Date of Award
Open Access Dissertation
Catalytic conversion of biomass-derived oxygenates to fuels and value-added chemicals is a promising strategy in the search for renewable and sustainable energy sources. Most relevant catalytic processes are carried out in an aqueous environment using supported transition metal catalysts. The reaction network consists of multiple series and parallel pathways leading to formation of hydrogen, alkanes, and lighter oxygenates. The final product distribution ultimately depends on the sequence and competition of C−C, C−O, C−H, and O−H bonds scissions. Ethylene glycol (EG) is the simplest model molecule of various biomass-derived polyols that has a C:O stoichiometry of 1:1 and contains all relevant C−C, C−O, C−H, and O−H bonds. While the reaction mechanism of EG reforming is to some degree understood at the metal–gas interface, lack of a well-established methodology for describing the influence of a complex liquid phase on a reaction across a solid–liquid interface has hindered similar theoretical studies in an aqueous environment.
In this dissertation, we show how first-principles calculations can be used for a systematic investigation of complex reaction pathways at a metal–water interface. We proposed a multistep strategy where the description of the influence of an aqueous environment on reaction kinetics and equilibria is successively refined. First, we developed a new computational approach for implicit solvation of periodic metal slabs by integrating planewave density functional theory (DFT) calculations with an implicit solvation model. Rapid convergence with size of the metal cluster and basis set was demonstrated for C−C cleavage in dehydrogenated EG at a Pt (111)/H2O interface. The method was then successfully applied for predicting experimentally reported CO frequency shifts in water at Pt (111)/H2O and Pd (111)/H2O interfaces. Next, we developed a hybrid quantum mechanics/molecular mechanics (QM/MM) method to allow an explicit description of water molecules at the metal–water interface, and applied it to construct the complete free energy profile for a model C−C cleavage reaction. Finally, we investigated the mechanism of EG reforming over Pt (111) in vapor and aqueous phases from first-principles calculations and developed microkinetic models for the respective phases. Initial dehydrogenation of EG was found to be rate-determining under operating conditions which is in agreement with experimental observations.
Faheem, M.(2014). Theoretical Investigation of Heterogeneous Catalysis at the Solid–Liquid Interface for the Conversion of Lignocellulosic Biomass Model Molecules. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/2770