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Applied Catalysis A: General 409– 410 (2011) 55– 65

Contents lists available at SciVerse ScienceDirect

Applied Catalysis A: General

j ourna l ho me page: www.elsev ier .com/ locate /apcata

atalytic reduction of NO by CO over copper-oxide supported mesoporous silica

rchana Patela, Pradeep Shuklaa, Thomas Ruffordb, Shaobin Wangc, Jiuling Chena,ictor Rudolpha, Zhonghua Zhua,∗

School of Chemical Engineering, The University of Queensland, St. Lucia, QLD 4072, AustraliaSchool of Mechanical and Chemical Engineering, University of Western Australia, Crawley, WA 6009, AustraliaDepartment of Chemical Engineering, Curtin University, GPO Box U1985, Perth, WA 6845, Australia

r t i c l e i n f o

rticle history:eceived 2 February 2011eceived in revised form4 September 2011ccepted 19 September 2011

a b s t r a c t

Copper oxide supported on four different types of mesoporous silica (SBA-15, MCM-41, MCM-48 andKIT-6) were prepared and examined for catalytic reduction of NO with CO in the temperature range of250–500 ◦C. Their structural and chemical properties were characterized by N2 adsorption, low angleand wide angle X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectronspectroscopy (XPS), FTIR and temperature programmed reduction (TPR). H2-TPR revealed that MCM-41

vailable online 24 September 2011

eywords:itric oxideatalytic reductionesoporous silica

and SBA-15 tend to possess a higher quantity of reducible copper species in contrast to MCM-48 andKIT-6. CuO supported on MCM-41 and SBA-15 exhibited higher activity in catalytic reduction of NO thanCuO supported on MCM-48 and KIT-6. The superior catalytic activity was attributed to homogeneousdispersion of CuO and availability of the reducible copper ions in the channels of mesoporous materials.

© 2011 Elsevier B.V. All rights reserved.

opper oxide

. Introduction

Oxides of nitrogen (NOx) emitted from the stationary and mobileources such as thermal power plants, nitric acid production plantsnd automobiles are major environmental pollutants generallyesponsible for major environmental problems such as acid rain,reenhouse effect, photochemical smog and ozone layer depletion.long with NOx, carbon monoxide (CO) simultaneously emitted

rom the power plants and automobiles also tend to damage thenvironment to a great extent. Despite significant investmentsade in pollution control technologies, efficient control for simul-

aneous NOx and CO removal remains a major challenge. Amongeveral technologies such as adsorption and absorption, selectiveatalytic reduction (SCR) is an important process for the control ofOx from coal fired power plants and automobiles. In addition, theresence of CO in the exhaust gas may be effectively exploited tovoid the need to inject reductants like ammonia, urea, hydrocar-ons and hydrogen during the process.

Copper oxide loaded on various supports are well known asfficient catalysts for NO reduction in the presence of a suitableeducing agent. Copper-containing zeolites [1,2] and CuO sup-

orted on titania, zirconia, cerium oxide and silica have beeneported to have good activity in NO reduction at high temperatures3–8]. Developing small sized and well dispersed copper catalysts

∗ Corresponding author.E-mail address: z.zhu@uq.edu.au (Z. Zhu).

926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2011.09.024

for enhancement of the reaction efficiency and conversion has beena forefront research challenge. To overcome this challenge, severalmesoporous materials have been utilized to support copper for NOreduction [3,9,10]. Due to high surface area, pore volume and uni-form pore channels, these mesoporous materials have been foundpromising applications as efficient adsorbents, catalysts or catalystsupports, and also in controlled drug delivery systems [11–14]. Theactivity of a supported catalyst primarily depends on the effectivedispersion of the active metal center in the support and the acces-sibility of the reactant molecules within the pores of the catalyst’ssupport. This has motivated several researchers to develop metaloxide supports having high surface area and wide pore openings. Inaddition to the surface area and pore size, the structure of the porechannels also significantly affects catalyst dispersion within and inturn determining its activity. In order to develop well encapsulatedcopper catalysts, various mesoporous supports have been inves-tigated. Cu–Al–MCM-41 showed a long-term stability and welldispersed Cu2+ ions for simultaneous removal of NOx and VOC pri-marily due to the well-ordered mesoporous long rang hexagonalarray [15]. Velu et al. [16] studied Cu/MCM-41 and Cu–Zn/MCM-41 for selective oxidation of alcohol. Cu2+ ions in MCM-41 werecompletely reduced to copper metal because the ions were eas-ily accessible to H2 gas in interior surface of mesopores. Severalresearches have also focused on the comparison between different

types of support to examine their catalytic property and reactionmechanism. Anpo et al. [17] studied CO2 reduction with H2O usingTi–MCM-41 and Ti–MCM-48 which have 2D straight and 3D inter-connected pore channels, respectively. They observed that both

