EPR characterization of Ti3+ ions at the metal-support interface in Pt/TiO2 catalysts
1988; Elsevier BV; Volume: 113; Issue: 1 Linguagem: Inglês
10.1016/0021-9517(88)90240-0
ISSN1090-2694
AutoresLaurent Bonneviot, Gary L. Haller,
Tópico(s)Catalysis and Oxidation Reactions
ResumoTitania-supported Pt catalysts prepared by cation exchange 0.44 and 1.0 wt% loading and reduced at low, medium, and high temperatures (513, 573, and 773 K, respectively) were studied by EPR. The spectra of four different Ti3+ ions have been resolved at 17 K; only two of these species are surface Ti3+ which are sensitive to CO or H2 adsorption. One of these surface species, characterized by an axial g tensor at g⊥ = 1.969 and g∥ = 1.936, is specific of the presence of platinum since it does not exist in reduced TiO2. Its signal undergoes a temperature-dependent broadening effect which is mostly removed at 17 K; this is related to its proximity to a platinum particle allowing an efficient relaxation mechanism through the conduction electrons of the metal. This ion can be titrated reversibly by hydrogen and irreversibly by carbon monoxide. Its intensity relative to other Ti3+ ions increases when the reduction temperature increases from 373 to 573 K. This trend is probably maintained for the reduction at 773 K, but no accurate conclusions can be drawn since the EPR signal undergoes a strong decrease in intensity and line broadening due to spin pairing and spin-spin relaxation mechanisms. These two effects are related to the increase of Ti3+ concentration in the vicinity of platinum, the metal promoting the reduction of the support. This species, located at the metal-support interface, may either account for the TiOx fragments of the support which migrate on the metal particles in the SMSI state or be a precursor of these fragments.
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