Hayek, Konrad

Penner, Simon

Rupprechter, Günther

Schlögl, Robert

Su, Dang Sheng

Wang, Di

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PLATINUM NANOCRYSTALS SUPPORTED BY SILICA, CERIA AND ALUMINA: 
METAL-SUPPORT INTERACTIONS DUE TO HIGH-TEMPERATURE 
REDUCTION IN HYDROGEN  
 
S. Penner1, D. Wang2, D. S. Su2, G. Rupprechter2, R. Schlögl2, K. Hayek1 
 
1Institut für Physikalische Chemie, Leopold-Franzens-Universität, Innrain 52a, 
A-6020 Innsbruck, Austria  
2Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany 
E-mail: simon.penner@uibk.ac.at 
 
Regular Pt nanoparticles, obtained by epitaxial deposition on (001) NaCl surfaces, were 
supported by thin films of amorphous silica, amorphous alumina and crystalline ceria, 
respectively, and subjected to reductive treatments in flowing hydrogen (1 bar) at increasing 
temperature up to 1073 K. The changes in morphology and composition were followed by 
(HR)TEM, electron diffraction and EELS, and possible structures were checked by image 
simulation. After reduction above 773 K similar structural changes are observed on all 
supports, despite their differing chemical properties. The observed platelet- and cube-like 
structures, some exhibiting double lattice periodicities in diffraction patterns and high 
resolution images, arise in part from surface reconstruction in the hydrogen and in part from 
alloy formation. Reduction of the ceria support above 973 K leads to stable Ce suboxide films 
with large periodicities. 
 
Introduction 
In heterogeneous catalysis, model systems are indispensable to elucidate the mechanism of 
structure-sensitive reactions. Regular metal particles on planar thin supports greatly facilitate 
the characterisation by electron microscopy, and provide a defined initial state for subsequent 
structural alterations due to oxidation and reduction. Noble metal particles grown epitaxially 
on NaCl and thereafter transferred to different supporting films are ideally suited to study the 
influence of different supports on identical nanoparticles. Lately, they have been extensively 
studied as model systems for supported catalysts [1,2].  
 
Experimental 
Pt particles epitaxially grown on (001) NaCl single crystals were covered with a supporting 
film of amorphous silica. Subsequently, the NaCl was dissolved in water and the films were 
mounted on gold grids [1]. After an oxidising treatment (O2, 673K, 1h) the samples were 
exposed to H2 at temperatures varying from 673 to 1073 K for 1h, either in a flow system or 
in a circulating batch reactor. The morphology and structure of the samples were examined by 
HRTEM and compared to the as-prepared state. The images were taken with a Zeiss EM 10C, 
with a Philips CM FEG 200 and with a JEOL 4000 microscope. 
 
Results and Discussion 
Fig. 1a shows a low-magnification image of the "initial state" of the Pt particles (which is 
identical in all catalysts) in the as-prepared Pt/SiO2 sample, together with the corresponding 
diffraction pattern. Most of the as-grown Pt particles exhibit square or rectangular outlines 
and were identified as truncated half octahedra with (001) faces perpendicular to the electron 
beam [1] (insert). In contrast, after a reductive treatment at 873 K a considerable number of Pt 
particles have undergone a change in either shape or orientation (Fig. 1b). Those with round 
edges also have increased in size, obviously due to agglomeration. Particles with sharp edges  
 
Fig. 1 a) Pt nanocrystals supported by SiO2, as grown with SAED pattern and HR image of a selected particle; 
b) after a 1 bar hydrogen treatment at 873 K for 1 h; c) ceria-supported Pt particles after the same treatment as 
in b); d) ceria-supported Pt after hydrogen treatment at 1073 K, with large lattice spacings of CeOx. 
 
are cube-or platelet-like and double periodicities are observed in both the diffraction pattern 
and the lattice fringes at high resolution (inserts in Fig. 1b). At the same time the formerly 
amorphous silica support has crystallised whereby the Pt particles possibly act as nuclei. 
Under the given experimental conditions silicide formation [3] must be taken into account, at 
least to some extent, but it does not explain all features of the diffraction patterns. Moreover, 
very similar results were obtained when Pt nanocrystals supported on alumina were reduced 
under identical conditions. Again, sharp edged platelet and cube-like structures were formed 
while amorphous alumina had crystallised (not shown here). Finally, platelets and cubes were 
also formed upon reduction from Pt particles in contact with crystalline ceria already upon 
hydrogen treatment at 773 K and above (Fig. 1c), whereby the epitaxial relation between the 
Pt particles and the ceria grains was retained. Upon further reduction the support is reduced to 
a chemically stable CeIII oxide with large lattice periodicities (Fig.1 d). 
At present, image simulation and EELS measurements are carried out in order to discriminate 
between alloy phases and superstructures due to surface reconstruction under hydrogen [4].  
 
This work was supported by the Austrian Science Fund (Project S 8105). 
 
References 
[1] G. Rupprechter, K. Hayek, L. Rendón, M. Jose-Yacamán, Thin Solid Films 260 (1995) 148; [2] G. 
Rupprechter, K. Hayek, H. Hofmeister, J. Catal. 173 (1998) 409; [3] R. Lamber, N. Jaeger, G. Schultz-
Ekloff, Surface Sci. 227 (1990) 268; [4] T. Wang, C. Lee, L. D. Schmidt, Surface Sci. 163 (1985) 181. 


Platinum nanocrystals supported by silica, ceria and alumina: metal-support interactions due to high-temperature reduction in hydrogen

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