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Liquid crystal phase templated mesoporous platinum alloy
Microporous and Mesoporous Materials 44±45 (2001) 159±163
www.elsevier.nl/locate/micromeso
Liquid crystal phase templated mesoporous platinum alloy George S. Attard *, Stephane A.A. Leclerc, Stephanie Maniguet, Andrea E. Russell, Iris Nandhakumar, Bernhard R. Gollas, Philip N. Bartlett Department of Chemistry, University of Southampton, Southampton SO17 1BJ, UK Received 28 January 2000; accepted 11 May 2000
Abstract The reduction of metal salts dissolved in the aqueous domains of the lyotropic liquid crystalline phases of surfactants provides a versatile route to a range of mesoporous metals with aligned pore systems having a high degree of spatial periodicity. Here we report that the co-reduction of binary mixtures of metal compounds dissolved in the hexagonal phase of a non-ionic surfactant leads to bimetallic alloy powders that have a long-ranged porous nanostructure and high speci®c surface areas. These results indicate that the use of liquid crystalline reaction mixtures could provide a new route to alloys of importance for applications in catalysis. Ó 2001 Published by Elsevier Science B.V. Keywords: Mesoporous materials; Pt±Ru alloys; Liquid crystal templating
1. Introduction Recently we reported that metals, such as platinum and tin, having a well-de®ned periodic mesoporous nanostructure can be obtained by the reduction of metal salts dissolved in the aqueous domains of the lyotropic liquid crystalline phases of oligoethylene oxide surfactants [1±3]. In these structured reaction systems the metal is formed in between the surfactant supramolecular assemblies that constitute the building blocks of lyotropic liquid crystalline phases. Consequently, the porous nanostructure of the resulting metal is eectively a cast of the supramolecular architecture of the phase in which it was synthesised. Although the
*
Corresponding author. Fax: +44-1703-593-781. E-mail address: [email protected] (G.S. Attard).
liquid crystal templating strategy has been used successfully to produce mesoporous metals, it is not possible to be certain, a priori, that the coreduction of binary mixtures of metal compounds will lead to intermetallic materials with a uniform distribution of the constituent metals. Indeed, it is often the case that nanoparticles produced by coreduction from isotropic solutions or from microemulsions have a surface layer that is enriched in one of the metals [4]. The ability to produce binary intermetallic materials that have a uniform dispersion of the constituent metals is of considerable importance for the development of new catalysts, since in principle for such systems, catalytic activity can be related to a well de®ned and reproducible surface chemistry [5,6]. Thus it is reasonable to expect that the production of binary alloys of platinum that combine high speci®c surface areas, controllable pore diameters and a uniform distribution of metal
1387-1811/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 1 8 0 - 9
160
G.S. Attard et al. / Microporous and Mesoporous Materials 44±45 (2001) 159±163
species will impact on a wide range of catalytic processes. 2. Experimental Hexagonal liquid crystal mixtures were prepared by mixing Brij76 (9.0 g), a commercially available non-ionic surfactant (Aldrich) that nominally corresponds to decaethyleneoxide monooctadecyl ether (C18 EO10 ) with hexachloroplatinic acid (3.0 g) and ruthenium trichloride (1.52 g) dissolved in 2.84 ml deionised water. The amounts of metal salts were chosen so as to produce an alloy with a nominal 1:1 atomic ratio of platinum to ruthenium. Reduction of the liquid crystal mixture was carried out by the addition of sodium borohydride (0.3 g dissolved in 6 ml deionised water) at room temperature for 2 h. In a separate experiment, a mixture of KOH (0.5 g) in 2.5 ml deionised water mixed with 2.5 ml formaldehyde (40% solution in water) was used in place of sodium borohydride as reducing agent. After reduction, the products were washed in water and solids were isolated by centrifugation. The washing/centrifugation procedure was repeated six times and after the ®nal wash, the solids were dried in an oven at 60°C for 12 h to aord a ®ne black powder. Transmission electron microscopy was used to visualise the nanostructure of the HI ±Pt/ Ru powders using a JEOL 2000 FX transmission electron microscope operating at a voltage of 200 kV. The speci®c surface areas of the materials were measured using nitrogen BET porosimetry. Low and high angle X-ray diraction data were obtained using CuKa1 radiation and EXAFS data were collected on Wiggler Station 9.2 of the Synchrotron Radiation Source at Daresbury, UK. The EXAFS components were analysed using EXCURVE98 [7]. 3. Results and discussion Transmission electron microscopy was employed to visualise the nanostructure of the HI ±Pt/ Ru powders. As shown in Fig. 1, the material prepared exhibit a nanostructure consisting of
Fig. 1. Transmission electron micrographs of HI ±Pt/Ru with a nominal Pt:Ru ratio of 1:1 prepared using sodium borohydride as reducing agent. The scale bar corresponds to 50 nm.
