Can Transition Metal Elements Boost Ammonia Synthesis in Iron Catalysts?

In recent years, substantial research has been conducted on the promotional effects of transition metal elements from the iron and adjacent iron groups. Notably, Cobalt (Co) exhibits low intrinsic ammonia synthesis activity, yet serves as a potent promoter significantly elevating the high-temperature activity of conventional dual-promoted molten iron catalysts. Kalenczuk's studies demonstrate that the addition of cobalt to Fe/AlO enhances its ammonia synthesis activity. Employing scanning electron microscopy, X-ray diffraction, and Mössbauer spectroscopy, Kalenczuk investigates the impact of cobalt co-catalyst on the surface interactions of nitrogen, hydrogen, and ammonia in cobalt-modified iron catalysts.

At 10 MPa and temperatures ranging from 350 to 450°C, research on traditional molten iron catalysts and a series of cobalt-containing variants indicates a notable enhancement in ammonia synthesis catalytic activity with the addition of cobalt. The peak activity is observed at a cobalt content of 5.5% by weight, attributed to cobalt's role in promoting nitrogen chemical adsorption and ammonia desorption, thereby augmenting catalytic activity.

Kalenczuk's findings further reveal that the addition of cobalt to Fe/MHC (where MHC denotes alkaline carbonate magnesium) increases its activity. Wei Keming and team have also developed cobalt-containing catalysts for ammonia synthesis, such as A201 and A202. A201, a low-temperature, highly active catalyst with dual iron-cobalt functionality, exhibits excellent thermal stability and resistance to poisoning. A202, under similar conditions, outperforms A201 in terms of ammonia synthesis activity, heat resistance, and resistance to poisoning.

Moreover, the shape of cobalt-containing catalysts influences their activity. Wang et al.'s research indicates that the effect of cobalt differs in irregular and spherical catalysts. In irregular catalysts, activity and surface area increase with cobalt content but decrease with the iron atom ratio. Conversely, in spherical catalysts, despite a reduction in surface area, there is an increase in specific activity, resulting in an overall activity improvement.

Research on alloy catalysts highlights manganese/iron alloy's high ammonia synthesis activity, while nickel addition diminishes iron's synthetic activity, and molybdenum has a critical maximum content. Reports suggest that the activity of iron-tungsten alloys surpasses that of pure iron catalysts, increasing with tungsten content. From these studies, it is evident that the addition of transition metal elements from the iron or adjacent iron groups is likely advantageous for enhancing the ammonia synthesis activity of iron catalysts.