Pt

XD

a

ARR2AA

KGAC

1

amt[hlcsanNt

rnhSimimg

0d

Colloids and Surfaces A: Physicochem. Eng. Aspects 358 (2010) 122–127

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

reparation of Au catalyst on modified SiO2 via adsorbed-layer nanoreactorechnique

in Jiang ∗, Hui Deng, Xinjie Wangepartment of Chemical Engineering, Zhejiang University, Zheda road 38#, Hangzhou 310027, Zhejiang, China

r t i c l e i n f o

rticle history:eceived 28 August 2009eceived in revised form2 December 2009

a b s t r a c t

Au nanoparticles were supported on modified SiO2 via adsorbed-layer nanoreactor technique. The mor-phology of the catalysts was analyzed by TEM, and the grain size of Au was determined by X-raydiffraction. Owing to the higher isoelectric point (IEP), the modifying component was significantly help-

ccepted 14 January 2010vailable online 22 January 2010

eywords:old catalystdsorbed-layer nanoreactor

ful for the preparation of much smaller Au nanoparticles on silica. However, the size order of Au wasdifferent with the order of IEP of modifying component. The catalysts showed an apparent Au parti-cle size-dependent activity on the oxidation of cyclohexane. The catalyst modified by Ni(OH)2 with thesmallest Au particles had the highest catalytic activity, while it had the lowest one when modified byMg(OH)2 with the biggest Au particles. The selectivity to cyclohexanol and cyclohexanone could maintainat a high value as the conversion increased, which was different from the results of catalysts prepared by

yclohexane oxidation other routes.

. Introduction

Au nanoparticles about 3–5 nm in diameter are catalyticallyctive for many chemical reactions, which has led to a dra-atic increase in interest [1–5]. Several methods are available

o obtain nano-scale Au catalyst, including impregnation (IMP)2], co-precipitation (CP) [3], deposition–precipitation (DP) [4],omogeneous deposition–precipitation (HDP) [5], etc. Au cata-

ysts obtained by different methods are different both in theharacteristic of Au and in the interaction between Au andupport. A new method could create some new features in cat-lyst structure and in catalysis performance. In this paper, aew process for preparing nanoparticles, named Adsorbed-layeranoreactor Technique, was introduced to synthesize Au nanopar-

icles.Adsorbed-layer Nanoreactor Technique using adsorbed-layer to

estrict the free growth of grains is favorable for preparing smallanoparticles with good dispersion. Via this method, our groupad synthesized nanoparticulate TiO2 [6], Ag [7] on the surface ofiO2 and CuO-Ag/SiO2 [8] nanocomposites. The particles formedn the adsorbed-layer were about several nanometers, which just

et the size range of Au to perform outstanding catalytic activ-ty. Therefore, we approached to prepare Au catalyst with this new

ethod. Considering the repulsion of SiO2 surface with low IEP toold hydroxychlorides (Au(OH)xCl−4−X (X = 0–4)), we modified sil-

∗ Corresponding author. Tel.: +86 571 87952828; fax: +86 571 87951227.E-mail address: jiangx@zju.edu.cn (X. Jiang).

927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2010.01.024

© 2010 Elsevier B.V. All rights reserved.

ica with materials with high IEP and investigated their influence onAu particles.

2. Experimental

2.1. Materials

Hydrophilic silica (SiO2; Degussa AEROSIL200, averagesize 12 nm, specific area 200 m2/g), ethanol (C2H5OH; AR,Hangzhou Xiaoshan Chemical Reagent Ltd.), magnesium nitrate(Mg(NO3)2·6H2O; AR, Sinopharm Chemical Reagent Co., Ltd.), zincnitrate (Zn(NO3)2·6H2O; AR, Shanghai Meixing Chemical ReagentLtd.), nickel nitrate (Ni(NO3)2·6H2O; AR, Shanghai HengxinChemical Reagent Ltd.), sodium hydroxide (NaOH; AR, HangzhouChemical Reagent Ltd.), gold(III) chloride (HAuCl4·4H2O; AR,Shanghai chemical reagent co., Ltd.), sodium borohydride (NaBH4;AR, Sinopharm Chemical Reagent (Shanghai) Co., Ltd.), cyclohex-ane (C6H12; AR, Sinopharm Chemical Reagent (Shanghai) Co., Ltd.),tert-butyl peroxide (C4H10O2; CP, Sinopharm Chemical ReagentCo., Ltd.).

