Low-Temperature Denitrification Performance of Cu2O/Activated Carbon Catalysts for Selective Catalytic Reduction of NOx by CO

WANG DefuHUANG BangfuLONG HongmingSHI ZheLIU LanpengLI Lu

1 Department of Metallurgical Engineering，Kunming University of Science and Technology，Kunming 650093，China 2 Key Laboratory of Clean Metallurgy of High-Efficiency and Complex Iron Resources of Yunnan Province，Kunming 650093，China 3 Key Laboratory of Metallurgical Emission Reduction &Resources Recycling，Anhui University of Technology，Ministry of Education，Ma’anshan 243002，China

Abstract：To improve the denitrification performance of carbon-based materials for sintering flue gas，we prepared a composite catalyst comprising coconut shell activated carbon (AC) modified by thermal oxidation air. The microstructure，the specific surface area，the pore volume，the crystal structure，and functional groups presented in the prepared Cu2O/AC catalysts were thoroughly characterized. By using scanning electron microscopy (SEM)，nitrogen adsorption/desorption isotherms，F(xiàn)ourier-transform infrared (FTIR) spectroscopy and X-ray diffractometry (XRD)，the effects of Cu2O loading and calcination temperature on Cu2O/AC catalysts were investigated at low temperature (150 ℃). The research shows that Cu on the Cu2O/AC catalyst is in the form of Cu2O with good crystalline performance and is spherical and uniformly dispersed on the AC surface. The loading of Cu2O increases the active sites and the specific surface area of the reaction gas contact，which is conducive to the rapid progress of the carbon monoxide selective catalytic reduction (CO-SCR) reaction. When the loading of Cu2O was 8% and the calcination temperature was 500 ℃，the removal rate of NOx facilitated by the Cu2O/AC catalyst reached 97.9%. These findings provide a theoretical basis for understanding the denitrification of sintering flue gas.

Key words:thermal oxidation；coconut shell activated carbon(AC)；Cu2O/AC；catalyst；carbon monoxide selective catalytic reduction(CO-SCR)；denitrification performance

Introduction

In recent years，NOx pollution in China has become a serious environmental problem.To achieve a sustainable development strategy，China has adopted measures to control NOx emissions，for example，setting very strict emission standards.Hence，flue gas denitrification has become one of the most important issues[1-4].At present，the selective catalytic reduction (SCR) process is the most widely used flue gas denitrification process.Low-temperature SCR technology has a few advantages，i.e.，significant energy saving emission reduction and low operating cost，thus gaining attention from many researchers in recent years.However，the industrialization of this technology still faces problems that include low catalyst activity，poor anti-poisoning performance，and unclear reaction mechanism in the low-temperature regime.Furthermore，the use of toxic and corrosive NH3as a reducing agent is still necessary in the denitrification process，which readily forms ammonium salts with SO2，H2O，and other compounds，in the flue gas causing clogging of the internal pores of the catalyst and a reduction in the denitrification efficiency.Carbon monoxide (CO) is an excellent reducing agent for use in SCR and is presented in the flue gas of the most combustion processes.The CO-SCR denitrification technology can efficiently convert NOx from the flue gas into benign N2while simultaneously removing CO，which has great application prospects[5].

A large number of studies that focus on different types of denitrification catalysts at low temperatures have been reported.Using existing catalysts，the reaction temperature required for low-temperature catalytic removal of NOx is 100-200 ℃，and the main catalyst support is activated carbon (AC).Coconut shell AC is a porous material with a large number of micropores，a high surface area，high specific adsorption capacity，and good thermal and chemical stabilities，making it a strong choice for a variety of different catalytic applications.However，when coconut shell AC itself is used as an adsorbent in CO-SCR，the denitrification efficiency is very low.At present，a better method is to modify the structure of the AC by various acid-base modifications to enhance the adsorption performance and then dope with metal elements to improve the catalyst structure and enhance the electronic exchange.Such doping modifications enhance the activity of the catalyst for desulfurization and denitrification[6-7].

