Supported ammonia synthesis catalysts based on potassium derivatives of anionic ruthenium, osmium and iron carbonyl clusters. The first catalysts on carbon support exhibiting activity in ammonia synthesis in the absence of a specially added electron promoter

Abstract

The first catalysts on carbon support exhibiting activity in ammonia synthesis in the absence of a specially added electron promoter have been found. The catalysts were prepared by depositing K 2[Ru 4(CO) 13], K 2[Os 3(CO) 11] and K 2[Fe 2(CO) 8] from THF solutions on the original graphite-like active carbon `Sibunit' followed by drying of the samples obtained at 20°C in vacuum. The highest efficiency is displayed by the K 2[Ru 4(CO) 13] catalyst which is active in ammonia synthesis starting from 250°C (1 atm). On passing from K 2[Ru 4(CO) 13] to K 2[Os 3(CO) 11] and then to K 2[Fe 2(CO) 8] the rate of ammonia synthesis lowers. The replacement of `Sibunit' by usual commercial active carbon SKT results in a sharp decrease in activity and stability of the catalysts. An introduction of metallic potassium into the catalysts on `Sibunit' carbon accelerates strongly the process of ammonia synthesis. A study of the K 2[Ru 4(CO) 13]+K and K 2[Fe 2(CO) 8]+K catalysts has shown that at constant transition metal and carbon contents in the system, the activity of both catalysts passes through a maximum with an increase in potassium quantity, in the case of ruthenium the position of the maximum being reached at considerably less amounts of potassium in the sample than in the case of iron. The K 2[Ru 4(CO) 13]+K catalyst is much less active than the K 2[Fe 2(CO) 8]+K catalyst at 200°C but considerably exceeds it in activity at higher temperatures (250–350°C).