56 A. Patel et al. / Applied Catalysis A: G

Nomenclature

� rate of reaction, moles of NO converted/m3 sRp particle diameter, mCs concentration of reactant A, mol/m3

Def effective diffusivity, m2/sdp pore diameter, mmp main peak

s4iodIibrttsptipttst4TslmitatemSpsht

N

tnso

2

2

tcuu

sp satellite shake up peak

upports showed similar catalyst dispersion, however, Ti–MCM-8 showed better activity resulting from better reactant transport

n the 3D pore channels. Jang et al. [18] reported a superior activityf Pt–MCM-48 in comparison to Pt–MCM-41 for NOx reduction,ue to the better dispersion of PtO2 phase in MCM-48 support.

n another investigation, Hadjiivanov et al. [19] reported the load-ng of CuO in MCM-41 and MCM-48 supports. It was reported thatetter reducibility and varied Cu2+ sites on the MCM-41 possiblyesulted in better dispersion and catalyst-support bonding. In addi-ion, Cu–MCM-41 showed better catalytic activity in comparisono Cu–MCM-48. In majority of the cases the efficient metal disper-ion within the support has been attributed to the pore size andore volume of the support. Despite the differences in the catalystype and the reaction mechanism, an interesting observation maden all the above reports is the application of support of differentore structure. MCM-41 tends to pose a 2D straight pore struc-ure whereas MCM-48 poses a 3D pore network structure. Most ofhe supports had the similar pore size and surface area, but stillhowed differences in catalytic activity. While the dispersion ofitania was similar on both the supports viz MCM-41 and MCM-8, the platinum dispersion was found better in case of MCM-48.he dispersion of copper, however, was found to be better in thetructure of MCM-14. These reports suggested that despite of simi-arity in the size and types of pores and channels of mesoporous

aterials, highly varying results could be obtained, thus imply-ng the surface chemistry of the support and its interaction withhe impregnated ion tends to significantly influence the dispersionnd the chemical phase of the active metal catalyst loaded insidehem rather than the physical structure itself. The present studyxamines the metal support interactions and catalyst dispersion inesoporous supports with different pore channels and pore sizes.

ynthesis of template materials provides a suitable route to pre-are support materials with highly variant pore structure. In thistudy we reported the synthesis of four different types of supportaving different pore structure and pore size for CuO loading andhe effect on NO reduction using CO as a reducing agent (Eq. (1)).

O + CO → (1/2)N2 + CO2 (1)

The mesoporous materials synthesized can be grouped in twoypes: 2D linear pore channels and 3D interconnected pore chan-els. Additionally, for each type of pore structure, two types ofupport were synthesized, one having a larger pore size and thether with a smaller pore size.

. Materials and methods

.1. Chemicals and catalyst synthesis

Tetraethyl orthosilicate (TEOS, 98%, Aldrich) was utilized as

he silica source in the synthesis. Pluronic P123 (BASF) andetyltrimethylammonium bromide (CTABr, 99%, Aldrich) weresed as the templates. Copper (II) nitrate (99.8%, Aldrich) wastilized as a CuO precursor for catalyst loading. Other important

eneral 409– 410 (2011) 55– 65

chemicals utilized in the synthesis were KCl (99%, Aldrich), HCl,and 32% NH3(aq).

Mesoporous silicas, SBA-15, MCM-41, MCM-48 and KIT-6, wereprepared from P123 or CTABr and TEOS based on the proceduresdescribed elsewhere [20–23]. In brief, for SBA-15 synthesis: 4 g ofP123, 9.44 g KCl and 90 ml of 2 M HCl were mixed and stirred for 2 h.6.44 g of TEOS was slowly added into the mixture and kept at 40 ◦Cfor 24 h. The solution was then transferred into an autoclave forhydrothermal treatment at 100 ◦C for 24 h. Finally, the precipitatewas filtered, dried and calcined at 500 ◦C for 6 h. In the typical syn-thesis of MCM-48: 2.4 g of CTABr, 50 ml of deionized water, 50 mlof ethanol and 12 ml of ammonia were mixed and stirred until thesolution became clear. Then, 3.4 g of TEOS was added and stirred for2 h. The precipitate was then removed, dried and calcined at 600 ◦Cfor 6 h. For MCM-41 preparation, 2.4 g of CTABr and 120 ml of deion-ized water were mixed and stirred until the solution became clear.8 ml of 32% ammonia solution were added followed by addition of10 ml of TEOS. After 24 h of stirring, the precipitate was filtered,dried and calcined at 500 ◦C for 5 h. KIT-6 was synthesized by mix-ing 6 g of P123, 217 g of deionized water, 11.8 g of concentrated HCland 6 g of butanol. The mixture was stirred for 1 h followed by addi-tion of 12.9 g of TEOS and stirred for 24 h at 35 ◦C. The solution wastransferred to an autoclave for hydrothermal treatment at 100 ◦Cfor 24 h. Precipitate was removed, dried and calcined at 500 ◦C for5 h [20,21].

