Page 359 - Adsorption, Ion Exchange & Catalysis- 2007, Elsevier - Copy

Else_AIEC-INGLE_cH005.qxd 7/12/2006 10:03 AM Page 356
356 5. Catalysis

can be shaped to provide large surface area, the unsupported catalysts. Actiated car- v
bon, silica gel, alumina, titanium dioxide, amorphous aluminosilicates, and zeolites are
widely used as catalyst supports, alumina having a particular place in environmental
v
v
applications. Synthetic zeolites hae been deeloped and used as supports since 1960,
w
because they hae open, well-controlled frameork structures that create intercon-
v
nected cavities. Their structure combined with the fact that the placement of the
catalytic atoms at specific sites in the zeolites is possible makes them ideal for selec-
v
v tie catalysis. Other important perspecties of huge practical importance are connected
to the so-called mesoporous catalysts, exhibiting unique properties and prepared by a
v
reolutionary synthesis method. Rationally designed, both microporous and meso-
porous inorganic catalysts are already facts for many industrial and environmentally
compatible technologies.
• Catalytic a g ents : Mainly metals and metal oxides are used as the catalytically acti e v
components that are dispersed onto the support. The transition group elements and sub-
v
group I are used eely in environmental applications. Ag, Cu, Fe, Ni, their
xtensi
oxides, and precious metals like Pt, Pd, and Rh are a common choice in catalysis.
• Promoters : Promoters are elements or compounds, such as cerium oxide or zirconia,
used as addities to improe the physical or chemical properties of the catalysts. The v v y
arious w ays: can act in v

• maintain the dispersion of the catalytic agents
• improve the thermal stability of the support
• vity enhance the catalytic acti

The most fundamental characteristic of a catalyst is its chemical composition, which is
vity and decisive for its specific usage (Table 5.1). The properties of a catalyst, namely acti
selectivity, hae been discussed in Chapter 2. The physical properties of the catalyst are
v
v
also important for its successful application. They are inestigated by both adsorption
methods and various instrumental techniques derived for estimating their porosity and sur-
face area.
The physical characteristics that are really important to a catalyst are surface area, par-
ticle size distrib and particle densityThese properties hae been eely dis- v xtensi v
ution,
.
T cussed in Section 3.9. In able 5.2, the surf pore v and mean pore radii are olume,
ace area,
presented for some common catalysts.
5.2 BASIC PRINCIPLES OF CATALYSIS

As discussed in Chapter 2, a heterogeneous catalyst accelerates a reaction that has fluid-
phase reactants and products, while itself remaining unchanged by the reaction. In general,
it does this by stabilizing reactie intermediates whose formation would be ener v getically
unfavorable in the fluid phase. The great advantage of heterogeneous catalysis over homo-
genous catalysis is that since the catalyst is in a separate phase from the reacting mixture,