Coupling a Main-Group Metal with a Transition Metal to Create Biatom Catalysts for Nitric Oxide Reduction

Developing efficient biatom catalysts for renewable energies is becoming increasingly worthwhile, yet it remains a great challenge. Herein, by means of large-scale first-principles calculations, we report a design principle of coupling main-group metal with transition metal (TM) to explore biatom catalysts for nitric oxide reduction reaction (NORR), namely, $\mathrm{Mg}/\mathrm{Al}/\mathrm{Ga}-\mathrm{TM}$ dimers anchored on nitrogen-doped graphene. We propose a comprehensive screening strategy to recognize promising candidates of such catalysts. Following this strategy, we screen $\mathrm{Mg}-\mathrm{Ni}@\mathrm{NC}$ and $\mathrm{Ga}-\mathrm{Cr}@\mathrm{NC}$ out of 24 candidates as promising biatom catalysts with high activity and selectivity for direct $\mathrm{NO}$-$\mathrm{to}$-${\mathrm{NH}}_3$ conversion. Such high catalytic activity is related to the synergetic interatomic interactions. Moreover, based on the interplay between main-group metal and TM centers, we propose a “donation-backdonation-redonation” mechanism to characterize $\mathrm{NO}$ activation. In addition, $\mathrm{\Delta }{\epsilon }_{s/p\to d}$ is identified as an efficient quantitative descriptor to prereduce the number of such catalyst candidates. Our work opens a strategy for rational design of biatom catalysts toward NORR.

Figure 1

(a) Two crystal structures of $\mathrm{Mg}/\mathrm{Al}/\mathrm{Ga}-\mathrm{TM}@\mathrm{NC}$. (b) Flowchart (top panel) of the five-step screening strategy for identifying HBCs toward $\mathrm{NO}$-$\mathrm{to}$-${\mathrm{NH}}_3$ conversion, and the corresponding color codes map (bottom panel). The gray, pink, red, yellow, and blue color codes in (b) refer to the excluded systems from each step, and the green color codes refer to the final identified candidates. (c) Binding energies of the most stable configuration of $\mathrm{Mg}/\mathrm{Al}/\mathrm{Ga}-\mathrm{TM}@\mathrm{NC}$.

Figure 2

(a) Adsorption energies of $\mathrm{NO}$ on (a) $\mathrm{Mg}-\mathrm{TM}@\mathrm{NC}$, (b) $\mathrm{Al}-\mathrm{TM}@\mathrm{NC}$ and (c) $\mathrm{Ga}-\mathrm{TM}@\mathrm{NC}$. (d) Schematic diagram of the “donation-backdonation-redonation” mechanism for $\mathrm{Mg}-\mathrm{TM}@\mathrm{NC}$.

Figure 3

(a) Scaling relationship between the transferred charge and $\mathrm{N}—\mathrm{O}$ bond length of adsorbed $\mathrm{NO}$ on $\mathrm{Mg}/\mathrm{Al}/\mathrm{Ga}-\mathrm{TM}@\mathrm{NC}$. Top views of the differential charge densities of (b) $\mathrm{Mg}-\mathrm{Ni}@\mathrm{NC}$, (c) $\mathrm{Mg}-\mathrm{Mg}@\mathrm{NC},$ and (d) $\mathrm{Ni}-\mathrm{Ni}@\mathrm{NC}$. The isosurface value is 0.004 e Å⁻³, and green and blue isosurfaces represent electron accumulation and depletion areas, respectively. (e) ΔG_*NO as a function of Δε_s→d. (f) ΔG_*NO as a function of Δε_p→d. (g) Schematic diagrams of the interactions between s-p orbitals of main-group metals and d orbitals of TM atoms. (h) Projected density of states of $\mathrm{Mg}$-$\mathrm{s}$ orbital for $\mathrm{Mg}-\mathrm{Mg}@\mathrm{NC}$. (i) Projected density of states of $\mathrm{Ni}$-$\mathrm{d}$ orbital for $\mathrm{Ni}-\mathrm{Ni}@\mathrm{NC}$ (top panel) and $\mathrm{Ni}$-$\mathrm{d}/\mathrm{Mg}$-$\mathrm{s}$ orbitals for $\mathrm{Mg}-\mathrm{Ni}@\mathrm{NC}$ (bottom panel). The Fermi level is set to 0 eV.

Figure 4

(a) Gibbs free energies of *HNO and *NOH on $\mathrm{Mg}/\mathrm{Al}/\mathrm{Ga}-\mathrm{TM}@\mathrm{NC}$. (b) Theoretical limiting potentials for 24 candidate systems through the most favorable reaction pathway. (c) Comparison of theoretical limiting potentials for the productions of ${\mathrm{NH}}_3,{\mathrm{N}}_2\mathrm{O},{\mathrm{N}}_2$ on $\mathrm{Mg}$-$\mathrm{Ni}@\mathrm{NC},\mathrm{Mg}$-$\mathrm{Cu}@\mathrm{NC}$, and $\mathrm{Ga}$-$\mathrm{Cr}@\mathrm{NC}$.