Among the metals used for catalyst modification，Cu in the form of Cu2O is a preferred catalyst active material for NO reactions，especially NO reduction[8-9].The active site at which NO molecule activation occurs is reported to be related to Cu+species[10]，and the Cu2O active sites facilitate redox interactions for reducing and oxidizing reactants[11-12].Park and Kim[13]used Cu-loaded AC to directly remove NO.It has also been found that metal oxide particles on Cu2O/AC act as active centers.Catalyst properties，such as the surface functional groups and oxygen-containing functional groups of the AC，have a direct influence on the chemical adsorption characteristics of SO2and NO[14].

In this study，an equal-volume ultrasonic impregnation method was used to prepare Cu2O/AC catalysts from coconut shell AC modified by thermal oxidation in the air.The physicochemical properties of the prepared Cu2O/AC catalysts were characterized by scanning electron microscopy (SEM)，Brunauer-Emmett-Teller (BET) nitrogen adsorption/desorption isotherms，F(xiàn)ourier-transform infrared (FTIR) spectroscopy，and X-ray diffractometry (XRD).The effects of different calcination temperatures and Cu2O loadings on the Cu2O/AC catalyst structure and denitrification performance were analyzed，and the mechanism for the observed changes in performance was elucidated in order to provide a reference for optimizing the preparation conditions of the catalyst.

1 Methodology

1.1 Materials

Analytically pure copper nitrate (Cu(NO3)2·3H2O) was obtained from Komiou Chemical Reagent Co.，Ltd.，Tianjin，China，and coconut shell AC(particle size of 20-40 mesh) was from Henan Gongyi Blue Sky Water Purification Technology Co.，Ltd.，Zhengzhou，China.Distilled water was used in all experiments.The gases required for this experiment (CO，NO，O2，and N2) were purchased from Guangruida Gas Co.，Ltd.，Kunming，China.

1.2 Preparation of Cu2O/AC

The coconut shell AC was first activated in an air atmosphere at 350 ℃ to obtain AC after oxidation.AC was added to solutions of Cu(NO3)2with Cu∶AC mass fractions of 2%∶1，4%∶1，6%∶1，8%∶1 and 10%∶1,and then subjected to ultrasonic agitation for 1 h in an ultrasonic water bath maintained at a temperature of 60 ℃.The processed mixtures were then transferred to a clean drum and air-dried in an oven at a constant temperature of 110 ℃ for 12 h.After drying，the obtained powders were calcined in the N2atmosphere at 350，400，450，500，and 550 ℃ each for 4 h until the nitrate was completely decomposed to prepare Cu2O/AC catalysts.

1.3 Catalyst characterization

The surface microstructural changes of Cu2O/AC after Cu2O loading were observed by SEM (Tescan VEGAS SBH Czech Tesken，USA).Nitrogen adsorption/desorption isotherms at 77 K (QDS-evo，Conta GBANG-8，USA) were analyzed using BET theory to determine the specific surface area，the pore volume，and the average pore diameter of Cu2O/AC catalysts，and the micropore volume was calculated by the Dubinin-Radushkevich method.Prior to N2adsorption，each sample was degassed under vacuum at 200 ℃ for 4 h.The changes in surface functional groups after Cu2O loading were investigated using FTIR spectroscopy (Nicolet iS 10，American Thermo Fisher Scientific，USA) in the wavenumber range of 4 000-400 cm-1.The crystal phase of the supported Cu2O was determined by XRD (TTR18Kw copper target，Nippon Science，Japan).

1.4 Catalytic activity tests

Catalytic activity evaluation was carried out in a fixed-bed reactor.The experimental procedure is shown in Fig.1.The denitrification activity was measured at 150 ℃ using 10 g Cu2O/AC.A thermocouple located in the tubular reactor was used to measure the reaction temperature.The reactor bed was flushed with N2for 1 h at 150 ℃ prior to each measurement.The furnace temperature was adjusted to the desired reaction temperature before a simulated flue gas was introduced into the reactor.The simulated flue gas comprised NO at a flow rate of 16 mL/min，CO at a flow rate of 60 mL/min，5% O2，and N2making up a total gas flow rate of 1 300 mL/min.The final tail gas was tested with a testo-340 flue gas analyzer (German Instruments).

