catalytic oxidation of no over cnf/acf-supported ceo 2 and cu...

Catalytic Oxidation of NO over CNF/ACF-Supported CeO2 and CuNanoparticles at Room TemperaturePriyankar Talukdar,† Bhaskar Bhaduri,† and Nishith Verma*,†,‡

†Department of Chemical Engineering and ‡Center for Environmental Science and Engineering, Indian Institute of TechnologyKanpur, 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 playeddual roles: (1) catalyzing the growth of the CNFs and (2) catalytically oxidizing NO to NO2. The synergistic interaction betweenthe Cu NPs and CeO2 enhanced the oxidation rate. The maximum NO conversion using the CeO2−Cu−CNFs/ACFs developedin this study was ∼80% for a 500 ppm NO concentration at room temperature (∼30 °C). A mathematical model was developedto explain the proposed mechanism for NO oxidation, incorporating the kinetic rate and mass-transfer effects in the tubularreactor. The CNF/ACF-supported CeO2 and Cu bimetal catalyst prepared in this study represents a promising candidate forabating NO emissions by oxidation at room temperature.

1. INTRODUCTIONSelective catalytic reduction (SCR) is commonly used for thecontrol of nitrogen oxide (NO), using ammonia (NH3) or ureaas 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 storageand handling of NH3. Furthermore, the fugitive emission ofNH3 is a concern. On the other hand, the decomposition ofurea at high reduction temperatures limits the efficiency of themethod.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 nickeltransition metals, supported on various substrates includingzeolites and activated carbons.2−4 Bimetallic oxides have alsobeen used for the catalytic oxidation of NO.5−7 The conversionin 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 carbonnanotubes (CNTs) have been studied as adsorbents andsupports for metal catalysts because of their high Brunauer−Emmett−Teller (BET) surface areas, uniformity in pore sizedistribution (PSD), and amenability toward surface function-alization.8−10 The superiority of CNFs over ACFs in catalyticand adsorption applications lies in the relatively higher stabilityof CNFs in acidic/basic media and chemical activity.11−14 ACF-or CNF-supported metal catalysts have also been used in theabatement of NO emissions by oxidation or reduction.15−17

The present study describes the development of CeO2(ceria)- and Cu nanoparticle- (NP-) dispersed CNFs/ACFsfor the removal of NO by oxidation at room temperature. TheCNFs/ACFs were prepared by growing CNFs on an ACFsubstrate using catalytic chemical vapor deposition (CVD).Ceria and Cu NPs were in situ incorporated into the ACFsprior to CVD. The Cu NPs played dual roles: (1) catalyzing the

growth of CNFs and (2) catalytically oxidizing NO to NO2.Ceria had a promotional effect on the catalytic activity of Cuthrough the release of nascent oxygen during the redox cycleand a synergistic interaction with the Cu NPs. The oxidationreaction was performed in a perforated tubular reactor wrappedwith CeO2−Cu−CNFs/ACFs using various oxygen (O2) andNO concentrations. The experiments were sequentiallyperformed on different materials: first the ACF substrate(without metals) and then single-metal-based materials,namely, Cu−ACFs, Cu−CNFs/ACFs, and CeO2−ACFs, andfinally, the CeO2−Cu−CNF/ACF bimetal-based material tounderstand the individual roles of Cu NPs, CNFs, and CeO2 inthe oxidation of NO. A kinetic mechanism for the oxidation ofNO to NO2 over the catalyst produced in this study wasproposed and incorporated in a transport-based mathematicalmodel to explain the experimental data for the oxidation of NOin 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 theadsorption/desorption of NO and O2 at the catalyst surface,followed by the catalytic oxidation of NO to NO2. Step Binvolves the synergistic interaction between CeO2 and Cu in aredox cycle, releasing the lattice oxygen, which oxidizes NO toNO2.Oxygen is dissociatively adsorbed on the vacant sites of

ACFs. It reacts with the NO adsorbed on the adjacent sites toproduce NO2. The adsorbed NO undergoes transformations inseveral steps to produce intermediate surface complexcompounds and NO2, leaving behind the active sites for

Received: May 19, 2014Revised: July 12, 2014Accepted: July 16, 2014Published: July 16, 2014

Article

pubs.acs.org/IECR

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

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successive adsorption. The redox reaction involving thesynergistic interaction between Ce3+ and Cu2+ produces latticeoxygen and restores the oxidation state (4+) of Ce in ceria.Such synergistic effects between different metals have beenobserved in several catalytic reactions.18,19 Figure 1 schemati-cally describes adsorption/desorption of the reacting speciesand the synergistic interaction between CeO2 and Cu NPs. Asimilar type of study incorporating the adsorption−desorptionreaction and synergistic interaction between ceria and Cu canalso be observed elsewhere.20 The proposed mechanism for thereaction, 2NO + O2 → 2NO2 is as followsStep A involves the adsorption−desorption of NO and O2 on

CNFs/ACFs and oxidation of NO to NO2.

