Metal–Support Interaction in Pt/VOx and Pd/VOx Systems: A Comparative (HR)TEM Study

Abstract

The manifestations of strong and reactive metal–support interaction after reduction at elevated temperatures in Pt/V2O3 and Pd/V2O3 systems were studied using a dedicated thin film model system. Pt and Pd particles were prepared by electron-beam evaporation on NaCl(001) growth templates and subsequently embedded in a crystalline V2O3 matrix, prepared by thermal evaporation of V metal in 10−4 mbar oxygen pressure. Template temperatures of 600 K were used to induce the formation of epitaxially-ordered metal–oxide systems. Engineering of the metal–support interface by distinct annealing treatments allows steering the extent and quality of metal–support interaction. Whereas for Pt/V2O3 catalysts, high-temperature reduction at 773 K in hydrogen causes the epitaxial formation of a well-ordered body-centered tetragonal Pt3V intermetallic phase, the Pd/V2O3 system is mostly unaffected by similar treatments and remains in a metal–oxide state. Nevertheless, oxidation at 773 K of both catalysts prior to the hydrogen treatments lifts the epitaxial relation between metal and oxide and in turn, subsequent reduction at high-temperatures (T ≥ 773 K) yields only polycrystalline Pt3V and Pd3V intermetallic phases without particular ordering with respect to the former growth substrate. Along with this formation of intermetallic phases goes a transformation of the support stoichiometry from V2O3 to VO. Catalyst regeneration by partial oxidative decomposition of the intermetallic state is only possible at high-temperatures (T ≥ 750 K), yielding mostly metal particles and vanadium oxides with oxygen contents higher than V:O = 1:2, in particular V3O7. Adjusting the extent of metal–support contact area by annealing treatments allows for easy steering the structure and morphology of well-defined intermetallic compounds in Pt–VOx and Pd–VOx systems. Well-defined Pt3V intermetallic compounds are formed by direct reduction, whereby lifting the ordering by pre-oxidation yields less-defined compounds with altered metal–support contact area and consequently, strong metal–support interaction.