Mesostructure Au/TiO2 nanocomposites for highly efficient catalytic reduction of p-nitrophenol

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Journal of Molecular Catalysis A: Chemical 358 (2012) 145– 151

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Mesostructure Au/TiO2 nanocomposites for highly efficient catalytic reduction ofp-nitrophenol

Adel A. Ismail a,c,∗, Amer Hakkib,1, Detlef W. Bahnemannb,1

a Advanced Materials Department, Central Metallurgical R. & D. Institute (CMRDI), P.O. Box: 87, Helwan 11421, Cairo, Egyptb Photocatalysis and Nanotechnology Unit, Institut für Technische Chemie, Leibniz Universität Hannover, Callinstrasse 3, 30167 Hannover, Germanyc Centre for Advanced Materials and Nanoengineering (CAMNE), Najran University, P.O. Box: 1988, Najran 11001, Saudi Arabia

a r t i c l e i n f o

Article history:Received 28 January 2012Received in revised form 1 March 2012Accepted 12 March 2012Available online 21 March 2012

Keywords:Au/TiO2

MesoporousCatalytic reductionp-Nitrophenol

a b s t r a c t

Mesoporous Au/TiO2 nanocomposites have been synthesized using two methods. (1) In situ preparation;(2) photochemical deposition of Au onto either highly ordered mesoporous TiO2 or disordered TiO2. Fol-lowing the photodeposition process, the Au nanoparticles are dispersed and uniform exhibiting diametersof ∼10 nm; however, following the in situ preparation, the Au particles are ranging diameters of approx.25–300 nm based on the Au content (0.3–5 wt%). The prepared catalysts have been tested for catalyticreduction of p-nitrophenol (p-NPh) in presence of sodium borohydride. Using the Au/TiO2 prepared byin situ method, the reaction rate of the catalytic reduction of p-NPh was found to be 6 times higher whenthe amount of Au decreases from 5 wt% to 0.3 wt%. However, Au photodeposited onto TiO2 is much better2 times higher than that in situ Au/TiO2 prepared. The larger catalytic activity of the Au/TiO2 nanocompos-ites prepared by photodeposition process is attributed to the higher dispersity and the small size of theAu particles (10 nm). The results indicated that a highly ordered mesoporous system is not a prerequisitefor high catalytic activity. However, the sample clacined at 350 ◦C must be considered as economicallymore viable catalysts as compared to that clacined at 500 ◦C since the preparation energy can be savedin the calcination step. The recycling tests indicated that Au/TiO2 nanocomposites was quite stable nosignificant decrease in catalytic reduction of p-NPh was observed even after being used repetitively for5 times, showing a good potential in practical application.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

In recent years much attention has been focused on the syn-thesis of nanosized noble metal particles because of their unusualphysical and chemical properties compared to bulk metals andtheir important applications in catalysis [1]. The discovery of theextremely high catalytic activity of Au NPs for the CO oxidationby Haruta et al. [2] has triggered intensive research on Au NPscatalysis to reveal its high catalytic activity for many chemical reac-tions [3]. Au nanoparticles (NPs) have been extensively investigatedfor their applications as catalysts for low-temperature oxidationreactions such as CO oxidation, selective propene oxidation, liquid-phase glycohol oxidation [4], and other catalytic and photocatalyticoxidation reactions [5–12]. On the other hand, AuNPs have alsobeen found to be very active for a number of reduction reactions[13–15]. Among these reduction reactions the catalytic reductionof p-nitrophenol (p-NPh) in the presence of NaBH4 has been used

∗ Corresponding author. Tel.: +20 2 25010643; fax: +20 2 25010643.E-mail addresses: [email protected], [email protected] (A.A. Ismail).

1 Tel.: +49 511 7625560; fax: +49 511 7622774.

frequently to check the catalytic activity of free or immobilized Aunanoparticles [16,17].

