Hydrocarbon synthesis from COH2 on supported iron: Effect of particle size and interstitials

Rates of hydrocarbon synthesis (HS) from COH2 (syngas) were measured for Fe supported on MgO (FeMgO) while Mössbauer effect spectroscopy (MES) was used for monitoring the structure of the metal. Three effects were observed as particle size d of H2 prereduced Fe changed between 1 and 17 nm. First, the site-time yield of CH4 synthesis (STYCH4, the number of CH4 molecules produced per CO-titrated site per second) increased by over an order of magnitude. Second, under conditions used (525K, H2CO = 3, atmospheric pressure), large Fe particles produced a lower fraction of C+2 in the hydrocarbon product (C+2 selectivity) and showed a greater tendency for deactivation and loss of C+2 selectivity than small particles. Third, as reaction in syngas proceeded, the dominant phase shifted from x-carbide to ε′-carbide as d decreased, and further decrease of d produced a decrease in the rate of ε′-carbide formation. The three trends for HS were the same qualitatively and similar quantitatively when the catalysts were converted to ε-nitride before reaction, instead of being used directly after reduction in H2. However, MES showed that the ε-nitride structure persisted even after 20 h in reacting syngas although measurements of CH4 evolved during reduction of used samples indicated that the surface of all catalysts was thoroughly carbided. Thus, for HS, particle size was more important than interstitials in determining activity and selectivity of supported Fe.