Synthesis of highly active and reusable supported gold nanoparticles andtheir catalytic applications to 4-nitrophenol reduction
Xin Huang,a,b Xuepin Liao*a,b and Bi Shi*b
Received 19th July 2011, Accepted 22nd July 2011DOI: 10.1039/c1gc15873b
Gold nanoparticles (AuNPs) are first prepared for the first time by a one-step, green synthesismethod using plant tannins as reductant as well as stabilizer. Subsequently, the resultant AuNPswere supported on g-Al2O3 to prepare a heterogeneous AuNP catalyst (Al2O3-BT-AuNPs). Theresultant Al2O3-BT-AuNPs catalyst was well characterized by N2 adsorption/desorption,ultraviolet diffusion reflection (UV-DR) spectroscopy and transmission electron microscopy(TEM). It was found that the Al2O3-BT-AuNPs catalyst was highly active and reusable in thecatalytic reduction of 4-nitrophenol to 4-aminophenol, and its catalytic activity was dependant onthe loading percentage of BT.
Introduction
In recent decades, gold nanoparticles (AuNPs) have receivedconsiderable attention because of their remarkable catalyticperformance in CO oxidation and NO reduction.1,2 To facilitatecatalyst recovery, AuNPs are usually dispersed onto solidmatrices to prepare heterogeneous AuNP catalysts. According tothe literature,3–8 various materials have been used as supportingmatrices, including carbon nanotubes, silica, titania, ceria andalumina, etc. Amongst them, alumina is one of the mostfrequently used supports due to its remarkable properties, suchas high surface area, porous structure and good mechanicalstrength.9,10 The impregnation method is a general strategyfor the heterogenization of AuNPs.11,12 However, the resultantcatalysts often suffer from a significant loss of catalytic activityduring recycling owing to weak interactions between the AuNPsand supporting matrices. To overcome these disadvantages,much effort has been paid to the development of new method-ologies for the heterogenization of AuNPs. For example,13 Liand co-workers have reported the synthesis of stable AuNPsencapsulated in a silica dendrimer organic–inorganic hybridcomposite as a recyclable catalyst for the oxidation of alcohols.In connection with our previous and ongoing research on planttannins, we provided herein a new strategy for preparing highlyactive and reusable supported AuNP catalysts.
Plant tannins extracted from plants are water soluble polyphe-nols with abundant adjacent phenolic hydroxyls, which are gen-erally divided into condensed tannin and hydrolyzable tannin.
aDepartment of Biomass chemistry and Engineering, Sichuan University,Chengdu, 610065, P. R. China. E-mail: [email protected]; Fax: +86 2885460356; Tel: +86 28 85400382bNational Engineering Laboratory of Clean Technology for LeatherManufacture, Sichuan University, Chengdu, 610065, P. R. China.E-mail: [email protected]; Fax: +86 28 85460356; Tel: +86 28 85400356
Bayberry tannin (BT) is a kind of typical condensed tannin,of which the molecular structure is shown in Fig. 1a. It can beseen that the BT mainly consists of polymerized flavan-3-ols thathave a large number of multiple phenolic hydroxyls located onthe B-rings. These phenolic hydroxyls can be easily oxidizedto benzoquinones, acting as reducing agents (Fig. 1b) whencontacted with electrophilic ions.14 In addition, the molecularbackbone of BT contains rigid aromatic rings, which couldprovide enough steric hindrance to metal species. To utilizethese unique properties of BT, we have developed a one-stepgreen strategy for the synthesis of homogenous AuNP colloidsat room temperature using BT as the reductant and stabilizer.15
Previous studies inspire us to believe that if the prepared BT-stabilized AuNPs (BT-AuNPs) can be supported on alumina, ahighly active heterogeneous AuNP catalyst could be prepared.
Fig. 1 The molecular structure of BT (M represents metal species).