+ −−

H IooNO X NO Xk

k

1

1

(i)

+ −−

H IooO 2X 2O Xk

k2

2

2

(ii)

− + − − +−

H IoooooNO X O X NO X Xk

k [Cu]2

3

3

(iii)

− − + +−

H Iooooo2NO X NO X NO Xk

k2

[Cu]3

4

4

(iv)

− + − − − +−

H IoooooNO X NO X NO NO X Xk

k3

[Cu]3

5

5

(v)

− − ⎯ →⎯⎯⎯⎯ +NO NO X 2NO Xk

3[Cu]

26

(vi)

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

→ +2CeO Ce O O(lattice)2 2 3 (vii)

+ → ++ + + +Ce Cu Ce Cu3 2 4 (viii)

+ → ++ + −Cu12

O Cu O (adsorbed)22

(ix)

Assuming a Langmuir−Hinshelwood type of rate mechanismwith reaction step vi as the rate-determining step and steps i−vin a quasi-equilibrium state, the overall rate of oxidation can bewritten as

= − = − −C

tC

tk

12

d

dd

d[NO NO X]NO NO

6 32

(1)

The equilibrium constants can be written in terms of thesteady-state concentrations of different adsorbed species, asfollows

= −K

[NO X][NO][X]1

(2)

= −K

[O X][O ] [X]2

1/2

21/2

(3)

=−

− −K

[NO X][X][NO X][O X]3

2

(4)

=−

−K

[NO X][NO][X][NO X]43

22

(5)

=− −− −

K[NO NO X][X][NO X][NO X]5

3

3 (6)

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

= + − + − + −

+ − + − −

[X ] [X] [NO X] [O X] [NO X]

[NO X] [NO NO X]0 2

3 3 (7)

Expressing the number density of adsorbed sites in terms of themajor species, the rate expression for the oxidation of NO toNO2 can be obtained as follows

− =′

′ + ′ + ′C

tk K

K K K12

dd

[NO][NO] [NO]

NO 6 12

2 3 42

(8)

where

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

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie502043e | Ind. Eng. Chem. Res. 2014, 53, 12537−1254712538

′ =K K K K K K [O ][X ]1 13

2 32

4 5 2 0 (9)

′ = +K K1 [O ]2 21/2

21/2

(10)

′ = + +K K K K K K K K K[O ] [O ]3 1 1 21/2

3 21/2

12

2 32

4 2 (11)

′ = ′ ′K K K K K K [O ]4 1 2 3 4 5 2 (12)

For the case of an external supply of O2, the concentration ofNO at the inlet of the reactor is insignificant in comparison toO2. Therefore, the concentration-squared term, namely, [NO]2

in the denominator of eq 8, can be neglected, and eq 8 can besimplified as follows

− =+

Ct

K

K12

dd

[NO]

1 [NO]NO c

2

c

1

2 (13)

where

=′

′=

′′

Kk KK

KKK

andc6 1

2c

3

21 2 (14)

Clearly, the rate expression for the noncatalytic oxidation,namely, NO oxidation, using the ACF substrate alone assumesa form similar to eq 13. However, the numerical value of therate constant k6 for the noncatalytic oxidation is relativelysmaller (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 tothat described above

− =+

Ct

K

K12

dd

[NO]

1 [NO]NO c

c

1

2 (15)

Reaction ii of step A is redundant and not considered inderiving the rate expression (eq 15). Reaction iii is modified, asthe required O concentration for the oxidation is available fromthe surface functional groups. We later revisit this aspect. Thenotable difference between eq 13 and eq 15 is the squaredependency of the oxidation rate on the NO concentration, ifthe reaction is performed using the externally supplied O2 andthe linear dependency if the reaction is performed without anyexternal O2. The term with the NO concentration squared inthe numerator of eq 15 arises because of the dissociativeadsorption of O2 species on the surface. The rate expression forNO oxidation on the ACF-supported CeO2 without an externalsupply of O2 can be shown to assume a form similar to that ofeq 15, with the rate having a linear dependency on the NOconcentration. In such a case, ceria is the source for latticeoxygen, as shown in the step B above.2.2. Model Development. Three mechanistic steps are

involved in the oxidation reaction performed in the tubularreactor: (1) mass transfer of NO from the gaseous phase to thesurface of carbon fibers, (2) diffusion within the pores of thefibers, and (3) catalytic oxidation on the surface of the pores.Therefore, the mathematical model developed in this study isbased on the corresponding conservation equations for (1) gasflow in the catalyst-packed tubular reactor, (2) diffusion withinthe pores of the material, and (3) oxidation on the surface ofthe pores. The detailed mathematical steps are not presentedhere for brevity and are described in our previous study.10 Theset of model governing equations was simultaneously solvedusing the Fortran (NAG library) subroutine D03PCF. The

