Origin of the catalytic activity of phosphorus doped MoS2 for oxygen reduction reaction (ORR) in alkaline solution: a theoretical study.
ABSTRACT: Phosphorus doped MoS2 nanosheets (P-doped MoS2) have been reported as excellent oxygen reduction reaction (ORR) catalysts with four-electron selectivity in alkaline solution. By performing density functional theory (DFT) calculations, we revealed the detailed reaction mechanism and the key reaction sites on surface of P-doped MoS2 for ORR catalysis. The double P-doped MoS2 (2P-MoS2) is calculated to be more stable than the single P-doped MoS2 (P-MoS2), and the configuration with two P atoms in neighboring sites exhibits the highest stability. The surface of P-doped MoS2 is found highly active for dissociation of O2. Comparative calculations reveal that P-MoS2 is unsuitable as ORR catalyst due to the high dissociation barrier of H2O (1.19 and 2.06?eV for the first and second adsorbed H2O), while the 2P-MoS2 shows good ORR catalytic activity with much lower dissociation barrier of H2O (0.62?eV). Furthermore, we elucidated that the ORR catalytic activity in 2P-MoS2 originates from the activated S2 atom, which provides an extra adsorption site for the first H2O and the following OH group benefited from the enhanced hydrogen bond interaction. Our results illustrate the mechanisms of doped MoS2 based catalysts and provide rational way for designing ORR catalysts with high activity.
Project description:The development of efficient, stable, and low-cost catalytic material for the oxygen reduction reaction (ORR) is currently highly desirable but challenging. In this work, based on first-principles calculation, the stabilities, catalytic activities and catalytic mechanisms of isolated Au atom supported on defective porous BN (p-BN) have been studied in detail. The results reveal that the defective p-BN anchor Au atom strongly to ensure the stability of Au/p-BN. Based on frontier molecular orbital and charge-density analysis, isolated Au atom supported on porous BN with VN defect (Au/p-BN-VN) is an effective ORR catalyst. Especially, the low barriers of the formation (0.38 eV) and dissociation (0.31 eV) of *OOH and the instability of H2O2 on Au/p-BN-VN catalyst suggest that ORR proceeds via 4-electron pathway. Along the favorable pathway, the reduction of O2 to *OOH is the rate-limiting step with the largest activation barrier of 0.38 eV and the maximum free energy change is 1.88 eV. Our results provide a useful guidance for the design and fabrication of new Au-base catalyst with high-efficiency and are beneficial for the developing of novel isolated metal atom catalysts for ORR.
Project description:Rapid depletion of fossil fuel and increased energy demand has initiated a need for an alternative energy source to cater for the growing energy demand. Fuel cells are an enabling technology for the conversion of sustainable energy carriers (e.g., renewable hydrogen or bio-gas) into electrical power and heat. However, the hazardous raw materials and complicated experimental procedures used to produce electro-catalysts for the oxygen reduction reaction (ORR) in fuel cells has been a concern for the effective implementation of these catalysts. Therefore, environmentally friendly and low-cost oxygen reduction electro-catalysts synthesised from natural products are considered as an attractive alternative to currently used synthetic materials involving hazardous chemicals and waste. Herein, we describe a unique integrated oxygen reduction three-dimensional composite catalyst containing both nitrogen-doped carbon fibers (N-CF) and carbon microspheres (N-CMS) synthesised from apricot sap from an apricot tree. The synthesis was carried out via three-step process, including apricot sap resin preparation, hydrothermal treatment, and pyrolysis with a nitrogen precursor. The nitrogen-doped electro-catalysts synthesised were characterised by SEM, TEM, XRD, Raman, and BET techniques followed by electro-chemical testing for ORR catalysis activity. The obtained catalyst material shows high catalytic activity for ORR in the basic medium by facilitating the reaction via a four-electron transfer mechanism.