According to the literature,16,17 plant tannins are capable offorming multiple hydrogen bonds with other matrices owingto the presence of abundant phenolic hydroxyls. Our recentstudies found that BT can be absorbed onto g-Al2O3 by theformation of multiple hydrogen bonds via its abundant phenolichydroxyls.18 On the other hand, BT has an excellent nucleophilicreaction activity, which can be cross-linked using aldehydesas the cross-linking agent.19 Thus, it is feasible in principle toprepare highly active and reusable heterogeneous AuNPs via theadsorption of BT-AuNPs onto g-Al2O3, followed by the cross-linking of BT-AuNPs using aldehydes as the cross-linking agent.As follow-up experiments to the green synthesis of BT-AuNPs,the present investigation mainly focuses on the heterogenizationof BT-AuNPs onto g-Al2O3. In the present investigation, aseries of g-Al2O3-supported BT-AuNP catalysts (Al2O3-BT-AuNP) were prepared and well-characterized by means ofN2 adsorption/desorption, ultraviolet diffusion reflection (UV-DR) spectra and transmission electron microscopy (TEM).Subsequently, 4-nitrophenol reduction was carried out overAl2O3-BT-AuNP catalyst using BH4
- as the hydrogen donor.
Experimental section
1.0 mL of 5.0 g L-1 Au3+ was added into 49.0 mL of a BTsolution (BT = 0.005, 0.01, 0.02 and 0.03 g) under constantstirring at room temperature, which allowed the preparation ofa homogenous BT-AuNP colloid. Then, 1.0 g of g-Al2O3 with200 mesh was suspended in the above solution and kept underconstant stirring for 24 h at 40 ◦C. The resultant mixture wastransferred into an oven at 80 ◦C to completely evaporate thewater and then transferred into 20.0 mL of a gluteraldehydesolution (2.5%, v/v) to cross-link the BT-AuNPs at 40 ◦Cfor 24 h. Finally, the Al2O3-BT-AuNP catalyst was collectedby filtration followed by vacuum drying at 50 ◦C for 24 h.For comparison, an Au-Al2O3 catalyst was prepared by theimpregnation method. In a typical preparation process, 1.0 gof g-Al2O3 was suspended in 50.0 mL of an Au3+ solutioncontaining 5.0 mg Au3+. The resultant mixture was stirred andthen transferred into an oven at 80 ◦C to completely evaporatethe water. After calcination at 600 ◦C for 2 h, the Au-Al2O3
catalyst was obtained.A certain amount of the Al2O3-BT-AuNP catalyst (containing
2.5 mmol Au) was mixed with 20.0 mL of a 4-NP solution(0.2 mmol), which had been bubbled with N2 for 20.0 min toremove the dissolved O2 in solution. Subsequently, 10.0 mmol ofNaBH4 was added into the mixture, which allowed the catalyticreduction to start. The conversion of 4-NP was monitored bymeasuring the characteristic adsorption peak of 4-NP in itsultraviolet visible spectrum at 400 nm. When the reaction wascompleted, the catalyst was collected by centrifugation and thenreused. As a control, the catalytic reduction of 4-aminophenolwas carried out using an Au-Al2O3 catalyst under the sameexperimental conditions.
Characterization
The specific surface area of samples were analyzed by N2 ad-sorption/desorption using a surface area and porosity analyzer.Ultraviolet diffusion reflection spectra (UV-DR) of the catalyst
were measured by a UV-3600 spectrophotometer (Shimadzu,Japan). Transmission electron microscopy (TEM) observationswere carried out on a FEI-Tecnai G2 microscope.
Results and discussion
Fig. 2 is the proposed preparation mechanism of the Al2O3-BT-AuNP catalyst. According to our developed one-step method,15
a series of BT-AuNPs were first prepared by the addition offixed amount of an Au3+ solution into BT solutions of differentconcentration. The AuNPs were stabilized by the phenolichydroxyls of BT and the reductively-formed benzoquinones.The obtained BT-AuNPs were then mixed with g-Al2O3 underconstant stirring for 24 h at 40 ◦C, allowing the adsorption ofBT-AuNPs onto g-Al2O3. The resultant mixture was transferredinto an oven at 80 ◦C to completely evaporate the water and thensuspended into a gluteraldehyde solution to cross-link the BT-stabilized AuNPs. The C6 and C8 positions of BT molecules havea high nucleophilic reaction activity and thus the gluteraldehydecan form bridges among them, which results in the formationof a BT-AuNP network around the g-Al2O3. Herein, a seriesof heterogeneous Al2O3-BTy-AuNPx catalysts (x and y are theloading percentages of BT and Au, respectively) were prepared,which included Al2O3-BT0.50-AuNP0.50, Al2O3-BT0.99-AuNP0.49,Al2O3-BT1.95-AuNP0.49 and Al2O3-BT2.89-AuNP0.48.
