Abstract
Natural gas, composed primarily of methane and ethane, is a key industrial feedstock. Methane is converted to syngas (CO + H2) for ammonia, methanol, and hydrogen production, while ethane is converted to ethylene, one of the highest-volume chemical products globally. Cracking of ethane (C2H6 → C2H4 + H2) is energy-intensive, produces carbon dioxide, and is limited by carbon deposition (coking) on reactor walls. Despite reduced costs from cracking, increasing demand for olefins, and processing limitations, have motivated the development of alternative catalytic pathways. The initial C-H bond cleavage is often the rate-limiting step in alkane reformation and influences overall reactivity. Heterogeneous catalysis enables hydrocarbon activation at lower energetic costs than gas-phase bond dissociation. For example, methane C-H bond activation is significantly lower on Pt(111), at 58.9 kJ/mol, compared to the gas phase (439 kJ/mol). Similar reductions are observed for ethane on catalytic surfaces.
In this thesis, the dissociative chemisorption of ethane on Rh(111) is investigated to determine the activation energy and pre-exponential factors for C-H bond cleavage. Dissociative sticking coefficients were measured over a wide temperature range (400 – 1000 K), revealing two distinct Arrhenius regimes corresponding to high-temperature (1000 – 700 K) and low-temperature (700 K – 400 K) behavior. This dual-channel behavior is consistent with site-specific reactivity, in which defect (step) sites dominate at low temperatures and terrace sites contribute at higher temperatures. The extracted kinetic parameters highlight the role of surface structure in controlling activation barriers for alkane dissociation on Rh(111). These results advance the fundamental understanding of hydrocarbon activation on transition-metal surfaces and inform the design of catalytic systems with greater resistance to deactivation and enhanced selectivity for ethylene.