2.2. Synthesis process

The synthesis of the catalyst was carried out through two steps,

the modification of SiO2 and the formation of Au. At the first step,0.5 g SiO2 baked at 120 ◦C for 2 h was well dispersed in 150 mL abso-lute ethyl alcohol. Then 1 mL water and 3 mmol NaOH dissolved in50 mL ethanol were added successively. Because of the selectiveadsorption of water molecule on the silica surface, a water-rich

hysico

abr1wWiiaawwtfS

awiantsrccmSe3ea

tt

2

a1atg

2

(

(

(

a

X. Jiang et al. / Colloids and Surfaces A: P

dsorbed-layer formed. The sodium hydroxide distributed itselfetween adsorbed-layer and ethanol bulk. The adsorption equilib-ium was attained in 12 h under stirring and 25 ◦C conditions. Then,.5 mmol M(NO3)2 (M = Mg, Zn or Ni) dissolved in 50 mL ethanolas slowly added dropwise to the reaction system in 30 min.e had measured the concentration of NaOH in bulk phase by

ntroducing the electrical conductivity method and sequentiallynvestigated the NaOH distribution between the adsorbed-layernd ethanol bulk [7]. The percentage of NaOH in the adsorbed-layerpproached to 40%. The concentration of alkali in adsorbed-layeras larger than that in alcohol phase, which leads to the over-helming occurrence of the reaction of modifying component in

he adsorbed water layer on the surface of SiO2. The precursor dif-used into the adsorbed-layer and reacted with OH−. As a result,iO2 was evenly modified by M(OH)2.

At the second step, the reacting temperature was shifted to 0 ◦C,nd then, 0.024 mmol HAuCl4·4H2O dissolved in 30 mL ethanolas added for adsorption. Zanella et al. [4] and Lee and Gavri-

lidis [2] studied the adsorption of gold hydroxychlorides on TiO2nd alumina, respectively, and suggested that the adsorption wasot purely a coulombic interaction but also involved the forma-ion of covalent bonds with the solid surface by reaction withurface hydroxyl groups. In our system, the gold hydroxychlo-ides with negative charges will adsorb on the modified silica byoulombic interaction firstly, and then interact with the modifyingomponent to form the species having Au-OH groups. Therefore,ost of the gold hydroxychlorides concentrated on the surface of

iO2. After 3-h adsorption, 0.024 mmol NaBH4 dissolved in 30 mLthanol was added dropwise in 30 min. After reacting for another0 min, the purple product Au-M(OH)2/SiO2 was gained by sev-ral centrifugation–redispersion–washing cycles, then was driedt room temperature.

The gold content in Au-Ni(OH)2/SiO2 was measured by elec-ronic energy spectrum. The measured value 1.01 wt% approachedo the theoretical value 0.91 wt%.

.3. Catalytic activity test

0.1 g grinded catalyst was dispersed into 25 g cyclohexane intri-flask without solvent, then was scattered in ultrasound for

0 min. The reaction was performed under mild conditions (70 ◦C,tmospheric pressure). 0.25 mL Tert-butyl peroxide was added intohe reaction system every 30 min. The products were analyzed byas chromatography (capillary column, ATOV-225).

.4. Catalyst characterization

A) Transmission Electron Microscopy (TEM): One drop ofan ultrasono-mixed, dilute alcohol suspension of the “as-prepared” samples was placed on a carbon-coated grid, andafter evaporation of the solvent electron micrographs of theparticles retained was taken. A JEM-2010(HR) transmissionelector microscope was used.

B) Energy-Dispersive X-Ray Microanalysis (EDAX): A smallamount of samples was placed on the sample platform andcompressed till flat. Then, two areas of the sample surface werescanned and analyzed by EDAX (Genesis 4000, Ametek, Mah-wah, NJ). The Au content was measured.

C) X-ray Diffraction (XRD): The as-prepared catalysts were ana-lyzed by XRD using a D/max-rA XRD instrument (XD-98, Philips,Eindhoven, The Netherlands) with CuK� radiation (1.5406 Å).

The accelerating voltage and the applied current were 40 kVand 30 mA, respectively.

The XRD analysis was carried out in two steps. First, we scannedrange of 2� from 10◦ to 80◦ with a rate of 4 ◦/min to observe

chem. Eng. Aspects 358 (2010) 122–127 123

the overall XRD peaks. Then, in the subsequent analysis, the scan,starting from 35◦ and ending at 48◦ at a rate of 1 ◦/min, was aimedto calculate the grain size of the Au (1 1 1).

