catalytic removal of toluene over three-dimensionally ordered macroporous eu1–xsrxfeo3
TRANSCRIPT
Chemical Engineering Journal 214 (2013) 262–271
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Catalytic removal of toluene over three-dimensionally orderedmacroporous Eu1–xSrxFeO3
Kemeng Ji, Hongxing Dai ⇑, Jiguang Deng, Haiyan Jiang, Lei Zhang, Han Zhang, Yijia CaoLaboratory of Catalysis Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering,Beijing University of Technology, Beijing 100124, PR China
h i g h l i g h t s
" 3DOM Eu1�xSrxFeO3 are prepared bythe PMMA-templating method.
" 3DOM Eu1�xSrxFeO3 are high insurface area and Oads content andgood in reducibility.
" 3DOM Eu0.6Sr0.4FeO3 performs wellin the combustion of toluene.
" Catalytic activity is governed by Oads
concentration and reducibility.
1385-8947/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cej.2012.10.083
⇑ Corresponding author. Tel.: +86 10 6739 6118; faE-mail address: [email protected] (H. Dai).
g r a p h i c a l a b s t r a c t
3DOM Eu1�xSrxFeO3 (x = 0, 0.4) with high surface areas are fabricated using the citric acid-assistedPMMA-templating method. It is found that large surface area, high oxygen adspecies concentration,and good low-temperature reducibility as well as high-quality 3DOM structure are responsible for thegood catalytic performance of 3DOM Eu0.6Sr0.4FeO3.
80
100
)
0.24
0.30
l/(g
s))
EuFeO3 Eu0.6Sr0.4FeO3
0
20
40
60
200 250 300 350 400 450Temperature (oC)
Tol
uene
con
vers
ion
(%
0.00
0.06
0.12
0.18
Tol
uene
rea
ctio
n ra
te (µ
mo
( , ) EFO-bulk( , ) EFO-3DOM( , ) ESFO-3DOM
200 nm 200 nm
50 nm
Eu0.6Sr0.4FeO3
-8.0
-6.0
-4.0
-2.0
0.0
1.6 1.9 2.2 2 .5
1000/T (K -1
)
ln k
EFO-bulk EFO-3DOM ESFO-3DOM
E a = 81.6 kJ/mol
A = 2.76 × 107 (s
-1)
E a = 96.0 kJ/mol
A = 1.39 × 108 (s
-1)
E a = 104.2 kJ/mol
A = 3.55 × 106 (s
-1)
a r t i c l e i n f o
Article history:Received 10 August 2012Received in revised form 29 September2012Accepted 4 October 2012Available online 10 November 2012
Keywords:Three-dimensionally ordered macroporousperovskite-type oxideSr-substituted europium ferriteTemplating preparation methodLow-temperature reducibilityToluene combustion
a b s t r a c t
Three-dimensionally ordered macroporous (3DOM) perovskite-type oxides EuFeO3 (EFO-3DOM) andEu0.6Sr0.4FeO3 (ESFO-3DOM) were prepared by the citric acid-assisted polymethyl methacrylate-templat-ing method. The physicochemical properties of the materials were characterized by means of numeroustechniques. Catalytic activities of these porous samples were evaluated for the combustion of toluene. Itis shown that the EFO-3DOM and ESFO-3DOM catalysts were of high-quality 3DOM architecture and sin-gle-phase orthorhombic crystal structure with a surface area of 16–31 m2/g. The sequence in surface oxy-gen species concentration and low-temperature reducibility decreased in terms of ESFO-3DOM > EFO-3DOM > EFO-bulk, in good agreement with the order in catalytic activity. The ESFO-3DOM catalyst exhib-ited the best performance, giving the T10%, T50%, and T90% of 233, 278, and 305 �C at a space velocity of20,000 mL/(g h), respectively. Apparent activation energies of the ESFO-3DOM, EFO-3DOM, and EFO-bulkcatalysts were ca. 82, 96, and 104 kJ/mol, respectively. The excellent catalytic activity of ESFO-3DOMmight be associated with its higher surface area and surface oxygen species concentration and betterlow-temperature reducibility as well as high-quality 3DOM structure.
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1. Introduction
Perovskite-type oxides (ABO3) show higher chemical, thermal,and structural stability than single oxides. Due to the low cost,anti-poisoning capacity, and high thermal stability, ABO3 has beengenerally believed to be a potential alternative to noble metals forthe combustion of soot and volatile organic compounds (VOCs)[1–5]. Among the various perovskite catalysts, LaMO3 (M = Mn,
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Co, Cr, Fe, and Ni) show good catalytic activities for the oxidationof hydrocarbons, carbon monoxide, and carbon particulates [2].Via the partial substitution of A- and B-site ions in ABO3 by foreignions (e.g. La1–xSrxFeO3 [6] and La0.9K0.1Co1�xFexO3�d [4]), it is feasi-ble to tailor-make the dimensions of the unit cell [7] and the cov-alency of the B–O bond, thus modifying the catalytic property ofsuch a material. It has been generally accepted that catalytic activ-ity of an ABO3 is associated with factors, such as oxygen nonstoi-chiometry, reducibility, surface area, and pore structure [8]. Thepartial substitution of A-site ions with heterovalent ions canproduce structural defects (e.g., oxygen vacancies) and stabilize
K. Ji et al. / Chemical Engineering Journal 214 (2013) 262–271 263
unusual oxidation states of the B-site ions [3], and hence improv-ing the catalytic activity. Moreover, since reaction rate is often di-rectly proportional to the surface area of a perovskite catalyst [9],the preparation of ABO3 with high surface areas is highly desirable.