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

3. EXPERIMENTAL SECTION

3.1. Materials. The phenolic resin precursor-based ACFswere 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 Sigma-Aldrich (Munich, Germany). Nitric acid (HNO3) waspurchased from Fisher Scientific (Pittsburgh, PA). Hydrogen(purity > 99.99%), nitrogen (purity > 99.99%), oxygen (purity> 99.99%), and acetylene (C2H2, AAS grade) gases werepurchased from Sigma Gases (Mumbai, India).

3.2. Pretreatment of ACFs. The ACF samples were firsttreated with 0.4 M HNO3 for approximately 2 h at 100 °C toremove any undesired species from the surface of the material.After the treatment, the ACFs were washed several times withdeionized (DI) water and dried in air for 6−7 h and then in anoven for another 12 h at 120 °C. Finally, the ACFs werevacuum-dried at 200 °C for 12 h to remove the entrapped gaseswithin the pores.

3.3. Preparation of CeO2- and Cu-NP-Dispersed ACFsand CNFs/ACFs. Cu(NO3)2·3H2O and Ce(NO3)3·6H2O saltsmixed in DI water were used as the precursors for CeO2 andCu NPs, respectively. The ACF samples were wrapped over theperforated glass tube contained in a tubular shell. The setupused for impregnation was described in a previous study.12 TheACF samples were impregnated with 100 mL of a mixturemade 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 solutionto increase the monodispersion of the salts in the solution, withminimal agglomeration and a relatively high metal loading onthe ACF surface. The total concentration of the impregnatingsalt solution was optimized at 0.4 M. The impregnation of theACFs with the solution at concentrations greater than 0.4 Mresulted in the blockage of the ACF pores with the excess saltcrystals. Similarly, impregnation using relatively greateramounts of ceria resulted in the underperformance of thematerial, as discussed later. To this end, the solution wascontinuously recycled for 24 h using a peristaltic pump rotatedat 105 rpm. After the impregnation, the samples were dried atroom temperature for approximately 6 h and then in the ovenat 100 °C for 12 h.The salt-impregnated ACF samples were calcined and

subjected to reduction using H2. Temperature-programmedreduction (TPR) was performed a priori using H2 (5%) and N2(95%) to determine the optimum calcination and reductiontemperatures, as discussed later. The ACFs were wrapped overa stainless steel (SS) mesh and placed within a tubular reactor(i.d. = 30 mm, L = 0.8 m) mounted inside in the programmablehorizontal electric furnace (HEF). Cu NPs were produced insitu in the ACFs, using calcination for 4 h at 200 °C in an inertatmosphere maintained by N2 gas at a constant flow rate of 40standard cm3 per min (sccm), followed by reduction at 300 °Cfor 2 h using H2 at a flow rate of 120 sccm. The reductiontemperature for CeO2 is relatively higher (>850 °C), andtherefore, it remained dispersed as CeO2−x in the substrate.21



The Cu-NP-dispersed ACFs were used as the substrate forCNFs. The CNFs were produced by CVD in the HEF, usingC2H2 as the carbon source. CVD was performed for 30 min at450 °C. The flow rate of C2H2 was constant at 50 sccm. Thegrowth of CNF followed the tip-growth mechanism. Somesamples of CeO2−Cu−CNFs/ACFs were ultrasonicated in 0.3M HNO3 for 5 min to remove the Cu NPs from the tips of thefibers. Such samples were used in the oxidation tests forcomparison purposes.3.4. Oxidation of NO. The setup for NO oxidation

consisted of gas-mixing, test, and analytical sections. Thestreams of three different gases (N2, O2, and NO) were allowedto mix at a desired concentration in the mixing chamber (L =135 mm, i.d. = 25 mm) constructed of SS. The flow rates ofindividual gases were maintained constant within an accuracy of±10 sccm using mass flow controllers (model PSFIC-I,Bronkhorst, Ruurlo, The Netherlands). The gaseous streamswere dried using silica gel purifiers to remove moisture and anyunwanted 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 aprogrammable proportional−integral−derivative (PID) con-troller. Before the activity measurements, the perforatedportion of the reactor was wrapped with ∼1 g of the preparedACF- or CNF/ACF-supported catalyst. The concentration ofNO in the gaseous stream was measured using a chemilumi-nescence NOx analyzer (model 42C, Thermo ElectronCorporation, Madison, WI).3.5. Surface Characterization. The prepared catalysts

