DOI: 10.1002/chem.201404307

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& Heterogeneous Catalysis

Silver Nanoparticles Supported on CeO2-SBA-15 by Microwave Irradiation Possess Metal–Support Interactions and Enhanced Catalytic Activity Xufang Qian,[a] Yasutaka Kuwahara,[a, b] Kohsuke Mori,[a, b] and Hiromi Yamashita*[a, b]

Chem. Eur. J. 2014, 20, 15746 – 15752


 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper Abstract: Metal–support interactions (MSIs) and particle size play important roles in catalytic reactions. For the first time, silver nanoparticles supported on CeO2-SBA-15 supports are reported that possess tunable particle size and MSIs, as prepared by microwave (MW) irradiation, owing to strong charge polarization of CeO2 clusters (i.e., MW absorption). Characterizations, including TEM, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure, were carried out to disclose the influence of CeO2 contents on the Ag particle size, MSI effect between Ag nanoparticles and CeO2-SBA-15 supports, and the strong MW absorption of

Introduction Particle size and metal–support interactions (MSIs) in supported nanoparticles have attracted much attention in heterogeneous catalysis.[1–4] Gold nanocatalysts loaded on metal oxide supports with MSIs have been the most widely studied case in fine-chemical synthesis, photocatalytic hydrogen production, environmental catalysis, and many organic synthesis reactions.[5–8] Silver, which is another d10 metal atom, is less expensive and a commercial catalyst for ethylene oxide and formaldehyde production.[9] In comparison with gold catalysis, the precise control of supported silver nanoparticles and related catalysis has attracted less interest.[10–14] Recently, some efforts regarding the synthesis of supported silver nanocatalysts and developing the catalytic properties in green organic synthesis have been made to break a major barrier of high cost and short supply for platinum-group, metal-based catalysts.[13, 15–20] Kaneda et al. reported that hydrotalcite- and hydroxyapatitesupported silver nanoparticles (7.6 nm) displayed effective catalytic activity for green organic synthesis, namely, oxidation of silane and deoxygenation of epoxides into alkenes.[15] Zhang et al. synthesized monodispersed silver nanoparticles (4 nm) through ligand-protected reduction and those supported on carbon materials, which directly catalyzed benzyl alcohols to form styryl ethers.[18] Fuku et al. synthesized different silver nanostructures, including nanoparticles, nanorods within the mesopores of SBA-15 by microwave (MW) irradiation, and compared the localized surface plasmon resonance (LSPR) effect for ammonia borane (AB) dehydrogenation under visible-light irradiation.[19] However, silver nanocatalysts with precise size control and MSIs have rarely been reported, to date.

[a] Dr. X. Qian, Prof. Dr. Y. Kuwahara, Prof. Dr. K. Mori, Prof. Dr. H. Yamashita Division of Materials and Manufacturing Science Graduate School of Engineering, Osaka University 2-1 Yamadaoka, Suita, Osaka, 565-0871 (Japan) E-mail: [email protected] [b] Prof. Dr. Y. Kuwahara, Prof. Dr. K. Mori, Prof. Dr. H. Yamashita Unit of Elements Strategy Initiative for Catalysts & Batteries Kyoto University, Kyoto 606-8501 (Japan) Supporting information for this article is available on the WWW under Chem. Eur. J. 2014, 20, 15746 – 15752

CeO2 clusters that contribute to the MSIs during Ag deposition. The Ag particle sizes were controllably tuned from 1.9 to 3.9 nm by changing the loading amounts of CeO2 from 0.5 to 2.0 wt %. The Ag nanoparticle size was predominantly responsible for the high turnover frequency (TOF) of 0.41 min1 in ammonia borane dehydrogenation, whereas both particle size and MSIs contributed to the high TOF of 555 min1 in 4-nitrophenol reduction for Ag/0.5CeO2-SBA-15, which were twice as large as those of Ag/SBA-15 without CeO2 and Ag/CeO2-SBA-15 prepared by conventional oilbath heating.

