Nanosized Au supported on three-dimensionally ordered mesoporous β-MnO2: Highly active catalysts for the low-temperature oxidation of carbon monoxide, benzene, and toluene

Nanosized Au supported on three-dimensionally ordered mesoporous β-MnO2: Highly active catalysts for the low-temperature oxidation of carbon monoxide, benzene, and toluene

Microporous and Mesoporous Materials 172 (2013) 20–29 Contents lists available at SciVerse ScienceDirect Microporous and Mesoporous Materials journa...

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Microporous and Mesoporous Materials 172 (2013) 20–29

Contents lists available at SciVerse ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Nanosized Au supported on three-dimensionally ordered mesoporous b-MnO2: Highly active catalysts for the low-temperature oxidation of carbon monoxide, benzene, and toluene Qing Ye a,⇑, Jiansheng Zhao a, Feifei Huo a, Dao Wang a, Shuiyuan Cheng a, Tianfang Kang a, Hongxing Dai b,⇑ a

Department of Environmental Science, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, PR China Laboratory 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 b

a r t i c l e

i n f o

Article history: Received 24 June 2012 Received in revised form 25 December 2012 Accepted 8 January 2013 Available online 16 January 2013 Keywords: Three-dimensionally ordered mesoporous b-MnO2-supported gold catalyst Low-temperature reducibility Synergistic action Carbon monoxide oxidation Volatile organic compound combustion

a b s t r a c t Three-dimensionally (3D) ordered mesoporous b-MnO2-supported Au nanocatalysts (Au/b-MnO2(urea), Au/b-MnO2(NaOH), and Au/b-MnO2ðNa2 CO3 Þ ) with an Au loading of 5 wt.% were prepared by the deposition–precipitation method using urea, NaOH and Na2CO3 as precipitating agent, respectively. The physicochemical properties of the materials were characterized by means of numerous analytical techniques, and their catalytic activities were evaluated for the complete oxidation of CO, benzene, and toluene. It is shown that the nature of precipitating agent had an important influence on the physicochemical properties of the b-MnO2 support, Au nanoparticles, and Au/b-MnO2 catalysts. Among the three Au/b-MnO2 samples, the Au/b-MnO2(NaOH) showed the highest surface atomic ratios of Mn3+/Mn4+, Oads/Olatt, and Au3+/Au0. The loading of gold could greatly modify the reducibility of Au/b-MnO2 via the strong interaction between the gold and the b-MnO2 support, and the Au/b-MnO2(NaOH) sample possessed the best low-temperature reducibility. Gold loading resulted in a significant enhancement in catalytic activity of b-MnO2. The three Au/b-MnO2 catalysts outperformed the Au-free b-MnO2 catalyst, among which the Au/b-MnO2(NaOH) one showed the best catalytic activity (T50% and T100% = 48 and 70 °C for CO oxidation, 200 and 250 °C for benzene oxidation, and 170 and 220 °C for toluene oxidation, respectively). It is concluded that factors, such as the better gold dispersion, higher surface Au3+ and oxygen adspecies concentrations, better low-temperature reducibility, stronger synergistic action between the gold and the support as well as the high-quality 3D ordered mesoporous structure of the support, might be responsible for the excellent catalytic performance of Au/b-MnO2(NaOH). Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Most of volatile organic compounds (VOCs) and carbon monoxide emitted from transportation and industrial activities are harmful to the atmospheric environment and human health. It is highly desired to eliminate these pollutants. The conventional incineration of organic pollutants is usually energy-intensive due to the requirements of high temperatures (>1000 °C). Catalytic oxidation of VOCs and CO has been generally accepted to be one of the most effective pathways for the removal of these pollutants at low temperatures (<500 °C). Such a catalytic strategy has advantages of good feasibility, low operation cost, and high elimination efficiency [1]. The key issue is the availability of an effective catalyst. ⇑ Corresponding authors. Tel.: +86 10 6739 1659; fax: +86 10 6739 1983 (Q. Ye), tel.: +86 10 6739 6118; fax: +86 10 6739 1983 (H. Dai). E-mail addresses: [email protected] (Q. Ye), [email protected] (H. Dai). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.01.007

In the past years, a number of materials, such as supported noble metals (e.g., Au, Pt, Pd, and Rh) and base transition-metal (e.g., Cr, Co, Mn, Ni, and Cu) oxides [1,2], have been used as catalyst for the combustion of VOCs and CO. The former are highly active at lower temperatures, but their applications are limited due to the high cost and the usually involved sintering, volatility, and susceptibly poisoning tendency. Therefore, there is a need to develop highly active catalysts that show good catalytic stability. Bulk and/or supported manganese oxides have long been used as an active material for the catalytic oxidation of carbon monoxide, methane, and hydrocarbons. Since single-component catalysts are usually hard to rival precious metal catalysts, attempts for enhancing catalytic performance have been made via the combination of two or more transition-metal elements. For example, Mn–Fe composite oxides showed a higher activity than the zeolite-supported Pt catalyst in catalyzing the oxidation of oxygenates [3]. One of our previous works had shown that loading a small amount of sil-

Q. Ye et al. / Microporous and Mesoporous Materials 172 (2013) 20–29

ver nanoparticles onto manganese oxide (i.e., Ag/MnOx) could significantly enhance the catalytic activity of MnOx for the oxidation of CO and benzene [4]. Gold has long been regarded as a very poor catalyst because of its chemical inertness and its tendency to facile aggregations induced by low melting and Tammann temperatures. An upsurge of interest in gold as a catalytic material originates from Haruta and coworkers’ report, in which the authors observed dramatic enhancement in performance of gold catalysts for low-temperature CO oxidation [5]. In addition to the use for CO oxidation, supported gold catalysts have also been demonstrated to be active for the selective oxidation of methane [6]. To the best of our knowledge, only few reports on the use of nanosized MnO2-supported Au as catalyst for benzene combustion have been seen in the literature. For example, Sinha et al. [7] investigated the catalytic combustion of VOCs (103.3 ppm toluene, 200 ppm acetaldehyde or a mixture of 19.5 ppm toluene, 27 ppm acetaldehyde, and 50 ppm hexane) over the Au/c-MnO2 catalysts. Wang et al. [8] studied the low-temperature oxidation of CO over the gold nanoparticles supported on manganese oxides (Au/ Mn2O3, Au/MnO2, and Au/Mn3O4). Recently, we have prepared numerous MnO2-supported transition-metal catalysts, and found that these materials performed well in the combustion of some typical VOCs (e.g., [4,9]). Herein, we report the preparation, characterization, and catalytic performance of nanosized Au supported on the 3D ordered mesoporous b-MnO2 derived with various precipitation agents for the complete oxidation of carbon monoxide, benzene and toluene.