were characterized for various physicochemical properties usingdifferent spectroscopic and analytical techniques, such asatomic 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 CHNOelemental analysis.The metal loading in the ACFs was determined from the

analysis of the impregnating solution, before and afterimpregnation of the ACFs, by AAS (Varian AA-240, PaloAlto, CA). The analysis was performed at a wavelength of 324.8nm in an air−acetylene flame. The BET surface area and PSDwere determined using an Autosorb-1C instrument (modelAS1-C, Quantachrome Instruments, Boynton Beach, FL).Approximately 40−45 mg of prepared sample was degassedat 150 °C for 8 h in a vacuum. A 30% N2−He gaseous mixturewas used as the probe for physisorption. The total pore volumewas measured from the amount of N2 adsorbed at a relativepressure close to unity (0.9994). The BET surface area wasmeasured by the multipoint BET method in the pressure rangebetween 0.05 and 0.35. The PSD was determined using thedesorption isotherm. The micropore and mesopore volumes

were calculated using density functional theory and theBarrett−Joyner−Halenda (BJH) method, respectively.TPR analysis of the samples was performed using the

Quantachrome instrument to determine the reducibility of thematerials by H2. Approximately 0.15 g of the sample wasdegassed at 120 °C to remove moisture. A gaseous mixture of5% H2 and 95% N2 was introduced at 10 sccm flow rate. Thetemperature of the samples was raised from 40 to 750 °C. Theamount of H2 consumed during the analysis was measuredusing 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) radiationwas used in the 2θ range of 20−70° at a scanning rate of 3°/min using a PANalytical X’Pert-Pro diffractometer. Theamounts (weight percentages) of various elements (carbon,hydrogen, nitrogen, and oxygen) present in the samples weredetermined using an elemental analyzer (Exeter Analytical Inc.,model CE 440). The degree of graphitization of the preparedsamples was estimated by Raman analysis. The Raman spectraof the samples were recorded using a confocal Ramaninstrument (model Alpha, WiTec GmbH, Ulm, Germany)with an Ar-ion laser as the source of excitation and a CCDdetector in the range of 120−3700 cm−1 at room temperature.The surface morphology and texture of the prepared materialswere observed using a field-emission scanning electronmicroscope (Supra 40 VP, Zeiss, Oberkochen, Germany).EDX spectra of the materials were obtained using OxfordINCAX-sight software.

4. RESULTS AND DISCUSSION4.1. Cu(II) Loading in the ACFs. The impregnation of

ACFs with 0.4 M solution of Cu(NO3)2 yielded a Cu loading of320 mg/g. The metal loading decreased to 181 mg/g when theACFs were impregnated with an aqueous mixture (100 mL)made up of 0.4 M Cu(NO3)2 and 0.4 M Ce(NO3)3 saltsolutions mixed in a 3:1 volume ratio. When the ACFs wereimpregnated with solutions prepared using the individual saltsolutions in volume ratios of 1:1 and 1:3, the Cu loadings were120 and 59 mg/g, respectively. In this study, all results arediscussed for the materials produced using the 3:1 volume ratioof the impregnating salt solutions.

4.2. BET and PSD. Table 1 summarizes the BET surfaceareas (m2/g) and PSDs of the materials at various stages ofpreparation. The BET surface area and total pore volume of thematerial decreased (∼598 m2/g) upon impregnation with thebimetallic salts, attributed to the blockage of the pores of theACFs during impregnation. Following calcination and reduc-tion, an increase in the BET surface area and pore volume wasobserved, attributed to the conversion of metal nitrates into thecorresponding metal oxides and, then, into the respectivemetallic state. The growth of the CNFs on ACFs caused adecrease in the BET surface area (∼355 m2/g). The decreasewas attributed to the formation of nanopores during CVD,

Table 1. BET Surface Areas and PSDs of the Prepared Materials

PSD (%)

sample SBET (m2/g) VT (cm3/g) micropores mesopores macropores

ACFs 1237 0.71 85.56 9.79 4.65Ce(NO3)3−Cu(NO3)2−ACFs 598 0.37 75.95 20.50 3.50CeO2−Cu−ACFs 1053 0.65 72.72 22.17 5.11Cu−CNFs/ACFs 478 0.39 71.28 23.72 5.00CeO2−Cu−CNFs/ACFs 355 0.24 67.35 29.28 3.37



which hindered the N2 probe molecules from accessing theACF pores during the BET surface area measurements. It ismentioned that the surface area and total pore volume areunderestimated by ∼40% when N2 is used as the physisorptionprobe at 77 K.22 Interestingly, the mesopore contents in thematerial increased with a simultaneous decrease in the micro-and macropore volumes during the various stages ofpreparation, 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-