cylindrical pores of uniform diameter disposed in an hexagonal arrangement with respect to one another. Measurements taken from the micrographs indicate that HI ±Pt/Ru has a pore diameter and a wall thickness at the point of of 37 1 A nearest contact between the pores of 34 1 A (Fig. 1). These values are comparable with those observed for HI ±Pt obtained from the hexagonal phase of Brij76 [8]. The speci®c surface area of the HI ±Pt/Ru powder was measured using nitrogen BET porosimetry. The adsorption/desorption isotherms (Fig. 2) are typical of mesoporous materials and show little hysteresis. The speci®c surface area of the material was determined as 86 m2 g 1 . Low angle X-ray diraction studies of the HI ±Pt/Ru alloy show a re¯ection at 1:47° 2h, which corresponds and a hole-to-hole to a lattice parameter of 60.3 A distance of 69.6 A. The reduction of isotropic aqueous binary mixtures of metal salts has been widely reported as a route to intermetallic nanoparticles with diameters ranging from a few nm to > 100 nm [9]. However, as is evident from the literature, coreduction of binary mixtures does not necessarily produce intermetallic materials with an homogeneous distribution of metal atoms. In particular, it is often the case that the reaction products consist of a physical mixture of nanoparticles of each of the two metals (i.e. no alloying). Even when al-
G.S. Attard et al. / Microporous and Mesoporous Materials 44±45 (2001) 159±163
161
Fig. 3. Wide angle X-ray diraction for HI ±Pt/Ru with a nominal Pt:Ru ratio of 1:1.
Fig. 2. Nitrogen adsorption/desorption isotherms for HI ±Pt/Ru with a nominal Pt:Ru ratio of 1:1.
loying is achieved successfully, it is often observed that the metal distribution is not uniform within a particle [4]. Wide angle X-ray diraction and EXAFS were therefore employed to investigate whether or not HI ±Pt/Ru were indeed intermetallic materials. The X-ray diraction data are summarised in Table 1. The diractogram for HI ±Pt/ Ru (Fig. 3) shows four re¯ections that are in excellent agreement with those reported for nanoparticulate Pt/Ru alloy with a nominal Pt:Ru ratio of 1:1 which was prepared by co-reduction in an isotropic reaction mixture [9]. Comparison of the wide angle diraction data of HI ±Pt/Ru, with that of bulk platinum or platinum black indicates that HI ±Pt/Ru has an fcc structure (Fm3m space
group). The contraction in the lattice parameter, evidenced by the shift of the re¯ections to shorter distances, is consistent with the smaller size of ruthenium compared with platinum. Con®rmation of the intermetallic nature of the HI ±Pt/Ru material, as well as of the uniform distribution of the metallic components was achieved using EXAFS. The k3 weighted Fourier transforms of the Pt LIII and Ru K edge EXAFS data are shown in Fig. 4a and b respectively. Structural parameters obtained by ®tting the data are summarised in Table 2. The ®t to the Pt LIII edge data HI ±Pt/Ru indicates that the ®rst shell Pt±Pt co-ordination number is 5.4 0:3 while the ®rst shell Pt±Ru coordination number is 2.9 0:2. The Pt±Pt coordination distance obtained from the ®t is 2.737 compared with a distance of 2.742 A for HI ±Pt A, prepared from Brij76 (data not shown). This reduction in co-ordination distance on alloying is consistent with previous EXAFS investigations of Pt/Ru particles supported on carbon [10], and with
Table 1 for bulk platinum [11], platinum black, Summary of wide angle X-ray diraction data obtained using CuKa1 radiation k 1:5405 A HI ±Pt and HI ±Pt/Rua hkl 111 200 220 311 a
d (A)
Bulk platinum (Fm3m) I (a.u.)
Platinum black d (A) I (a.u.)
HI ±Pt d (A)
I (a.u.)
d (A)
I (a.u.)
