Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

Figure 2:

Pt particle size distribution

from Tests 1

(left) and 2 (right) for

AR and PO specimens.

3.4. Effect of environment on the

Pt deposition behaviour

The Pt concentrations from two tests in reducing

(Test 1) and oxidising (Test 2) environments are

compared in Table 3. The Pt concentration was

found to be relatively homogenous over the 120

measurement points across the front and back

side of each specimen. The Pt loading presented in

Table 3 is normalised for a total Pt injection of 660

µg, which is taken from PSI’s standard reference

test [13]. For a better comparability of the Pt loading

results the normalising was necessary, as there

is a small but relevant difference in total amount of

Pt injected during both tests.

The Pt loading of the specimens from both tests

are in the same range, whereas the AR specimens

show lower Pt concentrations than the PO ones.

This might be explained by the higher density of

bigger oxide crystals and thus rougher and larger

surface of the PO specimens offering more «traps»

for the Pt particles to be attached to the surface.

Looking at the Pt particle distribution, a more

homogeneous distribution is observed under

reducing conditions, both on AR and PO specimens.

Figure 2 shows the Pt particle size distribution

of AR and PO specimens from both tests. The

particle sizes were calculated using SEM pictures

and ImageJ software [14] by thresholding the

matrix background and by increasing the contrast

of the particles. A clear difference in the particle

size distribution can be seen comparing both tests.

The average Pt particle size from both, AR and PO

specimens, was found to be around 9 ± 4 nm in

Test 1 and 30 ± 6 nm in Test 2.

Therefore, it seems that the O 2 and H 2 contents

in the feed water have a strong influence on the

Pt particle size and its distribution, but not on the

total Pt surface concentration. The reducing environment

leads to finer Pt particles and homogenous

distribution, whereas the oxidising environment

leads to larger Pt particles and inhomogeneous

distribution.

3.5. Effect of Pt injection rate on

the Pt deposition behaviour

To study the effect of Pt injection rate several tests

with different Pt injection rates but otherwise comparable

parameters have been performed. From

the data displayed in Figure 3, it is easy to observe

that the increase in Pt injection rate is paralleled

by an increase in average particle size. This can

be explained by the fact that at a lower injection

rate the Pt concentration in the high-temperature

water is lower thus preventing the particles from

growing too much by agglomeration after the

nucleation phase [15]. Above a Pt injection rate

of about 4 µg/h the average particle size seems

to remain constant at around 16 nm, whereas a

minimum size of about 2 nm was observed at the

lowest investigated rates. The average surface Pt

concentration is not affected by the injection rate

if the total amount of injected Pt is the same (Table

3). Changing the environment to more oxidising or

reducing conditions can shift the Pt particle size to

higher or lower values, respectively (Figure 3).

Figure 3:

Average Pt particle

sizes from tests with

different Pt injection

rates.

ENSI Erfahrungs- und Forschungsbericht 2012 143

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