The characteristic peaks of Au were fitted via Cauchy equation,from which the grain size of Au was calculated by Scherrer equation[9].

y = k0 + k1x + A1

[1 + ((x − x1)/B1)2]+ A2

[1 + ((x − x2)/B2)2](1)

D = K�

ˇ cos �(2)

x1 and x2 were the location parameters, specifying the location ofthe peaks. B1 and B2 were the scale parameters specifying the half-width at half-maximum (HWHM). A1 and A2 were the peak’s height.k0 and k1 were related to the baseline. K was related to the crystal-lite shape. � and � were the radiation wavelength and Bragg’s angle.ˇ was line broadening (ˇ = B1 − b0, where b0 was the half-widthsof the silicon standard). By fitting these two reflection peaks, wecould ascertain an accurate baseline. In this paper, our discussionwas based on grain size calculated from peak at 2� = 38.2◦ and thesoftware for fitting was MATLAB.

3. Results and discussion

3.1. Morphology of catalysts

The morphology of the catalyst was analyzed by TEM. In orderto draw a comparison, Au/SiO2 catalyst was prepared via support-ing Au directly on SiO2 without surface modification. TEM pictureof Au/SiO2 was showed in Fig. 1A. The fuscous balls with diametermore than 20 nm on SiO2 were Au particles. With low IEP (IEP = 3.7[10]), silica was negatively charged. The gold hydroxychlorides hadnegative charges and would be repulsed by the negative chargeson the silica. Therefore, most of the gold hydroxychlorides con-centrated in the ethanol bulk and then gradually formed into Auparticles. The lower dielectric constant of ethanol was favorablefor the formation of large, string like Au particles [11]. Because Auparticles were not generated in the water-rich adsorbed-layer, itwas difficult to prepare tiny Au by the adsorbed-layer nanoreactortechnique directly on the silica without surface modification.

To regulate the interaction between gold hydroxychloridesand the support, we introduced Mg(OH)2 (IEP > 12) [12], Zn(OH)2(IEP ≈ 9.2) [10] or Ni(OH)2 (IEP = 11.1) [10], to modify silica beforethe adsorption of gold precursor. The procedure has been describedin Section 2.2. Their TEM pictures were C, E and G in Fig. 1 and theparticle size distribution of Au were showed in D, F and H. The dark-est points in these pictures were Au and these particles were nearto spherical. The modifying components scattered on the silica sur-face uniformly and could not be distinguished with silica easily. Theparticle size of Au was much smaller than that in Fig. 1A and the con-glomeration was disappeared. It suggested the surface electronicproperty of the support surface might turn from negatively chargedto positively charged by the introduction of modifying components.The attraction between gold hydroxychlorides and modified SiO2ensured the Au particles formed mainly in the adsorbed-layer. Theadsorbed-layer restrained the growth of Au particle, meanwhilethe movement and aggregation of the formed Au particles sloweddown. The inset picture in Fig. 1G was a HRTEM picture of Au-Ni(OH)2/SiO2 with a d-spacing between adjacent lattice planes of

2.1 ´A.

Fig. 2 showed the XRD patterns. The peaks at 2� = 38.2◦, 44.4◦,

64.6◦ and 77.5◦ were corresponding to (1 1 1), (2 0 0), (2 2 0) and(3 1 1) of Au grains, respectively. Au grains in different catalystsmodified by different nanoparticles were all in several nanome-ters. A small amount of big Au particles appeared on Mg(OH)2 and

124 X. Jiang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 358 (2010) 122–127

Fig. 1. TEM pictures as well as particle size distribution of (A and B) Au/SiO2; (C and D) Au-Mg(OH)2/SiO2; (E and F) Au-Zn(OH)2/SiO2 and (G and H) Au-Ni(OH)2/SiO2. Insetshows HRTEM of Au-Ni(OH)2/SiO2.

