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 are exploring this decomposition reaction through multiscale microkinetic modeling for a number of transition metal catalysts including Cu, Pt, Ir, Ru, Pd, Rh, Co, Ni, Fe, W, and Mo to better understand the reaction mechanism. An understanding of the reaction mechanism and electronic properties of metals will give insight into how to tailor the catalysts to improve catalytic activity for this reaction.

The chemical reaction mechanism consists of 12 elementary reaction steps and 5 surface species, namely N, H, NH, NH2, and NH3. For many of the metals, with a large portion of the surface being covered by adsorbates, repulsive adsorbate-adsorbate interactions change the binding energies of the surface species, thereby changing the elementary reaction activation barriers and modifying the catalyst activity [1]. Coverage dependant atomic heats of chemisorption were calculated through density functional theory using the Vienna Ab-initio Simulation Package (VASP) for the various transition metal catalysts. Molecular binding energies and activation barriers were calculated through bond-order conservation (BOC) [2] using these coverage dependant binding energies. The activation barrier coverage dependencies resulting from this computationally inexpensive method were verified by nudge elastic band (NEB) calculations. Inclusion of the interaction parameters to the models resulted in reduced nitrogen coverages and a peak shift in the volcano curve. The conversions, sensitivity coefficients, and the most abundant reaction intermediate were plotted against the characteristic nitrogen heats of chemisorption for each metal, which has been found to be an adequate descriptor for this reaction.

VASP calculations and microkinetic models predict the surface monolayer Ni/Pt bimetallic system to be a potentially active ammonia decomposition catalyst. To test the accuracy of these predictions and how the binding energy of the nitrogen atom is modified by the bimetallic Ni/Pt system, temperature programmed desorption (TPD) experiments were carried out on bimetallic surfaces prepared on a Pt(111) substrate [3]. Quantitative comparison of experimental data and simulations will also be shown.

[1] A. B. Mhadeshwar, J. R. Kitchin, M. A. Barteau and D. G. Vlachos, Catalysis Letters 2004, 96, 13-22.

[2] E. Shustorovich and H. Sellers, Surface Science Reports 1998, 31, 5-119.

[3] J. G. Chen, C. A. Menning and M. B. Zellner, Surface Science Reports 2008, 63, 201-254.