2. Experimental 2.1. Catalyst preparation 3D ordered mesoporous silica KIT-6 and b-MnO2 were prepared according to the procedures similar to those reported by other researchers [10,11]. In a typical synthesis of 3D ordered mesoporous b-MnO2, 30 g of Mn(NO3)26H2O (98%, Aldrich) was dissolved in 20 mL of deionized water to form a saturated Mn(NO3)2 solution. 5 g of 3D ordered mesoporous KIT-6 powders was dispersed in 200 mL of dried n-hexane. After stirring at room temperature (RT) for 3 h, 5 mL of the saturated Mn(NO3)2 solution was added slowly under stirring. The mixture was magnetically stirred overnight, filtered, and dried at RT. The dried powders were calcined in air at a ramp of 1 °C/min to 400 °C and kept at this temperature for 3 h. The silica template was removed by treating the calcined sample with a hot NaOH aqueous solution (2 mol/L) twice, followed by washing with deionized water several times and then drying at 60 °C for 24 h. The 5 wt.% Au/b-MnO2 catalysts were prepared by the deposition–precipitation (DP) method using urea, sodium hydroxide or sodium carbonate as precipitating agent. 150 mL of tetrachloroauric acid aqueous solution (0.0109 mol/L) and 16 mL of urea aqueous solution (1.0 mol/L) were added under stirring to an aqueous suspension of 6 g of the 3D ordered mesoporous b-MnO2 support. The mixed solution was heated at 75 °C to accelerate the hydrolysis of urea and keep this temperature for 2 h. The pH value of the mixed solution was maintained at 6.7, so that a high dispersion of fine gold particles on the oxide support was achieved. An appropriate amount of 0.5 mol/L NaBH4 solution (NaBH4/Au3+ molar ratio = 2/1) was added dropwise to the obtained mixed solution containing HAuCl4 and b-MnO2 under stirring for 1 h at RT, in which most of the gold ions were reduced to metallic gold. The mixture was filtered, washed with deionized water at 60 °C several times for removal of the chloride ions, dried at 80 °C for 24 h, and finally calcined in air at 400 °C for 4 h. The obtained catalyst was

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denoted as Au/b-MnO2(urea). Using the DP method with sodium hydroxide or sodium carbonate as precipitating agent, 5 wt.% Au/ b-MnO2 catalysts (denoted as Au/b-MnO2(NaOH) and Au/bMnO2ðNa2 CO3 Þ , respectively) were also prepared. The detailed procedures were as follows: 1 g of the 3D ordered mesoporous b-MnO2 support was added to a HAuCl4 aqueous solution (0.0109 mol/L) at 80 °C. The pH value of the mixed solution was adjusted to 9.0 by dropwise adding a suitable amount of NaOH (1.0 mol/L) or Na2CO3 (1.0 mol/L) aqueous solution under stirring for 4 h. An appropriate amount of 0.5 mol/L NaBH4 solution (NaBH4/Au3+ molar ratio = 2/ 1) was added dropwise to the mixed aqueous solution containing HAuCl4 and b-MnO2 under stirring for 1 h, in which most of the gold ions were reduced to metallic gold. The suspension was filtered, washed with hot deionized water several times for the elimination of Na+ and Cl ions, and dried at 80 °C overnight. The dried powders were calcined in air at a heating rate of 1 °C/min from RT to 400 °C and held at this temperature for 4 h. 2.2. Catalyst characterization The exact gold content was analyzed using the energy-dispersive X-ray fluorescence (EDXRF) spectrometer (PanaliticalMagix, PW2403) and the results are shown in Table 1. Wide- and smallangle X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker-AXS D8 Advance diffractometer using Cu Ka radiation (k = 0.15406 nm) at a voltage and current of 50 kV and 30 mA, respectively. The crystallite sizes were calculated according to the Scherrer equation. Transmission electron microscopy (TEM) was employed to take the TEM images of the samples on a TecnaiG2F20 U-TWIN (America FEI) operated at 200 kV. N2 adsorption–desorption isotherms were measured at 196 °C on a Micromeritics ASAP 2010 instrument. Prior to measurements, the samples were degassed at 200 °C for 2 h. Surface areas of the samples were determined by the Brunauer–Emmett–Teller (BET) method. X-ray photoelectron spectroscopic (XPS) spectra of the 3D ordered mesoporous b-MnO2 and Au/b-MnO2 samples were recorded on an ESCALAB 250Xi spectrometer using Al Ka as excitation source. The binding energies (BEs) of the Au 4f, Mn 2p, and O 1s were calibrated against the C 1s signal (284.6 eV) of contaminant carbon. H2 temperature-programmed reduction (H2-TPR) was carried out in the RT-750 °C range on a Micromeritics AutoChem II 2920 instrument. About 100 mg of the sample was placed in a U-shaped quartz tube (i.d. = 4 mm), then treated in O2 (40 mL/ min) at 300 °C for 1 h, and finally cooled to RT. The H2-TPR profiles were recorded during the reduction of the samples in a 5% H2–95% Ar flow of 25 mL/min at a ramp of 10 °C/min. The thermal conductivity detector (TCD) responses were calibrated against that of the complete reduction of a standard CuO powdered sample (Aldrich, 99.995%). 2.3. Catalytic evaluation Catalytic activity measurements of the samples for the oxidation of CO, benzene or toluene were carried out in a continuous flow fixed-bed quartz microreactor at atmospheric pressure. 200 mg of the catalyst (40–60 mesh) and 200 mg of quartz sands (40–60 mesh) were well mixed and loaded to the microreactor. The feed gas composition was 1.08 vol.% CO + air. The total flow rate was 100 mL/min and the corresponding space velocity (SV) was 30,000 mL/(g h). Benzene and toluene was chosen as the representative VOCs. A N2 flow of 2.8 mL/min and 8.3 mL/min passed through a benzene- or toluene-containing tubular saturator at 30 °C, and then mixed with an air flow of 197.2 mL/min and 191.7 mL/min, respectively, thus giving a total flow rate of 200 mL/min and a SV value of 60,000 mL/(g h). In the feed gas mixture, the benzene or toluene vapor concentration was 2000 ppm.