impregnated ACF samples at various temperatures. The dottedcurve in the figure shows the TPR profiles of the CuO-dispersed ACFs without ceria. The peaks observed at around250 °C in each bimetallic-oxide-dispersed sample correspond tothe reduction of CuO to Cu. Therefore, the dispersion of ceriacaused a decrease in the reduction temperature from 330 to250 °C. In other words, the reducibility of the materialimproved with the incorporation of ceria. The peak observed atapproximately 570 °C is attributed to the partial reduction ofceria. It has been shown that the substantial reduction of CeO2takes place at high temperatures (∼1200 °C), although thereduction starts at approximately 400 °C.21 Moreover, the TPRspectra of the ACFs without salts show a small peak at around600−700 °C, attributed to the evolution of CO caused by thedecomposition of lactone and several other functional groupspresent in ACFs.11 The decomposition temperature ofCu(NO3)2 is ∼200 °C. When the samples were calcined at100 °C, a partial conversion of Cu(NO3)2 into CuO occurred.The EDX and CHNO analyses indicated that the materialcontained significant amounts of nitrogen after calcination at100 °C (data not shown for brevity), reconfirming the partialconversion of Cu(NO3)2. Furthermore, some metal oxidesremained in the oxide upon H2 reduction, leading to thenonuniform and less-dense growth of the CNFs. Based on theTPR data, the optimum calcination temperature and thecorresponding reduction temperature for the Cu and Ce

bimetallic-salt-impregnated ACF samples were optimized at200 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

attributed to the amorphous cokelike structure present inACFs, produced during the carbonization of the phenolicprecursor at high temperatures. The peaks observed at 2θangles of ∼29.5°, 37°, 47°, 56°, and 58° correspond to the(111), (200), (220), (311), and (222) planes, respectively, ofthe face-centered ceria having a cubic fluorite structure.21 Twocharacteristic peaks at 2θ values of ∼43° and 51° correspond tothe (111) and (200) planes, respectively, of the face-centered-cubic structure of the Cu NPs. The minor peak at a 2θ angle of39° is attributed to the face-centered-cubic structure of Cu2O.These XRD patterns confirm that most of the copper oxideswere converted to the metallic state when the CeO2−CuO−CNF/ACF samples were reduced at a reduction temperature of300 °C.The average crystallite sizes were calculated based on 2θ

angles using the Scherrer formula

τ λβ θ

= kcos1/2

where τ is the average size of crystalline domain, k is the shapefactor (0.9), λ is the X-ray wavelength (0.154 nm), β1/2 is theline broadening at half the maximum intensity (full width athalf-maximum, FWHM) in radians, and θ is the Bragg angle.The average crystallite sizes of CeO2 and the Cu NPs werecalculated to be 12.7 and 21.1 nm, respectively, based on the 2θangles shown in Figure 3.

4.5. Surface Morphology. Figure 4 shows the SEM imagesof ACFs and CeO2−Cu−CNFs/ACFs at low (5000×) andhigh magnifications (100000×). SEM images of the pre-CVDsamples, namely, CeO2−Cu−ACFs, were also taken forcomparison purposes. All images were recorded at a workingdistance of 3−4 mm. The SEM image at low magnificationrevealed that the external surface of the ACFs was smooth(Figure 4a), containing pores of different sizes (Figure 4b). Asobserved in Figure 4c,d, the metal NPs were approximatelyuniformly and homogeneously dispersed on the surface of theCeO2−Cu−ACFs. Some NPs were observed inside the pores ofthe ACFs. The average particle size of the NPs was determinedto be approximately 25 nm using ImageJ software. Figure 4e,f

Figure 2. TPR profiles of the CeO2−CuO-dispersed ACFs preparedby calcination of the corresponding nitrate-salt-impregnated samples atvarious temperatures. The dotted curve shows the TPR profile of aCuO-dispersed sample without ceria.

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



shows the approximately uniform and dense growth of CNFs inthe CeO2−Cu−CNFs/ACFs. The EDX analysis confirmed theshiny NPs as Cu, attached to the tips of the fibers. The CeO2

NPs remained adhered on the surface of the ACFs. The averagediameter of the nanofibers ranged between 30 and 50 nm.Figure 4g,h shows the EDX spectra of Cu−CNFs/ACFs andCeO2−Cu−CNFs/ACFs. No foreign elements except Cu andC 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 (notpresented here) confirmed that most of the Cu NPs were

dislodged from the tips of the fibers, without affecting theCNFs.

