RESUMO
Here, we study the influence of the Pt loading and the particle size of Pb0.25@Ptx/C catalysts on their specific activity toward ethanol oxidation in acid media. High angle annular dark field-scanning transmission electron microscopy and electron energy loss spectroscopy data indicate the formation of Pb0.25@Ptx/C core-shell structures, which are well dispersed on carbon support, with spherical shapes and small particle sizes (2.9-6.6 nm). Cyclic voltammetry experiments confirm characteristic profiles of polycrystalline Pt for Pb0.25@Ptx/C structures. The specific activity of the catalysts toward ethanol oxidation reaction greatly depends on the Pt content on Pb core, and consequently, depends on the size of the nanoparticles. The optimum activity occurs with the lowest Pt load in the shell and smaller particle size. Enhancements in specific activity result from the higher number of nanoparticles available for the ethanol oxidation reaction and the tensile strain effect of Pt atoms on the surface expanded in Pb0.25@Pt0.75/C. The lower activity observed for the catalysts with loads of 35 and 50% wt. (Pb0.25@Pt1.5 and Pb0.25@Pt2.25/C, respectively) in comparison to Pt/C, could be explain by the larger particle sizes obtained at these catalysts. Moreover, the Pb0.25@Pt0.75/C catalyst has high electrochemical stability and should be more stable in direct ethanol fuel cells systems than monolithic Pt catalysts. This is because the Pt shell in Pb0.25@Pt0.75/C exhibits lower chemical potential (p < 0) than at Pt/C and at the other core-shell catalysts studied; thus, reducing its tendency to dissolve. The developed core-shell nanostructure is thus a potential candidate as high-performance anode catalyst for application in direct ethanol fuel cells.
RESUMO
The formation of scales in the petroleum industry, such as those composed of calcium and barium sulfates, may reduce productivity since these sediments can partially or totally obstruct the pipes. The mitigation of these inorganic precipitates can be accomplished by using scale inhibitors or by non-intrusive physical technologies. Here, we investigated the influence of magnetic field on the incrustations of barium sulfate by analyzing the concentration of barium and sulfate ions, the solution flow rate, the capillary tube geometry, and the magnetic field intensity in a homemade experimental unit supported on the monitoring of the dynamic differential pressure. The results show that the saline concentration and the flow rate of the solutions and the geometry of the capillary tube have a significant influence on the dynamics of barium sulfate incrustation. The presence of the magnetic field tends to prolong the induction time of the barium sulfate precipitation. A semi-empirical model was used to describe the effect of the studied variables on the barium sulfate incrustation behavior. The X-ray diffraction data of the precipitated particles analyzed using the Rietveld method suggest that the use of the magnetic field favor the formation of more crystalline particles and with smaller crystallite size than those formed in the absence of a magnetic field. Optical and scanning electron microscopy measurements also corroborate with these findings. The results from this study suggest that magnetic fields can be of interest in practical crystallization processes of barium sulfate and successfully applied to decrease the speed of barium sulfate incrustation in pipelines.