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Catalysis Communications 8 (2007) 821–824

Enhancement of MTBE photocatalytic degradation by modificationof TiO2 with gold nanoparticles

Alexander Orlov, David A. Jefferson, Mintcho Tikhov, Richard M. Lambert *

Chemistry Department, Cambridge University, Lensfield Road, Cambridge CB2 1EW, UK

Received 27 February 2006; received in revised form 31 August 2006; accepted 31 August 2006Available online 16 September 2006

Abstract

The photocatalytic degradation of methyl tert-butyl ether by gold-modified TiO2 has been studied by a combination of high resolutionelectron microscopy, X-ray photoelectron spectroscopy and reactor measurements. The optimum gold loading corresponded to a meanAu particle size of 6 3 nm at which point the gold may no longer be metallic. Such catalysts exhibited a threefold rate enhancementcompared to unmodified TiO2, possibly due to injection of photo-excited electrons from semiconducting Au into the TiO2 conductionband. Since the presence of dissolved oxygen was crucially important to good performance, it is possible that an additional factor waslowering of the local work function, thus promoting electron transfer to dioxygen molecules adsorbed at the Au/TiO2 interface.� 2006 Elsevier B.V. All rights reserved.

Keywords: Gold; TiO2; Photocatalytic decomposition; MTBE; HREM; XPS

1. Introduction

Semiconductor photocatalysis is a dynamic field thatplays an important part in modern catalytic research. Inparticular, environmental applications of titanium dioxide,one of the most active and stable photocatalysts, haveattracted considerable attention [1]. Numerous attemptsto improve the catalytic performance of TiO2 haveincluded, among other methods, modification with noblemetals. Most such efforts have been centred on Pt/TiO2 sys-tems [2], although a limited number of studies on the Au/TiO2 and Ag/TiO2 systems have also been reported [3–9].

Methyl tert-butyl ether (MTBE) is a common pollutantof significant environmental impact and has become anissue of considerable concern in recent years. However,the photocatalytic approach for MTBE degradation ismuch less developed than that for other pollutants, withonly a few publications devoted to the subject [10–12].These earlier papers deal exclusively with the application

1566-7367/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.catcom.2006.08.040

* Corresponding author. Tel.: +44 1223 336467; fax: +44 1223 336362.E-mail address: rml1@cam.ac.uk (R.M. Lambert).

of unmodified TiO2. Recently we demonstrated that mod-ification of titania with gold nanoparticles can increase itsphotocatalytic activity towards 4-chlorophenol degrada-tion [13]. However, the question remains as whether thisstriking behaviour is specific to a particular class of mole-cules (i.e. aromatics) or is applicable to a wider range oforganic pollutants, such as MTBE, which is chemicallyvery different from 4-chlorophenol. Moreover, such issuesas effect of oxygen on the catalyst activity and the effectof gold loading on the catalyst activity towards aliphatichydrocarbons were not addressed in the previous work.We demonstrate here that Au particles can also induce asubstantial improvement in the photocatalytic performanceof TiO2 towards degradation of MTBE and show that oxy-gen plays an important role in catalyst activity.

2. Experimental

The photocatalytic system was based on a 1000 W Xelamp (ORIEL), the radiant flux from which was colli-mated, filtered once to remove infra red radiation, thenagain to eliminate radiation below 300 nm which can

Fig. 1. TEM image of 0.55 at.% Au modified TiO2.

822 A. Orlov et al. / Catalysis Communications 8 (2007) 821–824

causes photolysis. The resulting radiation enables compa-rability with earlier work and provided a closer approxima-tion to the solar spectrum where intensity at wavelengths<300 nm is insignificant. An on-line intensity monitorwas used to compensate for intensity variation due to agingof the lamp. The incident intensity at 365 nm was50 ± 5 mW/cm2 (UVITECH RX003). Reactions were car-ried out in a well-mixed heterogeneous batch reactor madefrom quartz, which incorporated ports for sampling, oxy-gen injection, and a thermocouple. The system temperaturewas stabilized at 20 �C by a water recirculation bath con-nected to the outer jacket of the reactor. Oxygen was deliv-ered to the reactor at constant rate of 18 ml/min by meansof a mass flow controller. The overall set-up closely resem-bled that introduced many years ago by Hoffman and co-workers [1] and subsequently widely used by many others.

