RESUMO
The activity/stability towards the ORR of Pt x Gd/C nanoalloys has been enhanced by controlling the atmosphere during the dealloying process. By minimising the formation of porous nanoarchitectures, the ORR activity is increased, and is accompanied by higher activity retention and attenuation of metal dissolution on cycling to high voltage.
RESUMO
Surface properties of porous aluminosilicates of the MCM-41 type have been tested by immersional calorimetry. Two series of materials, referred to as SiAlxCn, where x is the Si : Al mole ratio and n the chain length of the surfactant template, having (1) x=32 and n=8, 12, 14, 16, 18 and (2) n=14 and x=8, 32, infinity, were used. The results of thermogravimetric analysis on these samples served to evaluate the surface density of hydroxyl groups. This parameter is rather sensitive to the pore size than to the aluminium content in the solid matrix. Based on the experimentally measured enthalpies of immersion in n-heptane, water, and formamide per unit BET specific surface area, estimates could be made of the apolar, Lewis acid, and Lewis base contributions to the total surface enthalpy of MCM-41 materials. The samples studied have a predominant surface acidic character, which is markedly enhanced by incorporating aluminium into the silica matrix. Surface acidity is also modified by changes in the porous structure, although the trends are less noticeable here. Nevertheless, the total surface enthalpy of MCM-41 aluminosilicates appears to be small in comparison with typical inorganic oxides, such as silica or alumina. Copyright 2001 Academic Press.
