQ TP5080 auto-adsorption◦ apparatus.◦ First, the rate. Afterquartz a tube span is of added 30 min to at the 200 catalysts◦C, the and catalysts a helium were gas cooling of 30 mL/min to 80 ◦ C. was The introduced sample was and adsorbed the for 30temperature min under of pure the furnace ammonia was gas raised conditions from room of temperature 30 mL/min. to The200 °C physically with a 10 °C/min adsorbed heating ammonia to therate. surface After of a thespan catalyst of 30 min was at 200 then °C, purged the catalysts with were 30 mL cooling/min to helium 80 °C. The for sample 1 h. At was last, adsorbed the catalysts were heatedfor 30 min to under 500 ◦ Cpure at aammonia heating gas rate conditions of 5 ◦C /ofmin 30 mL/min. under a The helium physically gas flowadsorbed of 30 ammonia mL/min, to and a temperature-programmedthe surface of the catalyst ammonia was then desorptionpurged with curve30 mL/min was helium recorded. for 1 h. At last, the catalysts were heated to 500 °C at a heating rate of 5 °C/min under a helium gas flow of 30 mL/min, and a 3.3. Catalysttemperature Activity-programmed Evaluation ammonia desorption curve was recorded.

The3.3. deviceCatalyst was Activity used Evaluation for NH3 -SCR reaction activity evaluation process as shown in Figure 11. The catalytic activities of solid samples were evaluated in a fiedbed stainless steel micro-reactor loaded The device was used for NH3-SCR reaction activity evaluation process as shown in Figure 11. with 0.10The gcatalytic of catalyst activities powder of solid in 20 samples mesh were size. evaluated The electrically in a fiedbed heated stainless wire andsteel thermocouple micro-reactor in a fiedbedloaded stainless with 0.10 steel g micro-reactorof catalyst powder was in used 20 mesh to heatsize. The the mixedelectrically gas. heated In order wire to and simulate thermocouple the realistic exhaustin condition,a fiedbed stainless the composition steel micro- ofreactor NH3 ,was NO used and to O 2heatin thethe feedmixed gas gas. were In order 0.12%, to simulate 0.10% and the 3.60%, 1 respectively.realistic Theexhaust total condition flow rate, t washe composition 1.00 L/min of and NH the3, NO gas and hourly O2 in space the feed velocity gas were (GHSV) 0.12%, was 0.10% 5000 h− . and 3.60%, respectively. The total flow rate was 1.00 L/min and the gas hourly space velocity (GHSV) N2 was used as balance gas and NH3 was used as reductant. Catalytic activity was measured in a −1 steadywas flow 5000 mode h . N in2 awas fiedbed used as stainless balance steelgas and micro-reactor. NH3 was used Each as reductant. part of gas Catalytic was controlled activity wa bys mass measured in a steady flow mode in a fiedbed stainless steel micro-reactor. Each part of gas was flow controllers. Prior to each activity test, the catalysts were pretreated at 200 C for 2 h with flowing controlled by mass flow controllers. Prior to each activity test, the catalysts were pretreated◦ at 200 °C air, andfor cooled 2 h with to flowing the initial air, reactionand cooled temperature to the initial around reaction 100 temperature◦C. Thesteady-state around 100 °C deNO. The xsteadyactivity- test was thenstate conducted deNOx activity in the test reactionwas then temperatureconducted in the range reaction from temperature 100 to 350 range◦C at from intervals 100 to of350 50 °C◦ atC. Each temperatureintervals point of 50 was°C . Each kept temperature for 30 min topoint achieve was kept a balanced for 30 min reaction to achieve gas concentrationa balanced reaction and gas all of the data wereconcentration obtained and when all of the the SCR data reactionwere obtained reached when the the steady SCR reaction state. reached the steady state.

FigureFigure 11. Flow 11. Flow chart chart ofNH of NH3-SCR3-SCR reactivity reactivity evaluation. evaluation. 1. 1. N N2 22. 2.O2 O3.2 NO3. NO 4. NH 4.3 NH5. Mass3 5. Mass flow ﬂow controllerscontrollers 6. Gas 6. Gas generating generating system system 7. 7. Fiedbed Fiedbed stainless steel steel micro micro-reactor-reactor 8. The 8. Theflue gas ﬂue analyzer gas analyzer 9. Adsorbent.9. Adsorbent.

Catalysts 2020, 10, 135 13 of 17

An FT-IR gas analyzer (GASM ET DX 4000, Finland) was used for the purpose of analyzing the concentrations of NOx (NO + NO2) and N2O in the outlet gas. The following Equations (1) and (2) were used to estimate the NOx conversion and N2 selectivity:

NO NOxout NO conversion (%) = in − 100% (1) NOin ×

NOin NOxout 2N2Oout N2 selectivity (%) = − − 100% (2) NO NOxout × in − where NOin refers to the concentration of nitric oxide at the inlet of the reactor, NOxout refers to the concentration of NOx (NO + NO2) at the outlet of the reactor, N2Oout refers to the concentration of N2O at the outlet of the reactor. The catalytic activity experiments were done in triplicate. If the experimental data fluctuated greatly, it was discarded. If the fluctuation of the experimental data was less than 10%, it was averaged. Finally, the average was plotted in the figures. The error bar was plotted in the figures, which was used as an index to evaluate the difference between the average and measurement. The following Equations (3) and (4) were the experiment result average and error bar, respectively.

N + N + N N = 1 2 3 (3) a 3 P3 = Ni Na Error bar = i 1| − | (4) 3 where Na represents the average of three experiment results, N1, N2 and N3 represent the result of three experiments, respectively. Ni and Na represent the measured and averaged result, respectively.

4. Conclusions In this paper, an impregnation method was used to prepare the low-temperature SCR catalysts made of MnOx-Cu loaded on activated carbon. Then, the catalysts were packed into the ﬁxed bed reactor to evaluate the selective catalytic reduction of NO performance. The experimental results show that NO conversion is highest at 96.82% and it remains rather stable with a slight ﬂuctuation around 1% over time throughout the 25 h continuous procedure when the calcination temperature is ± 350 ◦C, and MnOx loading amount is 5%, and Cu loading amount is 3%, and reaction temperature is 200 ◦C, which can meet the needs of deNOx at a low temperature and provide reference for industrial production and application. The concentration of N2O as a by-product in the outlet gas is measured. The results show that the concentration of N2O is very low in the outlet gas of the reactor in the SCR reaction, and the selectivity to N2 is almost 99%. Although much progress has been made in this research, many problems need to be solved. As for the development of low-temperature SCR catalysts in the future, it is worthwhile to explore Mn-based catalysts with excellent resistance to sulfur, water and so on. Discovering new doping elements and novel supports may be promising research directions, and monolithic catalysts should be given more consideration from a commercial perspective.

Author Contributions: All authors have contributed in various degrees to the analytical methods used, to the research concept, to the experiment design, to the acquisition of data, or analysis and interpretation of data, to draft the manuscript or to revise it critically for important intellectual content. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Open Foundation of Shaanxi Key Laboratory of Lacustrine Shale Gas Accumulation and Exploitation (under planning), and the Major Science and Technology Projects of Shanxi Province (No.20181102017), and the Fundamental Research Funds for the Central Universities (No.2009QH03). Acknowledgments: In this section you can acknowledge any support given which is not covered by the author contribution or funding sections. This may include administrative and technical support, or donations in kind (e.g., materials used for experiments). Conflicts of Interest: The authors confirm that this article content contains no conflicts of interest. Catalysts 2020, 10, 135 14 of 17

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