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Kinetics, Catalysis, and Reaction Engineering

Low-temperature selective catalytic reduction of NO by CO in the presence of O2 over Cu:Ce catalysts supported by multi-walled carbon nanotubes Zahra Gholami, and Guohua Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01343 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Industrial & Engineering Chemistry Research

Low-temperature selective catalytic reduction of NO by CO in the presence of O2 over Cu:Ce catalysts supported by multi-walled carbon nanotubes

Zahra Gholami*, Guohua Luo* Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

*Corresponding author Tel: +86 10 62788994, Fax: +86 10 62772051, E-mail: [email protected], [email protected]

ABSTRACT The catalytic activity of a series of Cu-Ce catalysts supported on carbon nanotubes was studied for NO reduction by CO. The 20 wt.% Cu1:Ce3/CNT catalyst showed the highest NOx conversion of 96% at 220 °C in the presence of oxygen (O2/CO ≤ 0.6). The catalytic activity of the CNT-supported catalysts was significantly enhanced due to synergistic interactions between surface oxygen vacancies and Cu+ species in the CNT-supported catalysts. Shifting of redox equilibrium to right (Cu2+ + Ce3+ ↔ Cu+ + Ce4+) resulted in creation of more reduced state Cu+. In the presence of excess oxygen (O2/CO ≥ 0.6), the catalyst can effectively catalyze the CO−O2 reaction and the NO + CO reaction did not occur. Compared with Cu1:Ce3/CNT catalyst, Cu1:Ce3 catalyst supported on activated carbon showed lower activity due to the lower Cu+/Cu2+. A possible reaction mechanism was proposed, providing insight into the catalytic reactions between NO and CO. 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

Keywords: Selective catalytic reduction, NO reduction by CO, Copper-based catalyst, Carbon nanotubes, Activated carbon

1. INTRODUCTION Among the main air pollutants, nitrogen oxides (NOx) are produced by stationary and mobile sources. NOx are the main cause for a series of environmental problems, such as photochemical smog, acid rain, and ozone depletion in the recent years 1-2. The aim of NOx control technologies, such as selective catalytic reduction (SCR) and selective non-catalytic reduction, is to reduce NOx in the flue gas to harmless N2 3. At present, SCR has been considered as one of the most efficient removal techniques in the field of NOx removal. The SCR of NOx uses several reducing agents, such as hydrocarbons, H2, CO, and urea. Amongst the aforementioned agents, CO is an effective reagent for NOx reduction because CO is also produced during combustion and exists in the flue gas, and CO can catalytically react with NO, generating CO2, N2, and/or N2O 4-7. In this method, NOx and CO simultaneously decreases, and a considerably cheap and simplified feeding system for the NOx abatement process is expected to be obtained by using CO as the reducing agent. Different types of catalysts have been studied for the reduction of NOx by CO. Precious metal catalysts were found to be efficient for this reaction. However, the high cost and unavailability of these metals hinder their practical application. Therefore, the development of efficient nonprecious metal catalysts has attracted increased attention to NOx reduction research. Copper catalysts are some of the most promising catalysts in NOx reactions, especially for NO reduction 8-9

. Active sites, where the activation of NO molecules occurs, are supposedly associated with Cu+

species

10

. Copper oxide active sites reveal facile redox interaction with both reducing and

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oxidizing reactants

11-12

. Supported copper-based catalysts were not comparable with supported

noble-metal catalysts in low temperature reactions; hence, this problem should be addressed

13

.

The addition of a promoter to the metal oxide catalyst and the metal–promoter interaction can significantly enhance catalytic activity 14. Ceria (CeO2) was extensively used in CO + NO reactions because of its good redox property, ample surface oxygen vacancy (SOV), and high oxygen storage/release capacity

13-14

. CeO2 has attracted special attention, because oxygen buffering

occurs during Ce4+/Ce3+ redox cycle 15. The support also plays an important role in the dispersion of metals and enhancing catalytic properties by affecting the interaction of NO and CO with metal oxides. Supported copper-based catalysts are extensively studied using different support materials, such as alumina 16, zirconia 1718

and mesoporous silica 19. Carbon materials in the presence or absence of additional metals are

extensively used in NOx treatment 20-21. Carbon-based materials loaded with transition metals, such as Co 22, Cr 23, and Cu 24, also reveal high activity in NOx reduction by CO below 300 °C in the absence of O2. Carbon nanotubes (CNTs) are interesting and competitive supports for heterogeneous catalysts given their high mechanical strength, high thermal and electrical conductivity and adsorption, unique nanostructure, mechanical and thermal properties, and hydrophobicity

25-26

. Previous research

27-29

indicated that CNT is a good option as a support

because of the excellent properties of CNT in CO oxidation and NO reduction. The hydrophobic characteristics and inertness of the graphitic surface of CNTs needs to be modified to improve its surface chemical structure. Moreover, CNTs as support provides improved dispersion of metal nanoparticles and prevents nanoparticle agglomeration 30. Recently, bimetallic Cu–Fe catalysts supported on CNTs and γ-Al2O3 have been used for NO reduction by CO

31

. The presence of CNTs improves catalytic activity. In the absence of



oxygen, 100% NO conversion is obtained at 500 °C. However, the preparation of Cu–Fe/CNT catalyst involves complicated methodology. In addition, bimetallic catalysts containing 25 wt.% Cu–25 wt.% Fe and supported alumina are also prepared by co-impregnation, and NO conversion of 80%−85% is obtained in these catalysts at 500 °C. At 250 °C, NO conversion was 75% over Cu–Fe/CNT and 60% over Cu–Fe/alumina catalysts. Results revealed that metal dispersion on the support surface is mainly affected by the nature and properties of the support. Moreover, metal oxides supported on CNTs were more prone to reduction than those supported on alumina. However, the catalytic behavior in the presence of oxygen which is very important for the further industrial application of the catalyst has not been examined over these catalysts. Given the competition between NO and oxygen for CO, at high oxygen concentrations the NO + CO reaction is a nonselective reduction reaction, and CO preferentially reacts with oxygen rather than NO. The main deficiencies of NO + CO reactions that need to be considered are the high reaction temperature and the sharp decrease in NO reduction in the presence of oxygen. Moreover, the high cost and complexity of the catalyst preparation are other parameters that need to be considered in term of industrial application. The synthesis of a low-cost catalyst with high activity at low temperature and in the presence of oxygen is apparently crucial for the reduction of NOx via CO–SCR and leads to an efficient industrial NOx reduction process. In the present research, a series of copper-based catalysts were synthesized by a simple coimpregnation method and obtained samples have been investigated systematically using various characterization techniques. This study focused on evaluating the effects of texture, structure, and surface copper species of the synthesized catalysts on the catalytic activity of the NO + CO reaction. The effect of catalyst composition on the physicochemical properties and activity of the catalyst for NO + CO reaction in the presence of oxygen were investigated. Moreover, the effect of support


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on the catalytic performance has been investigated and catalytic performance of CNT-supported catalyst compared with that of activated carbon (AC) supported catalyst.

