Mechanism-driven CO2 Capture and Activation on Two-dimensional Transition-metal Diborides

TL;DR

This study uses first-principles density functional theory to investigate CO$_2$ adsorption and activation on hexagonal M$_2$B$_2$ 2D monolayers (M = Sc, Y, Ti, Zr, Nb). CO$_2$ binds strongly as a chemisorbed δ− species, bending and lengthening C–O bonds (to ~$1.27$–$1.29$ Å) while transferring electrons from the substrate, with IpCOHP$(E_F)$ decreasing from $-18.29$ eV for isolated CO$_2$ to about $-13.9$–$14.4$ eV upon adsorption. Adsorption energies range from $-1.84$ to $-2.16$ eV, with Sc$_2$B$_2$ and Ti$_2$B$_2$ showing the strongest binding and activation trends correlating with charge transfer. AIMD at 300 K reveals thermal sensitivity of the activated CO$_2$, including spontaneous dissociation on Ti$_2$B$_2$, while Nb$_2$B$_2$ remains comparatively stable. Overall, the work demonstrates that the metal center in M$_2$B$_2$ surfaces tunes CO$_2$ adsorption energetics and activation pathways, positioning these 2D diborides as promising candidates for CO$_2$ capture and catalytic conversion technologies.

Abstract

The urgent need to mitigate rising atmospheric CO2 levels motivates the search for stable, efficient, and tunable adsorbent materials. In this study, we employ first-principles density functional theory to investigate the adsorption of CO2 molecules on two-dimensional hexagonal transition-metal diboride monolayers, M2B2 (M = Sc, Y, Ti, Zr, Nb). The adsorption energies, structural distortions, and bonding characteristics are systematically analyzed to understand how the metal center governs CO2 activation. The calculated adsorption energies range from -1.84 to -2.16 eV (or -1.98 to -4.42 eV), with Ti2B2 and Sc2B2 exhibiting the strongest CO2 binding, while Y2B2, Zr2B2, and Nb2B2 show moderately strong chemisorption. Adsorption induces significant molecular activation, evidenced by elongated C-O bonds (1.27-1.29 Angstrom) and bent O-C-O angles (129-132 degrees), compared to the linear gas-phase configuration (1.17 Angstrom, 180 degrees). Charge analysis further reveals substantial electron transfer from the monolayer to CO2, consistent with strong chemisorption and structural deformation. Correspondingly, the shift toward less negative IpCOHP(Ef) values indicates a pronounced weakening of the internal C-O bonds, reflecting increased population of antibonding pi* orbitals. Ab initio molecular dynamics simulations show that the activated CO2 species is thermally sensitive: while most M2B2 surfaces retain stable adsorption at 300 K, Ti2B2 drives spontaneous CO2 dissociation into CO and O, revealing a temperature-assisted activation pathway. These findings highlight how the choice of transition metal tunes electronic interactions, adsorption energetics, and activation pathways on M2B2 surfaces. Overall, this work identifies two-dimensional transition-metal diborides as promising candidates for next-generation CO2 capture and activation technologies.

Figures (12)

• Figure 1: Top (left) and side (right) views of the optimized atomic structure of two-dimensional hexagonal $\mathrm{M_2B_2}$ ($\mathrm{M = Sc, Y, Ti, Zr, Nb}$) monolayer. The purple and green spheres represent the metal (M) and boron (B) atoms, respectively. The black lines indicate the primitive unit cell. The structure exhibits a planar boron honeycomb layer sandwiched between two metal atom layers.
• Figure 2: Phonon dispersion relations of the pristine two-dimensional $\mathrm{M_2B_2}$ ($\mathrm{M = Sc, Y, Ti, Zr, Nb}$) monolayers along the high-symmetry path $\Gamma$--K--M--$\Gamma$ in the Brillouin zone. Panels (a)–(e) correspond to $\mathrm{Sc_2B_2}$, $\mathrm{Y_2B_2}$, $\mathrm{Ti_2B_2}$, $\mathrm{Zr_2B_2}$, and $\mathrm{Nb_2B_2}$, respectively. The absence of imaginary phonon modes confirms the dynamic stability of all $\mathrm{M_2B_2}$ monolayers.
• Figure 3: Schematic representation of the zone-center vibrational modes of the hexagonal $\mathrm{M_2B_2}$ monolayers, illustrating the atomic displacement patterns for the symmetry modes $E_{1u}$, $E_{2g}$, $A_{1g}$, $B_{1g}$, $E_{1g}$, and $A_{2u}$. Red arrows indicate the direction and relative magnitude of atomic motions for metal (purple) and boron (green) atoms within each irreducible representation. These modes correspond to the phonon eigenvectors at the $\Gamma$ point and are used to analyze the vibrational and dynamical stability of the monolayers.
• Figure 4: Top (upper panels) and side (lower panels) views of the optimized geometries of CO$_2$ adsorption on two-dimensional $\mathrm{M_2B_2}$ ($\mathrm{M = Sc, Y, Ti, Zr, Nb}$) monolayers: (a) $\mathrm{Sc_2B_2}$, (b) $\mathrm{Y_2B_2}$, (c) $\mathrm{Ti_2B_2}$, (d) $\mathrm{Zr_2B_2}$, and (e) $\mathrm{Nb_2B_2}$. The red, brown, green, and colored (purple, dark green, blue, light green, and cyan) spheres represent oxygen, carbon, boron, and metal (M) atoms, respectively. The adsorption configurations show that CO$_2$ molecules are chemisorbed on the $\mathrm{M_2B_2}$ surfaces with different binding orientations, depending on the type of transition metal.
• Figure 5: Charge density difference plots for CO$_2$ adsorption on the surface at an isosurface level of 0.15. Yellow and cyan denote charge accumulation and depletion, respectively, indicating interfacial charge transfer upon adsorption.