A Rapid and Efficient Method to Deposit Gold Particles onto Catalyst Supports and Its Application for CO Oxidation at Low Temperatures

DOI: 10.1002/adfm.200601121

A Rapid and Efficient Method to Deposit Gold Particles on CatalystSupports and Its Application for CO Oxidation at LowTemperatures**

By Ziyi Zhong,* Jianyi Lin, Siew-Pheng Teh, Jaclyn Teo, and Frits M. Dautzenberg

1. Introduction

In the last two decades intensive attention has been paid toAu catalysis, and this is expected to lead to the development ofa new generation of catalysts.[1,2] Initially, Au catalysts were be-lieved to be barely active in catalysis; however, this is only thecase when the Au particles are large in size. When the particlesize becomes very small (usually ≤ 5 nm), high activities undermild conditions are revealed for a series of reactions.[3] Chemi-cal reactions in which Au catalysts are active include many syn-thetic reactions for some bulk and fine chemicals[4] as well asthe oxidation of CO at low temperatures. The catalytic activityof Au catalysts depends strongly on the particle size;[3] how-ever, the development of practical methods for the preparationof Au-based catalysts that allow good control over the particlesize and result in high stability still remains a challenge.

The conventional impregnation method usually employs therelatively cheap HAuCl4 as Au precursor, but has poor controlover the Au particle size.[5] Moreover, in this method Cl–1 an-ions are difficult to remove in subsequent washing and calcina-tion procedures because they are chemically bound to the Auatoms. Chloride not only causes agglomeration of Au particlesduring heat treatment, but also poisons the active sites.[6] Toavoid contamination with trace chloride several expensiveorganic Au precursors, such as Au(CH3)2(C5H7O2)[7] andAu(PPh3)(NO3),[8] have also been employed in the catalystpreparation, and better catalytic activity was subsequently ob-served. Other methods for the preparation of Au catalysts in-clude ion-exchange[9] as well as chemical vapor deposition,[10]

co-precipitation (CP),[11] and deposition–precipitation (DP)methods.[12] So far it appears that CP and DP methods are themost successful for preparing Au catalysts with high dispersionsand Au loadings, and with high activity and selectivity for theoxidation of CO, even in the presence of H2.[13,14] In the twomethods, HAuCl4 is converted to its hydroxide by the precipi-tation reaction and subsequently decomposed into gold oxideor metallic gold. However, both methods still cannot complete-ly avoid the adsorption of gold hydroxyl chlorides onto thecatalyst supports,[14] or the wrapping of the chloride in the pre-cipitate, which may cause deactivation of the Au catalysts. Inaddition, the ability to retain very good control over the parti-cle size in a series of indispensable procedures, which includeslong-term thermostatting, washing, drying, and calcination, stillposes a big challenge. Therefore, attention has also been paidto alternative approaches for the preparation of Au catalysts,for example, capping the small Au particles with ligand mole-cules, modifying the surface properties of the catalysts supports

1402 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17, 1402–1408

–[*] Dr. Z. Zhong, Prof. J. Lin, Dr. S.-P. Teh, J. Teo,

Dr. F. M. DautzenbergInstitute of Chemical Engineering and Sciences1 Pesek Road, Jurong Island, Singapore 627833 (Singapore)E-mail: [email protected]. J. Lin, Dr. F. M. DautzenbergSerenix Corporation5008 Chelterham Terrace, San Diego, CA 92130 (USA)

[**] This research was supported by the Agency for Science, Technology,and Research in Singapore (A-STAR, ICES03-414002). Z.Z. thanksGary Ng, Suriya Subramanian, Vivien Ng, Zhan Wang, and WeiqiangLim for technical assistance, and Dr. Keith Carpenter and Dr. P. K.Wong for their kind support to this project. Supporting Informationis available online from Wiley InterScience or from the authors.