2. EXPERIMENTAL Preparation of the catalyst involves two major steps, namely, pre-treatment of the support and the deposition of metals and promoters onto the support. 2.1. Pretreatment of CNTs and AC Surface modification and oxidation of the MWCNTs (CNano Technology Ltd., China, 95% < purity < 97%) will be performed by dispersion of required amount of MWCNTs in acid mixture HNO3 (65%, Beijing Modern Oriental Fine Chemicals Co., Ltd.) and H2SO4 (98%, Modern Oriental (Beijing) Technology Development Co., Ltd), of HNO3:H2SO4 ratio of 3:1 (v/v) at room temperature for 24 h with continues mixing. After the oxidation process, the mixture will be thoroughly washed with deionized water until pH become neutral and following by drying at 110 ºC for 12 h 32. In the present study, coconut shell activated charcoal with a specific surface area of 600–800 m2g−1 and a size of 50–200 mesh was selected as a substrate. AC was first soaked in 6 molL−1 hydrochloric acid (AR, Beijing Chemical Works) for 24 h. Then, acid treated AC was washed and filtered using deionized water until pH = 7. Then, AC was dried at 100 °C for 24 h 33.

2.2. Synthesis of CNT supported nanocomposites The Cu/Ce catalysts supported on MWCNT with different ratios of Cu:Ce (1:0, 3:1, 1:1, 1:3, 0:1) and 20 wt.% of metal loading were prepared by a co-impregnation method assisted by ultrasonication treatment. The requisite amounts of precursors including Cu(NO3)2.3H2O (>99%,



Beijing Modern Eastern Fine Chemicals Co., Ltd, China) and Ce(NO3)3.6H2O (>99%, Modern Oriental (Beijing) Technology Development Co., Ltd, China) were dissolved in deionized water. Then aqueous solution of metals was drop-wised to the support with constant stirring. The mixture will be exposed to an ultrasonic treatment for 3 h, and after that solutions were stirred for 24 h. Then, all impregnates were dried at 120°C overnight and then calcined at 350 °C under N2 atmosphere for 3 h. The AC supported Cu/Ce catalyst was prepared using the same procedure, while proper amount of acid-treated AC was used as support.

2.3. Catalyst Characterization The catalysts were characterized in terms of particle size, surface area, porosity, phase, surface composition and reducibility properties. Mettler Toledo thermal analysis system was used to measure weight changes of the sample when heated under a flow of nitrogen and also oxygen at a constant heating rate of 10 °C/min from 25 °C to 800 °C. The morphology of powder was analyzed by transmission electron microscopy (TEM, JEM-2100). The phase structures of the spent and fresh catalysts were analyzed using X-ray diffractometer (XRD, D8 ADVANCE Rigaku D/max-RB) with Cu Ka radiation (k = 1.5408 Å) at 60 kV and 80 mA. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific K-alpha photoelectron spectrometer. The temperature program desorption (TPD) of CO, and NO analysis were conducted in a flow reactor equipped with a thermal conductivity detector (TCD). 50 mg of catalyst was heated from room temperature to 300 °C and held for 60 min in He flow (30 mLmin-1), subsequently cooled to room temperature in He atmosphere. Then the gas was subjected to 5 vol% NO/He and 10 vol% CO/He for NO-TPD and CO-TPD, respectively and held for 30 min. Afterward, the catalyst was purged by He flow for 30 min to remove the physically adsorbed CO and NO. Then,


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the sample was heated to 800 °C (NO-TPD) and 600 °C (CO-TPD) with a rate of 10 °C min-1 in He flow. TCD was used for the continuously monitoring of NO and CO consumption.

2.4. Evaluation of Catalytic Performance for the Reduction of NO by CO A fixed bed tubular reactor (10 mm i.d.) mounted inside a vertical furnace was used to perform the NO + CO reaction. The experimental setup is shown in Figure1. Catalytic activity was measured using a 200 mg of catalyst at different temperature, from 140 ℃ to 260 °C. Reaction temperature was measured by a thermocouple located inside the tubular reactor. Before the reaction measurement, the catalyst was flushed in situ with helium at 200 °C for 1 h. Furnace temperature was adjusted to the required reaction temperature, and then the reactor was fed with the reactant gas mixture. The feed was composed of 250 ppm NO, 0–0.5 vol.% O2, and 5000 ppm CO in He. The total gas flow rate of gas was 300 mL/min and the GHSV (gas hourly space velocity) was 12600 h−1. At each measurement, the reaction condition was maintained until a steady state concentration of outlet gas was observed; this state usually takes 30 min to achieve. The product was analyzed using a chemical luminescence NO/NO2/NOx analyzer (CAI-600 HCLD). The concentrations of O2, CO, and CO2 were analyzed using a PG810 multi-gas detector.



Figure 1. Schematic of the fixed bed reactor for selective catalytic reduction of NO with CO. 1) mass flow controller; 2) mixing chamber; 3) fixed bed; 4) filter; 5) chemiluminescence analyzer; 6) furnace; 7) temperature controller.

3. RESULTS AND DISCUSSION 3.1. Characterization of catalysts TGA was conducted to assess the thermal stability of the catalyst in both oxygen and nitrogen atmospheres (Figure 2). This analysis was used also to investigate the presence of any decomposable materials in the un-calcined catalysts. Results showed that in the protective atmosphere (N2), the weight loss for the 20 wt.% Cu:Ce/CNT catalyst started at below 200 °C and continued up to 350 °C (~22% weight loss). In the same temperature range, the weight loss for the catalyst was ~50% under oxygen flow.