2.2650 1.9616 1.3873 1.1826
100 53 31 33
2.26 1.96 1.38 1.18
2.25 1.96 1.38 1.19
100 54 29 31
2.24 1.95 1.37 1.18
100 48 27 30
100 54 29 31
I denotes the intensity of the diraction peaks, and d their position in real space.
HI ±Pt/Ru
162
G.S. Attard et al. / Microporous and Mesoporous Materials 44±45 (2001) 159±163
Fig. 4. k3 weighted (v(k)k3 ) Fourier transform of EXAFS data for HI ±Pt/Ru (dark curves) and theoretical ®ts to data (pale curves); the ®tting parameters are de®ned in the legend to Table 2. (a) Data for Pt LIII edge, and (b) data for Ru K edge. The Fourier transforms have been phase corrected for backscattering by the Pt and Ru near-neighbour contributions respectively. HI ±Pt/Ru powder was mixed with boron nitride in an amount which gave an edge step of 0.5 in the ln(I0 =I) plot at the Pt LIII edge. The mixture was pressed into a wafer which was mounted in a cryostat. XAS data were collected using the transmission data collection mode with the sample at liquid nitrogen temperature. All data were collected on Wiggler station 9.2 of the synchrotron radiation source (SRC) at Daresbury, UK. The ring operated with 2.0 GeV energy and 100±250 mA current. The station was operated with a Si(2 2 0) double crystal monochromator, which was detuned to 50% intensity for the Pt LIII edge and to 70% for the Ru K edge, to minimise the presence of higher harmonics. Three ion chambers, optimised for the appropriate edge energy, were used in series to measure the intensities of the incident beam (I0 ), the beam transmitted by the sample (It ) and the beam subsequently transmitted by a Pt foil or Ru powder sample (Im ). The reference materials were used for internal calibration of the X-ray beam energy.
Table 2 Structural parameters for HI ±Pt/Ru obtained from theoretical ®ts of the EXAFS dataa 2 Sample (edge) Neighbour N R (A) 2r2 (A) HI ±Pt/Ru (Pt LIII )
Pt±Pt Pt±Ru Pt±Pt
5:4 0:3 2:9 0:2 1:4 0:7
2:737 0:002 2:709 0:003 3:81 0:02
0.013 0.014 0.015
HI ±Pt/Ru (Ru K)
Ru±Ru Ru±Pt Ru±O
3:2 0:3 2:7 0:4 1:2 0:3
2:668 0:006 2:714 0:008 1:95 0:02
0.013 0.014 0.011
EF (eV) 10:1
1.33
R EXAFS ) (%) 38
49
a The absorption spectra were processed using EXBROOK [7]. The zero point energy scale was taken to be the point of in¯ection in the absorption edge. The pre-edge region was ®tted by a straight line and extrapolated to zero energy. A polynomial spline was ®tted to the non-oscillatory component of the post-edge region and was extrapolated to zero energy. The dierence between the extrapolated values was taken to be the step height and the spectrum was normalised to this value. The EXAFS components v k were then isolated from the absorption spectra and subsequently analysed using EXCURVE98 [7], a least squares ®tting programme based on curvedwave theory, using a Z 1 2p core hole approximation. N is the co-ordination number, R is the distance to the near neighbour, 2r2 the Debye±Waller term, and EF is the shift in the Fermi energy. It was assumed that the Hedin±Lundqvist potential accounted for any amplitude reductions due to absorption processess which do not result in EXAFS features. R EXAFS , which is a measure of the goodness of the ®t between the EXAFS data (vexp ) and the theoretical model (vth ), is de®ned as: ( ) N X 1 exp jviexp vth j r 100 R EXAFS i i i
where N is the number of data points and r exp is the standard deviation for each data point.