To facilitate catalyst recovery, AuNPs are usually dispersed ontosolid matrices to prepare heterogeneous AuNPs catalysts. Variousmaterials have been used as supporting matrices, including carbonnanotubes, silica, titania, ceria and alumina [18–20]. Quite recently,Shi and co-workers [17,21], have reported the synthesis of stableAuNPs using a natural materials such as tannin or collagen. In allthese cases, the most intriguing point is the effect of Au particle sizeon the catalytic activity. Due to its thermal and chemical stability,non toxicity, and relatively low cost; TiO2 is still one of favorablecandidate as a support for metals nanoparticles.

In most cases, the metal nanocrystals were anchored on thesurfaces of the metal oxide as isolated “islands” to produce het-erointerfaces due to catalytic activity dependent on the size of themetal nanocrystals [16,22–24]. Metal particles should be dispersedevenly on the supports in order to prevent aggregation betweenparticles in close proximity, and to maintain metal–support con-tact areas. Numerous processes have been developed for effectivedispersion of metal nanoparticles on metal oxide support [25,26].In this regard, materials, such as TiO2, with regular and large meso-pores tend to become indispensable. These materials possess high

1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.doi:10.1016/j.molcata.2012.03.009


146 A.A. Ismail et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 145– 151

specific surface areas, regular and tunable pore sizes, large pore vol-umes, as well as stable and interconnected frameworks with activepore surface for easy modification or functionalization, meeting therequirements as promising adsorbents and/or catalysts [27]. Theimpregnation method is a general strategy for the preparation ofAuNPs [20,28]. However, the resultant catalysts often suffer from asignificant loss of catalytic activity during recycling owing to weakinteractions between the AuNPs and supporting matrices. Becauseof the photocatalytic activity of TiO2 the photoassisted depositionmethod can be used to achieve a successful deposition of the met-als nanoparticles from its salts aqueous solutions on the surface ofTiO2 upon illumination [29]. This method has been widely usedin order to improve the photocatalytic activity of the semicon-ductor via the trapping of the photogenerated electrons by thedeposited metal nanoparticles [30–32]. In our previous work wereported a successful one-step synthesis of hexagonal P6m meso-porous Pt/TiO2 nanocomposites at different Pt content (0–2 wt%)and anatase/rutile ratios. The effect of the crystallinity and theporosity on the photocatalytic oxidation of methanol in aqueoussuspensions was investigated and found to be strongly affected byboth of them [33].

Therefore, our strategy focused on the preparation of meso-porous Au/TiO2 nanocomposites employing two differentpathways: (1) The one step synthesis in which all precursorswere dissolved and allowed to react in one pot to achieve thein situ prepared Au/TiO2. (2) The second methodology in whichAu particles were photocatalytically deposited onto TiO2 calcinedat either 350 ◦C or 500 ◦C. The effect of the Au particle size andthe orderity of the mesoporous TiO2 on the catalytic activity ofthe Au/TiO2 composite have been investigated. The reductionof p-nitrophenol (p-NPh) to p-aminophenol (p-APh) by NaBH4has been chosen as a model reaction because this reaction israpid and it can be easily monitored via UV–vis spectroscopyby the decrease of the strong adsorption of 4-nitrophenolateanion at 400 nm, leading directly to the rate constant[34–38].

2. Experimental

2.1. In situ preparation of Au/TiO2 nanocomposites

The block copolymer surfactant EO106–PO70EO106 (F-127,EO = CH2CH2O , PO = CH2(CH3)CHO ) was obtained fromSigma, titanium tetraisopropoxide (TTIP), titanium tetrabutoxide(TBOT), glacial acetic acid, ethanol, methanol, and hydrogen tetra-chloroaurate tetrahydrate (HAuCl4·4H2O) were purchased fromAldrich and were used without further purification. TiO2 andAu/TiO2 nanocrystals were synthesized through a simple one-stepsol–gel process in the presence of the F127 triblock copolymeras structure directing agent [39]. To embed gold nanoparticleshomogenously within the titania framework, a multicomponentassembly approach was utilized, where the surfactant, titania,and gold were assembled. TiO2 and Au/TiO2 were prepared asfollows; at first, 1.29 g of the triblock copolymer F-127 wasdissolved in 11 g of methanol by stirring for 30 min. Then, asolution of 9.84 g TTIP in 1.44 mL HCl (37%) was added undervigorous stirring for 10 min, followed by a dropwise additionof the calculated amount of HAuCl4 aqueous solution (40 g L−1)to obtain 0.3, 0.5, 1, 3, and 5 wt% Au in the Au/TiO2 compos-ites and the mixture was stirred vigorously for 10 min. Finally,the sol was gelled at 40 ◦C in air for 30 min. The gel sampleswere calcined at 500 ◦C for 4 h with a heating rate of 2 ◦C/minand a cooling rate of 2 ◦C/min in air to remove the surfactantand to obtain nanocrystalline Au/TiO2 catalysts with disorderedmesostructure.

