MnO2 Nanostructured Catalysts and

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Facile Controlled Synthesis of Pt/MnO2 Nanostructured Catalysts and Their Catalytic Performance for Oxidative Decomposition of Formaldehyde Xuehua Yu,†,‡ Junhui He,*,† Donghui Wang,§ Yucai Hu,‡ Hua Tian,† and Zhicheng He† †

Functional Nanomaterials Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China ‡ School of Chemistry and Materials Science, Ludong University, Yantai, Shandong 264025, China § Research Institute of Chemical Defense, Beijing 100083, China

bS Supporting Information ABSTRACT: Pt/MnO2 nanostructured catalysts with cocoon-, urchin-, and nest-like morphologies were synthesized by a facile method. The synthesized MnO2 nanostructures and Pt/MnO2 catalysts were characterized by means of X-ray diffraction (XRD), N2 adsorptiondesorption, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). TEM analyses showed that Pt nanoparticles of 14 nm were evenly dispersed on the surface of three MnO2 nanostructures, and no Pt nanoparticle agglomeration occurred in the Pt/MnO2 catalysts. These Pt/MnO2 catalysts showed much higher catalytic activities than the corresponding MnO2 nanostructures for oxidative decomposition of formaldehyde. Comparison of Pt/MnO2 catalysts with varied Pt loadings and MnO2 morphologies revealed that 2 wt % is the optimal Pt loading, and 2 wt % Pt/nest-like MnO2 showed the highest catalytic activity for oxidative decomposition of formaldehyde (temperature for complete decomposition of HCHO is 70 °C). The high dispersion and small size of Pt nanoparticles and the synergistic effect between the Pt nanoparticle and MnO2 nanostructure are considered to be the main reasons for the observed high catalytic activity of Pt/nest-like MnO2.

1. INTRODUCTION Formaldehyde (HCHO) is known to cause nasal tumors and irritation to eyes, the respiratory tract, and skin even at low concentration and has been regarded as a major indoor pollutant emitted from widely used building and decorative materials in airtight buildings.1 Great efforts have been made to reduce the indoor concentration of HCHO to meet the stringent environmental regulations due to the increasing concern for human health. A number of methods have been proposed for elimination of HCHO, including physical adsorption,2,3 plasma technology,4 plant absorption,5,6 photocatalysis,710 and catalytic oxidation.1113 Because HCHO can be oxidized into CO2 and H2O by catalytic oxidation at low cost using simple technology, the catalytic oxidation is considered as one of the most important and promising technologies for HCHO elimination. However, the development of highly effective catalysts remains a major challenge for complete oxidation of HCHO into harmless CO2 and H2O at low temperature.14,15 For indoor air cleaning, low energy consumption requirement and low formaldehyde concentration call for catalysts that exhibit high activity for complete oxidation of HCHO at relatively low temperatures or even at ambient temperature. As transition metal r 2011 American Chemical Society

elements have varied valence states and special electronic configuration,16 transition metal oxide catalysts show high catalytic activity for degradation of volatile organic contaminants (VOCs).1720 Among them, manganese oxide (MnO2) is one of the most attractive materials because of its technological importance for catalytic, molecular adsorption, and electrochemical applications.2123 However, the properties of MnO2 are significantly influenced by its structure, morphology, and crystal form.2427 Sekine11 reported many kinds of metal oxides for decomposition of HCHO and found that MnO2 could decompose HCHO and release CO2 in a static reaction vessel even at relatively low temperature, suggesting that MnO2 could be used as an active component for HCHO removal in indoor air. Our group synthesized MnO2 with varied morphologies by a microemulsion method, among which birnessite MnO2 hollow nanospheres showed high catalytic activity for oxidative decomposition of HCHO.2830 Cryptomelane-type MnO2 (e.g., manganese oxide octahedral molecular sieve (OMS-2)) consists of chains of 2 2 edge-shared Received: September 16, 2011 Revised: November 19, 2011 Published: December 09, 2011 851

dx.doi.org/10.1021/jp208947e | J. Phys. Chem. C 2012, 116, 851–860

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MnO6 octahedra and has one-dimensional channels of 0.46 nm 0.46 nm.31 It is chemically composed of mixed valence manganese (Mn2+, Mn3+, and Mn4+) and physically consists of large open layers or channels.32 OMS-2 was investigated in previous reports as an oxidation catalyst for decomposition of HCHO.3335 For example, our group33 synthesized K-OMS-2 with varied morphologies including K-OMS-2 nanoparticles and nanorods. The K-OMS-2 nanoparticles showed rather high catalytic activity for complete oxidation of HCHO. Chen et al.35 prepared three manganese oxides with different square tunnel sizes, including pyrolusite of 0.23 nm 0.23 nm, cryptomelane of 0.46 nm 0.46 nm, and todorokite of 0.69 nm 0.69 nm. The effective tunnel diameter of cryptomelane is similar to the dynamic diameter of the HCHO molecule and was proposed to be a major factor that gave rise to its high catalytic activity. However, the manganese oxides and manganese composite oxides yet show somewhat low removal efficiency for HCHO at low temperature and thus limitations for indoor HCHO removal despite their low costs. Recently, the advantages of noble metal nanoparticles (NPs) supported catalysts have been recognized in heterogeneous catalysis for decomposition of VOCs. 3640 Dispersion of noble metal NPs on supports has many advantages, such as increasing the number of surface atoms and thus active sites, bringing synergistic effects between NP and support, preventing aggregation of NPs even at high particle densities, and lowering the cost lvarez-Galvan et al.41 developed a series of catalysts. Recently, A of Mn/Al2O3 and MnPd/Al2O3 catalysts for catalytic decomposition of HCHO and demonstrated that HCHO could be completely decomposed by MnPd/Al2O3 at 80 °C. Tang et al.42,43 reported that when Ag and Pt NPs were supported on MnOxCeO2 the catalysts had excellent catalytic activity for decomposition of HCHO at low temperature. Shen et al.4447 deposited Au NPs on Fe2O3, ZrO2, and CeO2 supports. The three-dimensionally ordered macroporous Au/CeO2 catalysts exhibited high catalytic activity, and HCHO could be completely oxidized into CO2 and H2O at 75 °C. Zhang et al.4850 synthesized TiO2 supported noble metal (Au, Rh, Pd, and Pt) catalysts and showed that the Pt/TiO2 could completely decompose HCHO into CO2 and H2O at ambient temperature. Similar results were also reported by Wang et al.51,52 However, few studies have been reported so far on catalytic oxidation of HCHO by noble metal NPs supported on MnO2. Due to both better catalytic performance of MnO2 than other transition metal oxides11,2830 and high catalytic activity of Pt NPs for catalytic oxidation of HCHO,4852 Pt/MnO2 catalysts are expected to show even enhanced catalytic performance probably through synergistic effects between MnO2 supports and Pt nanoparticles. In this paper, three MnO2 nanostructures with varied morphologies, including cocoon-, urchin-, and nest-like MnO2, were successfully synthesized and used as supports for loading Pt nanoparticles. Pt/MnO2 catalysts with varied amounts of Pt NPs were obtained using polyvinylpyrrolidone (PVP), sodium citrate, and ascorbic acid as dispersant, protective agent, and reducing agent, respectively. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), etc. were used to characterize the morphology and structure of catalysts. The surface area and pore diameter of three MnO2 nanostructures were determined by nitrogen adsorption desorption measurements. The catalytic performance of Pt/MnO2 catalysts with varied morphologies and Pt loadings was evaluated for catalytic decomposition of formaldehyde. The possible