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120

(a)

Weight Loss (%)

100 80 60 40 Under O2

20

Under N2

0 0

100

200

300 400 500 Temperature (°C)

600

700

800

120

(b)

100 Weight Loss (%)

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


80 60 40

Under N2

20

Under O2

0 0

100

200

300

400

500

600

700

800

Temperature (°C)

Figure 2: Thermogravimetric analysis of synthesized catalysts under oxygen and nitrogen flow for 20wt.% a) acid-treated CNT, b) Cu1:Ce3/CNT catalysts

From Figure 2 it can be seen that during the heating up to 800 °C under oxygen flow, the carbon gasification of CNT is the main cause of weight loss, which occurs at around 200 °C– 450 °C for the CNT supported catalyst. It has been reported that the carbon gasification occurs at around 600 °C–700 °C for multi-walled CNT (MWCNT)

29

, which is in good agreement with

obtained results for TGA analysis for the CNT in this research [Figure 2 (a)]. It can be assumed that the presence of copper and cerium oxides promotes carbon gasification at low temperature, as can been seen in Figure 2 (b). The introduction of functional groups such as -COOH, -OH and =CO groups by acid treatment of CNTs prior to use decrease the hydrophobicity of CNTs, resulting



in an enhanced affinity for metal precursors at the surface

34

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. The peaks belong to different

functional groups on the surface of the CNT supported catalyst also detected by XPS analysis of O 1s spectra of the catalyst (please refer to Figure 10). Given the negligible weight loss of the catalyst under N2 flow after 350 °C, calcination of the CNT-supported catalysts at 350 °C under nitrogen flow was expected to remove the displaceable water and counterions present in the catalyst. The XRD patterns of the CNT and Cu:Ce/CNT catalysts are shown in Figure 3. The peaks at 2θ of 25° and 43° for carbon nanotubes correspond to the graphite plane (002) reflection

35

,

which is evident for all prepared catalysts in this research. It can be seen that ceria exhibits several characteristic diffraction peaks at 2θ = 28.5°, 33.1°, 47.5°, and 56.8° corresponding to the cubic fluorite structure 36. XRD patterns of bulk CuO crystallites in the monoclinic tenorite phase can be detected in catalysts with Cu:Ce ratios exceeding 1:3 (Cu1:Ce1/CNT, Cu3:Ce1/CNT and Cu/CNT). For samples with lower Cu:Ce ratio (1:3), no peaks were visible for copper oxide crystal phases, indicating the high dispersion of amorphous copper species and the incorporation of copper into the ceria lattice

29, 36

. The XRD patterns also revealed the presence of ceria with fluorite-type

structure for ceria-containing catalysts, and the intensity of CeO2 peaks increased for samples with high ratios of Ce to Cu. No notable peaks were observed for metal crystals in the XRD pattern of Cu1:Ce3 catalyst before calcination [Figure 3 (b)]. This observation could be due to the effect of calcination on changing the structure of catalyst and formation of crystals on the catalyst’s surface as a result of thermal treatment. Moreover, the XRD pattern of the spent catalyst, which was used for 8 h in the NO + CO reaction, revealed the retained initial structure of the catalyst and slightly decreased intensity of the peaks. Results of catalytic activity test also revealed that the activity of the catalyst did not change after 8 h and remained constant.


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¤

* MWCNT × CeO2

(a)

Δ CuO ¤ Cu2O Δ

Δ

¤ Δ Δ

Intensity [a.u.]

Cu/CNT

Cu3:Ce1/CNT Cu1:Ce1/CNT

×

Cu1:Ce3/CNT

*

Ce/CNT

×

×

*

CNT

10

×

20

* MWCNT × CeO2

30 *

40

50 60 2 Theta [ ° ]

70

80

90

× (b)

Δ CuO ¤ Cu2O

×

× Δ

×

¤

¤

Cu1:Ce3/CNT

Intensity [a.u.]

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


Spent Cu1:Ce3/CNT catalyst Cu1:Ce3/CNT catalyst-Before calcination

Cu1:Ce3/AC

10

20

30

40

50

60

70

80

90

2 Theta [ ° ] Figure 3. XRD patterns of (a) Cu:Ce/CNT catalysts with different ratios of Cu:Ce, (b) Cu1:Ce3/CNT catalyst (fresh, spent and before calcination) and Cu1:Ce3/AC catalyst. 11 ACS Paragon Plus Environment


The dispersion of metal oxides on the MWCNT support was observed from TEM and HRTEM images (Figure 4, Figure S1). The outer diameters of MWCNTs of supported metal oxides are in the range of 10–20 nm, and the particle size slightly increased with increasing Cu/Ce ratio. Notably, metal oxides are mostly attached on the outer wall of MWCNT, and copper and cerium metal nanoparticles aggregate on the surface of carbon nanotubes. The EDS and XRF analysis results are shown in Table S1. The distribution and concentrations of copper and cerium was identified in the synthesized Cu:Ce catalysts with different molar ratios of Cu:Ce. It can be seen that the Cu:Ce ratio obtained from EDS analysis for Cu3:Ce1 (2:1) is not close to the nominal ratio (3:1). It might be due to the sensitivity of EDS analysis which affected by different parameters such as topography of sample, atomic number of the elements, etc., and EDS analysis results were just about rough idea of elemental composition in the samples. However, the Cu:Ce ratios obtained from XRF analysis were close to the nominal molar ratios of Cu and Ce for all samples. Figure S2 shows the nitrogen adsorption-desorption isotherms for CNTs and CNT supported catalysts. It can be seen that all these isotherms show a type IV hysteresis loops according to the IUPAC classifications, which is related to the capillary condensation taking place in mesopores, and the limiting uptake over a range of high P/P0 37. The Surface area of the CNT supported catalysts range from 168.2 m2/g to 146.9 m2/g. The surface areas and pore volume of the catalysts (Table S1) were found to be in the following order; CNT (179.6 m2/g, 1.76 cm3/g) > Cu1:Ce3/CNT (168.2 m2/g, 1.07 cm3/g) > Cu1:Ce1/CNT (153.8 m2/g, 0.99 cm3/g) > Cu3:Ce1/CNT (146.9 m2/g, 0.90 cm3/g). The observed decrease in surface area and pore volume may be ascribed to the partial blockage of the pore wall micropores. It is generally reported that a large surface area leads to an increase in the area of interface between catalyst and reactant and resulted in an increase in the catalytic performance.


