Page 132 - Principles of Catalyst Development
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120 CHAPTER 6
out reduction during preparation, where conditions are more uniform.
Pre-reduction for commercial operations has many advantages. Nonunifor-
mity due to impure hydrogen and poor flow control is avoided, start-up
time is decreased, process hydrogen is not needed, and catalyst damage
during reduction is eliminated. There are also benefits, even in laboratory
use. If a catalyst charge is reduced in a large batch and used for separate
testing and characterization, uniformity of the metal dispersion is more
dependable. Unfortunately, high dispersions of metals are often pyrophoric
and burn when exposed to air. They cannot be handled or shipped in the
reduced state but must first be passivated. Passivation is accomplished in
two ways. In the first method, the reduced catalyst charg~ is cooled and
remaining hydrogen removed with a flow of inert gas. Oxygen or air is
introduced into the inert stream at low concentration (less than 1 %) slowly
enough that the exothermic oxidation reaction does not cause hot bands.
Oxidation is restricted to the first few layers of the metal surface, effectively
protecting the bulk from further reaction. The catalyst can be handled safely.
Only a small amount of surface reduction is necessary after loading in the
reactor. Tests show that the original metal dispersions are retained. ([ 781
The second method is based on observations that the cause of pyrophor-
icity is not the reaction of oxygen with nickel but with adsorbed and occluded
hydrogen atoms. (178) Residual hydrogen is removed by heating in an inert
gas at temperatures lO-20°C above the reducing conditions. The catalyst
may then be exposed directly to the atmosphere without burning. Passivation
takes place through adsorption but without the painstaking steps used in
the first method.
Another example of activation is found in hydrodesulfurization proces-
ses. Prepared as dispersed molybdena on alumina and promoted with cobalt
or nickel, these catalysts are sulfided before use. In the plant, this is done
either with a sulfur-containing feed or by pretreatment with CS 2 or H 2S.
For laboratory operations, sulfiding with 10% H2S/ H2 mixtures is sufficient.
Care must be taken not to reduce in H2 prior to sulfiding since the reduced
state is less easily sulfided. Exact conditions, such as temperature, flow, or
time, depend on the properties of the catalyst and the method of preparation,
so that the sulfiding sequence is a factor in establishing activity. (179) Argu-
ments for presulfiding before handling and shipment are also valid, and
this practice is now becoming common in the industry.
6.5. EXTRACTION
Each method discussed so far uses some form of deposition to produce
dispersion. It is also possible to generate porosity by extracting a component