Impact of the dopant-induced ensemble structure of hetero-double atom catalysts in electrochemical NH3 production

Abstract

Using spin-polarized density functional theory (DFT) calculations, we examined electrochemical N₂ reduction (N₂RR) toward NH₃ production on hetero-RuM (M = 3d transition metals) double atom catalysts supported on defective graphene by means of analysis on the geometric ensemble structure, the N₂RR mechanism, the decoupling of strain, dopant and configurational effects and the d-orbital resolved density of states (ORDOS) (d_z², d_xz, d_yz, d_xy, and d_x²–y²) on the hetero-double atoms. In addition, we computationally screened novel catalysts by exploring 4d, 5d and p block metals as the hetero-M metals in the RuM system. First, we found the significantly enhanced N₂RR activity of inclined pentagon M (Fe, Mn, and Sc) double atom catalysts (RuFe has the highest activity) compared to the homo-Ru₂ double atom catalyst. Our DFT calculations on the interplay of strain, dopant and configurational effects in the inclined pentagon M (Fe, Mn, and Sc) double atom catalysts predicted that (1) the dopant effect was the promoter to improve the N₂RR activity of RuSc and RuMn, (2) the tensile strain (RuSc) tended to reduce the NH₃ productivity via the N₂RR, while the effect of compressive strain (RuFe and RuMn) was insignificant, and (3) the dopant-support interaction induced a unique inclined pentagon M double atom ensemble structure, which leads to the large reduction of the N₂RR activity of the hetero-RuSc double atom but the activity increases for the hetero-RuFe and RuMn cases. Finally, our DFT calculation on the analysis of the p–d (d_z², d_xz, d_yz, d_xy, and d_x²–y²) orbital overlap identified the key d orbitals in determining the descriptor (NH₂ adsorption energy) for representing the N₂RR. That is, the orbitals (d_z², d_xz, and d_yz) having an orientation toward the z direction in the horizontal Ru₂ double atom played an important role in determining the NH₂ adsorption process, while for the inclined pentagon M double atoms (RuFe, RuSc, and RuMn), the d_xz and d_xy orbitals were found to be essential for the modification of NH₂ adsorption energy. Finally, a descriptor based DFT search additionally discovered promising hetero-RuOs and RuIr double atom catalysts. This study highlights that the dopant engineering of hetero-double atom catalysts supported on defective graphene can significantly modify the electrochemical reactivity, particularly by the dopant type and geometric ensemble structure.