A rapid, simple, and efficient method for the preparation of highly dispersed supported Au catalysts has been developed. In thepreparation, NaBH4 is used to reduce the Au precursor, lysine is employed to cap the formed Au colloids, and a short sonica-tion time is applied to facilitate dispersion and deposition of the Au colloids onto the catalyst support, which has been mixedwith the precursor beforehand. The end-point pH value of the solution and the isoelectric points (IEPs) of the catalyst supportshave an influence on the size of the Au particles and their deposition. The optimum value for the end-point pH is 7.5–10.0, andthe IEP should be 5–10. The amino acid capping agent is easily removed at the catalyst activation stage at 200 °C, and the Auparticles are thermally stable against sintering, even at 500 °C for 3 h. It is also proven that the method is applicable to the prep-aration of supported Pt catalysts. The catalytic activity of the prepared Au catalysts for CO oxidation in the absence/presenceof H2 is comparable to that of a Au catalyst prepared by the co-precipitation (CP) method, and to that of the standard catalystfrom the World Gold Council (WGC). X-ray photoelectron spectroscopy (XPS) results show that only metallic Au exists in thecatalysts before and after activation, and also after the catalysis reaction.

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before the deposition of the Au particles, confining the Au pre-cursor/particles to small pores, or a combination of them. Mouand co-workers[15] used surface silyl to modify a number of cat-alyst supports and immobilized highly dispersed Au particleson them. It should be noted, however, that the introduction ofsilica at the interface between the Au particles and the catalystsupports usually leads to a low catalytic activity.[15] RecentlyCorma’s group[16] and Dai’s group[17] independently preparedhighly dispersed Au/silica catalysts by confining different Auprecursors in small pores. Grunwaldt et al.[18] first developed atwo-stage method, based on Au colloids, for the preparation ofAu/TiO2 and Au/ZrO2 catalysts employing tetrakis(hydroxy-methyl)-phosphonium chloride (THPC) as the reducing andcapping agent. Later, Porta et al. and Mirescu et al.[19] em-ployed poly(vinyalcohol) (PVA) and poly(diallyldimethylam-monium bromide) (PDDA) as protective agents, and preparedAu/C and Au/TiO2 catalysts.

The colloid method has two advantages compared with othermethods: i) it is much easier to achieve control over the Auparticle size in colloidal form than in supported Au catalysts,and ii) it can effectively avoid contamination with chloride.Different from the CP and DP method, in which the Au alwayscontains gold hydroxyl chloride,[14] Au exists in its metallicstate in the colloidal method. Because the capping moleculesalways cap the Au colloids, the Cl– anions are chemically sepa-rated from the Au colloids and can be easily removed by wash-ing; however, in the colloid method[18a,19] the P-containing andpolymer capping molecules are difficult to remove and, more-over, the efficiency of transferring the Au colloids from solu-tion onto the catalyst support remains an uncertain factor.

Herein, we report a one-step method for the preparation ofsupported Au (Pt) catalysts that is based on colloidal Au. Inthe preparation, lysine was used as capping agent and NaBH4

as reducing agent. After the Au colloids were formed theywere immediately deposited onto the catalyst supports, a pro-cess that was facilitated by ultrasonic irradiation (sonication).The entire preparation, including solution preparation andwashing procedures, could be finished within 1 h. This methodnot only allows good control over the particle size but is alsoapplicable to a variety of oxide supports, and the prepared Aucatalysts have high catalytic activity and stability for the oxida-tion of CO.

2. Results and Discussions

To obtain very small Au colloids and to efficiently transferthem onto the catalyst supports, three measures were takenduring the catalyst synthesis: i) fast nucleation to form tiny par-ticles; ii) capping of the small particles with ligand molecules,preventing their aggregation;[20,21a] and iii) the application ofultrasonic irradiation (sonication). In the preparation, NaBH4

was selected as the reducing agent because it can reduceHAuCl4 instantly, thereby leading to the formation of manysmall Au particles in a short time. The capping agents can ef-fectively reduce the surface energy by forming chemical bondswith the colloids, and may also introduce strong electric repul-

sion between the particles, thereby keeping them separate.[21]

Without use of the capping agent we observed that the Au par-ticle size was large, even with the application of sonication.This has also been described in literature reports.[22] Other ad-vantages of using the capping agent include that it can effec-tively reduce contamination with chloride, because the cappingagent molecules are far more effective ligands for metallic Auparticles than Cl–. Finally, during the preparation sonicationwas used to facilitate the deposition of the small metal particlesonto the catalyst supports.

The most commonly used and studied capping agents for Auparticles are organic thiols, which contain SH groups that caneffectively cap Au particles. However, these require a high-temperature calcination step to remove them.[15] Therefore, wechoose NH2-containing capping agents. Compared with thiols,the NH2-containing capping agents provide a relatively weakprotection for the Au particles but can be removed easily. A se-ries of amino acids and organic amines that have one or twoamino groups were screened. Among them, lysine (Lys) isunique (for reasons discussed later) and was finally chosen forthe catalyst preparation because it afforded the most homoge-neous Au particle size distribution. When Lys is used as cap-ping agent, the pH value of the suspension is critically impor-tant because the Au particle size is pH-dependent.