2.2. Preparation of highly ordered TiO2 nanocomposites

1.6 g of F127, 2.3 mL of CH3COOH and 0.74 mL of HCl were dis-solved in 30 mL of ethanol and then added to 3.5 mL of TBOT [40,41].The mixture was stirred vigorously for 60 min and transferred intoa Petri dish. Ethanol was subsequently evaporated at 40 ◦C and arelative humidity of 40% for 12 h followed by the transfer of thesample into a 65 ◦C oven and ageing for an additional 24 h. The as-made mesostructured hybrids were calcined at 350 ◦C in air for 4 hat a heating rate of 1 ◦C/min and a cooling rate of 2 ◦C/min to removethe surfactant and to obtain highly ordered mesostructured TiO2.

2.3. Photochemical deposition of gold on mesoporous TiO2

Au was photochemically deposited onto both orderedmesostructure TiO2 calcined at 350 ◦C and disordered mesostruc-ture TiO2 calcined at 500 ◦C as follows: 1.0 g of mesoporous TiO2was suspended by stirring in 200 mL aqueous methanol solution(1.0 vol%) containing calculated amount of HAuCl4·4H2O. Theresulting solution was irradiated with UV(A) light by a Philips (Hg)lamp (illumination intensity 2.0 mW cm−2) for 12 h. The obtainedpowder was separated by centrifugation, washed three times withdistilled water and dried at 110 ◦C for 12 h. The catalysts preparedby this method are denoted as Au/TiO2-350 and Au/TiO2-500.

2.4. Characterization

Wide angle XRD (WXRD) data were acquired on a BrukerAXS D4 Endeavour X diffractometer using Cu K�1/2, �˛1 = 154.060pm, �˛2 = 154.439 pm radiation and small angle X-ray diffraction(SXRD) patterns were recorded on a Bruker D8 advance. The nitro-gen adsorption and desorption isotherms at 77 K were measuredusing a Quantachrome Autosorb 3B after vacuum-drying the sam-ples at 200 ◦C overnight. The sorption data were analyzed usingthe Barrett–Joyner–Halenda (BJH) model with Halsey equation[42]. Transmission electron microscopy (TEM) measurements wereconducted at 200 kV with a JEOL JEM-2100F-UHR field-emissioninstrument equipped with a Gatan GIF 2001 energy filter and a1k-CCD camera in order to obtain EEL spectra.

2.5. Catalytic reduction of p-nitrophenol

The catalytic activities of the prepared catalysts wereinvestigated using the reduction of p-nitrophenol (p-NPh) to p-aminophenol (p-APh) in the presence of NaBH4 as a model reaction.In all catalytic runs, the experimental conditions were kept con-stant at molar ratio Au:p-NPh:NaBH4 of 1:10:1000. In a typical run,to a suspension of (x) wt% Au/TiO2 nanocomposites (Au 3 �mol)dispersed in H2O (10 mL), 10 mL of freshly prepared aqueous solu-tion of NaBH4 (300 mM) was added and the mixture was stirredfor 15 min at adjusted temperature (298 K). Then 10 mL of p-nitrophenol (3 mM) was added to the mixture and the suspendedsolution was stirred at the same temperature. The reaction progresswas monitored by assessing a small portion of the reaction mix-ture at a regular time and measuring the extinction of the reactionmixture, after dilution 10 times, at 400 nm as a function of time.Measurements were carried out using a Varian Cary 100 ScanUV–vis spectrophotometer.

