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HREM study of Pt-clusters on K-LTL crystal surfaces
H.G. Karge and J. Weitkamp (Eds.) 52
Zeolite Science 1994: Recent Progress and Discussions
Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All rights reserved. H R E M S t u d y o f Pt-clusters on K - L T L C r y s t a l S u r f a c e s Osamu Terasakil), Tetsu Ohsuna 2) & Denjiro Watanabe 2) 1) Department of Physics, Tohoku University, Sendai 980-77, JAPAN 2) Department of Materials Science, Iwaki Meisei University, Iwaki 970, JAPAN
Zeolites containing noble-metal particles have attracted attention as catalysts. The accurate characterization of particles is essential for the understanding of their chemical properties. Highresolution electron microscopy (HREM) is a powerful method for this purpose, but it is not easy to confirm that the particles are inside the channels of the zeolite. This is because that zeolites are electron-beam sensitive and that the strong contribution from the framework masks the contrast from the clusters, when electrons are incident with the direction of the channels. There are several ways to overcome this difficulty: image processing 1), HREM observation of serial ultrathin-sectioned specimens 2) and the Z-contrast method3). Normally, if we can not find the particles on the surface by looking at the direction perpendicular to the channel, we are able to say that the particles are inside. A question then arises, how small particles can be observed along this axis. Consider Pt / K-LTL as an example. The LTL sample ( Si/A1 = 2.4) was kindly supplied by Tosoh Coorp., Japan and Pt was evaporated onto the crystals by the sputtering method. The amount of Pt was controlled by changing the distance between the zeolite powder and the Ptplates, and by the duration of sputtering. HREM images were taken by 400 keV TEM along both the [001] and the <100> directions. The cylindrically shaped K-LTL crystals are electron beam sensitive and show characteristic morphology change ( e.g. to form a waist4)) under the electron beam, also at relatively small electron dose. With the electron beam parallel to the channels, we can observe a contrast from Pt-clusters which are on the (001) plane, if the particles are larger than approximately 30 A. It is however rather difficult to verify that this contrast is caused by the particles, unless the framework is destroyed by the beam. Pt-clusters, which are sticking to the side-wall of the crystals, i.e., { 100}, show a strong tendency to align along the channels facing to the surface (this is observed after morphology change in the beam). HREM images (Fig. 1) taken with the electron beam perpendicular to the channels show clearly where the Pt-particles are before (a) and after (b) a serious damage by the electrons. Pt-particles are stationary on the surface during the electron beam irradiation and this is different from the previous observation of metallic particles5). The Pt-particles, with a diameter larger than 10 ~, are situated on the (001) surface and in the projection shown in Fig.la they can be seen situated above the row of channel openings at the surface. It is clear from all observations that there is a spatial correlation between the channels and Pt-particles (three of them are shown by arrows in Fig 1.a). The shape and darkness of the contrast from the different rows of Pt-particles
53 indicates that the occupancy is not the same for each row and a slight off-set can be seen in some cases. In Fig. lb one can see the lattice fringes of Pt-particles, and it is quite clear from the images that the K-LTL crystal changes its morphology and the Pt-particles are migrated by the beam influence. Therefore one must be careful to discuss the particle size after the destruction of the framework, although it is easier to observe contrast from the particles in this way. REFERENCES 1) V.Alfredsson, O.Terasaki & J-O.Bovin: J.Solid State Chem.,105(1993), 223. 2) J-O.Bovin, V.Alfredsson, G.Karlsson, Z.Blum & O.Terasaki: Proc. MSA'94. 3) S.B.Roce, J.Y.Koo, M.M.Disco & M.M.J.Treacy: Ultramicroscopy 34(1990),108. 4) M.M.J.Treacy & J.M.Newsam: Ultramicroscopy 23(1987), 411. 5) R.Wallenberg, J-O.Bovin & D.J.Smith: Naturwiss. 72(1985), S.539.
