Bi-Layer Single Atom Catalysts Boosted Nitrate-to-Ammonia Electroreduction with High Activity and Selectivity

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 (UL) 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 dxz/dyz and pz orbital hybridizations. Notably, the presence of axial dz2 orbitals introduces a novel avenue for fine-tuning d-band characteristics and reactivity through dz2-dz2 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 (εd), dxz + dyz orbital occupation numbers, and UL to describe NO3RR catalytic efficacy. Optimal BSACs should possess concurrent appropriate εd and dxz + dyz occupation numbers. Remarkably, Ti-Mo and Ti-Ta BSACs emerge as exceptional NO3RR catalyst candidates, both displaying a remarkably low UL of −0.13 V. The hybridization between dz2-dz2 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.