Abstract
Density functional theory was employed to investigate the effect of the hydrogen in the adsorption and direct dissociation of CO on Fe (100) surface. The preadsorption of hydrogen with coverages of 0, 1/3 and 2/3 monolayer (ML) was used in the present investigation. In the case of 1/3 ML of hydrogen, two configurations of adsorption were studied. The presence of hydrogen shows a major transference of electronic density from Fe surface to CO adsorbed, increasing the adsorption energy of CO from 2.00 eV in clean surface, to 2.76 eV in 2/3 ML of hydrogen. Furthermore, the activation barrier for direct dissociation of CO was 1.13 eV and for the recombination energy 2.28 eV on clean Fe (100) surface. In the same way, the activation barrier for CO in the presence of coadsorbed hydrogen was slightly affected presenting values of 1.06 eV and 1.16 eV to 1/3 ML configurations and 0.98 eV for 2/3 ML of hydrogen. Finally, the recombination energy decreases to 1.63 eV and 1.49 eV for 1/3 ML configurations and to 1.23 eV for 2/3 ML of coadsorbed hydrogen. These results indicate that the CO adsorption and dissociation are favored in the presence of hydrogenated surfaces.
Highlights
The interaction between carbon monoxide and transition metal surfaces has been subject of many experimental and theoretical investigations and it is an important step in many industrial processes such as Fischer-Tropsch synthesis (FTS) [1]
The adsorption of H2 and CO processes are the first steps for understanding the influence of the hydrogen in the mechanisms of adsorption and direct CO dissociation on iron surfaces
Where Eads is the adsorption energy of the molecule, Esurf is the total energy of the clean slab or the slab with preadsorbed hydrogen, Emol is the total energy of the molecule at vacuum and Esys is the total energy of the molecule/slab [17]
Summary
The interaction between carbon monoxide and transition metal surfaces has been subject of many experimental and theoretical investigations and it is an important step in many industrial processes such as Fischer-Tropsch synthesis (FTS) [1]. Despite of the interest in CO dissociation on iron catalysts for the chemical industry, and the several experimental works reported about the adsorption and dissociation of CO on iron surfaces [12] [13], theoretical calculations are relatively limited. Sorescu et al have studied several diffusion pathways and dissociation channels, reporting a CO stretching frequency of 1246 cm−1 for a CO molecule tilted on a fourfold hollow site with an angle between the CO molecular axis and the surface normal of 50 ̊. Besides of the direct dissociation of CO, the experimental results of Ojeda et al [21] have provided evidence of two parallel CO activation pathways on CO-covered Fe catalysts, the direct and the H-assisted, both routes show a trend to form CH2 monomers. Our ultimate goal is to understand the hydrogen influence on the CO adsorption and dissociation, and provide an explanation of hydrogen influence in the initial steps of Fischer-Tropsch reactions on Fe (100)
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