MCM-41 catalysts

Colloids and Surfaces A: Physicochem. Eng. Aspects 379 (2011) 136–142

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

TPD and TPSR studies of formaldehyde adsorption and surface reaction activity over Ag/MCM-41 catalysts Dan Chen, Zhenping Qu ∗ , Weiwei Zhang, Xinyong Li, Qidong Zhao, Yong Shi Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, China

a r t i c l e

i n f o

Article history: Received 30 June 2010 Received in revised form 27 October 2010 Accepted 18 November 2010 Available online 8 December 2010 Keywords: Formaldehyde Ag/MCM-41 Catalytic oxidation TPD TPSR

a b s t r a c t The adsorption and surface reaction activity of formaldehyde were studied on the Ag/MCM-41 catalysts with different silver loadings by temperature-programmed desorption (TPD) and temperatureprogrammed surface reaction (TPSR) methods. It appeared that the silver loading had strong influence on the adsorption and surface reaction activity of HCHO. The addition of silver active species provided new adsorption site for the HCHO at low temperature, and its desorption temperature moved to lower temperature with the increase of silver loading to 8 wt%. With the further increase of silver loading, the desorption temperature of HCHO shifted to higher temperatures, which could be due to the aggregation of silver particles on the surface of the support. Moreover the quantity of adsorbed HCHO at higher silver loading (>8 wt%) changed inversely with the increase of silver content. TPSR experiments indicated that the surface reaction activity for HCHO oxidation was proportional with the adsorption performance of HCHO over Ag/MCM-41 catalysts with different silver loadings, and 8Ag/MCM-41 catalyst showed highest surface reaction activity for HCHO oxidation. Thus it is reasonable to suggest that an appropriate silver loading and particle dispersion would be essential to obtain high catalytic activity for HCHO oxidation at low temperatures. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Formaldehyde (HCHO) is a serious pollutant of the indoor air, which is often released in room decorating and refurbishing process, as well as in furniture production [1]. HCHO is known to cause nasal tumors, irritation of the mucous membranes of the eyes and respiratory tract, and skin irritation in the most volatile organic compounds (VOCs). The formaldehyde concentration in indoor air of civil residences, schools, hospitals and workshops is strictly regulated. Due to growing concern over the major presence of this pollutant inside buildings, the abatement of HCHO has significant practical interest at low temperature, especially at room temperature. Conversion of HCHO into H2 O and CO2 using heterogeneous catalysts has proven to be an efficient and practical technology for controlling HCHO emission for indoor air purification [2]. The success of this approach greatly relies on the properties of the catalyst. The desired catalyst should be able to work at lower temperatures and has higher catalytic activity [3]. Precious metal catalysts, such as Pt, Au-based catalysts [2,4–15], have been widely studied and show high catalytic activities for HCHO oxidation. Comparatively, much less information is currently

∗ Corresponding author. Tel.: +86 15542663636. E-mail addresses: [email protected], [email protected] (Z. Qu). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.11.062

available on the performance of Ag-based catalysts in the same reaction [16–18]. As a matter of fact, silver catalysts showed great catalytic activity even at lower temperature in many reactions, for example, hydrogenation of unsaturated aldehydes [19–22], partial oxidation of methanol to formaldehyde [23,24], and oxidative coupling of methane to ethane and ethylene [25], CO oxidation [26,27], NOx elimination [28–30] and so on. Although silver is a noble metal, its resource is more abundant compared with that of platinum, and thus its price is much lower. Therefore, silver catalysts have great potential to be the outstanding catalyst in catalytic elimination of the VOCs. Also, it seems that little attention has been devoted to the HCHO oxidation over mesoporous zeolite carrier. Zeolite with a hexagonal structured mesopore network (MCM-41 and SBA-15) has been extensively studied since the first report dealing with their synthesis [31,32]. Their high specific surface area, porous volume and adjustable pore size diameter make them ideal support for the preparation of highly dispersed heterogeneous catalysts [33]. Their remarkable high surface area favoring a high dispersion of the active component on the surface of the catalytic support as well as the improved accessibility of active sites in comparison with zeolites should have a favorable impact on catalytic activity [34,35]. Therefore, MCM-41 as one of the mesopore zeolite is one of the most attractive supports for metal or metal oxide nanoparticles, moreover it is also a superior material for the adsorption-catalysis dual functional system.

D. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 379 (2011) 136–142

(100)

Intensity(a.u.)

In view of the investigation of the formaldehyde catalytic oxidation, most attention was focused on the catalytic activity in the fixed bed at present, and many techniques (XRD, XPS, EXAFS, EPR and FTIR spectroscopy) have been employed for its characterization. However, temperature programmed techniques, such as temperature programmed desorption (TPD) and temperature programmed surface reaction (TPSR), have been less often used to study the characteristics of HCHO adsorption–desorption and surface reaction on the catalysts [12]. In the present study, MCM-41 with a mesoporous structure and large specific surface area was used as support, and silver was loaded onto the support by an incipient-wetness impregnation method. The adsorption properties and surface reaction activity of the formaldehyde at moderate temperatures over Ag/MCM-41 with different silver loadings were investigated by the TPD, TPSR techniques, and it was found that the silver loading and the metal dispersion on the surface of the support have a strong influence on the adsorption of HCHO and its surface reaction activity combining the XRD and TEM results.

137

(110) (200)

b a

1

2

3

4

5

6

2Theta(degree) Fig. 1. SAXRD patterns of MCM-41 (a) and Ag/MCM-41 (b).

2. Experimental 2.1. Materials The water used in the experiments was deionized with a Pureflow system. Inorganic and organic chemicals were of analytical grade. The mesoporous MCM-41 silica used in the experiments was obtained from Shanghai Zhuoyue Chemical Agent Co. (China). The AgNO3 was obtained from Tianjing Chemical Agent Co. (China). 2.2. Preparation of xAg/MCM-41 material The MCM-41 supported Ag catalysts were prepared by an incipient-wetness impregnation method. Series xAg/MCM-41 (x is the silver weight percent) catalysts were prepared by varying the amount of the AgNO3 . They were named according to the silver content. The wet samples obtained were dried at room temperature overnight and then dried at 100 ◦ C for about 24 h. The catalysts were sieved into 20–40 mesh powders and pretreated in flowing Ar/O2 (30 vol.% O2 , 50 ml/min) at 500 ◦ C for 2 h before testing. 2.3. Catalyst characterization Specific areas were computed from these isotherms by applying the Brunauer–Emmett–Teller (BET) method in Quantachrom quadrasorb SI. Before measurement, the samples were treated by degassing at 300 ◦ C for 4 h. The pore diameter distributions were calculated from desorption branches using the BJH (Barrett–Joyner–Halenda) methods. Powder X-ray diffraction (XRD) measurements of the catalysts were carried out on a Rigaku D/max-(b powder diffract meter using CuK␣ radiation ( = 0.1542 nm) and operating at 40 kV and 200 mA. The patterns were taken over the 2 range from 10◦ to 80◦ and a position-sensitive detector using a step size of 0.02◦ . The mean crystallite sizes were estimated using the Scherrer equation. A peak broadening due to the instrumental broadening of 2 = 38.0◦ was taken into account. UV–vis diffuse reflectance spectra were recorded in air on a SHIMADZU UV-2450 UV–vis spectrophotometer. Reference spectra were collected with pressed BaSO4 disks. The following parameters were used to collect data: 5.0 spectra band width, 0.5 nm data pitch, 800–190 nm measurement range, and 200 nm/min scanning speed. Transmission electron microscopy (Tecnai G2 F30 S-Twin) operated at 300 kV was used to study the morphology of catalyst samples. The sample was supported on a copper mesh for the TEM analysis.

