Search

Recently, heteroatom co-doped carbon material catalysts have shown extraordinary oxygen reduction reaction (ORR) catalytic activity, but a lack of understanding of the specific mechanism of the ORR has hindered their further development. In this study, we studied the geometric structure, stability, electronic properties, catalytic sites and detailed ORR pathways of P, N co-doped graphene (PN-Gra) by density functional theory (DFT) calculations, trying to reveal the effect of N and P co-doped graphene on the catalytic activity of the ORR. Our calculations indicate that the PN-Gra structure remains stable at high temperature (1000 K) and P, N atoms with their adjacent four carbon atoms constitute the active region to adsorb O2 more efficiently than single phosphorus or nitrogen doped catalysts. Combining energy barrier and free energy diagram analysis, the most favorable pathway is O2* → O* + O* → OH* + O* → 2OH* → OH* + H2O → 2H2O, which is thermodynamically favorable and the associated energy barrier is low, with an overpotential of 0.96 V. After comprehensive analysis and comparison, P, N co-doping does improve the catalytic ability due to the synergistic effect compared to single doping. We believe that this study provides researchers a way to understand the reaction mechanism and ways to improve the catalytic properties of carbon-based materials. [ABSTRACT FROM AUTHOR]

We selected representative carbon substrates (MnN 4 -Gra and MnN 4 -C60) to study the thermodynamics of toxic gas molecule adsorption processes. Gas molecules are physically adsorbed on MnN 4 -Gra and MnN 4 -C60, which are easily desorbed spontaneously at low temperatures, so only chemisorbed toxic gas molecules are studied here. With the results of the vibrational frequency analysis of the DFT calculation, reveal that ΔG ads (T) increases with increasing T, when ΔG ads (T) changes from negative to positive at Td, where Td is defined as the desorption temperature. According to the second law of thermodynamics, the adsorption of gas molecules on the MnN 4 -Gra or MnN 4 -C60 surface will spontaneously occur when T is below Td. The Td of gas molecules adsorbed on MnN 4 -Gra (or MnN 4 -C60) increased in the order of SO 2 (210 K) < NH 3 (450 K) < CO(770 K) < NO 2 (980 K) [SO 2 (310 K) < H 2 S(400 K) < N 2 O(510 K) < CO/NH 3 (690 K)]. • The most stable adsorption configuration of the molecule on MnN 4 -CS can be determined by molecular electrostatic potential and adsorption energy. • The adsorption properties of MnN 4 -CS are mainly affected by doped atoms, curvature and size of the substrate. • MnN 4 -Gra, MnN 4 -GNR and MnN 4 -GNS are preliminarily determined as suitable for ORR catalysts. MnN 4 -Gra, MnN 4 -GNR, MnN 4 -CNT are suitable as catalysts for CO oxidation. • We subsequently determined the normal operating temperature of the gas sensor by Gibbs free energy and molecular dynamics. Toxic gases destroy the ecological environment and endanger human health, which enlightened the researchers in development of new materials and technologies that effectively collect and capture toxic gases, and convert to harmless gases with the aid of catalyst. Density functional theory (DFT) calculations are performed to evaluate the changes in adsorption energy, charge transfer, sensitivity and Gibbs free energy for MnN 4 -decorated graphene (MnN 4 -Gra), graphene nanoribbons (MnN 4 -GNR), graphene nanosheets (MnN 4 -GNS), carbon nanotubes (MnN 4 -CNT) and C60 fullerenes (MnN 4 -C60) after adsorbing gas molecules, including NO, NO 2 , CO, SO 2 , NH 3 , H 2 S, N 2 O, O 2 , H 2 O and CO 2. The most stable adsorption configuration of the molecule on MnN 4 -CS can be determined by molecular electrostatic potential and adsorption energy. The adsorption properties of MnN 4 -CS are mainly affected by doped atoms, curvature and size of the substrate. Among five tested materials, MnN 4 -C60 has good adsorption energy for toxic gases and is suitable for collecting and trapping gases. MnN 4 -Gra, MnN 4 -GNR and MnN 4 -GNS are evaluated as catalysts for oxygen reduction reaction (ORR), MnN 4 -Gra, MnN 4 -GNR, MnN 4 -CNT are suitable as catalysts for CO oxidation. Also the normal operating temperatures of gas sensors were determined by Gibbs free energy and molecular dynamics (MD). This investigation provides valuable information for designing high-performance gas sensors. [ABSTRACT FROM AUTHOR]