The synergistic acceleration of the ammonia synthesis over the K2[Fe2(CO)8] + K catalysts on carbon “Sibunit” by rhodium Original Research Article

The effect of Rh on the ammonia synthesis over the K2[Fe2(CO)8]+K and K2[Ru4(CO)13]+K catalysts on the graphite-like active carbon “Sibunit” has been investigated. The catalysts were prepared by depositing K2[Fe2(CO)8] and K2[Ru4(CO)13], respectively, onto the “Sibunit” carbon-supported Rh, followed by thermal decomposition of the deposited cluster and treatment of the resulting sample with metallic potassium. The catalysts containing no potassium metal have been studied as well. It has been found that the presence of Rh in the Ru catalysts either little affects the ammonia synthesis rate or even somewhat decreases it. By contrast, the introduction of Rh in the K2[Fe2(CO)8]+K systems leads to a synergistic acceleration of the process of the ammonia synthesis. The strongest accelerating effects of Rh are observed at lowered temperatures. Thus, at 200°C, 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°C results in a gradual weakening of the Fe-Rh synergism. A remarkable feature of the Fe-Rh catalyst found is its increased activity in the ammonia formation at 150°C. The analogous Rh catalysts produce no ammonia at all at 150–200°C in the absence of Fe and their efficiency at 250–400°C is either equal to zero or very low.