Platinum vapor pressure, high temperature

The catalyst should have a very low vapor pressure at the operating temperature in order to avoid loss of catalytic material. The vapor pressure of palladium at 1300°C is about an order of magnitude lower than that of platinum. Computer modeling [24] shows that at 1300 °C 50% of platinum will be lost over 30 min and 50% of palladium over 30 h. Clearly, neither of these metals would be usable at this temperature. Loss of oxygen is a problem with oxides such as C03O4 at high temperatures and can also be a problem with mixed oxides such as the perovskites, due to the presence of simple oxides as impurities. 

Phosphoric acid concentrated to 100% is used as an electrolyte in this cell that operates between 150°C and 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor and carbon monoxide poisoning of the platinum electrocatalyst in the anode becomes severe. The relative stability of concentrated phosphoric acid is high compared to other common acids. Consequently, the PAFC is capable of operating at the high end of the acid temperature range (100°C-220°C). In addition, the use of concentrated acid (100%) minimizes the water vapor pressure, so water management in the cell is not difficult. The matrix universally used to retain the acid is silicon carbide and the electrocatalyst in both the anode and the cathode is platinum. 

For metals a sublimation temperature is chosen that corresponds to a vapor pressure of 10 Torr. This vapor pressure is not sufficient for all cases. Platinum and boron, e.g., have a vapor pressure of 10" Torr at 2100°C, yet platinum evaporates four times faster than boron at this temperature. The same difference in evaporation rate between metals and nonmetals is observed for osmium and carbon. Both have a vapor pressure of 10" Torr at 2650°C but their evaporation rates are as different as in the case of platinum and boron. Apparently the activation energy for evaporation of nonmetals is higher than for metals. Ease of evaporation or a high vapor pressure does not guarantee fast deposition rates even for metals. Although magnesium and zinc are volatile, they are difficult to deposit because they do not condense easily as their closed outer electron shell confers helium-like properties on their gaseous atoms. 

CVD is typically used to form thin films but it can also be used to make nanoparticles. Pt-Ru particles 2 nm in diameter have been formed by vapor deposition using commercially available single metal preeursors, namely platinum(II)-acetylacetonate and ruflienitnn(lll)-aeetylaeetonate [48]. The precursors were adsorbed onto carbon black by sublimation and subsequently decomposed at 320 °C in H2 or N2. While particle size was virtually independent of sublimation temperature, the Pt Ru ratio decreased as sublimation temperature increased from 170 to 240 °C. A maximal Pt Ru ratio, equaling the Pt Ru ratio of the precursor salts, was reached at 220 °C. The composition is affected by the sublimation temperature, as the vapor pressures of the two precursors are influenced by the sublimation temperature. This example illustrates how the vapor pressure of the precursors can act as a limitation upon CVD alloy formation [21]. CVD is also limited by processing temperature and mass-transfer kinetics [21]. One final disadvantage of CVD to note is that precursors can be highly toxic and therefore difficult to work with [49]. 

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