Figure 1 shows transmission electron microscopy (TEM) im-ages of 3 wt % Au/a-Fe2O3 catalysts prepared at various pH val-ues. At pH values below 6, most Au particles were severely ag-gregated, and the particle size ranged from 10–50 nm (Fig. 1A).By contrast, in the range of pH 6.5–10.5, most of the Au particleswere below 5 nm in size and were highly dispersed (Fig. 1B–D).

Adv. Funct. Mater. 2007, 17, 1402–1408 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.afm-journal.de 1403

pH=3.4

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100nm

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Figure 1. TEM images for 3 wt % Au on a-Fe2O3 catalysts prepared at dif-ferent pH values and using lysine as the capping agent. A) pH 3.4,B) pH 6.5, C) pH 8.6, D) pH 9.4.

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Z. Zhong et al./Deposition of Gold Particles onto Catalyst Supports

It appears that the range of pH 7.5–10.0 gives rise to the mostuniform Au particles. Coincidently, this pH range is almost thesame as that of highly dispersed Au colloids in solution,[23] indi-cating that the key factor to achieving control over the Au parti-cle size is to reduce their aggregation opportunities in solutionbefore their deposition onto the catalyst supports. In other words,by controlling the aggregation state of the Au colloids in solution,a similar Au particle size distribution can be obtained for the sup-ported catalyst. A similar result was observed for Pt. Most of thePt particles were in the range of 3–5 nm (see Fig. SI-1, Support-ing Information). In the absence of the catalyst supports, thesolution precipitated within several minutes after the reductionreaction when the pH was below 4. By contrast, when the pHwas above 6.5 a stable solution could be maintained for 2 weeksand longer (see Fig. SI-2).[23b] Therefore the size, that is, the ex-tent of aggregation, of the Au and Pt particles can be easily regu-lated through adjusting the pH in the synthesis.

This can be explained by considering the electrodynamics ofAu colloids.[23a] Lys has two terminal amino groups: NH2-I,which is the a-NH2 group neighboring the COOH group, andNH2-II, which is further away from the COOH group. Com-pared with NH2-I, NH2-II is more active and will bind to theAu surface first.[23a] At very low pH values the carboxylic acidgroup is protonated, and the a-NH2 group can crosslink otherAu particles, leading to the formation of aggregates or precipi-tation. By contrast, at high pH values the neighboring COOHgroups are deprotonated and become COO–, which will limitcrosslinking by the a-NH2 group (actually existing as a-NH3

+,i.e., Lys is in its zwitterionic form) to the other Au particles.When the pH reaches a value of ca. 8, which is close to the pK2

of Lys, most of the a-NH3+ groups will become deprotonated,

and thus the capped Au colloids have many COO– groups pro-truding from the surface. The electric repulsion between thembecomes very strong (pH 8–10). This phenomenon was first ob-served and discussed in our previous report.[23a,b] In this pHrange, linear Au colloid aggregates were formed, as evidencedby the formation of a black Au colloid solution (SI-2), but soni-cation could break up the linear aggregates[23b] and drive theisolated particles to deposit onto the catalysts supports. It isalso worth noting that, different from thiol molecules that haveSH groups, amino acids bind to the Au particles with only inter-mediate strength. Our Fourier transform IR results (data notshown) revealed that they were removed below 200 °C, thus,they will not contaminate the catalyst and the growth of the Auparticles in high-temperature calcination employed by othermethods can be avoided.[10–13]

The effect of the Au particle size on the catalytic activity to-wards CO oxidation is obvious. Figure 2 shows the CO oxida-tion activity (1 % CO in air) of catalysts prepared at variousend-point pH values. The temperature required to achieve cer-tain similar CO conversion value decreases with increasingend-point pH value. When the pH value reaches 8, the pre-pared catalysts exhibit very high activities and can completelyconvert CO into CO2 at room temperature (RT), because ofthe very small Au particle size.