X. Jiang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 358 (2010) 122–127 125

FM

ZpowbIrIsal

3

hImcatsiti

usc

ma

kl

TS

size obtained in the last section. The relationship between catalyticactivity and Au particle size was drawn in Fig. 6. The smaller the par-ticles were, the higher the catalytic activity was. When Au particlesreached 6 nm, the decrease in reaction rate became more obvious.

ig. 2. XRD patterns of samples with different modifying component. ((A) Au-g(OH)2/SiO2; (B) Au-Zn(OH)2/SiO2; (C) Au-Ni(OH)2/SiO2).

n(OH)2 while there were no big ones on Ni(OH)2. The statisticalarticle sizes obtained from TEM and grain sizes obtained from XRDf Au were shown in Table 1. Although the sizes from TEM and XRDere different, they showed same tendency. Au particles were the

iggest on Mg(OH)2 and the smallest on Ni(OH)2. Comparing theEP of modifying components, we found the size order was not cor-espondent with the order of IEP. It might suggest that, when theEP of one modifying component reached a certain value, the repul-ion became attraction and the reaction mainly took place in thedsorbed-layer. Hereafter the IEP of the modifying component hadittle to do with the processes of generation and growth of particles.

.2. Catalytic activity

Since the pioneer work of Haruta [3,13], gold nanoparticlesave been studied as catalysts for a number of reactions [14–19].

n particular, Au particles dispersed on ZSM-5 [20], mesoporousaterials [21–23], and Al2O3 [24] were explored for liquid-phase

yclohexane oxidation. As well known, the products, cyclohexanolnd cyclohexanone are substantially more reactive than the reac-ant. Thus, it is difficult to receive high conversion and selectivityimultaneously under mild conditions. The cyclohexane oxidations one of the most challenging and promising subjects, becausehis process produces an important intermediate in the petroleumndustrial chemistry. We chose this system to test our catalysts.

To test the catalytic activity of modifying component, we firstsed M(OH)2/SiO2 as catalyst. After reacting for 10 h, the conver-ion of cyclohexane was less than 0.05%, suggesting the modifyingomponent had no catalyst activity for oxidation of cyclohexane.

The catalytic activity of Au/SiO2 was also tested and the experi-ent showed Au/SiO2 had little catalytic activity. The poor catalytic

ctivity was probably attributed to the big size of Au (Fig. 3).Fig. 4 showed the conversion of cyclohexane catalyzed by three

inds of catalysts with different modifying components. The cata-yst with Ni(OH)2 as modifying component had the highest catalytic

able 1ize of Au of different samples.

Catalyst Grain size of Au/nma Particle size of Au/nmb

Au-Mg(OH)2/SiO2 5.8 5.9Au-Zn(OH)2/SiO2 5.0 5.4Au-Ni(OH)2/SiO2 3.3 4.9

a Obtained from XRD.b Obtained from TEM.

Fig. 3. UV–vis spectra of Au-M(OH)2/SiO2 ((A) Au-Mg(OH)2/SiO2, (B) Au-Zn(OH)2/SiO2 and (C) Au-Ni(OH)2/SiO2).

activity and the one with Mg(OH)2 had the lowest. The conversioncurves kept straight in the first several hours and then became flatgradually, especially for the curve of catalyst modified by Ni(OH)2.The TEM analysis of the catalysts after reaction showed that thedecrease in reaction rate was resulted from the change of catalyst.Fig. 5 showed the Au-Ni(OH)2/SiO2 catalyst after reaction. Compar-ing to Fig. 1D, Au particles grew obviously, from 4.9 nm to 8.7 nm.The increase in Au size during the reaction led to the decrease ofthe reaction rate. The Au particles on Ni(OH)2 were the smallest andthe aggregation of these Au particles was the most serious whichresulted in the most obvious change in catalytic activity.

To evaluate the influence of modifying component on catalysis,we got the initial reaction rate from the data of the first severalhours. In this period, the conversion of cyclohexane was low andthe concentration changed little. In the process of calculation, itwas feasible not to take the change in the concentration of reac-tant into account. Therefore, the catalysis data in the first fewhours (first 4 h for Au-Mg(OH)2/SiO2 and Au-Zn(OH)2/SiO2, first3 h for Au-Ni(OH)2/SiO2) was linear fitted to obtain the reactionrate. The rate constant of Au-Mg(OH)2/SiO2, Au-Zn(OH)2/SiO2 andNi(OH)2/SiO2 were 0.35, 0.63, 0.78 mol h−1 gAu

−1, respectively. Thesequence of the catalytic activity was the same as that of Au particle

Fig. 4. Effect of reaction time on cyclohexane conversion.

126 X. Jiang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 358 (2010) 122–127

Fig. 5. TEM picture as well as particle size distribution of (A and B) Au-Ni(OH)2/SiO2

catalyst after reaction.

Fig. 6. Au grain size-dependent of catalytic activity.

