Catalytic Oxidation of NO over CNF/ACF-Supported CeO2 and Cu ...

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Catalytic Oxidation of NO over CNF/ACF-Supported CeO2 and Cu Nanoparticles at Room Temperature Priyankar Talukdar,† Bhaskar Bhaduri,† and Nishith Verma*,†,‡ †

Department of Chemical Engineering and ‡Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India ABSTRACT: The present study describes the development of carbon micro- and nanofibers (CNFs) in situ dispersed with ceria (CeO2) and Cu metal nanoparticles (NPs) for the oxidative removal of NO at room temperature. CNFs/activated carbon fibers (ACFs) were prepared by growing CNFs on an ACF substrate using chemical vapor deposition at 450 °C. The Cu NPs played dual roles: (1) catalyzing the growth of the CNFs and (2) catalytically oxidizing NO to NO2. The synergistic interaction between the Cu NPs and CeO2 enhanced the oxidation rate. The maximum NO conversion using the CeO2−Cu−CNFs/ACFs developed in this study was ∼80% for a 500 ppm NO concentration at room temperature (∼30 °C). A mathematical model was developed to explain the proposed mechanism for NO oxidation, incorporating the kinetic rate and mass-transfer effects in the tubular reactor. The CNF/ACF-supported CeO2 and Cu bimetal catalyst prepared in this study represents a promising candidate for abating NO emissions by oxidation at room temperature.

1. INTRODUCTION Selective catalytic reduction (SCR) is commonly used for the control of nitrogen oxide (NO), using ammonia (NH3) or urea as a reducing agent.1 There are a few drawbacks in this method. The reduction temperature is relatively high (150−600 °C). There are operational difficulties associated with the storage and handling of NH3. Furthermore, the fugitive emission of NH3 is a concern. On the other hand, the decomposition of urea at high reduction temperatures limits the efficiency of the method. NO can be removed by catalytic oxidation at room temperature or relatively lower temperatures (50−350 °C) using different catalysts such as platinum (Pt) and gold (Au) noble metals, or the oxides of vanadium, iron, cobalt, and nickel transition metals, supported on various substrates including zeolites and activated carbons.2−4 Bimetallic oxides have also been used for the catalytic oxidation of NO.5−7 The conversion in these studies, however, has been shown to be limited to 50% or lower at room temperature (∼30 °C) and 80% at 350 °C. Recently, activated carbon fibers (ACFs) and relatively newer forms of carbon such as carbon nanofibers (CNFs) and carbon nanotubes (CNTs) have been studied as adsorbents and supports for metal catalysts because of their high Brunauer− Emmett−Teller (BET) surface areas, uniformity in pore size distribution (PSD), and amenability toward surface functionalization.8−10 The superiority of CNFs over ACFs in catalytic and adsorption applications lies in the relatively higher stability of CNFs in acidic/basic media and chemical activity.11−14 ACFor CNF-supported metal catalysts have also been used in the abatement of NO emissions by oxidation or reduction.15−17 The present study describes the development of CeO2 (ceria)- and Cu nanoparticle- (NP-) dispersed CNFs/ACFs for the removal of NO by oxidation at room temperature. The CNFs/ACFs were prepared by growing CNFs on an ACF substrate using catalytic chemical vapor deposition (CVD). Ceria and Cu NPs were in situ incorporated into the ACFs prior to CVD. The Cu NPs played dual roles: (1) catalyzing the © 2014 American Chemical Society

growth of CNFs and (2) catalytically oxidizing NO to NO2. Ceria had a promotional effect on the catalytic activity of Cu through the release of nascent oxygen during the redox cycle and a synergistic interaction with the Cu NPs. The oxidation reaction was performed in a perforated tubular reactor wrapped with CeO2−Cu−CNFs/ACFs using various oxygen (O2) and NO concentrations. The experiments were sequentially performed on different materials: first the ACF substrate (without metals) and then single-metal-based materials, namely, Cu−ACFs, Cu−CNFs/ACFs, and CeO2−ACFs, and finally, the CeO2−Cu−CNF/ACF bimetal-based material to understand the individual roles of Cu NPs, CNFs, and CeO2 in the oxidation of NO. A kinetic mechanism for the oxidation of NO to NO2 over the catalyst produced in this study was proposed and incorporated in a transport-based mathematical model to explain the experimental data for the oxidation of NO in the tubular reactor.