CuO loading in the supports were done by a wet impregnationmethod. For preparation of 5% CuO loaded on silica supports, 1 gof mesoporous silica was added in aqueous solution of copper (II)nitrate and stirred for 24 h. The solution was dried and calcined at350 ◦C for 6 h resulting in the formation of copper oxide particleson the supports.

2.2. Characterization of the synthesized catalysts

The synthesized supports and the catalysts were critically exam-ined for their structural and chemical properties. Powder XRDanalysis of the samples was carried out on a Rigaku miniflex diffrac-tometer operated at 40 kV. Small angle X-ray scattering pattern wasrecorded on a Bruker D8 Advanced Research Diffractometer witha Cu-K� radiation source at 40 kV and 30 mA. Sample imaging wasdone to observe the pore morphology and catalyst dispersion in thesupport using a JEOL JEM1010 electron microscope.

The adsorption–desorption of nitrogen at −196 ◦C was mea-sured using a Micromeritics Tristar 3000. Prior to the analysis thesamples were degassed for 8 h at 200 ◦C. The specific surface areas(SBET) of silica supports and Cu catalysts were calculated by theBrunauer–Emmett–Teller (BET) equation. Total pore volumes (Vp)were evaluated at relative pressures (P/P0) close to unity. Poresize distributions were calculated from adsorption branches of theisotherms (for P/P0 > 0.35) using the Barrett, Joyner and Halenda(BJH) method.

Bulk elemental composition of the catalysts was measured withthe help of inductive coupled plasma optical emission spectrome-try (ICP-OES). In a typical ICP-OES measurement 20 mg of samplewas digested with 5 ml of nitric acid and 2.5 ml of hydrofluoric acidby microwave using a CEM MDS 2000 digester, and the elementalconcentrations were determined using a Varian Vista Pro ICP-OES.

The FTIR spectroscopic analysis was done to determine the func-tional groups of the samples by a Perkin–Elemer spectrum 100 FTIRinstrument using a KBr pellet technique at room temperature. Afixed amount of dried KBr was homogenized with the samples in amortar. A fixed of sample was mixed with KBr and disks with radius

1 cm and thickness ≈0.1 cm were prepared using a hydraulic press.All the spectra were recorded over 4000–450 cm−1

.

Surface chemistry of catalyst samples was examined by X-rayphotoelectron spectroscopy (XPS) analysis. XPS data were acquired

is A: G

upi2paehtdrcqtfoHdcc

2

iaaiwga1Nd

A. Patel et al. / Applied Catalys

sing a Kratos Axis ULTRA X-ray Photoelectron Spectrometer incor-orating a 165 mm hemispherical electron energy analyzer. The

ncident radiation was Monochromatic Al K� X-rays (1486.6 eV) at25 W (15 kV, 15 mA). Survey (wide) scans were taken at analyzerass energy of 160 eV and multiplex (narrow) high resolution scanst 40 eV. Survey scans were carried out over 1200–0 eV bindingnergy range with 1.0 eV steps and a dwell time of 100 ms. Narrowigh-resolution scans were run with 0.05 eV steps and 250 ms dwellime. Base pressure in the analysis chamber was 1.0 × 10−9 Torr anduring sample analysis 1.0 × 10−8 Torr. Temperature programmededuction (TPR) experiments were performed with H2 using a Bel-at instrument. Typically, 50 mg of catalyst were loaded, along withuartz wool, into a U-shaped quartz cell of 9 mm in diameter. Prioro the TPR experiments the catalyst was heated in situ at 350 ◦Cor 1 h, and then cooled to 25 ◦C in Ar. The reduction was carriedut from room temperature to 900 ◦C in a 30 ml min−1 flow of 5%2 in Argon at a heating rate of 10 ◦C min−1. The water produceduring the catalyst reduction was trapped in a zeolite column. Theonsumption of H2 was measured continuously using a thermalonductivity detector (TCD).

.3. Evaluation of catalyst activity for NO reduction

Catalytic activity experiments for NO reduction were performedn a fixed bed quartz reactor (i.d. 0.9 cm) mounted in the verticallyligned tube furnace with approximately 0.1 g of catalyst. Temper-ture of the catalyst bed was measured by a thermocouple probenserted in the catalyst bed. Catalyst sample was kept on the quartz

ool plug placed at the center of the quartz tube. Flow rate of theases were controlled by a rotameter. Before activity tests, the cat-

lysts loaded reactor was flushed by flowing Argon at 300 ◦C for

h. The catalytic reduction of NO with CO was performed withO–CO–He gas mixture containing 500 ppm NO and 500 ppm CO atiscreet temperature steps in the temperature range of 250–500 ◦C.