Project description:Synthesis of metal-free carbon-based electrocatalysts for oxygen reduction reaction (ORR) to replace the conventional platinum-based catalysts has currently become a hot topic of research. This work proposes an activation-assisted carbonization strategy for the fabrication of nitrogen-doped nanoporous carbon microfibers (Me-CFZ-900) with a high BET surface area (~?929.4?m2?g-1) via using melamine as a promoter/nitrogen source and bamboo-carbon biowastes as the carbon source with the help of a zinc chloride activator. Electrochemical tests showed that the Me-CFZ-900 material has exhibited excellent ORR electrocatalytic activity and long-term stability, and also displayed a quasi-four-electron ORR pathway in alkaline electrolyte. We also find that the graphitic-N may be the catalytically active site for the ORR, but the formation of planar-N can further help to promote the ORR activity for our catalysts. The results open a new space and provide a new idea to prepare valuable porous nanocarbon materials on the basis of carbonaceous solid wastes for catalysis of a wide range of electrochemical reactions in the future.
Project description:Based on first-principles density functional theory calculations, we investigated a modified routine using hydroxyl adsorption that recently demonstrated the controlled growth of MoS2 monolayers. The new growth approach impedes the deposition of a second MoS2 layer; however, the hydroxyl adsorption and its effects have been mostly unexplored. Through this study, we first explored the adsorption behaviors of the hydroxyl radical (OH) on monolayer MoS2 and briefly discussed its effects on the stability and electronic structure. Monolayer MoS2 repels charged OH-, whereas the adsorption of the neutral OH radical is energetically favorable; the corresponding adsorption energies are 0.09 eV and -1.35 eV, respectively. The diffusion barrier of the OH radical on MoS2 is 0.52 eV, indicating that the molecule can quickly diffuse. Next, the study demonstrated that for multiple OH adsorptions, a concerted reaction including OH dissociation and H2O formation is more energetically favorable than the adsorption of two OH molecules by 2.50 eV, which in turn results in a mixed adsorption configuration of O and OH. In addition, we revealed that the OH adsorption creates a mid-gap state and facilitates the reconstruction of the MoS2 edge.
Project description:To increase Pt utilization for oxygen reduction reaction (ORR) in fuel cells, reducing particle sizes of Pt is a valid way. However, poisoning or surface oxidation limits the smallest size of Pt particles at 2.6?nm with a low utility of 20%. Here, using density functional theory calculations, we develop a core-shell Al13@Pt42 cluster as a catalyst for ORR. Benefit from alloying with Al in this cluster, the covalent Pt-Al bonding effectively activates the Pt atoms at the edge sites, enabling its high utility up to 70%. Valuably, the adsorption energy of O is located at the optimal range with 0.0-0.4?eV weaker than Pt(111), while OH-poisoning does not observed. Moreover, ORR comes from O2 dissociation mechanism where the rate-limiting step is located at OH formation from O and H with a barrier of 0.59?eV, comparable with 0.50?eV of OH formation from O and H2O on Pt(111).
Project description:Nanocrystalline multivalent metal spinels are considered as attractive non-precious oxygen electrocatalysts. Identifying their active sites and understanding their reaction mechanisms are essential to explore novel transition metal (TM) oxides catalysts and further promote their catalytic efficiency. Here we report a systematic investigation, by means of soft X-ray absorption spectroscopy (sXAS), on cubic and tetragonal CoxMn3-xO₄ (x = 1, 1.5, 2) spinel oxides as a family of highly active catalysts for the oxygen reduction reaction (ORR). We demonstrate that the ORR activity for oxide catalysts primarily correlates to the partial covalency of between O 2p orbital with Mn4+ 3d t2g-down/eg-up, Mn3+ 3d eg-up and Co3+ 3d eg-up orbitals in octahedron, which is directly revealed by the O K-edge sXAS. Our findings propose the critical influences of the partial covalency between oxygen 2p band and specific metal 3d band on the competition between intermediates displacement of the ORR, and thus highlight the importance of electronic structure in controlling oxide catalytic activity.