Fig. 1 HREM images (a)before and (b)after a serious damage by the electrons.
Zeolite Science 1994: Recent Progress and Discussions
Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All rights reserved. H R E M S t u d y o f Pt-clusters on K - L T L C r y s t a l S u r f a c e s Osamu Terasakil), Tetsu Ohsuna 2) & Denjiro Watanabe 2) 1) Department of Physics, Tohoku University, Sendai 980-77, JAPAN 2) Department of Materials Science, Iwaki Meisei University, Iwaki 970, JAPAN
Zeolites containing noble-metal particles have attracted attention as catalysts. The accurate characterization of particles is essential for the understanding of their chemical properties. Highresolution electron microscopy (HREM) is a powerful method for this purpose, but it is not easy to confirm that the particles are inside the channels of the zeolite. This is because that zeolites are electron-beam sensitive and that the strong contribution from the framework masks the contrast from the clusters, when electrons are incident with the direction of the channels. There are several ways to overcome this difficulty: image processing 1), HREM observation of serial ultrathin-sectioned specimens 2) and the Z-contrast method3). Normally, if we can not find the particles on the surface by looking at the direction perpendicular to the channel, we are able to say that the particles are inside. A question then arises, how small particles can be observed along this axis. Consider Pt / K-LTL as an example. The LTL sample ( Si/A1 = 2.4) was kindly supplied by Tosoh Coorp., Japan and Pt was evaporated onto the crystals by the sputtering method. The amount of Pt was controlled by changing the distance between the zeolite powder and the Ptplates, and by the duration of sputtering. HREM images were taken by 400 keV TEM along both the [001] and the <100> directions. The cylindrically shaped K-LTL crystals are electron beam sensitive and show characteristic morphology change ( e.g. to form a waist4)) under the electron beam, also at relatively small electron dose. With the electron beam parallel to the channels, we can observe a contrast from Pt-clusters which are on the (001) plane, if the particles are larger than approximately 30 A. It is however rather difficult to verify that this contrast is caused by the particles, unless the framework is destroyed by the beam. Pt-clusters, which are sticking to the side-wall of the crystals, i.e., { 100}, show a strong tendency to align along the channels facing to the surface (this is observed after morphology change in the beam). HREM images (Fig. 1) taken with the electron beam perpendicular to the channels show clearly where the Pt-particles are before (a) and after (b) a serious damage by the electrons. Pt-particles are stationary on the surface during the electron beam irradiation and this is different from the previous observation of metallic particles5). The Pt-particles, with a diameter larger than 10 ~, are situated on the (001) surface and in the projection shown in Fig.la they can be seen situated above the row of channel openings at the surface. It is clear from all observations that there is a spatial correlation between the channels and Pt-particles (three of them are shown by arrows in Fig 1.a). The shape and darkness of the contrast from the different rows of Pt-particles
53 indicates that the occupancy is not the same for each row and a slight off-set can be seen in some cases. In Fig. lb one can see the lattice fringes of Pt-particles, and it is quite clear from the images that the K-LTL crystal changes its morphology and the Pt-particles are migrated by the beam influence. Therefore one must be careful to discuss the particle size after the destruction of the framework, although it is easier to observe contrast from the particles in this way. REFERENCES 1) V.Alfredsson, O.Terasaki & J-O.Bovin: J.Solid State Chem.,105(1993), 223. 2) J-O.Bovin, V.Alfredsson, G.Karlsson, Z.Blum & O.Terasaki: Proc. MSA'94. 3) S.B.Roce, J.Y.Koo, M.M.Disco & M.M.J.Treacy: Ultramicroscopy 34(1990),108. 4) M.M.J.Treacy & J.M.Newsam: Ultramicroscopy 23(1987), 411. 5) R.Wallenberg, J-O.Bovin & D.J.Smith: Naturwiss. 72(1985), S.539.
Fig. 1 HREM images (a)before and (b)after a serious damage by the electrons.