7/30/2019 1-s2.0-S1387181105004579-main

1/6

Gold nano-particles stabilized in mesoporous MCM-48 as activeCO-oxidation catalyst

M. Bandyopadhyay a, O. Korsak a, M.W.E. van den Berg b, W. Grunert b,A. Birkner c, W. Li d, F. Schuth d, H. Gies a,*

a Institut fur Geologie, Mineralogie und Geophysik, Lehrstuhl Kristallographie, Fakultat fur Chemie, Ruhr-Universitat Bochum,

Universitatsstr., D-44780 Bochum, Germanyb Lehrstuhl fur Technische Chemie, Ruhr-Universitat Bochum, Universitatsstr., D-44780 Bochum, Germany

c Lehrstuhl fur Physikalische Chemie 1, Ruhr-Universitat Bochum, Universitatsstr., D-44780 Bochum, Germanyd MPI fur Kohlenforschung, Kaiser-Wilhelmplatz 1, D-45470 Muhlheim, Germany

Received 31 May 2005; received in revised form 15 September 2005; accepted 18 September 2005Available online 28 November 2005

Abstract

Gold in nano-crystal size is known as highly active CO-oxidation catalyst. Using simple deposition techniques gold has been depositedas $3 nm particles inside the channels of mesoporous silicaTiO2MCM-48. In the presence of gold nano-particles the catalyst convertsCO to CO2 at 50% level at 20 C. The composite is stable against sintering up to at least 200 C. XANES and EXAFS confirm thecoexistence of elementary and ionic gold during the catalytic activity. 2005 Elsevier Inc. All rights reserved.

Keywords: Au/TiO2MCM-48; Au/TiO2; CO oxidation; Au XANES; Au EXAFS

1. Introduction

Gold is thought to be the noblest of all metals [1].Ground breaking work by Haruta et al. [24], showed thatgold as catalyst exhibits considerable activity when it ishighly dispersed on metal oxides supports [57]. Two differ-ent classes of oxides could be used for the support [8].Active supports, such as TiO2 and Fe2O3, which can be eas-ily reduced, or inert supports, like Mg(OH)2, Al2O3, and

silica. These supports can also give active catalysts, typi-cally when some reducible elements are also introduced[9], although in some cases even for Au on the unmodifiedsupport material, low-temperature CO-oxidation activityhas been reported. For all catalytic applications it is impor-tant that the small gold particles are highly dispersed on theoxide support [7]. Au/oxides catalysts have been widely

applied to many important chemical reactions such asCO oxidation [2], hydrogenation of unsaturated hydrocar-bons [10], reduction of NO

x[11], epoxidation of C3H6 [12],

selective CO oxidation in a hydrogen-rich steam [13], com-bustion of methane [14], etc. Recently, Hua et al. [15] haveused Au/iron oxide catalysts for the water gas shiftreaction.

The catalytic performances of different systems withgold on oxide supports is strongly influenced by the prepa-

ration method, the specific synthesis parameters such aslight protection, the Au particle size, and more [5,9,16].The depositionprecipitation (DP) process is consideredto be the best method to synthesize highly active gold cat-alysts [17] because of the good control of the particle sizeby adjustment of pH during particle formation. In addi-tion, some other methods like chemical vapor deposition[18] (CVD) or co-sputtering [19] are widely used for cata-lyst formation. An extensive study on Au/Al2O3 catalystprepared by both DP and CVD methods reveals that finelydispersed Au/Al2O3 with Au particle size

7/30/2019 1-s2.0-S1387181105004579-main

2/6

easily prepared by the CVD method, whereas the tradi-tional DP method leads to formation of larger gold parti-cles (>7 nm) on the Al2O3 support [20]. A series of activeAu/TiO2, Au/Co3O4, Au/ZrO2 catalysts were studied lead-ing to the conclusion that, with similar gold particle sizes,Au/TiO2 is the most active system. However, a draw back

of all nano-dispersed Au on oxide supports is its readyaggregation to larger particles already at moderate temper-atures, usually at $60 C.