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(a)

5 nm

(b)

10 nm

(c)

10 nm

(d)

10 nm

Figure 4. HRTEM images of (a) acid treated CNT, (b) Cu1:Ce3/CNT, (c) Cu1:Ce1/CNT, (d) Cu3:Ce1/CNT

XPS analysis was used to study the surface composition and elemental oxidation states of the samples. Figure 5(a) exhibits the XPS spectrum of Cu 2p in catalysts with different Cu:Ce ratios. For the Cu 2p spectra of samples, the components at 932.1 and 951.5 eV can be assigned to the Cu 2p3/2 and 2p1/2 peaks of Cu2O, respectively. The spectra at 933.6 and 954.1 eV can be assigned to the Cu 2p3/2 and Cu 2p1/2 peaks of CuO, respectively 1,15,38. Shake-up satellite peaks at around 942.0 and 961.9 eV were ascribed to Cu 2p3/2 and Cu 2p1/2, respectively. From the XPS data in Table 1, the Cu+/Cu2+ ratios of the Cu and Cu3:Ce1 catalysts were 1.49 and 2.33, respectively. Increased amount of cerium in the catalyst structure also increased the amount of Cu+, with the Cu+/Cu2+ ratio reaching 4.01 in Cu1:Ce3 catalysts. The formation of Cu+ may be due to the higher electronegativity of Cu (1.9) than that of Ce (1.1), thus the tendency to capture 13 ACS Paragon Plus Environment


electrons of Cu is stronger than that of Ce, therefore, Cu2+ can accept electrons to create more Cu+ and Cu (Cu2+ + e-  Cu+, E = +0.19V, Cu2+ + 2e-  Cu0, E = +0.34V) 39,40, and resulted in the shifting of redox reaction to right. Table 1. XPS data of the synthesized catalysts. Catalyst Cu(at%) Ce(at%) O(at%) C(at%) aCu+/Cu2+ bCe3+/Ce4+ cCu:Ce dCu:Ce Cu/CNT 0.35 3.84 95.81 1.49 1:0 1:0 Cu3:Ce1/CNT 0.92 0.68 4.02 94.39 2.33 0.61 1.35:1 3:1 Cu1:Ce1/CNT 1.05 1.04 4.84 93.07 2.74 0.58 1:1 1:1 Cu1:Ce3/CNT 0.90 1.97 7.83 89.30 4.01 0.55 1:2.2 1:3 Cu1:Ce3/AC 0.95 1.87 9.75 87.42 2.65 0.88 1:2 1:3 Spent0.8 1.7 4.18 93.32 3.65 0.55 1:2.1 1:3 Cu1:Ce3/CNT f Before calc.0.69 1.16 6.38 91.77 0.44 2.5 1:1.7 1:3 Cu1:Ce3/CNT a Area ratio of Cu+/Cu2+ estimated by considering the deconvolution peak areas of Cu+ and Cu2+. b Area ratio of Ce3+/Ce4 estimated by considering the deconvolution peak areas of Ce3+ and Ce4+, Ce3+/Ce4 = Aareas (Ce3+)/Aareas (Ce4+) = Aareas (v0+v'+ u0 + u') / Aareas (v+v''+v'''+u+u''+u'''). c Cu:Ce ratio determined by XPS, d Cu:Ce ratio according to the nominal composition, f Before calcination Cu1:Ce3/CNT.

The formation of Cu+ together with the presence of Ce3+ species is revealing that the redox equilibrium (Cu2+ + Ce3+ ↔ Cu+ + Ce4+) is shifting to right, which has significant impact on the catalytic activity

15

. XPS analysis of the spent catalyst revealed that the Cu+/Cu2+ ratio did not

change, thus maintaining the structure during the reaction. The chemical state of the elements on the surface of the catalyst before calcination were also evaluated, and the high amount of Cu2+ revealed that copper mostly existed the CuO form Owing to redox equilibrium, the amount of Cu+ increased after thermal treatment. The presence of (Cu+, Cu2+) and (Ce3+, Ce4+) redox couples in Cu/CNT and Ce/CNT catalysts, respectively recommend that there is an electron transfer also between copper or ceria to the CNT supports 41. Same electron transfers also may be available in Cu:Ce catalysts. However, the rate of the electron transfers between CNT support and Cu and Ce might be lower compare to the electron transfer in the redox equilibrium between copper and ceria (Cu2+ + Ce3+ ↔ Cu+ + Ce4+). 14 ACS Paragon Plus Environment

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Cu1:Ce3/AC

Cu2+

Cu+

2+

Cu

Cu+

(a)

Cu2+ shake-up satellite

Cu1:Ce3/CNT-Spent

Intensity (a.u.)

Cu1:Ce3/CNT-Before calcination Cu1:Ce3/CNT Cu1:Ce1/CNT Cu3:Ce1/CNT Cu/CNT

965

960

955

Cu1:Ce3/AC

u'''

950

945 940 Binding Energy (eV) u''

u'

u0,u v'''

935

v''

930

v'

v 0, v

925

(b)

v'' Cu1:Ce3/CNT-Before calcination

Cu1:Ce3/CNT-Spent

Intensity (a.u.)

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


Cu1:Ce3/CNT


925

915

905 895 Binding Energy (eV)

Figure 5. XPS spectra of Cu2p (a) and Ce3d (b) for the catalysts.


885

875


To verify the surface oxidation state as a function of composition, we further studied the representative bands of Ce3+ and Ce4+ ions. For all catalysts, the high-resolution spectrum of Ce 3d was numerically fitted with eight components, with the assignment defined in Figure 5(b). The bands labeled (v0, u0) and (u', v') represent the 3d104f 1 initial electronic state corresponding to Ce3+, whereas the peaks labeled (u, v), (u'', v''), and (u''', v''') represent the 3d104f 0 state of Ce4+ ions 1,15,42

. Results confirmed that the amount of Cu+ species on the surface of Cu1:Ce3 catalyst was

larger than that of other catalysts. The amount of Ce3+ in the catalyst was inversely proportional to the increasing Ce content. The lowest amount of Ce3+/Ce4+ species was observed on the surface of the Cu1:Ce3 catalyst. The CO–TPD analysis was used to study the CO adsorption on the catalyst surface. As shown in Figure 6(a), two peaks are located at around 100 °C−140 °C and 400 °C−460 °C in the CO–TPD profile of the Cu−Ce catalysts, showing different types of activated adsorption sites with different binding strengths on the catalysts surface. However, only one peak was observed at around 460 °C for the Cu/CNT catalyst. This difference can be due to the presence of different types of activated adsorption sites with different binding strengths. CO was difficult to adsorb on pure CuO at low temperature. Therefore, desorption peaks at approximately 460 °C for the Cu/CNT catalyst were ascribed to the desorption of CO from the Cu0 and/or Cu+ sites on the surface 43,44. This occurence has been confirmed by the XPS results (Cu+/Cu2+ = 1.49). The adsorption and desorption of CO at high temperatures and the low amount of Cu+ can cause the low activity of the Cu/CNT catalyst in NO reduction by CO at low temperature. The addition of cerium influenced the CO desorption behavior of the samples. A new broad peak of CO desorption was observed at 100 °C–140 °C. This low-temperature peak of the Cu:Ce/CNT catalysts was attributed to the desorption of CO on the Cu0 and/or Cu+ sites on the surface, which is in


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agreement with the increased Cu+ content as calculated by XPS. There is a broad peak of CO desorption at ~135 °C in the CO–TPD profile of Cu3:Ce1/CNT, and this peak shifted to lower temperature with the increase in Ce content. The Cu1:Ce3/CNT catalyst showed the lowest desorption temperature compared with the other Cu:Ce catalysts, which can be attributed to the high content of Cu+ on the catalyst surface. This effect can be the cause of the slightly higher catalytic activity of Cu1:Ce3/CNT compared with those of the other Cu:Ce catalysts in the CO oxidation and NO + CO reaction.