Fig. 1. TEM images of all mesoporous supports (a) SB

eneral 409– 410 (2011) 55– 65 57

A reactor feed gas flow rate of 80 ml min−1 was selected to ensurethe reaction rate was not limited by external diffusion. At each tem-perature the reaction was carried out until steady state obtained.Concentrations of NO/NO2 were measured using a chemilumines-cence NOx analyzer (Thermo 42i HL). The catalytic activity tests atrelevant temperatures were repeated multiple times using the cat-alyst prepared from the same batch. The evaluated experimentaldata were used to calculate rate of reaction at each temperature.The rate of reaction was calculated on the base of NO disappear-ance. Steady state NO concentration at each temperature was usedto calculate rate of reaction.

3. Results and discussion

3.1. Structural properties of the mesoporous silica materials

The structure of the pristine support was observed using TEM.The 2D layered pore structure of SBA-15 can be easily identifiedfrom the TEM shown in Fig. 1a. Similarly, the long channeled struc-ture of MCM-41 having structural analogies to SBA-15 can also beseen from the TEM images in Fig. 1b. Typically, both materials havesimilar two dimensional ordered layer of hexagonal pore struc-ture; however, the BET analysis, as discussed later, suggests thatSBA-15 tends to have larger pore channel diameter as compared toMCM-41. The remaining two materials, MCM-48 and KIT-6, havethree dimensional pore structures. MCM-48 has a cubic repetitionof the unit structure with pore mouth opening in all six directions.KIT-6 shares structural analogies with MCM-48 but tends to posseslarger pore size opening. The TEM image of the MCM-48 shownin Fig. 1c depicts a series of parallel channels of the particle lying

along the length of the face tube on the specimen plate. In contrast,the image of KIT-6 (as seen in Fig. 1d) clearly shows an orderedcubic pore structure. The structure of the supports can also be jus-tified from the low angle XRD spectra for all the samples as seen

A-15, (b) MCM-41, (c) MCM-48 and (d) KIT-6.

58 A. Patel et al. / Applied Catalysis A: General 409– 410 (2011) 55– 65

f1tciascld

gciIdfiwalpKitT2K

3m

m

TT

Fig. 3. N2 adsorption–desorption isotherms of (a) mesoporous silica supports and

Fig. 2. Small angle X-ray scattering pattern of mesoporous silica supports.

rom Fig. 2, wherein the first peak indexed at (1 0 0) plane in SBA-5 appeared at a smaller Bragg angle or larger d1 0 0 as comparedo MCM-41. The three peaks indexed at (1 0 0), (1 1 0) and (2 0 0)an be easily resolved for SBA-15 while significantly less intensiven MCM-41, conforming to the p6mm space group for hexagonalrrays of mesopores [22,23]. The intensive (2 1 1) and the less inten-ive peaks indexed for (2 2 0) and (4 2 0) planes obtained for KIT-6onfirm to the cubic Ia3d structure. The similar but significantlyess intensive peaks in MCM-48 also confirm the presence of threeimensional cubic pores [20,21,23].

The results from nitrogen sorption were found to be inood agreement with the pore-size and pore structure as dis-ussed above. As observed from Fig. 3, the adsorption–desorptionsotherms of all mesoporous silica supports clearly exhibited typeV isotherm from the IUPAC classification. SBA-15 and KIT-6epicted capillary condensation at very high P/P0 ratio (∼0.7) con-rming a larger pore diameter in contrast to MCM-41 and MCM-48,hich showed capillary condensation at comparatively smaller rel-

tive pressure thereby indicating smaller pore diameter. Table 1ists the specific surface area and the range of pore structuralarameters of the pristine and copper loaded supports. SBA-15 andIT-6 were found to have small surface area but larger pore volume

n comparison to MCM-41 and MCM-48. The BJH pore size distribu-ions of the synthesized mesoporous materials are shown in Fig. 4.he pore size distributions showed maxima for pore size between

and 5 nm for MCM-41 and MCM-48 while 5–10 nm for SBA-15 toIT-6.

.2. Structural and chemical properties of catalyst loaded

esoporous silica materials

Copper oxide loading in the supports resulted in significantodifications of the pore opening and porosity. The catalyst loading

able 1extural property of mesoporous silicas and Cu–mesoporous silica.