the wide angle X-ray diraction data. The ®t to the Ru K edge data indicates a Ru±Ru ®rst shell coordination number of 3.2 0:3 and a Ru±Pt ®rst
shell co-ordination number of 2.7 0:4. The latter is in excellent agreement with the value obtained from the ®t to the Pt LIII edge data, con-
G.S. Attard et al. / Microporous and Mesoporous Materials 44±45 (2001) 159±163
®rming that HI ±Pt/Ru consists of a well-mixed 1:1 alloy of the two metals. The presence of an oxygen neighbour in the ®rst co-ordination shell of Ru indicates that HI ±Pt/Ru is slightly oxidised, with the oxide forming preferentially at the ruthenium sites. 4. Conclusion By conducting the reduction of binary mixtures of metal salts within the aqueous domains of an hexagonal lyotropic liquid crystalline phase we were able to produce nanostructured mesoporous bimetallic alloys. These data illustrate the viability of employing a liquid crystal templated route to the production of materials of considerable interest for applications in catalysis. Acknowledgements The authors thank J. Evans and M.T. Weller for discussions on the EXAFS data and R.J. Mathew, J. Yao and F. Mosselmans (CCLRC Daresbury) for technical assistance with the EXAFS experiments and B. Cressey for the TEM
163
studies. This work was funded by City Technology Ltd. and by the Engineering and Physical Sciences Research Council. References [1] G.S. Attard, C.G. G oltner, J.M. Corker, S. Henke, R.H. Templer, Agnew. Chem. Intl. Edn. Engl. 36 (1997) 1315. [2] G.S. Attard, P.N. Bartlett, N.R.B. Coleman, J.M. Elliott, J.R. Owen, J.H. Wang, Science 278 (1997) 838. [3] A.H. Whitehead, J.M. Elliott, J.R. Owen, G.S. Attard, J. Chem. Soc. Chem. Commun. (1999) 331. [4] F.C.M.J.M. vanDelft, B.E. Nieuwenhuys, J. Siera, R.M. Wolf, ISIJ Int. 29 (1989) 550. [5] H.A. Gasteiger, N. Markovic, P.N. Ross, E.J. Cairns, Electrochim. Acta. 39 (1994) 1825. [6] S. Mukerjee, S. Srinivasan, M.P. Soriaga, J. McBreen, J. Electrochem. Soc. 142 (1995) 1409. [7] N. Binsted, Excurv98: CCLRC Daresbury Laboratory Computer Programme, CCLRC Daresbury Laboratory, 1998. [8] N.R.B. Coleman, Thesis, University of Southampton, 1999. [9] M. Watanabe, M. Uchida, S. Motoo, J. Electroanal. Chem. 229 (1987) 395. [10] J. McBreen, S. Mukerjee, J. Electrochem. Soc. 142 (1995) 3399. [11] T. Barth, G. Lunde, Z. Phys. Chem. (Munich) 121 (1926) 78.
www.elsevier.nl/locate/micromeso
Liquid crystal phase templated mesoporous platinum alloy George S. Attard *, Stephane A.A. Leclerc, Stephanie Maniguet, Andrea E. Russell, Iris Nandhakumar, Bernhard R. Gollas, Philip N. Bartlett Department of Chemistry, University of Southampton, Southampton SO17 1BJ, UK Received 28 January 2000; accepted 11 May 2000
Abstract The reduction of metal salts dissolved in the aqueous domains of the lyotropic liquid crystalline phases of surfactants provides a versatile route to a range of mesoporous metals with aligned pore systems having a high degree of spatial periodicity. Here we report that the co-reduction of binary mixtures of metal compounds dissolved in the hexagonal phase of a non-ionic surfactant leads to bimetallic alloy powders that have a long-ranged porous nanostructure and high speci®c surface areas. These results indicate that the use of liquid crystalline reaction mixtures could provide a new route to alloys of importance for applications in catalysis. Ó 2001 Published by Elsevier Science B.V. Keywords: Mesoporous materials; Pt±Ru alloys; Liquid crystal templating
1. Introduction Recently we reported that metals, such as platinum and tin, having a well-de®ned periodic mesoporous nanostructure can be obtained by the reduction of metal salts dissolved in the aqueous domains of the lyotropic liquid crystalline phases of oligoethylene oxide surfactants [1±3]. In these structured reaction systems the metal is formed in between the surfactant supramolecular assemblies that constitute the building blocks of lyotropic liquid crystalline phases. Consequently, the porous nanostructure of the resulting metal is eectively a cast of the supramolecular architecture of the phase in which it was synthesised. Although the
*
Corresponding author. Fax: +44-1703-593-781. E-mail address: [email protected] (G.S. Attard).
