Computational Studies of Reactions within the Solid State and on Solid Surfaces
The growing climate crisis makes it imperative that we reduce our energy consumption through process optimization and that we develop clean renewable sources of energy. This thesis addresses both challenges. The first part investigates the possibility of optimizing mechanically driven processes such as stress-induced phase transitions in the solid state by tuning the applied stresses to reduce reaction barriers. Computational studies can help elucidate the changes in reaction paths caused by the applied stress, but accurate calculations are costly because it is necessary to re-optimize the reaction path for every new stress. Here, we develop a simple model to predict the reaction barrier and enthalpy for any low force from quantum mechanically calculated parameters. These parameters are calculated from stationary point structures on the minimum energy path with no force applied. We show how to use this model to obtain the reaction energetics of a manifold of stresses, thereby allowing the identification of stresses that will most efficiently lower the reaction barrier and reduce energy consumption. The model’s predictions are assessed through molecular dynamics simulations. The second part examines the energetics and possible reaction pathways on NiSe2, NiSe, and Ni3Se2 surfaces using density functional theory. These materials have been demonstrated experimentally as cheap, efficient catalysts for the oxygen evolution reaction (OER) which is a rate-limiting step in the production of hydrogen from water creating a bottleneck in replacing fossil fuels with hydrogen fuels cells. Our computational investigation of different Miller index surfaces and surface terminations provide insight into these materials’ activities and restructurings under OER conditions. Our results show that the (101) surface of NiSe gives the lowest reaction potential energy; however, owing to the electronic structure of the OER intermediates, SeO desorption may be a competing reaction. Doping these nickel compounds with cobalt has been shown experimentally to improve the material’s catalytic activity. Here, we shall discuss the effects of cobalt levels and surficial symmetry on the stability of NiSe2 and Ni3Se2. We also show how the dopant levels and distribution affect the energetics of the OER and possible SeO desorption on NiSe2 surfaces.
URI for this recordhttp://hdl.handle.net/1974/28719
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