Paper HC+SS-TuM5
Synergistic Effects of Pd and PdO Domains on Thin Film TbOx(111)/Pt(111)

Tuesday, October 23, 2018, 9:20 am, Room 201A

Among the rare earth oxides (REOs), the terbium oxides exhibit favorable properties in selective oxidation catalysis due to the flexibility in the storage and release of oxygen within the lattice. Of particular note is the ease of structural rearrangement into well-ordered intermediates between the Tb₂O₃ and TbO₂ stoichiometries, providing a novel, dynamic surface interface for the promotion of oxidation reactions. We investigated the stabilization and reactivity of metallic Pd domains grown on top of ultrathin c-Tb₂O₃(111)/Pt(111) films in ultrahigh vacuum (UHV) and subsequently oxidized by plasma-generated gaseous atomic oxygen. XPS shows that while both the film and metallic domains are almost fully oxidized to both TbO_2­ and PdO by atomic oxygen, subsequent annealing to ~600 K can significantly reduce the TbO_x supporting film while leaving the PdO largely unreduced. Our results provide evidence that the presence of Pd structures on the TbO_x surface greatly promotes the thermal reduction of TbO₂. Further annealing of the system at ~900 K results in PdO decomposition and agglomeration of metallic Pd domains as evidenced by the diminution of the XPS Pd 3d peaks.

The oxidation/reduction behavior of the Pd/TbO_x(111) system has enabled the study of oxidative reactions on three characteristic interfaces: PdO on TbO₂(111), PdO on Tb_nO_2n-m(111), and Pd on c-Tb₂O₃(111). TPD and TPRS experiments show that adsorbed CO and C₃H₈ only react with the stabilized PdO domains, with C₃H₈ desorption at ~200 K being characteristic of adsorbed propane σ-complexes observed previously on PdO(101) surfaces. Continual reduction of PdO with adsorbed CO and C₃H₈ also show that when thermal reduction is limited to ~600 K, the underlying TbO_x support will continually replenish the reduced PdO domains with oxygen. This is also noted by the substantially higher conversion of adsorbed CO to CO₂ compared with that seen on pure PdO(101) as conversion would be less limited if reactive oxygen is supplied from both PdO and the TbO_x support. This behavior suggests a strong synergy between the surface Pd/PdO domains and the underlying TbO_x film, such as a Mars-van Krevelen interaction in which TbO_x readily transfers O-atoms to Pd and thereby sustains oxidation chemistry.