Microwave Irradiation-Selective Catalytic Reduction of NO to N2 by

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Microwave Irradiation-Selective Catalytic Reduction of NO to N2 by Activated Carbon at Low Temperature Kang Peng, Jicheng Zhou, Wentao Xu, Zhimin You, Wei Long, Min Xiang, and Mide Luo Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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Microwave Irradiation-Selective Catalytic Reduction of

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NO to N2 by Activated Carbon at Low Temperature

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Kang Peng, Jicheng Zhou*, Wentao Xu, Zhimin You, Wei Long, Min Xiang, Mide Luo

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Key Laboratory of Green Catalysis and Chemical Reaction Engineering of Hunan Province,

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School of Chemical Engineering, Xiangtan University, Xiangtan 411105, PR China

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*Corresponding Author

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Tel/fax: +86-731 58298173. E-mail: [email protected].

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ABSTRACT

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Activated carbon (AC) supported ceria and mixed metal oxides MeOx-CeO2 (Ni,

10

Mn, Fe) were investigated for microwave irradiation-selective catalytic reduction

11

(MW-SCR) of NO in the microwave catalytic reaction mode (MCRM). NO

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conversion almost reached 100% for 5% (21% NiOx)-CeO2/AC at 300 °C, 5% (21%

13

MeOx)-CeO2/AC (Me=Mn, Fe) and 5% CeO2/AC at 350 °C in the MCRM. The

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temperature of NO removal by SCR using AC as reductant in the MCRM was usually

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lowers 150-200 °C than that of in the conventional reaction mode (CRM). These

16

results suggested that microwave irradiation could efficiently improve the activity of

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NO removal at low temperature and decrease the reaction temperature of NO with AC.

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MW-SCR of NO by AC primarily produced N2 and CO2, and O2 existence had no

19

influence the activity of NO removal by AC in the MCRM. These results show that

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microwave irradiation exhibit the microwave catalytic effect and microwave selective

21

effect. This method of MW-SCR is a viable and promising method for flue-gas such 1

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as NO, N2O and SO2 control.

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KEY WORDS: microwave irradiation; MW-SCR; microwave selective effect;

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flue-gas control;

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1. INTRODUCTION

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NOx are very harmful for ecosystem and humanity, and result in serious

6

environmental problems such as acid rain, ozone layer destruction, and photochemical

7

smog. Therefore, several NOx emission control strategies have been developed, such

8

as direct catalytic decomposition,1,2 selective catalytic reduction (SCR),3-7 and

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selective non-catalytic reduction (SNCR).8,9 Among various NOx removal

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technologies, the most extensive investigation for the NOx removal is the SCR with

11

ammonia.3-5 But stoichiometric control of NH3 must be maintained to avoid the

12

emission of unreacted NH3. Another problem of the SCR technology is the

13

transportation and storage of NH3. Utilization of AC as reducing agents instead of

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NH3 is a promising method for NOx removal and has many advantages. For example,

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carbon materials have the advantage of cheap and rich source without the problem of

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production, transportation and store. In addition, carbon materials as good catalyst

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support have many characteristics, such as relativity larger specific surface area,

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microporous structure, and a variety of surface functional groups.10 Therefore, it is

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great interest and significance to develop carbon materials supported catalysts SCR of

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NO, and the carbon materials act as both catalysts support and reducing agents.

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However, SCR of NO using AC as reductant exhibits appreciable activity only at 2


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temperatures in excess of 500 °C in the conventional reaction mode (CRM).7,11-13 But

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when temperature higher than 400 °C, the O2-carbon reaction become more and more

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clear with increasing of temperature in the presence of excess oxygen.14 Therefore, it

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couldn’t satisfy the requirements for practical application. Additionally, decrease

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carbon consumption but holding a high level of NO conversion in the presence of

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oxygen is the main problem to be overcome for practical purpose.15 Therefore, it is

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urgent to develop new strategy for NO removal with high activity at low temperature

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range and effectively prevent O2-carbon reaction.

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Microwave technique is one of effective methods in control of pollution.

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Microwave has been applied to enhance the reaction of NO with carbon (without

11

catalyst).16 The reaction of SO2 with carbons is significantly promoted by microwave

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plasma.17 CH4 reduction of NO with microwave radiation in the presence of excess O2

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over In-Fe2O3/HZSM-5 catalyst was studied.18 However, a limited number of

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literatures reported about microwave irradiation-selective catalytic reduction of NO to

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N2 using AC as reductant by catalysis at low temperature under excess O2.19,20

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Microwave irradiation can effectively decrease the heterogeneous catalytic reaction

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temperature,18,19,21 hence a high-temperature reaction can work effectively at a low

18

temperature. Furthermore, microwave has a beneficial effect on the activation of polar

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reactant molecules to enhance reaction selectivity.1,2,18,19,21-25 Hence AC can react

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selectively with polar NO rather than non-polar O2 and produce non-polar products N2

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and CO2, providing a potential method for selective catalytic reduction of NO to N2 3


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by AC at low temperature in the MCRM.