Sample Surfacearea (m2/g)

Average porediameter (nm)

Pore volume(cm3/g)

Cu (wt%)

SBA-15 642 5.5 0.94 –MCM-41 816 2.6 0.63 –MCM-48 1278 2.4 0.86 –KIT-6 803 5.1 0.96 –CuO/SBA-15 506 6.2 0.91 4.17CuO/MCM-41 829 4.2 0.52 4CuO/MCM-48 893 4 0.54 4.21CuO/KIT-6 659 7 0.97 4.3

(b) CuO supported on mesoporous silica (•) desorption isotherm and (�) adsorptionisotherm.

caused a drop in the pore volume of all the supports except KIT-6.This effect was more pronounced in the support having smallerpore size, with MCM-48 having the largest drop in the pore vol-ume followed by MCM-41. Typically, the catalyst particles tend tooccupy the porous space inside the support media and possiblyblockage of pore mouth thus resulting in the drop in the pore vol-ume. This feature is complemented by a shift in the average poresize suggesting that the occupied catalyst particles in the porestend to block the openings of the smaller pore mouth. As seen fromthe BJH pore size distribution of CuO loaded mesoporous silica inFig. 5, the pore size distributions became broader after loading CuOon mesoporous supports. Similar observation was also noticed inour previous work [24]. The TEM images shown in Fig. 6 of sup-ported copper catalysts suggested well dispersed catalyst particlesin SBA-15 and MCM-41 supports, in contrast a few bulk agglom-erates of CuO clearly visible in the KIT-6 based supports. Basedon the physical characterization, it is evident that the pore vol-

ume and surface area have a lesser dominance on the catalystdispersion in the support, since KIT possessing the highest pore vol-ume and MCM-48 having the largest surface area had rather poor

A. Patel et al. / Applied Catalysis A: General 409– 410 (2011) 55– 65 59

ributi

cjMo4bia

sst3[sIlsTraattft

Fig. 4. BJH Pore size dist

atalyst dispersion. A similar observation was reported by Had-iivanov et al. [19], who observed better dispersion of copper in

CM-41 as compared to MCM-48 pertaining to the larger amountf Cu2+ species formed in Cu–MCM-41 as compared to Cu–MCM-8 sample. In addition, it was also reported that the Cu2+ speciesonded with the MCM-41 support differed in the electrophilic-

ty resulting in different co-ordination of the metal. Such featuresssist in better metal dispersion and metal-support bonding.

FTIR spectroscopy is a useful technique for investigating theurface chemistry and the metal incorporation in the frameworktructure of the mesoporous particles. Fig. 7 shows the FTIR spec-ra of all the four samples. A large and broad band peaked around490 cm−1 is assigned to surface silanol and adsorbed water groups25]. The three major peaks observed in all four copper loadedamples around 1070 cm−1, 795 cm−1 and 455 cm−1 represents theR absorption of Si–O–Si [26,27]. The vibration observed at theowest frequency (455 cm−1) is due to the rocking mode corre-ponding to the out of Si–O–Si plane motion of the oxygen atom.he band observed at 788 cm−1 represents the bending vibrationesulting from the oxygen atom motion between the Si–O–Si planend along Si–O–Si angle bisector. Finally the largest band observedt 1072 cm−1 occurs from the vibration due to the stretching of

he Si–O–Si bond. The peak in the region 900–1000 are of par-icular interest since band centered around 965 cm−1 originatedrom Si–O vibrations in the Si–O–M group, where M correspondso metal incorporated in the framework structure [28]. The band is

on of mesoporous silica.

typically assigned to the lattice defects caused due to metal incor-poration and formation of O–M linkage. The intensity of this peak isobserved to be slightly higher in case of SBA-15 and MCM-41 basedsupports. Finally, the C–H stretching bands observed between 2800and 3000 cm−1 is attributed to the methyl group indicating that asmall amount of surfactant still remains inside the silica framework[28].

The surface composition and the chemical state of the supportedcopper samples were further examined by means of XPS. The com-plete spectrum is shown in Fig. 8 which shows the presence of majorelements consisting of Cu, silicon and oxygen. The weak carbonpeak observed is once again attributed to the residual organic inthe structure. The high resolution XPS spectra focused on copperspecies are shown in Fig. 9. Both MCM-41 and MCM-48 supportedcopper catalysts showed a major peak at BE (2p3/2) = 932.9 eV andBE (2p3/2) = 933.09 eV respectively. In addition, both the samplesshowed a small shoulder peak at BE (2p3/2) = 935.2 eV and BE(2p3/2) = 933.8 eV respectively. The similar primary and shoulderpeaks were clearly observed in SBA-15 and KIT-6 supported sam-ples as shown in Table 2. In addition, all the four samples showeda shake up peak associated with primary peak at around 943 eV.The BE value for the primary peaks observed in case of CuO/SBA-15

and CuO/KIT-6 along with their respective shake-up peaks confirmsthe presence of Cu2+ phase [29]. The prominent shoulder peaks alsofound in those two samples which are at higher BE (approximate935 eV) is likely due to the coordination of the Si–OH groups with

60 A. Patel et al. / Applied Catalysis A: General 409– 410 (2011) 55– 65

Fig. 5. BJH pore size distribution of CuO supported on mesoporous silica.