In addition to zeolites, M41S [21] materials are thoughtto be particularly useful as carriers or matrices for func-tional molecules or nano-particles because of their highthermal and chemical stability. Their mesoporous structurecould be explored as a host or matrix to immobilize catalyt-ically active species on or for providing nano-size confine-ment inside the pore system [2226]. Some literature isavailable on highly dispersed metal nano-particles onSBA-15 type material [27,28]. Recently, we reported onthe deposition of Ti salts within the pore system of cubic

MCM-48 and their subsequent decomposition to Ti-oxideclusters [26]. The use of MCM-48 with quantum-sized tita-nia particles inside the porous matrix as a support for Auparticles is the main interest of our present study. Herewe have explored the use of TiO2/MCM-48 as a matrixfor the formation of thermally stable nano-sized goldmetal particles and the use of the composite as CO-oxidationcatalyst.

2. Experimental

2.1. Synthesis

TiMCM-48 was prepared by a post-synthetic wetimpregnation method according to the literature procedure[26]. In a typical synthesis procedure 500 mg of the stan-dard MCM-48, which was dried overnight in vacuum at180 C, was reacted with 200 ml of solution of tetrabutyl-orthotitanate (Merck) in dry acetone (0.05 m). The mixturewas stirred for 6 h at room temperature. As tetrabutylorth-otitanate is moisture sensitive, the whole procedure wascarried out in dry N2 atmosphere. Finally, the solid wasfiltered off carefully, washed with acetone, dried at roomtemperature and then calcined at 300 C for 5 h. For succ-essive impregnations the respective procedure was repeatedseveral times. Three times impregnated TiO2/MCM-48containing $15 wt.% titania was used for Au loading.The depositionprecipitation method was chosen for goldinsertion [29]. During catalyst preparation 100 ml of anaqueous solution of HAuCl4 3H2O (Aldrich, 8 wt.% Auwith respect to support) was heated at 70 C. The initialpH was around 2.5. The pH was adjusted to neutral value7 by drop-wise addition of dilute NaOH. After that 0.1 g ofTiMCM-48 was dispersed in the solution. The pH of thesolution became 56 and the pH was then readjusted to 7again by addition of NaOH. The suspension was stirredfor 1 h at same temperature. Finally the solid was filtered,

washed with water, and freeze-dried overnight under

vacuum. The whole procedure was carried out in absenceof light.

2.2. Characterization

X-ray powder diffraction experiments were carried out

using a Siemens D5000 diffractometer, which was operatingin transmission mode (modified Debye Scherrer geometry)with monochromated CuKa1 radiation (k = 1.54059 A).For long exposure times, a Huber Guinier imaging platecamera G670 was also used with CuKa1 radiation. In bothcases the sample was loaded in a glass capillary.

A Hitachi H-8100 scanning and transmission electronmicroscope operating at accelerating voltages up to200 kV with a single crystal LaB6 filament was used forthe TEM studies. The specimens were prepared by placinga drop of the dilute solution of the calcined powdersamples in ethanol on a carbon-coated copper grid. The

samples were allowed to dry at room temperature.X-ray absorption spectra (Au LIII-edge at 11919.0 eV)were measured at Hasylab X1 station (Hamburg, Ger-many). This beamline was equipped with a Si(311) dou-ble-crystal monochromator that was used detuned to 50%of the maximum intensity in order to exclude higher har-monics present in the X-ray beam. The samples were mixedwith polyethylene and pressed into wafers of sufficientthickness. The spectra l(k) were measured in transmissionmode using ionization chambers, with the sample wafer atliquid nitrogen temperature. A gold metal foil (between thesecond and the third ionization chamber) was measured atthe same time for energy calibration purposes. Data treat-

ment was carried out using the software package VIPER[30]. For background subtraction a Victoreen polynomialwas fitted to the pre-edge region. A smooth atomic back-ground l0(k), was evaluated using smoothed cubic splines.The radial distribution function FT[k2v(k)] was obtainedby Fourier transformation of the k2-weighted experimentalfunction v(k) = (l(k) l0(k))/l0(k) multiplied by a Besselwindow.

2.3. Catalytic activity measurements

For the CO oxidation reaction a plug-flow reactor with

inner diameter of 4.5 mm was used. Measurements werecarried out under dynamic conditions while ramping thetemperature. The heating rate during these experimentswas 2 C/min. The reaction gas contained 1% CO, 20%O2 and N2 as balance which was passed over 50 mg of thecatalyst (Au/Ti/MCM-48) with a flow rate of 67 ml/min,corresponding to a space velocity of 80,000 ml/h gcat.

