(112a) Ruthenium on Praseodymium Oxide Catalysts for Ammonia Synthesis

Due to the enormous emissions and energy usage, traditional ammonia synthesis is a strong candidate for new, greener approaches. Further, with the desire for ammonia to be used as a hydrogen carrier, this push is even further exacerbated. Ruthenium as an active metal is one solution for these problems, as it is much more active than iron at more modest temperatures and pressures. Unfortunately, due to the high weight-loadings of previous ruthenium-based catalysts and the resultant cost, these have not been industrially introduced on a large scale. However, with new research into lanthanide-based supports and the desire to run these reactors intermittently on renewable energy, these catalysts are returning as options.

Here, we have explored close to forty promoters for their effect on a starting Ru/PrO_x catalyst, finding that barium and cesium were best for maximizing activity. Following this, optimization of several pretreatment conditions and promoter weight loadings were undertaken, resulting in a catalyst that produced more than 65 mmol/g_cat/hr at 400°C and 30 bar, which is the most active catalyst compared to the literature on a per-ruthenium basis. Rigorous kinetic evaluation and characterization were undertaken, allowing us to determine that the role of barium (as barium oxide) was to act as a hydrogen scavenger and donator, reducing hydrogen poisoning. Further, it was determined that the unpromoted catalyst was a nanocluster/single atom catalyst in nature and used the associative reaction mechanism instead of the traditional dissociative pathway. Finally, using in situ DRIFTS, it was shown that the first step in the associative mechanism is the hydrogenation of the nitrogen molecule rather than the cleavage of one of the triple bonds as originally thought.