Fig. 2 Schematic diagram showing the preparation mechanism ofAl2O3-BT-AuNPs.
N2 adsorption/desorption analysis was employed to examinethe porous structure of the Al2O3-BT-AuNPs catalysts. Ta-ble 1 summarizes the corresponding structural parameters ofthe Al2O3-BT-AuNP catalysts. It was found that the porousstructure of g-Al2O3 was well preserved in these Al2O3-BT-AuNPs catalysts with a low loading percentage of BT. Forexample, Al2O3-BT0.50-AuNP0.50 catalyst with the lowest loadingpercentage of BT has the highest specific surface area (294.36m2 g-1), which is comparable to that of pure g-Al2O3 (305.73m2 g-1). The specific surface area and average pore size of theAl2O3-BT0.99-AuNPs0.49 catalyst was 290.34 m2 g-1 and 5.05 nm,
respectively, which were also close to those of pure g-Al2O3.However, a higher loading percentage of BT leads to a loss ofspecific surface area of the catalyst (entries 4 and 5). The specificsurface area of the catalyst decreased from 290.34 to 257.22m2 g-1 when its loading amount of BT was increased from 0.99to 2.89%. The decreased specific surface area should be causedby BT blocking the porosity of g-Al2O3.
Subsequently, we measured the ultraviolet diffusion reflectionspectra (UV-DR) spectrum of the Al2O3-BT-AuNPs catalystsand treated them with the Kubelka–Munk function.20,21 Asshown in Fig. 3, all the catalysts show a surface plasma resonance(SPR) peak for the AuNPs around 520 nm,22 suggesting thesuccessful immobilization of AuNPs onto the g-Al2O3. The M–L intensity of the catalyst increased along with the increaseof BT loading percentage from 0.92 to 1.35. Additionally,the absorbance intensity in the wavelength range 350–450 nmgradually increased with increasing loading percentage of BT,which was due to the strong absorbance of ultraviolet light byBT.
Fig. 3 Kubelka–Munk function-treated ultraviolet diffusion reflection(UV-DR) spectra of the as-prepared Al2O3-BT-AuNPs catalysts.
The morphology and particle size of the as-prepared Al2O3-BT-AuNPs catalysts were analyzed by transmission electronmicroscopy (TEM), and the corresponding results are shownin Fig. 4. As for Al2O3-BT0.50-AuNPs0.50, the AuNPs with anaverage diameter of 23 ± 8 nm were well dispersed on the g-Al2O3. Additionally, smaller AuNPs were found to predominatein catalysts with a higher loading percentage of BT. For example,the average diameter of the AuNPs in Al2O3-BT2.89-AuNPs0.48 isas small as 14 ± 3 nm (Fig. 4b). These TEM results are consistentwith our previous work where BT-stabilized AuNP colloidswith smaller particle sizes could be synthesized by the use of a
Fig. 4 TEM images of the Al2O3-BT0.50-AuNPs0.50 and Al2O3-BT2.89-AuNPs0.48 catalysts.
high concentration of BT due to the strengthened reducing andstabilizing abilities.15 Therefore, it is understandable that whenthose BT-AuNP colloids were immobilized onto the g-Al2O3,the obtained heterogeneous Al2O3-BT-AuNPs catalyst shouldhave smaller well dispersed AuNPs.
To evaluate the catalytic activity of the as-prepared Al2O3-BT-AuNPs catalysts, the reduction of 4-nitrophenol (4-NP) to4-aminophenol (4-AP) was carried out as a benchmark reactionusing NaBH4 as the reducing agent,23 where the AuNPs of theAl2O3-BT-AuNPs catalysts act as electronic relay systems totransfer electrons donated by BH4
- to the nitro group of 4-NP. Prior to the catalytic reaction, a pre-determined amountof Al2O3-BT-AuNPs catalyst (containing 2.5 mmol Au) wasmixed with a 4-NP aqueous solution (0.2 mmol) that hadbeen bubbled with N2 for 20 min to remove the dissolved O2
in solution. Subsequently, an excess amount of NaBH4 (10.0mmol) was added to the mixture, allowing the catalytic reductionto start. The amount of NaBH4 was 50-times than that of 4-NP, which eliminates the influence of the donor BH4
- on thecatalytic reduction. After mixing with NaBH4, the characteristicadsorption peak of 4-NP in the ultraviolet-visible spectrumshifted from 317 to 400 nm and decreased as the reactionproceeded. Meanwhile, two new adsorption peaks appearedat 235 and 295 nm, suggesting the formation of 4-AP. Fig. 5illustrates the absorbance vs. reaction time plots for the reductionof 4-NP to 4-AP using the different Al2O3-BT-AuNPs catalysts.We found that the catalytic activity of the Al2O3-BT-AuNPs
Fig. 5 Absorbance vs. reaction time plots for the reduction of 4-NP to4-AP using different Al2O3-BT-AuNPs catalysts.