mechanisms of high catalytic activity of Pt/MnO2 catalysts are discussed based on the experimental results.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals used were of analytical grade and used without further purification. Ultrapure water with a resistivity higher than 18.2 MΩ 3 cm was used in all experiments and was obtained from a three-stage Millipore Mill-Q Plus 185 purification system (Academic). 2.2. Preparation of MnO2 Nanostructures and Pt/MnO2 Catalysts. 2.2.1. Synthesis of Cocoon-Like MnO2 Nanostructures. Cocoon-like MnO2 nanostructures were prepared by the redox reaction of Mn7+ and Mn2+ at room temperature.53 In a typical procedure, 2.54 g of MnSO4 3 H2O was dissolved in 150 mL of deionized water. Under vigorous stirring, 100 mL of aqueous KMnO4 (0.1 M) was added by dripping. After the mixture was vigorously stirred at room temperature for an additional 6 h, a black product was obtained. It was centrifuged and washed several times with deionized water and absolute alcohol to remove any possible residual reactants. Finally, the product was dried at 60 °C for 12 h. 2.2.2. Synthesis of Urchin-Like MnO2 Nanostructures. Urchinlike MnO2 nanostructures were synthesized by the reflux method under acidic conditions.54 In a typical procedure, 1.69 g of MnSO4 3 H2O was dissolved in 100 mL of deionized water. Under vigorous stirring at 80 °C in a water bath, 1 mL of concentrated sulfuric acid was added by dripping. Then 66 mL of aqueous KMnO4 (0.1 M) was added by dripping. After the mixture was vigorously stirred at 80 °C for an additional 24 h, a black product was obtained. It was centrifuged and washed several times with deionized water and absolute alcohol to remove any possible residual reactants. Finally, the product was dried at 60 °C for 12 h. 2.2.3. Synthesis of Nest-Like MnO2 Nanostructures. Nest-like MnO2 nanostructures were prepared by a hydrothermal method under acidic conditions. In a typical procedure, 0.55 g of KMnO4 was dissolved in 14 mL of deionized water, and then the KMnO4 aqueous solution was transferred into a 40 mL stainless-steel autoclave with a Teflon-liner. Under vigorous stirring, a 21 mL auqeous solution with 0.89 g of MnSO4 3 H2O and 0.3 mL of concentrated sulfuric acid was added by dripping. The autoclave was sealed and maintained at 120 °C for 12 h. After it was allowed to cool to room temperature, a black product was obtained. It was centrifuged and washed several times with deionized water and absolute alcohol to remove any possible residual reactants. Finally, the product was dried at 60 °C for 12 h. 2.2.4. Synthesis of Pt/MnO2 Catalysts. Pt/MnO2 catalysts were prepared by reduction of chloroplatinic acid with ascorbic acid as reducing agent.55 In a typical procedure, 0.068 g of PVP (PVP:H2PtCl6 = 20:1, molar ratio) and 0.18 g of sodium citrate (sodium citrate:H2PtCl6 = 20:1, molar ratio) were dissolved in 100 mL of deionized water and transferred into a three-necked flask. Under vigorous stirring at 80 °C in a water bath, 0.8 mL of aqueous chloroplatinic acid (H2PtCl6 3 6H2O) (20 mg/mL) was added by dripping. After stirring for an additional 5 min, 10 mL of aqueous solution with 0.108 g of ascorbic acid (ascorbic acid: H2PtCl6 = 20:1, molar ratio) was added dropwise to the mixture solution. One hour later, 0.3 g of MnO2 powder (dispersed in 50 mL of deionized water by ultrasound) was added into the three-necked flask. After reaction at 80 °C for 4 h in a water bath, a precipitate was filtered and washed several times with deionized 852

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water and absolute alcohol to remove any possible residual reactants. Finally, the product was dried at 60 °C for 12 h. To get different Pt loadings for Pt/MnO2 catalysts, the mass ratio of Pt and MnO2 was changed (0.5 wt %, 1 wt %, 3 wt %), while the mole ratios of other starting materials were kept the same (i.e., PVP:H2PtCl6 = 20:1, sodium citrate:H2PtCl6 = 20:1, ascorbic acid:H2PtCl6 = 20:1). 2.3. Characterization. XRD patterns of as-prepared products were collected on a Bruker D8 Focus X-ray diffractometer using Cu Kα radiation with a voltage and current of 40 kV and 40 mA, respectively. SEM observations were carried out on a Hitachi S-4300 field emission scanning electron microscope operated at 10 kV. TEM images were obtained using a JEOL JEM-2100 transmission electron microscope. A typical TEM sample was prepared by adding several droplets of a nanoparticles/ethanol mixture onto a carbon-coated copper grid. Nitrogen adsorption desorption measurements were performed on a Quadrasorb SI automated surface area and pore size analyzer at 196 °C, using the volumetric method. Specific surface areas and pore volumes were calculated by the BrunauerEmmettTeller (BET) method. 2.4. Catalytic Activities. Catalytic activities of as-prepared catalysts for HCHO oxidation were performed in a fixed-bed reactor under atmospheric pressure. The catalyst (100 mg, 4060 mesh) was loaded in a quartz tube reactor (length = 500 mm, diameter = 4 mm). Gaseous HCHO was generated by passing a purified air flow over HCHO solution in an incubator kept at 0 °C, leading to a feed gas with 460 ppm of HCHO. The total flow rate was 50 mL/min in a gas hourly space velocity (GHSV) of 20 000 mL/(gcat h). Effluents from the reactor were analyzed with an online Agilent 6890 gas chromatograph equipped with FID and Ni catalyst converter which was used for converting carbon oxides quantitatively into methane in the presence of hydrogen before the detector. No other carbon-containing compounds except CO2 were detected in the effluents for all tested catalysts. Thus, the HCHO conversion was expressed in the yield of CO2 and calculated as follows33

Figure 1. XRD patterns of three MnO2 nanostructures and Pt/MnO2 catalysts: (a) cocoon-like MnO2, (b) 2 wt % Pt/cocoon-like MnO 2, (c) urchin-like MnO2 , (d) 2 wt % Pt/urchin-like MnO 2, (e) nest-like MnO2, and (f) 2 wt % Pt/nest-like MnO2.