Based on acoustic cavitation, sonication has a variety ofphysical and chemical effects and can be used for the synthesis

of nanometer-sized particles.[24] However, the sonication condi-tions should be selected properly because otherwise the sono-chemical method could lead to the formation of small particleswith various shapes and a wide size distribution.[25] In addition,sonication can facilitate the deposition of colloidal particlesonto a catalyst support, because when during sonication in liq-uid solution bubbles are created and subsequently collapse,high-speed jets and waves with a scale of several hundreds ofmeters per second are produced. The high-speed jets can effec-tively push particles to hit the catalyst supports, so as to pro-duce supported catalysts or nanocomposites.[26] Although inthe range of pH 8–10 Au colloids still can form linear aggre-gates when using Lys as capping agent, sonication can effec-tively break them.[23b] Excess sonication, however, will providesufficient energy to overcome the electrostatic repulsionamong the colloidal particles and cause their aggregation.[27]

We observed that after long-time exposure to sonication(above 10 min) a black precipitate was formed in a Au colloi-dal solution, even when the pH was above 6.5. As a result, wechose a short sonication time (20–60 s) in our syntheses. Highlydispersed Au catalysts could be prepared under this condition.

The isoelectric point (IEP) of the catalyst support is anotherparameter that obviously influences the size distribution of theAu particles. For low-IEP supports, such as silica (IEP ≈ 2) andcarbon, large Au particles and low loadings are usually ob-tained. When the pH value is above the IEP the catalyst sup-ports are negatively charged on the surface, and thus the nega-tively charged colloidal particles are difficult to access fordeposition onto the support oxide surface. Instead, these Auparticles will start to aggregate in the solution after sonicationis stopped. However, for catalyst supports with IEP values inthe range of 5–10 highly dispersed Au particles are obtained.Similar to the DP method, the best pH value should be nearthe IEP of the catalyst support, at which point the catalyst sup-port surface is almost neutral.[14,28] Shown in Figure 3 are Auparticles supported on a-Fe2O3 (Fig 3A and B), on CeO2

1404 www.afm-journal.de © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17, 1402–1408

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nve

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Figure 2. Catalytic oxidation of 1 % CO in air on 3 wt % Au/a-Fe2O3(A) cat-alysts prepared at various pH values, indicated in the figure(GHSV = 15 500 h–1). GHSV: gas hourly space velocity.

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(Fig. 3C, IEP = 9)) and Pt particles on TiO2 (Fig 3D, IEP = 6).These deposited Au and Pt particles are quite uniform in size.Similar results were obtained for Au supported on TiO2,Al2O3, and CuO (TEM images not shown here). Also, no ob-vious difference was observed on various TiO2 and a-Fe2O3

samples from different resources. Obviously, although we stillcan not say that this method is universal, it is indeed suitablefor a variety of catalyst supports with IEP values above 5.

It should be pointed out that this method can provide highloading values for Au; however, when the Au loading was high-er than 6 wt % several large particles were observed becauseof the increased aggregation opportunities for the colloids inthe solution. We also attempted to deposit the Au particles ona-Fe2O3 by employing repetitive impregnation procedures witha low concentration of the precursor solution (each time withan increase in Au loading by ca. 2 wt %). Unfortunately, afterthree successive depositions the Au particles on the catalystwere also severely aggregated, probably because the Lys mole-cules were capable of crosslinking the deposited Au particleswith the Au colloids in the solution, thereby causing many Aucolloids in the solution to be deposited onto the previously de-posited Au particles. The Au colloids left in the solution wereminimal, and could be easily recovered. In a control experi-ment, we added HCl solution to the colloidal Au solution untilpH 2 (without any catalyst support, because otherwise theamount of recovered Au would be too small to be observed),and a black precipitate was obtained. By harvesting the precip-itate through centrifugation and calcining it at ca. 200 °C, theAu precipitate complex was decomposed and metallic Au, al-most 100 %, was recovered.

The prepared catalysts exhibited good thermal stabilityagainst calcination. In Figure 4, the Au/a-Fe2O3(H) catalystwas calcined at 300 and 500 °C for 3 h, respectively. No obvious

growth in size for most of the Au particles is observed. Mostprobably these small particles are anchored onto the catalystsupports by reaction with surface hydroxyl groups,[26a] and thesonication may provide energy to enhance this reaction. Also,the low mobility of the Au particles can be partially attributedto the very low concentration of chloride in the catalysts.[6] Forsome Au particle aggregates formed in the solution, their inter-action with the supports is weak, and thus they are sinteredeasily.

