(699a) Ab-Initio Studies of Palladium –Niobium Alloys for Hydrogen Separation

Coal, having the most abundant reserves of any fossil fuel by approximately 847 billion tonnes of proven reserves worldwide, provides at least 27% of the world's total energy consumption. Moreover, its lower price compared to oil and natural gas, makes coal an economical source to be used for power generation. Gasification of coal used in power generation allows for the application of palladium-based membrane technology by giving highly concentrated 40-55% vol. CO2 streams compared to the 15% vol. CO2 flue gas produced by typical pulverized coal combustion processes. The use of membrane technology not only reduces the carbon emissions by separating CO2 but also produces the clean energy carrier hydrogen. As a membrane material, palladium has been the main focus by many researchers since it has a high selective permeability and catalytic activity to H2. However, a high affinity of Pd to interact with sulfur species causes sulfur poisoning of the membrane even at ppm concentrations of sulfur making this option technically and economically unfavorable. It is possible to overcome these barriers in addition to increasing the functionality of the membranes by alloying Pd with other metals. Nb is a good candidate for this reason since it has the highest hydrogen permeability among any other Group V metals. Unfortunately, hydrogen embrittlement is an important drawback in the pure Nb systems. In this study, Pd-Nb alloys were investigated to understand the mechanisms involved in surface site stabilities, reactivity, and subsurface hydrogen diffusion. Binding energies of hydrogen and sulfur species in alloys of Pd and Nb were calculated by using the Vienna ab initio Simulation software package employing density functional theory. Surface and bulk properties of materials composed of transition metals were obtained through a density of states analysis and the reactivity was predicted using the ?d-band center shift? model of Hammer and NØrskov. Moreover the binding mechanisms associated with hydrogen and sulfur have been investigated by calculating the local density of states (LDOS) of the s- p- and d-orbital states of the individual atoms.