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To choose an appropriate catalyst for the microwave process is of great

3

importance, such a catalyst has to be excellent receptor of microwave energy, without

4

changing its structure and properties under intense microwave radiation.26 Metallic

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oxides such as CeOx, NiOx, MnOx and FeOx are inexpensive and absorbing

6

microwaves well, which may be them suitable for use as microwave catalyst. To the

7

best of our knowledge, SCR of NO over activated carbon supported transition metal

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oxides CeO2 and MeOx-CeO2/AC (Me=Ni, Mn, Fe) catalysts in the MCRM has not

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been investigated yet. Herein, we reported use AC as reductant MW-SCR of NO over

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activated carbon supported transition metal oxides CeO2 and MeOx-CeO2/AC (Me=Ni,

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Mn, Fe) catalysts at low temperature. With our work, we attempt to evaluate the

12

potential to apply this method as an effective NOx control strategy, and illuminate the

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microwave selective effect and the mechanism of MW-SCR reduction reaction

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temperature in the MW-SCR of NO by AC.

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2. EXPERIMENTAL SECTION

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2.1. Catalyst Preparation

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AC (coconut shell activated carbon, ignition point is 380 °C, Fujian Xinsen

18

Carbon Co., Ltd.) supported catalysts were prepared by the urea homogeneous

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precipitation method. A certain amount of AC with pretreated was added to the metal

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nitrates solution, and then kept 1-hour in the presence of ultrasonic oscillation at room 4


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temperature. After a certain amount of urea was added to the solution and constant

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stirring for 9-hours at 90 °C, and the temperature of the mixture was kept at 90 °C for

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an additional 1-hour for aging. The product was dried overnight at 120 °C. AC

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supported metal oxides catalysts were obtained after calcining in microwave

5

irradiation for 40-minutes at 550 °C with N2 protection. The final catalysts are labeled

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as x% (y% MeOx)-CeO2/AC (Me=Ni, Mn and Fe), where x represents the weight

7

percentage of mixed metal oxides in the catalyst, and y represents the weight

8

percentage of NiOx, MnOx or FeOx in mixed metal oxides. For comparison, sample of

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CeO2/AC was also prepared by the same method.

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2.2. Characterization of Catalysts

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The specific surface area and pore structure of AC supported metal oxides and

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AC was characterized by N2 sorption isotherms with TriStar II 3020 at -196 °C. X-ray

13

diffraction (XRD) of samples was obtained on a Rigaku D/max-II/2500 X-ray powder

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diffractometer, Cu Ka radiation was employed and the working voltage and current

15

were 40 kV and 30 mA, respectively. XPS measurements were conducted on a

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Thermo Scientific K-Alpha spectrometer with a monochromatic Al Kα radiation.

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2.3. Measurement of Activity

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The reaction between NO and AC was carried out in a microwave catalytic

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reactor at atmospheric pressure. The microwave catalytic reactor rated input power is

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2000W. The microwave catalytic reactor was developed to investigation the role of 5


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the microwave irradiation in a continuous flow of gas-solid catalytic reaction systems.

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The microwave catalytic reactor system employed in this work was described earlier.1

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The microwave energy is supplied by a 2.45 GHz microwave generator where the

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power could be varied continuously in the range of 0-1000W. The activity evaluations

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were performed in a quartz tube (i.d. 10 mm and 540 mm in length) at the center of

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the cavity. The AC or AC loaded metal oxides were filled in the middle of the quartz

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tube and both ends were sealed by using asbestos. In the activity evaluation

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experiments, the reaction bed was controlled at special temperature by adjusting the

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microwave power. The microwave power of different catalysts at different

10

temperature was show in table S1. The catalyst bed temperature was measured

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precisely by a modified thermocouple probe inserted into the catalyst bed. The

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reactant gas was composed of NO, O2 (when used), water vapor (when used) and the

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balance N2. The catalyst bed contained 6 g of the catalyst. The total feed rate of the

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reactant gases were maintained at W/F = 2.25 g·s·cm-3 (SV=960 h-1), where the W

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and F are represents the catalyst weight and the total flow rate of reactant gases

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respectively. The NO and NO2 concentration of the out gas was analyzed by an online

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NOx analyzer (42C, Thermo Environmental Instruments Co., Ltd., U. S.). In addition,

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the analysis system has GC (Agilent 7890A) with two thermal conductivity detectors,

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a column of Poropak Q for N2O analysis and a column of 5A zeolite for CO analysis,

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and the analysis system has 900D Portable 4 component Infrared analysis instrument

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(Beijing Huayun Analytic Instrument Institute) for CO analysis. In this reaction, N2 6


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was the desired product, NO2 was the by-product, and both very undesirable products

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N2O and CO were no detected. Therefore, the NO conversion and NO selectivity to

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N2 could be calculated by the formulas as follow:

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X NO =

Cin ( NO ) -C out ( NO ) Cin ( NO )

× 100% , SNO =

Cin ( NO ) -Cout ( NO ) -C out ( NO 2 ) Cin ( NO ) -C out ( NO )

× 100%

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XNO represents the NO conversion, SNO represents the NO selectivity toward N2,

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Cin(NO) represents the NO concentration before the reaction, Cout(NO) and Cout(NO2)

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are the outlet concentration of NO and NO2.

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3. RESULTS

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3.1. Physicochemical Properties of Catalysts

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The BET surface area, micropore area, total pore volume and micropore volume

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of the AC and AC supported mixed metal oxides sample were summarized in table 1.