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Table 1 Actual and surface gold loadings, BET surface areas, pore volumes, average pore diameters, Au particle sizes, H2 consumption, and reduction temperatures of the b-MnO2 and Au/ b-MnO2 catalysts. Catalyst

b-MnO2 Au/bMnO2(urea) Au/bMnO2(NaOH) Au/b-

Au loading (wt.%)

BET surface area (m2/g)

Total pore volume (cm3/g)

– 4.00a (4.02)b

131 95

0.33 0.23

4.05a (4.50)b

120

0.31

MnO2ðNa2 CO3 Þ

a

4.75 (5.02)

b

Average pore diameter (nm)

DAuc (nm)

TEM

XRD

12.4 9.8

– 8–15

– 13.2

11.9

3–5

5.2

113

H2 consumption (mmol/g)

Reduction temprature (°C) Peak 1

Peak 2

10.1 7.3

250 140

350 230

9.2

130

210

0.3011.3 5–8

8.3

8.9

230 160 a b c

Determined from XRF results. Determined from XPS results. The data were estimated according to the Scherrer equation using the FWHM of the (1 1 0) line of b-MnO2.

The effluent gases from the microreactor were analyzed on-line by a Shimadzu GC-14C gas chromatograph equipped with a thermal conductivity detector (TCD) and a TDX-01 packing column for CO analysis or by a Techcomp GC-7900 gas chromatograph equipped with a flame ion detector (FID) and a SE-30 capillary column for organic compound analysis. The conversion of CO, benzene or toluene was calculated according to the changes of CO, benzene or toluene concentration in the inlet and outlet gas mixtures. For the changes in SV, we altered the mass of the catalyst. 3. Results and discussion 3.1. Crystal phase composition As shown in Table 1, the actual gold contents in the catalysts were not equal to 5.0 wt.%, indicating that the actual gold content depended on the synthesis method. One can observe that the Au loading of Au/b-MnO2(NaOH) and Au/b-MnO2ðNa2 CO3 Þ were almost the same (ca. 4.0 wt.%), whereas that of Au/b-MnO2(urea) was 4.75 wt.%. The significant loss of gold during catalyst preparation may be related to their iso-electric point. The iso-electric point of b-MnO2 is in the region of 7.3–7.7 [12]. At a higher pH value, the surface would be negatively charged, resulting in an electrostatic repulsion of gold-containing anions. This probably accounted for the drop in gold content at pH = 9 with NaOH or Na2CO3 as precipitant agents. For the preparation of the Au/b-MnO2(urea) sample, the pH value was 6.7, close to the iso-electric point of b-MnO2, therefore resulting in the maintaining of higher Au loading. Fig. 1 shows the wide- and small-angle XRD patterns of manganese oxide and supported Au catalysts as well as the small-angle XRD pattern of KIT-6. It can be observed from Fig. 1(A) that the XRD peaks at 2h = 28.8°, 37.3°, 42.8°, and 56.6° of the manganese oxide support were indexed to the (1 1 0), (1 0 1), (2 0 0), and (2 1 1) crystal planes of tetragonal b-MnO2 phase (JCPDS PDF# 24-0735), respectively. Similar XRD patterns of manganese oxide were reported by other researchers [13]. After the loading of Au, the XRD peaks (Fig. 1A(c–e)) assignable to the tetragonal b-MnO2 phase were also detected, but the intensity of these XRD signals altered. This result indicates the retaining of tetragonal b-MnO2 crystal structure and the slight change in crystallinity. It might result from the treatment of the support in basic conditions (pH = 9) and/or the reduction with NaBH4. Such observations were in consistence with those reported by Wu et al. [14], who detected no transformation of TiO2 crystal phase and crystal growth of TiO2 after Pt loading. For the Au/b-MnO2 samples, however, additional two XRD signals at 2h = 38.0° and 44.4° attributable to the (1 1 1)

and (2 0 0) crystal planes of metallic Au0 phase (JCPDS PDF# 040784) were recorded. The XRD intensity of Au0 phase of the Au/ b-MnO2(urea) sample was higher than that of the Au/b-MnO2(NaOH) and Au/b-MnO2ðNa2 CO3 Þ samples, implying that the particle size of the former was bigger. The average crystal sizes of Au0 were estimated according to the Scherrer equation using the full width of half maximum (FWHM) of the (1 1 0) peak and the results are summarized in Table 1. The crystal sizes of the Au/b-MnO2(NaOH) and Au/b-MnO2ðNa2 CO3 Þ samples were ca. 5.2 and 8.3 nm, respectively, whereas that of the Au/b-MnO2(urea) sample was ca. 13.2 nm. Therefore, the XRD results clearly demonstrate that the use of NaOH or Na2CO3 as precipitant via the DP preparation route favored the generation of an Au/b-MnO2 catalyst, on which a better dispersion of gold particles could be achieved. As can be seen from the small-angle XRD patters of the samples, the three peaks at 2h = 0.9° (d = 9.8 nm), 1.04° (d = 8.5 nm), and 1.70° (d = 5.1 nm) of the KIT-6 sample (Fig. 1B(a)) were assigned to the (2 1 1), (2 2 0), and (3 3 2) crystal planes, respectively, characteristics of 3D ordered mesoporous KIT-6 [15]. However, the bMnO2 support obtained via the nanocasting method displayed only one diffraction peak ascribable to the (2 1 1) reflection (Fig 1B(b)), indicating that the 3D ordered mesoporous framework was retained; the peak slightly shifted to a higher 2h value and the peak intensity became weaker, suggesting that the ordered mesopores of b-MnO2 were somewhat shrunk and partially collapsed. After Au loading, no significant changes in small-angle XRD pattern (Fig. 1B(c–e)) were observed, indicating the presence of 3D ordered mesoporous structure in the Au/b-MnO2 samples. That is to say, the b-MnO2 support and Au/b-MnO2 catalysts possessed 3D ordered mesoporous structures. 3.2. Pore structure and surface area Fig. 2 illustrates the nitrogen adsorption–desorption isotherms and the pore-size distributions (that were calculated from the desorption branch of the isotherms) of the b-MnO2 and Au/bMnO2 catalysts. It is observed that either the b-MnO2 support or the Au/b-MnO2 catalysts showed a type-IV isotherm with H1-type hysteresis loop in the p/p0 range of 0.4–0.95, indicative of formation of uniform mesopores. Such a deduction was substantiated by the appearance of two averaged pore sizes at 3.7 and 12.5 nm (Fig. 2(B)): the 3.7 nm reflected the minimum wall thickness of KIT-6, whereas the 12.5 nm corresponded either to the wall junctions in KIT-6 or to the pores between the particles. The discrepancies in hysteresis loop of isotherms and peak intensity of pore-size distributions suggest the presence of a difference in pore parame-