Recently, interests in the fabrication of three-dimensionally or-dered macroporous (3DOM) perovskites are rapidly growing due totheir large surface areas and potential catalytic applications. Someadvances in preparing 3DOM-structured ABO3 (e.g., 3DOM-struc-tured LaFeO3 [10], La1–xSrxFeO3 [11], Au/LaFeO3 [12], LaCoxFe1–xO3
[13], and La1–xKxCoO3 [14]) have been achieved. Our group havealso investigated the fabrication, characterization, and physico-chemical properties of 3DOM-structured ABO3 (e.g., LaMnO3 [15]and SrFeO3–d [16]) by using the polymethyl methacrylate(PMMA)-templating strategy.
Multiferroics are a kind of versatile materials and attract moreand more attention. Ferrite nanoparticles are considered as apromising material that offers several advantages over their metal-lic counterpart in a variety of applications [17]. EuFeO3 has beenthoroughly investigated on the crystal structure and magneticproperty, several researchers reported the good photocatalytic per-formance of EuFeO3 in the degradation of water-soluble dyes underUV-light irradiation [18] and of rhodamine B under visible-lightirradiation [19,20]. However, no reports on the catalytic applica-tion of Eu1–xSrxFeO3 with porous structure and high surface areasfor the combustion of VOCs have been seen in the literature. More-over, the Eu1–xSrxFeO3 materials derived from the conventionalmethods (e.g., the solid-state reaction [21], sol–gel process [18],and mechanical alloying [22]) are usually nonporous and relativelylow in surface area (<10 m2/g), unfavorable for the catalytic com-bustion of VOCs. Therefore, it is highly desirable to develop aneffective strategy to prepare such a material with 3DOM structurethat exhibit a high catalytic activity for VOCs combustion. It hasbeen reported that doping 40% of Sr into the A site of ABO3 can giverise to an A0.6Sr0.4BO3 catalyst that shows the best catalytic activity[2,23]. Hence in this work, we report the preparation, characteriza-tion, and catalytic properties of 3DOM-structured Eu1–xSrxFeO3
(x = 0 and 0.4) for toluene combustion. It is found that the3DOM-structured Eu0.6Sr0.4FeO3 catalyst performed well for theaddressed reaction.
2. Experimental
2.1. Catalyst preparation
The well-arrayed hard template PMMA microspheres with anaverage diameter of ca. 300 nm were synthesized according tothe procedures described elsewhere [24]. The 3DOM-structuredEuFeO3 (EFO-3DOM) and Eu0.6Sr0.4FeO3 (ESFO-3DOM) sampleswere fabricated via a citric acid-assisted PMMA-templating route.The total quantity of the metal nitrates was 0.02 mol, and thequantity of citric acid was varied as the specific preparation meth-od (i.e., citric acid/total metal molar ratio = 1/1 and 0.6/1 for theEFO-3DOM and ESFO-3DOM samples, respectively). In a typicalpreparation, 2.0 mL of ethylene glycol (EG), 4.0 mL of methanol(MeOH), 5.0 mL of deionized water, and a certain amount of citricacid were mixed to give a uniform solution under stirring. Stoichi-ometric amounts of metal nitrates were dissolved into the abovemixed solution under stirring to form a homogeneous solution.Then, 2.0 g of the PMMA template was soaked in this homogeneoussolution for 4 h. After being filtered and dried, the solids werefirstly calcinated in a N2 flow of 20 mL/min at a ramp of 1 �C/minfrom room temperature (RT) to 500 �C and kept at this temperaturefor 3 h, and then cooled to 50 �C in the same atmosphere. The as-obtained materials were heated in an air flow of 20 mL/min at aramp of 1 �C/min from RT to 750 �C and held at this temperature
for 4 h, thus obtaining the EFO-3DOM and ESFO-3DOM catalysts.The bulk EFO (EFO-bulk) sample was prepared by a citrate acid-complexing method as follows: 0.02 mol of the stoichiometric me-tal nitrates and a certain amount of citric acid (the citric acid/totalmetals molar ratio = 1/1) were dissolved in 10 mL of deionizedwater under stirring, and the obtained mixture solution was evap-orated at 80 �C until a gel was formed, and then kept in an oven at120 �C overnight. The resulting powders were calcined in a mufflefurnace at a ramp of 10 �C/min from RT to 500 �C and kept at thistemperature for 3 h, and further heated at a ramp of 10 �C/minfrom 500 to 750 �C for 4 h to generate the final product. All ofthe purchased chemicals (Beijing Chemical Agents Company) wereA.R. in purity and used without further purification.
2.2. Catalyst characterization
The catalysts were characterized by means of techniques, suchas X-ray diffraction (XRD), thermogravimetric analysis (TGA) anddifferential scanning calorimetric (DSC) analysis, Fourier transforminfrared (FT-IR) spectroscopy, scanning electron microscopy (SEM),transmission electron microscopy (TEM), selected-area electrondiffraction (SAED), nitrogen adsorption–desorption (BET) measure-ments, X-ray photoelectron spectroscopy (XPS), hydrogen temper-ature-programmed reduction (H2-TPR). The detailedcharacterization procedures are described in the Supplementarydata.
