Computational catalysis for carbon capture and utilization [electronic resource]
Keywords: Energy Resources Engineering.

Living in a world of high energy demand, energy independence is important for surviving the future economic uncertainties. Oil, natural gas and coal have been the world's primary energy resource for a number of decades. However, coal remains the world's largest abundant fossil fuel and coal reserves are more distributed around the world compared to oil or natural gas. Generally, coal is mainly used as a direct fuel source for power generation utilities. Burning fossil fuels such as coal results in the release of heat (from which energy is extracted) and inevitably emits gaseous products, water and carbon dioxide (CO2), and other trace contaminants (i.e., particulate matter, SOx, and NOx). According to the National Oceanic & Atmospheric Administration (NOAA), the current average global atmospheric CO2 concentration as of September 2012 is close to 390 ppm. This level has risen sharply from preindustrial levels of 280 ppm a century ago as energy demands have increased globally. To stabilize the CO2 concentration in the atmosphere to reach a level that would avoid dangerous impacts to the environment, a portfolio of strategies including carbon capture and storage and/or utilization (CCS and/or U) and alternative energy sources with no carbon emissions - solar, wind, geothermal, hydro and nuclear - is required. However, it will likely take decades to supply the rising global energy demand solely with non-carbonized energy resources. This is because technologies used to exploit non-carbonized energy sources efficiently and economically are still either in development or in an early stage of commercialization. Consequently, fossil fuels will remain the primary source to supply the global energy needs for the foreseeable future. Carbon capture combined with storage and/or utilization serves as the only approach to reduce the amount of CO2 emission in the near term. The current study consists of two projects that target carbon capture and utilization from an atomistic view by aid of computational chemistry. Throughout recent decades, computational chemistry has become a promising tool to serve as a theoretical framework for providing a fundamental description of surface chemical reactions. Considerable information regarding surface chemical processes including equilibrium structures, adsorption energies, reaction paths and activation energies are required to aid in the understanding of the activity or selectivity for a particular reaction on a particular surface. Calculations are based on a quantum-mechanical description of the interactions between electrons and between electrons and atomic nuclei. The advantages of theoretical studies are not limited to a better understanding of the surface science processes, but also are a crucial component to material design for future technologies. The first project is to develop a catalytic nitrogen (N2)-selective membrane technology with potential applications of indirect CO2 capture and ammonia synthesis. The N2-selective membrane technology benefits from the driving force of N2 in flue gas (~73 wt.%) streams for indirect CO2 capture as it provides atomic nitrogen on the permeate side of the membrane during separation. Metallic membranes made from Earth-abundant Group V metals, i.e., vanadium (V) and its alloy with ruthenium (Ru) are considered for catalytic selective N2 separation. Similar to a traditional palladium (Pd)-based H2-selective membrane for hydrogen purification, N2 molecules preferentially adsorb on the catalytic membrane and dissociate to two nitrogen atoms. The atomic nitrogen diffuses through the crystal lattice by hopping through the interstitial crystal sites of the bulk metal, ultimately leading to atomic nitrogen on the permeate side of the membrane. This study has been focused on the nitrogen interactions only at the membrane surface and throughout several subsurface layers. The adsorption energies of N2 as well as atomic N on the V surface (V(110)) and Ru-alloyed V surface (Ru-doped V(110)), are calculated and compared with the traditional catalyst for ammonia synthesis, i.e., iron (Fe). The nitrogen dissociation pathway and its corresponding activation barrier are also determined. Additionally, the diffusion of atomic N from the V(110) surface to its subsurface layers is investigated to determine the rate-limiting step of nitrogen transportation across membrane surface. It is found that the N2 molecule and atomic N bind on the V(110) surface very strong compared to adsorption on the Fe surfaces. Though the activation energy (ca. 0.4 eV) for nitrogen dissociation on the V(110) surface is greater than that of the Fe surfaces, it is comparable to that of the Ru surfaces. Atomic N slightly prefers to stay on the V(110) surface rather than in the subsurface layers. Coupling this with relatively high activation barrier for subsurface diffusion (ca. 1.4 eV), it is likely that the subsurface diffusion of nitrogen is the rate-limiting step of nitrogen transportation across membrane surface. Alloying Ru with V reduces the adsorption energy of atomic N on the Ru-doped V(110) surface and in the subsurface layers. Therefore, it is expected to facilitate the nitrogen transport across the membrane surface. The second project is associated with conversion of the synthesis gas, i.e., carbon monoxide (CO) and hydrogen (H2) obtained from the fossil fuel combustion to higher valuable products such as gasoline or other chemicals via Fischer-Tropsch (FT) synthesis. This study is primarily focused on the CO adsorption on the surface of the iron-cobalt alloy - FeCo(100). This alloy of FeCo is chosen because this material has experimentally shown high activity towards FT synthesis and has demonstrated promise for suppressing carbide formation, thereby slowing the catalyst degradation rate. The CO adsorption on the FeCo(100) surface has been investigated and compared to the previous theoretical study of CO adsorption on the FeCo(110) surface. The analysis of the local density of states (LDOS) and charge density profile of the CO-adsorbed systems have been applied to examine the CO adsorption mechanism on the FeCo(100) surface, which is the first step in the FT synthesis process. The range of computed adsorption energies from this study falls between the CO adsorption energies on pure Fe and Co surfaces. Moreover, CO prefers to adsorb on the top site of the Co surface of FeCo alloys, whereas CO has stronger adsorption on pure Fe than on the pure Co surface. This change in metal preference for adsorption (i.e., from Fe in a pure system to Co in the FeCo alloy surface in the current investigation) is due to the shift in the d-band center of the alloyed material. This implies that alloying Fe with Co changes the electronic structure properties of the pure metal and ultimately affects the CO adsorption energy. This work represents an example of how the electronic structure properties of a metal might be tuned for optimal CO-surface reactivity via alloying.