New Pt,Rh-CeOx Catalytic Systems Prepared from Intermetallic Compounds
P.A. Zosimova * , A.V. Smirnov, S.N. Nesterenko, I.I. Ivanova
Chemistry Department, M.V.Lomonosov Moscow State University, Moscow, Russia
Nano-sized materials have great potential for the development of new catalysts. Noblemetals supported on transition or rare earth oxides showed the promising results in various
catalytic processes. It was established that oxide support influences on dispersed metal component
changing its properties, in particular, resistance to the sulfur poisoning. This effect stronglydepends on the homogeneity of the metal component distribution.
The aim of this study was to verify the effect of ceria support on the sulfur tolerance of Ptand Rh-containing catalysts in the model reaction of toluene hydrogenation. The new approach inthe preparation of mixed Pt (Rh)/cerium oxide catalysts included oxidative treatment of noblemetal/cerium intermetallic precursors. This preparation technique leads to formation ofultradispersed noble metal particles in cerium oxide.
The Pt-Ce and Rh-Ce intermetallic precursors were synthesized by arc-melting ofstoichiometric mixtures of pure components on a water-cooled copper hearth under 5∗104 Pa ofargon. The alloys were crushed and oxidized in the airflow at 550°C. The samples werecharacterized using in situ XRD; SEM/EDS; TEM/EDS; TPD H2 and XPS techniques. Catalytic
activity and sulfur resistance of the samples were tested in toluene hydrogenation reaction in atubular continuous flow quartz reactor under atmospheric pressure at 150°C and weight hourly
space velocity (WHSV) of 0.7h-1. Prior to the catalytic runs, the samples were activated at 450oCin a flow of H2. Methylcyclohexane was the only reaction product. To study the sulfur tolerance ofthe catalysts, the mixture of H2S (970ppm) in N2 was added to the feed.
The XRD analysis showed that after oxidation of Pt-Ce intermetallic compound the sample
consisted of metalic Pt and CeO2 phases, while in the case of Rh-Ce sample metallic Rh, Rh2O3and CeO2 were detected. Activation at 450 °C in H2 flow led to formation of Pt/CeOx and Rh/CeOx
materials. The particle sizes of noble metals in these systems calculated by Scherrer equation were
13 and 18 nm, respectively. The broadening of the CeO2 peaks in the XRD patterns indicated anaverage ceria particle size of 7-10 nm.
The initial intermetallic compounds were inactive in toluene hydrogenation. The oxidized
Pt-Ce samples showed the catalytic activity comparable with the activity of pure platinum, while
the activity of Rh-Ce system was significantly higher. The later sample demonstrated also thehighest sulfur tolerance. The deactivation curves of oxidized samples consisted of two regions withdifferent slopes indicating the existence of two types of active sites with different tolerance to H2S.
This conclusion was confirmed by TEM experiments over the oxidized Pt-Ce sample, which
revealed the presence of ceria rich and Pt rich areas along with the areas, where Pt and ceria may
form combined structures with very fine distribution of Pt and ceria. The SEM/EDS analysis of thesample, partially deactivated by H2S, showed that sulfur was concentrated preferably on Pt richareas. These results suggested that rapid catalyst deactivation was due to the poisoning of puremetallic sites, while mixed Pt-Ce or Rh-Ce sites were more stable to deactivation.

*P.A. Zosimova, Phone: +7-495-939-2054, E-mail: PolinaZosimova@phys.chem.msu.ru