2.3. Catalytic evaluation
The catalytic activity of each sample for the complete oxidationof toluene was evaluated in a continuous flow fixed-bed quartz mi-cro-reactor (i.d. = 4 mm) at atmospheric pressure. 50 mg of the cat-alyst (40–60 mesh) was diluted with 200 mg of quartz sands (40–60 mesh) for the minimization of hot spot existence. The total flowrate of the reactant feed (1000 ppm toluene + O2 + N2 (balance))was of 33.3 mL/min, giving a toluene/O2 molar ratio of 1/400 anda space velocity (SV) of ca. 20,000 mL/(g h). The 1000 ppm toluenewas fed to the micro-reactor by passing a N2 flow through a tolu-ene-filled container immersed in an ice–water isothermal bath.Prior to each run, the catalyst was treated with the reactant mix-ture at 100 �C for 1.5 h to avoid overestimation (caused by adsorp-tion) of toluene conversion. The outlet gases were analyzed on-lineby a gas chromatograph (Shimadzu GC-2010) equipped with aflame ionization detector (FID) and a thermal conductivity detector(TCD), using Chromosorb 101 column for organic gas separationand a Carboxen 1000 column for permanent gas separation. Thevariation of SV was fulfilled by changing the catalyst mass at afixed flow rate. The balance of carbon throughout the catalytic sys-tem was evaluated to be ca. 99.5%. The toluene conversion (X%) andreaction rate (r (lmol/(g s))) can be calculated using the followingequations:
Xð%Þ ¼ ð½C7H8�in � ½C7H8�outÞ=½C7H8�in � 100
r ¼ ½C7H8�in � X=mcat ¼ SV � X � 1:24256� 10�7
where the [C7H8]in and [C7H8]out denotes the inlet and outlet tolu-ene concentrations, respectively.
3. Results and discussion
3.1. Crystal phase structure
Fig. 1 shows the XRD patterns of the EFO and ESFO samples. Byreferring to the XRD pattern of the standard EuFeO3 sample (JCPDSPDF# 47-0066), one can deduce that the EFO-bulk, EFO-3DOM, and
20 30 40 50 60 70 80
2 Theta (Deg.)
Inte
nsit
y (a
.u.)
(a)
(b)
(002
)(11
0)
(111
)
(020
)
(112
)(0
20)
(021
)
(103
)(0
22)
(202
)(1
13) (0
04)(
220)
(221
)(02
3)
(131
)
(132
)(0
24) (3
12)(
204)
(223
)
(133
)
(224
)
(314
)
(241
)(1
16)
(c)
Fig. 1. XRD patterns of (a) EFO-bulk, (b) EFO-3DOM, and (c) EFSO-3DOM.
264 K. Ji et al. / Chemical Engineering Journal 214 (2013) 262–271
ESFO-3DOM samples were single-phase and possessed an ortho-rhombic perovskite structure. The variation in peak intensity isindicative of the difference in crystallinity, the EFO-bulk sampleshowed the best crystallinity. The crystallite size (49.4 nm) of theEFO-bulk sample was much bigger than that (25.2–25.7 nm) ofthe two 3DOM-structured samples (Table 1). The TGA/DSC results(Fig. S1 of the Supplementary material) reveal that calcining theEFO and ESFO sample precursors at 750 �C was appropriate forthe generation of a single-phase perovskite-type oxide structure,which has been confirmed by the FT-IR investigations (Fig. S2 ofthe Supplementary material).
3.2. Morphology, pore structure, and surface area
Fig. 2 shows the SEM images of the three samples. It is observedthat the nonporous EFO-bulk sample was composed of agglomer-ated particles with a mean diameter of ca. 60 nm, in rough agree-ment with the XRD result (Table 1). The EFO-3DOM and ESFO-3DOM samples displayed a regularly aligned macroporous archi-tecture, with the macropore sizes being 190–200 nm for the formersample and 170–180 nm for the latter sample. Furthermore, themacropores were interconnected each other through small win-dows (80–90 nm and 70–80 nm in diameter for the 3DOM-struc-tured EFO and ESFO samples, respectively). It is worth notingthat, because of the shrinkage of microspheres during calcination,all of the pore sizes estimated from the SEM images were 35–45% smaller than the initial size (ca. 300 nm) of PMMA microbeads.Another noticeable aspect is the wall structure of the 3DOM sam-ples, which could be clearly observed from the TEM images shownin Fig. 3. The macropore wall was composed of crystallite grains
Table 1Preparation parameters, crystallite sizes (D), BET surface areas, and pore volumes of the E
Catalyst Hard template Fe3+/citric acid(mol/mol)
Calcination condition
EFO-bulk – 1:2 500 �C (air, 3 h) ? 750 �C (aEFO-3DOM PMMA 1:1.2 500 �C (N2, 3 h) ? 750 �C (aESFO-3DOM PMMA 1:2 500 �C (N2, 3 h) ? 750 �C (a
a The data were calculated according to the Scherrer equation using the FWHM of th
and the wall thickness of the 3DOM-structured samples was inthe range of 20–30 nm. Moreover, some randomly distributednanovoids with diameters of 2–8 nm were present in the macropo-rous walls of the two 3DOM-structured samples (Fig. 3b, c, e, andf). Ueda and coworkers attributed the formation of nanovoids onthe skeletons of 3DOM-structured LaFeO3 to the use of EG in thesolution during the sample preparation process [10]. Similar re-sults were also reported in our previous works [15,25]. As can beseen from the high-resolution TEM images (Fig. 3c and f) of the3DOM-structured EFO and ESFO samples, there were the presenceof clear lattice fringes with lattice spacings (d values) of ca. 0.27and 0.16 nm corresponding to the (112) and (312) planes of theEFO sample (JCPDS PDF# 47-0066) and ca. 0.27 nm correspondingto the (112) plane of the ESFO sample. In addition, the recording ofmultiple bright electron diffraction rings in the SAED patterns (in-sets of Fig. 3c and f) reveals that both the 3DOM-structured sam-ples were polycrystalline.
