Abstract:

First-principles calculations based on density functional theory (DFT) have been used to investigate how Ni doping in the supporter impacts CH₄/CO₂ reforming over Ni/MgO catalysts. Ni8 clusters supported on different Ni_xMg_yO₂₇(100) slabs (x+y= 27) have been selected to model Ni/MgO catalysts with different Ni : Mg ratio. The CH₄/CO₂ reforming mechanisms on different Ni8/Ni_xMg_yO₂₇(100) slabs indicate that the energy barriers of CH₄ dissociated adsorption and CH_x oxidation both increase with an increase in the Ni : Mg ratio of the supporter. CH would be easily generated from the pyrolytic carbon. The electron structure analysis shows that the direction of electron transfer changes to "between metal and supporter" with increasing Ni : Mg ratio of the supporter, but not to "from supporter to metal" on pure Ni/MgO. When the Ni cluster is negative, electron transfer occurs between adsorbed species and Ni atoms in the surface layer of the Ni cluster, and CH₂ is the main species for CH_x oxidation. When the Ni cluster is positive, its Hirshfeld is stable, electron transfer occurs between adsorbed species and Ni atoms in the surface layer of the NiO-MgO solid solution, and CH is the main oxidative species. The electron deficiency of the Ni cluster is the reason for the poor catalytic performance of Ni/MgO with a high Ni doping ratio.

Key words: CH₄/CO₂ reforming, Ni/MgO, Ni:Mg ratio, Electron transfer, DFT