3. Results and discussion

The gold particles were introduced into the mesoporousTiO2/MCM-48 matrix using the DP method. After three

impregnation runs the final Au content amounted to

M. Bandyopadhyay et al. / Microporous and Mesoporous Materials 89 (2006) 158163 159

7/30/2019 1-s2.0-S1387181105004579-main

3/6

$3 wt.% Upon loading with gold the colour of the whitesample changed to grayish. Low-angle powder XRD pat-terns show that the parent structure of the mesoporoushost is maintained after loading with titania and consecu-tively with Au (Fig. 1(a) and (b)). However, because ofthe strong reduction of scattering contrast between wall

and pore due the impregnation of nano-particles [31], theresulting X-ray diffraction signal from the mesoporous car-

rier MCM-48 is only very weak. The high angle shoulder inthe (211)-peak originating from the (220)-peak can just beseen. For the confirmation of the analysis complementaryinformation from TEM-experiments is needed and will bediscussed later on. In addition, a strong contribution ofthe nano-particles to the SAXS-signal can be seen in the

low angle region. In the wide-angle XRD pattern given inFigs. 1 and 2, there are clear signals of gold metal at

2 8 14 20 26 32 38 44 50 56 62

0

30000

60000

90000

120000

150000

180000

210000

240000

2 ()

Intens

ity

(counts

)

Au/TiO2-MCM-48

2 3 4 5 6 7 8 9 10 11 12

0

30000

60000

90000

120000

150000

180000

210000

240000

2 ()

I

ntens

ity

(counts

)

Au/TiO2-MCM-48

a

b

Fig. 1. XRD diagram of Au/TiO2MCM-48 showing (a) the complete pattern up to 62 2h and (b) the low angle part between 2 and 12 2h. The insert in(a) is enlarged by a factor of 50 and clearly shows the diffraction pattern of nano-particles of gold. In (b) the diffraction pattern still shows the integrity ofthe MCM-48 mesoporous support. Because of the considerable decrease of scattering contrast through TiO2 and Au loading, only weak signals are seen.At $2.64 2h the major signal appears, at $3.04 2h the typical shoulder of MCM-48 material can be seen, and, finally, at $3.65 2h a very weak signal of

the second set of signals shows up.

160 M. Bandyopadhyay et al. / Microporous and Mesoporous Materials 89 (2006) 158163

7/30/2019 1-s2.0-S1387181105004579-main

4/6

around 38.3, 44.4, 64.7, 78.0 2h. Using the Scherrer for-mula for an estimation of the particle size, the peak halfwidth of all reflections gave an averaged value of around27 A for metallic gold. Thus, the particles are small enoughto fit inside the porous matrix of TiO2/MCM-48 as thepore diameter of TiMCM-48 is 2829 A [26]. However,

as obvious from the unusual shape of the diffraction signal,there is signal overlap from particles of different size. Fewlarger particles contribute to the generally very broad sig-nal giving a rather sharp and high signal on a broad baseleading to an overestimation of the particle size, since theFWHM is measured to small. To confirm the interpreta-tion of the XRD experiments, complementary TEM studieswere carried out. First, the integrity of every sample was

checked with TEM together with its qualitative titaniaand gold content using EDX. The results of the analysesalways showed the typical lattice fringes of the well orderedand well maintained silica host structure of MCM-48. TheTEM image (Fig. 3) also shows uniform and highly dis-persed gold particles inside the channel system.

The XAS spectra of the Au LIII-edge can be seen inFig. 4. Model analysis of the first coordination spherearound the central gold atom yields a AuAu coordinationnumber of 7.6 0.2 (see Table 1). By assuming sphericallyshaped Au particles, the apparent crystallite size can beestimated from the coordination number derived fromEXAFS data, as shown by Borowski [32]. Doing so, appar-ent crystallite sizes ranging from 11 to 12 A are found.

Fig. 2. Powder XRD patterns of Ti impregnated (blue) and Ti/Au impregnated MCM-48 (red). The fist maximum at ca. 23 2h is due to the amorphoussilica matrix, the following 4 maxima can be indexed as (111), (200), (220) and (311) for fcc gold metal. (For interpretation of the references in colour inthis figure legend, the reader is referred to the web version of this article.)

Fig. 3. TEM image and EDX analysis of the Au/TiO2MCM-48 composites freshly prepared.

M. Bandyopadhyay et al. / Microporous and Mesoporous Materials 89 (2006) 158163 161

7/30/2019 1-s2.0-S1387181105004579-main

5/6

Fitting the data with models containing just AuAu-contri-butions were unsuccessful. The XANES shown in Fig. 4(a)show that the measured Au foil differs somewhat from thesample measured. From this, and the non-zero intensity inthe Fourier transform of the Au LIII-edge at lower distancethan AuAu it can be concluded that a light atom must bepresent near the gold. The model shown in Table 1 incor-porates an oxygen atom at 1.91 A and a chlorine atom at2.40 A. The correlation coefficient for this model was 7.6.It was the best fit thus far, however, a possible Ti contribu-tion should be further investigated.