Fig. 4 shows the N2 adsorption–desorption isotherms and pore-size distributions of the three samples. A type II isotherm with atype H3 hysteresis loop in the relative pressure (p/p0) range of0.9–1.0 as well as a small H2 type hysteresis loop in the p/p0 rangeof 0.2–0.9 was observed in the two 3DOM-structured samples. TheEFO-bulk sample displayed the same adsorption–desorption iso-therm but much weaker hysteresis loops (Fig. 4a). The low-pres-sure portion of the almost linear middle section of the isotherm,ascribable to the unrestricted mono- or multilayer adsorption,indicates that these samples possessed macroporous structures[26]. The big rise in adsorption volume at elevated relative pres-sure was the characteristic of textural mesopores or nanovoidsexisting within the macropore walls [24,27]. The appearance of asmall H2-type hysteresis loop in the p/p0 range of 0.2–0.9 of eachsample also suggests the formation of mesopores or nanovoids[26,28]. The presence of mesopores or nanovoids would make agreat contribution to the high BET surface area. Such a deductionwas substantiated by the pore-size distributions (inset of Fig. 4).It is observed that, except that the EFO-bulk sample exhibited onlya broad pore size distribution centered at 20 nm, there were twopore size distributions for the 3DOM-structured EFO and ESFOsamples: one narrow peak was centered at 2–5 nm, and anotherbroad peak was centered at 67 and 35 nm, respectively. The3DOM-structured ESFO sample possessed a much larger surfacearea (31.3 m2/g) than that (15.9 m2/g) of the 3DOM-structuredEFO sample, and both were much larger than that (2.9 m2/g) ofthe bulk EFO sample obtained by the citric acid-complexing meth-od. The surface area of the 3DOM-structured ESFO sample wasmarkedly higher than that (14.9 m2/g) of EuFeO3 prepared viathe citrate decomposition at 700 �C [29] and that (2.6 m2/g) ofEuFeO3 obtained by the microwave-assisted combustion methodat 800 �C [30]. In addition, the pore volumes of the 3DOM-struc-tured samples were approximately equal (0.06–0.07 cm3/g), butmuch larger than that (0.014 cm3/g) of the nonporous EFO-bulksample (Table 1). The porous structure of 3DOM-structured EFOand ESFO are expected to reduce the transport resistance com-pared to the bulk EFO sample [31].
FO and ESFO catalysts.
D (nm)a Surface area (m2/g) Pore volume(cm3/g)
Macropore(>50 nm)
Mesopore(<50 nm)
Total
ir, 4 h) 49.4 1.3 1.6 2.9 0.014ir, 4 h) 25.7 6.2 9.6 15.8 0.070ir, 4 h) 25.2 10.5 20.8 31.3 0.063
e (112) line.
20 µm
(a)
200 nm
(b)
1 µm
(c)
100 nm
(d)
1 µm
(e)
100 nm
(f)
Fig. 2. SEM images of (a, b) EFO-bulk, (c, d) EFO-3DOM, and (e, f) ESFO-3DOM.
50 nm
(d)
50 nm
(a)
20 nm
(e)
20 nm
(b)
5 nm
(f)
0.27 nm
(112)
(112)
0.27 nm
5 nm
(c)
(112)
0.27 nm
(312)
0.16 nm
Fig. 3. TEM (a, b, d, e) and high-resolution TEM (c, f) images as well as the SAED patterns (insets) of (a–c) EFO-3DOM and (d–f) ESFO-3DOM.
K. Ji et al. / Chemical Engineering Journal 214 (2013) 262–271 265
0
25
50
75
100
0.0 0.2 0.4 0.6 0.8 1.0
Relative pressure p/p0
Vol
ume
adso
rbed
(cm
3 /g, S
TP
)
(b)
(c)
(a)
0 30 60 90Pore diameter (nm)
dV/d
(lo
gD)
(a)
(b)
(c)
Fig. 4. Nitrogen adsorption–desorption isotherms and pore-size distributions(insets) of (a) EFO-bulk, (b) EFO-3DOM, and (c) EFSO-3DOM.