2. THEORETICAL STUDY 2.1. Kinetics. The oxidation of NO on CeO2−Cu−CNFs/ ACFs consists of two simultaneous steps. Step A involves the adsorption/desorption of NO and O2 at the catalyst surface, followed by the catalytic oxidation of NO to NO2. Step B involves the synergistic interaction between CeO2 and Cu in a redox cycle, releasing the lattice oxygen, which oxidizes NO to NO2. Oxygen is dissociatively adsorbed on the vacant sites of ACFs. It reacts with the NO adsorbed on the adjacent sites to produce NO2. The adsorbed NO undergoes transformations in several steps to produce intermediate surface complex compounds and NO2, leaving behind the active sites for Received: Revised: Accepted: Published: 12537

May 19, 2014 July 12, 2014 July 16, 2014 July 16, 2014 dx.doi.org/10.1021/ie502043e | Ind. Eng. Chem. Res. 2014, 53, 12537−12547

Industrial & Engineering Chemistry Research

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Figure 1. Schematic diagram of the adsorption−desorption reaction and synergistic interaction between ceria and Cu.

successive adsorption. The redox reaction involving the synergistic interaction between Ce3+ and Cu2+ produces lattice oxygen and restores the oxidation state (4+) of Ce in ceria. Such synergistic effects between different metals have been observed in several catalytic reactions.18,19 Figure 1 schematically describes adsorption/desorption of the reacting species and the synergistic interaction between CeO2 and Cu NPs. A similar type of study incorporating the adsorption−desorption reaction and synergistic interaction between ceria and Cu can also be observed elsewhere.20 The proposed mechanism for the reaction, 2NO + O2 → 2NO2 is as follows Step A involves the adsorption−desorption of NO and O2 on CNFs/ACFs and oxidation of NO to NO2.

Assuming a Langmuir−Hinshelwood type of rate mechanism with reaction step vi as the rate-determining step and steps i−v in a quasi-equilibrium state, the overall rate of oxidation can be written as dC 1 dC NO2 = − NO = k6[NO−NO3−X] 2 dt dt

The equilibrium constants can be written in terms of the steady-state concentrations of different adsorbed species, as follows

K1 =

[NO−X] [NO][X]

k1

NO + X HooI NO−X k −1

k −2

NO−X + O−X HoooooI NO2 −X + X k −3

k −4

k −5

k6[Cu]

NO3−NO−X ⎯⎯⎯⎯⎯→ 2NO2 + X

(iii)

K4 =

(iv)

K5 =

(vii)

Ce3 + + Cu 2 + → Ce 4 + + Cu+

(viii)

1 O2 → Cu 2 + + O−(adsorbed) 2

[NO2 −X]2

(5)

[NO3−NO−X][X] [NO3−X][NO−X]

(6)

[X 0] = [X] + [NO−X] + [O−X] + [NO2 −X]

(vi)

2CeO2 → Ce2O3 + O(lattice)

[NO3−X][NO][X]

where Ki = ki/k−i represents the ratio of forward to backward rate constants. The overall site balance can be written as

(v)

+ [NO3−X] + [NO3−NO−X]

Step B involves the release of nascent oxygen and synergistic interaction between ceria and Cu NPs.

Cu+ +

(4)

K3 =

k5[Cu]

NO3−X + NO−X HoooooI NO3−NO−X + X

[NO2 −X][X] [NO−X][O−X]

(ii)

k4[Cu]

2NO2 −X HoooooI NO3−X + NO + X

(3)

K 21/2 =

k 3[Cu]

(2)

[O−X] [O2 ]1/2 [X]

(i)

k2

O2 + 2X HooI 2O−X

(1)

(7)

Expressing the number density of adsorbed sites in terms of the major species, the rate expression for the oxidation of NO to NO2 can be obtained as follows −

(ix)

k6K1′[NO]2 1 dC NO = 2 dt K 2′ + K3′[NO] + K4′[NO]2

(8)

where 12538

dx.doi.org/10.1021/ie502043e | Ind. Eng. Chem. Res. 2014, 53, 12537−12547

Industrial & Engineering Chemistry Research K1′ = K13K 2K32K4K5[O2 ][X 0]

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constants Kc1 and Kc2 of eq 15 were used as adjustable model parameters.

(9)

K 2′ = 1 + K 21/2[O2 ]1/2

(10)

K3′ = K1 + K1K 21/2K3[O2 ]1/2 + K12K 2K32K4[O2 ]

(11)

K4′ = K1′K 2K3′K4K5[O2 ]

(12)