Using the freshly prepared sample in a CO-oxidationexperiment, the conversion set in at $30 C reaching50% conversion at 20 C (Fig. 5). In consecutive runsthe temperature for 50% conversion remained almost con-stant. After a short recovery period after the second run,the 50% conversion temperature for a third cycle was againlower than for the 2nd cycle, at $20 C. In additionalexperiments, which were conducted after storing the freshlyprepared sample in the dark for one week, similar valuesfor the conversion were obtained. It is important to men-tion that only those samples were active which have beensynthesized under protection from daylight. Those whichwere exposed to daylight were active only with 50% conver-sion at $160 C. In attempts to increase the dispersion of

gold inside the mesoporous channels by controlling the

pH even more precisely, a sample has been synthesized witha 50% conversion temperature around 30 C reflectingthe increased activity of the composite material. EXAFS-and XANES-studies in operando are currently carried out.

In order to determine the thermal stability of the nano-confined gold particles the sample was heated to $200 Cand studied with X-ray diffraction, TEM and CO-oxida-tion catalysis. Different from gold nano-particles depositedon dense supports, the nano-particles located in the confin-ing pore system are stable towards sintering up to at least200 C. There was no difference in particle distributionand particle size of the deposited gold before and after

catalysis and heating. In catalysis-runs the heat treated

0 4 8

-1

0

1

4 6 8 1 0 1 2 1 4 1 6-1

0

1

11900 11950 12000 12050

FT ( ), absoluteFT (), imaginary partfitted

FT((k)k2)

r,

(d)

(c)

(b)(a)

k, eV

BFTfitted

k, eV

4 6 8 10 12 14 16-1

0

1(raw)

Normalisedabsorption(-)

E, eV

Au FoilAu/TiO2 @ MCM-48

0.2

(k)k2)

(k)k2)

2 6

Fig. 4. k2

-weighted Au LIII-edge absorption spectra of Au/TiO2-MCM-48, showing (a) the XANES compared to Au-foil, (b) the measured v(k), (c) themodel v(k) and (d) the Fourier transform and model Fourier transform.

Table 1Parameters of the model fit of the Au-EXAFS

No. Element R (A) CN 103r2 (A2)

1 O 1.913 0.019 0.3 0.1 1.0 0.02 Cl 2.400 0.023 0.3 0.2 10.0 1.13 Au 2.852 0.002 7.6 0.2 5.9 0.1

The table lists the Au-element distance, coordination number, and DebyeWaller factor.

-40 -30 -20 -10 0 10 20

0

20

40

60

80

100

black-first

red-second

blue-third

CO

Conversion%

Temp.oC

Fig. 5. CO-conversion as function of temperature. Three consecutive runsare shown. The fresh sample (black) showed the best performance, in thesecond run (red) the 50% conversion temperature increased, however, aftera recovery period, the 50% conversion temperature dropped againshowing that the catalyst is still very active. (For interpretation of thereferences in colour in this figure legend, the reader is referred to the webversion of this article.)

162 M. Bandyopadhyay et al. / Microporous and Mesoporous Materials 89 (2006) 158163

7/30/2019 1-s2.0-S1387181105004579-main

6/6

sample showed the same activity at similar temperatures asthe freshly prepared sample. This shows that the structuraland electronic properties of the gold particles relevant forcatalysis are not changed and an active form of the goldparticles is preserved inside the nano-confining MCM-48support.

In conclusion, the study shows that gold nano-particlescan be deposited inside mesoporous titania-modifiedMCM-48. The composite is catalytically active even at tem-peratures as low as 30 C. In the active catalyst, both,metallic and ionic Au was observed. If the Au ions detectedare relevant for the CO oxidation activity is subject toongoing research. The encapsulation in mesoporousMCM-48 prevents the gold nano-particles from sinteringup to at least 200 C without apparent loss in catalyticactivity, a temperature which would lead to the deactivationof the catalyst deposited on a bulk TiO2 powder. BesidesCO-oxidation, other gold-catalyzed reactions should alsobe feasible, e.g. the epoxidation of alkenes in gas phase.