2.4. Adsorption and surface reaction experiments The adsorption and surface reaction of HCHO were performed in a fixed catalytic reactor system at the middle of which 0.1 g catalyst (20–40 mesh) was packed. The reaction was performed at temperatures ranged from room temperature (RT) to 500 ◦ C. A thermocouple was placed in the middle of the catalyst bed for the temperature measurement. Gaseous HCHO was generated by flowing Ar over trioxymethylene (99.5%, Acros Organics) in an incubator kept in ice water mixture. A HCHO adsorption breakthrough curve was obtained for each run to ensure the saturation of the catalyst surface by the mass spectrum (Ametek, LC-D200M). The catalysts were purged with high purity argon for 1 h to fully remove physically adsorbed HCHO, and then the temperature was ramped at 10 ◦ C/min from RT to 200–500 ◦ C. The effluent from quartz reactor was analyzed by MS: HCHO (m/z = 30), O2 (m/z = 32), CO2 (m/z = 44), and H2 O (m/z = 18). The exhaust line from the reactor to the mass spectrometer was maintained at ∼120 ◦ C to prevent the condensation of the formaldehyde and reaction products. Temperature programmed desorption (TPD) of HCHO was carried out in a continuous flow of Argon, and the temperature programmed surface reaction (TPSR) experiments were conducted in flow of O2 /Ar (30 vol.% O2 ). 3. Results and discussion 3.1. Structure of the xAg/MCM-41 catalysts 3.1.1. XRD The SAXRD patterns recorded at low diffraction angles are presented in Fig. 1, which reveals the regular structure of the MCM-41 silica materials. For pure MCM-41 material one observed a prominent peak and two other reflection peaks which can be attributed to (1 0 0), (1 1 0) and (2 0 0) indices of hexagonal arrangement of pores. No obvious variation of the XRD peaks for Ag/MCM-41 catalyst was observed. That is, the addition of silver did not modify the hexagonal pore structure of MCM-41. High angle X-ray diffraction patterns (2 = 10–80◦ ) of the calcined Ag/MCM-41 catalysts are shown in Fig. 2. For all the samples, a broad peak at about 2 ≈ 22.0◦ is ascribed to the amorphous silica. The peaks around 38.0◦ , 44.3◦ , 64.4◦ and 77.6◦ , which are respectively corresponding to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of the structure of metallic silver are observed for Ag/MCM-41. No lines due to silver oxides were observed. As expected, the increase of silver loading lead to the gradual increase of the intensities of

D. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 379 (2011) 136–142

(220) (311) ∗ ∗ x=16 x=12 x=8 x=4

∗ 20

x=2 40

60

80

2Theta(degree) Fig. 2. XRD patterns of xAg/MCM-41 (x: the silver loading).

each peak. The half-width of the Ag (1 1 1) peak decreased with the increase of the silver loading, which indicated that the silver particle size increased with the silver loading. The size of about 2–15 nm calculated by the Scherrer equation from half-width of the Ag (1 1 1) from the XRD pattern was obtained. 3.1.2. N2 adsorption–desorption isotherms Fig. 3 shows the nitrogen adsorption–desorption isotherms (a) and the corresponding BJH pore size distribution (b) of the MCM41 material and silver catalysts (xAg/MCM-41). It showed type IV isotherms with a sharp increase of nitrogen uptake at the intermediate relative pressures, which was caused by capillary condensation of nitrogen inside the mesopores. When the silver was loaded, the hysteresis was enhanced, and the unclosed hysteresis loop at P/Po > 0.8 became obvious, indicating the existence of the interparticle pores [36]. It can also be obviously observed that the pore structure was transformed from diplopore to simple pore as shown in Fig. 3b. The diplopore was still retained when small amount (2 wt%) silver supported on MCM-41 silica. However, the pore was jammed with the continuous increasing of the silver loading. The observed changes of the specific surface area and pore volume as well as pore size are shown in Table 1. An increase in the BET surface and pore volume area at low silver loading (x = 2, 4) may be ascribed to the increasing mesoporosity. The formation of interparticle voids due to incorporated silver nanoparticles was responsible for the textural mesoporosity on the length scale of the silica particles [36] as shown in TEM (Fig. 4). Moreover, the BJH pore volume increased obviously. Thus it was suggested that the incorporated small amount Ag only occupied a very limited space and that almost all of the nanopore channels of the host silica remained open. This space is important to allow guest reactant molecules to diffuse into the host silica for some catalytic reaction which consistent with the HCHO-TPD results as shown in next Fig. 6 [37]. When the loading increased continually (from x = 8 to x = 16), the surface area fell sharply which might be the agglomeration of Ag nanoparticles. 3.1.3. TEM The morphology and silver particle size of Ag/MCM-41 catalysts after O2 treatment at 500 ◦ C for 2 h were studied by TEM, as shown in Fig. 4. Fig. 4a clearly shows that the mesopore channels were still retained when silver was loaded on the support. For all the samples, the silver particles were between 2 and 20 nm in diameter which was similar to the result obtained from XRD. The high dispersion of silver particles was observed on silver catalysts with low loading (≤8 wt%), however the sharp aggregation of silver particles formed