Herein, we report, for the first time, that Ag nanoparticles supported on CeO2-SBA-15 prepared by a facile and efficient MW-assisted reduction process possess tunable particle size (1.9–3.9 nm) and MSIs. The deposition of Ag nanoparticles was carried out by using 1-hexanol as a solvent and reductant, laurate groups as protective ligands, and CeO2-SBA-15 with 0.5– 2.0 wt % of CeO2 as the supports under MW irradiation. The MSIs only appear in Ag nanoparticle supported samples prepared with MW irradiation and CeO2 clusters, which are attributed to the high charge polarization (i.e., MW absorption) of CeO2 in the MW irradiation process. AB dehydrogenation and 4-nitrophenol (4-NP) reduction by using AB as the H2 source demonstrated that particle size and MSI effects were highly responsible for the enhanced catalytic activity.

Results and Discussion After silver nanoparticle deposition, all samples retain the characteristic type IV curves with sharp capillary condensation steps and H1-type hysteresis loops in the relative pressure (p/ p0) range of 0.6–0.8, which are typical features of highly ordered and well-reserved cylinder-type mesopores (Figure 1 A). The corresponding narrow PSDs calculated by the Barrett– Joyner–Halenda (BJH) method are around 7.0 nm (Figure 1 B). The textural properties of Ag/SBA-15 and Ag/xCeO2-SBA-15 (x = 0.5–2.0) are listed in Table 1. The specific BET surface area

Figure 1. N2 sorption isotherms (A) and pore size distributions (PSDs; B) of Ag/SBA-15 (a), Ag/0.5CeO2-SBA-15 (b), Ag/1CeO2-SBA-15 (c), and Ag/2CeO2SBA-15 (d) prepared by a 1-hexanol reduction method assisted by MW heating in the presence of sodium laurate, and Ag/0.5CeO2-SBA-15(o) (e) prepared by a conventional oil-bath heating method. The isotherms for b)–e) were offset vertically by 120, 240, 360, and 480 cm3 g1, respectively.


