The ammonia decomposition reaction has recently received increased attention due to the possibility of ammonia being used as a hydrogen storage medium in a possible hydrogen economy. We have explored this decomposition reaction through multiscale microkinetic modeling for a number of transition metal catalysts, including Pt, Pd, Ir, Ni, Rh, Co, Ru, Re, and Mo, to better understand the reaction mechanism. An understanding of the reaction mechanism and electronic properties of these metals has given insight into how to tailor catalysts to improve catalytic activity for this reaction.
The mechanism consists of 12 elementary reaction steps and 5 surface species, namely N, H, NH, NH2, and NH3. For many of the metals, a large portion of the surface is covered by adsorbates. For these metals, repulsive adsorbate-adsorbate interactions were expected to change the binding energies of the surface species, thereby changing the elementary reaction activation barriers and modifying the catalytic activity . Coverage dependant atomic heats of chemisorption were calculated through DFT using the Vienna Ab-initio Simulation Package (VASP) for the various transition metal catalysts. Coverage dependant molecular binding energies were calculated and activation barriers were calculated through the bond-order conservation (BOC) method .
Inclusion of the interaction parameters to the models resulted in reduced nitrogen coverages and a peak shift in the volcano curve. The conversions were plotted against the characteristic nitrogen heat of chemisorption for each metal, which was found to be an adequate descriptor for this reaction. The volcano curve of the conversions calculated through the microkinetic models are in good agreement with experimental data of single metal catalysts by Ganley and coworkers . The maximum activity was found at a nitrogen heat of chemisorption of approximately 134 kcal/mol.
A DFT study of nitrogen binding energies on Pt-3d bimetallic surfaces showed several promising bimetallic catalysts. Surface science experiments were performed to assess the microkinetic model and DFT results, using the Ni-Pt-Pt surface, which had a calculated binding energy near the predicted optimum, as a test case. The Ni-Pt-Pt surface was found to be more active at decomposing ammonia at low temperatures and desorbed nitrogen at lower temperatures than a Ru(0001) surface, currently the most active single metal catalyst . Additional experimental results of other promising Pt based bimetallics will also be presented.
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