3. Results and discussions

3.1. Catalysts investigations

In situ preparation of mesoporous Au/TiO2 nanocomposites (Seesupplementary materials) at varied Au concentrations was char-acterized by small angle X-ray scattering, Nitrogen adsorption


A.A. Ismail et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 145– 151 147

654321

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ty/

(Counts

)

0

5000

10000

15000

20000

25000

Au/TiO2-350

1 wt% Au/TiO2

2θ/ o

Fig. 1. SAXS patterns of mesoporous Au/TiO2 nanocomposites calcined at 500 ◦C for4 h and Au photodeposited on highly ordered mesoporous TiO2 calcined at 350 ◦C.

isotherms, and TEM. The small angle X-ray scattering (SAXS) pat-terns of highly ordered mesoporous calcined at 350 ◦C are shownin Fig. 1. The sample shows two well-resolved peaks, which canbe indexed to the (1 0) and (2 0) Bragg reflections confirmingan ordered 2D-hexagonal mesostructure of the P6m space group[39]. The observed high intensities and the sharpness of the peaksprove that a long-range order exists in the TiO2. With increas-ing calcination temperature up to 500 ◦C, SAXS indicated thatthe mesostructural regularity of the Au/TiO2 nanocomposites wasnearly collapsed entirely because of the crystal growth during cal-cination (Fig. 1).

It is evident that after the collapses of the hexagonal orderingeven the pore channels themselves start to collapse and disor-dered mesostructures of the crystalline TiO2 are obtained. After theremoving of the template, the structural regularity declines but thelattice parameters calculated from the d10 value decrease only from12.96 to 11.74 and 11.12 nm as a result of calcinations at 350 and500 ◦C, respectively indicating an approx. 9.4 and 14.3% contractionof the structure.

The X-ray diffraction (XRD) patterns in Fig. 2 show the in situprepared Au/TiO2 calcined at 500 ◦C for 4 h and the Au/TiO2-350samples. In order to confirm the bulk composition for each sample,the XRD patterns were compared with the JCPDS-ICDD standardsfor anatase (21-1272), rutile (21-1276) and Au (04-0784). Thediffractograms for TiO2 are essentially equivalent, exhibiting peaksat 25.4◦, 36.4◦, 48.1◦, 54.2◦ and 62.8◦ that are consistent withthe (1 0 1), (0 0 4), (2 0 0), (2 1 1) and (2 1 3) planes associated withtetragonal anatase, evidencing that the TiO2 phase easily nucleatesduring heating and remains intact upon calcination. In addition, asignal at 27.4◦ and 41.3◦ are observed corresponding to the (1 1 0)and (1 1 1) planes rutile. In contrast, the catalysts with higher Aucontent exhibited a peak at 2� = 38.1◦ and 44.5◦ that correspondsto the (1 1 1) and (2 0 0) Au planes. The peak at 38.1 with 100%relative intensity peak counts which confirm the Au0. This resultsuggests that the presence of Au facilitated the transition of anataseto rutile during calcination at 500 ◦C. The requisite temperature forTiO2 phase transformation can be affected by the inclusion of arange of additives [43] that catalyze or inhibit the transformation.Our findings suggest that the presence of Au lowers the temper-ature requirements for rutile formation, where Au content is acritical factor. A switch from anatase to rutile is accompanied by

2θ/ degree

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.

