(487b) First Principles Study of Ethane Dehydrogenation On a Model Catalyst Surface

The use of heterogeneous metal-based catalysts offers the possibility of converting alkanes to more useful products such as synthesis gas [1]. Synthesis gas is potentially needed in both stationary and mobile applications such as fuel cells, petroleum refining processes, as reducing agents in exhaust-gas after-treatment systems and for the manufacture of ammonia, methanol and commodity chemicals and so the demand for compact and energy saving procedures for production synthesis gas is increasing fast in recent years.

High temperature, low contact time oxidation reactions of alkanes on noble metal surfaces have emerged as a potentially important new technology, emphasizing the importance of understanding adsorption and fast reactions on catalytic surfaces [2-3]. Ethane is the simplest saturated hydrocarbon having a C-C bond and is an appropriate model species for the study of higher hydrocarbons interaction with metal surface and subsequent conversion to synthesis gas in the presence of oxygen. Ethane can be efficiently converted to synthesis gas via different processes of which one potential method is catalytic partial oxidation (CPOX). CPOX of ethane at high temperature and short contact time over Pt-supported catalyst lead to the production of ethylene while under similar operating conditions Rh-supported catalyst leads to a lower amount of ethylene and large amount of synthesis gas at the same time [3].

Many experimental and theoretical works for methane CPOX [4-6] have been carried out on various catalytic systems; in comparison only few studies have been performed for reaction of ethane with noble metal surfaces [7]. Our present study deals with a systematic theoretical investigation of ethane dehydrogenation steps on Rh (111) surface, which play a key role towards synthesis gas formation.

For the present study, we have used plane wave density functional theory (DFT), which in the recent years has become a very powerful method for analyzing reaction mechanisms and explaining the factors contributing to catalysis. The objective of the present work is to complement experimental data available and to provide additional information, so that complex surface processes over the catalytic partial oxidation of ethane could be better understood.

Some experiments have characterized small hydrocarbon species on various metal surfaces [8-9], but there remains considerable uncertainty about the structures and energetics of the chemisorbed intermediates and the role they play in various reaction pathways. To assist in developing improved catalysts, it is useful to know these quantities for a complete set of reaction intermediates. For this purpose, geometries, binding energies, and binding site preferences for the intermediate chemisorbed hydrocarbon species and hydrogen produced during ethane dehydrogenation are investigated and rationalized to provide insights into the crucial elementary steps. For each elementary step, the reaction energies and reaction barriers are evaluated. A meticulous analysis of the results of simulations based on density-functional theory allows setting up the mechanism, which determines the reactivity of a model catalyst at the nanometric scale.

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