Identifying of Pure and Defected Ti2C Materials Using Gas Probe Molecules: First Principles Calculations

Employing first principles calculations, we systematically investigated the geometrical and electronic structures of pure, titanium defected (DTi) and carbon defected (DC) Ti2C materials. We found the defected Ti2C exhibits stronger metallic property than pure Ti2C due to the enhanced density of Ti-d orbital state near the Fermi level. We then studied the adsorption as well as the infrared spectrum (IR) response of the four kinds of gas molecules (CH4 , NH3 , CO and NO) on pure, DTi and DC Ti2C surfaces. Simulations show that CO and NO molecules are chemically adsorbed on all Ti2C surface with similar adsorption sites. However, CH4 and NH3 molecules would be dissociated on Ti2C surface. Negative values of crystal orbital Hamilton population as well as the PDOS calculations show that the red shift in IR spectra of CO and NO molecules originates from the decreasing bonding strength of probe molecules. The present work provides rich insight for the adsorption and identification for different Ti2C materials.

Tunable Electric and Magnetic Properties of Transition Metal@Nx Cy -Graphene Materials by Different Metal and Defect Types

Transition metal@N_x C_y -graphene (TM@N_x C_y -GR) materials have been widely used as redox reaction catalysts in the field of fuel cells due to their low cost and high performance. In the present work, we systematically investigate the effect of different metal and defect types on the electro-magnetic properties of TM@N_x C_y -GR materials using first principles calculations. Our simulations show that TM@N₃ -GR (the minimum defect size) and TM@N₇ -GR (the maximum defect size) materials always possess metallic property regardless the metal type. However, doping different TM can regulate the medium defects (TM@N₂ C₂ -GR-I and TM@N₂ C₂ -GR-II) among metallicity, half-metallicity and semi-conductivity. In addition, we found that different TM and defect type largely affects the magnetic moment. The spin density and projected density of state calculations show that the net charges of the defect structure are mainly located near the hole, and the magnetic regulation comes from the coupling of TM-d orbital with carbon (nitrogen)-s(p) orbitals. The present study provides abundant valuable information for the TM@N_x C_y -GR materials designs and applicants in the future.

Novel joint catalytic properties of Fe and N co-doped graphene for CO oxidation

Using density functional theory, we have performed detailed calculations of the joint catalytic activity of graphene co-doped with Fe and N atoms. The Fe atom can be located on single vacancy graphene and acts as the active site. Due to the strong attraction of the Fe ion, the O-O bond length of the O2 molecule is elongated, which decreases the bonding energy between the O atoms. The energy barrier of CO oxidization is 0.84 eV. When N atoms are doped into the graphene, the interactions between the Fe ions and O2 molecules are stronger, and the O-O bond lengths are elongated further, which makes the desorption of the quasi-CO2 molecule easier. The energy barriers are reduced to 0.62 eV, 0.49 eV, and 0.33 eV for graphene doped with one, two and three N atoms, respectively. The O atom remaining on the Fe ion can form a CO2 molecule with an additional CO molecule. The produced CO2 molecule can be released with a small or even zero energy barrier by adsorbing an O2 molecule. The adsorbed O2 molecule is then involved in the next reaction process, and the material can be used as a recycled catalyst.

Diameter-dependent structural and electronic property of fused porphyrin nanotubes: A density functional study

We have systematically carried out a density functional theory-based investigation to understand the structural and electronic properties of various fused metalloporphyrin nanotubes (MPNT; M = Sc and Ti) by varying their diameters ranging from 7.91 Å to 18.70 Å for ScPNT and 7.90 Å to 18.59 Å for TiPNT. Binding energies and curvature energies are calculated to access the binding strength and stability of the nanotubes (NTs). From band structure and density of states, it is observed that the ScPNTs are metallic in nature and TiPNTs are semiconductors with small band gaps. The energy gap increases with increasing tube diameter. Our study also indicates that the transition metal atoms play an important role in determining the electrical nature (metallic or semiconducting) of the NTs. Furthermore, work functions for the fused NTs are found to decrease with increasing tube diameter. These results may have direct relevance to the technological applications in terms of band gap engineering or controlled thermionic emission.

Identifying Iron-Nitrogen/Carbon Active Structures for Oxygen Reduction Reaction under the Effect of Electrode Potential

Transition-metal-nitrogen/carbon (TM-N/C) materials are promising alternatives to Pt-based oxygen reduction reaction (ORR) electrocatalysts of fuel cells. Identifying the highly active sites is the prerequisite for the design of high-performance electrocatalysts, in which the density functional theory (DFT) calculation is an important tool. However, the DFT simulation was usually conducted with a charge-neutral model, which is far away from the working condition, that is, under certain potentials. Herein, by using the DFT method with the explicit consideration of electrode potential, we systematically compared the activities of the Fe-N/C moieties previously proposed in the literature and identified the best one. This study not only demonstrates the significance of the electrode potential in computational electrochemistry but also suggests a feasible experimental strategy to increase the ORR performance of Fe-N/C electrocatalysts by creating edges defects and coordinating with the axial ligands on the Fe center, which is of practical significance for exploring the advanced non-precious-metal-based ORR electrocatalysts and related devices.

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 (U_L) 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 Å apart. 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.