TiO2 modified by gold nanoparticles was prepared bythe deposition precipitation method established by Har-uta et al. [14]. After adjusting the pH of an aqueous solu-tion of HAuCl4 to pH 7, the titania support (P25, fromDegussa Corporation) was added and the mixture stirredfor several hours. The gold loading was varied by varyingthe concentration of HAuCl4 in aqueous solution. Thesolid product was washed, calcined in air for 5 h at300 �C, ground and sieved to give catalyst grains below45 lm. Because Au-modified TiO2 particles tend toagglomerate after drying, a reliable reference samplewas produced by subjecting unmodified P25 TiO2 to thesame heating and mechanical procedures. In a typicalexperiment, the reactor was loaded with 100 ml of200 mg/l of MTBE solution and 100 mg of catalyst. Con-trol experiments were performed to account for (i) vola-tilisation of MTBE from aqueous solution and (ii)photolytic reactions in the absence of the catalyst: volatil-ization accounted for <15% of MTBE depletion and allthe data were corrected accordingly. The system was stir-red in the dark for approximately 1.5 h to establish sorp-tion/desorption equilibria before switching the lamp on.Progress of the reaction was followed by observing therate of decay of the reactant. After starting the photocat-alytic reaction, aliquots containing MTBE were periodi-cally withdrawn from the reactor over the course of thereaction, centrifuged at 2000 rpm and transferred to vialsfor analysis by the headspace method (GCFID Agilent6850). Analysis was carried out using splitless injectionwith a HP-1 methyl siloxane 30 m · 320 lm · 0.25 lmcolumn, helium flow of 4.2 ml/min, FID temperature of250 �C, oven temperature of 70 �C and inlet temperatureof 150 �C. High resolution electron microscopy (HREM)images were recorded on a JEOL JEM-3011 electronmicroscope operated at 300 keV. Particle size was deter-mined from TEM images by counting at least 50 parti-cles. Surface area measurements were carried out withMicromeretics Gemini 2360 surface area analyser. X-rayphotoelectron spectroscopy (XPS) experiments were per-formed in a VG ADES 400 electron spectrometer systemoperated at a base pressure of 1 · 10�10 Torr. XP spectra

were obtained using Mg Ka radiation and fixed analyserpass energy of 50 eV.

3. Results and discussion

A number of photocatalysts with a range of gold load-ings were prepared, representative samples being character-ized by HREM. The average gold particle size varied from3.6 nm (lowest Au loading) to 4.8 nm (highest loading) andFig. 1 shows a representative image obtained with a0.55 at.% Au catalyst. Energy-dispersive X-ray spectros-copy indicated no detectable impurities in every case: onlyTi and Au were observed.

XPS results showed that samples indeed containedgold, as confirmed by a detectable Au 4f signal. Fig. 2shows the variation in Au 4f XP signal intensity withincrease in gold loading, as expected. The non-linearintensity dependence is consistent with TEM results whichreveal significant agglomeration of the Au at high goldloadings. XPS results also indicated the extent of TiO2

surface coverage by Au. No significant changes in the Ti2p XP signal with gold loading were observed, as expectedsince most of the TiO2 surface was not in contact with thegold nanoclusters. BET surface area measurements on thecatalyst samples and on pure untreated P25 titania showedthat the deposition precipitation technique had no mea-surable effect on the total surface area which was �48m2/g in all cases.

The dependence of catalytic activity for the photodeg-radation of MTBE on gold loading is shown in Fig. 3.These results are striking in that they show a � threefoldincrease in catalytic activity at 0.55 at.% Au: this is a very

Fig. 2. Integrated Au 4f intensity (normalized by Ti 2p integrated signal)plotted against gold loading.

Fig. 3. Photocatalytic degradation of MTBE as a function of goldloading. The inset shows time dependent concentration profile of MTBEon 0.55 at.% Au modified TiO2.

Fig. 4. Effect of dissolved oxygen on photocatalytic degradation of MTBEon (A) 0.55 at.% gold modified TiO2 in oxygen saturated aqueoussolution; (B) 0.55 at.% gold modified TiO2 in oxygen-free solution;(C) TiO2 in oxygen-free aqueous solution and (D) TiO2 in oxygensaturated aqueous solution.

A. Orlov et al. / Catalysis Communications 8 (2007) 821–824 823

large enhancement in performance compared to unmodi-fied titania. Moreover, it is much larger than the activitygains reported in the literature for other methods of tita-nia modification [7], although it must be borne in mindthat the photocatalytic reactions studied are different inthese cases. Measured UV–Vis spectra did not provideany insight into the role of Au in enhancing the photocat-alytic activity Comparisons with earlier work on the pho-tocatalytic degradation of MTBE in the aqueous phase byunmodified titania [11] are not helpful either as the datawere obtained under very significantly conditions. Inany case, our own reference data for pure P25 titania pro-vide all the reference information that is needed. Notethat at the highest gold loadings the activity of our cata-lysts decreased substantially, approaching that of unmod-ified titania.

