Single-Atom Anchored g-C3N4 Monolayer as Efficient Catalysts for Nitrogen Reduction Reaction.

Electrochemical N2 reduction reaction (NRR) is a promising approach for NH3 production under mild conditions. Herein, the catalytic performance of 3d transition metal (TM) atoms anchored on s-triazine-based g-C3N4 (TM@g-C3N4) in NRR is systematically investigated by density functional theory (DFT) calculations. Among these TM@g-C3N4 systems, the V@g-C3N4, Cr@g-C3N4, Mn@g-C3N4, Fe@g-C3N4, and Co@g-C3N4 monolayers have lower ΔG(*NNH) values, especially the V@g-C3N4 monolayer has the lowest limiting potential of -0.60 V and the corresponding limiting-potential steps are *N2+H++e-=*NNH for both alternating and distal mechanisms. For V@g-C3N4, the transferred charge and spin moment contributed by the anchored V atom activate N2 molecule. The metal conductivity of V@g-C3N4 provides an effective guarantee for charge transfer between adsorbates and V atom during N2 reduction reaction. After N2 adsorption, the p-d orbital hybridization of *N2 and V atoms can provide or receive electrons for the intermediate products, which makes the reduction process follow acceptance-donation mechanism. The results provide an important reference to design high efficiency single atom catalysts (SACs) for N2 reduction.

Theoretical exploration of the structural, electronic and optical properties of g-C3N4/C3N heterostructures.

Integration of graphene-like carbon nitride materials is essential for nanoelectronic applications. Using density-functional theory (DFT), we systematically investigate the structural, electronic and optical properties of a s-triazine-based g-C3N4/C3N heterostructure under different modified conditions. The g-C3N4/C3N van der Waals heterostructure (vdWH) formed has an indirect bandgap with type-II band alignment and the band structures can be tuned from type-II band alignment to type-I band alignment by applying biaxial strains and external electric fields (Efield). Compared to single transition metal (TM) atoms at g-C3N4/C3N surfaces, the TM atoms anchored in the interlayer region exhibit more stability, and the corresponding bandgaps are changed from 0.19 eV to 0.61 eV. In addition, the g-C3N4/C3N heterostructure has a strong absorption coefficient in the ultraviolet-visible light region along the x direction. It is found that compressive strain has a large influence on the absorption coefficient of the g-C3N4/C3N system. With the increased compressive strain, the absorption spectra in the visible light region disappeared. Tensile strain has a slight effect on the absorption range, but causes a red shift of the absorption spectrum. In comparison, the light absorption coefficient of the g-C3N4/C3N system remains almost unchanged under the Efield conditions. In summary, the formation of a s-triazine-based g-C3N4/C3N heterostructure has shown potential for applications in nanoelectronic and optoelectronic devices.

Tuning the adsorption and interaction of CO and O2 on graphene-like BC3-supported non-noble metal atoms.

Research into suitable substrate-supported single-atom catalysts has become a major challenge for electrochemical sensors and energy devices. Firstly, we investigate the adsorption properties of metal atoms (MA = Fe, Co, Ni, Cu and Al) on pristine and defective BC3 sheets through using first-principles calculations. It is found that the MA-doped BC3 configurations (MA-BC3) are quite stable at high temperature and the positively charged MAs as surface active sites can effectively regulate the stability of reactive gases. Secondly, the adsorption of individual O2 molecules is more stable than that of CO molecules, which can modify the electronic and magnetic properties of MA-BC3 systems. Moreover, the possible reaction processes of CO oxidation on the Fe-BC3 substrate are comparably analyzed through the Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanisms. In the LH mechanism, the coadsorbed O2 and CO as starting materials start to form an OOCO complex with a smaller energy barrier (0.38 eV), which is an energetically more favorable process than that of the OOCO (0.65 eV) or CO3 complex (0.42 eV) formed through ER mechanisms. This result indicates that the functionalized MA-BC3 sheets have low cost and high activity.

Theoretical study on geometric, electronic and catalytic performances of Fe dopant pairs in graphene.

The formation geometries, electronic structures and catalytic properties of monovacancy and divacancy graphene sheets with two embedded Fe dopants (2Fe-MG and 2Fe-DG) have been systematically investigated using the first-principles calculations. It was found that the configuration of 2Fe-DG is slightly more stable than that of 2Fe-MG sheets and the two doped Fe atoms in graphene (2Fe-graphene) as active sites could regulate the stability of gas molecules. In addition, the adsorption of O2 and CO molecules could modulate the electronic and magnetic properties of 2Fe-graphene systems. Moreover, the adsorption behaviors of reactants could determine the reaction pathway and energy barrier of the catalytic oxidation of CO. On the 2Fe-graphene substrates, the adsorptive decomposition of O2 molecules (<0.20 eV) and the subsequent Eley-Rideal (ER) reaction (2Oads + 2CO → CO2) (<0.60 eV) have low energy barriers. In comparison, the CO3 complex is quite stable and its formation needs to overcome a higher energy barrier (>0.90 eV). Hence, the dissociation of O2 as an initial step is an energetically more favored process. These results provide valuable guidance for the design of functionalized graphene-based devices.