Ammonia decomposition on NiPt supported on γ-alumina : a study of stability of real catalysts
Date
2012
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Ammonia decomposition is an important reaction due to its impact that it will have on the hydrogen economy. Hydrogen has been found to be a possible source of alternative energy with the application of hydrogen fuel cells. However, the major difficulty with hydrogen, as an alternative energy source, is its low energy density. Hydrogen is a low-density gas, and in order for its use in automobiles to be economically plausible, large tanks under high pressure would have to be employed. The result would limit space available in automobiles for other purposes and pose significant safety issues. As an alternative ammonia has been proposed as a means to store hydrogen chemically, due to its increased energy density. At moderate pressures (~9 atm) ammonia is a liquid, and could be adapted into the current liquid fuel infrastructure. In addition, the decomposition of ammonia would not produce carbon monoxide, a known poison of fuel cell electrodes. Thus, the ammonia decomposition reaction is one piece of the mechanism that could make hydrogen a viable alternative energy source. The work of Hansgen et al. examined ammonia decomposition through computational studies and surface science experiments on monolayer bimetallic surfaces. The Ni-Pt-Pt(111) configuration of NiPt was found to show favorable results for ammonia decomposition. In a reducing environment, the subsurface configuration of Pt-Ni-Pt(111) is thermodynamically preferred, and the surface configuration of Ni-Pt-Pt(111) is most stable in an oxidizing environment. The focus of this work is on reactor experiments at ambient pressure with supported catalysts. Experiments performed using supported catalysts will validate predictions made by computations and surface science experiments. The experiments in this work will bridge the pressure and materials gap from surface science experiments to real supported catalysts. NiPt catalysts were synthesized and characterized by CO chemisorptions and EXAFS. Batch reactor and flow reactor experiments were performed to measure catalytic activity. There were no observations made of increased activity with the bimetallic catalyst. This is evidence of Ni-terminated surface not present in the reactor environment, indicating that the DFT calculations and surface science experiments performed previously within the group do not accurately approximate the real catalyst. Microkinetic modeling was used to find the expected results in the batch reactor and flow reactor. It was predicted that the Ni never segregates to the surface of the catalyst due to the large amount of hydrogen present on the surface of the catalyst and an insignificant amount of nitrogen bound to the surface.