catalysts were highly dependant on the loading percentage ofBT on g-Al2O3. The Al2O3-BT0.50-AuNPs0.50 catalyst showed thehighest catalytic activity, where the catalytic reduction of 4-NPto 4-AP was completed in about 5 min. Along with the increaseof loading percentage of BT, the reduction rate of 4-NP graduallydecreased. For the Al2O3-BT0.99-AuNPs0.49 catalyst, the timeneeded for the complete reduction of 4-NP was about 8.0 min,but the time needed increased to 20.0 min when the Al2O3-BT2.89-AuNPs0.48 catalyst was employed. However, it should benoted that the activity of these Al2O3-BT-AuNPs catalysts is stillappreciable compared with other reported biomass-stabilizedheterogeneous AuNP catalysts.24,25 These catalytic results seemnot to be consistent with the TEM analyses because Al2O3-BT-AuNPs catalysts with higher loading percentages of BT havesmaller AuNPs, which should exhibit better catalytic activitydue to the higher fraction of surface atoms. According to theliterature,16 tannin molecules can interact with each other viathe formation of hydrogen bonds and/or hydrophobic bonds.We believe that these intermolecular interactions among tanninmolecules will be strengthened along with the increase of tanninconcentration, and accordingly supramolecular BT shell maybe gradually formed on the surface of AuNPs when preparingthe AuNP colloids. Under such conditions, the dense BT shellstabilized by AuNPs would lead to steric hindrance of thesubstrate to access the AuNPs. As a result, the reduction rate of4-NP over the Al2O3-BT-AuNPs catalyst is gradually sloweddown when the loading percentage of BT in the Al2O3-BT-AuNPs catalyst is increased. Actually, a similar hindrance effectof stabilizers to metal nanoparticles has also been reported byother researchers.26,27
The reusability of the Al2O3-BT-AuNPs catalyst was alsoinvestigated. As shown in Fig. 6, the Al2O3-BT0.50-AuNPs0.50
catalyst could be reused four times without any significant loss ofsubstrate conversion. In the fourth run, the conversion yield of4-NP was still as high as 95.1%. TEM analysis of the reusedcatalyst does not show any obvious aggregation of AuNPs,as shown in Fig. 7. On the contrary, the Au-Al2O3 catalystprepared by the impregnation method exhibited a relatively poorreusability under the same experimental conditions. In the fourthrun, the conversion yield of 4-NP was only 80.7% when theAu-Al2O3 catalyst was employed. These facts indeed suggested
Fig. 6 The conversion yields of 4-NP during recycles of Al2O3-BT0.50-AuNPs0.50 and Au-Al2O3 catalysts.
Fig. 7 TEM image of the Al2O3-BT0.50-AuNPs0.50 catalyst after beingreused four times.
that the as-prepared BT-AuNP-Al2O3 catalyst exhibited a betterreusability than the Au-Al2O3 catalyst.
Conclusions
In summary, BT-stabilized AuNPs can be stably-immobilizedonto porous g-Al2O3 by the formation of multiple hydrogenbonds with g-Al2O3 and self cross-linking interactions amongBT-stabilized AuNPs. The as-prepared heterogeneous Al2O3-BT-AuNPs catalyst can be used in the highly active reduction of4-nitrophenol to 4-aminophenol. Moreover, the catalyst exhib-ited a better reusability than a conventional Au-Al2O3 catalyst.It should be noted that this strategy might be extended forthe preparation of other heterogeneous nanoparticle catalystsbecause BT is capable of chelating with many metal species,such as Pt and Pd.
Acknowledgements
We acknowledge the financial support provided by the NationalNatural Science Foundation of China (20776090) and theA Foundation for the Author of National Excellent DoctorDissertation of P. R. China (FANEDD200762). We thank DrMing Liu (Test Centre of Sichuan University) for their assistancewith TEM analyses.
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