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It can be seen that the BET surface area of AC was considerable large and no

13

obviously varied after loading metal oxides. The micropore area, total pore volume

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and micropore volume of 5wt% CeO2/AC, 5wt% (21wt% NiOx)-CeO2/AC and 5wt%

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(21wt% MnOx)-CeO2/AC had slightly increased after CeO2, NiOx-CeO2 and

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MnOx-CeO2 metal oxides loading on AC. Maybe the plugged pores were partially

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opened in the process of calcining. However, the micropore area, total pore volume

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and micropore volume of 5wt% (21wt% FeOx)-CeO2/AC had slightly decreased after

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AC loaded FeOx-CeO2 mixed metal oxides. Maybe the free pores of AC were partially

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occupied during the precipitation process. 7


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Table 1

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Fig. 1 displays the XRD patterns of AC and AC supported metal oxides catalysts.

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It is clear that the typical diffraction peaks of carbon can be observed on AC support.

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With supported cerium on AC, new peaks appeared at 2θ=28.5°, 33.1°, 47.5°, 56.3°,

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indicating the existence of crystallized CeO2 in 5% CeO2/AC catalyst. In addition, the

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diffraction peaks of CeO2 didn’t disappear when nickel oxides, iron oxides and

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manganese oxides are introduced into 5% (21% NiOx)-CeO2/AC, 5% (21%

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FeOx)-CeO2/AC and 5% (21% MnOx)-CeO2/AC catalysts, respectively. It is worth

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noting that the typical diffraction peaks of corresponding to nickel oxides, iron oxides,

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manganese oxides couldn’t be observed on 5% (21% NiOx)-CeO2/AC, 5% (21%

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FeOx)-CeO2/AC, 5% (21% MnOx)-CeO2/AC catalysts, respectively. Suggesting that

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metal oxides species were finely dispersed on AC or they are too small to be detected

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by XRD analysis.

14 15

Fig. 1

3.2. Catalytic Activing

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Fig. 2a compares the NO conversion vs reaction temperature over AC and AC

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supported metal oxides in the MCRM. The result shows that NO conversion

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decreased in the following sequence: 5% (21% NiOx)-CeO2/AC > 5% (21%

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MnOx)-CeO2/AC > 5% CeO2/AC > 5% (21% FeOx)-CeO2/AC > AC. It can be seen

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that NO conversion was increased by loaded metal oxides on AC. More than 95% of 8


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NO conversion at 300 °C can be achieved in 5% CeO2/AC and 5% (21%

2

MeOx)-CeO2/AC (Me=Ni, Mn). When temperature was increased above 350 °C, the

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reduction ability of NO in 5% (21% MeOx)-CeO2/AC (Me=Ni, Mn, Fe) was closed to

4

each other, their NO conversion almost reached 100%. As demonstrated in fig. 2b,

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with increasing temperature, NO conversion slowly increased and exhibited greatest

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NO conversion was 96.5% in the MCRM and 69.3% in the CRM at 450 °C. It is

7

noting that 5% CeO2/AC and 5% (21% FeOx)-CeO2/AC catalysts exhibited high

8

adsorption capacity for NO below 200 °C, which was higher as compared to 5% (21%

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MeOx)-CeO2/AC (Me=Ni, Mn) for NO adsorption capacity. Interestingly, the NO

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conversion over 5% (21% MeOx)-CeO2/AC (Me=Ni, Mn) is higher than that of 5%

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CeO2/AC and 5% (21% FeOx)-CeO2/AC above 300 °C. Moreover, the sequence of

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NO conversion of these catalysts was not consistent with the values of SBET and Vmic.

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These results suggest that the BET surface area and micropore volume of the used AC

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are high enough for metal oxides loaded AC catalysts and thus the NO removal

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activities of metal oxides loaded AC catalysts had no direct relationships with the pore

16

structures.

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As shown in fig. 2c, the N2 selectivity over AC, 5% CeO2/AC and 5% (21%

18

MeOx)-CeO2/AC (Me=Ni, Fe, Mn) slightly decreased with temperature increase, but

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their N2 selectivity still higher than 90% at 100-450 °C. Compare the NO conversion

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and N2 selectivity of all catalysts, we chose the best catalytic activity catalyst of 5%

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(21% NiOx)-CeO2/AC studied the effect of O2 and water vapor in the MCRM in the 9


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following experiments.

2

Fig. 2

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To determine the oxidation states and oxygen species of catalysts before and

4

after reaction, 5% CeO2/AC and 5% (21% MeOx)-CeO2/AC (Me=Ni, Mn, Fe)

5

catalysts were examined by XPS analysis before and after reaction. Surface atomic

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concentration of Ni, Fe, Mn, Ce and O are summarized in table 2 and the XPS spectra

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of Ni 2p, Fe 2p, Mn 2p, Ce 3d and O 1s are shown in fig. S1 to fig. S4. For CeO2/AC

8

catalyst, the Ce species was mainly existed with Ce4+ on the 5% CeO2/AC catalyst

9

surface and the lattice oxygen, surface-adsorbed oxygen and C-O combined oxygen

10

was coexisted on the surface of the 5% CeO2/AC catalyst before and after reaction

11

and hadn’t obviously changed after reaction. The atomic ratio of Ce3+/(Ce3++Ce4+)

12

hadn’t obviously change after reaction. For 5% (21% NiOx)-CeO2/AC catalyst, the Ni

13

species was mainly existed with Ni2+ before reaction and hadn’t transformed to Ni3+

14

after reaction. The Ce3+ and Ce4+ was coexist on the surface of the 5% (21%

15

NiOx)-CeO2/AC catalyst before reaction and a part of Ce3+ was transform to Ce4+ after

