Page 169 - Principles of Catalyst Development

CATALYST CHARACTERIZATION 157
TABLE 7.6. Results of Difiusiyity Measurement on CoMo/ AI 20 3 HDS Catalysts

wt% C Sg (m 2 g-') (J Deff (cm 2 s-') T dependence

0 270 0.617 0.055 T3/2
8.0 219 0.545 0.049
9.0 180 0.542 0.036
15.3 160 0.297 0.035
19.9 49 0.203 0.028 TI/2

broadened by diffusion as it passes through the particles. Dimensions of
the eluted pulse are used to calculate the van Deempter curve. Measurements
at different velocities then lead to an estimation of D eff • Table 7.6 gives
results of measurements on carbon-containing hydrodesulfurization
catalysts. (218)
The relative change in tortuosity between the fresh and heavily fouled
catalyst is only 0.9, whereas the surface area drops by a factor of 5, diffusivity
by 2. The most revealing result is the temperature dependence, T3/2 for
fresh, TI/2 for fouled, indicating a change from bulk to Knudsen diffusion.
A fouling model was suggested from these results, that is, uniform deposition
on the surface, reducing pore diameters evenly, with no preferential blocking
of the pores.

7.4. SURFACE PROPERTIES

A catalyst is a surface-active agent. Measurement of surface phenomena
in catalysis has occupied scientists since the beginning of research in the
field. Surface chemists have borrowed the techniques of colloid chemistry
to probe the surface with molecules characterized by va.rious absorption
spectroscopies, such as ultraviolet and infrared. Electron microscopy
developed into a sensitive tool leading to breakthroughs in understanding
surface morphology. But during the last two decades, a revolution in surface
technology has occurred. Surface physicists have evolved new generations
of high-technology methods to study composition, structure, and interac-
tions on the surface itself. For the first time in catalysis, we now have the
opportunity to observe surface phenomena.l219.220)
These techniques often involve photon and electron bombardment and
emission, with a vast number of possibilities. A multiple of methods and
acronyms have appeared, bringing confusion for the uninitiated. Clean
well-characterized surfaces are studied under conditions of ultrahigh
vacuum to preserve purity. Research of this type is now beginning to make