Platinum Metals Rev., 1978, 22, (2), 56
The nature of a platinum surface exposed to the air we breathe is, as Oscar Wilde said of truth, rarely pure and never simple. The metal will almost certainly be covered with a more or less continuous, adherent, and protective film of an oxide or mixture of oxides of platinum. Depending on the previous history of the surface—whether it has been heated since it was formed, the temperatures to which it was raised, the rates of heating and cooling, and the chemical composition of the atmosphere—the film may consist of PtO2, Pt3O4, possibly PtO, or a mixture of these with inclusions of fine platinum particles. All this is to say nothing of possible contamination by sulphur compounds and O-H complexes.
The constitution of the surface is of more than academic interest for it largely determines the nature and extent of its catalytic activity at low temperatures up to, perhaps, a few hundreds of degrees centigrade. The erratic behaviour of early platinum catalysts for igniting domestic gas fires and burners was undoubtedly due to the variable composition of the platinum surface films.
It is thus of interest that the existence of PtO as a constituent of these films—for some time a matter of controversy—has recently been established and its electron diffraction and X-ray diffraction spectra recorded. The work, carried out by M. and A. Hecq at the State University at Mons in Belgium, is reported in J. Less-Common Metals, 1977, 56, (1), 133–136.
Films of PtO, stable at room temperature, were formed by reactive sputtering by an electrical discharge in a mixture of 10 per cent oxygen and 90 per cent argon at a pressure of 10−1 to 10−2 torr, using a platinum target. The technique was devised by the authors for studying oxide films on tin, and it is believed that the oxide is formed in the plasma as neutral particles and that these are then collected on the substrate.
In these experiments, a current density of 0.3 mA/cm2 was used at 2000V and the PtO deposited on the substrate at 200°C. Films 400Å thick were formed at a rate of 35 Å/min. The films decompose on heating to 400°C in argon.
As a result of this work, it should be possible more certainly than heretofore to establish quantitatively the presence of PtO whenever it may occur on the surfaces of platinum catalysts.