3. EXPERIMENTAL SECTION 3.1. Materials. The phenolic resin precursor-based ACFs were procured from Nippon Kynol Inc. (Osaka, Japan). Copper(II) nitrate trihydrate [Cu(NO3)2·3H2O] (purity > 95%) was purchased from Fisher Scientific (Pittsburgh, PA). Sodium dodecyl sulfate (SDS) was purchased from Merck (Darmstadt, Germany). Cerium(III) nitrate hexahydrate [Ce(NO3)3·6H2O] (purity > 99%) was purchased from SigmaAldrich (Munich, Germany). Nitric acid (HNO 3) was purchased from Fisher Scientific (Pittsburgh, PA). Hydrogen (purity > 99.99%), nitrogen (purity > 99.99%), oxygen (purity > 99.99%), and acetylene (C2H2, AAS grade) gases were purchased from Sigma Gases (Mumbai, India). 3.2. Pretreatment of ACFs. The ACF samples were first treated with 0.4 M HNO3 for approximately 2 h at 100 °C to remove any undesired species from the surface of the material. After the treatment, the ACFs were washed several times with deionized (DI) water and dried in air for 6−7 h and then in an oven for another 12 h at 120 °C. Finally, the ACFs were vacuum-dried at 200 °C for 12 h to remove the entrapped gases within the pores. 3.3. Preparation of CeO2- and Cu-NP-Dispersed ACFs and CNFs/ACFs. Cu(NO3)2·3H2O and Ce(NO3)3·6H2O salts mixed in DI water were used as the precursors for CeO2 and Cu NPs, respectively. The ACF samples were wrapped over the perforated glass tube contained in a tubular shell. The setup used for impregnation was described in a previous study.12 The ACF samples were impregnated with 100 mL of a mixture made up of 0.4 M Ce(NO3)3·6H2O and 0.4 M Cu(NO3)2· 3H2O salt solutions in a 1:3 volume ratio. Approximately 0.3% (w/w) SDS surfactant was added to the impregnating solution to increase the monodispersion of the salts in the solution, with minimal agglomeration and a relatively high metal loading on the ACF surface. The total concentration of the impregnating salt solution was optimized at 0.4 M. The impregnation of the ACFs with the solution at concentrations greater than 0.4 M resulted in the blockage of the ACF pores with the excess salt crystals. Similarly, impregnation using relatively greater amounts of ceria resulted in the underperformance of the material, as discussed later. To this end, the solution was continuously recycled for 24 h using a peristaltic pump rotated at 105 rpm. After the impregnation, the samples were dried at room temperature for approximately 6 h and then in the oven at 100 °C for 12 h. The salt-impregnated ACF samples were calcined and subjected to reduction using H2. Temperature-programmed reduction (TPR) was performed a priori using H2 (5%) and N2 (95%) to determine the optimum calcination and reduction temperatures, as discussed later. The ACFs were wrapped over a stainless steel (SS) mesh and placed within a tubular reactor (i.d. = 30 mm, L = 0.8 m) mounted inside in the programmable horizontal electric furnace (HEF). Cu NPs were produced in situ in the ACFs, using calcination for 4 h at 200 °C in an inert atmosphere maintained by N2 gas at a constant flow rate of 40 standard cm3 per min (sccm), followed by reduction at 300 °C for 2 h using H2 at a flow rate of 120 sccm. The reduction temperature for CeO2 is relatively higher (>850 °C), and therefore, it remained dispersed as CeO2−x in the substrate.21

For the case of an external supply of O2, the concentration of NO at the inlet of the reactor is insignificant in comparison to O2. Therefore, the concentration-squared term, namely, [NO]2 in the denominator of eq 8, can be neglected, and eq 8 can be simplified as follows K c1[NO]2 1 dC NO − = 2 dt 1 + K c2[NO]

(13)

where K c1 =

k6K1′ K 2′

and

K c2 =

K3′ K 2′

(14)

Clearly, the rate expression for the noncatalytic oxidation, namely, NO oxidation, using the ACF substrate alone assumes a form similar to eq 13. However, the numerical value of the rate constant k6 for the noncatalytic oxidation is relatively smaller (or the activation energy is larger), as shown later. For the case of no external supply of O2, the kinetic rate expression can be obtained by following a procedure similar to that described above −

K c1[NO] 1 dC NO = 2 dt 1 + K c2[NO]

(15)

Reaction ii of step A is redundant and not considered in deriving the rate expression (eq 15). Reaction iii is modified, as the required O concentration for the oxidation is available from the surface functional groups. We later revisit this aspect. The notable difference between eq 13 and eq 15 is the square dependency of the oxidation rate on the NO concentration, if the reaction is performed using the externally supplied O2 and the linear dependency if the reaction is performed without any external O2. The term with the NO concentration squared in the numerator of eq 15 arises because of the dissociative adsorption of O2 species on the surface. The rate expression for NO oxidation on the ACF-supported CeO2 without an external supply of O2 can be shown to assume a form similar to that of eq 15, with the rate having a linear dependency on the NO concentration. In such a case, ceria is the source for lattice oxygen, as shown in the step B above. 2.2. Model Development. Three mechanistic steps are involved in the oxidation reaction performed in the tubular reactor: (1) mass transfer of NO from the gaseous phase to the surface of carbon fibers, (2) diffusion within the pores of the fibers, and (3) catalytic oxidation on the surface of the pores. Therefore, the mathematical model developed in this study is based on the corresponding conservation equations for (1) gas flow in the catalyst-packed tubular reactor, (2) diffusion within the pores of the material, and (3) oxidation on the surface of the pores. The detailed mathematical steps are not presented here for brevity and are described in our previous study.10 The set of model governing equations was simultaneously solved using the Fortran (NAG library) subroutine D03PCF. The 12539



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Table 1. BET Surface Areas and PSDs of the Prepared Materials PSD (%) sample

SBET (m2/g)