The conversion rate of NO is calculated by

whereCinrepresents the NOx concentration at the inlet of the reactor(mL/min)，Coutrepresents the NOx concentration at the outlet of the reactor (mL/min)，andηrepresents the denitration efficiency (%).

2 Results and Discussion

2.1 Morphological analysis

The surface morphology of the AC catalyst before and after loading with Cu2O was characterized by SEM，as shown in Fig.2.

1-N2；2-NO；3-CO；4-O2；5-tube furnace inlet；6-fixed bed reactor；7-furnace temperature control device；8-gas buffer bottle；9-exhaust gas discharge device；10-fume analyzerFig.1 Schematic diagram of catalyst activity test equipment

Fig.2 SEM micrographs of different catalysts：(a) AC；(b) 2% Cu2O/AC；(c) 4% Cu2O/AC；(d) 6% Cu2O/AC；(e) 8% Cu2O/AC；(f) 10% Cu2O/AC

It can also be seen that the surface morphology of the catalyst changes after Cu2O impregnation.The micrograph of the AC before loading [shown in Fig.2(a)] shows a distinct porous structure with few surface voids，mostly pore-shaped structures and smooth pore walls.After loading Cu2O，the main pore-shaped structure of the AC carrier is maintained，but some additional features can be observed.At 2% Cu2O loading，it can be seen from Fig.2(b) that a small number of particles are dispersed in the pores of the AC.As the loading increases in Figs.2(c)-(e)，it can be seen that Cu2O is uniformly dispersed on the surfaces of the AC channels，and the catalyst particles exhibit a spherical shape with uniform size.It greatly increases the active sites and the specific surface area in contact with the reaction gas，and thus promotes a rapid CO-SCR reaction.Excessive Cu2O loading causes Cu2O to agglomerate on the AC surface，forming a larger mass as shown in Fig.2(f)，which will reduce the specific surface area of the catalyst and block the pores and is not conducive to gas adsorption，resulting in reduced denitrification efficiency.

2.2 BET analysis

The specific surface area of the catalyst affects its SCR denitration activity to a certain extent.A larger specific surface area can provide more places for the loading of Cu2O，thereby increasing the contact area of the reaction gases CO and NO and the catalyst surface，and promoting the denitration reaction.Therefore,the specific surface area,the pore volume and the pore diameter of the coconut shell activated carbon (AC0)，AC after air thermal oxidation，and XCu2O/AC catalysts were measured.In this experiment，Cu2O with different mass fractions was loaded on AC and calcined at different temperatures.The specific surface area and the pore volume after calcination at a temperature of 500 ℃ are shown in Table 1.Table 1 also shows that the specific surface area and the pore volume of the catalyst increase with increasing loading，because the addition of the active component of the loaded Cu2O leads to a decrease in the pore size but does not affect the reduction activity.Moreover，new pores will be formed from the original pores to improve the catalytic ability.After the Cu2O loading reaches a certain limit，the original pores will be blocked，resulting in the loss of the specific surface area and the pore volume[15]，which can also be verified by the SEM observations.

Table 1 Specific surface area,pore volume and pore diameter of XCu2O/AC at 500 ℃

2.3 FTIR analysis

The chemical properties of the surface of AC are mainly determined by the type and the number of surface functional groups that are mainly oxygen-containing functional groups such as carboxyl，hydroxyl，carbonyl，and ketone and ethers[16].AC used here was calcined after loading Cu2O.During this process，changes in the type and the number of functional groups occured，and gases such as H2O，CO，and CO2were released.

