(532eh) Recent Progress on Optimizing Haber-Bosch Ruthenium Catalysts through Multiscale Modelling

Ammonia is one of the most important compounds used in chemical industry and a precursor in agriculture for fertilizers. More than a century after the discovery of the Haber-Bosch process, it is still produced from and at high temperatures (500 °C) and pressures (around 200 bar). Targeted intelligent design and optimisation of catalysts has been the focus of research in recent years. This is helped by multiscale modelling, which includes quantum level calculations (density functional theory - DFT), microkinetic modeling and reactor modeling (computational fluid dynamics - CFD). It has proven capable of aiding in the process design and optimization.

The performance of catalysts is not dependent solely on the chemical composition but on the (nano)particle size, faceting, doping and alloy, while the entire process is governed also by reaction conditions and reactor design. We focused on Ru catalysts for ammonia synthesis. We used DFT calculations to obtain the thermodynamics, energetics and kinetics at different facets and to correlate them with structure. Using microkinetic modeling and kinetic Monte Carlo at the mesoscale, we observed these effects on the kinetics and the reaction conditions effect. Using the established correlations, the performance of differently sized, shaped and truncated nanoparticles was described experimentally and tested theoretically. Lastly, the system was described using CFD to account for realistic reactor conditions (flows, pressure fields, conversions).

It was found that by controlling nanoparticle size and shape, the reaction can be greatly accelerated or, conversely, run at milder conditions. Nanocubes are superior in performance and their optimum size for greatest activity was found.