VT (cm3/g)

micropores

mesopores

macropores

ACFs Ce(NO3)3−Cu(NO3)2−ACFs CeO2−Cu−ACFs Cu−CNFs/ACFs CeO2−Cu−CNFs/ACFs

1237 598 1053 478 355

0.71 0.37 0.65 0.39 0.24

85.56 75.95 72.72 71.28 67.35

9.79 20.50 22.17 23.72 29.28

4.65 3.50 5.11 5.00 3.37

were calculated using density functional theory and the Barrett−Joyner−Halenda (BJH) method, respectively. TPR analysis of the samples was performed using the Quantachrome instrument to determine the reducibility of the materials by H2. Approximately 0.15 g of the sample was degassed at 120 °C to remove moisture. A gaseous mixture of 5% H2 and 95% N2 was introduced at 10 sccm flow rate. The temperature of the samples was raised from 40 to 750 °C. The amount of H2 consumed during the analysis was measured using a thermal conductivity detector (TCD) controller. XRD analysis was performed to determine the crystal size and pattern of Cu and CeO2. Cu Kα (λ = 1.54178 Å) radiation was used in the 2θ range of 20−70° at a scanning rate of 3°/ min using a PANalytical X’Pert-Pro diffractometer. The amounts (weight percentages) of various elements (carbon, hydrogen, nitrogen, and oxygen) present in the samples were determined using an elemental analyzer (Exeter Analytical Inc., model CE 440). The degree of graphitization of the prepared samples was estimated by Raman analysis. The Raman spectra of the samples were recorded using a confocal Raman instrument (model Alpha, WiTec GmbH, Ulm, Germany) with an Ar-ion laser as the source of excitation and a CCD detector in the range of 120−3700 cm−1 at room temperature. The surface morphology and texture of the prepared materials were observed using a field-emission scanning electron microscope (Supra 40 VP, Zeiss, Oberkochen, Germany). EDX spectra of the materials were obtained using Oxford INCAX-sight software.

The Cu-NP-dispersed ACFs were used as the substrate for CNFs. The CNFs were produced by CVD in the HEF, using C2H2 as the carbon source. CVD was performed for 30 min at 450 °C. The flow rate of C2H2 was constant at 50 sccm. The growth of CNF followed the tip-growth mechanism. Some samples of CeO2−Cu−CNFs/ACFs were ultrasonicated in 0.3 M HNO3 for 5 min to remove the Cu NPs from the tips of the fibers. Such samples were used in the oxidation tests for comparison purposes. 3.4. Oxidation of NO. The setup for NO oxidation consisted of gas-mixing, test, and analytical sections. The streams of three different gases (N2, O2, and NO) were allowed to mix at a desired concentration in the mixing chamber (L = 135 mm, i.d. = 25 mm) constructed of SS. The flow rates of individual gases were maintained constant within an accuracy of ±10 sccm using mass flow controllers (model PSFIC-I, Bronkhorst, Ruurlo, The Netherlands). The gaseous streams were dried using silica gel purifiers to remove moisture and any unwanted impurities. The test section comprised a perforated (0.5-mm holes) SS tubular reactor (L = 150 mm, i.d. = 14 mm) vertically mounted in a tubular furnace equipped with a programmable proportional−integral−derivative (PID) controller. Before the activity measurements, the perforated portion of the reactor was wrapped with ∼1 g of the prepared ACF- or CNF/ACF-supported catalyst. The concentration of NO in the gaseous stream was measured using a chemiluminescence NOx analyzer (model 42C, Thermo Electron Corporation, Madison, WI). 3.5. Surface Characterization. The prepared catalysts were characterized for various physicochemical properties using different spectroscopic and analytical techniques, such as atomic absorption spectroscopy (AAS), Brunauer−Emmett− Teller (BET) surface area and pore size distribution (PSD) measurements, temperature-programmed reduction (TPR), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), and CHNO elemental analysis. The metal loading in the ACFs was determined from the analysis of the impregnating solution, before and after impregnation of the ACFs, by AAS (Varian AA-240, Palo Alto, CA). The analysis was performed at a wavelength of 324.8 nm in an air−acetylene flame. The BET surface area and PSD were determined using an Autosorb-1C instrument (model AS1-C, Quantachrome Instruments, Boynton Beach, FL). Approximately 40−45 mg of prepared sample was degassed at 150 °C for 8 h in a vacuum. A 30% N2−He gaseous mixture was used as the probe for physisorption. The total pore volume was measured from the amount of N2 adsorbed at a relative pressure close to unity (0.9994). The BET surface area was measured by the multipoint BET method in the pressure range between 0.05 and 0.35. The PSD was determined using the desorption isotherm. The micropore and mesopore volumes

4. RESULTS AND DISCUSSION 4.1. Cu(II) Loading in the ACFs. The impregnation of ACFs with 0.4 M solution of Cu(NO3)2 yielded a Cu loading of 320 mg/g. The metal loading decreased to 181 mg/g when the ACFs were impregnated with an aqueous mixture (100 mL) made up of 0.4 M Cu(NO3)2 and 0.4 M Ce(NO3)3 salt solutions mixed in a 3:1 volume ratio. When the ACFs were impregnated with solutions prepared using the individual salt solutions in volume ratios of 1:1 and 1:3, the Cu loadings were 120 and 59 mg/g, respectively. In this study, all results are discussed for the materials produced using the 3:1 volume ratio of the impregnating salt solutions. 4.2. BET and PSD. Table 1 summarizes the BET surface areas (m2/g) and PSDs of the materials at various stages of preparation. The BET surface area and total pore volume of the material decreased (∼598 m2/g) upon impregnation with the bimetallic salts, attributed to the blockage of the pores of the ACFs during impregnation. Following calcination and reduction, an increase in the BET surface area and pore volume was observed, attributed to the conversion of metal nitrates into the corresponding metal oxides and, then, into the respective metallic state. The growth of the CNFs on ACFs caused a decrease in the BET surface area (∼355 m2/g). The decrease was attributed to the formation of nanopores during CVD, 12540