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(c)

(d)

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A: Anatase

R: Rutile

Au: GoldAR

A Au

Au Au

A

AAA

AA AR

R

A AuA

Fig. 2. XRD patterns for Au photodeposited on TiO2 calcined at 350 ◦C (a) andAu/TiO2 nanocomposites at different Au content, 0 wt% (b), 1 wt% (c), 3 wt% (d), and5 wt% (e) obtained from calcination at 500 ◦C for 4 h. Shifted for sake of clarity.

a decrease in surface area, as noted elsewhere [44] and shown inTable 1. It is common to explain the modification in the transi-tion temperature due to an oxygen deficiency (OD). It has beenreported that the modification of the anatase to rutile phase transi-tion temperature was due to an excess in oxygen vacancies. It wasargued that, when the dopants were introduced into the matrix,due to their own stoichiometry, an OD was produced. This OD pro-vides the space required for the anions and cations of the titaniato produce the anatase to rutile phase transition. Once the tran-sition has taken place, the crystalline structure is contracted toproduce the densified rutile phase. This has been the mechanismused to explain why the OD promotes the anatase–rutile phasetransformation [43]. Analyzing the width at half maximum of thereflections at 2� = 25.4◦ employing Scherrer’s equation [45] resultsin TiO2 nanoparticle sizes with a maximum of 10 nm (Table 1).

Nitrogen adsorption isotherms of the mesoporous in situpreparation 1 wt% Au/TiO2 and Au/TiO2-350 samples are shownin Fig. 3. Typical reversible type IV adsorption isotherms are

Relative pressure (P/Po)

1.00.80.60.40.20.0

50

100

150

200

1 wt% Au/TiO2

Au/TiO2-350

Vo

lum

e ab

sorb

ed (

cm3/g

, S

TP

)

Fig. 3. Nitrogen adsorption–desorption isotherms and BJH pore-size distributionplot (inset) for mesoporous 0.5 wt% Au/TiO2 after calcinations at 500 ◦C for 4 h andfor mesoporous Au/TiO2-350 samples.



Table 1Textural properties of mesoporous Au/TiO2 nanocomposites synthesized by in situ and photodeposition and for catalytic reduction of p-nitrophenol to p-aminophenol.

Entry Catalysts PS Au (nm) SAa (nm2) TSAa (cm2) k/103 (s−1) r (107 × mol L−1 s−1) SBET (m2 g−1) Vp (cm3/g) Dp (nm)

1 5% Au/TiO2 300 282,600 6.11 0.7 8 91 0.189 9.12 3% Au/TiO2 200 125,600 9.16 1.6 18 97 0.211 9.63 1% Au/TiO2 75 17,663 24.43 3.6 39 98 0.233 9.64 0.5% Au/TiO2 50 7850 36.65 3.7 41 107 0.259 10.85 0.3% Au/TiO2 25 1963 73.29 4.2 46 129 0.267 11.06 1% Au/TiO2-500 – – – 7.1 74 173 0.286 10.07 1% Au/TiO2-350 10 314 183.23 6.5 68 245 0.31 7.1

k, rate constant; r, reaction rate; SBET, surface area; PS Au, average particle size of Au nanoparticle; SA, surface area of the Au nanoparticle calculated assuming a sphericalshape; TSA, the total surface area of the used catalyst dose; Vp, pore volume, Dp, pore diameter.

a Assuming Au nanoparticles are spherelike and using the particle size, the volume of each Au nanoparticles was calculated from the formula (v = 4/3�r3). Consideringthe mass density of gold is d = 19.32 g cm−3, the average weight of one Au nanoparticle is derived as (weight = v × d). The number of Au nanoparticles used in each run wascalculated by dividing the weight of gold used in each catalyst dose by the wight of a single Au particle. Then the total surface of the Au in the used catalyst dose was calculatedby multiplying the number of Au particles by the surface area of a single particle which was calculated from the formula (S = 4�r2).

found for both samples. For Au/TiO2-350 sample, the sharp-ness of the inflection resulting from capillary condensation atrelative pressures p/p0 between 0.45 and 0.7 is characteristicfor mesopores ordered in two-dimensional hexagonal symmetry.However, for 1 wt% Au/TiO2 calcined at 500 ◦C the inflection isshifted to relative pressures p/p0 between 0.60 and 0.88 char-acteristic interparticle mesopores (disordered mesostructures).The Barrett–Joyner–Halenda (BJH) analysis shows that the 1 wt%Au/TiO2 and Au/TiO2-350 mesoporous network exhibits mean poresizes of about 10 and 7 nm, respectively (Fig. 3 inset). BET surfaceareas, the pore volumes, and the pore diameters of all samplesare summarized in Table 1. It can be noticed from this table that,with increasing gold loading, In situ preparation, the surface areasdecrease from 173 to 90 m2 g−1, reflecting the increase in size of theanatase nanoparticles, whereas the surface area of Au/TiO2-350 is245 m2 g−1.