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3.3. Surface composition, oxygen species, and Mn oxidation state Shown in Fig. 4 are the O 1s, Mn 2p, and Au 4f XPS spectra of the b-MnO2 and Au/b-MnO2 samples. The asymmetrical O 1s signal could be decomposed to two components: one at BE = ca. 529.6 eV and the other at BE = ca. 531.6 eV (Fig. 4(A)), the former was assignable to the surface lattice oxygen (Olatt) species, whereas the latter to the surface adsorbed oxygen (Oads) species [17]. The estimated surface Oads/Olatt atomic ratios of the samples are listed in Table 2. It is found that loading Au using different precipitants resulted in the Au/b-MnO2 samples having different surface Oads/ Olatt atomic ratios, and the surface Oads/Olatt atomic ratio decreased in the order of Au/b-MnO2(NaOH) (0.66) > Au/b-MnO2ðNa2 CO3 Þ (0.49) > Au/b-MnO2(urea) (0.36) > b-MnO2 (0.31). The formation of surface oxygen species was due to the presence of surface oxygen vacancies on b-MnO2, which implies that there might be the coexistence of Mn3+ and Mn4+ ions in/on the b-MnO2 and Au/bMnO2 samples. This deduction was supported by the Mn 2p XPS results of these samples. As shown in Fig. 4(B), there were two asymmetrical signals at BE = ca. 642 and 654 eV for each of the b-MnO2 and Au/b-MnO2

Au (200)

Au (111)

(A)

Intensity (a.u.)

(e)

(d)

20

30

50

(301)

(002)

60

(310)

(211)

40

(220)

(111)

(b)

(210)

(200)

(101)

(110)

(c)

70

80

o

2 Theta ( )

(211)

(B)

(e) (d) (c) (211)

(b)

(332)

(220)

Intensity (a.u.)

ter of the samples. Table 1 summarizes the textural parameters and surface areas of the as-prepared b-MnO2 support and Au/bMnO2 samples. The 3D ordered mesoporous b-MnO2 support has a surface area of 131 m2/g and a pore volume of 0.33 cm3/g, larger than those (0.6 m2/g and 0.01 cm3/g, respectively) of the conventionally derived b-MnO2 sample [16]. The loading of Au caused the surface area and pore volume to decrease, but the Au/bMnO2ðNa2 CO3 Þ and Au/b-MnO2(NaOH) still retained higher surface areas (113–120 m2/g and pore volumes (0.30–0.31 cm3/g). Compared with the b-MnO2 support, the Au/b-MnO2(urea) sample exhibited significant drops in surface area and pore volume, a result possibly due to formation of larger Au nanoparticles that block a more amount of mesopores of the sample. Fig. 3 presents the TEM images of KIT-6, b-MnO2, and Au/bMnO2 samples. It can be seen that the KIT-6 sample showed a high-quality 3D ordered mesoporous structure (Fig. 3(a)). The bMnO2 support replicated the structure of 3D ordered mesoporous KIT-6 (Fig. 3(b)). After Au loading, the obtained Au/b-MnO2(NaOH) and Au/b-MnO2ðNa2 CO3 Þ samples (Fig. 3(e and g)) displayed a better quality of 3D ordered mesoporous architecture than the Au/b-MnO2(urea) sample (Fig. 3(c)), giving rise to surface areas of the former two samples higher than that of the latter one. From the high-resolution TEM images, one can observe clear lattice fringes of the b-MnO2 phase in the b-MnO2 and Au/b-MnO2 samples (Fig. 3(b, d, f, and h)), and the estimated d values of the exposed (1 0 1) and (1 1 0) crystal planes were 0.24 and 0.31 nm, respectively, rather close to those (0.2407 and 0.3110 nm, respectively) of the (1 0 1) and (1 1 0) crystal planes of the standard bMnO2 sample (JCPDS PDF# 24-0735). Furthermore, well-resolved lattice fringes of the exposed Au (2 0 0) crystal plane in the Au/bMnO2 samples (Fig. 3(d, f, and h)) were recorded after the loading of Au, the d value (0.20 nm) of the Au (2 0 0) crystal plane was not far away from that (0.204 nm) of the standard Au0 sample (JCPDS PDF# 04-0784). This result suggests that the most possibly exposed lattice plane for gold was the Au (2 0 0) plane. One can also observe from Fig. 3(d, f, and h) that the Au0 particles were homogeneously dispersed on the mesopore surfaces of b-MnO2, and the Au0 particle sizes in the Au/b-MnO2(urea), Au/b-MnO2(NaOH), and Au/b-MnO2ðNa2 CO3 Þ samples were in the ranges of 8–15, 3–5, and 5–8 nm, respectively. This result was in good agreement with that revealed by the XRD investigations (Fig. 1(A) and Table 1). Therefore, the precipitation agent had an important effect on the quality of 3D ordered mesopore structure and the particle size of Au0 in the Au/b-MnO2 sample.