Table 2Cyclohexane oxidation without solvent.

Catalyst Time/h Rate constant/mol h−1 gAu−1

Au/SiO2 8 0.19Au-Mg(OH)2/SiO2 8 0.35Au-Zn(OH)2/SiO2 8 0.63Au-Ni(OH)2/SiO2 8 0.780.25% Au/graphitea 17 0.850.5% Au/graphitea 17 0.551% Au/graphitea 17 0.172% Au/graphitea 17 0.17

a Data from literature [19].

Fig. 7. Relationship between cyclohexane conversion and the total selectivity tocyclohexanol and cyclohexanone.

Except above common features, gold catalysts prepared byAdsorbed-layer Nanoreactor Technique showed some distinguish-ing features. In a recent paper, Hutchings and co-workers [25] haveinvestigated the catalytic activity of Au/graphite catalyst on theselective oxidation of cyclohexane under similar experimental con-ditions. They found that, at a higher conversion of cyclohexane,the total selectivity to cyclohexanol and cyclohexanone was quitelow. They used a range of additives, such as 1,4-difluorobenzene,chlorobenzene, fluorobenzene, etc., as inhibitors to keep a lowerconversion. The selectivity maintained at a high value. The con-clusion of their study was the selectivity was solely a functionof conversion, which in turn was a function of reaction time.Table 2 listed their data as well as our results. Our catalysts showedmuch higher selectivity to cyclohexanol and cyclohexanone thantheir Au/graphite catalyst without any additives, meanwhile thecatalytic activity kept the same level. The relationship betweencyclohexane conversion and total selectivity to cyclohexanol andcyclohexanone was shown in Fig. 7. In the first stage, the selectivityfell down with the increase of conversion, and then the selectivitykept stable. This trend was significantly different with the contin-uous decrease of selectivity in Hutchings article.

4. Conclusions

Gold catalysts were prepared by the adsorption-layer nanoreac-tor technique on the silica surface modified by Mg(OH)2, Zn(OH)2 or

Ni(OH)2. The negative charges on silica were considered the maincause to block in-situ synthesis of Au on silica. The modifying com-ponent changed the surface charge properties of support, therefore,played an important role in the synthesis of tiny Au. The parti-cle size from TEM and grain size from XRD of Au were in order

Selectivity

Cyclohexanol Cyclohexanone Total

45.9 40.9 86.851.3 40.2 91.532.2 33.3 65.535.6 34.5 70.1

9.5 5.8 15.311.0 6.2 17.214.4 8.7 23.1

5.9 4.0 9.9

hysico

ooatac

A

Ge

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

X. Jiang et al. / Colloids and Surfaces A: P

f Mg(OH)2 > Zn(OH)2 > Ni(OH)2. The catalytic activity of selectivexidation of cyclohexane showed an inverse trend with Au sizend smaller Au particles were easier to grow up. The selectivityo cyclohexanol and cyclohexanone could maintain in a high values conversion increased, which was different from the results ofatalysts prepared by other routes.

cknowledgement

Financial support from National Science Foundation of Chinarants (Contract 20876137 and 20776126) is gratefully acknowl-dged.

eferences

[1] F. Somodi, I. Borb�th, M. Hegedus, A. Tompos, I.E. Sajó, Á. Szegedi, S. Rojas, J.L.G.Fierro, J.L. Margitfalvi, Modified preparation method for highly active Au/SiO2

catalysts used in CO oxidation, Appl. Catal. A 347 (2008) 216–222.[2] S.J. Lee, A. Gavriilidis, Supported Au catalysts for low-temperature CO oxidation

prepared by impregnation, J. Catal. 206 (2002) 305–313.[3] M. Haruta, Novel catalysis of gold deposited on metal oxides, Catal. Surv. Jpn.

1 (1997) 61–73.[4] R. Zanella, S. Giorgio, C.R. Henry, C. Louis, Alternative methods for the prepa-

ration of gold nanoparticles supported on TiO2, J. Phys. Chem. B 106 (2002)7634–7642.

[5] N.S. Patil, B.S. Uphade, D.G. McCulloh, S.K. Bhargava, V.R. Choudhary, Styreneepoxidation over gold supported on different transition metal oxides preparedby homogeneous deposition-precipitation, Catal. Commun. 5 (2004) 681–685.

[6] X. Jiang, T. Wang, Y.W. Wang, Preparation of TiO2 nanoparticles on the surfaceof SiO2 in binary liquids, Colloids Surf. A 234 (2004) 9–15.

