Catalytic Properties of Nitrogen- and Oxygen-Modified Earth-Abundant Materials
Creators: Denny, Steven Robert
Publisher: name: Columbia University
Year: 2021
Subjects: subject: Chemical engineering
subject: FOS: Chemical engineering
subject: Catalysis
subject: Water--Electrolysis
subject: Renewable energy sources--Technological innovations
subject: Ethanol
subject: Glycerin
subject: Tetrahydrofuran
subject: Thin films
subject: Synthesis gas

description: The replacement or loading-reduction of precious metal catalysts with low-cost, earth-abundant materials is an important step for the development of next-generation industrial chemical processes. By decreasing the potential cost of an electrolyzer device or enabling new pathways of upgrading biomass-derived oxygenates (BDOs), the nitrides and oxides of earth-abundant transition metals may be strategically utilized for the modular use of renewable power or the transition of conventional chemical feedstocks to renewable sources. This thesis uses a combination of electrochemical and surface science techniques to study the catalytic properties of nitrogen- and oxygen-modified earth-abundant materials for the electrolysis of water, and the surface reactions of three BDOs: ethanol, glycerol, and tetrahydrofurfuryl alcohol (THFA). The development of stable, active, and low-cost electrocatalysts with reduced platinum group metal (PGM) loading for the hydrogen evolution reaction (HER) is an important step towards the grid-level implementation of electrolyzers. In this thesis, the electrochemical stability of tungsten nitride (WN) and niobium nitride (NbN) was characterized over broad pH-potential regimes, from which pseudo-Pourbaix diagrams were developed. In addition, unmodified and platinum-modified WN and NbN thin films were assessed for HER, where monolayer (ML) Pt-modification led to Pt-like HER activity in acid electrolyte. The selective bond scission of oxygen-rich and functionally complex biomass-derived oxygenates offers a unique opportunity to convert renewable biomass, as opposed to fossil fuels, into important industrial feedstocks. The fundamental surface reactions of three BDOs, either lignin-derived or biofuel-derived, was studied in this thesis. Ethanol reforming was studied on unmodified and Pt-modified Mo2N thin film surfaces for the production of synthesis gas, or syngas, a CO and H₂ gas feedstock for Fischer-Tropsch synthesis. Mo₂N, and the Mo₂C carbide analogue, have exhibited strong oxophilicity for the reaction of simple and complex alcohols that results in unselective C-O bond scission. Pt-modification was used to selectively conserve the C-O bond for CO production, and cleave the C-H and C-C bonds for H2 generation. Pt-modification shifted the reaction pathway from undesired decomposition on Mo₂N to reforming, while inhibiting undesired pathways such as ethylene or methane production. The hydrodeoxygenation (HDO) of glycerol, the primary manufacturing byproduct of biodiesel, to propylene was studied on WOx-modified Pt(111) surfaces. Two WOx active sites were observed for the deoxygenation of glycerol: Brønsted acid WOx sites for dehydration and oxygen vacancy sites for hydrodeoxygenation (HDO). While the undesired dehydration of glycerol to acrolein was most active on surfaces with thick WOx coverages, propylene production via the HDO pathway was more facile at intermediate coverages. Lastly, the ring-opening of THFA, a promising biomass-derived platform oxygenate, was studied on WOx/Pt(111) surfaces. The desired ring-opened product, 1,5-PeD, was also used as a probe molecule to study binding and desorption on WOx/Pt(111) surfaces. This work indicates that WOx-modification weakens the interaction between the ring-opened intermediate and the surface, to an extent that facilitates the hydrogenation of the 1,5-PeD-like intermediate and the desorption of gas-phase 1,5-PeD.