Page 80 - Principles of Catalyst Development
P. 80
CATALYTIC MATERIALS 67
TABLE 4.5. (continued)
Group Element Compound Type Eg,eY
Nb NbO P
Nb 20 S n 3.3
Ta Ta20S n 3.6
(YB Ti TiO P
Ti02 n 3.1
Ti 2O) P 2.2
Zr Zr0 2 n 2.3
Hf Hf0 2 n,p 4.4
IIIB Sc Sc 2O) p,n 2.2
y Y20) n 2.9
La La20) n 2.6
Al I'-AI 2O) n 7.3
Zn ZnO n 3.3
ZnS n 3.6
Mg MgO n,p 8.7
Ca CaO n,p 7.5
Ce Ce02 n
Th Th0 2 n 3.5
overall trend does not follow any specific quantitative correlation. Certainly,
other factors such as dispersion and impurities must influence the results,
but dimensions and symmetry at adsorptive sites are also important.
Refinement beyond mere type of semiconductivity is necessary.
Third, for a given crystal structure, the position of the Fermi level plays
an important role. For example, in three NiO-containing samples in Table
4.6, the influence of doping is clearly evident. Addition of Li+ ions to the
Ni2+ lattice creates more acceptor sites, decreasing the Fermi level and
increasing activity. With Cr + incorporation, donor levels are produced, the
3
Fermi level increases, and activity decreases.
! ...................... /. ! M +
/...
~ CONDUCTION BAND
e __________________________ ; e
M E 11
-D- F 0
'Q o
~-o.-o-o-o-o _0_0.01:)' I
I I
, I
VALENCE BAND I
3 I
10 nm I
Figure 4.16. Electron transfer at the semiconductor surface.