A spin promotion effect in catalytic ammonia synthesis

Ammonia plays an important role as the basis for nitrogen fertilizer production and is also being considered as a potential energy carrier in a sustainable future. The need for efficient ammonia synthesis is as urgent as ever. Over the past two decades, many attempts to find new catalysts for ammonia synthesis at mild conditions have been reported and, in particular, many new promoters of the catalytic rate have been introduced beyond the traditional K and Cs oxides. We provide an overview of recent experimental results for non-traditional promoters and develop a comprehensive model to explain how they work. The model, which is based on extensive density functional theory calculations, has two components. First, we establish what is the most likely structure of the active sites in the presence of the different promoters. We then show that there are two components dictating the catalytic activity. One is an electrostatic interaction between the adsorbed promoter and the N-N dissociation transition state. In addition, we identify a new promoter effect for magnetic catalysts giving rise to an anomalously large lowering of the activation energy that can make otherwise inactive metals like Co as active as the best Ru and Fe-based catalysts.