16

reaction. The oxygen species was existed with lattice oxygen and surface-adsorbed

17

oxygen before reaction and a part of lattice oxygen and surface-adsorbed oxygen

18

transform to C-O combined oxygen after reaction. For 5% (21% FeOx)-CeO2/AC

19

catalyst，the Fe species was mainly existed with Fe2+ before reaction and a part of Fe2+

20

was transform to Fe3+ after reaction. The Ce3+ and Ce4+ was coexist on the surface of

21

the 5% (21% FeOx)-CeO2/AC catalyst before reaction and amount of Ce4+ was 10


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transform to Ce3+ after reaction. A vast of C-O combined oxygen was existed on the

2

surface of the 5% (21% FeOx)-CeO2/AC catalyst before and after reaction and hadn’t

3

obviously changed after reaction. For 5% (21% MnOx)-CeO2/AC catalyst, the Mn3+

4

and Mn4+ was coexisted on the surface of the 5% (21% MnOx)-CeO2/AC catalyst

5

before reaction and the Mn3+ was all transform to Mn4+ after reaction. The Ce species

6

was mainly existed with Ce4+ before reaction and hadn’t obviously changed after

7

reaction. A vast of C-O combined oxygen was existed on the surface of the 5% (21%

8

MnOx)-CeO2/AC catalyst before and after reaction and hadn’t obviously changed

9

after reaction.

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Table 2

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3.3. Effects of O2 on NO Conversion and N2 Selectivity over 5% (21%

12

NiOx)-CeO2/AC Catalyst

13

As the exhaust gas usually contains a trance amount of O2, the improvement of

14

O2 tolerance is one of the challenges for SCR. The reduction of NO into N2 by AC

15

over 5% (21% NiOx)-CeO2/AC was carried out in the presence of excess oxygen. As

16

show in fig. 3, NO conversion had slightly increased and N2 selectivity had slightly

17

decreased over 5% (21% NiOx)-CeO2/AC in the MCRM when introduced oxygen.

18

Specifically, NO conversion was more than 99.3% and N2 selectivity was more than

19

95.7% in O2 concentration from 0 to 10%. The effects of O2 for the durability of 5%

20

(21% NiOx)-CeO2/AC catalyst reduction of NO at 300 °C as show in fig. 4. At the 11


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beginning 5.5-hours test, NO conversion was more than 99.8% didn’t decrease and N2

2

selectivity was more than 95.6% in the MCRM. However, the NO conversion slightly

3

decreased after 5.5-hours test, sharply decreased after 6-hours test and fell to 76.3%

4

after 6.5-hours test. The N2 selectivity gradually increased after 1.5-hours test and it is

5

higher than 99% after 4.5-hours test.

6

Fig. 3

7

Fig. 4

8

In order to identify the surface species and chemical state change of elements in

9

the 5% (21% NiOx)-CeO2/AC catalyst sample through the SCR reaction in the

10

presence of 5% O2. The fresh and used samples were examined by XPS spectroscopy.

11

The XPS spectra of Ni 2p, Ce 3d and O 1s are presented in fig. 5. The surface atomic

12

ratios of Ni2+/(Ni2++Ni3+), Ce3+/(Ce3++Ce4+) and different oxygen species are listed in

13

table 3. For fresh sample, the spectrum of Ni 2p main peaks and its satellite at 855.1

14

eV and 862.4 eV were corresponding to Ni2+.27 Ni 2p for used sample shows three

15

characteristic primary peaks at 854.7 eV and 855.6 eV were corresponding to

16

Ni2+,27,28 at 856.8 eV was corresponding to Ni3+.27 The corresponding satellite peak

17

was observed at 861.8 eV associated to Ni2+.29 The complex spectra of Ce 3d were

18

decomposed into six components. The peaks at 885.8 eV and 898.9 eV were ascribed

19

to the primary photoemission of Ce3+ and the other peaks at 882.8 eV, 901.6 eV, 906.3

20

eV and 917.2 eV were associated with Ce4+ final states over fresh sample.30-33 The 12


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peaks at 899.0 eV was ascribed to the primary photoemission of Ce3+ and the other

2

peaks at 883.2 eV, 886.8 eV, 901.6 eV, 906.7 eV and 917.4 eV were associated with

3

Ce4+ final state over used sample.30,31,34 For oxygen species, the bands labeled as Oα at

4

530.7 eV was ascribed to the lattice oxygen of metal oxides and the labeled as Oβ at

5

533.0 eV was attributed to the surface-adsorbed oxygen.28,35,36 Comparing to the fresh

6

sample, the corresponding used sample contained more surface-adsorbed oxygen

7

species. In addition, compared with fresh and used sample, a part of Ni2+ and Ce3+

8

species were transformed to Ni3+ and Ce4+, which may be related to the SCR activity

9

slightly decreased in the SCR reaction process. In the SCR reaction proceeding,

10

maybe a part of from NO dissociated oxygen adsorbed on catalyst surface (discussion

11

eq 5), and then reaction with Ni2+ and Ce3+ led to the activity of SCR gradually

12

decrease.

13

Fig. 5

14

Table 3

15

3.4. Effect of Water Vapor on NO Conversion and N2 Selectivity over 5% (21%

16

NiOx)-CeO2/AC Catalyst

17

It is well known that exhaust gas usually contains a large amount of water vapor.