liquid crystal templating strategy has been used successfully to produce mesoporous metals, it is not possible to be certain, a priori, that the coreduction of binary mixtures of metal compounds will lead to intermetallic materials with a uniform distribution of the constituent metals. Indeed, it is often the case that nanoparticles produced by coreduction from isotropic solutions or from microemulsions have a surface layer that is enriched in one of the metals [4]. The ability to produce binary intermetallic materials that have a uniform dispersion of the constituent metals is of considerable importance for the development of new catalysts, since in principle for such systems, catalytic activity can be related to a well de®ned and reproducible surface chemistry [5,6]. Thus it is reasonable to expect that the production of binary alloys of platinum that combine high speci®c surface areas, controllable pore diameters and a uniform distribution of metal
1387-1811/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 1 8 0 - 9
160
G.S. Attard et al. / Microporous and Mesoporous Materials 44±45 (2001) 159±163
species will impact on a wide range of catalytic processes. 2. Experimental Hexagonal liquid crystal mixtures were prepared by mixing Brij76 (9.0 g), a commercially available non-ionic surfactant (Aldrich) that nominally corresponds to decaethyleneoxide monooctadecyl ether (C18 EO10 ) with hexachloroplatinic acid (3.0 g) and ruthenium trichloride (1.52 g) dissolved in 2.84 ml deionised water. The amounts of metal salts were chosen so as to produce an alloy with a nominal 1:1 atomic ratio of platinum to ruthenium. Reduction of the liquid crystal mixture was carried out by the addition of sodium borohydride (0.3 g dissolved in 6 ml deionised water) at room temperature for 2 h. In a separate experiment, a mixture of KOH (0.5 g) in 2.5 ml deionised water mixed with 2.5 ml formaldehyde (40% solution in water) was used in place of sodium borohydride as reducing agent. After reduction, the products were washed in water and solids were isolated by centrifugation. The washing/centrifugation procedure was repeated six times and after the ®nal wash, the solids were dried in an oven at 60°C for 12 h to aord a ®ne black powder. Transmission electron microscopy was used to visualise the nanostructure of the HI ±Pt/ Ru powders using a JEOL 2000 FX transmission electron microscope operating at a voltage of 200 kV. The speci®c surface areas of the materials were measured using nitrogen BET porosimetry. Low and high angle X-ray diraction data were obtained using CuKa1 radiation and EXAFS data were collected on Wiggler Station 9.2 of the Synchrotron Radiation Source at Daresbury, UK. The EXAFS components were analysed using EXCURVE98 [7]. 3. Results and discussion Transmission electron microscopy was employed to visualise the nanostructure of the HI ±Pt/ Ru powders. As shown in Fig. 1, the material prepared exhibit a nanostructure consisting of
Fig. 1. Transmission electron micrographs of HI ±Pt/Ru with a nominal Pt:Ru ratio of 1:1 prepared using sodium borohydride as reducing agent. The scale bar corresponds to 50 nm.
cylindrical pores of uniform diameter disposed in an hexagonal arrangement with respect to one another. Measurements taken from the micrographs indicate that HI ±Pt/Ru has a pore diameter and a wall thickness at the point of of 37 1 A nearest contact between the pores of 34 1 A (Fig. 1). These values are comparable with those observed for HI ±Pt obtained from the hexagonal phase of Brij76 [8]. The speci®c surface area of the HI ±Pt/Ru powder was measured using nitrogen BET porosimetry. The adsorption/desorption isotherms (Fig. 2) are typical of mesoporous materials and show little hysteresis. The speci®c surface area of the material was determined as 86 m2 g 1 . Low angle X-ray diraction studies of the HI ±Pt/Ru alloy show a re¯ection at 1:47° 2h, which corresponds and a hole-to-hole to a lattice parameter of 60.3 A distance of 69.6 A. The reduction of isotropic aqueous binary mixtures of metal salts has been widely reported as a route to intermetallic nanoparticles with diameters ranging from a few nm to > 100 nm [9]. However, as is evident from the literature, coreduction of binary mixtures does not necessarily produce intermetallic materials with an homogeneous distribution of metal atoms. In particular, it is often the case that the reaction products consist of a physical mixture of nanoparticles of each of the two metals (i.e. no alloying). Even when al-
G.S. Attard et al. / Microporous and Mesoporous Materials 44±45 (2001) 159±163
161
Fig. 3. Wide angle X-ray diraction for HI ±Pt/Ru with a nominal Pt:Ru ratio of 1:1.