Fig. 6. TEM images of CuO supported on mesoporous silica (a) CuO/SBA-15, (b) CuO/MCM-41, (c) CuO/MCM-48 and (d) CuO/KIT-6.

A. Patel et al. / Applied Catalysis A: General 409– 410 (2011) 55– 65 61

tShg

Table 2Cu 2p high resolution spectra XPS spectra (mp – main peak, sp – satellite shake uppeak).

Sample Cu 2p3/2 mp B.E. Cu 2p3/2 sp Spitting (eV) Isp/Imp

CuO/SBA-15 933.3 943.2 9.9 0.33CuO/MCM-41 933.1 942.3 4.5 0.27CuO/MCM-48 933.4 941.5 8.1 0.39

Fig. 7. FTIR spectra of CuO supported on mesoporous silica after calcination.

he copper ions and copper ions linked with silica support [30].imilar peaks were observed in case of copper loading in phosphateetero-structures due to copper ion exchange with the phosphateroups [31].

CuO/SBA-15

Binding energy (eV)

020040060080010001200

Inte

nsity

(CP

S)

CuO/MCM-48

Binding energy (eV)

020040060080010001200

inte

nsity

(CP

S)

Fig. 8. Wide scan XPS profile of CuO supported on

CuO/KIT-6 934.0 943.4 9.5 0.56

In contrast to Cu/SBA-15 and Cu/KIT-6, the MCM supportedcatalysts showed much smaller shoulder peaks. In addition, thelower BE of their primary 2p3/2 peak indicates the coexistence ofsmall amount of Cu+ species on the surface. This is further exam-ined with the intensity ratio of satellite and primary Cu 2p3/2as shown in Table 2. For standard CuO phase, this ratio is closeto 0.55 and decreased to 0 for Cu0 phase [32]. As seen fromTable 2 the MCM-41 supported catalyst showed a significantlylower Isp/Imp ratio as compared to CuO/SBA-15 and CuO/MCM-48.CuO/KIT-6 showed around the same value as reported in literature[33,34].

The powder XRD patterns of CuO/mesoporous-silica are shownin Fig. 10. The peaks at 2� = 35.5◦, 38.5◦ and 48.5◦ indicated the pres-

ence of CuO phase (JCPDS 44-0706). Very small amount of metalliccopper crystalline phase was also observed in some CuO/silicasamples (2� = 50.5◦). The slight amount of metal Cu could have

CuO/MCM-41

Binding energy (eV)

020040060080010001200

Inte

nsity

(CP

S)

CuO/KIT-6

Binding energy (eV)

020040060080010001200

Inte

nsity

(CP

S)

silica samples after calcinations at 350 ◦C.

62 A. Patel et al. / Applied Catalysis A: General 409– 410 (2011) 55– 65

CuO/MCM-41

Binding energy (eV)

960955950945940935930

Inte

nsity

(CP

S)

CuO/SBA-15

Binding energy (eV)

960955950945940935930

Inte

nsity

(CP

S)

CuO/MCM-48

Binding energy (eV)

960955950945940935930

Inte

nsity

(CP

S)

7400

7600

7800

8000

8200

8400

8600

CuO/KIT-6

Binding energy (eV)

960955950945940935930

Inte

nsity

(CP

S)

ctra o

boobs

Fc

Fig. 9. Cu 2p core level spe

een resulted due to the reduction of copper ion in the presencef slight organic precursor during the calcinations. The absence

f metallic copper peaks in case of CuO/SBA-15 and CuO/KIT-6ased catalyst which has the least residual organic moity in thetructure (as seen from FTIR) further justifies the assumption.

ig. 10. X-ray diffraction pattern of CuO supported on mesoporous silica after cal-ination.

f CuO supported on silica.

The diffractions of copper oxide were well resolved in case ofCuO/MCM-41 and CuO/MCM-48 samples but are less predominantin the CuO/SBA-15 and CuO/KIT-6 samples. The average crystal sizeof CuO calculated from the Scherrer equation was 16 ± 2 nm and26 ± 2 nm for CuO/SBA-15 and CuO/KIT-6 respectively. Whereascrystalline size of CuO on CuO/MCM-41 was 19 ± 2 nm and onCuO/MCM-48 was 28 ± 2 nm. The evidence of CuO crystallites onall CuO/mesoporous-silica being larger than that of the averagepore size of respective mesoporous silica material suggests thatthe crystallites tend to grow beyond the pore structure, poten-tially on the pore surface thereby blocking the small pore openingscompletely as confirmed from BET results. The crystallite sizewas found to be in the order SBA-15 > MCM-41 > MCM-48 > KIT-6.The larger crystallite in CuO/KIT-6 and CuO/MCM-48 samples iseven confirmed from the bulk CuO particulates observed in TEMimages.

