Catalysis of Imperfections: Importance of the Nature and Abundance of Defects in Oxides and Chalcogenides for Diverse Applications

Abstract

Catalysis is ubiquitous in modern civilization, with far-reaching applications in the chemical, energy, automobile, electronic and pharmaceutical industries. Recent advances in computational infrastructure and state-of-the-art experimental techniques are leading to an exponential growth of our understanding of catalysis. active sites in catalysts provide its essential functionality and are analogous to the role of mitochondria for life functions. This thesis aims to provide case-studies of synergistic combinations of experimental spectroscopic techniques, and computationally efficient ab-initio methods such as Density Functional Theory (DFT)to study and predict the intrinsic nature of these active-sites, and in turn their catalytic activity.

The first example relates to the abatement of nitrate and nitrite ions, which are globally detected surface and ground water contaminants with adverse health effects. In/In₂O₃ deposited on Pd nanoparticles (NPs) shows excellent room temperature nitrate catalytic reduction activity to nitrogen gas. We provide evidence that metallic Pd active sites are responsible for H₂ activation and spillover to reduce In₂O₃ and form the active sites for nitrate ion adsorption and reduction to nitrite. A volcano-type activity relation exists for the overall nitrate reduction activity and the In surface content of the bimetallic catalysts particles.

Next, a hybrid Mo₂C/MoS₂ catalyst prepared via carburization of vertically aligned nanosheets of MoS₂ was investigated. It exhibits exceptional electrochemical performance for the hydrogen evolution reaction (HER) and outperforms the parent electrocatalysts (Mo₂C and MoS₂). Experimental and computational evidence points towards the introduction of CH_X species at the S defects on the S-edge of MoS₂, and we attribute the remarkable catalytic activity to these novel active sites that exhibit a thermoneutral differential Gibbs free energy of H adsorption.

Finally, MoO3 is studied as a potential hydrodeoxygenation (HDO) catalyst to upgrade pyrolysis vapor to value added fuels and chemicals. We propose that O defects on the surface provide the active site for HDO reactions involving C-O scission and H₂ splitting. Transition metal promotion and carburization to a mixed oxy-carbide phase greatly enhance the O defect site density and provide additional metallic sites for H₂ splitting.