ARTICLE pubs.acs.org/IECR
Gold Nanoparticles Intercalated into the Walls of Mesoporous Silica as a Versatile Redox Catalyst Lifang Chen,† Juncheng Hu,*,‡ Zhiwen Qi,† Yunjin Fang,† and Ryan Richards*,§ †
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Key Laboratory of Catalysis and Materials of Hubei Province, South-Central University for Nationalities, Wuhan 430074, China § Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401, United States ‡
bS Supporting Information ABSTRACT: A nanoscaled reactor framework of well-dispersed gold particles intercalated into the walls of mesoporous silica (GMS) was prepared by functionalizing silica with thioether groups. The GMS maintains mesoporous structure with uniform pores of 5.6 nm and possesses high surface area of more than 800 m2 3 g 1. Very recently, we reported that the GMS catalyst was very active for the oxidation of alkanes and alcohols but was also durable and recyclable. Here, we show that the catalyst is also very active for the reduction of p-nitrophenol (PNP) and methylene blue (MB) displaying catalytic activity in the reduction of PNP with Knor‑PNP of 45.9 mmol 1 3 s 1. The unobstructed ordered mesoporous structure of the GMS catalyst and the small size of gold nanoparticles are the main factors leading to high catalytic activities. Further, for reduction of MB, the catalytic rate of the catalyst decreases by less than 6% when recycled 10 times. Therefore, the nanoreactor framework catalyst is very robust and is readily separable and reusable, demonstrating attractive potential for practical applications.
1. INTRODUCTION Noble metal nanoparticles supported on oxide supports exhibit high catalytic activity and efficiency for a broad range of chemical reactions including hydrogenation, oxidation reduction, and reforming.1 5 Metal particles should be well dispersed on the inert supports in order to prevent aggregation of neighboring particles, and to achieve very small and stable particles ( Krc‑ONP > Krc‑MNP. These differences in Krc might be due to the differences in reactivities of different nitrophenols. We have found that the rate constants (Krc‑PNP) of the borohydride reduction of PNP by different catalysts vary with the amount of catalyst employed.41,43,44 Thus, for coherent comparison the catalytic activities of different catalysts are presented in terms of the normalized rate constant (Knor‑PNP), which was obtained by normalizing the Krc‑PNP values with respect to the total amount (millimoles) of catalysts used (see Table 2) and shows an opposite trend. The Knor‑PNP value for Au@silica yolk/shell structure is 8 times larger than that of core/ shell particles and 25 times larger than those of Au particles anchored on other supports. This indicates that the Knor‑PNP value for a nanoreactor framework of Au@ silica yolk/shell structure is expected to show attractive catalytic behaviors for PNP reduction (Table 2, entry 2). Au@silica yolk/shell particles are generally 43 ( 7 nm in diameter, while the GMS sample is very small, about 2 nm in diameter for gold nanoparticles. As shown in Table 2, GMS displays higher catalytic activity in the reaction than Au@silica yolk/shell particles and other catalysts. Moreover, 1 mol of gold nanoparticles in GMS can catalyze 1320 mol of PNP reduction from the molar ratio of PNP and gold nanoparticles. This indicates that GMS catalyst exhibits a super highly reductive capability compared with other catalysts in Table 2. This result has been attributed to GMS possessing a nanoreactor framework structure, which has a high surface area and small gold nanoparticles in diameter, and an ordered mesoporous structure effectively interconnected with void defects, a better transport of reactant and product, desirable for mass and heat transfer, feasibly leading to high catalytic activity. It is known that reusability is the main advantage of using a heterogeneous catalyst rather than a homogeneous catalyst for industrial applications. The catalyst particles were readily separated from the reaction mixture by simple centrifugation at 6000 rpm. The precipitates could be redispersed in water by brief sonication and recycled again for reduction after the elution of the product, as illustrated in Figure 6. Knor‑PNP only decreased from 45.9 to 43.6 mmol 1 s 1 for the fifth reuse, indicating the GMS could be stable and recycled. 3.3. Reduction of Methylene Blue. Methylene blue (MB) has a basic dye skeleton of a thiazine group and is generally used
Figure 5. Plot of ln(Ct/C0) versus time for reduction of (a) MNP, (b) ONP, and (c) PNP.
