The synergistic acceleration of the ammonia synthesis over the K-2[Fe-2(CO)(8)]+K catalysts on carbon "Sibunit" by rhodium

The effect of Rh on the ammonia synthesis over the K-2[Fe-2(CO)(8)]K+ and K -2[Ru-4(CO)(13)]+K catalysts on the graphite-like active carbon "Sibunit" h as been investigated. The catalysts were prepared by depositing K-2[Fe-2(CO )(8)] and K-2[Ru-4(CO)(13)]. respectively, onto the "Sibunit" carbon-suppor ted Rh. followed by thermal decomposition of the deposited cluster and trea tment of the resulting sample with metallic potassium. The catalysts contai ning no potassium metal have been studied as well. It has been found that t he presence of Rh in the Ru catalysts either little affects the ammonia syn thesis rate or even somewhat decreases it. By contrast, the introduction of Rh in the K-2[Fe-2(CO)(8)] + K systems leads to a synergistic acceleration of the process of the ammonia synthesis. The strongest accelerating effect s of Rh are observed at lowered temperatures. Thus, at 200 degreesC, the Fe -Rh catalyst (Fe/carbon = 5.6 wt.%, the Fe:Rh molar ratio is 1.6:1) proves to be 3.5 times more active than the corresponding Fe sample. A rise in the reaction temperature to 250, 300 and then to 350 degreesC results in a gra dual weakening of the Fe-Rh synergism. A remarkable feature of the Fe-Rh ca talyst found is its increased activity in the ammonia formation at 150 degr eesC. The analogous Rh catalysts produce no ammonia at all at 150-200 degre esC in the absence of Fe and their efficiency at 250-400 degreesC is either equal to zero or very low. (C) 2001 Elsevier Science B.V. All rights reser ved.