Electron microscopy investigations of metal-support interaction effects in M/Y2O3 and M/ZrO2 thin films (M=Cu, Ni)

Abstract

Model systems of the clean and pure oxides Y2O3 and ZrO2, as well as Cu/Cu2O and Ni/NiO particles embedded in the respective oxides have been used to study the reduction behavior of the oxides and the eventually associated metal-support interaction effects in oxide-supported systems. Particular emphasis has also been given to the influence of the phase transformation in ZrO2-containing systems on metal-support interaction. Whereas Y2O3 has been found to be an outstandingly structurally and thermally stable oxide even upon reduction in hydrogen up to 1073 K, ZrO2 was found to undergo a series of phase transformations from amorphous ZrO2 to polycrystalline tetragonal ZrO2 (∼673 K) and subsequently to monoclinic ZrO2 (above 873 K). Both phase transformations were found to be basically dependent on gas partial pressure and annealing rate. However, substantial reduction of the oxides did not take place during the phase transformations. In turn, both Cu- and Ni-containing systems were not observed to be substantially affected by any (strong) metal-support interaction effects such as encapsulation by sub-stoichiometric oxides or reductive formation of intermetallic phases, at least up to temperatures of 1073 K. Equally, for the ZrO2-containing systems, also the phase transformations occurring at elevated temperatures did not cause structural or thermo-chemical alterations of the Cu or Ni-particles. Differences in the metal-support interaction between Cu- and Ni-particles have only been obtained in the structural “reference” systems, that is, if supported on SiO2. Whereas Cu/Cu2O particles on SiO2 are basically unaffected by the reductive treatment at elevated temperatures, a Ni3Si2 intermetallic phase is formed if SiO2-supported Ni/NiO particles are treated in hydrogen at 673 K and above.