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Full Paper state Ag species. However, low shifts of 0.2, 0.2, and 0.4 eV are observed for Ag/xCeO2-SBA-15 Sample[a] Ag[b] Particle size[c] SBET[d] Vt[e] DBJH[f] TOF [min1] supports with 0.5, 1.0, and [wt %] [nm] [m2 g1] [cm3 g1] [nm] AB dehydrogenation 4-NP reduction 2.0 wt % of CeO2, respectively; SBA-15 – – 783 1.1 7.0 – – this indicates the presence of Ag/SBA-15 0.9 2.1 633 0.9 7.0 0.22 319 positively charged Ag species 0.8 1.9 621 0.9 7.0 0.41 555 Ag/0.5CeO2SBA-15 (Agnd + ).[21] Possible assumptions 0.9 2.7 649 1.0 7.0 0.22 328 Ag/1CeO2SBA-15 with regard to the low shift for 1.0 3.9 657 1.0 7.3 0.08 229 Ag/2CeO2SBA-15 3.5 617 1.0 7.2 0.18 233 Ag/0.5CeO2SBA-15(o) 0.9 the binding energy of Ag/xCeO21.0 – – – – 0.20 – Ag/SBA-15(ref.)[g] SBA-15 prepared under MW irra[a] Ag/SBA-15 and Ag/xCeO2-SBA-15 catalysts prepared by MW heating in the presence of sodium laurate, and diation are as follows: 1) the forAg/0.5CeO2-SBA-15(o) prepared by oil-bath heating. [b] Ag loading amount measured by ICP. [c] Average partimation of Ag oxide species due cle size of Ag nanoparticles calculated by measuring more than 100 nanoparticles on TEM images. [d] SBET, BET to the high reactivity of Ag surface areas calculated by the adsorption branch of the N2 isotherm. [e] Vt, total pore volume. [f] DBJH, pore dinanoparticles exposed to air; ameter determined by using the BJH model. [g] Data from ref. [19]. and 2) Agnd + species stabilized through interactions with CeO2 (SBET) and total pore volume (Vt) of the catalysts are calculated clusters (i.e., MSI effect) under MW irradiation. Generally, an oxto be 617–657 m2 g1 and 0.9–1.0 cm3 g1, respectively. In comidation state forms more easily for bare and ultrafine Ag nanoparison with pristine SBA-15, the SBET value of silver-deposited particles due to high reactivity with O2, whereas a larger low samples is smaller due to the incorporation of silver nanopartishift (0.4 eV), that is, higher valance state for Ag nanoparticles cles, whereas the pore size remains unchanged; this indicates (4.4 nm) on 2CeO2-SBA-15, relative to the 0.2 eV low shift of Ag nanoparticles (1.9 nm) on 0.5CeO2-SBA-15. This phenomenon that the mesopores are not clogged. reflects that CeO2 clusters with an oxygen vacancy in the TEM images and the particle size distributions of Ag nanomesoporous support probably get electrons from the Ag nanoparticles supported on pristine SBA-15 and CeO2-SBA-15 by MW heating and oil-bath heating are provided in Figure 2. particles, which results in the presence of surface Agnd + on Ag Highly dispersed Ag nanoparticles with a mean particle size of nanoparticles synthesized under MW irradiation. Interestingly, 2.1 nm are deposited in the mesoporous channels of pristine the phenomenon of binding energy shift for Ag 3 d5/2 rarely SBA-15 without aggregation (Figure 2 A). Silver nanoparticles appears on Ag/0.5CeO2-SBA-15(o) synthesized by oil-bath heatconfined in the mesopores can be clearly seen in the enlarged ing. The above results suggest that both the CeO2 clusters and TEM image (Figure 2 B). Through the modification of CeO2 MW irradiation contribute to MSIs and tunable particle sizes from 1.9 to 3.9 nm. To further clarify the electronic structure of (0.5 wt %) on pristine SBA-15, ultrafine Ag nanoparticles with Ag, XAFS spectra of Ag were measured (Figure 3 B). Ag foil and particle sizes of 1.9 nm can be observed over a large domain, AgO show clearly different K-edge XANES spectra that correwhich are well-confined within the mesoporous frameworks spond to different electronic structures of Ag.[22] Ag/SBA-15, (Figure 2 C and D). The Ag particle sizes for samples Ag/1CeO2SBA-15 and Ag/2CeO2-SBA-15 increase to 2.7 and 3.9 nm, reAg/xCeO2-SBA-15 (x = 0.5, 1, and 2), and Ag/0.5CeO2-SBA-15(o) spectively, as the contents of CeO2 increase to 1 and 2 wt %, reshow very similar XANES spectra to that of Ag foil, which respectively (Figure 2 E and F). The Ag nanoparticle dispersions flects the metallic state of Ag nanoparticles; this is consistent on these two supports are as high as those on 0.5CeO2-SBAwith the XPS results. The FT-EXAFS spectra show a peak at ap15. Additionally, a slight color change from bright yellow to proximately 2.7  that is assigned to the AgAg bond for all of deep yellow appears, along with variation in the CeO2 content the supported Ag nanoparticles, whereas the peak assigned to from 0 to 2 wt %; this change should be caused by the variathe AgO bond at around 1.7  appears only for AgO (Figtion in sizes of the Ag nanoparticles (Figure S1 in the Supporture 3 C). ing Information). Ag nanoparticles with an average particle Herein, the size of the Ag nanoparticles increases as amount size of 3.5 nm and decreased dispersion were observed for Ag/ of CeO2 increases from 0.5 to 2.0 wt %. Moreover, MSIs were 0.5CeO2-SBA-15(o), which was prepared by conventional oilonly found for Ag nanoparticles deposited on CeO2-SBA-15 by bath heating (Figure 2 G and H). UV/Vis diffuse reflectance MW heating (5 min, maximum temperature (Tmax)  135 8C) spectroscopy showed a typical absorbance at l  400 nm rather than by conventional oil-bath heating (5 min, constant owing to the surface plasmon resonance property of Ag nanotemperature 135 8C). To this end, the oxidation state of Ce was particles supported in mesopores (Figure S2 in the Supporting measured by Ce LIII-edge XANES before and after MW irradiaInformation).[19] tion (Figure 4). Two main peaks at 5737.0 and 5730.2 eV appear for commercial CeO2 ; these correspond to 2 p! The Ag 3 d X-ray photoelectron spectroscopy (XPS) results for all samples a show symmetric full-width at half-maximum 4 f15 d1L and 2 p!4 f05 d1 transitions of Ce4 + , respectively. Only (FWHM; Figure 3 A). The Ag 3 d5/2 and 3 d3/2 binding energies of one peak at 5726.0 eV is observed for an aqueous solution CeCl3 (10 mm), which is included as a reference; this is asAg/SBA-15 prepared under MW irradiation and Ag/ 0.5CeO2SBA-15(o) heated in an oil bath are 368.0 and 374.0 eV, signed to the 2 p!4 f15 d1 transition of Ce3 + (Figure 4). Two respectively; these results indicate predominantly metallic peaks similar to that of CeO2 are observed in the XANES region Table 1. Textural properties of pristine SBA-15 and Ag/xCeO2-SBA-15 with different Ag particle sizes and loading amounts, and the turnover frequencies (TOFs) of AB dehydrogenation and 4-NP reduction.