TEM images of in situ preparation revealed that Au/TiO2nanocomposites corresponding anatase TiO2 nanoparticles(Fig. 4a), which mainly consisted of nanoscale cubes and rhom-bohedral [46]. The TEM images of the mesoporous Au/TiO2-350

nanocomposites show a well-defined 2D hexagonal mesostruc-ture, evincing the formation of a highly ordered mesostructure(Fig. 4b) [47], which is consistent with the analysis of the SAXSpattern. Already upon calcination at 350 ◦C the TiO2 nanocrystalsare randomly oriented within the amorphous walls as indicatedby the characteristic lattice fringes (Fig. 4b) and the particle size ofTiO2 nanocrystals has been measured to be 5 nm. Dark field-STEM(DF-STEM) (Fig. 4c–f) detected Au nanoparticles and the imagesshow that Au nanoparticles are dispersed. The resulting particleswere nearly spherical with an average diameter of 300 nm at 5 wt%Au/TiO2 which decreased to 25 nm at 0.3 wt% Au/TiO2. Energydispersive X-ray (EDX) analysis reveals the presence of Au, Ti andO and confirms that the final Au and Ti content in the compositematerials is consistent with the Au:Ti ratio used in the starting solmixtures.

3.2. Catalytic reduction of p-nitrophenol

The catalytic performance of Au/TiO2, prepared either in situor photocatalytically, were tested for the reduction of p-NPh to

Fig. 4. Representative TEM micrographs of a sample calcined at 500 ◦C for 4 h (a). TEM image of two-dimensional hexagonal mesoporous Au photodeposited TiO2 calcinedat 350 ◦C (b), and the dark-field TEM image of Au photodeposited TiO2 calcined at 350 ◦C (c). DF-TEM of 1, 3 and 5 wt% Au/TiO2 nanocomposites calcined at 500 ◦C for 4 h (d),(e) and (f), respectively.



600550500450400350300250

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Fig. 5. Time-dependent UV–vis spectral changes of the reaction mixture of (p-NPh)by sodium borohydride catalyzed by mesoporous Au/TiO2 nanocomposites. Themain peak at 400 nm (nitrophenolate ions) is decreasing with reaction time (blackarrow), whereas a second peak at 300 nm (p-APh) is slowly increasing. The twoisosbestic points appear at 280 and 314 nm. The experimental conditions were keptconstant at molar ratio Au:p-NPh:NaBH4 of 1:10:1000 using 1%Au/TiO2-500 as acatalyst.

p-APh as a model reaction with an excess amount of NaBH4.It has become one of the model reactions for evaluating thecatalytic activity of various metal nanoparticles. The p-NPh isinert to NaBH4 and the reaction did not occur in the absenceof Au/TiO2 catalysts, even for a period of 24 h. However, Au NPscan effectively catalyze the reduction of p-NPh by acting as anelectron relay system; electron transfer takes place between p-NPh and BH4