FTIR is an effective method to characterize the functional groups on the surface of AC.The FTIR diagrams of AC0and air thermal oxidation modified AC are shown in Fig.3(a).The firing at 350，400，450，500，and 550 ℃ yielded five 8% Cu2O/AC FTIR curves as shown in Fig.3(b).It is shown in Fig.3 that the basic types of 8%Cu2O/AC functional groups at different temperatures are the same.The broad peak at about 3 440 cm-1belongs to —COOH，the stretching vibration peak of —OH，and —NH in the chemical adsorption of water[17-18].The absorption peaks at about 1 590，1 385，and 1 110 cm-1correspond to the asymmetric vibration absorption peak of the ester group on the surface of AC，the bending vibration peak of —OH，and the stretching vibration peak of C—O in the C—O—C bond，respectively[19-23].It can be seen from Fig.3(a) that the number of AC functional groups after modification has significantly increased，and the absorption strength is enhanced，indicating that the air has a significant activation effect after modification by thermal oxidation.Figure 3(b) shows that as the temperature increases，the —OH stretching vibration absorption peak in the carboxyl group at about 3 440 cm-1and the adsorbed water in the chemical are in an enhanced state，and the asymmetric vibration absorption peak of the lactone group at about 1 630 cm-1becomes more pronounced.With the increase of the calcination temperature，the intensities of absorption peaks such as carboxyl groups and lactone groups on the AC surface continue to increase，providing more adsorption sites for the denitrification reaction.

Fig.3 FTIR diagrams of different catalysts：(a) AC0 and AC；(b) 8% Cu2O/AC at different temperatures

2.4 XRD analysis

The XRD patterns of AC0and air thermal oxidation modified AC are shown in Fig.4(a).The XRD patterns of Cu2O/AC catalysts obtained by impregnating and baking at different concentrations of Cu(NO3)2at 500 ℃ are shown in Fig.4(b).It can be seen from Fig.4(a) that the XRD diffraction peaks of all catalysts are broad and diffuse，because C in activated carbon mainly exists in an amorphous form.There is almost no obvious diffraction peak in 2% Cu2O/AC，indicating that the loading of Cu is too small and it is highly dispersed on the surface of AC.With the increase of the load，it can be seen that Cu2O/AC with different loads has obvious characteristic diffraction peaks of Cu2O at 2θof 43.2°，50.4°，61.3°，and 74.1°，corresponding to (200，211，220)，and (311) crystal planes.It shows that Cu on the Cu2O/AC catalyst is in the form of Cu2O and the crystallization performance is better.Studies have shown that the interaction of Cu(NO3)2metal precursors and AC surface functional groups makes Cu+easily migrate into the AC pores[24]，thereby slowing the aggregation and growth of Cu2O on the AC surface and improving the dispersion of Cu2O on AC.

Fig.4 XRD spectra of catalysts：(a) AC0 and AC；(b) different Cu2O/AC catalysts

2.5 Effect of Cu2O loading on denitrification efficiency

The Cu2O loading is an important factor that affects the activity of the catalyst.Therefore，this experiment investigated the relationship between Cu2O loading and CO-SCR low-temperature denitrification performance.

Fig.5 Effect of Cu2O loading on denitrification efficiency

2.6 Effect of calcination temperature on denitrification efficiency

Based on the results shown in Fig.5，the effects of different calcination temperatures on the denitrification efficiency of 8% Cu2O/AC were studied.The calcination temperature has a significant impact on the physical and chemical properties and the catalytic activity of the catalyst.The calcination temperature will affect the specific surface area，the pore volume，the pore diameter distribution，surface functional groups，nitrate decomposition，and the metal oxide distribution of the catalyst[25-26].

Fig.6 Effect of calcination temperature on 8% Cu2O/AC denitrification efficiency

3 Conclusions

(1) Cu on the Cu2O/AC catalyst is in the form of spherical Cu2O，which is uniformly dispersed on the surface of the AC channels and has good crystallinity and uniform size.These characteristics increase the active sites and the specific surface area in contact with the reaction gas to promote rapid CO-SCR.The number of pores and the type and the number of functional groups increase with increasing Cu loading.

(2) When the Cu loading exceeds 8% and the temperature exceeds 500 ℃，Cu2O agglomeration occurs，which results in blockage of the pores，coverage of the active sites，reduction of the specific surface area，and changes in the type and the number of functional groups.

(3) The optimum Cu loading and calcination temperature are 8% and 500 ℃，respectively，which leads to a large specific surface area of Cu2O/AC，a small pore size，a large number and a variety of oxygen-containing functional groups，good crystallinity，and high dispersion.Therefore，Cu2O/AC exhibites the best CO-SCR denitrification activity at low temperature (150 ℃)，and the denitrification efficienty is 97.9%.These findings can provide a theoretical basis for the denitrification of sintering flue gas.