3.1.4. UV–vis The co-existence of different kinds of silver species can be discriminated by UV–vis spectroscopy as shown in Fig. 5. The DR spectrum of the MCM-41 support is also shown for comparison. These samples showed the similar bands, and the appearing and increasing in intensity in the major peaks at 220, 280, 410 nm were observed. The adsorption band at 220 nm appeared when the silver was loaded, which can be the presence of the isolated Ag ions electronic transition 4d10 → 4d9 5s1 [38,39]. The adsorption band of average intensity at 280 nm should be assigned to the presence of the Agı+ n clusters [40,41]. Moreover it was found that the intensity at 280 nm increased with the silver loading. However, XRD analysis did not identify any silver oxides. Thus it was reasonable to consider that the adsorption band at 280 nm should be the welldispersed silver oxides (below XRD detection limit) or the other mixed oxides which need further investigations. In addition, the broad bands centered at 353 nm and 410 nm are the characteristic absorbance of metallic silver particles, which may be several nanometers or much larger [39,42,43]. More metallic silver species have been known to form on the support with the silver loading (as evidenced in XRD patterns), thus which brought the increase of the band intensity at 410 nm. To further understand the effects of the silver loading and the structure on the HCHO adsorption and

a x=16 3 -1

(200) ∗

Intensity(a.u.)

larger ones on silver catalysts with high loading (>8 wt%), whose results are similar with that obtained in XRD patterns.

*Ag

Volumeadsorbed ( cm g )

(111) ∗

x=12 x=8 x=4 x=2 x=0

0.0

0.2

0.4

0.6

0.8

1.0

Relativepressure (p/p0)

b Poresizedistribution(cm3 g-1 nm-1 )

138

x=16

1.0

x=12 x=8 0.5

x=4 x=2 0.0

x=0 2

4

6

Porediameter (nm)

8

10

Fig. 3. Pore diameter distribution curves (a) and N2 adsorption–desorption isotherms (b) for xAg/MCM-41 catalysts.

D. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 379 (2011) 136–142

139

Table 1 BET, pore volume and pore diameter of the xAg/MCM-41. Sample

BET surface (m2 /g)

Pore volume (ml/g)

Pore diameter (nm)

Particle sizea (nm)

0Ag/MCM-41 2Ag/MCM-41 4Ag/MCM-41 8Ag/MCM-41 12Ag/MCM-41 16Ag/MCM-41

538.8 599.0 589.0 455.9 457.4 406.4

0.6391 0.7452 0.7302 0.5434 0.5242 0.4685

2.440 2.468 2.465 2.454 2.468 2.467

– 2.1 2.9 9.8 12.5 13.0

a

d = (k · )/(FW · cos ).

surface reaction activity over Ag/MCM-41 catalysts HCHO-TPD and TPSR experiments were carried out. 3.2. HCHO temperature programmed desorption (HCHO-TPD) Fig. 6 shows the desorption of HCHO (m/z = 30) profiles of the Ag/MCM-41 catalysts with different silver loadings in TPD test. The corresponding profile of MCM-41 was also shown for comparison purposes. All samples were pretreated with oxygen at 500 ◦ C before HCHO adsorption. In the case of MCM-41, small amounts of HCHO were desorbed at slightly high temperature (the desorption peak is higher than 100 ◦ C). Also, HCHO-TPD profiles in Fig. 6 showed that the desorbed amount of HCHO increased sharply when the silver contents were 2 wt% and 4 wt% compared with the MCM41, which could have some connection with their increasing pore volume and BET (Table 1). However, the specific surface areas fell evidently when the silver content was above 8 wt% as shown in Table 1, which resulted in the corresponding decreasing of the desorbed amount of HCHO. It was reasonable to think that the decrease

of the desorption amount was partly due to the aggregation of the silver particles and the reduction of SBET at high silver loading. As discussed above, the appropriated pore volume was important to allow guest reactant molecules to diffuse into the host silica in order to activate in the catalytic reaction [37]. The adsorbed reactant molecules could be located in silver nanoparticles in/on the silica, and this kind of adsorbed HCHO molecule could not be physical adsorption form as it desorbed only after heating. Thus the location and higher dispersion of silver nanoparticles enhanced the HCHO adsorption ability. 2Ag/MCM-41 and 4Ag/MCM-41 catalysts show the largest adsorption capacity for HCHO (Fig. 6), but the desorption temperature window was broader than that of the other catalysts, which should not be avail for the HCHO catalytic oxidation. Interestingly, a new low-temperature HCHO desorption peak appeared after silver was loaded on the MCM-41 silica in the HCHO-TPD profiles, as clearly shown in the curve-fitting results of Fig. 7. The summarized data for the curve-fitting procedure is shown in Table 2. From Fig. 7 and Table 2, the MCM-41 support had only two desorption