Figures 5 and 6 show the catalytic activity of the Au cata-lysts as a function of the flow rate and reaction temperaturein the absence and presence of H2, respectively. There are twoapplications for this reaction: the oxidation of 1 % CO in airis mainly used to model the function of a mask filter, whichcan prevent CO poisoning during a fire accident, while theoxidation of CO in the presence of H2 is used to model theH2 purification reaction in a polymer electrolyte membranefuel cell (PEMFC), because trace CO in H2 can poison fuel-cell electrodes.[4] As for 1 % CO in air, the CO conversion onthe 1 wt % Au/TiO2 catalyst was 100 % even at 0 °C, but whenthe temperature was lowered to ca. –63 °C the CO conversiondecreased to 2.4 % (Fig. 5A). At room temperature, the COconversion could be maintained at 100 % for at least 200 hwithout obvious deactivation (Fig. 5B). The minimum COconcentration was reduced to ca. 10–20 ppm, as monitored bya CO sensor, which is already in the safe range for humanbeings. We also compared the catalytic activity of the 5 wt %Au/a-Fe2O3(H) catalyst with a standard catalyst purchasedfrom the World Gold Council (i.e., 5 wt % Au/a-Fe2O3(WGC)). In each test, 10 mg of each catalyst was usedand the flow rate was adjusted to 200 mL min–1. The normal-ized activity at 23 °C for the 5 wt % Au/a-Fe2O3(H) (H: homeprepared) catalyst was 4.85 × 10–4 mol CO gAu

–1 s–1, whereas thecorresponding activity for the 5 wt % Au/a-Fe2O3(WGC)catalyst was 4.32 × 10–4 mol CO gAu

–1 s–1, indicating that the two


A B

C D

50nm 5nm

20nm 10nm

Figure 3. TEM images for 5 wt % Au on A,B) a-Fe2O3(H), C) CeO2, andD) 2 wt % Pt on TiO2(H), prepared at pH 8.0–8.5 and using lysine as thecapping agent.

A B

100nm100nm

Figure 4. 5 wt % Au/a-Fe2O3(A) calcined at A) 300 °C and B) 500 °C for3 h. The ramp rate was 10 °C min–1. Most of the Au particles showed noobvious increase in size, except in some areas (marked by a circle in (B)).Scale bars: 100 nm.

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Figure 5. Oxidation of CO in air (1 % CO in air) on a 1 wt % Au/TiO2(H) at various temperatures and flow rates. A) Temperatures of 0 and –63 °C wereachieved by placing the quartz tube containing the catalyst in ice and dry ice, respectively, for 30 min. The temperature was measured with a thermocou-ple inserted into the catalyst bed in the quartz tube. B) Room-temperature reaction. A continous flow of 1 % CO gas was maintained for 220 h. In eachexperiment, 50 mg catalyst was used.

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Figure 6. A) Selective oxidation of CO in the presence of H2 (GHSV = 23 000 h–1). B) TEM image of the 4 wt % Au/a-Fe2O3(A) catalyst. C) Au particle sizedistribution of the catalyst in (B). D) XPS spectrum of the Au/a-Fe2O3(A) catalyst activated at 200 °C in air.

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catalysts are basically at the same level in activity. However,in the presence of H2 and CO2 (the reactant gas was 77 %H2 + 2 % O2 + 1 % CO + 20 % CO2), the maximum CO con-version for the 1 wt % Au/TiO2(H) catalyst was only ca. 55 %.In the latter case, the supported Au/a-Fe2O3(H) catalystexhibited a much higher catalytic activity. As for the 4 wt %Au/a-Fe2O3(H) catalyst, when the reactant gas mixture was77 %H2 + 2 %O2 + 1 %CO + 20 %Ar a 100 % conversion wasachieved in the temperature range of 20–80 °C (Fig. 6A).However, the selectivity of O2 for CO oxidation decreasedwith increasing reaction temperature, because more oxygencould react with H2 at higher temperatures (Fig. 6A). Theconversion of CO to CO2 is more difficult in the presence ofboth H2 and CO2 (the reactant gas mixture was 77 %H2 + 2 % O2 + 1 % CO + 20 % CO2). The CO conversion wasca. 99 % at 50 °C. Similarly, a further increase of the reactiontemperature led to a decrease of the CO conversion becauseof the H2 oxidation reaction (results not shown here). How-ever, an increase of the Au loading to above 5 wt % couldslightly further increase the CO conversion. The Au catalyston the commercial a-Fe2O3(A) (A: purchased from Aldrich)support was even better than on a-Fe2O3(H) in this reaction.For all the supported Pt catalysts, a much poorer catalytic ac-tivity was observed. These observed catalytic activities for COoxidation are very close to the results in literature.[13] Themeasured X-ray photoelectron spectroscopy (XPS) spectra(Fig. 6D) shows that the Au particles only exists in their me-tallic state,[29,30] and similar results were observed for the cata-lysts before the activation and after the reaction, indicatingthat metallic Au, not its oxidized state, is the active compo-nent for this reaction.