HCHO conversion ð%Þ ¼ CO2 yield ð%Þ ¼

½CO2 out vol % 100 ½HCHOin vol %

where [CO2]out is the CO2 concentration in the effluents (vol %) and [HCHO]in is the HCHO concentration of the feed gas (vol %).

3. RESULTS AND DISCUSSION 3.1. XRD Analysis of MnO2 Nanostructures and Pt/MnO2 Catalysts. Figure 1 presents the XRD patterns of as-prepared

MnO2 nanostructures and Pt/MnO2 catalysts. Figure 1a and b are the XRD patterns of cocoon-like MnO2 nanostructures and 2 wt % Pt/cocoon-like MnO2 catalyst, respectively. The weak diffraction peaks indicate that their crystallinity is low, and the loading reaction did not change the crystal form of cocoon-like MnO2. Unfortunately, these two XRD patterns are difficult to match a certain crystal form because of the weak diffraction peaks. Figure 1c and d show the XRD patterns of urchin-like MnO2 and 2 wt % Pt/urchin-like MnO2 catalyst, respectively. The crystal form of urchin-like MnO2 matches cryptomelane-M (JCPDS Card No. 44-1386) and akhtenskite (JCPDS Card No. 30-0820). As compared with the XRD patterns of urchin-like MnO2, the diffraction peaks of 2 wt % Pt/urchin-like MnO2 shift

Figure 2. SEM (a,c,e) and TEM (b,d,f) images of three MnO2 nanostructures: (a,b) cocoon-like MnO2, (c,d) urchin-like MnO2, (e,f) nestlike MnO2; insets in d and f are corresponding HRTEM images. 853



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Table 1. Surface Areas, Pore Sizes, and Total Pore Volumes of Three MnO2 Nanostructures pore size (nm) surface area samples

(m2/g)

cocoon-like MnO2

247.6

urchin-like MnO2 nest-like MnO2

62.3 56.9

total pore volume Dmic -0.578 0.573

Dmes 3.84 16.9 18.1

(cm3/g) 0.324 0.124 0.219

3.2.2. Nitrogen AdsorptionDesorption Measurements of MnO2 Nanostructures. The surface area, pore size, and total pore volume of three MnO2 nanostructures were revealed by N2 adsorptiondesorption measurements. Figure 3 shows that three MnO2 nanostructures have a typical type IV adsorption desorption isotherm based on the IUPAC.56 The BET surface areas of cocoon-, urchin-, and nest-like MnO2 were calculated to be 247.6, 62.3, and 56.9 m2/g, respectively. The corresponding total pore volumes were calculated to be 0.324, 0.124, and 0.219 cm3/g, respectively (Table 1). The average mesopore sizes of three MnO2 nanostructures were estimated by the BJH method, and they are 3.84, 16.9, and 18.1 nm, respectively (Table1). The mesopores size distributions of three MnO2 nanostructures are shown in Figure S1 (Supporting Information). The mesopores of 16.9 and 18.1 nm are considered to be accumulation holes in the nest-like MnO2. The average micropore sizes of urchin- and nestlike MnO2 nanostructures were calculated by the HK method to be 0.578 and 0.573 nm, respectively. The micropores were not observed in the cocoon-like MnO2. These results agree well with those of XRD, SEM, and TEM analyses. 3.3. Structural Features of Pt/MnO2 Catalysts. 3.3.1. TEM Observations of Pt/Urchin-Like MnO2 Catalysts. TEM images of Pt/urchin-like MnO2 catalysts with varied Pt loadings are shown in Figure 4. Clearly, Pt NPs of narrow size distribution are highly dispersed on the nanorod surface of urchin-like MnO2. The sizes of Pt NPs are in the range of 14 nm. The average Pt NP sizes were estimated to be 3.0 ( 1.4, 1.7 ( 0.54, 2.0 ( 0.37, and 2.6 ( 0.55 nm for the Pt/urchin-like MnO2 catalysts with a Pt loading of 0.5 wt %, 1 wt %, 2 wt %, and 3 wt %, respectively (Figure 4c, f, i, and l). HRTEM images of the Pt/urchin-like MnO2 catalysts suggest that Pt NPs are highly dispersed on the nanorod surface of urchin-like MnO2 even at a Pt loading as high as 3 wt %. The average sizes of Pt NPs of varied Pt loadings on three MnO2 nanostructures (Table S1, Supporting Information) show that the size of Pt NPs first increases and then decreases with increase of Pt loading. The increased Pt NP size at high Pt loading should be caused by high precursor (H2PtCl6 3 6H2O) concentration during the formation of Pt NPs but before the loading process. The observation of larger Pt NPs at a Pt loading of 0.5 wt % might result from agglomeration of Pt nuclei during the synthesis of Pt NPs due to a smaller amount of PVP and sodium citrate used in the reaction mixture. 3.3.2. Comparison of Pt/MnO2 Catalysts with Varied MnO2 Nanostructures. Pt/MnO2 catalysts of an identical amount of Pt loading but with varied MnO2 nanostructures as support were observed by TEM to compare the effects of supports on the loading of Pt nanoparticles. Figure 4(g, h, i) and Figure 5 are TEM images and Pt NP histograms of 2 wt % Pt/cocoon-like MnO2, 2 wt % Pt/urchin-like MnO2, and 2 wt % Pt/nest-like MnO2, respectively. As shown in Figure 5a and d and Figure 4g, Pt NPs

Figure 3. N2 adsorptiondesorption isotherms of three MnO2 nanostructures.

