to other investigators experimental results of a 40 kJ/mol activation energy of water off of
a Pt(1 11) surface [37]. Both fits returned value for average stay time of a water molecule
on the surface of To = 2.5E-15 sec, lies within the range of experimentally calculated
values [36], but is a little shorter than expected for time between molecular escape
attempts off of a surface.
Some researches consider the inverse of the crystal lattice vibrational frequency as a
reasonable estimate of the time between molecular surface escape attempts. Using this
assumption would result in a value of To = 1.OE-13 sec. Using this set value for attempt
time results in the model fits in Figure 6-6.
a).12 1.0 o = 1.0 E-13 sec b)0.12 1.0 *o 1.0 E-13 sec
. 0.10 0.8 Ea= 36.3 kJ/mol 0.10 0.8 Ea (kJ/mol)
c c -o- 35.1
0 0.08 6 0.08 -- 29.6
S0.6 0 -0-- 35.7
8 0.06 8. 0.06 -*- 38.0
C0.4 0.4
o o
0.04- 0.04 "
0.02 02 0.2 0.02
0.02 002 0
0.00 0.00 _
300 320 340 360 380 300 320 340 360 380
temperature (K) temperature (K)
o 123 Pa (01% RH) a 1233Pa(10%RH) a 2837 Pa (23% RH) 4933 Pa (40% RH)
model 123 Pa model 1233 Pa model 2837 Pa model 4933 Pa
Figure 6-6 Model fit of eqn 6.4 to experimental data with a set value of To=lE-13 sec for
a) a universally fit value of Ea, and b) a variable fit of Ea.
In Figure 6-6a a universal value was fit for the activation energy. This returned an
activation energy of Ea = 36.3 kJ/mol. Figure 6-6b shows the comparison between eqn
6.4 and the data for variable fit values for activation energy for each water vapor
pressure. Allowing the activation energy to vary for each vapor pressure (Figure 6-6b)
did return a closer model line and data fit. The average value of these four fits was 34.6