REACTIONS: THERMODYNAMIC ASPECTS, KINETICS, AND CATALYSIS - Biomass as a Sustainable Energy Source for the Future - page 153

2. Ammonia production based on the reaction

N 2 +3H 2 !

2NH 3

From a catalytic point of view, the key factor of this reaction is the chemical
inertia of the N

mol −1

N bond that needs 670,000 J to be broken (vs. the

≡

mol −1 ). Moreover, from a stoichiometric point
of view, for the uncatalyzed reaction, a collision of four molecules (which is not
very probable) is needed. With the use of iron-based catalysts, an activation
energy of 125,000 J

H - H bond that needs 420,000 J

mol −1 is reached.
3. Cracking: Complex organic molecules such as kerosene or heavy hydrocarbons
are broken down into simpler molecules such as light hydrocarbons by the break-
ing of carbon carbon bonds in the precursors. The rate of cracking and the end

products are strongly dependent on the temperature and the presence of catalysts.

-

The catalyzed reaction steps take place very close to the solid surface. These steps may
be between gas molecules adsorbed on the catalyst surface, or the reaction may involve
the topmost atomic layers of the catalyst. The influence of the solid does not effectively
extend more than one atomic diameter into the gas phase, and the direct involvement of
atoms below the topmost catalyst layers is not usually possible (Twigg, 1996). In
Table 5.1, a list of pros and cons of homogeneous and heterogeneous catalysis is given.

5.3.3 Catalyst Deactivation

One of the major problems related to the operation of heterogeneous catalysis is deac-
tivation , the loss of catalyst activity with time-on-stream. This process may be both

TABLE 5.1 Advantages and disadvantages of the use of homogeneous versus
heterogeneous catalysis
Homogeneous catalysis Heterogeneous catalysis

Pros Cons Pros Cons

￿ Fast transport
phenomena

￿ Difficult separation of
catalyst and products

￿ Catalyst and
products easy
to separate

￿ Only the surface
is available

￿ Uniform catalyst ￿ Corrosion (if acid

catalysts are used)

￿ No corrosion ￿ The surface can

be poisoned

￿ High selectivity ￿ Low selectivity at low

temperatures

￿ Possible at any
temperature

￿ Lower
selectivity

￿ Temperature easily
controlled for
exothermic
reactions

￿ High costs for separation
and purification of
catalyst and products

￿ Heat removal
problems