to smaller diffraction angles. The loading process of Pt NPs adopted the same temperature as the synthesis of urchin-like MnO2, which might have resulted in the change of crystal parameters of 2 wt % Pt/urchin-like MnO2 catalyst. Figure 1e and f are the XRD patterns of nest-like MnO2 and 2 wt % Pt/nest-like MnO2 catalyst, respectively. The feature peaks were observed at 2θ of 12.8° (101), 17.8° (200), 28.6° (201), 37.4° (211), 41.6° (310), 49.5° (411), 59.9° (512), 65° (020), and 69.7° (514) and could be readily indexed to cryptomelane-M with a space group of I2/m(12) (JCPDS Card No. 44-1386). In contrast to 2 wt % Pt/urchin-like MnO2 catalyst, the loading process of Pt nanoparticles did not influence the crystal parameters of nest-like MnO2. An overview of the whole XRD patterns of as-prepared MnO2 nanostructures indicates that the crystallinity of MnO2 nanostructures improves with an increase of their synthesis temperature. However, the loading process has no significant influence on the crystal form of MnO2 nanostructures. 3.2. Structural and Textural Features of MnO2 Nanostructures. 3.2.1. Morphologies of MnO2 Nanostructures. Figure 2 shows SEM and TEM images of three MnO2 nanostructures. The MnO2 nanostructure obtained at room temperature has a cocoon-like morphology and a size of ca. 500 nm (Figure 2a). A magnified TEM image (Figure 2b) shows that it has a lamellar structure on the surface of microspheres. The MnO2 nanostructure obtained under a water bath of 80 °C has an urchin-like morphology with many long MnO2 nanorods radiating from its center (Figure 2c). Meanwhile, the surface of urchin-like MnO2 consists of a large number of short MnO2 nanorods that interweave with each other. Figure 2d shows TEM and HRTEM images of long nanorods. The lattice distance of 0.49 nm is approximately equal to the pore size of OMS-2, which consists of chains of 2 2 edge-shared MnO6 octahedra and has onedimensional channels of 0.46 nm 0.46 nm.31 Figure 2e shows SEM images of the MnO2 nanostructure obtained by the hydrothermal method under acidic conditions. The obtained MnO2 has a nest-like shape with diameter in the range of 13 μm. The TEM image (Figure 2f) shows that the nest-like MnO2 consists of nanorods with widths of 1050 nm and lengths of 0.2 1.5 μm. The HRTEM image (Figure 2f, inset) also shows a lattice distance of 0.49 nm, which is approximately equal to the pore size of OMS-2. The lattice distance (0.49 nm) agrees well with the lattice spacing of the (200) crystal plane observed in the XRD patterns (Figure 1e). 854



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Figure 4. TEM images and histograms of Pt nanoparticles in Pt/urchin-like MnO2 catalysts. Pt loading: a,b,c, 0.5 wt %; d,e,f, 1 wt %; g,h,i, 2 wt %; j,k,l, 3 wt %.

are highly dispersed on the surface of three MnO2 nanostructures, i.e., on the lamellar structure surface of cocoon-like MnO2 and the nanorod surfaces of urchin-like MnO2 and nest-like MnO2. The Pt NP sizes range from 1.0 to 3.5 nm for the three Pt/MnO2 catalysts, as depicted in Figure 5c and f and Figure 4i. The average Pt NP sizes were estimated to be 2.0 ( 0.44, 2.0 ( 0.37, and 1.9 ( 0.31 nm for 2 wt % Pt/cocoon-like MnO2, 2 wt % Pt/urchin-like MnO2, and 2 wt % Pt/nest-like MnO2, respectively. Clearly, the Pt NPs have narrow size distributions on the surface of varied MnO 2 nanostructures. HRTEM images (Figure 5b and e and Figure 4h) show that the density of Pt NPs on the nest-like MnO2 is lower than those of the cocoon- and urchin-like MnO2. The nest-like MnO2 has a loose structure, and Pt NPs (as shown by arrows in Figure 5d) may be loaded not only

on the outer nanorods surface but also on the inner nanorods surface through their accumulation pores. In contrast, Pt NPs may be loaded preferably on the outer lamellar structures or nanorod surfaces of the other two MnO2 nanostructures due to their denser inner structures. It is interesting that the average sizes of Pt NPs of Pt/MnO2 catalysts are similar to each other, so the size of Pt NPs is not significantly influenced by different MnO2 nanostructures. The current method, in which PVP, sodium citrate, and ascorbic acid act as dispersant, protective agent, and reducing agent, respectively, may have advantages in improving catalytic activity. First, complete dispersion of Pt NPs on the surface of MnO2 nanostructures can be readily achieved, and thus high utilization of Pt NPs can be realized. Second, Pt NPs can be 855



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Figure 5. TEM images and histograms of Pt nanoparticles in 2 wt % Pt/cocoon-like MnO2 (a,b,c) and 2 wt % Pt/nest-like MnO2 (d,e,f) catalysts.

protect Pt NPs against their aggregation, and thus the ratio of exposed surface atoms to total atoms would increase, enhancing the catalytic activity. 3.4. Catalytic Decomposition of HCHO on As-Prepared Catalysts. The catalytic properties of Pt/urchin-like MnO2 with varied Pt loadings were investigated. As shown in Figure 6A, the catalytic performance of Pt/urchin-like MnO2 increases with an increase of Pt loading up to 2 wt %. The conversion of HCHO decreases with further increase of Pt loading. The catalytic activities of Pt/urchin-like MnO2 with varied Pt loadings follow the order: 2 wt % Pt/urchin-like MnO2 > 1 wt % Pt/urchin-like MnO2 > 3 wt % Pt/urchin-like MnO2 > 0.5 wt % Pt/urchin-like MnO2 . urchin-like MnO2. To gain further insight into the effect of Pt loading on the activity of Pt/urchin-like MnO2, the dependence of HCHO conversion at 20 °C and the temperature for complete conversion of HCHO were plotted on the Pt loading, and results are depicted in Figure 6B. Clearly, the conversion of HCHO at 20 °C increases with the Pt loading up to 2 wt % and then decreases with a further increase of the Pt loading. The temperature for complete conversion of HCHO is 80 °C when the Pt loading is 2 wt %. Comparison of the catalytic activities of urchin-like MnO2 and Pt/urchin-like MnO2 indicates that Pt NPs play an important role in the catalytic decomposition of HCHO. Considering both the TEM images (Figure 4) and the catalytic performance (Figure 6) of Pt/urchin-like MnO2 with varied Pt loadings, it is clear that the catalytic performance of Pt/urchin-like MnO2 can be significantly influenced by the size of Pt NPs. Previous results indicated that the catalytic activity of the metal NP largely depends on the ratio (NS/NT) of its exposed surface atoms (NS) to its total atoms (NT),60,61 and the size of the metal NP plays an important role in determining its catalytic activity.62 According to the following equation reported by He et al.60

Figure 6. (A) Catalytic performance of urchin-like MnO2 nanostructures with varied Pt loadings (a, 0 wt %; b, 0.5 wt %; c, 1 wt %; d, 2 wt %; e, 3 wt %). (B) Dependence of the catalytic performance of Pt/urchinlike MnO2 at 20 °C (a) and the temperature for complete conversion of HCHO (b) on the Pt loading.