266 K. Ji et al. / Chemical Engineering Journal 214 (2013) 262–271
3.3. Surface composition, Fe oxidation state, and oxygen species
Oxygen surface diffusion and exchange are crucial to the cata-lytic activity of a material in the oxidation process. The chemicalstates and compositions of Fe and O elements on the surface ofthe samples were investigated by the XPS technique, and the re-sults are shown in Fig. 5 and Table 2. Each of the Eu 3d XPS spectraof the samples (Fig. 5A) contained two main peaks at BE = ca.1164.3 and 1134.6 eV with the corresponding satellite peaks atBE = 1174 and 1144 eV, respectively, assignable to the 3d3/2 and
707 710
Binding en
Inte
nsit
y (a
.u.)
710.4 eV
(a)
(b)
(c)
1120 1140 1160 1180
Binding energy (eV)
Inte
nsit
y (a
.u.)
(A) Eu 3d1134.6 eV
1164.3 eV
1144.0 eV
1155.0 eV
(a)
(b)
(c)1174.0 eV
1125.0 eV
Fig. 5. (A) Eu 3d, (B) Fe 2p3/2, and (C) O 1s XPS spectra o
Table 2Surface compositions, H2 consumption, and catalytic activities of the EFO and ESFO catalyratio = 1/400, and SV = 20,000 mL/(g h).
Catalyst Surface composition H2 con
Fe/Ma Fe3+/Fe4+ Oads/Olatt <500 �C
EFO-bulk 0.29 (0.50) 0.72 1.05 0.54EFO-3DOM 0.35 (0.50) 0.84 1.20 0.59ESFO-3DOM 0.32 (0.50) 1.01 1.53 1.30
a M refers to (Eu + Fe) or (Eu + Sr + Fe) and the data in parenthesis are the nominal Feb The data were obtained by quantifying the reduction peaks of the H2-TPR profiles.
3d5/2 final states of Eu3+. Moreover, a small amount of Eu2+ waspresent on the surface of each of the EFO and ESFO samples be-cause there were the shake-down satellites of Eu2+ at BE = ca.1125 and 1155 eV [32,33]. It can be seen from Fig. 5B that therewas a similar asymmetrical Fe 2p3/2 XPS signal for each of the sam-ples, which could be deconvoluted into two components atBE = 710.4 and 712.2 eV, attributable to the surface Fe3+ and Fe4+
species [3], respectively. It has been reported that there was theco-presence of Fe3+ and Fe5+ in La0.5SrxCeyFeO3 [34] and La1–x–ySrx-
CeyFeO3 [35]. In light of these reports and the rather close BEs ofFe4+ and Fe5+, one cannot preclude the contribution from Fe5+ tothe peak at BE = 712.4 eV in the ESFO sample. The surface Fe3+/Fe4+ molar ratios decreased in the order of ESFO-3DOM(1.01) > EFO-3DOM (0.84) > EFO-bulk (0.72). On the basis of elec-troneutrality principle, such an order implies that the ESFO-3DOM sample would possess the highest oxygen vacancy density.Merino et al. [1] and Kahoul et al. [36] reported that the partialsubstitution of La3+ by Ca2+ in the La1–xCaxCoO3 catalysts gave riseto the increase in oxygen vacancy density. This phenomenon wascorroborated by the O 1s XPS results. From Fig. 5C, one can observethat the asymmetrical O 1s XPS signal could be decomposed intotwo components at BE = ca. 528.9 and 531.3 eV, ascribable to thesurface lattice oxygen (Olatt) and adsorbed oxygen (Oads, e.g., O2
�,O� or O2
2�) species [37], respectively. It should be noted that nosignificant amounts of surface carbonate species were formed onthe surface of the EFO-bulk, EFO-3DOM (1.20), and ESFO-3DOMsamples (Fig. S3 of the Supplementary material). The surface Oads/Olatt molar ratios of the three samples increased in the sequence ofEFO-bulk (1.05) < EFO-3DOM (1.20) < ESFO-3DOM (1.53). In addi-tion, one can see from Table 2 that the smaller surface Fe/M atomicratios than the stoichiometric value (0.5) implies the presence ofEu and/or Sr enrichments on the surfaces of the EFO and ESFO sam-ples. Such an enrichment phenomenon was also reported by Falconet al. [3] who observed the Sr enrichment on the surface of SrFeO3.
713 716
ergy (eV)
712.2 eV
(B) Fe 2p 3/2
526 529 532 535
Binding energy (eV)
Inte
nsit
y (a
.u.)
(a)
(b)
(c)
528.9 eV531.3 eV
(C) O 1s
f (a) EFO-bulk, (b) EFO-3DOM, and (c) ESFO-3DOM.
sts under the conditions of toluene concentration = 1000 ppm, toluene/oxygen molar
sumptionb (mmol/g) Toluene oxidation activity (�C)
>500 �C T10% T50% T90%
4.75 300 360 4205.42 276 322 3535.03 233 278 305
/M atomic ratios.
