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Keywords:

  • adsorption;
  • density functional calculations;
  • hydrogenation;
  • iron;
  • surface chemistry

Abstract

The thermodynamics and kinetics of the surface hydrogenation of adsorbed atomic carbon to methane, following the reaction sequence C+4 H⇄CH+3 H⇄CH2+2 H⇄CH3+H⇄CH4, are studied on Fe(100) by means of density functional theory. An assessment is made on whether the adsorption energies and overall energy profile are affected when zero-point energy (ZPE) corrections are included. The C, CH and CH2 species are most stable at the fourfold hollow site, while CH3 prefers the twofold bridge site. Atomic hydrogen is adsorbed at both the twofold bridge and fourfold hollow sites. Methane is physisorbed on the surface and shows neither orientation nor site preference. It is easily desorbed to the gas phase once formed. The incorporation of ZPE corrections has a very slight, if any, effect on the adsorption energies and does not alter the trends with regards to the most stable adsorption sites. The successive addition of hydrogen to atomic carbon is endothermic up to the addition of the third hydrogen atom resulting in the methyl species, but exothermic in the final hydrogenation step, which leads to methane. The overall methanation reaction is endothermic when starting from atomic carbon and hydrogen on the surface. Zero-point energy corrections are rarely provided in the literature. Since they are derived from C[BOND]H bonds with characteristic vibrations on the order of 2500–3000 cm−1, the equivalent ZPE of 1/2  is on the order of 0.2–0.3 eV and its effect on adsorption energy can in principle be significant. Particularly in reactions between CHx and H, the ZPE correction is expected to be significant, as additional C[BOND]H bonds are formed. In this instance, the methanation reaction energy of +0.77 eV increased to +1.45 eV with the inclusion of ZPE corrections, that is, less favourable. Therefore, it is crucial to include ZPE corrections when reporting reactions involving hydrogen-containing species.