Layered SiC sheets: A promising metal-free catalyst for NO reduction.

Recently, the catalytic reduction is shown to be an effective method to remove the harmful NO. In terms of the high cost and limited supply of the traditional transition metal-based catalysts, the novel metal-free catalyst is highly desirable for NO reduction. Here, density functional theory (DFT) computations were performed to explore the potentials of layered SiC sheets as a metal-free catalyst for NO reduction. From our DFT results, it can be predicted that layered SiC sheets exhibit superior catalytic activity toward NO reduction. In particular, a dimer mechanism is shown to be more favorable than the direct dissociation one for NO reduction on this metal-free catalyst and a three-step mechanism is involved in this process: (1) the formation of a (NO)2 dimer on layered SiC sheet, followed by (2) its dissociation into N2O+Oad, and (3) the recovery of catalyst by subsequent NO. The trans-(NO)2 dimer might be a necessary intermediate, in which the calculated barrier for the rate-determining step along the energetically most favorable pathway is 0.722 eV. The high reactivity of layered SiC sheets may be attributed to the certain amount of charge transfer from the catalyst to (NO)2 dimer, which shortens the NN bonding and thus stabilizes these systems due to the extra electrons on the dimers. This excellent catalytic activity provides a useful guidance to design the next generation catalysts for NO reduction with lower cost and higher activity.

Theoretical prediction of the mechanisms for defect healing or oxygen doping in a hexagonal boron nitride (h-BN) sheet with nitrogen vacancies by NO2 molecules.

Healing defects in hexagonal boron nitride (h-BN sheet) or doping it with oxygen can modify or restore its physical properties, which would increase its range of potential applications. Thus, it is very important to find an efficient method of healing or a BN sheet or doping it with oxygen. In this work, using density functional theory (DFT) calculations, we identified a mechanism for healing h-BN sheets with nitrogen vacancies (VN) or doping BN sheets with oxygen using NO2 molecules. The results indicate that such reactions involve three steps: (1) the chemisorption of NO2, (2) the incorporation of the N or O atom of NO2 into the defective h-BN sheet, and (3) the removal of the adsorbed O atom or NO molecule. We found that the proposed mechanism is theoretically possible and has the following advantages. First, the barrier is about 0.60 eV for the formation of the O-doped h-BN sheet. For the healing process, because the energy released during NO2 chemisorption (-4.94 eV) completely offsets the subsequent barrier (1.17 eV), a perfect h-BN sheet can easily be achieved by using NO2 and an h-BN sheet with VB defects as reactants. Second, no catalyst is needed, and thus there is no need for a purification step to remove the catalyst. Third, NO2, a toxic gas, can be used as a reactant and will then be reduced to O2 or NO. Fourth, NO2 shows high selectivity for vacancy defect sites. Our findings show that this is an effective theoretical method of synthesizing O-doped h-BN sheets or of healing defective h-BN sheets, which should prove useful in the design of h-BN sheet-based devices.

Theoretical study of oxidation of monovacancies in hexagonal boron nitride (h-BN) sheet by oxygen molecules.

By performing density functional theory (DFT) calculations, we study the oxidation of monovacancies in hexagonal boron nitride (h-BN) sheet by oxygen molecules. In particular, the changes in structural, electronic, and magnetic properties of defective h-BN sheet are mainly addressed. The results indicate that the local magnetic moments of h-BN sheet would remain intact if its dangling bonds around monovacancy are not fully saturated. In contrast, because the dangling bonds of defective h-BN sheet in the lowest energy configurations are completely saturated, the magnetism of the system is mainly contributed by the adsorbed O2. Moreover, these oxidized h-BN sheets are converted to semiconductor with various band gaps, which depends on the adsorption site and chemical bonding of O2 on substrates. Our results may be useful to provide guidance to experimentally study the behavior of h-BN sheet under exposure to the oxygen plasma, which is necessary to estimate its stability in air.

[The influence of Co-Cr-Mo alloys on biological behaviour of L929 mouse fibroblasts].

PURPOSE: To investigate the influence of dental Co-Cr-Mo alloys on biological behaviour of L929 mouse fibroblasts. METHODS: Leaching liquor of medical pure titanium, Co-Cr-Mo alloys, Co-Cr alloys and Ni-Cr alloys was prepared and used to culture the L929 cells for 24 h, 48 h and 72 h, respectively; then the growth of L929 cells was observed under inverted phase contrast microscope, and the cytotoxicity grades of 4 kinds of materials were evaluated by using CCK-8 test. The apoptosis of L929 cells was measured by flow cytometry(FCM) and acridine orange staining was used to observe the L929 cells adhered on the surface of samples under fluorescent microscope. The data was statistically analyzed with SPSS17.0 software package. RESULTS: Under inverted phase contrast microscope, at each time point, L929 cells grew in a good condition except in the Ni-Cr alloys group that minor karyopyknosis was indicating slight celluar cytotoxicity. Optical density (OD), apoptosis rate and cell adhesion number in Co-Cr-Mo alloys group at each time point were significantly less than in medical pure titanium group (P<0.05), and higher than in Co-Cr alloys group (P<0.05) and Ni-Cr alloys group (P<0.05). During observation period, the cytotoxicity grades of medical pure titanium group, Co-Cr-Mo alloys group and Co-Cr alloys group was grade 1, while Ni-Cr alloy group was grade 2, i.e. mild cytotoxicity. CONCLUSIONS: Co-Cr-Mo alloys have no adverse effect on the biological behavior of L929 cells, which may be suitable for dental clinical application with good biocompatibility.