The redox property is the most critical assessment to judge thecatalytic activity of the synthesized supported CuO catalysts. Thereducibility of the catalyst depends primarily on the availability ofthe active sites potentially arising due to the defects located in theCuO crystalline particles. In addition, TPR experiment being a bulkprocess, the rate and extent of reduction also depend on the particlesize [35]. The larger particle tends to provide diffusion resistancehence may result in a broad peak. Fig. 11 shows TPR profiles of

CuO/silica samples by H2. The colour change of the samples fromlight gray to dark purple indicated complete reduction of CuO tometallic copper. In a typical CuO reduction process two reductionstages were generally observed, namely low temperature reduction

A. Patel et al. / Applied Catalysis A: General 409– 410 (2011) 55– 65 63

SBA-1

(t

C

C

C

tbaCposilctfsfifbos

The catalytic activity of four prepared CuO/silica catalysts inNO reduction by CO was evaluated at varying temperatures underkinetic control conditions. Several preliminary experiments werecarried out and the flow rate of the gas in the reactor was

Table 3TPR peak temperature and H2 consumption of CuO supported on mesoporous silica.

Sample Temperature of peakmaximum (◦C)

Total H2 consumption(mmol H2/mmol Cu)

First peak Second peak

Fig. 11. H2-TPR profiles of fresh CuO supported on

LTR) and high temperature (HTR) reduction associated with Cu2+

o Cu+ and Cu+ to Cu0, respectively [36].

uO + H2 → Cu0 + H2O (170 ◦C) (2)

u2+ + (1/2)H2 → Cu+ + H+ (> 170 ◦C) (3)

u+ + (1/2)H2 → Cu0 + H+ (> 335 ◦C) (4)

The presence of bulk particles of CuO would often result in a longail or shoulder peak. The low temperature reduction tends to occurelow 170 ◦C whereas the high temperature reduction happensround 335 ◦C for all the synthesized samples. For CuO/SBA-15 anduO/KIT-6, the first large peak is attributed to the reduction of dis-ersed Cu2+ species to Cu+ due to the abstraction of intracrystallinexygen atom within the CuO structure [36–38]. The dispersed Cu2+

pecies potentially results from the smaller CuO crystallites form-ng an intermolecular bond with the parent support SiO2 structure,eaving behind a partially charged species. The second smaller peaklose to 350 ◦C is associated with the reduction of larger bulk par-icles of CuO aggregates [39]. A third reduction peak was observedor both catalysts above 500 ◦C, which is associated with the secondtep reduction of Cu+ species to Cu0 [36–39]. The reduction pro-les of CuO/MCM-41 and CuO/MCM-48 catalysts showed similar

eatures to that of CuO/SBA-15, with two reduction peaks observedetween 200–350 ◦C. Amount of H2 consumed for the reductionf catalysts is shown in Table 3. Higher amount of H2 was con-umed for the reduction of CuO supported on MCM-41 and SBA-15

5, MCM-41, MCM-48 and KIT-6, calcined at 350 ◦C.

as compared to CuO/MCM-48 and CuO/KIT-6. Due to the presenceof smaller crystallite size and higher interaction with the supportstructure as discussed from XRD and FTIR, the CuO/SBA-15 andCuO/MCM-41 based samples shows higher activity as compared tothe remaining samples. The highly active sites in CuO/SBA-15 andCuO/MCM-41 would critically assist in promoting NOx reductionby CO as via oxygen transfer mechanism as discussed in Section3.3.

3.3. Catalytic activity of supported CuO catalysts and the effect ofpore structure on the catalytic activity

CuO/SBA-15 288 565 0.00649CuO/MCM-41 288 360 0.00692CuO/MCM-48 269 332 0.0055CuO/KIT-6 232 385 0.00548

64 A. Patel et al. / Applied Catalysis A: G

Fs5

oaW

N

wor

fPnsc4w

C

abptssdstmlpsms4otci(rhSIt

ig. 12. Profiles of NO conversion as a function of reaction temperature of CuOupported on mesoporous slica, total flow rate: 80 ml min−1, NOin: 500 ppm, COin:00 ppm.

ptimized to ensure minimal film resistance. Additionally, thebsence of internal pore diffusion resistance was checked by theeistz–Prater parameter (Eq. (5)) [40,41].

W–P = �Rp2

CsDeff(5)

here, NW–P = Weistz–Prater parameter. In all the tests, the valuef the Weistz–Prater parameter was less than 0.3, showing that theeaction was not controlled by internal pore diffusion.