18

The improvement of water vapor tolerance is one of challenges for SCR catalysts. NO

19

conversion and N2 selectivity over 5% (21% NiOx)-CeO2/AC catalyst was tested in

20

the feed gas containing 5%-15% water vapor. As summarized in table 4, NO 13


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conversion over 5% (21% NiOx)-CeO2/AC didn’t change in the presence of 5%-15%

2

water vapor, but the N2 selectivity had slightly decreased when introduced water

3

vapor and gradually decreased in water vapor concentration from 0 to 15%. However,

4

the N2 selectivity was kept more than 92% even the water vapor concentration

5

reached 15%. The catalytic activity wasn’t influenced by water vapor could be the

6

H2O molecules is a strong polar molecules and has a good microwave-adsorbing

7

ability. Therefore, H2O molecules can interact with the catalyst in the microwave field

8

when they are adsorbed on the catalyst, which leads to a strong coupling and then

9

resulting in the H2O molecules desorption from the adsorption sites, or weakening the

10

adsorption of H2O molecules on the catalyst surface.18

11 12

Table 4

4. DISCUSSION

13

It is worthwhile to note that NO could be efficiently removed by solid AC

14

reductant at low temperature in the MCRM. NO conversion almost reached 100% for

15

5% (21% NiOx)-CeO2/AC at 300 °C, 5% (21% MeOx)-CeO2/AC (Me=Mn, Fe) and 5%

16

CeO2/AC at 350 °C in the MCRM. By comparison, the temperature of NO removal

17

using AC as reductant over AC loaded transition metals (Cr, Fe, Co, Ni and Cu)

18

catalysts were higher 500 °C in the CRM.13 With increasing temperature, NO

19

conversion over AC exhibited a sharp increase in the temperature range from 200 to

20

350 °C and the highest NO conversion was 96.5% at 450 °C in the MCRM. However,

21

NO conversion over AC was only 69.3% at 450 °C in the CRM. The NO reduction 14


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1

with AC exhibited a sharp increase in the vicinity of 650 °C in the CRM.7 The

2

temperature of SCR of NO by AC in the MCRM was usually lowers 150-200 °C than

3

in the CRM. These results suggest that microwave irradiation can efficiently decrease

4

the reaction temperature of NO with AC and improve the activity of NO removal. The

5

similar conclusion was reported by previous studies,19,21,22,37 microwave irradiation

6

can efficiently decrease the temperature of chemical reaction.

7

No CO was detected and the N2 selectivity was always very high over all

8

catalysts in the MCRM. The oxygen concentration had no influence on the activity of

9

5% (21% NiOx)-CeO2/AC microwave catalysts MW-SCR of NO in the MCRM.

10

These results may be explained by microwave selective effect under microwave

11

irradiation. It is known that high frequency electromagnetic field is beneficial for the

12

activation of polar molecules. Therefore, it is speculate that the polar NO, CO, N2O

13

and NO2 adsorbed on the catalysts can be effectively activated by the high frequency

14

electromagnetic field by absorption microwave energy. But the nonpolar N2, O2 and

15

CO2 adsorbed on the catalysts or in the gas phase couldn’t be directly activated by the

16

high frequency electromagnetic field. Microwave enhance reaction selectivity has

17

been demonstrated by previous investigation,25,38-40 the non-polar molecules products

18

yield improved by microwave irradiation.

19 20

On the basis of our results and discussion above, the possible mechanism for the NO reduction by AC in the MCRM is suggested as follows.

15


Energy & Fuels

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1 2

Page 16 of 30

adsorption

NO  → NOσ

(1)

MW NOσ (polar)  → NO* → N* +O* → NOσ

(2)

3

MW C x  → C*x

(3)

4

2N* → N 2σ

(4)

5

2O* → O 2σ

(5)

6

nO* +C*x → C x O n → C x −1O n − 2 + CO 2σ

(6)

7

MW nO* +C*x → C x O n → C x −1O n−1 + COσ (polar)  → CO* → C* +O*

(7)

8

MW N* +2O* → NO2σ (polar)  → NO2* → N* +2O*

(8)

9

MW 2N* +O* → N 2 Oσ ( polar )  → N 2O* → 2N* +O*

(9)

desorption

10

N 2σ  → N2

11

CO 2σ  → CO 2

12

COσ  → CO

13

NO 2σ  → NO 2

14

N 2 Oσ  → N 2O

(10)

desorption

(11)

desorption

(12)

desorption

(13)

desorption

(14)

15

AC and AC supported metal oxides absorbing microwave ability was very well

16

(fig. S5). The catalysts bed reached reaction temperature and catalysts “active sites”

17

hold better catalytic activity by microwave electromagnetic filed through absorption

18

microwave energy under microwave irradiation. As show in fig. 6, NO was firstly 16


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Energy & Fuels

1

physically adsorbed on the surface of catalyst, and then can be chemisorbed on

2

“active sites” (eq 1). Chemisorbed NOσ can be activated effectively by the microwave

3

electromagnetic filed by absorption microwave energy because NO is a strongly polar

4

molecule (eq 2).18,23,24 The reason of AC with NO reaction temperature was decreased

5

may be the reactant molecules and catalysts were activated by absorption a portion of

6

microwave energy in the MCRM. Then activated NO* dissociated to activated O* and

7

N* (eq 2). Two activated nitrogen combine produce N2σ on catalyst active sites (eq 4)

