Osmium, catalyst synthesis activity

Successful ammonia conversion required discovery of a catalyst, which would promote a sufficiently rapid reaction at 100-300 atm and 400-500°C to utilize the moderately favorable equilibrium obtained under these conditions. Without this, higher temperatures would be required to obtain sufficiently rapid rates, and the less favorable equilibrium at higher temperatures would necessitate higher pressures as well, in order to obtain an economic conversion to ammonia. The original synthesis experiments were conducted with an osmium catalyst. Haber later discovered that reduced magnetic iron oxide (Fe304) was much more effective, and that its activity could be further enhanced by the presence of the promoters alumina (AI2O3 3%) and potassium oxide (K2O 1%), probably from the introduction of iron lattice defects. Iron with various proprietary variations still forms the basis of all ammonia catalyst systems today. 

In the original work on catalytic ammonia synthesis, Haber [41] had used an osmium catalyst, but this metal was much too expensive to be the basis of the large-scale industrial plants. In the long search for alternatives to the Mittasch catalyst, alkali-promoted ruthenium was found to exhibit specific activity, which is even superior to the iron catalyst [42] and which was subsequently developed to an industrial catalyst [43]. The Mittasch catalyst is cheap and the alumina promoter provides a high specific surface area. This situation is different with Ru catalysts that are prepared as small particles on a suitable support. Figure 1.1 was a t)q)ical electron microscopic picture from such a catalyst particle on MgAl204 (spinel) support [44]. 

Although iron catalysts were already used in ammonia synthesis at that time, excellent industrial catalysts had not been found yet. Haber energetically screened about 20,000 catalysts. In the March of 1909, he found that osmium exhibits excellent activity in ammonia synthesis. He obtained 8% ammonia concentration on... 

Of the platinum group metals only ruthenium and osmium show significant ammonia synthesis activity, though only in the presence of alkali promoters. This can be seen from the performance of the potassium metal promoted carbon-supported catalysts in Table Although osmium was one of the earliest and... 

In 1905 Haber reported a successful experiment in which he succeeded in producing NH3 catalytically. However, under the conditions he used (1293 K) he only found minor amounts of NH3. He extrapolated his value to lower temperatures (at 1 bar) and concluded that a temperature of 520 K was the maximum temperature for a commercial process. This was the first application of chemical thermodynamics to catalysis, and precise thermodynamic data were not then known. At that time Haber regarded the development of a commercial process for ammonia synthesis as hopeless and he stopped his work. Meanwhile, Nernst had also investigated the ammonia synthesis reaction and concluded that the thermodynamic data Haber used were not correct. He arrived at different values and this led Haber to continue his work at higher pressures. Haber tried many catalysts and found that a particular sample of osmium was the most active one. This osmium was a very fine amorphous powder. He approached BASF and they decided to start a large program in which Bosch also became involved. 

Because it forms stronger bonds with ligands, osmium complexes are generally less active catalysts than analogous ruthenium complexes. The exception is in oxidation chemistry where certain osmium complexes are better and more selective catalysts. Because of this, osmium tetroxide and other osmium 0X0 complexes are widely used in the synthesis of complex organic molecules. 

Ruthenium and Osmium. - Much attention has been paid in the last few years to Ru as a hydrogenation catalyst and N activator in the synthesis of NH3. CO adsorption at the (100) face of Ru monocrystals has been studied by Thomas and Weinberg and Pfnizr et A continuous red... 

Though in moderate yield, the reported conversion of ethanol to ethyl acetate by complex 14 and a related osmium dimer complex [37] by Gusev generated interest in the catalytic synthesis of ethyl acetate from ethanol because it is a widely used fine chemical. Very recently, Beller et al. [38] screened the catalytic activity of various known pincer complexes for this transformation and found thatTakasago s complex 15, known as Ru-MACHO catalyst, is very efficient and the reaction in the presence of a base resulted in very good TON (Table 1.4). 

Apart from the Fe catalyst, osmium and mthenium catalysts have been applied in ammonia synthesis reaction. The mthenium catalyst was found to be more active than the Fe catalyst, so it can perform at milder conditions than the Fe catalyst (Dahl et al., 2001 Logadottir et al., 2001 Rossetti, Pemicone, Ferrero, Fomi, 2006). However, Ru is more expensive than Fe, and the lifetime of the Ru catalyst is shorter than that of the Fe catalyst. 

However, osmium is a very rare resource and osmium oxide volatilize easily. Therefore, it was necessary to develop cheaper catalysts with excellent performances for ammonia synthesis. Since then, Haber identified that uranium is active for ammonia synthesis. However, in 1912, he was appointed as the director of Institute of Physical Chemistry and Electrochemistry in the Kaiser Wilhelm Institute, indicating the end of the research activity of Haber in the field of ammonia synthesis. Since then, Bosch and Mittasch became the principal researchers in BASF to continue the industrialization process for the ammonia synthesis. Bosch was the leader of the whole research group, and Mittasch became the main investigator for the exploration of catalysts. 

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