Signal (a.u.)

(a)

Cu1:Ce3/CNT Cu1:Ce1/CNT Cu3:Ce1/CNT Cu/CNT

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Temperature (°C) (b)

Signal (a.u.)

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Cu1:Ce3/CNT Cu1:Ce1/CNT Cu3:Ce1/CNT Cu/CNT

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350 450 550 Temperature (°C)

650

750

Figure 6. (a) CO-TPD profiles of catalysts with different Cu:Ce ratio, (b) NO-TPD profiles of catalysts with different Cu:Ce ratio. 17 ACS Paragon Plus Environment


The NO–TPD profiles of the catalysts are shown in Figure 6(b). NO desorption peaks were observed at around 100 °C–200 °C and 350 °C–750 °C. For the Cu/CNT catalyst, two sharp peaks were observed at 460 °C and 615 °C, indicating that NO was adsorbed on different active sites. The addition of Ce improved the adsorption capacity for NO at low temperature. NO desorption peaks at ~150 °C were attributed to the desorption of physically adsorbed NO from copper and cerium ions 45,46. Desorption peaks at above 400 °C were attributed to the decomposition of nitrite and nitrate species, which exhibit high thermal stability 47,48. As a result, more nitrite and nitrate species on the Cu1:Ce3/CNT catalyst’s surface can participate in the low temperature NO + CO reaction compared with the surface of the catalyst without cerium. The NO–TPD result suggest that catalysts with increased amount of cerium can adsorb and dissociate NO at low temperatures, resulting in enhanced catalytic activity. Hence, the Cu1:Ce3/CNT catalyst may possess more unpaired electrons than other samples. Back-donation and releasing of these unpaired electrons into the empty antibonding orbital of the adsorbed NO species can weaken the N–O bond, leading to the dissociation of adsorbed NO species 44.

3.2. Catalytic performance tests 3.2.1. Effect of catalyst composition Figure 7 shows the catalytic performance of the Cu:Ce/CNT catalysts with different ratios of Cu:Ce. The catalysts for NO reduction by CO were prepared by co-impregnation. The NOx conversion of Cu/CNT and Ce/CNT was negligible and the catalytic performance improved with increasing cerium oxide. The reduction behavior of the Cu:Ce/CNT catalyst with increased Ce content was superior to that of other catalysts possibly because of sufficient contact among components. Co-impregnation is beneficial for the generation of low-valence-state copper species


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(Cu+/Cu0) and the formation of additional oxygen vacancies during reaction. This effect was conducive to the adsorption and dissociation of CO and NO species, respectively, and effectively promotes catalytic performance. Results of the present study revealed that the Cu1:Ce3/CNT catalyst exhibited the highest NOx conversion rate of 96%. 120 Cu/CNT Cu3:Ce1/CNT

100

NOx conversion (%)

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



80

Ce/CNT

60 40 20 0

140

160

180

200 220 Temperature (°C)

240

260

Figure 7. NOx conversion over Cu:Ce/CNT, reaction conditions: NO=250 ppm, CO=5000 ppm, total flow rate: 300 ml/min; Temperature: 140 °C–260 °C, catalyst: 200 mg, GHSV=12600 h-1.

Yao et al. 1 used Ce0.67Sn0.33O2, Ce0.67Zr0.33O2, and Ce0.67Ti0.33O2 solid solutions as a support to prepare supported copper oxide catalysts for the removal of NO by CO. They found that CuO/Ce0.67Zr0.33O2 is more easily reduced compared with other samples possibly owing to the differences in the electronegativity of the dopant. The large gradient of electronegativity between Cu and other components (Ce, Zr, Ti) is reportedly useful for obtaining Cu+ and Cu0 species. Li et al. 15 investigated reduction of NO by CO in CuO–Ce0.9Zr0.1O2 catalysts prepared by a simple co-



precipitation method. Variation in Ce/Zr ratio in this catalyst resulted in different interactions between Ce and Cu, which affects the formation of Cu+ species.

3.2.2. Effect of reaction temperature Temperature dependency of NO catalytic reduction by CO over different CNT supported catalysts is shown in Figure 7. Low NOx conversion of Cu1:Ce3/CNT catalyst was observed at low temperature (140 °C), whereas enhancement was observed at temperature exceeding 180 °C. The effect may be due to the reduction of the catalyst by CO at increased temperature, which led to the formation of increased surface oxygen vacancies; moreover, increased temperature provided sufficient energy for dissociation of NO on SOV

35,38

. In addition, during CO oxidation in the

CuO–CeO2 catalyst, CO molecules can be efficiently adsorbed by Cu+, resulting in enhanced reaction 49,50. The chemisorption of CO on Cu+ sites and increased oxygen vacancies on the surface of the catalyst led to an increase in the catalyst reactivity. Possible reactions over the prepared catalysts are summarized below 51: 2NO  N2 + O2

(1)

2NO + CO  N2O + CO2

(2)

N2O + CO  N2 + CO2

(3)

NO + CO  ½ N2 + CO2

(4)

CO + ½ O2  CO2

(5)

At a temperature of 180 °C and above, surface reduction led to the gradual enhancement of NO reduction to N2. The NO + CO reaction was highly active, and superb NO reduction performance was expected. From another perspective, N2O can be considered as the intermediate for NO reduction into N2, and reaction 4 can be the overall reaction of reactions 2 and 3. The most


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interesting results were obtained with Cu1:Ce3/CNT catalyst at 220 °C and 240 °C, because NO reduction by CO was already occurring in this temperature range, with increasing selectivity toward N2. The CNT support was not oxidized, thereby preventing undesired carbon gasification 20

. An increasing in reaction temperature to 260 °C resulted in a decrease in NOx conversion. This

effect can be due to the destruction of the catalyst structure and the gasification of carbon at high temperature, which was in accordance with the TGA analysis’ results.