Fig. 4. TEM micrograph of the representative as-prepared 4Ag/MCM-41 (a); 4Ag/MCM-41 (b); 8Ag/MCM-41 (c); and 12Ag/MCM-41 (d).

140

D. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 379 (2011) 136–142 Table 2 The summarized data of the curve-fitting procedure in Fig. 7.

410

Absorbance (a.u.)

220 280

x=16

x=12 x=8 x=4 x=2 x=0 200

300

400

500

600

700

800

900

Wavelengh (nm) Fig. 5. UV–vis spectra of support and Ag/MCM-41 catalysts after O2 pretreatment.

x=2 x=4 x=0 x=8 x=16

50

100

150

200

250

300

Temperature (oC) Fig. 6. HCHO (m/z = 30) desorption profiles in TPD over xAg/MCM-41 catalysts (x: the silver loading).

x=0 x=2

×1/5

x=4

×1/5

x=8 x=12 x=16 0

50

100

150

200

250

300

Temperature (◦ C)

Temperature (◦ C)

Temperature (◦ C)

0Ag/MCM-41 2Ag/MCM-41 4Ag/MCM-41 8Ag/MCM-41 12Ag/MCM-41 16Ag/MCM-41

– 90 84 77 97 113

101 117 101 104 117 140

135 146 135 126 175 188

peaks (101, 135 ◦ C). However three desorption peaks exist on all Ag/MCM-41 catalysts. The loaded silver would provide new active adsorption site for HCHO adsorption for the Ag/MCM-41 catalysts. It could also be found from the fitting results that the new desorption peak for HCHO moved to lower temperature from 90 ◦ C to 77 ◦ C when the silver loading was changed from 2 wt% to 8 wt%. Moreover the desorption window for HCHO became narrow over 8 wt% Ag/MCM-41. When the silver loading further increased, the desorption temperature shifted to higher temperatures (from 77 ◦ C to 100 ◦ C), meanwhile its desorption amount sharply decreased. Large crystalline silver nanoparticles seem to lose the active adsorption sites for HCHO at low temperatures which need future investigation certainly. As we all know, the adsorption of reactants on catalysts is usually the first and essential step of a catalytic reaction, and the desorption temperature for reactant over catalysts should have strong influence on the catalytic activity of the catalysts. Usually, the lower desorption temperature, the higher catalytic activity at lower temperatures. The new adsorption sites for HCHO adsorption at low temperatures will promote the HCHO oxidation which will be approved by the next TPSR experiments. 3.3. Temperature programmed surface reaction (TPSR) for HCHO oxidation

x=12

0

Sample

×4 350

400

Temperature (oC) Fig. 7. Curve-fitting results of HCHO (m/z = 30) desorption profiles in TPD over xAg/MCM-41 catalysts.

The catalysts were still pretreated with oxygen at 500 ◦ C for 2 h before adsorption. After the saturated adsorption of HCHO/Ar mixture at room temperature, the catalyst was heated to 200 ◦ C at a heating rate of 10 ◦ C/min under 30 vol.% O2 in Ar and the effluent gases were continuously monitored with mass spectrometer as a function of temperature. The gas oxygen would react with the HCHO adsorbed on the silver active sites because no oxygen consumption (m/z = 32) was observed on MCM-41 silica (data not shown) even at high temperatures in TPSR experiment. That is, the active sites for HCHO surface reaction was not support sites but silver sites. It can be observed the obvious gas oxygen consumption (m/z = 32) on all Ag/MCM-41 catalysts and meanwhile large amounts of CO2 (m/z = 44) and H2 O (m/z = 18) were produced during TPSR experiments as shown in the inset of Fig. 8 (16 wt% Ag/MCM-41). The initial reaction temperature was used to represent the activity of HCHO oxidation for all catalysts in this work. Fig. 8 also compared the oxygen consumption profiles (m/z = 32) during TPSR experiments over xAg/MCM-41 catalysts. As shown in Fig. 8, the activity of HCHO surface reaction increased with the silver loading when the silver loading was less than 8 wt%. However the activity decreased with the sequential increasing of the silver loading. Thus the 8Ag/MCM-41 catalyst showed better surface reaction activity for HCHO oxidation displaying in the lowest initial O2 consumption temperature. As discussed above, we supposed that more low temperature adsorption sites for HCHO would be beneficial for the high HCHO catalytic activity. It has been known from Table 2 that the 8 wt% Ag/MCM-41 catalyst has the lowest temperature desorption peak (77 ◦ C) in HCHO-TPD profiles, and meanwhile the highest surface reaction activity was observed at 8 wt% Ag/MCM-41 catalyst (Fig. 8). Thus it is reasonable to be suggested the appropriate silver loading

D. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 379 (2011) 136–142

141

Doctoral Program of Higher Education (No. 200801411111), the Program for New Century Excellent Talents in University (NCET09-0256), the Program for Changjiang Scholars and Innovative Research Team in University (IRT0813) and the 123 Project of Liaoning environment Education Research supported by Geping Green finance for students. References

Fig. 8. O2 (m/z = 32) consumption profiles in TPSR over xAg/MCM-41 catalysts.

should be beneficial for the HCHO low-temperature catalytic oxidation. It has been well known that catalytic performances can be very sensitive to the size of the active phase [44], moreover we also found the different silver particle sizes formed over Ag/MCM-41 with different silver loadings. Higher dispersion of silver particles was observed on the surface of the support at lower silver loadings and lower dispersion of silver particles at higher silver loadings. The investigation about the effect of particle size of silver catalysts on the HCHO adsorption and catalytic activity is being processed now in our group. 4. Conclusions The silver supported mesoporous MCM-41 catalysts with a method of impregnation were found to exhibit superior adsorption performance and catalytic activity for the removal of formaldehyde from indoor air at moderate temperature. As indicated by XRD, metallic Ag formed after silver supported mesoporous MCM-41 catalysts were pretreated with O2 at 500 ◦ C 2 h and Agı+ n clusters existed observed from UV–vis. The size of the silver particles increased with the increasing of the silver loading. The silver loading showed a strong effect on the adsorption and surface catalytic activity for HCHO oxidation. The desorption temperature for HCHO adsorbed on silver active site moved to lower temperature with the increasing of the silver loading from 2 to 8 wt%. However the opposite order was observed with the further increase of the silver loading, meanwhile the desorption amount for HCHO sharply decreased, which should be due to the great aggregation of silver particles at high silver loadings. The new adsorption sites for HCHO adsorption at low temperatures due to the addition of silver species were suggest to promote the HCHO surface oxidation activity proved by TPSR experiments, and 8Ag/MCM-41 catalyst showed highest surface reaction activity for HCHO oxidation. Thus an appropriate silver loading and particle dispersion will be essential to obtain high catalytic activity for HCHO oxidation at low temperatures. Acknowledgements This work was supported financially by the National Nature Science Foundation of China (No. 20807010), the National High Technology Research and Development Program of China (863 Program) (No. 2009AA062604), the Specialized Research Fund for the

