Volatile platinum oxides

During atmospheric pressure operation, once the platinum has become active by this crystallization process the gauze gradually deactivates as platinum is lost as a volatile platinum oxide. Loss per tonne of nitric acid produced is proportional to the gauze temperature and increases from 50-100 mg of platinum, at atmospheric pressure and 800°C, to about 400 mg of platinum, at 8-atm pressure and 900°C. Gauzes are usually changed when about 5% of the metal has been lost and the rhodium content has increased to more than 12%. 

Alloys with iridium Iridium alloys with platinum in all proportions, and alloys containing up to about 40% iridium are workable, although considerably harder than pure platinum. The creep resistance of iridium-platinum alloys is better than that of rhodium-platinum alloys at temperatures below 500°C. Their stability at high temperatures, however, is substantially lower, owing to the higher rate of formation of a volatile iridium oxide. 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

Platinum loadings, reducing, 19 628 Platinum metals plating, 9 822-823 Platinum oxides, volatilized, 17.T80 Platinum-palladium thermocouple, 24 461 Platinum reforming catalysts, rhenium and, 21 695-696... 

The oxidation behavior of platinum metals is of importance for their applications at high temperatures. These metals form solid oxides and also volatilize as oxides at higher temperature in an oxidizing atmosphere. Most of the gaseous oxides are stable only at high temperatures and usually contain the metal in its highest oxidation states. Palladium is an exception since it dissolves oxygen in the solid state and only forms the solid oxide PdO which dissociates at temperatures above 800 °C. 

Schmidt et al. (139) postulated that in the presence of excess oxygen, platinum was transported as volatile oxides through the gas phase and boundary layer. This mechanism could not adequately explain the reconstruction observed far into the excess ammonia regime. It was suggested that under these and other conditions, other volatile platinum species formed. Moreover, these species might decompose by reaction in the boundary layer, leading eventually to the platinum replating itself. 

Other workers (165) used X-ray photoelectron spectroscopy (XPS) to examine the influence of ammonia oxidation on the surface composition of alloy gauzes. After several months on stream, the surface was covered by the same types of highly faceted structures noted by others. As illustrated in Fig. 14, XPS analysis provides evidence that the top microns, and in particular the top 100 A of the surface, were enriched in rhodium. This enrichment was attributed to the preferential volatilization of platinum oxide. The rhodium in the surface layers was present in the oxide form. Other probes confirm the enrichment of the surface in rhodium after ammonia oxidation (166). Rhodium enrichment has been noted by others (164, 167), and it has been postulated that in some cases it leads to catalyst deactivation (168). 

Anderson also accepted the model of etching by platinum oxide formation and volatilization, yet the results clearly demonstrate that the surface area increase and etching rate correlate with local reaction rate and not oxygen concentration. No control experiments (e.g., with the sample in pure oxygen) were conducted to test the model. 

In sum, according to one postulated mechanism of catalytic etching, particularly of platinum, etching occurs via the transport of volatile metal oxide species. It has always been understood, even by the model s proponents, that it does not fully explain the observations. It has been repeatedly suggested that other volatile species may exist, or that other unaccounted-for processes are responsible for the observed behavior. Indeed, as discussed below, several recent studies suggest that other volatile species and other unaccounted-for processes may very well explain some or all catalytic etching. 

High Temperature Properties. There are marked differences in the abihty of PGMs to resist high temperature oxidation. Many technological appHcations, particularly in the form of platinum-based alloys, arise from the resistance of platinum, rhodium, and iridium to oxidation at high temperatures. Osmium and mthenium are not used in oxidation-resistant appHcations owing to the formation of volatile oxides. High temperature oxidation behavior is summarized in Table 4. 

The more important cases of oxide volatilisation occur in the platinum metals " and with the refractory metals at high temperatures. In these systems, unlike the aforementioned, it is the higher valence oxide which is the more volatile so that at sufficiently high temperature the metal may be oxide free. Gulbransen has shown that the rate of oxidation is then con-... 

In the solvent-extraction process, the platinum metal concentrate is solubilized in acid using chlorine oxidant. Ruthenium and osmium are separated by turning them into the volatile tetroxides. 

Iwasita T, Vielstich W. 1986. On-line mass spectroscopy of volatile products during methanol oxidation at platinum in acid solutions. J Electroanal Chem 201 403-408. 

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