3. Conclusions

We have developed a simple and versatile method for thepreparation of the highly dispersed and supported Au cata-lysts. The small Au particles are formed by fast reduction ofHAuCl4 with NaBH4 in the solution, and protected from ag-gregation by capping with Lys molecules. During the catalystpreparation, a short sonication time is applied to facilitatethe dispersion and deposition of the Au colloids on the cata-lyst supports. The other two factors that influence the size ofthe Au particles and their distribution are the end-point pHvalue in the solution and the IEPs of the catalyst supports.The optimum end-point pH value is 7.5–10.0, and the IEPshould be 5–10. As proven, the method is also applicable tothe preparation of supported Pt catalysts. The amino acidused as capping agent is easily eliminated by calcination dur-ing the activation stage at 200 °C, and the Au catalysts exhib-it good thermal stability against sintering, even at 500 °C for3 h, and very high activity towards CO oxidation in theabsence/presence of H2. Additional catalysts will be pre-pared by this method, and more applications for them willbe explored in the future.

4. Experimental

All chemicals were purchased from Aldrich, except for P-25 anataseTiO2, which was a gift of Degussa, and the standard catalyst of 5 wt %Au/a-Fe2O3, which was purchased from the WGC (UK). The gas mix-ture of 77 % H2 + 2 % O2 + 1 % CO + 20 % Ar was purchased from Sin-gapore National Oxygen. Anatase TiO2(H) was prepared by hydrolysisof Ti isopropoxide in water in the presence of octylamine followed bycalcination at 200 °C for 3 h. a-Fe2O3(H) was prepared by precipitationreaction of Fe(NO3)3 with NaOH at pH 9 followed by thorough wash-ing with deionized water and calcination at 500 °C for 2 h, or purchasedfrom Aldrich (a-Fe2O3(A)). The measured surface areas were230 m2 g–1 for TiO2(H), 50 m2 g–1 for a-Fe2O3(H), and 30 m2 g–1 fora-Fe2O3(A). Different from Grunwaldt’s two-stage colloid method, inour method the Au colloids were produced in situ in the presence ofthe catalyst supports. In a typical catalyst preparation, 0.50 g a-Fe2O3

was put in 10 ml deionized water, and 3.0 ml of 0.01 M HAuCl4 and3.0 ml of 0.01 M lysine (Lys) were added subsequently. The pH of thesuspension was adjusted to 5–6 with 0.10 M NaOH. The suspension wassubjected to sonication (Vibracell 500 Watt Ultrasonic processor, 40 %energy efficiency) for 20 s, and during the sonication freshly preparedNaBH4 (0.1 M, 5–10 times the Au molar number) was injected instantlyand the pH value at the end point was measured. The suspension im-mediately turned dark in color and was washed with deionized water 4times by using centrifuge. In the case of Pt catalysts, H2PtCl4 was usedas precursor.

The measurement of catalytic oxidation of CO was carried out in afixed-bed microreactor. Prior to the test, the catalyst (50 mg, unlessmentioned otherwise) was treated in air at 200 °C for 1 h. After beingcooled down, reactant gas containing CO was passed through the cata-lyst bed. The outlet gas was analyzed on-line with a gas chromatograph(Shimadzu-14B).

The catalysts were characterized by transmission electron microsco-py (TEM, Tecai TF20 Super Twin, 200 kV), powder X-ray diffraction(XRD, Bruker D8 Advance X-ray diffractometer with Cu Ka radia-tion), surface area measurements (Brunauer–Emmett–Teller (BET)surface area, Quantachrome Autosorb-6B Surface Area & PoreSizeAnalyzer), X-ray photoelectron spectroscopy (XPS) analysis (VGESCALAB 250 spectrometer), and the binding energies were cor-rected for surface charging by taking the C1s peak of contaminant car-bon as a reference at 285.0 eV.

Received: November 22, 2006Revised: February 6, 2007

Published online: April 3, 2007

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