completely loaded on the MnO2 nanostructures as proved by the colorless filtrate in the Buchner flask in the process of vacuum filtration, thus reducing the loss of Pt and enhancing the utilization of the H2PtCl6 3 6H2O precursor (Figure S2, Supporting Information). Third, PVP has N-containing functional groups and exhibits a high affinity to Pt NPs.5759 Addition of PVP could

N ¼ 856

4 AW πR 3 d= 3 NA

ð1Þ



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Table 2. Number Ratio (NS/NT) of Exposed Surface Atoms (NS) to Total Atoms (NT) of Pt Nanoparticles Pt loading (wt%)

average size (nm)

NS

NT

NS/NT (%)

0.5

3.0

361

936

38.6

1 2

1.7 2.0

102 148

170 277

60.3 53.5

3

2.6

267

609

43.5

where N is the number of atoms in the nanoparticle; R is the radius of the nanoparticle; d is the metal bulk density (21.45 g/cm3 for Pt); AW is the metal atomic weight; and NA is the Avogadro constant. Assuming that the Pt nanoparticle is spherical with closely packed Pt atoms (Figure S3, Supporting Information), the following equation was derived for the NS/NT ratio of exposed surface atoms (NS) to total atoms (NT) of Pt nanoparticle by using eq 1 and referring to the literature.60 pffiffiffi NS R 3 ðR 3rÞ3 ¼ ð2Þ NT R3

Figure 7. Catalytic performance of three MnO 2 nanostructures: (a) cocoon-like MnO2, (b) urchin-like MnO2, and (c) nest-like MnO2.

where R is the radius of the Pt nanoparticle, and r is the radius of the Pt atom (0.13 nm for Pt). The NS/NT values of Pt nanoparticles were estimated, and the results are shown in Table 2. By comparison with Figure 6, the change of catalytic activity is basically in agreement with the change of NS/NT. According to the values of NS/NT, the catalytic activity of 1 wt % Pt/urchin-like MnO2 should be higher than that of 2 wt % Pt/urchin-like MnO2. However, the actual order of catalytic activities is opposite. As shown in Figure 6(Ac and Ad), the catalytic activities of 1 wt % Pt/urchin-like MnO2 and 2 wt % Pt/urchin-like MnO2 are similar below or at 40 °C, while the catalytic activity of 2 wt % Pt/urchin-like MnO2 is higher than that of 1 wt % Pt/urchin-like MnO2 when temperature exceeds 40 °C. Two major factors, including surface atoms of Pt NPs and the synergetic effect between Pt NP and MnO2 nanostructure, are supposed to contribute to the improvement of catalytic activity of Pt/urchin-like MnO2. The surface atoms of Pt NPs may be the determining factor for the catalytic activity at low temperatures (below 40 °C). However, the synergetic effect between the Pt NP and MnO2 nanostructure may become the determining factor for the catalytic activity at high temperatures (above 40 °C). High loading of Pt could provide more active sites for catalytic reaction. The enhancement of the synergetic effect and the increase of active sites are expected to contribute to the enhanced catalytic activity of 2 wt % Pt/urchin-like MnO2 as compared with 1 wt % Pt/urchin-like MnO2. The low catalytic activity of 3 wt % Pt/urchin-like MnO2 may be explained by the fact that the large size of Pt NPs decreases the number of surface Pt atoms and weakens the synergetic effect. To further investigate the influence of Pt NPs and the synergetic effect between the Pt NP and MnO2 nanostructure, the catalytic activities of three MnO2 nanostructures with varied morphologies and crystal forms were evaluated in oxidation of HCHO, and the results are shown in Figure 7. Clearly, the morphologies and crystal forms of MnO2 nanostructures have significant influence on the catalytic performance. The catalytic activities follow the order: nest-like MnO2 > urchin-like MnO2 > cocoon-like MnO2. However, the catalytic activities of three MnO2 nanostructures are much lower than those of the corresponding Pt/MnO2 catalysts (Figure 8).

Figure 8. Catalytic performance of three Pt/MnO2 catalysts: (a) 2 wt % Pt/cocoon-like MnO2, (b) 2 wt % Pt/urchin-like MnO2, and (c) 2 wt % Pt/nest-like MnO2.

As shown in Figure 7, the HCHO conversions over three MnO2 nanostructures are close to zero below 60 °C. Their catalytic activities are enhanced with an increase of temperature. The nest-like MnO2 shows the best catalytic activity among three MnO2 nanostructures at temperatures above 80 °C. As discussed above, the BET surface areas of cocoon-, urchin-, and nest-like MnO2 are 247.6, 62.3, and 56.9 m2/g, respectively. Although the BET surface areas of nest- and urchin-like MnO2 are much lower than that of cocoon-like MnO2, they have better catalytic performance. Both XRD and TEM results revealed that the urchin- and nest-like MnO2 have a channel structure in the nanorods of MnO2 (Figure 2). The HK method also gave micropore sizes of 0.578 and 0.573 nm for urchin- and nest-like MnO2, respectively (Table 1). Such a channel structure of MnO2 may also play a key role in controlling the catalytic activity for decomposition of HCHO. Wang et al.63 reported the structure and properties of 2 2 channels by adsorbing different gases with different molecular diameters and demonstrated that molecules with diameters below 0.265 nm could be inserted into the channels of OMS-2, whereas those molecules with diameters above 0.33 nm were excluded from the channels. The dynamic diameter of HCHO is 0.243 nm, which can be inserted into the 857



