Page 65 - Principles of Catalyst Development

52 CHAPTER 4

It is now known that d-electrons and their orbitals, hybrid and other-
wise, are responsible for the bonding within the metal and at the surface.
The type of bond in the bulk leads to properties such as crystal structure
and dimensions, melting temperature, mechanical strength, magnetic state,
and electrical conductivity. Surface bonds determine adsorption and surface
mechanisms. The ability of a molecule to bond with the surface depends
upon two factors(23 1 : (1) geometric or ensemble, and (2) electronic or ligand.
Much has been learned in recent years about the structures of metal
surfaces,(91) which do not always parallel the crystallography of the bulk
material. (921 Well-defined single-crystal surfaces provide us with an atomic
view that is helpful in deciphering similar structures existing on dispersed
metals. (93,94.95)
Ensemble effects are useful when adsorption requires a special grouping
of surface atoms. To explain this, let us examine the simple example of
ethylene adsorption on nickel, which occurs in a di-adsorbed mode.(96) Two
nickel atoms, the right distance apart, are needed to bond a pair of carbon
atoms. The bonds must be stable, but not too strong or subsequent reaction
is difficult. Figure 4.2 shows symmetry and distances for low index planes
of the face centered cubic nickel surface.
Two bond distances, 0.25 and 0.35 nm, are found. An ethylene molecule
chemisorbed acrosss the 0.25 nm spacing results in Ni-C-C angles of 105°,
close to the tetrahedral 109° and thus fairly stable and strong. On the other
hand, the O.35-nm distance results in an angle of 123°, which produces a

[}05' ~~ '''''I.l'
0 0 0 oE\O

? 035 0

0,25 ,.," 123°0 0
C" 0 0 0
~
0
o o

[100]

o NICKEL

ETHYLENE
~
Figure 4.2. Surface planes of nickel.