− through the Au NPs. The reaction can be readilymonitored by using UV–vis spectroscopy. Fig. 5 shows a typicalUV–vis absorption change of the reaction mixture by the addi-tion of Au/TiO2-500. The p-nitrophenolate anion formation fromp-NPh (pKa = 7.15) in the initial step upon addition of NaBH4 isindicated when the peak at 317 nm (due to p-NPh) is red shiftedto 400 nm. The isosbestic points demonstrate with high precisionthat p-NPh is fully converted to p-APh and no side reaction takesplace [34–38]. The reduction can be visualized by the graduallydecreases of the 400 nm peak with the concomitant appearanceof a new peak at 300 nm after the catalytic reaction as a result ofp-APh formation [34–38]. Usually an induction time in which noreduction takes place is observed as a result of the presence of dis-solved oxygen in water [48]. The oxygen will react at a faster ratewith the NaBH4 than nitrophenol. To avoid this, in many cases,nitrogen is purged before adding NaBH4. In our case, however,such a phenomenon is not involved because the catalyst was pre-activated with NaBH4 for 15 min before adding of p-NPh, and thusthe reaction was carried out under atmospheric conditions. In caseof bare TiO2, a slight decrease of the 400 nm peak is observed.However, this decrease is not associated with the concomitant evo-lution of a peak at 300 nm indicating that the process does notinvolve any reduction, rather it is a mere adsorption of the p-nitrophenolate ion (data not shown) [49]. Since the concentrationof NaBH4 largely exceeds the concentration of p-NPh, the reductionrate can be assumed to be independent of NaBH4 concentration. So,in this case, a pseudo-first-order rate kinetics with regard to thep-nitrophenolate concentration could be used to evaluate the cat-alytic rate [48]. Linear relation of ln(Ct/C0) versus reaction time wasobserved in all reaction runs (see Fig. 6). In all catalytic runs, theexperimental conditions were kept constant at molar ratio Au:p-NPh:NaBH4 of 1:10:1000. The rate constants and reaction rates (see

Fig. 6. Plot of ln(Ct/C0) versus reaction time in the presence of mesoporous Au/TiO2

nanocomposites prepared by either in situ method at different Au content 0, 5, 3, 1,0.5, 0.3 wt% (a, b, c, d, e, and f, respectively) or photodeposition method, Au/TiO2-350(g) and Au/TiO2-500 (h). The experimental conditions were kept constant at molarratio Au:p-NPh:NaBH4 of 1:10:1000.

Table 1) were estimated from the slopes of the linear relations ofln(Ct/C0) versus reaction time.

3.2.1. Effect of the catalyst surface areaThe heterogeneous catalysis reaction may occur in four steps:

(1) adsorption of the reactant molecule to the surface, (2) diffusionof the molecule to the active site and formation of the surface com-plex, (3) reaction of the complex to form the adsorbed product, and(4) finally desorption of the product. So the adsorption and diffu-sion play an important role in the heterogeneous catalysis system.The results presented in (Table 1) indicate that the reaction ratesof p-NPh reduction over mesoporous Au/TiO2 are increased withdecreasing the percentage of the gold in Au/TiO2 nanocomposites.This may be attributed to many reasons; firstly, a higher intrin-sic activity of the mesoporous catalyst enabling rapid transportanionic reactants, i.e., p-nitrophenolate and NaBH4 to the activesites due to the facile diffusion of the anionic reactants throughthe porous network, thus increasing the catalytic efficiency. Sec-ondly, the increased catalytic activity of mesoporous TiO2 can beexplained by its high surface area which should be beneficial forthe adsorption of the p-NPh molecules. Finally, the decrease ofthe Au particle size is reflected in an increase of the metal sur-face area. However, the results presented in Table 1 show thatthe surface area of TiO2 has no real effect on the catalytic activ-ity. It can be seen from Table 1 (entries 1–3) that the reaction ratesincreased from 8 × 10−7 mol L−1 s−1 to 39 × 10−7 mol L−1 s−1 whilethe BET surface area only changed from 91 m2 g−1 to 98 m2 g−1.Moreover, the reaction rates using either Au/TiO2-350 or Au/TiO2-500 (entries 6 and 7) are almost equal which indicates that thecrystallinity of the support does not play a role on the activityof the catalyst. We suggest that the well-ordered mesostructureof Au/TiO2-350 and disordered Au/TiO2-500 supports the trans-port properties of all reactants involved in the catalytic processand, thus, enhances the overall activity. The good catalytic perfor-mance of both Au/TiO2-350 and Au/TiO2-500 indicates that a highlyordered mesoporous system is not a prerequisite for high catalyticactivity.