3.2.3. Effect of oxygen concentration The influence of oxygen concentration was evaluated on the NO reduction performance of the Cu1:Ce3/CNT catalyst at 220 °C (Figure 8). Possible reactions comprise direct decomposition of NO and NO reduction by CO. In the absence of oxygen, only NO and CO are involved in the NO related reaction. But when oxygen is added to the feed gas, the competitive reactions (reactions 4 and 5) occurred and CO conversion was considerably higher than that in the absence of oxygen, and this outcome is partially due to CO oxidation in reaction 5. When oxygen concentration is below the stoichiometric value required for CO oxidation (< ~0.3%), after consumption of oxygen the excess CO can be used efficiently for the reduction of NO, and high NO conversion can be obtained. In the presence of high oxygen concentration (> ~0.3%), the reaction is in an oxidative atmosphere, and most of the CO is oxidized by oxygen and the conversion of NO remains almost zero. At the investigated oxygen concentration levels above 0.3% almost no NOx conversion was observed, and NO conversion decreased considerably and continuously with increasing oxygen concentration. The oxygen inhibition effect resulting from the consumption of the reducing agent is a problem in the reduction of NO by CO or by hydrocarbons over different catalysts 20,52.



Figure 8. Effect of oxygen concentration on NO–CO reaction over Cu1:Ce3/CNT catalysts, reaction conditions: NO=250 ppm, CO=5000 ppm, O2=0.1-5%, total flow rate: 300 ml/min; catalyst: 200 mg, temperature: 220 °C, GHSV=12600 h-1.

High catalytic activity at O2/CO ratio, up to 0.6, revealed that the Cu1:Ce3/CNT catalyst was comparable with other reported results for reduction NO by CO in the presence of oxygen. Deng et al.

44

reported that over CuO-supported on CeO2-doped TiO2, a NO conversion rate of 100%

was obtained for the NO + CO reaction in the absence of oxygen at 350 °C. Wen et al. 53 reported that at 300 °C and with an O2/CO ratio of 0.36, over Cu/Ce/Mg/Al mixed oxide catalyst obtained 100% NOx conversion rate. Li et al. 54 studied NO reduction by CO over an Fe-based catalyst at 700 °C; they observed that the highest NO conversion of 100% was obtained in the absence of oxygen.; increase in O2/CO ratio to 0.4 resulted in decreased NO conversion to 55%, whereas increased oxygen concentration above this value led to a sharp decrease in NO conversion to zero. 22 ACS Paragon Plus Environment

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Owing to the competition between NO and oxygen for CO, the NO + CO reaction is a non-selective reduction reaction in the presence of oxygen. At increased oxygen concentrations, CO preferentially reacts with oxygen. The metal/support interface and electron transfer are important factors in the catalytic activity and it is important to have an optimum structure of the interface between the metals and the supports 41,55. In addition to the redox equilibrium between copper and ceria (Cu2+ + Ce3+ ↔ Cu+ + Ce4+), there is also an electron transfer between the support and metals (Cu and Ce). During the reaction in the presence of O2, the electron transfers between metals (Cu and Ce) and O2 molecule also occur and weakens the O−O bond, and results in the formation of more active O2− species 41, which may easily react with CO. Catalytic performance may be attributed to the nature of the catalyst, such as high surface area and mesoporous structure, and uniform dispersion of metal species. It is worth mentioning that type of support also plays an important rule, and it may significantly affect the catalytic activity. The Cu/TiO2 catalyst showed good stability for reduction of NO with CO in the presence of excess oxygen

56,57

, and the NO conversion of 50% obtained while the feed gas contained

CO=NO=400 ppm, and 2 vol.% of oxygen at 200 °C. However, they have reported that addition of ceria to the Cu/TiO2 catalyst resulted in a decrease in NO conversion to 35% which could have attributed to the enhancement of the catalytic activity toward CO oxidation due to the increasing of the electron transfer between the metals and oxygen and formation of more active oxygen species to react with CO. A negligible amount of NO2 (less than 2 ppm) was formed during the reaction over the Cu1:Ce3/CNT catalyst at different O2 concentrations. Nitrogen dioxide was formed (about 10−30 ppm) when oxygen was added to the feedstock, and then NO2 reached a constant concentration of less than 2 ppm at all O2 concentrations after ~10 min. In the presence of oxygen, a rapid



homogeneous gas-phase oxidation of NO to NO2 occurred, forming NO2. NO2 subsequently reacted with CO to produce N2

51

. In the presence of oxygen, the formation of strongly bonded

oxidized species leads to the formation of NO2, nitrite, and nitrate groups. These components decomposed, resulting in the evolution of NO gas. Hence, oxygen can oxidize the catalyst surface and gas-phase NO to NO2, favoring NO adsorption 51. Catalyst stability is an important parameter in industrial application. Therefore, catalyst stability of Cu1:Ce3/CNT was evaluated under identical reaction conditions. The catalyst activity remained constant, and formation of NO2 was negligible during 8 h of reaction.

3.2.4. Effect of support Metal precursors (Cu and Ce) are also dispersed on AC as other carbonaceous porous solid carriers, and its activity in NO–CO reaction has been studied and compared with CNT-supported catalyst. NOx conversion over CNT-supported (96%) catalyst was higher than the conversion over Cu1:Ce3 catalysts supported on AC (41%) (Figure 9). Carbonaceous supports are good potential alternatives to overcome this problem. Activated carbon is expected to demonstrate superior results to those of metallic supports owing to its stability at high temperature and resistance to basic or acidic media. However, in present research, in comparison with AC, CNTs exhibit superior mass transition efficiency, thermal stability, as well as better dispersion of nanoparticles 58,59. In addition, the mesoporous structure of the support plays an important role in the improvement of catalytic performance by allowing the penetration of macromolecules and their adsorption on the surface of the catalyst. Acid treatment with HNO3 and H2SO4 is the most common technique for surface modification; this method introduces functional oxygen-containing groups 60, resulting in breaks in nanotubes and removal of amorphous carbon. Capillary forces facilitate the access of metal salt


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solutions during impregnation, and acid modification allows the metals to bind to the surface of CNTs. 100 Cu1:Ce3/CNT

80 NOx conversion (%)

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


60 40 Cu1:Ce3/AC

20 0 0

20

40

60

80

100

120

Time (min) Figure 9. Effect of support on NO–CO reaction over Cu1:Ce3/CNT and Cu1:Ce3/AC catalysts, reaction conditions: NO=250 ppm, CO=5000 ppm, total flow rate: 300 ml/min; catalyst: 200 mg, temperature: 220 °C.

