RESUMO
In this work, we present a comprehensive study on the role of metal species in MOF-based Me-N-C (mono- and bimetallic) catalysts for the hydrogen evolution reaction (HER). The catalysts are investigated with respect to HER activity and stability in alkaline electrolyte. On the basis of the structural analysis by X-ray diffraction, X-ray-induced photoelectron spectroscopy, and transmission electron microscopy, it is concluded that MeN4 sites seem to dominate the HER activity of these catalysts. There is a strong relation between the amount of MeN4 sites that are formed and the energy of formation related to these sites integrated at the edge of a graphene layer, as obtained from density functional theory (DFT) calculations. Our results show, for the first time, that the combination of two metals (Co and Mo) in a bimetallic (Co,Mo)-N-C catalyst allows hydrogen production with a significantly improved overpotential in comparison to its monometallic counterparts and other Me-N-C catalysts. By the combination of experimental results with DFT calculations, we show that the origin of the enhanced performance of our (Co,Mo)-N-C catalyst seems to be provided by an improved hydrogen binding energy on one MeN4 site because of the presence of a second MeN4 site in its close vicinity, as investigated in detail for our most active (Co,Mo)-N-C catalyst. The outstanding stability and good activity make especially the bimetallic Me-N-C catalysts interesting candidates for solar fuel applications.
RESUMO
Influence of metal oxide (MO) supports on nanoparticle (NP) catalysts is still under investigation. Theoretical studies demonstrate that active defect sites on the surface of a MO support can affect the structure and activity of metal clusters. In the present work, we show that even defect-free surfaces of MOs can cause considerable restructuring and accumulation of interfacial charges on Pt NPs of size 1 nm (Pt55). Independent of the type of MO support, we find that supported Pt55 behaves like a conductor since the binding energy of a test adsorbate on top of it is similar to that on an intact Pt55. However, adsorption energy at binding sites close to the perimeter of the nanoparticle/support interface can vary by 1.8 eV depending on the distance between the adsorbate and surface cations (possibility of forming ionic bonds) as well as the amount and sign of charges (ionization energy) of interfacial Pt atoms.
RESUMO
Mechanism of Li diffusion at the LiCoO2(101[combining overline]4) surface and in bulk LiCoO2 is studied using density functional theory calculations. We find that there is almost no barrier for the diffusion of Li between the two topmost surface layers. The results show that Li intercalation occurs by the diffusion of Li ions from the first layer to the divacancy of Li sites created by removal of two neighboring Li ions in the first and second layer. However, Li deintercalation occurs by the diffusion of Li ions from the second layer to the missing row of topmost Li sites. The energy barrier for the process of intercalation/deintercalation of Li between the second and third surface layers is also lower than that in the bulk. This finding indicates that nanosized LiCoO2 with a large surface area/volume ratio is a promising cathode material for fast charging/discharging Li-ion batteries.
RESUMO
This paper presents the results of an experimental study to understand the dominant mechanism in bond strength between dental resin agent and zirconia ceramic by investigating the effects of different surface treatments. Effects of two major mechanisms of chemical and micromechanical adhesion were evaluated on bond strength of zirconia to luting agent. Specimens of yttrium-oxide-partially-stabilized zirconia blocks were fabricated. Seven groups of specimens with different surface treatment were prepared. 1) zirconia specimens after airborne particle abrasion (SZ), 2) zirconia specimens after etching (ZH), 3) zirconia specimens after airborne particle abrasion and simultaneous etching (HSZ), 4) zirconia specimens coated with a layer of a Fluorapatite-Leucite glaze (GZ), 5) GZ specimens with additional acid etching (HGZ), 6) zirconia specimens coated with a layer of salt glaze (SGZ) and 7) SGZ specimens after etching with 2% HCl (HSGZ). Composite cylinders were bonded to airborne-particle-abraded surfaces of ZirkonZahn specimens with Panavia F2 resin luting agent. Failure modes were examined under 30× magnification and the effect of surface treatments was analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). SZ and HSZ groups had the highest and GZ and SGZ groups had the lowest mean shear bond strengths among all groups. Mean shear bond strengths were significantly decreased by applying a glaze layer on zirconia surfaces in GZ and SGZ groups. However, bond strengths were improved after etching process. Airborne particle abrasion resulted in higher shear bond strengths compared to etching treatment. Modes of failure varied among different groups. Finally, it is concluded that micromechanical adhesion was a more effective mechanism than chemical adhesion and airborne particle abrasion significantly increased mean shear bond strengths compared with another surface treatments.