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observed difference in catalytic activity for oxidative decomposition of HCHO. First, OMS-2 is a microporous material consisting of mixed valence manganese oxides (e.g., Mn2+, Mn3+, and Mn4+).6466 The catalytic activity is possibly associated with the capacity of manganese to form varied oxidation states (e.g., the redox reaction of Mn2+/Mn3+ or Mn3+/Mn4+) and with “oxygen mobility”’ in the oxide lattice.65,66 Second, as described in Figure S4 (Supporting Information), the addition of Pt NPs could have a notable influence on the redox properties of the manganese oxide nanostructures. It is supposed that Pt nanoparticles may enhance the reactivity of manganese oxides by modifying the rate of vacancy exchange between the bulk and the oxide surface and by facilitating the migration of oxygen vacancies from the bulk to the oxide surface.69 Meanwhile, the addition of Pt NPs would significantly promote the adsorption of HCHO on Pt/MnO2 catalysts, consequently enhancing their catalytic activities.6971 Third, the micropores and high total pore volume of nest-like MnO2 could contain more HCHO molecules to enhance their contact with Pt NPs and thus their decomposition on Pt/nestlike MnO2 at low temperatures. The synergetic effect between Pt NP and MnO2 nanostructure may also play an essential role in catalytic decomposition of HCHO. In fact, several previous works have reported the synergetic effect between noble metal NPs and supports. For example, Tang et al.40,41 demonstrated the synergistic effect between noble metal NPs and supports by investigating catalytic activities of Ag/MnOx-CeO2 and Pt/MnOx-CeO2 catalysts for decomposition of HCHO. Shen et al.46,47 recently showed that different morphologies of CeO2 supports dramatically influenced the catalytic activity of the Au/CeO2 catalyst for decomposition of HCHO and pointed out that the pore structures of CeO2 supports contributed to the enhancement of Au/CeO2 catalytic activity. In the current work, as discussed above (Figure S4 (Supporting Information), Figure 7, Figure 8, and Table 3), the synergistic effect between Pt nanoparticles and MnO2 nanostructures enhanced the catalytic activity of as-prepared catalysts. When a comparison is made on supported catalysts for decomposition of HCHO, it should be considered that the experimental conditions may deviate from each other concerning HCHO concentration, catalyst dosage, and detecting system. Therefore, fine activity comparison of different catalytic systems is usually difficult. Nevertheless, a rough comparison, including the temperature for complete decomposition of HCHO, is still feasible in the present study. Compared with the literature, the catalytic activities of the current 2 wt % Pt/MnO2 were greatly enhanced, with a 100% formaldehyde conversion at temperatures around 70 °C for 2 wt % Pt/nest-like MnO2, 80 °C for 2 wt % Pt/urchinlike MnO2, and 90 °C for 2 wt % Pt/cocoon-like MnO2, much lower than those reported previously.37,41,4447 The cycling stability is an important issue for practical applications of catalysts. Further investigation into this property is currently being carried out in our laboratory. The stability of Pt/nest-like MnO2 was preliminarily tested at 70 °C, at which HCHO was completely decomposed. The catalytic activity only slightly deceased from 100% to 94% after continuous catalytic reaction of 10 h (Figure S5, Supporting Information), pointing to a good performance stability.

Table 3. Catalytic Performance of Pt/MnO2 Catalysts at 20 °C and Temperature for Complete Conversion of HCHO MnO2

Pt loading

nanostructures

(wt%)

conversion at 20 °Ca

T100%b

cocoon-likeMnO2

0 2

0 24.3%

>200 °C 90 °C

urchin-like MnO2

0 2

nest-like MnO2

0 2

0 30.9% 0 41.6%

>200 °C 80 °C 200 °C 70 °C

Conversion of HCHO at 20 °C. b Temperature for complete decomposition of HCHO. a

channels of OMS-2. Thus, the channel size of OMS-2 is more suitable for HCHO adsorption and subsequent oxidation reaction than the other crystal form of MnO2 nanostructures. Moreover, it has been widely accepted that OMS-2 is a microporous material consisting of mixed valence manganese oxides, e.g., Mn2+, Mn3+, and Mn4+.6466 The redox reaction of Mn2+/Mn3+ or Mn3+/Mn4+ and ‘‘oxygen mobility’’ in the oxide lattice are expected to contribute to the catalytic activity and would be enhanced at high temperatures.67,68 This trend is clearly demonstrated in Figure 7 for three MnO2 nanostructures. The above results suggest that the crystal form, morphology, and pore size of MnO2 nanostructures are at least not less important than their BET surface area for catalytic decomposition of HCHO. The catalysts with 2 wt % of Pt on varied MnO2 nanostructures were evaluated in oxidative decomposition of HCHO, and the results are shown in Figure 8. The catalytic activity nearly follows the order: 2 wt % Pt/nest-like MnO2 > 2 wt % Pt/urchin-like MnO2 > 2 wt % Pt/cocoon-like MnO2. For 2 wt % Pt/urchin-like MnO2, the catalytic activity becomes fluctuant with an increase of temperature. As shown in Figure 8 (curve b), the catalytic activity of 2 wt % Pt/urchin-like MnO2 is lower at 40 °C and higher at 50 °C, respectively, than those of the other two catalysts. However, comparing the catalytic performances of Pt/MnO2 catalysts, 2 wt % Pt/nest-like MnO2 has the best catalytic activity. It is noted that the activities of the Pt/MnO2 catalysts are significantly higher than those of the corresponding MnO2 nanostructures, with the highest enhancement in activity being achieved over 2 wt % Pt/nest-like MnO2. The temperature at which the HCHO conversion reaches 100% is 70 °C for 2 wt % Pt/nest-like MnO2, and it is lower than those of Pt/cocoon-like MnO2 and Pt/urchin-like MnO2. The catalytic performance at 20 °C and the temperature for complete conversion of HCHO over each catalyst are also listed in Table 3 for further comparison of three Pt/MnO2 catalysts and the corresponding MnO2 nanostructures. Again, the structural features of MnO2 nanostructures are seen to have a significant influence on the activity of Pt/MnO2 catalysts. Pt/nest-like MnO2 (2 wt%) shows the highest catalytic activity among the three Pt/MnO2 catalysts: the conversion of HCHO at 20 °C reaches as high as 41.6%, and the temperature for complete decomposition of HCHO is as low as 70 °C. As discussed above (Figure 5c and f and Figure 4i), the average sizes of Pt NPs are 2.0, 2.0, and 1.9 nm for 2 wt % Pt/cocoonlike MnO2, 2 wt % Pt/urchin-like MnO2, and 2 wt % Pt/nest-like MnO2, respectively. According to eq 2, the NS/NT ratios of three Pt/MnO2 catalysts were calculated to be 53.5%, 53.5%, and 55.6%, respectively, which are indeed similar. Thus, the MnO2 nanostructures should have made an essential contribution to the

4. CONCLUSIONS In summary, Pt nanoparticles were deposited on three MnO2 nanostructures with varied morphologies (cocoon-, urchin-, 858



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and nest-like MnO2) by a facile method, which adopts ascorbic acid, PVP, sodium citrate as reducing agent, dispersant, and protective agent, respectively, in the formation of Pt nanoparticles. Pt nanoparticles of 14 nm were homogeneously dispersed on the surface of three MnO2 nanostructures, and no Pt nanoparticle agglomeration was observed in the Pt/MnO2 catalysts. The synthesized Pt/MnO2 showed higher catalytic activity than the corresponding MnO2 nanostructures for decomposition of HCHO. Varied Pt loadings resulted in varied Pt particles sizes and eventually in varied catalytic performances of the Pt/MnO2 catalysts. Small size Pt nanoparticles played a key role for enhancement of catalytic activity. Pt/nest-like MnO2 (2 wt %) had the highest catalytic activity among the Pt/MnO2 catalysts, and the temperature for complete decomposition of HCHO reached as low as 70 °C. The high catalytic activities of Pt/MnO2 catalysts resulted from the valence states, the MnO 2 porous structures, Pt nanoparticles, as well as the synergistic effect between the Pt nanoparticle and MnO2 nanostructure. These Pt/MnO2 catalysts are promising for indoor decomposition of formaldehyde due to their easy synthesis, low cost, and excellent catalytic performance.