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bimetallic-salt-impregnated ACF samples were optimized at 200 and 300 °C, respectively. 4.4. XRD. Figure 3 shows the XRD patterns of CeO2−Cu− CNFs/ACFs. The peak observed at a 2θ angle of 25° is

which hindered the N2 probe molecules from accessing the ACF pores during the BET surface area measurements. It is mentioned that the surface area and total pore volume are underestimated by ∼40% when N2 is used as the physisorption probe at 77 K.22 Interestingly, the mesopore contents in the material increased with a simultaneous decrease in the microand macropore volumes during the various stages of preparation, namely, calcination, reduction, and CVD. 4.3. TPR. Figure 2 shows the TPR profiles of the CeO2− CuO-dispersed ACFs prepared by calcination of the salt-

Figure 3. XRD spectra of CeO2−Cu−CNFs/ACFs.

attributed to the amorphous cokelike structure present in ACFs, produced during the carbonization of the phenolic precursor at high temperatures. The peaks observed at 2θ angles of ∼29.5°, 37°, 47°, 56°, and 58° correspond to the (111), (200), (220), (311), and (222) planes, respectively, of the face-centered ceria having a cubic fluorite structure.21 Two characteristic peaks at 2θ values of ∼43° and 51° correspond to the (111) and (200) planes, respectively, of the face-centeredcubic structure of the Cu NPs. The minor peak at a 2θ angle of 39° is attributed to the face-centered-cubic structure of Cu2O. These XRD patterns confirm that most of the copper oxides were converted to the metallic state when the CeO2−CuO− CNF/ACF samples were reduced at a reduction temperature of 300 °C. The average crystallite sizes were calculated based on 2θ angles using the Scherrer formula

Figure 2. TPR profiles of the CeO2−CuO-dispersed ACFs prepared by calcination of the corresponding nitrate-salt-impregnated samples at various temperatures. The dotted curve shows the TPR profile of a CuO-dispersed sample without ceria.

impregnated ACF samples at various temperatures. The dotted curve in the figure shows the TPR profiles of the CuOdispersed ACFs without ceria. The peaks observed at around 250 °C in each bimetallic-oxide-dispersed sample correspond to the reduction of CuO to Cu. Therefore, the dispersion of ceria caused a decrease in the reduction temperature from 330 to 250 °C. In other words, the reducibility of the material improved with the incorporation of ceria. The peak observed at approximately 570 °C is attributed to the partial reduction of ceria. It has been shown that the substantial reduction of CeO2 takes place at high temperatures (∼1200 °C), although the reduction starts at approximately 400 °C.21 Moreover, the TPR spectra of the ACFs without salts show a small peak at around 600−700 °C, attributed to the evolution of CO caused by the decomposition of lactone and several other functional groups present in ACFs.11 The decomposition temperature of Cu(NO3)2 is ∼200 °C. When the samples were calcined at 100 °C, a partial conversion of Cu(NO3)2 into CuO occurred. The EDX and CHNO analyses indicated that the material contained significant amounts of nitrogen after calcination at 100 °C (data not shown for brevity), reconfirming the partial conversion of Cu(NO3)2. Furthermore, some metal oxides remained in the oxide upon H2 reduction, leading to the nonuniform and less-dense growth of the CNFs. Based on the TPR data, the optimum calcination temperature and the corresponding reduction temperature for the Cu and Ce

τ=

kλ β1/2 cos θ

where τ is the average size of crystalline domain, k is the shape factor (0.9), λ is the X-ray wavelength (0.154 nm), β1/2 is the line broadening at half the maximum intensity (full width at half-maximum, FWHM) in radians, and θ is the Bragg angle. The average crystallite sizes of CeO2 and the Cu NPs were calculated to be 12.7 and 21.1 nm, respectively, based on the 2θ angles shown in Figure 3. 4.5. Surface Morphology. Figure 4 shows the SEM images of ACFs and CeO2−Cu−CNFs/ACFs at low (5000×) and high magnifications (100000×). SEM images of the pre-CVD samples, namely, CeO2−Cu−ACFs, were also taken for comparison purposes. All images were recorded at a working distance of 3−4 mm. The SEM image at low magnification revealed that the external surface of the ACFs was smooth (Figure 4a), containing pores of different sizes (Figure 4b). As observed in Figure 4c,d, the metal NPs were approximately uniformly and homogeneously dispersed on the surface of the CeO2−Cu−ACFs. Some NPs were observed inside the pores of the ACFs. The average particle size of the NPs was determined to be approximately 25 nm using ImageJ software. Figure 4e,f 12541