It was found that the presence of oxygen was a majorfactor in determining the photocatalytic rate, for both

unmodified and for gold-modified titania. This is illus-trated in Fig. 4 which shows how in both case the pseudofirst order kinetic rate constants are drastically increasedin the presence of oxygen, the effect being most pronouncedfor the (optimum) 0.55 at.% Au catalyst. In the absence ofoxygen, the unmodified and Au-modified catalysts areequally poor.

It has been proposed [5–8] that relatively large metallicgold (larger than 5 nm) nanoparticles in contact withTiO2 act as a sinks for photo-generated electrons, thusreducing the e�/h+ recombination rate and enhancing thephotocatalytic rate. This mechanism might explain whyin our case the activity observed for photodegradation ofMTBE at higher Au loadings is marginally higher than thatof pure TiO2. Our earlier work [13] indicates that the mostlikely cause of the rate decrease observed at the highest Auloadings is elimination of hole-trapping OH groups at theTiO2 surface. At very high metal loadings (not used here)shadowing of the titania by very large Au particles couldact to reduce the TiO2 surface area available for MTBEadsorption and photo-excitation [5], thus strongly sup-pressing catalytic activity.

The most interesting results are those obtained with thevery small (<3 nm) gold particles for which the explanationabove is not likely to be valid. In this size range the Au par-ticles are expected to be semiconducting rather than metal-lic [15] and one may invoke a mechanism based onsemiconductor-semiconductor contact in which photo-excited electrons are injected from Au into the TiO2 con-duction band creating separated electrons and holes whichthen undergo charge transfer reactions with adsorbates.However, little is known about the charge transfer dynam-ics of electrons and holes with molecules adsorbed at semi-conductor surfaces—clearly an area for future theoreticaland experimental investigation [16].

824 A. Orlov et al. / Catalysis Communications 8 (2007) 821–824

A second positive effect of nanoscopic Au on the pho-tocatalytic rate could arise as follows. Sykes et al.reported [15] that deposition of Au nanoparticles onpolycrystalline TiO2 resulted in a substantial decreasein work function. It is generally agreed [1] that the ratelimiting step in photocatalytic reactions of this type iselectron transfer from the TiO2 surface to adsorbed O2.It is also suggested that oxygen adsorbs at the interfacebetween Au and TiO2 [14,17]. Thus lowering the localwork function of titania in the vicinity of such interfacialoxygen adsorption sites should result in a significantincrease in the rate of electron transfer to O2 to yieldhighly oxidising peroxy or superoxy species that wouldsubstantially enhance the rate of photocatalytic reaction.Support for this view comes from our results obtained inthe presence and absence of O2. When oxygen is presentthe Au-modified catalyst has an activity three timeshigher than that of the unmodified catalyst. However,when oxygen is absent, the activity of Au-modified cata-lyst is the same as that of unmodified catalyst. This is inline with the mechanism proposed above (see Fig. 4)based on the idea that lowering of the local work func-tion by Au should enhance the rate of formation ofstrongly oxidizing species generated by electron transferto adsorbed dioxygen.

Comparison of the present results and earlier work [13]on the photocatalytic degradation of 4-chlorophenol byAu-promoted titania shows that the optimum Au particlesize for rate enhancement is essentially the same in thetwo cases. This suggests that gold modified titania shouldbe efficient for the photocatalytic decomposition of widevariety of pollutants.

4. Conclusions

1. Gold nanoparticles substantially enhance the of photo-catalytic performance of titania for the degradation ofMTBE. At the optimum gold loading, which corre-sponded to an average Au particle size of �3 nm, thekinetic rate constant for MTBE degradation was threetimes greater than that for unmodified titania.

2. This rate enhancement may be due to injection of photo-excited electrons from semiconducting Au into the TiO2

conduction band.3. Since the presence of dissolved oxygen was crucially

important to good performance, it is possible that anadditional factor was lowering of the local work func-tion, thus promoting electron transfer to dioxygen mol-ecules adsorbed at the Au/TiO2 interface.

Acknowledgements

This work was supported under Research Grant GR/R03082/01 awarded by the UK Engineering and PhysicalSciences Research Council. A.O. thanks the EnvironmentalResearch and Education Foundation for the award of anInternational Scholarship, and Schlumberger CambridgeResearch for additional financial support.

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