In addition, it would be interesting the take advantage ofthe size selectivity of the mesoporous support for the cata-lytic reaction. Studies concerning these properties are cur-rently carried out.

References

[1] B. Hammer, J.K. Norskov, Nature 376 (1995) 238.[2] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989)

301.[3] M. Haruta, N. Yamada, T. Kobayashi, H. Kageyama, M. Genet, B.

Delmon, J. Catal. 144 (1993) 175.[4] M. Haruta, T. Kobayashi, H. Sano, N. Yamada, Chem. Lett. 1987

(1987) 405.[5] M. Haruta, Catal. Today 36 (1997) 153.[6] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1637.[7] W. Vogel, D.A.H. Cunningham, K. Tanaka, M. Haruta, Catal. Lett.

40 (1996) 175.[8] M.M. Schubert, S. Hackenberg, A.C. van Veen, M. Muhler, V. Plzak,

R.J. Behm, J. Catal. 197 (2001) 113.

[9] A. Wolf, F. Schuth, Appl. Catal. A 226 (2002) 1.[10] F. Cosandey, T.E. Madey, Surf. Rev. Lett. 8 (2001) 73.[11] E. Seker, E. Gulari, R.H. Hammerle, C. Lambert, J. Leerat, S.

Osuwan, Appl. Catal. A 226 (2002) 183.[12] B.S. Uphade, Y. Yamada, T. Akita, T. Nakamura, M. Haruta, Appl.

Catal. A. 215 (2001) 137.[13] G.K. Bethke, H.H. Kung, Appl. Catal. A 194 (2000) 43.[14] R.J.H. Grisel, D.T. Kooyaman, B.E. Nieuwenhuys, J. Catal. 191

(2000) 430.[15] J. Hua, K. Wie, Q. Zheng, X. Lin, Appl. Catal. A 259 (2004) 121.[16] M.M. Schubert, V. Plzak, J. Garche, R.J. Behm, Catal. Lett. 76

(2001) 143.[17] S. Tsubota, D.A.H. Cunningham, Y. Bando, M. Haruta, in: G.

Poncelet, J. Martens, B. Delmon, P.A. Jacobs, P. Grange (Eds.),Stud. Surf. Sci. Catal. 91 (1995) 227.

[18] M. Okumura, K. Tanaka, A. Ueda, M. Haruta, Solid State Ionics 95(1997) 143.

[19] T. Kobayashi, M. Haruta, S. Tsuota, H. Sano, Sens. Actuators B1(1990) 222.

[20] Y.J. Chen, C.T. Yeh, J. Catal. 200 (1) (2001) 59.[21] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck,

Nature 359 (1992) 710.[22] R. Kohn, M. Froba, Catal. Today 68 (2001) 227.[23] A. Corma, Q. Kan, F. Rey, Chem. Commun. 579 (1998).[24] M. Morey, A. Davidson, H. Eckert, G.D. Stucky, Chem. Mater. 8

(1996) 486.[25] H. Gies, S. Grabowski, M. Bandyopadhyay, W. Grunert, O.P.

Tkachenko, K.V. Klementiev, A. Birkner, Micropor. Mesopor.Mater. 60 (2003) 31.

[26] M. Bandyopadhyay, A. Birkner, M.W.E. van den Berg, K.V.Klementiev, W. Schmidt, W. Grunert, H. Gies, Chem. Mater. 17(2005) 3820.

[27] C.M. Yang, P.H. Liu, Y.F. Ho, C.Y. Chiu, K.J. Chao, Chem. Mater.15 (1) (2003) 275.

[28] C.M. Yang, M. Kalwei, F. Schuth, K.J. Chao, Appl. Catal. 254(2003) 289.

[29] A.K. Sinha, S. Seelan, T. Akita, S. Tsubota, M. Haruta, Appl. Catal.

A 240 (2003) 243.[30] K.V. Klementiev, VIPER for Windows (Visual Processing in EXAFSResearches), freeware. Available from: .

[31] B. Marler, U. Oberhagemann, S. Vortmann, H. Gies, Micropor.Mater. 6 (1996) 375383.

[32] M.J. Borowski, J. Phys. IV France 7 (1997) 259260.

M. Bandyopadhyay et al. / Microporous and Mesoporous Materials 89 (2006) 158163 163

http://www.desy.de/~klmn/viper.htmlhttp://www.desy.de/~klmn/viper.htmlhttp://www.desy.de/~klmn/viper.htmlhttp://www.desy.de/~klmn/viper.html