Platinum Metals Rev., 1969, 13, (1), 28
The Solid Oxides of Platinum and Rhodium
Formation in High Pressures of Oxygen
At everyday temperatures and in air at normal pressures the solid oxides of the platinum metals are notably of uncertain composition. According to the method of preparation, samples are frequently mixtures of one or more oxides with free platinum and are often partly hydrated. Such uncertainties stem inevitably from the feeble nature of the binding forces in these materials when so close to dissociation.
In view of the importance of these oxides as catalysts, it is surprising that so little has been done in the past to characterise them more strictly. A recent paper (1) by Olaf Muller and Rustum Roy, working in the Materials Research Laboratory of the Pennsylvania State University describes, for instance, the first work that appears ever to have been reported on the use of high pressures of oxygen to stabilise the compounds in reasonably well-crystallised forms for structural examination.
The original intention was to determine the phase equilibria in the system Pt–O at high oxygen pressures. It was soon obvious, however, that it is an almost impossible task to define the phase limits with any certainty owing to sluggishness of the reactions at the temperatures under study. Apparent shifts of up to 50°C were observed in the phase boundaries, depending upon the starting materials used and upon the direction to which the ‘equilibrium ‘ temperature was approached.
Platinum black and such compounds as Pt(NH3)2(NO2)2 were quite unreactive below about 700°C and at the best produced mixtures of platinum metal with small amounts of mixed oxide phases even after several days under pressure. The most consistent results were obtained by heating small quantities of PtI2 enclosed in platinum (generally 10 mg or less but sometimes up to 0.5 gm), for 1 to 2 days to 400° to 900°C at pressures up to 3500 atm of oxygen. At the end of a run, the “bomb” was cooled in water before releasing the pressure. The samples were examined by X-ray diffraction to determine the structures of the phases produced, but chemical examination was not as thoroughly performed as might have been desired. The authors obviously lacked experience in platinum metal analysis and appear to have been daunted by the insolubility of some of the oxides in aqua regia.
Three platinum oxides were found: α PtO2 (hexagonal), a new phase, β PtO2 (CaCl2 structure) and Pt3O4 (NaxPt3O4 structure). Tetragonal PtO and body-centred cubic Pt3O4 which have previously been reported in the literature were not found.
The evidence is that hexagonal α PtO2 is the oxide stable at all pressures below about 570°C, the temperature at which it decomposes at atmospheric pressure. It is not easily produced in well-crystallised form even at the highest oxygen pressure; and it is possible that many months of reaction time might be needed for the growth of perfect specimens. There is a suggestion that the presence of sodium nitrate as a flux may promote crystallisation.
At higher temperatures the new β PtO2 is formed provided that the oxygen pressure is sufficiently high—above about 600 atmospheres at 700°C and 1000 atmospheres at 800°C. It is black, relatively stable and completely insoluble in mineral acid mixtures and it would be of interest to know more of its properties—particularly in the field of its catalytic activity.
At intermediate pressures, Pt3O4 is the stable oxide. For its preparation in bulk it is suggested that commercial platinum black or fine mesh platinum powder should be heated to 850°C under 200 to 300 atmospheres of oxygen and any unreacted platinum removed by treatment with aqua regia. Its structure appears to be identical with that of Nax Pt3 O4, the implication being that removal of the sodium atoms leaves empty positions in the lattice.
The work on the Rh-O system described by the authors is less extensive but is remarkable as indicating RhO2, a phase not previously described, as the most stable structure. It is easily prepared by heating Rh2O3. 5H2O at 700 to 850°C at high pressures of oxygen. It is shown to be stable at all temperatures up to about 1000°C at oxygen pressures above about 20 atmospheres and it is, moreover, the stable phase at atmospheric temperatures below about 700°C. It is a black compound, highly insoluble even in hot aqua regia, and obviously deserves further study. The two other oxide phases, both forms of Rh2O3, have relatively narrow fields of existence at 650 to 900°C and above 950°C respectively.
The interests of the authors as shown by this paper lie in the interpretation of the X-ray data on their samples. The methods of preparation chosen may, however, be of value in a much wider field. High pressure techniques may well help to resolve many outstanding problems of platinum metal chemistry by stabilising transient reactions which otherwise elude study.
- 1Olaf Muller and Rustum Roy, Formation and Stability of the Platinum and Rhodium Oxides at High Oxygen Pressures and the Structures of Pt 3 O 4, β-PtO 2, and RhO 2 . J. Less-Common Metals, 1968, 16, ( 2 ), 129 .