Table 1. Catalytic Data for Different Nitrophenol Reductions Using GMS type of nitrophenol GMS used (mmol)
Krc (s 1)
Knor (mmol
ortho
7.6 10
5
2.02 10
3
26.58
meta
7.6 10
5
1.53 10
3
20.13
para
7.6 10
5
3.49 10
3
45.92
1
s 1)
Table 2. Recent Studies on the Reduction of p-Nitrophenol by Different Gold Catalysts entry
a
material support
NaBH4/PNP/Au (mol/mol/mol)
K (s 1)
Knor (mmol
1
s 1)
ref
1
silica/core@shell
750/2.1/1
4.6 10
4
0.29
39
2
silica/yolk@shell
750/2.1/1
3.9 10
3
2.44
39
3
poly(amidoamine)
1200/1/1
2.0 10
3
0.67
40
4 5
anion-exchange resin mesoporous silica
2.7 10 3.5 10
4
0.089 45.9
a
/0.27/1 13200/1320/1
3
37 this work
No data. 13646
dx.doi.org/10.1021/ie200606t |Ind. Eng. Chem. Res. 2011, 50, 13642–13649
Industrial & Engineering Chemistry Research
Figure 6. Normalized rate constant (Knor‑PNP) in different cycles of catalytic reduction of PNP using GMS.
Figure 7. Plot of ln(Ct‑MB/C0‑MB) versus time for reduction of MB and (inset) time-dependent UV vis spectral changes of the reaction mixture of MB and NaBH4 catalyzed by GMS.
as an oxidation reduction indicator in chemistry and biology.45 The oxidized form of MB exhibits an intense absorption band in the region 200 700 nm with an absorption maximum (λmax) at 660 nm in aqueous solution (Figure S4a in the Supporting Information). On the other hand, the reduced form of the dye, leucomethylene blue (MBH), shows a λmax at 264 nm (Figure S4b in the Supporting Information). Thus, the progress of the reduction of MB can be carried out by a UV vis spectrophotometer measuring the changes in the specified absorbance maxima. To investigate its catalytic activity, GMS was employed for the reduction of MB in the presence of NaBH4. With the progress of the reaction, a steady decrease of the absorbance of the dye was noted, as shown in Figure 7, inset. The absorption bands for MB decrease gradually without showing any changes in shape or position of the peaks, which indicates that the MB is reduced without any other side reactions. After 5 min, the absorbance flattened and the reaction completely finished. Figure 7 reveals that there is a linear relationship with a slope (Krc‑MB) of 0.016 s 1 between the ln(Ct‑MB /C0‑MB) and time under constant concentration of NaBH4, which infers that the reduction is a first-order reaction to the concentration of MB. Meanwhile,
ARTICLE
Figure 8. Curve of reaction constant of MB vs cycle times catalyzed by GMS.
Ct‑MB and C0‑MB are MB concentrations at times t and 0, respectively, were measured from the relative intensities of the respective absorbances at 660 nm of MB ion, At‑MB and A0‑MB. In the absence of GMS, under the same experimental conditions, no such decrease in absorbance of the dye was observed in the experimental time scale, indicating that photochemical reduction by indoor lighting and/or degradation does not take place. Therefore, it can be concluded that the GMS can also successfully catalyze the reduction of MB at the present experimental conditions due to its ordered mesoporous structure without pore blockage and small size of Au nanoparticles in GMS catalyst. The GMS catalyst can be easily recycled and initiate the next reduction after rinsing with distilled water. The reaction was studied using recycled catalysts. Along with the increase of the recycle times, the catalysts possess high activity all the time, though their activity diminishes slowly. The relationship of the normalized rate constant (Knor‑MB), normalized Krc‑MB, with respect to the total amount (millimoles) of GMS used versus cycle times is shown in Figure 8, which may be expressed as 1.15n, where Knor‑MB is the normalized Knor‑MB = 211.78 reaction rate constant and n is the cyclic time. The catalytic rates of the catalysts only decrease 6% when the cyclic time is 10 and decrease less than 18% when the cyclic time is up to 30. This nanoreactor framework structure of GMS has many advantages such as an ordered and stable mesoporous wall structure hindering the aggregation of neighboring gold nanoparticles, ordered mesoporous channels interconnected with void defects offer large surface area, better transport of reactant and product, desirability for mass and heat transfer in the catalytic reaction leading to high catalytic activity, and reusability. The slight activity decrease may be due to a considerable amount of GMS lost during centrifugation for purification compared to the virgin GMS as shown in Figure 8. The use of gold as a catalyst requires to focusing on achieving very small and stable gold particles (
