Abstract:

Designing efficient single-atom catalysts (SACs) with high selectivity for the electrocatalytic reduction of nitrate to ammonia formation is both crucial and challenging. This challenge arises due to the intricate and competitive electronic interactions among intermediates, metal active centers, and coordination environments. In this work, we present a comprehensive investigation detailing how to enhance the activity and selectivity of the electrocatalytic nitrate reduction reaction (NO₃RR) by transitioning from single-layer SACs to bilayer SACs (BSACs). This enhancement is achieved through axial d–d orbital hybridization, as elucidated by a systematic study of 27 SACs and BSACs utilizing density functional theory (DFT) calculations. Considering potential pathways involving O-terminal, N-terminal, NO-terminal, and NO-dimer configurations, our calculations reveal that among monolayer SAC candidates, Ti-Pc and V-Pc exhibit low limiting potentials (U_L) of −0.24 and −0.48 V, respectively. Furthermore, analyses of formation energy, dissolution potential, and ab initio molecular dynamics results demonstrate the robust stability of these catalysts under reaction conditions. In these single-layer transition metal (TM)-Pc complexes, the d-band energy levels and occupation numbers are influenced by d_xz/d_yz and p_z orbital hybridizations. Notably, the presence of axial d_z² orbitals introduces a novel avenue for fine-tuning d-band characteristics and reactivity through d_z²–d_z² interactions. Building on these insights, the formation of BSACs using Ti-Pc and V-Pc as substrates, facilitated by axial d–d orbital hybridization, offers a distinctive approach to modulating the catalytic performance of NO₃RR. Significantly, we establish a two-dimensional volcano correlation encompassing the d-band center (ε_d), d_xz + d_yz orbital occupation numbers, and U_L to describe NO₃RR catalytic efficacy. Optimal BSACs should possess concurrent appropriate ε_d and d_xz + d_yz occupation numbers. Remarkably, Ti-Mo and Ti-Ta BSACs emerge as exceptional NO₃RR catalyst candidates, both displaying a remarkably low U_L of −0.13 V. The hybridization between d_z²–d_z² orbitals heightens charge transfer and structural stability within double-layer metals. The scarcity of contiguous metal sites introduces a substantial energy barrier hindering NO₂, NO, and N₂ formation, effectively suppressing NO₃RR by-products. In summation, this investigation imparts valuable insights into effectively enhancing nitrate reduction on SACs and BSACs, offering valuable guidance for advancing electrocatalyst development.

Key words: Nitrate reduction, First-principles calculation, Single-atom catalysts, Bilayer single-atom catalysts, Axial orbital hybridization