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Figure 4. SEM images of (a,b) as-received ACFs, (c,d) CeO2−Cu−ACFs, and (e,f) CeO2−Cu−CNFs/ACFs and EDX spectra of (g) Cu−CNFs/ ACFs and (h) CeO2−Cu−CNFs/ACFs.

dislodged from the tips of the fibers, without affecting the CNFs. 4.6. Elemental Analysis. The elemental (C, H, N, and O) contents of the samples were determined using an elemental analyzer, and the results are summarized in Table 2. The O content in the salt-impregnated sample was much higher than (approximately twice) that in the ACF substrate because of the incorporation of Ce(NO3)3·6H2O and Cu(NO3)2·3H2O in the ACFs during impregnation. The N content increased by approximately 8 times after the impregnation. The O content in the calcined and reduced samples marginally decreased because

shows the approximately uniform and dense growth of CNFs in the CeO2−Cu−CNFs/ACFs. The EDX analysis confirmed the shiny NPs as Cu, attached to the tips of the fibers. The CeO2 NPs remained adhered on the surface of the ACFs. The average diameter of the nanofibers ranged between 30 and 50 nm. Figure 4g,h shows the EDX spectra of Cu−CNFs/ACFs and CeO2−Cu−CNFs/ACFs. No foreign elements except Cu and C were detected in the Cu−CNFs/ACFs, whereas Cu and Ce, along with C and O, were detected in the CeO2−Cu−CNFs/ ACFs. Further, the SEM images of the sonicated samples (not presented here) confirmed that most of the Cu NPs were 12542



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Table 2. Results of Elemental Analysis of the Prepared Materials sample

C (%)

H (%)

N (%)

O (%)

ACFs Cu−CNFs/ACFs Ce(NO3)3−Cu(NO3)2−ACFs CeO2−Cu−ACFs CeO2−Cu−CNFs/ACFs

70.81 86.35 46.09 55.79 65.28

1.35 2.90 1.55 1.37 1.83

0.56 0.72 4.10 0.95 0.51

27.48 10.03 48.26 41.89 32.28

of the reduction of CuO to Cu. The O contents in the prepared materials were expectedly found to be in the following order: CeO2−Cu−CNFs/ACFs > Cu−CNFs/ACFs > ACFs. As discussed later, the carboxylic functional groups contain significant amounts of oxygen, responsible for the oxidation of NO to NO2 without requiring an external supply of O2. 4.7. Raman Spectroscopy. Figure 5 shows the Raman spectrum of CeO2−Cu−CNFs/ACFs. There are two sharp

Figure 6. Effect of O2 concentration on NO oxidation using ACF substrate (T = 30 °C, P = 1 bar, W = 1 g, NO = 1000 ppm, Q = 37.5 sccm).

first 10 min), with the NO concentration reaching a steadystate level. In the absence of O2, the NO conversion, calculated as (NOinlet − NOout)/NOinlet, was ∼8%, attributed to the oxygen-containing surface functional groups present in the ACFs. The conversion, however, increased significantly with the external supply of O2 and also with increasing O2 concentrations. The steady-state conversion was ∼28% using 20% O2. Figure 6 also shows the model-predicted results for the breakthrough concentrations of NO. A reasonably good agreement can be observed between the experimental data and model predictions. The model simulations were performed at the experimental conditions. As previously mentioned in the text, the model developed in this study considered the diffusion effects in the packed bed and within the pores of the material. The particle-size-based Reynolds number (Re), Schmidt number (Sc), and Sherwood number (Sh) were calculated as 0.0079, 0.0144, and 2.014, respectively. The small value of Re is attributed to the small diameter (2.8 × 10−6 m) of the fibers. The gas-to-fiber mass-transfer coefficient calculated from Sh was ∼653 m/s. The relatively higher mass-transfer coefficient indicated insignificant diffusion resistance within the packed bed. The intraparticle diffusion coefficient was calculated to be ∼2.8 × 10−8 m2/s, which is of the same order of magnitude as that reported in the literature.24 The model parameters Kc1 and Kc2 were adjusted to fit the experimental data. The numerical value of Kc1 was increased from 7 × 10−8 for the case of the reaction without O2 to 1.2 × 10−6 m3/mol·s for the case with 20% O2, which is consistent with eqs 9−15. The numerical value of Kc2 was kept approximately constant with increasing O2 concentration. Table 3 lists the numerical values of the model parameters adjusted to explain the experimental data shown in Figure 6 and in Figures 7 and 8, as discussed later. The physically adsorbed NO is oxidized to NO2 by the oxygen present in the surface functional groups or an external source. The greater the O2 concentration, the smaller the NO concentration at the exit of the reactor. The initial increase observed in the NO concentration is attributed to the “roll-up” characteristics of the adsorbing binary gaseous mixture, with the NO physically displaced from the surface by O2.9,25 The relatively higher O2 concentration levels in the reactor resulted

Figure 5. Raman spectrum of CeO2−Cu−CNFs/ACFs.