Fig. 12 shows a comparison of the catalytic activity of CuO/silicaor NO reduction in terms of conversion and rate of NO reduction.reliminary investigations suggested that for all the four catalysts,o significant activity was observed below 250 ◦C and NO conver-ion increased with temperature. The isotherm revealed the highestatalytic activity was obtained for CuO/MCM-41, with approximate2% conversion of NO at 485 ◦C. The activity trend of the catalystsas found to be in the following order:

uO/MCM-41∼CuO/SBA-15 > CuO/KIT-6 > CuO/MCM-48

Taking into account the crystallite size information from XRDnd metal support interactions from FTIR, the activity trend coulde assigned to the smaller crystallite size and better catalyst dis-ersion in the support. Khodakov el al. and others [42,43] reportedhe importance of the pore size of MCM-41, SBA-15 and amorphousilica for cobalt loading and the size of cobalt crystallite inside theupport structure. It was observed that the pore size of the supportsetermined the growth of the cobalt particles and MCM-41 basedupport produced the smallest size of cobalt oxide particle insidehe support structure. However, in the present work, the pore size of

esoporous supports seemed to have a little effect on the encapsu-ated catalyst size. The crystallite size measured from the diffractioneaks were found to be larger than the pore channel diameter thusuggesting that the metal crystallite tends to grow outside the poreouth. Among the four support materials, MCM-41 and SBA-15

howed smaller CuO particle in comparison to KIT-6 and MCM-8, despite the fact that MCM-48 has much smaller pore mouthpening as compared to SBA-15. Based on the observation of allhe supports, it could be concluded that formation of copper oxiderystallites depends significantly on coordination of copper ionsn the structure, which further depends on the surface chemistrypossibly involving Si–OH group) and the topology of the supportather than the pore size itself. The coordination of Cu in Si–OH

as been examined by XPS. A similar conclusion was deduced byzegedi et al. for cobalt loading on different silica supports [44].n addition, it has been previously reported by several investiga-ors that 3D open pore channels in the support would enhance

eneral 409– 410 (2011) 55– 65

dispersion of the catalyst inside the support and also provide eas-ier access of reactant molecules to catalyst particles thus possiblyenhancing the reactivity [21,45–47]. However, again a contradict-ing observation was found in the present reaction system. The 2Dstraight pore channel supported catalyst clearly outperfromed thecatalyst impregnated inside the 3D open pore channels.

The above made observations point towards the dominance ofchemical characteristic of the support for determining the reac-tivity of the encapsulated catalyst over their physical texture. Thesimultaneous CO oxidation and NO reduction take place due to theredox behavior of the CuO, which in turn takes place due to the easeof mobility of the intracrystalline oxygen atom in the CuO crystal-lite. The CuO crystallite in case of CuO/SBA-15 and CuO/MCM-41potentially shares a strong interaction with SiO2 and thus resultingin partial charge transfer taking place from the Cu atom to the SiO2support. This would then cause a slight weakening of the intracrys-talline oxygen bonding with copper in the CuO crystallite, thusgiving it higher mobility and in turn higher reactivity to the SBA-15and MCM-41 loaded catalysts. The influence of SiO2 support struc-ture on the charge sharing with Cu atom and the oxygen mobilityshould be investigated at a molecular scale. But such a work isbeyond the scope of this article. Nevertheless, the effect of ease ofmobility of intracrystalline oxygen atom in case of CuO/SBA-15 andCuO/MCM-41 on the macroscale is evident from the TPR informa-tion as discussed earlier, wherein easy loss of oxygen atom resultsin enhanced H2 consumption in case of the SBA-15 and MCM-41based supports. Finally, it can also be suggested that the reactionequilibrium of NO reduction in the presence of CO is not neces-sarily controlled by the kinetic of NO and CO availability on thesurface, and thus modifying the support structure to suite bettertransport would be less advantageous over the support which havechemical interactions with the catalyst. The detailed kinetic studyis currently planned for future investigations.

4. Conclusions

CuO supported on four mesoporous silicas with varying porestructure were prepared by wet impregnation method and wereexamined for NO reduction with CO. It was concluded that, largersurface area, pore size and a 3D pore structure of the support maynot necessarily provide better CuO dispersion in the support. Thesupport structure tends to play a major role in determining thequantity of the active phase during the catalyst impregnation. H2-TPD results suggest a higher amount of reducible Cu species in SBA-15 and MCM-41 supports, which results in better activity of Cu-SBA-15 and Cu-MCM-41 for NO reduction in comparison to thosewith 3D pore structure.

Acknowledgements

Financial support for this research was provided by the Aus-tralian Research Council and Indigo Technologies Pty. Ltd (throughARC linkage Project LP0775429).

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