Search

This paper reports on oxygen reduction activity and methanol tolerance of PtV/C prepared by a polyol method and the effects of heat treatments in reducing environments at low temperatures (up to 300 °C). The as-prepared catalyst showed a low content of alloyed vanadium, but it was considerably more active for the oxygen reduction reaction than Pt/C and had superior methanol tolerance. Dispersive X-ray absorption spectroscopy measurements around the Pt L edge indicated that the presence of vanadium in the catalyst increased the Pt 5d-band vacancy at the time that it seems to diminish the Pt coverage by adsorbed OH species. Heat treatments at 150, 200, and 300 °C carried out in strongly reducing conditions produced minor changes in the degree of alloying together with a moderate particle growth. The largest incorporation of vanadium in the alloyed phase was observed after heating at 150 °C while a decrease in the alloying and Pt segregation to the surface were observed for higher treatment temperatures. In general, heat treatments were detrimental for oxygen reduction activity, suggesting that the presence of vanadium oxides might play a role. In addition, the decreased in methanol tolerance might be associated to Pt segregation to the surface. [ABSTRACT FROM AUTHOR]

The microstructures of Pt/C and PtM/C (M = Ni, Cu, or Ag) electrocatalysts were studied using X-ray diffraction and transmission electron microscopy (TEM). The electrochemically active surface areas of the prepared materials were estimated by cyclic voltammetry in 1 M HSO. The materials, with metal contents ranging from 30 to 35 wt.%, were synthesized by chemically reducing the metal precursors in water-ethylene glycol solutions. The actual composition of the bimetallic nanoparticles corresponds to a theoretical (1:1) composition for the PtAg/C catalysts, whereas in the PtNi/C and PtCu/C materials, a portion of the alloying component exists in an oxide form. Decreasing the average metallic crystallite sizes from 3.5 to 1.6 nm does not increase the electrochemically active surface area. This apparent contradiction is because a majority of the PtNi and PtCu nanoparticles consist of 2-4 disordered crystallites. In addition, a portion of the PtNi or PtCu nanoparticle surface is covered by nickel or copper oxides, respectively. PtAg nanoparticles, which have a smaller size relative to other bimetallic particles according to the TEM data, are characterized by an intense platinum surface segregation. The agglomeration processes are lowest for the PtAg nanoparticles. [ABSTRACT FROM AUTHOR]