peaks, one at 1352 cm −1 (width = 261.651 cm −1 ) corresponding to the D-band and the other at 1598 cm−1 (width = 82.419 cm−1) corresponding to the G-band. The Dand G-bands represent the degree of disorder and graphitization, respectively, in the material. The ratio of ID to IG for the areas under the D- and G-bands describes the graphitic characteristics of the material. The lower this value, the greater the presence of sp2 carbon in the samples. The D- and G-bands in the spectrum shown in Figure 5 were fitted with a Gaussian− Lorentzian mixed-type of curve, and the ID/IG ratio was determined to be 3.536. The data indicate the amorphous characteristics of the material. The peak at ∼460 cm−1 is attributed to the CeO2 crystals having a cubic fluorite structure.23 4.8. NO Oxidation. All reactions were performed at room temperature and atmospheric pressure, using a gaseous mixture of N2, NO, and O2 at a total gas flow rate of 37.5 sccm and a NO concentration of 1000 ppm. The O2 concentration was varied between 0 and 20% (v/v) in different tests. The different catalysts prepared in this study were the single-metal Cu− ACFs, Cu−CNFs/ACFs, and CeO2−ACFs and the bimetallic CeO2−Cu−CNFs/ACFs. The ACF substrate without metals was also tested for comparison purposes. 4.8.1. ACF. Figure 6 shows the breakthrough data for NO for different O2 concentrations, obtained during the reaction performed on the ACF substrate without metals. The breakthrough of NO occurred instantaneously (within the 12543



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Figure 7b describes the results for Cu−CNFs/ACFs under identical reaction conditions as for the Cu−ACFs. The NO conversion was measured as ∼22% using the Cu-NP-grown CNFs on the ACFs, without O2. Therefore, the CNFs promoted the oxidation of NO. The catalytic activity of CNFs, attributed to the exposed free edges of the hexagons containing the heteroatoms and sp2 carbon in the material, is reported in the literature.13 Moreover, the reacting NO and O2 were more exposed to the Cu NPs attached at the tips of the fibers than to the Cu NPs dispersed in the ACFs. The effect of the active CNF surface on the oxidation was, however, reduced with increasing O2 concentration. In this case, the catalytic activity of the Cu NPs was dominant. Figure 7 compares the model-predicted breakthrough curves with the experimental data for Cu−ACFs and Cu−CNFs/ ACFs. The numerical value of the model parameter Kc1 was higher for Cu−CNFs/ACFs than Cu−ACFs, signifying a relatively larger number of active sites available in the CNFs. Kc2 was kept approximately constant. A reasonably good agreement was observed between the model-predicted values and the experimental data. 4.8.3. CeO2−ACFs and CeO2−Cu−CNFs/ACFs. The oxidation tests were performed on CeO2−ACFs (without Cu) to ascertain the role of ceria in the oxidation of NO to NO2. As observed from Figure 8a, the NO conversion was less than 15% when O2 was not supplied externally. The oxygen required for the oxidation was provided by the redox reaction between CeO2 and Ce2O3 and the oxygen-containing surface functional groups. The oxygen released from the lattice of ceria promoted the conversion of NO to NO2. The conversion increased to ∼38% when O2 was supplied externally. However, it was less than that for Cu−ACFs (Figure 7a). Thus, the role of ceria was predominantly as the oxygen provider. The performance of CeO2−Cu−CNFs/ACFs was superior to that of all other materials prepared in this study. The NO

Table 3. Numerical Values of the Model Parameters sample ACFs

Cu−ACFs

Cu−CNFs/ACFs

CeO2−ACF

CeO2−Cu−CNFs/ACFs

O2 (%)

Kc1 (m3/mol·s)

0 10 20 0 10 20 0 10 20 0 10 20 0 10 20

0.07 0.36 0.71 0.10 1.15 3.25 0.25 1.97 5.00 0.13 0.51 1.20 0.50 2.33 6.10

Kc2

3

(m /mol)

38.2 38.9 39.7 38.2 38.4 38.6 38.9 39.6 40.3 32.8 37.6 39.8 37.4 38.6 41.1

in the early roll-up in NO concentrations. The steady-state concentration observed below the inlet concentration level is attributed to the catalytic activity of the ACF surface.9 4.8.2. Cu-NP-Dispersed Samples. The reactions were performed on the single-metal Cu−ACF and Cu−CNF/ACF samples to ascertain the role of Cu NPs and CNFs for the oxidation of NO to NO2. The NO conversion increased marginally from 8% on ACFs to approximately 10% on Cu− ACFs, without the use of an external O2 source (Figure 7a). With an external supply of O2, the conversion, however, increased significantly. The conversion was ∼52%, in comparison to ∼28% on the ACFs, when using 20% O2. Thus, the catalytic activity of Cu NPs in the oxidation of NO was obvious, which also significantly increased with increasing O2 concentration.