(a) 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

o

2 Theta ( ) Fig. 1. (A) Wide-angle and (B) small-angle XRD patterns of (a) KIT-6, (b) b-MnO2, (c) Au/b-MnO2(urea), (d) Au/b-MnO2(NaOH), and (e) Au/b-MnO2ðNa2 CO3 Þ .

samples, decomposable to the two components at BE = ca. 641.3 and ca. 642.4 eV (Mn 2p3/2) and those at BE = ca. 652.9 and ca. 654.0 eV (Mn 2p1/2), respectively. The Mn 2p spin–orbit splitting of our b-MnO2 and Au/b-MnO2 samples was 11.6 eV, rather close to that (11.7 eV) of the MnO2 sample reported by other authors [18]. The signals at BE = ca. 642.4 and 641.3 eV could be assigned to the surface Mn4+ and Mn3+ species [18], respectively. By quantitatively analyzing the XPS spectra of the samples, one can obtain the surface Mn3+/Mn4+ atomic ratios, as summarized in Table 2. Obviously, the nature of the precipitant had a remarkable influence on the surface Mn3+/Mn4+ atomic ratio of the as-prepared sample. The highest surface Mn3+/Mn4+ atomic ratio (0.44) was achieved on the Au/b-MnO2(NaOH) sample, higher than that (0.13) on the bMnO2 support. Based on the principle of electroneutrality, we hence conclude that the surface oxygen vacancy density was the highest on the b-MnO2 surfaces of the Au/b-MnO2(NaOH) sample. Usually, oxygen molecules are adsorbed at the vacant oxygen sites of an oxide material. Therefore, we believe that the oxygen adspecies should locate at the surface oxygen vacancies of b-MnO2 as well as the interface between the Au0 and the b-MnO2. This result is in good consistence with the result of O 2p XPS studies. The use of precipitating agent with a stronger basicity (e.g., NaOH) favored the enhancements of Mn3+ concentration and surface oxygen

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catalysts (Table 2), the Au/b-MnO2(NaOH) showed the highest Au3+/Au0 atomic ratio (0.49).

Volume adsorbed (cm3/g)

(A) 300 250

3.4. Reducibility

200

Fig. 5(A) illustrates the H2-TPR profiles of the b-MnO2 and Au/bMnO2 catalysts, and their reduction temperatures and H2 consumptions are summarized in Table 1. The reduction profile of the b-MnO2 support (Fig. 5(a)) exhibited two well-resolved reduction peaks centered at 250 and 350 °C, in good agreement with the TPR curves of bulk MnO2 samples [21]. The former reduction peak could be ascribed to the reduction of MnO2?Mn3O4 and the latter one was due to the further reduction of Mn3O4–MnO. The reduction of MnO to metallic Mn0 was not detected even up to 750 °C, which was due to its bigger negative value of reduction potential [21]. For the b-MnO2 sample, the H/Mn molar ratio estimated from the amount of hydrogen consumed during the reduction process was ca. 1.76, lower than the theoretical H/Mn molar ratio (2.0) calculated according to the (MnO2 + H2 ? MnO + H2O) reaction. This result implies that there was the presence of Mn ions (Mn3+ and Mn4+) with mixed oxidation states, which was substantiated by the results of XPS investigations (Fig. 4(B) and Table 2). Compared to the reduction behavior of the b-MnO2 support, the introduction of Au brought about a remarkable alteration in reduction peak shape, and the reduction peaks greatly shifted to the low-temperature region, with the reduction peaks being centered at 130–160 and 210–230 °C, respectively, which corresponded to the reduction of MnO2 ? Mn3O4 and Mn3O4 ? MnO [21]. This result indicates that Au loading resulted in the enhancement of low-temperature reducibility of the Au/b-MnO2 samples (Fig. 5A(b–d)). An explanation on such a phenomenon is that the noble metal (e.g., Au) could promote the reduction of MnO2?Mn3+ and/or Mn2+ via the spillover of active hydrogen adspecies on the noble metal surface during the H2-TPR experiments [22]. The introduction of Au to b-MnO2 induced the rise in oxygen vacancy density of b-MnO2 in/on the supported gold sample, also facilitating the reduction process. Of course, the reduction of small amounts of gold oxide species occurred below 200 °C, and their reduction peaks were overlapped with the first reduction peak of b-MnO2 [21]. Obviously, the reduction behaviors of Au/b-MnO2 were markedly influenced by the nature of the precipitant adopted during the catalyst preparation process, and the Au/b-MnO2(NaOH) catalyst showed the best lowtemperature reducibility. From Table 1, one can see that the H2 consumptions of Au/b-MnO2(NaOH) and Au/b-MnO2ðNa2 CO3 Þ were higher than that of Au/b-MnO2(urea) below 300 °C. This observation might be due to the presence of stronger interaction between the Au and b-MnO2 nanodomains in Au/b-MnO2(NaOH) and Au/bMnO2ðNa2 CO3 Þ than in Au/b-MnO2(urea). Such a strong interaction was also found in the Ag/MnO2 nanowires/nanorods [23]. It is a better method to compare the reducibility of a catalyst in terms of the initial H2 consumption rate per mole of the reducible metal (Mn), where no occurrence of phase transformation takes place and the reduction is less than 25%. Fig. 5(B) shows the initial H2 consumption rates of the catalysts versus inverse temperature. Apparently, the initial H2 consumption rate decreased in the order of Au/b-MnO2(NaOH) > Au/b-MnO2ðNa2 CO3 Þ > Au/b-MnO2(urea) > b-MnO2, coinciding with the sequence of their catalytic performance (shown below).