[1] J.J. Spivey, Complete catalytic oxidation of volatile organics, Ind. Eng. Chem. Res. 26 (1987) 2165–2180. [2] L. Wang, M. Sakurai, H. Kameyama, Study of catalytic decomposition of formaldehyde on Pt/TiO2 alumite catalyst at ambient temperature, J. Hazard. Mater. 167 (2009) 399–405. [3] Y. Shen, X. Yang, Y. Wang, Y. Zhang, H. Zhu, L. Gao, M. Jia, The states of gold species in CeO2 supported gold catalyst for formaldehyde oxidation, Appl. Catal. B: Environ. 79 (2008) 142–148. [4] C. Zhang, H. He, K.-I. Tanaka, Catalytic performance and mechanism of a Pt/TiO2 catalyst for the oxidation of formaldehyde at room temperature, Appl. Catal. B: Environ. 65 (2006) 37–43. [5] C. Li, Y. Shen, M. Jia, S. Sheng, M.O. Adebajo, H. Zhu, Catalytic combustion of formaldehyde on gold/iron-oxide catalysts, Catal. Commun. 9 (2008) 355–361. [6] C.B. Zhang, H. He, K. Tanaka, Perfect catalytic oxidation of formaldehyde over a Pt/TiO2 catalyst at room temperature, Catal. Commun. 6 (2005) 211–214. [7] X.L. Tang, B.C. Zhang, Y. Li, Y.D. Xu, Q. Xin, W.J. Shen, Structural features and catalytic properties of Pt/CeO2 catalysts prepared by modified reduction–deposition techniques, Catal. Lett. 97 (2004) 163–169. [8] C. Zhang, H. He, A comparative study of TiO2 supported noble metal catalysts for the oxidation of formaldehyde at room temperature, Catal. Today 126 (2007) 345–350. [9] X.F. Tang, J.L. Chen, X.M. Huang, Y. Xu, W.J. Shen, Pt/MnOx –CeO2 catalysts for the complete oxidation of formaldehyde at ambient temperature, Appl. Catal. B: Environ. 81 (2008) 115–121. [10] G. Zhao, Y. Tang, R. Chen, R. Geng, D. Li, Potential and current oscillations during formaldehyde oxidation on platinum particles dispersed in three-dimensional pore networks of TiOx /Ti, Electrochim. Acta 53 (2008) 5186–5194. [11] J.X. Peng, S.D. Wang, Correlation between microstructure and performance of Pt/TiO2 catalysts for formaldehyde catalytic oxidation at ambient temperature: effects of hydrogen pretreatment, J. Phys. Chem. C 111 (2007) 9897–9904. [12] Y.B. Zhang, Y.N. Shen, X.Z. Yang, S.S. Sheng, T. Wang, M.F. Adebajo, H.Y. Zhu, Gold catalysts supported on the mesoporous nanoparticles composited of zirconia and silicate for oxidation of formaldehyde, J. Mol. Catal. A: Chem. 316 (2010) 100–105. [13] J.X. Peng, S.D. Wang, Performance and characterization of supported metal catalysts for complete oxidation of formaldehyde at low temperatures, Appl. Catal. B: Environ. 73 (2007) 282–291. [14] T. Kecskes, J. Rasko, J. Kiss, FTIR and mass spectrometric studies on the interaction of formaldehyde with TiO2 supported Pt and Au catalysts, Appl. Catal. A: Gen. 273 (2004) 55–62. [15] J. Rasko, T. Kecskes, J. Kiss, Formaldehyde formation in the interaction of HCOOH with Pt supported on TiO2 , J. Catal. 224 (2004) 261–268. [16] C.F. Mao, M.A. Vannice, Formaldehyde oxidation over Ag catalysts, J. Catal. 154 (1995) 230–244. [17] X. Tang, J. Chen, Y. Li, Y. Li, Y. Xu, W. Shen, Complete oxidation of formaldehyde over Ag/MnOx –CeO2 catalysts, Chem. Eng. J. 118 (2006) 119–125. [18] J. Geng, Y. Bi, G. Lu, Morphology-dependent activity of silver nanostructures towards the electro-oxidation of formaldehyde, Electrochem. Commun. 11 (2009) 1255–1258. [19] A.K. Datye, D.J. Smith, The study of heterogeneous catalysts by high-resolution transmission electron-microscopy, Catal. Rev. Sci. Eng. 34 (1992) 129–178. [20] Y. Cao, W.L. Dai, J.F. Deng, The oxidative dehydrogenation of methanol over a novel Ag/SiO2 catalyst, Appl. Catal. A: Gen. 158 (1997) L27–L34. [21] Q. Liu, Y. Cao, W.L. Dai, J.F. Deng, The oxidative dehydrogenation of methanol over a novel low-loading Ag/SiO2 –TiO2 catalyst, Catal. Lett. 55 (1998) 87–91. [22] K.H. Lim, A.B. Mohammad, I.V. Yudanov, K.M. Neyman, M. Bron, P. Claus, N. Rosch, Mechanism of selective hydrogenation of alpha, beta-unsaturated aldehydes on silver catalysts: a density functional study, J. Phys. Chem. C 113 (2009) 13231–13240. [23] X. Bao, M. Muhler, B. Pettinger, R. Schlogl, G. Ertl, On the nature of the active state of silver during catalytic-oxidation of methanol, Catal. Lett. 22 (1993) 215–225. [24] W.L. Dai, C. Yong, L.P. Ren, X.L. Yang, J.H. Xu, H.X. Li, H.Y. He, K.N. Fan, Ag–SiO2 –Al2 O3 composite as highly active catalyst for the formation of formaldehyde from the partial oxidation of methanol, J. Catal. 228 (2004) 80–91. [25] X. Bao, M. Muhler, R. Schlogl, G. Ertl, Oxidative coupling of methane on silver catalysts, Catal. Lett. 32 (1995) 185–194. [26] Z.P. Qu, M.J. Cheng, W.X. Huang, X.H. Bao, Formation of subsurface oxygen species and its high activity toward CO oxidation over silver catalysts, J. Catal. 229 (2005) 446–458. [27] D. Tian, G.P. Yong, Y. Dai, X.Y. Yan, S.M. Liu, CO oxidation catalyzed by Ag/SBA15 catalysts prepared via in situ reduction: the influence of reducing agents, Catal. Lett. 130 (2009) 211–216.