XRD and XPS analyses were conducted to determine the differences in structure between Cu1:Ce3/CNT and Cu1:Ce3/AC catalysts; these structural differences caused large differences in catalytic activity in the reduction of NO by CO. The XRD patterns of the AC- and CNT-supported catalysts are shown in Figure 3(b). The peaks at 2θ= 26° for CNTs and AC are attributed to the graphene structures of carbon adsorbents, which is more pronounced in CNT-supported samples. In comparison with AC, MWCNTs exhibit higher crystallinity and ordering between graphene layers. AC is an amorphous carbon material with a short range of crystallinity

61

. XRD results

revealed that the peaks belonging to CeO2 are visible for both CNT- and AC-supported catalysts at ~29°, 34°, 48°, and 57°, but the peaks belonging to CuO and Cu2O were not visible for ACsupported catalyst at 35.5°, 38°, and 43.5°. 25 ACS Paragon Plus Environment


XPS analysis was also performed to compare the composition and chemical state of the sample surface in Cu1:Ce3/AC and those in CNT-supported catalysts. As shown in Figure 5, the Cu 2p and Ce 3d spectra of the catalysts were deconvoluted into component peaks. The ratios of these peaks are shown in Table 1. In the Cu 2p spectra of the AC-supported catalyst, the components at 932 and 951.6 eV can be ascribed to the Cu 2p3/2 and Cu 2p1/2 peaks of Cu2O, respectively. The peaks at 933.6 and 954.1 eV are assigned to the Cu 2p3/2 and Cu 2p1/2 peaks of CuO, respectively

1,15,37,62

. For the AC-supported catalyst, the peaks assigned to Cu2+ was more

prominent compared with the Cu2+ peaks in CNT-supported catalyst, and the Cu+/Cu2+ ratio for AC-supported catalyst (2.65) was lower than the ratio for the CNT-supported catalyst (4.01) (please refer to table 1). The findings were in agreement with the XRD results. Moreover, higher intensity of copper species was observed in CNT-supported catalysts compared with AC-supported catalysts. The Ce 3d XPS spectra of Cu1:Ce3/AC (Figure 5) were deconvoluted into component peaks, which are ascribed to the Ce 3d5/2 and Ce 3d3/2 spin-orbit components. The predominant valence of Ce in the Cu1:Ce3/AC catalyst was +3, and the calculated content ratio of Ce3+/Ce4+ was 0.88 (Table 1), which was higher than the Ce3+/Ce4+ ratio in the CNT-supported catalyst (0.55). The O 1s XPS spectra of Cu1:Ce3/CNT and Cu1:Ce3/AC catalysts are shown in Figure 10. The peak belongs to C=O bonds in carbonyl and carboxylic acids observed at 531.4 eV (peak 4). The peak at 532.4 eV (peak 5) attributed to O=C–O was assigned to carboxylic group, anhydride, lactone, and ester. The peaks at 533.5 (peak 6) and 534.8 eV (peak 7) were ascribed to the C−O bonds in hydroxyl group and ether and those in carboxyl groups, respectively 63-65. Oxygen ions in CeO2 reportedly exhibited intense peaks at 528.6−530.1 eV. In Figure 12, the peak corresponding


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to CeO2 can be seen clearly (peak 1 at 528.6 eV). The peaks at 530.5 (peak 2) and 529.4 eV (peak 3) were assigned to Cu2O and CuO, respectively 66.

C-O(H) Cu1:Ce3/CNT

O-C=O

C=O, C-OH

Cu2O

CuO CeO2

C-O

Intensity (a.u.)

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


7

539

537

3

5

Cu1:Ce3/AC

535

6

2

4

533 531 Binding energy (eV)

1

529

527

525

Figure 10. XPS spectra of O 1s for Cu1:Ce3/CNT and Cu1:Ce3/AC catalysts.

Table 2. Relative contents of functional groups of the O 1s spectra. Catalyst Cu1:Ce3/CNT Cu1:Ce3/AC

Total O (at.%) 5.24 9.75

Peak1 35.04 9.64

Peak2 1.94 9.9

Peak3 22.15 35.25

Peak4 16.52 3.14

Peak5 15.77 31.39

Peak6 5.31 5.29

Peak 7 2.18 3.13

The total oxygen contents of Cu1:Ce3/CNT and Cu1:Ce3/AC were 5.24 at.%, and 9.75 at.%, respectively. The main type of functionality was O=C–O, which was present at 15.77 and 31.39 at.% for CNT- and AC-supported catalysts, respectively (Table 2). Activated carbon contains rich surface oxygen-containing groups, such as carboxyl, quinine carbonyl, and phenolic hydroxyl groups. These groups reduce the surface basicity/acidity, hydrophobicity of carbon, electronic properties, and the ionic exchange capability. Thus, the dispersion of active species on the supported catalyst and the interaction between the support and the active sites are affected by 27 ACS Paragon Plus Environment


oxygen-containing groups. Furthermore, AC contains more acidic groups than CNTs, and the oxidabizability of oxygen-containing groups confer increased strength to AC in oxidizing environments. Based on the results of the O 1s XPS spectra of the samples, Cu2O is the main oxidation state of copper at the surface of both AC- and CNT-supported catalysts, the ratio of peaks corresponding to Cu2O/CuO (Peak3/Peak2) in CNT-supported catalysts (11.42) exceeds that in AC-supported catalysts (3.56). The findings are in agreement with the results obtained from the XPS analysis of the Cu 2p spectra in these catalysts. Table S2 shows a comparison of catalytic performance in the NO + CO reaction between the Cu:Ce/CNT catalyst prepared in the present study and other catalysts reported in literatures at 220 °C. Other catalysts were not active at 220 °C, whereas the prepared catalyst from the present study exhibited approximately 96% conversion. Alumina, cerium oxide, and titanium oxides are mostly used as support for these catalysts. CNTs have been used as support for the catalytic reduction of NO by CO. Dasireddy et al. 31 reported that the Cu−Fe catalyst supported on CNTs shows high NO conversion rate. Cu–Fe/CNT exhibited a lower reduction peak temperature compared with Cu–Fe/Al2O3. However, the Cu−Fe/CNT catalyst was only able surpass 95% NO conversion rate at a high temperature of 450 °C. According to another study, the presence of Ce in the catalyst structure and the use of CNT as a support result in superior dispersion of metals on the catalyst surface. In effect, this situation results in decreased reduction temperature, increased SOVs, facilitated dissociation of NO species, and increased Cu+ species for the adsorption of CO molecules. Therefore, compared with the other reported catalyst, the Cu:Ce/CNT catalyst prepared in the present study shows higher NO reduction at lower temperature.


