Formation of NaCl- and Cu 2O-type oxides of platinum and palladium on carbon and alumina support films

Abstract

Vacuum deposition of ultrathin films of platinum and palladium on carbon and alumina supports at a substrate temperature of 350–400 °C and a working pressure of 10 −5 Torr is shown to give rise not only to metallic particles but also to the formation of two new cubic oxides, namely, an NaCl-type (space group Fm3m, Z = 4) and a Cu 2O-type oxide (space group Pn3m, Z = 2) with lattice parameters: PtO a = 0.515 ± 0.005 nm PdO a = 0.565 ± 0.005 nm Pt 2O a = 0.395 ± 0.005 nm Pd 2O a = 0.428 ± 0.005 nm Electron microscopic observations reveal that the NaCl-type oxide is in the form of thin, well-spread single crystals, whereas the fine dispersed particles correspond to metal and/or Cu 2O-type oxides. It seems that the partial pressure of oxygen at 10 −5 Torr is enough for finely divided platinum or palladium to form oxides at 350–450 °C. The existence of these new oxides in the bulk state has yet to be established, however. The formation of Pt 2O and Pd 2O suggests a valency of one for platinum and palladium in certain situations. Alternatively, these oxides may exhibit non-stoichiometry.