Methane, formaldehyde and methanol formation pathways from carbon monoxide and hydrogen on the (0 0 1) surface of the iron carbide χ-Fe5C2

Abstract

a b s t r a c tFormation of CH x (O) monomers and C 1 products (CH 4 , CH 2 O, and CH 3 OH) on C-terminated v-Fe 5 C 2 (0 0 1) (Hägg carbide) surfaces of different carbon contents was investigated using periodic DFT simulations. Methane (CH 4 ) as well as monomer (CH x ) formation follows a Mars-van Krevelen-like cycle starting with the hydrogenation of surface carbidic carbon, which is regenerated by subsequent CO dissociation, while oxygen is removed as H 2 O. In cases where surface carbon is readily available, the apparent barrier for CH 4 formation was found to be $95 kJ/mol. However, different rate-determining steps show that different propagation mechanisms may be possible for actual chain growth, depending on the carbon content of the surface. Hydrogen addition to CO forms formyl (HCO), which is a precursor for both H-assisted CO activation and oxygenate formation. Further hydrogenation of HCO yields adsorbed formaldehyde and methoxy, rather than hydroxymethyl (HCOH) that would give C-O bond splitting. Full hydrogenation to gas-phase methanol faces a high barrier, suggesting that CH x O species may be involved in higher oxygenate formation in a full Fischer-Tropsch mechanism or that the C-O bond does not break until the CHO fragment has been incorporated in a C 2 species, a route for which precedents are available in the literature.

Reflections on the Fischer-Tropsch synthesis: Mechanistic issues from a surface science perspective

“…4 indicate that CO can in fact play a crucial role in steering the selectivity of, in particular, hydrocarbon surface reactions, and hence it is of crucial importance to further explore this in a systematic manner to further test the hypotheses brought forward in the present article. For the case of FT on iron it should be realized that catalysts are active in carbidic form, and recent computational work [13][14][15] has been based on the assumption that lattice carbidic carbon can be involved in the reaction, while C-vacancies would enable dissociation of CO without the need for explicit surface step sites, making the situation distinctly different from the proposed reaction sequence on the metallic surfaces…”

“…As a consequence, the influx of monomer species into the smaller ensembles that do exist is comparatively smaller. Assuming that the residence time of a growing chain is not affected by particle size the consequence is that selectivity, defined as: ␣ = r growth r growth + r term (14) goes down. This is even worsened when the activity of active sites drops as well, for example because it becomes progressively more difficult to remove oxygen from the surface.…”

“…alkyls, alkenyls, etc) have been made, where organometallic chemistry has been used for inspiration [11,12]. In the case of iron, the active phase is a carbide, where the lattice carbon atoms in the surface region are likely to be involved in the reaction mechanism [13][14][15][16][17]. Nevertheless, similar mechanistic issues play a role as in the mechanism over metallic FTS catalysts.…”

Production and stability of low amount fraction of formaldehyde in hydrogen gas standards

“…This adsorption is thought to facilitate the hydrogenation reaction. There have been several studies on the kinetics of hydrogenation on various catalytic surfaces such as platinum, nickel, palladium, ruthenium, cobalt, rhodium, osmium, iridium, and iron [31][32], copper [33], iron carbides [34], copper-zinc [35], etc. All surfaces show significant catalytic activity with the activation barrier dropping, for example, to 107 kJ mol -1 on copper [33] or to 125 kJ mol -1 on nickel(111) [32].…”

Experimental Investigation of Methanol Conversion Factor to Formaldehyde in a Pilot Reactor

“…Therefore, studies on methane oxidation show that methane with significant amount under 600 °C does not oxidize, while, as already mentioned, formaldehyde begins to decompose at temperatures below this. Formaldehyde is used because of elimination of bacteria as an antiseptic solution [16][17][18][19].…”