4.6. Elemental Analysis. The elemental (C, H, N, and O)contents of the samples were determined using an elementalanalyzer, and the results are summarized in Table 2. The Ocontent in the salt-impregnated sample was much higher than(approximately twice) that in the ACF substrate because of theincorporation of Ce(NO3)3·6H2O and Cu(NO3)2·3H2O in theACFs during impregnation. The N content increased byapproximately 8 times after the impregnation. The O content inthe calcined and reduced samples marginally decreased because

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.



of the reduction of CuO to Cu. The O contents in the preparedmaterials were expectedly found to be in the following order:CeO2−Cu−CNFs/ACFs > Cu−CNFs/ACFs > ACFs. Asdiscussed later, the carboxylic functional groups containsignificant amounts of oxygen, responsible for the oxidationof 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

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 D-and G-bands represent the degree of disorder and graphitiza-tion, respectively, in the material. The ratio of ID to IG for theareas under the D- and G-bands describes the graphiticcharacteristics of the material. The lower this value, the greaterthe presence of sp2 carbon in the samples. The D- and G-bandsin the spectrum shown in Figure 5 were fitted with a Gaussian−Lorentzian mixed-type of curve, and the ID/IG ratio wasdetermined to be 3.536. The data indicate the amorphouscharacteristics of the material. The peak at ∼460 cm−1 isattributed to the CeO2 crystals having a cubic fluoritestructure.23

4.8. NO Oxidation. All reactions were performed at roomtemperature and atmospheric pressure, using a gaseous mixtureof N2, NO, and O2 at a total gas flow rate of 37.5 sccm and aNO concentration of 1000 ppm. The O2 concentration wasvaried between 0 and 20% (v/v) in different tests. The differentcatalysts prepared in this study were the single-metal Cu−ACFs, Cu−CNFs/ACFs, and CeO2−ACFs and the bimetallicCeO2−Cu−CNFs/ACFs. The ACF substrate without metalswas also tested for comparison purposes.4.8.1. ACF. Figure 6 shows the breakthrough data for NO for

different O2 concentrations, obtained during the reactionperformed on the ACF substrate without metals. Thebreakthrough of NO occurred instantaneously (within the

first 10 min), with the NO concentration reaching a steady-state level. In the absence of O2, the NO conversion, calculatedas (NOinlet − NOout)/NOinlet, was ∼8%, attributed to theoxygen-containing surface functional groups present in theACFs. The conversion, however, increased significantly withthe external supply of O2 and also with increasing O2concentrations. The steady-state conversion was ∼28% using20% O2.Figure 6 also shows the model-predicted results for the

breakthrough concentrations of NO. A reasonably goodagreement can be observed between the experimental dataand model predictions. The model simulations were performedat the experimental conditions. As previously mentioned in thetext, the model developed in this study considered the diffusioneffects in the packed bed and within the pores of the material.The particle-size-based Reynolds number (Re), Schmidtnumber (Sc), and Sherwood number (Sh) were calculated as0.0079, 0.0144, and 2.014, respectively. The small value of Re isattributed to the small diameter (2.8 × 10−6 m) of the fibers.The gas-to-fiber mass-transfer coefficient calculated from Shwas ∼653 m/s. The relatively higher mass-transfer coefficientindicated insignificant diffusion resistance within the packedbed. The intraparticle diffusion coefficient was calculated to be∼2.8 × 10−8 m2/s, which is of the same order of magnitude asthat 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 thereaction without O2 to 1.2 × 10−6 m3/mol·s for the case with20% O2, which is consistent with eqs 9−15. The numericalvalue of Kc2 was kept approximately constant with increasing O2

concentration. Table 3 lists the numerical values of the modelparameters adjusted to explain the experimental data shown inFigure 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 externalsource. The greater the O2 concentration, the smaller the NOconcentration at the exit of the reactor. The initial increaseobserved in the NO concentration is attributed to the “roll-up”characteristics of the adsorbing binary gaseous mixture, with theNO physically displaced from the surface by O2.

9,25 Therelatively higher O2 concentration levels in the reactor resulted

Table 2. Results of Elemental Analysis of the PreparedMaterials

sample C (%) H (%) N (%) O (%)

ACFs 70.81 1.35 0.56 27.48Cu−CNFs/ACFs 86.35 2.90 0.72 10.03Ce(NO3)3−Cu(NO3)2−ACFs 46.09 1.55 4.10 48.26CeO2−Cu−ACFs 55.79 1.37 0.95 41.89CeO2−Cu−CNFs/ACFs 65.28 1.83 0.51 32.28

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

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



in the early roll-up in NO concentrations. The steady-stateconcentration observed below the inlet concentration level isattributed to the catalytic activity of the ACF surface.9

4.8.2. Cu-NP-Dispersed Samples. The reactions wereperformed on the single-metal Cu−ACF and Cu−CNF/ACFsamples to ascertain the role of Cu NPs and CNFs for theoxidation of NO to NO2. The NO conversion increasedmarginally 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%, incomparison to ∼28% on the ACFs, when using 20% O2.Thus, the catalytic activity of Cu NPs in the oxidation of NOwas obvious, which also significantly increased with increasingO2 concentration.

