RESUMO
Chiral bipyrimidine-bridged dinuclear LnIII complexes of general formula [(µ-bipym){((+)-tfacam)3Ln}2] and [(µ-bipym){((-)-tfacam)3Ln}2], have been prepared from the assembly of Ln(AcO)3·nH2O (LnIII = Dy, Gd), (+)/(-)-3-(trifluoroacetyl)camphor enantiopure ligands ((+)/(-)-Htfacam) and bipyrimidine (bipym). The structure and chirality of these complexes have been supported by single-crystal X-Ray diffraction and circular dichroism. The study of the magnetic properties of the GdIII complexes revealed a very weak antiferromagnetic interaction between the GdIII ions through the bipyrimidine bridging ligand. Ab initio CASSCF calculations indicated that the ground Kramers doublet (KD) of both DyIII centers is almost purely axial with the anisotropy axis located close to the two tfacam-ligands at opposite sides of each DyIIIatom, which create an axial crystal field. In keeping with this, ac dynamic measurements indicated slow relaxation of the magnetization at zero field with U eff = 55.1 K, a pre-exponential factor of τo = 2.17·10-6 s and τQTM = 8 µs. When an optimal dc field of 0.1 T is applied, QTM is quenched and U eff increases to 75.9 K with τo = 6.16 × 10-7 s. The DyN2O8 coordination spheres and SMM properties of [(µ-bipym){((+)-tfacam)3Ln}2] and their achiral [(Dy(ß-diketonate)3)2(µ-bpym)]analogous have been compared and a magneto-structural correlation has been established, which has been supported by theoretical calculations. In contrast to the GdIII compounds, the magnetic exchange interaction between the DyIII ions has been calculated to be very weak and, generally, ferromagnetic in nature. Relaxation mechanisms for [(µ-bipym){((+)-tfacam)3Ln}2] and previously reported analogous have been proposed from ab initio calculations. As the magnetic exchange interaction found to be very weak, the observed magnetization blockade in these systems are primarily dictated by the single ion anisotropy of DyIII ions.
RESUMO
The development of upscalable oxygen evolving electrocatalysts from earth-abundant metals able to operate in neutral or acidic environments and low overpotentials remains a fundamental challenge for the realization of artificial photosynthesis. In this study, we report a highly active phase of heterobimetallic cyanide-bridged electrocatalysts able to promote water oxidation under neutral, basic (pH < 13), and acidic conditions (pH > 1). Cobalt-iron Prussian blue-type thin films, formed by chemical etching of Co(OH)1.0(CO3)0.5·nH2O nanocrystals, yield a dramatic enhancement of the catalytic performance toward oxygen production, when compared with previous reports for analogous materials. Electrochemical, spectroscopic, and structural studies confirm the excellent performance, stability, and corrosion resistance, even when compared with state-of-the-art metal oxide catalysts under moderate overpotentials and in a remarkably large pH range, including acid media where most cost-effective water oxidation catalysts are not useful. The origin of the superior electrocatalytic activity toward water oxidation appears to be in the optimized interfacial matching between catalyst and electrode surface obtained through this fabrication method.
RESUMO
The effect of the support (activated carbon or titanium dioxide) on the catalytic activity and selectivity to nitrogen of Pt-Sn catalysts in nitrate reduction was studied. The effects of the preparation conditions and the Pt:Sn atomic ratio were also evaluated. It was observed that the support plays an important role in nitrate reduction and that different preparation conditions lead to different catalytic activities and selectivities. Generally, the catalysts supported on activated carbon were less active but more selective to nitrogen than those supported on titanium dioxide. The monometallic Pt catalyst is active for nitrate reduction only when supported on titanium dioxide, which is explained by the involvement of the support in the reaction mechanism. The catalysts were characterized by different techniques, and significant changes on metal chemical states were observed for the different preparation conditions used. Only metallic Pt and oxidized Sn were observed at low calcination and reduction temperatures, but some metallic Sn was also present when high temperatures were used, being also possible the formation of Pt-Sn alloys.