[7] X. Jiang, S. Chen, C.W. Mao, Synthesis of Ag/SiO2 nanocomposite material byadsorption phase nanoreactor technique, Colloids Surf. A 320 (2008) 104–110.

[8] X. Jiang, H. Deng, Influence of copper species on silver grain size in Ag–CuO–SiO2

ternary nanocomposites, Colloids Surf. A 330 (2008) 180–183.[9] R. Jenkins, R.L. Snyder, Introduction of X-ray Powder Diffractometry, Wiley,

New York, 1996.

[

[

chem. Eng. Aspects 358 (2010) 122–127 127

10] J.D. Grunwaldt, M. Maciejewski, O.S. Becker, P. Fabrizioli, A. Baiker, Comparativestudy of Au/TiO2 and Au/ZrO2 catalysts for low-temperature CO oxidation, J.Catal. 186 (1999) 458–469.

11] S.L. Westcott, S.J. Oldenburg, T.R. Lee, N.J. Halas, Formation and adsorption ofclusters of gold nanoparticles onto functionalized silica nanoparticle surfaces,Langmuir 14 (1998) 5396–5401.

12] A.P. Oliveira, M.L. Torem, The influence of some metallic cations on deinkingflotation, Colloids Surf. A 110 (1996) 75–85.

13] M. Haruta, Adsorption of CO on gold supported on TiO2, Catal. Today 36 (1997)115–123.

14] D.B. Akolekar, S.K. Bhargava, Investigations on gold nanoparticles in meso-porous and microporous materials, J. Mol. Catal. A: Chem. 236 (2005) 77–86.

15] H. Falsig, B. Hovlbaek, I.S. Kristensen, T. Jiang, T. Bligaard, C.H. Christensen, I.K.Nørskov, Trends in the catalytic CO oxidation activity of nanoparticles, Angew.Chem. Int. Ed. 47 (2008) 4835–4839.

16] X. Zhang, A. Corma, Supported gold(III) catalysts for highly efficient three-component coupling reactions, Angew. Chem. Int. Ed. 47 (2008) 4358–4361.

17] A. Corma, C. González-Arellano, M. Lglesias, F. Sánchez, Gold nanoparticles andgold(III) complexes as general and selective hydrosilylation catalysts, Angew.Chem. Int. Ed. 46 (2007) 7820–7822.

18] T. Ishida, M. Haruta, Gold catalysts: towards sustainable chemistry, Angew.Chem. Int. Ed. 46 (2007) 7154–7156.

19] A. Abad, A. Corma, H. Garciá, Catalyst parameters determining activity andselectivity of supported gold nanoparticles for the aerobic oxidation of alcohols:the molecular reaction mechanism, Chem. Eur. J. 14 (2008) 212–222.

20] R. Zhao, D. Ji, G.M. Lu, G. Qian, L. Yan, X.L. Wang, J.S. Suo, A highly efficientoxidation of cyclohexane over Au/ZSM-5 molecular sieve catalyst with oxygenas oxidant, Chem. Commun. 7 (2004) 904–905.

21] G.M. Lu, D. Ji, G. Qian, Y.X. Qi, X.L. Wang, J.S. Suo, Gold nanoparticles inmesoporous materials showing catalytic selective oxidation cyclohexane usingoxygen, Appl. Catal. A 280 (2005) 175–180.

22] G.M. Lu, R. Zhao, G. Qian, Y.X. Qi, X.L. Wang, J.S. Suo, A highly efficient catalystAu/MCM-41 for selective oxidation cyclohexane using oxygen, Catal. Lett. 97(2004) 115–118.

23] K.K. Zhu, J.C. Hu, R. Richards, Aerobic oxidation of cyclohexane by gold nanopar-

ticles immobilized upon mesoporous silica, Catal. Lett. 100 (2005) 195–199.

24] L.X. Xu, C.H. He, M.Q. Zhu, S. Fang, A highly active Au/Al2O3 catalyst for cyclo-hexane oxidation using molecular oxygen, Catal. Lett. 114 (2007) 202–205.

25] Y.J. Xu, P. Landon, D. Enache, A.F. Carley, M.W. Roberts, G.J. Hutchings, Selectiveconversion of cyclohexane to cyclohexanol and cyclohexanone using a goldcatalyst under mild conditions, Catal. Lett. 101 (2005) 175–179.