Hayek, K.

Penner, S.

Rupprechter, G.

Schlögl, R.

Su, D.

Wang, D.

English

MPG.PuRe

TRANSMISSION ELECTRON MICROSCOPY OF SILICA-SUPPORTED PTPARTICLESD. Wang1, S. Penner2, G. Rupprechter1, *D. S. Su2, K. Hayek2, R. Schlögl11Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin,Germany, and 2Institut für Physikalische Chemie, Leopold-Franzens-Universität, A-6020Innsbruck, AustriaIn heterogeneous catalysis studies, the catalyst surface needs to be well defined duringthe catalytic process. However, it is difficult to apply UHV compatible methods andelectron microscopy to a „real“ catalyst. Therefore, model systems are necessary toelucidate the mechanism of structure-sensitive reactions. Metal particles grown onplanar thin supports, thus facilitating the characterisation by electron microscopy, wereextensively studied as model systems for supported catalysts. In the present work, Ptparticles epitaxially grown on (001) NaCl single crystals were covered with asupporting film of amorphous silica. Subsequently, the NaCl was dissolved in distilledwater and the films were mounted on gold grids for electron microscopy [1]. After anoxidising treatment (O2, 673K, 1h) the samples were exposed to H2 at 873K for 1h. Themorphology and structure of the samples were then examined by HRTEM andcompared to the as-prepared state.Fig. 1(a) is a low-magnification image of the as-prepared sample; Fig 1 (b) shows thereduced catalyst. Both images were taken with a Philips CM200 FEG electronmicroscope. Most of the as-prepared Pt particles exhibit a square or rectangular shape[1]. In contrast, after the reductive treatment, a considerable amount of Pt particles haveundergone a change in either shape or orientation. Those with round edges also haveincreased in size, obviously due to the agglomeration of some metal particles. Furtherinformation about the structure modification can be deduced from the diffractionpatterns of the sample before and after the reductive treatment, shown in Figs 2 (a) and(b). The sharp diffraction spots in Fig 2(a) reveal that, due to their orientation withrespect to the original NaCl substrate, most of the as-prepared Pt particles are in their[001] zone, and exhibit parallel lattice planes. In contrast, the diffraction pattern of thesample after the reductive treatment shows series of rings together with somebroadened, but distinct spots. Thus, some particles have changed in azimuthalorientation, although they are still in [001] zone. Apart from (200) and (220), also (111)and (311) spots belonging to a [110] zone can be detected which suggests that the baseplanes of some particles have changed during the treatment. Moreover, weaker spots areobserved corresponding to (100) and (110) reflections (denoted in Fig. 2(b) by arrows),which are forbidden for an fcc structure. Also in the high resolution image a doubleperiodicity of the lattice fringes (3.92 Å) is observed in many particles.Hence, the shape and microstructure of the particles differ considerably before and aftertreatment. Changes in particle structure after reductive treatments under comparableconditions have been explained previously by the approach to a hydrogen-inducedequilibrium shape [2], but also a platinum silicide formation has to be taken intoaccount [3]. Since the diffraction pattern is not consistent with silicide formation anexplanation in terms of a modulated superstructure is favoured in this case. Theformation mechanism of this structure and its relevance for catalytic activity are stillunder investigation.Figure 1. Low magnification image of Pt supported on SiO 2 (a) before and (b) after thetreatment in H2 at 873 K.Figure 2. Electron diffraction patterns of Pt supported on SiO 2 (a) before and (b) afterthe treatment in H2 at 873 K.References[1] Rupprechter, G., Hayek, K., Rendón, L., Jose-Yacamán, M. (1995) Thin Solid Films260, 148; Rupprechter, G., Hayek, K., Hofmeister, H. (1998) J. Catal. 173, 409.[2] Wang, T., Lee, C., Schmidt, L. D. (1985) Surface Sci. 163, 181.[3] Lamber, R., Jaeger, N., Schultz-Ekloff, G. (1990) Surface Sci. 227, 268.                                                * dangsheng@fhi-berlin.mpg.de(a) (b)100 nm(a) (b)200220 220200

Transmission electron microscopy of silica-supported Pt particles

Abstract

Similar works

Full text

Available Versions

MPG.PuRe

MPG.PuRe