Page 205 - Principles of Catalyst Development

194 CHAPTER 8
reaction of nickel with common supports to produce nickel aluminates and
silicates. Steam forms oxides from cobalt and iron, nitrogen results in
nitrides, and carbon-containing atmospheres give carbides. These are not
surface phenomena, similar to poisoning, but bulk compound formation,
easily identified with x-ray diffraction. (263)

8.3.6. Sintering
Sintering is a well-known phenomenon in metallurgy and ceramic
science. Much of what we know about this topic comes from these
disciplines. (2641 However, special features apply to catalysis because of the
extremely small crystallite sizes, porous supports, reactive atmospheres, and
relatively lower temperatures. As discussed in Chapter I, surface and bulk
atoms become mobile at temperatures of about one-third and one-half of
the melting point, respectively. Since supports are high-melting ceramic
oxides and active components are low-melting, dispersed metals, oxides,
or sulfides, different sintering mechanisms apply to each.
Progressive steps in sintering of a support is shown in Fig. 8.5. Small
gains (50-500 ,urn) with high internal mesoporosity are compressed together
during formulation. Grain boundary flow and necking is part of the process
which imparts strength to the particle. Sintering has already commenced.
At this point, the particle is better described in terms of porosity. Diffusion
of the material occurs first across bridges between small pores, where surface
tension forces are highest. I nitially, small pores collapse leading to decreased

Figure 8.5. Increase of pore size during sintering of the support.