Fig. 2. Nitrogen adsorption/desorption isotherms for HI ±Pt/Ru with a nominal Pt:Ru ratio of 1:1.
loying is achieved successfully, it is often observed that the metal distribution is not uniform within a particle [4]. Wide angle X-ray diraction and EXAFS were therefore employed to investigate whether or not HI ±Pt/Ru were indeed intermetallic materials. The X-ray diraction data are summarised in Table 1. The diractogram for HI ±Pt/ Ru (Fig. 3) shows four re¯ections that are in excellent agreement with those reported for nanoparticulate Pt/Ru alloy with a nominal Pt:Ru ratio of 1:1 which was prepared by co-reduction in an isotropic reaction mixture [9]. Comparison of the wide angle diraction data of HI ±Pt/Ru, with that of bulk platinum or platinum black indicates that HI ±Pt/Ru has an fcc structure (Fm3m space
group). The contraction in the lattice parameter, evidenced by the shift of the re¯ections to shorter distances, is consistent with the smaller size of ruthenium compared with platinum. Con®rmation of the intermetallic nature of the HI ±Pt/Ru material, as well as of the uniform distribution of the metallic components was achieved using EXAFS. The k3 weighted Fourier transforms of the Pt LIII and Ru K edge EXAFS data are shown in Fig. 4a and b respectively. Structural parameters obtained by ®tting the data are summarised in Table 2. The ®t to the Pt LIII edge data HI ±Pt/Ru indicates that the ®rst shell Pt±Pt co-ordination number is 5.4 0:3 while the ®rst shell Pt±Ru coordination number is 2.9 0:2. The Pt±Pt coordination distance obtained from the ®t is 2.737 compared with a distance of 2.742 A for HI ±Pt A, prepared from Brij76 (data not shown). This reduction in co-ordination distance on alloying is consistent with previous EXAFS investigations of Pt/Ru particles supported on carbon [10], and with
Table 1 for bulk platinum [11], platinum black, Summary of wide angle X-ray diraction data obtained using CuKa1 radiation k 1:5405 A HI ±Pt and HI ±Pt/Rua hkl 111 200 220 311 a
d (A)
Bulk platinum (Fm3m) I (a.u.)
Platinum black d (A) I (a.u.)
HI ±Pt d (A)
I (a.u.)
d (A)
I (a.u.)
2.2650 1.9616 1.3873 1.1826
100 53 31 33
2.26 1.96 1.38 1.18
2.25 1.96 1.38 1.19
100 54 29 31
2.24 1.95 1.37 1.18
100 48 27 30
100 54 29 31
I denotes the intensity of the diraction peaks, and d their position in real space.
HI ±Pt/Ru
162
G.S. Attard et al. / Microporous and Mesoporous Materials 44±45 (2001) 159±163
Fig. 4. k3 weighted (v(k)k3 ) Fourier transform of EXAFS data for HI ±Pt/Ru (dark curves) and theoretical ®ts to data (pale curves); the ®tting parameters are de®ned in the legend to Table 2. (a) Data for Pt LIII edge, and (b) data for Ru K edge. The Fourier transforms have been phase corrected for backscattering by the Pt and Ru near-neighbour contributions respectively. HI ±Pt/Ru powder was mixed with boron nitride in an amount which gave an edge step of 0.5 in the ln(I0 =I) plot at the Pt LIII edge. The mixture was pressed into a wafer which was mounted in a cryostat. XAS data were collected using the transmission data collection mode with the sample at liquid nitrogen temperature. All data were collected on Wiggler station 9.2 of the synchrotron radiation source (SRC) at Daresbury, UK. The ring operated with 2.0 GeV energy and 100±250 mA current. The station was operated with a Si(2 2 0) double crystal monochromator, which was detuned to 50% intensity for the Pt LIII edge and to 70% for the Ru K edge, to minimise the presence of higher harmonics. Three ion chambers, optimised for the appropriate edge energy, were used in series to measure the intensities of the incident beam (I0 ), the beam transmitted by the sample (It ) and the beam subsequently transmitted by a Pt foil or Ru powder sample (Im ). The reference materials were used for internal calibration of the X-ray beam energy.
