To

Fig.2. Typical In-situ X-ray diffractograms of alloy-electrocatalysts during the anodic polarization (240°C, 0.8 mh/cm2. ca. 0.9V )

Fig.2. Typical In-situ X-ray diffractograms of alloy-electrocatalysts during the anodic polarization (240°C, 0.8 mh/cm2. ca. 0.9V )

Effecl of temperature and potential for corrosion of the alloy-electrocalalysts —Considering that the main peak between 36° and 44° of the corroded electrocatalyst consisted of tiiree components, PtfCo'Ni], Pt3[Co*Ni] and Pt, the observed peak was deconvoluted into three peaks. In order to evaluate the changes in the composition of the alloy during the corrosion process, the relative integrated intensity of each component was determined. The crystallite size for each component was also calculated using Scherrer's equation (Tablel). As shown in Fig.3, the relative intensity of PtfCo'Ni] decreased significantly at 200°C by the anode polarization at 0.7 or 0.9V for 4Sh. At 240°C, this original alloy phase was almost disappeared. Concomitantly, a formation of pure Pt was accelerated with raising temperature or potential. On the other hand, the relative intensity of Pt3[Co*Ni] was almost constant independent of the temperature and the potential, indicating that PtjfCo'Ni] phase is stable compared with other alloy phases. This is also supported by the results shown in Table 1 that the crystallite size of PtijCo'Ni] was not so changed during the corrosion process at 200°C or 240°C although it increased to some extent at the initial stage.

took

am as-received

0.9V 200*C

0.9V 240"C

Fig.3 Phase changes in supported alloy particles with the treatment conditions of different temperatures and potentials for 48 hours.

Table 1. Crystal I ¡to size changes in different phases on supported alloy catalysts treated under different temperatures and potentials for 48 hours.

At 200°C, the crystallite size of Pt increased from 72A to 93A,i.e.,1.3tiines, with increasing the potential form 0.7 V to 0.9V.The same increase in the potential (0.7—0.9V) at 240°C resulted in the growth of the Pt crystallite size from 91A to 127 A, i.e.,1.4 times. At a given potential, the crystallite size of Pt at 240°C was larger than that at 200JC. The effect of temperature elevation from 200°C to 240°C at a given potential is approximately equivalent to that of potential from 0.7 V to 0.9 V at a constant temperature for the corrosion process. On the other hand, the crystallite size of Pt[Co'Ni] at

Table 1. Crystal I ¡to size changes in different phases on supported alloy catalysts treated under different temperatures and potentials for 48 hours.

Phase

Crystallite size(A)

as-received

0 0

Post a comment