(6) Terelaka, K.; Trybulaa, S.; Majchrzaka, M.; Ottb, M.; Hasse, H. Chem. Eng. Process. 2005, 44, 671. (7) Akbarzadeh, R.; Umbarkar, S. B.; Sonawane, R. S.; Takle, S.; Dongare, M. K. Appl. Catal. A: Gen. 2010, 374, 103. (8) Li, Y. X.; Jiang, Y.; Peng, S. Q.; Jiang, F. Y. J. Hazard. Mater. 2010, 182, 90. (9) Photong, S.; Boonamnuayvitaya, V. Appl. Surf. Sci. 2009, 255, 9311. (10) Fu, P. F.; Zhang, P. Y.; Li, a, J. Appl. Catal. B: Environ. 2011, 105, 220. (11) Sekine, Y. Atmos. Environ. 2002, 36, 5543. (12) Seiichiro, I.; Daisuke, U.; Kazunori, U.; Tomoyasu, I. Catal. Lett. 1994, 24, 377. (13) Tang, X. F.; Li, Y. G.; Huang, X. M.; Xu, Y. D.; Zhu, H. Q.; Wang, J. G.; Shen, W. J. Appl. Catal. B: Environ. 2006, 62, 265. (14) Zhou, L.; He, J. H.; Zhang, J.; He, Z. C.; Hu, Y. C.; Zhang, C. B.; He, H. J. Phys. Chem. C 2011, 115, 16873. (15) Torres, J. Q.; Giraudon, J. M.; Lamonier, J. F. Catal. Today 2011, 176, 277. (16) Reed, J.; Ceder, G. Chem. Rev. 2004, 104, 4513. (17) Gluhoi, A. C.; Bogdanchikova, N.; Nieuwenhuys, B. E. J. Catal. 2005, 229, 154. .; Cherkezova-Zheleva, Z.; Dimitrova, A.; (18) Popova, M.; Szegedi, A Mitov, I. Appl. Catal. A: Gen. 2010, 38, 26. (19) Wang, R. H.; Li, J. H. Environ. Sci. Technol. 2010, 44, 4282. (20) Sinha, A. K.; Suzuki, K.; Takahara, M.; Azuma, H.; Nonaka, T.; Suzuki, N.; Takahashi, N. J. Phys. Chem. C 2008, 112, 16028. (21) Feng, Q.; Kanohb, H.; Kenta, O. J. Mater. Chem. 1999, 9, 319. (22) Liu, D. W.; Garcia, B. B.; Zhang, Q. F.; Guo, Q.; Zhang, Y.;H.; Sepehri, S.; Cao, G. Z. Adv. Funct. Mater. 2009, 19, 1015. (23) Xu, C. L.; Zhao, Y. Q.; Yang, G. W.; Li, F. S.; Li, H. L. Chem. Commun. 2009, 48, 7575. (24) Zhu, H. T.; Luo, J.; Yang, H. X.; Liang, J. K.; Rao, G. H.; Li, J. B.; Du, Z. M. J. Phys. Chem. C 2008, 112, 17089. (25) Subramanian, V.; Zhu, H. W.; Vajtai, R.; Ajayan, P. M.; Wei, B. Q. J. Phys. Chem. B 2005, 109, 20207. (26) Strobel, P.; Thiery, F.; Darie, C.; Proux, O.; Ibarra-Palos, A.; Bacia, M.; Soupart, J. B. J. Mater. Chem. 2005, 15, 4799. (27) Jin, L.; Chen, C. H.; Crisostomo, V. M. B.; Xu, L. P.; Son, Y. C.; Suib, S. L. Appl. Catal. A: Gen. 2009, 355, 169. (28) Chen, H.; He, J.; Zhang, C.; He, H. J. Phys. Chem. C 2007, 111, 18033. (29) Zhu, J.; He, J. Acta Chim. Sin. 2010, 68, 961. (30) Zhou, L.; Hu, Y.; He, J. Acta Chim. Sin. 2011, 69, 666. (31) Luo, J.; Zhang, Q. H.; Huang, A. M.; Suib, S. L. Microporous Mesoporous Mater. 2000, 3536, 209. (32) Villegas, J. C.; Garce, L. J.; Gomez, S.; Durand, J. P.; Suib, S. L. Chem. Mater. 2005, 17, 1910. (33) Tian, H.; He, J. H.; Zhang, X. D.; Zhou, L.; Wang, D. H. Microporous Mesoporous Mater. 2011, 138, 118. (34) Tang, X. F.; Huang, X. M.; Shao, J. J.; Liu, J. L.; Li, Y. G.; Xu, Y. D.; Shen, W. J. Chin. J. Catal. 2006, 27, 97. (35) Chen, T.; Dou, H. Y.; Li, X. L.; Tang, X. F.; Li, J. H.; Hao, J. M. Microporous Mesoporous Mater. 2009, 122, 270. (36) Burgos, N.; Paulis, M.; Antxustegi, M. M.; Montes, M. Appl. Catal. B: Environ. 2002, 38, 251. lvarez-Galvan, M. C.; Pawelec, B.; de la Pe~ (37) A na O’Shea, V. A.; Fierro, J. L. G.; Arias, P. L. Catal. Commun. 2003, 4, 223. (38) Garcia, T.; Solsona, B.; Cazorla-Amoros, D.; Linares-Solano, A.; Taylor, S. H. Appl. Catal. B: Environ. 2006, 62, 66. (39) Navascues, N.; Escuin, M.; Rodas, Y.; Irusta, S.; Mallada, R.; Santamaría, J. Ind. Eng. Chem. Res. 2010, 49, 6941. (40) Liotta, L. F. Appl. Catal. B: Environ. 2010, 100, 403. lvarez-Galvan, M. C.; Pawelec, B.; de la Pe~ (41) A na O’Shea, V. A.; Fierro, J. L. G.; Arias, P. L. Appl. Catal. B: Environ. 2004, 5, 83. (42) Tang, X. F.; Chen, J. L.; Li, Y. G.; Li, Y.; Xu, Y. D.; Shen, W. J. Chem. Eng. J. 2006, 118, 119. (43) Tang, X. F.; Chen, J. L.; Huang, X. M.; Xu, Y. D.; Shen, W. J. Appl. Catal. B: Environ. 2008, 81, 115.