Table 2 Structural parameters for HI ±Pt/Ru obtained from theoretical ®ts of the EXAFS dataa 2 Sample (edge) Neighbour N R (A) 2r2 (A) HI ±Pt/Ru (Pt LIII )
Pt±Pt Pt±Ru Pt±Pt
5:4 0:3 2:9 0:2 1:4 0:7
2:737 0:002 2:709 0:003 3:81 0:02
0.013 0.014 0.015
HI ±Pt/Ru (Ru K)
Ru±Ru Ru±Pt Ru±O
3:2 0:3 2:7 0:4 1:2 0:3
2:668 0:006 2:714 0:008 1:95 0:02
0.013 0.014 0.011
EF (eV) 10:1
1.33
R EXAFS ) (%) 38
49
a The absorption spectra were processed using EXBROOK [7]. The zero point energy scale was taken to be the point of in¯ection in the absorption edge. The pre-edge region was ®tted by a straight line and extrapolated to zero energy. A polynomial spline was ®tted to the non-oscillatory component of the post-edge region and was extrapolated to zero energy. The dierence between the extrapolated values was taken to be the step height and the spectrum was normalised to this value. The EXAFS components v k were then isolated from the absorption spectra and subsequently analysed using EXCURVE98 [7], a least squares ®tting programme based on curvedwave theory, using a Z 1 2p core hole approximation. N is the co-ordination number, R is the distance to the near neighbour, 2r2 the Debye±Waller term, and EF is the shift in the Fermi energy. It was assumed that the Hedin±Lundqvist potential accounted for any amplitude reductions due to absorption processess which do not result in EXAFS features. R EXAFS , which is a measure of the goodness of the ®t between the EXAFS data (vexp ) and the theoretical model (vth ), is de®ned as: ( ) N X 1 exp jviexp vth j r 100 R EXAFS i i i
where N is the number of data points and r exp is the standard deviation for each data point.
the wide angle X-ray diraction data. The ®t to the Ru K edge data indicates a Ru±Ru ®rst shell coordination number of 3.2 0:3 and a Ru±Pt ®rst
shell co-ordination number of 2.7 0:4. The latter is in excellent agreement with the value obtained from the ®t to the Pt LIII edge data, con-
G.S. Attard et al. / Microporous and Mesoporous Materials 44±45 (2001) 159±163
®rming that HI ±Pt/Ru consists of a well-mixed 1:1 alloy of the two metals. The presence of an oxygen neighbour in the ®rst co-ordination shell of Ru indicates that HI ±Pt/Ru is slightly oxidised, with the oxide forming preferentially at the ruthenium sites. 4. Conclusion By conducting the reduction of binary mixtures of metal salts within the aqueous domains of an hexagonal lyotropic liquid crystalline phase we were able to produce nanostructured mesoporous bimetallic alloys. These data illustrate the viability of employing a liquid crystal templated route to the production of materials of considerable interest for applications in catalysis. Acknowledgements The authors thank J. Evans and M.T. Weller for discussions on the EXAFS data and R.J. Mathew, J. Yao and F. Mosselmans (CCLRC Daresbury) for technical assistance with the EXAFS experiments and B. Cressey for the TEM
163
studies. This work was funded by City Technology Ltd. and by the Engineering and Physical Sciences Research Council. References [1] G.S. Attard, C.G. G oltner, J.M. Corker, S. Henke, R.H. Templer, Agnew. Chem. Intl. Edn. Engl. 36 (1997) 1315. [2] G.S. Attard, P.N. Bartlett, N.R.B. Coleman, J.M. Elliott, J.R. Owen, J.H. Wang, Science 278 (1997) 838. [3] A.H. Whitehead, J.M. Elliott, J.R. Owen, G.S. Attard, J. Chem. Soc. Chem. Commun. (1999) 331. [4] F.C.M.J.M. vanDelft, B.E. Nieuwenhuys, J. Siera, R.M. Wolf, ISIJ Int. 29 (1989) 550. [5] H.A. Gasteiger, N. Markovic, P.N. Ross, E.J. Cairns, Electrochim. Acta. 39 (1994) 1825. [6] S. Mukerjee, S. Srinivasan, M.P. Soriaga, J. McBreen, J. Electrochem. Soc. 142 (1995) 1409. [7] N. Binsted, Excurv98: CCLRC Daresbury Laboratory Computer Programme, CCLRC Daresbury Laboratory, 1998. [8] N.R.B. Coleman, Thesis, University of Southampton, 1999. [9] M. Watanabe, M. Uchida, S. Motoo, J. Electroanal. Chem. 229 (1987) 395. [10] J. McBreen, S. Mukerjee, J. Electrochem. Soc. 142 (1995) 3399. [11] T. Barth, G. Lunde, Z. Phys. Chem. (Munich) 121 (1926) 78.