8

and then desorbed timely (eq 10). Activated oxygen with activated carbon to form the

9

carbon-oxygen complexes, then the carbon-oxygen complexes decompose to form

10

CO2σ (eq 6) and then release timely (eq 11). However, CO, NO2 and N2O are strongly

11

polar molecule and can be activated effectively by the microwave electromagnetic

12

filed and then dissociated (eq 7-9).18,23,24 So the products of CO, NO2 and N2O would

13

be less likely produce for the microwave selective effect (eq 12-14). In addition, the

14

O2 adsorbed on the catalysts or in the gas phase couldn’t be directly activated by the

15

high frequency electromagnetic field at such low temperature for the microwave

16

selective effect.23 So the reactions of NO with O2 to produce NO2 and AC with O2 to

17

produce CO2 would be less likely at such low temperature. Consequently, the AC with

18

NO reaction temperature decreased may be attributed to the reactant molecules and

19

catalysts absorption a portion of microwave energy in the MCRM. These results show

20

that microwave irradiation exhibited microwave catalytic effect. Further, the AC with

21

NO reaction activation energy in the CRM is 64 kJ/mol, and in the MCRM is as low 17


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

as 24.06 kJ/mol.41 The AC with NO reaction activation energy decrease is also

2

illustrate the microwave catalytic effect. MW-SCR of NO by AC primarily produce

3

N2 and CO2, and O2 existence has no influence the activity of NO removal by AC in

4

the MCRM because of microwave selective effect.

5 6

Fig. 6

5. CONCLUSIONS

7

Microwave irradiation could efficiently decrease the reaction temperature of NO

8

with AC and improve the activity of NO removal. The temperature of NO removal by

9

SCR using AC as reductant in the MCRM was usually lower 150-200 °C than that of

10

in the CRM. NO conversion almost reached 100% for 5% (21% NiOx)-CeO2/AC at

11

300 °C, 5% (21% MeOx)-CeO2/AC (Me=Mn, Fe) and 5% CeO2/AC at 350 °C in the

12

MCRM. Microwave irradiation exhibited the microwave catalytic effect and

13

microwave selective effect in the MCRM. MW-SCR of NO by AC primarily

14

produced N2 and CO2, and O2 existence had no influence the activity of NO removal

15

by AC in the MCRM. MW-SCR is a viable and promising method for flue-gas control.

16

Although only 5% CeO2/AC and 5% (21% MeOx)-CeO2/AC (Me=Ni, Mn, Fe)

17

microwave catalysts and the NO removal through MW-SCR were used as a case study.

18

We believe that this new process can be extended to other microwave catalysts and

19

chemical reactions, such as N2O, SO2 removal by MW-SCR.

18


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Energy & Fuels

1

ASSOCIATED CONTENT

2

Supporting Information

3

The power supply of different catalysts at different temperature (Table S1),

4

Photo-peak binding energy coordination oxidation states (Table S2), XPS spectra for

5

5% CeO2/AC catalyst before and after reaction (Fig. S1), XPS spectra for 5% (21%

6

NiOx)-CeO2/AC catalyst before and after reaction (Fig. S2), XPS spectra for 5% (21%

7

FeOx)-CeO2/AC catalyst before and after reaction (Fig S3), XPS spectra for 5% (21%

8

MnOx)-CeO2/AC catalyst before and after reaction (Fig. S4), Microwave heating

9

behavior of AC and AC supported different metal oxides catalysts in the MCRM at

10

the microwave power of 150 W (Fig. S5).

11

ACKNOWLEDGEMENTS

12

This work was supported by the Natural Science Foundation of China (21676227，

13

20976147), the Key Project of Hunan Provincial Natural Science Foundation of China

14

(09JJ3021), the Key Laboratory Open Foundation of Higher Education Institutions of

15

Hunan Province (12K047), the Natural Science Foundation of Hunan Province

16

(2017JJ3298). The authors deeply appreciate the support by Synotherm Corporation

17

for microwave experimental apparatus.

18

REFERENCES

19

(1) Xu, W.; Zhou, J.; You, Z.; Luo, Y.; Ou, Y. Microwave irradiation coupled with physically

20

mixed MeOx(Me=Mn, Ni) and Cu-ZSM-5 catalysts for the direct decomposition of nitric oxide

21

under excess oxygen. ChemCatChem 2015, 7 (3), 450-458. 19


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towards oxygen inhibition removal for highly-effective NO decomposition by microwave catalysis

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iron-modified Ce/TiO2 catalyst for selective catalytic reduction of NOx with ammonia. J. Phys.

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Chem., C 2012, 116 (48), 25319-25327.

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Eng. Chem. Res. 2012, 51 (36), 11667-11673.

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Commun. 2006, 7 (9), 678-684.

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Improvements in NOx reduction by carbon using bimetallic catalysts. Fuel 2001, 80 (14),

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2001-2005.

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(16) Cha, C. Y.; Kim, D. S. Microwave induced reactions of sulfur dioxide and nitrogen oxides in char and anthracite bed. Carbon 2001, 39 (8), 1159-1166. (17) Wang, X.; Wang, A.; Wang, X.; Zhang, T., Microwave plasma enhanced reduction of SO2 to sulfur with carbon. Energy Fuels 2007, 21 (2), 867-869. (18) Wang, X.; Zhang, T.; Xu, C.; Sun, X.; Liang, D.; Lin, L. Microwave effects on the selective reduction of NO by CH4 over an In-Fe2O3/HZSM-5 catalyst. Chem. Commun. 2000, (4), 279-280.