K. Ji et al. / Chemical Engineering Journal 214 (2013) 262–271 267
3.4. Reducibility
H2-TPR experiments are conducted to study the reducibility ofthe perovskite catalysts. The H2-TPR profiles and their quantitativeanalysis results are shown in Fig. 6 and Table 2, respectively. For thebulk and 3DOM EuFeO3 samples, there were two reduction peakscentered at 384 and 804 �C, and 368 and 794 �C, respectively. Theweak reduction peak could be assigned to the reduction of Fe4+ toFe3+ as well as the removal of a small amount of Oads, whereas thebig reduction peak could be ascribed to the reduction of Fe3+ toFe2+ or even metallic Fe0 [3,38]. Compared to the EFO-bulk sample,the EFO-3DOM sample was reducible at a lower temperature; fur-thermore, the H2 consumptions (0.59 and 5.42 mmol/g) at lowand high temperatures were higher for the EFO-3DOM sample thanthose (0.54 and 4.75 mmol/g) for the EFO-bulk sample (Table 2). Asfor the ESFO-3DOM sample, there were reduction peaks centered at244, 410, 720, and 805 �C. The reduction of the ESFO-3DOM samplecould take place at a much lower temperature than that of the EFOsamples. This phenomenon was also observed in the case of La1–x-
CaxCoO3 [1]. The H2 consumption below 500 �C of the ESFO-3DOM sample was 1.30 mmol/g, much higher than those of theEFO samples (Table 2). A similar result was observed in the3DOM-structured La1–xKxCoO3 catalysts [14]. To make a bettercomparison on the reducibility of the EFO and ESFO samples, weuse the initial H2 consumption rates (where less 25% oxygen inthe sample was consumed for the first reduction peak and no crys-tal phase transformation took place [39,40]) to evaluate theirreducibility, as shown in Fig. 6B. It can be clearly seen that the initialH2 consumption rate increased in the order of EFO-bulk < EFO-3DO-M < EFSO-3DOM. In other words, the low-temperature reducibilitydecreased in the sequence of EFSO-3DOM > EFO-3DOM > EFO-bulk.
3.5. Catalytic performance
In the blank experiment (only quartz sands were loaded), nosignificant conversion of toluene was observed below 450 �C, indi-cating that under the adopted reaction conditions there is nooccurrence of homogeneous reactions. Fig. 7A shows the tolueneconversion and toluene reaction rate versus reaction temperatureover the EFO and ESFO catalysts for toluene combustion underthe conditions of toluene concentration = 1000 ppm, toluene/O2
molar ratio = 1/400, and SV = 20,000 mL/(g h). It can be clearly ob-served that toluene conversion and toluene reaction rate increasedwith the rise in temperature, and the porous EFO and ESFO cata-
100 300 500 700 900
Temperature (oC)
H2
cons
umed
(a.
u.)
384 oC
794 oC
368 oC804 oC
244 oC410 oC
720 oC
805 oC
(a)
(b)
(c)
(A)
Fig. 6. (A) H2-TPR profiles and (B) initial H2 consumption rate as a function of
lysts performed much better than the bulk EFO catalyst. Thechanging trend of the TOF (lmol/molFe s) versus temperature(Fig. S4 of the Supplementary material) was the same as those oftoluene reaction rate versus temperature of the porous EFO andESFO and bulk EFO catalysts. It is worth pointing out that toluenewas completely oxidized to CO2 and H2O over the as-preparedEFO and ESFO catalysts, and there was no detection of productsof incomplete oxidation, as confirmed by the good carbon balanceof ca. 99.5% in each run. According to the Weisz–Prater criterion,when the effectiveness factor g P 0.95 and reaction order n = 1,the dimensionless Weisz–Prater parameter (uWP) value is less than0.3, which can be considered as a sufficient condition for the ab-sence of significant pore diffusion limitations [41]. At toluene con-version 620%, we carried out the Weisz–Prater analysis andcalculated the uWP values, and find that the uWP values (0.02–0.06) obtained in our present work were much less than 0.3. There-fore, no significant mass transfer limitations existed in our cata-lytic system.
It is convenient to compare the catalytic activities of these sam-ples by adopting the reaction temperatures T10%, T50%, and T90% (cor-responding to the toluene conversion = 10%, 50%, and 90%), assummarized in Table 2. Among the EFO and ESFO samples,3DOM-structured ESFO with the highest BET surface area(31.3 m2/g) showed the best catalytic activity, and the T10%, T50%,and T90% were ca. 233, 278, and 305 �C, respectively, which were67, 82, and 115 �C lower than those achieved over the bulk EFO cat-alyst derived by the conventional citric acid-complexing method(surface area = 2.9 m2/g). The catalytic performance of the ESFO-3DOM sample for toluene combustion at SV = 20,000 mL/(g h)was much better than that (T10% = 220 �C, T50% = 292 �C, andT90% = 340 �C) of 3DOM-structured SrFeO3–d [16], that (T10% >250 �C, T50% > 300 �C, and T90% > 320 �C) of mesoporous yLaCoO3/SBA-15 (y = 10–50 wt%) [42], and that (T10% = 262 �C, T50% =310 �C, and T90% = 320 �C) of La0.6Sr0.4CoO3 [43], but inferior to that(T10% = 169 �C, T50% = 222 �C, and T90% = 243 �C) of 3DOM-structuredLaMnO3 [15] and that (T10% = 89 �C, T50% = 139 �C, and T90% = 230 �C)of mesoporous Cr2O3 [44].