(d)

150

(c)

100

(b) 50

(a)

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0)

Desorption Dv (log d)(cm3/g)X103

(B)

30 25 (d) 20 (c)

15 10

(b)

5 (a) 0 0

5

10

15

20

25

30

35

40

Pore diameter (nm) Fig. 2. (A) Nitrogen adsorption–desorption isotherms and (B) pore-size distributions of 3D ordered mesoporous (a) b-MnO2, (b) Au/b-MnO2(urea), (c) Au/bMnO2(NaOH), and (d) Au/b-MnO2ðNa2 CO3 Þ .

vacancy density. The strong interaction of Mn with Au might be also one of the reasons for the highest surface Mn3+ concentration of the Au/b-MnO2(NaOH) sample. The formation of surface oxygen vacancies on the Au/b-MnO2(NaOH) sample was beneficial for the oxidation of CO and VOCs, which provides a good interpretation for the higher catalytic activity of Au/b-MnO2(NaOH) at low temperatures (shown Section 3.5). From Fig. 4(C), one can observe that there were two asymmetrical Au 4f XPS signals at BE = ca. 84 and 88 eV, which could reasonably be decomposed into components at BE = 84.4 and 86.8 eV due to Au 4f7/2 and at BE = 88.1 and 90.5 eV due to Au 4f5/2, respectively. According to the XPS results of supported gold catalysts reported in the literature [19], we could attribute the components at BE = ca. 84.4 and 88.1 eV to the surface metallic Au0 species, whereas the ones at BE = ca. 86.8 and 90.5 eV to the surface Au3+ species. Therefore, there was the co-presence of Au0 and Au3+ on the surfaces of the Au/b-MnO2 samples. Usually, gold oxide (Au2O3) can decompose completely into Au0 and O2 at a temperature higher than 300 °C [20]. In our Au/b-MnO2 catalysts (derived after calcination in air at 400 °C), however, part of gold existed in trivalency. The generation of Au3+ species might be an indication of strong interaction between the gold nanoparticles and the b-MnO2 support, and such a strong interaction would cause part of the gold to be oxidized by the active oxygen adspecies on the b-MnO2 surface into Au3+. Among the three supported Au

3.5. Catalytic performance The blank experiments (only quart sands were loaded in the microreactor) indicate that no significant conversions of CO below 200 °C and of benzene and toluene below 400 °C were detected. That is to say, no homogeneous reactions of CO, benzene or toluene with oxygen took place under the reaction conditions adopted in

Q. Ye et al. / Microporous and Mesoporous Materials 172 (2013) 20–29

25

Fig. 3. TEM images of (a) KIT-6, (b) b-MnO2, (c, d) Au/b-MnO2(urea), (e, f) Au/b-MnO2(NaOH), and (g, h) Au/b-MnO2ðNa2 CO3 Þ .

the present studies. Fig. 6 shows the catalytic activities of the bMnO2 and Au/b-MnO2 samples for the oxidation of CO, benzene, and toluene. It is observed from Fig. 6(A) that the CO conversion increased with the rise in reaction temperature over all of the catalysts. The loading of Au resulted in the significant enhancement in CO conversion, demonstrating the promotional effect of gold on catalytic activity. The Au/b-MnO2(NaOH) and Au/b-MnO2ðNa2 CO3 Þ samples exhibited better catalytic performance than the Au/bMnO2(urea) sample. It is convenient to use the temperatures (T50% and T100%, at which 50% and 100% reactant conversions were achieved, respectively) to evaluate the catalytic activity. The T50% and T100% values were ca. 48 and 70 °C over Au/b-MnO2(NaOH), ca. 60 and 100 °C over Au/b-MnO2ðNa2 CO3 Þ , ca. 110 and 150 °C over Au/b-MnO2(urea), and ca. 145 and 180 °C over b-MnO2, respectively. It has been generally accepted that the Au particle size has an influence on the activity of Au catalysts [24]. From the characterization results (Figs. 1 and 3 and Table 1), one can see that the Au/bMnO2(NaOH) and Au/b-MnO2ðNa2 CO3 Þ catalysts possessed higher surface areas and smaller Au nanoparticles than the Au/b-MnO2(urea) catalyst. Both Au0 and Aud+ have been reported to be required for high activity of a gold catalyst. Bond and Thompson [25] thought that the Aud+ ions existed as a 2D layer at the Au/metal oxide interface, and the latter could act as ‘‘chemical glue’’ that binds the particle to the support [26]. The authors considered that the Au0 species adsorbed CO, whereas the Aud+ species cemented the metallic particle on the support and activated the surface hydroxyl groups, which could react with the adsorbed CO to form adsorbed carboxylate group attached to the periphery. The adsorbed carboxylate group was then attacked by an oxygen species (e.g., O2) from the support, thus liberating the OH ion to re-engage in the catalytic cycle. The Au3+/Au0 atomic ratio [27] can be a key factor for CO oxidation, although other factors may also play roles. The peripheral gold atoms in small supported particles may readily transform into ions [28], resulting in the simultaneous presence of both gold atoms and gold ions in the supported gold catalysts. After investigating the oxidation states of gold in a-Fe2O3-supported gold catalysts highly active for CO oxidation, Hutchings et al. [29] observed that the most active catalysts contained cationic gold with small amounts of zero-valence gold, whereas no

evidence of cationic gold was found in the almost inactive gold catalysts. These results are consistence with those obtained by Guzman and Gates [30]. Hence, it is clear that cationic gold has an important role to play in the catalytic oxidation of CO. As revealed in Fig. 4(C) and Table 2, there were larger amounts of surface Au3+ species on Au/b-MnO2(NaOH) and Au/b-MnO2ðNa2 CO3 Þ , whereas a larger amount of Au0 species was present on Au/b-MnO2(urea). Furthermore, the role of support cannot be ignored. In addition to the roles of Au particle size and shape, the oxide support could act as a stabilizer of the Au dispersion, a modifier of the Au electronic state, and a participant in the activation of oxygen molecules [31]. Tripathi et al. [32] and Grisel and Nieuwenhuys [33] believed that the transition-metal oxide MOx (e.g., Fe3O4 and MnOx) support could activate O2, and CO was adsorbed on Au or at the Au/MOx perimeter. The adsorbed CO reacted with O⁄ species from MOx, probably along the border of the Au particles. The consumed O⁄ species was then replenished by gas-phase O2. The activated O⁄ could react with either another oxygen vacancy or a second CO molecule. It is apparent that a correlation between the catalytic performance and reducibility of the oxide support would be expected. Therefore, the Au particle size, nature and concentration of surface Au species on b-MnO2 could influence the performance of the Au/b-MnO2 catalysts. Among the VOCs (i.e., aromatics, aldehydes, alcohols, carboxylic acids, ketones, ethers, esters, and light alkanes, etc.) that can induce atmospheric pollutions, aromatics are relatively hard to be eliminated. Hence, we selected benzene and toluene as the representatives of aromatics and examined the performance of the supported Au catalysts. Fig. 6(B and C) shows the benzene and toluene conversions as a function of temperature over the b-MnO2 support and Au/b-MnO2 catalysts at SV = 60,000 mL/(g h). It is worth pointing out that CO2 and H2O were the only products. Compared to the Au/b-MnO2 catalysts, the b-MnO2 support showed a poorer catalytic activity, over which the benzene and toluene conversions at 400 °C were only 72.0% and 82.7%, respectively. After loading 5.0 wt.% Au to the b-MnO2 support, the benzene and toluene conversions increased markedly. For the Au/MnO2(NaOH) catalyst, the T50% and T100% values were ca. 200 and 250 °C for benzene oxidation, and 170 and 220 °C for toluene oxidation, respectively. For

