Реферат: KN-2 Fischer-Tropsch (FT) synthesis is increasingly important technology to convert a wide range of carbon sources into liquid fuels. Despite its practical importance, many aspects about the exact nature of the elementary reaction steps underlying FT mechanism remain unclear. Modern insights into the relation between surface topology and reactivity dictate that direct CO dissociation occurs on step-edge sites of transition metals such as Co, Rh and Ru [1]. The alternative is hydrogen-assisted CO dissociation [2]. Another debate concerns chain growth paths via CHx species or direct CO insertion followed by C-O bond activation [3]. Recent theoretical studies emphasize strong binding of oxygen to Ru and Co. Then removal of water can be the rate-controlling step [4]. Here, we combine steady state isotopic transient kinetic analysis (SSITKA) and chemical transient kinetic analysis (CTKA) with first-principles based microkinetics simulations to better understand FT mechanism on Co catalysts. The salient findings of the present transient kinetic study are as follows: First-principles calculations stress the importance of step-edge sites for facile CO dissociation and chain-growth on these sites [4]. During methanation (260 ºC), the CO consumption rate is limited by C and O hydrogenation rather than CO dissociation. When C species are deposited on a Co nanoparticle surface followed by evacuation to remove all CO, chain growth proceeds upon hydrogenation. This shows that adsorbed CO is not a necessary prerequisite for chain growth. During FT synthesis (220 ºC), scrutinizing the labelled C content in higher hydrocarbons shows that the hydrocarbons formation rate is mainly limited by monomer formation rather than C-C coupling. Propagation is fast compared to CO dissociation. Under FT synthesis conditions, the CO consumption rate is not limited anymore by C and O hydrogenation; the reason is that, due to increased surface coverage, the lack of surface vacancies causes CO dissociation to become the rate-controlling step. Lack of surface vacancies is a necessary condition for high chain growth probability due to the reversibility of the fast chain-growth process.

Библиографическая ссылка: Hensen E.J.M.
On the Mechanism of the Fischer-Tropsch Reaction
X International Conference “Mechanisms of Catalytic Reactions” 02-06 Oct 2016