Figure 7. Effect of O2 concentration on NO oxidation using Cu-dispersed samples without ceria: (a) Cu−ACFs and (b) Cu−CNFs/ACFs (T = 30 °C, P = 1 bar, W = 1 g, NO = 1000 ppm, Q = 37.5 sccm). 12544



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Figure 8. Effect of O2 concentration on NO oxidation using (a) CeO2−ACFs and (b) CeO2−Cu−CNFs/ACFs (T = 30 °C, P = 1 bar, W = 1 g, NO = 1000 ppm, Q = 37.5 sccm).

conversion increased to ∼30% without using O2 and to ∼70% when 20% O2 was used (Figure 8b). Clearly, the enhanced activity of the material can be attributed to the combined effects of the CNFs, Cu NPs, and CeO2 on the oxidation. In the previous section, we discussed the catalytic characteristics of the CNFs and Cu NPs and a relatively greater exposure of the Cu NPs attached to the tips of the CNFs responsible for the increased oxidation rate. The synergistic interaction between CeO2 and Cu NPs, in situ incorporated in the matrix of the CeO2−Cu−CNFs/ACFs, resulted in the production of lattice oxygen having a relatively higher mobility, also discussed previously. Similarly to the previous cases, the model predictions were found to be in reasonably good agreement with the data for CeO2−ACFs and CeO2−Cu−CNFs/ACFs. The model parameters Kc1 and Kc2 were adjusted in each case to obtain the best fit and are listed in Table 3. The variation in the parameters was consistent with eqs 8−15. 4.8.4. Sonicated Cu−CNFs/ACFs and CeO2−Cu−CNFs/ ACFs. We previously mentioned that some samples of Cu− CNFs/ACFs and CeO2−Cu−CNFs/ACFs were ultrasonicated to dislodge the Cu NPs from the tips of the CNFs. As observed in Figure 9, NO conversion decreased in the sonicated samples (refer to Figures 7b and 8b for the corresponding data on the respective unsonicated samples). The decrease in the activity reconfirmed the catalytic role of the Cu NPs in the oxidation of NO to NO2. The NO conversion for the sonicated CeO2−Cu− CNFs/ACFs was expectedly found between those for the unsonicated CeO 2 −ACFs and CeO 2 −Cu−CNFs/ACFs (shown in Figure 8). Figure 10 presents the comparative performances of the prepared materials used for the NO oxidation. The performances of the materials with and without CeO2 and Cu NPs were found to be in the following order, as previously discussed: CeO2−Cu−CNFs/ACFs > Cu−CNFs/ACFs > Cu−ACFs > CeO2−ACFs > ACFs.

Figure 9. NO oxidation over sonicated Cu−CNFs/ACFs (□) and CeO2−Cu−CNFs/ACFs (○) (T = 30 °C, P = 1 bar, W = 1 g, NO = 1000 ppm, Q = 37.5 sccm, O2 = 20%).

Figure 11 depicts the NO2 concentrations at the exit of the reactor. The comparative data for different materials are consistent with those for the NO concentrations at the exit under the identical operating conditions (Figure 10), indicating the catalytic activities of the materials for the same reaction time. 4.8.5. Effect of NO Concentration. The oxidation reactions were performed at different NO concentrations (500, 750, and 1000 ppm) using CeO2−Cu−CNFs/ACFs, the best material prepared in this study. As observed in Figure 12, the conversion increased with decreasing NO concentration and was ∼80% for a 500 ppm NO concentration. The NO conversion achieved in this study can be compared to the approximately same conversion achieved using Mn−Co−Ce−Ox at 150 °C7 and 12545



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Ce0.80Zr0.20O2 at 400 °C,5 and the ∼43% at 200 ppm using Pt− Pd/Al2O3 at 400 °C.3 In another study, the NO conversion was reported as ∼90% at 450 ppm NO using WO3/Pt/Al2O3 at 220 °C.2

5. CONCLUSIONS A novel CeO2- and Cu-NP-dispersed CNF/ACF material was produced, in which the bimetals were in situ incorporated during the synthesis stage. The produced material was applied for the control of NO emissions by oxidation at room temperature. Approximately 80% conversion was achieved for of a NO concentration of 500 ppm in an oxygen-rich (20%) atmosphere. The relatively larger conversion achieved using CeO2−Cu−CNFs/ACFs is attributed to the combined catalytic effects of the CNFs and Cu NPs and the synergistic interaction between ceria and the Cu NPs. A mathematical model was developed for predicting the breakthrough concentration profiles of NO in the tubular reactor packed with the catalyst materials, considering a Langmuir−Hinshelwood-type kinetic mechanism and mass-transfer effects within the reactor. The model results were found to be in good agreement with the experimental data. The CeO 2−Cu−CNF/ACF material developed in this study is a potential catalyst for the effective removal of NO by oxidation at room temperature.

Figure 10. Comparative performance of the prepared materials in the oxidation of NO (T = 30 °C, P = 1 bar, W = 1 g, NO = 1000 ppm, Q = 37.5 sccm, O2 = 20%).

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: 91-512-2596352/7704. Fax: 91-512-2590104. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors thank Kynol, Inc. (Tokyo, Japan) for providing the ACFs. Figure 11. NO2 outlet concentration with time for different materials during the oxidation of NO (T = 30 °C, P = 1 bar, W = 1 g, NO = 1000 ppm, Q = 37.5 sccm, O2 = 20%).

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Figure 12. Effect of NO concentration on NO oxidation using CeO2− Cu−CNFs/ACFs (P = 1 bar, W = 1 g, Q = 37.5 sccm, O2 = 20%).

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