26

Q. Ye et al. / Microporous and Mesoporous Materials 172 (2013) 20–29

Fig. 4. (A) O 1s, (B) Mn 2p, and (C) Au 4f XPS spectra of (a) b-MnO2, (b) Au/bMnO2(urea) (c) Au/b-MnO2(NaOH) and (d) Au/b-MnO2ðNa2 CO3 Þ .

the Au/MnO2ðNa2 CO3 Þ catalyst, the T50% and T100% values were ca. 220 °C and 300 °C for benzene oxidation, and 200 and 260 °C for toluene oxidation, respectively. In the case of the Au/b-MnO2(urea) catalyst, however, the corresponding T50% and T100% values were 280 °C and 390 °C for benzene oxidation, and 230 and 320 °C for toluene oxidation. Apparently, the catalytic performance decreased

in the sequence of Au/MnO2(NaOH) > Au/MnO2ðNa2 CO3 Þ > Au/bMnO2(urea) > b-MnO2. It is noted that toluene can be more easily oxidized than benzene. This may be related to the ionization potential of the VOCs molecules [34]. Becker and Foster [35] concluded that toluene was a more reactive reactant than benzene over precious metal catalysts, because the ionization potential (9.24 eV) of benzene is lower than that (8.82 eV) of toluene. A similar conclusion was also drawn by Bedia et al. [36] who investigated the catalytic combustion of non-halogenated volatile organic compounds. The intrinsic catalytic activities in terms of the specific reaction rate (molCO/(or toluene)/(gcat s) or molCO/(or toluene)/(gAu s)) of our catalysts for the oxidation of CO and toluene were compared with those of the catalysts reported in the literature [37–39], as summarized in Table S1 of the Supplementary material. It is observed that the specific reaction rate at 100 °C of b-MnO2 was 2.0  107 molCO/(gcat s), which is the same order of magnitude as those at 100 °C (7.2  108–1.0  106 molCO/(gcat s) of the MnxOy catalysts reported by Carabineiro et al. [37], but lower than those at 100 °C (6.8  107–1.5  106 molCO/(gcat s) of the MnOx catalysts reported by Wang et al. [38]. Addition of Au to manganese oxide increased significantly the catalytic performance for CO oxidation. The specific reaction rates at 100 °C (2.9  105–9.1  105 molCO/ (gcat s) of the Au/b-MnO2 catalysts are comparable to those at 100 °C (1.3  105–2.4  104 molCO/(gcat s) of the Au/MnOx catalysts reported by Wang et al. [38]. For the combustion of toluene, the specific reaction rate at 260 °C (3.0  107 moltoluene/(gcat s) of the b-MnO2 catalyst and those at 260 °C (2.6  105– 3.4  105 moltoluene/(gcat s) of the Au/b-MnO2 catalysts were also within the same order of magnitude as those at 260 °C (9.1  108–1.1  106 moltoluene/(gcat s) of the Mnx (x = 1, 2, 4, and 10) catalysts and those at 260 °C (1.7  105–9.5  105 moltoluene/(gcat s) of the Au/MnOx catalysts reported by Bastosa et al. [39]. It has been generally accepted that the sites active for the total oxidation of benzene and toluene differ from those associated with CO oxidation. This may indicate that the active sites for the oxidation of benzene and toluene may not merely be the sites at the interface between the gold nanoparticles and the support, as is proposed to be the case for CO oxidation [40]. The mechanism of VOC oxidation over the gold-based ceria and other reducible metal oxide catalysts has been proposed by Scire et al. [41], Minico et al. [42], and Centeno et al. [43]. These authors believed that the oxidation of VOCs over these catalysts would follow the Mars and van Krevelen mechanism [44]. The key steps of this mechanism are the supply of active oxygen species by the readily reducible oxide and its re-oxidation by gas-phase oxygen molecules. It is well known that manganese oxide has a particular ability to undergo deep and rapid redox cycles (MnO2 M MnO2x + (x/2)O2) through the interaction with reducing or oxidizing agents in the reaction system [45]. In this way, manganese oxide can control and maintain the adequate oxidation state of gold active sites and enhance the lattice oxygen mobility, hence leading to an enhancement in catalytic activity. There might be a direct relationship between the reducibility and the catalytic performance of a material. The lattice oxygen mobility of b-MnO2 would play an important role in the good performance of this type of catalysts. The Au/bMnO2(NaOH) and Au/b-MnO2ðNa2 CO3 Þ catalysts outperformed the Au/ b-MnO2(urea) catalyst for the total oxidation of benzene and toluene. This result may be related to the higher low-temperature H2 consumptions, stronger low-temperature reducibility, and better lattice oxygen mobility (which were due to the presence of more amounts of surface oxygen vacancies). On the other hand, hydrocarbons can interact with Au surfaces via its p bond [46]. The activation of oxygen is a difficult step on the gold surface [47]. However, a high dispersion of gold on the support surface favors

27

Q. Ye et al. / Microporous and Mesoporous Materials 172 (2013) 20–29 Table 2 Binding energies of surface elements and molar ratios of surface Mn3+/Mn4+, Au3+/Au0, and Oads/Olatt atomic ratios of the b-MnO2 and Au/b-MnO2 catalysts. Catalyst

3+

MnO2 Au/b-MnO2(urea) Au/b-MnO2(NaOH) Au/b-MnO2ðNa2 CO3 Þ

(A)