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Figure 3. Ag 3 d XPS results (A), and Ag K-edge X-ray absorption near-edge spectroscopy (XANES; B) and Fourier transform extended X-ray absorption fine structure (FT-EXAFS; C) spectra of Ag/SBA-15 (a), Ag/0.5CeO2-SBA-15 (b), Ag/1CeO2-SBA-15 (c), Ag/2CeO2SBA-15 (d), and Ag/0.5CeO2-SBA-15(o) (e). AgO and Ag foil reference samples are also given in B) and C).

Figure 2. TEM images of Ag/SBA-15 (A, B), Ag/0.5CeO2-SBA-15 (C, D), Ag/ 1CeO2-SBA-15 (E), Ag/2CeO2-SBA-15 (F), and Ag/0.5CeO2-SBA-15(o) (G, H). Insets in A), C), and E)–G): corresponding particle size distributions.

for 0.5CeO2-SBA-15, fresh and used Ag/0.5CeO2-SBA-15, and fresh Ag/0.5CeO2-SBA-15(o), whereas the peak corresponding to the 2 p!4 f05 d1 transition becomes broad and also shifts to lower energy, which indicates the formation of more Ce3 + species relative to CeO2 (Figure 4 a–d).[23] Additionally, a highenergy shift of the pre-edge in the enlarged region of 5722– 5729 eV reflects that Ce3 + species in 0.5CeO2-SBA-15 transform into Ce4 + after Ag deposition under MW irradiation for 5 min,[24] whereas the oxidation state of Ce remains unchanged for Ag/0.5CeO2-SBA-15(o) under oil-bath heating (Figure 4, inset). The above results reveal that the possible driving force is MSI formation for Ag/CeO2-SBA-15 prepared under MW irraChem. Eur. J. 2014, 20, 15746 – 15752

Figure 4. Normalized Ce LIII-edge XANES spectra of reference samples CeO2 and CeCl3, 0.5CeO2-SBA-15 (a), fresh Ag/0.5CeO2-SBA-15 (b), Ag/0.5CeO2-SBA15 used in the AB dehydrogenation reaction (c), and Ag/0.5CeO2-SBA-15(o) (d). Inset: an enlarged view of the 5722–5728 eV region for all of the spectra.

diation (Tmax = 135 8C). In the presence of laurate groups, positively charged Ag + were protected by the ligand and diffused within the mesoporous channels, and then uniform Ag nanoparticles were reduced and deposited on the pristine silica (SBA-15) surface with silanol groups as a solution in 1-hexanol (Scheme 1A). In the case of CeO2-SBA-15, the deposition of Ag nanoparticles preferably occurred on CeO2 clusters due to ion defects (Ce3 + species), which play an important role in charge


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Figure 5. A) Initial H2 production from AB dehydrogenation and B) plot of time versus ln (C/C0,4-NP) for 4-NP reduction with different catalysts: Ag/SBA15 (a), Ag/0.5CeO2-SBA-15 (b), Ag/1CeO2-SBA-15 (c), Ag/2CeO2-SBA-15 (d), and Ag/0.5CeO2-SBA-15(o) (e).

Scheme 1. The deposition of Ag nanoparticles on pristine SBA-15 and xCeO2-SBA-15 supports with 0.5, 1.0, and 2.0 wt % CeO2 by a MW heating process.

