Unveiling the Origin of Alkali Metal (Na, K, Rb, and Cs) Promotion in CO2 Dissociation over Mo2C Catalysts

Abstract

Molybdenum carbide (Mo2C) is a promising and low-cost catalyst for the reverse water−gas shift (RWGS) reaction. Doping the Mo2C surface with alkali metals can improve the activity of CO2 conversion, but the effect of these metals on CO2 conversion to CO remains poorly understood. In this study, the energies of CO2 dissociation and CO desorption on the Mo2C surface in the presence of different alkali metals (Na, K, Rb, and Cs) are calculated using density functional theory (DFT). Alkali metal doping results in increasing electron density on the Mo atoms and promotes the adsorption and activation of CO2 on Mo2C; the dissociation barrier of CO2 is decreased from 12.51 on Mo2C surfaces to 9.51–11.21 Kcal/mol on alkali metal-modified Mo2C surfaces. Energetic and electronic analyses reveal that although the alkali metals directly bond with oxygen atoms of the oxides, the reduction in the energy of CO2 dissociation can be attributed to the increased interaction between CO/O fragments and Mo in the transition states. The abilities of four alkali metals (Na, K, Rb, and Cs) to promote CO2 dissociation increase in the order Na (11.21 Kcal/mol) < Rb (10.54 Kcal/mol) < Cs (10.41 Kcal/mol) < K (9.51 Kcal/mol). Through electronic analysis, it is found that the increased electron density on the Mo atoms is a result of the alkali metal, and a greater negative charge on Mo results in a lower energy barrier for CO2 dissociation.

Recent Advances in Carbon Dioxide Adsorption, Activation and Hydrogenation to Methanol using Transition Metal Carbides

“…The E ads of CO 2 are −1.78 eV on Na−Mo 2 C, −1.68 eV on Cs−Mo 2 C, −1.64 eV on K−Mo 2 C, and −1.66 eV on Rb−Mo 2 C (Table 3). CO 2 binds on the top site of one Mo atom in X—Mo 2 C(001) facet, while the oxygen atoms make two Mo−O bonds with neighboring Mo atoms in bent form (MMM), similar to the MMC configurations stated above [114] . Mulliken charge analysis shows that the Mo atom of undoped bare Mo 2 C(001) facet was positive.…”

“…CO 2 binds on the top site of one Mo atom in X-Mo 2 C(001) facet, while the oxygen atoms make two MoÀ O bonds with neighboring Mo atoms in bent form (MMM), similar to the MMC configurations stated above. [114] Mulliken charge analysis shows that the Mo atom of undoped bare Mo 2 C(001) facet was positive. After doping of the alkali metals, the Mulliken charge on Mo atoms close to doped alkali metal becomes negative.…”

“…Liu et al, [114] in 2022, performed density functional theory (DFT) calculations to study the doping effects of alkali metals such as X=Na, K, Rb, and Cs on CO 2 dissociation over the X-Mo 2 C (001) facet using PBE exchange and correlation functional. [113] They observed that, after the alkali metal doping, the charge on the adjacent Mo atom became negative due to charge transfer from doped alkali metal to Mo.…”

“…This means that the greater the negative charge on the Mo atom, the lower will be the dissociation energy barrier. [114]…”

“…The CO 2 dissociation barrier fell from 0.543 eV on the Mo 2 C facet to 0.412–0.486 eV on the alkali metal‐modified Mo 2 C facet. This means that the greater the negative charge on the Mo atom, the lower will be the dissociation energy barrier [114] …”