Figure 7b describes the results for Cu−CNFs/ACFs underidentical reaction conditions as for the Cu−ACFs. The NOconversion was measured as ∼22% using the Cu-NP-grownCNFs on the ACFs, without O2. Therefore, the CNFspromoted the oxidation of NO. The catalytic activity ofCNFs, attributed to the exposed free edges of the hexagonscontaining the heteroatoms and sp2 carbon in the material, isreported in the literature.13 Moreover, the reacting NO and O2were more exposed to the Cu NPs attached at the tips of thefibers than to the Cu NPs dispersed in the ACFs. The effect ofthe active CNF surface on the oxidation was, however, reducedwith increasing O2 concentration. In this case, the catalyticactivity 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 washigher for Cu−CNFs/ACFs than Cu−ACFs, signifying arelatively larger number of active sites available in the CNFs.Kc2 was kept approximately constant. A reasonably goodagreement was observed between the model-predicted valuesand the experimental data.

4.8.3. CeO2−ACFs and CeO2−Cu−CNFs/ACFs. The oxida-tion tests were performed on CeO2−ACFs (without Cu) toascertain the role of ceria in the oxidation of NO to NO2. Asobserved from Figure 8a, the NO conversion was less than 15%when O2 was not supplied externally. The oxygen required forthe oxidation was provided by the redox reaction betweenCeO2 and Ce2O3 and the oxygen-containing surface functionalgroups. The oxygen released from the lattice of ceria promotedthe conversion of NO to NO2. The conversion increased to∼38% when O2 was supplied externally. However, it was lessthan that for Cu−ACFs (Figure 7a). Thus, the role of ceria waspredominantly 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 O2 (%) Kc1 (m3/mol·s) Kc2 (m

3/mol)

ACFs 0 0.07 38.210 0.36 38.920 0.71 39.7

Cu−ACFs 0 0.10 38.210 1.15 38.420 3.25 38.6

Cu−CNFs/ACFs 0 0.25 38.910 1.97 39.620 5.00 40.3

CeO2−ACF 0 0.13 32.810 0.51 37.620 1.20 39.8

CeO2−Cu−CNFs/ACFs 0 0.50 37.410 2.33 38.620 6.10 41.1

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).



conversion increased to ∼30% without using O2 and to ∼70%when 20% O2 was used (Figure 8b). Clearly, the enhancedactivity of the material can be attributed to the combined effectsof the CNFs, Cu NPs, and CeO2 on the oxidation. In theprevious section, we discussed the catalytic characteristics of theCNFs and Cu NPs and a relatively greater exposure of the CuNPs attached to the tips of the CNFs responsible for theincreased oxidation rate. The synergistic interaction betweenCeO2 and Cu NPs, in situ incorporated in the matrix of theCeO2−Cu−CNFs/ACFs, resulted in the production of latticeoxygen having a relatively higher mobility, also discussedpreviously.Similarly to the previous cases, the model predictions were

found to be in reasonably good agreement with the data forCeO2−ACFs and CeO2−Cu−CNFs/ACFs. The model param-eters Kc1 and Kc2 were adjusted in each case to obtain the best fitand are listed in Table 3. The variation in the parameters wasconsistent 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 ultrasonicatedto dislodge the Cu NPs from the tips of the CNFs. As observedin Figure 9, NO conversion decreased in the sonicated samples(refer to Figures 7b and 8b for the corresponding data on therespective unsonicated samples). The decrease in the activityreconfirmed the catalytic role of the Cu NPs in the oxidation ofNO to NO2. The NO conversion for the sonicated CeO2−Cu−CNFs/ACFs was expectedly found between those for theunsonicated CeO2−ACFs and CeO2−Cu−CNFs/ACFs(shown in Figure 8).Figure 10 presents the comparative performances of the

prepared materials used for the NO oxidation. The perform-ances of the materials with and without CeO2 and Cu NPs werefound to be in the following order, as previously discussed:CeO2−Cu−CNFs/ACFs > Cu−CNFs/ACFs > Cu−ACFs >CeO2−ACFs > ACFs.

Figure 11 depicts the NO2 concentrations at the exit of thereactor. The comparative data for different materials areconsistent with those for the NO concentrations at the exitunder the identical operating conditions (Figure 10), indicatingthe catalytic activities of the materials for the same reactiontime.