5

4+

Mn

Mn

641.0 641.0 641.3 641.3

642.5 642.6 642.5 642.4

160

Au 4f5/2 0

0.13 0.18 0.44 0.23

Au3+/Au0 atomic ratio

O 1s Olatt

Oads

– 0.11 0.49 0.33

529.6 529.6 529.7 529.6

531.8 531.7 531.5 531.6

3+

Au

Au

– 84.4 84.5 84.4

– 86.8 86.8 86.8

Oads/Olatt atomic ratio

0.31 0.36 0.66 0.49

230

(d)

4 130

210

2

H2 consumption rate (molH /molMn.s)X103

Mn3+/Mn4+ atomic ratio

Mn 2p3/2

3

(c) 230

2

140

(b) 250

1

350

(a)

0 0

100

200

300

400

500

600

700

800

2.60

2.64

2.68

o

Temperature ( C) 8 (c)

7 6

2

Initial H2 consumption (molH /molMn.s)X104

(B)

5

(d)

4 (b)

3 2 1 0 2.36

(a)

2.40

2.44

2.48

2.52

2.56 -1

1000/T (K ) Fig. 5. (A) TPR profiles and (B) initial H2 consumption rates versus inverse temperature of (a) b-MnO2, (b) Au/b-MnO2(urea) (c) Au/b-MnO2(NaOH), and (d) Au/ b-MnO2ðNa2 CO3 Þ .

the formation of a large number of specific catalytic sites for the activation of hydrocarbons, thus attaining a high catalytic activity. It is worth mentioning that the presence of 3D ordered mesopores in the support was favorable for the generation of Au/b-MnO2 with a higher Au dispersion and for the adsorption and diffusion of reactant molecules. In addition, it was also suggested that the oxidation state of gold played a key role in the catalytic combustion of VOCs. Minico et al. [48] reported that some representative VOCs (e.g., 2-propanol, ethanol, methanol, acetone, and toluene) to CO2 over the gold/iron oxide catalyst decreased greatly when the calcination temperature increased, which was due to the disappearance of oxidized gold species in the catalyst. Wang and Ro [49] pointed out that the oxidized gold exhibited a higher activity than the metallic gold for the total oxidation of methanol. Waters et al. [50] observed that the gold with higher oxidation states could promote the cata-

Fig. 6. (A) CO conversion at SV = 30,000 mL/(g h), (B) benzene conversion at SV = 60,000 mL/(g h), and (C) toluene conversion at SV = 60,000 mL/(g h) as a function of temperature over the b-MnO2 and Au/b-MnO2 catalysts.

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Q. Ye et al. / Microporous and Mesoporous Materials 172 (2013) 20–29

240,000 mL/(g h) for benzene or toluene oxidation at the same reaction temperature. 4. Conclusions

Fig. 7. CO, benzene or toluene conversion as a function of on-stream reaction time over the Au/b-MnO2(NaOH) catalyst under the reaction conditions of (i) 1.08 vol.% CO + air and SV = 30,000 mL/(g h) or (ii) 2000 ppm benzene (or toluene) + 20 vol.% O2 + 79.8 vol.% N2 and SV = 60,000 mL/(g h).

3D ordered mesoporous b-MnO2-supported nanosized Au catalysts (5 wt.% Au/b-MnO2) were prepared using the deposition–precipitation method with urea, NaOH or Na2CO3 as precipitant agent. The nature of the precipitant added during the loading of gold had an important impact on the physicochemical properties of b-MnO2, Au nanoparticles, and Au/b-MnO2 catalysts. Among the Au/b-MnO2 samples, the Au/b-MnO2(NaOH) sample possessed the highest surface Mn3+/Mn4+, Oads/Olatt, and Au3+/Au0 atomic ratios. The loading of gold improved the reducibility of Au/b-MnO2 via the strong interaction between the gold and the b-MnO2 nanodomains, with the Au/b-MnO2(NaOH) sample exhibiting the best low-temperature reducibility. Under the conditions of (i) CO concentration = 1.08% and SV = 30,000 mL/(g h) or (ii) benzene or toluene concentration = 2000 ppm and SV = 60,000 mL/(g h), the Au/b-MnO2 catalysts showed better performance than the b-MnO2 support. Over the best-performing Au/b-MnO2(NaOH) catalyst, the T50% and T100% values were 48 °C and 70 °C for CO oxidation, 200 and 250 °C for benzene oxidation, and 170 °C and 220 °C for toluene oxidation, respectively. Based on the characterization results and activity data, we conclude that the excellent catalytic performance of Au/ b-MnO2(NaOH) was associated with the better gold dispersion, higher oxidized gold species and oxygen adspecies concentrations, better low- temperature reducibility, and stronger synergism between the gold and the support as well as the high-quality 3D ordered mesoporous structure of the b-MnO2 support. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20777005) and Natural Science Foundation of Beijing (No. 8082008). Appendix A. Supplementary data

Fig. 8. Effect of SV on the CO, benzene or toluene conversion over the Au/bMnO2(NaOH) catalyst under the reaction conditions of (i) 1.08 vol.% CO + air or (ii) 2000 ppm benzene (or toluene) + 20 vol.% O2 + 79.8 vol.% N2.

lytic combustion of methane. Casaletto et al. [51] has also claimed that the higher catalytic performance for CO oxidation over Au/ CeO2 with respect to Au/SiO2 was attributed to a better stabilization of the Au3+ species by cerium ions. All of the above results and discussion demonstrate that the presence of Au with a higher oxidation state would be beneficial for the enhancement in catalytic activity. To examine the catalytic stability of the Au/b-MnO2 sample, we measured the CO, benzene and toluene conversions within 50 h of on-stream reaction and the result is shown in Fig. 7 over Au/bMnO2(NaOH) catalyst. It is observed that during the 50-h on-stream reaction time, no significant change in CO, benzene or toluene conversion was detected, indicating that the Au/b-MnO2 catalyst was durable. The effect of SV on the catalytic activity of the Au/bMnO2(NaOH) sample for the oxidation of CO, benzene, and toluene was also investigated, as shown in Fig. 8. As expected, the CO, benzene or toluene conversion decreased with the rise in SV from 30,000 to 120,000 mL/(g h) for CO oxidation or from 60,000 to

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