Elementary Steps In Catalytic Ammonia Synthesis And Decomposition On Fe111

Author : Danielle Ann Hansgen
Publisher :
ISBN 13 : 9781124882987
Total Pages : pages
Book Rating : 4.8/5 (829 download)

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Download or read book Rational Catalyst Design for the Ammonia Decomposition Reaction written by Danielle Ann Hansgen and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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. In this thesis, the ammonia decomposition reaction was explored 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, NH 2, and NH 3 . For many of the metals, a large portion of the surface is covered by adsorbates. For these metals, repulsive adsorbate-adsorbate interactions were found to change the binding energies of the surface species, thereby changing the elementary reaction activation barriers and modifying the catalytic activity. Coverage dependent atomic heats of chemisorption were calculated through density functional theory (DFT) using the Vienna ab-initio Simulation Package (VASP) for the various transition metal catalysts. Coverage dependent molecular binding energies and activation barriers were calculated through the bond-order conservation (BOC) method. The catalytic conversions and turnover frequencies were correlated with the characteristic low coverage atomic nitrogen binding energy on each metal, which was found to be an adequate descriptor for this reaction. The volcano curve of the turnover frequencies 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 binding energy of approximately 134 kcal/mol. A DFT study of nitrogen binding energies on Pt-3d bimetallic surfaces showed a number of potentially active surfaces, including the Ni-Pt-Pt(111), Co-Pt-Pt(111), and Fe-Pt-Pt(111) surfaces while the corresponding subsurface configurations, Pt-Ni-Pt(111), Pt-Co-Pt(111), and Pt-Fe-Pt(111), were predicted to be inactive. The parent metals, Ni, Pt, Fe and Co were predicted to have varying activities. This set of surfaces, with a wide range of predicted activities, provided a strong basis for qualitative assessment of the microkinetic model and DFT predictions. Surface science experiments were performed on these surfaces to experimentally assess the decomposition activity. The surface configurations were found to have high decomposition activity, based on the high amount of nitrogen formed on the surface after exposure to ammonia, and the subsurface configurations did not decompose ammonia as predicted by the models, providing evidence that microkinetic modeling combined with DFT can be invaluable to predicting catalyst activity. Ni-Ru(0001) and Ni-WC surfaces were also predicted to be active, with DFT calculated nitrogen binding energies near the optimal range. The three Ni-monolayer surfaces, Ni-Ru(0001), Ni-WC and Ni-Pt-Pt(111), each with similar predicted activity, were experimentally tested and compared. While the amount of nitrogen formed on each of the surfaces was very similar, indicating similar barriers for NH x decomposition, the nitrogen desorption temperatures were very different. The differences indicated additional important considerations for catalyst design such as (1) synergistic effects which include surface heterogeneity, and (2) deactivation mechanisms such as catalyst instability and nitride formation.