Cooperative Spin Transition of Monodispersed FeN3 Sites within Graphene Induced by CO Adsorption.

The significance of identifying the fundamental mechanism of interactions between adjacent catalytic active centers has long been underestimated. Utilizing density functional theory calculations, we demonstrate controllable cooperative interaction between two nearby Fe centers embedded on nitrogenated graphene aided by CO adsorption. The interconnected adjacent Fe atoms respond cooperatively to CO molecules with communicative structural self-adaption and electronic transformation. The adsorbed CO changes not only the spin of the active site it is attached to but also that of its adjacent site. Consequently, the two adjacent Fe atoms feature unique oscillatory long-range spin coupling. Our theoretical investigation suggests cooperative communication between adjacent active sites on a single-atom catalyst is nontrivial.

Strong current polarization and perfect negative differential resistance in few-FeN4-embedded zigzag graphene nanoribbons.

Ferromagnetic devices have special significance in spintronics. Here, we investigate the electronic structures and transport properties of the experimentally achievable FeN4-embedded armchair and zigzag graphene nanoribbons (FeN4-AGNR and FeN4-ZGNR). The first principles results show that FeN4 induces room-temperature stable ferromagnetic ground states in both AGNRs and ZGNRs, but only significant changes in the band structure of the latter, inducing strong current polarization (nearly 100%) and spin-dependent negative differential resistance (NDR) in the FeN4-ZGNR based devices. We find that the performance of the NDR can be easily enhanced by embedding more FeN4 structures. Its peak-to-valley current ratio (PVCR) rises rapidly and reaches 104 when only 4 FeN4 structures are used. It is revealed that the localized f electrons of the Fe atom and the p electrons of the C atoms at the ribbon edges have the same spin orientation, resulting in a ferromagnetic ground state with a larger magnetic moment, FeN4 induces conductive states around the Fermi level, which are responsible for the observed NDR, and the quite different conductivity of the frontier orbitals in the spin-down and spin-down systems contributes to the strong current polarization. Such intrinsic properties suggest prospective device applications of the FeN4-ZGNRs in spintronics.

Conversion of Dinitrogen to Ammonia by FeN3-Embedded Graphene.

Nitrogen fixation is one of the most important issues but a long-standing challenge in chemistry. Here, we propose FeN3-embedded graphene as the catalyst for nitrogen fixation from first-principles calculations. Results show that in view of the chemical coordination, the FeN3 center is highly spin-polarized with a localized magnetic moment substantially to promote N2 adsorption and activate its inert N-N triple bond. The synergy between the graphene and FeN3 equips the system with novel features for the catalytic conversion of the activated N2 into NH3 via a six-proton and six-electron process, following three possible reaction pathways at room temperature. Our findings provide a rational paradigm for catalytic nitrogen fixation that would be conducive to ammonia production.

Uniform and perfectly linear current-voltage characteristics of nitrogen-doped armchair graphene nanoribbons for nanowires.

Metallic nanowires with desired properties for molecular integrated circuits (MICs) are especially significant in molectronics, but preparing such wires at a molecular level still remains challenging. Here, we propose, from first principles calculations, experimentally realizable edge-nitrogen-doped graphene nanoribbons (N-GNRs) as promising candidates for nanowires. Our results show that edge N-doping has distinct effects on the electronic structures and transport properties of the armchair GNRs and zigzag GNRs (AGNRs, ZGNRs), due to the formation of pyridazine and pyrazole rings at the edges. The pyridazine rings raise the Fermi level and introduce delocalized energy bands near the Fermi level, resulting in a highly enhanced conductance in N-AGNRs at the stable nonmagnetic ground state. Especially for the family of AGNRs with widths of n = 3p + 2, their semiconducting characteristics are transformed to metallic characteristics via N-doping, and they exhibit perfectly linear current-voltage (I-V) behaviors. Such uniform and excellent features indicate bright application prospects of the N-AGNRs as nanowires and electrodes in molectronics.

Cooperative Single-Atom Active Centers for Attenuating the Linear Scaling Effect in the Nitrogen Reduction Reaction.

Cooperative effects of adjacent active centers are critical for single-atom catalysts (SACs) as active site density matters. Yet, how it affects scaling relationships in many important reactions such as the nitrogen reduction reaction (NRR) is underexplored. Herein we elucidate how the cooperation of two active centers can attenuate the linear scaling effect in the NRR through a first-principle study on 39 SACs comprised of two adjacent (∼4 Å apart) four N-coordinated metal centers (MN4 duo) embedded in graphene. Bridge-on adsorption of dinitrogen-containing species appreciably tilts the balance of adsorption of N2H and NH2 toward N2H and thus substantially loosens the restraint of scaling relationships in the NRR, achieving low onset potential (V) and direct N≡N cleavage (Mo, Re) at room temperature, respectively. The potential of the MN4 duo in the NRR provides new insight into circumventing the limitations of scaling relationships in heterogeneous catalysis.

Impact of Active Site Density on Oxygen Reduction Reactions Using Monodispersed Fe-N-C Single-Atom Catalysts.

Exploring the impact of active site density on catalytic reactions is crucial for reaching a more comprehensive understanding of how single-atom catalysts work. Utilizing density functional theory calculations, we have systematically investigated the neighboring effects between two adjacent Fe-N-C sites of monodispersed Fe-N-C single-atom catalysts on oxygen reduction reaction (ORR). While the thermodynamic limiting potential (UL) is strongly dependent on the intersite distance and the nature of adjacent active sites in FeN3, it is almost invariable in FeN4 until two FeN4 sites are ∼4 Šapart. Further, under certain conditions, an otherwise unfavorable physisorbed-O2-initiated 2e- pathway becomes feasible due to charge transfer between reactive species and graphene support. Our results cast new insight into the rational design of high-density single-atom catalysts and may create an alternative route to manipulate their catalytic activities.

Cooperative Nitrogen Activation and Ammonia Synthesis on Densely Monodispersed Mo-N-C Sites.

The production of ammonia from nitrogen reduction reaction (NRR) under mild conditions is one of the most challenging issues in modern chemistry. The linear scaling relationship between the adsorption energies of -N2H and -NH2 on a single active site is a well-established bottleneck. By investigating a series of densely monodispersed Mo-N-C sites embedded in graphene using first-principles calculations, we found that previously underappreciated neighboring effects between adjacent active sites may help break the limit: they not only improve the energetics of potential determining steps of NRR but also promote an alternative associative mechanism based on a cooperative bridge-on adsorption of N2 by two Mo-N-C sites of ∼6 Šapart. Further, a barrier of 0.71 eV for N-N bond dissociation is achieved by proper ratio of coordinated C/N atoms of Mo. Our work suggests the cooperation of two adjacent active sites may offer an alternative strategy of nitrogen fixation.