Page 49 - Principles of Catalyst Development

STRUCTURE OF CATALYSTS 35
TABLE 2.4. Ethane Hydrogenolysis with
Different Supports a

Rate
[moles m- 2 hr-'(metal) x 10 6 ]

Support 10% Ni 10% Co
lSI 230
Si0 2
57 185
AI 20 3
7 10
Si0 2-AI 20 3
" Reference 58.

species is generated on a metal site and then migrates over the support to
other sites where further reaction occurs. (62)
The most favored explanation is electron transfer, in which electrons
are moved from or to the active component due to electron donating or
accepting sites on the support. Electron density in the active component
changes much like in alloys. This effect was observed much more dramati-
cally in the case of reducible supports such as Ti0 2 , which provide a large
enough concentration of altervalent ions for electron transfer. However,
the effect has not been conclusively proven. (63)

2.3.3. Promoters

A promoter is some third agent which when added, often in small
amounts, results in desirable activity, selectivity or stability effects. It is
perhaps in this regard that catalysis still deserves the term "black art" and
the catalyst designer "alchemist". Promoters are like spices in cooking, a
pinch is added here and there because we know it makes the recipe taste
better. Indeed, as in cooking, many promotional effects are discovered
accidentally. We then glorify them with "scientific" explanations. There is
much work to be done in this part of catalysis.
Promoters are designed to assist either the support or the active com-
ponent. One important example of support promotion is control of stability.
Support oxides may occur in several different phases, some undesirable.
With AI2 0 J , for example, the preferred phase is 'Y- AI 2 0 J • (64) A defect spinel,
this phase has high surface area, a certain degree of acidity, and forms solid
solutions with transition oxides such as NiO and CoO. When heated,
'Y-AI203 transforms into a-AI 20 3 , which has an hexagonal structure and
low surface area. The transition begins measurably at about 900°C, a
temperature not usually encountered during process conditions but possible