Identification of Active Sites in Heterogeneous Catalysis and Surface Chemistry Study of Alkanes Adsorption

• Main Author: Chakradhar, Ashish
• Format: Others
• Published: North Dakota State University 2018
• Online Access: https://hdl.handle.net/10365/27339

Heterogeneous catalysis is considered the basis of the chemical industry. Surface science investigations on model catalysts have made significant contributions in gaining molecular level understanding of heterogeneously catalyzed reactions. Surface science studies involving the interactions of coal combustion gases such as CO and CO2 on electron beam lithography (EBL)-fabricated CuOx/SiO2 nanoclusters to identify active sites and kinetics data characterizing the alkane adsorption on surfaces of alkaline earth metal oxides, metalloids, and metals are investigated in this dissertation. Diverse surface phenomena such as surface-adsorbate interactions, adsorbate-adsorbate interactions, chemical rearrangements of adsorbed reaction intermediates, identification of active sites, and formation of products have been studied utilizing surface science techniques. Thermal desorption spectroscopy (TDS) and molecular beam scattering (MBS) were utilized to study the adsorption kinetics and dynamics, respectively, of probe molecules on catalyst surfaces. The catalyst surfaces were characterized by various surface science techniques such as Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), low energy electron diffraction (LEED), and scanning electron microscopy (SEM). Active sites on the catalyst surface of EBL-fabricated Cu/CuOx nanoclusters (methanol synthesis model catalyst) were identified for CO and CO2 adsorption. Experimentally, it was proven that CO2 adsorbs preferentially along the clusters' rim site and CO on both the rim and terrace sites. Identifying the active sites on a catalyst surface forms the basis of systematic catalyst design strategies. Bond activation in alkanes is a crucial step in the catalytic processing of alkanes in application such as the catalytic combustion of natural gas, exhaust gas remediation, and the selective oxidation of alkanes to high demand products. Adsorption of small chain alkanes such as n-butane, n-pentane, and n-hexane on CaO(100) surface resulted in bond activation to form mostly methane and ethylene via hydrogen abstraction. This production of hydrogen gas has significant economic and environmental benefits. Whereas, the adsorption of n-butane on Sb(111) and silica-supported Mo clusters was found to be molecular and non-activated. However, a strong hydrophobic property of Sb(111) surface was characterized by studying co-adsorption of n-butane and water. In addition, the adsorption sites for n-butane on Sb(111) and Mo nanoclusters were characterized.