polarization during the MW absorption process.[25] This probably results in high local thermal energy around CeO2 clusters, and thus, ligand-protected Ag + can be reduced and deposited on CeO2 clusters (Scheme 1B–D). Therefore, the particle sizes were precisely tuned solely by changing the amount of CeO2, and MSIs were also formed in an efficient (5 min) MW irradiation process at a low temperature (Tmax = 135 8C). CeO2 clusters were inappropriate for the uniform dispersion of Ag nanoparticles and MSI formation in the case of oil-bath heating (Scheme 1E). The effects of particle size and MSIs were evaluated by means of two model reactions, namely, AB dehydrogenation and 4-NP reduction. AB contains 19.6 wt % of hydrogen, which exceeds that of gasoline, and has attracted much attention as a promising chemical hydrogen-storage material.[19, 26–29] H2 production from aqueous AB under ambient conditions in Ar (20 8C) was carried out to evaluate the catalytic activity of different supported Ag nanoparticles (Figure S3 in the Supporting Information). Almost negligible H2 production was monitored by gas chromatography for 0.5CeO2-SBA-15 without Ag nanoparticles; this indicates the stability of AB in aqueous solution and that the Ag nanoparticles are the active component. Ag/ 0.5CeO2-SBA-15, with a particle size of 1.9 nm, is the most efficient catalyst and H2 production can be completed within 50 min (Figure S3b in the Supporting Information). The time course of initial H2 production under ambient conditions shows that the rate of H2 production of Ag/0.5CeO2-SBA-15 (3.3 mol % min1) is nearly two times faster than that of Ag/ SBA-15 (1.9 mol % min1) and more than two times that of Ag/ 0.5CeO2-SBA-15(o) (1.5 mol % min1; Figure 5). The TOF value of Ag/0.5CeO2-SBA-15 (0.41 min1) is nearly 2 times larger than those of Ag/SBA-15 (0.22 min1) and our previous result of Ag/ SBA-15 (0.20 min1),[19] and the order follows Ag/0.5CeO2-SBA15 (0.41 min1) > Ag/SBA-15 = Ag/1CeO2-SBA-15 (0.22 min1) > Chem. Eur. J. 2014, 20, 15746 – 15752

(0.18 min1) > Ag/2CeO2-SBA-15 Ag/0.5CeO2-SBA-15(o) 1 (0.08 min ), which shows good agreement with the particles size variation of Ag (Table 1). Additionally, all of the catalysts showed negligible variation in textural properties (Table 1), which could be ignored in correlation with catalytic activity. The above phenomenon suggests that the hydrolysis of AB occurs on the surface of the Ag nanoparticles, in accordance with the fact that smaller Ag nanoparticles possesses a larger number of surface atoms as well as more low-coordinated metal sites, that is, vertices and edges. In addition, a recycling test shows that the initial H2 production rate of Ag/0.5CeO2SBA-15 retains 91 % of its activity after the third run, which is higher than that of Ag/0.5CeO2-SBA-15(o) (71 %; Figure S4 in the Supporting Information). The weak interactions between Ag nanoparticles and the support synthesized by oil-bath heating may result in the aggregation and detachment of Ag nanoparticles in the mesoporous support. The reduction of aromatic nitro compounds to amines in a facile and well-controlled way is very important in organic synthetic chemistry.[30] Herein, the chemical reduction of 4-NP was chosen as a model reaction, in which AB was chosen as a new, mild H2 source to replace NaBH4 due to its unstable and toxic properties.[31] 4-NP shows a strong absorption band at l = 317 nm under neutral conditions and remains stable without Ag catalysts and AB, whereas a new absorption band at l = 400 nm appears when AB is added to the aqueous solution of 4-NP (Figure S5 in the Supporting Information). The above phenomenon is caused by the formation of 4-NP anions through H + dissociation from 4-NP in a basic solution of AB (Figure S5 in the Supporting Information). UV/Vis spectra show that the absorption band intensity of 4-NP anions at l = 400 nm reaches a maximum value 130 s later, and then gradually decreases to almost 0 from 130 to 610 s (in 8 min); this is accompanied by the appearance of a new band at l = 296 nm, which is assigned to 4-aminophenol (Figure S5 in the Supporting Information). Figure 5 B shows the time dependence of