Fig. 7B shows the effect of SV on the catalytic activity of theESFO-3DOM sample. As expected, the toluene conversion increasedwith the drop in SV value (i.e., the extension in contact time). Inaddition, the catalytic stability of the ESFO-3DOM sample wastested in a consecutive reaction experiment (Fig. 7C), in whichthe catalyst experienced first two runs of catalytic tests, and then6 h of on-stream reaction at 300 �C and 20,000 mL/(g h), and finally
0.4
0.7
1.0
1.3
1.5 1.9 2.3
1000/T (K )-1
Init
ial H
2 co
nsum
ptio
n ra
te (
µmol
/(m
ols)
(a)(b)
(c)
(B)
inverse temperature of (a) EFO-bulk, (b) EFO-3DOM, and (c) EFSO-3DOM.
0
20
40
60
80
100
200 250 300 350 400 450Temperature (oC)
Tol
uene
con
vers
ion
(%)
0.00
0.06
0.12
0.18
0.24
0.30
Tol
uene
rea
ctio
n ra
te (µ
mol
/(g
s))
( , ) EFO-bulk( , ) EFO-3DOM( , ) ESFO-3DOM
0
20
40
60
80
100
200 250 300 350Temperature (oC)
Tol
uene
con
vers
ion
(%)
SV (mL/(g h)) 10 000 20 000 40 000
ESFO-3DOM
200 3200
20
40
60
80
100
320 320 3202 4 6
the 1st run
Tol
uene
con
vers
ion
(%)
Temperature (oC)
the 2nd run
the 3rd run
Time (h) Temperature (oC)
the 4th run
260 260260260
ESFO-3DOM
6 h on-stream
reaction at 300 oC
260260260
(A) (B)
(C)
Fig. 7. (A) Toluene conversion and the corresponding reaction rate versus reaction temperature over the EFO-bulk, EFO-3DOM, and ESFO-3DOM catalysts, (B) effect of SV onthe catalytic activity of the ESFO-3DOM catalyst, and (C) the catalytic stability of the ESFO-3DOM catalyst at SV = 20,000 mL/(g h) under the conditions of tolueneconcentration = 1000 ppm and toluene/O2 molar ratio = 1/400.
268 K. Ji et al. / Chemical Engineering Journal 214 (2013) 262–271
two runs of catalytic tests. Each run lasted ca. 5 h. The overall testtime was ca. 26 h. The result reveals that no obvious activity deac-tivation took place within 26 h of the catalytic process. Therefore,we believe that the sample is catalytically stable.
-8.0
-6.0
-4.0
-2.0
0.0
1.5 1.7 1.9 2.1 2.3 2.5
1000/T (K-1)
ln k
EFO-bulk EFO-3DOM ESFO-3DOM
(A)
Fig. 8. Arrhenius plots for total oxidation of toluene over (A) EFO-bulk, EFO-3DOM, an
It is well known that the catalytic performance of an ABO3
material is associated with the surface area, defect nature and den-sity, oxygen adspecies, and reducibility. A higher surface area isbeneficial for the improvement of catalytic activity [9,45]. The
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.8 1.9 2.0 2.1 2.2
1000/T (K -1)
ln k
SV (mL/(g h)) 10 000 20 000 40 000
(B) ESFO-3DOM
d ESFO-3DOM at SV = 20,000 mL/(g h) and (B) ESFO-3DOM at different SV values.
Tabl
e3
Rate
cons
tant
s(k
),re
acti
onra
tes
(r),
acti
vati
onen
ergi
es(E
a),p
re-e
xpon
enti
alfa
ctor
s(A
),an
dco
rrel
atio
nco
effi
cien
ts(R
2)
ofth
eEF
Oan
dES
FOca
taly
sts
for
the
oxid
atio
nof
tolu
ene
inth
ete
mpe
ratu
rera
nge
of20
0–30
0�C
.
Cat
alys
tSV
(mL/
(gh
))k
(�10�
2s�
1)/
r(�
10�
3l
mol
/(g
s))a
Kin
etic
para
met
er
200
�C21
0�C
220
�C24
0�C
260
�C27
0�C
280
�C29
0�C
300
�CE a
(kJ/
mol
)A
(�10
7s�
1)
R2
EFO
-bu
lk20
,000
––
0.30
1/0.
746
–1.
83/4
.47
––
6.95
/16.
2–
104.
031
.50.
9992
EFO
-3D
OM
20,0
00–
0.60
4/1.
49–
2.35
/5.7
2–
7.76
/17.
9–
–26
.3/5
1.7
96.0
13.9
0.99
92ES
FO-3
DO
M40
,000
1.42
/3.4
8–
3.31
/7.9
57.
41/1
7.1
14.7
/31.
8–
4.31
/74.
8–
17.3
/158
81.6
2.76
0.99
97ES
FO-3
DO
M20
,000
2.67
/6.4
6–
6.50
/15.
213
.8/3
0.1
27.7
/53.
9–
110/
130
–58
0/21
281
.13.
570.
9995
ESFO
-3D
OM
10,0
004.
06/9
.69
–9.
29/2
1.1
19.3
/40.
341
.8/7
3.3
–16
6/15
5–
1940
/236
82.0
1.61
0.99
94
aTh
eda
tabe
fore
the
‘‘/’’
sym
bol
are
the
kva
lues
,wh
erea
sth
ose
afte
rth
e‘‘/
’’sy
mbo
lar
eth
er
valu
es.