’ ASSOCIATED CONTENT

bS

Supporting Information. The pore size distributions of three MnO2 nanostructures, the average sizes of Pt nanoparticles under varied Pt loadings on three MnO2 nanostructures, photographs of filtration kit for separation of Pt/MnO2 nanostructured catalyst and evidence for complete loading of Pt on MnO2 support, illustration of an ideal Pt nanoparticle, and the schematic illustration of mechanism for Pt/MnO2 catalyzed decomposition of HCHO. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel./Fax: (+86) 10 82543535. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 20871118), the Knowledge Innovation Program of the Chinese Academy of Sciences (CAS) (Grant Nos. KSCX2-YW-G-059 and KGCX2-YW-111-5), “Hundred Talents Program” of CAS, and the National Basic Research Program of China (Grant No. 2010CB934103). It was also partially supported by Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry (TIPC), CAS. ’ REFERENCES (1) Salthammer, T.; Mentese, S.; Marutzky, R. Chem. Rev. 2010, 110, 2536. (2) Domingo-García, M.; Fernandez-Morales, I.; Lopez-Garzon, F. J.; Moreno-Castilla, C.; Perez-Mendoza, M. Langmuir 1999, 15, 3226. (3) Matsuo, Y.; Nishino, Y.; Fukutsuka, T.; Sugie, Y. Carbon 2008, 46, 1162. (4) Liang, W. J.; Li, J.; Li, J. X.; Zhu, T.; Jin, Y. Q. J. Hazard. Mater. 2010, 175, 1090. (5) Xu, Z. J.; Qin, N.; Wang, J. G.; Tong, H. Bioresour. Technol. 2010, 101, 6930. 859



ARTICLE

(44) Li, C. Y.; Shen, Y. N.; Jia, M. L.; Sheng, S. S.; Adebajo, M. O.; Zhu, H. Y. Catal. Commun. 2008, 9, 355. (45) Zhang, Y. B.; Shen, Y. N.; Yang, X. Z.; Sheng, S. S.; Wang, T. N.; Adebajo, M. F.; Zhu, H. Y. J. Mol. Catal. A: Chem. 2010, 316, 100. (46) Zhang, J.; Jin, Y.; Li, C. Y.; Shen, Y. N.; Han, L.; Hu, Z. X.; Di, X. W.; Liu, Z. L. Appl. Catal. B: Environ. 2009, 91, 11. (47) Shen, Y. N.; Yang, X. Z.; Wang, Y. Z.; Zhang, Y. B.; Zhu, H. Y.; Gao, L.; Jia, M. L. Appl. Catal. B: Environ. 2008, 79, 142. (48) Zhang, C. B.; He, H.; Tanaka, K. Catal. Commun. 2005, 6, 211. (49) Zhang, C. B.; He, H.; Tanaka, K. Appl. Catal. B: Environ. 2006, 65, 37. (50) Zhang, C. B.; He, H. Catal. Today 2007, 126, 345. (51) Wang, L. F.; Zhang, Q.; Sakurai, M.; Kameyama, H. Catal. Commun. 2007, 8, 2171. (52) Wang, L. F.; Sakurai, M.; Kameyama, H. J. Hazard. Mater. 2009, 167, 399. (53) Devaraj, S.; Munichandraiah, N. J. Phys. Chem. C 2008, 112, 4406. (54) Wang, R. H.; Li, J. H. Environ. Sci. Technol. 2010, 44, 4282. (55) Yu, X. Y.; He, J. H.; Hu, Y. C.; Tian, H.; Zhou, L. Acta Chim. Sin. 2011, 69, 2445. (56) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Piertti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (57) Liu, M. H.; Han, M. F.; William, W. Y. Environ. Sci. Technol. 2009, 43, 2519. (58) Borodko, Y.; Habas, S. E.; Koebel, M.; Yang, P. D.; Frei, H.; Somorjai, G. A. J. Phys. Chem. B 2006, 110, 23052. (59) Song, H.; Rioux, R. M.; Hoefelmeyer, J. D.; Komor, R.; Niesz, K.; Grass, M.; Yang, P. D.; Somorjai, G. A. J. Am. Chem. Soc. 2006, 128, 3027. (60) He, J.; Ichinose, I.; Kunitake, T.; Nakao, A.; Shiraishi, Y.; Toshima, N. J. Am. Chem. Soc. 2003, 125, 11034. (61) He, J.; Muto, E.; Kunitake, T.; Liu, S.; Zhang, S.; Liu, X. Chem. Phys. Lett. 2008, 454, 274. (62) Liu, H. L.; Liu, Y. L.; Li, Y. W.; Tang, Z. Y.; Jiang, H. F. J. Phys. Chem. C 2010, 114, 13362. (63) Wang, Z. M.; Tezuka, S.; Kanoh, H. Chem. Mater. 2001, 13, 530. (64) Chen, X.; Shen, Y. F.; Suib, S. L.; O’Young, C. L. Chem. Mater. 2002, 14, 940. (65) Opembe, N. N.; King’ondu, C. K.; Espinal, A. E.; Chen, C. H.; Nyutu, E. K.; Crisostomo, V. M.; Suib, S. L. J. Phys. Chem. C 2010, 114, 14417. (66) Iyer, A.; Galindo, H.; Sithambaram, S.; King’ondu, C.; Chen, C. H.; Suib, S. L. Appl. Catal. A: Gen. 2010, 375, 295. (67) Gandhe, A. R.; Rebello, J. S.; Figueiredo, J. L.; Fernandes, J. B. Appl. Catal. B: Environ. 2007, 72, 129. (68) Yu, L.; Sun, M.; Yu, J.; Yu, Q.; Hao, Z. F.; Li, C. S. Chin. J. Catal. 2008, 29, 1127. (69) Wang, L. C.; Liu, Q.; Huang, X. S.; Liu, Y. M.; Cao, Y.; Fan, K. N. Appl. Catal. B: Environ. 2009, 88, 204. (70) Hu, R. R.; Yan, C. F.; Xie, L. Y.; Cheng, Y.; Wang, D. Z. Int. J. Hydrogen Energy 2011, 36, 64. (71) Hu, R. R.; Xie, L. Y.; Ding, S.; Hou, J.; Cheng, Y.; Wang, D. Z. Catal. Today 2008, 131, 513.

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