142

D. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 379 (2011) 136–142

[28] A. Musi, P. Massiani, D. Brouri, J.M. Trichard, P.D. Costa, On the characterisation of silver species for SCR of NOx with ethanol, Catal. Lett. 128 (2009) 25–30. [29] J. Shibata, K.-I. Shimizu, Y. Takada, A. Shichi, H. Yoshida, S. Satokawa, A. Satsuma, T. Hattori, Structure of active Ag clusters in Ag zeolites for SCR of NO by propane in the presence of hydrogen, J. Catal. 227 (2004) 367–374. [30] M. Boutros, J.M. Trichard, P. Da Costa, Silver supported mesoporous SBA-15 as potential catalysts for SCR NOx by ethanol, Appl. Catal. B: Environ. 91 (2009) 640–648. [31] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, A new family of mesoporous molecular sieves prepared with liquid crystal templates, J. Am. Chem. Soc. 114 (1992) 10834–10843. [32] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores, Science 279 (1998) 548–552. [33] D.E. De Vos, M. Dams, B.F. Sels, P.A. Jacobs, Ordered mesoporous and microporous molecular sieves functionalized with transition metal complexes as catalysts for selective organic transformations, Chem. Rev. 102 (2002) 3615–3640. [34] A. Corma, From microporous to mesoporous molecular sieve materials and their use in catalysis, Chem. Rev. 97 (1997) 2373–2420. [35] A. Sayari, Catalysis by crystalline mesoporous molecular sieves, Chem. Mater. 8 (1996) 1840–1852.

[36] H.Y. Liu, D. Ma, R.A. Blackley, W.Z. Zhou, X.H. Bao, Highly active mesostructured silica hosted silver catalysts for CO oxidation using the one-pot synthesis approach, Chem. Commun. 23 (2008) 2677–2679. [37] L. Li, J.L. Shi, L.X. Zhang, L.M. Xiong, J.N. Yan, A novel and simple in-situ reduction route for the synthesis of an ultra-thin metal nanocoating in the channels of mesoporous silica materials, Adv. Mater. 16 (2004) 1079–1082. [38] Z. Li, A. Chou, F.-M. Tao, Ab initio study of the intermolecular potential surface of He–NH3 , Chem. Phys. Lett. 313 (1999) 313–320. [39] A. Keshavaraja, X. She, M. Flytzani-Stephanopoulos, Selective catalytic reduction of NO with methane over Ag–alumina catalysts, Appl. Catal. B: Environ. 27 (2000) L1–L9. [40] K.A. Bethke, H.H. Kung, Supported Ag catalysts for the lean reduction of NO with C3 H6 , J. Catal. 172 (1997) 93–102. [41] A.N. Pestryakov, A.A. Davydov, Active electronic states of silver catalysts for methanol selective oxidation, Appl. Catal. A: Gen. 120 (1994) 7–15. [42] T. Ung, L.M. Liz-Marzán, P. Mulvaney, Controlled method for silica coating of silver colloids. Influence of coating on the rate of chemical reactions, Langmuir 14 (1998) 3740–3748. [43] L. Kundakovic, M. Flytzani-Stephanopoulos, Deep oxidation of methane over zirconia supported Ag catalysts, Appl. Catal. A: Gen. 183 (1999) 35–51. [44] A.T. Bell, The impact of nanoscience on heterogeneous catalysis, Science 299 (2003) 1688–1691.