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Full Paper ln (C/C0,4-NP), in which C and C0,4-NP denote the 4-NP concentration at time t and t0, respectively. A nonlinear variation of ln (C/ C0,4-NP) appears in the initial period (dashed rectangle in Figure 5 B) for all of the catalysts, which indicates the presence of an induction time due to activation of the catalysts.[32] After the induction time, a rapid and linear evolution is observed for all of the catalysts; this indicates a typical first-order reaction. Two apparent rate constants (kapp,1, black lines, and kapp,2, gray lines) were calculated by fitting 1) all of the data points (kapp,1), and 2) the data points excluding those in the induction time (kapp,2 ; Figure 5 B, inset). In the presence of sufficient AB (H2 source), kapp,1 and kapp,2 follow the order of Ag/0.5CeO2-SBA15 > Ag/1CeO2-SBA-15 > Ag/SBA-15 > Ag/2CeO2-SBA-15 > Ag/ 0.5CeO2-SBA-15(o). The TOF values for the Ag catalysts also follow the same order, namely, Ag/0.5CeO2-SBA-15 (555 min1) > Ag/1CeO2-SBA-15 (328 min1) > Ag/SBA-15 1 (319 min ) > Ag/2CeO2-SBA-15 (229 min1)  Ag/0.5CeO2-SBA15(o) (233 min1). In comparison with the TOF of AB dehydrogenation, the size of the Ag nanoparticles is not the only decisive factor in 4-NP reduction, whereas MSIs between the Ag nanoparticles and CeO2 in combination with Ag nanoparticle size are responsible for the catalytic activity. Regarding the reduction mechanism of 4-NP in the presence of AB, the process spontaneously includes the catalytic hydrolysis of AB and reduction of 4-NP (Figure S6 in the Supporting Information). Based on the catalytic results, the catalytic hydrolysis of AB is dependent on particle size, whereas the different catalytic activity order for 4-NP reduction should also be attributed to the surface electronic structure of the Ag nanoparticles. An electrically more positive surface of Ag due to the strong interactions of Ag with the O atoms of CeO2 facilitates the adsorption of negatively charged reactant intermediate, which may be responsible for the higher TOF values for Ag nanoparticles prepared under MW irradiation.

Conclusion Supported silver nanoparticles with tunable particle sizes (1.9– 3.9 nm) and MSIs on CeO2-SBA-15 supports have been successfully prepared through an efficient MW heating method. We found, for the first time, that the strong charge polarization of CeO2 clusters during the MW heating process contributed to the precise control of Ag particle size and MSIs relative to those obtained through oil-bath heating. Catalytic activity evaluation also demonstrated that the deposition of Ag nanoparticles on CeO2-SBA-15 through MW heating provided higher TOF numbers than those of Ag/SBA-15 without CeO2 and Ag/ CeO2-SBA-15 prepared through conventional oil-bath heating, in which particle size and MSIs were observed to be responsible for the enhanced catalytic performances.

Experimental Section Synthesis of supports and catalysts Pristine mesoporous silica SBA-15 was prepared according to the literature.[33] In a typical procedure, P123 (5.0 g; Aldrich) was disChem. Eur. J. 2014, 20, 15746 – 15752

solved in a 2 m solution of HCl and water (32.5 g) to give a clear mixture. Tetraethoxysilane (TEOS; 10.4 g; Nacalai,  95 %) was added dropwise to the above solution under vigorous stirring at 38 8C for 24 h, and then the suspension was transferred to a stainless-steel autoclave with a Teflon container and hydrothermally treated at 100 8C for 24 h. The precipitate was washed with water, collected by filtration, and dried at 100 8C overnight. The templates were removed by calcination at 550 8C for 5 h in air. CeO2-SBA-15 carriers with different CeO2 contents were synthesized through an impregnation method. Pristine SBA-15 was first degassed at 180 8C overnight before use. Briefly, degassed SBA-15 (1.0 g) was dispersed into an aqueous solution (20 mL) containing 0.025, 0.05, or 0.1 g of Ce(NO3)3·6 H2O ( 98 %, Wako) to control the amounts of CeO2 (0.5, 1.0, and 2.0 wt %, respectively). Subsequently, the above mixture was dried by rotary evaporation under vacuum at 60 8C, and then calcined at 500 8C with a heating rate of 4 8C min1 for 4 h. The obtained carriers were designated as xCeO2-SBA-15, in which x was the percentage of CeO2 (i.e., 0, 0.5, 1.0, and 2.0 wt %). Ag/xCeO2-SBA-15 catalysts were synthesized by an alcohol reduction method assisted by MW heating in the presence of ligand protection.[19] Typically, xCeO2-SBA-15 carriers (0.396 g) were dispersed in 1-hexanol (40 mL; Nacalai), which acted as the reducing agent, then an aqueous solution of AgNO3 (0.037 mmol; Nacalai) and sodium laurate (C12H23NaO2 ; Nacalai) was added to the above suspension, following by MW irradiation (500 W, (2450  30) MHz) under an argon atmosphere. The obtained Ag nanocatalysts deposited on mesoporous carriers (designated as Ag/xCeO2-SBA-15) were dried at 343 K overnight after filtration and washing with acetone and distilled water several times. Control sample Ag/0.5CeO2SBA-1(o) was prepared by means of similar procedures, but through heating on oil bath at 135 8C, which was the highest temperature reached during MW irradiation.