3.2.2. Effect of the Au particle sizeThe results of the in situ prepared catalyst (Table 1, entries

1–5) indicates that the rate constants increased from 7 × 10−3 s−1

to 7 × 10−2 s−1 as the Au nanoparticle diameters decreased from300 nm to 25 nm. It was reported that a decrease in the particle size



Scheme 1. Schematic illustration of the proposed mechanism to explain theenhanced catalytic activity of mesoporous Au/TiO2 nanocomposites for catalyticreduction of p-nitrophenol to p-aminophenol.

leads to an increase in the fraction of low coordination metal sitessuch as vertices, edges, and kinks, which can promote adsorption ofthe reactants and facilitate the reaction. However, this is for smallparticles of less than 5 nm diameter [50], but in case of the in situprepared catalysts, which has a diameter > 25 nm, there will not bea significant increase of the active surface atom fractions. However,the apparent reaction rate can be related to the total surface of thenanoparticles. The total surface of the used catalyst increased from6 cm2 to 183 cm2 when the Au particle diameter decreased from300 nm to 10 nm, respectively. It was reported that the adsorp-tion of substrates on Au particle is driven by chemical interactionbetween the particle surface and the substrates. Here nitropheno-late ions get adsorbed onto the Au particle surface when present inthe aqueous medium. This caused a blue shift of the plasmon band[51]. A strong nucleophile such as NaBH4, because of its diffusivenature and high electron injection capability, transfers electrons tothe substrate via metal particles. This helps to overcome the kineticbarrier of the reaction. The total reaction steps can be summarizedin Scheme 1.

For industrial point of view, the possibility of catalyst recov-ery and reuse in catalytic processes is of major importance, sinceit contributes significantly to lowering the operational cost ofthe catalytic processes and wastewater treatment. To explore theadvantage of Au/TiO2-350 nanocomposites and their applicabil-ity, reuse cycles of newly catalysts was tested for the reductionof p-NPh (Fig. 7). Experiments were performed where the catalystAu/TiO2-350 was recovered and reused by keeping all other param-eters constant. The results revealed that Au/TiO2-350 shows a verygood activity for five catalytic runs without any significant loss inthe p-NPh conversion. The conversion yield of p-NPh after 20 minwas still as high as 98% even at the fifth run. It can be conclude thatAu/TiO2-350 nanocomposites has acceptable stability and it may be

54321

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50

60

70

80

90

100

Co

nv

ersi

on

(%

)

Recycles

Fig. 7. Repeated cycles up to 5 times The conversion yield of p-NPh after 20 min overAu/TiO2-350 nanocomposites. The experimental conditions were kept constant atmolar ratio Au:p-NPh:NaBH4 of 1:10:1000.

reusable for at least 5 runs, showing a good potential for practicalapplications.

4. Conclusions

Gold-titania nanocomposites at different Au nanoparticles weresynthesized, characterized, and tested as reduction catalysts forp-NPh with NaBH4. The reaction with Au nanoparticles (25 nm)was fastest and its reaction rate was about 6 times higher thanthat of Au nanoparticles (300 nm). These experimental resultsshow that Au particle sizes plays a crucial role for the efficientnanoparticle-catalyzed electron transfer reactions. It is concludedthat the network structure has to be taken into account for theunderstanding of the strong enhancement of catalytic activity bysuch a small amount of Au. Au/TiO2 nanocomposites exhibit highcatalytic activity toward p-NPh reduction at room temperature.This mesoporous system catalyzed the reduction of p-NPh, whichexhibited a size-dependent reaction property. The good catalyticperformance of either Au/TiO2-350 or Au/TiO2-500 indicates that ahighly ordered mesoporous system is not a prerequisite for highcatalytic activity. However, Au/TiO2-350 must be considered aseconomically more viable catalysts as compared to Au/TiO2-500since the preparation energy can be saved in the calcination step.The recycle tests of Au/TiO2-350 nanocomposites confirm that theprepared catalysts are very stable and high efficient catalytic reduc-tion of p-nitrophenol.

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Mesostructure Au/TiO2 nanocomposites for highly efficient catalytic reduction of p-nitrophenol

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