K. Ji et al. / Chemical Engineering Journal 214 (2013) 262–271 269
presence of oxygen deficiencies favors the activation of gas-phaseoxygen molecules for the generation of active oxygen adspecies.The higher the oxygen vacancy density, the better is the catalyticperformance of a perovskite sample [8,46]. The strong redox abilityof ABO3 guarantees the recyclability of B-site ions (with at leasttwo oxidation states), thus facilitating the oxidation of organiccompounds [47]. The much better reducibility of the ESFO-3DOMsample than the EFO samples renders the former to have good re-dox ability. Toluene combustion took place over the perovskiteoxide catalysts [48,49] through the suprafacial catalytic mecha-nism proposed by Voorhoeve et al. [50], in which surface oxygeninteracts with reactants at low temperatures (<400 �C). Comparedto the bulk and nonporous EFO sample, the 3DOM-structured EFOand ESFO samples showed a higher surface area and a moreamount of surface oxygen vacancies, which would be beneficialfor the adsorption and activation of oxygen molecules [1,51–53].The presence of highly porous structures in EFO-3DOM andESFO-3DOM could facilitate the adsorption and diffusion of reac-tant molecules, thus promoting the enhancement in catalytic per-formance. Therefore, it is concluded that the excellent catalyticperformance of ESFO-3DOM might be associated with its highersurface area and surface oxygen species concentration and betterlow-temperature reducibility as well as its good-quality 3DOMstructure.
3.6. Kinetic parameters
It has been reported that the oxidation of VOCs over perovskite-type oxides in the presence of excess oxygen obey a first-orderreaction mechanism [15,54], i.e., r = kc = [Aexp(�Ea/RT)]c, in whichthe rate constant k (s�1), apparent activation energy Ea (kJ/mol),and pre-exponential factor A could be calculated from the toluenereaction rates, toluene conversions at different reaction tempera-tures, and the slopes of the Arrhenius plots, respectively. Fig. 8Aand B shows the linear Arrhenius plots (the correlation coefficients(R2) were more than 0.999) of the EFO and ESFO catalysts for tolu-ene combustion at SV = 20,000 mL/(g h) and of the ESFO-3DOMcatalyst at different SV values and toluene conversion <25%,respectively. From Fig. 8A, one can estimate the Ea values of theEFO-bulk, EFO-3DOM, and ESFO-3DOM catalysts to be ca. 82, 96,and 104 kJ/mol, respectively; from Fig. 8B, the apparent activationenergies of the were estimated to be in the range of 81.1–82.0 kJ/mol. These apparent activation energies were higher than that(57–72 kJ/mol) of 3DOM-structured LaMnO3 [15] and that(79.8 kJ/mol) of mesoporous Cr2O3 [44], indicating that the3DOM-structured EFO and ESFO catalysts showed lower catalyticactivities than the 3DOM-structured LaMnO3 and mesoporouschromia. As shown in Table 3, when the SV changed from 10,000to 40,000 mL/(g h) over the ESFO-3DOM catalyst, the Ea valueswere rather close (81.1–82.0 kJ/mol), but the pre-exponential fac-tor A value first increased and then decreased with the rise in SV.Therefore, there might be a suitable SV value so that a high colli-sion frequency could be obtained on the surface of the catalyst.
4. Conclusions
By using PMMA as hard template, citric acid as the compelxingagent, and metal nitrates as metal source, 3DOM-structured EuFe-O3 (EFO-3DOM) and Eu0.6Sr0.4FeO3 (ESFO-3DOM) catalysts couldbe generated. The EFO-3DOM and ESFO-3DOM catalysts displayeda high-quality 3DOM architecture, and showed a single-phaseorthorhombic crystal structure, with the surface area being 15.8and 31.3 m2/g, respectively. The surface oxygen species concentra-tion and low-temperature reducibility of the three samples de-creased in the order of ESFO-3DOM > EFO-3DOM > EFO-bulk,
270 K. Ji et al. / Chemical Engineering Journal 214 (2013) 262–271
coinciding with their catalytic performance sequence. Under theconditions of toluene concentration = 1000 ppm, toluene/O2 molarratio = 1/400, and SV = 20,000 mL/(g h), the ESFO-3DOM catalystperformed the best, giving the T10%, T50%, and T90% of 233, 278,and 305 �C, respectively. The apparent activation energies of theESFO-3DOM, EFO-3DOM, and EFO-bulk catalysts were ca. 82, 96,and 104 kJ/mol, respectively. It is concluded that surface area, sur-face oxygen species concentration, and low-temperature reducibil-ity as well as 3DOM structure are responsible for the excellentcatalytic activity of the ESFO-3DOM material.
Acknowledgments
The work was supported by the NSF of China (20973017 and21077007), the NSF of Beijing Municipality (2102008), the Disci-pline and Postgraduate Education (005000541212014), the Crea-tive Research Foundation of Beijing University of Technology(00500054R4003 and 005000543111501), the National High-TechResearch and Development (863) Key Program of Ministry of Sci-ence and Technology of China (2009AA063201), and the FundingProject for Academic Human Resources Development in Institu-tions of Higher Learning under the Jurisdiction of Beijing Munici-pality (PHR201007105 and PHR201107104). We also thank Prof.Chak Tong Au (Department of Chemistry, Hong Kong Baptist Uni-versity) and Mrs. Jianping He (State Key Laboratory of AdvancedMetals & Materials, University of Science and Technology Beijing)for doing the XPS and SEM analyses, respectively.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2012.10.083.
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