Surface and bulk characterization of Rh/ZrO 2 prepared by absorption of Rh 4 (CO) 12 clusters on ZrO 2 powder
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Surface and bulk characterization of RhyyZrO2 prepared by absorption of Rh4 (CO)12 clusters on ZrO2 powder L. E. Depero INFM and Department of Materials Chemistry and Physics, University of Brescia, Brescia, Italy
R. Bertoncello Department of Inorganic, Metallorganic and Analytical Chemistry, University of Padua, Padua, Italy
T. Boni Department of Materials Science, CISE S.p.A., Milano, Italy
P. Levrangi, I. Natali Sora, and E. Tempesti INFM and Department of Materials Chemistry and Physics, University of Brescia, Brescia, Italy
F. Parmigiania) Department of Physics, Polytechnic of Milan, P.zza Leonardo da Vinci 32, 20133 Milan, Italy (Received 3 June 1995; accepted 24 April 1996)
Rh nanoparticles supported on ZrO2 powder were prepared by adsorbing Rh4 (CO)12 clusters from hexane solution under Ar atmosphere. Four samples with Rh content ranging from 0.25 Rh wt. % up to 4.10 Rh wt. % were studied by x-ray diffraction (XRD), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), scanning Auger microscopy (SAM), and time of flight secondary ion mass spectroscopy (ToF-SIMS). TEM measurements show, for all the samples, Rh particles ˚ The 4.10 Rh wt. % sample also shows some agglomerates with a size of about 50 A. of Rh nanoparticles and only for this sample Rh metallic phase was detected by XRD. The profile analysis of the XRD lines indicates an average size of the Rh crystallites ˚ XPS studies show only a single spectral component for the Zr3d 5/2 of about 60 A. core line at 182.2 eV. Instead, at least, two components at 307.2 eV and 308.5 eV are detected for the Rh3d 5/2 core line. These results suggest that Zr is present only as oxidized state, whereas nonoxidized and oxidized Rh are both observed. A nonoxidized Rh state is also suggested by the XPS valence band electron removal spectrum which exhibits a significant emission within the ZrO2 band gap assigned to Rh 4d bands. A further support to this finding arises from scanning Auger maps of OKVV and Rh M45 N23 V where nonoxidized Rh is observed. Finally scanning Auger maps using the OKVV and the Zr M45 N23 V emissions show surface regions where only the Zr Auger lines are detected, whereas on pure ZrO2 powder this effect is not observed. Since it is possible to rule out from the XPS Zr3d core line spectra, the presence of metallic zirconium hydrogen spillover mechanisms is invoked to explain this result.
I. INTRODUCTION
Nanoscale metal particles exhibit intermediate properties between the single atom and the solid state. In nanosized metal clusters only a small number of atoms have the bulk coordination, while most of them are less coordinatively saturated. The lost translational symmetry as well as the coordinative unsaturation of surface atoms are at the origin of some peculiar behavior.1,2 Since the electronic properties of these systems depend on the relative number of coordinatively unsaturated and nonbulklike coordinated atoms, the chemical and physical properties of metal clusters a
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