Page 70 - Principles of Catalyst Development

CATALYTIC MATERIALS 57
TABLE 4.2. Orbital Angles on Surface Planes

Plane

Orbital [ 100] [110] [ III]
e. dc' 90° 45° 36°16'
e. d,2_ 1 2 0° 45° 36°16' (2)
12" d" 0° 30° (2) 30°
t2R. d" 45° (2) 30° (2) 30°
12g d" 45°(2) 90° 30°

research is necessary to assign quantitative values for heats of adsorption
of these modes. However, the theory in principle explains (1) different
modes and heats of adsorption, (2) differences in reactivities between planes,
(3) patterns in different metals, and (4) alloy effects.
It is known that heat of adsorption decreases with coverage.(24)
Although there are many explanations, site heterogeneity is most plausible.
Adsorption occurs first on the strongest sites until they saturate, followed
by bonding with the weaker sites. The fact that reaction may occur only on
selective locations complicates the interpretation of areal rates deduced
from measurement of full-coverage chemisorption.
Careful measurements of reactivities on planes have revealed differen-
ces. For examples, Table 4.3 shows such a dependence in ammonia syn-
thesis.( 107)

c::::::>c::> e g (EM PTY)
- _ 129 (FILLED)

TOP VIEW SIDE VIEW
[100]

Figure 4.8. Orbitals for surface atoms.