Platinum catalysts potassium effect

We are rather familiar with the effects that small amounts of additives can have on surfaces. For example, one part per million of carbon monoxide in a gas stream can poison a platinum catalyst surface and inhibit oxidation reactions. Similarly, five parts per million of potassium ferricyanide can stop the crystallization of sodium chloride from brine solution, The example given in the previous section is another illustration of such surface effects a few parts per million of sodium polyacrylate can produce a repulsion between oxide surfaces, thus inhibiting adhesion. 

With the addition of Sn as a second metal the composition was closer to that predicted for pure Cg cyclization. Rhenium had the opposite effect (107). Suppression of the acidity of the alumina support (of platinum or palladium) by incorporation of sodium nitrate (126) decreases first the C5 - Cg ring enlargement activity of the catalyst (124, 127). Potassium ions (94-94b) or iec-butylamine (70) have a similar effect. 

The importance of catalysts in chemical reactions cannot be overestimated. In the destruction of ozone previously mentioned, chlorine serves as a catalyst. Because of its detrimental effect to the environment, CFCs and other chlorine compounds have been banned internationally. Nearly every industrial chemical process is associated with numerous catalysts. These catalysts make the reactions commercially feasible, and chemists are continually searching for new catalysts. Some examples of important catalysts include iron, potassium oxide, and aluminum oxide in the Haber process to manufacture ammonia platinum and rhodium in the Ostwald synthesis of nitric... 

In aqueous acetic acid, the disproportionation of the platinum still occurs quite rapidly, and it can be suppressed further by adding mineral acid. Hydrochloric acid is often used, but this has a disadvantage in that the exchange rate is inversely proportional to the chloride ion concentration. Perchloric acid has been found to be more satisfactory (55). The platinum(II) catalyst most used is sodium or potassium tetrachloropla-tinate(II). An aromatic compound added to the reaction mixture also inhibits disproportionation of the platinum(II) complex—benzene, pyrene, and other aromatics have been used. A comparative study of the effect of various aromatics on the H—D exchange in alkanes has been carried out (55). Even under optimum conditions, the disproportionation [Eq. (4)] still takes place, and the catalytic platinum(II) is slowly removed from the reaction mixture. To get useful rates of exchange in alkanes, temperatures of 100° to 120°C have to be used, and the disproportionation rate increases with temperature. 

From these results, it is observed that the way selected to add potassium is very important with respect to the resultant catalyst stability. When potassium is added on the monometallic Pt/y-A Os system previous to the reaction with SnBit), the reaction between platinum and SnBu4 results, apparently, non affected, however, the stability of these catalysts is relatively low, lower than the one of the systems in which potassium is added on the bimetallic catalyst at the final preparation stage. Consequently, it is more effective to prepare well defined bimetallic phases, and afterwards neutralizing with potassium the superficial acid sites. 

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