Project description:The catalytic oxidation of CO molecule on a thermodynamically stable Cu4 cluster doped MoS2 monolayer is investigated by density functional theory (DFT) where the reaction proceeds in a new formation order of COOOCO* (O2*?+?2CO*???COOOCO*), OCO* (COOOCO*???CO2?+?OCO*), and CO2 (OCO*???CO2) desorption with the corresponding reaction barrier values of 0.220?eV, 0.370?eV and 0.119?eV, respectively. Therein, the rate-determining step is the second one. This low barrier indicates high activity of this system where CO oxidation could be realized at room temperature (even lower). As a result, the Cu4 doped MoS2 could be a candidate for CO oxidation with lower cost and higher activity without poisoning and corrosion problems.
Project description:A layered nanoreactor (zinc hydroxide gallate/nitrate nanohybrid) has been designed as a nano-vessel to confine the gallate/nitrate reaction inside zinc hydroxide layers for production of metal/nitrogen-doped carbon catalysts. Metals (Fe2+, Co2+ and Ni2+) doped and bare zinc hydroxide nitrates (ZHN) were synthesized as the ?-phase hydroxide hosts. By an incomplete ion-exchange process, nitrate anions between the layers of the hosts were then partially replaced by the gallate anions to produce the layered nanoreactors. Under heat-treatment, the reaction between the remaining un-exchanged nitrate anions and the organic moiety inside the basal spacing of each nanohybrid plate resulted in obtaining highly porous 3D metal/nitrogen-doped carbon nanosheets. These catalysts were then used as extremely efficient electrocatalysts for catalyzing oxygen reduction reaction (ORR). This study is intended to show the way to get maximum electrocatalytic activity of the metal/N-doped carbon catalysts toward the ORR. This exceptionally high ORR performance originates from the increased available surface, the best pore size range and the uniform distribution of the active sites in the produced catalysts, all provided by the use of new idea of the layered nanoreactor.
Project description:Hydrogen evolution reaction (HER) is more sluggish in alkaline than in acidic media because of the additional energy required for water dissociation. Numerous catalysts, including NiO, that offer active sites for water dissociation have been extensively investigated. Yet, the overall HER performance of NiO is still limited by lacking favorable H adsorption sites. Here we show a strategy to activate NiO through carbon doping, which creates under-coordinated Ni sites favorable for H adsorption. DFT calculations reveal that carbon dopant decreases the energy barrier of Heyrovsky step from 1.17?eV to 0.81?eV, suggesting the carbon also serves as a hot-spot for the dissociation of water molecules in water-alkali HER. As a result, the carbon doped NiO catalyst achieves an ultralow overpotential of 27?mV at 10?mA?cm<sup>-2</sup>, and a low Tafel slope of 36?mV?dec<sup>-1</sup>, representing the best performance among the state-of-the-art NiO catalysts.
Project description:The development of non-noble metal catalysts for hydrogen revolution in alkaline media is highly desirable, but remains a great challenge. Herein, synergetic ultrathin NiO/MoS2 catalysts were prepared to improve the sluggish water dissociation step for HER in alkaline conditions. With traditional electrode assembly methods, MoS2:NiO-3:1 exhibited the best catalytic performance; an overpotential of 158 mV was required to achieve a current density of 10 mA/cm2. Further, a synergetic ultrathin NiO/MoS2/nickel foam (NF) electrode was assembled by electrophoretic deposition (EPD) and post-processing reactions. The electrode displayed higher electrocatalytic ability and stability, and an overpotential of only 121 mV was needed to achieve a current density of 10 mA/cm2. The improvement was ascribed to the better catalytic environment, rather than a larger active surface area, a higher density of exposed active sites or other factors. DFT calculations indicated that the hybrid NiO/MoS2 heterostuctured interface is advantageous for the enhanced water dissociation step and the corresponding lower kinetic energy barrier-from 1.53 to 0.81 eV.