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(19) Xu, W.; Zhou, J.; Li, H.; Yang, P.; You, Z.; Luo, Y. Microwave-assisted catalytic reduction

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of NO into N2 by activated carbon supported Mn2O3 at low temperature under O2 excess. Fuel

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Process. Technol. 2014, 127, 1-6.

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(20) Ma, S.; Jin, X.; Wang, M.; Jin, Y.; Yao, J.; Liu, W. Experimental study on removing NO

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from flue gas using microwave irradiation over activated carbon carried catalyst. Sci. China

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Technol. Sc. 2011, 54 (12), 3431-3436.

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(21) Zhou, J.; Xu, W.; You, Z.; Wang, Z.; Luo, Y.; Gao, L.; Yin, C.; Peng, R.; Lan, L. A new

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type of power energy for accelerating chemical reactions: the nature of a microwave-driving force

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for accelerating chemical reactions. Sci. Rep. 2016, 6, 25149.

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(22) Xu, W.; Zhou, J.; Su, Z.; Ou, Y.; You, Z. Microwave catalytic effect: a new exact reason for

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microwave-driven heterogeneous gas-phase catalytic reactions. Catal. Sci. Technol. 2016, 6 (3),

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698-702.

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(23) Tang, J.; Zhang, T.; Liang, D.; Xu, C.; Sun, X.; Lin, L. Microwave discharge-assisted catalytic conversion of NO to N2. Chem. Commun. 2000, (19), 1861-1862. (24) Tang, J.; Zhang, T.; Ma, L.; Li, N.; Liang, D.; Lin, L. Microwave-assisted purification of automotive emissions. J. Catal. 2002, 211 (2), 560-564. (25) Kappe, C. O.; Pieber, B.; Dallinger, D. Microwave effects in organic synthesis: myth or 21


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reality? Angew. Chem. Int.l Edit. 2013, 52 (4), 1088-1094. (26) Tang, J.; Zhang, T.; Liang, D.; Yang, H.; Li, N.; Lin, L. Direct decomposition of NO by microwave heating over Fe/NaZSM-5. Appl. Catal., B 2002, 36 (1), 1-7. (27) Salagre, P.; Fierro, J. L. G.; Medina, F.; Sueiras, J. E., Characterization of nickel species on several γ-alumina supported nickel samples. J. Mol. Catal., A 1996, 106 (1), 125-134.

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(28) Murugan, R.; Vijayaprasath, G.; Mahalingam, T.; Ravi, G. Enhancement of room

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temperature ferromagnetic behavior of rf sputtered Ni-CeO2 thin films. Appl. Surf. Sci. 2016, 390,

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583-590.

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(29) Peck, M. A.; Langell, M. A. Comparison of nanoscaled and bulk NiO structural and environmental characteristics by XRD, XAFS, and XPS. Chem. Mater. 2012, 24 (23), 4483-4490. (30) Anandan, C.; Bera, P. XPS studies on the interaction of CeO2 with silicon in magnetron sputtered CeO2 thin films on Si and Si3N4 substrates. Appl. Surf. Sci. 2013, 283, 297-303.

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(31) Singh, P.; Srivatsa, K. M. K.; Barvat, A.; Pal, P. X-ray photoelectron spectroscopic studies

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of CeO2 thin films deposited on Ni-W (100), c-Al2O3 (0001) and Si (100) substrates. Curr. Appl.

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Phys. 2016, 16 (10), 1388-1394.

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(32) Cai, W.; Chen, F.; Shen, X.; Chen, L.; Zhang, J. Enhanced catalytic degradation of AO7 in the CeO2–H2O2 system with Fe3+ doping. Appl. Catal., B 2010, 101 (1–2), 160-168.

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parameter and valency states in nanocrystalline cerium oxide. Appl. Phys. Lett. 2005, 87 (13),

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133113-133113-3.

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(35) Zhou, G.; Liu, H.; Cui, K.; Jia, A.; Hu, G.; Jiao, Z.; Liu, Y.; Zhang, X. Role of surface Ni and Ce species of Ni/CeO2 catalyst in CO2 methanation. Appl. Surf. Sci. 2016, 383, 248-252. (36) Zhou, G.; Lan, H.; Wang, H.; Xie, H.; Zhang, G.; Zheng, X. Catalytic combustion of PVOCs on MnOx catalysts. J. Mol. Catal., A 2014, 393, 279-288.

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Microwave irradiation as a sustainable tool for catalytic carbonylation reactions. Inorg. Chim.

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(38) Obermayer, D.; Gutmann, B.; Kappe, C. O. Microwave chemistry in silicon carbide

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reaction vials: separating thermal from nonthermal effects. Angew. Chem. 2009, 121 (44),

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8471-8474.

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(39) Gerbec, J. A.; Magana, D.; Washington, A.; Strouse, G. F. Microwave-enhanced reaction rates for nanoparticle synthesis. J. Am. Chem. Soc. 2005, 127 (45), 15791-15800. (40) Zhang, X.; Hayward, D. O.; Mingos, D. M. P. Effects of microwave dielectric heating on heterogeneous catalysis. Catal. Lett. 2003, 88 (1), 33-38. (41) Ma S. C.; Yao J. J.; Jin X. Zhan B. Kinetic study on desulfurization and denitrification using microwave irradiation over activated carbon. Sci. China. Tech. Sci. 2011, 54, 2321-2326.

