Electron microscopy of supported metal particles II. Further studies of Pt/TiO

Abstract

Hydrogen and carbon monoxide chemisorption on Pt on titanium oxide powders [Pt/TO(P)] and transmission electron microscopy of Pt on titanium oxide films [Pt/TO(F)] have been investigated for a common series of thermal treatments in various gaseous atmospheres. Following reduction at 875 K in H2, Pt/TO(P) did not adsorb H2 or CO, and X-ray diffraction examination showed that the Pt had not agglomerated. This behavior is attributed to a strong metal-support interaction (SMSI). The adsorption of H2 and CO could be partially restored if the sample in the SMSI state were treated with H2O vapor at 525 K or O2 at 875 K, the latter treatment being more effective. Transmission electron microscopy studies of Pt/TO(F) treated in the same way as the Pt/TO(P) showed that in the SMSI state the Pt was in the form of hexagonal thin pillbox structures grown on a lower oxide of titania, Ti4O7. When Pt in the SMSI state was oxidized, there was an appreciable increase in the growth rate of the Pt particles, comparable to that found on conventional supports. Examination of the individual Pt particles showed that while their shape remained hexagonal, their thickness increased following oxidation so they were no longer present as thin pillbox structures. Electron diffraction analysis demonstrated that the support had also undergone a structural change, Ti4O7 reverting to TiO2. Reduction of the sample to induce the SMSI state showed that although a few large Pt crystallites remained, there was significant redispersion of the metal. Following re-reduction, all particles had again assumed the pillbox structure of the SMSI state and the support had reverted, once again, to Ti4O7. A schematic model is presented to illustrate the sequence of events during cycling into and out of the SMSI state.