Characterizations BET surface area measurements were performed by using a BELSORP max instrument (Bel Japan, Inc.) at 77 K. The sample was degassed in vacuum at 343 K for 24 h prior to analysis. UV/Vis diffuse reflectance spectra of powdered samples were collected using a Shimadzu UV-2450 spectrophotometer. The reference was BaSO4, and the absorption spectra were obtained by using the Kubelka– Munk function. Surface electronic states were analyzed by XPS (Perkin–Elmer PHI 5000C, AlKa). All binding energies were calibrated by using the contaminant carbon (C 1 s = 284.6 eV) as a reference. Inductively coupled plasma optical emission spectrometry (ICPOES) measurements were performed using a Nippon Jarrell-Ash ICAP-575 Mark II. The TEM image was obtained with a Hitachi Hf2000 FE-TEM equipped with a Kevex energy-dispersive X-ray detector operated at 200 kV. Ag K-edge and Ce LIII-edge X-ray absorption fine structure (XAFS) were recorded using a fluorescence-yield collection technique at the beam line 01B1 station with an attached Si (111) monochromator at SPring-8, JASRI, Harima, Japan (prop. No. 2014 A1045).

Catalytic reactions Hydrolytic dehydrogenation of AB: The catalytic reactions were carried out in an aqueous suspension with various catalysts. Briefly, Ag/SBA-15 powder (20 mg) was dispersed in distilled water (5 mL) in a Pyrex tube. After bubbling with Ar for 30 min, AB (20 mmol) was injected into the mixture in the dark at room temperature (20 8C). The amount of H2 in the gas phase was measured by using a Shimadzu GC-8A gas chromatograph equipped with a MS-5A column.


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Full Paper Chemical reduction of 4-NP by AB: Aqueous solutions of 4-NP (15.5 mm) and AB (18.5 mm) were freshly prepared. Before the reaction, catalyst (20 mg) was uniformly dispersed into water (30 mL). The catalytic reaction was implemented in a quartz cuvette, wherein water (2.2 mL), catalyst solution (0.3 mL), and 4-NP (40 mL) were mixed together. Afterwards, AB solution (0.4 mL) was injected into the mixture and the intensity of the absorption band at l = 400 nm for 4-NP was recorded on a UV/Vis spectrometer at different time intervals.

Acknowledgements This work was supported by Strategic China-Japan Research Cooperative Program 2012 from JST. Part of this work was also performed under management of ‘Elements Strategy Initiative for Catalysts & Batteries (ESICB)’ supported by MEXT. X.F.Q. acknowledges a JSPS Postdoctoral Fellowship for Foreign Researchers (P12075). Keywords: microwave chemistry · nanoparticles · silver · supported catalysts · surface plasmon resonance [1] S. J. Tauster, S. C. Fung, R. L. Garten, J. Am. Chem. Soc. 1978, 100, 170 – 175. [2] V. T. T. Ho, C.-J. Pan, J. Rick, W. N. Su, B. J. Hwang, J. Am. Chem. Soc. 2011, 133, 11716 – 11724. [3] L. R. Baker, G. Kennedy, M. Van Spronsen, A. Hervier, X. J. Cai, S. Y. Chen, L. W. Wang, G. A. Somorjai, J. Am. Chem. Soc. 2012, 134, 14208 – 14216. [4] M. Cargnello, V. V. T. Doan-Nguyen, T. R. Gordon, R. E. Diaz, E. A. Stach, R. J. Gorte, P. Fornasiero, C. B. Murray, Science 2013, 341, 771 – 773. [5] S. K. Beaumont, G. Kyriakou, R. M. Lambert, J. Am. Chem. Soc. 2010, 132, 12246 – 12248. [6] X. Y. Liu, M. H. Liu, Y. C. Luo, C. Y. Mou, S. D. Lin, H. K. Cheng, J. M. Chen, J. F. Lee, T. S. Lin, J. Am. Chem. Soc. 2012, 134, 10251 – 10258. [7] B. Mei, C. Wiktor, S. Turner, A. Pougin, G. van Tendeloo, R. A. Fischer, M. Muhler, J. Strunk, ACS Catal. 2013, 3, 3041 – 3049. [8] Z. F. Bian, T. Tachikawa, P. Zhang, M. Fujitsuka, T. Majima, J. Am. Chem. Soc. 2014, 136, 458 – 465. [9] R. A. Van Santen, H. Kuipers, Adv. Catal. 1987, 35, 265 – 321. [10] A. R. Gonzalezelipe, J. Soria, G. Munuera, J. Catal. 1982, 76, 254 – 264. [11] S. R. Seyedmonir, D. E. Strohmayer, G. L. Geoffroy, M. A. Vannice, H. W. Young, J. W. Linowski, J. Catal. 1984, 87, 424 – 436.