Theoretical investigation on catalytic mechanisms of oxygen reduction and carbon monoxide oxidation on the MnNx system

Integrating all structures of the MnN_x system, MnN₄ shows the best ORR and COOR catalytic performance.

Graphdiyne doped with sp-hybridized nitrogen atoms at acetylenic sites as potential metal-free electrocatalysts for oxygen reduction reaction

Exploring metal-free electrocatalysts with high efficiency and lower cost for oxygen reduction reaction (ORR) is necessary to realize the commercialization of fuel cells. In this paper, the ORR mechanisms on nitrogen-doped graphdiyne (GDY) are investigated using the first principles calculations. It is found that the GDY doped with sp-hybridized N at acetylenic sites can activate molecular oxygen (O₂). The kinetically most favorable reaction pathway is O₂  →  OOH  →  O  +  H₂O  →  OH  →  H₂O, which is an efficient four-electron ORR process. The first reaction step O₂  →  OOH is the rate determining step (RDS), and the energy barrier is 0.61 eV. The energy barrier of RDS is smaller than that of pure Pt (0.80 eV). Therefore, these results illustrate that sp-hybridized N-doped GDY is a promising carbon-based metal-free ORR catalyst.

Effect of nitrogen-doping configuration in graphene on the oxygen reduction reaction

In this study, we investigate the oxygen reduction reaction (ORR) reactivity of nitrogen-doped graphene by using density functional theory (DFT), a computational quantum mechanical technique. Four doping configurations and five models were comprehensively studied: quaternary nitrogen (NQ), pyrrolic nitrogen (N5), two forms of pyridinic nitrogen (N6, N6nH) and three-pyridinic nitrogen (3N6). Models for possible sites during each step of ORR were set up and visualized to provide a platform to calculate the free energy of the reaction pathway to determine the suitability of each doping scenario. Associative mechanisms were displayed by all models except N5, which showed dissociative mechanism. The calculated free energy pathways demonstrate that the ranking of the reactivity for ORR by different nitrogen configurations from most reactive to least reactive is N6, NQ, N6nH, 3N6, and N5. Spin density and charge density aid in describing levels of reactivity.

Systematic exploration of N, C configurational effects on the ORR performance of Fe–N doped graphene catalysts based on DFT calculations

Metal single-atom catalysts (MSATs), such as Fe–N coordination doped sp²-carbon matrices, have emerged as a promising oxygen reduction reaction (ORR) catalyst to replace their costly platinum (Pt) based counterparts in fuel cells. In this work, we employ density functional theory (DFT) to systematically discuss the electronic-structure and surface-stress effects of N, C configurations on Fe–N doped graphene in single and double vacancy. The formation energy (E_f) of Fe–N-gra dropped off with the increase of N atoms incorporated for both single and double vacancy groups. The theoretical overpotentials on Fe–N–C sites were calculated and revealed that moderate N-doping levels and doping configuration could enhance the ORR activity of Fe–N coordination structures in the double vacancy and that doping N atoms is not effective for ORR activity in single vacancy. By exploring the d-band centers, we found that ligand effects and surface tension effects contribute to the modification of the d-band centers of metal Fe atoms. An optimum Fe–N–C ORR catalyst should exhibit moderate surface stress properties and an ideal N, C ligand configuration. This study provides new insight into the effects of N atom doping in Fe–N-gra catalysts and could help guide the rational design of high-performance carbon-based ORR electrocatalysts.

An optimum Fe–N–C ORR catalyst should exhibit a moderate surface stress property and an ideal N, C ligand configurations that results in a moderate interaction between the ORR intermediates and its surface sites.

Improvement of O2 adsorption for α-MnO2 as an oxygen reduction catalyst by Zr4+ doping

Zr4+ doped α-MnO2 nanowires were successfully synthesized by a hydrothermal method. XRD, SEM, TEM and XPS analyses indicated that Mn3+ ions, Mn4+ ions, Mn4+δ ions and Zr4+ ions co-existed in the crystal structure of synthesized Zr4+ doped α-MnO2 nanowires. Zr4+ ions occupied the positions originally belonging to elemental manganese in the crystal structure and resulted in a mutual action between Zr4+ ions and Mn3+ ions. The mutual action made Mn3+ ions tend to lose their electrons and Zr4+ ions tend to get electrons. Cathodic polarization analyses showed that the electrocatalytic activity of α-MnO2 for oxygen reduction reaction (ORR) was remarkably improved by Zr4+ doping and the Zr/Mn molar ratio notably affected the ORR performance of the air electrodes prepared by Zr4+ doped α-MnO2 nanowires. The highest ORR current density of the air electrodes prepared by Zr4+ doped α-MnO2 nanowires in alkaline solution appeared at Zr/Mn molar ratio of 1 : 110, which was 23% higher than those prepared by α-MnO2 nanowires. EIS analyses indicated that the adsorption process of O2 molecules on the surface of the air electrodes prepared by Zr4+ doped α-MnO2 nanowires was the rate-controlling step for ORR. The DFT calculations revealed that the mutual action between Zr4+ and Mn3+ in Zr4+ doped α-MnO2 nanowires enhanced the adsorption process of O2 molecules.

A density functional study on the oxygen reduction reaction mechanism on FeN2-doped graphene

The rational design of heteroatom doped graphene as a highly active and non-noble oxygen reduction reaction (ORR) electrocatalyst is significant for the commercial applications of fuel cells.