23


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Page 24 of 30

1

Table 1. Values of SBET, micropore area (Smic), total pore volume (Vtotal), total micropore volume

2

(Vmic) of the catalysts. Sample

SBET (m2/g)

Smic (m2/g)

Vtotal (cm3/g)

AC

1434

850

0.627

0.336

5wt% CeO2/AC

1486

912

0.654

0.361

5wt% (21wt% NiOx)-CeO2/AC

1427

911

0.641

0.361

5wt% (21wt% FeOx)-CeO2/AC

1414

817

0.622

0.322

5wt% (21wt% MnOx)-CeO2/AC

1480

928

0.640

0.367

3

24


Vmic (cm3/g)

Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

1

Table 2. Surface atomic concentration (%) Sample

Ni

Mn

Fe

Ce

O

Ni2+

Ni3+

Mn3+

Mn4+

Fe2+

Fe3+

Ce3+

Ce4+

Oα

Oβ

Oγ

Before reaction

-

-

-

-

-

-

0.55

4.78

12.73

23.85

58.09

After reaction

-

-

-

-

-

-

1.03

7.28

18.20

24.32

49.17

Before reaction

4.86

-

-

-

-

-

3.16

3.78

28.96

59.24

-

After reaction

2.24

-

-

-

-

-

0.67

5.87

8.81

29.00

53.41

Before reaction

-

-

-

-

2.23

0.38

1.30

4.12

8.47

23.72

59.78

After reaction

-

-

-

-

2.16

4.93

2.78

2.37

6.31

15.14

66.31

Before reaction

-

-

2.62

1.04

-

-

0.48

5.39

5.71

17.84

66.92

After reaction

-

-

-

2.54

-

-

0.77

3.66

3.47

19.70

69.86

5% CeO2/AC

5% (21% NiOx)-CeO2/AC

5% (21% FeOx)-CeO2/AC

5% (21% MnOx)-CeO2/AC

2

Oα: lattice oxygen;

3

Oβ: surface-adsorbed oxygen;

4

Oγ:C-O combined oxygen. 25


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Page 26 of 30

Table 3. The surface atomic ratio of fresh and used sample. Atomic ratio (%) Samples Ni2+/(Ni2++Ni3+)

Ce3+/(Ce3++Ce4+)

Oα/( Oα+Oβ)

Fresh sample

100.00

46.08

32.81

Used sample

74.48

8.74

28.23

2 3

Table 4. Effect of water vapor on NO conversion and NO selectivity to N2 under 5% (21%

4

NiOx)-CeO2/AC catalyst in the MCRM. Temperature (°C)

200

250

300

350

400

Absence of H2O

31.67

48.89

99.29

99.91

99.90

Presence of 5% H2O

31.11

48.33

99.92

99.92

99.90

Presence of 10% H2O

30.89

47.78

99.91

99.92

99.92

Presence of 15% H2O

30.00

47.44

99.91

99.88

99.89

Absence of H2O

100.00

100.00

99.98

98.54

97.17

Presence of 5% H2O

97.86

97.70

97.74

97.57

96.37

Presence of 10% H2O

92.81

93.95

96.42

95.63

95.07

Presence of 15% H2O

92.59

92.74

93.19

93.67

93.31

Conversion (%)

Selectivity (%)

5

Reaction condition: weight of catalyst: 6 g; feed: 900 ppm NO, N2 as the balance; total flow rate:

6

160ml/min.

26


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1 2

Fig. 1. XRD patterns of the catalysts; AC (a), 5% CeO2/AC (b), 5% (21% NiOx)-CeO2/AC (c), 5%

3

(21% MnOx)-CeO2/AC (d), 5% (21% FeOx)-CeO2/AC (e).

27


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

2

3 4

Fig. 2. Effects of reaction temperature on NO conversion (a) and N2 selectivity (c) under AC

5

supported different metal oxides in the MCRM, effects of reaction temperature on NO conversion

6

over AC under different reactor systems (b). (Reaction conditions: weight of catalyst: 6 g; feed:

7

900 ppm NO; N2 as the balance; total flow rate: 160ml/min.)

28


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Energy & Fuels

1 2

Fig. 3. Effects of O2 concentrations on NO conversion and N2 selectivity under 5% (21%

3

NiOx)-CeO2/AC catalyst in the MCRM. (Reaction conditions: weight of catalyst: 6 g; feed: 1000

4

ppm NO, N2 as the balance; total flow rate: 160ml/min; reaction temperature: 300 °C.)

5

6 7

Fig. 4. Effects of O2 for the durability of 5% (21% NiOx)-CeO2/AC catalyst in the MCRM.

8

(Reaction conditions: weight of catalyst: 6 g; feed: 1000 ppm NO, 5% O2, N2 as the balance; total

9

flow rate: 160ml/min; reaction temperature: 300 °C.)

10

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Energy & Fuels

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Fig. 5. XPS spectra for fresh and used sample (a: Ni 2p, b: Ce 3d, c: O 1s).

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Fig. 6. Illustration of NO removal by MW-SCR over Me-CeO2/AC catalysts.

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Microwave Irradiation-Selective Catalytic Reduction of NO to N2 by

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