Chem. Eur. J. 2014, 20, 15746 – 15752

[12] X. M. Li, A. Vannice, J. Catal. 1995, 151, 87 – 95. [13] P. Claus, H. Hofmeister, J. Phys. Chem. B 1999, 103, 2766 – 2775. [14] H. Y. Liu, D. Ma, R. A. Blackley, W. Z. Zhou, X. H. Bao, Chem. Commun. 2008, 2677 – 2679. [15] T. Mitsudome, S. Arita, H. Mori, T. Mizugaki, K. Jitsukawa, K. Kaneda, Angew. Chem. Int. Ed. 2008, 47, 7938 – 7940; Angew. Chem. 2008, 120, 8056 – 8058. [16] K. Mori, A. Kumami, M. Tomonari, H. Yamashita, J. Phys. Chem. C 2009, 113, 16850 – 16854. [17] K. Mori, A. Kumami, H. Yamashita, Phys. Chem. Chem. Phys. 2011, 13, 15821 – 15824. [18] Q. Zhang, S. F. Cai, L. S. Li, Y. F. Chen, H. P. Rong, Z. Q. Niu, J. J. Liu, W. He, Y. D. Li, ACS Catal. 2013, 3, 1681 – 1684. [19] K. Fuku, R. Hayashi, S. Takakura, T. Kamegawa, K. Mori, H. Yamashita, Angew. Chem. Int. Ed. 2013, 52, 7446 – 7450; Angew. Chem. 2013, 125, 7594 – 7598. [20] K. Fujiwara, Y. Deligiannakis, C. G. Skoutelis, S. E. Pratsinis, Appl. Catal. B 2014, 154, 9 – 15. [21] S. J. Chang, M. Li, Q. Hua, L. J. Zhang, Y. S. Ma, B. J. Ye, W. X. Huang, J. Catal. 2012, 293, 195 – 204. [22] K. Shimizu, K. Sugino, K. Sawabe, A. Satsuma, Chem. Eur. J. 2009, 15, 2341 – 2351. [23] A. Bensalem, F. Bozonverduraz, M. Delamar, G. Bugli, Appl. Catal. A 1995, 121, 81 – 93. [24] II. Soykal, H. Sohn, D. Singh, J. T. Miller, U. S. Ozkan, ACS Catal. 2014, 4, 585 – 592. [25] H. J. Wu, L. D. Wang, Y. M. Wang, S. L. Guo, Appl. Surf. Sci. 2012, 258, 10047 – 10052. [26] Q. Xu, M. Chandra, J. Power Sources 2006, 163, 364 – 370. [27] M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey, K. I. Goldberg, J. Am. Chem. Soc. 2006, 128, 12048 – 12049. [28] J. M. Yan, X. B. Zhang, T. Akita, M. Haruta, Q. Xu, J. Am. Chem. Soc. 2010, 132, 5326 – 5327. [29] H. F. Cheng, T. Kamegawa, K. Mori, H. Yamashita, Angew. Chem. Int. Ed. 2014, 53, 2910 – 2914; Angew. Chem. 2014, 126, 2954 – 2958. [30] A. M. Tafesh, J. Weiguny, Chem. Rev. 1996, 96, 2035 – 2052. [31] X. Wang, D. P. Liu, S. Y. Song, H. J. Zhang, J. Am. Chem. Soc. 2013, 135, 15864 – 15872. [32] Y. Mei, G. Sharma, Y. Lu, M. Ballauff, M. Drechsler, T. Irrgang, R. Kempe, Langmuir 2005, 21, 12229 – 12234. [33] D. Y. Zhao, J. L. Feng, Q. S. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, G. D. Stucky, Science 1998, 279, 548 – 552.

Received: July 9, 2014 Published online on October 21, 2014


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Silver nanoparticles supported on CeO2-SBA-15 by microwave irradiation possess metal-support interactions and enhanced catalytic activity.

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