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3.2.5. Evaluation of catalyst performance for NO oxidation and CO oxidation reactions Further investigation was conducted to increase our understanding of the reaction mechanism. The catalytic activity of Cu1:Ce3/CNT in individual CO and NO oxidation reactions at different temperature was evaluated. The results are shown in Figure 11. CO

CO2

120

CO conversion

(a)

5000

100

4000

80

3000

60

2000

40

1000

20

0

CO conversion (%)

Concentration (ppm)

6000

0 27

40

50

60

70

80

90 100 120 140 160 180 200 220 240 Temperature (°C)

NO conversion

(b)

30

250

25

200

20

150

15

100

10

50

5

0

0 25

50

75

100

125

150

175

220

NO conversion (%)

NO

300 NO concentration (ppm)

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


240

Temperature (°C)

Figure 11. a) CO oxidation (CO=5000 ppm), b) NO oxidation (NO=250 ppm) over Cu1:Ce3/CNT catalysts, reaction conditions: O2=2%, total flow rate: 300 ml/min; catalyst: 200 mg, GHSV=12600 h-1.



As seen in Figure 11(a), the CO conversion rate at temperature exceeding 90 °C reached 100%, and CO was completely converted to CO2. Hence, the catalyst is highly efficient in CO oxidation. Increased reaction temperature facilitated the reduction of Cu–O–C species by CO and created a large amount Cu+ species, which was favorable for CO adsorption. In the presence of oxygen, these adsorbed CO species can be easily oxidized and converted to carbon dioxide. NO oxidation results reveal that the catalyst is not active for NO oxidation at low temperature, and NO conversion reaching only 18% at 240 °C [Figure 11(b)]. The dissociation of NO plays an important role in reduction of NO by CO. Dissociation or desorption of NO molecules occurred when the reaction temperature was increased, resulting in the opening of active sites for CO adsorption 44, 67. Dissociation of NO at temperature levels exceeding 200 °C was also in accordance with the obtained results in the NO + CO reaction. NO conversion can be significantly increased by increasing the temperature to 180 °C and reach the maximum value at 240 °C (Figure 7). This effect can be due to the oxygen vacancies created by the dissociation and adsorption of NO molecules at temperature above 200 °C.

3.3. Possible mechanism for reduction of NO by CO To understand the nature of the NO + CO reaction, a possible mechanism was proposed as shown in Figure 12. When Cu:Ce/CNT catalyst were exposed to the NO and CO mixed gases, NO molecules were preferentially adsorbed on the catalyst’s surface because their unpaired electrons prevented the adsorption of CO species

1,36

. Dissociation of NO is an important step for NO

reduction by CO, and dissociation can be promoted by oxygen vacancies 43, 66. Increased reaction temperature resulted in the dissociation or desorption of NO species; thus, active sites were


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released for CO species adsorption. Simultaneously, Cu–O–C species were reduced by CO, creating abundant oxygen vacancies and Cu+ species, which are favorable for CO adsorption. Dissociation of NO species resulted in the formation of oxygen radicals, which can react with the adsorbed CO molecules on Cu+ species; N radicals can combine together to form N2 or combine with another NO species and form N2O. Reduced Cu+ was present in the catalyst as a result of the shift in redox equilibrium to right (Cu2+ + Ce3+ ↔ Cu+ + Ce4+). NO reduction by CO was enhanced by the synergistic effect of SOV and Cu+ species 1, resulting in the good catalytic activity of Cu:Ce/CNT catalysts. The reduction of NO by CO in the presence of oxygen exceeding the stoichiometric value needed for CO oxidation did not follow the proposed mechanism. In the presence of large amounts of oxygen, most of the CO was oxidized to CO2, the NO + CO reaction did not occur, and NO conversion remained almost zero. This outcome was due to the competition between oxygen and NO in reacting with CO.

CO2 Cu2+

N2O/N2 NO/N2

O

O

N

C

3+

Ce3+ Ce

Oxygen vacancy

Ce4+

Ce4+ Ce3+

Cu+

N

C

Cu+

N

NO

O

O

O

CO2

Cu2+ N2O/N2

MWCNT

Figure 12. Possible reaction mechanism for NO reduction by CO over the Cu:Ce/CNT catalyst. 31 ACS Paragon Plus Environment


4. CONCLUSION In this study, a series of CNT-supported Cu:Ce catalysts were prepared by co-impregnation to investigate the influence of different ratios of active metals on catalyst structure and catalytic performance in NO reduction by CO. On the basis of the obtained results, it can be concluded that: 1) the high catalytic activity of 96% at 220 °C for NO + CO reaction was observed over the Cu1:Ce3/CNT catalyst due to the large SOV concentration, superior reducing capability, and high Cu+ species content; 2) the synergistic effect between SOV and Cu+ species in Cu:Ce/CNT catalysts play an important role in the NO + CO model reaction. The redox equilibrium from the interaction between cerium and copper resulted in the formation of Cu+ species on the surface of catalyst; 3) NOx conversion over CNT-supported catalyst was higher than that of Cu1:Ce3/AC catalyst. CNTs with mesoporous structure exhibit better mass transition efficiency, thermal stability, as well as better dispersion of nanoparticles. A temperature of around 200 ℃ to 240 °C is favorable for NO + CO reaction in the presence of oxygen over Cu1:Ce3/CNT catalyst. Compared with Cu1:Ce3/AC catalyst, CNT-supported catalyst showed more prominent amount of Cu+, which plays an important role in catalytic activity in the reduction of NO; 4) the presence of oxygen at equal or less than the stoichiometric value needed for CO oxidation did not affect the catalyst performance in the reduction of NO by CO. However, NO reduction by CO at different oxygen concentrations, exceeding the stoichiometric value needed for CO oxidation, resulted in considerable and continuous decrease in NO conversion with increasing oxygen concentration. In the presence of high oxygen concentration (O2/CO>0.6), the Cu:Ce/CNT catalyst can effectively catalyze the CO−O2 reaction. The Cu1:Ce3/CNT catalyst found to be very active for CO oxidation reaction and the CO conversion rate at 100 °C reached 100%, and CO was completely converted to CO2. However, the catalyst was not active for NO oxidation at low temperature.


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Acknowledgement This work was supported by the Research Fellowship for International Young Scientists, National Natural Science Foundation of China (NSFC) [Grant Number 21750110436].

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org at DOI ………… Table S1. EDS and XRF analysis results and textural properties of CNT and Cu:Ce catalysts supported on CNT; Table S2. Catalytic activity of different catalyst for reduction of NO by CO at 220 °C; Figure S1. TEM images of (a) acid treated CNT, (b) Cu1:Ce3/CNT, (c) Cu1:Ce1/CNT, (d) Cu3:Ce1/CNT; Figure S2. Nitrogen adsorption–desorption isotherms of CNT and Cu:Ce/CNT catalysts.

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