4.8.5. Effect of NO Concentration. The oxidation reactionswere performed at different NO concentrations (500, 750, and1000 ppm) using CeO2−Cu−CNFs/ACFs, the best materialprepared in this study. As observed in Figure 12, the conversionincreased with decreasing NO concentration and was ∼80% fora 500 ppm NO concentration. The NO conversion achieved inthis study can be compared to the approximately sameconversion achieved using Mn−Co−Ce−Ox at 150 °C7 and

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).

Figure 9. NO oxidation over sonicated Cu−CNFs/ACFs (□) andCeO2−Cu−CNFs/ACFs (○) (T = 30 °C, P = 1 bar, W = 1 g, NO =1000 ppm, Q = 37.5 sccm, O2 = 20%).



∼75% at 100 ppm using MnOx−CeO2 at 300 °C.4 On theother hand, the conversion achieved in this study is significantlylarger than the ∼60% achieved at 500 ppm NO concentrationusing Mn−Ce−Ox at 150 °C,6 the ∼25% at 500 ppm using

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. CONCLUSIONSA novel CeO2- and Cu-NP-dispersed CNF/ACF material wasproduced, in which the bimetals were in situ incorporatedduring the synthesis stage. The produced material was appliedfor the control of NO emissions by oxidation at roomtemperature. Approximately 80% conversion was achieved forof a NO concentration of 500 ppm in an oxygen-rich (20%)atmosphere. The relatively larger conversion achieved usingCeO2−Cu−CNFs/ACFs is attributed to the combined catalyticeffects of the CNFs and Cu NPs and the synergistic interactionbetween ceria and the Cu NPs. A mathematical model wasdeveloped for predicting the breakthrough concentrationprofiles of NO in the tubular reactor packed with the catalystmaterials, considering a Langmuir−Hinshelwood-type kineticmechanism and mass-transfer effects within the reactor. Themodel results were found to be in good agreement with theexperimental data. The CeO2−Cu−CNF/ACF materialdeveloped in this study is a potential catalyst for the effectiveremoval of NO by oxidation at room temperature.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: 91-512-2596352/7704. Fax: 91-512-2590104. E-mail:[email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank Kynol, Inc. (Tokyo, Japan) for providing theACFs.

■ REFERENCES(1) Li, J.; Chang, H.; Ma, L.; Hao, J.; Yang, R. T. Low-temperatureselective catalytic reduction of NOx with NH3 over metal oxide andzeolite catalystsA review. Catal. Today 2011, 175, 147.(2) Dawody, J.; Skoglundh, M.; Fridell, E. The effect of metal oxideadditives (WO3, MoO3, V2O5, Ga2O3) on the oxidation of NO andSO2 over Pt/Al2O3 and Pt/BaO/Al2O3 catalysts. J. Mol. Catal. A:Chem. 2004, 209, 215.(3) Irani, K.; Epling, W. S.; Blint, R. Effect of hydrocarbon species onNO oxidation over diesel oxidation catalysts. Appl. Catal. B 2009, 92,422.(4) Cui, M. S.; Li, Y.; Wang, X. Q.; Wang, J.; Shen, M. Q. Effect ofpreparation method on MnOx-CeO2 catalysts for NO oxidation. J.Rare Earths 2013, 31, 572.(5) Guillen-Hurtado, N.; Atribak, I.; Bueno-Lopez, A.; Garcia-Garcia,A. Influence of the cerium precursor on the physico-chemical featuresand NO to NO2 oxidation activity of ceria and ceria−zirconia catalysts.J. Mol. Catal. A: Chem. 2010, 323, 52.(6) Li, H.; Tang, X.; Yi, H.; Yu, L. Low-temperature catalyticoxidation of NO over Mn-Ce-Ox catalyst. J. Rare Earths 2010, 28, 64.(7) Li, K.; Tang, X. L.; Yi, H. H.; Ning, P.; Kang, D. J.; Wang, C.Low-temperature catalytic oxidation of NO over Mn−Co−Ce−Oxcatalyst. Chem. Eng. J. 2012, 92, 99.(8) Bandosz, T. J. Activated Carbon Surfaces in EnvironmentalRemediation; Elsevier: Amsterdam, 2006.(9) Adapa, S.; Gaur, V.; Verma, N. Catalytic oxidation of NO byactivated carbon fiber (ACF). Chem. Eng. J. 2006, 116, 25.(10) Das, D.; Gaur, V.; Verma, N. Removal of volatile organiccompound by activated carbon fiber. Carbon 2004, 42, 2949.

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

Figure 11. NO2 outlet concentration with time for different materialsduring the oxidation of NO (T = 30 °C, P = 1 bar, W = 1 g, NO =1000 ppm, Q = 37.5 sccm, O2 = 20%).

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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