Page 51 - Principles of Catalyst Development
P. 51
STRUCTURE OF CATALYSTS 37
Promotion of the active component may be either structural or
electronic. (67) For metals, there are many well documented examples.
Ammonia synthesis catalysts consist of iron doubly promoted with alumina
and potassium. (2) It was long thought that the role of alumina was to prevent
iron sintering upon reduction and that potassium poisoned acid sites inad-
vertently introduced by the alumina. (68) Now, there are indications the
alumina promotes the iron by stabilizing [111] surface planes which are
500 times more active than other planes. (67) The potassium also doubles as
an electronic modifier by donating electrons from the ionized state. (14) These
improve 7T-bonding of nitrogen, leading to faster dissociation and higher
rates. Similar effects have been reported for hydrogenation of carbon
monoxide on nickel. (67)
Another successful promotion is in catalytic reforming, where addition
of rhenium to platinum leads to vastly improved performance, due to
decreased hydrogenolysis of the hydrocarbons. (J 1) The rhenium functions
perhaps as an electronic promoter, but its exact role is still being debated.
Chemically, the rhenium preferentially bonds with low coordination sites
on crystal plane corners, edges and steps. Since these atoms appear to be
involved with the hydrogenolysis, coke-forming reactions are suppressed. (67)
With oxidic active components, promoter interactions can also be
structural or electronic. A good example is CoMo/ A1 20 3 , discussed in detail
in the next section.
Table 2.5 lists common promoters and their mode of action. In principle,
any additive which enhances or inhibits catalytic functions can be classed
as a promoter.
4. EXAMPLE-HDS CATALYSTS
Here we demonstrate the unique role of all three components in the
case of CoMo/ Al 20 3 catalysts, used in hydrodesulfurization processes.
Many different types of organic sulfur compounds are involved and the
reaction follows a series (15)
+H,
R-S R~ -- R (2.5)
Hydrogenolysis of C-S bonds is followed by hydrogenation. Petroleum
fractions are desulfurized for many reasons; to protect catalysts, to improve
product quality, and to prevent environmental pollution. (69) Feedstocks vary
from light naphthas to heavy residua and require reactors of increasing