Mechanistic pathway for methane formation over an iron-based catalyst

The methanation reaction mechanism under Fischer-Tropsch conditions is investigated with the Steady State Isotopic Transient Kinetic Analysis (SSITKA) technique over a precipitated iron-based catalyst. The (CH4)-C-13 transients resulting from a (CO)-C-12 ->(CO)-C-13 switch (330 degrees C, 1.2 bar, and H-2/CO = 15) provided kinetic information for the methanation reaction. Six methanation models were screened and only three of these could describe the methane transient. These models were subsequently extended to account for the Fischer-Tropsch higher hydrocarbon products by considering C-C coupling reactions and the kinetic rate parameters were estimated. The result was two indistinguishable mechanisms which could describe the methane transient as well as the experimental steady-state concentrations. Both mechanisms have two active pools of carbon (C-alpha and C-beta) on the catalyst surface with both leading towards the formation of methane. The C-beta pool is 25 to 50 times less active than the Ca pool for methanation and occupies 92% of the total CHx coverage (0.25 ML). The C-C coupling reaction was shown to involve both the C-alpha and C-beta pools. The concentration of molecularly adsorbed CO on the Fe-based catalyst is shown to be extremely low, with an estimated surface coverage of 9 x 10(-4) ML (C) 2008 Elsevier Inc. All rights reserved.